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LIBBARY 

OF 


Nee  Bc-ire  fas  est  omnia. 


MADLERS    TELESCOPIC  VIEW   OF    THE    MOON 


POPULAR  LECTURES 


ON 


SCIENCE    AND    ART; 


VOLUME    L 


— > 


POPULAR  LECTURES 


SCIENCE    AND    ART; 


DELIVERED   IN   THE   PRINCIPAL 


CITIES  AND  TOWNS  OF  THE  UNITED  STATES/ 


BY 


DIONYSIUS    LARDNER, 


DOCTOR  OF  CIVIL  LAW,  FELLOW  OF  THE  ROYAL  SOCIETIES  OF  LONDON. AND  EDINBURGH- 
"    OX  THK  ROTAL  IRISH  ACADEMT,  MEMBER  OF  THE  PRINCIPAL  EUROPEAN  SOCIETIES 
FOR  THK  ADVANCEMENT  OF  SCIENCE,  AND  FORMERLY  PROFESSOR  O*  ASTROS- 
OMT  ANE  NATURAL  PHILOSOPHY  IN  THK  UNIVERSITY  OF  LONDON. 


"  The  most  obvious  means  of  elevating  the  people,  is  to  provide  for  them  works  on  popular  and  prac- 
tical science,  freed  from  mathematical  symbols  and  technical  terms,  written  in  simple  and  perspicuous 
language,  and  illustrated  by  facts  and  experiments  which  are  level  to  the  capacity  of  ordinary  minds." 

LONDON  QUARTERLY  REVIEW. 


IN    TWO    VOLUMES. 
VOL.  I. 

FIFTEENTH  EDITION. 

NEW-YORK: 

BLAKE MAN     AND     MASON. 
1859. 


,'UI  IAJU1 


ft 


MA    (IMA 


' 


Entered,  according  to  Act  of  Congress,  in  the  year  1845, 

BY    GREELEY    &    McELRATH, 

Jn  the  Clerk's  Office  of  the  District  Court  of  the  United  States,  in  and  for  the  Southern 
District  of  New  York. 


M  J*  'a'  j<n:  f 


ITEREOTYFXD   BY   REDFIELD   *  SAVAGE, 
13  Chambers  Street,  V.  T. 


PUBLISHERS'   ADVERTISEMENT, 


THE  publishers  announce  that  Dr.  LARDNER,  having  brought  to  a 
close  his  public  lectures  in  this  country,  they  have  availed  themselves 
of  the  opportunity  thus  presented  to  induce  him  to  prepare  for  publica- 
tion the  present  complete  and  authentic  edition  of  these  discourses. 
The  general  interest  which  they  excited  in  every  part  of  this  country  is 
universally  felt  and  acknowledged.  Probably  no  public  lecturer  ever 
continued  for  the  same  length  of  time  to  collect  around  him  so  nu- 
merous audiences.  Nor  has  there  been  any  exception  to  this  favorable 
impression.  Visit  after  visit  has  been  made  to  all  the  chief  cities  ;  and, 
on  every  succeeding  occasion,  audiences  amounting  to  thousands  have 
assembled  to  hear  again  ana'  again  these  lessons  of  useful  knowledge. 
The  same  simplicity  of  language,  perspicuity  of  reasoning,  and  felicity 
of  illustration,  which  rendered  the  oral  discourses  so  universally  ac- 
ceptable, are  preserved  in  these  miscellanies,  which  are,  as  nearly  as 
possible,  identical  with  the  lectures  as  they  were  delivered. 

The  publishers  feel  that  in  these  volumes  they  present  to  the  Ameri- 
can public  a  most  agreeable  offering,  and  an  interesting  and  useful 
miscellany  of  general  information,  which  will  also  afford  that  large  class 
of  persons,  who  have  attended  the  lectures,  an  agreeable  means  of 
reviving  the  impressions  from  which  they  have  already  derived  so  much 
profit  and  pleasure. 

NEW  YORK,  April,  1855. 


THE  PUBLISHERS. 


"  In  prirnis,  honnnis  est  propria  VERI  mquisitio  atque  investigatio.  Itaque  cum  sumum  negotiis  neces- 
sariis,  curis  quo  vacui,  turn  avemus  aliquid  videre,  audire,  ac  dicere,  cognitionemque  rerum,  aut  oc: -It- 
arum  aut  admirabilium,  ad  ben6  beateque  vivendum  necessariam  ducimus  ;  ex  quo  intelligitur,  ^uod 
VERUM,  simplex,  sincerumque  sit,  id  esse  naturae  hominis  aptissimum.  Huic  veri  videndi  curiditati  ai- 
juncta  est  appetitio  quxdam  principals,  ut  remini  parere  animus  beng  a  natura  informatus  velit,  ni.il 
praecipienti,  aut  docenti,  aut  utHitatis  causajustg  et  legitimfe  imperanti:  ex  quo  animi  magnitude  exis;it, 
et  bumanarum  rerum  contemtio." — Cicero,  de  OJficiis,  lib.  1,  <>  13. 

Above  all  things,  man  is  distinguished  by  his  pursuit  and  investigation  of  TRUTH,  and  nen-c,  when  free 
from  needful  business  and  cares,  we  delight  to  see,  to  hear,  and  to  communicate,  and  consider  a.  knowl- 
edge of  many  admirable  and  abstruse  things  necessary  to  the  good  conduct  and  happiness  cf  our  Kves.    . 
Whence  it  is  clear,  that  whatsoever  is  TRUE,  simple,  and  direct,  the  same  is  most  congenial  to  oil'-  na-    ) 
ture  as  men.    Closely  allied  with  this  earnest  longing  to  see  and  know  the  truth,  is  a  kind  of  d'gr.ifed   ) 
and  princely  sentiment  which  forbids  a  mind,  naturally  well  constituted,  to  submit  its  faculties  to  any 
but  those  who  announce  its  precept  and  doctrine,  or  to  yield  obedience  to  apy  orders  but  SMch  as  ars  at 
once  just,  lawful,  and  founded  on  utility     From  this  source  spring  greatness  of  mind  and  contempt  of 
worldly  advantages  and  troubles 


PREFACE. 


IN  presenting  to  the  American  public  the  collection  of  sci- 
entific miscellanies  which  forms  the  contents  of  these  volumes, 
it  may  be  proper  to  explain  the  circumstances  which  gave 
occasion  to  them  in  their  original  form  of  oral  discourses,  the 
character  of  the  audiences  to  which  they  were  addressed,  and 
of  the  readers  to  whose  information  and  amusement  it  is  hoped 
they  may  contribute  in  their  present  more  permanent  state. 

Engaged  for  a  large  portion  of  my  life  in  the  practical  ap- 
plication of  the  physical  sciences  to  the  uses  of  life,  and  more 
especially  to  those  scientific  industries  which  derive  their  effi- 
cacy from  the  agency  of  steam,  I  had  always  looked  forward 
with  the  liveliest  interest  to  a  time  when  I  might  be  enabled 
to  visit  a  country  which  had  taken  so  prominent  a  part  in  the 
advancement  of  these  arts,  and  which  had  formed  from  an 
early  period  so  grand  a  theatre  for  their  development,  as  the 
United  States.  To  the  claims  which  that  country  presented 
to  the  attention  of  every  intelligent  and  inquiring  tourist,  ari- 
sing from  its  important  commercial  relations  with  the  old 


PREFACE. 


1 


world,  from  its  peculiar  political  institutions,  and  from  the 
grandeur  of  its  territorial  extent  and  physical  resources,  I  was 
as  sensible  as  other  travellers.  But  in  addition  to  these,  the 
enterprising  character  of  its  population,  and  the  inventh  3  spirit 
which  so  universally  prevailed  there  in  the  me  shames  and 
physical  arts,  rendered  the  country  which  had  been  tks  cradle  ; 
of  steam  navigation  more  than  commonly  attractive  to  rne.  • 
Had  I,  like  most  tourists,  been  contented  to  have  made  ?,  short 
visit  to  America,  flying  through  the  states  as  fast  as  steam- 
boats and  railways  could  transport  me,  without  remaining  in 
any  one  place  a  sufficient  time  to  see  more  than  the  external 
forms  of  things,  and  scarcely  even  that  I  might  easily  have 
accomplished  my  purpose.  But  these  travellers  were  beacons 
to  warn,  rather  than  examples  to  be  followed.  I  knew  that  it 
were  worse  than  useless  to  cross  the  Atlantic,  until  I  could  do 
so  with  the  power  of  remaining  in  America  for  such  a  time  as 
might  enable  me  to  acquire  a  knowledge  of  its  population, 
their  character  and  habits,  the  physical  features  and  industrial 
resources  of  the  country,  and  the  practical  working  of  its  po- 
litical institutions  in  all  their  various  phases.  The  full  attain- 
ment of  such  an  object  would  require,  not  a  summer's  tour,  or 
a  winter's  residence,  but  a  sojourn  of  several  years,  to  be  judi- 
ciously distributed  among  different  parts  of  that  vast  country 
in  the  proportion  of  their  relative  interest  and  importance. 

Prepared  to  carry  out  these  views,  I  departed  for  America 
in  the  autumn  of  1840,  and  entered  the  splendid  bay  of  New 
York  on  the  evening  of  the  29th  September.  I  determined  to 
divide  the  first  year  of  my  residence  between  the  two  chief 
cities,  New  York  and  Philadelphia.  After  remaining  for  a 
few  days  in  the  former  city,  at  the  Globe  hotel,  I  accordingly 
( -stablishecl  myself  in  Philadelphia,  where  I  remained  for  seven 
months;  after  which  I  removed  to  New  Yorl:,  whore  I  resided 


PREFACE. 


about  the  same  period.  I  now  prepared  to  commence  what 
might  properly  be  called  the  grand  tour  of  the  states ;  and  be- 
ing accompanied  by  my  family,  the  consequent  expenses  of 
travelling  for  so  long  a  time,  and  through  such  distant  coun- 
tries, became  a  subject  of  consideration.  Besides  this  view 
of  my  projected  tour,  another  presented  itself.  Might  I  not 
render  myself  useful  to  the  public,  while  gleaning  information 
from  them  ?  and  in  the  act  of  being  useful  to  them,  might  I 

O  '  O 

not  multiply  and  enlarge  the  means  of  obtaining  the  informa- 
tion of  which  I  was  in  quest  ?  Since  my  arrival,  I  had  often 
been  solicited  to  deliver  in  one  or  other  of  the  chief  cities  pop- 
ular lectures  on  scientific  subjects,  such  as  I  had  occasionally 
given  in  England.  I  had  already  observed  that  the  American 
public  in  New  York  and  Philadelphia  manifested  more  than 
ordinary  taste  for  that  species  of  oral  instruction.  Societies 
under  various  denominations  existed  in  these  cities  and  else- 
where, whose  chief  object  was  to  get  up  weekly  lectures  on 
miscellaneous  and  unconnected  subjects,  delivered  by  various 
individuals  invited  for  the  purpose  by  the  directors  of  such 
societies.  These  lectures,  although  for  the  most  part  since 
discontinued,  were  at  that  time  popular  and  numerously  at- 
tended. The  success  of  these  projects  was  the  more  encour- 
aging when  the  quality  of  the  article  so  greedily  enjoyed  by 
the  public  was  considered.  It  is  true,  that  among  the  numer- 
ous discourses  thus  brought  together  from  all  parts  of  the 
Union,  some  were  found  eminently  possessing  the  qualities 
which  such  discourses  ought  to  have,  and  which  were  well 
deserving  of  success.  But  these  were  like  angels'  visits,  few 
and  far  between — 

Apparent  rari  nantes  in  gurgite  vasto. 

In  general,  the  history  of  such  productions  might  be  thus 


traced :    The  committee  of  the society  of ,  in 

the  state  of ,  having  determined  to  make  up  a  course 

of  weekly  lectures  to  run  through  the  ensuing  season,  send 
applications  to  all  persons  whose  names  they  imagine  will 
prove  attractive  to  their  subscribers.  The  real  fitness  of  the 
individuals  by  their  talents,  acquirements,  or  habits,  to  fulfil 
the  duty  of  a  public  instructor  is  little  regarded.  But  the  title 
of  the  Honorable  A.  B.,  senator  from  the  state  of  C.  D.,  or,  if 
senators  cannot  be  found,  the  Honorable  E.  F.,  member  of  the 
house  of  representatives,  is  regarded  as  a  qualification  of  the 
first  order.  In  any  case  an  honorable  is  mcst  important. 
The  selection  being  made,  a  missive  in  due  form  is  despatched 
by  the  president  of  the  society,  inviting  the  honorable  legisla- 
tor to  deliver  a  lecture  in  the  course  of  the  ensuing  season 

before  the  members  of  the society,  on  such  subject  as 

the  honorable  legislator  may  please  to  select.  To  this  an  an- 
swer arrives  in  due  time,  graciously  accepting  the  proffered 
invitation,  and  informing  the  committee  that  the  subject  on 
which  the  honorable  legislator  will  descant  for  the  edification  t 

of  the  members  of  the society  will,  for  example,  be  the 

life  and  character  of  Dr.  Johnson.  When  the  important  even- 
ing, in  the  fulness  of  time,  arrives,  the  lecturer  is  ushered  in 
solemn  form  by  the  members  of  the  committee  to  the  pulpit, 
where  a  decanter  of  water,  a  glass  goblet,  and  a  pair  of  wax 
candles,  are  duly  provided,  and  the  members  of  the  society 
are  entertained  for  an  hour  and  one  half  with  selections  from 
Boswell's  Life  of  Johnson,  in  the  formation  of  which  the 
use  of  the  scissors  bears  an  unconscionable  ratio  to  that  of 
the  pen. 

Such  was  the  process  by  which  courses  of  lectures  were 
usually  got  up.  Now  and  then,  however,  these  societies 
would  obtain  the  aid  of  one  of  those  self-styled  professors  who 


PREFACE.  11    ; 


made  a  business  of  popular  lecturing.  In  such  cases,  how-  1 
ever,  the  instruction  offered  to  the  audience  was  but  a  shade  ! 
better  than  that  afforded  by  the  amateurs  to  whom  I  have  just 
referred.  The  information  of  these  teachers  is  usually  but 
skin  deep.  Their  study,  if  so  it  can  be  called,  is  made  ex- 
pressly for  their  lectures,  and  the  measure  of  their  own  infor- 
mation is  strictly  limited  by  the  demands  of  their  audience. 
They  have  learned  for  the  occasion  so  much  about  the  matter 
in  hand  as  they  shall  have  to  say,  and  no  more.  Like  certain 
storekeepers  in  Broadway  and  Chesnut-street,  they  exhibit 
their  entire  stock  in  their  windows. 

Although  such  was  the  general  character  of  the  popular 
lectures  given  in  the  chief  cities  at  the  time  to  which  I  refer, 
there  were,  nevertheless,  occasional  exceptions.  Public 
teachers,  eminently  qualified,  were  from  time  to  time  induced 
to  extend  the  benefits  of  their  labors  from  the  professional 
chairs  of  the  universities,  colleges,  and  public  schools,  to  the 
more  mixed  and  popular  assemblies  of  the  literary  societies  of 
the  towns  and  cities  of  the  Union,  or  to  deliver  courses  to 
classes  brought  together  by  the  talents  and  reputation  of  the 
lecturer.  In  such  case,  I  observed  that  the  superior  value  of 
the  instruction  offered  was  duly  appreciated  by  the  public,  and 
that  large  and  attentive  audiences  were  collected,  notwith- 
standing the  unavoidable  imposition  of  a  much  higher  fee  of 
admission. 

Encouraged  by  all  these  circumstances,  I  proceeded  to  pre- 
pare the  necsssary  means  of  illustration  adapted  for  large  and 
popular  audiences,  and  commenced  my  proceedings  by  a 
public  lecture  given  in  the  lecture-room  of  Clinton-Hall,  in 
New  York,  in  November,  1841.  The  result  having  proved  to 
be  successful,  I  removed  to  the  theatre  at  Niblo's  gardens,  \ 
where  an  advantageous  arrangement  was  made  with  the  pro-  ! 


12  PREFACE. 


prietor,  and  the  lectures  were  continued  every  evening-  until 
Christmas.  The  months  of  January  and  February,  1842,  were 
passed,  at  Boston,  where  the  lectures  were  given  at  the  Melo- 
deon  and  at  the  Tremont  theatre.  The  unprecedented  num- 
bers collected  in  the  latter  building  to  attend  the  lectures  will 
not  be  forgotten  by  those  who  were  present  on  these  occa- 
sions, and  the#  afforded  a  satisfactory  proof  that  the  discourses 
delivered  were  adapted  to  the  wants  and  the  tastes  of  the  pop- 
ulation of  that  part  of  the  Union. 

The  reputation  which  this  species  of  entertainment  had  thus 
acquired  now  brought  invitations  from  the  other  chief  cities 
of  the  Union,  and  after  having  passed  the  months  of  January 
and  February  in  Boston,  I  went  to  Philadelphia,  where  dis- 
courses were  delivered  in  the  Chesnut-street  theatre  on  the 
alternate  evenings  during  the  month  of  March. 

Between  this  time  and  the  close  of  the  year  1844,  I  visited 
every  considerable  city  and  town  of  the  Union,  from  Boston  to 
New  Orleans  and  from  New  York  to  St.  Louis.  Most  of  the 
principal  cities  were  twice  visited,  and  several  courses  were 
given  in  Boston,  New  York,  and  Philadelphia.  Nor  did  the 
appetite  for  this  species  of  intellectual  entertainment  appear  to 
flag  by  repetition.  The  audiences  at  Palmo's  theatre,  New 
York,  in  August,  1844,  were  even  more  crowded  than  they 
had  been  at  Niblo's  in  1841 ;  those  in  the  Melodeon  at  Bos- 
ton, in  October,  1844,  were  as  numerous  as  they  had  been  at 
the  Tremont  theatre  in  January,  1842;  and  the  crowds  assem- 
bled in  the  great  saloon  of  the  Philadelphia  museum,  in  De- 
cember, 1843,  and  January,  1844,  were  much  greater  than 
even  the  audiences  of  the  Chesnut-street  theatre,  in  March, 
1842. 

My  purpose  in  mentioning  these  circumstances  is  not  the 
gratification  which  such  results  might  afford  to  my  vanity,  al- 


PREFACE. 


13 


though  I  see  no  reason  why  I  might  not  without  impropriety 
express  the  pleasure  which  they  afforded  to  me.     I  wish  to 
produce  them  as  affording  a  very  striking  characteristic  of  the 
American  people.    \It  was  usual  on  each  evening  to  deliver 
from  two  to  four  of  the  essays  which  compose  the  contents 
of  the  present  volumes,  and  the  duration  of  the  entertainment 
was  from  two  to  three  hours.     On  every  occasion  the  most 
>  profound  interest  was  evinced  on  the  part  of  the  audience,  and 
the  most  unremitting  and  silent  attention  was  given.     These 
assemblies  consisted  of  persons  of  both  sexes  of  every  age, 
from  the  elder  class  of  pupils  in  the  schools  to  their  grand- 
fathers and  grandmothers.     Frequently,  as  at  the  Tremont 
theatre,  at  the   Chesnut-street  theatre  in  1842,  and  at  Pal- 
mo's  (New  York)  in  1844,  the  audiences  amounted  to  twelve 
hundred,  and  sometimes,  as  at  the  Philadelphia  museum  in 
1843,  they  exceeded  two  thousand.     Nor  was  the  manifesta- 
v  tion  of  this  interest  confined,  as  might  be  imagined,  to  the 
'  northern  Atlantic  cities,  where  education  is  known  to  be  at- 
!  tended  to,  and  where,  as  in  New  England,  the  diffusion  of 
{  useful  knowledge  is  regarded  as  a  paramount  duty  of  the 
!  state.     The  same  crowded  assemblages  were  collected  for  a 
long  succession  of  nights  in  the  largest  theatres  of  each  of  the 
southern  and  western  cities- — in  the  Charleston  theatre ;  the 
Mobile  theatre ;  the  St.  Charles  theatre,  New  Orleans ;  the 
Vicksburg  and  Jackson  theatres,  Mississippi ;  the  St.  Louis 
theatre,  Missouri ;    and  in  the  theatres  of  Cincinnati,  Pitts- 
burg,  and  other  western  and  central  cities. 

It  cannot  be  denied,  that  such  facts  are  symptomatic  of  a 

very  remarkable  condition  of  the  public  mind,  more  especially 

j  among  a  people  who  are  admitted  to  be,  more  than  any  other 

j  nation,  engrossed  by  money-getting  and  by  the  more  material 

<  pursuits  of  life.     The  less  pretension  to  eloquence  and  the 


14 


PREFACE. 


attractive  graces  of  oratory  the  lecturer  can  offer,  the  more 
surprising  is  the  result,  and  the  more  creditable  to  the  intelli- 
gence of  the  American  people.  It  is  certain  that  a  similar  in- 
tellectual entertainment,  clogged,  as  it  necessarily  was,  with 
a  pecuniary  condition  of  admission,  would  fail  to  attract  an 
audience  even  in  the  most  polished  and  enlightened  cities  of 
Europe. 

It  is  proper  to  state  here,  that  the  lectures  as  orally  given 
though  similar  in  substance  with  those  which  appear  in  the 
present  volumes,  differed  considerably  in  form  and  expression 
This  must  necessarily  be  the  case.  The  oral  discourses  were 
strictly  extemporaneous,  in  the  only  sense  in  which  didactic 
discourses  ever  are  so.  They  were  delivered  from  the  stage 
of  the  theatre  without  reference  to  any  written  notes  or  mem- 
oranda. The  general  outline  of  the  subject,  the  leading  argu- 
ments, and  the  most  important  illustrations  and  examples, 
alone  were  previously  registered  in  the  memory  of  the  \ 
speaker.  The  language  in  which  these  were  clothed,  and  the 
more  minute  details  of  the  subjects,  arguments,  examples, 
and  illustrations,  were  left  to  the  suggestion  and  inspiration 
of  the  moment.  Nor  was  this  course  adopted  merely  to  save 
labor,  or  from  any  necessity  arising  from  the  over-pressure  of 
business.  It  was  pursued  because  it  was  found,  from  lono 
practical  experience  in  public  lectures,  to  be  the  best.  The 
style  of  the  speaker  is  more  animated  than  it  could  be  when 
the  discourse  is  uttered  verbally  from  memory.  The  mastery 
which  he  has,  or  ought  to  have,  over  his  subject,  and  the  rich 
and  various  stores  of  illustration  on  which  he  draws,  enables 
him  to  adapt  his  mode  of  reasoning  and  style  of  illustration 
to  the  varying  character  and  capacity  of  his  audience,  and 
hence  it  will  happen  often  that  the  same  lecture,  delivered  on 
two  different  occasions  and  to  two  different  audiences,  will  bt 

^"fc--'"w'^'*K-'''1*-'>h*'^-^^»*''*1^''^^  t 


PREFACE. 


15 


given  in  different  language,  style,  and  with  different  illustra- 
tions. Those  who  have  attended  more  'than  once  the  same 
lectures  delivered  by  me,  will  recognise  the  truth  of  this  ob- 
servation. 

But  a  written  didactic  discourse  ought  to  differ  materially 
from  an  oral  discussion  of  the  same  subject.  A  reader  and  a 
hearer  are  placed  under  very  different  conditions.  The  one 
can  proceed  with  such  deliberation  as  the  readiness  or  slow- 
ness of  his  capacity  and  the  greater  or  less  abstruseness  of 
the  subject  may  require.  He  may  retrace  his  steps  as  often 
as  he  may  find  necessary,  returning  again  and  again  on  the 
same  sentences.  The  other  must  catch  the  spirit  and  sense 
as  fast  as  the  words  fall  from  the  lips  of  the  speaker.  The 
style  of  a  written  essay  is  like  that  of  a  cabinet  picture,  that  of 
an  oral  discourse  like  scene  painting.  The  effect  of  the  one  is 
produced  by  elaborate  finish,  that  of  the  other  by  bold  and 
rough  lines  which  seize  the  most  inattentive  and  unskilled 
eye. 

These  distinctions,  however  true  and  important,  are  rarely 
attended  to  by  those  on  whom  the  duty  of  public  instruction 
devolves.  Lectures  accordingly,  even  when  they  proceed 
from  those  who  by  acquirement  are  most  competent  to  in- 
struct, are  often  either  nothing  more  than  demonstrations  of 
scientific  propositions  and  principles,  or  written  discourses, 
generally  read  from  the  manuscript,  or,  as  much  more  rarely 
happens,  committed  to  memory,  and  delivered  verbatim  as 
written. 

The  qualifications  of  a  good  public  lecturer  for  popular  audi- 
ences are  seldom  found  combined  in  the  same  person,  although 
none  of  them  can  be  regarded  as  very  exalted  intellectual 
gifts.  Such  a  teacher  must  above  all  things  possess  a  knowl- 
edge of  his  subject  much  more  profound  than  that  which  he 


16 


PREFACE. 


is  required  to  impart.  He  must  have  a  familiarity  with  all  its 
details,  such  as  can  only  be  acquired  by  long  experience  in 
teaching.  The  same  experience  can  alone  make  him  know 
the  difficulties  of  comprehension  which  his  hearers  will  feel, 
and  render  him  familiar  with  those  means  of  illustration  and 
exposition  which  will  give  him  the  easiest,  surest,  and  most 
expeditious  avenues  to  their  understandings.  He  must  pos- 
sess such  command  of  his  subject,  and  such  fluency  of  lan- 
guage, as  will  render  him  altogether  independent  of  written 
memoranda  or  notes,  and  enable  him  to  speak  directly  from 
his  thoughts  and  his  understanding,  and  not  merely  repeat 
words  which  he  has  previously  committed  to  memory.  Clear- 
ness and  order  must  be  conspicuous  in  his  reasonings,  and  his 
illustrations  must  not  only  be  apposite,  but  adapted  to  the 
character,  capacity,  and  acquirements  of  his  audience.  He 
must  be  endowed  by  nature  with  voice  sufficiently  powerful, 
and  articulation  sufficiently  distinct,  to  render  every  syllable  he 
utters  easily  and  immediately  audible  to  the  most  remote  of 
his  hearers,  and  his  manner  and  appearance  must  be  exempt 
from  any  peculiarities  calculated  to  excite  repugnance.  Such 
a  teacher  will  be  sure  to  command  success  with  a  popular 
audience,  and  his  labors  will  be  beneficial  to  his  hearers  and 
profitable  to  himself. 

That,  in  the  delivery  of  the  lectures  comprised  in  these  vol- 
umes, I  was  enabled  to  present  this  combination  of  qualifica- 
tions I  do  not  pretend  ;  but  I  can  state,  with  perfect  truth,  that 
ever  since  I  commenced  my  duties  as  a  public  teacher,  it  has 
been  my  aim  to  acquire  these  qualifications  to  the  utmost  ex- 
tent to  which  my  natural  gifts  enabled  me  to  attain  them,  and 
it  is  to  the  diligence  with  which  these  endeavors  were  directed, 
and  the  perseverance  with  which  they  were  continued,  that  I 
ascribe  the  success  which  has  attended  my  efforts  as  a  popu- 


W*X^-N^-X^-\ 

PREFACE.  17 


lar  lecturer,  both  in  Europe  and  America.     I  may  therefore  < 
be  allowed  to  express  a  hope,  that  this  statement  may  prove 
useful  to  others  who  may  be  induced  to  adopt  a  like  course 
with  the  same  public  object. 

The  misce  laneous  nature  of  the  contents  of  the  present 
volumes,  and  the  absence  of  any  logical  connexion  or  ar- 
rangement among  them,  render  some  further  explanation  ne- 
cessary respecting  the  mode  in  which  the  lectures  were  given. 
The  audiences  being  composed,  for  the  most  part,  of  persons 
engaged  in  the  pursuits  of  business,  the  exercise  of  profes- 
sions, and  the  other  active  occupations  of  life,  no  regular  cr 
consecutive  attendance  on  any  series  of  lectures  could  be 
looked  for.  Occasional  attendances,  as  leisure,  convenience, 
or  inclination,  might  induce,  were  all  that  could  be  expected. 
It  was,  therefore,  necessary  that  the  discourses  delivered  on 
each  evening  should  be,  as  far  as  possible,  separate  and  inde- 
pendent, intelligible,  useful,  and  entertaining  of  themselves, 
without  reference  to  any  others  previously  given,  so  that  no 
one  might  be  deterred  from  availing  themselves  of  any  one 
evening's  lecture  merely  because  they  had  not  been  enabled 
to  attend  the  preceding  ones.  The  same  consideration  ren- 
dered it  unnecessary  to  observe  any  fixed  order  in  the  subjects 
of  the  lectures.  It  was  usual  to  extend  the  evening's  enter- 

O 

tainment  to  a  length  not  previously  customary  with  public 
lectures.     From  two  and  one  half  to  three  hours  was  not  an 
unusual  length.     This  time,  however,  was  not  devoted  to  a 
single  subject.     A  succession  of  two,  three,  and  sometimes 
four  subjects,  would  often  be  produced,  having  no  connexion 
whatever  with  each  other.     Thus  astronomy,  electricity,  and  I 
the  steam-engine,  would  be  successively  noticed,  short  inter-  j 
vals  of  rest  being  left  between  them,  as  between  the  perform-  I 

ances  in  a  dramatic  theatre.     Unusual  and  unpromising  as 
VOL.  i.— a 


r  as 

•*w«^rNM»"* 


r 

•     i  q  PREFACE. 


J  such  a  project  may  seem  to  have  been,  it  was  nevertheless  j 
j  perfectly  successful,  not  in  one,  or  in  two,  or  in  three  cities, 
\  but  in  every  part  of  the  Union.     This  will  explain   much 
that  might  otherwise  appear  strange  in  the  subject  and  con- 
tents of  these  volumes.     The  miscellaneous  character  of  the 
subjects  discussed— the  rejection  of  all  attempt  at  system- 
atic arrangement — and  the  varying  length  of  the  articles  - 
all  correspond  with  the  lectures  as  they  were  delivered  to  the 

public. 

It  is  scarcely  necessary  to  observe  that  the  same  series  of 
discourses  was  not  given  in  all  places  which  I  visited,  nor 
was  the  entire  collection  contained  in  the  present  volumes 
given  in  any  one  place.  Most  of  these  essays  were,  however, 
on  some  one  or  other  of  my  visits  to  New  York,  Philadelphia, 
and  Boston,  given  in  those  cities. 

A  considerable  number  of  these  essays  were  prepared  ex- 
pressly for  my  lectures,  among  which  may  be  mentioned  all 
those  on  astronomical  subjects,  with  one  or  two  trifling  excep- 
tions, and  several  of  those  on  steam.  The  substance  of  some 
have  been  incorporated  in  one  or  other  of  my  former  works, 
but  in  every  case  they  have  been  more  or  less  modified  and 
adapted  to  their  present  purpose. 

The  object  of  this  miscellany  is  not  to  enlighten  those  who 
devote  themselves  to  the  regularly-disciplined  study  of  those 
sciences  and  arts  which  are  here  so  slightly  and  popularly 
iched.     My  purpose  has  been  to  instruct  and  inform,  and 
at  the  same  time  rationally  to  amuse,  those  who  have  neither 
time,  inclination,  nor  opportunity,  to  cultivate  mathematics,  bj 
(  which  alone  a  strict  professional  knowledge  of  astronomy, 
•(  mechanics,  and  physics,  can  be  acquired.     To  have  attempted 
to  adapt  the  work  to  both  classes — to  those  who  merely  seek 
lor  general  information  on  these  subjects,  without  pursuing 


PREFACE. 


19 


them  through  their  strict  scientific  details,  and  to  those  whose 
object  is  to  obtain  a  profound  knowledge  of  them — would 
have  assuredly  led  to  the  production  of  a  work  which  would 
have  been  useless  to  both  classes.  It  would  have  been  unin- 
telligible to  the  popular  reader,  and  insufficient  for  the  scien- 
tific student. 

Mathematical  reasoning  and  technical  phraseology  have, 
therefore,  been  almost  if  not  altogether  excluded  from  these 
essays.  Instead  of  the  rigid  demonstrations  of  which  the 
propositions  and  principles  are  susceptible  by  the  aid  of  the 
language  and  symbols  of  the  pure  mathematics,  other  proofs 
are  substituted,  expressed  in  ordinary  language,  based  on  or- 
dinary notions,  and  coming  within  ordinary  comprehension. 
Illustrations  which  would  be  inadmissible  in  strictly  scientific 
essays,  are  here  freely  used,  and  even  profusely  resorted  to. 
The  same  position,  where  it  presents  any  difficulty  or  ab- 
struseness,  is  presented  to  the  reader  successively  under  dif- 
ferent aspects,  and  elucidated  by  different  illustrations;  so  that 
understanding,  which  may  not  be  reached  by  one,  will  proba- 
bly be  struck  by  another.  Subjects  also  are  occasionally 
selected  for  discussion,  such,  foj  example,  as  the  plurality  of 
worlds,  which,  though  quite  admissible  here,  would  scarcely 
find  a  fit  place  in  a  strictly  scientific  work. 

Great  pains  have  been  taken  by  me,  and  no  expense  has 
been  spared  by  the  publishers,  in  supplying  these  volumes 
with  instructive  and  useful  diagrams.  Those  which  I  used  in 
my  public  lectures,  have  been  reduced  in  scale,  and  engraved 
for  this  purpose.  The  telescopic  views  of  the  planets  have 
been  taken  from  the  drawings  of  the  observers  of  highest 
reputation;  and  some  of  the  views  of  the  lunar  surface,  copied 
from  Madler's  drawings,  now  appear  for  the  first  time  (so  far 
as  I  am  informed)  in  this  country. 


2Q  PREFACE. 

In  the  lectures  on  the  steam-engine,  I  used  large  sectional 
models  as  illustrations.  In  lieu  of  these,  the  present  vol- 
umes are  illustrated  with  an  extensive  collection  of  plans 
and  sections  of  steam-engines  and  their  various  parts,  made 
on  a  scale  as  large  as  the  size  of  these  pages  admitted. 
Among  these,  may  be  mentioned,  as  more  especially  de- 
serving of  attention,  the  series  of  eight  large  drawings  of 
the  locomotive-engines  of  Messrs.  "  Stephenson  and  Com- 
pany." 

It  may  be  proper  to  observe  here,  that,  as  these  discourses 
were  designed  for  the  use  of  the  general  reader,  the  prac- 
tice I  have  found  beneficial  in  my  lectures,  of  using  round 
numbers  in  preference  to  the  exact  numerical  value,  has 
been  persevered  in.  Round  numbers  have  the  advantage 
of  being  easily  impressed  on  the  memory ;  and  for  the  pur- 
poses of  the  readers  for  whose  use  these  volumes  are  in- 
tended, they  have  all  the  necessary  utility.  Thus,  for  ex- 
ample, the  distance  of  the  earth  from  the  sun  is  generally 
stated  as  a  hundred  millions  of  miles.  This  is  easily  re- 
membered. Nor  is  it  of  any  real  importance  for  the  objects 
of  general  information,  that  the  real  distance  is  more  ex- 

o  ' 

actly  ninety-five  millions  of  miles.  Again,  the  pressure  of 
the  atmosphere  is  a  varying  quantity,  changing  not  only 
daily  and  hourly  everywhere,  but  even  at  the  same  time  dif- 
fering in  different  places.  It  would  be  impossible  to  fix  in 
the  memory  its  average  values  at  each  season  of  the  year, 
and  at  different  places  ;  but  it  is  very  useful  and  satisfac- 
tory to  know  that  it  may  be  assumed  generally  to  be  at  the 
rate  of  about  fifteen  pounds  on  every  square  inch  of  surface 
exposed  to  its  action. 

\      These  volumes  have  been  designed  for  general  informa- 
i  tion  and  amusement,  rather  than  for  the  purposes  of  refer- 


PREFACE.  21 

i  ____ .^ _ 

j  ence  or  systematic  instruction.  Nevertheless,  the  publishers 
i  have  caused  a  copious  index  to  be  made  for  the  work :  the 
j  same  facility  of  reference  is  afforded  as  if  the  usual  order 
!  were  observed  in  the  arrangement  and  classification  of  the 
j  subjects. 

DION.  LARDNER. 

MAY,  1846. 


" 

^ 

( 


.SEVERAL  of  the  lectures  delivered  by  Dr.  LAIV_.TS  ?. : ;  the  r.^y  of 
New  York  were  reported  for  "  The  New  York  T-.ibt.jK,"  and  were 
afterward  published  in  pamphlet  form.  The  last  edWon  of  th^s^  lec- 
tures was  introduced  by  a  "  Sketch  of  the  Prog  ess  of  Physical  Sci- 
ence," written  by  Dr.  THOMAS  THOMSON,  of  London.  The  publishers 
of  this  complete  edition  of  Dr.  LARDNEK'S  lectures  deem  the  following 
extracts  from  that  treatise,  respecting  the  physical  -jc:cv-.ces  of  the  anc'ents, 
an  appropriate  introduction  to  these  volumes  : — 

The  cradle  of  the  human  race  was  beyond  dispute  the  southern  por- 
tion of  Asia  —  a  delightful  climate,  where  the  original  inhabitants  of  the 
earth  first  lived  and  multiplied.  Chaldea  and  India  had  attained  a  high 
degree  of  civilization  long  before  the  Greeks  and  Romans  had  begun  to 
emerge  from  a  state  of  barbarism  ;  but  we  know  comparatively  little  of 
the  attainments  in  science  which  these  nations  had  reached.  We 
are  equally  ignorant  of  the  progress  which  mathematical  and  physical  ( 
inquiries  had  made  in  China — not  one  of  the  treatises  on  mathematics, 
arithmetic,  and  astronomy,  in  the  Chinese  language,  having  been  trans- 
lated into  any  of  the  languages  of  modern  Europe.  But  the  resem- 
blance between  the  Chinese  and  the  ancient  Egyptians  is  so  very  stri- 
king, and  so  complete,  that  it  is  difficult  to  avoid  suspecting  that  they 


;'  24  INTRODUCTION. 

. — 1 


had  a  common  origin.     If  this  were  so,  China,  from  its  contiguity  to 
India  and  Chaldea,  and  from  the  delicious  nature  of  its  climate,  must 
have  been  first  furnished  with  inhabitants.     And  the  Egyptians,  if  ever  j 
they  were  a  colony  of  Chinese,  must  have  been  transplanted  into  Egypt  j 
long  before  the  commencement  of  history.     It  was  from  Egypt  that  the  j 
Greeks  drew  the  first  rudiments  of  their  mathematical  and  physical  sci-  ( 
ence ;  and  the  scientific  acquisitions  of  that  singular  people  constitute 
everything  that  we  know  respecting  the  progress  which  the  ancients  had 
made  in  the  investigation  of  nature. 

From  the  genial  climate  of  the  early  inhabitants  of  the  east,  and  the 
nature  of  the  life  which  they  led,  it  was  natural  to  expect  that  the  mag- 
nificent spectacle  of  the  heavens  would  speedily  attract  their  attention. 
We  are  certain  that  the  Chaldeans  made  astronomical  observations  at 
least  as  early  as  the  twenty-seventh  and  twenty-eighth  years  of  the  era  of 
Nabonasser ;  that  is  to  say,  seven  hundred  and  nineteen  and  seven  hun- 
dred and  twenty  years  before  the  commencement  of  the  Christian  era  : 
for  Ptolemy  makes  use  of  three  observations  of  the  eclipses  of  the  moon, 
which  took  place  during  these  years,  and  which  he  found  in  their  rec- 
ords. Diogenes  Laertius  informs  us  that  the  Egyptians  had  preserved 
in  their  annals  an  account  of  three  hundred  and  seventy-three  eclipses  of 
the  sun,  and  eight  hundred  and  thirty-two  of  the  moon,  which  had  hap- 
pened before  the  arrival  of  Alexander  the  Great  in  their  country.  Now 
these  eclipses  required  between  twelve  hundred  and  thirteen  hundred 
years  to  happen.  Alexander's  visit  to  Egypt  took  place  in  the  year  331 
before  the  Christian  era.  If  we  add  this  number  to  the  length  of  time 
during  which  the  Egyptians  continued  to  observe  the  eclipses  of  the  sun 
and  moon,  we  obtain  sixteen  hundred  and  thirty-one  years  before  the 
commencement  of  the  Christian  era  for  the  period  at  which  the  Egyp- 
tians began  to  record  their  observations.  This  period  is  rather  more 
than  a  century  after  the  death  of  Moses,  and  is  about  twenty-four  years 
before  the  institution  of  the  Olympic  games  ;  constituting  but  a  small 
part  of  the  forty-eight  thousand,  eight  hundred  and  sixty-three  years  du- 
ring which  they  boasted  that  they  had  been  engaged  in  making  astro- 
(  nomical  observations ;  but  this  was  obviously  a  fable,  invented  for  the  pur- 
t  pose  of  raising  themselves  in  the  opinion  of  the  Macedonian  conqueror,  j 


INTRODUCTION.  25 


What  progress  the  Chaldeans  and  Egyptians  had  made  in  astronomy, 
\  it  is  hard  to  say.     They  certainly  had  become  acquainted  with  the  plan- 

>  ets  ;   but  whether  the  Egyptians  had  discovered,  as  Macrobius  assures 
us,  that  Mercury  and  Venus  revolve  round   the   sun,  is  not  so   clear. 
Their  notions  respecting  the   length  of  the  solar  year,  and  the  mean 
length  of  the  lunation,  must  have  been  a  near  approximation  to  the  truth. 
This  is  evident  from  the  famous  Chaldean  period  called  Saros.     It  con- 

!  sisted  of  two  hundred  and  twenty-three  lunar  months,  at  the  end  of 

>  which  the  sun  and  moon  were  in  the  same  situation  with  respect  to  each 
J  other  as  when  the  period  began.     This  period  includes  a  certain  num- 

ber of  eclipses  of  each  luminary,  which  are  repeated  every  saros  in  the 
same  order. 

The  Chaldeans  appear  to  have  divided  the  day  into  twelve  hours,  and 
to  have  constructed  sun-dials  for  pointing  out  the  hour.  The  sun-dial 
of  Ahaz  is  mentioned  in  the  Old  Testament,  on  the  occasion  of  the  re- 
covery of  Hezekiah  ;  but  nothing  is  said  about  its  construction.  Un- 
doubtedly, however,  such  sun-dials  would  require  a  certain  knowledge 
of  gnomonics  —  which,  therefore,  the  Chaldeans  must  have  possessed. 

That  the  Egyptians  had  made  some  progress  in  mathematics  admits 
of  no  doubt,  as  the  Greeks  inform  us  that  they  derived  their  first  knowl- 
edge of  that  branch  of  science  from  the  Egyptian  priests.  But  that  the 
mathematical  knowledge  of  the  people  could  not  have  been  very  exten- 
sive, is  evident  from  the  ecstasy  into  which  Pythagoras  was  thrown 
when  he  discovered  that  the  square  of  the  hypotenuse  of  a  right-angled 
triangle  is  equal  to  the  square  of  the  two  sides  :  for  ignorance  of  this 
very  elementary,  but  important  proposition,  necessarily  implies  very 
little  knowledge  even  of  the  most  elementary  parts  of  mathematics. 

It  was  in  Greece  that  pure  mathematics  first  made  decided  progress. 
P 
1  The  works  of  three  Greek  mathematicians  still  remain,  from  which  we 

I  have  obtained  information  of  all  or  almost  all  the  mathematical  knowl- 

>  ed^e    attained   by  the  Greeks.     These   are   Euclid,  Appolonius,   and 
[  Archimedes. 

Euclid  lived  in  Alexandria  during  the  reign  of  the  first  Ptolemy. 
Nothing  whatever  is  known  respecting  the  place  of  his  nativity  ;  though 
it  is  certain  he  lived  in  Greece,  and  that  he  died  in  Egypt,  after  the 


•    1 

)    -2G  INTRODUCTION. 

j  foundation  of  the  celebrated  Alexandrian  school.  He  collected  all  the  I 
elementary  facts  known  in  mathematics  before  his  time,  and  arranged  j 
them  in  such  an  admirable  order— beginning  with  a  few  simple  axioms, 
and  deducing  from  them  his  demonstrations,  every  subsequent  demon- 
stration depending  on  and  rigidly  deduced  from  those  that  immediately 
precede  it — that  no  subsequent  writer  has  been  able  to  produce  any-  > 
thing  superior  or  even  equal.  His  "Elements"  still  continue  to  be  j 
taught  in  our  schools,  and  could  not  be  dispensed  with,  unless  we  were  ) 
to  give  up  somewhat  of  that  rigor  which  has  been  always  so  much  ad-  ( 
J  mired  in  the  Greek  geometricians.  Perhaps,  however,  we  carry  this  j 
admiration  a  little  too  far.  The  geometrical  axioms  might  be  somewhat  ) 
enlarged,  without  drawing  too  much  upon  the  faith  of  beginners.  And  < 
were  the  method  followed,  considerable  progress  might  be  made  in  ) 
mathematics  without  encountering  some  of  those  difficult  demonstrations  ) 
that  are  apt  to  damp  the  ardor  of  beginners. 

The  elements  of  Euclid  consist  of  thirteen  books.  In  the  first  four 
he  treats  of  the  properties  of  lines,  parallel  lines,  angles,  triangles,  and 
circles.  The  fifth  and  sixth  treat  of  proportions  and  ratios.  The  sev- 
enth, eighth,  ninth,  and  tenth,  treat  of  numbers.  The  eleventh  and 
twelfth  treat  of  solids  ;  and  the  thirteenth  of  solids  :  also  of  certain  pre- 
liminary propositions  about  cutting  lines  in  extreme  and  mean  ratio.  It 
is  the  first  four  books  of  Euclid  chiefly  that  are  studied  by  modern  ge- 
ometricians. The  rest  have  been,  in  a  great  measure,  superseded  by 
more  modern  improvements. 

Appolonius  was  born  at  Perga  in  Pamphylia,  about  the  middle  of  the 
second  century  before  the  Christian  era.  Like  Euclid,  he  repaired  to  ( 
Alexandria,  and  acquired  his  mathematical  knowledge  from  the  succes-  s 
sors  of  that  geometrician.  The  writings  of  Appolonius  were  numerous  , 
and  profound  ;  but  it  is  upon  his  "  Treatise  on  the  Conic  Sections,"  in  \ 
eight  books,  that  his  celebrity  as  a  mathematician  chiefly  depends. 

The  conic  sections,  which,  after  the  circle,  are  the  most  important  of  / 
all  curves,  were  discovered  by  the  mathematicians  of  the  Platonic  school ;  ( 
though  who  the  discoverer  was  is  not  known.  A  considerable  number  j 
of  the  properties  of  these  curves  were  gradually  developed  by  the  Greek  !• 
geometricians.  And  the  first  four  books  of  Appolonius  are  a  collection  < 


f 


INTRODUCTION.          _  27 


of  everything  known  respecting  these  curves  before  his  time.  The  last 
four  books  contain  his  own  discoveries.  In  the  fifth  book  he  treats  of 
the  greatest  and  smallest  lines  which  can  be  drawn  from  each  point  of 
their  circumference,  and  many  other  intricate  questions,  which  required 
the  greatest  sagacity  and  the  most  unremitting  attention  to  investigate. 
The  sixth  book  is  not  very  important  nor  difficult ;  but  the  seventh  con- 
tains many  very  important  problems,  and  points  out  the  singular  analogy 
that  exists  between  the  properties  of  the  various  conic  sections.  The 
eighth  book  has  not  come  down  to  us.  The  fifth,  sixth,  and  seventh 
books,  were  discovered  by  Borelli,  in  Arabic,  in  the  library  of  the  grand- 
duke  of  Tuscany.  He  got  them  translated,  and  published  his  translation, 
with  notes  and  illustrations,  in  the  year  1661.  Dr.  Halley  published  an 
t  edition  of  Appolonius  in  1710,  and  has  supplied  the  eighth  book  from 
the  account  given  by  Pappus  of  the  nature  of  its  contents. 

Archimedes  was,  beyond  dispute,  the  greatest  mathematician  that  an- 
tiquity produced.  He  was  born  in  Sicily,  about  the  year  2S7  before  the 
Christian  era,  and  is  said  to  have  been  a  relation  of  Hiero,  king  of  Syr- 
acuse. So  ardent  a  cultivator  was  he  of  the  mathematics,  that  he  was 
accustomed  to  spend  whole  days  in  the  deepest  investigations,  and  was 
wont  to  neglect  his  food,  and  forget  his  ordinary  meals,  till  his  attention 
was  called  to  them  by  the  care  of  his  domestics.  His  studies  were  par- 
ticularly directed  to  the  measurement  of  curvilinear  spaces  ;  and  he  in- 
vented a  most  ingenious  method  of  performing  such  measurement,  well 
known  by  the  name  of  the  "  Method  of  Exhaustions." 

When  it  is  required  to  measure  the  space  bounded  bv  curve  lines, 
the  length  of  a  curve,  or  the  solid  bounded  by  curve  surface.1  he  inves- 
tigation does  not  fall  within  the  range  of  elementary  geoiT.—1 -y.  Recti- 
linear figures  are  compared  on  the  same  principle  as  superposition  ;  but 
this  principle  can  not  be  applied  to  curvilinear  figures.  It  occurred  to 
Archimedes,  that,  by  inscribing  a  rectilinear  figure  within,  and  another 
without  the  figures,  two  limits  would  be  obtained,  the  one  greater  and 
the  other  smaller  than  the  area  required.  It  was  evident  that,  by  in- 
creasing the  number,  and  diminishing  the  sides  of  these  figures,  these 
two  limits  were  made  continually  to  approach  each  other.  Thus  they 
came  nearer  and  nearer  to  the  curve  area  which  was  intermediate  be- 


28  INTRODUCTION. 


tween  them.  He  observed,  by  thus  increasing  the  number  of  sides  for  a  ? 
great  number  of  times  successively,  that  he  approached  a  certain  as- 
signable rectilinear  area,  and  could  come  nearer  to  it  than  any  difference 
how  small  soever.  It  was  evident  that  this  rectilinear  area  was  the 
real  size  of  the  curvilinear  area  to  be  measured.  It  was  in  this  way  that 
he  found  that  two  thirds  the  rectangle  under  the  abscissa  and  crdinate 
of  a  parabola,  is  equal  to  the  area  contained  by  the  abscissa  and  ordi- 
nate,  and  that  part  of  the  circumference  of  the  parabola  lying  between 
them.  In  the  same  way  he  obtained  an  approximate  measure  of  the 
area  of  the  circle,  demonstrating  that  if  the  radius  be  unity,  the  circum- 
ference is  less  than  3}§,  and  greater  than  3}f.  His  two  books  on  the 
sphere  and  cylinder  were  conducted  by  a  similar  method  of  reasoning. 
He  measures  the  surface  and  solidity  of  these  bodies,  and  terminates  his 
treatise  by  demonstrating  that  the  sphere  (both  in  surface  and  solidity)  is 
two  thirds  of  the  circumscribed  cylinder. 

In  the  same  spirit  his  "  Treatise  on  Conoids  and  Spheroids"  was 
conducted.  These  names  he  gave  to  solids  formed  by  the  revolutions 
of  the  conic  sections  round  their  axis.  We  pass  over  his  researches  on 
the  "  Spiral  el  Archimedes,"  as  it  is  usually  called,  though  in  reality  dis- 
covered by  Conon,  one  of  his  friends  ;  but  must  notice  the  treatise  enti- 
tled "  Psany  nites,"  or  "  Arenarius."  Some  persons  had  affirmed  that 
no  number,  however  great,  was  sufficient  to  express  the  number  of 
grains  of  sand  situated  on  the  seashore.  This  induced  Archimedes  to 
write  his  treatise,  in  which  he  demonstrated  that  the  fiftieth  term  of  a 
ducuple  increasing  progression  is  more  than  sufficient  to  express  all  the 
grains  of  sand  contained  in  a  sphere,  having  for  its  diameter  the  distance 
bf> \ween  the  earth  and  the  sun,  and  totally  filled  with  grains  rf  sand. 
7  :ie  treatise  is  short,  but  abstruse,  in  consequence  of  its  imperfect  method 
o  expressing  numbers  employed  by  the  Greeks.  Were  our  figures 
?  bstituted  for  the  Greeks  letters,  the  reasoning  would  be  sufficiently 
.<  nple  and  clear. 

Archimedes  did  not  confine  himself  to  pure  mathematics :  he  turned 
his  attention  likewise  to  mechanics,  and  may  in  some  measure  be  con- 
sidered as  the  founder  of  that  important  branch  of  physical  science.     He  $ 
first  laid  down  the  true  principles  of  statics  and  hydrostatics.     The  fqr-  / 


INTRODUCTION.  29 


mer  he  treats  in  his  work  entitled  "  Isorropica,"  or  "  De  Equiponderan- 
tibus."  His  statics  are  founded  on  the  ingenious  idea  of  the  centre  of 
gravity,  which  he  first  conceived,  and  which  has  been  so  advantageously 
employed  by  modern  writers  on  statics.  By  means  of  this  principle, 
and  a  few  simple  axioms,  he  demonstrates  the  reciprocity  of  the  weight, 
and  the  distance  in  the  lever  and  in  balances,  with  unequal  arms.  He 
determined  the  centre  of  gravity  of  various  figures,  particularly  of  the 
parabola,  with  great  ingenuity. 

His  discoveries  in  hydrostatics  were  the  consequence  of  a  query  put 
to  him  by  King  Hiero.  This  monarch  had  given  a  certain  quantity  of 
gold  to  a  jeweller,  to  fabricate  a  crowh,  and  he  suspected  that  the  artist 
had  purloined  a  portion  of  the  gold,  and  substituted  silver  in  its  place. 
Archimedes  was  requested  to  point  out  a  method  of  determining  how 
much  gold  had  been  purloined,  and  how  much  silver  substituted.  The 
method,  it  is  said,  occurred  to  him  all  at  once,  while  in  the  bath ;  and  he 
was  so  transported  with  joy,  that  he  ran  naked  through  the  streets  of 
Syracuse,  crying  out,  tvpnxa,  ev^a,  —  "I  have  found  it!  I  have  found 
it !"  The  discovery  with  which  he  was  so  deservedly  delighted  was 
this  :  "  Every  body  plunged  into  a  fluid  loses  as  much  of  its  weight  as 
is  equal  to  the  weight  of  a  quantity  of  the  fluid  equal  in  bulk  to  the  body 
plunged  in."  This  discovery  furnished  him  with  the  method  of  deter- 
mining the  specific  gravity  of  pure  gold  and  pure  silver.  These  being 
known,  he  had  only  to  take  the  specific  gravity  of  the  crown,  which 
(supposing  no  alteration  in  volume  when  the  two  metals  are  melted  to- 
gether) would  enable  him  to  discover  how  much  gold  and  how  much 
silver  it  contained. 

The  first  principle  being  known,  Archimedes  deducted  from  it  various 
other  well-known  hydrostatical  principles,  which  he  consigned  in  the 
first  book  of  his  treatise  "  de  Incidentibus  in  Fluido."  The  second 
book  of  that  treatise  is  occupied  with  various  difficult  questions  respect- 
ing the  situation  and  stability  of  certain  bodies  immersed  in  a  fluid. 

The  ancients  ascribe  to  him  the  invention  of  forty  remarkable  me- 
chanical contrivances  ;  but  nothing  more  than  some  obscure  notices  of 
two  or  three  of  them  have  come  down  to  us.  His  sphere,  a  machine 
by  which  he  represented  the  movements  of  the  stars  and  planets,  is  one 


— — — / 

t 

of  the  most  celebrated.     It  has  been  noticed  by  grave  philosophers,  and  j 

sung  by  poets,  as  may  be  seen  in  the  following  epigram  of  Claudian  : —  ; 

; 

"Jupiter,  in  parvo  com  cerneret  ffithera  vitro, 

Risit,  et  ad  snperos  talia  verba  dedit : 
Hnccine  mortalis  progressa  potentia  cur® 
Ecce  Syracusii  ludimar  arte  eenis." 

Archimedes  wrote  a  description  of  this  machine,  under  the  name  of 
"  Sphaeropffiia ;"  but  it  is  lost,  and  with  it  everything  respecting  the  na- 
ture of  the  sphere  has  perished. 

The  burning  mirrors,  by  which  he  is  said  to  have  set  fire  to  the  Ro- 
man vessels  in  the  harbor  of  Syracuse,  were  long  considered  as  fabulous. 
But  BufFon  showed  how,  by  placing  a  number  of  small  mirrors  so  that 
every  one  of  them  should  reflect  the  image  of  the  sun  to  the  same  point, 
heat  enough  might  be  produced  to  kindle  wood  at  the  distance  of  one 
hundred  and  forty  feet. 

The  protracted  defence  of  Syracuse  against  the  Romans,  chiefly  in 
consequence  of  the  wonderful  mechanical  inventions  of  Archimedes,  is 
too  well  known  to  be  enlarged  on  here. 

If  we  except  the  discoveries  of  Archimedes  in  statics  and  hydrostatics, 
hardly  any  other  branch  of  physical  science  was  much  cultivated  by  the 
ancients.  They  have  made,  indeed,  considerable  progress  in  the  knowl- 
edge of  acoustics,  so  far  as  music  is  concerned.  In  optics  they  can 
scarcely  be  said  to  have  made  any  progress  of  consequence  ;  and,  in  as- 
tronomy, very  little  till  the  time  of  Hipparchus,  who  may  be  considered 
as,  in  some  measure,  the  founder  of  that  sublime  science. 

Dr.  Thomson  lays  down  two  methods  by  which  the  physical  sciences 
are  advanced :  observation  and  experiment ;  and  the  application  of  math- 
ematical reasoning  to  deduce  new  facts  from  principles  already  estab- 
lished. We  give  his  remarks  on  observation  and  experiment,  in  which 
he  exhibits  an  analysis  of  the  theory  of  Bacon  on  this  subject : — 

It  was  not  to  be  expected  that  mankind  should  at  first  make  any  rapid 

progress  in  investigating  the  laws  which  regulate  the  changes  that  take 

place  in  the  material  world.     The  objects  were  too  numerous  and  too 

varied,  and  escaped  his  attention  by  their  very  regularity.     Everywhere 

?  in  the  early  ages  of  the  world,  we  meet  with  descriptions  of  prodigies 


INTRODUCTION.  .   31 


>  and  wonders,  while  die  regular  operations  of  nature  scarcely  attracted 
5  attention.     The  method  of  investigating  nature  by  observation  and  ex- 
'  periment  was  scarcely  thought  of,  except  by  two  individuals,  who,  by 
I  means  of  them,  made  some  progress  in  mechanics  and  hydrostatics,  and 
in  astronomy  :  these  were  Archimedes  and  Hipparchus.     The  mechani- 
cal discoveries  of  Archimedes  were  slightly  extended  by  Ctesibius  and 
Hero,  by  Anthemius,  and  by  Pappus  ;  while  the  astronomical  observa- 
tions begun  by  Hipparchus  were  continued  by  Ptolemy. 

But  at  the  revival  of  letters,  in  the  sixteenth  century,  a  spirit  of  obser- 
vation and  inquiry  awoke,  which  nothing  could  damp,  and  men  began  to 
pry  into  the  secrets  of  nature,  by  the  way  of  experiment.  Galileo,  in 
Italy,  and  Gilbert,  in  England,  especially  the  former,  constitute  remark- 
able examples  of  the  successful  investigation  by  experiment.  But  it  was 
Francis  Bacon,  Lord  Verulam,  who  first  investigated  the  laws  according 
to  which  such  experimental  investigations  should  be  conducted,  who 
pointed  out  the  necessity  of  following  these  laws  in  all  attempts  to  ex- 
tend the  physical  sciences,  and  who  foretold  the  brilliant  success  that 
would  one  day  repay  those  who  should  adopt  the  methods  which  he 
pointed  out.  This  he  did  in  his  "  Novum  Organum,"  published  in  the 
early  part  of  the  seventeenth  century. 

Before  laying  down  the  rules  to  be  followed  in  his  new,  or  inductive 
process,  Bacon  enumerated  the  causes  of  error,  which  he  divided  into 
four  sets,  and  distinguished,  according  to  the  fashion  of  the  times,  by  the 
following  fanciful  but  expressive  names: — 

Idols  of  the  tnbe  ; 
Idols  of  the  den  ; 
Idols  of  the  forum ; 
Idols  of  the  theatre. 

The  idvls  of  the  tribe  are  the  causes  of  error,  founded  on  human  na- 
ture in  general.  Thus  all  men  have  a  propensity  to  find  in  nature  a 
greater  degree  of  order,  simplicity,  and  regularity,  than  is  actually  indi- 
cated by  observation.  This  propensity,  usually  distinguished  by  the 
title  of  spirit  of  system,  is  one  of  the  greatest  enemies  to  its  progress  that  \ 
science  has  to  struggle  with. 

The  idols  of  the  den  are  those  that  spring  from  the  peculiar  character 


INTRODUCTION. 


,1  of  v.e  individual.     Each  individual,  according  to  Bacon,  has  his  own 

/  dirk  cave  or  den,  into  which  the  light  is  imperfectly  admitted,  and  in 

}  the  obscurity  of  which  an  idol  lurks,  at  whose  shrine  the  truth  is  often 

>  sacrificed.     Some  minds  are  best  adapted  to  catch  the  differences,  others 

/  the   rsse  nblances   of  things.      Some  proceed   too  rapidly,   others  too 

siowly.     Almost  every  person  has  acquired  a  partiality  for  some  branch 

of  science,  to  which  he  is  prone  to  fashion  and  force  every  other. 

The  idols  of  the  forum  are  those  which  arise  out  of  the  intercourse  of 
society,  and  especially  from  language,  by  means  of  which  men  commu- 
nicate with  each  other.  It  is  well  known  that  words,  in  some  measure, 
govern  thought,  and  that  we  cannot  think  accurately  unless  we  are  able 
to  express  ourselves  accurately.  The  same  word  does  not  convey  the 
same  idea  to  different  persons.  Hence  many  disputes  are  merely  verbal, 
though  the  disputants  may  not  be  aware  of  the  circumstance. 

The  idols  of  the  theatre  are  the  deceptions  which  have  taken  their  rise 
from  the  systems  of  different  schools  of  philosophy.  These  errors  af- 
fected the  philosophy  of  the  ancients  more  than  that  of  the  moderns. 
But  they  are  not  yet  without  their  effect,  and  often  act  powerfully  upon 
individuals  without  their  being  aware  of  their  effect. 

After  an  historical  view  of  science  from  its  dawn  among  the  Greeks 
to  his  own  time,  and  pointing  out  the  little  progress  which  it  had  made, 
in  consequence  of  the  improper  way  in  which  it  had  been  cultivated, 
Bacon  proceeds,  in  his  second  book,  to  point  out  the  true  way  of  ad- 
vancing science  by  induction. 

The  first  object  ought  to  be,  to  prepare  a  history  of  the  phenomena 
to  be  explained,  in  all  their  modifications  and  varieties.  This  history  is 
to  comprehend  not  only  all  such  facts  as  spontaneously  offer  themselves, 
but  all  the  experiments  instituted  for  the  sake  of  discovery,  or  for  any  of 
the  purposes  of  the  useful  arts.  It  ought  to  be  composed  with  great 
care;  the  facts  should  be  accurately  related  and  distinctly  arranged  — 
their  authenticity  carefully  ascertained,  and  those  that  are  doubtful  should 
le  marked  as  uncertain,  with  the  grounds  for  the  judgment  formed. 
Tin*  record  of  facts  Bacon  calls  natural  history. 

The  next  object  is,  a  compaiison  of  the  different  facts,  to  find  out  the 
cause  of  the  phenomenon. 


INTRODUCTION.  33   ' 


The  method  of  induction  here  laid  down  is  applicable  to  all  investi-  ! 
gations  where  experience  is  the  guide,  whether  in  the  moral  or  natural  I 
world. 

It  is  obvious  that  all  facts,  even  supposing  them  truly  and  accurately 
recorded,  are  not  of  equal  value  in  the  discovery  of  truth.  Some  of 
them  show  the  thing  sought  for  in  its  highest  degree,  others  in  its  lowest; 
some  show  it  simple  and  uncombined,  while  others  are  confused  with  a 
variety  of  circumstances.  Some  facts  are  easily  interpreted,  others  are 
very  obscure,  and  are  understood  only  in  consequence  of  the  light  thrown 
on  them  by  the  former.  This  led  Bacon  to  consider  the  comparative 
value  of  facts  as  means  of  discovery.  He  enumerates  twenty-seven  dif- 
ferent species  ;  but  we  shall  satisfy  ourselves  here  with  noticing  a  few  of 
the  most  important  of  them  : — 

1.  Instantice  solitaries  are  examples  of  the  same  quality  existing  in  two 
bodies,  which  have  nothing  else  in  common  ;  or  of  a  quality  differing  in 
two  bodies,  which  are  in  all  other  respects  the  same. 

2.  The  instantice  migrantes  exhibit  some  nature  or  property  of  bodies 
passing  from  one  condition  to   another,  either   from  less  to   greater,  or 

(  from  greater  to  less.     Thus,  glass  while  entire  is  colorless,  but  becomes 
>  white  when  reduced  to  powder. 

3.  The  instantice  ostensivce  show  some  particular  nature  in  its  highest 
state  of  power  or  energy.     In  this  way  the  thermometer  shows  the  ex- 
pansive power  of  heat,  and  the  barometer  the  weight  of  air. 

4.  The  instcntia  analogies  consist  of  facts  between  which  an  anal- 
ogy or'ressmU-r.ce  is  visible  in  some  particulars,  notwithstanding  great 
diversity  in  all  the  rest.     Such   are  the  telescope  and  microscope  in 
works  of  art,  compared  with  the  eye  in  the  works  of  nature. 

5.  The  instantice  crucis   is  the  division   of  this   experimental   logic  | 
which  is  the  most  frequently  resorted  to  in  the  practice  of  inductive  in- 
vestigation.    When,  in  such  an  investigation,  the  understanding  is,  as  it 
were,  placed  in  equilibrio  between  two  or  more  causes,  each  of  which 
accounts  equally  well  for  the   appearances,  so   far  as   they  are  known, 
nothing  remains  but  to  look  out  for  a  fact  which  can  be  explained  by  the  <J 
one  of  these  causes,  and  not  by  the  other.     If  such  a  fact  can  be  fouDd, 
the  uncertainty  is  removed,  and  the  true  cause  becomes  apparent.     Such  ' 

VOLi.  I.— 3 


facts  perform  the  office  of  a  cross,  erected  at  the  meeting  of  two  roads,  to  $ 
direct  the  traveller  which  way  he   is   to  go.     On  this  account,  Bacon  j 
gave  them  the  name  of  instantia  crucis.     Suppose  it  were  inquired  into  ; 
why  metals  become   heavier  when  calcined,  various  explanations  might  \ 
be  conceived.     But  the  cxpcrimentum  crucis  of  Lavoisier  removed  the 
ambiguity.     He  enclosed  a  quantity  of  tin  in  a  large  glass  vessel,  which 
was  hermetically  sealed.     Heat  being  then  applied,  the  tin  melted  and 
was  partly  calcined.     The  process  being  finished,  the  weight  of  the  glass 
and  its  contents  were  found  unchanged.     But  the  glass  being  opened,  a 
quantity  of  air  rushed  in,  amounting  in  weight  to  ten  grains ;  and  the  tin 
was  found  to  have  increased  in  weight  to  ten  grains.     It  was  obvious 
from  this,  that  by  the  calcination  of  the  tin  a  portion  of  the  air  had  been 
absorbed,  which  had  occasioned  the  increase  of  the  weight. 

In  cases  where  an  exyerimentum  crucis  cannot  be  resorted  to,  there  is 
often  a  great  want  of  conclusive  evidence.  This  is  the  case  in  agricul- 
ture, in  medicine,  in  political  economy,  &c.  To  make  one  experiment 
similar  to  another  in  all  respects  but  one,  is  what  the  cxpcrimentum  crucis 
and  the  principle  of  induction  in  general  requires.  But  this,  in  the  sci- 
ences just  named,  can  seldom  be  accomplished.  Hence  the  great  diffi- 
culty of  separating  the  causes,  and  allotting  to  each  its  due  proportion  of 
the  effect.  Men  deceive  themselves  in  consequence  of  this  continually, 
and  think  they  are  reasoning  from  fact  and  experience,  when  in  reality 
they  are  drawing  their  conclusions  from  a  mixture  of  truth  aid  false- 
hood. Facts  so  incorrectly  apprehended  only  serve  to  render  srror 
more  incorrigible. 

Of  the  twenty-seven  classes  into  which  instantia  are  arranged  by 
Bacon,  fifteen  address  themselves  immediately  to  the  understanding  ; 
five  serve  to  correct  or  inform  the  senses  ;  and  seven  to  direct  the  hand 
in  raising  the  superstructure  of  art  on  the  foundation  of  science.  The 
examples  which  we  have  selected  are  from  the  first  of  these  divisions. 
The  other  two  are  of  inferior  importance,  and  may  be  omitted  in  this 
imperfect  summary. 

Such  are  the  rules  laid  down  by  B&con  for  prosecuting  the  sciences  by 
induction.     The  effects  which  were  ultimately  produced  by  the  "  Novum  ' 
i|  Organiun"  must  have  been  very  great.     It  may  be  questioned,  indeed,  > 


INTRODUCTION.  35 


whether  those  who  have  contributed  most  effectually  to  the  advancement  ( 
of  the  sciences,  have  rigidly  adhered  to  Bacon's  rules.  And,  in  gen-  > 
eral,  such  a  rigid  adherence  is  unnecessary  ;  because  so  much  assistance  5 
can,  in  general,  be  derived  from  what  knowledge  has  been  already  ac- 
quired, that  a  rigid  natural  historical  detail  of  all  the  phenomena  becomes 
unnecessary.  It  was  only  in  the  infancy  of  science  that  such  details 
were  requisite.  Boyle  often  draws  them  up  in  his  inquiries  into  the 
cause  of  various  phenomena,  and  his  investigations  were  of  considerable 
use  in  forwarding  those  branches  of  science  which  he  cultivated.  Bacon 
also  was  mistaken  in  conceiving  that,  by  investigation,  mankind  may  be- 
come acquainted  with  the  essences  of  the  powers  and  qualities  residing  in 
bodies.  So  far  as  science  has  hitherto  advanced,  no  one  essence  has 
been  discovered,  either  as  to  matter  or  as  to  any  of  its  more  extensive 
modifications.  Thus  we  are  still  in  doubt  whether  heat  and  electricity 
be  qualities  or  substances.  Yet  we  have  discovered  many  important 
properties  or  laws,  by  means  of  which  heat  and  electricity,  whether 
properties  or  substances,  are  regulated.  And  from  this  knowledge, 
probably,  we  derive  as  much  advantage  as  could  be  obtained  from  a 
complete  knowledge  of  their  essence. 

By  experiment  or  observation  all  the  new  facts  in  every  science  are 
acquired.  By  the  application  of  mathematical  reasoning  to  these  facts, 
they  are  reduced  to  the  requisite  simplicity,  and  the  general  orinciples 
which  regulate  every  particular  science  determined. 


ANALYTICAL    INDEX. 


[NOTE.— The  volumes  are  indicated  by  the  numerical  letters  i.,  ii..  and  the  pages  hy  fi&urei.  A 
few  changes  in  the  pages  of  vol.  i.,  require  the  reader  who  uses  this  index,  to  deduct  6  pages  after 
page  266.  vol.  i.,  and  30  pages  after  page  328,  vol  i.] 


A. 


Action  and  reaction,  ii.  197-204. 

^Epinus,  his  works,  i.  133. 

Air,  elasticity  of,  ii.  41-60;   substance  and 

color  of,  i.  193;  weight  of,  i.  194;  inertia 

of,  i.  195;  impenetrability  of,  i.  196 ;  ii.  31. 
Air,  elasticity  and  compressibility  of,  i.  198 ; 

ii.  31. 

Air-drawn  dagger,  illusion  of,  i.  264. 
Air-pump,  the,  ii.  47-56,  423. 
Alcohol  thermometer,  ii.  138. 
Aldebaran,  ii.  338. 
Ampere  on  electro-magnetism,  ii.  122 ;  his 

theory  of  terrestrial  magnetism,  ii.  125. 
Analysis  of  the  heavens,  ii.  378. 
Anecdote  of  Napoleon,  i.  369. 
Animal  and  vegetable  life  sustained  by  the 

atmosphere,  i.  59. 

Ancient  method  of  directing  lightning,  ii.  99. 
Animalcules,  their  minute  organization,  &c., 

ii.  25. 

Animal  electricity,  i.  364. 
Annual  motion  of  the  earth,  i.  480. 
Annual  variation  of  the  electricity  of  the 

air,  ii.  154. 
Apparatus  for  observing   the  electricity  of 

the  atmosphere,  ii.  149,  150. 
<  Appearance  accompanying  meteors,  i.  460. 
;  Arago  shows  how  comets  may  be  made  to  as- 
sume different  degrees  of  brightness,  i.  517. 
Ara-jo's  observations  on  silent  lightning,  i. 

552;    his  calculation  of  the  quantity  of 

lightning  drawn  down  by  a  conductor,  ii. 

104. 

Arcturus,  ii.  339. 
Arms  and  feet,  motions  and  positions  of,  ii. 

234. 
Arms  of  the  lever,  ii.  247. 


j  Artificial  freezing,  Leslie's  method  of,  ii.  171. 
Artificial  light,  heat  of,  ii.  193. 
Artificial  magnets,  construction  of,  ii.  113. 
Astronomical  and  arithmetical  calculations, 
i.  183. 


Atmosphere,  the,  i.  58-64,  193-202;  limited 
height  of,  i.  198 ;  ordinary  state  of,  ii.  151. 

Atmosphere  of  the  planets,  i.  60;  of  Saturn, 
i.  246 ;  of  Ceres  and  Pallas,  i.  207. 

Atmosphere,  various  states  of  (vide  Atmo- 
spheric Electricity),  ii.  149. 

Atmospheric  air,  i.  193-202. 

Atmospheric  currents  at  Jupiter,  i.  241. 

Atmospheric  electricity,  i.  137;  ii.  149-160. 

Atmospheric  engine  invented  by  Newcomen, 
ii.  411. 

Atmospheric  pressure,  i.  295,  296 ;  probably 
first  discovered  from  the  effects  of  suction 
by  the  mouth,  i.  285 ;  the  pump  cannot 
act  in  the  absence  of  atmospheric  pres- 
sure, 11,  53  ;  effects  of  atmospheric  pres- 
sure at  boiling  point,  ii.  303 ;  upon  the 
boiling  of  water,  ii.  305. 

Atmospheric  tides,  i.  409. 

Atoms,  or  molecules,  ii.  22. 

Atoms,  ultimate,  ii.  26. 

Attraction  and  repulsion  of  electric  cur- 
rents, law  of,  ii.  120. 

Aurora  Borealis,  the,  i.  89-100;  the  effect 
of  atmospheric  electricity,  i.  137. 

Aurora,  phenomenon  of,  noticed  by  the 
ancients,  i.  K)0. 

Auroral  character  of  falling  stars,  i.  98. 


r 

5  38 


ANALYTICAL  INDEX. 


B. 


Ball  lightning,  i.  460,  538,  540,  548. 
Balance  of  torsion,  i.  136. 
Balance-wheel  of  a  watch,  ii.  267. 
Barking  trees,  ii.  78. 
Barometer,  the,  i.  285-304;  how  to  secure 

the  requisites  for  a  good  one,  i.  288;   di- 
agonal barometer,  i.  292;  wheel  barom- 
eter, i.  292. 
Barometer-gauge,   the,   ii.   49;    applied  to 

steam-engine,  ii.  506. 
Barometric  column,  diurnal  variation  of  the, 

i.  410. 

Barton's  piston,  ii.  488,  489. 
Beccaria's  observations  on  electricity,  i.  127. 
Becquerel's  objections  to  Biot's  theory  of 

the  aurora,  i.  97 ;  his  experiments  in  the 

higher  stratum  of  air,  ii.  157. 
Beer  and  Miidler's  telescopic  views  of  the 

moon,  i.  320. 
Beer's  observations  on  the  planet  Mars,  i. 

153. 
Berard's  experiments  on  the  subject  of  the 

radiation  of  heat,  i.  443. 
Bessel's  discovery  of  the  Parallax,  i.  589. 
Binary  stars,  ii.  365. 
Biot's   excursion  to   the   Shetland    isles   to 

observe  the  aurora,  i.  90 ;  his  theory  and 

explanation  of  it,  i.  95. 
Biela's  comet,  i.  425. 
Bituminous  matter  accompanying  a  lightning 

discharge,  i.  551. 
Bladder  burst  by  atmospheric  pressure,  ii. 

52 ;  by  elasticity  of  air.  ii.  52. 
Blinkensop's  patent  locomotive  engine,  ii. 

531 ;  his  patent  for  the  application  of  the 

rack  rail,  ii.  531. 
Blood,  globules  of  the,  ii.  25. 
Blue  sky,  the  cause  of,  i.  194. 
Boiler,  steam,  ii.  496-513. 
Boilers  and  their  appendages,  ii.  407. 
Boiling   points   and  latent    heats   in   other 

liquids  than  water,  ii.  312. 
Boiling,  the  process  of,  ii.  298. 
Bread  panic  in  London,  i.  160. 
Breathing  i.  299. 
Brewster's  investigations  on  the  subject  of 

the  theory  of  colors,  i.  577. 

Broken  planets,  fragments  of,  i.  206. 
Brunton's  self-regulating  furnace,  ii.  513. 
Burning-glass,  ii.  192. 


C. 


Calorific  effects  of  the  srn's  rays,  i.  490. 

Calorific  powers  cf  the  secindary  pile,  i.  377. 

Canton's  experiments  in  eiectric.ty,  :.  130. 

Capstan,  ii.  ?~>1. 

Captive  balloons,  ii.  1C4. 
$   Carriage,  centre  of  pravitv  of  a,  ii.  233. 
j  Cart-.irlght's  engin3,  .:i.  485 ;  hie  piston,  ^87. 
)  Castor,  ii.  342. 

I   Cavendish's  experiment  o.i  the  weights  of 
bodies,  i.  ; 


Celestial  globe,  uses  of,  ii.  342. 

Central  eclipse  of  the  sun,  i.  69,  83. 

Centre  of  gravity,  ij.  221-240;  how  found, 
ii.  223. 

Centrigrade  thermometer,  ii.  138. 

Ceres  discovered  by  Piazzi,  i.  206. 

Ceres  and  Pallas,  magnitude  and  appear- 
ance of,  i.  208. 

Centrifugal  force,  on  what  it  depends,  ii.  466. 

Charged  clouds,  action  of  on  light  bodies, 
i.  607. 

Chemical  action,  effects  of,  discovered  by 
Davy,  i.  372. 

Chemical  changes  operated  by  lightning,  ii. 
65. 

Chemical  combination,  ri.  321. 

Chronometer,  ii.  264 ;  uses  of  the,  i.  569- 
570. 

Clairaut  applies  the  principles  of  gravitation 
to  Halley's  comet,  i.  182;  his  researches, 
i.  182 ;  predicts  the  discovery  of  the  planet 
Herschel,  i.  184. 

Climate  and  temperature  of  places  changed 
by  the  presence  or  absence  of  the  atmo- 
sphere, i.  64. 

Clock,  floral,  i.  56. 

Clouds,  i.  60 ;  ii.  175 ;  character  and  elec- 
tric charge  of,  i.  532. 

Clouds,  luminous,  i.  545,  546. 

Coal,  analysis  of,  ii.  493. 

Cocks  and  valves,  ii.  474. 

Cold  fusion,  Franklin's,  ii.  66. 

Cold,  supposed  rays  of,  i.  453. 

Colors,  theory  of,  i.  575-582. 

Combustion,  i.  334  ;  ii.  321-328,  494 ;  with- 
out flame,  ii.  324;  of  gas  in  flues,  ii.  41'^. 

Combustion  and  combustibles,  supporters  of, 
ii.  323. 

Combination  cf  levers,  ii.  252. 

Comet,  Halley's,  i.  171-190. 

Comets  of  1811  and  1680,  i.  523;  of  1769 
and  1843,  i.  524  ;  of  1844,  i.  527;  motion 
of  comets,  i.  173  ;  how  they  may  be  rec- 
ognised, i.  173. 

Comets'  dimensions  enlarged  as  they  recede 
from  the  source  of  heat,  i.  517. 

Comets,  periodic,  i.  423-134. 

Comets,  physical  constitution  of,  i.  513-52S. 

Common  bellows,  i.  299. 

Comparative  brightness  of  the  stars  in  rela- 
tion to  the  sun,  i.  593. 

Compression  of  steam  without  loss  of  heat, 
effect  of,  ii.  310. 

Composition  and  resolution  of  ftrce,  i.  207- 
218. 

Compressibility,  ii.  29. 

Concave  reflectors,  i.  263. 

Condenser,  the,  discovered  by  Wilkie  and 
^Epinus,  and  perfected  by  Volta,  i.  134; 
ii.  59. 

Condensation,  i.  331. 

Condensation  of  steam  in  the  cylinder,  ii.  4C1. 

Condensation,  separate,  ii.  422. 

Condensing  syringe,  the,  ii.  56». 

Conducting  bodies,  effects  of,  on  lightning, 
ii.  73  ;  protection  afforded  by,  ii.  74. 


ANALYTICAL  INDEX. 


39 


Conduction  of  heat,  i.  333;  ii.  179-184. 

Conducting  powers  of  bodies,  ii.  181. 

Conic  sections,  i.  174. 

Conical  valves,  ii.  475. 

Connecting  rod  and  crank,  ii.  459,  469. 

Constellations,  forms  of,  ii.  332. 

Constellation  Cassiopeia,  ii.  336. 

Consumption  of  steam,  variation  in  the,  ii. 
512. 

Contemplation  of  the  firmament,  and  the 
reflections  thereby  produced,  i.  51. 

Converging  and  diverging  rays  of  light,  ii. 
348. 

Convex  reflector,  i.  263. 

Cooling,  process  of,  by  evaporation,  ii.  172, 
174. 

Cornish  system  of  inspecting  steum-engine, 
ii.  522. 

Corpuscular  theory  of  light,  i.  224,  231. 

Correspondence  betweea  the  tides  and  the 
phases  of  the  moon,  i.  211 ;  between  elec- 
tric and  magnetic  variations,  ii.  155. 

Coulomb  lays  the  foundation  of  electro-stat- 
ics, i.  136. 

Coulomb's  researches  on  artificial  magnets, 
ii.  115. 

Crane   the,  ii.  255,  261. 

Crowbar  and  handspike,  ii.  247. 

Crown  and  bevelled  wheels,  ii.  262. 

Cruikshank's  experiments  in  galvanism,  i. 
37i. 

Cryophoius,  Dr.  Wollaston's,  ii.  174. 

CrvMnllLzHtion  of  salts,  ii.  26. 
aU,  ii.  27. 

Cube,  the,  ii.  224. 

Cupping,  ii.  55. 

Cycles   of  nineteen  years,  i.  417;    of  nine 

years,  i.  419. 
Cylindrical  cock,  the,  ii.  481. 

D. 

Dalton's  law  of  liquids,  ii.  166. 

Damper,  self-regulating,  ii.  513. 

Dampness,  dangerous  effects  of,  ii.  173. 

Dancers,  position  of,  ii.  235. 

Danger  from  lightning  during  storms,  ii. 
101. 

Davy's  researches  on  the  subject  of  galvan- 
ism, i.  371  ;  his  celebrated  Bakerian  lec- 
ture; prize  awarded  him  by  the  French 
academy,  i.  379 ;  discovery  of  the  trans- 
ferring power  of  the  pile  in  chemical  ac- 
tion, i.  379 ;  his  electro-chemical  theory, 
i.  379. 

Day  and  night,  inequalities  of,  i.  485. 

Days  and  nights  of  the  planets,  i.  56. 

Death  of  Prof.  Richmann,  i.  120. 

Deceptive  oral  disk  in  the  horizon,  ii.  91. 

Decomposition  of  water,  i.  370;  of  distilled 
water,  i.  380;  of  potash  and  soda,  i.  385. 

Deluse,  the,  was  it  produced  by  Whiston's 
comet,  i.  429 ;  Mosaic  account  of  the,  ii. 
77. 

Density,  ii.  28;  of  the  earth,  i.  490. 


Description  of  auroras  seen  at  Fort  Enter- 
prise during  the  polar  voyage  of  Captain 
Franklin,  1/99. 

Dew,  ii.  175. 

Diagonal  barometer,  i.  292. 

Dick's  observations  on  the  last  appearance 
of  Halley's  comet,  i.  188. 

Disability,  ii.  29. 

Dilatation  or  expansion,  i.  328. 

Dip  of  the  magnetic  needle,  ii.  113. 

Dipping-needle,  invention  of,  ii.  113. 

Discovery  of  barium,  strontium,  calcium,  and 
magnesium,  i.  395 ;  of  induction  by  Frank- 
lin, i.  131. 

Disk  of  the  sun  concealed  by  the  disk  of  the 
moon,  i.  83. 

Distribution  of  the  electricity  of  the  air,  ii. 
156. 

Diurnal  motion  of  the  earth,  i.  485 ;  of  Ju- 
piter, i.  238. 

Diurnal  rotation,  ii.  332 ;  of  the  electricity 
of  the  atmosphere,  ii.  153. 

Diurnal  variation  of  the  magnetic  needle, 
ii.  115. 

Diverging  and  converging  rays  of  light,  ii. 
348. 

Dog-star,  the,  or  Sirius,  ii.  338. 

Double  stars,'ii.  351,  362,  373. 

Double-acting  engine,  ii.  448,  467,  468. 

Double  suns,  ii.  369. 

Dry  Voltaic  piles,  i.  400 ;  dry  pile  regarded 
as  an  extended  Voltaic  series,  i.  401. 

Dufaye's  experiments  in  electricity,  i.  107. 

Duty  of  a  steam-boiler,  ii.  520. 

Dynamics  and  statics,  ii.  243. 


E. 


Earth,  the,  i.  55,  477-498;  appearance  of, 
as  seen  from  the  moon,  i.  317;  annual 
motion  of,  i.  480 ;  diurnal  motion  of,  i. 
485;  negative  state  of  the,  ii.  156. 

Ebullition,  ii.  297-318. 

Echo,  the  cause  of  rolling  thunder,  i.  554. 

Eclipse,  solar,  how  formed,  i.  69. 

Eclipses,  solar  and  lunar,  i.  79—86. 

Eclipses  of  Jupiter's  moons,  i.  244. 

Ecli  ptic,  the,  whence  it  derives  its  name,  i.  85. 

Ecliptic  limits,  i.  85. 

Effect  of  light  on  the  retina  of  the  eye,  ii. 
347. 

Effects  of  lightning,  ii.  63-82 ;  popular  im- 
pressions of  the  effects  of  thunder,  ii.  78; 

Effects  of  steam,  ii.  400,  401. 

Elastic  force,  water  raised  by,  ii.  53. 

Elastic  and  inelastic  fluids,  ii.  403. 

Elasticity  and  compressibility  of  air,  i^ 
198. 

Elasticity  of  air,  ii.  41-60;  of  fluids,  ii.  32; 
of  steam,  ii.  306;  of  different  gases,  ii. 
404. 

Electricity,  i.  103-140;  resinous  and  vit- 
reous, i.  108;  distribution  of  the  elec- 
tricity of  the  air,  ii.  156. 

Electric  acid,  i.  379. 

Electricity,  atmospheric,  ii.  149-160. 


ANALYTICAL  INDEX. 


Electric  currents  circulating  round  the 
globe,  ii.  121 ;  their  effect  on  animal  and 
vegetable  substances,  i.  386. 

Electrical  experiments  in  Ei-gland  and 
France,  i.  112. 

Electrics  and  non-electrics,  i.  105. 

Electric  kites,  ii.  103,  104. 

Electric  matter,  discharge  of  i'rom  the  sur- 
face cf  the  earth,  ii.  78. 

Electric  and  magnetic  variations,  corres- 
pondence between,  ii.  155. 

Electrical  machine  of  Otto  Guericke,  i.  105. 

Electric  phenomena  observed  by  the  ancients, 
i.  103. 

Electro-chemical  theory,  i.  379. 

Electrical  state  of  the  atmosphere  favorable 
to  the  process  of  barking  trees,  ii.  78. 

Electrized  clouds,  mutual  attraction  and  re- 
pulsion of,  i.  533. 

Electroscope,  Saussure's,  ii.  150. 

Electro-magnetism,  ii.  119—128. 

Electro-statics,  foundation  of  laid  by  Cou- 
lomb, i.  136. 

Embroidery,  gilding  of,  ii.  24,  25. 

Emperor  Augustus's  sealskin  cloak  as  a 
lightning  protector,  ii.  100. 

Encke's  comet,  i.  423. 

Engine-makers,  ii.  520. 

Enumeration  of 


Equator  and  poles,  definition  of,  i.  562. 

Equestrian  feats  explained,  ii.  216. 

Equilibrium,  stable,  unstable,  and  neutral, 
ii.  227,  231. 

Ericsson's  propeller,  i.  275  ;  his  plan  of  con- 
verting a  steamer  into  a  sailing  craft,  or 
a  sailing  vessel  into  a  steamer,  i.  278. 

Errors  of  the  sense  of  feeling,  ii.  86. 

Evaporation,  i.  331 ;  ii.  163-176. 

Evaporation  proportioned  to  horse-power,  ii. 
519. 

Evolution  of  heat  by  compressed  air,  ii.  33. 

Excitability  of  the  London  public,  i.  159. 

Expansive  action  of  steam,  ii.  436. 

Eye,  foramen  or  pupil  of,  i.  54;  structure 
'of,  i.  223. 


f  a^roni-s  experiments  in  galvanism,  i.  355. 

Fahrenheit's  thermometer,  ii.  138. 
\    Fallacies,  popular,  ii.  85-96. 

_7araday's  hypothesis  of  the  aurora,  i.  98; 
Ms    researches  in  electro-magnetism,   ii. 
P       123. 

'   Feats  of  the  fire-kin?  explained,  ii.  90. 
>  Feeders,  self-regulatinjr,  for  steam-boiler,  ii. 
,       504,  505. 

\  Fell  in:,'  timber,  tae  time  for,  i.  502. 
\  Fisure,  ii.  21. 
';  Filtration,  ii.  28. 
j*   Fin'-cscapes,  ii.  273. 

(  First  electric  shocks,  singular  effects  of,  i. 
(        HO. 

\  Fishes,  hrw  Uiey  adhere  to  rocks,  i.  299. 
»-..-^-*-  -* 


Flaccid  bladder  swells  by  the  expansion  of 

air,  ii.  52. 

;  Flame,  effects  of,  i.  138  ;  flame  produced  by 
chemical   combination,  ii.   321;    illumin- 
ating powers  of  flame,  ii.  324. 
Flattering-glass  explained,  i.  265. 
Flat  plate,  the,  ii.  225. 
Flies,  how  they  adhere  to  ceilings,  i.  299. 
Floral  clock,  i.  56. 

Fluids,  elasticity  of,  ii.  32 ;  mechanical  prop- 
erties of,  ii.  402. 
Fly-wheel,  the,  ii.  461. 
Force,  ii.  22;  philosophy  of,  ii.  208;  single 

equivalent  force,  ii.  223. 
Force,  composition  and  resolution  of,  ii.  207- 

218. 

Force  and  weight,  ii.  244. 
Forked  lightning,  i.  538. 
Form  of  the  earth,  i.  477. 
Form  and  structure  of  the  steam-boiler  illus- 
trated, ii.  496. 

Form  and  motion  of  light,  i.  484. 
Form  and  rotation  of  the  sun,  i.  72. 
Forms  of  constellations,  ii.  332. 
Four-way  cock,  ii.  482. 
Fragments  of  broken  planets,  i.  206. 
Franklin's  attention  is  drawn  to  the  subject 
of  electricity,  i.  113;  his  experiments  and 
letters,  i.  114;    his  celebrated  theory  of 
positive  and  negative  electricity,  i.  115; 
analyzes  the  phenomena  of  the   Leyden 
jar,    i.    116;    suggests    the    analogy    and 
probable  identity  of  lightning  and  elec- 
tricity, i.  119;   considered  wild  and  vis- 
ionary  by  the  Royal  society  of  London, 
i.   121 ;    establishes  such   identity  by  his 
memorable  kite  experiment,  i.    122;    his 
right  to  the  discovery  denied  by  M.  Arago, 
i.  123;  his  claim  vindicated,  i.   124;   his 
cold  fusion,  ii.  66. 
Freezing  and  boiling  points,  determination 

of,  ii.  136. 

Freezing  point,  i.  329. 
Friction,  probable  influence  of,  ii.  152. 
Fulcrum,  the,  ii,  247. 
Fulgurites  and  vitrifications,  ii.  67-69. 
Fulminary  tubes,  ii.  67. 
Fusible  plugs,  ii.  511. 
Fusion  and  contraction  of  metals,  ii.  65. 
Fusee  of  a  watch,  ii.  257. 
Fusion,  the  point  of,  ii.  188. 


G. 


Galileo's  observations  of  Jupiter,  i.  243;  his  J 
investigations  on  the  subject  of  stmo-  * 
spheric  pressure,  i.  286. 

Galvanism,  i.  361-402. 

Galvanometer,  or  multiplier,  ii.  124. 

Galvani  an  astronomical  professor  at  Bo- 
losna,  i.  362;  hi?  experiments  on  the  fros, 
i.  262-263;  opposed  by  Volta,  i.  364. 

Gap  in  the  solar  system,  i.  205. 

Gas,  combustion  of  in  flues,  ii.  498. 

Gasometer,  the,  i.  303. 

Gases,  ii.  494. 


r 


ANALYTICAL  INDEX. 


Gauge  to  ascertain  the  level  of  water  in 
steam-boilers,  ii.  502. 

Geographical  surface  of  the  planets,  i.  61. 

Gilbert's  discoveries  in  electricity,  i.  104. 

Glass,  the  cheapest,  but  not  the  best  mate- 
rial for  mirrors,  i.  265. 

Globules  of  the  blood,  ii.  25. 

Governor,  the,  ii.  463. 

Grate  and  ash-pit  for  steam-boiler,  construc- 
tion of,  ii.  499. 

Gravity,  centre  of,  ii.  221-240. 

Great  Bear,  ii.  333. 

Great  comet  seen  in  1456,  i.  178. 

Great  frost  in  London,  i.  166. 

Great  power  of  steam,  ii.  401. 

Green  sea,  the  cause  of,  i.  194. 

Grey's  discoveries  in  electricity,  i.  105. 

Grey  and  Wheeler's  experiments  in  elec- 
tricity, i.  106. 

Grotthus's  hypothesis  of  galvanism,  i.  378. 

Groups  of  the  planets,  inner  and  outer,  i. 
56. 

H. 

Hadley's  sextant,  i.  566. 

Halley's  comet,  i.  171-190;  his  description 
of  a  total  eclipse  of  the  sun,  i.  83  ;  his  re- 
searches on  the  subject  of  comets,  i.  180. 

Harding  discovers  Juno,  i.  206. 

Harris's  explanation  of  ba-11-lightning,  i.  541 ; 
his  lightning-conductors  for  ships,  ii.  104. 


Hawksbee's  experiments  in  electricity,  i.  105. 
Heat,  i.  325-334  ;  radiation  of,  i.  437-456 ; 

heat  evolved  by  compressed  air,  ii.  33  ;  in 

the  process  of  combustion,  ii.  495 ;   con- 
duction of,  ii.  179-184. 
Heat  of  artificial  light,  ii.  193. 
Heat  and  light,  relation  of,  ii.  187-194. 
Heat  lightning,  i.  545. 
Heavens,  how  to  observe  the,  ii.  331-353; 

Herschel's  analysis  of,  ii.  378. 
Hecla,  experiment  with  the,  ii.  565. 
Heights,  measurement  of,  i.  297. 
Hemispheres,  northern  and  southern,  i.  562. 
Hemp-packed  piston,  ii.  484. 
Herschel,  or  Uranus,  its  diameter,  bulk,  and 

distance  from  the  sun,  i.  253. 
Herschel's  observations  of  the  planet  Mars, 

i.  152  ;  his  observations  on  Sirius,  ii.  338 ; 

his  catalogue  of  nebulae,  ii.  392. 
Hish  mountains  on  the  planets  Mercury  and 

Venus,  i.  148. 

)  Hook's  theory  of  combustion,  ii.  327. 
Horse-power  of  steam-engines,  ii.  516. 
How  comets  may  be  recognised,  i.  173. 
Howard's   improvement   in  the   process  of 

sugar-refining,  ii.  170. 
How  to  observe  the  heavens,  ii.  331-353. 
Human  body,  temperature  of  the,  ii.  88. 
Hull's  patent  for  towing  ships  against  wind  | 

and  tide,  ii.  443. 


Humboldt's    observations  of   land-spout  in 

the  Steppes  of  South  America,  i.  600. 
Hunter's  screw,  ii.  291,  292. 
Hunting-cog,  ii.  264. 
Hydrogen  gas  in  coal,  ii.  494. 
Hygrometers,  ii.  168. 


Identity  of  lightning  and  electricity,  i.  119, 

549. 

Illusion  of  the  air-drawn  dagger,  i.  264. 
Image  of  an  object  in  a  plane  reflector,  i. 

262 ;  image  of  the  banks  of  a  lake  or  river, 

i.  265. 

Impediments  to  motion,  ii.  34. 
Impenetrability,  ii.  21 ;  of  air,  i.  196. 
Incandescence,  ii.  188. 
Inclined  plane,  wedge  and  screw,  the,  ii. 

283-294. 

Jmcompressibility  of  liquids,  ii.  32. 
Indicator  invented  by  Watt,  ii.  508. 
Induction  discovered  by  Franklin,  i.  131; 

induction    between  the  clouds   and    the 

earth,  ii.  72. 

Inductive  action  of  lightning,  ii.  71. 
Inequalities  of  day  and  night,  i.  485. 
Inertia  (vide  Action  and  Reaction),  ii.  197 ; 

in  a  single  body,  ii.  198;  consequence  of 

in  two  or  more  bodies,  ii.  199. 
Ink-bottles,  i.  301. 
Ink-bottle,  pneumatic,  ii.  174.     • 
Inundations  from  subterranean  sources,  ii.  77. 
Invention  of  the  Leyden  vial,  i.  110;    of 

lightning  conductors,  i.  125. 
Invisible  rays  of  heat,  i.  439. 
Isolated  clouds  discharge  lightning,  i.  534. 


J. 


Juno  discovered  by  Dr.  Harding,  i.  206. 
Jupiter,  i.  237-244  ;   diurnal  rotation  of,  i. 

238;  belts  and  telescopic  appearance  of, 

i.  239  ;  appearance  of  the  sun  at,  i.  242; 

his  satellites,  i.  243  ;   the  variety  of  his 

months,  i.  244. 

K. 

Kepler  show  a  correspondence  between  the 

tides  and  the  phases  of  the  moon,  i.  211. 
Knee-joint,  effect  of  the,  ii.  234. 


L. 


La  Couronne  des  Tasses,  i.  367. 

Lalande,  i.  183. 

Land-spout  at  Montpellier,  France,  i.  599  ; 

atf  Escalades,  i.  600;  at  Marchefroid,  i. 

601;  Ossonval,  i.  601. 
Laplace's  experiments  in  electricity,  i.  139  ; 

his  nebular  hypothesis,  ii.  395. 
Lardner's  experiments  on  the  Great  Western 

railway  in  England,  ii.  562. 
Latent  heat,  i.  331  ;  of  steam,  ii.  300. 
Lateral  or  divided  discharges  of  lightning, 

ii.  107. 


ANALYTICAL  INDEX. 


Lateral  shock  discovered  by  Dr.Wilson,  i.  1 12. 

Latitude,  parallel  of,  i.  562. 

Latitudes  and  longitudes,  the,  i.  561-572; 
how  determined,  i.  564. 

Lavoisier  and  Laplace's  theory  of  combus- 
tion, i.  138;  ii.  32(5. 

Leather-suckw,  effects  of,  i.  299. 

Lepaute,  Madame,  i.  183. 

Leslie's  differential  thermometer,  i.  444;  his 
method  of  artificial  freezing,  ii.  171. 

Level  of  water  in  a  steam-boiler,  how  indi- 
cated, ii.  502. 

Lever  and  wheelwork,  the,  ii.  243-268. 

Lever,  three  kinds  of  the,  ii.  247 ;  rectangu- 
lar, ii.  250. 

Levers,  combination  of,  ii.  252. 

Lexell's  comet,  causes  of  its  appearance  and 
disappearance,  i.  427. 

Leyden  vial,  invention  of  tho,  i.  110. 

Light,  i.  223-234 ;  velocity  of,  i.  225 ;  waves 
of  measured  by  Newton,  i.  228 ;  a  pencil 
of,  i.  259 ;  light  of  the  sun  three  hundred 
thousand  times  greater  than  that  of  the 
moon,  i.  63;  light  of  comets,  i.  515. 

Light  and  heat,  uniform  supply  of,  i.  53; 
relations  of,  i.  234. 

Light  and  sound,  alliance  between,  i.  230. 

Li  .'iiliiina:,  the  effects  of,  ii.  63-82;  protec- 
tion from,  ii.  99-108;  forked,  zigzag, 
sheet,  and  ball,  i.  538;  rising  from  the 

N  earth  like  a  rocket,  ii.  78,  79 ;  from  the 
ashes,  sm6ke,  and  vapor  of  volcanoes,  i. 
535. 

Lightning  conductors,  i.  125;  ii.  75;  point- 
ed and  blunt,  ii.  104;  for  powder  mag- 
azines, ii.  106. 

Li«h;nin?  and  electricity,  identity  of,  i.  118, 
122,  549. 

Limbs  of  animals  considered  as  levers,  ii. 
248. 

Limited  height  of  the  atmosphere,  i.  198. 

Line  or  lines  of  least  resistance,  ii.  108. 

Liquids  not  absolutely  incompressible,  ii.  32 ; 
non-conductors,  ii.  183. 

Living  body  a  conductor  of  electricity,  ii. 
101. 

Locomotive  engine,  the,  ii.  528;  experimental 
trial  of  on  the  Liverpool  and  Manchester 
railway,  ii.  535;  progressive  improve- 
ment of,  ii.  537 ;  description  of  the  ten- 
der, ii.  543  ;  power  of  the  locomotive,  ii. 
554. 

Load  between  two  bearers,  ii.  251. 

Local  and  periodical  changes  of  the  mag- 
netic variation,  ii.  115. 

London  water  and  air  panic,  i.  160. 

Longitude,  how  determined,  i.  567. 

,ig-glass,   effects  of  the,  analyzed,   i. 
264. 

Leper's  propeller,  i.  278. 

Loss  of  steam-power,  sources  of,  ii.  51S. 

Lottin's  observations  of  the  aurora  at  Bosse- 
kop,  on  the  coast  of  West  Finuiark,  in 
1838-'39,  i.  91. 

Lower  stratum  of  air,  character  of,  ii.  156. 

Luminiferous  ether,  i.  22 1. 


Luminous   coating  of  the  sun,  its  thickness 

measured  by  Herschel,  i.  75. 
Luminous  rain,  ii.  81. 
Luminous  sleet,  ii.  82. 
Luminous  spots  on  the  dark  hemisphere  of 

the  moon,  i.  83. 

Lunar  attraction,  theory  of,  i.  410. 
Lunar  crater,  i.  321. 
Lunar  influences,  i.  501—510. 
Lunar  mountains,  heights  of,  i.  319. 
Lunar  surface,  physical  condition  of,  i.  316 


M. 


Machines,  mechanic  powers  of,  ii.  245. 

Madler's  observations  and  telescopic  views 
of  Mars,  i.  153;  his  telescopic  view  of 
Jupiter,  i.  241. 

Magdeburgh  hemispheres,  the,  ii.  54. 

Magnetic  attraction,  ii.  Ill;  known  to  the 
ancients,  ii.  112;  laws  of  discovered  by 
Coulomb,  ii.  114. 

Magnetic  effects  of  lightning  (vide  electro- 
magnetism),  ii.  122. 

Magnetic  equator,  ii.  116. 

Magnetic  meridian,  ii.  111. 

Magnetic  needle,  dip  of  the,  ii.  113. 

Magnetic  polarity,  ii.  Ill,  112. 

Magnetic  poles,  northern  and  southern,  ii. 
116. 

Magnets,  artificial,  method  of  making,  ii.  114. 

Magnetism,  ii.  111-116;  influence  of  heat 
upon.  ii.  115. 

Magnetism,  electro,  ii.  119-128. 

Magnetizing  power  of  the  electric  current  at 
different  distances,  ii.  126. 

Magnitude,  ii.  20;  magnitude  of  the  sun,  i. 
69;  change  in  the  sun's  magnitude  im- 
possible, i.  481 ;  magnitude  of  the  earth, 
i.  479;  of  the  stars,  i.  592. 

Major  planets,  the,  i.  237-256. 

Malus's  discoveries  in  the  philosophy  of 
light,  i.  233. 

Mars,  his  distance  from  the  earth,  diurnal 
xotation,  &c.,  i.  15  J  ;  his  atmosphere  and 
physical  constitution,  i.  152;  has  he  a  sat- 
ellite? i.  153;  appearance  of  the  sun  at 
Mars,  i.  155. 

Masses  of  metal  melted  by  lightning,  ii.  66. 

Maskelvne's  experiments  on  the  weishts  of 
bodies,  i.  487,  488. 

Matter  and  its  physical  properties,  ii.  10- 
38 ;  matter  incapable  of  spontaneous 
change,  ii.  33. 

Measurement  of  heights,  i.  296,  297. 

Mechanical  effects  of  lightning,  ii.  69 ;  of 
steam,  ii.  436,  437 ;  mechanical  force  of 
steam,  ii.  419. 

Meltin?  and  boiling  points,  i.  329. 

Mercurial  thermometer,  ii.  132. 

Mercury,  its  diameter,  bulk,  &c.,  i.  143. 

Mercury  and  Venus,  their  diurnal  motion, 
seasons,  climate,  and  zones,  ii.  145,  146; 
their  orbits  and  transits,  geographical  sur- 
face, &c.,  147-150. 

Meridian  of  a  place,  i.  562. 


ANALYTICAL  INDEX. 


43 


Meridian,  standard,  i.  563. 

Metallic  contact,  accidental  discovery  of  the 
effects  of,  i.  363. 

Metallic  pistons,  ii.  485. 

Metallic  reflectors,  i.  265. 

Mine?,  the  drainage  of,  ii.  441. 

Meteor  at  Dreux  and  Mantes  in  France,  i. 
601;  meteor  seen  and  described  by  Pel- 
tier, i.  602;  meteor  of  November,  1833,  i. 
466;  of  August,  1838,  i.  469. 

Meteoric  phenomena,  various  instances  of, 
i.  474. 

Meteoric  stones  and  shooting-stars,  i.  459- 
474. 

Micrometer,  description  of,  ii.  352. 

Micrometer-screw,  ii.  293. 

Micrometric  wire,  ii.  24. 

Milky-way,  the,  ii.  378. 

Minor  planets,  the,  i.  143-156. 

Molecules,  or  atoms,  ii.  22. 

Moon,  the,  i.  307-322. 

Moon  and  the  weather,  i.  405-420. 

Moon's  influence  on  the  tides,  i.  212;  on 
the  weather,  i.  315. 

Moonlight,  ii.  193 ;  physical  qualities  of,  i.  3 12. 

Motion  of  comets,  i.  173 ;  motion  not  esti- 
mated by  speed  and  velocity  alone,  ii.  199- 
201 ;  motion  absolute  and  relative  illus- 
trated, ii.  218. 

Motion  and  pressure,  ii.  207. 

Morrison's  weather  almanac,  i.  165. 

Mosaic  account  of  the  Deluge,  ii.  77. 

Mountain  Tycho,  appearance  of,  i.  319. 

Mountains  of  the  moon,  i.  318. 

Multiplier,  or  galvanometer,  ii.  124. 

Mutual  attraction  or  repulsion  of  electrized 
clouds,  i.  533. 

Murray's  slides,  ii.  476. 


Napoleon's  invitation  to  Volta  to  visit  Paris, 
i.  367;  his  liberality,  i.  368. 

Neap  tides,  i.  215. 

Nebulae  and  clusters  of  stars,  ii.  383 ;  neb- 
ulas in  the  constellation  of  the  Swan  and 
the  Great  Bear,  ii.  384,  385 ;  nebulae  re- 
solvable into  stars,  ii.  387;  nebulae  in 
Orion,  ii.  388 ;  catalogue  of  nebulae,  ii. 
392;  planetary  nebulae,  ii.  395. 

Nebular  hypothesis  of  Laplace,  ii.  395. 

Nebulosity,  the,  i.  519. 

Negative  state  of  the  earth,  ii.  156. 

Needles  and  steel  bars  magnetized  by  means 
of  the  electric  currents,  ii.  121. 

Newcomen  and  Cawley's  patent  for  an  en- 
gine, ii.  411. 

Newcomen's  conception  of  the  atmospheric 
engine,  ii.  411. 

Norman  discovers  the  dip  of  the  magnetic 
needle,  ii.  113. 

New  metals :  potassium,  sodium,  barium, 
strontium,  calcium,  &,c.,  i.  395. 

New  planets,  the,  i.  205-208. 

Newton's  speculations  on  the  subject  of 
comets,  i.  179,  425 ;  his  researches  on  the 


subject  of  the  weights  of  bodies,  i.  487 ; 
his  explanation  of  the  prismatic  spectrum, 
i.  577 ;  his  three  propositions,  or  the  laws 
of  motion,  ii.  203. 


Nitric  acid  formed  by  the  electric  spark,  ii. 

65 ;   produced  during  a  thunder-storm,  ii. 

65. 

Non-conductors  of  heat,  ii.  183. 
Nucleus,  the,  i.  520. 


0. 


Object,  image  of  an,  in  a  plane  reflector,  i. 
262. 


Oersted's  experiments  in  electro-magnetism 

at  Copenhagen,  ii.  120. 
Olbers  discovers  Pallas  and  Vesta,  i.  206. 
Orbit  of  Halley's  comet,  its  magnitude,   i. 


187. 

Orbit  of  the  moon,  i.  321. 
Orbits  and  transits  of  Venus  and  Mercury,  ( 

i.  147. 
Orbitual  motion  of  comets,  i.  513 ;  of  double  * 

stars,  ii.  365. 

Ordinary  state  of  the  atmosphere,  ii.  151.        ( 
Orion,  ii.  336. 
Otto  Guericke's  electrical  machine,  i.  105. 


P. 


Paddle-wheels  of  steamboats,  ii.  255 ;  de- 
fects of  common  ones  for  Atlantic  steam- 
navigation,  i.  272. 

Paiitzch,  a  peasant  near  Dresden,  first  dis- 
covers Halley's  comet  on  its  reappearance, 
i.  184. 

Pallas,  i.  206,  208. 

Pa  pin  produces  a  vacuum  by  the  condensa- 
tion of  steam,  ii.  441. 

Papin's  invention  for  rowing  vessels  age.'  *st 
wind  and  tide,  ii.  442. 

Parallax,  the  annual,  ii.  365. 

Parallel  forces,  ii.  221. 

Parallel  of  latitude,  i.  562. 

Parallel  motion,  ii.  454. 

Parallelogram  of  forces,  ii.  208. 

Paratonnerres,   or  lightning  conductors.    :: 
102. 

Paschal's  experiment  on  atmospheric  p-  .'- 

Pegassus,  ii.  341. 
sure,  i.  287. 

Peltier's  experimental  illustration  of  th: 
phenomena  of  water  and  land  spouts,  i.  635. 

Pendulum,  the,  ii.  265;  illustrated  and  ex- 
plained, ii.  266. 

Perihelion  and  aphelion,  i.  482. 

Period  and  orbit  of  Encke's  comet,  i  473, 
424 ;  of  Biela's  comet,  i.  426. 

Periodic  comets,  i.  423-434. 

Periodic  motion  of  double  stars. ;..  3:7, 


ANALYTICAL  INDEX. 


Periodic  stars,  ii.  358. 

Permament  gases,  nature  of,  ii.  315. 

Phases  of  the  moon,  i.  309. 

Philosophy  offeree,  ii.  208. 

Phosphorescence,  ii.  194. 

Physical  constitution  of  comets,  i.  513-528; 

of  Mars,  i.  152. 
Piuzzi  discovers  Ceres,  i.  206. 
Pion,  ii.  336. 
Piston,  application  of  the,  to  steam-engine 

illustrated,  ii.  486. 

Pistons,  ii.  484;  metallic  pistons,  ii.  485. 
Piston-rod  and  beam,  connexion  of,  in  dou- 
ble-acting engine,  ii.  453-457. 
Plan  of  the  working  machinery  of  an  engine, 

ii.  547. 

Planes  of  cleavage,  ii.  27. 
Planetary  nebulae,  ii.  391. 
Planets,  are  they  inhabited?  i.  52;  their 

pnalogy  to  the  earth,  i.  53. 
Planet  Herschel,  discovery  of  predicted  by 

Clairaut,  i.  184. 
Plug-frame,  ii.  415. 
Plurality  of  worlds,  i.  51-64. 
Pneumatic  trough  in  the  chemical  laborato- 
ries, i.  302. 

Pointed  and  blunt  lightning  conductors,?!.  104. 
Pointers,  the,  ii.  334. 
Poison's  analytical  works,  i.  139. 
Polarity  of  the  magnet,  illustrations  of,  ii.  113. 
Pole-star,  the,  ii.  332. 
Pontecoulant  predicts  a  third  appearance  of 

Halley's  comet,  i.  186. 
Pools,  disappearance  of,  i.  607. 
Popular  fallacies,  ii.  85-96. 
Popular  impressions  respecting  the  effects 

of  thunder,  ii.  78. 

Porosity,  ii.  28 ;  all  bodies  have  pores,  ii.  29. 
Positive  and  negative  electricity,  i.  115. 
Potash  and  soda,  decomposition  of,  i.  385. 
Powder-magazines,  lightning  conductors  for. 

ii.  106. 

Power  of  a  locomotive,  ii.  554. 
Priming  and  lugging  of  the  tides,  i.  216. 
Princeton  steamer,  i.  280. 
Principle  of  heat,  most  ordinary  sources  of, 

ii.  183,  184. 

Principle  of  the  steam-engine,  ii.  314. 
Prism,  the,  i.  577. 

Prismatic  spectrum,  the,  i.  438,  577. 
Procyon,  ii.  338. 
Prognostications    of   the   weather    by    the 

ancients,  i.  406. 

Proper  motions  of  the  stars,  ii.  370. 
Proportion  of  the  diameter  to  the  stroke  of 

the  cylinder  of  steam-engine,  ii.  521. 
Prospects  of  steam-navigation,  i.  269-282. 
Protection  from  lightning,  ii.  99-108. 
Pulley,  the,  ii.  271-280. 
Pulsations  of  the  eye,  i.  230. 

Q. 

Quadrupeds,  motion  of,  ii.  236. 
QuicKsilver  passing  through  the  porus  of 
wood,  ii.  28. 


R. 


Rack  rail,  ii.  531. 

Radiation,  i.  333. 

Radiation  of  heat,  i.  437-456. 

Radiation,  reflection,  and  absorption  of  heat, 
i.  446. 

Radius-rod,  the,  ii.  469. 

Railway,  Liverpool  and  Manchester,  ii. 
534. 

Railways,  ii.  527. 

Rain,  luminous,  ii.  81. 

Range  of  the  tides,  i.  218,  219. 

Range  of  vision,  ii.  357. 

Ratchet-wheel,  ii.  255. 

Rays  of  heat  exist  unaccompanied  by  light, 
i.  438. 

Rays  of  light,  diverging  and  converging,  ii. 
348. 

Records  of  mining,  ii.  523. 

Rectangular  lever,  ii.  250. 

Red  moon,  the,  i.  502. 

Reflection,  irregular,  i.  260 ;  at  plane  sur- 
faces, i.  260;  its  laws,  i.  261 ;  at  curved 
surface,  i.  263. 

Reflection  of  light,  i.  259-266  ;  of  liquids,  i. 
265. 

Reflectors,  concave  and  convex,  i.  263. 

Refraction  at  plane  surfaces,  i.  576. 

Refraction  of  a  ray  of  light,  i.  575. 

Regulus,  ii.  338. 

Relation  of  heat  and  light,  ii.  187-194. 

Relative  brightness  of  the  stars,  ii.  346. 

Resinous  electricity  discovered  by  Dufaye, 
i.  108. 

Resistance  produced  by  friction,  ii.  262 ;  ex- 
periments on  resistance,  ii.  263. 

Rest  and  motion,  i.  361. 


Revolving  shafts  in  spinning  machinery,  ii. 

259. 

Richmann,  death  of,  i.  126. 
Rigel,  ii.  336. 

Ritter's  secondary  pile,  i.  376. 
Roads  regarded  as  inclined  planes,  ii.  284. 
Rolling  thunder  caused  by  echo,  i.  554. 
Rotatory  motion  of  the  planets,  i.  56. 


S. 


Sabine's  observations  of  luminous  clouds,  i 

547. 

Safety-valve,  the,  ii.  511. 
Salts,  crystallization  of,  ii.  26. 
Sand  fused  by  artificial  heat,  ii.  69. 
Satellites  of  Saturn,  i.  251. 
Saturn,  his  diurnal  rotation,  i.  245;  his  at 

mosphere   and   rings,   i.    246;    when  his 

rings  will  be  visible  at  the  earth,  i.  249; 

his  satellites,  i.  251 ;  variety  of  his  months, 

i.  251. 

Saussure's  electroscope,  ii.  150. 
Savery's  engine,  ii.  405. 
Sawmill  at  Southampton,  England,  ii.  259. 


ANALYTICAL  INDEX. 


45 


Schxibler's  experiments  on  the  influences  of 
Junar  phases,  i.  414;  his  observations  of 
the  electricity  of  the  air,  ii.  154,  158. 

Science,  predictions  of,  i.  171. 

Screens,  effects  of,  i.  451. 

Screw,  the,  ii.  288. 

Screw,  wedge, and  inclined  plane, ii.  283-294. 

Seasons,  the,  i.  490 ;  seasons  of  the  planets, 
i.  58. 

Self-regulating  feeders  for  a  steam-boiler,  ii. 
504. 

Self-regulating  damper,  ii.  513. 

Sensations,  ii.  19. 

Senses,  fallacious  indications  of  the,  ii.  85. 

Sense  of  feeling,  errors  of,  ii.  86. 

Seward's  slides,  ii.  479. 

Shadow  of  the  earth,  i.  80 ;  of  the  moon, 
i.  82. 

Sheet  lightning,  i.  539. 

Shooting  stars  and  meteoric  stones,  i.  459- 
474. 

Silent  lightning,  i.  545. 

Single-acting  engine,  ii.  428. 

Single  cock,  the,  ii.  481. 

Single  clack-valve,  the,  ii.  474. 

Siphon-gauge,  the,  ii.  49. 

Sirius,  or  the  Dog-star,  ii.  338. 

Sleet,  luminous,  ii.  82. 

Slide-valves,  ii.  476. 

Smeaton's  tackle,  ii.  275. 

Smellins,  deceptions  of,  ii.  95. 

Soap-bubbles,  thickness  of,  ii.  24. 

Solar  eclipse,  i.  83.          , 

Solar  system,  the,  i.  53,  172;  ii.  239;  mo- 
tion of,  ii.  371. 

Solomon's  temple  supposed  never  to  have 
been  struck  with  lightning,  ii.  106. 

Sound  and  light,  alliance  between,  i.  230. 

Sound  cannot  be  transmitted  in  the  absence 
of  air,  ii.  56. 

Space  beyond  the  limits  of  the  solar  system, 
i.  585. 

Spanish  bartons  explained,  ii.  277,  278. 

Specific  heat,  i.  332. 

Spectrum,  the,  how  produced,  i.  578. 

Speed  of  lightning,  i.  541. 

Spheroidal  form  of  the  earth  proved,  i.  495- 
498. 

Spheroid,  oblate  and  prolate,  ii.  224. 

Spica,  ii.  338. 

Spontaneous  change,  matter  incapable  of,  ii. 
33. 

Spontaneous  motion,  ii.  36. 

Spots  on  the  sun.  i.  73. 

Spring  tides,  I.  215. 

Spur-wheels,  ii.  262. 

Stars,  immense  distance  of  the,  i.  589 ;  dif- 
ferent magnitudes  or  orders  of  stars,  i. 
590-592 ;  relative  brightness  of,  ii.  346 ; 
double  stars,  ii.  351,  365,373;  periodic 
stars,  ii.  358;  temporary  stars,  ii.  360; 
binary  stars,  ii.  365;  the  visible  stars,  i. 
585-596;  ii.  350. 

Statics  and  dynamics,  ii.  243. 

Steam,  elasticity  of,  ii.  306 ;  compression  of 
steam  without  loss  of  heat,  ii.  310;  great 


power  of  steam,  ii.  401 ;  steam  a  common 
property  of  all  liquids,  ii.  405 ;  mechan- 
ical force  of  steam,  ii.  419;  variation  in 
the  consumption  and  production  of  steam, 
ii.  512. 

Steam-boiler,  ii.  496-513. 

Steam-engine,  the  (five  lectures),  ii.  399- 
568 ;  Watt's  inventions  and  improvements 
of  the  steam-engine,  ii.  423-441 ;  principle 
of  the  steam-engine  (see  Ebullition),  ii. 
314. 

Steam-gauge,  ii.  506. 

Steam-jacket,  ii.  424. 

Steam-navigation,  prospects  of,  i.  269-282 ; 
art  of,  applied  to  ocean-voyages,  i.  343.  • 

Steamship 

Steam  space  and  water  space  in  steam-boil- 
er, ii.  501. 

Steam-vessels  for  national  defence,  i.  274. 

Steelyard,  the,  ii.  250. 

Stellar  universe,  the,  ii.  357—396. 

Stephenson;s  engines  at  Killingworth,  ii.  533. 

Storm  converted  into  a  land-s-pout,  i.  602. 

Storm-clouds,  height  of,  i.  536. 

Straight  wand,  the,  ii.  225. 

Straps  or  cords,  ii.  258. 

Subterranean  sources,  inundations  from,  ii. 
77. 

Suction  by  the  mouth  (the  effects  of)  the 
means  of  discovering  atmospheric  pres- 
sure, i.  285. 

Suctiun-pipe,  the,  ii.  407. 

Sugar-refining,  Howard's  improvement  in 
the  process  of,  ii.  170. 

Sulphureous  odor  developed  by  lightnin?,  ii. 
64. 

Sulzer's  experiment  in  galvanism,  i.  364. 

Sun,  the,  i.  67-76  ;  magnitude  of  the  sun,  i. 
69;  its  density,  form,  and  rotation,  i.  72; 
central  eclipse  of  the  sun,  i.  83  ;  sun's  in- 
fluence at  Venus  and  Mercury,  i.  149;  its 
appearance  at  Mars,  i.  355;  it  is  the  com- 
mon centre  of  the  planets,  i.  172;  sun's 
influence  on  the  tides,  i.  214;  combined 
influence  of  the  sun  and  moon,  i.  216 ;  the 
sun's  appearance  as  seen  from  Jupiter,  i. 
2-12;  as  seen  from  Saturn,  i.  245,246; 
calorific  effects  of  the  sun's  rays,  i.  490  ; 
horizontal ;  appearance  of  the  sun  and 
moon,  ii.  9 1 ;  heat  of  the  sun's  rays,  ii.  193. 

S'.m-and-planet  wheels,  ii.  447. 

Supporters  of  combustion  and  combustlDes. 
ii!  323. 

Supposed  rays  of  cold,  i.  453. 

Surface  of  the  planets,  i.  61. 

Sword  and  belt  of  Orion,  ii.  336. 

Symmer's  theory  of  electricity,  i.  135. 
I  Syringe,   the   exhausting,   ii.   41 ;    the  con- 
densing syringe,  ii.  56. 

Systems  of  pulleys,,  ii.  274. 


Table  showing  the  te-np?ratJire  at  which 
water  wi!1.  j.'l  unce.  ,Uficc  ir.t  press.ires 
of  the  atmo.<pheie,  ii.  3i  5  ;  table  exhibit- 
ing tie  me.i.aixical  po*e'  of  water  con- 


<  46 


ANALYTICAL  INDEX. 


*"*w-s^*»*. 


verted  into  steam  at  various  pressures,  ii. 
517;  table  showing  the  improvement  of 
Cornish  engines,  ii.  5'J3  ;  table  of  observa- 
tions on  the  height  of  storm-clouds  by  31. 
Arago,  i.  537. 

Tacking  a  vessel,  process  of,  ii.  215. 

Tails  of  comets,  i.  521. 

Tr-ste,  deceptions  cf,  ii.  95. 

Teeth  of  wheel?,  ii.  259. 

Telescope,  limited  powers  of,  i.  51  ;  unable 
to  magnify  a  star,  i.  592 ;  philosophy  of 
xhe  telescope,  ii.  346 ;  ell'ect  on  lixed  stars, 
ii.  346. 

Telescope,  astronomical,  i.  480. 

Temperature  of  the  sun's  r.urface,  i.  75. 

Temporary  stars,  ii.  360. 

Terrestrial  attraction  the  combined  action 
of  parallel  forces,  ii.  2'2'2. 

Terrestrial  magnetism,  Ampere's  theory  of, 
ii.  124,  125. 

The  earth,  i.  477-498. 

Theorem  regulating  pressure  and  motion,  ii. 

Theory  of  colors,  i.  575-582. 

Thermometer,  the,  i.  329 ;  ii.  131-146. 

Thermometer,  mercurial,  advantages  of,  ii. 
132. 

Thermo-electricity,  ii.  126. 

Thermo-electric  piie,  ii.  127. 

Thermo-electric  scale  of  metals,  ii.  127. 

Throttle- valve,  ii.  462. 

Thunder,  i.  547-549 ;  distance  at  which  it 
may  be  heard,  i.  553 ;  cause  of  thunder, 
i.  f.54;  popular  impressions  respecting  the 
effects  of  thunder,  ii.  78. 

Thunder-bursts,  i.  545. 

Thunder-clouds,  common,  i.  532. 

Thunder-storms,  i.  531-558. 

Tid:il  wave,  the  great,  i.  217. 

Tides,  the,  i.  211-220;  correspondence  be- 
tween the  tides  and  the  phases  of  the 
moon,  i.  211;  the  moon's  influence  on  the 
tides,  i.  212,  213;  the  sun's  influence,  i. 
214,  215;  combined  influence  of  the  sun 
and  moon,  i.  216;  velocity  of  the  tides,  i. 
218;  range  of  the  tides,  i.  218,  219. 

Time  of  day,  how  found  on  land,  i.  567,568; 
at  sea,  i.  569. 

Tints,  variety  of,  how  produced  by  the  sim- 
ple component  colors,  i.  581. 

Toaldo,  the  meteorologist,  i.  418. 

Toothed  wheel,  the,  ii.  292,  293. 
i  Torricelli,  a  pupil  of  Galileo,  discovers  at- 
mospheric pressure,  i.  286. 
)  Total  eclipse  of  the  sun,  Halley's  description 
<       of,  i.  83. 

Transferring  power  of  the  Voltaic  pile,  i.  379. 
|    1  .ansmission  of  sound,  i.  553. 
i  Transparent  and  opaque  bodies,  i.  450. 
•   Treadmill,  the,  ii.  255. 

Turning-lathe,  the,  ii.  248. 

Twilight  at  Venus  and  Mercury,  i.  150. 

T-*':-way  cock,  ii.  482. 

U. 

Ultimate  atom-,  '•.  °6. 


Ultra-r.odiacal  planets,  i.  207. 
Undulatory  theory  of  light,  i.  224,  232. 
Uniform  supply  of  light  and  heat  from  the 

sun,  i.  53. 

Upward  flashes  of  lightning,  ii.  72. 
Ursa  Major  or  Great  Bear,  ii.  333. 
Ursa  Minor,  ii.  334. 
Useful  arts,  examples  in,  ii.  171,  172. 


Vacuity  between  our  system  and  the  stars, 

i.  586. 

Vacuum,  maxim  of  the  ancients  that  "Ma- 
ture abhors  a  vacuum,"  i.  285,  286;  a 
perfect  one  cannot  be  produced,  ii.  W ; 
vacuum  produced  by  the  condensation  of 

steam,  ii.  441. 

Valves  of  double-acting  engines,  ii.  448. 
Valves,  slides,  and  cocks,  ii.  474. 
Vapor,  condensation  of,  ii.  313. 
Vaporization,  i.  331 ;  ii.  299. 
Vaporization  and  condensation,  ii.  299. 
Variable  stars,  how  to  observe  them,  ii.  350. 
Variation  of  atmospheric  pressure,  i.  2i!ti ; 

of  the  magnetic  needle,  ii.  113. 
Variations,  local,   of  the  electricity  of  the 

air,  ii.  155. 
Velocity  of  the  tides,  i.  218. 
Vent-peg,  the,  i.  300. 
Venus,  its  diameter,  position,  &c.,  i.  145. 
Vernier,  the,  for  noting  very  small  changes 

in  the  barometer,  i.  294. 
Vesta,  i.  207. 

Visible  stars,  the,  i.  585-596. 
Vision,  theory  of  illustrated  by  a  rotating 

disk,  i.  542;  deceptions  of  vision,  ii.  9o  ; 

range  of  vision,  ii.  357. 
Vitreous  electricity  discovered  by  Dufaye,  i. 

108. 

Vitrifications  and  fulgurites,  ii.  67-69. 
Volcanic  lightning,  i.  535. 
Volcanic  thunder-clouds,  i.  535. 
Volta's   experiments   in   electricity,  i.  138; 

his  theory  of  contact,  i.  364;  of  the  origin 

of  atmospheric  electricity,  ii.  151. 
Voltaic    pile,    invention   of,    &c.,   i.    366 ; 

physical  effects  of  the  pile,  i.  368;   ento- 
mological effects,  i.  368;  mode  of  action, 

i.  390. 
Voltaire's  investigations  on  the  subject  of 

comets,  adopts   Newton's  conjectures,  i. 

179. 

Volume  and  weight  of  thr  mn,  i.  70. 
Voyages  to  the  north  pole   ••  56. 

W. 

Wagon-boiler,  the,  for  steam-engine,  ii.  496. 

Walking  engine,  ii.  532. 

War-steamers,  i.  280. 

Waste  steam,  resistance  of  t)ie,  ii.  554. 

Water,  decomposition  of,  i.  370;  water  raised 

by  elastic  force,  ii.  53. 
Water-spouts  and  whirlwinds,  i.  599-608;   ( 

spouts  witnessed  by  Capt.  Beechy,  i.  60?     J 


ANALYTICAL  INDEX. 


47 


W:,t-T  wheels,  ii.  255. 

Watch,  general  view  of,  ii.  267. 

W.itch-sprin:r,  ii.  257. 

Watson  and  Bevitr's  experiments  in  elec- 
tricity, i.  111. 

Watt's  inventions  and  improvements  iu  the 
steam-engine,  ii.  423-440 ;  his  air-pump, 
ii.  423 ;  his  experimental  apparatus,  ii. 
425  j  his  first  patent  for  a  steam-engine, 
ii.  428;  his  steam-indfcator,  ii.  5^)8;  his 
counter  for  the  steam-ensine,  ii.  510. 

U'enther  almanacs,  i.  159-168. 

Waves  of  lisht,  minuteness  of,  i.  229. 

UY'l-re,  the,  ii.  287. 

\\~- -<i  je,  screw,  and  inclined  plane,  ii.  283- 
294. 

Weight  of  air,  i.  94;  of  the  earth,  i.  487. 

AVi  i-.jht  and  force,  ii.  244. 

Wdls's  theory  of  dew,  i.  456. 

Westerly  winds,  their  effects  on  sailing  ves- 
sels,,!". 341. 

Wht-atstone's  experiments  on  the  speed  of 
lightning,  i.  541. 

Wh> •(•!,  the,  applied  to  the  steam-engine,  ii. 
447* 

Wheel  and  axle,  ii.  253. 

Wheel  barometer,  i.  292. 

Wheelwork,  ii.  253. 

Wheelwork  and  the  lever,  ii.  243-268. 

WheweH's  researches  oa  the  subject  of  the 
tides,  i.  217. 


Whirlwinds  and  water-spouts,  i.  599-608. 
Whiston's  co.Tiet,  ami  his  theory,  i.  428.    * 
White's  pulley,  ii.  276. 
Williams's  patent  for  a  method  of^consuming 

unburnt  gases,  ii.  499. 
Wilson  discovers  the  lateral  shock,  i.  112. 
I  Wind,  action  of  on  sails  of  vessels,  ii.  213. 
|  Wind  and  water-mills,  Sineaton's  improve- 
ments ot,  ii.  443. 
i  Windlass,  the,  ii.  254. 
j  Wings  of  insects,  ii.  24. 
|  Wollaston's  cryophorus,  ii.  174. 
Wollaston's    micrometric  wire,   ii.  23,   24; 
his  inrettifationi  on  the  subject  of  the 
comparative  ebrishtness  »nd  magnitude  of 
the  stars,  i.  5t(3. 
Woolf's  piston,  ii.  485. 
Working-machinery  of  a  locomotive  engine, 

plan  of,  ii.  547. 
Worlds,  plurality  of,  i.  51-64. 


Y. 


louns;  s   discoveries  in    the   philosophy  of 
light,  i.  233. 

Z. 

Zigzag  lightning,  i.  538,  556. 
j  Zodiacal  constellations,  ii.  338. 


THE    PLURALITY    OF    WORLDS. 


Contemplation  of  the  Firmament — Reflections  thereby  suggested. — Limi'e'*.  Powers  of  the  Tele- 
scope.— What  it  can  do  for  us. — Its  effect  on  the  Appearances  of  the  Hancut.-  -Are  the  Planets  in- 
habited?— Circumstantial  Evidence. — Analogies  of  the  Planets  to  the  Earth. — Plan  of  the  Solar 
System.— Uniform  Supply  of  Light  and  Warmth. — Expedient  for  securing  it^Different  Distances  of 
the  Planets  do  not  necessarily  infer  different  Temperatures — nor  different  Degrees  of  Light. — Ad- 
mirable Adaptation  of  the  Rotation  of  the  Earth  to  the  Organization  of  its  Inhabitants. — The  same 
Provision  exists  on  the  Planets. — Minor  and  Major  Planets. — Short  Days  on  the  latter. — The  Sea- 
sons.— Similar  Arrangement  on  the  Planets. — The  Atmosphere. — Similar  Appendage  to  the  Plan- 
eU. — Many  uses  of  the  Atmosphere. — Clouds. — Rain,  Hail,  and  Snow. — Mountains  on  the  Plan- 
et*.— Land  and  Water. — Weights  of  Bodies  on  the  Planets  analogous  to  Weight  on  the  Earth. — 
Appearances  of  the  San. — Conclusion. 


'  The  Heaven§  declare  the  glory  of  God : 
And  the  Firmament  showeth  his  handy- work." 

Fixix.1. 


• 
I 


THE  PLURALITY  OF  WORLDS. 


WHEN  we  walk  forth  on  a  serene  night  and  direct  our  view  to  the  aspect 
of  the  heavens,  there  are  certain  reflections  which  will  present  themselves  to 
every  mind  gifted  with  the  slightest  power  of  contemplation.  Are  those 
shining  orbs  which  so  richly  decorate  the  firmament  peopled  with  creatures 
endowed  like  ourselves  with  reason  to  discover,  with  sense  to  love,  and  with 
imagination  to  expand  toward  their  limitless  perfection  the  attributes  of  Him 
of  "  whose  fingers  the  heavens  are  the  work  ?"  Has  He  who  "  made  man  lower 
than  the  angels  to  crown  him,"  with  the  glory  of  discovering  that  light  in  which 
he  has  "  decked  himself  as  with  a  garment,"  also  made  other  creatures  with 
like  powers  and  like  destinies  ;  with  dominion  over  the  works  of  his  hands, 
and  having  all  things  "  put  in  subjection  under  their  feet  ?"  And  are  those  re- 
splendent globes  which  roll  in  silent  majesty  through  the  measureless  abysses 
of  space,  the  dwellings  of  such  beings  ?  These  are  questions  which  will  be 
asked,  and  which  will  be  answered.  These  are  inquiries  against  which  nei- 
ther the  urgency  of  business  nor  the  allurements  of  pleasure  can  block  up  the 
avenues  of  the  mind.  These  are  questions  that  have  been  asked,  and  that 
will  continue  to  be  asked,  by  all  who  view  the  earth  as  an  individual  of  that 
little  cluster  of  worlds  called  the  solar  system. 

Those  whose  information  on  topics  of  this  nature  is  limited,  would  be  prompt- 
ed, in  seeking  the  satisfaction  of  such  inquiries,  to  look  immediately  for  direct 
evidence ;  and  consequently  to  appeal  to  the  telescope.  Such  an  appeal 
would,  however,  be  fruitless.  Vast  as  are  the  powers  of  that  instrument,  and 
great  the  improvements  which  have  been  conferred  upon  it,  it  still  falls  infi- 
nitely short  of  the  ability  to  give  direct  evidence  on  such  inquiries.  What 
will  a  telescope  do  for  us  in  regard  to  the  examination  of  the  heavenly  bodies, 
or  indeed  of  any  distant  object  ?  It  will  accomplish  this,  and  nothing  more  : 
it  will  place  us  at  a  less  distance  from  the  object  to  which  we  direct  our  view  ; 
it  will  enable  us  to  approach  it  within  a  certain  limit  of  distance,  and  to  behold 
it  as  we  should  do  without  a  telescope  at  the  lesser  distances.  But,  strictly 


THE  PLURALITY  OF  WORLDS. 


speaking,  it  cannot  accomplish  even  this  ;  for  to  suppose  it  did,  would  be  to 
imagine  it  to  possess  all  the  admirable  optical  perfection  of  the  eye.  That 
instrument,  however  nearly  it  approaches  the  organ  of  vision  in  its  qualities, 
is  still  deficient  in  some  of  the  attributes  which  have  been  conferred  upon  the 
eye  by  its  Maker.  It  is"  found  that  in  proportion  as  we  augment  the  magnify- 
ing power  of  the  telescope,  we  diminish  both  the  quantity  of  light  upon  the 
object  we  behold,  and  also  the  distinctness  of  its  features  and  outlines.  These 
and  some  other  circumstances  peculiar  to  the  telescope,  which  need  not  be 
particularly  detailed  now,  impose  a  limit  on  the  magnifying  powers  that  are 
practically  available  in  inquiries  of  this  kind. 

Let  us,  however,  suppose  that  we  could  resort  to  the  use  of  a  telescope  hav- 
ing the  magnifying  power  of  a  thousand  in  examining  any  of  the  heavenly 
bodies  :  what  would  such  an  instrument  do  for  us  ?  It  would  in  fact  place  us 
a  thousand  times  nearer  to  the  object  that  we  are  desirous  to  examine,  and  thus 
enable  us  to  see  that  object  as  we  should  see  it  at  that  diminished  distance 
without  a  telescope  at  all.  Such  is  the  extent  of  the  aid  which  we  should 
derive  from  the  telescope.  Now,  let  us  see  what  this  aid  would  effect.  Take 
the  case  of  the  moon,  the  nearest  body  in  the  universe  to  the  earth.  The  dis- 
tance of  that  object  is  about  240,000  miles  ;  the  telescope  would  then  place  us 
about  240  miles  from  it.  Could  we  at  the  distance  of  240  miles  distinctly,  or 
even  indistinctly,  see  a  man,  a  horse,  an  elephant,  or  any  other  natural  object  ? 
Could  we  discern  any  artificial  structure  ?  Assuredly  not !  But  take  the  case 
of  one  of  the  planets.  When  Mars  is  nearest  to  the  earth,  its  distance  is 
about  50,000,000  of  miles.  Such  a  telescope  would  place  us  at  a  distance  of 
50,000  miles  from  it.  What  object  could  we  expect  to  see  at  50,000  miles' 
distance  ?  The  planet  Venus,  when  nearest  the  earth,  is  at  a  distance  some- 
thing less  than  30,000,000  of  miles,  but  at  that  distance  her  dark  hemisphere 
is  turned  toward  us ;  and  when  a  considerable  portion  of  her  enlightened  hem- 
isphere is  visible,  her  distance  is  not  less  than  that  of  Mars.  All  the  other  plan- 
ets, when  nearest  to  the  earth,  are  at  much  greater  distances.  As  the  stars 
lie  infinitely  more  remote  than  the  most  remote  planet,  it  is  needless  here  to 
add  anything  respecting  them. 

It  is  plain,  then,  that  the  telescope  cannot  afford  any  direct  evidence  on 
the  question  whether  the  planets,  like  the  earth,  are  inhabited  globes.  Yet, 
although  science  has  not  given  direct  answers  to  these  questions,  it  has  sup- 
plied a  body  of  circumstantial  evidence  bearing  upon  them  of  an  extremely  in- 
teresting nature.  Modern  discovery  has  collected  together  a  mass  of  facts 
connected  with  the  position  and  motions,  the  physical  character  and  conditions, 
and  the  parts  played  in  the  solar  system  by  the  several  globes  of  which  that 
system  is  composed,  which  forms  a  body  of  analogies  bearing  on  this  inquiry, 
even  more  cogent  and  convincing  than  the  proofs  on  the  strength  of  which  we 
daily  dispose  of  the  property  and  lives  of  our  fellow-citizens,  and  hazard  our 
own. 

In  considering  the  earth  as  a  dwelling-place  suited  to  man  and  to  the  crea- 
tures which  it  has  pleased  his  Maker  to  place  in  subjection  to  him,  there  is  a 
mutual  fitness  and  adaptation  observable  among  a  multitude  of  arrangements 
which  cannot  be  traced  to,  and  which  indeed  obviously  cannot  arise  from,  any 
general  mechanical  law  by  which  the  motions  and  changes  of  mere  material 
masses  are  observed  to  be  governed.  It  is  in  these  conveniences  and  luxuries 
with  which  our  dwelling  has  been  so  considerately  furnished,  that  we  see  the 
beneficent  intentions  of  its  Creator  more  immediately  manifested,  than  by  any 
great  physical  or  mechanical  laws,  however  imposing  or  important.  If — having  £ 
a  due  knowledge  of  our  natural  necessities — of  our  appetites  and  passions— of  ) 
our  susceptibilities  of  pleasure  and  pain — in  fine,  of  our  physical  organization —  I 

.•+s~*rf 


THE  PLURALITY  OF  WORLDS. 


53 


we  were  for  the  first  time  introduced  to  this  glorious  earth  with  its  balmy  atmo- 
sphere— its  pure  and  translucent  waters — the  life  and  beauty  of  its  animal  and 
vegetable  kingdoms — with  its  attraction  upon  the  matter  of  our  own  bodies  just 
sufficiently  great  to  give  them  the  requisite  stability,  and  yet  not  so  great  as  to 
deprive  them  of  the  power  of  free  and  rapid  motion — with  its  intervals  of  light 
and  darkness,  giving  an  alternation  of  labor  and  rest  nicely  corresponding  with 
our  muscular  power — with  its  grateful  succession  of  seasons,  and  its  moderate 
extremes  of  temperature,  so  justly  suited  to  our  organization :  with  all  this 
fitness  before  us.  could  we  hesitate  to  infer  that  such  a  place  must  have  been 
provided  expressly  for  our  habitation  ?  If,  then,  the  discoveries  of  modern 
science  disclose  to  us  in  each  planet,  which,  like  our  own,  rolls  in  regulated 
periods  round  the  sun,  provisions  in  all  respects  similar — if  they  are  proved  to 
be  habitations  similarly  built,  ventilated,  warmed,  illuminated,  and  furnished — 
supplied  with  the  same  alternations  of  light  and  darkness  by  the  same  expe- 
dient— with  the  same  pleasant  succession  of  seasons — the  same  geographical 
diversity  of  climates — the  same  agreeable  distribution  of  land  and  water — can 
we  doubt  that  such  structures  have  been  provided  as  the  abodes  of  beings  in 
all  respects  resembling  ourselves  ?  The  strong  presumption  raised  by  such 
proofs  is  converted  into  a  moral  certainty,  when  it  is  shown  from  physical  anal- 
ogies of  irresistible  force  that  such  bodies  are  the  creation  of  the  same  Hand 
that  raised  the  round  world  and  launched  it  into  space.  Such,  then,  is  the  na- 
ture of  the  evidence  which  science  offers  on  this  interesting  question.  Let  us 
endeavor  to  strip  it  of  such  technical  forms  of  language  and  reasoning  as  are 
intelligible  only  to  the  scientific,  and  to  present  it  so  as  to  be  easily  and 
agreeably  comprehended. 

If  we  look  at  a  plan  of  the  solar  system,  the  first  glance  will  impress  us  with 
an  idea  that  the  earth  is  an  individual  of  a  class  ;  that  that  class  is  the  planets  ; 
that  the  sun  is  an  object  provided  for  different  purposes,  and  the  same  may  be 
said  of  the  satellites.  We  take  this  impression  from  the  simple  fact  that  the 
planets,  including  the  earth  among  the  number,  move  round  the  sun  as  a  centre 
in  circles  all  in  the  same  direction,  and  nearly  in  the  same  plane ;  while  the 
satellites  or  moons  (in  a  manner  which  we  shall  hereafter  notice)  revolve  re- 
spectively round  the  planets.  The  impression  is  irresistible  that  the  planets, 
including  the  earth,  form  a  class  ;  but  let  us  see  the  purposes  in  the  economy 
of  nature  which  are  fulfilled  by  this  common  character  given  to  the  motion  of 
the  planets  and  the  position  of  the  sun.  We  find,  upon  considering  the  quali- 
ties of  organized  bodies,  and  especially  the  species  of  the  animals  and  vegeta- 
bles upon  the  earth,  that  the  maintenance  of  their  physical  well-being  is  essen- 
tially dependant  on  the  uniformity  and  regularity  with  which  they  are  supplied 
with  the  two  great  physical  principles  of  light  and  heat.  Should  these,  or 
either  of  them,  be  subject  to  any  extreme  variations,  such  vicissitudes  would 
be  incompatible  with  the  organization  of  the  species.  There  is  a  cold  on  one 
hand  and  a  heat  on  the  other,  under  which  no  organized  body  could  continue 
to  exist,  and  there  are  still  narrower  limits  within  which  it  is  necessary  to 
confine  the  temperatures  they  are  exposed  to  in  order  to  secure  the  perfec- 
tion of  their  physical  health.  There  are  also  degrees  of  light,  the  intensity 
of  which  would  be  incompatible  with  the  continued  perfection  of  the  organs  of 
vision. 

We  see,  then,  how  essential  to  the  well-being  of  the  infinite  varieties  of  crea- 
tures that  people  this  globe,  a  uniform  regulation  of  light  and  heat  is.  How, 
then,  is  this  great  and  important  end  attained  1  If  we  had  a  fire  which  at  once 
supplied  light  and  heat  in  our  neighborhood,  and  that  circumstances  obliged  us 
continually  to  shift  our  position  in  regard  to  it,  but  at  the  same  time  so  to  order 
our  movements  as  to  receive  from  it  a  uniform  intensity  of  light  and  heat,  how 


54 


THE  PLURALITY  OF  WORLDS. 


should  we  move  ?  Should  we  not  take  care  to  keep  always  at  the  same  dis- 
tance from  it  ?  And  to  accomplish  this,  should  we  move  in  any  other  path 
than  that  of  a  circle,  having  the  fire  in  the  centre  ?  This,  however,  is  precisely 
what  is  accomplished  by  the  annual  motion  of  the  earth.  It  traverses  its  course 
round  the  central  fire  of  the  system,  keeping  always  nearly  at  the  same  distance 
from  the  inexhaustible  fountain  of  light  and  warmth.  By  this  simple  expedient 
of  observing  a  circular  path,  with  the  sun  in  the  centre,  this  necessary  object 
is  attained. 

Now,  in  examining  the  movements  of  all  the  other  planets,  we  find  that  the 
same  expedient  is  provided :  that  they  severally,  in  their  periodical  courses, 
like  the  earth,  preserve  uniform  distances  from  the  sun — moving  round  that 
body  in  circles,  of  which  it  is  the  common  centre. 

Seeing,  then,  that  this  motion  in  the  case  of  the  earth  is  a  means  whereby  an 
important  end  is  attained,  analogy  justifies  the  conclusion  that  it  is  to  be  re- 
garded likewise  as  a  means  for  the  attainment  of  a  similar  end  in  each  of  the 
planets.  But  it  will  probably  be  said  that  the  planets  are  at  different  distances 
from  the  sun  ;  that  the  most  remote  of  them  is  nearly  twenty  times  farther  from 
that  luminary  than  the  earth,  while  the  nearest  of,  them  is  little  more  than  one 
third  the  earth's  distance  ;  therefore,  that  although  it  must  be  admitted  that  each 
planet  (considered  per  se]  is  supplied  uniformly  with  light  and  warmth  by  this 
circular  motion  ;  yet  the  intensity  of  these  principles  to  which  the  several 
planets  are  exposed,  comparing  one  with  another,  is  so  extremely  different  as 
to  destroy  all  analogy  between  them. 

In  answer  to  this,  we  are,  however,  to  consider  that  the  influence  of  light  and 
heat  upon  a  planet  does  not  depend  solely  on  its  distance  from  the  sun.  The 
heat,  as  is  well  known,  produced  by  the  solar  rays,  depends  on  the  density  of 
the  air  which  surrounds  the  objects  affected  by  it.  Thus  we  find  the  tempera- 
ture, at  great  elevations  in  our  own  atmosphere,  considerably  lower  than  at  the 
mean  surface  of  our  globe  ;  because  at  these  elevations  the  air  becomes  so  thin 
as  to  be  incapable  of  collecting  and  retaining  the  sun's  heat.  We  can  there- 
fore easily  imagine,  provided  the  existence  of  their  atmospheres  be  conceded, 
that  their  density  has  been  so  regulated,  that  the  nearest  planets  to  the  sun, 
which  receive  the  greatest  intensity  of  its  rays,  may  not,  after  all,  be  more 
heated  than  the  most  remote  ones,  which  are  exposed  to  the  least  intensity  of 
its  rays  :  just  as  we  find  that  the  temperature  of  the  summits  of  lofty  mount- 
ains at  the  tropics  is  as  low  as  the  temperature  of  some  of  the  polar  latitudes. 
It  is  plain,  then,  how  the  effects  of  the  various  distances  of  the  planet  from 
the  sun  may  be  equalized  and  compensated.  The  means  of  accomplishing  this 
are  provided  in  the  form  of  atmospheres,  as  we  shall  presently  see. 

But  let  us  turn  to  the  consideration  of  the  solar  light.     The  intensity  of  the 

sun's  light  varies  with  his  distance  exactly  in  the  same  proportion  as  that  of 

his  heat ;  and  the  brightness  of  a  day  in  the  most  remote  planet  would  be  less 

than  that  of  a  day  in  the  nearest  in  the  same  proportion  as  the  sun's  heat  would 

be  less.     It  may  therefore  be  objected  that  there  might  be    scarcely  daylight 

enough  in  the  planet  Herschel  to  serve  the  purposes  of  social  and   civil  life. 

Such  might  undoubtedly  be  the  case  if  we  were  to  deny  the  possibility  of  any 

variation,  however  minute,  in  the  organs  of  vision  ;  but  without  denying  this, 

let  us  consider  how  the  matter  would  stand.     The  perception  which  the  eye 

of  any  creature  acquires  of  light,  depends  (cateris  paribus)  upon  the  magnitude 

of  the  circular  aperture  or  foramen,  in  front  of  the  eye,  called  the  pupil,  which 

has,  externally,  the   appearance  of  a  circular  black  spot ;  but  which  is,   in 

)  reality,  a  circular  hole  through  which  the  light  is  admitted  to  the  interior  of  the 

I  chamber  of  vision,  there  to  affect  the  membranous  coating  which  transmits  its 

j  influence  to  the  brain  and  causes  the  sensation.     It  must  be  evident,  even  to 

W->-~. 


THE  PLUEAL1TY  OF  WORLDS. 


55 


the  least  informed,  that  the  brightness  of  light  will  then  depend  upon  the  mag- 
nitude of  this  foramen.  Granting  that  there  are  two  eyes,  in  one  of  which  the 
pupil  is  twice  as  large  as  it  is  in  the  other,  the  organ  being  in  all  other  respects 
the  same,  then  it  is  evident  that  one  would  admit  twice  as  much  light  as  the 
other.  If,  then,  the  large  pupil  was  exposed  to  light  of  only  one  half  the  in- 
tensity or  brightness  of  that  to  which  the  smaller  one  is  exposed,  then  the  two 
lights  would  appear  to  these  eyes  of  the  same  brilliancy,  although  in  fact,  one 
would  be  only  half  as  bright  as  the  other.  What,  then,  shall  we  say  of  the 
planets  ?  Grant  that  the  pupils  of  the  eyes  of  all  creatures  endowed  with 
vision  upon  them  are  enlarged  in  their  opening  according  as  the  planets  are 
more  removed  from  the  sun  and  diminished  as  they  are  nearer  to  that  luminary, 
and  the  whole  difficulty  arising  from  the  varying  intensity  of  light  will  vanish. 
The  inhabitants  of  all  the  planets  will,  in  fact,  enjoy  days  of  the  same  bright- 
ness, notwithstanding  the  extreme  difference  of  their  distances  from  the  sun. 

In  considering  closely  the  physical  powers  of  locomotion  and  strength  con- 
ferred upon  animals  on  the  surface  of  the  earth,  we  find  that  they  have  certain 
limitations ;  that  animals  are  capable  of  exercising  the  powers  of  locomotion 
for  certain  periods  of  time,  varying,  it  is  true,  among  individuals,  but  still  in 
the  main  comprised  within  certain  narrow  limits.  We  find  that  after  the  lapse 
of  certain  intervals,  bodily  repose  is  wanted.  But  besides  the  disposition  to  ac- 
tivity and  locomotion  and  the  alternate  want  of  rest,  animals  in  general  have 
also  other  physical  wants  and  capabilities  of  enjoyment  which  are  periodical. 
Thus  they  are  capable  of  wakefulness  for  certain  periods,  after  which  recurs  the 
physical  want  of  sleep. 

Now  upon  a  general  survey  of  the  creation,  it  is  found  that  the  average  pe- 
riods which  must  regulate  the  intervals  of  labor  and  rest,  of  wakefulness  and 
sleep,  corresponds  in  the  main  with  those  which  regulate  the  alternations  of 
light  and  darkness.  In  the  vegetable  kingdom  we  find  prevailing  also  peri- 
odical functions,  certainly  not  so  obvious  and  apparent,  but  not  on  that  ac- 
count the  less  interesting,  which  are  ascertained  to  have  the  same  close 
alliance  with  the  period  that  regulates  the  returns  of  light  and  darkness. 

Plants  undergo  certain  changes  and  suffer  certain  effects,  in  the  presence 
of  solar  light,  which  are  different  from,  and  in  some  respects  contrary  to,  those 
which  they  undergo  in  its  absence.  These  changes  are  essential  to  the  vege- 
table health  of  the  creature ;  without  them  the  tribes  of  plants  would  be 
extinct.  The  duration  of  these  operations  is  just  as  essential  as  their  alterna-. 
tions.  Light  must  be  present  a  certain  time  and  neither  more  nor  less  ;  and  its 
absence  must  be  equally  regulated  by  limits,  otherwise  the  plant  must  perish. 
There  is,  then,  it  is  evident,  an  essential  relation  between  the  functions  and 
qualities  of  the  vegetable  kingdom — between  the  power  of  activity,  the  suscep- 
tibility of  enjoyment  and  the  physical  wants  of  animals,  and  the  periods  which 
separate  light  from  darkness  ;  but  what  are  those  periods  ?  What  is  the 
mechanical  expedient  to  which  He  has  resorted  to  accomplish  his  inscru- 
table purposes,  who  divided  the  light  from  the  darkness,  and  "  saw  that  it 
was  good"  Nothing  can  be  more  simple.  Nothing  can  be  more  beautiful. 
Nothing  can  be  more  admirably  perfect.  While  the  globe  of  the  earth  makes 
its  annual  course  round  the  sun,  it  has  at  the  same  time  a  spinning  motion,  on 
a  certain  diameter,  as  an  axis,  in  virtue  of  which  it  successively  exposes  all 
parts  of  its  surface  to  the  light  and  warmth  of  the  sun.  Each  complete  rota- 
tion is  accomplished  in  the  space  which  we  call  twenty-four  hours  ;  subject  to 
a  variation  which  we  shall  notice  hereafter.  All  points  on  our  earth  are  alter- 
nately exposed  to  and  withdrawn  from  the  solar  light ;  the  average  intervals 
being  twelve  hours. 

Now  when  we  reflect  on  the  close,  the  exact  correspondence  between  these 


MM^^l^MM 

56  THE  PLURALITY  OF  WORLDS. 

intervals  and  the  indispensable  wants  of  all  organized  creatures,  can  we  for  a  ? 
moment  doubt  that  the  earth  was  made  to  turn  upon  its  axis  in  that  particular  ] 
time  rather  than  any  other,  because  it  was  more  conducive  than  otherwise  to 
the  well  being  of  the  countless  myriads  of  species,  the  production  of  the  Divine 
hand,  for  whose  enjoyment  the  earth  was  made  ?  Had  the  time  of  rotation  been 
materially  less  than  it  is,  our  periods  of  activity  and  labor  would  be  too  short  to 
prepare  us  for  the  return  of  darkness,  and  had  the  time  of  rotation  been  greater, 
we  should  have  needed  rest  before  the  return  of  the  natural  epoch  designed  for 
it.  As  it  is,  the  natural  vicissitudes  are  nicely  adapted  to  our  wants  ;  and  yet  our 
organization  is  in  no  way  connected  physically  with  the  rotation  of  the  earth, 
by  any  relation  of  the  nature  of  cause  and  effect,  and  to  suppose  such  an 
adaptation  fortuitous,  would  be  an  outrage  upon  all  principles  of  probability. 
This  mutual  fitness  is,  then,  another  of  the  many  proofs  which  offer  themselves 
that  the  earth  as  a  dwelling,  and  man  as  a  dweller,  has  been  each  expressly 
designed  for  the  other. 

Many  practical  examples  may  be  given  of  this  correspondence  between  the 
time  of  rotation  of  the  earth  upon  its  axis  and  the  periodical  functions  of  the 
organized  world.  Thus,  Linnaeus  proposed  the  use  of  what  he  termed  a  flo- 
ral clock,  which  was  to  consist  of  plants  which  opened  and  closed  their  blos- 
soms at  particular  hours  of  the  day.  Thus,  the  day-lily  opens  at  five  in  the 
morning,  the  common  dandelion  at  six,  the  hawkweed  at  seven,  the  ma- 
rigold at  nine,  and  so  on  ;  the  closing  of  the  blossoms  marking  corresponding 
hours  in  the  afternoon.  Nor  was  this  to  be  regarded  as  a  specific  effect  of  light 
upon  the  plants,  for  when  the  flowers  were  introduced  into  a  dark  chamber 
they  were  found  to  open  and  close  their  blossoms  at  the  same  times. 

The  necessity  of  observing  a  correspondence  between  the  intervals  of  activ- 
ity and  repose,  the  taking  of  food,  &c.,  and  the  period  of  light  and  darkness, 
was  practically  shown  in  the  case  of  voyages  made  to  the  north  pole,  where 
navigators  attained  those  latitudes  in  which  the  sun  never  rises  for  several 
weeks,  in  which  cases  it  was  found  necessary  to  make  the  crews  of  the  ships 
adhere  with  the  utmost  punctuality  to  the  habit  of  retiring  at  nine  o'clock  and 
rising  at  a  quarter  before  six.  Under  these  circumstances  they  enjoyed  a 
state  of  salubrity  very  remarkable,  notwithstanding  the  trying  severity  of  cli- 
mate to  which  they  were  exposed. 

Seeing  then, — that  the  expedient  of  making  the  globe  of  the  earth  turn  upon 
its  axis  in  twenty-four  hours  is  one  productive  of  such  multifarious  benefits, 
and  so  intimately  related  to  the  organized  species  of  our  globe,  that  were  it  to 
turn  otherwise  than  it  does,  in  a  greater  or  less  time,  an  entire  derangement  of 
the  animal  or  vegetable  economy  would  ensue, — it  becomes  an  interesting  ques- 
tion to  ascertain  whether  the  other  planets  are  provided  with  a  similar  expedi- 
ent ;  and  if  so,  to  what  extent  the  application  of  such  expedient  corresponds 
with  the  case  of  the  earth.  We  accordingly  find  that  all  the  planets  without 
exception  have  a  motion  of  rotation  on  certain  diameters  as  an  axis  while  they 
make  their  periodical  revolutions  round  the  sun,  and  that  the  diameter  in  which 
they  so  rotate  has  been  selected  in  such  a  manner  as  to  secure  to  each  of  them 
regular  alternations  of  light  and  darkness  in  every  part  of  their  surfaces  ;  in 
fact,  they,  like  the  earth,  have  days  and  nights.  But  are  those  days  and  nights 
regulated  by  the  same  intervals  as  ours  ?  for  that  is  an  important  question ; 
such  intervals  being,  as  we  have  shown,  a  key  to  the  organizations  and  func- 
tions of  the  creatures  upon  them  respectively. 

We  shall  on  another  occasion  show  that  the  planets  consist  of  two  groups 
which,  although  characterized  by  common  qualities,  are  still  distinct  in  several 
particulars.  The  inner  group  consists  of  Mercury,  Venus,  Mars,  and  the  Earth; 
the  outer  group  consists  of  Jupiter,  Saturn,  and  Herschel.  There  are  circum-  ) 


THE  PLURALITY  OP  WORLDS.  57 


stances  which  prepare  us  to  expect  some  discrepancies  in  the  provisions  made 
in  these  two  groups  ;  but  everything  leads  us  to  anticipate  a  uniformity  in 
each  of  them  respectively.  We  shall  on  another  occasion  show  that  the 
three  planets,  Mercury,  Venus,  and  Mars,  which  with  our  own  form  the 
inner  group,  do  all  turn  on  their  axes  ;  that  they  have  all  a  diurnal  motion 
completed  in  the  same  time,  or  very  nearly  so,  as  that  of  the  earth.  Thus 
these  several  planets  not  only  have  days  and  nights,  but  have  days  and  nights 
precisely  similar  to  our  own.  They  are  regulated  by  the  same  average  dura- 
tion ;  and  He  that  gave  them  those  alternations  has  seen  it  good  to  "  divide  the 
light  from  the  darkness"  after  the  same  fashion. 

If,  then,  the  duration  of  our  days  and  nights  be  evidently  regulated  with  a 
view  to  the  accommodation  and  well-being  of  the  organized  creatures  to  which 
the  earth  has  been  appropriated,  we  are  surely  warranted  by  all  analogy  in  con- 
cluding that  the  adaptation  of  the  same  expedients  in  the  planets,  Mercury, 
Venus,  and  Mars,  have  been  directed  to  the  same  beneficent  purposes,  and  that 
the  creatures  upon  them,  as  upon  the  earth,  are  so  organized  as  to  require  the 
same  intervals  of  labor  and  rest,  of  activity  and  repose,  of  wakefulness  and 
sleep. 

In  the  outer  group  the  times  of  rotation  are  different,  yet  among  them  a  sim- 
ilar uniformity  prevails.  Jupiter  and  Saturn  revolve  on  their  axes  in  about  ten 
hours'.  The  telescope  has  not  informed  us  of  the  time  of  rotation  of  Herschel; 
but  it  is  probably  not  different  from  the  two  cognate  planets.  It  appears  then 
that  the  intervals  of  light  and  darkness  in  these  remote  bodies,  instead  of  being 
regulated  by  intervals  of  twelve  hours,  is  determined  by  average  intervals  of 
five  hours.  A  corresponding  difference  of  organization  and  functions  may  of 
course  be  inferred  to  prevail  upon  them ;  but  still  it  will  be  observed  that  the 
difference  between  them  and  the  inner  group,  lies  merely  in  the  duration  of 
intervals  of  light  and  darkness  ;  those  intervals  being  in  the  main  preserved. 
There  is  no  planet,  then,  in  which  are  not  provided  days  and  nights. 

In  considering  the  expedient  by  which  days  and  nights  are  secured  to  the 
planets,  it  is  interesting  to  contemplate  the  particular  position  of  the  diameters 
on  which  they  have  been  made  to  turn.  There  are  a  great  variety  of  different 
diameters  upon  which  the  earth  might  have  spun  while  it  revolves  round  the 
sun.  It  might,  for  example,  have  turned  on  a  diameter  at  right  angles  to  its 
annual  orbit.  If  it  had  been  so  we  should  have  had  equal  days  and  nights 
throughout  the  entire  year,  and  at  every  part  of  the  earth.  It  might  again  have 
turned  upon  a  diameter  lying  in  the  plane  of  its  annual  orbit.  In  such  a  case  we 
should  not  have  had  alternations  of  days  and  nights  at  all ;  we  should  have  had 
the  sun  constantly  visible  for  six  months,  and  absent  for  other  six  months,  mod- 
ified in  a  very  complex  manner,  however,  by  other  vicissitudes ;  in  fact  we  should 
have  had  changes  of  light  and  darkness  utterly  unfit  for  our  wants.  In  the 
first  case  we  should  have  been  deprived  of  seasons  and  of  the  means  of  main- 
taining any  convenient  chronology.  Thus,  in  either  case,  we  should  be  strip- 
ped of  many  of  the  benefits  and  utilities  arising  from  the  present  arrangement. 
Again,  the  earth  might  have  turned  upon  an  axis  nearly  perpendicular  to  the  plane 
of  its  annual  orbit ;  or  in  nearly  that  plane  ;  it  might,  in  fact,  be  inclined  in 
any  position,  between  those  extremes.  Had  it  stooped  down  nearly  to  the  eclip- 
tic, consequences  would  have  ensued  almost  as  fatal  as  those  which  any  position 
in  the  plane  of  the  ecliptic  would  have  inferred.'  We  find,  however,  in  fact, 
that  a  position  has  been  given  to  this  axis  slightly  inclined  from  the  perpendicu- 
lar. In  virtue  of  this  inclination  the  northern  hemisphere  leans  toward  the 
sun  during  one  half  of  the  year,  and  the  southern  hemisphere  during  the  other. 
We  enjoy  the  grateful  succession  of  seasons  ;  it  is  thus  that  spring,  summer, 
autumn,  and  winter,  follow  each  other  with  pleasant  variety,  marking  in  their 


58 


THE  PLURALITY  OF  WORLDS. 


progress  by  obvious  phenomena  the  course  of  time.  Yet  this  inclination  or 
stooping  of  the  axis  is  so  regulated  that  the  extremes  of  the  seasons  are  con- 
fined within  such  moderate  limits  as  are  necessary  and  conducive  to  the 
physical  well-being  of  the  numerous  tribes  which  people  the  earth. 

It  is  true  that  this  succession  of  seasons  was  not  indispensably  necessary  to 
the  continuance  of  the  races  that  inhabit  the  earth,  for  had  the  axis  been  per- 
pendicular to  the  orbit  so  as  to  render  days  and  nights  perpetually  and  every- 
where equal,  the  organized  world  would  still  have  continued  to  exist.  Thus 
we  see  that  the  seasons  are  a  provision  received  from  the  Divine  hand,  par- 
taking more  of  the  character  of  a  luxury  than  of  an  absolute  physical  want. 
We  could  have  done  without  them,  but  not  so  well.  We  are  therefore  pre- 
pared on  examining  the  other  planets  to  expect  a  greater  difference  to  prevail 
among  them  in  this  respect  than  in  regard  to  the  other  provisions,  such  as 
days  and  nights,  without  which  the  organized  world  could  not  have  continued. 

On  examining  the  position  of  axes  on  which  the  several  planets  revolve,  we 
find  them  to  be  such  as  might  be  anticipated.  Some  of  them  correspond  almost 
minutely  with  that  of  the  earth.  Thus  the  seasons  in  Mars  are  regulated  by 
exactly  the  same  extremes  as  those  upon  the  earth ;  the  summer  and  winter 
ranging  between  similar  limits  of  heat  and  cold.  The  same  is  true  of  the 
planet  Saturn.  In  the  case  of  Jupiter,  on  the  other  hand,  we  find  the  axis 
nearly  perpendicular  to  the  orbit,  so  as  to  produce  scarcely  any  perceptible 
effect  in  the  form  of  seasons.  Great  difficulties  have  been  encountered  in 
ascertaining  the  position  of  the  axes  of  the  planets  Mercury  and  Venus.  There 
appears  reason  for  believing  that  they  are  inclined  at  very  great  angles  from 
the  perpendicular,  and  consequently  that  the  extremes  of  the  seasons  are  pro- 
portionally great ;  in  short,  if  the  position  of  the  axes  of  these  planets  be  rightly 
determined  a  very  complicated  succession  of  seasons  would  prevail  upon  their 
surfaces  ;  however,  until  observations  of  a  most  decisive  character  shall  be  ob- 
tained, it  is  vain  to  speculate  upon  these  bodies. 

The  atmosphere  which  surrounds  our  globe  is  an  appendage  which  does  not 
arise  from  any  known  physical  law,  yet  it  is  one  which  has  an  obvious  and 
important  relation  to  the  animal  and  vegetable  kingdoms.  That  respiratory 
beings  depend  upon  it  for  the  maintenance  of  vitality  is  obvious.  The  me- 
chanical and  chemical  apparatus  of  the  breathing  organs  is  expressly  con- 
structed to  be  the  object  of  its  operation.  Its  relation  to  vegetable  life  is  no 
less  important.  But  besides  these  qualities,  without  which  life  would  become 
extinct  on  the  surface  of  the  globe,  the  atmosphere  administers  to  our  con- 
venience and  pleasures  in  other  ways.  It  is  the  medium  by  which  sound  is 
transmitted ;  and  as  the  apparatus  of  the  lungs  is  adapted  to  operate  chemi- 
cally upon  it,  so  as  to  impart  to  the  blood  the  principle  by  which  that  fluid  sus- 
tains life,  so  the  exquisite  mechanism  of  the  ear  is  constituted  to  receive  the 
effects  of  its  pulsations  and  convey  them  to  the  sensorium  to  produce  the  per-  > 
ception  of  sound.  Again,  the  mechanism  of  the  organs  of  voice  is  adapted  to 
impress  on  the  atmosphere  those  pulsations,  and  thereby  to  convey  its  intona- 
tions to  the  correspondingly  susceptible  organization  of  the  ear.  Without  '.he 
atmosphere,  therefore,  even  supposing  we  could  live  in  its  absence,  however 
perfect  might  be  our  organs  of  speech  and  hearing,  we  should  possess  them  in 
vain.  Voice  we  might  have,  but  no  word  could  we  utter ;  listeners  we  might 
be,  but  no  sound  could  we  hear ;  endowed  with  the  full  powers  of  hearing  and 
speaking,  we  should  nevertheless  be  deaf  and  dumb. 

Another  important  manner  in  which  the  atmosphere  administers  to  our  con- 
venience, is  by  diffusing  in  an  agreeable  manner  the  solar  light,  and  mitigating  ^ 
its  intensity.     In  this  respect,  the  atmosphere  may  be  considered  as  perform-  I 
ing  in  regard  to  the  sun  what  the  imperfect  transparency  of  a  ground-glass  ) 


THE  PLURALITY  OF  WORLDS. 


shade  performs  for  the  glare  of  the  lamp.  In  the  absence  of  an  atmosphere,  the 
light  of  the  sun  would  only  illuminate  objects  on  which  its  direct  rays  would 
fall ;  we  should  have  no  other  degrees  of  light  but  the  glare  of  intense  sun- 
shine, or  the  most  impenetrable  darkness.  Shade,  there  would  be  none ;  the 
apartment  whose  casement  did  not  face  the  sun,  at  the  mid-day  would  be  as  at 
midnight.  The  presence  of  a  mass  of  air  extending  from  the  surface  of  the 
earth  upward  to  a  height  of  from  thirty  to  forty  miles,  becomes  strongly  illumi- 
nated by  the  sun.  This  air  reflects  the  solar  light  on  every  object  exposed  to 
it,  and  as  it  spreads  over  every  part  of  the  earth's  surface,  it  conveys  with  it 
the  reflected,  but  greatly  mitigated  light  of  the  sun. 

When  the  evening  sun  withdraws  its  light,  the  atmosphere  continuing  to  be 
illuminated  by  its  beams,  supplies  the  gradual  declining  twilight  which  termi- 
nates in  the  shade  of  night.  Before  it  rises,  in  like  manner,  the  atmosphere 
is  the  herald  of  its  coming,  and  prepares  us  for  its  splendor  by  the  gray  dawn 
and  increasing  intensity  of  morning  twilight.  In  the  absence  of  an  atmosphere, 
the  moment  of  sunset  would  be  marked  by  an  abrupt  and  instantaneous  transi- 
tion from  the  blaze  of  solar  light  to  the  most  impenetrable  darkness  ;  and  for 
the  same  reason,  the  morning  would  be  characterized  by  an  equally  abrupt 
change  from  absolute  darkness  to  broad,  unmitigated  sunshine. 

In  the  absence  of  an  atmosphere  we  could  have  no  clouds  ;  day  would  be 
one  unvaried  wearisome  glare  of  the  sun.  The  bright  azure  sky,  so  grateful 
to  the  sight,  is  nothing  more  than  the  natural  color  of  the  air  reflected  to  the 
eye.  The  air  which  fills  a  room  is  not  perceived  to  be  blue  only  because  it  is 
not  present  in  sufficient  quantity  to  excite  in  the  eye  any  perception  of  its 
color ;  just  as  a  glass  of  sea-water  seems  translucent  and  colorless,  while  the 
same  water  viewed  through  a  considerable  depth,  appears  with  its  proper  hue 
of  green. 

When  we  look  up,  therefore,  through  forty  miles  of  atmosphere,  we  behold 
it  of  its  proper  tint  of  blue.  In  the  absence  of  the  atmosphere  the  great  vault 
of  the  heavens  would  present  one  unvaried  and  eternal  black,  the  stars  dimly 
twinkling  here  and  there,  the  whole  forming  a  most  funereal  contrast  with  the 
bright  orb  which  would  be  seen  holding  its  solitary  course  through  this  eternal 
expanse  of  darkness. 

The  atmosphere  produces  effects  on  the  temperature  of  our  habitation  which 
are  not  less  important.  It  retains  and  diffuses  warmth,  whether  proceeding 
from  the  sun  above,  or  from  sources  of  internal  heat  within  the  globe  itself. 
What  situation  with  respect  to  temperature  we  should  be  placed  in  by  its  ab- 
sence, or  even  by  a  considerable  diminution  of  its  quantity  or  density,  may  be 
easily  inferred  by  considering  the  state  of  those  parts  of  the  earth  which  are 
placed  at  such  an  altitude  as  to  leave  below  them  a  large  portion  of  the  atmo- 
sphere. The  summits  of  lofty  ridges,  such  as  those  of  the  Alps,  the  Andes, 
and  the  Himalaya,  are  examples  of  this.  No  intensity  of  direct  solar  heat  can 
compensate  for  the  absence  of  a  sufficiently  dense  atmosphere,  and  even  within 
the  tropics  water  can  not  exist  in  a  liquid  form  at  elevations  above  14,000  feet. 
The  summits  of  the  Andes  are  clothed  in  everlasting  snow. 

Had  we,  therefore,  been  unprovided  with  an  atmosphere,  or  even  had  our 
atmosphere  been  so  rare  and  attenuated  as  it  is  at  an  elevation  of  three  miles 
(scarcely  one  tenth  of  its  whole  height),  the  waters  of  our  oceans  would  have 
been  solid.  Vegetation  could  never  have  existed,  and  in  spite  of  the  light 
and  genial  warmth  of  the  sun — in  spite  of  the  grateful  changes  of  season — in 
spite  of  the  beautiful  and  simple  provision  by  which  spring  succeeds  winter, 
and  is  followed  by  summer  and  autumn,  the  earth  would  have  been  a  barren 
and  arid  waste,  enveloped  in  a  shell  of  eternal  ice,  devoid  of  life,  motion,  form, 
and  beauty. 


Seeing,  then,  how  necessary  to  the  existence  of  an  animal  and  vegetable 
world  an  atmosphere  is — how  indispensable  its  presence  is  to  a  society  of  crea- 
tures whose  means  of  intercommunication  is  sound — and  yet  bearing  in  mind  at 
the  same  time  that  this  atmosphere  is  not  essential  to  any  of  the  great  mechan- 
ical functions  of  the  earth  in  the  economy  of  the  solar  system — considering 
also  that  without  its  presence  the  part  which  that  earth,  as  a  whole,  performs 
in  the  society  of  the  planets,  would  be  the  same  as  it  now  is — can  we  come  to 
any  other  conclusion  than  that  this  atmosphere  was  cast  around  the  earth  ex- 
pressly with  a  view  of  the  well-being  of  its  occupants— to  afford  them  a  genial 
warmth — to  give  them  diffused  and  gentle  light — to  convey  the  varieties  of 
sound — to  promote  and  facilitate  social  felicity,  by  supplying  the  means  of 
intercommunication  by  language — to  preserve  the  seas  liquid — and  supplying 
propitious  winds  to  stimulate  the  intercourse  of  nations  and  knit  together  the 
races  of  beings  who  occupy  its  most  distant  points  by  the  kindly  bonds  of  re- 
ciprocal beneficence  ?  If  then  such,  and  such  only,  be  admitted  to  be  the  pur- 
poses and  uses  of  our  atmosphere,  the  question  whether  other  planets,  in  situa- 
tions resembling  ours,  are  occupied  by  similar  beings,  must  be  materially  influ- 
enced by  the  result  of  an  investigation  as  to  whether  or  not  these  planets  are 
supplied  with  like  atmospheres. 

Telescopic  observations  have  most  clearly  and  satisfactorily  answered  this 
question.  The  atmosphere  around  the  planets  are  as  palpable  to  sight  as  the 
clouds  which  float  on  our  own.  Venus  and  Mercury  are  enveloped  in  thick 
atmospheres:  in  the  former  the  air  is  especially  conspicuous,  nay,  \ve  can 
even  see  the  morning  and  evening  twilight  in  that  distant  world.  The  atmo- 
sphere of  Mars  is  likewise  apparent.  We  see  the  clouds  floating  on  it.  Ju- 
piter and  Saturn  afford  not  less  unequivocal  manifestations  of  atmospheres  ; 
and  if  we  have  not  the  same  clear  and  satisfactory  evidence  in  the  case  of  H/r- 
schfil,  we  have  abundant  reason  for  the  want  of  it,  in  its  enormous  distance  and 
the  hitherto  deficiency  of  telescopic  power. 

The  ascertained  existence  of  clouds  in  the  planets  proves  more  than  the 
mere  presence  of  atmospheres  upon  them.  An  atmosphere  is  necessary  to  sup- 
port clouds,  but  must  not  be  identified  with  them.  Clouds  are  no  more  parts 
of  the  atmosphere  than  the  mud  and  sand  which  float  in  a  turbid  river  are 
parts  of  its  waters.  Water  is  converted  into  vapors  by  the  agency  of  the  sun 
and  wind.  This  vapor,  when  it  escapes  from  the  surface  of  the  liquid,  is  gen- 
erally lighter,  bulk  for  bulk,  than  that  part  of  the  atmosphere  contiguous  to  it. 
It  rises  into  more  exalted  regions,  where,  by  the  agency  of  cold,  and  by  electri- 
city, it  is  made  to  resume  its  liquid  state,  but  in  such  minute  particles  that  it 
floats  and  forms  those  semi-opaque  masses  called  clouds.  Clouds  are,  then,  in 
fact,  water  existing  in  a  very  minute  state  of  mechanical  division,  and  affected 
in  peculiar  ways  by  electricity. 

When  these  particles  are  caused  to  coalesce  into  drops  or  spherules  of  wa- 
ter— an  efl'ect  which  may  arise  from  temperature  or  electricity,  or  both  combined 
— their  weight  renders  their  further  suspension  impossible,  and  they  descend  to 
the  surface  in  the  form  of  rain  ;  or  if  the  cold  be  so  great  as  to  congeal  the  par- 
ticles before  they  coalesce  into  globules,  they  descend  in  the  form  of  snow  ;  or. 
finally,  if  by  the  sudden  evolution  of  heat  caused  by  electrical  influences  their 
solidification  is  effected  into  drops,  they  come  down  in  the  form  of  hail. 

Thus  wherever  the  existence  of  clouds  is  made  manifest,  there  WATER  must 
exist;  there  EVAPORATION  must  go  on;  there  ELECTRICITY,  with  its  train  of  kin- 
dred phenomena,  must  reign;  Mere  RAINS  must  fall;  there  HAIL  and  SNOW 
must  descend. 

That  healthful  and  refreshing  winds  agitate  the  atmospheres  of  the  group  of 
worlds  in  the  centre  of  which  our  suu  presides,  and  of  which  he  is  the  common  I 


THE  PLURALITY  OF  WORLDS. 


band — that  showers  refresh  their  surfaces — that  their  climates  and  seasons  are 
modified  by  evaporation — that  their  continents  are  bounded  by  seas  and  oceans 
— that  intercourse  is  facilitated  by  winds  which  convert  the  surfaces  of  their 
waters  into  highroads  for  nations — these  and  a  thousand  other  consequences  of 
what  has  been  here  explained,  all  tending  to  one  conclusion — that  these  vari- 
ous globes  are  placed  in  the  system  for  the  same  purpose  as  the  earth — that 
they  are  in  fact,  the  dwellings  of  beings  in  all  respects,  even  from  their  lowest 
physical  wants  to  their  highest  social  advantages,  like  ourselves,  crowd  upon 
the  mind  so  thickly  that  we  can  scarcely  give  them  expression  in  a  clear  and 
intelligible  order. 

It  may  be  asked  whether  by  immediate  observation  we  may  not  perceive 
the  geographical  surfaces  of  the  planets,  so  as  to  declare  by  direct  survey 
their  divisions  of  land  and  water,  mountain  and  valley,  and  other  varieties  of 
surface. 

Even  the  most  superficial  view  of  the  subject  will  render  apparent  some 
great  difficulties  which  must  obstruct  such  an  inquiry  with  respect  to  most  of 
the  planets.  The  very  presence  of  those  atmospheres  and  the  clouds  with  which 
they  are  loaded,  offers  a  serious  obstruction  to  any  observations  having  for 
their  object  to  ascertain  the  geographical  character  of  their  surfaces.  The 
great  distance  of  some  of  them  is  a  formidable  obstacle  to  such  an  inquiry ; 
still,  where  some  peculiar  circumstances  favor  the  observation,  something  has 
been  done  in  this  investigation. 

Venus  and  Mars,  the  two  planets  in  the  system  which  come  nearest  to  the 
path  of  the  earth,  are  evidently  the  most  eligible  objects  for  such  an  inquiry, 
and  sufficient  has  been  ascertained,  especially  with  regard  to  the  latter  planet, 
to  draw  very  closely  indeed  the  ties  of  analogy  by  which  the  planets  are  asso- 
ciated with  the  earth. 

Notwithstanding  the  dense  atmosphere  and  thick  clouds  with  which  Venus 
and  Mercury  are  constantly  enveloped,  the  existence  of  mountains  of  great  eleva- 
tion upon  them  has  been  discovered ;  but  it  is  upon  the  planet  Mars  that  the 
most  surprising  advances  have  been  made  in  this  department  of  telescopic  in- 
quiry. The  Prussian  astronomers,  Beer  and  Madler,  have  devoted  their  labors 
for  many  years  back  to  the  examination  of  Mars,  and  the  result  has  put  us  m 
possession  of  a  map  of  the  geography  of  that  planet,  almost  as  exact  and  well 
defined  as  that  which  we  possess  of  our  own.  In  fact,  the  geographical  outlines 
of  land  and  water  have  been  made  apparent  upon  it.  Thus  we  see  that  in  the 
other  planets  on  which  the  clouds  clear  away  sufficiently  to  disclose  to  our  view 
their  geographical  nature,  the  surface  is  the  same  as  our  own ;  and  analogy 
justifies  the  conclusion  that,  if  we  could  get  an  equally  clear  view  of  the  sur- 
faces of  the  other  planets,  we  sh6uld  find  upon  them  the  same  characteristics. 
Connected  with  the  observations  of  these  Prussian  astronomers,  as  well  as 
those  of  the  younger  Herschel  on  the  planet  Mars,  there  is  a  circumstance  too 
interesting  to  be  passed  without  noticing  it  here.  They  have  discovered,  on 
the  polar  regions  of  that  planet,  an  extensive  deposition  of  snow,  which  is 
found,  in  a  great  degree,  to  melt  away  during  the  summer,  and  to  be  reproduced 
during  the  winter. 

In  tracing  the  analogies  whieh  prove  the  suitableness  of  the  planets  for  in- 
habitable  globes,  and  which  connect  them  by  ties  of  kindred  with  the  earth, 
one  of  the  most  important  and  interesting  is  dependant  upon  the  quantity  of 
matter  composing  these  planets,  compared  with  their  volumes  or  bulks.  Let  us 
see  how  this  affects  the  condition  of  the  organized  creatures  that  dwell  upon 
them. 

All  organized  beings,  whether  animal  or  vegetable,  are  endowed  with  a  cer- 
tain limited  amount  of  bodily  strength.  In  the  case  of  animals,  which  have 


62  THE  PLURALITY  OF  WORLDS. 

powers  of  locomotion,  this  strength  is  regulated  with  reference  to  their  weight, 
and  the  extent  and  quantity  of  motion  necessary  for  their  well-being  on  the 
surface  of  the  globe.  The  structure  of  every  animal  is  such,  in  the  first  place, 
as  to  give  it  strength  to  support  and  move  its  own  body  ;  but  this  is  not  enough  ; 
it  must  have  a  further  amount  of  disposable  force,  to  enable  it  to  supply  its  own 
wants  by  the  pursuit  of  its  prey  ;  by  the  collection  of  its  food  ;  by  the  erection  of 
its  dwelling ;  and,  in  general,  by  its  labor  in  the  supply  of  its  physical  wants. 
In  the  case  of  vegetables,  the  strength  must  be  sufficient  to  support  its  weight, 
and  resist  those  external  disturbances  to  which  it  is  exposed — such  as  -the  ac- 
tion of  winds  and  other  natural  effects.  But  what,  let  us  ask,  regulates  this 
necessary  quantity  of  strength  ?  What  is  the  chief  resistance  which  it  has  to 
overcome  1  We  answer,  mainly  the  weight  of  the  creature  itself.  But  again  ; 
what  is  this  weight  ?  It  is  a  force  produced  by  what  ?  By  the  combined  at- 
tractions of  the  whole  mass  of  matter  composing  the  globe  of  the  earth,  exer- 
cised upon  the  matter  composing  the  creature  itself;  thus  the  weight  of  a  man 
is  merely  the  amount  of  the  attraction  of  the  globe  of  the  earth  exercised  upon 
the  matter  composing  the  body  of  the  man.  The  amount  of  this  attraction, 
therefore,  depends  upon  the  quantity  of  matter  in  the  earth ;  but  not  on  that 
alone  :  it  is  a  universal  law  of  nature,  that  the  energy  of  the  attraction  exerted 
by  matter,  is  increased  with  the  proximity  of  the  attracted  body  to  the  centre 
of  the  attracted  mass.  Now  if  the  matter  composing  the  globe  of  the  earth 
were  condensed  into  half  its  present  bulk,  all  bodies  placed  upon  the  surface, 
being  proportionally  nearer  the  centre,  would  be  attracted  with  greater  energy; 
and,  on  the  other  hand,  if  the  matter  of  the  earth  were  swelled  into  a  larger 
bulk,  the  distance  of  objects  on  the  surface  from  the  centre  being  proportion- 
ally increased,  the  energy  of  the  attraction  would  be  diminished.  In  the  one  case 
the  weights  of  all  bodies  would  be  augmented,  and  in  the  other  they  would  be 
diminished.  The  weights,  then,  of  bodies  placed  on  the  surface  of  the  earth, 
depend  conjointly  in  the  mass  of  matter  composing  the  earth,  and  on  its 
density. 

It  is  evident,  then,  that  the  adaptation  which  we  see  usually  to  prevail  between 
the  strength  of  animals  and  plants  and  their  weights,  is,  in  reality,  an  exquisite 
harmony  which  is  maintained  between  the  strength  of  these  infinitely  various 
tribes  of  organized  creatures,  and  the  mass  and  density  of  the  globe  upon  which 
they  are  placed  ;  the  slightest  disturbance  or  change  in  this  relation  would 
utterly  derange  the  fitness  of  things,  and  would  render  the  globe  unfit  for  its 
creatures,  and  its  creatures  unfit  for  the  globe.  The  amount  of  attraction,  or, 
to  use  the  more  familiar  term,  the  weight  of  the  body  on  the  surface  of  the 
globe,  is,  then,  an  index,  so  to  speak,  to  the  organization  of  the  creatures  placed 
upon  the  globe.  If  we  would,  then,  inquire  respecting  the  probable  organiza- 
tion of  the  dwellers  upon  the  planets,  one  of  the  means  of  our  inquiry  would 
be  to  ascertain  what  would  be  the  weights  of  bodies  upon  their  surfaces.  Physi- 
cal science  enables  us  perfectly  to  accomplish  this.  The  masses  of  matter 
composing  all  the  planets  have  been  discovered  with  a  great  degree  of  precision. 
Their  magnitudes  have  also  been  measured.  Now,  to  ascertain  the  weights  of 
bodies  placed  upon  the  surface  of  any  of  them,  it  is  only  necessary  to  consider 
their  masses  and  their  magnitudes.  The  weight  of  a  body  placed  upon  any 
planet  is  greater  or  less,  caeteris  parib us,  than  the  weight  of  a  body  placed  upon  the 
earth,  just  in  proportion  as  the  mass  of  matter  in  the  planet  is  greater  or  less  than 
the  mass  of  matter  in  the  earth.  If  the  distance  from  the  surface  to  the  centre  of 
the  planet  be  double  the  corresponding  distance  in  the  case  of  the  earth,  then 
the  weight  of  bodies  upon  its  surface  would,  on  that  account  alone,  be  four 
times  less  than  in  the  case  of  the  earth.  But  if,  at  the  same  time,  the  mass  of 
matter  in  the  planet  were  sixteen  times  greater  than  the  mass  of  matter  in  the 


THE  PLURALITY  OF  WORLDS. 


earth,  then,  the  weight  of  bodies  on  the  planet,  on  that  account  alone,  would  be 
sixteen  times  greater.  The  weight,  then,  on  the  one  score,  would  be  sixteen 
times  greater,  and  on  the  other,  four  times  less  ;  the  result  being  that  the  actual 
weight  under  such  circumstances,  would  be  four  times  greater  than  upon  the 
earth.  Such  are  the  principles  by  which  may  be  calculated  the  weights  of 
bodies  upon  the  surfaces  of  the  different  planets.  It  has  been  found  that  the 
weights  of  bodies  on  the  surfaces  of  Mercury,  Venus,  and  Saturn,  are  nearly 
the  same  as  upon  the  earth ;  that  upon  Mercury  they  are  one  half  less,  and  on 
Jupiter  three  times  more.  Thus  it  is  apparent  that  there  are  no  very  extreme 
deviations  in  weight,  comparing  the  surface  of  one  planet  with  another,  and 
hence  we  are  led  to  infer  the  probability  of  an  organization  not  very  different 
upon  the  several  planets. 

We  have  already  explained  by  how  easy  means  the  great  variety  of  light 
and  warmth  conveyed  to  the  different  planets  by  the  sun  may  be  practically 
equalized,  by  the  adaptation  of  the  organization  of  the  eye,  and  the  regulation 
of  the  density  of  the  atmosphere.  Since,  however,  this  difference  in  the  physi- 
cal condition  of  the  planets  excites  usually  much  attention,  it  may  be  well  here, 
before  closing  this  discourse,  to  enlarge  somewhat  further  on  this  point. 

The  principles  of  optics  prove  that  the  sun's  light  will  be  less  upon  the 
planet  Mars  than  upon  the  earth,  in  the  proportion  of  one  to  two.  Jupiter  will 
receive  about  twenty-five  times,  and  Saturn  about  one  hundred  times  less 
warmth  than  the  earth  does,  while  the  diminution  in  the  case  of  the  most  re- 
mote planet,  Herschel,  will  be  nearly  four  hundred  fold ;  on  the  other  hand, 
Venus  and  Mercury,  being  nearer  to  the  sun  than  the  earth,  the  one  will  re- 
ceive twice,  and  the  other  seven  times,  as  much  light  and  warmth  as  the  earth 
does.  The  apparent  magnitude  of  the  sun  to  these  planets  will  be  in  the  same 
proportion.  To  Jupiter  it  will  have  an  apparent  diameter  five  times  less  than 
to  the  earth.  To  Saturn  the  diameter  will  be  ten  times  less,  and  to  the  planet 
Herschel  nearly  twenty  times  less. 

The  apparent  magnitude  of  the  sun  as  we  behold  it  is  measured  by  an  angle 
of  about  thirty  minutes  ;  consequently,  to  the  inhabitants  of  the  planet  Herschel 
it  will  appear  under  an  angle  less  than  two  minutes,  or  about  three  times  the 
size  of  Jupiter  when  that  planet  appears  the  largest  and  brightest.  We  should, 
however,  form  a  very  erroneous  estimate  of  the  actual  light  of  the  sun  under 
these  circumstanes  by  these  comparisons.  It  shines  by  its  own  light,  whereas 
the  objects  with  which  it  is  attempted  to  be  compared  shine  with  reflected 
light.  The  full  moon  has  the  same  apparent  magnitude  as  the  sun,  the  differ- 
ence being  that  the  one  shines  with  direct,  and  the  other  with  reflected  light ; 
how  much  is  lost  in  splendor  on  this  score  may  be  judged,  when  we  state  that 
the  light  of  the  full  moon  is  three  hundred  thousand  times  less  than  that  of  the 
sun  ;  we  may  also  form  some  guess  at  the  effect  of  the  sun's  light,  even  at  the 
most  remote  planet,  Herschel,  when  it  is  stated  that  it  gives  a  light  equal  nearly 
to  that  of  a  thousand  full  moons. 

If  we  could  actually  behold  the  da%  of  Saturn  and  Herschel  on  the  one 
lined,  and  of  Mercury  and  Venus  on  the  other,  we  should  be  surprised  how 
disproportionate  to  their  numerical  representation  their  apparent  splendor  would 
be.  The  eye  is  a  bad  photometer.  In  a  solar  eclipse,  in  which  half  the  sun's 
disk  is  covered,  we  are  scarcely  sensible  of  diminished  light ;  and  even  when 
the  eclipse  is  nearly  total — when  only  a  thin  crescent  of  the  sun  remains  un- 
covered— there  is  still  the  broad  light  of  day,  though  very  sensibly  diminished 
in  splendor.  A  thick  covering  of  clouds  upon  the  firmament  produces  an  im- 
mense numerical  diminution  of  the  light  of  day,  yet  we  suffer  no  inconveni- 
ence in  being  exposed  to  all  the  varying  degrees  of  splendor  between  that  and 
ihe  unclouded  radiance  of  a  summer's  sun. 


64 


THE  PLURALITY  OF  WORLDS. 


How  various  may  be  the  circumstances  of  climate  and  temperature  in  places 
receiving  exactly  the  same  influences  from  the  sun's  rays,  will  be  apparent  by 
a  reference  to  the  tropical  regions  of  our  own  globe.  There  under  the  same 
influences  of  the  same  solar  heat,  we  have  in  different  elevations  every  variety 
of  climate  and  temperature.  On  the  general  surface,  near  the  elevation  of  the 
sea,  we  have  the  fierce  climate  of  the  torrid  zone  ;  we  have  only  to  ascend 
the  mountains  to  a  certain  height,  to  behold  the  trees,  fruits,  and  flowers,  of  the 
temperate  zone  ;  while  at  a  still  greater  elevation,  we  encounter  all  the  atmo- 
spheric phenomena  and  vegetable  productions  of  the  frigid  zone.  In  the  low 
valleys  of  the  Andes  are  rich  bananas  and  palms,  while  the  elevated  parts  of 
the  range  produce  oaks,  firs,  and  the  tribes  common  to  the  north  of  Europe. 
The  oak  flourishes  on  them  at  elevations  varying  from  six  to  ten  thousand 
feet.  At  fifteen  thousand  feet  of  height  vegetation  disappears,  save  the  lichens, 
and  then  we  enter  the  solitude  of  everlasting  snow,  in  which  every  living  thing 
disappears. 

How  easy,  then,  and  how  natural,  is  it  not,  to  conceive  that  atmospheric  ar- 
rangements like  those  which,  under  a  tropical  sun,  produce  at  certain  eleva- 
tions the  moderate  temperature  of  our  own  climate — at  others,  less  or  greater, 
the  fierce  heat  of  the  line,  or  the  rigor  of  the  poles — may  be  the  means  of 
modifying  the  varieties  of  effect  which  would  be  produced  in  different  planets 
by  their  different  distances  from  the  sun  ! 

Such  is,  then,  the  brief  view  which  we  offer  of  that  vast  body  of  analogy 
which  leads  the  intelligent  and  reflecting  mind,  that  loves  to  see  the  most  ex- 
alted attributes  of  Divine  power  manifested  throughout  all  parts  of  creation,  to 
the  conclusion  that  the  planets  are  worlds,  fulfilling  in  the  economy  of  the  uni- 
verse the  same  functions,  and  are  created  by  the  same  Divine  hand,  for  the 
same  moral  purposes,  and  with  the  same  destinies,  as  the  earth. 


r-*-^^>*> 


THE    8  U  N . 


The  most  Interesting  Object  in  the  Firmament. — Its  Distance. — How  Measured. — Its  Magnitude. — 
How  Ascertained. — Its  Bulk  and  Weight. — Its  Density. — Form. — Time  of  Rotation. — Spots. — 
Its  Physical  Constitution. — Nature  of  the  Spots. — Luminous  Coating. — Its  Thickness. — Probable 
Temperature  of  the  Surface  of  the  Sun. — Nature  of  its  Luminous  Matter. 
> 


"\ 

THE  SUN.  67 


THE    SUN. 


ALTHOUGH  perhaps  the  moon  is  the  object  among  the  heavenly  bodies 
which  presents  the  subject  of  most  interesting  inquiry  to  the  world  in  general, 
yet,  to  the  thoughtful  and  contemplative  mind,  the  Sun  is  undoubtedly  one  of 
vastly  superior  interest.  The  sun — the  fountain  of  light  and  life  to  a  family 
of  circumvolving  worlds — the  inexhaustible  store  of  genial  warmth  by  which  the 
countless  tribes  of  organized  beings  that  people  these  globes  are  sustained — 
the  physical  bond  whose  predominating  attraction  gives  stability,  uniformity, 
and  harmony,  to  the  movements  of  the  entire  planetary  system  :  to  collect  to- 
gether in  a  brief  compass  the  information  which  modern  scientific  research  has 
supplied  relating  to  this  body,  cannot  be  otherwise  than  an  interesting  and 
agreeable  task. 

DISTANCE    OF    THE    SUN. 

When  we  direct  our  inquiries  to  any  object  in  the  heavens,  the  first  ques- 
tions which  present  themselves  naturally  to  us  are,  "  What  is  its  distance, 
magnitude,  motion,  and  position  ?"     When  we  say  that  the  distances  of  the 
bodies  composing  the  solar  system  can  be  measured  with  the  same  degree 
of  relative  accuracy  with  which  we  ascertain  the  distances  of  bodies  on  the 
)  surface  of  the  earth,  those  who  are   unaccustomed  to  investigations  of  this 
I  kind  usually  receive  the  statement  with  a  certain  degree  of  doubt  and  incredu- 
1  lity ;  they  cannot  conceive  how  such  spaces  can  be  accurately  measured,  or 
J  indeed  measured  at  all.     Thus,  when  they  are  told  that  the  sun  is  at  a  distance 
i  from  the  earth  amounting  to  nearly  100,000,000  of  miles,  the  mind  instantly  re- 
j  volts  from  the  idea  that  such  a  space  could  be  exactly  ascertained  and  esti- 
'  mated.     Yet,  let  us  ask,  why  this  difficulty?  whence  this  incredulity  ?     Is  it 
{  because  the  distance  thus  measured  is  enormously  great  ?     Greater  transcend- 
)  ently  than  any  distance  we  are  accustumed  to  contemplate  upon  our  own  globe  1 
j  To  this  we  reply  that  the  magnitude  of  a  distance  or  space  does  not  constitute 
}  of  itself  any  difficulty  in  its  admeasurement.     Nay,  on    the   contrary,  it   is 


THE  SUN. 


often  the  case  that  we  are  able  to  measure  large  distances  with  greater  ac- 
curacy than  small  ones  ;  this  is  frequently  so  in  the  surveys  conducted  on  the 
surface  of  our  own  globe.  If,  then,  the  greatness  of  the  magnitudes  does  not 
.  constitute  of  itself  any  difficulty,  to  what  are  we  to  ascribe  the  doubt  entertained 
|  by  the  popular  mind  in  regard  to  such  measurement  ?  It  will,  perhaps,  be 
replied  that  the  object,  whose  distance  we  claim  to  have  measured,  is  inacces- 
sible to  us  ;  that  we  cannot  travel  over  the  intermediate  space,  and  therefore 
<  cannot  be  conceived  to  measure  it.  But  again,  let  us  ask  whether  this  cir- 
cumstance of  being  inaccessible  constitutes  any  real  difficulty  in  the  measure- 
ment of  the  distance  of  an  object  ?  The  military  engineer,  who  directs  his 
projectiles  against  the  buildings  within  a  town  which  is  besieged,  can,  as  we 
well  know,  level  them  so  as  to  cause  a  shell  to  drop  on  any  individual  building 
which  may  have  been  chosen.  To  do  this,  he  must  know  the  exact  distance 
of  the  building  from  the  mortar.  Yet  the  building  is  inaccessible  to  him  ;  the 
walls  of  the  town,  the  fortifications,  and  perhaps  a  river,  intervene.  Yet  he 
finds  no  difficulty  in  measuring  the  distance  of  this  inaccessible  building.  To 
accomplish  this,  he  lays  down  a  space  upon  the  ground  he  occupies,  called  the 
base  line,  from  the  extremities  of  which  he  takes  the  bearings  or  directions  of 
the  building  in  question.  From  these  bearings,  and  from  the  length  of  the 
base  line,  he  is  enabled  to  calculate  by  the  most  simple  principles  of  geometry 
and  arithmetic  the  distance  of  the  building  in  question.  Now  imagine  the 
building  in  question  to  be  the  sun,  and  the  base  line  to  be  the  whole  diameter 
of  the  globe  of  the  earth .  in  what  respect  would  the  problem  be  altered  ?  The 
building  within  the  town  is  inaccessible — so  is  the  sun  ;  the  base  line  of  the 
engineer  is  exactly  known — so  is  the  diameter  of  the  earth  ;  the  bearings  of 
the  building  from  the  ends  of  the  base  line  are  known — so  are  the  bearings  of 
the  sun's  centre  from  the  extremes  of  the  earth's  diameter.  The  problems  are, 
in  fact,  identical  ;  they  differ  in  nothing  except  the  accidental  and  unimportant 
circumstance  of  the  magnitudes  of  the  lines  and  angles  that  enter  the  question. 
In  short,  the  measurement  of  distances  of  objects  in  the  heavens  is  effected 
upon  principles  in  all  respects  similar  to  those  which  govern  the  measurement 
of  distances  upon  the  earth  ;  nor  are  they  attended  with  a  greater  difficulty,  or 
more  extensive  sources  of  error. 

By  such  means,  then,  it  has  been  ascertained  that  the  distance  of  the 
sun  from  the  earth  is  about  100,000,000  of  miles.  The  distance  is  more  .ex- 
actly 95,000,000  of  miles  ;  but  let  me  counsel  those,  who  for  the  mere  pur- 
pose of  general  information,  and  without  any  strictly  or  scientific  object,  study 
subjects  of  this  nature,  to  be  content  to  confine  themselves  generally  to  round 
numbers — they  are  more  easily  remembered,  and  answer  all  purposes  as  well ; 
for  this  reason  I  shall,  in  the  course  of  these  discourses,  generally  adopt,  in 
the  expression  of  distances,  magnitudes,  motions,  and  times,  the  nearest  round 
numbers. 


MAGNITUDE    OF    THE    SUN. 

Having  explained  the  distance  of  the  sun,  let  us  now  see  how  its  magnitude 
can  be  ascertained.  There  is  one  general  principle  by  which  the  magnitudes 
of  all  the  heavenly  bodies  can  be  ascertained  when  their  distance  is  known. 
This  is,  in  fact,  accomplished  by  the  device  of  comparing  them  with  some  ob- 
ject of  known  magnitude  anJ  which  at  any  known  distance  will  have  the  same 
apparent  size.  As  this  is  important,  considered  as  a  general  principle  applied 
to  all  objects  in  the  heavens,  it  may  not  be  uninteresting  to  develop  it  some- 
what fully  in  its  application  to  the  present  object,  the  sun. 


THE  SUN.  69 


The  common  observation  of  every  one  who  directs  his  view  to  the  heavens, 
will  inform  him  of  the  fact  that  the  sun  and  full  moon  appear  to  be  of  the  same 
size.  The  mere  effect  of  ordinary  visual  observation  is,  perhaps,  enough  to 
establish  this  ;  but  if  more  be  desired,  instruments  expressly  adapted  to  meas- 
ure the  apparent  magnitudes  of  objects  may  be  applied.  We  are  also  con- 
firmed in  the  fact  by  the  consideration  of  the  well-known  phenomena  of  solar 
eclipses.  A  solar  eclipse  is  produced  by  the  interposition  of  the  globe  of  the 
moon  between  the  eye  and  the  globe  of  the  sun.  The  eclipse  is  said  to  be 
central  when  the  centre  of  the  moon  is  directly  in  line  between  the  eye  and 
the  centre  of  the  sun.  When  this  takes  place  we  find  that  the  globe  of  the 
moon  generally  covers,  pretty  exactly  that  of  the  sun.  Owing,  however,  to  a 
slight  variation  in  the  apparent  size  of  these  bodies,  from  a  cause  that  we  shall 
explain  on  another  occasion,  the  moon  at  one  time  a  little  more  than  covers  the 
sun  and  at  another  time  a  little  less.  In  short,  the  average  apparent  magnitude 
of  these  bodies  are  the  same,  the  one  exactly  covering  or  concealing  the  other. 

But  we  have  already  stated  that  the  distance  of  the  moon  is  only  a  quarter 
of  a  million  of  miles.  It  appears,  then,  that  the  distance  of  the  sun  is  four 
hundred  times  greater  than  that  of  the  moon  ;  yet  these  two  globes  appear  to 
the  eye  to  be  of  the  same  magnitude.  The  sun,  notwithstanding  its  being  four 
hundred  times  farther  off,  appears  just  as  large  as  the  moon.  What,  then,  are 
we  to  infer  respecting  its  real  magnitude  ?  If  the  sun  were  really  equal  in 
magnitude  to  the  moon,  it  would  assuredly  appear  four  hundred  times  less  at  four 
hundred  times  a  greater  distance  :  but  as  at  that  greater  distance  it  does  not  ap- 
pear less  or  greater,  but  of  the  same  magnitude,  the  irresistible  conclusion 
level  to  the  apprehension  of  any  understanding,  is,  that  the  sun  must  in  reality 
be  four  hundred  times  greater  in  its  diameter  than  the  moon.  If  it  were  less, 
at  four  hundred  times  the  moon's  distance,  it  would  appear  less  than  that  of  the 
moon  ;  if  it  were  greater,  at  that  distance  it  would  appear  greater.  It  follows, 
then,  that  whatever  be  the  magnitude  of  the  diameter  of  the  moon,  the  diame- 
ter of  the  sun  must  assuredly  be  four  hundred  times  greater.  Now  it  has  been 
ascertained  by  absolute  measurement  that  the  diameter  of  the  moon  measures 
about  two  thousand  miles.  If  we  multiply  this  by  four  hundred  we  shall  ob- 
tain eight  hundred  thousand  miles,  which  is,  therefore,  the  diameter  of  the  sun. 

These  calculations  have  been  made  roughly  and  in  round  numbers  ;  more  ac- 
curately, the  diameter  of  the  sun  measures  888,000  miles,  but  as  we  recom- 
mend the  adoption  of  round  numbers,  we  shall  call  the  sun's  diameter 
900,000  miles.  Such  is  the  stupendous  mass  placed  in  the  centre  of  the  sys- 
tem which,  by  its  attraction,  coerces  the  movements  of  the  planets. 

Such  magnitudes  are  so  far  beyond  all  the  ordinary  standards  with  which  we 
are  familiar,  that  the  imagination  is  confounded  in  its  efforts  to  form  to  itself  any 
distinct  conception  of  them.  Let  us  see  whether  we  may  not  find  some  illus- 
tration which  will  aid  the  understanding  in  conceiving  the  dimensions  of  this 
immense  globe.  We  know  that  the  earth  is  a  globe  whose  diameter  is  eight 
thousand  miles,  and  that  the  moon  holds  its  monthly  course  around  it  at  the  dis- 
tance of  about  a  quarter  of  a  million  of  miles.  Let  us  suppose  the  centre 
of  the  earth  at  E.,  placed  at  the  centre  of  the  sun.  Let  the  moon,  M., 
hold  its  monthly  course  around  it,  the  distance  from  M.  to  E.  will  then  be 
about  two  hundred  and  fifty  thousand  miles,  but  the  surface  of  the  sun.  S.,  is 
at  a  distance  from  its  centre  E.  a  little  less  than  four  hundred  and  fifty  thou- 
sand miles.  Consequently  it  follows  that  the  earth  and  its  moons  would  thus 
be  not  only  continued  within  the  globe  of  the  sun,  but  the  surface  of  the  sun 
would  even  then  be  two  hundred  thousand  miles  outside  the  monthly  orbit  oi 
the  moon.  The  sun  would,  in  fact,  contain  the  moon  and  earth  within  it,  and 
have  a  couple  of  hundred  thousand  miles  to  spare  ! 


70  THE  SUN. 


VOLUME    OF    THE    SUX. 

But  we  have  hitherto  only  spoken  of  the  diameter  of  the  sun  ;  let  us  now 
consider  its  bulk.  When  we  know  the  diameters  of  two  globes  we  can  always, 
by  an  easy  operation  of  arithmetic,  estimate  theirbulks.  Thus,  if  one  globe  have 
a  diameter  double  another,  the  bulk  of  the  former  will  be  eight  times  that  of 
the  latter.  If  the  diameter  be  ten  times  greater,  the  bulk  will  be  a  thousand 
fold  greater,  and  so  on.  Now  we  know  that  the  diameter  of  the  sun  is  about 
one  hundred  and  twelve  times  greater  than  that  of  the  earth,  from  which  we 
infer,  by  the  same  principles  of  arithmetic,  that  the  bulk  of  the  sun  must  be 
very  nearly  one  million  four  hundred  thousand  times  the  bulk  of  the  earth.  To 
make  a  globe  like  the  sun,  it  would  then  be  necessary  to  roll  one  million 
four  hundred  thousand  globes  like  the  earth  into  one !  It  is  found  by  consid- 
ering the  bulks  of  the  different  planets,  that  if  all  the  planets  and  satellites  in 
the  solar  system  were  moulded  into  a  single  globe,  that  globe  would  still  not 
exceed  the  five  hundredth  part  the  globe  of  the  sun  :  in  other  words,  the  bulk 
of  the  sun  is  five  hundred  times  greater  than  the  aggregate  bulk  of  all  the  rest 
of  the  bodies  of  the  system. 

WEIGHT    OF    THE    SCX. 

The  astronomer,  however,  is  called  upon  to  execute  processes  more  difficult 
and  yet  no  less  indispensable,   than  the  mere  measurement  of  distances  and 
magnitudes.     If  we   desire  to  know  the  quantities  of  matter  composing  those 
distant  orbs,  we  must  not  merely  measure   their  magnitudes  and  fathom  their 
distances,  but  we  must  wing   our  flight,  in  imagination,   across  those  vast  'lis- 
(  tances  which  separate  us  from  them  and  weigh  their  stupendous  masses.     If 
(  the  popular  student  finds  it  difficult  to  believe   and  comprehend  how  we  can 
measure  distances  and  magnitudes  such  as  those  of  the  heavenly  bodies,  how 
much  more  will  he  be  confounded  when  he  is  assured  that  we  have  at  our  dis- 
posal a  balance  of  the  most  unerring  exactitude  :'.n  which  we  can  place  those 
vast  orbs  and  poise  them  !     The  globe  of  the  sun  itself,  transcendency  greater 
than  the  earth  and  all  the  planets  put  together,  is  weighed  with  as  great  relative 
S  precision,  as  that  with  which  the  chemist  in  his  analysis,  estimates  the  weights 
of  the  constituents  of  the  bodies  which  pass  under  his  hands.     As  the  general 


THE  SUN. 


71 


principles  by  which  the  weights  of  the  bodies  of  the  universe  are  ascertained 
is  in  spirit  the  same  for  all,  it  may  be  worth  while  here  to  explain  the  method, 
once  for  all,  in  its  application  to  the  sun. 

When  a  body  revolves  in  a  circle,  we  know  from  common  and  familiar  ex- 
periments that  it  has  a  tendency  to  fly  from  the  centre  of  'the  circle,  which 
tendency  is  greater  the  more  rapidly  the  body  revolves  and  the  greater  its  dis- 
tance from  the  centre.  The  boy  who  whirls  a  stone  in  a  sling  is  conscious 
of  this  physical  truth.  The  stone,  as  it  revolves,  stretches  the  string  with  a 
certain  definite  force  ;  this  force  is  not  in  the  gravity  of  the  stone,  for  it  would 
be  equally  manifested  if  the  stone  revolved  in  a  horizontal  plane.  It  is  that 
tendency  which  we  have  just  adverted  to,  and  which  is  technically  called  cen- 
trifugal force.  If  you  increase  the  velocity  with  which  the  stone  is  whirled 
round,  you  will  find  the  string  will  be  more  and  more  tightly  stretched,  and 
you  may  augment  the  velocity  to  such  an  extent  as  to  break  the  string.  If  you 
lengthen  or  shorten  the  string,  preserving  the  same  velocity  of  rotation,  you  will 
find  that  the  tendency  to  stretch  the  string  will  be  proportionally  increased  or 
diminished  ;  in  short,  a  fixed  rule  or  law,  as  it  is  called,  will  be  easily  discov- 
ered by  a  series  of  simple  experiments  which  wil1  enable  us  to  predict  how 
much  the  string  will  be  stretched,  provided  we  know  the  distance  of  the 
revolving  weight  from  the  centre  of  the  circle  and  the  time  it  takes  to  make 
each  revolution. 

To  apply  this  general  principle,  then,  to  the  case  before  us,  let  it  be  consid- 
ered that  the  moon  in  its  monthly  course  revolves  in  a  circle  round  the  centre 
of  the  earth.  We  know  its  distance  and  we  know  the  time  which  it  takes  to 
make  each  revolution,  we  are  therefore  in  a  condition  to  declare  with  what 
force  it  would  stretch  a  string,  tying  it  to  the  centre  of  the  earth.  That  the 
moon  exercises  such  a  force  cannot  then  be  doubted.  But  on  what,  it  will  be 
asked,  is  that  force  expended  ?  There  is  no  string,  rod,  or  any  other  material 
or  tangible  connection  between  tho  moon  and  the  centre  of  the  earth.  And 
yet  the  moon  is  held  as  firmly  and  steadily  in  its  circular  course  round  the 
earvh,  as  if  it  were  tied  to  the  centre  by  a  string.  In  the  absence  of  the  string 
there  must  then  be  some  physical  agency  which  plays  its  part ;  there  must  be 
something  to  resist  that  tendency  which  the  string,  if  there,  would  have  resist- 
ed. That  something  was  discovered  by  Newton  to  be  the  attraction  of  the 
earth's  GRAVITATION  exercised  upon  the  moon  and  holding  the  moon  in  its  cir- 
cular orbit,  in  the  same  manner  that  it  would  be  held  by  the  string  which  has 
been  just  described.  As  we  know,  by  the  simple  mechanical  law  above  ex- 
plained, the  force  with  which  that  string  would  be  stretched  by  the  moon  in 
this  case,  we  are  enabled  by  the  same  principle  to  say  what  is  the  amount 
of  attractive  force  which  the  earth  exercises  upon  the  moon  to  keep  it  in  its 
monthly  orbit. 

In  this  manner,  in  general,  we  are  enabled  to  estimate  the  force  of  attraction 
which  a  central  mass  exercises  upon  another  body  revolving  in  a  circle  round 
it  at  a  known  distance,  and  in  a  known  time. 

While,  on  the  one  hand,  we  know  the  distance  and  time  of  the  moon's  revo- 
lution round  the  earth,  we  also  know  the  distance  and  time  of  the  earth's  revo- 
lution round  the  sun.  We  are  thus,  allowing  for  the  difference  of  the  two 
distances,  in  a  condition  to  compare  the  actual  amount  of  attraction  which  the 
earth  and  the  sun  respectively  exercise  upon  bodies  revolving  round  them,  and 
we  find,  accordingly,  that  the  attraction  exercised  by  the  sun  upon  any  body 
is  greater  than  the  attraction  that  would  be  exercised  by  the  earth  upon  the 
same  body  in  a  like  position,  in  the  proportion  of  three  hundred  and  fifty  thou- 
sand to  one.  But  as  these  attractions  are,  in  fact,  produced  by  the  respective 
masses  of  matter  composing  the  sun  and  the  earth,  it  follows  that  the  weight 


72  THE  SUN. 

of  the  sun,  or  what  is  the  same,  the  mass  of  matter  composing  it,  is  three  hun- 
dred and  fifty  thousand  times  greater  than  the  the  mass  of  matter  or  weight 
of  the  earth. 

To  make  a  globe  as  heavy  as  the  sun,  it  would  then  be  necessary  to  agglom- 
merate  into  one  three  hundred  and  fifty  thousand  globes  like  the  earth 

DENSITY    OF    THE    SUN. 

Having  ascertained  the  weights  and  bulks  of  the  bodies  of  the  universe,  we 
are  in  a  condition  to  determine  their  densities,  and  thus  to  obtain  some  clue  to  a 
knowledge  of  their  constituent  materials.  We  have  seen  that  while  the  bulk 
of  the  sun  is  about  one  million  and  four  hundred  thousand  times  greater  than 
that  of  the  earth,  its  weight  is  greater  in  the  much  less  proportion  of  three  hun- 
dred and  fifty  thousand  to  one.  Let  us  see  to  what  inference  this  leads  in  re- 
gard to  the  nature  of  the  matter  that  composes  the  sun.  If  the  materials  of  the 
sun  were  similar  to  those  of  the  earth,  its  weight  would  necessarily  be  greater 
than  that  of  the  earth  in  the  same  proportion  as  its  bulk,  and  in  that  case,  of 
course,  the  weight  of  the  sun  would  be  one  million  and  four  hundred  thousand 
times  that  of  the  earth.  But  it  is  not  nearly  so  great  as  this  ;  on  the  contrary, 
it  is  much  less.  Consequently,  it  follows  that  the  constituent  materials  of 
the  sun  are  lighter  than  those  of  the  earth  in  the  proportion  of  about  four  to 
one.  The  density  of  the  sun  is,  therefore,  very  nearly  equal  to  that  of  water, 
and,  consequently,  the  weight  of  the  solar  orb  is  equal  to  the  weight  of  a  globe 
of  the  same  magnitude  composed  altogether  of  water. 

FORM    AND    ROTATION    OF    THE    SUN. 

Although  to  minds  unaccustomed  to  the  rigor  of  scientific  research,  it 
might  appear  sufficiently  evident,  without  further  demonstration,  that  the  sun 
is  globular  in  its  form,  yet  the  more  exact  methods  pursued  in  the  investiga- 
tion of  physics  demand  that  we  should  find  more  conclusive  proof  of  the  sphe- 
ricity of  the  solar  orb  than  the  mere  fact  that  the  disk  of  the  sun  is  always  cir- 
cular. It  is  barely  possible,  however  improbable,  that  a  flat  circular  disk  of 
matter,  the  face  of  which  should  always  be  presented  to  ills  earth,  might  be 
the  form  of  the  sun  ;  and  indeed  there  are  a  great  variety  of  other  forms  which, 
by  a  particular  arrangement  of  their  motions,  might  present  to  the  eye  a  circu- 
lar appearance  as  well  as  a  globe  or  sphere.  To  prove,  then,  that  a  body  is 
globular,  something  more  is  necessary  than  the  mere  fact  that  it  always  appears 
circular. 

When  a  telescope  is  directed  to  the  sun,  wo  discover  upon  it  certain  marks 
or  spots,  of  which  we  shall  speak  more  fully  presently.  We  observe  that 
these  marks,  while  they  preserve  the  same  relative  position  with  respect  to 
each  other,  move  regularly  from  one  side  of  the  sun  to  the  other.  They  disap- 
pear, and  continue  to  be  invisible  for  a  certain  time,  come  into  view  again  on  the 
other  side,  and  so  once  more  pass  over  the  sun's  disk.  This  is  an  effect  which 
would  evidently  be  produced  by  marks  on  the  surface  of  a  globe,  the  globe 
itself  revolving  on  an  axis,  and  carrying  these  marks  upon  it.  That  this  is,  in 
fact,  the  case,  is  abundantly  proved  by  the  fact  that  the  periods  of  rotation  for 
all  these  marks  are  found  to  be  exactly  the  same,  viz.,  about  twenty-five  and  a 
half  days.  Such  is,  then,  the  time  of  rotation  of  the  sun  upon  its  axis,  and  that 
it  is  a  globe  remains  no  longer  doubtful,  since  the  globe  is  the  only  body  which, 
while  it  revolves  with  a  motion  of  rotation,  could  always  present  the  circular 
appearance  to  the  eye.  The  axis  on  which  the  sun  revolves  is  very  nearly 
perpendicular  to  the  plane  of  the  earth's  orbit,  and  the  mot'  .n  of  rotation  of  the 


THE  SUN. 


73 


sun  upon  the  axis  is  in  tha  same  direction  as  the  motion  of  the  planets  round 
the  sun,  that  is  to  say,  from  west  to  east. 

SPOTS    ON    THE    SUN. 

One  of  the  earliest  fruits  of  the  invention  of  the  telescope  was  the  discovery 
of  the  spots  upon  the  sun,  and  the  examination  of  these  has  gradually  led  to  a 
knowledge  of  the  physical  constitution  of  the  centre  of  our  system. 

When  we  submit  a  solar  spot  to  telescopical  examination,  we  discover  its 
appearance  to  be  that  of  an  intensely  black  irregularly-shaped  patch,  edged  with 
a  penumbral  fringe,  the  brightness  of  the  general  surface  of  the  sun  gradually 
fading  away  into  the  blackness  of  the  spot.  When  a  spot  is  watched  for  a  con- 
siderable time,  it  is  found  to  undergo  a  gradual  change  in  its  form  and  magni- 
tude ;  at  first  increasing  gradually  in  size,  until  it  attains  some  definite  limit  of 
magnitude,  when  it  ceases  to  increase,  and  soon  begins,  on  the  contrary,  to 
diminish ;  and  its  diminution  goes  on  gradually,  until  at  length  the  bright  sides 
closing  in  upon  the  dark  patch,  it  dwindles  first  to  a  mere  point,  and  finally 
disappears  altogether.  The  period  which  elapses  between  the  formation  of 
the  spot,  its  gradual  enlargement,  subsequent  diminution,  and  final  disap- 
pearance, is  very  various.  Some  spots  appear  and  disappear  very  rapidly, 
while  others  have  lasted  for  weeks  and  even  for  months.  The  magnitudes 
of  the  spots  are  in  proportion  to  the  magnitude  of  the  sun  itself.  At  the 
distance  of  the  sun,  a  spot,  the  magnitude  of  which  would  be  barely  visible, 
must  have  a  diameter  of  four  hundred  and  sixty  miles,  and  an  area  of  one 
hundred  and  sixty-six  thousand  square  miles,  which  is,  therefore,  the  smallest 
space  on  the  surface  of  the  sun  which  would  be  distinctly  seen.  Among  the 
many  spots  which  have  been  recorded,  one  was  observed  by 'Mayer,  the 
area  of  which  was  about  fifteen  hundred  millions  of  miles  square,  or  about 
thirty  times  the  surface  of  the  earth. 

Spots  have  been  occasionally  seen  on  all  parts  of  the  sun,  but  that  region  on 
which  they  are  found  generally  to  prevail,  is  one  which  corresponds  with  the 
tropical  parts  of  the  earth,  that  is,  a  space  extending  about  thirty  degrees  on 
either  side  of  the  solar  equator. 


74 


THE  SUN. 


PHYSICAL    CONSTITUTION    OF    THE    SUN. 

What  are  the  spots  ?  Two,  and  only  two,  suppositions  have  been  proposed 
to  explain  them.  One  supposes  them  to  be  scoriae,  or  dark  scales  of  incombus- 
tible matter  floating  on  the  general  surface  of  the  sun.  The  other  supposes 
them  to  be  excavations  in  the  luminous,  matter  which  coats  the  sun,  the  dark 
part  of  the  spot  being  a  part  of  the  solid  non-luminous  nucleus  of  the  sun.  In 
this  latter  supposition  it  is  assumed  that  the  physical  constitution  of  the  sun  is 
a  solid  non-luminous  globe,  covered  with  a  coating  of  a  certain  thickness  of  lu- 
minous matter.  This  latter  supposition  has  been  in  a  great  measure  demon- 
strated by  continued  and  accurate  observations  on  the  spots. 

That  the  spots  are  excavations,  and  not  mere  black  patches  on  the  surface, 
is  proved  by  the  following  observations :  If  we  select  a  spot  which  is  at  the 
centre  of  the  sun's  disk,  having  some  definite  form,  such  as  that  of  a  circle,  and 
watch  the  appearance  of  the  same  spot  when,  by  the  motion  of  the  sun  upon 
its  axis  it  is  carried  toward  the  edge,  we  find,  first,  that  the  circle  becomes  an 
oval.  This,  however,  is  what  would  be  expected  even  if  the  spot  were  a 
circular  patch,  inasmuch  as  a  circle  seen  obliquely  is  foreshortened  into  an  oval. 
But  we  find  that  as  the  spot  moves  toward  the  side  of  the  sun's  limb,  the  black 
patch  gradually  disappears,  the  penumbral  fringe  on  the  inside  of  the  spot  be- 
comes invisible,  while  the  penumbral  fringe  on  the  outside  of  the  spot  increases 
in  apparent  breadth,  so  that  when  the  spot  approaches  the  edge  of  the  sun,  the 
only  part  that  is  visible  is  the  external  penumbral  fringe.  Now  this  is  ex- 
actly what  would  oocur  if  the  spot  were  an  excavation.  The  penumbral  fringe 
is  produced  by  the  shelving  of  the  sides  of  the  excavation,  sloping  down  to  its 
dark  basis.  As  the  spot  is  carried  toward  the  edge  of  the  sun,  the  height  of 
the  inner  side  is  interposed  between  the  eye  and  the  bottom  of  the  excavation,  so 
as  to  conceal  the  latter  from  view.  The  surface  of  the  inner  shelving  side  also 
takes  the  direction  of  the  line  of  vision  or  very  nearly,  diminishes  in  apparent 
breadth,  and  ceases  to  be  visible,  while  the  surface  of  the  shelving  side  next 
the  edge  of  the  sun  becomes  nearly  perpendicular  to  the  line  of  vision,  and, 
consequently,  appears  of  its  full  breadth. 

In  short,  all  the  variations  of  appearance  which  the  spots  undergo,  as  they 
move  across  the  sun's  disk,  changing  their  distances  and  positions  with  regard 
to  the  sun's  centre,  are  exactly  those  changes  of  appearance  which  would  be 
produced  by  an  excavation,  and  not  at  all  those  which  a  dark  patch  on  the 
solar  surface  would  undergo. 

It  may  be  considered  then  as  proved,  that  the  spots  on  the  sun  are  excava- 
tions ;  and  that  the  apparent  blackness  is  produced  by  the  fact  that  the  part 
constituting  the  dark  portion  of  the  spot  is  either  a  surface  totally  destitute  of 
light  or  by  comparison  so  much  less  luminous  than  the  general  surface  of  the 
sun  as  to  appear  black.  This  fact  combined  with  the  appearance  of  the  penum- 
bral edges  of  the  spots  have  led  to  the  supposition,  which  appears  scarcely  to 
admit  of  doubt,  that  the  solid,  opaque  nucleus,  or  globe  of  the  sun,  is  invested  \ 
with  two  atmospheres,  that  which  is  next  the  sun  being  like  our  own,  non-  > 
luminous,  and  the  superior  one  being  that  in  which  alone  light  and  heat  are 
evolved ;  at  all  events,  whether  these  strata  be  in  the  gaseous  state  or  not,  the 
existence  of  two  such,  one  placed  above  the  other,  the  superior  one,  being  lu- 
minous, seems  to  be  exempt  from  doubt. 

By  observing  the  magnitude  of  the  spots,  and  the  rate  at  which  they  increase 
and  diminish,  the  velocity  of  their  edges  has  been  ascertained,  and  this  velocity 
has  been  found  to  be  such  as  can  scarcely  be  attributed  to  matter  except  in  the 
gaseous  form. 

We  are  not  warranted  in  assuming  that  the  black  portion  of  the  spots  are 


THE  SUN. 


75 


really  surfaces  deprived  of  light,  for  the  most  intense  artificial  light  which  can 
be  produced,  such,  for  example,  as  that  of  a  piece  of  quick-lime  exposed  to  the 
action  of  the  compound  blow-pipe,  when  seen  projected  on  the  sun's  disk, 
appears  as  dark  as  the  spots  themselves ;  an  effect  which  must  be  ascribed 
to  the  infinitely  superior  splendor  of  the  sun's  light.  All  that  can  be  legiti- 
mately inferred  respecting  the  spots,  then,  is,  not  that  they  are  destitute  of 
light,  but  that  they  are  incomparably  less  brilliant  than  the  general  surface  of 
the  sun. 

The  thickness  of  the  luminous  coating  which  covers  the  sun,  was  attempted 
to  be  measured  by  Sir  William  Herschel,  by  means  of  observations  made  on 
the  spots,  and  the  result  of  his  inquiry  was  that  its  depth  varied  from  two  to 
three  thousand  miles.  The  under  and  non-luminous  stratum,  by  reflecting  a 
considerable  portion  of  the  rays  which  fall  upon  it  from  the  luminous  stratum 
above,  not  only  increases  the  light  which  the  luminous  stratum  disperses  through 
space,  but  serves  as  a  canopy  to  screen  the  solid  body  of  the  sun  from  the 
overpowering  effects  of  the  light  and  heat  of  the  superior  stratum.  Herschel 
even  supposed  that  the  density  of  the  lower  stratum  might  be  such  as  to  main- 
tain a  temperature  on  the  actual  surface  of  the  solid  globe  of  the  sun  not  higher 
than  that  upon  our  earth.  However  this  may  be,  there  seems  to  be  little  doubt 
that  the  actual  temperature  at  the  visible  surface  of  the  sun,  that  is  to  say, 
upon  its  luminous  coating,  must  be  much  more  elevated  than  any  artificial  heat 
we  are  able  to  produce. 

According  to  Sir  John  Herschel,  we  have  various  indications  of  this. 

First,  from  the  law  of  the  decrease  of  radiant  heat  and  light,  which  being  in 
the  inverse  proportion  of  the  squares  of  the  distances,  it  follows  that  the  heat 
received  on  a  given  area  exposed  at  the  distance  of  the  earth,  and  on  an  equal 
area  at  the  visible  surface  of  the  sun,  must  be  in  the  proportion  of  the  apparent 
magnitude  of  the  sun  to  the  whole  extent  of  the  firmament,  that  is,  in  the  pro- 
portion of  about  one  to  three  hundred  thousand.  A  far  less  intensity  of  solar 
radiation  collected  in  the  focus  of  a  burning-glass,  is  sufficient  to  evaporate 
gold  or  platinum. 

Secondly,  from  the  facility  with  which  the  sun's  heat  passes  through  glass, 
a  property  possessed  by  artificial  heat  in  a  very  small  degree,  and  always  in 
the  direct  proportion  of  its  intensity. 

Thirdly,  from  the  fact  that  the  most  vivid  flames  and  intense  artificial  light 
appear,  as  we  have  already  stated,  only  as  black  spots  when  held  between  the 
disk  of  the  sun  and  the  eye. 

The  idea  that  the  heat  of  the  sun  arises  from  any  process  analogous  to  that 
of  common  combustion,  seems  to  be  beset  with  insuperable  difficulties.  How 
can  we  suppose  the  inexhaustible  supply  of  the  materials  necessary  to  sup- 
port so  enormous  and  interminable  a  conflagration?  There  are  two  other 
sources  of  heat  which  may  be  imagined,  that  are  not  subject  to  the  same  dif- 
ficulty. Bodies  submitted  to  friction  evolve  heat  without  any  change  in  the 
condition  of  their  constituent  parts.  Also  when  a  galvanic  current  is  trans- 
mitted through  certain  conducting  substances,  they  become  heated  with  more 
)  or  less  intensity  and  sometimes  to  such  a  degree  as  to  emit  light  of  the  most 
intense  brilliancy,  and  yet  in  this  process  they  suffer  no  other  physical  change 
than  that  of  temperature.  It  is  therefore  possible  to  suppose  either  of  these 
causes,  but  especially  the  latter,  to  be  in  constant  operation  on  the  sun,  with 
sufficient  energy  to  educe  the  light  and  heat  which  it  affords. 

The  actual  physical  character  of  the  luminous  matter  which  coats  the  sun 
had  not  been  ascertained  until  a  recent  period.  According  to  the  report  of 
the  astronomical  lectures  of  Arago,  lately  delivered  in  Paris,  it  would  seem 
that  that  philosopher  has  succeeded  in  solving  this  problem.  As  we  have  not 


76  THE  SUN. 

had  access  to  the  original  papers  containing  this  investigation,  we  can  only 
speak  of  it  from  the  imperfect  information  supplied  by  that  report.  It  would 
seem  from  it  that  Arago  reasons  in  the  following  manner :  There  are  two 
states  in  which  light  is  capable  of  existing ;  the  ordinary  state,  and  the  state 
of  polarization.  It  has  been  proved  by  Fourier,  that  all  bodies  rendered  in- 
candescent by  heat,  which  are  in  the  solid  or  liquid  state,  emit  polarized 
light ;  while  bodies  which  are  gaseous,  when  rendered  incandescent,  invariably 
emit  light  in  its  ordinary  state.  Thus  the  physical  condition  of  a  body  may 
be  distinguished  when  it  is  incandescent,  by  examining  the  light  which  it 
affords.  There  are  polariscopic  instruments  by  which  we  are  enabled  to  dis- 
tinguish these  different  states  of  light.  On  applying  these  tests  to  the  direct 
light  of  the  sun,  it  has  been  found  to  be  in  the  unpolarized,  or  ordinary  condi- 
tion. Hence  it  has  been  inferred  by  Arago,  that  the  matter  from  which  this 
light  proceeds  must  be  in  the  gaseous  state.  It  will  doubtless  be  readily  un- 
derstood that  gas,  when  incandescent,  is  that  which  is  commonly  called  flame. 
If  Arago's  reasoning,  then,  be  rightly  reported,  and  his  observations  correct, 
it  follows  that  the  globe  of  the  sun  is  a  solid,  opaque,  non-luminous  orb,  in- 
vested with  an  ocean  of  flame. 

Certain  observations  made  by  Bouguer,  led  that  astronomer  to  suppose  that 
the  sun  is  surrounded  by  an  atmosphere  of  considerable  extent  above  the  sur- 
face of  the  luminous  coating.  The  ground  of  this  supposition  was  the  impres- 
sion that  the  splendor  of  the  sun's  light  near  the  borders  of  the  disk  was  less 
than  near  the  centre  ;  an  effect  which  could  not  be  produced  if  the  luminous 
coating  had  nothing  above  it  imperfectly  transparent.  On  the  contrary,  the 
brightness  toward  the  borders,  owing  to  the  obliquity  of  the  direction  of  the 
surface  to  the  line  of  vision  would  be  greater,  inasmuch  as  a  greater  extent  of 
luminous,  surface  would  be  comprised  within  the  same  visual  angle.  The 
more  accurate  observations,  however,  of  Arago,  made  with  delicate  polariscopic 
instruments  disprove  this  by  showing  that  the  brightness  is  the  same  on  all 
parts  of  the  sun's  disk 


ECLIPSES. 


Lonar  and  Solar  Eclipses. — Their  Causes. — Shadow  of  the  Earth. — And  Moon. — Magnitude  of 
Eclipses. — When  they  can  happen. — Central  Solar  Eclipse. — Great  Solar  Eclipse  described  by 
Halley. — Ecliptic  Limits. 


ECLIPSES. 


ECLIPSES. 


Or  all  the  occasional  astrononomical  phenomena,  those  which  have  attract- 
ed most  popular  attention  are  LUNAR  and  SOLAR  ECLIPSES.  We  shall  on  the 
present  occasion  explain  the  principal  circumstances  attending  l^hem. 

When  a  luminous  body,  radiating  light  in  all  directions  around  it,  throws 
these  rays  upon  an  opaque  body,  that  body  prevents  a  portion  of  the  rays  from 
penetrating  into  the  space  behind  it.  That  portion  of  the  space  from  which 
the  light  is  thus  excluded  by  the  interposition  of  the  opaque  body,  is  called  in 
astronomy  the  SHADOW  of  that  body. 

The  shape,  magnitude,  and  extent,  of  the  shadow  of  an  opaque  body,  will 
depend  partly  on  the  shape  and  magnitude  of  the  opaque  body  itself,  and  partly 
on  that  of  the  body  from  which  the  light  proceeds. 

In  the  cases  before  us,  the  form  of  the  bodies  are  globes.  If  the  globe  of 
the  SUN  were  equal  in  magnitude  to  the  globe  of  the  earth,  the  shadow  of  the 
latter  would  be  a  cylinder,  the  base  of  which  would  be  equal  to  a  great  circle 
of  the  earth,  and  such  shadow  would  be  interminable,  since  its  sides  would  be 
parallel.  This  will  be  evident  by  an  inspection  of  the  annexed  figure,  1 ,  in  which 
S.  represents  the  sun,  and  E.  the  earth  ;  the  rays  <S.  E.  forming  the  sides  of 
the  shadow,  being  parallel,  could  never  meet,  and  consequently  the  shadow 
would  be  infinite,  since  light  can  never  penetrate  into  the  space  between  them. 
If,  on  the  other  hand,  the  sun  were  a  globe  less  in  magnitude  than  the  earth, 
then  the  shadow  of  the  latter  would  have  diverging  sides  as  represented  in  the 
annexed  figure,  2,  which  would  widen  as  they  proceed  from  the  earth,  and  would 
be  interminable  ;  but  the  sun  having  in  reality  a  diameter  about  one  hundred 
and  twelve  times  greater  than  that  of  the  earth,,  the  rays  which  proceed 
from  the  upper  and  lower  limb  of  the  sun,  and  which  touch  the  earth  at  a  and  b, 
fig.  3,  will  converge  to  certain  point  at/,  behind  the  earth,  and  will  form  a  conical 
space,  whose  base  will  be  at  a  b,  and  whose  apex  will  be  at  /.  From  the  space 
enclosed  by  this  cone  the  light  of  the  sun  is  entirely  excluded,  and  it  is  there- 
fore properly  the  shadow  of  the  earth.  But  there  is  also  a  certain  space  be- 
hind the  earth  from  which  the  sun's  light  is  only  partially  excluded,  and  which 


80 


ECLIPSES. 


Fisr.  1. 


forms  what  is  called  the  earth's  penumbra.  The  ray  m  a,  fig.  4,  from  the  top  of  the 
sun's  disk. passes  to  the  point/,  while  the  ray  n  a  from  the  lowest  point  of  the 
sun's  disk  passes  to  the  point  c.  The  space  between  a  /and  a  c  will  be  par- 
tially illuminated  by  the  sun.  If  a  spectator  were  placed  anywhere  in  that 
space,  he  would  see  a  portion  of  the  upper  limb  of  the  sun,  and  would  see  more 
of  it  the  nearer  he  might  be  to  c,  and  less  of  it  the  nearer  he  might  be  to/. 

As  he  would  see  the  sun,  he  would  of  course  receive  a  portion  of  its  light. 
Thus  that  part  of  the  space  included  between  a  /  and  a  c,  which  is  near  a/, 
receives  light  from  a  small  portion  of  the  upper  limb  of  the  sun,  while  that  part 
which  is  near  c  c  receives  light  from  nearly  the  whole  of  the  sun  ;  and  in  short, 
proceeding  from  a  f  to  a  c,  the  light  received  from  the  sun  will  be  gradually 
increased.  • 

Fig.  4. 


In  like  manner,  the  ray  m  b  proceeding  from  the  upper  limb  of  the  sun  and 
continued  to  d,  will  include  between  it  and  the  ray  b  f  a.  space  which  is  only 
partially  illuminated,  and  will  be  subject  to  the  same  observations  as  we  have 
made  respecting  the  space  between  a  /  and  a  c. 

When  any  object  which  receives  its  light  from  the  sun  passes  between  fhs 
lines  a  c  and  b  d,  it  will  be  either  wholly  or  partially  deprived  of  the  sun's  light. 
If  it  be  outside  the  limits  6/and  a/  it  will  be  only  partially  obscured ;  but  if 
it  be  within  these  limits,  it  will  be  altogether  darkened. 

The  length  of  the  line  of  being  incomparably  less  than  the  distance  of  any 
body  in  the  universe  from  the  earth  except  the  moon,  but  being  on  the  contrary 
considerably  greater  than  the  distance  of  the  moon,  it  follows  that  the  only 
body  in  the  system  which  can  be  deprived  of  light  by  the  earth's  shadow  is  the 
moon,  and  that  whenever  that  object  is  in  opposition  to  the  sun,  and  at  the  same 
time  so  near  the  ecliptic  as  to  be  included  between  the  lines  ac  and  b  d,  it  will 


be  partially  deprived  of  the  sun's  light ;  but  if  it  be  so  much  nearer  as  to  be  in- 
cluded between  the  lines  a  /and  b  /  it  will  be  wholly  deprived  of  the  sun's 
light.  Thus  the  causes  of  a  partial  or  total  eclipse  of  the  moon  are  ex- 
plained. 

If  the  plane  of  the  moon's  orbit  coincided  with  that  of  the  ecliptic,  the  moon 
would  pass  behind  the  centre  of  the  earth  in  the  direction  of  the  line  E  f  form- 
ing the  axis  of  the  shadow,  every  revolution,  and  consequently  there  would  be 
a  total  lunar  eclipse  every  month  ;  but  as  the  moon's  orbit  is  inclined  at  an 
angle  of  five  degrees  to  the  plane  of  the  ecliptic,  the  distance  of  the  moon  from 
that  plane  is  greater  than  the  distance  of  lines  of  a  c  and  b  d  from  E  f,  except 
when  the  moon  is  near  to  that  point  where  its  orbit  crosses  the  ecliptic,  which 
is  called  the  moon's  node.' 

No  lunar  eclipses  happen,  therefore,  except  when  either  of  the  moon's  nodes 
is  nearly  in  opposition  to  the  sun. 

When  a  lunar  eclipse  does  happen,  the  moon  will  first  enter  the  penumbra 
at  a  c,  and  will  be  very  slightly  obscured.  As  it  approaches  a/,  it  is  more  and 
more  deprived  of  the  sun's  light,  until  finally  it  enters  the  shadow  afb,  where 
it  is  altogether  obscured.  At  the  end  of  the  eclipse,  as  it  must  pass  through 
the  penumbra,  it  will  recover  the  sun's  light  by  slow  degrees. 

The  length  of  the  line  E  f  being  about  800,000  miles,  and  the  distance  of 
the  moon  from  the  earth  being  less  than  250,000,  the  moon  when  it  passes 
through  the  shadow  will  be  about  500,000  miles  within  the  point  /  and  will 
consequently  pass  through  the  shadow  at  a  part  of  considerable  breadth. 

In  expressing  the  magnitude  of  the  eclipse,  whether  of  the  sun  or  of  the 
moon,  it  is  customary  to  suppose  the  diameters  of  these  bodies  divided  into 
twelve  equal  parts,  called  digits,  and  the  magnitude  of  the  eclipse  is  ex- 
pressed by  stating  the  proportion  of  the  diameter  of  the  disk  which  is  obscured. 
Thus  when  half  the  disk  is  obscured,  we  say  that  the  eclipse  measures  six 
digits,  and  so  on. 

From  what  has  been  stated,  it  is  evident  that  an  eclipse  of  the  moon  will 
not  be  affected  in  its  appearance  by  the  position  of  the  observer  on  the  surface 
of  the  earth.  Wherever  he  may  be,  the  eclipse  will  appear  to  him  the  same  ; 
but  if  it  should  happen  that  while  the  moon  is  passing  through  the  shadow,  the 
person  desirous  to  observe  it  is  in  a  portion  of  the  earth  which  at  that  time  is 
turned  toward  the  sun,  the  eclipse  will,  of  course,  be  invisible  to  him.  In 
short,  it  will  only  be  visible  from  that  hemisphere  of  the  earth  that  is  turned 
from  the  sun  at  the  time  of  its  occurrence. 

The  moon,  like  the  earth,  receiving  the  sun's  light,  projects  behind  it  a  conical 
shadow  and  a  diverging  penumbra :  if  this  shadow  or  penumbra  fall  upon  any 
portion  of  the  earth's  surface,  they  will  deprive  such  portion  wholly  or  partially 
of  the  sun's  light,  and  there  will  be  a  solar  eclipse  of  a  corresponding  species. 
When  the  moon  is  between  the  sun  and  earth,  the  length  of  its  shadow  is  about 
equal  to  its  distance  from  the  earth,  and  consequently  the  point  of  the  shadow 
would  just  reach  the  surface  of  the  earth  ;  but  as  the  moon's  distance  is  subject 
to  a  slight  variation,  it  sometimes  happens  that  the  length  of  the  moon's  shadow 
is  a' little  more  and  sometimes  a  little  less  than  its  distance  from  the  earth.  If 
.  the  length  of  the  shadow  be  greater  than  its  distance  from  the  earth,  then  the 
(  shadow  will  cover  a  small  portion  of  the  earth's  surface,  to  all  places  within 
which  there  will  be  a  total  solar  eclipse.  The  circumstances  affecting  a  solar 
eclipse  are  represented  in  the  annexed  figure,  where  S  is  the  centre  of  the 
sun's  disk,  W  is  its  upper  limb,  and  V  its  lower  limb  ;  c  d  is  the  moon,  and  e 
the  point  of  its  shadow  ;  d  h  and  c  g  are  the  sides  of  its  penumbra,  and  a  b  is 
the  portion  of  the  earth  on  which  the  penumbra  falls.  An  observer  placed  be- 
tween e  and  g,  will  see  the  upper  limb  of  the  sun  only,  the  lower  limb  being 

6 


82 


ECLIPSES. 


eclipsed.  An  observer,  on  the  other  hand,  between  d  e  and  d  h,  would  see  the 
lower  limb  only,  the  upper  limb  being  eclipsed  ;  and  the  eclipse  would  be 
greater  to  each  of  these  observers  the  nearer  their  position  would  be  to  the 
point  e.  To  observers  between  h  and  Y  or  g  and  Y,  there  would  be  no  eclipse, 
for  no  part  of  the  moon  would  be  interposed  between  them  and  any  part  of  the 
sun. 

If  the  vertex  of  the  cone  of  the  moon's  shadow  is  farther  from  the  moon  than 
the  surface  of  the  earth,  then  there  will  be  a  small  portion  of  the  earth's  sur- 
face at  e  within  the  shadow  ;  and  to  an  observer  within  any  portion  of  that 
surface,  the  sun  will  be  totally  eclipsed ;  but  if  the  vertex  of  the  shadow  do 
not  reach  the  earth,  then  an  observer  at  e  will  see  a  ring  of  the  sun,  not  cov- 
ered by  the  moon,  surrounding  the  globe  of  the  moon,  and  the  phenomenon 
will  be  what  is  called  an  annular  eclipse. 

These  circumstances  will  render  easily  intelligible  all  the  ordinary  circum- 
stances of  solar  eclipses.  It  will  be  readily  understood,  that  while  a  lunar 
eclipse  is  the  same  to  all  observers  on  the  earth,  a  solar  eclipse  will  vary  in 
its  magnitude  and  character  with  the  position  of  the  observer ;  the  same  solar 
eclipse  which  at  one  part  of  the  earth  is  total  or  annular,  at  other  parts  of  the 
earth  is  partial  in  various  degrees,  and  at  other  parts  again  is  not  exhibited 
at  all. 

A  natural  consequence  of  the  diffusion  of  knowledge  is,  that  while  it  lessens 
the  vague  sense  of  wonder,  with  which  singular  phenomena  in  nature  are  be- 
held, it  increases  the  feeling  of  admiration  at  the  harmonious  laws,  the  devel- 
opment of  which  renders  effects  apparently  strange  and  unaccountable  easily 
intelligible.  It  will  be  easily  imagined  what  a  sense  of  astonishment,  and 
even  terror,  the  sudden  disappearance  of  an  object  like  the  sun  or  moon  must 
have  produced  in  an  age  when  the  causes  of  eclipses  were  known  only  to  the 
learned.  Such  phenomena  were  regarded  as  precursors  of  divine  vengeance. 
History  informs  us  that  in  ancient  times  armies  have  been  destroyed  by  the 
effects  of  the  consternation  spread  among  them  by  the  sudden  occurrence  of  an 
eclipse  of  the  sun.  Commanders  who  happened  to  possess  some  scientific 
knowledge,  have  taken  advantage  of  it  to  work  upon  the  credulity  of  those 
around  them  by  menacing  them  with  prodigies  the  near  approach  of  which 
they  were  well  aware  of,  illustrating  thus,  in  a  singular  and  perverted  manner, 
the  maxim  that  knowledge  is  power.  Happily,  in  the  present  day  information 
is  too  generally  diffused  to  permit  the  bulk  of  mankind  to  be  thus  played  upon. 

Of  all  the  various  phenomena  presented  by  eclipses,  that  which  is  transcend- 


ECLIPSES.  83 


antly  the  most  remarkable  and  interesting  is  a  central  eclipse  of  the  sun.  If 
it  be  total,  the  spectacle  it  offers  is  most  imposing :  the  light  of  dav  is  grad- 
ually withdrawn  to  such  a  degree  that  the  brighter  planets,  such  as  Venus  and 
Jupiter,  and  the  stars  of  the  first  magnitude,  become  visible  to  the  naked  eye. 
We  see,  however,  a  faint  light  of  the  sun  behind  the  disk  of  the  moon.  Some- 
times, as  has  been  stated,  when  the  apparent  magnitude  of  the  moon  is  a  little 
less  than  that  of  the  sun,  the  disk  of  the  moon  conceals  the  entire  disk  of  the 
sun,  except  only  a  thin  luminous  ring  surrounding  it.  This  is  a  phenomenon 
of  very  rare  occurrence,  and  only  to  be  seen  at  particular  places  on  the  earth. 
An  instance  of  it  occurred  on  the  7th  of  September,  1820.  It  commenced  to 
be  visible  at  the  north  latitude  of  80°,  in  Hudson's  bay,  near  the  eastern  coast 
of  New  North  Wales.  It  was  visible  next  in  the  direction  of  the  northeast 
of  Greenland,  at  the  mouth  of  the  Wesel,  at  Bremen,  in  the  gulf  of  Venice,  and 
in  Arabia  deserta,  and  ceased  near  the  Persian  gulf.  While  this  eclipse  was 
produced  in  these  different  places,  the  observers  who  were  on  the  same  me- 
ridians, but  further  south,  saw  only  a  partial  eclipse,  and  others,  still  further 
south,  saw  no  eclipse  at  all.  the  contrary  took  place  with  observers  on  the  same 
meridians  farther  north,  to  all  of  whom  the  eclipse  was  annular. 

It  was  during  a  phenomenon  of  this  kind  that  Schroter  imagined  he  saw  the 
solar  light  coming  through  an  immense  opening  in  the  moon.  Other  observers, 
however,  who  saw,  or  imagined  they  saw,  luminous  spots  on  the  dark  hemi- 
sphere of  the  moon,  in  a  solar  eclipse,  ascribed  them  to  lunar  volcanoes.  As 
to  the  existence  of  these  luminous  spots  on  the  dark  hemisphere  of  the  moon, 
rendered  manifest  in  a  total  eclipse  of  the  sun,  we  have  the  testimony  of  so 
many  astronomers,  among  whom,  besides  Schroter,  may  be  mentioned  Sir 
William  Herschel  and  Kater,  that  we  can  scarcely  doubt  their  reality.  The 
causes  which  may  produce  them  have  only  been  explained  in  the  two  ways 
above  mentioned,  namely,  either  by  the  supposed  existence  of  active  volcanoes, 
on  the  moon,  or  perforations  through  the  moon,  through  which  the  sun's  light 
passes. 

The  following  description  of  a  total  eclipse  of  the  sun.  given  by  Halley,  who 
observed  it,  is  quoted  by  Arago,  and  will  be  read  with  interest : — 

"  I  send  you,  according  to  promise,  my  observations  of  the  solar  eclipse, 
though  I  fear  they  will  not  be  of  much  use  to  you.  Not  being  furnished  with 
the  necessary  instruments  for  measuring  time,  I  confined  my  views  to  examin- 
ing the  spectacle  presented  by  nature  under  such  extraordinary  circumstances, 
a  spectacle  which  has  hitherto  been  neglected  or  imperfectly  studied.  I  chose 
for  my  point  of  observation  a  place  called  Haradowhill,  two  miles  from  Ames- 
bury,  and  east  of  the  avenue  of  Stonehenge,  of  which  it  closes  the  vista.  In 
front  is  that  celebrated  edifice  upon  which  I  knew  that  the  eclipse  would  be 
directed.  I  had,  moreover,  the  advantage  of  a  very  extensive  prospect  in 
every  direction,  being  on  the  loftiest  hill  in  the  neighborhood,  and  that  nearest 
to  the  centre  of  the  shadow.  To  the  west,  beyond  Stonehenge,  is  another 
rather  steep  hill,  rising  like  the  summit  of  a  cone  above  the  horizon.  This  is 
Clay  hill,  adjoining  Westminster,  (?)  and  situated  near  the  central  line  of  dark- 
ness which  was  to  set  out  from  this  point,  so  that  I  could  be  aware  in  time  of 
its  approach.  I  had  with  me  Abraham  Soirges  and  Stephen  Evans,  both  na- 
tives of  the  country,  and  able  men.  The  sky,  though  overcast,  gave  out  some 
straggling  rays  of  the  sun,  that  enabled  me  to  see  around  us.  My  two  com- 
panions looked  through  the  blackened  glasses,  while  I  made  some  .reconnais- 
sance of  the  country.  It  was  half-past  five  by  my  watch  when  they  informed 
me  that  the  eclipse  was  begun.  We  watched  its  progress,  therefore,  with  the 
naked  eye,  as  the  clouds  performed  for  us  the  service  of  colored  glasses.  At  ( 
the  moment  when  the  sun  was  half  obscured,  a  very  evident  circular  rainl-ow  ) 

1  **^s*^ 


ECLIPSES. 


formed  at  its  circumference,  with  perfect  colors.  As  the  darkness  increased, 
we  saw  the  shepherds  on  all  sides  hastening  to  fold  their  flocks,  for  they  ex- 
pected a  total  eclipse  of  an  hour  and  a  quarter  duration. 

"  When  the  sun  assumed  the  appearance  of  the  new  moon,  the  sky  was  tol- 
erably clear,  but  it  was  soon  covered  with  deeper  clouds.  The  rainbow  then  van- 
ished, the  steep  hill  I  have  named  became  very  obscure,  and  on  each  side,  that 
is,  north  and  south,  the  horizon  exhibited  a  blue  tint,  like  that  which  it  possesses 
in  summer  toward  the  close  of  day.  Scarcely  had  we  time  to  count  ten,  when 
Salisbury  spire,  six  miles  to  the  south,  was  enveloped  in  darkness.  The  hill 
disappeared  entirely,  and  the  deepest,  night  spread  around  us-  We  lost  sight 
of  the  sun,  whose  place  till  then  we  had  been  able  to  distinguish  in  the  clouds, 
but  whose  trace  we  could  now  no  more  discover  than  if  it  had  never  existed. 

"  By  my  watch,  which  I  could  scarcely  discern  by  some  light  that  reached 
us  from  the  north,  it  was  thirty-five  minutes  past  six.  Shortly  before,  the  sky 
and  the  earth  had  assumed,  literally  speaking,  a  livid  tint,  for  it  was  a  mixture 
of  black  and  blue,  only  the  latter  predominated  on  the  earth  and  at  the  horizon. 
There  was  also  much  black  diffused  through  the  clouds,  so  that  the  whole  pic- 
ture presented  an  awful  aspect,  that  seemed  to  announce  the  death  of  nature. 

"  We  were  now  enveloped  in  a  total  and  palpable  darkness,  if  I  may  be  al- 
lowed the  expression.  It  came  on  rapidly,  but  I  watched  so  attentively,  that 
I  could  perceive  its  progress.  It  came  upon  us  like  rain,  falling  on  our  left 
shoulders  (we  were  looking  to  the  west),  or  like  a  great  black  cloak  thrown 
over  us,  or  like  a  curtain  drawn  from  that  side.  The  horses  we  held  by  the 
bridle  seemed  deeply  struck  by  it,  and  pressed  to  us  with  marks  of  extreme 
surprise.  As  well  as  I  could  perceive,  the  countenances  of  my  friends  wore  a 
horrible  aspect.  It  was  not  without  an  involuntary  exclamation  of  wonder  I 
looked  around  me  at  this  moment.  I  distinguished  colors  in  the  sun,  but  the 
earth  had  lost  all  its  blue,  and  was  entirely  black.  A  few  rays  shot  through 
the  clotids  for  a  moment,  but  immediately  afterward  the  earth  and  the  sky  ap- 
peared totally  black.  It  was  the  most  awful  sight  I  had  ever  beheld  in  my 
life. 

"  Northwest  of  the  point  whence  the  eclipse  came  on,  it  was  impossible  for 
me  to  distinguish  in  the  least  degree  the  earth  from  the  sky,  for  a  breadth  of 
sixty  degrees  or  more.  We  looked  in  vain  for  the  town  of  Amesbury,  situated 
below  us ;  scarcely  could  we  see  the  ground  under  our  feet.  I  turned  fre- 
quently during  the  total  darkness,  and  observed  that,  at  a  considerable  distance 
to  the  west,  the  horizon  was  perfect  on  both  sides,  that  is,  to  the  north  and  to 
the  south ;  the  earth  was  black,  and  the  lower  part  of  the  sky  clear ;  the  ob- 
scurity, which  extended  to  the  horizon  in  those  points,  seemed  like  a  canopy 
over  our  heads,  adorned  with  fringes  of  a  lighter  color,  so  that  the  upper  edges 
of  all  the  hills,  which  I  recognised  perfectly  by  their  outlines,  formed  a  black 
line.  I  saw  perfectly  that  the  interval  between  light  and  darkness,  observable 
in  the  earth,  was  between  Mortinsol  (?)  and  St.  Anne  ;  but  to  the  south  it  was 
less  distinctly  marked. 

"  I  do  not  mean  to  say  that  the  line  of  shadow  passed  between  these  two 
hills,  which  were  twelve  miles  distant  from  us ;  but  as  far  as  I  could  distin- 
guish the  horizon,  there  was  none  behind,  and  for  this  reason :  My  elevated 
position  enabled  me  to  see  the  light  of  the  sky  behind  the  shadow ;  still,  that 
yellowish  green  line  of  light  I  saw  was  broader  toward  the  north  than  toward 
the  south,  where  it  was  of  a  tan  color.  At  this  period  it  was  too  black  behind 
us,  that  is,  to  the  east,  looking  toward  London,  to  enable  me  to  see  the  hills 
beyond  Andover,  for  the  anterior  extremity  of  the  shadow  lay  beyond  that 
place.  The  horizon  was  then  divided  irvto  four  parts,  differing  in  extent,  in 
light,  and  in  darkness.  The  broadest  and  least  black  was  to  the  northwest,  and 


ECLIPSES. 


the  longest  and  brightest  to  the  southwest.  The  only  change  I  could  perceive 
during  the  whole  time  the  phenomenon  lasted,  was  that  the  horizon  divided 
into  two  parts — one  clear,  the  other  obscure.  The  northern  hemisphere  then 
acquired  more  length,  brightness,  and  breadth,  and  the  two  opposite  parts  coa- 
lesced. 

"  Like  the  shadow  in  the  beginning  of  the  eclipse,  the  light  approached  from 
the  north,  and  fell  on  our  right  shoulders.  I  could  not,  indeed,  distinguish  on 
that  side  either  defined  light  or  shadow  upon  the  earth,  which  I  watched  atten- 
tively ;  but  it  was  evident  that  the  light  returned  but  gradually,  and  with  oscil- 
lation :  it  receded  a  little,  advanced  rapidly,  till  at  last,  with  the  first  brilliant 
point  that  appeared  in  the  sky,  I  saw  plainly  enough  an  edge  of  light  that 
grazed  our  sides  for  a  considerable  time,  or  brushed  our  elbows  from  west  to 
east.  Having  good  reason,  therefore,  to  suppose  the  eclipse  ended  for  us,  I  I 
looked  at  my  watch,  and  found  that  the  hand  had  traversed  three  minutes  and 
a  half.  The  hill-tops  then  resumed  their  natural  color,  and  I  saw  a  horizon  at 
the  point  previously  occupied  by  the  centre  of  the  shadow.  My  companions 
cried  out  that,  they  again  saw  the  steep  hill  toward  which  they  had  been  look- 
ing attentively.  It  still,  indeed,  remained  black  to  the  southeast,  but  I  will  not 
say  that  the  horizon  was  difficult  to  discover.  Presently  we  heard  the  song  of 
the  larks  hailing  the  return  of  light,  after  the  profound  and  universal  silence  in 
which  everything  had  been  plunged.  The  earth  and  sky  appeared  then  as 
they  do  in  the  morning  before  sunrise.  The  latter  was  of  a  grayish  tint,  in- 
clining to  blue ;  the  former,  as  far  as  my  eye  could  reach,  was  deep  green  or 
russet. 

"  As  soon  as  the  sun  appeared,  the  clouds  grew  denser,  and  for  several  min- 
utes the  light  did  not  increase,  just  as  happens  at  a  cloudy  sunrise.  The  in- 
stant the  eclipse  became  total,  till  the  emersion  of  the  sun,  we  saw  Venus,  but 
no  other  stars.  We  perceived  at  this  moment  the  spire  of  Salisbury  cathedral. 
The  clouds  not  dispersing,  we  could  not  push  our  observations  further  :  they 
cleared  up,  however,  considerably  toward  evening.  I  have  hastened  home  to 
write  this  letter.  So  deep  an  impression  has  this  spectacle  made  upon  my 
mind,  that  I  shall  long  be  able  to  recount  all  the  circumstances  of  it  with  as 
much  precision  as  now.  After  supper,  I  made  a  sketch  of  it  from  memory,  on 
the  same  paper  on  which  I  had  previously  drawn  a  view  of  the  country. 

"  I  will  own  to  you  I  was,  methinks,  the  only  person  '"1  Eiv  ^nd  who  did 
not  regret  the  presence  of  clouds  :  they  added  much  to  the  -oleua.iitv  of  the 
spectacle — incomparably  superior,  in  my  opinion,  to  that  of  17ic  which  1  saw 
perfectly  from  the  top  of  the  belfrey  of  Boston,  in  Lincolnshire,  where  the  sky 
was  very  clear.  There,  indeed,  I  saw  the  two  sides  of  the  shadow  coming 
from  afar,  and  passing  to  a  great  distance  behind  us  ;  but  this  eclipse  exhibited 
great  variety,  and  was  more  awfully  imposing  ;  so  that  I  cannot  but  congratu- 
late myself  on  having  had  opportunities  of  seeing,  under  such  different  circum- 
stances, these  two  rare  accidents  of  nature." 

The  ECLIPTIC  derives  its  name  from  the  fact,  that  the  shadow  of  the  earth 
always  lying  in  it,  no  object  can  be  eclipsed  unless  it  be  very  near  to  it.  If  we 
imagine  a  line  drawn  from  the  centre  of  the  sun  through  the  centre  of  the 
earth,  and  continued  beyond  the  earth,  that  line  will  be  the  axis  of  the  earth's 
shadow,  and  the  diameter  of  the  conical  shadow  must  be  everywhere  less  than 
the  diameter  of  the  earth.  The  moon  can  not  touch  the  shadow,  if  the  distance 
of  its  nearer  limb  from  the  ecliptic  be  greater  than  the  diameter  of  the  earth. 

The  ecliptic  limits,  is  a  term  expressing  the  greatest  distances  of  the  moon  from 
its  node  at  which  it  is  possible  that  an  eclipse,  either  lunar  or  solar,  can  hap- 
pen. This  distance  for  eclipses  of  the  moon  is  twelve  degrees,  and  for  eclip- 
ses of  the  sun  seventeen  degrees. 


86 


Whenever  the  moon  is  less  than  seventeen  degrees  from  its  node  at  a  time 
when  it  is  in  conjunction  with  the  sun,  there  must  be  a  solar  eclipse ;  and 
whenever  it  is  less  than  twelve  degrees  from  its  node  at  the  time  of  full  moon, 
there  must  be  a  lunar  eclipse.  Within  these  limits  the  less  the  distance  of 
the  moon  from  its  node,  the  greater  will  be  the  number  of  digits  eclipsed, 
whether  of  the  sun  or  moon. 


THE    AURORA    BOREALIS 


}  Origin  of  the  Name. — Produced  by  Electricity. — General  Phenomena  of  Auroras. — Various  Exam- 
ples of  this  Meteor. — Riot's  Excursion  to  the  Shetland  Isles  to  ohserve  the  Aurora. — Lottin's  Ob- 

(  servalious  in  13:38-'9. — Various  Auroras  seen  by  him. — Theory  of  Biot  to  explain  these  Meteors. — 
Objections  to  it. — Hypothesis  of  Faraday. — Auroras  seen  on  the  Polar  Voyage  of  Captain 
Franklin. 


J 


THE  AUEORA  BOREALIS.  89 


THE    AURORA   BOREALIS. 


THE  AURORA  BOREALIS  is  a  luminous  phenomenon,  which  appears  in  the 
heavens,  and  is  seen  in  high  latitudes  in  both  hemispheres.  The  term  AURO- 
RA BOREALIS,  or  NORTHERN  LIGHTS,  has  been  applied  to  it  because  the  oppor- 
tunities of  witnessing  it  are,  from  the  geographical  character  of  the  globe,  much 
more  frequent  in  the  northern  than  in  the  southern  hemisphere.  The  term 
AURORA  POLARIS  would  be  a  more  proper  designation. 

This  phenomenon  consists  of  luminous  rays  of  various  colors,  issuing  from 
every  direction,  but  converging  to  the  same  point,  which  appear  after  sunset 
generally  toward  the  north,  occasionally  toward  the  west,  and  sometimes,  but 
rarely,  toward  the  south.  It  frequently  appears  near  the  horizon,  as  a  vague 
and  diffuse  light,  something  like  the  faint  streaks  which  harbinger  the  rising 
sun  and  form  the  dawn.  Hence  the  phenomenon  has  derived  its  name,  the 
NORTHERN  MORNING.  Sometimes,  however,  it  is  presented  under  the  form  of 
a  sombre  cloud,  from  which  luminous  jets  issue,  which  are  often  variously  col- 
ored, and  illuminate  the  entire  atmosphere. 

A  meteor  so  striking  as  the  aurora  could  not  fail  at  an  early  period  to  attract 
the  attention  of  scientific  inquirers,  and  to  give  rise  to  various  theories.  Some 
supposed  it  to  be  the  refraction  of  the  solar  rays  ;  others  ascribed  it  to  the 
effects  of  the  magnetic  fluid.  Euler  identified  it  with  the  tails  of  comets. 
Mairan  supposed  it  to  proceed  from  the  intermixture  of  the  far-extending  atmo- 
sphere of  the  sun  with  that  of  the  earth.  When,  however,  the  luminous  effects 
of  artificial  electricity  were  shown — when  the  electric  light  transmitted  through 
rarefied  air  was  exhibited — and  when  the  identity  of  lightning  with  electricity 
was  established,  these  various  hypotheses  were  by  common  consent  abandoned  ; 
and  the  explanation  proposed  by  Eberhart,  of  Halle,  and  Paul  Frisi,  of  Pisa, 
which  ascribed  the  phenomenon  to  electricity  transmitted  through  regions  in 
which  the  atmosphere  is  in  a  highly  rarefied  state,  was  adopted.  Any  doubt 
which  might  have  hung  round  this  explanation  was  dispelled  when  the  rela- 
tions between  magnetism  and  electricity  were  demonstrated ;  and  although  the  (. 
complete  explanation  of  the  details  of  the  aurora  has  not  been  accomplished, 


90 


THE  AURORA  BOREALIS. 


the  electricity  and  magnetism  of  the  earth  and  its  atmosphere  must  now  be 
regarded  as  its  source. 

In  his  treatise  on  these  meteors,  Mairan  describes  their  appearance  and  the 
succession  of  changes  »to  which  they  are  subject  with  great  minuteness  and 
precision.  The  more  conspicuous  auroras  commence  to  be  formed  soon  after 
the  close  of  twilight.  At  first  a  dark  mist  or  foggy  cloud  is  perceived  in  the 
north,  and  a  little  more  brightness  toward  the  west  than  in  the  other  parts  of  the 
heavens.  The  mist  gradually  takes  the  form  of  a  circular  segment,  resting  at 
each  corner  on  the  horizon.  The  visible  part  of  the  arc  soon  becomes  sur- 
rounded with  a  pale  light,  which  is  followed  by  the  formation  of  one  or  several 
luminous  arcs.  Then  come  jets  and  rays  of  light  variously  colored,  which 
issue  from  the  dark  part  of  the  segment,  the  continuity  of  which  is  broken  by 
bright  emanations,  which  indicate  a  movement  of  the  mass,  which  seems  agi- 
tated by  internal  shocks,  during  the  formation  of  these  luminous  radiations, 
which  issue  from  it  as  flames  do  from  a  conflagration.  When  this  species  of 
fire  has  ceased,  and  the  aurora  has  become  extended,  a  crown  is  formed  at  the 
zenith,  to  which  these  rays  converge.  From  this  time  the  phenomenon  dimin- 
ishes in  its  intensity,  exhibiting,  nevertheless,  from  time  to  time — sometimes  on 
one  side  of  the  heavens  and  sometimes  on  another — jets  of  light,  a  crown  and 
colors  more  or  less  vivid.  Finally  the  motion  ceases,  the  light  approaches 
gradually  to  the  horizon  ;  the  cloud,  quitting  the  other  parts  of  the  firmament, 
settles  in  the  north.  The  dark  part  of  the  segment  becomes  luminous,  its 
brightness  being  greatest  near  the  horizon,  and  becoming  more  feeble  as  the 
altitude  augments,  until  it  loses  its  light  altogether. 

The  aurora  is  sometimes  composed  of  two  luminous  segments,  which  are 
concentric,  and  separated  from  each  other  by  one  dark  space,  and  from  the 
earth  by  another.  Sometimes,  though  rarely,  there  is  only  one  dark  segment, 
which  is  symmetrically  pierced  round  its  border  by  openings,  through  which 
light  or  fire  is  seen,  as  represented  in  fig.  1.  A  meteor  of  this  kind  was  ob- 


served  by  Mairan  himself  at  Breuille-Pont,  on  the  19th  of  October,  1726. 
This  meteor  was  seen  at  the  same  time  in  distant  parts  of  Europe,  such  as 
Warsaw,  Moscow,  St.  Petersburg,  Rome,  Naples,  Lisbon,  and  Cadiz.  The 
least  height  which  is  compatible  with  its  observed  position  in  these  places 
would  be  about  fifty  leagues  above  the  surface  of  the  earth. 

In  the  year  1817,  M.  Biot  made  a  voyage  to  the  Shetland  isles,  where  he 
had  frequent  and  favorable  opportunities  of  observing  these  phenomena ;  and 
the  known  habits  of  accuracy  and  skill  in  experimental  investigation  of  that 
philosopher  must  confer  great  value  on  the  results  of  his  observations.  A  re- 
markable aurora  was  seen  by  him  on  the  27th  of  August,  1817. 

Several  thin  jets  of  light  were  first  seen  to  rise  at  the  northeast  to  a  small 
height.  Having  played  for  some  time,  they  were  extinguished  ;  but,  after  an 
hour  and  a  half,  they  reappeared,  with  increased  extent  and  brilliancy,  in  the 
same  part  of  the  sky.  They  soon  began  to  form  above  the  horizon  a  regular 


AURORA  BOREALIS. 


91 


arc,  like  a  rainbow,  which  was  not  complete  at  first,  but  by  degrees  increased 
its  amplitude,  and,  after  some  moments,  was  completed,  by  the  sudden  forma- 
tion of  the  remainder,  which  rose  in  a  moment,  accompanied  by  a  multitude  of 
jets  of  light,  which  issued  from  all  points  of  the  northern  horizon.  The  vertex 
of  the  bow  then  reached  very  nearly  to  the  zenith.  This  bow  was  at  first  ilet  t- 
ing  and  undecided  in  its  character,  as  if  the  matter  of  which  it  was  composed 
had  not  yet  taken  a  stable  arrangement ;  but  all  this  agitation  quickly  subsided, 
and  then  it  remained  hanging  in  the  heavens  in  all  its  beauty  for  more  than  an 
hour,  having  a  progressive  motion  barely  sensible  toward  the  southeast,  where 
it  seemed  to  be  carried  by  a  light  wind  which  was  then  felt  from  the  north- 
east. M.  Biot  had  thus  full  time  to  contemplate  it ;  and  he  observed  its  posi- 
tion with  the  instruments  he  had  provided  for  astronomical  purposes.  He 
found  that  it  embraced  an  extent  upon  the  horizon  of  128°  42',  and  that  its 
centre  was  placed  precisely  in  the  direction  of  the  magnetic  meridian.  The 
whole  extent  of  the  firmament  traversed  by  this  grand  arc,  on  the  northwestern 
side,  was  continually  intersected,  in  every  direction,  by  jets  of  light,  the  forms, 
motions,  colors,  and  continuance  of  which,  strongly  attracted  his  attention. 
Each  of  these  jets,  when  it  first  appeared,  was  a  simple  line  of  whitish  light : 
its  magnitude  and  splendor  were  augmented  rapidly,  presenting  sometimes  sin- 
gular variations  of  direction  and  curvature.  When  it  attained  its  entire  devel- 
opment, it  was  contracted  to  a  thin  straight  thread,  the  light  of  which  was 
extremely  vivid  and  brilliant,  and  of  a  decided  red  tint.  After  this  it  grew 
gradually  fainter,  and  became  extinct  frequently  at  the  same  place  precisely 
where  it  commenced  its  appearance.  This  permanence  of  a  great  number  of 
jets,  each  in  the  same  apparent  place,  while  their  brightness  exhibited  an  infi- 
nite variety  of  degrees,  renders  it  probable,  in  the  opinion  of  Biot,  that  their 
light  is  not  reflected,  but  direct,  and  that  it  is  developed  in  the  place  where  it 
is  seen.  This  inference  is  further  confirmed  by  the  circumstance  that  no  trace 
of  polarization  could  be  discovered  in  it.  All  these  meteors,  and  the  bow  with- 
in which  their  play  was  confined,  must  have  occupied  a  region  above  the 
clouds,  since  the  latter  occasionally  intercepted  their  light. 

One  of  the  most  recent  and  detailed  descriptions  of  the  aurora  borealis  is 
due  to  M.  Lottin,  an  officer  of  the  French  navy,  and  a  member  of  the  scientific 
commission  sent  some  years  ago  to  the  north  seas. 

During  the  winter  of  1838-'9,  M.  Lottin  observed  the  auroras  at  Bossekop, 
in  the  bay  of  Alien,  on  the  coast  of  West  Fin  mark,  in  the  latitude  of  70°  N. 
Between  September,  1838,  and  April,  1839,  being  an  interval  of  two  hundred 
and  six  days,  he  observed  one  hundred  and  forty-three  auroras :  they  were 
most  frequent  during  the  period  which  the  sun  remained  below  the  horizon, 
that  is,  from  the  17th  of  November  to  the  25th  of  January.  During  this  night 
of  seventy  times  twenty-four  hours,  there  were  sixty-four  auroras  visible,  with- 
out counting  those  which  were  rendered  invisible  by  a  clouded  sky,  but  the 
presence  of  which  was  indicated  by  the  disturbance  they  produced  on  the  mag- 
netic needle. 

Without  entering  into  the  details  of  the  individual  appearances  of  these  me- 
teors, we  shall  here  briefly  describe  the  appearances  and  the  succession  of 
changes  which  they  usually  presented. 

Between  the  hours  of  four  and  eight  o'clock  in  the  afternoon,  a  light  sea- 
fog,  which  almost  constantly  prevailed,  extending  to  the  altitude  of  from  four 
to  six  degrees,  became  colored  on  its  upper  border,  or  rather  was  fringed  with 
the  light  of  the  aurora,  which  was  then  behind  it ;  this  border  became  gradu- 
ally more  regular,  and  took  the  form  of  an  arc  of  a  pale  yellow  color,  the  edges 
of  which  were  diffuse,  an  d  the  extremis  s  rested  on  the  horizon.  This  bow 
swelled  upward  more  or  less  slowly,  its  vertex  being  constantly  on  the  mag- 


92 


THE  AURORA  BOREALIS. 


netic  meridian,  or  very  nearly  so.  It  was  not  easy  to  determine  this  with  pre- 
cision, because  of  the  motion  of  the  bow,  and  the  great  magnitude  of  the  circle, 
of  which  it  formed  but  a  small  segment :  blackish  streaks  divided  regularly  the 
luminous  matter  of  the  arc,  and  resolved  it  into  a  system  of  rays  ;  these  rays 
were  alternately  extended  and  contracted  ;  sometimes  slowly,  sometimes  in- 
stantaneously ;  sometimes  they  would  dart  out,  increasing  and  diminishing  sud- 
denly in  splendor.  The  inferior  parts,  or  the  feet  of  the  rays,  presented  always 
the  most  vivid  light,  and  formed  an  arc  more  or  less  regular.  The  length  of 
these  rays  was  very  various,  but  they  all  converged  to  that  point  of  the  heavens 
indicated  by  the  direction  of  the  southern  pole  of  the  dipping  needle,  as  indi- 
cated in  fi^r.  2.  Sometimes  they  were  prolonged  to  the  point  where  their 

Fig.  2. 


directions  intersected,  and  formed  the  summit  of  an  enormous  dome  of  light,  as 
represented  in  fig.  3. 

Fig.  3. 


The  bow  then  would  continue  to  ascend  toward  the  zenith  :  it  would  suffer 
an  undulatory  motion  in  its  light — that  is  to  say,  that  from  one  extremity  to  the 
other  the  brightness  of  the  rays  would  increase  successively  in  intensity.  This 
luminous  current  would  appear  several  times  in  quick  succession,  and  it  would 
pass  much  more  frequently  from  west  to  east  than  in  the  opposite  direction. 
Sometimes,  but  rarely,  a  retrograde  motion  would  take  place  immediately  af- 
terward ;  and  as  soon  as  this  wave  of  light  would  run  successively  over  all  the 
rays  of  the  aurora  from  west  to  east,  it  would  return,  in  the  contrary  direction, 
to  the  point  of  its  departure,  producing  such  an  effect  that  it  was  impossible  to 
say  whether  the  rays  themselves  were  actually  affected  by  a  motion  of  transla- 
tion in  a  direction  nearly  horizontal,  or  if  this  more  vivid  light  was  transferred 
from  ray  to  ray,  the  system  of  rays  themselves  suffering  no  change  of  position. 

The  bow,  thus  presenting  the  appearance  of  an  alternate  motion  in  a  direc- 
tion nearly  horizontal,  had  usually  the  appearance  of  the  undulations  or  folds 
of  a  riband  or  flag  agitated  by  the  wind,  as  represented  in  fig.  4.  Sometimes 


THE  AURORA  BOREALI3. 


one  and  sometimes  both  of  its  extremities  would  desert  the  horizon,  and  then 
its  folds  would  become  more  numerous  and  marked,  the  bow  would  change  its 
character,  and  assume  the  form  of  a  long  sheet  of  rays  returning  into  itself, 
and  consisting  of  several  parts  forming  graceful  curves,  as  represented  in  fig.  5. 

Fig.  5. 


The  brightness  of  the  rays  would  vary  suddenly,  sometimes  surpassing  in 
splendor  stars  of  the  first  magnitude  ;  these  rays  would  rapidly  dart  out,  and 
curves  would  be  formed  and  developed  like  the  folds  of  a  serpent ;  then  the 
rays  would  effect  various  colors,  the  base  would  be  red,  the  middle  green,  and 
the  remainder  would  preserve  its  clear  yellow  hue.  Such  was  the  arrange- 
ment which  the  colors  always  preserved;  they  were  of  admirable  transparency, 
the  base  exhibiting  blood-red,  and  the  green  of  the  middle  being  that  of  the 
pale  emerald ;  the  brightness  would  diminish,  the  colors  disappear,  and  all  be 
extinguished,  sometimes  suddenly,  and  sometimes  by  slow  degrees.  After 
this  disappearance,  fragments  of  the  bow  would  be  reproduced,  would  continue 
their  upward  movement,  and  approach  the  zenith  ;  the  rays,  by  the  effect  of 
perspective,  would  be  gradually  shortened  ;  the  thickness  of  the  arc,  which 
presented  then  the  appearance  of  a  large  zone  of  parallel  rays  (fig.  6),  would 
be  estimated  ;  then  the  vertex  of  the  bow  would  reach  the  magnetic  zenith,  or 
the  point  to  which  the  south  pole  of  the  dipping  needle  is  directed.  At  that 
moment  the  rays  would  be  seen  in  the  direction  of  their  feet.  If  they  were 
colored,  they  would  appear  as  a  large  red  band,  through  which  the  green  tints 
of  their  superior  parts  could  be  distinguished ;  and  if  the  wave  of  light  above 
mentioned  passed  along  them,  their  feet  would  form  a  long  sinuous  undulating 
zone,  while,  throughout  all  these  changes,  the  rays  would  never  suffer  any  os- 
cillation in  the  direction  of  their  axis,  and  would  constantly  preserve  their 
mutual  parallelisms. 

While  these  appearances  are  manifested,  new  bows  are  formed,  either  com- 
mencing in  the  same  diffuse  manner,  or  with  vivid  and  ready-formed  rays  : 
they  succeed  each  other,  passing  through  nearly  the  same  phases,  and  arrange 
themselves  at  certain  distances  from  each  other.  As  many  as  nine  have  been 


THE  AURORA  BOREALIS. 


Fig.  6. 


counted,  forming  as  many  bows,  having  their  ends  supported  on  the  earth,  and, 
in  their  arrangement,  resembling  the  short  curtains  suspended  one  behind  the 
other  over  the  scene  of  a  theatre,  and  intended  to  represent  the  sky.  Some- 
times the  intervals  between  these  bows  diminish,  and  two  or  more  of  them 
close  upon  each  other,  forming  one  large  zone,  traversing  the  heavens,  and  dis- 
appearing toward  the  south,  becoming  rapidly  feeble  after  passing  the  zenith. 
But  sometimes,  also,  when  this  zone  extends  over  the  summit  of  the  firmament 
from  east  to  west,  the  mass  of  rays  which  have  already  passed  beyond  the  mag- 
netic zenith  appear  suddenly  to  come  from  the  south,  and  to  form  with  those 
from  the  north  the  real  boreal  corona,  all  the  rays  of  which  converge  to  the 
zenith.  This  appearance  of  a  crown,  therefore,  is  doubtless  the  mere  effect  of 
perspective  ;  and  an  observer,  placed  at  the  same  instant  at  a  certain  distance 
to  the  north  or  to  the  south,  would  perceive  only  an  arc. 

The  total  zone,  measuring  less  in  the  direction  north  and  south  than  in  the 
direction  east  and  west,  since  it  often  leans  upon  the  earth,  the  corona  would 
be  expected  to  have  an  elliptical  form  ;  but  that  does  not  always  happen  :  it 
has  been  seen  circular,  the  unequal  rays  not  extending  to  a  greater  distance 
than  from  eight  to  twelve  degrees  from  the  zenith,  while  at  other  times  they  { 
reach  the  horizon. 

Let  it.  then,  be  imagined,  that  all  these  vivid  rays  of  light  issue  forth  with 
splendor,  subject  to  continual  and  sudden  variations  in  their  length  and  bright- 
ness ;  that  these  beautiful  red  and  green  tints  color  them  at  intervals  ;  that 
waves  of  light  undulate  over  them  :  that  currents  of  light  succeed  each  other ; 
and,  in  fine,  that  the  vast  firmament  presents  one  immense  and  magnificent 
dome  of  light,  reposing  on  the  snow-covered  base  supplied  by  the  ground — 
which  itself  serves  as  a  dazzling  frame  for  a  sea,  calm  and  black  as  a  pitchy 
lake — and  some  idea,  though  an  imperfect  one,  may  be  obtained  of  the  splen- 
did spectacle  which  presents  itself  to  him  whv>  witnesses  the  aurora  from  the 
bay  of  Alten. 

The  corona,  when  it  is  formed,  only  lasts  for  some  minutes :  it  sometime* 
forms  suddenly,  without  any  previous  bow.  There  are  rarely  more  than  t\v  > 
on  the  same  night ;  and  many  of  the  auroras  are  attended  with  no  crown  at  all. 

The  corona  becomes  gradually  faint,  the  whole  pheaomenon  being  to  the 
south  of  the  zenith,  forming  bows  gradually  paler,  and  generally  disappearing 
before  they  reach  the  southern  horizon.  All  this  most  commonly  takes  place  in  the 
first  half  of  the  night,  after  which  the  aurora  appears  to  have  lost  its  intensity  : 
the  pencils  of  rays,  the  bands  and  the  fragments  of  bows,  appear  and  disappear  at 
intervals  ;  then  the  rays  become  more  and  more  diffused,  and  ultimately  morge 
into  the  vague  and  feeble  light  which  is  spread  over  the  heavens  grouped  like  j 


THE  AURORA  BOREALIS. 


95 


little  clouds,  and  designated  by  the  name  of  auroral  plates  (plaques  aurorales}. 
Their  milky  light  frequently  undergoes  striking  changes  in  its  brightness,  like 
motions  of  dilatation  and  contraction,  which  are  propagated  reciprocally  be- 
tween the  centre  and  the  circumference,  like  those  which  are  observed  in  ma- 
rine animals  called  Medusae.  The  phenomena  become  gradually  more  faint, 
and  generally  disappear  altogether  on  the  appearance  of  twilight.  Sometime?, 
however,  the  aurora  continues  after  the  commencement  of  daybreak,  when  the 
light  is  so  strong  that  a  printed  book  may  be  read.  It  then  disappears,  some- 
times suddenly  ;  but  it  often  happens  that,  as  the  daylight  augments,  the  aurora 
becomes  gradually  vague  and  undefined,  takes  a  whitish  color,  and  is  ultimately 
so  mingled  with  the  cirrho-stratus  clouds  that  it  is  impossible  to  distinguish  it 
from  them. 

Among  the  various  theories  and  hypotheses  which  have  been  proposed  to 
explain  auroras,  that  which  appears  most  entitled  to  attention  has  been  suggested 
by  M.  Biot. 

The  first  question  which  naturally  urges  itself  upon  the  consideration  of  the 
scientific  inquirer  is,  whether  the  phenomenon  is  to  be  regarded  as  meteoro- 
logical or  astronomical ;  in  other  words,  whether  it  takes  place  within  the  limits 
of  our  atmosphere,  and  partakes  in  common  with  that  fluid  in  the  diurnal  motion 
of  the  earth,  or  is  situate  in  a  region  beyond  the  limits  of  the  atmosphere,  being 
seen  through  it,  like  the  stars,  planets,  comets,  and  other  celestial  objects.  The 
relation  which  the  form  of  aurora  invariably  bears  to  the  direction  of  the  mag- 
netic meridian  raises  a  prima  facie  presumption  in  favor  of  the  phenomenon  be- 
ing atmospheric  ;  but  all  doubt  on  this  question  has  been  removed  by  the  obser- 
vations of  M.  Biot,  from  which  it  appears  that  the  apparent  place  of  the  aurora 
in  relation  to  celestial  objects  is  not  fixed ;  that  its  altitude  and  azimuth  do  no-t 
undergo  those  hourly  changes  to  which  celestial  objects  are  subject ;  and  that 
they  undergo  no  motion,  in  reference  to  the  zenith  or  horizon,  such  as  would  be 
produced  by  the  diurnal  rotation  of  the  earth.  It  must  then  be  taken  as  demon- 
strated, that  the  aurora  borealis  is  a  phenomenon  placed  within  the  limits  of 
our  atmosphere,  and  that  it  is  connected  with  the  atmosphere  or  with  some  mat- 
ter suspended  in  it,  partaking  of  the  diurnal  motion  common  to  the  atmosphere 
and  the  globe. 

The  fact  that  the  rays  or  columns  of  light  are  always  paralled  to  the  dipping 
needle,  and  that  the  bows,  coronse,  and  other  visible  forms  which  the  phenom- 
ena afiect,  are  always  symmetrically  placed  with  respect  to  the  magnetic  me- 
ridian, demonstrate  that  the  cause  of  the  phenomena,  whatever  it  may  be,  has 
an  intimate  relation  with  that  of  terrestrial  magnetism. 

M.  Biot  conceives  that  the  luminous  columns  composing  the  aurora  have  not 
in  reality  the  position  or  form  which  they  appear  to  the  eye  to  have  ;  but  that 
their  apparent  form  is  merely  the  result  of  perspective.  He  considers  that  the 
phenomenon  is  produced  by  an  infinite  number  of  luminous  columns,  parallel 
to  the  dipping  needle  and  to  each  other,  arranged  side  by  side  at  nearly  the 
same  height  from  the  surface  of  the  earth ;  these  systems  of  columns  being 
placed  at  unequal  distances  from  the  eye,  and  see"  Mirier  different  v?3'?s  of 
obliquity,  are  projected  into  various  figures,  which  are  subject  to  variation 
arising  "from  the  varying  splendor  of  their  component  rays. 

It  has  been  attempted,  on  various  occasions,  to  determine  the  height  of  auroras 
by  the  same  method  which  has  been  applied  with  such  accurate  results  to  the 
determination  of  the  distances  of  the  sun,  moon,  and  other  celestial  objects. 
This  method  consists  in  the  comparison  of  two  observations  of  the  exact  ap- 
parent  place  in  the  heavens  observed  at  the  same  moment  in  distant  parts  of  the 
earth.  Many  causes,  however,  conspire  to  render  this  method  inapplicable  to 
auroras ;  among  which  may  be  mentioned  the  difficulty  of  making  the  two  ob- 


96 


THE  AURORA  BOREALIS. 


serrations  at  the  same  instant  of  time,  and  the  total  impossibility  of  the  two  ob- 
servers being  certain  of  directing  their  observations  to  precisely  the  same  point  of 
the  aurora.  To  such  causes  must  be  ascribed  the  widely-varying  estimates  of 
the  height  of  auroras ;  obtained  in  this  manner — estimates  which  vary  from 
fifty  to  three  hundred  miles  from  the  surface  of  the  earth.  Meanwhile,  what- 
ever be  their  height,  it  is  evidently  subject  to  continual  variation,  even  in  the 
same  aurora,  as  is  rendered  apparent  by  the  sudden  changes  which  the  phe- 
nomenon undergoes,  and  by  the  progressive  motion  of  its  arcs. 

Great  differences  have  existed  among  meteorologists  respecting  the  sounds 
which  are  said  to  proceed  from  auroras.  The  inhabitants  of  the  northern  re- 
gions, where  these  appearances  most  prevail,  are  unanimous  in  declaring  that 
they  are  frequently  accompanied  by  hissing  and  cracking  noises  in  the  air, 
like  those  produced  by  artificial  fireworks.  Persons  engaged  in  the  whale- 
fisheries  make  the  same  statements.  M.  Biot  found  the  inhabitants  of  the 
Shetland  islands  unanimous  on  the  question ;  and  M.  Lottin  found  the  same 
impression  among  the  far-distant  inhabitants  of  Siberia.  On  the  other  hand, 
during  the  sojourn  of  M.  Biot  in  the  Shetland  isles,  he  witnessed  several  great 
auroras,  but  heard  no  sound.  During  M.  Lottin's  expedition,  he  witnessed 
one  hundred  and  forty-three  auroras,  in  not  one  of  which  was  he  sensible  of 
any  sound.  The  only  strictly  scientific  observer  who  appears  to  have  person- 
ally experienced  such  sounds  is  Cavallo,  who  states  that  he  has  distinctly 
heard  them  on  several  occasions,  but  limits  his  testimony  to  this  general  form, 
assigning  neither  time  nor  place.  Such  discordancy  of  evidence  can  oaly  be 
reconciled  by  the  supposition  that  such  sounds  are  audible  on  rare  occasions, 
when  the  region  in  which  the  aurora  is  developed  is  within  a  very  limited  dis- 
tance of  the  observer ;  and  if  the  existence  of  such  sounds  be  thus  admitted, 
it  must  be  also  admitted  that  the  height  of  the  aurora  is,  at  least  in  such  cases, 
infinitely  less  than  is  commonly  estimated  ;  and  if,  in  particular  cases,  its 
height  be  so  small,  it  is  probably  in  all  others  proportionally  under  the  highest 
estimates  which  have  been  made  of  it. 

From  a  comparison  of  all  the  observed  effects,  it  may  then  be  assumed  as 
nearly,  if  not  conclusively,  proved,  that  the  aurora  borealis  is  composed  of  real 
clouds,  proceeding  generally  from  the  north,  and  formed  of  extremely  attenuated 
and  luminous  matter  floating  in  the  atmosphere,  which  frequently  arrange  them- 
selves in  series  of  lines  or  columns  parallel  to  the  dipping  needle.  What  tho 
nature  of  the  matter  is  composing  such  clouds  must,  in  the  present  state  of 
science,  rest  upon  mere  conjecture.  The  following  is  the  substance  of  the 
theory  of  M.  Biot  on  this  subject  already  referred  to : — 

Among  material  substances,  certain  metals  alone  are  susceptible  of  magnet- 
ism. Since,  then,  the  luminous  matter  composiag  the  aurora  obeys  the  magnetic 
influence  of  the  earth,  it  is  very  probable  thai  the  luminous  clouds  of  which  it 
consists  are  composed  of  metallic  particles  reduced  to  an  extremely  minute  and 
subtile  form.  This  being  admitted,  another  consequence  will  immediately  ensue. 
Such  metallic  clouds,  if  the  expression  be  allowed,  will  be  conductors  of  elec- 
tricity, more  or  less  perfect,  according  to  the  degree  of  proximity  of  their  con- 
stituent particles.  When  such  clouds  arrange  tliemselves  in  columnar  forms, 
and  connect  strata  of  the  atmosphere  at  different  elevations,  if  such  strata  be 
unequally  charged  with  electricity,  the  electrical  equilibrium  will  be  re-estab- 
lished through  the  intervention  of  the  metallic  columns,  and  light  and  sound 
will  be  evolved  in  proportion  to  the  imperfect  conductability  of  the  metallic 
clouds  arising  from  the  extremely  rarefied  state  of  the  metallic  vapor,  or  fine 
dust,  of  which  they  are  constituted.  All  the  results  of  electrical  experiment* 
countenance  these  suppositions,  when  the  phenomena  are  produced  in  tk« 
more  elevated  regions,  where  the  air  is  highly  rarefied,  little  resistance  being 


97  ; 

opposed  to  the  motion  of  the  electric  fluid :  light  alone  is  evolved  without  son-  I 
sible  sound,  as  is  observed  when  electricity  is  transmitted  through  exhausted  ; 
tubes  ;  but  when  the  aurora  is  developed  in  the  lower  strata  of  the  atmosphere,  ? 
it  would  produce  the  hissing  and  cracking  noise  which  appears  to  be  heard  on  ; 
some  occasions.     If  the  metallic  cloud  possess  the  conducting  power  in  a  hiah  f 
degree,  the  electric  current  may  pass  through  it  without  the  evolution  of  either  / 
light  or  sound :  and  thus  the  magnetic  needle  may  be  affected  as  it  would  be 
by  an  aurora  at  a  time  when  no  aurora  is  visible.     If  any  cause  alters  the  con- 
ductability  of  those  columnar  clouds  suddenly  or  gradually,  a  sudden  or  gradual 
change  in  the  splendor  of  the  aurora  would  ensue. 

According  as  those  clouds  advance  over  more  southern  countries,  the  direc- 
tion of  their  columns  being  constantly  parallel  to  the  dipping  needle,  they  take 
gradually  a  more  horizontal  position,  and  consequently  the  strata  of  atmosphere 
at  their  extremities  become  gradually  less  distant,  and  consequently  more 
nearly  in  a  state  of  electrical  equilibrium ;  hence  it  follows,  that  as  the  latitude 
diminishes,  the  appearance  of  aurora  becomes  more  and  more  rare,  until,  in  the 
lower  latitudes,  where  the  columns  are  nearly  parallel  to  the  horizon,  such 
phenomena  are  never  observed. 

This  ingenious  and  beautiful  theory  still,  however,  requires,  before  its  va- 
lidity can  be  admitted  by  the  rigid  canons  of  modern  physics,  that  the  main 
fact  on  which  it  rests  should  be  proved  :  it  is  necessary  that  it  should  be 
hown  that  such  metallic  clouds  as  are  here  supposed,  and  on  the  agency  of 
vhich  the  whole  theory  is  based,  should  be  accounted  for.  This  demand  is 
accordingly  answered  by  M.  Biot. 

The  magnetic  pole,  or  its  vicinity,  is  evidently  the  point  from  which  these 
olumnar  masses  of  meteoric  light  proceed.  Therefore,  the  extremely  minute 
ays  composing  these  columns  must  issue  from  the  earth  in  that  region.  Now 
t  is  well  known  that  that  part  of  the  globe  is,  and  always  has  been,  character- 
zed  by  the  prevalence  of  frequent  and  violent  volcanic  eruptions,  and  several 
•olcanoes  have  been,  and  still  are,  in  activity  round  the  place  where  the  mag- 
netic pole  is  situate.  These  eruptions  are  always  accompanied  by  electric 
>henomena.  Thunder  issues  from  the  volcanic  clouds  ejected  by  the  craters  ; 
and  these  clouds  of  volcanic  dust,  thus  charged  with  electricity,  are  projected 
o  great  heights,  and  carried  to  considerable  distances  through  the  air,  carrying 
vith  them  all  the  electricity  taken  from  the  crater. 

These  vast  eruptions,  issuing  from  depths  so  unfathomable  that  they  seem  al- 
most to  penetrate  the  globe,  and  issuing  with  such  violence  from  the  gulfs  by 
vhich  they  are  projected  into  the  atmosphere,  must  necessarily  produce  strong 
•ertical  currents  of  air,  by  which  the  volcanic  dust  will  be  carried  to  an  eleva- 
ion  exceeding  that  of  common  clouds.  Travellers  who  have  visited  Iceland 
lave  often  seen  suspended  over  it,  during  eruptions,  a  species  of  volcanic  fog. 
Such  clouds  are  known  to  be  of  a  sulphureous  and  metallic  nature,  painfully  irri- 
ating  the  eyes,  mouth,  and  nostrils.  Moreover,  the  existence  of  dry  fogs,  dif- 
using  a  fetid  and  sulphureous  odor,  was  ascertained  in  1783,  when  all  Europe 
was  enveloped  in  a  fog  of  that  description. 

To  this  it  may  be  added,  that  more  recent  observations  have  rendered  it 
lighly  probable,  if  not  certain,  that  metallic  matter,  and  more  particularly  iron 
n  a  pure  and  uncombined  state,  is  frequently  precipitated  from  the  clouds  in 
hundcr-storms. 

To  the  theory  of  M.  Biot  it  is  objected  by  M.  Becquerel,  that  the  existence 
of  metal  in  that  uncombined  form,  in  which  alone  it  has  the  conducting  power, 
n  volcanic  eruptions,  has  not  been  proved ;  that  the  matter  ejected  from  vol- 
canoes consists  of  vitrified  substances,  silicates,  aluminates,  and  other  sub- 
stances, which  are  non-conductors,  but  that  pure  metal  is  never  found. 

T 


THE  AURORA  BOREALIS 


At  the  time  when  M.  Biot  promulgated  his  theory,  it  was  necessary  for  him 
to  assign  an  adequate  source  whence  the  electricity  was  derived,  to  which 
he  ascribed  the  aurora ;  and  he  accordingly  supposed  it  to  proceed  from  the 
polar  volcanoes.  In  the  progress  of  electrical  discovery,  so  many  new  sources 
of  electricity  have,  however,  been  since  disclosed,  that  this  part  of  his  hypothe- 
sis has  become  needless. 

The  following  hypothesis  has  been  suggested  by  Professor  Faraday  (Erp. 
Research.  192)  : — 

"  I  hardly  dare  venture,  even  in  the  most  hypothetical  form,  to  ask  whether 
the  aurora  borealis  and  australis  may  not  be  the  discharge  of  electricity ,  thus 
urged  toward  the  poles  of  the  earth,  whence  it  is  endeavoring  to  return 
by  natural  and  appointed  means  above  the  earth  to  the  equatorial  regions.  The 
non-occurrence  of  it  in  very  high  latitudes  is  not  at  all  against  the  supposition ; 
and  it  is  remarkable  that  Mr.  Fox,  who  observed  the  deflections  of  the  magnetic 
needle  at  Falmouth,  by  the  aurora  borealis,  gave  that  direction  of  it  which  per- 
fectly agrees  with  the  present  view."  The  manner  in  which  the  electricity 
above  alluded  to  is  urged  toward  the  poles,  belongs  to  another  division  of  our 
subject,  "  Magneto  Electricity."  If  the  above  view  is  correct,  may  it  not  help 
us  in  the  difficult  question  of  atmospheric  electricity  1 

The  mode  adopted  to  illustrate  the  electrical  nature  of  the  aurora,  is  to  ex- 
haust a  tall  glass  tube  by  means  of  the  air-pump,  and  then  to  pass  a  succession 
of  electric  sparks  down  the  interior  of  the  tube,  from  the  prime  conductor  of 
the  machine.  The  effects  produced  by  a  powerful  machine  are  most  brilliant ; 
a  close  inspection  shows  that  the  whole  tube  is  at  times  filled  with  a  mass  of 
miniature  flashes  of  lightning  ;  the  color  varies  from  the  usual  bright  electrical 
light  to  a  vivid  violet.  The  most  exalted  effects  have  been  produced  by  means 
of  the  hydro-electric  machine.  The  tension  of  this  machine  is  equal  to  a 
spark  of  twelve  or  fourteen  inches  in  the  atmosphere,  and  therefore  of  power 
to  pass  readily  through  four  or  five  feet  of  partial  vacuum,  and  its  quantity  is 
equivalent  to  a  charge  of  eighty  feet  of  coated  surface  in  ten  seconds.  A  pe- 
culiar effect  attending  this  powerful  discharge  is,  that  sometimes  the  aurora 
appears  with  a  bright  line  of  light  proceeding  from  each  end  of  the  tube,  and  a 
revolving  spiral  embracing  the  lower  part. 

The  falling  star  is  an  experiment  of  the  auroral  character  often  introduced 
in  books  on  electricity.  Cavallo  says  (vol.  ii.,  p.  101),  "  When  the  receiver  is 
not  exhausted,  the  discharge  of  a  jar  through  some  part  of  it  will  appear  like  a 
small  globule  exceedingly  bright."  Whence  we  often  hear  it  said,  that  the  dis- 
charge of  a  battery  will  produce  a  ball  of  light  passing  from  one  end  to  the 
other  of  the  exhausted  receiver.  If  this  really  were  the  case,  ij;  would  be  a 
most  important  experiment ;  for  if  the  ball  were  seen  to  pass  from  one  end  to 
the  other,  it  would  follow  that  its  direction  had  been  actually  seen  ;  and,  if  so, 
the  one-fluid  theory  would  have  been  demonstrated.  But  very  little  reflection 
will  suffice  to  show  the  impossibility  of  such  an  appearance ;  for,  admitting 
the  actual  existence  of  a  ball,  though  we  are  more  inclined  to  suppose  that  any 
such  thing  would  be  like  an  oblong  spheroid,  the  extreme  velocity  of  electricity 
would  take  it  to  the  end  of  its  course  before  the  impression  of  its  first  appear- 
ance on  the  retina  had  subsided ;  just,  indeed,  as  the  rotating  wheel,  having 
red  radii,  appears  entirely  red  during  the  period  of  rapid  rotation  ;  and  so,  in- 
stead of  seeing  a  ball,  if  such  really  were  there,  the  eye  would  recognise  a 
continuous  line  of  light.  And  this  is  actually  the  case.  We  have  ourselves 
repeated  the  experiment  under  very  favorable  circumstances,  and  in  the  pres- 
ence of  very  competent  witnesses,  and  one  and  all  agreed  in  perceiving  in 
every  case  a  distinct  continuous  line  of  light,  but  no  appearance  of  a  ball  or 
falling  star. 


THE  AURORA  BOREALIS.  99 


An  extraordinary  experiment,  illustrative  of  the  theory  of  the  aurora  similar 
to  that  suggested  by  Faraday,  with  t«e  addition  that  "  electricity  is  radiated  in 
a  peculiar  manner  fr>»m  magnetized  bodies,"  was  introduced  by  Mr.  Nott  at  the 
meeting  of  the  British  Association  at  Cork  (1843).  He  rotated  a  steel  globe, 
and  passed  magnets  from  the  equator  to  the  poles,  till  the  globe  was  perfectly 
magnetized.  He  then  insulated  the  globe,  and  placed  an  insulated  ring  around 
its  equatorial  regions.  He  connected  the  ring  with  the  prime  conductor  of  the 
resinous  plate  of  his  "  rheo-electric  machine,"  and  one  pole  of  the  globe  with  the 
conductor  of  the  vitreous  plate.  It  is  necessary  to  mention,  that  the  machine 
alluded  to  consists  of  two  parallel  plates,  one  plass,  the  other  resin,  rotating  on 
the  same  axis,  and  provided  with  separate  rubbers.  The  circuit,  including  the 
rubbers  and  conductors,  is  completed  in  various  ways  ;  the  machine  is  described 
as  producing  a  current  of  electricity  of  tension  analogous  to  that  of  the  pile. 
In  the  present  experiment,  when  the  machine  is  rotated,  a  truly  beautiful  and 
luminous  discharge  takes  place  between  the  unconnected  pole  of  the  globe  and 
the  ring.  A  dense  atmosphere  is  more  favorable  to  the  success  of  the  experi- 
ment than  a  dry  one.  It  had  then  the  appearance  of  a  ring  of  light,  the  upper 
part  of  which  was  brilliant,  and  the  under  dark :  above  the  ring,  all  around  the 
axis  were  foliated  diverging  flames,  one  behind  the  other. 

In  Captain  Franklin's  narrative,  the  auroras  observed  at  Fort  Enterprise,  in 
North  America,  are  described  by  Lieutenant  Wood  as  follows  : — 

They  rise  with  their  centres  sometimes  in  the  magnetic  meridian,  and 
sometimes  several  degrees  to  the  eastward  or  westward  of  it.  The  number 
visible  seldom  exceeds  five,  and  is  seldom  limited  tu  one.  The  altitude  of  the 
lowest,  when  first  seen,  is  never  less  than  four  degrees.  As  they  advance 
toward  the  zenith,  their  centres  (or  the  parts  most  elevated)  preserve  a  course 
in  the  magnetic  meridian,  or  near  to  it ;  but  the  eastern  and  western  extremi- 
ties vary  their  respective  distances,  and  the  arches  become  irregularly  broad 
streams  in  the  zenith,  each  dividing  the  sky  into  two  unequal  parts,  but  never 
crossing  one  another  until  they  separate  into  parts.  Those  parts  which  were 
bright  at  the  horizon,  increase  their  brilliancy  in  the  zenith,  and  discover  the 
beams  of  which  they  are  composed,  where  the  interior  motion  is  rapid.  This 
interior  motion  is  a  sudden  glow,  not  proceeding  from  any  visible  concentra- 
tion of  matter,  but  bursting  out  in  several  parts  of  the  arch,  as  if  an  ignition  of 
combustible  matter  had  taken  place,  and  spreading  itself  rapidly  toward  each 
extremity.  In  this  motion  the  beams  are  formed.  They  have  two  motions : 
one  at  right  angles  to  their  length,  or  sidewise,  and  the  other  a  tremulous  and 
short  vibration,  in  which  they  do  not  exactly  preserve  their  parallelism  to  each 
other.  The  wreaths,  when  in  the  zenith,  present  the  appearance  of  corona 
boreales.  The  second  motion  is  alwajTs  accompanied  by  colors  ;  for  it  must 
be  observed  that  beams  are  often  formed  without  any  exhibition  of  colors,  and 
I  have  not  in  that  case  perceived  the  vibratory  motion. 

The  northern  lights  are  sometimes  tinged  with  the  various  prismatic  colors, 
among  which  orange  and  green,  but  more  frequently  the  different  shades  of 
red,  predominate.  Maupertius  describes  one  seen  by  him  in  Lapland,  by  which 
an  expensive  region  of  the  heavens  toward  the  south  appeared  tinged  with  so  live- 
ly a  red,  that  the  whole  constellation  of  Orion  seemed  as  if  dyed  in  blood.  Some 
observers  of  this  meteor  have  affirmed  that  they  have  heard  a  rustling  or  crack- 
ling sound  proceed  from  it ;  doubts  have,  however,  been  entertained  on  this  point, 
from  the  circumstance  that  no  such  noises  were  heard  by  Scoresby,  Richard- 
son, Franklin,  Parry,  and  Hood,  who  observed  the  polar  lights  with  great  care, 
under  the  most  favorable  circumstances,  in  very  high  latitudes.  But  the  testimo- 
ny of  other  observers  is  so  positive  a  kind,  as  to  leave  no  reasonable  doubt  that  ^ 
the  phenomenon  has,  at  least  in  particular  instances,  been  accompanied  by  sounds.  > 


From  the  accounts  which  have  been  collected  of  the  polar  lights,  it  would 
seem  that  the  phenomenon  was  less  frequent  in  former  ages  than  it  is  now  ;  but 
it  must  be  kept  in  mind  that  meteoric  observations  have  not  always  been  so 
much  attended  to  as  at  present.  Aristotle  describes  the  phenomenon  with  suf- 
ficient accuracy  in  his  book  of  meteors.  Allusions  are  also  made  to  it  by 
Pliny,  Cicero,  and  Seneca ;  so  that  it  must  have  been  witnessed  by  the  an- 
cients, even  in  the  climates  of  Greece  and  Italy.  The  descriptions  of  armies 
fighting  in  the  air,  and  similar  observations,  in  the  dark  ages,  doubtless  owed 
their  origin  to  the  striking  and  fantastic  appearances  of  the  northern  lights. 
It  is  remarkable,  however,  that  no  mention  is  made  by  any  English  writer  of 
an  aurora  borealis  having  been  observed  in  England  from  the  year  1621  to 
1707.  Celsius  says  expressly  that  the  oldest  inhabitants  of  Upsala  considered 
the  phenomenon  a  great  rarity  before  1716.  In  the  month  of  March  in  that 
year,  a  very  splendid  one  appeared  in  England,  and  by  reason  of  its  brilliancy 
attracted  universal  attentipn.  It  has  been  described  by  Dr.  Halley  in  the  Philo- 
sophical Transactions,  No.  347.  Since  then  the  meteor  has  been  much  more 
common.  A  complete  account  of  all  the  appearances  of  auroras  recorded  previ- 
ous to  1754  may  be  found  in  the  work  of  Mairan,  "  Traite  de  I'Aurore  Boraele." 

The  aurora  is  not  confined  to  the  northern  hemisphere,  similar  appearances 
being  observed  in  high  southern  latitudes.  An  aurora  was  observed  by  Don 
Antonio  d'Ulloa  at  Cape  Horn  in  1745  ;  one  appeared  at  Cuzco,  in  1744  ;  and 
another  is  described  by  Mr.  Forster  (who  accompanied  Captain  Cook  in  his 
last  voyage  round  the  world),  which  was  seen  by  him  in  1773,  in  latitude  58° 
south,  and  resembled  entirely  those  of  the  northern  hemisphere,  excepting  that 
the  light  exhibited  no  tints,  but  was  of  a  clear  white.  Similar  testimony  is 
given  by  subsequent  navigators. 

There  is  great  difficulty  in  determining  the  exact  height  of  the  aurora  bo- 
realis above  the  earth,  and  accordingly  the  opinions  given  on  this  subject  by 
different  observers  are  widely  discordant.  Mairan  supposed  the  mean  height 
to  be  one  hundred  and  seventy-five  French  leagues  ;  Bergman  says  four  hun- 
dred and  sixty,  and  Euler  several  thousand  miles.  From  the  comparison  of  a 
number  of  observations  of  an  aurora  that  appeared  in  March,  1826,  made  at 
different  places  in  the  north  of  England  and  south  of  Scotland,  Dr.  Dalton,  in 
a  paper  presented  to  the  Royal  Society,  computed  its  height  to  be  about  one 
hundred  miles.  But  a  calculation  of  this  sort,  in  which  it  is  of  necessity  sup- 
posed that  the  meteor  is  seen  in  exactly  the  same  place  by  the  different  ob- 
servers, is  subject  to  very  great  uncertainty.  The  observations  of  Dr.  Rich- 
ardson, Franklin,  Hood,  Parry,  and  others,  seem  to  prove  that  the  place  of  the 
aurora  is  far  within  the  limits  of  the  atmosphere,  and  scarcely  above  the  region 
of  the  clouds  ;  in  fact,  as  the  diurnal  rotation  of  the  earth  produces  no  change 
in  its  apparent  position,  it  must  necessarily  partake  of  that  motion,  and  conse- 
quently be  regarded  as  an  atmospherical  phenomenon. 


L 


ELECTRICITY, 


Electric  Phenomena  observed  by  the  Ancients. — Thales. — Gilbert  de  Magnete. — Otto  Guerieke's 
Electric  Machine. — Hawkesbee's  Ex'iennis-i.-is. — Stephen  Grey's  Discoveries  on  Electrics  and 
Non-Electrics. — Wheeler  and  Grey'<  dxocnments. — Dufaye  discovers  the  Resinous  and  Vitre- 
ous Electricities. — Invention  of  the  Le /(«•»='  t-iiial. — Singular  Effects  of  the  first  Electric  Shocks. — 
Experiments  of  Watson  and  Bevis. — Experiments  on  Conductors. — Franklin's  Experiments  and 
Letters. — His  Celebrated  Theory  of  Positive  and  Negative  Electricity. — His  Experiments  ou  the 
LeycJen  Phial. — His  Discovery  of  the  Identity  of  Lightning  and  Electricity. — Reception  of  his 
Suggestions  by  the  Royal  Society. — His  Kite  Experiment. — His  Right  to  this  Discovery  denied 
by  Arago. — His  Claim  vindicated. — Invention  of  Conductors. — Death  of  Richmann. — Beccaria's 
Observations. — Canton's  Experiments. — Discovery  of  Induction. — Invention  of  the  Condenser. — 
Works  of  .AJpinus. — Theory  of  Symmer. — Experiments  of  Coulomb. — Balance  of  Torsion. — 
Electricity  of  the  Atmosphere. — Effects  of  Flame. — Experiments  of  Volta. — Lavoisier  and  La- 
(  place. — Analytical  Work  of  Poissoa. 


ELECTRICITY.  JQ3 


ELECTRICITY. 


ALTHOUGH  it  has  been  reserved  for  modern  times  to  bring  to  perfection  the 
methods  of  investigation  pursued  in  physical  researches,  these  great  divisions 
of  human  knowledge  ha've  nevertheless  been  always  progressive.  If  the  la- 
bors of  the  ancients  were  obstructed,  their  advancement  retarded,  and  their 
productions  disfigured  by  fantastical  theories  ;  the  facts  they  accumulated,  the 
phenomena  they  described,  and  the  observations  they  recorded,  have  formed  a 
bequest  of  the  highest  value  to  the  better  disciplined  inquirers  and  observers 
of  later  days.  Astronomy,  the  mechanics  of  solid  and  fluid  bodies,  and  the 
physics  of  the  imponderable  agents,  light  and  heat,  received  severally  more  or 
less  attention  at  an  early  epoch  of  the  progress  of  human  knowledge ;  and  the 
results  of  ancient  researches  in  some  of  these  branches  of  science,  astronomy 
for  example,  form  an  important  element  of  the  knowledge  we  now  possess. 
Electricity,  however,  is  a  remarkable  exception  to  this  state  of  progressive 
movement.  To  that  particular  division  of  physics  antiquity  has  contributed 
absolutely  nothing.  The  vast  discoveries  which  have  accumulated  respecting 
this  extraordinary  agent,  by  which  its  connexion  with  and  influence  upon  the 
whole  material  universe,  its  relations  to  the  phenomena  of  organized  bodies, 
the  part  it  plays  in  the  functions  of  animal  and  vegetable  vitality,  its  subservi- 
ence to  the  uses  of  man  as  a  mechanical  power,  its  intimate  connexion  with 
the  chemical  constitution  of  material  substances,  in  fine,  its  application  in  al- 
most every  division  of  the  sciences,  and  every  department  of  the  arts,  have 
been  severally  demonstrated,  are  exclusively  and  peculiarly  due  to  the  spirit 

)  of  modern  research,  and  in  a  great  degree  to  the  labors  of  the  present  age. 

•       The  beginnings  of  science  have  often  the   appearance  of  chance.     A  felici- 
tous  accident  throws  a  certain  natural  fact  under  the  notice  of  an  inquiring  and  ) 
philosophic  mind.     Attention  is  awakened  and  investigation  provoked.     Simi-  < 
lar  phenomena  under  varied  circumstances  are  eagerly  sought  for ;  and  if  in  ( 

(  the  natural  course  of  events  they  do  not  present  themselves,  circumstances  are  < 

|  designedly  arranged  so  as  to  bring  about  their  production.     The   seeds   of  / 

V 


104 


ELECTRICITY. 


science  are  thus  sown,  and  soon  begin  to  germinate.  Whether  such  primary 
facts  are  really  fortuitous,  or  ought  not  rather  to  be  viewed  as  the  prompting 
of  HIM,  whose  will  is  that  intellectual  progression  shall  be  incessant,  it  is  cer- 
tain that  they  not  only  give  the  first  impetus  to  science,  but  their  occasional 
and  timely  occurrence  in  its  progress  produces  frequently  greater  effects  on  the 
celerity  of  its  advancement  than  the  most  exalted  powers  of  the  human  mind, 
unsupported  by  such  aid,  have  ever  accomplished.  It  may  then  be  imagined 
that  if  any  such  hints  were  offered  by  ordinary  phenomena,  an  agent  so  all- 
pervading  as  electricity  could  scarcely  have  eluded  notice,  or  failed  to  command 
attention,  during  a  succession  of  ages  which  witnessed  the  growth  and  exten- 
sion of  so  many  other  parts  of  natural  knowledge.  On  the  contrary,  the  class 
of  effects  in  which  electricity  originated  was  observed  by  and  well  known  to 
the  early  philosophers  of  Greece.  THALES,  six  centuries  before  the  Christian 
era,  was  acquainted  with  the  property  of  amber,  from  which  electricity  derives 
its  name ;  *  and  Theophrastus  and  Pliny,  as  well  as  other  writers,  Greek  and 
Roman,  mention  the  property  of  this  and  certain  other  substances,  in  virtue  of 
which,  when  submitted  to  friction,  they  acquire  the  power  to  attract  straws, 
and  other  light  bodies,  as  a  magnet  attracts  iron.  In  the  spirit  which  charac- 
terized the  times,  such  effects  were  regarded  with  feelings  of  superstition.  A 
soul  was  ascribed  to  amber,  and  it  was  held  sacred. 

Nor  were  these  the  only  phenomena  which  presented  themselves  to  the  an- 
cients, and.  afforded  them  a  clue  to  the  foundation  of  this  part  of  physics. 

Various  other  scattered  facts  are  recorded,  which  prove  that  nature  did  not 
conceal  her  secrets  with  more  than  usual  coyness  in  this  case.  The  luminous 
appearance  attending  the  friction  of  those  substances  which  exhibited  electrical 
effects  was  observed.  The  Roman  historians  record  the  frequent  appearance 
of  a  flame  at  the  points  of  the  soldiers'  javelins,  at  the  summits  of  the  masts  of 
ships,  and  sometimes  even  on  the  heads  of  the  seamen. f  The  effects  of  the 
torpedo  and  electrical  fishes  are  referred  to  by  Aristotle,  Galen  and  Oppian  ; 
and  at  a  period  less  remote,  Eustathius,  in  his  Commentary  on  the  Iliad  of 
Homer,  mentions  the  case  of  Walimer,  a  Gothic  chief,  the  father  of  Theodo- 
ric,  who  used  to  eject  sparks  from  his  body  ;  and  further  refers  to  a  certain 
ancient  philosopher,  who  relates  of  himself  that  on  one  occasion,  when  chan- 
ging his  dress,  sudden  sparks  were  emitted  from  his  person  on  drawing  off  his 
clothes,  and  that  flames  occasionally  issued  from  him,  accompanied  by  a 
crackling  noise.l 

Such  phenomena  attracted  little  attention,  and  provoked  no  scientific  research. 
Vacant  wonder  was  the  most  exalted  sentiment  they  raised  ;  and  they  accord- 
ingly remained,  while  twenty  centuries  rolled  away,  barren  and  isolated  facts 
upon  the  surface  of  human  knowledge.  The  vein  whence  these  precious  frag- 
ments were  detached,  and  which,  as  we  have  shown,  cropped  out  sufficiently 
often  to  challenge  the  notice  of  the  miner,  continued  unexplored  and  undiscov- 
ered ;  and  its  splendid  treasures  were  reserved  to  reward  the  toil  and  crown 
the  enterprise  of  our  generation. 

The  work  of  classification  and  generalization  was  first  commenced  upon  the 
phenomena  of  electricity  by  Gilbert,  an  English  physician,  in  a  work  entitled 
De  Magnete,  published  in  the  beginning  of  the  seventeenth  century.  In  this 
treatise,  the  substances  then  known  to  be  susceptible  of  electrical  excitement 
were  enumerated,  and  several  of  the  circumstances  which  affect  the  production 
of  electixca..  phenomena,  such  as  the  hygrometric  state  of  the  atmosphere, 
were  explained.  Between  that  period  and  the  earlier  part  of  the  last  century 

*  "HA«Tj59i/,  amber. 

t  Cajsar,  de  Bell.  Afr.  cap.  vi.  Liv.  cap.  xxxii.     Plut  Vita  Lys.    Plin.  sec.  Hist  Mun.  lib.  ii. 

I  Eudtatk.  in  Iliad,  E. 


— > 

ELECTRICITY.  105 

the  science  was  not  advanced  by  any  capital  discoveries.  In  that  interval, 
however,  Otto  Guericke,  celebrated  as  the  inventor  of  the  air-pump,  contrived 
the  first  electrical  machine.  This  apparatus  consisted  of  a  globe  of  sulphur, 
mounted  upon  a  horizontal  axis,  from  which  it  received  a  motion  of  rotation, 
by  means  of  a  common  handle  or  winch.  The  operator  turned  this  handle 
with  one  hand,  while  with  the  other  he  applied  a  cloth  to  the  globe,  the  friction 
of  whiih  produced  thi,  electrical  state. 

Aided  by  such  apparatus,  this  philosopher  discovered,  that  after  a  light  sub- 
stance has  been  attracted  by  and  brought  into  contact  with  an  electrified  body, 
it  will  not  be  again  attracted,  but,  on  the  contrary,  will  be  repelled  by  the  same 
body  ;  but  that  after  it  has  been  touched  by  the  hand,  its  primitive  condition  is 
restored,  and  it  is  again  attracted.  He  also  showed  that  a  body  becomes  elec- 
tric by  being  brought  near  to  an  electrified  body  without  touching  it ;  but  offer- 
ed no  explanation  of  this  fact,  which,  as  will  be  seen  hereafter,  indicated  one 
of  the  most  important  principles  of  electrical  science. 

Whether  it  was  that  all  his  attention  was  altogether  engrossed  by  the  re- 
searches which  he  prosecuted  with  such  splendid  results  in  astronomy,  the 
higher  mechanics,  and  optics,  or  that  facts  had  not  yet  accumulated  in  sufficient 
number  and  variety  to  impress  him  with  a  just  notion  of  the  importance  of  elec- 
tricity as  a  general  physical  agent,  Newton  bestowed  on  it  no  attention.  One 
experiment  only  proceeding  from  him  is  recorded,  in  which  he  shows  that  when 
one  surface  of  a  plate  of  glass  is  electrified,  the  attraction  will  be  transmitted 
through  the  glass,  and  will  be  manifested  by  its  effect  on  any  light  substances 
placed  on  the  other  side  of  it. 

In  the  beginning  of  the  eighteenth  century,  Hawkesbee  made  a  series  of 
experiments  on  electrical  light  produced  in  rarefied  air  ;  but  as  no  consequences 
were  deduced  from  them  affecting  the  progress  of  the  science,  we  shall  not 
further  iiotice  them.  In  the  construction  of  the  apparatus  for  producing  elec- 
tricity, he  substituted  a  glass  sphere  for  the  globe  of  sulphur  proposed  by  Otto 
Guericke.  This  was  a  considerable  improvement ;  and  yet  the  experimental- 
ists who  followed  abandoned  it,  and  used  no  more  convenient  apparatus  than 
glass  tubes,  which  were  held  in  one  hand,  and  rubbed  with  the  other.  To 
this  circumstance  Dr.  Priestley  ascribes  in  a  great  degree,  the  slow  progress 
made  by  the  immediate  successors  of  Hawkesbee  in  electrical  discoveries. 

About  the  year  1730  commenced  that  splendid  series  of  discoveries  which 
has  proceeded  with  accelerated  speed  to  the  present  day,  and  now  forms  the 
body  of  electrical  science.  Mr.  Stephen  Grey,  a  pensioner  of  the  Charter 
House,  impelled  by  a  passionate  enthusiasm,  engaged  in  a  course  of  experi- 
mental researches,  in  which  were  developed  some  general  principles,  which 
produced  important  effects  on  subsequent  investigations. 

The  most  considerable  discovery  of  Mr.  Grey  was,  that  all  material  substan- 
ces might  be  reduced,  in  reference  to  electrical  phenomena,  to  two  classes, 
electrics  and  non-electrics ;  the  former,  including  all  bodies  then  supposed  to  be 
capable  of  electric  excitation  by  friction ;  and  the  other,  those  which  were  in- 
capable of  it.  He  also  discovered  that  non-electrics  were  capable  of  acquiring 
the  electric  state  by  contact  with  excited  electrics.  As  the  experiments 
which  led  to  these  conclusions  were  of  the  highest  interest,  we  shall  hero 
state  them. 

Desiring  to  make  some  experiments  with  an  excited  glass  tube,  he  procured 
one  about  three  feet  and  a  half  long,  and  an  inch  and  a  quarter  in  diameter.  To 
keep  the  interior  free  from  dust,  he  stopped  it  at  the  ends  with  corks.  When 
this  tube  was  excited,  he  happened  to  present  one  of  the  corks  to  a  feather, 
and  was  surprised  to  observe  that  the  feather  was  first  attracted,  and  then  re- 
\  polled  by  the  cork,  in  the  way  it  was  wont  to  be  by  the  glass  tube  itself.  He 


ELECTRICITY. 


concluded  from  this,  that  the   electric  virtue   conferred  on  the  tube  by  friction 
passed  spontaneously  to  the  cork. 

It  then  occurred  to  him  to  inquire  whether  this  transmission  of  electricity 
would  be  made  to  other  substances  besides  cork.  With  this  view  he  obtained 
a  deal  rod  about  four  inches  in  length,  to  one  end  of  which  he  attached  an  ivory 
ball,  and  inserted  the  other  in  the  cork,  by  which  the  glass  tube  was  stopped.  On 
exciting  the  tube,  he  found  that  the  ivory  ball  attracted  and  repelled  the  feather 
even  more  vigorously  than  the  cork.  He  then  tried  longer  rods  of  deal ;  next 
rods  of  brass  and  iron  wire,  with  like  results.  He  then  attached  to  one  end 
of  the  tube  a  piece  of  common  packthread,  and  suspended  from  the  lower  end 
of  this  thread  the  ivory  ball  and  various  other  bodies,  all  of  which  he  found 
capable  of  acquiring  the  electric  state  when  the  tube  was  excited.  Experi- 
ments of  this  kind  were  made  from  the  balconies  of  his  house  and  other  ele- 
vated stations. 

With  a  true  philosophic  spirit,  he  now  determined  to  inquire  what  circum- 
stances attending  the  manner  of  experimenting  produced  any  real  effect  upon 
the  results ;  and,  first,  whether  the  position  or  direction  of  the  rods,  wires,  or 
cords,  by  which  the  electricity  was  transmitted  from  the  excited  tube,  affected 
the  phenomena.  For  this  purpose  he  extended  a  piece  of  packthread  in  a  ho- 
rizontal direction,  supporting  it  at  different  points  by  other  pieces  of  similar  cord, 
which  were  attached  to  nails  driven  into  a  wooden  beam,  and  which  were  there- 
fore in  a  vertical  position.  To  one  end  of  the  horizontal  cord  he  attached  the 
ivory  ball,  and  to  the  other  he  tied  the  end  of  the  glass  tube.  On  exciting  the 
tube  he  found  that  no  electricity  was  transmitted  to  the  ball,  a  circumstance 
which  he  rightly  ascribed  to  its  escape  by  the  vertical  cords,  the  nails  support- 
ing them  and  the  wooden  beam. 

Soon  after  this,  Grey  was  engaged  in  repeating  his  experiments  at  the  house 
of  Mr.  Wheeler,  who  was  afterward  associated  with  him  in  these  investiga- 
tions, when  that  gentleman  suggested  that  threads  of  silk  should  be  used  to 
support  the  horizontal  line  of  cord  instead  of  pieces  of  packthread.  It  does 
not  appear  that  this  suggestion  of  Wheeler  proceeded  from  any  knowledge  or 
suspicion  of  the  electric  properties  of  silk ;  and  still  less  does  it  appear  that 
Grey  was  acquainted  with  them  ;  for,  in  assenting  to  the  proposition  of  Wheeler, 
he  observed,  that  "  silk  might  do  better  than  packthread  on  account  of  its  small- 
ness,  as  less  of  the  virtue  would  probably  pass  off  by  it  than  by  the  thickness 
of  the  hempen  line  which  had  been  previously  used." 

They  accordingly  extended  a  packthread  through  a  distance  of  about  eighty 
feet  in  a  horizontal  direction,  supporting  it  in  that  position  by  threads  of  silk. 
To  one  end  of  this  packthread  they  attached  the  ivory  ball,  and  to  the  other 
the  glass  tube.  When  the  latter  was  excited,  the  ball  immediately  became 
electric,  as  was  manifested  by  its  attraction  upon  metallic  leaf  held  near  it. 
After  this,  they  extended  their  experiments  to  lines  of  packthread  still  longer 
when  the  silk  threads  used  for  its  support  were  found  to  be  too  weak,  and  were 
broken.  Being  under  the  erroneous  impression  that  the  escape  of  the  elec- 
tricity was  prevented  by  the  fineness  of  the  silk,  they  now  substituted  for  it 
thin  brass  wire,  which  they  expected,  being  still  smaller  than  the  silk,  would 
more  effectually  intercept  the  electricity  ;  and  which,  from  its  nature,  would 
have  all  the  necessary  strength.  The  experiment,  however,  completely  failed. 
No  electricity  was  conveyed  to  the  ivory  ball,  the  whole  having  escaped  by 
the  brass  wire,  notwithstanding  its  fineness.  They  now  saw  that  the  silk 
threads  intercepted  the  electricity,  because  they  were  silk,  and  not  because 
they  were  small. 

Having  thus  accidentally  discovered  the  insulating  property  of  silk,  they 
proceeded  to  investigate  its  generalization,  and  found  that  the  same  property 


ELECTRICITY. 


107 


was  enjoyed  by  resin,  hair,  glass,  and  some  other  substances.  In  fact,  it  soon 
becams  apparent  that  this  property  belonged  in  a  greater  or  less  degree  to  all 
those  substances  which  were  then  known  to  be  capable  of  being  rendered 
electrical  by  friction,  and  which  were  denominated  electrics. 

Grey  now  extended  his  inquiry  to  fluids  and  animal  bodies.  Having  at  that 
time  no  other  test  of  the  electrical  state  of  a  body  than  its  attraction  for  light 
substances  placed  on  a  stand  under  it,  the  application  of  such  a  test  to  liquids 
presented  at  first  some  difficulty.  This  was  soon  surmounted  by  the  expe- 
dient of  blowing  a  soap-bubble  from  the  bole  of  a  tobacco-pipe.  The  bubble 
was  held  suspended  over  some  leaf  metal,  and  on  bringing  the  excited  tube  to 
the  small  end  of  the  pipe,  the  bubble  immediately  became  electrical. 

It  was  in  the  prosecution  of  these  experiments  that  he  discovered  that,  when 
the  electrified  tube  was  brought  near  to  any  part  of  a  non-electric  body,  without 
touching  it,  the  part  most  remote  from  the  tube  became  electrified.  He  thus 
fell  upon  the  fact  which  afterward  led  to  the  principle  of  INDUCTION.  The 
science,  however,  was  not  yet  ripe  for  that  great  discovery,  and  Grey  accord- 
ingly continued  to  apply  the  principle  of  inductive  electricity,  without  the  most 
remote  suspicion  of  the  rich  mine  whose  treasures  lay  beneath  his  feet. 

In  another  series  of  experiments,  Grey  was  also  unfortunate  in  missing  a 
subsequent  discovery  on  which  he  just  touched.  He  found  that  certain  electric 
bodies  were  capable  of  becoming  permanently  excited  without  the  previous 
process  of  attrition.  He  took  nineteen  different  substances,  among  which  were 
resin,  gum-lac,  shell-lac,  sulphur,  and  pitch,  and  the  remainder  of  which  were 
various  compounds  of  these.  The  sulphur  he  melted  in  a  glass  vessel,  the 
others  in  a  spherical  iron  ladle.  When  they  became  solid,  and  cooled,  and 
were  removed  from  the  moulds  in  which  they  were,  in  this  manner,  cast,  he 
found  them  to  be  electrified,  and  that,  on  preserving  them  from  exposure  to  the 
air,  by  wrapping  them  in  paper  or  wool,  this  electrified  state  continued  for  an 
indefinite  time.  In  the  case  of  sulphur,  he  found  that  not  only  the  sulphur 
was  electrical,  but  also  the  glass  from  which  it  was  removed.  Had  he  carried 
these  inquiries  further,  and  looked  carefully  into  the  circumstances  of  the  at- 
traction exhibited  by  the  sulphur  and  the  glass,  he  could  not  have  failed  in 
discovering  the  existence  of  the  two  opposite  electricities,  and  would  probably 
have  also  found  the  reason  why  the  iron  ladle  did  not  exhibit  electrical  signs 
as  well  as  the  glass.  This,  however,  escaped  him,  and  the  honor  of  the  dis- 
covery was  reserved  for  a  contemporary  philosopher. 

In  his  investigations  respecting  the  power  of  liquids  to  receive  electricity 
from  excited  glass,  Grey  exhibited,  in  a  manner  which  at  that  period  appeared 
striking,  the  attraction  of  the  glass  tube  for  liquids.  We  shall,  however,  pass 
over  these  and  some  other  experiments  of  less  importance,  since  they  did  not 
conduce  to  the  development  of  any  general  principle. 

Contemporary  with  Grey  was  the  celebrated  Dufaye,  who,  though  not  im- 
pelled by  the  same  enthusiasm,  nor  exhibiting  the  same  unwearied  activity  in 
multiplying  experiments,  was  endowed  with  mental  powers  of  a  much  higher 
order,  and  consequently  was  not  slow  to  perceive  some  important  consequences 
flowing  from  the  experiments  of  Grey,  which  had  eluded  the  notice  of  that 
philosopher.  Dufaye,  in  the  first  place,  extended  the  class  of  substances  called 
electrics — showing  that  all  substances  whatever,  except  the  metals  and  bodies 
in  the  soft  or  liquid  state,  were  capable  of  being  electrified  by  friction  with  any 
sort  of  cloth,  and  that,  to  secure  the  result,  it  was  only  necessary  to  warm  the 
body  previously.  He  also  showed  that  the  property  of  receiving  electricity 
by  contact  with  an  excited  electric  was  much  more  general  than  was  supposed 
by  Grey,  and  that  most  substances  exhibited  that  property  in  a  greater  or  less 
degree,  when  supported  by  glass  well  warmed  and  dried.  Dufaye  also  showed 


ELECTRICITY. 


that  the  conducting  power  of  the  packthread  used  in  the  experiments  of  Grey 
depended  on  the  moisture  contained  in  it,  and  that  the  conducting  power  \vas 
considerably  increased  by  wetting  it.  By  this  expedient  he  transmitted  elec- 
tricity along  a  cord  to  the  distance  of  about  thirteen  hundred  feet. 

It  had  been  previously  ascertained  that  when  any  substance  charged  with 
electricity  communicated  the  electric  principle  to  another  body  near  it,  but  not 
in  contact  with  it,  the  electricity  passed  from  the  one  body  to  the  other  in  the 
form  of  a  spark,  accompanied  by  a  snapping  or  cracking  noise,  like  that  of  a 
slight  explosion.  It  had  also  been  discovered  by  Grey  and  Wheeler  that  the 
bodies  of  men  and  animals  would  become  charged  with  electricity  if  placed  in 
the  usual  manner  in  contact  with  an  excited  glass  tube,  provided  they  were 
suspended  by  silk  cords,  so  as  to  prevent  the  escape  of  the  electricity.  Du- 
faye,  therefore,  reasoned  that  a  man  being  so  suspended  by  silk  cords,  the 
electricity  imparted  to  his  person  could  not  escape  ;  and  being  charged  by  the 
excited  glass  tube,  sparks  of  fire  ought  to  issue  from  his  body,  if  any  body  ca- 
pable of  receiving  the  electricity  were  presented  to  it.  To  reduce  this  to  the 
immediate  test  of  experiment,  Dufaye  suspended  his  own  person  by  silk  lines  ; 
and  being  electrified,  the  Abbe  Noflet,  who  assisted  him  in  these  experiments, 
presented  his  hand  to  his  body,  when  immediately  a  spark  of  fire  issued  from 
the  person  of  the  one  philosopher  and  entered  the  body  of  the  other.  Although 
such  a  result  had  been  predicted  as  a  consequence  of  the  arrangement,  the  as- 
tonishment was  not  the  less  great  at  its  occurrence.  Nollet  states  that  he  can 
never  forget  the  surprise  of  both  Dufaye  and  himself  when  they  witnessed  the 
first  explosion  from  the  body  of  the  former. 

The  celebrity  of  Dufaye  rests,  however,  not  on  his  experiments,  but  on  the 
sagacity  which  led  him  to  evolve  natural  laws  of  a  high  degree  of  generality 
from  his  own  experiments,  and  from  those  of  the  philosophers  who  preceded 
him.  He  reproduced  in  a  more  definite  form  the  principles  of  attraction  and 
subsequent  repulsion,  which  had  previously  been  announced  by  Otto  Guericke. 
"  I  discovered,"  says  Dufaye,  "  a  very  simple  principle,  which  accounts  for  a 
great  part  of  the  irregularities,  and,  if  I  may  use  the  term,  the  caprices,  that 
seem  to  accompany  most  of  the  experiments  in  electricity."  This  principle 
was,  first,  that  excited  electrics  attract  all  bodies  in  their  natural  state  ;  second, 
that  after  a  body  is  so  attracted,  and  has  touched  the  excited  electric,  then  such 
body  is  repelled  by  the  excited  electric ;  third,  that  if,  after  being  so  repelled, 
such  body  touches  any  other,  it  will  be  again  attracted,  and  again  repelled  «by 
the  excited  electric,  and  so  on. 

But  a  discovery  of  a  much  higher  order  was  due  to  Dufaye.  "  Chance," 
says  he,  "  threw  in  my  way  another  principle  more  universal  and  remarkable 
than  the  preceding  one,  and  which  casts  a  new  light  upon  the  subject  of  elec- 
tricity. The  principle  is,  that  there  are^vo  distinct  kinds  of  electricity,  very 
difierent  from  one  another ;  one  of  which  I  shall  call  vitreous,  and  the  other 
resinous  electricity.  The  first  is  that  of  glass,  rock-crystal,  precious  stones, 
hair  of  animals,  wool,  and  many  other  bodies.  The  second  is  that  of  amber, 
copal,  gum-lac,  silk-thread,  paper,  and  a  vast  number  of  other  substances.  The 
characteristic  of  these  two  electricities  is,  that  they  repel  themselves  and  at- 
tract each  other.  Thus  a  body  of  the  vitreous  electricity  repels  all  other 
bodies  possessed  of  the  vitreous,  and,  on  the  contrary,  attracts  all  those  of  the 
resinous  electricity.  The  resinous  also  repels  the  resinous,  and  attracts  the 
vitreous.  From  this  principle  one  may  easily  deduce  the  explanation  of  a 
great  number  of  other  phenomena,  and  it  is  probable  that  this  truth  will  lead 
us  to  the  discovery  of  many  other  things." 

This  was  a  discovery  of  the  highest  order,  and  in  its  consequences  fully 
justified  the  anticipation  that  "  it  would  lead  to  the  discovery  of  many  other 


ELECTRICITY. 


109 


things."  It  is  the  basis  of  the  only  theory  of  electricity  which  has  been  found 
sullicient  to  explain  all  the  phenomena  of  the  science,  and  with  the  subsequent 
hypothesis  of  Symmer,  and  the  laws  of  attraction  developed  by  the  researches 
of  Coulomb,  it  has  brought  the  most  subtle  and  incontrollable  of  all  physical 
agents  under  the  subjection  of  the  rigorous  canons  of  mathematical  calcula- 
tion. 

A  new  question  now  arose  respecting  any  body  which  has  been  rendered 
electrical,  whether  by  immediate  excitation,  or  by  contact  with  another  body 
already  excited.  It  was  not  enough  to  ascertain  that  it  was  electrified  ;  but  it 
was  necessary  to  know  with  which  of  the  two  kinds  of  electricity  it  was  in- 
vested. The  test  of  this  proposed  by  Dufaye  was  the  same  which  has  ever 
since  his  time  been  adhered  to.  He  electrified  a  light  substance  freely  sus- 
pended with  a  known  species  of  electricity ;  say,  for  example,  with  resinous 
electricity.  If  this  substance  was  repelled  on  bringing  it  near  another  electri- 
fied body,  then  the  electricity  of  that  body  was  known  to  be  resinous ;  but  if, 
on  the  contrary,  it  was  attracted,  then  the  electricity  of  the  other  body  was 
known  to  be  vitreous. 

Dr.  Desaguliers,  whose  works  in  other  parts  of  physical  science  are  well 
known,  devoted  some  attention  to  electricity  from  the  close  of  the  labors  of 
Grey  till  the  year  1742,  but  the  researches  of  this  philosopher  contributed 
nothing  to  the  extension  of  the  science.  He  methodized  the  elements  which 
had  already  accumulated,  and  improved  in  some  important  instances  the  no- 
menclature. He  denominated  all  substances  in  which  electricity  may  be  ex- 
cited electrics  per  se,  and  defined  in  a  distinct  manner  their  characters.  He 
also  first  applied  the  term  conductor  to  bodies  which  freely  transmitted  electrici- 
ty, and  showed  that  animal  substances  owed  this  property  to  the  fluids  which 
they  contain.  He  however  failed  to  discover  that  moisture  was  the  conducting 
agent  in  many  other  bodies  which  at  that  time  were  used  to  propagate  elec- 
tricity to  a  distance. 

The  subject  of  electricity  now  began  to  attract  the  attention  of  the  Germans, 
and  the  first  consequence  was  considerable  improvement  in  the  power  and  effi- 
ciency of  electrical  apparatus.  The  globe  of  glass  revolving  on  a  horizontal 
axis,  which  had  originated  with  Hawkesbee,  but  had,  ever  since  that  time, 
greatly  to  the  detriment  of  the  science,  been  abandoned  in  favor  of  the  glass 
tube,  was  now  resumed  by  Professor  Boze  of  Wittemburg,  who  added,  for  the 
first  time,  the  prime  conductor  to  the  machine.  This  conductor  consisted  of  an 
oblong  cylinder,  or  tube,  of  iron  or  tin.  It  was  at  first  supported  by  a  man, 
who  was  insulated  by  standing  on  cakes  of  rosin  ;  but  it  was  subsequently  sus- 
pended by  silken  cords. 

The  method  of  exciting  the  globe  or  tube  hitherto  generally  practised,  and, 
indeed,  long  afterward  persevered  in,  was  to  rub  them  with  the  hand,  taking 
care  that  it  was  dry  -and  warm.  Winkler,  a  professor  in  the  university  of 
Leipsic,  substituted  the  more  convenient  expedient  of  a  cushion  fixed  in  con- 
tact with  the  globe,  and  gently  pressed  upon  its  surface  by  springs,  or  any 
similar  means.  Gordon,  a  Scottish  Benedictine  monk,  who  was  professor  of 
philosophy  at  Erfurt,  abandoned  the  use  of  the  globe,  and  substituted  for  it  a 
cylinder  of  glass,  having  its  geometrical  axis  horizontal,  and  supported  on 
pivots  so  as  to  revolve  on  that  axis.  The  cylinders  he  used  were  eight  inches 
long,  and  four  inches  in  diameter.  Thus  the  electrical  machine  assumed  a 
form  very  nearly  identical  with  the  cylindrical  machines  of  the  present  day. 

The  effects  produced  by  these  improved  and  powerful  apparatus  are  related 
to  have  been  extraordinary.  Various  inflammable  substances,  such  as  spirits, 
heated  oil,  pitch,  and  wax,  were  fired.  Appearances  of  electrical  light  issuing 
from  points,  and  the  experiment  since  known  as  the  electrical  bells,  were  the 


110  ELECTRICITY. 


productions  of  this  epoch.  The  spark  drawn  from  the  conductor  by  the  finger 
is  described  as  being  so  intense  as  to  burst  the  skin,  draw  blood,  and  produce 
a  wound.  Other  effects  on  the  animal  system  are  related,  in  which  there  is 
probably  some  exaggeration. 

The  year  1746  forms  a  remarkable  epoch  in  the  history  of  electricity,  being 
signalized  by  the  invention  of  the  LEYDEN  PHIAL.  The  merit  of  this  discovery 
is  disputed,  being  claimed  for  Professor  Muschenbroek,  Cuneus,  a  native  of 
Leyden,  and  Kleist,  a  monk  of  that  place.  Probably  all  these  individuals  were 
engaged  in  the  proceedings  in  which  the  discovery  originated.  Dr.  Priestley, 
a  contemporary  writer,  gives  an  account  of  this  invention,  apparently  obtained 
by  personal  inquiry,  of  which  the  following  is  the  substance  : — 

Professor  Muschenbroek  and  his  associates  having  observed  that  electrified 
bodies  exposed  to  the  atmosphere  speedily  lost  their  electric  virtue,  which  was 
supposed  to  be  abstracted  by  the  air  itself,  and  by  vapor  and  effluvia  suspended 
in  it,  imagined  that  if  they  could  surround  them  with  any  insulating  substance, 
so  as  to  exclude  the  contact  of  the  atmosphere,  they  could  communicate  a  more 
intense  electrical  power,  and  could  preserve  that  power  for  a  longer  time. 
Water  appeared  one  of  the  most  convenient  recipients  for  the  electrical  influ- 
ence, and  glass  the  most  effectual  and  easy  insulating  envelop.  It  appeared, 
therefore,  very  obvious,  that  water  enclosed  in  a  glass  bottle  must  retain  the 
electricity  given  to  it,  and  that  by  such  means,  a  greater  charge  or  accumula- 
tion of  electric  force  might  be  obtained  than  by  any  expedient  before  resorted 
to. 

In  the  first  experiments  made  in  conformity  with  these  views,  no  remarkable 
results  were  obtained.  But  it  happened  on  one  occasion  that  the  operator  held 
the  glass  bottle  in  his  right  hand,  while  the  water  contained  in  it  communi- 
cated by  a  wire  with  the  prime  conductor  of  a  powerful  machine.  When  he 
considered  that  it  had  received  a  sufficient  charge,  he  applied  his  left  hand  to 
the  wire  to  disengage  it  from  the  conductor.  He  was  instantly  struck  with  the 
convulsive  shock  with  which  electricians  are  now  so  familiar,  and  which  has 
been  since,  and  is  at  present,  so  frequently  suffered  from  motives  of  curiosity 
or  amusement.  \ 

It  is  curious  to  observe  how  much  effects  on  the  organs  of  sense  depend  on  I 
the  previous  knowledge  of  them,  which  may  or  may  not  occupy  the  minds  of   \ 
those  who  sustain  them.     Those  who  now  think  so  lightly  of  the  shock,  pro-  i 
duced  even  by  a  powerful  Leyden  phial,  would  be  surprised  at  the  letter  in  ) 
which  Muschenbroek  gave  Reaumer  an  account  of  the  effect  produced  upon  him  < 
by  the  first  experiment.     He  states,  that  "  he  felt  himself  struck  in  his  arms, 
shoulders,  and  breast,  so  that  he  lost  his  breath,  and  was  two  days  before  he  re- 
covered from  the  effects  of  the  blow  and   the  terror."     He  declared,  that  "  he 
would  not  take  a  second  shock  for  the  whole  kingdom  of  France." 

Nor  was  Muschenbroek  singular  in  his  extraordinary  estimate  of  the  effects 
of  the  shock.  M.  Allamand,  who  made  the  experiment  with  a  common  beer 
glass,  stated  that  he  lost  the  use  of  his  breath  for  some  moments,  and  then  felt 
so  intense  a  pain  along  his  right  arm  that  he  feared  permanent  injury  from  it. 
Professor  Winkler,  of  Leipsic,  stated,  that  the  first  time  he  underwent  the  ex- 
periment he  suffered  great  convulsions  through  his  body ;  that  it  put  his  blood 
into  agitation  ;  that  he  feared  an  ardent  fever,  and  was  obliged  to  have  recourse 
to  cooling  medicines  !  That  he  also  felt  a  heaviness  in  his  head,  as  if  a  stone 
were  laid  upon  it.  Twice  it  gave  him  bleeding  at  the  nose,  to  which  he  was 
not  subject.  The  lady  of  this  professor,  who  appears  to  have  been  as  little 
wanting  in  the  curiosity  which  is  ascribed  to  her  own,  as  in  the  courage  as- 
sumed for  the  other  sex,  took  the  shock  twice,  and  was  rendered  so  weak  by 
it  that  she  could  hardly  walk.  In  a  week,  nevertheless,  her  curiosity  again  got 


ELECTRICITY. 


Ill 


the  better  of  her  discretion,  and  she  took  a  third  shock,  which  immediately  ' 
produced  bleeding  at  the  nose. 

No  sooner  were  these  experiments  made  known,  than  the  amazement  of  ' 
all  classes  of  people  of  every  age,  sex,  and  rank,  was  excited  at  what  was  re-  \ 
gardcd  as  "  a  prodigy  of  nature  and  philosophy."  Philosophers  everywhere  ' 

(  repeated  the  experiment,  but  none  succeeded  in  explaining  its  effects.     After  ! 

I  the  first  emotions  of  astonishment  had  abated,  the  circumstances  which  influ- 

<  enced  the  force  of  the  shock  were  examined.  Muschenbroek  observed  that 
if  the  glass  were  wet  on  the  outer  surface  the  success  of  the  experiment  was 
impaired ;  and  Dr.  Watson  proved  that  the  force  of  the  shock  was  increased 
by  the  thinness  of  the  glass  of  which  the  bottle  containing  the  water  was  made. 
He  also  observed,  that  the  force  of  the  charge  did  not  depend  on  the  power  of 
the  electrical  machine  by  which  the  phial  was  charged.  Dr.  Watson  also 
showed  that  the  shock  could  be  transmitted,  undiminished,  through  the  bodies 
of  several  men  touching  each  other. 

By  further  repeating  and  varying  the  experiment,  Watson  found  that  the  force 
of  the  charge  depended  on  the  extent  of  the  external  surface  of  the  glass  in 
contact  with  the  hand  of  the  operator  ;  and  it  occurred  to  Dr.  Bevis  that  the 
hand  might  be  efficient  merely  as  a  conductor  of  electricity,  and  in  that  case 
the  object  might  be  more  effectually  and  conveniently  attained  by  coating  the 
exterior  of  the  phial  with  sheet-lead  or  tin-foil.  This  expedient  was  completely 
successful ;  and  the  phial,  so  far  as  related  to  its  external  surface,  assumed  its 
present  form. 

Another  important  step  in  the  improvement  of  the  Leyden  jar  was  also  due 
to  the  suggestion  of  Dr.  Bevis.  It  appeared  that  the  force  of  the  charge  in- 
creased with  the  magnitude  of  the  jar,  but  not  in  proportion  to  the  quantity  of 
water  it  contained.  It  was  conjectured  that  it  might  depend  on  the  extent  of 
the  surface  of  glass  in  contact  with  water  ;  and  that  as  water  was  considered 
to  play  the  part  merely  of  a  conductor  in  the  experiment,  metal,  which  was  a 
better  conductor,  would  be  at  least  equally  effectual.  Three  phials  were  there- 
fore procured  and  filled  to  the  usual  height  with  shot  instead  of  water.  A  me- 
tallic communication  was  made  between  the  shot  contained  in  them  respectively. 
The  result  was  a  charge  of  greatly  augmented  force.  This  was,  in  fact,  the 
first  electric  battery. 

Dr.  Bevis  now  saw  that  the  seat  of  the  electric  influence  was  the  surface  of 
contact  of  the  metal  and  the  glass,  and  rightly  inferred  that  the  form  of  a  bot- 
tle or  jar  was  not  in  any  way  connected  with  the  principle  of  the  experiment. 
He  therefore  took  a  common  pane  of  glass,  and  having  coated  the  opposite 
faces  with  tin-foil,  extending  on  each  surface  within  about  an  inch  of  the  edge, 
he  was  able  to  obtain  as  strong  a  charge  as  from  a  phial  having  the  same  ex- 
tent of  coated  surface.  Dr.  Watson  being  informed  of  this,  coated  large  jars 
made  of  thin  glass  both  on  the  inside  and  outside  surface  with  silver  leaf,  ex- 
tending nearly  to  the  top  of  the  jars,  the  effects  of  which  fully  corroborated 
the  anticipations  of  Dr.  Bevis,  and  established  the  principle  that  the  force  of 
the  charge  was  in  proportion  to  the  quantity  of  coated  surface. 

The  results  of  all  these  experiments  led  to  the  inference  that,  in  the  dis- 
charge of  the  phial,  the  electricity  passed  through  the  circle  of  conducting 
matter  which  was  extended  between  the  inside  and  the  outside  coating  of  the 
jar.  If  that  circle  were  anywhere  interrupted  by  the  presence  of  non-conduct- 
ing matter,  or  electrics  per  se,  as  they  were  then  called,  no  discharge  took  place. 
Also,  if  any  portion  of  the  circle  were  formed  of  living  animals,  each 
animal  sustained  the  shock.  To  carry  the  demonstration  of  this  further, 
Dr.  Watson  placed,  at  several  points  in  the  circuit,  spoons  filled  with  spirits 

\  between  the  extremities  of  iron  bars,  but  not  in  contact  with  them.     In  such 


112 


ELECTRICITY. 


rases  the  spirits  in  all  the  spoons  were  inflamed  apparently  at  the  instant  of  the 
discharge. 

Many  of  these  properties  were  simultaneously  discovered  by  Mr.  Wilson, 
who  experimented  in  Dublin.  He  coated  the  external  surface  of  the  jar  in  the 
first  experiments  by  plunging  it  in  water.  He  also  made  several  experiments 
with  a  view  to  affect  by  a  shock  one  part  of  the  human  body  without  affecting 
the  otner  parts.  But  the  most  remarkable  discovery  of  this  electrician  was 
the  Inttrnl  shock.  He  observed,  that  a  person  standing  near  the  circuit  through 
which  the  shock  is  transmitted,  would  sustain  a  shock  if  he  were  only  in  con- 
tact with  any  part  of  the  circuit,  or  even  placed  very  near  it. 

Those  who  are  conversant  with  the  science,  arid  aware  of  the  important 
principle  of  induction,  will  see,  with  much  interest,  how  nearly  many  of  the 
philosophers  engaged  in  these  researches  touched,  from  time  to  time,  on  that 
property,  and  yet  missed  the  honor  of  its  discovery.  Without  it,  the  explica- 
tion of  the  phenomenon  of  the  charge  and  discharge  of  the  Leyden  phial  was 
impossible.  The  lateral  shock  just  adverted  to,  and  observed  by  Wilson,  was 
almost  a  glaring  instance  of  it;  but  a  still  more  striking  manifestation  of  the 
theory  of  the  Leyden  phial  was  afforded  by  an  experiment  of  Mr.  Canton,  who 
showed  that  if  a  charged  phial  be  insulated,  the  internal  and  external  coat  inns 
would  give  alternate  sparks,  and  then,  by  continuing  the  process,  the  phial 
might  be  gradually  discharged.  Canton  just  touched  on  the  discovery  of  dis- 
simulated electricity. 

While  these  investigations  were  proceeding  in  England,  the  philosophers  of 
France  were  not  wanting  in  that  zeal  and  activity  which  they  have  always 
manifested  for  the  advancement  of  physical  science.  The  Abbe  Nollet,  M.  de 
Monnier,  and  others,  prosecuted  similar  experimental  researches,  and  arrived 
at  the  knowledge  of  several  of  the  important  circumstances  developed  in  Eng- 
land. Nollet  showed  that  a  phial  containing  rarefied  air  admitted  of  being 
charged  as  readily  as  one  which  contained  water,  and  stated  that  the  water 
in  the  Leyden  experiment  served  no  purpose  except  to  conduct  the  electricity 
to  the  glass. 

From  this  time  to  the  period  at  which  Dr.  Franklin  commenced  his  researches, 
no  important  progress  was  made  in  the  science,  although  at  no  former  period 
were  experiments  on  so  grand  a  scale  projected  and  executed ;  nor  was  public 
attention  ever  before  so  powerfully  attracted  to  any  scientific  subject.  Nume- 
rous and  extensive  experiments  were  made,  both  in  England  and  France,  to 
determine  the  distance  through  which  the  electric  shock  could  be  transmitted, 
the  nature  of  the  substances  through  which  it  could  be  propagated,  and  the 
rate  at  which  it  moved.  At  Paris,  M.  Nollet  transmitted  it  through  a  chain  of 
180  soldiers  ;  and  at  the  convent  of  the  Carthusians  he  formed  a  chain  meas- 
uring 5,100  feet,  by  means  of  iron  wires  extending  between  every  two  persons, 
and  the  whole  company  gave  a  sudden  spring,  and  sustained  the  shock  at  the 
same  instant. 

But  it  was  in  England  that  the  experiments  on  this  subject  were  made  on 
the  most  magnificent  scale.  Mr.  Martin  Folkes,  then  president  of  the  Royal 
Society,  Lord  Charles  Cavendish,  Dr.  Bevis,  and  several  other  fellows  of  the 
Society  formed  a  committee  to  witness  these  experiments,  the  chief  direction 
and  management  of  them  being  undertaken  by  Dr.  Watson.  A  circuit  was 
first  formed  by  a  wire  carried  from  one  side  of  the  Thames  to  the  other  over 
Westminster  bridge.  One  extremity  of  this  wire  communicated  with  the  in- 
terior of  a  charged  jar ;  the  ether  was  held  by  a  person  on  the  opposite  bank 

the  river.  This  person  held  in  his  other  hand  an  iron  rod,  which  he  dipped 
into  the  river.  On  the  other  side,  near  the  jar,  stood  another  person,  holding 
in  one  hand  a  wire  communicating  with  the  exterior  coating  of  the  jar,  and  in 


ELECTRICITY.  113 


the  oilier  hand  an  iron  rod.  This  rod  he  dipped  into  the  river,  when  instantly 
the  shock  was  received  by  both  persons,  the  electric  fluid  having  passed  over 
the  bridge,  through  the  body  of  the  person  on  the  other  side,  through  the  wa- 
tei  across  the  river,  through  the  rod  held  by  the  other  person,  and  through  his 
body  to  the  exterior  coating  of  the  jar.  Familiar  as  such  a  fact  may  now  ap- 
pear, it  is  impossible  to  convey  an  adequate  idea  of  the  amazement,  bordering 
on  incredulity,  with  which  it  was  at  that  time  witnessed. 

The  next  experiment  was  made  at  Stoke  Newington,  near  London,  where  a 
circuit  of  about  two  miles  in  length  was  formed,  consisting,  as  in  the  former 
case,  partly  of  water  and  partly  of  wire.  In  one  case  there  were  about  2,800 
feet  of  wire,  and  8,000  feet  of  water.  The  result  was  the  same  as  in  the  case 
of  the  experiment  at  Westminster  bridge.  In  this  case,  on  repeating  the  ex- 
periment, the  rods,  instead  of  being  dipped  in  the  water,  were  merely  fixed  in 
the  soil  at  about  twenty  feet  from  the  water's  edge,  when  it  was  found  that  the 
shock  was  equally  transmitted.  This  created  a  doubt  whether,  in  the  former 
case,  the  shock  might  not  have  been  conveyed  through  the  ground  between  the 
two  rods,  instead  of  passing  through  the  water,  and  subsequent  experiments 
proved  that  such  was  the  case. 

The  same  experiments  were  repeated  at  Highbury,  and  finally  at  Shooter's 
Hill,  in  August,  1747.  At  the  latter  place  the  wire  from  the  inside  of  the  jar 
was  6,732  feet,  and  that  which  touched  the  outside  coating  was  3,868  feet  long. 
The  observers  placed  at  the  extremity  of  these  wires,  were  two  miles  distant 
from  each  other.  The  circuit,  therefore,  consisted  of  two  miles  of  wire,  and 
two  miles  of  soil  or  ground,  the  latter  being  the  space  between  the  two  observ- 
ers. The  result  of  the  experiment  proved  that  no  observable  interval  elapsed 
between  the  moments  at  which  each  observer  sustained  the  shock.  In  this 
experiment  the  wires  were  insulated  by  being  supported  on  rods  of  baked 
wood. 

We  shall  here  pass  over  a  variety  of  experiments  made  in  England,  France, 
and  Germany,  on  the  effects  of  electricity  on  organized  bodies,  and  on  some 
proposed  medical  applications  of  that  agent ;  such  researches  not  having  led 
to  any  general  principles  affecting  the  real  advancement  of  the  science. 

Passing  from  the  analysis  of  the  confused  experimental  labors  of  his  imme- 
diate predecessors,  labors  which  contributed  so  little  to  the  development  of  the 
nature  and  laws  of  electrical  phenomena,  to  the  researches  of  Franklin,  is  like 
the  transition  from  the  mists  and  obscurity  of  morning  twilight  to  the  unclouded 
splendor  of  the  noontide  sun.  It  was  in  the  summer  of  the  year  1747,  that  a 
fortuitous  circumstance,  happily  for  the  progress  of  knowledge,  first  drew  the 
attention  of  this  truly  great  and  good  man,  and  (as  he  afterward  proved)  acute 
philosopher,  to  the  subject  of  electricity.  Mr.  Peter  Collinson,  a  fellow  of  the 
Royal  Society  of  London,  and  a  gentleman  who  took  much  interest  in  scien- 
tific affairs,  made  a  communication  to  the  Literary  Society  of  Philadelphia,  ex- 
plaining what  had  been  recently  done  in  England  in  electrical  experiments,  and 
with  his  letter  he  sent  a  present  of  one  of  the  glass  tubes  then  commonly  used 
to  excite  electricity,  with  directions  for  its  use.  Previous  to  this  time,  Frank- 
lin does  not  appear  to  have  ever  given  his  attention  to  physical  science.  Never- 
theless, he  now  commenced  repeating  the  European  experiments  with  all  the 
ardor  of  an  enthusiast,  and  extending,  varying,  and  adapting  them  to  the  de- 
velopment of  great  general  laws,  with  all  the  skill  and  sagacity  of  a  practised 
experimental  philosopher.  Within  the  brief  period  of  four  months  after  the 
arrival  of  the  tube,  he  commenced  a  series  of  letters  to  Mr.  Collinson,  in  which 
are  related  a  body  of  discoveries,  which  for  the  high  generality  of  the  laws 
they  unfolded,  the  surpassing  beauty  and  clearness  of  the  experimental  demon- 
strations on  which  they  were  based,  and  their  intimate  connexion  with  the 


114  ELECTRICITY. 


uses  of  ife,  are  well  worthy  to  be  put  in  juxtaposition  with  the  discoveries  of 
Newton  respecting  the  analysis  and  properties  of  light.  How  different,  how- 
ever, was  the  position  of  these  two  great  discoverers  and  benefactors  of  the 
human  race  !  One  brought  to  bear  on  the  subject  of  his  inquiry  a  mind  early 
disciplined  in  scientific  investigation,  a  memory  stored  with  profound  mathe- 
(  matical  erudition,  faculties  rendered  more  acute  and  strong  by  the  severe  studies  ' 
'  exacted  from  all  aspirants  to  academical  honor  and  office  in  the  universities  of 
the  old  countries,  zeal  awakened,  emulation  stimulated,  and  enthusiasm  kindled 
by  associates,  among  whom  were  included  all  that  was  most  distinguished  in 
the  physical  sciences  ;  the  other,  first  a  tallow-chandler's  apprentice,  and  next 
a  poor  printer's  boy,  unschooled,  undisciplined,  self-informed,  having  nothing  to 
aid  him  but  the  inborn  energy  of  his  mind,  separated  by  an  ocean  three  thou- 
sand miles  wide  from  the  countries  which  alone  were  the  seats  of  the  sciences, 
and  where  alone  those  aids  and  encouragements  derivable  from  the  society  of 
others  engaged  in  like  inquiries  could  be  obtained.  Such  was  the  individual 
whose  researches  we  must  now  briefly  notice.  The  series  of  letters  in  which 
he  embodied  the  details  of  his  experiments,  and  developed  the  laws  which  re- 
sulted from  them,  were  continued  from  1747  to  1754,  and  were  subsequently 
collected  and  published. 

"  Nothing,"  says  Priestley,  "  was  ever  written  upon  the  subject  of  electricity 
which  was  more  generally  read  and  admired  in  all  parts  of  Europe  than  these 
letters.  There  is  hardly  any  European  language  into  which  they  have  not 
been  translated  ;  and,  as  if  this  were  not  sufficient  to  make  them  properly 
known,  a  translation  of  them  has  lately  been  made  into  Latin.  It  is  not  easy 
to  say  whether  we  are  most  pleased  with  the  simplicity  and  perspicuity  with 
which  these  letters  are  written,  the  modesty  Avith  which  the  author  proposes 
every  hypothesis  of  his  own,  or  the  noble  frankness  with  which  he  relates  his 
mistakes  when  they  were  corrected  by  subsequent  experiments."* 

In  the  analysis  of  Franklin's  discoveries,  it  is  necessary  to  distinguish  care- 
fully fact  from  hypothesis,  and  to  separate  the  great  natural  laws  which  he 
brought  to  light,  the  truth  and  reality  of  which  can  never  be  shaken,  based,  as 
they  are,  on  innumerable  observed  phenomena,  from  the  theory  by  which  these 
phenomena  and  their  laws  are  attempted  to  be  explained  by  him  ;  which  latter, 
though  marked  by  great  sagacity  and  ingenuity,  and  adequate  to  the  explica- 
tion of  most  of  the  ordinary  effects  of  electricity,  has  been  found  insufficient  to 
represent  the  results  of  subsequent  researches,  and  has  been  generally  super- 
seded by  another  theory,  which  will  be  noticed  hereafter. 

The  first  step  made  by  this  philosopher  in  the  brilliant  series  of  discoveries 
by  which  he  rendered  his  name  so  memorable,  was  one  which  produced  a 
material  influence  on  his  subsequent  proceedings,  since  it  formed  the  founda- 
tion of  his  celebrated  hypothesis  of  positive  and  negative  electricity,  which 
served  him  as  the  link  by  which  many  scattered  facts  might  be  grouped  and 
connected,  and  as  a  clue  to  the  development  of  new  and  unobserved  phe- 
nomena. To  reduce  to  the  most  brief,  simple,  and  general  terms,  the  expres- 
sion of  the  first  fruit  of  his  observations,  it  may  be  said  to  consist  in  the  es- 
tablishment of  the  general  principle,  that  when  electricity  is  excited  by  the 
mutual  friction  or  attrition  of  any  two  bodies,  both  these  bodies  become  elec- 
trified ;  and  if  both  are  insulated  they  will  continue  to  be  so  electrified.  They 
will,  however,  be  in  different  electrical  states,  since,  if  moveable,  they  would 
attract  and  not  repel  each  other ;  but,  nevertheless,  each  of  them  will  exhibit 
in  relation  to  other  bodies  not  electrified,  the  same  properties.  Thus  sparks 
may  be  drawn  indifferently  from  either  ;  and  each  of  them  may  be  de-ekctriscd, 


ELECTRICITY. 


or  discharged  of  their  electricity,  by  being  put  in  metallic  communication  with 
the  ground.     These  general  facts  he  proved  by  direct  experiment. 

He  placed  two  persons,  A.  and  B.,  on  insulating  supports.  In  the  hand  of 
A.  he  put  a  glass  tube,  which  being  rubbed  by  A.  became  electrified.  This 
tube  was  then  touched  at  every  part  of  the  rubbed  surface  by  B. ;  after  which 
the  same  process  was  several  times  repeated,  the  tube  being  deprived  of  its  > 
electricity  as  often  as  it  was  touched  by  B.  A  third  person,  C.,  not  insulated, 
now  presented  his  ringer  or  a  metallic  sphere  to  B.,  from  whom  a  spark  was 
drawn  ;  and  by  repeating  this,  or  by  touching  the  person  of  B.,  the  latter  was 
deprived  of  the  electricity  he  had  received  from  the  tube.  This  was  no  more 
than  was  expected.  But  on  subjecting  A.  to  the  same  process,  the  very  same 
effects  were  produced.  It  appeared,  therefore,  that  both  A.  and  B.  were  elec- 
trified. 

Being  again  electrified,  as  before,  by  the  friction  of  the  tube,  instead  of  A. 
and  B.  being  successively  touched  by  C.,  they  were  made  to  touch  each  other, 
both  remaining  insulated.  After  this  both  were  found  to  be  as  completely 
de-elcctrised  and  restored  to  their  ordinary  state  as  when  they  had  been  touched 
by  C. 

A  cork  ball,  suspended  by  a  silk  thread,  being  electrified  by  contact  with  the 
excited  glass  tube,  was  repelled  when  brought  near  the  person  of  B.,  but  it  was 
attracted  when  brought  near  the  person  of  A. 

From  these  experiments  it  appeared  the  electrical  states  of  A.  and  B.  were 
different.  Franklin  called  the  state  of  B.,  and  consequently  that  of  the  glass 
tube  from  which  he  drew  the  electricity,  positive  and  that  of  A.  negative.  The 
one  was  said  to  be  positively,  the  other  negatively  electrified.  The  cloth  with 
which  A.  rubbed  the  glass  tube  was,  like  A.,  negatively  electrified — it  attracted 
the  cork  ball ;  and  the  glass  tube,  like  B.,  was  positively  electrified — it  re- 
pelled the  cork  ball. 

The  generality  of  this  result  was  established  by  a  great  variety  of  experi- 
ments. In  all  cases  it  appeared  that  the  opposite  electrical  charges  of  the  two 
bodies  submitted  to  friction,  or  of  any  insulated  bodies  in  communication  with 
them,  had  the  same  reciprocally  neutralizing  power  ;  in  virtue  of  which,  when 
brought  into  contact,  or  when  a  metallic  communication  was  established  be- 
tween them,  all  signs  of  electricity  would  disappear. 

Such  is  a  strict  statement  of  the  facts  as  evolved  in  the  experiments.  The 
hypothesis  proposed  by  Franklin  for  their  explication  was  as  follows :  All 
bodies  in  their  natural  state  are  charged  with  a  certain  quantity  of  electricity, 
in  each  body  this  quantity  being  of  definite  amount.  This  quantity  of  elec- 
tricity is  maintained  in  equilibrium  upon  the  body  by  an  attraction  which  the 
particles  of  the  body  have  for  it,  and  does  not  therefore  exert  any  attraction 
for  other  bodies.  But  a  body  may  be  invested  with  more  or  les?  electricity 
than  satisfies  its  attraction.  If  it  possess  more,  it  is  ready  to  give  i,p  the  surplus 
to  any  body  which  has  less,  or  to  share  it  with  any  body  in  its  natural  state ; 
if  it  have  less,  it  is  ready  to  take  from  any  body  in  its  natural  state  a  part  of  its 
electricity,  so  that  each  will  have  less  than  their  natural  amount.  A  body 
having  more  than  its  natural  quantity  is  electrified  positively  or  plus,  and  one 
which  has  less  is  electrified  negatively  or  minus. 

When  two  bodies  are  submitted  to  mutual  attrition  and  oecome  electrified, 
one  parts  with  a  portion  of  its  proper  electricity,  which  is  received  by  the 
other.  The  latter  then  has  more  than  its  natural  amount,  and  is  positively  elec- 
trified ;  the  former  has  less,  and  is  negatively  electrified. 

In  the  instance  above  stated,  when  A.  rubs  the  glass  tube,  he  loses  a  portion 
of  his  natural  electricity,  and  is  negatively  electrified  ;  while  the  tube  receives 
what  he  loses,  and  becomes  positively  electrified.  When  B.  touches  the  tube, 


116 


ELECTRICITY. 


he  takes  from  it  nearly  all  the  electricity  with  which  it  is  charged  over  and 
above  its  natural  amount ;  for  his  body  being  of  so  much  greater  magnitude 
thnn  the  tube,  the  proportion  which  will  remain  on  the  tube  will  be  insig- 
nificant. 

s  If  when  A.  nibs  the  tube  he  were  not  insulated,  he  would  not  be  electrified, 
because,  as  fast  as  his  body  would  lose  its  proper  amount  of  electricity,  the 
deficiency  would  be  made  up  from  the  earth,  with  which  he  is  in  free  electri- 
cal communication  ;  whereas  in  the  former  case  being  insulated,  this  supply 
could  not  be  obtained.  Hence,  in  this  theory,  the  earth  is  regarded  as  the 
common  reservoir  of  electricity,  from  which  bodies  negatively  electrified  re- 
ceive what  they  want,  and  to  which  bodies  positively  electrified  give  up  their 
surplus,  except  in  the  case  in  which  the  one  or  the  other  are  insulated. 

Such,  in  general,  was  the  Franklinian  theory;  which,  however,  will  be 
more  fully  developed  hereafter. 

Assuming  these  hypothetical  principles,  Franklin  next  proceeded  to  analyze 
the  phenomena  of  the  Leyden  jar.  His  first  experiments  were  directed  to  es- 
tablish the  fact,  that  when  the  jar  is  charged,  the  inside  is  electrified  positively, 
and  the  outside  negatively.  A  charged  jar  was  placed  on  an  insulating  sup- 
port, and  a  metallic  wire  bent  into  the  form  of  a  circular  arc  was  then  placed 
with  one  end  in  contact  with  the  outer  coating.  The  other  end  was  capable 
of  being  brought  into  contact  with  the  hook  of  the  wire  inserted  through  the 
cork,  and  thereby  put  in  metallic  communication  with  the  water  contained  in 
the  jar.  This  bent  wire  being  supported  by  a  handle  of  sealing-wax  was 
itself  insulated,  and  no  electricity  could  pass  in  the  experiment,  otherwise  than 
between  the  inside  of  the  jar  and  the  coating  on  the  outside.  On  bringing  the 
upper  extremity  of  the  bent  wire  into  contact  with  the  hook,  the  jar  was  in- 
stantly discharged,  both  the  inside  and  the  outside  being  restored  to  their 
natural  state.  Franklin  inferred  from  this,  that  before  the  discharge  the  in- 
terior of  the  jar  was  positively  electrified,  and  the  exterior  coating  negatively 
electrified,  in  an  equal  degree  ;  that  is  to  say,  that  the  interior  of  the  jar  con- 
tained an  excess  of  electricity  over  and  above  its  natural  amount,  and  the  ex- 
terior coating  fell  short  of  its  natural  amount  by  a  quantity  equal  to  that  excess. 

Various  other  experiments  were  made  to  verify  this  doctrine.  Two  metallic 
knobs  were  placed  near  each  other,  one  communicating  with  the  external 
coating,  and  the  other  with  the  water  within  the  jar.  A  small  cork  ball  sus- 
pended by  a  silk  thread  was  placed  between  those  two  knobs.  The  ball  was 
alternately  attracted  and  repelled,  "  playing  incessantly  from  one  to  the  other, 
the  bottle  was  no  longer  electrised ;  that  is,  it  fetched  and  carried  fire  from 
the  top  (inside)  to  the  bottom  (outside)  of  the  bottle,  till  equilibrium  was  re- 
stored."* 

i  had  been  observed  by  electricians  in  Europe,  that  a  jar  could  not  be 
charged  if  its  external  coating  were  insulated  ;  that,  in  fact,  it  was  a  necessary 
condition  that  a  communication  between  that  coating  and  the  ground  should  be 
provided  and  maintaiaed  by  some  conducting  matter,  such  as  a  metallic  wire. 
Franklin  assumes,  that  no  electricity  can  be  conveyed  to  the  inside  without 
causing  the  expulsion  of  an  equal  quantity  from  the  outside  ;  but  if  the  jar  be 
insulated,  no  means  of  escape  being  left  for  the  electricity  on  the  outside,  no 
accumulation  can  take  place  on  the  inside. f 

In  these  experiments,  we  find  also  a  description  of  the  method  of  charging 
a  series  of  jars,  now  called  the   charge  by  cascade.     "  Suspend  two  or  more 
phials  on  the  prime  conductor,  one  hanging  on  the  tail  of  another,  and  a  wire 
irom  the  last  to  the  floor.     An  equal  number  of  turns  of  the  wheel  will  charge  . 
them  all  equally,  and  every  one  as  much  as  one  alone  would  have  been ;  what  i 

*  Franklin's  Works  (Letters),  vol.  v.,  p.  192.     Boston.  1837.  t  Letters,  p.  190. 

~v-~-~ -^v^^^^^^v^^^ ^^x> ^-^S^-^^^^^^^^^^^^V^-N. I 


ELECTRICITY. 


117 


is  drawn  out  of  the  tail  of  the  first  serving  to  charge  (the  inside  of)  the  second  ; 
what  is  driven  out  of  the  second  charging  the  third,  and  so  on."* 

In  this  way  he  constructed  an  electrical  battery.  After  charging  a  series 
of  jars  he  separated  them,  putting  the  insides  in  metallic  communication  with 
each  other,  and  the  outsides,  in  like  manner,  in  metallic  communication.  By 
si;ch  means  he  obtained  discharges  sufficiently  powerful  to  kill  the  smaller 
animals. 

But  the  experiment  which  appeared  to  be  most  conclusive  in  the  support 
it  gave  to  his  hypothesis  of  the  transfer  of  the  electricity  from  the  exterior  to 
the  interior  of  the  jar  in  the  process  of  charging  it,  was  the  following :  A  jar 
was  suspended  by  its  hook  on  the  prime  conductor  of  the  machine,  so  that  a 
metallic  communication  was  maintained  between  the  conductor  and  the  inside 
of  the  jar.  Meanwhile,  the  rubber  was  insulated.  On  working  the  machine, 
the  jar  was  found  to  receive  no  charge.  A  metallic  wire  was  now  rolled  round 
the  outer  coating  of  the  jar,  and  carried  thence  to  the  rubber,  so  as  to  make  a 
communication  between  them,  both  being  still,  in  other  respects,  insulated. 
The  jar  was  now  charged  with  ease,  which  was  explained  by  the  supposition 
that  the  electric  fluid  passed  from  the  outside  coating  by  the  wire  to  the  rubber, 
and  thence  by  the  glass  globe  and  prime  conductor  to  the  inside  of  the  jar.f 

According  to  the  hypothesis  above  stated,  there  is  no  essential  distinction, 
so  far  as  relates  to  the  charge,  between  the  external  coating  and  the  internal 
contents  of  the  jar ;  the  one  ought  to  be  as  easily  charged  as  the  other.  This 
was  accordingly  found  to  be  the  case.  A  jar  was  placed  on  an  insulating  sup- 
port, and  while  the  external  coating  was  put  in  communication  with  the  prime 
conductor  of  the  machine,  the  wire  extending  from  the  interior  was  put  in  com- 
munication with  the  rubber.  The  electricity  of  the  outer  coating  was  now 
positive,  and  that  of  the  inside  negative  ;  and  the  jar  was  discharged,  and  pro- 
duced the  same  effects  as  before. 

The  next  important  investigation  was  as  to  the  place  in  which  the  electricity 
of  the  jar  was  contained.  To  determine  this,  Franklin  charged  a  jar,  and  in- 
sulated it.  He  then  removed  the  cork,  and  the  wire  by  which  the  electricity 
was  conveyed  from  the  machine  to  the  inside  of  the  jar.  On  examining  these, 
he  found  them  free  from  electricity.  He  next  carefully  decanted  the  water 
from  the  charged  jar  into  another  insulated  vessel.  On  examining  this  it  was 
found  to  be  free  from  electricity.  Other  water  in  its  natural  state  was  now 
introduced  into  the  charged  jar  to  replace  that  which  had  been  decanted  ;  and 
on  placing  one  hand  on  the  outside  coating,  and  the  other  in  the  water,  he  re- 
ceived the  shock  as  forcibly  as  if  no  change  had  been  made  in  the  jar  since  it 
was  first  charged.^ 

A  piece  of  glass  was  then  placed  between  two  plates  of  lead  extending  nearly 
to  its  edge  on  every  side.  One  of  these  plates  of  lead  being  touched  by  the 
hand,  the  other  was  charged  with  electricity  as  usual.  The  plates  were  then 
removed  from  the  glass,  and,  being  examined,  were  found  to  be  in  their  natural 
state.  On  presenting  the  finger  to  the  glass  where  the  lead  had  covered  it, 
little  sparks  were  received  ;  and  on  displacing  the  lead  and  touching  it  at  both 
surfaces,  a  violent  shock  was  received. 

From  this  he  inferred  that  the  glass  was  the  substance  in  which  the  electri- 
city was  deposited  ;  and  the  metallic  coating,  or  the  water,  or  other  conductor, 
applied  to  it,  "  served  only,  like  the  armature  of  the  loadstone,  to  unite  the 
forces  of  the  several  parts,  and  bring  them  at  once  to  any  point  desired  ;  it  be 
ing  the  property  of  a  non-electric  [conductor],  that  the  whole  body  instantly  re 
ceivesj  or  gives,  what  electrical  fire  is  given  to,  or  taken  from  any  one  of  its  parts. "|| 

From  a  very  early  period  of  the  progress  of  electrical  observations,  the  anal 

*  Letters,  p.  199.  t  Letters,  p.  253.  J  Letters,  p.  201.  ||  Letters,  p.  202. 


ogy  between  electricity  and  lightning  had  been  noticed,  and  conjectures  as  to 
their  identity  were  expressed  ;  and  in  some  cases  distinct  predictions  hazarded, 
that  the  time  would  arrive  which  would  fully  establish  their  identity.  Dr. 
Wall,  in  a  paper  published  in  the  "  Philosophical  Transactions,"  speaking  of 
the  electricity  of  amber,  said  that  he  had  no  doubt,  "  that  by  using  a  longer  and 
larger  piece  of  amber,  both  the  cracklings  and  the  light  would  be  much  greater. 
This  light  and  crackling  seems  in  some  degree  to  represent  thunder  and 
lightning."* 

Mr.  Grey,  whose  experiments  have  been  already  referred  to,  says,  speaking 
of  electrical  effects :  "  These  are  at  present  but  in  minimi s.  It  is  probable 
that,  in  time,  there  may  be  found  out  a  way  to  collect  a  greater  quantity  of  elec- 
tric fire,  and  consequently  to  increase  the  force  of  that  power,  which,  by  sev- 
eral of  these  experiments,  si  licet  magnis  componere  parva,  seems  to  be  of  the 
same  nature  with  that  of  thunder  and  lightning." 

But  of  all  the  anticipations  which  are  pretended  to  of  the  grand  discovery 
of  the  philosopher  of  Philadelphia,  that  which  is  by  far  the  most  remarkable 
proceeded  from  his  contemporary  and  competitor,  the  Abbe  Nollet.  Immedi- 
ately after  the  first  exhibition  of  the  experiments  proving  the  identity  of  elec- 
tricity and  lightning,  the  abbe  urged  his  claim  to  a  share  of  the  merit  of  having 
suggested  them.  In  a  paper,  dated  Paris,  June  6,  1752,  the  abbe,  after  noti- 
cing the  experiments,  observes  that  he  "  is  more  interested  than  any  one  to 
come  at  the  facts,  which  prove  a  true  analogy  between  lightning  and  electricity, 
since  these  experiments  establish  incontestably  a  truth  which  he  had  conceived, 
and  which  he  ventured  to  lay  before  the  public  more  than  four  years  ago." 

In  the  fourth  volume  of  his  Lecons  de  Physique  is  found  the  following  pas- 
sage :  "  If  any  one  should  undertake  to  prove,  as  a  clear  consequence  of  the 
phenomenon,  that  thunder  is,  in  the  hands  of  nature,  what  electricity  is  in  ours 
— that  those  wonders  which  we  dispose  at  our  pleasure  are  only  imitations  on 
a  small  scale  of  those  grand  effects  which  terrify  us,  and  that  both  depend  on 
the  same  mechanical  agents — if  it  were  made  manifest  that  a  cloud  prepared 
by  the  effects  of  the  wind,  by  heat,  by  a  mixture  of  exhalations,  &c.,  is  in  re- 
lation to  a  terrestrial  object,  what  an  electrified  body  is  in  relation  to  a 
body  near  it  not  electrified,  I  confess  that  this  idea,  well  supported,  would 
please  me  much ;  and  to  support  it,  how  numerous  and  specious  are  the  rea- 
sons which  present  themselves  to  a  mind  conversant  with  electricity.  The 
universality  of  the  electric  matter,  the  readiness  of  its  action,  its  instrumen- 
tality, and  its  activity  in  giving  fire  to  other  bodies ;  its  property  of  striking 
bodies  externally  and  internally,  even  to  their  smallest  parts  (the  remarkable 
example  we  have  of  this  effect  even  in  the  Leyden  jar  experiment,  the  idea 
which  we  might  truly  adopt  in  supposing  a  greater  degree  of  electric  power) ; 
all  these  points  of  analogy  which  I  have  been  for  some  time  meditating,  begin 
to  make  me  believe  that  one  might,  by  taking  electricity  for  the  model,  form  to 
oneself,  in  regard  to  thunder  and  lightning,  more  perfect  and  more  probable 
ideas  than  any  hitherto  proposed."! 

The  volume  containing  this  passage  was  printed  and  published  toward  the 
close  of  the  year  1748,  as  appears  by  the  register  of  the  Academy  of  Sciences, 
in  which  the  order  to  print  it  bears  date  on  the  9th  of  August  in 'that  year.  It 
will  presently  appear  that  Franklin's  first  publication  of  the  same  views  was 
in  a  letter  addressed  to  Mr.  Collinson,  despatched  in  1749.  So  far,  therefore, 
as  relates  to  these  speculations,  the  priority  of  publication  must  be  conceded 
to »  Nollet.  It  seems,  however,  improbable  that  Franklin,  residing  in  Philadel- 
phia, could  have  seen  Nollet's  volume  between  the  date  of  its  publication  and 


'  Priestley,  History  of  Electricity,  p.  11. 
t  Nollet,  Lemons  de  Physique,  torn  iv.,  p. 


315,  8me.  edition. 


ELECTRICITY.  119 


the  despatch  of  his  letter,  an  interval  not  exceeding  a  few  months ;  and  the 
probability  is,  therefore,  that  these  views  occurred  simultaneously  to  the  Amer- 
ican and  the  French  philosopher. 

From  the  moment  that  Franklin  first  engnged  in  electrical  inquiries,  nis 
views  were  constantly  bent  on  the  discovery  of  some  useful  purpose  to  which 
the  science  could  be  applied.  Cut  lono?  was  a  question  never-absent  from 
his  thoughts.*  This  craving  after  utility  was  the  great  characteristic  of  his 
mind,  and  might  be  regarded  as  being  carried  almost  to  a  fault.  To  bring  the 
properties  of  matter  and  the  phenomena  of  nature  into  subjection  to  the  uses 
of  civilized  life,  is  undoubtedly  one.  of  the  great  incentives  to  the  investigation 
of  the  laws  of  the  material  world  ;  but  it  is  assuredly  a  great  error  to  regard  it  as 
either  the  only  or  the  principal  motive  to  such  inquiries.  There  is  in  the  per- 
ception of  truth  itself — in  the  contemplation  of  connected  propositions,  leading 
by  the  mere  operation  of  the  intellectual  faculties,  exercised  on  individual 
physical  facts,  to  the  development  of  those  great  general  laws  by  which  the 
universe  is  maintained — an  exalted  pleasure,  compared  with  which  the  mere 
attainment  of  convenience  and  utility  in  the  economy  of  life  is  poor  and  mean. 
There  is  a  nobleness  in  the  power  which  the  natural  philosopher  derives  from 
the  discovery  of  these  laws,  of  raising  the  curtain  of  futurity,  and  displaying 
the  decrees  of  nature,  so  far  as  they  affect  the  physical  universe  for  count- 
less ages  to  come,  which  is  independent  of  all  utility.  There  is  a  lofty  and 
disinterested  pleasure  in  the  mere  contemplation  of  the  harmony  and  order  of 
nature,  which  is  above  and  beyond  mere  utility.  While,  however,  we  thus 
claim  for  truth  and  knowledge  all  the  consideration  to  which,  on  their  own  ac- 
count, they  are  entitled,  let  us  not  be  misunderstood  as  disparaging  the  great 
•benefactors  of  the  human  race,  who  have  drawn  from  them  those  benefits 
which  so  much  tend  to  the  wellbeing  of  man.  When  we  express  the  enjoy- 
ment which  arises  from  the  beauty  and  fragrance  of  the  flower,  we  do  not  the 
less  prize  the  honey  which  is  extracted  from  it,  or  the  medicinal  virtues  it 
yields.  That  Franklin  was  accessible  to  such  feelings,  the  enthusiasm  with 
which  he  expresses  himself  throughout  his  writings  in  regard  to  natural  phe- 
nomena abundantly  proves.  Nevertheless,  rtstful  application  was,  undoubtedly, 
ever  uppermost  in  his  thoughts  ;  and  he  probably  never  witnessed  any  physical 
fact,  or  considered  for  a  moment  any  law  of  nature,  without  inwardly  proposing 
to  himself  the  question,  "  In  what  way  can  this  be  made  beneficial  in  the 
economy  of  life  ?" 

The  analogy  and  probable  identity  of  lightning  and  electricity  were  first  sug- 
gested and  demonstrated  by  Franklin  in  a  letter  addressed  to  Collinson,  which 
appears  without  a  date,  and  which  has  by  some  been  referred  to  the  date  (1750) 
of  that  which  immediately  follows  it  in  the  published  collection  of  letters.  It 
appears,  however,  by  a  subsequent  letter,!  addressed  to  the  same  gentleman  in 
1753,  that  he  was  occupied  in  the  investigation  of  this  question  from  1747  to 

*  After  he  had  succeeded  in  making  the  discoveries  which  have  been  already  explained,  and 
beside*  inventing-  a  little  moving  power,  which  he  called  an  electrical  jack,  he  expressed  to  Mr. 
Collin=on.  in  his  nsunl  playful  manner,  his  disappointment  at  being  unable  to  find  any  application  of 
the  science  beneficial  to  mankind.  "  Chagrined  a  little  that  we  ha\je  hitherto  been  able  to  produce 
nothing1  in  this  wav  of  nse  to  mankind,  and  the  hot  weather  coming  on  when  electrical  experiments 
are  nrt  an  agreeable,  it  is  proposed  to  put  an  end  to  them  for  this  season,  somewhat  humorously,  in  a 
partv  of  pleasure  on  the  banks  of  the  Schnvlkill.  Spirits,  at  the  same  time,  are  to  be  fired  by  a 
spark  sent  from  side  to  side  throueh  the  river  without  any  other  conductor  than  the  water;  an  exper- 
iment which  we  some  time  since  performed  to  the  amazement  of  many.  A  turkey  is  to  be  killed  for 
dinner  bv  tho  electrical  shock,  and  roasted  by  the  electrical  jack,  before  a  fire  kindled  by  the  electri- 
fied bof/Ic,  when  the  healths  of  all  the  famo'us  electricians  of  England,  Holland,  France,  and  Ger- 
many, nre  to  bo  drunk  in  electrified  bumpers,  under  the  discharge  of  guns  from  the  electrical  battery." 
—  l.r''erx.  p.  210. 

t  "  In  my  former  paper  on  this  subject,  written  first  in  1747,  enlarged  and  sent  to  England  in  1749, 
I  considered  the  sea  as  the  great  source  of  lightning,"  dec. — Letters,  p.  300. 


120  ELECTRICITY. 

]  749  ;  that  the  paper  now  referred  to  was  first  written  in  the  former  year,  but 
that  it  was  enlarged  and  improved  and  sent  to  England  in  1749,  which  must, 
therefore,  be  taken  as  its  date.  In  this  letter  he  enters  very  fully  into  his  rea- 
sons for  considering  the  cause  of  electricity  and  lightning  to  be  the  same  phys- 
ical agent,  differing  in  nothing  save  the  intensity  of  its  action ;  and  he  truly 
observes,  that  the  difference  in  degree,  however  enormous,  is  no  argument 
against  the  identity  of  the  agents,  but  that,  on  the  contrary,  an  almost  infinite 
difference  might  be  naturally  looked  for.  "  When  a  gun -barrel  in  electrical 
experiments  has  but  little  electrical  fire  in  it,  you  must  approach  it  very  near 
with  your  knuckle  before  you  can  draw  a  spark.  Give  it  more  fire,  and  it  will 
give  a  spark  at  greater  distance.  Two  gun-barrels  united,  and  as  highly  elec- 
trified, will  give  a  spark  at  a  still  greater  distance.  But  if  two  gun-barrels 
electrified  will  strike  at  two  inches  distance,  and  make  a  loud  snap,  to  what  a 
great  distance  may  ten  thousand  acres  of  electrified  cloud  strike  and  give  its 
fire,  and  how  loud  must  be  that  crack  !"* 

The  analogies  which  he  stated  as  affording  presumptive  evidence  of  the 
identity  of  lightning  and  electricity  may  be  briefly  enumerated.  The  electrical 
spark  is  zigzag,  and  not  straight ;  so  is  lightning.  Pointed  bodies  attract  elec- 
tricity ;  lightning  strikes  mountains,  trees,  spires,  masts,  and  chimneys.  When 
different  paths  are  offered  to  the  escape  of  electricity,  it  chooses  the  best  con- 
ductor ;  so  does  lightning.  Electricity  fires  combustibles  ;  so  does  lightning. 
Electricity  fuses  metals  ;  so  does  lightning.  Lightning  rends  bad  conductors 
when  it  strikes  them  ;  so  does  electricity  when  rendered  sufficiently  strong. 
Lightning  reverses  the  poles  of  a  magnet ;  he  proved  by  direct  experiment  that 
electricity  had  the  same  effect.  A  stroke  of  lightning  when  it  does  not  kill,  c 
often  produces  blindness  ;  he  rendered  a  pigeon  blind  by  a  shock  of  electricity 
intended  to  kill  it.  Lightning  destroys  animal  life  ;  he  killed  a  hen  and  a  tur- 
key by  electrical  shocks. 

Having  ascertained  by  experiment  the  property  of  points  in  attracting  and 
discharging  electricity,  Franklin,  acknowledging  his  inability  to  give  a  satis- 
factory theory  of  this  effect,  set  himself  to  inquire  how  "  this  power  of  points 
might  possibly  be  of  some  use  to  mankind."  To  discover  this,  he  suspended 
a  large  conductor,  by  silk  lines,  from  the  ceiling,  and  charged  it  with  electricity, 
so  as  to  enable  it  to  give  a  spark  at  the  distance  of  two  inches,  "  strong  enough 
^  to  make  one's  knuckle  ache."  Under  these  circumstances,  he  found  that  if  a 
I  person  presented  the  point  of  a  needle  to  the  conductor  at  more  than  a  foot 
1  distance,  no  electricity  could  be  retained  upon  it,  all  passing  off  by  the  needle 
as  fast  as  it  was  supplied.  He  also  found,  that  if,  after  it  was  strongly  electri- 
fied, the  needle  was  presented  at  the  same  distance,  the  conductor  would  in- 
stantly lose  its  electricity.  That  the  electricity,  in  this  case,  really  passed  off 
by  the  point,  he  ascertained  by  observing  that,  in  the  dark,  the  light  was  visi- 
ble on  the  point  of  the  needle  ;  and  also  because,  when  the  person  presenting 
the  needle  was  himself  insulated,  or  stuck  the  needle  in  a  bundle  of  sealing 
wax,  the  electricity  no  longer  escaped. 

The  next  experiment  is  so  remarkable  in  itself,  and  so  characteristic  of  the 
mind  of  Franklin,  that  we  shall  give  it  in  his  own  words  : — 

"  Take  a  pair  of  large  brass  scales,  of  two  or  more  feet  beam,  the  cords  of 
the  scries  being  silk.  Suspend  the  beam  by  a  packthread  from  the  ceiling,  so 
that  the  bottom  of  the  scales  may  be  about  a  foot  from  the  floor ;  the  scales 
will  move  round  in  a  circle  by  the  untwisting  of  the  packthread.  Let  an  iron 
punch  (a  silvfrsinith's  iron  punch,  an  inch  thick,  is  what  I  use)  be  put  on  the 
end  upon  the  floor,  in  such  a  place  as  that  the  scales  may  pass  over  it  in  ma- 
king their  circle  ;  then  electrify  one  scale  by  applying  the  wire  of  a  charged 

*  Letters,  p.  218. 


ELECTRICITY.  121 


phial  to  it.  As  they  move  round,  you  see  that  scale  draw  nigher  to  the  floor, 
and  'lip  more  when  it  comes  over  the  punch ;  and,  if  that  be  placed  at  a  proper 
distance,  the  scale  will  snap,  and  discharge  its  fire  into  it.  But  if  a  needle  be 
stuck  on  the  end  of  the  punch,  its  point  upward,  the  scale,  instead  of  drawing 
nigh  to  the  punch  and  snapping,  discharges  its  fire  silently  through  the  poin|, 
and  rises  higher  from  the  punch.  Nay,  even  if  the  needle  be  placed  upon  the 
floor  near  the  punch,  its  point  upward,  the  end  of  the  punch,  though  so  much 
higher  than  the  needle,  will  not  attract  the  scale  and  receive  its  fire ;  for  the 
nrcille  will  get  it,  and  convey  it  away,  before  it  comes  nigh  enough  for  the 
punch  to  act. 

"  Now,  if  electricity  and  lightning  be  the  same,  the  conductor  and  scales 
may  represent  electrified  clouds.  If  a  tube  (conductor)  of  only  ten  feet  long 
will  strike  and  discharge  its  fire  on  the  punch  at  two  or  three  inches  distance, 
and  electrified  cloud  of  perhaps  ten  thousand  acres  may  strike  and  discharge 
on  the  earth  at  a  proportionally  greater  distance.  The  horizontal  motion  of  the 
scales  over  the  floor  may  represent  the  motion  of  the  clouds  over  the  earth,  and 
the  erect  iron  punch  a  hill  or  high  building ;  and  then  we  see  how  electrified 
clouds,  passing  over  hills  or  high  buildings  at  too  great  a  height  to  strike,  may 
be  attracted  lower  till  within  their  striking  distance.  And,  lastly,  if  a  needle 
fixed  on  the  punch  with  its  point  upright,  or  even  on  the  floor  below  the  punch, 
will  draw  the  fire  from  the  scale  silently  at  a  much  greater  than  the  striking 
distance,  and  so  prevent  its  descending  toward  the  punch ;  o-r  if  in  its  course 
it  would  have  come  nigh  enough  to  strike,  yet,  being  first  deprived  of  its  fire, 
it  cannot,  and  the  punch  is  thereby  secured  from  the  stroke  :  /  say,  if  these 
things  are  so,  may  not  the  knowledge  of  this  power  of  points  be  of  use  to  mankind 
in  preserving  houses,  churches,  ships,  <$fc.,from  the  stroke  of  lightning,  by  direct- 
ing us  to  fix,  on  the  highest  parts  of  those  edifices,  upright  rods  of  iron,  made 
sharp  as  a  needle,  and  gilt  to  prevent  rusting  ;  and,  from  the  foot  of  those  rods, 
a  wire  down  the  outside  of  the  building  into  the  ground,  or  down  round  one  of  the 
shrouds  of  a  ship,  and  down  her  side  till  it  reaches  the  water?  Would  not  these 
pointed  rods  probably  draw  the  electrical  fire  silently  out  of  a  cloud  before  it  came 
nigh  enough  to  strike,  and  thereby  secure  us  from  that  most  sudden  and  terrible 
mischief? 

"  To  determine  this  question,  whether  the  clouds  that  contain  lightning 
be  electrified  or  not,  I  would  propose  an  experiment  to  be  tried,  where  it 
may  be  done  conveniently.  On  the  top  of  some  high  tower  or  steeple,  place  a 
kind  of  sentry-box,  big  enough  to  contain  a  man  and  an  electrical  stand.  From 
the  middle  of  the  stand  let  an  iron  rod  rise,  and  pass,  bending,  out  of  the  door, 
and  then  upright  twenty  or  thirty  feet,  pointed  very  sharp  at  the  end.  If  the 
electrical  stand  be  kept  clear  and  dry,  a  man  standing  on  it,  when  such  clouds 
are  passing  low,  might  be  electrified,  and  aflbrd  sparks,  the  rod  drawing  fire  to 
him  from  a  cloud.  If  any  danger  to  the  man  be  apprehended,  let  him  stand 
on  the  floor  of  his  box,  and  now  and  then  bring  near  to  the  rod  the  loop  of  a 
wire  that  has  one  end  fastened  to  the  leads,  he  holding  it  by  a  wax  handle  ;  so 
the  sparks,  if  the  rpu  is  electrified,  will  strike  from  the  rod  to  the  wire,  and  not 
afl'ect  him."* 

When  this  and  other  papers  by  Franklin,  illustrating  similar  views,  were 
sent  to  London,  and  read  before  the  Royal  Society,  they  are  said  to  have  been 
considered  so  wild  and  absurd  that  they  were  received  with  laughter,  and  were 
not  considered  worthy  of  so  much  notice  as  to  be  admitted  to  a  place  in  the 
"  Philosophical  Transactions."!  They  were,  however,  shown  to  Dr.  Fother- 
gill,  who  considered  them  of  too  much  value  to  be  thus  stifled ;  and  he  wrote  a 

Letters,  p.  235.  t  Franklin's  works  (memoirs),  vol.  L,  p.  299. 


122  ELECTRICITY. 

t 

preface  to  them,  and  published  them  in  London.  They  subsequently  went 
through  five  editions. 

After  the  publication  of  these  remarkable  letters,  and  when  public  opinion 
in  all  parts  of  Europe  had  been  expressed  upon  them,  an  abridgment  or  ab- 
stract of  them  was  read  to  the  society  on  the  6th  of  June,  1751.  It  is  a  re- 
markable circumstance  that,  in  this  notice,  no  mention  whatever  occurs  of 
Franklin's  project  of  drawing  lightning  from  the  clouds.  Possibly  this  was  the 
part  which  had  before  excited  laughter,  and  was  omitted  to  avoid  ridicule. 

Franklin  was  under  an  impression  that  a  pointed  rod  could  not  be  ex- 
pected to  attract  the  lightning,  unless  it  were  placed  at  a  very  great  height  in 
the  atmosphere  ;  and  to  render  the  result  of  his  projected  experiment  more  cer- 
tain, he  determined  to  wait  for  the  completion  of  a  spire  then  being  erected  in 
Philadelphia:  Meanwhile,  however,  a  different  and  more  promising  expedi- 
ent occurred  to  him  ;  which  was,  to  send  up  the  pointed  wire  upon  a  kite,  by 
the  string  of  which  the  lightning  might  be  brought  within  his  reach.  He  soon 
succeeded  in  realizing  this,  the  most  bold  and  grand  conception  which  ever 
presented  itself  to  the  imagination  of  an  experimental  philosopher. 

He  prepared  his  kite  by  making  a  small  cross  of  two  light  strips  of  cedar, 
the  arms  of  sufficient  length  to  extend  to  the  four  corners  of  a  large  silk  hand- 
kerchief stretched  upon  them.  To  the  extremities  of  the  arms  of  the  cross  he 
tied  the  corners  of  the  handkerchief.  This  being  properly  supplied  with  a 
tail,  loop,  and  string,  could  be  raised  in  the  air  like  a  common  paper  kite,  and. 
being  made  of  silk,  was  more  capable  of  bearing  rain  and  wind.  To  the  up- 
right arm  of  the  cross  was  attached  an  iron  point,  the  lower  end  of  which  was 
in  contact  with  the  string  by  which  the  kite  was  raised,  which  was  a  hempen 
cord.  At  the  lower  extremity  of  this  cord,  near  the  observer,  a  ke\  was  fast- 
ened ;  and,  in  order  to  intercept  the  electricity  in  its  descent,  and  prevent  it 
from  reaching  the  person  who  held  the  kite,  a  silk  riband  was  tied  to  the  ring 
of  the  key,  and  continued  to  the  hand  by  which  the  kite  was  held. 

Furnished  with  this  apparatus,  on  the  approach  of  a  storm,  he  went  out  upon 
the  commons  near  Philadelphia,  accompanied  by  his  son,  to  whom  alone  he 
communicated  his  intentions,  well  knowing  the  ridicule  which  would  have  at- 
tended the  report  of  such  an  attempt,  should  it  prove  to  be  unsuccessful.  Hav- 
ing raised  the  kite,  he  placed  himself  under  a  shed,  that  the  riband  by  which 
it  was  held  might  be  kept  dry,  as  it  would  become  a  conductor  of  electricity 
when  wetted  by  rain,  and  so  fail  to  afford  that  protection  for  which  it  was  pro- 
vided. A  cloud,  apparently  charged  with  thunder,  soon  passed  directly  over 
the  kite.  He  observed  the  hempen  cord,  but  no  bristling  of  its  fibres  was  ap- 
parent, such  as  was  wont  to  take  place  when  it  was  electrified.  He  presented 
his  knuckle  to  the  key,  but  not  the  smallest  spark  was  perceptible.  The  agony 
of  his  expectation  and  suspense  can  be  adequately  felt  by  those  only  who  have 
entered  into  the  spirit  of  such  experimental  researches.  After  the  lapse  of 
some  time,  he  saw  that  the  fibres  of  the  cord  near  the  key  bristled,  and  stood  on 
end.  He  presented  his  knuckle  to  the  key,  and  received  a  strong  bright  spark. 
It  was  lightning.  The  discovery  was  complete,  and  Franklin  felt  that  he  was 
immortal. 

A  shower  now  fell,  and,  wetting  the  cord  of  the  kite,  improved  its  conducting 
power.  Sparks  in  rapid  succession  were  drawn  from  the  key,  a  Leyden  jar 
was  charged  by  it,  and  a  shock  given  ;  and,  in  fine,  all  the  experiments  which 
were  wont  to  be  made  by  electricity  were  reproduced  identical  in  all  their  con- 
comitant circumstances. 

This  experiment  was  performed  in  the  month  of  June,  1752.  It  will  be  re- 
membered that  Franklin's  letters  to  Mr.  Collinson  had  been  previously  pub- 
lished, translated,  and  widely  circulated  in  different  languages  throughout  Eu- 


ELECTRICITY. 


123 


rope ;  and  in  these  letters,  not  only  the  object  of  the  experiment  and  the  prin- 
ciple it  was  designed  to  establish  were  fully  explained,  but  minute  and  circum- 
stantial directions  were  given  as  to  the  manner  of  executing  it.  Persons  en- 
gaged in  physical  inquiries  in  different  parts  of  Europe  were  invited,  and  pre- 
pared to  submit  it  to  a  trial  when  convenient  opportunities  offered.  Among 
these  was  a  French  electrician,  M.  Dalibard,  who,  in  the  spring  of  1752,  pre- 
pared means  of  making  the  experiment,  at  Marly-la- Ville,  a  place  situate  about  six 
leagues  from  Paris.  He  succeeded  on  the  10th  of  May,  about  a  month  before 
th<5  experiment  of  Franklin,  and  made  a  report  of  his  proceedings  to  the  Acad- 
emy of  Sciences  at  Paris  on  the  13th,  in  which  he  states  that  the  experiment 
had  been  made  at  the  suggestion  and  according  to  the  method  laid  down  by 
Franklin.*  The  experiment  of  Franklin,  in  Juno,  was  made  before  he  could 
have  been  informed  of  that  of  Dalibard.  The  same  experiment  was  repeated 
on  the  18th  of  May  by  M.  de  Lor.  at  his  house  in  the  Estrapade,  at  Paris  ;  and 
an  account  of  it,  as  well  as  that  of  M.  Dalibard,  was  communicated  to  the 
Royal  Society  of  London  by  the  Abbe  Mazeas,  in  a  letter  dated  20th  May,  two 
days  after  the  latter  experiment,  in  which  the  abbe  ascribes  all  the  credit  of 
the  experiment  to  Franklin. f 

The  right  of  Franklin  to  the  credit  of  having  established  the  identity  of  light- 
ning and  electricity  has  been  denied,  and  the  honor  claimed  for  the  French 
philosophers  Nollet  and  Dalibard.  This  claim  was  advanced,  not  when  Eu- 
rope from  east  to  west,  and  from  north  to  south,  was  filled  with  amazement 
and  admiration  at  the  philosophic  boldness  of  the  "  Philadelphian  experiment" 
(as  it  was  universally  called),  or  the  profound  sagacity  with  which  it  was  con- 
ceived, with  which  its  minute  details  were  prescribed,  and  its  results  foretold 
— not  when  its  illustrious  author  was  elected  by  acclamation  a  member  of  the 
learned  societies  of  Europe,  and  received  the  academical  degree  from  the  most 
ancient  and  honored  of  universities — but  after  the  lapse  of  nearly  a  century,  after 
the  story  of  Franklin's  kite  had  passed  from  the  transactions  of  philosophical  so- 
cieties, and  the  memoirs  of  institutes  of  sciences,  into  the  primers  of  children. 
In  short,  it  was  so  recently  as  the  year  1831,  that,  in  his  admirable  Eloge  of 
Volta,  M.  Arago,  taking  a  retrospect  of  electrical  discovery,  maintained  that 
after  the  conjecture  of  Nollet,  on  the  identity  of  lightning  and  electricity,  an 
experiment  to  ascertain  the  fact  was  almost  useless.  And  the  reasons  he  as- 
signed for  such  inutility  were,  that  the  experiment  had  been  first  made  when 
flame  appeared  on  the  spears  of  soldiers,  and  the  masts  of  ships  ;J  but  that,  if 
any  credit  be  claimed  for  the  actual  exhibition  of  the  fact  by  immediate  experi- 
ment, that  credit  is  due  to  M.  Dalibard. 

if  such  a  statement,  supported  by  such  a  reason,  had  proceeded  from  a  quar- 
ter less  entitled  to  respect  than  the  "  perpetual  secretary  of  the  Academy  of 

*  "  En  suivant  la  route  que  M.  Franklin  nous  a  tracee,  j'ai  obtenu  one  satisfaction  complete." — 
Memoir  de  M.  Dalibard,  quoted  in  Franklin's  works,  vol.  v.,  p.  288. 

t  See  Phil.  Trans.,  vol.  xvii.  1752. 

t  "  Les  premieres  vues  de  Franklin  sur  1'analogie  de  I'electricite  et  dn  tonnerre  n'etaient,  comme 
l«s  idees  anterieures  de  Nollet  que  de  simples  conjectures.  Toute  la  difference,  entre  les  deux  phy- 
Biciens,  se  reduisait  alors  a  un  projet  d'experience,  dont  Nollet  n'avait  pas  parler Sans  por- 
ter attaint  a  la  gloire  de  Franklin,  je  dois  remarquer  que  I'experisnce  proposee  etait  prcsque  inutile. 
Les  soldats  de  la  cinquieme  legion  Romaine  1'avaient  deja  faite  pendant  la  guerre  d'Atrique.  le  jour 
ou,  comme  Cesar  le  rapporte,  le  fer  de  tous  les  javelots  parut  en  feu  a  la  suite  d'un  orage.  11  en  e*i 
de  m&me  des  nombreux  navigateurs  a  qui  Castor  et  Pollux  s'fitaient  montres,  soil  aux  pointes  me- 

talliques  des  mats  ou  des  vergues,  soil  sur  d'autres  parties  saillantes  de  leurs  navires Au 

reste,  soil  que  plusieurs  de  ces  circonstances  fusseut  ignorees,  soil  qu'on  ne  les  trouvat  pas  demon- 
stratives, des  essais  directs  semblereut  necessaires,  et  c'est  a  Dalibard,  notre  compatriote,  que  la  sci- 
ence en  a  eteredevable.  Le  10  Mai,  1752,  pendant  un  orage,  la  grande  tige  de  metal  pointue  qu'il 
avail  etablie  dans  un  jardin  de  Marly-la- Ville  donuait  de  petites  etincelles,  comme  le  fait  le  conduc- 
teur  de  la  machine  feiectrique  ordinaire,  quand  on  en  approche  un  fil  de  fer.  Franklin  ne  realisa 
eotte  meme  expferience  aux  Etats-Unis,  a  1'aide  d'un  cerf  volant,  qu'un  mois  plus  tard." — Eloge  de 
Volta,  p.  12. 


Sciences,"  the  astronomer  royal  of  France,  the  man  who  stands,  if  not  first,  in- 
contestably  in  the  first  rank  of  living  meteorologists — in  a  word,  than  M.  Arago 
— no  one  would  think  it  entitled  to  a  serious  answer.  It  would  be  classed 
among  those  strange  obliquities  of  historic  vision  which  have  led  some  persons 
to  see  in  Richard  and  Macbeth,  not  tyrants  and  murderers,  but  mild  and  virtu- 
ous princes,  cruelly  wronged  by  the  calumnies  of  tradition. 

Nollet  conjectured  the  probable  identity  of  lightning  and  electricity,  but  gave 
not  the  most  distant  hint  of  any  possible  method  by  which  the  probability  could 
he  experimentally  tested.  Franklin  boldly  maintained  the  identity  of  these 
agents,  gave  numerous  and  cogent  reasons  to  support  that  position,  and  more- 
over prescribed  with  minute  details  two  distinct  methods  by  which  lightning 
could  be  brought  into  the  hands  of  the  observer,  and  submitted  to  the  same  ex- 
perimental examination  as  electricity  had  undergone.  One  of  these  two  meth- 
ods was,  in  scrupulous  accordance  with  his  directions,  applied  in  France  ; 
and  the  other,  within  a  few  weeks,  was  adopted  by  himself  in  America.  The 
results  of  both  were  precisely  what  Franklin  had  foretold.  Both  were  com- 
pletely successful. 

But,  rejoins  M.  Arago,  the  whole  affair  of  the  experiment  was  useless,  for 
it  had  already  been  effected.  The  flame  on  the  javelins  of  the  Roman  senti- 
nels of  the  fifth  legion  was  sufficient  as  an  experiment,  not  to  mention  Castor 
and  Pollux,  so  often  seen  by  sailors  on  their  mast-tops  !  What  would  so  se- 
vere a  reasoner  as  M.  Arago  say  to  another  who  should  maintain,  without  fur- 
ther experiment,  that  either  of  these  luminous  appearances  was  identical  with 
lightning  1  —  and  if  that  were  conceded,  where  would  have  been  found  the 
proof  that  these  meteors,  and  the  lightning  with  which  they  would  be  granted 
to  be  identified,  were  due  to  the  same  physical  agent  as  that  manifested  by  the 
friction  of  glass  and  resin  ? 

If  however,  says  M.  Arago  again,  the  experiment  were  necessary  or  useful, 
science  owes  it  to  M.  Dalibard,  who  executed  it  at  Marly-la- Ville  a  month  be- 
fore Franklin,  with  his  kite,  made  it  at  Philadelphia.  This  statement  is  not 
attended  with  the  circumstantial  accuracy  which  M.  Arago  is  accustomed  to 
observe.  The  fact,  as  stated  by  M.  Dalibard  himself,  was,  that  he  took  Frank- 
lin's printed  directions  as  to  the  manner  of  performing  his  (Franklin's)  project- 
ed experiment,  and  followed  them  to  the  letter  in  preparing  his  apparatus  at 
Marly-la-Ville.  Having  accomplished  this,  he  put  the  directions  for  making 
the  observations  into  the  hands  of  one  Coiffier,  an  old  retired  soldier,  who  fol- 
lowed the  trade  of  a  carpenter,  and  who  probably  also  erected  the  apparatus 
itself,  and  desired  Coiffier  to  make  the  experiment  in  the  manner  prescribed 
by  Franklin,  if  a  storm  should  occur  at  a  time  when  he  (Dalibard)  was  absent. 
The  first  storm  did  occur  when  Dalibard  was  at  Paris.  Coiffier  presented  a  piece 
of  metal  to  the  rod,  and  received  several  sparks.  He  then  ran  for  the  cure, 
who,  with  him,  repeated  the  experiment,  and  immediately  wrote  a  full  descrip- 
tion of  it,  with  which  he  despatched  Coiffier  himself  to  Paris  to  M.  Dalibard. 

Thus  it  appears  that  so  far  from  science  being  indebted  to  M.  Dalibard  for 
the  earliest  exhibition  of  this  capital  experiment,  that  philosopher  had  no  other 
share  in  it,  save  that  of  having  caused  the  erection  of  the  conducting  rod  and 
other  apparatus  according  to  Franklin's  directions.  In  the  actual  performance 
of  the  first  experiment,  he  had  no  share  whatever. 

Let  us  now  see  how  the  account  of  credit  stands  on  the  score  of  this  memo- 
rable discovery : — 

In  1708,  Dr.  Wall  mentions  a  resemblance  of  electricity  to  thunder  and  light- 
ning. 

In  1735,  Mr.  Grey  conjectures  their  identity,  and  that  they  differ  only  in  \ 
degree. 


ELECTRICITY. 


In  1748,  the  Abbe  Nollet  reproduces  the  conjecture  of  Grey,  attended  with 

more  circumstantial  reasons. 

In  17-19,  Franklin  strongly  maintains  their  identity,  and  accurately  describes 
two  ways  of  experimentally  testing  it,  and  sends  his  instructions  to  Eu- 
rope, to  enable  others  with  better  local  opportunities  than  he  possessed  to 
try  it. 

In  1752,  MM.  Dalibard  and  Delor,  in  France,  make  the  preparations  prescri- 
bed according  to  one  of  Franklin's  methods  ;  and  Franklin  makes  in  Phil- 
adelphia preparations  according  to  the  other  method. 
On  lOlh  May,  1752,  Coiffier  and  the  curate  make  the  experiment  as  directed 

by  Franklin,  and  obtain  the  results  foretold  by  Franklin. 
In  June,  1752,  Franklin  makes  the  same  experiment  in   Philadelphia,  ac- 
cording to  the  other  method,  with  like  results. 

If  the  credit  of  the  discovery  is  due  to  him  who  first  conjectured  the  identity 
of  lightning  and  electricity,  then  it  is  due  to  Mr.  Stephen  Gray. 

If  it  be  due  to  him  who  showed  the  method  of  making  the  capital  experi- 
ment by  which  the  identity  must  be  either  established  or  refuted,  it  belongs  to 
Franklin. 

If  it  be  due  to  the  persons  at  whose  expense  Franklin's  apparatus  was 
first  constructed,  then  it  must  be  shared  between  Franklin,  Dalibard,  and 
Delor. 

If  it  be  due  to  him  who  first,  in  person,  performed  the  experiment  proposed 
by  Franklin,  it  must  be  accorded  to  the  carpenter  and  dragoon  Coiffier. 

We  shall  now  dismiss  this  matter,  to  which  more  space  has  been  allotted 
than  it  is  entitled  to,  merely  observing,  that  much  as  living  philosophers  must 
be  surprised  at  the  claim  advanced  in  favor  of  M.  Dalibard,  that  electrician 
himself,  could  he  rise  from  his  tomb,  would  see  with  infinitely  more  astonish- 
ment an  honor  sought  for  him  to  which  he  never  himself  aspired,  or  supposed 
he  had  the  slightest  title. 

Franklin  having  established,  beyond  the  possibility  of  dispute,  the  identity 
of  lightning  and  electricity,  proceeded,  in  accordance  with  that  characteristic 
attribute  of  his  mind  already  noticed,  to  turn  this  discovery  to  the  benefit  of 
mankind,  and  proposed  the  general  adoption  of  those  pointed  metallic  rods  now 
so  commonly  erected  at  the  summits  of  buildings  to  protect  them  from  the  effects 
of  lightning.  The  principle  of  this  apparatus,  as  now  constructed  for  edifices 
and  ships,  differs  in  nothing  essential  from  that  proposed  by  its  celebrated  in- 
ventor. 

This  part  of  the  labors  of  Franklin  in  electricity  cannot  be  dismissed  with- 
out a  passing  notice  of  the  dispute  which  was  maintained  in  England  respect- 
ing the  comparative  advantages  of  conductors  with  pointed  ends  as  proposed  by 
Franklin,  or  with  round  or  blunted  ends  as  suggested  by  some  others.  It  were 
for  the  honor  of  science  that  this  discreditable  controversy  had  never  taken 
place.  It  forms,  a  rare,  if  not  a  solitary  example,  of  the  prostitution  of  philos- 
ophy to  gratify  the  meanest  passions  of  an  obstinate  and  imbecile  prince.  The 
persevering  tenacity  with  which  the  British  monarch  fastened  his  last  grasp 
on  his  American  subjects  about  to  wrest  themselves  from  his  power,  and  assert 
their  independence,  is  well  known.  By  his  pursuit  of  that  object,  after  all 
reasonable  hope  of  securing  it  had  expired,  the  treasures  of  his  kingdom  were 
lavished,  and  the  blood  of  his  people  flowed  in  mutual  slaughter.  Bad  as  were 
these  consequences,  they  were  nevertheless  the  ordinary  consequences  of  war. 
But  the  vindictive  spirit  of  the  court  passed  from  the  field  and  council-board  to 
the  peaceful  halls  of  science  ;  and  because  Franklin,  the  agent,  representative, 
and  counsellor  of  the  American  people,  had  proposed  the  use  of  pointed  con- 
ductors, a  party  of  parasites  was  found,  who,  to  gratify  George  III.,  advocated 


126  ELECTRICITY. 


b/unt  conductors  ;  and  to  crown  this  most  egregious  absurdity,  blunt  conductors 
were  actually  erected  upon  the  royal  palace  !  * 

Franklin  next  directed  his  inquiries  to  the  quantity  and  nature  of  the  elec- 
tricity with  which  the  clouds  in  various  states  of  the  atmosphere  were  charg- 
ed. To  facilitate  his  experimental  inquiries  on  this  subject,  he  erected  in  his 
house  in  Philadelphia  a  pointed  iron  rod,  which  he  was  enabled  to  insulate  at 
pleasure.  This  rod  was  put  in  communication  with  a  system  of  bells,  which 
alternately  attracted  and  repelled  their  hammers  when  electrified.  Whenever 
a  cloud  charged  with  electricity  passed  over  the  house  within  such  a  distance 
as  to  affect  the  conductor,  these  bells  would  ring  and  inform  him  of  the  oppor- 
tunity of  prosecuting  his  experiments. 

Having  satisfied  himself  that  the  clouds  were  frequently  in  an  electrified 
state  when  there  was  no  thunder  or  lightning,  his  next  inquiry  was,  whether 
they  were  electrified  positively  or  negatively.  This  was  a  question  of  more 
interest  to  him,  because,  according  to  his  theory,  if  their  .electricity  were  neg- 
ative, the  earth,  "in  thunder-strokes,  would  strike  into  the  clouds,  and  not  the 
clouds  into  the  earth."  To  determine  this,  he  "took  two  phials  and  charged 
one  of  them  with  lightning  from  the  iron  rod,  and  gave  the  other  an  equal 
charge  (of  electricity)  from  the  prime  conductor.  When  charged  he  placed 
them  on  a  table  within  three  or  four  inches  of  each  other,  a  small  cork  ball 
being  suspended  by  a  fine  silk  thread  from  the  ceiling,  so  as  to  play  between 
the  wires.  If  both  bottles  then  were  electrified  positively,  the  ball  being  attract- 
ed and  then  repelled  by  the  one  must  be  repelled  by  the  other.  If  the  one 
positively  and  the  other  negatively,  then  the  ball  would  be  attracted  and  repel- 
led by  each,  and  continue  to  play  between  them,  so  long  as  any  considerable 
charge  remained.''! 

From  experiments  with  this  apparatus  lie  concluded  that  clouds  were  some- 
times positively  and  sometimes  negati'  ny  electrified,  but  oftener  negatively. 
Electrical  instruments  had  not  yet,  however,  advanced  to  such  a  state  of  im- 
provement as  to  enable  a  mind,  even  acute  as  his,  to  make  much  further  dis- 
covery in  atmospheric  electricity  ;  a>  d  although  the  details  of  his  experiments 
arid  his  theoretical  speculations  regarding  them  must  always  be  read  with 
profound  interest,  yet  no  further  principles  of  importance  appear  to  have  been 
evolved  from  them. 

If  it  be  true  that  the  Royal  Society  laughed  at  his  speculations  and  refused 
to  them  a  place  in  their  Transactions,  they  were  not  slow  to  retract  and  repair 
their  error.  They  conferred  upon  him  their  highest  honor  (the  Copley  medal), 
and  unanimously  elected  him  an  honorary  member  of  their  society,  in  1753. 

An  experiment  so  remarkable  as  the  attraction  of  lightning  from  the  clouds, 
could  not  fail  to  be  verified  and  repeated  by  many  enthusiastic  lovers  of  science. 
One  of  the  first  instances  of  this  zeal  was  rendered  memorable  by  its  fatal  re- 
sult. Professor  George  William  Richmann,  of  St.  Petersburg,  was  preparing 
an  essay  on  electricity  ;  and  in  order  to  obtain  the  most  certain  and  accurate 
knowledge  of  the  phenomena,  he  placed  a  conductor  on  his  house,  making  a 
metallic  communication  between  it  and  his  study,  where  he  provided  means  for 
repeating  Franklin's  experiments.  On  the  6th  of  August,  1753,  while  Rich- 
inann  attended  a  meeting  of  the  Petersburg  Academy  of  Science,  distant  thun- 
der was  heard,  on  which  he  went  to  his  house,  accompanied  by  Sokolow,  the 
engraver,  who  being  engaged  to  illustrate  his  work,  desired  to  see  those  elec- 

*  "  The  king's  changing  his  pointed  conductors  for  lihinf.  ones  is  a  matter  of  small  import;) 
me.     II  1  Lad  a  wish  about  them,  it  would  l>o,  that  he  would  reject  them  altogether  a.s  mvIl.x-Ma 
For  it  is  only  since  he  thought  himself  and  his  family  sale   from  the  thunder  of  heaven  that  he  ha 
dared  to  use  his  own  thunder  in  destroying  his  innocent  subjects."  —  Franklin's  Works  viii   227 

t  Lclterb,  p.  302. 


ELECTRICITY. 


127 


trical  appearances  which  he  would  have  to  represent  in  the  plates.  While 
Richmann  was  describing  to  Sokolow  the  nature  of  the  apparatus,  a  thunder- 
clap was  heard  louder  and  more  violent  than  any  which  had  been  remembered 
at  St.  Petersburg.  Richmann  stooped  toward  the  electrometer  of  the  appara- 
tus to  observe  the  force  of  the  electricity,  and  "  as  he  stood  in  that,  posture,  a 
great  white  and  bluish  fire  appeared  between  the  rod  of  the  electrometer  and 
his  head.  At  the  same  time  a  sort  of  steam  or  vapor  arose,  which  entirely  be- 
numbed the  engraver,  and  made  him  sink  on  the  ground."  Several  parts  of 
the  apparatus  were  broken  in  pieces  and  scattered  about.  The  doors  of  the 
room  were  torn  from  their  hinges,  and  the  house  shaken  in  every  part.  The 
wife  of  the  professor,  alarmed  by  the  shock,  ran  to  the  room,  and  found  her 
husband  sitting  on  a  chest,  which  happened  to  be  behind  him  when  he  was 
struck,  and  leaning  against  the  wall.  He  appeared  to  have  been  instantly 
struck  dead.* 

During  1752  and  the  succeeding  years  the  subject  of  atmospheric  electricity 
engaged  the  attention  of  persons  devoted  to  physical  science  in  different  parts 
of  Europe.  The  climate  of  England  being  less  favorable  to  such  researches 
than  more  southern  latitudes,  fewer  opportunities  of  observation  were  offered ; 
nevertheless,  Canton,  Wilson,  and  Bevis,  soon  repeated  and  verified  the  Phila- 
delphia experiments.  Canton  showed  that  the  clouds  were  electrified,  some- 
times negatively  and  sometimes  positively,  and  carried  such  observations  fur- 
ther than  Franklin.  A 

But  the  most  acute  and  indefatigable  follower  of  Franklin  at  this  time,  in  at- 
mospheric electricity,  was  Beccaria,  who,  in  1753,  published  a  treatise  on 
electricity  at  Turin,  and  a  series  of  letters  on  the  same  subject,  at  Bologna,  in 
1758.  He  erected  numerous  conducting  rods  in  different  places  of  observa- 
tion, and  elevated  kites  according  to  Franklin's  method.  By  raising  these  to 
various  heights,  he  observed  the  electricity  of  different  atmospheric  strata,  and 
he  improved  this  mode  of  observation  by  interlacing  the  strings  with  metallic 
wire.  To  keep  his  kites  constantly  insulated,  and  at  the  same  time  to  give 
them  more  or  less  string,  he  rolled  the  string  upon  a  reel,  which  was  supported 
by  pillars  of  glass,  and  his  conductors  were  placed  in  metallic  communication 
with  this  reel. 

This  profound  philosopher,  and  acute  and  accurate  observer,  has  left  in  the 
history  of  electricity  traces  of  his  genius  second  only  to  those  with  which 
Franklin  and  Volta  impressed  it.  Beccaria  was  the  first  who  diligently  studied 
and  recorded  the  circumstances  attending  the  phenomena  of  a  thunder-storm. 
He  observes  that  the  first  appearance  of  a  thunder-storm  (which  generally  hap- 
pens when  there  is  little  or  no  wind)  is  one  dense  cloud  or  more,  increasing 
rapidly  in  magnitude,  and  ascending  into  the  higher  regions  of  the  atmosphere. 
The  lower  edge  is  black  and  nearly  horizontal,  but  the  upper  is  finely  arched 
and  well  defined.  Many  of  these  clouds  often  seem  piled  one  upon  the  other, 
all  arched  in  the  same  manner  ;  but  they  keep  constantly  uniting,  swelling,  and 
extending  their  arches.  When  such  clouds  rise,  the  firmament  is  usually 
sprinkled  over  with  a  great  number  of  separate  clouds  of  odd  and  bizarre  forms, 
which  keep  quite  motionless.  When  the  thunder-cloud  ascends,  these  are 
drawn  toward  it ;  and  as  they  approach  they  become  more  uniform  and  regular 
in  their  shapes,  till,  coming  close  to  the  thunder-cloud,  their  limbs  stretch  mu- 
tually toward  one  another,  finally  coalesce,  and  form  one  uniform  mass.  But 
sometimes  the  thunder-cloud  will  swell  and  increase  without  the  addition  of 
these  smaller  adscititious  clouds.  Some  of  the  latter  appear  like  white  fringes 
a',  the  skirls  of  the  thunder-cloud  or  under  the  body  of  it,  but  they  continually 
grow  darker  and  darker  as  they  approach  it. 

*  Phil.  Trans.,  vol.  xlix.,  p.  Cl. 


128  ELECTRICITY 


When  the  thunder-cloud,  thus  augmented,  has  attained  a  great  magnitude, 
its  lower  surface  is  often  ragged,  particular  parts  being  detached  toward  the 
earth,  but  still  connected  with  the  rest.  Sometimes  the  lower  surface  swells 
into  large  protuberances,  tending  uniformly  toward  the  earth  ;  and  sometimes 
one  whole  side  of  the  cloud  will  have  an  inclination  to  the  earth,  which  the 
extremity  of  it  will  nearly  touch.  When  the  observer  is  under  the  thunder- 
cloud after  it  has  grown  large  and  is  well  formed,  it  is  seen  to  sink  lower  and 
to  darken  prodigiously,  and,  at  the  same  time,  a  great  number  of  small  clouds 
are  observed  in  rapid  motion,  driven  about  in  irregular  directions  below  it. 
While  these  clouds  are  agitated  with  the  most  rapid  motions,  the  rain  generally 
falls  in  abundance ;  and  if  the  agitation  be  very  great,  it  hails. 

While  the  thunder-cloud  is  swelling  and  extending  itself  over  a  large  tract 
of  country,  the  lightning  is  seen  to  dart  from  one  part  of  it  to  another,  and  often 
to  illuminate  its  whole  mass.  When  the  cloud  has  acquired  a  sufficient  ex- 
tent, the  lightning  strikes  between  the  cloud  and  the  earth  in  two  opposite 
places,  the  path  of  the  lightning  lying  through  the  whole  body  of  the  cloud  and 
its  branches.  The  longer  this  lightning  continues,  the  rarer  does  the  cloud 
grow,  and  the  less  dark  in  its  appearance,  till  it  breaks  in  different  places  and 
shows  a  clear  sky.  When  the  thunder  is  thus  dispersed,  those  parts  which 
occupy  the  upper  regions  of  the  atmosphere  are  spread  thinly  and  equally,  and 
those  that  are  beneath  are  black  and  thin  also,  but  they  vanish  gradually  with- 
out being  driven  awayAp  the  wind. 

The  instruments  for  electrical  observation  used  by  Beccaria  never  failed  to 
give  indications  corresponding  to  the  successive  changes  in  progress  in.  the 
atmosphere  above  his  observatory.  The  stream  of  fire  from  his  conductor  was 
generally  uninterrupted  while  the  thunder-cloud  was  directly  above  it.  The 
same  cloud  in  its  passage  electrified  his  conductor  alternately  with  positive  and 
negative  electricity.  The  electricity  of  the  conductor  continued  to  be  of  the 
same  kind  so  long  as  the  thunder-cloud  was  simple  and  uniform  in  its  direc- 
tion ;  but  when  the  lightning  changed  its  place,  a  change  in  the  species  of 
electricity  ensued.  A  sudden  change  of  this  kind  would  also  happen  after  a 
violent  flash  of  lightning  ;  but  the  change  would  be  gradual  when  the  lightning 
was  moderate,  and  the  progress  of  the  thunder-cloud  slow.* 

But  among  the  labors  of  this  philosopher,  that  rendered  by  modern  discov- 
eries most  memorable  was  one  which  by  his  contemporaries  and  their  imine.- 
diate  successors  was  regarded  as  an  ingenious  and  over-refined  conjecture, 
rather  than  what  it  afterward  proved  to  be,  the  distant  shadow  of  a  coming  dis- 
covery detected  by  the  far-sighted  rrh'nd  of  this  acute  and  extraordinary  man. 
Franklin  had  been  the  first  to  magnetize  fine  sewing-needles  by  the  electric 
spark.  Dalibard  observed  that  the  extremity  of  the  needle  at  which  the  spark 
from  the  excited  glass  entered  had  northern  polarity,  and  both  Franklin  and 
Dalibard  discovered  that  a  spark  of  equal  force  given  to  the  other  end  of  the 
needle  deprived  it  of  the  magnetic  virtue.  From  these  and  from  similar  ex- 
periments made  by  himself,  Beccaria  inferred  that  the  polarity  of  the  magnetic 
needle  was  determined  by  the  direction  in  which  the  electric  current  had 
passed  through  it.  He  assumed  the  magnetic  polarity  acquired  by  ferrugin- 
ous bodies  which  had  been  struck  by  lightning,  as  a  test  of  the  direction  of  the 
electric  current  in  passing  through  them,  and  thence  inferred  the  species  of 
electricity  with  which  the  thunder-cloud  had  been  charged. f 

Extending  this  analogy  to  the  earth  itself,  Beccaria  conjectured  that  terres- 
trial magnetism  was,  like  thai  of  the  needle  magnetized  by  Franklin  and  Dali- 

*  Beccaria,  Lcttere  dell'  Elettricismo.     Bologna,  1758  .-  p.  146,  et  seq. 

t  "  I  poli  del  mattoue  teste  descritto,  provano  che  anche  in  certi  corpi  che  abbiano  certa  porzione  \ 
di  ferro,  ilfulmine  imprime  un  scgiio  jxrmanente  della  sua  direzione."— Beccaria,  Lettere,  p.  261.        I 


ELECTRICITY.  121    ' 


bard,  the  mere  effect  of  permanent  currents  of  natural  electricity,  established 
and  maintained  upon  its  surface  by  various  physical  causes  ;  that,  as  a  violent 
current,  like  that  which  attends  the  exhibition  of  lightning,  produces  instanta- 
neous and  powerful  magnetism  in  substances  capable  of  receiving  that  quality, 
so  may  a  more  gentle,  regular,  and  constant  circulation  of  the  electric  fluid 
upon  the  earth  impress  the  same  virtue  on  all  such  bodies  as  are  capable  of 
it.  Observation  proves  that  a  vast  quantity  of  this  fluid  circulates  between 
different  parts  of  the  atmosphere  in  storms  ;  that  a  quantity  not  inconsiderable 
circulates  in  the  time  of  ordinary  rain  ;  and  that  even  when  the  weather  is  se- 
rene arid  the  heavens  unclouded,  some  quantity  is  still  observable.  "  Of  such 
fluid,  thus  ever  present,"  observes  Beccaria,  "  I  think  that  so'me  portion  is  con- 
stantly passing  through  all  bodies  situate  on  the  earth,  especially  those  which 
are  metallic  and  ferruginous  ;  and  I  imagine  it  must  be  those  currents  which 
impress  on  fire-irons,  and  other  similar  things,  the  power  which  they  are  known 
to  acquire  of  directing  themselves  according  to  the  magnetic  meridian  when 
they  are  properly  balanced."* 

He  observed,  that  to  say  we  are  insensible  to  this  current  around  us,  is  no 
good  argument  against  its  existence  ;  for  that  its  uniformity,  constancy,  and 
universality,  would  necessarily  render  it  imperceptible,  since  all  bodies  must 
partake  of  it  in  common.  His  hypothesis  to  account  for  the  variation  and  dip 
is  not  the  least  remarkable  part  of  this  extraordinary  anticipation.  He  consid- 
ers that  the  electro-magnetic  currents  have  not  all  a  pommon  centre,  but  may 
have  several  situate  in  our  northern  hemisphere.  The  aberration  of  their  com- 
mon centre  from  the  true  terrestrial  pole  may  probably  be  the  cause  of  the 
variation  of  the  compass.  The  periodical  change  to  which  the  position  of  this 
common  centre  is  subject  would  correspond  with  and  cause  the  periodical 
change  of  that  variation,  and  the  obliquity  of  these  currents  may  be  the  cause 
of  the  dip.f 

That  the  anticipation  of  the  fundamental  principle  of  electro-magnetism,  and 
terrestrial  magnetism,  should  have  been  complete  in  all  its  details,  could  scarce- 
ly have  happened  at  that  epoch  without  something  approaching  to  inspiration  ; 
but  it  will  be  readily  admitted  that  these  guesses  of  Beccaria,  when  compared 
with  the  discovery  of  Orested  and  the  theory  of  Ampere,  form  one  of  the  most 
striking  episodes  in  the  history  of  science. 

The  analogy  between  lightning  and  the  electric  spark,  arising  from  the  pe- 
culiar noise  or  explosion  with  which  each  was  attended,  had  been  noticed  by 
many  electricians.  Beccaria.  however,  investigated  and  demonstrated  its  cause,  ; 
by  showing  that  it  proceeded  from  a  pulsation  produced  in  the  air  by  the  sudden 
displacement  of  that  portion  of  it  through  which  the  electric  fluid  passes.  This 
displacement  being  transmitted  through  the  atmosphere  in  exactly  the  same 
manner  as  vibrations  are  produced  by  a  sonorous  body,  the  sound  accompany- 
ing an  electric  discharge,  and  the  thunder  which  attends  the  atmospheric  elec- 

*  "  Di  tale  fuoco,  io  penso  che  alcana  parte  perpetuamente  discorra  per  tutti  i  corpi  situati  sopra 
la  terra,  massimamente  per  i  metallic!  e  ferigni.     Penso  che  esso  sia,  il  quale  attraversando  le  padelle, 
Is  aiolle,  le  palette  ed  altri  si  fatti  bislnnghi  ferri,  i  quali  d'ordinario  pendono  o  posano  verticalmente,    j 
imprima  loro  la  virtil  di  situarsi  nella  meridiana  rnagnetica,  allora  che  sono  convenientemente  bili-   > 
cati." — Lettere,  p.  266. 

t  "  Questa  sistematica  elettrico-magnetica  circolazione,  secondp  me,  non  procederebbe  da  an  solo 
panto  setteiHrionale,  ma  avrebbe  infinite  sorgenti  in  divers!  punti  del  nostro  settentrionale  emisfero, 
forse  successivamente.  pill  folte  ne  luoghi  piu  vicini  ad  alcun  panto  settentrionale ;  e  la  frequenza,  la 
posizione,  o  piuttosto  la  direzione  del  corso  loro  mi  si  rappresenterebbono  dalla  posizione,  frequen- 
za, e  diverzione,  con  che  si  dispongono  intorno  alii  emisferi  di  una  sferica  calamita  le  ordinatissime 
fiize  della  limitura  di  ferro.  E  giasta  ana  tale  ipotesi,  1'aberrazione  del  centro  comane  di  tutte  le 
varie  sorgenti,  che  estenderebbono  la  loro  azione  ad  una  data  ragione,  dal  \ero  punto  settentrionale 
mi  spiegherebbe  1'aberrazione  della  calaraita ;  il  periodo  di  quella  aberrazione  mi  spiegherebbe  il 
periodo  di  questa  declinazione ;  i'obbliquita,  con  che  qaelle  sorgenti  spiccierebbono  da  terra,  e  si 
direggerebbono  verso  mezzo  di,  mi  spiegherebbe  e  la  inclinazione  degli  aghi,  e  la  particolare  fa- 
ciliti  con  che  si  calamitano  i  ferri  si  fattamente  inclinati." — Lettere,  p.  268. 

9  ^ j 


130  ELECTRICITY. 


tricky,  ensue.  Beccaria  verified  this  hypothesis  by  experiment.  He  con- 
structed a  glass  siphon,  in  one  leg  of  which  air  was  enclosed  above  a  column 
of  mercury,  and  compressed  by  the  column  in  the  other  leg  of  the  siphon.  On 
discharging  a  Leyden  jar  through  the  air  thus  enclosed,  the  column  of  mercury 
in  the  other  leg  was  suddenly  elevated,  and  recovered  its  position  after  several 
oscillations.*  This  fact  was  also  noticed  by  Kinnersley,  the  friend  and  asso- 
ciate of  Franklin,  but  not  until  a  later  period. 

This  was  afterward  corroborated  by  Bouguer  and  De  la  Condamine,  when 
they  encountered  a  violent  thunder-storm  on  one  of  the  highest  mountains  of 
Peru.  The  cloud  from  which  the  thunder  proceeded  was  placed  at  but  a  small 
distance  above  their  heads.  The  thunder  heard  by  them  consisted  only  of 
single  cracks,  or  explosions,  like  those  which  attend  the  discharge  of  electric 
batteries ;  an  effect  manifestly  produced  by  the  proximity  of  the  cause  of  the 
sound,  and  the  highly  rarefied  state  of  the  air  at  that  great  elevation. 

Contemporaneously  with  Beccaria,  Franklin,  and  Canton,  the  subject  of  at- 
mospheric electricity  engaged  the  attention  of  Lemonnier,  who  erected  an  ap- 
paraWis  according  to  Franklin's  method  at  St.  Germain-en-Laye,  with  which  he 
showed  that  sparks  were  received  from  the  conductor  not  only  in  times  of 
storm,  but  also  when  the  heavens  were  cloudless.  He  also  first  showed  that 
the  electricity  of  the  air  underwent  every  twenty-four  hours  periodical  varia- 
tions of  intensity. 

Beccaria  determined  the  law  of  these  variations,  and  was  the  first  who  dem- 
onstrated that  at  all  seasons,  at  all  heights,  and  in  every  state  of  the  wind,  the 
electricity  of  an  unclouded  atmosphere  is  positive.  He  found  no  indications 
of  electricity  in  the  air  in  high  winds,  when  the  firmament  was  covered  with 
black  and  scattered  clouds,  having  a  slow  motion  in  a  humid  state  of  the  air  ; 
but  in  the  absence  of  actual  rain,  he  found  that  in  changeable  squally  weather, 
attended  with  occasional  showers  of  snow,  hail,  or  rain,  the  electricity  was  very 
variable,  both  as  to  its  quantity  and  quality,  being  sometimes  feeble  and  some- 
times intense,  sometimes  positive  and  sometimes  negative. 

Contemporaneously  with  Beccaria  in  Italy,  Canton  prosecuted  inquiries  in 
many  respects  similar  in  England,  and  in  various  matters  of  minor  importance 
these  philosophers  arrived  at  the  same  results.     The  most  considerable  dis- 
covery due  to  Canton  was,  that  the  electricity  developed  in  the  friction  of  the 
same  substance  is  not  always  of  the  same  kind.     It  will  be  remembered  that 
Duiaye  gave  the  names  vitreous  and  resinous  to  the  two  fluids,  on  the  supposi-  i| 
tion  that  each  was  invariably  produced  by  the  friction  of  the  classes  of  bodies  Ji 
signified  by  these  terms.     Canton,  however,  showed  that  glass  itself  was  ca-  i 
pahle  of  being  electrified  negatively,  and  would  be  always  so  electrified,  if  the 
rubber  used  were  the  fur  of  a  cat.     Canton  also  (as  well  as  Beccaria)  proved 
that  a  volume  of  air  in  a  quiescent  state  might  be  charged  with  electricity.    To 
Canton  is  also  due  the  discovery  of  the  virtue  of  the  amalgam  of  tin  and  mer- 
cury, still  used  with  so  much  effect  to  augment  the  development  of  electricity 
on  glass. 

The  progress  of  the  science  had  now  attained  a  point  at  which  the  great 
principle  of  induction  could  scarcely  fail  to  force  itself  upon  the  notice  of  those 
engaged  in  electrical  researches.  A  natural  law  of  the  highest  order,  embra- 
cing within  the  range  of  its  application  nearly  the  whole  domain  of  electrical 
phenomena,  its  discovery  and  development,  forms  an  epoch  in  the  history  of 
the  science,  scarcely  second  in  importance  even  to  that  by  which  Franklin 
brought  meteorology  within  the  legislation  of  electricity.  How  much,  then, 
will  the  veneration  in  which  the  memory  of  the  philosopher  of  the  West  is 

*  Beccaria,  Elettricismo  Artificiale.     Turin,  1753 :  p.  227. 


^**»~s^t*t 


ELECTRICITY. 


held  be  increased,  if  it  can  be  demonstrated,  contrary  to  what  has  been  gener- 
ally maintained  by  the  historians  of  the  science,  that  to  him  is  justly  owing  the 
honor  of  the  discovery  of  this  physical  principle  ! 

Some  of  the  more  obvious  phenomena  of  induction  were  noticed  so  early  in 
the  progress  of  electrical  science  as  the  researches  of  Mr.  Grey ;  and  many 
other  effects  proceeding  from  it  presented  themselves  to  subsequent  experi- 
mental inquiries,  but  attracted  no  attention,  and  led  to  no  consequences.  Tho 
first  series  of  experiments,  conducted  so  as  to  develop  in  an  unequivocal  man- 
ner this  principle,  were  laid  before  the  Royal  Society  by  Canton,  on  the  Gth 
of  December,  1753.*  They  consisted  chiefly  in  rendering  insulated  conduc- 
tors electrical,  by  bringing  near  to  one  end  an  excited  glass  tube,  or  stick  of 
wax,  and  exhibiting  the  varying  state  of  cork-balls  suspended  on  the  conductor 
by  the  alternate  approach  and  removal  of  the  excited  electric. 

These  experiments  having  been  communicated  to  Franklin,  he  pursued  the 
inquiry,  and  succeeded  in  expressing,  in  clear  and  unequivocal  terms,  the  prin- 
ciple of  induction  ;  that  is  to  say,  in  demonstrating  that  a  body  charged  with 
either  kind  of  electricity  will,  on  approaching  a  conductor  in  its  natural  state, 
render  that  part  of  such  conductor  which  is  nearest  to  it  electrical ;  that  its 
electricity  will  be  contrary  to  that  of  the  approaching  electrified  body  ;  that  on 
removing  the  electrified  body,  the  conductor  would  be  restored  to  its  natural 
ftate  :  all  which  effects  Franklin  showed  would  follow  from  his  theory,  by  as- 
suming that  the  electric  fluid  is  self-repulsive,  and  attracted  by  the  matter  of 
the  conductor. 

The  experiments  and  reasoning  which  appear  to  establish  Franklin's  right 
to  the  honor  of  this  discovery  are  so  concise,  that  they  may  be  stated  here 
nearly  in  his  own  words. 

Let  a  metallic  conductor,  about  five  feet  long  and  four  inches  in  diameter, 
be  suspended  by  dry  silk  lines,  so  as  to  be  insulated.  From  one  end  of  it  sus- 
pend a  tassel  consisting  of  fifteen  or  twenty  threads  in  a  damp  state,  so  as  to 
give  them  a  conducting  power.  Present  an  electrified  glass  tube  within  five 
or  six  inches  of  the  opposite  end,  and  keep  it  in  that  position  for  a  few  sec- 
onds. The  threads  of  the  tassel  will  diverge,  and  when  the  tube  is  withdrawn 
they  will  collapse. 

While  the  tube  is  held  near  the  opposite  end  of  the  conductor  and  the 
threads  are  divergent,  present  the  finger  to  the  end  of  the  conductor  at  which 
the  tassel  is  suspended.  A  spark  will  be  received,  and  the  threads  of  the 
tassel  will  collapse. 

Let  the  tube  be  then  removed.  The  threads  of  the  tassel  will  again  di- 
Terge. 

Let  the  tube  be  again  presented  as  before.  The  threads  will  again  collapse, 
and  so  on. 

Finally,  let  the  tube  be  presented  to  the  tassel.  The  divergence  of  the 
threads  will  immediately  increase,  and  continue  to  increase,  as  the  tube  is 
brought  nearer  to  the  tassel. 

These  phenomena  are  accounted  for  by  Franklin  in  the  following  manner: 
'  By  taking  the  spark  from  the  end  of  the  conductor,  you  rob  it  of  part  of  its 
natural  quantity  of  electrical  matter,  which  part  so  taken  away  is  not  supplied 
by  the  glass  tube,  and  the  conductor  remains  negatively  electrified.  On  with- 
drawing the  tube,  the  electric  matter  on  the  conductor  recovers  its  equilibrium, 
or  equal  diffusion  ;  and  the  conductor  having  lost  some  of  its  natural  ••luctricity, 
the  threads  connected  with  it  lose  part  of  theirs,  and  so  are  electriiiod  nega- 
tively, and  repel  each  other. 

•  Phil.  Trana.,  vol.  xlviii.,  p.  350. 


132 


ELECTRICITY. 


"  When  the  tube  is  again  presented  to  the  opposite  end  of  the  conductor,  the 
part  of  the  natural  electricity  which  the  threads  had  lost  is  again  restored  to 
them  by  the  repulsion  of  the  tube  forcing  the  electric  fluid  toward  them  from 
other  parts  of  the  conductor,  and  thus  restoring  them  to  their  natural  state. 
When  the  tube  is  once  more  withdrawn,  the  fluid  is  again  equally  diffused,  and 
tho  threads,  as  before,  are  negatively  electrified. 

"  Finally,  when  the  tube  is  presented  to  the  threads  already  diverging  with 
negative  electricity,  still  more  of  their  natural  electricity  is  repelled  by  the  ex- 
cited tube,  and  the  threads  are  more  strongly  negative  than  before,  and  their 
divergence  is  consequently  augmented." 

Pursuing  the  principle  thus  developed  still  further,  Franklin  now  having  re- 
stored the  conductor  to  its  natural  state,  presented  the  excited  glass  tube  to  the 
tassel.  The  threads  immediately  diverged. 

Maintaining  the  tube  in  that  position  with  one  hand,  he  presented  the  finger 
of  the  other  to  the  tassel.  The  threads  receded  from  the  finger  as  if  repelled 
by  it. 

This  was  explained  on  the  same  principle.  When  the  excited  tube  is  pre- 
sented to  the  tassel,  part  of  the  natural  electricity  of  the  threads  is  driven  out 
of  them  into  the  conductor,  and  they  are  negatively  electrified,  and  therefore 
repel  each  other.  When  the  finger  is  presented  to  the  tassel  (being  then  close 
to  the  glass  tube),  part  of  its  natural  electricity  is  driven  back  through  the 
hand  and  body,  and  the  finger  becomes,  as  well  as  the  threads,  negatively  elec- 
trified, and  so  repels,  and  is  repelled  by  them.  To  confirm  this,  hold  a  slender 
light  lock  of  cotton,  two  or  three  inches  long,  near  a  conductor  positively  elec- 
trified. You  will  see  the  cotton  stretch  itself  out  toward  the  conductor.  At- 
tempt to  touch  it  with  the  finger  of  the  other  hand,  and  it  will  be  repelled  by 
the  finger.  Approach  it  with  a  positively-charged  wire  of  a  bottle,  and  it  will 
fly  to  the  wire.  Bring  it  near  a  negatively-charged  wire  of  a  bottle,  it  will 
recede  from  that  wire  in  the  same  manner  that  it  did  from  the  finger,  which 
demonstrates  that  the  finger  was  negatively  electrified  as  well  as  the  cotton.* 

The  great  principle  thus  thrown  before  the  scientific  world  by  Franklin,  was 
immediately  taken  up  and  pursued  through  its  consequences  by  Wilke  and 
^Epinus,  who  carried  on  their  researches  together  at  Berlin.  The  most  im- 
portant result  of  their  combined  labors  was  the  invention  of  the  instrument, 
which,  as  subsequently  improved  under  the  hands  of  Volta,  became  the  CON- 
DENSER now  so  useful  in  electroscopical  investigations. 

In  applying  the  principle  of  induction  to  the  phenomena  of  the  Leyden  jar, 
and  to  the  same  effects  as  exhibited  by  the  oppositely  electrified  surfaces  of  a 
coated  plate  of  glass,  these  philosophers  saw  that  the  negative  state  of  one  sur- 
face of  the  glass  was,  according  to  the  Franklinian  theory,  the  necessary  con- 
sequence of  the  positive  state  of  the  other.  This  contrary  state  of  the  elec- 
tricities could  only  be  maintained  on  the  supposition  that  glass  was  imperme- 
able by  the  electric  fluid ;  and  Wilke  and  jEpinus  reasoned,  that  to  whatever 
extent  air  or  any  other  body  might  be  similarly  impermeable,  to  the  same  ex- 
tent might  it  be  charged  on  its  opposite  surfaces.  To  realize  this  conception 
with  a  plate  of  air,  they  coated  two  large  boards  of  equal  size  with  tin-foil,  and 
suspended  them  one  over  the  other,  leaving  a  space  of  about  an  inch  in  thick- 
ness between  them.  This  space  was,  in  fact,  a  plate  of  air,  of  which  the  up- 
per and  lower  surfaces  were  in  contact  with  the  metallic  coating  of  the  boards. 
The  lower  board  communicated  with  the  ground,  and  a  charge  of  positive 
electricity  was  given  to  the  upper  one.  The  lower  one  then  became  charged 
with  negative  electricity ;  and  when  a  person  touched  at  the  same  time  the 


ELECTRICITY.  133 


coating  of  the  two  boards,  the  equilibrium  was  re-established,  and  he  received 
the  shock  produced  by  the  passage  of  the  electric  fluid  from  the  one  to  the  other. 

Many  curious  experiments  were  exhibited  with  this  apparatus.  They  found 
that  the  two  boards,  when  electrified,  strongly  attracted  each  other,  and  would 
have  rushed  together  if  they  had  not  been  prevented  by  the  strings.  Some- 
times, when  the  charge  was  strong,  the  intervening  plate  of  air  was  not  suf- 
ficiently impermeable  to  resist  the  mutual  attraction  of  the  opposite  electricities, 
and  a  spontaneous  discharge  would  take  place  through  it.  They  considered 
these  two  plates  to  represent  the  state  of  the  clouds  and  the  earth  during  a 
thunder-storm  ;  the  clouds  being  always  charged  with  one  kind  of  electricity, 
and  the  earth  with  the  other,  while  the  body  of  atmosphere  between  them  was 
analogous  to  the  stratum  of  air  between  the  two  boards.  When  the  charges 
of  the  earth  and  clouds  become  so  strong  that  the  air  can  no  longer  resist  the 
passage  of  the  electric  fluid  through  it,  a  spontaneous  discharge  ensues,  the 
fluid  is  seen  in  its  passage  by  the  light  it  evolves,  and  the  violent  displacement 
of  the  air  produced  in  its  passage  causes  the  thunder. 

From  these  experiments,  ./Epinus  inferred  that  the  phenomena  of  the  Leyden 
jar  was  not  owing,  as  Franklin  supposed,  to  any  peculiar  attraction  of  the 
glass  for  the  electric  fluid  ;  for,  since  a  plate  of  air  might  be  charged  as  well 
as  a  plate  of  glass,  that  property  must  be  common  to  them,  and  was  not  pecu- 
liar to  the  glass.  He  inferred,  therefore,  that  this  impermeability  was  a  prop- 
erty of  all  non-conductors  ;  and,  since  they  can  all  receive  electricity  to  a  cer- 
tain degree,  it  must  consist  in  the  difficulty  and  slowness  with  which  the  elec- 
tric fluid  moves  in  their  pores,  whereas,  in  perfect  conductors,  it  meets  with 
no  obstruction  at  all.* 

./Epinus  brought  to  the  investigation  of  the  Franklinian  theory  of  electricity 
those  mathematical  attainments  in  which  its  illustrious  founder  was  deficient. 
The  manner  in  which  that  theory  had  been  assailed  by  its  opponents,  and  de- 
fended by  its  partisans,  was  such  as  might  have  allowed  interminable  contro- 
versy. ./Epinus  first  reduced  its  principles  to  exact  mathematical  statement, 
with  a  view  to  ascertain  whether  the  consequences  deducible  from  them,  by 
rigorous  calculation,  should  be  in  accordance  with  the  observed  phenomena, 
not  only  in  their  general  character,  but  in  their  numerical  quantity.  He  as- 
sumed, according  to  Franklin's  hypothesis,  that  the  molecules  of  the  electric 
fluid  were  self-repulsive,  and  that  they  were  attracted  by  those  of  the  bodies 
on  which  they  were  diffused.  He  found,  however,  that  the  phenomena  could 
not  be  explained  on  these  suppositions,  unless  it  were  also  assumed  that  be- 
tween the  matter  composing  the  masses  of  different  bodies  there  existed  a  mu- 
tually repulsive  force,  acting  at  sensible  distances.  At  first  he  recoiled  from  an 
assumption  in  direct  opposition  to  the  known  properties  of  matter ;  but  the  ne- 
cessity of  its  admission,  in  order  to  give  consistency  and  validity  to  the  Frank- 
linian theory,  appears  at  length  to  have  reconciled  him  to  it. 

Ths  investigation  of  the  physical  relation  between  the  principle  of  heat  and 
that  of  electricity,  had  attracted  the  attention  of  experimental  philosophers  at  a 
very  early  period  in  the  history  of  electrical  research.  Beccaria  suspected 
that  heat  might  itself  be  an  immediate  means  for  the  development  of  electricity, 
and  made  some  experiments  to  illustrate  this.  He  soon,  however,  relinquished 
the  inquiry,  concluding  that,  in  cases  where  the  appearance  of  electricity  fol- 
lowed the  application  of  heat,  the  effect  was  due  to  evaporation,  or  other 
physical  agents,  which  ensued.  Priestley  observed  that  heat  had  some  relation 
to  the  conducting  power  of  bodies,  since,  by  the  elevation  of  temperature,  that 
quality  was  improved. 

*  jEpini  Tentamcn,  &c.    Petersburg,  1759,  p.  82,  83. 


134  ELECTRICITY. 


A  mineral  substance,  brought  from  the  east  by  the  Dutch  navigators,  called 

S  by  the  natives  of  Ceylon,  where  chiefly  it  was  found,  Tournamal,  and   since 

?  known    as    Tourmaline,  exhibited,  under   certain    circumstances,   a  property 

\  similar  to  that  of  amber,  and  other  electrics.     But  the  power  was  excited  in  it 

/  by  mere  elevation  of  temperature.     Lemery,  the  Due  de  Noia,  Wilson,  Priestley, 

s  and  others,  made  experiments  on  this  mineral,  and  published  results,  in  which 

/  there  were  much  discordance  and  contradiction.     ./Epinus  first  showed  that  the 

(  attraction  and  repulsion  exerted  by  this  gem  when  exposed  to  heat  were  owing 

)  to  the  development   of  electricity  upon  it ;  and  that,  when  so  excited,  its  op- 

S  posite  sides  or  ends  had  contrary  kinds  of  electricity,  one  being  always  nega- 

?  tive  and  the  other  positive.     This  was  the  first  case  of  the  distinct  exhibition 

S  of  electrical  polarity.     Canton  observed  that  the  development  of  the  electric 

/  fluid  upon  it  was  produced  only  by  change  of  temperature,  and  that  whenever 

S  the  gem  was  broken  each  fragment  exhibited  the  same  electrical  polarity. 
}       At  this  period  effects  were  observed,  which,  if  chemical  science  had  attained 

\  a  sufficiently  advanced  state,  could  not  fail   to  have   led   to  the  discovery  of 

(  electro-chemistry.     Beccaria,  by  the  electric  spark,  decomposed  the  sulphuret 

)  of  mercury,  and  recovered  the  metals,  in   some  instances,  from  their  oxides.* 

c  Watson  found  that  an  electric  discharge  passing  through  fine  wire  rendered  it 

5  incandescent,  and  that  it  was  even  fused  and  burned.     Canton,  repealing  these 

<  experiments  with  brass  wire,  found  that,  after  the   fusion   by  electricity,  drops 

>  of  copper  only  were  found,  the  zinc  having  apparently  evaporated.     Beccaria 

<  observed  that  when  the  electric  spark  was  transmitted  through  water,  bubbles 
)  of  gas  rose  from  the  liquid,  the  nature  or  origin  of  which  he  was  unable  to  de- 

<  termine.     Had  he  suspected  that  water  was  not  what  it  was  then  supposed  to 

>  be,  a  simple   elementary  substance,  the  discovery  of  its   composition   could 

<  scarcely  have  eluded  his  sagacity. 

)       After  general  laws  have  once  been  developed,  and  their  application  to  par- 

(  ticular  phenomena  has  become  familiar,  it  appears  wonderful  that  even  quick- 

>  sighted  and  acute  observers  should  have  had  such  effects  continually  repro- 
(  duced  under  their  eyes,  without  even  making  an  approach  to  the  discovery  of 

>  their  causes.     Franklin  found  that  the  frequent  application  of  the  electric  spark 

<  had  eaten  away  iron  ;  on  which  Priestley' observed,  that  it  must  be  the   effect 
)  of  some  acid,  and  suggested  the  inquiry,  whether  electricity  might  not  probably 

<  redden  vegetable  blues  ?     Priestley  also  observed  that  in  transmitting  electricity 
)  through  a  copper  chain,  a  black  dust  was  left  on  the  paper  which  supported 

<  the  chain  at  the  points  where  the  links  touched  it ;  and,  on  examining  this 
)  dust,  he  found  it  to  contain  copper. 

<  Some  years  after  the  invention  of  the  Leyden  jar,  when  the  necessity  of 

>  some  sufficient  indicator  of  the  presence  of  electricity,  and  some  visible  meas- 
(  ure  of  its  power  became  apparent,  the  invention  of  electrometers  engaged  the 
/  attention  of  electricians.     After  several  abortive  attempts  on  the  part  of  others, 
^  the  Abbe  Nollet  proposed  the  simple  expedient  of  suspending  two  threads, 
?  which,  when  electrified,  would  separate  by  their  mutual  repulsion.     Cavallo 
S  afterward  improved  upon  this,  by  substituting  two  pith  balls,  suspended  in  con- 
j  tact  by  fine  metallic  wires— an  apparatus  still  used.     After  this,  various  formr 
j  of  electroscopic  instruments  were  suggested  and  constructed  by  Volta,  Saus- 
?  sure,  and  others,  all  depending  on  the  principle  that  the  intensity  of  the  elec 

S  trie  fluid  was  manifested  by  the  force  of  its  attraction  or  repulsion  exerted  upon 

?  light  substances  to  which  it  was  imparted. 

j       The  principle  of  induction  applied  to  the  air-condenser  by  Wilke  and  ^Epi- 

?  nus,  was  taken  up  by  Volta,  and  applied,  first,  to  the  constructor  of  the  ELEC- 

*  Leltere  del  Elettricismo,  $  341,  p.  282. 


ELECTRICITY. 


135 


TROPHORUS,  and  subsequently  to  the  common  CONDENSEK,  which,  combined 
with  the  electroscope,  became  in  electricity  an  instrument  of  investigation 
analogous  in  its  character  and  importance  to  the  compound  microscope  in  optics. 

The  manner  in  which  the  electrified  fluid  is  distributed  upon  insulated  elec- 
trified conductors  next  became  the  subject  of  inquiry.  Beccaria  showed  that 
its  distribution  is  superficial,  and  that  the  internal  parts  of  the  electrified  body 
are  in  their  natural  state.  It  was  shown  that,  whether  the  electrified  conduc- 
tor were  hollow  or  solid,  the  electricity  contained  on  it  was  the  same.  Le- 
monnier  first  showed  that  the  form  of  the  conductor  had  an  influence  on  the  quan- 
tity and  the  distribution  of  the  fluids. 

In  1778  Volta  published  a  memoir  on  this  subject,  in  which  he  proved,  that 
of  two  cylinders  of  equal  superficial  dimensions,  that  which  had  the  greater 
length  would  receive,  cater  is  paribus,  the  stronger  charge,  and  inferred  that 
great  advantage  would  arise  from  the  substitution  of  a  system  of  small  cylin- 
ders for  the  large  conductors  of  electrical  machines.  About  the  same  period, 
he  showed  how  inflammable  gases  could  be  ignited  in  close  glass  receivers  by 
the  electric  spark,  the  apparatus  for  which  purpose  soon  grew  into  his  eudiom- 
eter, for  the  analysis  of  gases.  Soon  after  this,  the  same  apparatus  supplied 
the  means  of  inflaming  a  mixture  of  oxygen  and  hydrogen  gas,  which  led  to 
the  discovery  of  the  composition  of  water. 

In  the  year  1759  appeared,  in  the  "  Philosophical  Transactions,"  a  series  of 
papers  by  Mr.  Robert  Symmer,  which  are  entitled  to  be  recorded  in  the  histo- 
ry of  electricity ;  not  so  much  on  account  of  what  they  describe,  as  for  the 
theoretical  views  developed  in  them.  The  experiments  of  Symmer  consisted 
chiefly  in  exhibiting,  by  striking  examples,  the  effect  of  the  mutual  attraction 
of  bodies  electrified  by  opposite  kinds  of  electricity.  These  results  led  him 
to  doubt  the  sufficiency  of  the  Franklinian  theory,  then  and  long  afterward  uni- 
versally received,  to  explain  satisfactorily  the  phenomena ;  and  he  was  led  to 
consider  whether  the  hypothesis  of  Dufaye  might  not  be  so  modified  as  to  ex- 
plain them  more  adequately.  Dufaye,  as  has  been  already  stated,  assumed  the 
existence  of  two  independent  electric  fluids,  which  he  supposed  to  be  latent 
in  two  distinct  classes  of  bodies,  the  one  in  bodies  of  a  vitreous,  and  the  other 
in  bodies  of  a  resinous  nature ;  and  that  these  fluids,  while  they  were  each 
self-repulsive,  were  mutually  attractive  of  each  other. 

It  was  obvious  that  such  an  hypothesis  was  quite  inconsistent  with  the  known 
phenomena  of  electricity,  even  limited  as  they  were  in  variety  at  the  period 
now  referred  to.  Symmer  retained  the  supposition  of  Dufaye  so  far  as  regard- 
ed the  assumed  existence  of  two  distinct  fluids  mutually  attractive,  but  he  main- 
tained that  these  fluids  were  not  independent  of  each  other.  On  the  contrary, 
he  assumed  that  they  were  always  co-existent  in  bodies  not  electrified ;  that, 
by  their  natural  attraction,  they  held  each  other  in  subjection ;  that  every  body 
in  its  natural  state  contained  equal  quantities  of  these  fluids,  each  molecule  of 
the  vitreous  fluid  being  combined  with  a  molecule  of  the  resinous  fluid,  the 
compound  molecule  thus  formed  exciting  neither  attraction  nor  repulsion  on  the 
other  parts  of  the  natural  fluid. 

This  theory  of  two  fluids  was  left  by  its  author  unsupported  by  any  exten- 
sive application  to  the  phenomena  which  could  be  expected  to  shake  the  con- 
fidence then  generally  given  to  the  hypothesis  of  Franklin ;  and  although  it  is 
noticed  at  some  length  in  his  history  of  electricity  by  Dr.  Priestley,  it  obtained 
no  countenance  or  support  until  further  advances  in  electrical  experiments  ren- 
dered apparent  the  defects  of  the  theory  of  a  single  fluid.  It  may  be  here  ob- 
served, that  the  French  writers  generally  ascribe  the  theory  of  two  fluids  to 
Dufaye,  and  are  silent  as  to  Symmer's  share  in  it ;  with  what  justice  will  be 
apparent  from  what  has  been  above  stated. 


136  ELECTRICITY. 


In  the  year  1770,  Dr.  Priestley  published  his  works  on  electricity.  This 
philosopher  did  not  contribute  materially  to  the  advancement  of  the  science  by 
the  development  of  any  new  facts  ;  but  in  his  History  of  Electricity  he  collected 
and  arranged  much  useful  information  respecting  the  progress  of  the  science. 
At  this  period  the  Honorable  Henry  Cavendish,  whose  name  has  been  distin- 
guished in  other  departments  of  physics,  engaged  in  some  original  investiga- 
tions respecting  electricity.  The  discovery  of  the  composition  of  water,  by 
transmitting  an  electric  spark  through  a  mixture  of  oxygen  and  hydrogen  gases, 
has  been  generally  ascribed  to  him.*  Cavendish  conceived  the  notion  of  re- 
ducing the  phenomena  of  electricity  to  mathematical  analysis,  and  had  pro- 
ceeded with  a  memoir  on  that  subject,  which  was  completed  before  he  learned 
that  ^Epinus  had  produced  a  work  with  the  same  object.  On  comparing  his 
own  paper  with  the  Tentamen  of  ^Epinus,  he  found  that  they  were  nearly  simi- 
lar. Nevertheless,  Cavendish  published  his  memoir. 

The  year  1785  formed  an  important  epoch  in  the  history  of  electrical  sci- 
ence, marking,  as  it  did,  the  commencement  of  those  labors  by  which  Coulomb 
i;iid  the  foundations  of  ELECTRO-STATICS.  This  great  experimental  philoso- 
pher was  the  first  who  really  brought  the  phenomena  of  electricity  within 
the  reach  of  numerical  calculation,  and  thereby  prepared  the  way  for  his  fol- 
lowers in  the  same  field  to  reduce  this  most  subtle  of  all  physical  agents  to  the 
rigorous  sway  of  mathematics.  It  is  to  Coulomb  we  owe  it  that  statical  elec- 
tricity is  now  a  branch  of  mathematical  physics. 

The  immediate  instrument  by  which  this  vast  object  was  attained  was  the 
balance  of  torsion,  which  he  had  already  used  with  signal  success  in  other  deli- 
cate physical  inquiries.  This  apparatus,  which  will  be  fully  explained  in  the 
following  pages,  consisted  of  a  needle  suspended  in  a  horizontal  position  by  an 
exceedingly  fine  wire  or  filament  of  silk  attached  to  its  centre  of  gravity.  The 
attraction,  or  other  force  of  which  the  intensity  is  to  be  measured,  is  made  to 
act  on  one  end  of  this  needle,  so  as  to  twist  the  filament  by  which  it  is  sus- 
pended ;  and  it  is  resisted  in  its  effort  to  effect  this  by  the  reaction  proceeding 
from  the  torsion  so  produced.  This  reaction,  and  therefore  the  force  which 
produces  it,  and  is  in  equilibrium  with  it,  was  proved  by  Coulomb  to  be  pro- 
portionate to  the  angle  described  by  the  needle  round  its  centre  of  motion. 
Such  was  the  sensibility  of  this  exquisite  instrument,  that  it  was  found  to  be 
perceptibly  affected  by  a  force  not  exceeding  the  twenty-millionth  part  of  a 
grain. 

"With  this  instrument  Coulomb  measured  the  force  with  which  electrified 
bodies  attract  and  repel  each  other ;  and  the  first  result  of  this  investigation 
was  the  discovery,  that  the  law  of  this  attraction  and  repulsion  was  the  same 
which  Newton  showed  to  prevail  among  the  great  bodies  of  the  universe.  In 
fact,  he  showed  that  two  bodies,  oppositely  electrified,  attract  each  other  with 
a  force  which,  c&teris  paribus,  is  the  same  at  equal  distances,  and  which  aug- 
ments in  the  same  proportion  as  that  in  which  the  square  of  the  distance  is  di- 
minished. Also  if  two  bodies  be  similarly  electrified,  they  will  repel  each 
other  by  a  force  which  increases  according  to  the  same  proportion  when  the 
distance  between  them  is  diminished. 

By  attaching  a  very  small  circular  disk  of  paper  coated  with  metallic  foil  to 
an  insulating  handle,  Coulomb  found  that  by  touching  with  the  face  of  the  disk 
an  electrified  surface,  and  then  submitting  the  disk  itself  thus  electrified  by 
contact  to  the  test  of  the  balance  of  torsion,  he  could  determine  the  depth  of 
the  electric  fluid  on  the  surl'ace'touched  by  the  disk.  In  this  manner  was  he 
enabled  to  gauge  or  sound  the  electricity  on  the  surface  of  bodies,  so  as  to  com- 

•  This  claim  has  been  recently  called  in  question.— See  Larduer  on  the  Steam-Engine.  Seventh 
Edition,  p.  303. 


ELECTRICITY. 


137 


pare  numerically  its  depth  on  different  bodies,  or  on  different  parts  of  the  same 
body. 

With  this  instrument  he  measured  the  proportion  in  which  electricity  was 
shared  between  insulated  conductors  when  brought  into  contact,  and  also  the 
law  according  to  which  its  depth  varied  on  different  parts  of  the  same  insulated 
conductor.  These  results  acquired,  at  a  later  period,  still  greater  importance, 
supplying,  as  they  did,  tests  by  which  the  mathematical  analysis  of  the  science 
could  be  tried. 

The  same  apparatus  supplied  the  means  of  investigating  the  law  according  to 
which  an  insulated  electrified  conductor  had  its  charge  gradually  diminished  by 
dissipation  in  the  surrounding  air,  and  by  the  escape  of  the  fluid  by  the  imper- 
fect insulation  of  the  supports. 

The  results  of  the  observations  of  Coulomb  on  the  distribution  of  the  elec- 
tric fluid  on  the  surfaces  of  conductors  illustrated  satisfactorily  the  doctrine  of 
points,  which  formed  so  prominent  a  part  of  Franklin's  researches.  The  the- 
oretical solution  of  this  problem  was  not,  however,  effected  till  a  later  period. 

The  demonstration  of  the  identity  of  lightning  and  electricity  naturally  di- 
rected the  attention  of  philosophers  to  the  solution  of  other  meteorological  phe- 
nomena by  means  of  the  same  agency.  The  explanation  of  the  aurora  borealis 
had  long  exercised  the  sagacity  and  baffled  the  attempts  of  those  devoted  to 
physical  researches.  Some  ascribed  this  appearance  to  solar  light  refracted 
in  the  higher  regions  of  the  air,  others  assigned  it  to  the  agency  of  the  mag- 
netic fluid.  Euler  imagined  it  to  proceed  from  the  same  ether  which  formed 
the  tails  of  comets  ;  Mairan  conceived  it  to  arise  from  the  mixture  of  the  at- 
mosphere of  the  sun  with  that  of  the  earth ;  but  when  the  properties  of  elec- 
tric light  became  known,  and  when  its  appearance  in  rarefied  air  had  been  ob- 
served, all  these  hypotheses  were  by  common  consent  abandoned,  and  no 
doubt  was  entertained  that,  whatever  might  be  the  details  of  the  natural  process 
by  which  it  was  produced,  the  aurora  borealis  was  an  effect  of  atmospheric 
electricity.  Eberhart,  professor  at  Halle,  and  Paul  Frisi  at  Pisa,  were  the  first 
who  proposed  an  explanation  of  it,  founded  on  the  following  facts  :  "1.  Elec- 
tricity transmitted  through  rarefied  air  exhibits  a  luminous  appearance,  precise- 
ly similar  to  that  of  the  aurora  borealis." — "  2.  The  strata  of  atmospheric  air 
become  rarefied  as  their  altitude  above  the  surface  of  the  earth  is  increased." 
Hence  they  argued  that  the  aurora  is  nothing  more  than  electrical  discharges 
transmitted  through  parts  of  the  upper  regions  of  the  atmosphere,  so  rarefied 
as  to  produce  that  peculiar  luminous  appearance  which  they  exhibit.  This 
theory,  which  was  embraced  and  improved  in  its  details  by  Canton,  Beccaria, 
Wilke,  Franklin,  and  other  contemporary  electricians,  has  received  further 
countenance  from  more  recent  researches. 

Attempts  were  also  made  to  explain  on  electrical  principles  other  meteorolo- 
gical effects ;  such  as  waterspouts,  whirlwinds,  rain,  fogs,  hail,  &c.,  but  no 
satisfactory  conclusions  resulted  from  these  investigations,  and  the  discussion 
of  such  phenomena  forms  a  part  of  the  meteorological  inquiry  of  the  present 
time. 

While  the  series  of  experimental  researches  which  have  just  been  related 
were  in  progress,  many  attempts  were  made  to  trace  electricity  in  the  phenom- 
ena of  vegetable  and  animal  life,  and  more  especially  to  apply  it  as  a  medical 
agent  in  cases  of  organic  disease  in  the  animal  system.  None  of  these  at- 
tempts, however,  led  to  any  consequences  sufficiently  important  to  entitle  them 
to  attention  in  this  brief  sketch. 

After  electroscopes  had  been  much  improved,  and  in  their  application  to  at- 
mospheric electricity  had  derived  great  power  from  the  addition  of  a  long 
pointed  conductor,  extending  from  the  diverging  balls  to  a  height  of  several 


138  ELECTRICITY. 


feet,  Volta  engaged  in  the  investigation  of  the  electric  state  of  the  air.  He 
substituted  for  the  suspended  balls  two  blades  of  dry  straw,  hanging  in  contact 
and  communicating  with  the  lower  end  of  the  conducting  rod.  In  addition  to 
this,  he  had  recourse  to  another  apparently  strange  and  unusual  expedient.  He 
placed  on  the  point  of  the  rod  a  taper,  so  as  to  cause  this  conductor  to  termi- 
nate in  a  flame.  He  contended  that  the  flame  attracted  to  the  point  of  the 
conductor  three  or  four  times  as  much  electricity  as  would  be  collected  in 
its  absence.  This  was  explained  by  the  effect  of  the  vertical  current  of  air 
which  the  flame  maintained  directly  over  it,  which  established  a  better  com- 
munication between  the  metallic  conductor  and  the  strata  of  air  above  it. 

Assuming  this  property  of  flame,  Volta  argued,  that  since  fires  robbed  the  at- 
mosphere above  them  of  electricity  faster  and  more  effectually  than  metallic 
points,  it  must  follow  that,  to  prevent  coming  storms,  or  to  mitigate  their  force, 
the  best  expedient  would  be  to  light  enormous  fires  in  the  middle  of  extensive 
plains,  or,  better  still,  on  elevated  stations.  If  the  effects  of  the  lamp  on  the 
atmospheric  electrometer  were  admitted,  there  would  be  nothing  unreasonable 
in  the  supposition  that  large  fires  may,  in  a  short  interval  of  time,  rob  immense 
volumes  of  air  and  vapor  of  their  electricity. 

Volta  wished  to  submit  this  theory  to  an  experiment  on  a  large  scale,  but 
Avas  not  able  to  carry  the  design  into  effect.  M.  Arago  suggested,  that  by  ma- 
king suitable  meteorological  observations  in  those  parts  of  Staffordshire  and 
other  English  counties  which  abound  in  vast  iron  furnaces,  where  fires  of  ex- 
traordinary magnitude  are  maintained  night  and  day,  and  comparing  the  results 
with  similar  observations  made  in  adjoining  agricultural  districts,  the  conjec- 
ture of  Volta  might  be  tested.* 

Observations  of  this  kind  have  accordingly  been  recently  made  both  in  Eng- 
land arid  in  certain  parts  of  Italy,  the  results  of  which  will  be  explained  at  the 
proper  place  in  this  volume. 

It  has  been  already  stated,  that  direct  observations  proved  that  the  atmo- 
sphere, in  its  ordinary  condition,  is  always  charged  with  positive  electricity. 
The  beginning  of  the  year  1780  was  signalized  by  a  capital  experiment,  by 
which  it  was  proved  that  the  source  whence  this  vast  amount  of  the  electric 
fluid  was  derived,  or,  to  speak  more  correctly,  the  cause  of  the  disturbance  of 
the  general  equilibrium  of  the  globe,  which  gives  a  surplus  of  the  positive  fluid 
to  the  air,  and  leaves  the  earth  surcharged  with  negative  fluid,  and  which,  in  its 
effects,  assumes  all  the  terrific  forms  of  the  tempest  and  the  hurricane,  and 
piobably  of  many  other  violent  convulsions  which  are  occasionally  exhibited  in 
the  war  of  the  elements,  is  to  be  found  in  the  process  of  natural  evaporation, 
which  continually  maintains  its  silent  and  imperceptible  progress  upon  the  sur- 
faces of  ocean,  lake,  and  river,  and  even  upon  those  of  organized  bodies.  That 
heat  passes  off  in  a  latent  form  by  such  means,  and  equalizes  and  moderates 
the  general  temperature  around  us,  was  well  known ;  but  it  was  not  suspected 
that  the  elements  of  the  storm,  the  coruscations  of  meteoric  light,  and  the  splen- 
dors of  he  aurora,  were  due  to  the  same  cause. 

Volta  states,  that  in  the  year  1778  this  idea  occurred  to  him,  and  that  he  j 
conceived  the  notion  of  an  experiment  by  which  it  might  be  brought  to  an  im-  ] 
mediate  trial.    Let  a  metallic  dish  filled  with  water  be  placed  on  an  insulating  ) 
support,  and  exposed  in  the  open  air  until  it  evaporates,  the  dish  being  main-  < 
tained  in  communication  with  a  sufficiently  sensible  condensing  electroscope.  | 
If,  in  evaporating,  the  positive  fluid  be  carried  off,  the  dish  will,  after  the  evap- 
^  oration,  be  negatively  electrical,  and  the  electroscope  will  show  it ;  if  no!,  the 
(  electroscope  will  give  no  sign.     Various  circumstances  prevented  Volta  from 
j  trying  this  experiment  until  the  month  of  March,  1780,  when,  being  in  Paris, 

*  Eloge  de  Volta,  p.  18. 

lw^ 


ELECTRICITY. 


139 


lie  succeeded,  in  company  with  some  members  of  the  Academy  of  Sciences. 
There  appears,  nevertheless,  to  remain  some  doubt  as  to  the  share  which 
Volta  really  had  in  this  famous  experiment,  since,  in  the  account  of  it  pub- 
lished by  Lavoisier  and  Laplace,  it  is  related  as  performed  by  them,  and  Volta 
is  mentioned  incidentally  as  being  present  on  the  occasion.* 

After  the  phenomena  of  electricity  had,  by  the  labors  of  Coulomb,  been  re- 
duced to  exact  numerical  estimation,  this  branch  of  physics  was  in  a  state  to 
permit  its  being  brought  within  the  pale  of  mixed  mathematics.  To  accom- 
plish this  it  was  necessary  to  express,  by  mathematical  formulae,  the  intensity 
of  the  electric  fluid  on  different  parts  of  insulated  conductors  of  given  forms, 
placed  either  separately,  or  in  such  a  position  as  to  exercise  an  electrical  in- 
fluence upon  each  other  without  contact,  or,  finally,  when  placed  in  actual  con- 
tact. To  establish  such  formulae,  it  was  necessary  to  assume  some  definite 
hypothesis  as  the  law  of  electrical  action.  The  Franklinian  theory  of  a  single 
fluid  appeared  to  be  incapable  of  affording  the  means  of  explaining,  with  numer- 
ical precision,  the  state  of  such  bodies.  It  is  true  that  this  long-received  hy- 
pothesis was  sufficient  to  account,  in  a  general  way,  for  the  electrical  state  of 
bodies  under  the  ordinary  circumstances  of  their  mutual  action ;  but  when  rig- 
orous numerical  accuracy  was  demanded — when  not  merely  the  general  cir- 
cumstances of  the  dense  accumulation  of  electricity  in  one  part  of  the  surface, 
its  more  feeble  intensity  at  another,  its  total  abstinence  from  a  third  place, 
or  the  presence  of  negative  electricity  on  a  certain  side  of  a  conductor,  and  pos- 
itive electricity  on  another,  were  severally  demanded  ;  but  when  it  was  required 
to  determine  the  exact  numerical  measure  of  the  depth  of  the  fluid  at  each  particu- 
lar spot  on  a  given  insulated  conductor,  placed  under  given  conditions  with  ref- 
erence to  others,  so  that  such  numerical  measure,  so  obtained  by  calculation, 
might  be  compared  with  the  actual  depth  observed  by  the  instruments  invented 
and  applied  by  Coulomb,  then  this  theory  appeared  to  fail ;  at  least,  none 
of  its  advocates  produced  any  such  calculations.  Laplace  investigated,  on 
mathematical  principles,  the  distribution  of  electricity  on  ellipsoids  of  revolu- 
tion, assuming,  as  the  basis  of  his  reasoning,  the  hypothesis  of  two  fluids.  Biot 
also  investigated  the  same  problem  applied  to  spheroids  of  small  eccentricity  ; 
but  the  general  subjugation  of  this  portion  of  electrical  science  to  mathematical 
analysis  is  due  to  Poisson. 

This  illustrious  analyst  took  as  the  basis  of  his  investigations  the  theory  of 
two  fluids  proposed  by  Symmer  and  Dufaye,  with  such  modifications  and  addi- 
tions as  were  suggested  by  the  researches  of  Coulomb.  He  regarded  the 
mutual  attractions  and  repulsions  exhibited  by  electrified  bodies,  not  as  real 
forces  exercised  by  those  bodies,  but  as  altogether  due  to  the  electric  fluids 
with  which  they  are  charged.  The  laws  of  attraction  and  repulsion  devel- 
oped by  Coulomb  are  therefore  assumed  as  those  of  the  electric  fluids.  The 
particles  of  each  of  these  fluids  are  assumed  to  repel  each  other  with  a  force 
varying  according  to  that  law,  while  the  particles  of  each  fluid  attract  those  of 
the  contrary  fluid  by  a  force  governed  by  the  same  law.  These  conditions 
are  sufficient  to  supply  the  mathematical  formulae  necessary  to  the  determi- 
nation of  the  depth  and  quality  of  the  electric  fluid  on  every  part  of  the  surface 
of  a  body  of  given  figure  placed  under  any  given  electrical  conditions.  The 
electric  fluids  of  either  kind  would,  by  virtue  of  their  self-expansive  property, 
escape  from  the  surface  of  the  body  on  which  they  rest ;  but  this  is  prevented 
by  the  pressure  of  the  surrounding  air,  which  retains  them  in  their  position  so 
long  as  their  expansive  force  is  less  than  that  pressure.  On  bodies  of  elonga- 
ted forms,  or  those  which  have  edges,  corners,  or  points,  it  is  shown,  as  a  con- 
sequence of  this  theory,  that  the  electric  fluid  accumulates  in  greater  depths 

•  Eloge  de  Volta,  p.  21. 


140  ELECTRICITY. 


about  the  ends,  edges,  corners,  or  points,  than  in  other  places.  Its  expansive 
force  at  such  parts  is  therefore  greater  than  elsewhere,  and  will  exceed  the 
atmospheric  pressure,  and  escape  when  at  other  parts  of  the  surface  it  is 
retained. 

This  theory  will  be  explained  in  the  present  work,  as  far  as  its  development 
is  consistent  with  the  object  of  this  volume.  It  will  not,  therefore,  be  need- 
ful to  enlarge  upon  it  further  in  this  place.  It  may,  however,  be  asked  why  it 
is,  seeing  that  the  theory  of  two  fluids  is  sufficient  for  the  explanation  of  all 
the  phenomena  to  which  it  has  yet  been  applied,  and  that,  on  the  other  hand, 
the  theory  of  a  single  fluid  fails  to  afford  any  satisfactory  or  accurate  explana- 
tion of  so  many  phenomena,  the  latter  theory,  nevertheless,  still  has  followers, 
and  that  even  among  electricians,  whose  opinions  cannot  be  regarded  other- 
wise than  with  sentiments  of  respect,  it  is  still  clung  to  as  the  hypothesis  best 
entitled  to  reception  and  confidence  ?  It  is  not  easy  to  assign  any  sufficient  rea- 
son for  this,  unless  one  can  be  found  in  the  profound  and  abstruse  nature  of  the 
mathematical  principles  by  the  aid  of  which  alone  the  effects  are  capable  of  be- 
ing expressed.  When  it  is  remembered  that,  until  very  recently,  electricity  was 
regarded  as  exclusively  a  part  of  experimental  physics  ;  that  researches  in  it 
were  chiefly  carried  on  by  persons  engaged  in  chemical  investigations  ;  that, 
from  the  nature  of  their  studies  and  pursuits,  such  persons  rarely  cultivated 
even  the  elements  of  mathematics,  and  almost  never  pursued  analytical  science 
into  those  more  profound  parts  which  are  now  indispensable  for  the  solution  of 
the  class  of  problems  which  electricity  has  presented — it  cannot  be  matter  of 
much  surprise  that  reasoning  which  is  incapable  of  being  expressed  save  by 
symbols  of  which  the  force  and  import  must  be  unintelligible  to  the  great  mass 
of  such  persons,  should  fail  to  carry  conviction  to  their  understanding.  To 
arrive  at  such  conviction,  they  must  either  commence  their  education  anew,  or 
be  content  to  receive  those  new  doctrines  on  their  faith  in  the  assurance  of 
those  who  are  capable  of  investigating  them.  Either  side  of  such  an  alterna- 
tive is  never  very  willingly  embraced. 

Having  now  followed  the  progress  of  discovery  in  this  part  of  electrical  sci- 
ence to  that  point  at  which  all  subsequent  researches  must  be  regarded  as  the 
labor  of  our  contemporaries,  the  province  of  the  historian  ceases.  Whatever 
has  been  effected  more  recently  will  properly  form  a  part  of  the  subject  matter 
of  the  volume  here  presented  to  the  reader,  of  which  it  is  hoped  that  a  brief 
exposition  and  analysis  of  the  researches  of  contemporary  philosophers  will 
form  not  the  least  interesting  and  useful  portion. 


L. 


THE    MINOR   PLANETS. 


Classification  of  the  Planets. — Mercury. — Transit  over  the  Sun.— Relative  Position  with  /egard  to 
the  Sun. — Difficulty  of  observing  it. — Venus. — Diurnal  Motion  of  Venus  and  Mercury  indicated 
by  the  Shadows  of  Mountains. — Direction  of  the  Axis  of  Rotation. — Seasons,  Climates,  and 
Zones. — Orbits  and  Transits  of  Mercury  and  Venus. — Mountains  on  Mercury  and  Venus. — Influ- 
ence of  the  Sun  at  Mercury  and  Venus. — Twilight  on  Mercury  and  Venus. — Mars. — Atmosphere 
of  Mars. — Physical  Constitution  of  Mars. — Has  Mars  a  Satellite  ? — Appearance  of  the  Sun  at 
Mars. — Its  Close  Analogy  to  the  Earth. 


.J 


THE    MOOR    PLANETS. 


THERE  is  no  subject  of  inquiry  to  which  the  improved  powers  of  the  tel- 
escope have  been  directed  with  greater  effect  than  the  investigation  of  the 
physical  condition  of  the  several  planets  composing  the  solar  system.  We 
shall  on  the  present  occasion  take  a  review  of  some  of  these  bodies,  and  shall 
state  the  chief  circumstances  which  have  been  discovered  respecting  them. 

In  a  general  survey  of  the  system,  the  planets  composing  it  will  naturally 
be  classed  in  three  distinct  groups,  the  first  of  which  we  shall  call  the  minor 
planets,  the  second  the  new  planets,  and  the  third  the  major  planets. 

Proceeding  from  the  sun  outward  in  the  system,  the  four  planets  which  are 
nearest  to  that  luminary  are  Mercury,  Venus,  the  Earth,  and  Mars.  Between 
these  bodies  there  prevails  a  striking  analogy.  We  find  that  they  are  not 
very  different  in  magnitude  ;  that  they  correspond  closely,  so  far  as  we  can 
discover,  in  their  geographical  character ;  that  they  receive  in  not  very  differ- 
ent proportions  the  influence  of  the  sun.  The  close  alliance  between  them 
has  also  occurred  to  other  astronomical  writers,  inasmuch  as  they  are  some- 
times called  the  terrestrial  planets,  from  their  analogy  to  the  earth. 

OF    THE    PLANET    MERCURY. 

The  planet  Mercury  revolves  at  a  distance  from  the  sun  of  about  thirty-six 
millions  of  miles,  completing  his  periodical  revolution  in  about  eighty-eight  days, 
or  something  less  than  three  of  our  months.  The  diameter  of  this  pl.-met  is 
about  three  thousand  two  hundred  miles,  or  four  tenths  of  that  of  the  earth,  and 
consequently  its  volume  or  bulk  is  about  a  sixteenth  of  that  of  our  jjlobe.  As 
Mercury  revolves  round  the  sun  in  an  orbit  enclosed  within  that  of  the  earth,  it 
follows  that  his  illuminated  hemisphere,  which  is  always  presented  to  the  sun 
in  the  course  of  each  revolution,  must  assume  every  possible  variety  of  position 
in  regard  to  the  earth.  Thus  when  Mercury  is  between  the  sun  and  earth  as 
at  A,  in  what  is  called  inferior  conjunction,  his  dark  hemisphere  is  turned  tow- 


144 


THE  MINOR  PLANETS. 


ard  us,  and  he  is  invisible,  except  in  the  case  which  sometimes  occurs,  in 
which  he  is  so  exactly  in  line  of  the  direction  of  the  sun  as  to  be  between  the 
eye  and  some  portion  of  the  solar  disk.  In  that  case  the  planet  is  seen  as  a 
circular  black  spot  on  the  dislf  of  the  sun,  and  the  appearance  of  its  motion 
upon  that  disk  is  called  a  transit  of  Mercury, 

When  the  planet,  on  the  other  hand,  is  on  the  opposite  side  of  the  sun,  at 
B,  its  illuminated  hemisphere  is  presented  directly  in  the  line  of  vision  ;  but 
in  that  case,  the  planet  being  in  exactly  the  same  quarter  of  the  heavens  as  the 
sun  is,  would  necessarily  rise  and  set  with  the  sun,  and  its  appearance  being 
obscured  by  the  immeasurably  superior  splendor  of  the  sun,  it  would  not  be 
seen.  When  the  planet  is  in  an  intermediate  position  on  either  side  of  the  sun 
in  its  periodical  course,  its  illuminated  hemisphere  being  presented  as  it  always 
is,  directly  to  the  sun,  will  only  be  partially  turned  to  the  earth,  and  the  planet 


THE  MINOR  PLANETS.  145 

will  he  seen  under  a  corresponding  variety  of  phases,  in  short,  it  will  undergo 
all  the  changes  which  the  moon  presents  in  its  monthly  course  round  the  earth, 
as  represented  in  the  figure. 

When  near  the  point  behind  the  sun,  it  will  be  nearly  full,  or  gibbous ;  and 
when  near  the  point  where  its  dark  hemisphere  is  turned  to  the  earth,  it  will 
be  a  crescent.  In  a  certain  intermediate  position  it  will  be  halved,  and  will  pass 
through  all  the  other  phases. 

In  making  its  circuit  round  the  sun,  it  will  be  seen  alternately  at  the  east  and 
at  the  west  of  that  luminary,  separating  from  it  in  each  direction  to  an  extent 
limited  by  the  magnitude  of  its  orbit  round  the  sun.  When  it  is  at  the  west  of 
the  sun,  it  sets  before  the  sun,  and  rises  before  the  sun.  It  cannot,  in  that  case, 
be  seen  in  the  evening  ;  but  if  it  be  separated  from  the  sun  by  a  sufficient  dis- 
tance, it  will  rise  so  early  as  to  anticipate  the  light  of  the  morning  which 
precedes  the  sun's  rays,  and  may  then  be  seen  as  a  morning  star.  On  the  other 
hand,  when  it  is  at  the  east  of  the  sun,  it  rises  after  the  sun.  and  sets  after  it. 
It  cannot,  therefore,  be  seen  in  the  morning  ;  but  provided  it  be  sufficiently  dis- 
tant from  the  sun  to  remain  above  the  horizon  until  the  darkness  is  sufficient  to 
render  it  visible,  it  will  be  seen  as  an  evening  star. 

The  orbit  of  Mercury  is  so  limited  in  its  breadth,  compared  with  the  distance 
of  the  earth  from  the  sun,  that  even  when  that  planet  is  at  its  greatest  apparent 
distance  from  the  sun,  it  sets  in  the  evening  long  before  the  end  of  twilight ;  and 
when  it  rises  before  the  sun,  the  latter  luminary  rises  so  soon  after  it  that  it  is  never 
free  from  the  presence  of  so  much  solar  light  as  to  render  it  extremely  difficult 
to  see  the  planet  with  the  naked  eye.  In  short,  Mercury  is  seldom  seen  at  all, 
except  with  a  telescope.  It  is  said  that  Copernicus  himself  never  saw  this 
planet. 

OF    THE    PLANET    VENUS. 

The  planet  VEXUS  is,  on  many  accounts,  more  favorably  circumstanced  for 
telescopic  observation  than  Mercury.  Its  diameter  is  nearly  equal  to  that  of 
the  earth,  and  nearly  three  times  as  great  as  that  of  Mercury.  Its  distance 
from  the  sun  being  about  seventy  millions  of  miles,  it  separates  itself  in  its  pe- 
riodical course  so  widely  from  the  sun,  that  when  it  is  east  of  the  sun  it  re- 
mains above  the  horizon  in  the  evening  after  night-fall ;  and  when  it  is  west  of 
the  sun' it  rises  in  the  morning  so  long  before  the  hour  of  sunrise  that  it  is  dis- 
tinctly visible.  Owing  to  the  absence  of  the  solar  light,  it  forms,  therefore, 
the  object  with  whiofc  every  one  is  familiar,  under  the  names  of  the  morning  and 
evening  star.  It  is  subject,  by  the  operation  of  the  same  causes,  to  the  sajue 
variety  of  appearances  as  Mercury.  When  it  is  nearly  between  the  earth  and 
the  sun  it  appears  a  thin  crescent,  and  when  beyond  the  sun  it  appears  full ;  ar.l 
in  the  intermediate  positions  exhibits,  like  Mercury,  all  the  variety  of  phases 
of  the  moon. 

DIURNAL    MOTION    OF    VENUS    AND    MERCURY. 

One  of  the  most  interesting  objects  of  telescopic  inquiry  regarding  the  con- 
dition of  the  planets  is,  the  question  as  to  their  diurnal  rotation.  In  general, 
the  manner  in  which  we  should  seek  to  ascertain  this  fact  would  be,  by  exam- 
ining with  powerful  telescopes  the  marks  observable  upon  the  disk  of  the  planet. 
If  the  planet  revolves  upon  an  axis,  these  marks,  being  carried  round  with  it 
would  appear  to  move  across  the  disk  from  one  side  to  the  other ;  they  would 
disappear  on  one  side,  and,  remaining  for  a  certain  time  invisible,  would  reap- 
pear on  the  other,  passing,  as  before,  across  the  visible  disk.  Let  any  one 

10 


146 


THE  MINOR  PLANETS. 


stand  at  a  distance  from  a  common  terrestrial  globe,  and  let  it  be  made  to  re- 
volve upon  its  axis :  the  spectator  will  see  the  geographical  marks  delineated 
on  it  pass  across  the  hemisphere  which  is  turned  toward  him.  They  will  suc- 
cessively disappear  and  reappear.  The  same  effects  must,  of  course,  be  ex-  $ 
pected  to  be  seen  upon  the  several  planets,  if  they  have  a  motion  of  rotation 
resembling  the  diurnal  motion  of  our  globe.  If  this  species  of  observation  be 
attempted  with  respect  to  the  planets  MERCURY  and  VENUS,  we  shall  immedi- 
ately find  the  investigation  obstructed  by  an  unexpected  difficulty.  Their  disks 
present  no  permanent  marks  or  characteristics.  They  are,  it  is  true,  diversified 
more  or  less  by  lights  and  shadows,  but  we  soon  discover  that  these  varieties 
of  feature  are  not  of  a  permanent  kind ;  but,  on  the  contrary,  that  they  are 
continually  shifting  and  changing,  like  the  clouds  that  float  in  an  atmosphere. 
]t  has,  in  fact,  been  ascertained,  that  these  appearances  in  the  inferior  planets 
are  produced  by  clouds,  with  which  the  thick  atmosphere  that  invest  them  are 
continually  loaded.  These  clouds  are  so  continuous  that  they  never  permit  us 
to  see  the  geographical  character  of  the  planets  Mercury  and  Venus  at  all. 

For  a  long  period  this  circumstance  seemed  to  render  futile  all  attempts  to 
ascertain  the  rotation  of  these  planets  accurately.  At  length,  however,  a  cir- 
cumstance, apparently  accidental,  led  CASSINI  and  SCHROTER  to  the  discovery 
of  the  fact  of  the  rotation  of  VENUS  on  its  axis. 

This  discovery  was  effected  by  observing  that  the  points  of  the  horns  of  the 
crescent  of  Venus  were  at  certain  moments  cut  off  square,  and  after  a  certain 
time  would  recover  their  sharpness.  This  was  found  to  take  place  nearly  at 
the  same  time  each  successive  evening  and  morning.  The  cause  was  soon 
ascertained.  In  a  certain  part  of  the  surface  of  the  planet  a  lofty  mountain 
flung  its  shadow  across  the  region  which  formed  a  point  to  the  horn.  The 
diurnal  rotation  of  the  planet  soon  carried  this  point  into  another  position,  so 
that  the  shadow  disappeared  and  allowed  the  horn  of  the  crescent  to  recover  its 
sharpness.  Each  time  that  the  horn  became  thus  blunted,  it  was  ascertained 
that  the  mountain  had  returned  to  the  same  position,  and  consequently  that  the 
planet  must  have  completed  one  revolution  on  its  axis. 

It  is  a  remarkable  fact,  that  the  same  circumstance  was  found  to  take  place 
in  the  instance  of  the  planet  MERCURY,  and  the  result  has  been,  that  these  two 
planets  have  been  ascertained  to  have  a  diurnal  rotation;  that  of  MERCURY 
being  completed  in  24  hours,  5  minutes,  28  seconds,  and  that  of  VENUS  in  23 
hours,  21  minutes,  7  seconds.  Thus  it  appears  the  alternations  of  day  and 
night  in  these  planets  are  regulated  by  the  same  intervals  as  the  earth. 

DIRECTION   OF    THE    AXIS    OF    ROTATION. SEASONS,    CLIMATES,    AND    ZONES. 

The  position  of  the  axis  on  which  a  planet  revolves,  is  ascertained  by  ob- 
serving the  direction  of  the  apparent  motion  of  the  permanent  marks  upon  its 
disk — the  axis  being  necessarily  perpendicular  to  such  motion.  Since,  how- 
ever, the  rotation  of  MERCURY  and  VENUS,  as  we  have  just  explained,  do  not 
show  the  apparent  motion  of  any  of  these  permanent  marks,  the  circumstances 
which  led  to  the  discovery  of  their  rotation,  did  not  indicate  the  position  of  the 
axes  on  which  they  turned.  It  is  said,  however,  that  observations  have  been 
made  which  justify  the  conclusion  that  the  axis  on  which  the  planet  VENUS 
turns,  has  a  position  in  reference  to  its  orbit  very  different  indeed  from  that  of 
the  earth.  Let  it  be  remembered,  that  the  axis  of  the  earth  leans  from  the 
perpendicular  through  an  angle  of  231°,  in  consequence  of  which  the  polar  cir- 
cles and  tropics  have  corresponding  limits.  It  is  this  arrangement  which 
divides  the  surface  of  our  globe  into  the  temperate  and  frigid  zones ;  the  tem- 
perate being  those  which  lie  between  the  tropics  and  the  polar  circles,  in  which 


THE  MINOR  PLANETS.  147 


the  sun  is  never  vertical,  on  the  one  hand,  nor,  on  the  other  hand,  is  ever  absent 
for  twenty-four  successive  hours.     How  different  must  be  the  circumstances  at- 
tending the  planet  Venus,  if  it  be  true,  as  there  seems  reason  to  believe,  that  the 
axis  of  that  planet,  instead  of  being  inclined  23^°  from  the  perpendicular,  is 
inclined  75°  from  it.     The  polar  circles  would  include  a  portion  of  each  hem- 
isphere, the  extent  of  which  would  be  five  sixths  of  its  entire  breadth.     Thus 
the  greater  portion  of  such  a  globe  would  be  subject  to  vicissitudes  somewhat 
similar  to  those  which  are  incidental  to  our  frigid  zone,  but  the  changes  would  ( 
be  much  more  complicated.     Within  a  certain  space  of  such  a  planet,  the  sun  ] 
would  at  one  season  of  the  year  pass  through  the  zenith,  and  the  circumstances 
of  the  day  would  resemble  those  between  our  own  tropics  ;  while  at  another  pe-  7 
riod  of  the  year,  the  sun  would  never  rise  for  twenty-four  hours.     In  fact,  the  ! 
polar  circle  would  overlay  the  tropics,  and  the  phenomena  of  each  zone  would 
alternately  prevail  at  different  seasons. 

The  position  of  the  axis  of  Mercury  is  not  ascertained,  but  there  is  reason 
to  believe  that,  like  that  of  Venus,  it  is  inclined  at  a  very  large  angle  from  the 
perpendicular. 

ORBITS  AND  TRANSITS  OF  MERCURY  AND  VENUS. 

The  motion  of  the  planets  Mercury  and  Venus,  like  that  of  the  other  bodies 
of  the  system,  is  very  nearly  in  the  plane  of  the  ecliptic.  The  orbit  of  Mer- 
cury makes  with  the  piane  of  the  ecliptic  an  angle  of  7°,  and  that  of  Venus 
an  angle  of  less  than  4° ;  the  consequence  of  which  is,  that  these  planets  are 
never  seen  much  above  or  below  the  ecliptic.  The  apparent  diameter  of  the 
sun  is  about  half  a  degree  ;  consequently  the  greatest  distance  to  which  Venus 
can  depart  from  the  ecliptic,  will  be  less  than  eight  diameters  of  the  sun  ;  and 
the  greatest  distance  of  the  planet  Mercury  from  it  will  be  fourteen  diameters 
of  the  sun.  The  points  at  which  these  planets  are  seen  upon  the  ecliptic  are 
called  the  NODES  of  the  orbits ;  and  if  at  the  time  they  pass  near  these  nodes 
they  happen  to  be  in  inferior  conjunction,  they  may  be  directly  between  the 
eye  of  the  observer  on  the  earth  and  the  sun's  disk.  In  that  case,  they  would 
be  seen  as  a  black  spot  moving  in  the  sun's  disk.  In  order  that  this  remarka- 
ble phenomenon,  which  is  called  a  transit,  should  take  place,  it  is  obviously 
necessary  that  the  distance  of  the  disk  of  the  planet  from  the  place  of  the  sun's 
centre  should  be  less  than  half  the  sun's  apparent  diameter ;  that  is,  less  than 
fifteen  minutes  of  a  degree.  If,  then,  the  distance  of  either  of  the  inferior 
planets  from  the  ecliptic  at  the  time  they  are  in  inferior  conjunction  be  less  than 
fifteen  minutes,  there  must  be  a  transit ;  and  the  less  that  distance  is,  the  greater 
the  extent  of  the  sun's  disk  over  which  the  planet  will  be  seen  moving.  If  the  < 
planet  be  exactly  in  its  node  at  the  time  of  the  inferior  conjunction,  then  it  will  ) 
/  be  passing  directly  across  the  centre  of  the  sun. 

It  will  be  evident  that  the  part  of  the  sun's  disk  in  which  the  planet  is  seen  j 
i  projected  in  a  transit,  will  also  depend  on  the  position  of  the  observer  upon  the 
I  earth.     It  may  happen  that,  from  some  parts  of  the  earth,  the  planet  would  not 
|  be  projected  upon  the  solar  disk  at  all ;  and,  in  short,  at  different  parts  of  the 

!  earth,  the  line  of  its  projected  course  will  necessarily  be  different.  These 
effects  will  depend  on  the  extent  of  the  earth,  and  its  distance  from  the  sun 
and  the  planet. 

These   phenomena  have,  therefore,   supplied  a  very  happy  expedient  by 

which  the  distance  of  the  sun  from  the  earth  may  be  exactly  ascertained.     The- 

transit  of  Venus  is  especially  applicable  to  this  investigation,  and  has  been 

used  with  signal  success.     When  the  transit  of  the  planet  occurred  in  1765, 

)  observers  were  sent  by  different  European  governments  to  the  most  favorable  ( 


148 


THE  MINOR  PLANETS. 


parts  of  the  earth  for  observing  it :  some  to  Otaheite,  some  to  Cajaneburgh  in 
Swedish  Lapland,  and  elsewhere.  The  result  of  their  observations  proved 
that  the  distance  of  the  sun  from  the  earth  is  ninety-five  millions  of  miles. 

The  intervals  between  the  successive  transits  at  each  node  are  8  and  113 
years.  The  following  are  the  series  of  transits  to  take  place  for  the  next  four 
centuries  : — 


1874. 
1882. 
2004. 
2012. 


.Dec.  9 4  8  A.  M. 

.Dec.  6 4  16  P.  M. 

.June  8 8  51  A.  M. 

.June  6 1  17  A.  M. 


2117 Dec.  11 2  57  A.  M. 

2125 Dec.  18 3     9  p.  M. 

2247 June  11 0  21  P.  M. 

2255 June  9 4  44  A .  M. 

The  duration  of  a  transit  depends  on  the  part  of  the  sun's  disk  on  which  the 
planet  is  projected.  It  may  last  so  long  as  seven  hours,  if  the  planet  pass 
across  the  centre  of  the  disk  of  the  sun. 

The  last  transit  of  Mercury  took  place  on  the  7th  of  November,  1835.  It 
was  visible  in  this  country  but  not  in  Europe,  the  sun  having  set  there  before 
its  commencement.  The  next  transit  will  happen  in  the  present  year,  1845, 
on  the  8th  of  May :  it  will  commence  at  nineteen  minutes  past  four  in  the 
afternoon,  and  will  terminate  at  nine  minutes  before  eleven  at  night,  Green- 
wich time.  At  New  York  it  will  begin  and  end  four  hours  and  fifty-six  min- 
utes earlier  ;  it  will  therefore  begin  at  twenty-three  minutes  past  eleven  in  the 
forenoon,  and  will  terminate  at  five  minutes  before  six  in  the  afternoon.  The 
entire  transit  will  therefore  be  visible  in  the  United  States. 

The  transits  of  Mercury  during  the  present  century  will  be  as  follows  : — 

1845 May  8 7  54  P.  M. 

1848 Nov.  9 1  38  P.  M. 

1S61 Nov.  12 7  20  P.  M. 

<  1862 Nov.  5 6  44  A.  M. 

187S May  6 6  38  p.  M. 

1881 Nov.  8 0  40  A.  M. 

189 1 May  10 2  45  A.  M. 

1894 Nov.  10 6  17P.M. 

The  times  here  given  are  the  mean  times  at  Greenwich  of  the  middle  of  the 
(  transit. 


MOUNTAINS  ON  MERCURY  AND  VENUS. 

It  is  supposed  that  mountains  of  extraordinary  elevation  prevail  both  in 
Mercury  and  Venus.     Those  upon  Venus  are  estimated  to  be  about  four  times  > 
higher  than  upon  the  earth. 

Sir  William  Herschel  was  unable  to  distinguish  any  permanent  marks  on 
Mercury.  Schroter,  however,  has  been  more  successful.  This  astronomer 
has  discovered  mountains  on  the  surface  of  the  planet,  and  has  even  succeeded 
in  ascertaining  the  height  of  some  of  them.  One  of  them  he  found  to  rise  to 
an  altitude  of  5,600  feet,  and  another  to  the  scarcely  credible  height  of  nearly 
eleven  miles,  being  nearly  four  times  the  height  of  JElna.  or  the  peak  of  Ten- 
eriffe,  and  more  than  double  the  height  of  the  loftiest  mountain  on  the  earth. 
It  is  remarkable  that  the  highest  mountains  in  Mercury  are  situated  in  the 
southern  hemisphere  of  the  planet. 

Schroter,  to  whose  observations  we  are  indebted  for  much  of  the  knowledge 


THE  MINOR  PLANETS. 


149 


that  we  possess  of  the  planet  Venus,  showed  the  existence  of  several  mount- 
ains on  that  planet,  the  height  of  some  of  which  he  estimated  to  amount  to  > 
twenty-two  miles.     There  were  three  which  he  estimated  :  the  first  at  nineteen 
miles,  or  five  times  the  height  of  Chimborazo ;  the  second  at  eleven  and  a  half 
miles  ;  and  the  third  at  ten  and  three  quarters  miles. 

INFLUENCE    OF    THE    SUN   AT    MERCURY    AND    VENUS. 

The  distance  of  the  earth  from  the  sun  being  greater  than  that  of  Mercury 
in  the  ratio  of  100  to  39,  or  nearly  5  to  2,  the  apparent  diameter  of  the  sun  as 
seen  from  Mercury  will  be  greater  than  as  seen  from  the  earth  in  the  same 
ratio.  If  E  represent  the  apparent  magnitude  of  the  sun  as  seen  from  the 
earth,  M  will  represent  it  as  seen  from  Mercury. 


The  intensity  of  the  sun's  light  being  in  the  proportion  of  the  area  of  its  ap-  t 
parent  disk,  will  be  greater  at  Mercury  than  at  the  earth  in  the  ratio  of  25  to  ' 
>  4,  or  nearly  as  6  to  1.     If  the  heat  depended  solely  on  the  sun's  rays,  it  would  < 


be  in  the  same  proportion  greater  than  at  the  earth,  but  this  may  be  modified  ( 
by  many  causes  in  operation  on  the  planet  and  in  its  atmosphere. 


The  distance  of  the  earth  from  the  sun  is  greater  than  that  of  Venus  in  the 
ratio  of  10  to  7  nearly,  and  consequently  the  apparent  diameter  of  the  sun  as 
seen  from  Venus  will  be  greater  in  the  same  ratio  than  as  seen  from  the  earth. 
If  E  represent  the  apparent  magnitude  of  the  sun  as  seen  from  the  earth,  V  will 
represent  its  apparent  magnitude  as  seen  from  Venus. 

The  intensity  of  the  sun's  light  at  Venus  will  be  about  twice  its  intensity  at 
the  earth. 

TWILIGHT    ON    VENUS    AND    MERCURY. 

The  existence  of  an  extensive  twilight  in  these  planets  has  been  well  ascer- 
tained. By  observing  the  concave  edge  of  the  crescent  which  corresponds  to 
the  boundary  of  the  illuminated  and  dark  hemispheres  of  the  planets,  it  is  found 
that  tl  e  enlightened  portion  does  not  terminate  suddenly,  but  there  is  a  grad- 


THE  MINOR  PLANETS. 


151 


ual  fading  away  of  the  light  into  the  darkness,  produced  by  the  band  of  atmo- 
sphere illuminated  by  the  sun  which  overhangs  a  part  of  the  dark  hemisphere, 
and  produces  upon  it  the  phenomena  of  twilight. 

When  we  examine  the  dark  hemisphere  of  the  planet  Venus,  there  is  ob- 
served upon  occasions  a  faint  reddish  and  grayish  light,  which  is  visible  on 
parts  too  distant  from  the  illuminated  hemisphere  to  be  produced  by  the  light 
of  the  sun.  It  is  supposed  that  these  effects  are  indications  of  the  play  of  some 
atmospheric  phenomena  in  this  planet  similar  to  the  aurora  borealis. 

OF    THE    PLANET    MARS. 

Proceeding  outward  in  the  solar  system  from  the  sun,  the  first  planet  which 
we  find  revolving  beyond  the  earth  and  including  the  annual  path  of  the  earth 
within  its  periodical  course  is  the  planet  MARS.  This  body  makes  its  revolu- 
tion round  the  sun  at  a  distance  of  nearly  one  hundred  and  fifty  millions  of 
miles  from  that  luminary,  and  completes  its  revolution  in  six  hundred  and  eighty- 
six  days,  or  a  little  less  than  two  years. 

When  the  earth  is  between  Mars  and  the  sun,  the  distance  of  the  planet 
from  the  earth  is  less  than  fifty  millions  of  miles,  and  as  it  is  then  seen  in  the 
meridian  at  midnight,  the  circumstances  are  extremely  favorable  to  telescopic 
observation.  Although  its  distance  from  the  earth  at  that  epoch  is  greater  than 
that  of  Venus  when  near  inferior  conjunction,  yet  as  Venus  in  that  position 
has  her  dark  hemisphere  turned  to  the  earth,  while  the  enlightened  hemisphere 
of  Mars  is  turned  fully  toward  us,  the  observations  made  on  the  latter  are  more 
satisfactory. 

The  diameter  of  Mars  is  about  half  that  of  our  globe,  and  it  has  been  found 
by  the  observations  of  Arago  that  its  polar  diameter  is  little  less  than  its  equa- 
torial, and  that  consequently,  like  the  earth,  it  is  an  oblate  spheroid. 

As  the  planet  includes  the  orbit  of  the  earth  within  its  periodical  course 
round  the  sun,  the  hemisphere  which  it  presents  to  the  sun  is  always  very 
nearly,  although  not  exactly,  presented  to  the  earth  ;  the  consequence  of  which 
is  that  Mars  is  always  seen  with  a  full  phase,  or  very  slightly  gibbous.  It  has 
the  appearance  of  a  reddish  star 

DIURNAL    ROTATION    OF    MARS. 

On  examining  with  a  sufficiently  powerful  telescope  the  disk  of  Mars,  it  is 
found  to  be  characterized  by  features  of  lights  and  shadows,  like  those  which 
prevail  on  the  other  planets.  These  were  observed  at  a  very  early  period  in 
the  progress  of  astronomical  discovery.  There  are  diagrams  given  in  the  first 
volume  of  the  "Philosophical  Transactions,"  showing  telescopic  views  of  this 
planet. 

By  attentively  watching  these  marks,  they  have  been  observed  to  move  in 
parallel  lines  east  and  west — to  disappear  at  one  side  of  the  disk,  and  to  re- 
appear after  equal  intervals  at  the  other  side.  Hence  it  was  discovered  at  a 
very  early  epoch  by  CASSINI  that  Mars  has  a  diurnal  motion  upon  its  axis  in  a 
time  very  little  different  from  that  of  the  earth.  Cassini's  estimation  of  the 
time  of  rotation  of  this  planet  was  twenty-four  hours  and  forty  minutes.  A 
more  accurate  estimate  proves  it  to  be  twenty-four  hours  thirty-nine  minutes, 
and  twenty-one  seconds.  The  axis  on  which  it  turns,  and  which  is  perpen- 
dicular to  the  lines  in  which  the  marks  on  the  disk  move,  is  at  an  angle  of 
about  thirty  degrees  from  the  perpendicular  to  its  orbit.  Wher  it  is  remem- 
bered that  the  earth's  axis  is  inclined  at  an  angle  of  twenty-three  and  a  half 
degrees,  and  that  it  is  this  inclination  which  produces  the  succession  of  sea- 


152 


THE  MINOR  PLANETS. 


sons,  and  which  divides  the  earth  into  zortes  and  climates,  it  will  be  easily  in- 
ferred that  the  same  phenomena  prevails  in  Mars — the  limits  of  the  seasons 
being  little  more  extreme  than  those  which  prevail  in  the  earth. 


ATMOSPHERE    OF    MARS. 

The  existence  of  an  atmosphere  upon  Mars  is  proved  by  the  gradual  dimi- 
nution which  the  light  of  a  star  suffers  as  his  disk  approaches  it,  and  by  the 
variable  character  of  the  lights  and  shadows  apparent  upon  the  disk.  The 
ruddy  appearance  of  the  planet  has  been  explained  by  the  supposition  of  an 
atmosphere  of  great  density  around  it ;  but  more  accurate  telescopic  observa- 
tions have  led  Herschel  and  others  rather  to  incline  to  the  opinion  that  this 
redness  must  be  ascribed  to  a  peculiar  color  prevailing  on  the  surface  of  the 
planet,  like  that  of  the  red  sandstone  districts  upon  the  earth.  A  slight  appear- 
ance of  belts  has  always  been  noticed  on  this  planet,  which  affords  another 
indication  of  an  atmosphere,  as  will  be  more  clearly  understood  when  the  belts 
of  Jupiter  and  Saturn  shall  be  explained. 

PHYSICAL    CONSTITUTION    OF    MARS. 

Telescopic  inquiry  has  been  directed  to  determine  the  physical  condition  of 
this  planet,  and  with  a  degree  of  success  greater  perhaps  than  that  which  has 
attended  similar  inquiries  respecting  any  other  body  in  the  solar  system,  except 
the  sun  and  moon.  Sir  William  Herschel,  and  after  him  his  son,  Sir  John 
Herschel,  ascertained  the  form  and  position  of  a  variety  of  the  features  of  light 
and  color  on  the  disk  ;  but  it  has  been  reserved  for  the  Prussian  astronomers, 
BEER  and  MADLER,  to  carry  this  inquiry  to  a  much  greater  degree  of  detailed 
accuracy. 

Sir  John  Herschel  made  a  series  of  observations  on  Mars  within  the  last 
fourteen  years,  and  supplied  a  telescopic  drawing  of  one  hemisphere  of  the 
planet.  We  annex  a  figure  exhibiting  this  sketch. 


He  stated  that  the  outlines  here  exhibited  were  found  to  be  permanent  and 
unvariable,  and  must  therefore  be  regarded  as  geographical  and  not  atmospheric 
features.  It  is  true  that  they  were  not  always  visible,  being  sometimes  obscured, 
or  varied  by  what  seems  to  be  clouds  ;  but  when  visible  they  were  always  the 
same.  Some  portions  appeared  of  a  rsddish  color,  while  others  had  a  greenish 
tint.  He  supposes  the  red  portions  to  be  land  whose  geological  character  im- 


THE  MINOR  PLANETS. 


parts  to  them  that  peculiar  color.  The  greenish  portions  he  inferred  to  be 
seas. 

Among  the  features  apparent  on  this  planet,  what  attracted  most  attention 
are  certain  white  spots  seen  around  the  polar  regions.  These  were  among  the 
very  first  permanent  marks  discovered  on  the  planet,  and  are  represented  even 
in  the  first  rude  drawing  given  of  its  telescopic  appearances  in  the  proceedings 
of  the  Royal  Society.  In  the  observations  of  Herschel — both  father  and  son — • 
they  have,  however,  been  more  rigorously  examined  and  described ;  and  still 
more  so  in  the  investigations  of  Beer  and  Madler. 

It  has  been  ascertained  from  the  changes  they  undergo  that  they  must  be 
produced  by  deposites  of  snow  in  the  polar  regions.  Herschel  observed  that 
when  the  pole  had  been  turned  from  the  sun  during  the  winter,  and  first  re- 
appeared in  the  spring  of  the  planet,  the  whiteness  was  most  extensive  and 
vivid  ;  and  that  when  the  same  pole  was  exposed  to  the  influence  of  the  sun 
during  the  summer,  which  is  double  the  length  of  the  summer  upon  the  earth, 
this  whiteness  gradually  diminished,  and  always  disappeared.  Such  indica- 
tions cannot  be  mistaken,  and  admit  of  no  other  explanations  save  what  I  have 
now  adverted  to. 

The  elaborate  observations  of  Beer  and  Madler  have  supplied  various  tele- 
scopic views  of  this  planet.  In  their  work  upon  this  subject  they  have  pub- 
lished forty  views  of  hemispheres  made  by  planes  passing  nearly  through  the 
poles,  which  is  the  only  view  presented  to  the  observer  by  the  planet.  Hav- 
ing, by  combining  together  many  observations,  made  as  it  were  a  survey  of 
the  entire  surface  of  the  globe  of  Mars,  they  have  given  two  views,  one  of  its 
northern  and  the  other  pf  its  southern  hemisphere. 

We  have  obtained  copies  of  these  views,  and  have  affixed  them  here. 
Two  of  the  views  of  this  planet,  bounded  by  a  circle  passing  nearly  through 
its  poles,  are  annexed.  The  views  of  the  hemispheres  are  given  on  page  12. 


HAS    MARS    A    SATELLITE  ? 

Analogy  naturally  suggests  the  probability  that  the  planet  Mars  might  have 
a  moon.     These  attendants  appear  to  be  supplied  to  the  planets  in  augmented 
numbers  as  they  recede  from  the  sun  ;   and  if  this  analogy  were  complete,  it 
would  justify  the  inference  that  Mars  must  at  least  have  one,  being  more  re-  i 
mote  from  the  sun  than  the  earth,  which  is  supplied  with  a  satellite.     No  \ 
<  moon  has  ever  been  discovered  in  connexion  with  Mars.     It  has,  however,  * 
been  contended  that  we  are  not  therefore  to  conclude  that  the  planet  is  desti-  < 
tute  of  such  an  appendage ;  for  as  all  secondary  planets  are  much  less  than  ) 
their  primaries,  and  as  Mars  is  by  far  the  smallest  of  the  superioi  planets,  its  < 
satellite,  if  such  existed,  must  be  extremely  small.     The  second  satellite  of   | 
Jupiter  is  only  the  forty-third  part  of  the  diameter  of  the  planet ;  and  a  satellite  ^ 
which  would  only  be  the  forty-third  part  of  the  diameter  of  Mars,  would  be  S 
under  one  hundred  miles  in  diameter.     Such  an  object  could  scarcely  be  dis-  ( 

^^ ^^•^^>-V^^~^V^^^N.^^^^>^~^^X.V^-V^^^^X^^V^V^^^>^VXX>^>W.  ^fj 


THE  MINOR  PLANETS. 


155 


covered,  even  by  powerful  telescopes,  especially  if  it  did  not  recede  far  from 
the  disk  of  the  planet. 


APPEARANCE    OF   THE    SUN    AT    MARS. 
M 


The  distance  of  Mars  from  the  sun  being  greater  than  that  of  the  earth  in 
the  proportion  of  three  to  two,  it  follows  that  the  apparent  magnitude  of  the  sun 
to  the  inhabitants  of  Mars  will  be  less  than  to  the  inhabitants  of  the  earth  in 
the  same  proportion.  In  the  annexed  diagram,  if  E  represents  the  appearance 
of  the  sun  to  the  earth,  M  will  represent  its  appearance  at  Mars. 

The  light  which  it  affords  will  be  in  the  same  proportion  as  its  apparent 
magnitude  ;  and  as  the  superficial  magnitude  of  the  disk  will  be  about  half 
that  which  it  presents  to  the  earth,  it  follows  that  the  intensity  of  the  sun's 
light  at  Mars  will  be  less  in  the  same  proportion.  But,  for  the  reasons  which 
have  been  elsewhere  stated,  no  safe  inference  can  be  made  respecting  the 
effect  of  the  sun  on  the  temperature  of  the  planets. 

The  close  analogy  in  which  this  planet  stands  to  the  earth  will  be  apparent  ( 


156 


THE  MINOR  PLANETS. 


to  those  who  have  considered  the  facts  and  phenomena  now  described.  It  is 
a  globe  whose  diurnal  motion  is  such  as  to  give  it  days  of  the  same  length  ; 
its  seasons  succeeding  each  other  in  the  same  manner,  and  are  limited  by  the 
same  extremes  of  temperature.  Its  latitudes  are  diversified  by  the  same  torrid, 
temperate,  and  frigid  zones,  and  the  same  varieties  of  climate.  Its  surface  is 
characterized  by  a  like  distribution  of  land  and  water ;  and,  like  the  earth,  it 
has  its  continents,  islands,  and  seas.  It  is  invested  with  an  atmosphere,  sup- 
plying doubtless  all  the  interesting  objects  and  advantages  which  result  from 
our  own. 


WEATHER   ALMANACS 


Merits  of  Weather  Almanacs. — Excitability  of  the  London  Public. — Frighi  troiaced  by  Biela's 
Comet. — London  Water  Panic. — London  Air  Panic. — London  Bread  Panic. — Rage  for  Weather 
Almanacs — Patrick  Murphy's  Pretensions. — Examination  of  the  Predictions  of  the  Weather  Al- 
manac.— Their  Absurdity. — Comparison  of  the  Predictions  with  the  Event — Morrison's  Weather 
Almanac — Charlatanism  of  these  Publications. — Great  Frost  of  1838  in  London. — Other  Visita 
tiocs  of  Cold. 


159 


WEATHER   ALMANACS. 


NOTE. — The  subject  of  weather  almanacs  having  occasionally  been  introduced  in  an  abridged 
fo.-m  in  my  lectures,  I  have  thoueht  it  best  to  give  it  here  in  the  form  in  which  I  originally  presented 
•;  in  London,  when  a  rage  for  this  sort  of  scientific  charlatanism  prevailed  in  an  extraordinary  de- 
gree. The  following  appeared  in  the  spring  of  if  38. 

IF  the  weather  almanacs  presented  no  other  claims  to  our  attention  than 
those  which  rest  upon  their  intrinsic  importance,  they  would  assuredly  never 
have  been  noticed  by  us.  We  should  as  soon  think  of  discussing  their  merits 
among  our  scientific  discourses,  as  of  reviewing  the  performances  of  the 
penny  theatres,  or  the  buffoonery  of  the  booths  at  Bartholomew  fair.  When, 
however,  we  are  told  that  the  circulation  of  some  of  these  publications  is 
reckoned  by  hundreds  of  thousands,  and  that  at  a  price  which  would  im- 
pose a  narrow  limit  on  the  sale  on  any  ordinary  brochure  of  equal  bulk — 
and  when  we  know,  as  we  do,  that  this  enormous  circulation  is  not  either 
exclusively  or  principally  confined  to  the  lower  and  less-informed  clas- 
ses, but  extends  to  those  who  are,  or  ought  to  be,  the  best  educated  and  most 
enlightened — we  feel  that,  however  much  beneath  scientific  criticism  such 
productions  may  be,  they  have  acquired  some  claims  to  attention  from  the  suc- 
cess with  which  they  have  wrought  upon  the  credulity  of  the  "  most  thinking 
people"  in  the  world. 

It  is  astonishing,  in  this  age  of  the  diffusion  of  knowledge,  how  susceptible 
the  public  mind  is  of  excitement  on  any  topic,  the  principles  of  which  do  not 
lie  absolutely  on  the  surface  of  the  most  ordinary  course  of  elementary  educa- 
tion. It  was  only  in  the  year  1832  that  a  general  alarm  spread  throughout 
France,  lest  Biela's  comet,  in  its  progress  through  the  solar  system,  should 
strike  the  earth  ;  and  the  authorities  in  that  country,  with  a  view  to  tranquillize 
the  public,  induced  M.  Arago,  the  astronomer  royal,  to  publish  an  essay  on 
comets,  written  in  a  familiar  and  intelligible  style,  to  show  the  impossibility 
of  such  an  event. 

Several  panics  in  England,  connected  with  physical  questions,  have  oc- 
curred within  our  memory.  There  prevailed  in  London  a  "  water  panic,"  during 


160 


WEATHER  ALMANACS. 


•which  the  public  was  persuaded  that  the  water  supplied  to  the  metropolis  was 
destructive  to  health  and  life.  While  this  lasted,  the  papers  teemed  with  an- 
nouncements of  patent  filtering  machines  ;  solar-microscope  makers  displayed 
to  the  terrified  Londoners  troops  of  thousand-legged  animals  disporting  in  their 
daily  beverage  ;  publishers  were  busy  with  popular  treatises  on  entomology  ; 
and  tl-fi  public;  was  seized  with  a  general  hydrophobia.  It  was  in  vain  that 
Bra?  df  analyzed  the  water  at  the  Royal  Institute,  and  Faraday  attempted  to 
rea«-  r  Londor  into  its  senses.  Knowledge  ceased  to  be  power  ;  philosophy 
lost  u  authoilty.  Time  was,  however,  more  efficacious  than  science  ;  and  the 
parcrysms  of  the  disease  having  passed  through  their  appointed  phases,  the 
peop!^  WC:P  convalescent.  There  was  at  another  time  a  panic  against  atmo- 
spV.^'tc  air>  cuiing  which  the  inhabitants  of  the  great  metropolis  (in  a  literal 
sei  «-B)  scarcely  dared  to  breathe.  The  combustion  of  coal  was  denounced  as 
tl."}  ^reat  erl  in  this  case.  Calculations  were  circulated  of  the  number  of 
cub'.c  feet  if  sulphurous  gas  taken  into  the  lungs  of  each  adult  inhabitant  per 
annum  ;  ',h  j  pioperties  of  carbonic  acid  were  discussed  behind  counters  ;  patent 
furnacec  v^srs  plentifully  invented  and  advertised  for  sale ;  and  parliament  was 
urged  to  pass  a  bill  for  the  purification  of  the  atmosphere,  and  to  compel  all 
who  used  files  to  consume  their  own  smoke. 

A  few  years  ago,  the  people  of  London  were  seized  with  a  persuasion  that 
bakers  used  a  poisonous  substance  to  bleach  the  necessary  article  of  food 
which  they  manufactured,  and  forthwith  a  bread  panic  arose.  A  joint-stock- 
digestive-brown-bread  company  was  immediately  formed.  "  Fancy  baker,"  a 
title  previously  assumed  as  a  recommendation  to  their  customers'  favor,  was 
painted  over  ;  brown  loaves  usurped  the  place  of  French  rolls  ;  and  the  lacquey, 
whose  master  adhered  to  his  old  taste  in  defiance  of  poison,  as  he  sought  for 
white  loaves,  hummed —  • 

"  Tell  me  where  is  fancy  bread." 

In  1838,  the  public  turned  its  attention  to  meteorology,  and  the  causes 
which  govern  the  changes  of  weather  was  the  all-absorbing  topic.  Some  of 
the  intelligent  conductors  of  the  daily  and  weekly  press  seriously  descanted 
on  the  great  advantages  which  would  accrue  to  the  farmer,  the  gardener,  the 
manufacturer,  the  mariner,  and  others,  from  the  certain  prediction  of  the  weather, 
and  looked  forward,  evidently  not  without  hope,  to  an  early  period  when,  by  a 
new  principle  of  science  discovered  by  a  Mr.  Murphy,  and  he  said,  "  probably 
known  only  to  himself" — 

"  Careful  observers  may  foretell  the  hour, 
By  sure  prognostics,  when  to  dread  a  shower." 

Among  the  gifted  individuals  to  whom  it  has  been  vouchsafed  to  see  the 
shadows  which  coming  events  cast  before  them,  and  who  have  conferred  on 
the  public  the  inestimable  benefit  of  their  knowledge,  the  most  conspicuous  was 
a  gentleman  who  took  the  appellation  and  appendages  of  P.  Murphy,  Esquire, 
M.  N.  S.  What  praenomen  is  indicated  by  P.,  we  are  not  certainly  informed, 
but  we  believe  it  to  be  that  of  the  patron  saint  of  the  Emerald  isle,  of  which 
this  weather-seer  is  said  to  be  a  native.  Indeed,  there  is  abundant  proof  of  his 
country,  in  the  prevalence  throughout  his  writings  of  that  peculiar  species  of 
modesty  which  is  generally  considered  characteristic  of  the  "  Land  of  Song." 
We  have,  however,  looked  in  vain  among  the  many  combinations  of  letters 
expressing  the  various  learned  societies  in  this  and  other  countries  for  the  sig- 
nification of  M.  N.  S.  We  have  found  societies  designated  by  every  letter  in 


WEATHER  ALMANACS. 


161 


the  alphabet,  from  the  Astronomical  to  the  Zoological,  the  letter  N  alone  ex- 
cepted. 

After  all,  the  name  of  Patrick  Murphy  may  be  unwarrantably  assumed. 
Francis  Moore,  physician,  has  long  been  so  ;  and  a  table,  miscalled  Herschel's 
weather-table,  obtained  confidence  from  its  unauthorized  adoption  of  the  name 
of  that  eminent  astronomer.  Perhaps  the  weather  almanac  has  as  little  rela- 
tion to  the  veritable  Patrick  Murphy  as  Herschel's  weather-table  had  to  the 
great  telescopic  observer  ;  and  as  it  was  beneath  the  dignity  of  Sir  William 
even  to  disavow  such  trash  as  the  weather-table,  so  Sir  Patrick  may  possibly 
rely  on  the  dignity  of  his  station,  and  his  reputation  among  the  numerous  mem- 
bers of  the  N Society,  as  a  sufficient  refutation  of  this  imposture. 

Until  the  appearance  of  the  weather  almanac,  the  pretenders  to  prediction 
confined  their  forebodings  to  the  general  character  of  the  weather  at  particular 
epochs.  In  the  weather  almanac  there  was,  however,  a  distinct  prediction  for 
every  successive  day  of  the  year.  Every  possible  variety  of  weather  was  re- 
duced under  one  or  other  of  three  denominations — -fair,  rain,  and  changeable ; 
one  or  other  of  these  words  being  affixed  to  each  day  of  the  year.  For  some 
days  there  was  added  one  or  other  of  the  words  frost,  wind,  storm,  or  thunder. 

A  precaution  was  taken  in  the  preface  to  explain  the  meaning  in  which  these 
several  terms  are  intended  to  be  received. 

Fair,  means  a  day  in  which  drought  is  expected  to  predominate. 

Rain,  a  day  in  which  rain  is  expected  to  predominate. 

Changeable,  a  day  in  which  it  is  uncertain  whether  drought  or  rain  will  pre- 
dominate. 

To  be  enabled  fairly  to  compare  the  predictions  with  the  facts,  it  is  necessary 
that  these  explanations  of  the  terms  fair,  rain,  and  changeable,  be  clearly  un- 
derstood. 

Does  rain,  we  would  ask,  include  snow,  hail,  and  sleet  1  We  must  presume 
that  it  does,  since  these  vicissitudes  are  not  otherwise  expressed  in  the  al- 
manac. 

Does  drought  signify  anything  more  than  the  absence  of  rain,  snow,  or  sleet  ? 
We  shall  presume  that  it  does,  because  otherwise  this  very  common  state  of 
the  weather  would  have  no  designation  in  the  nomenclature  of  the  weather 
almanac,  and  we  should  have  a  prediction  of  a  severe  frost  in  January,  without 
any  prediction  of  the  thaw  which  follows  it. 

The  term  "  predominate,"  used  in  these  explanations,  we  take  to  refer  to 
duration.  Thus,  if  in  twenty-fours,  rain  fall  for  less  than  twelve  hours,  the 
day  is  to  be  designated  fair,  since  drought  predominates ;  and  if  rain  fall  for 
more  than  twelve  hours,  then  the  day  is  to  be  designated  rain,  since  rain  pre- 
dominates. 

The  causes  which  govern  the  phenomena  of  weather  being  physical  agen- 
cies independent  of  the  will  or  interference  of  any  being  save  of  Him  "  who 
rules  the  storm,"  are  as  fixed  and  as  certain  in  their  operation,  and  as  regular 
in  the  production  of  their  effects,  as  those  which  maintain  and  regulate  the 
motions  of  the  solar  system.  The  moment  of  the  rising  or  setting  of  the  sun 
on  any  given  day  of  the  ensuing  year  is  therefore,  in  the  nature  of  things,  not 
more  certain  than  the  atmospheric  phenomena  which  will  take  place  on  that 
day.  The  doubt  and  uncertainty  which  attend  these  events  belong  altogether 
to  our  anticipations  of  them,  and  not  to  the  things  themselves.  If  our  knowl- 
edge of  meteorology  were  as  advanced  as  our  knowledge  of  astronomy,  we 
should  be  in  a  condition  to  declare  the  time,  duration,  and  intensity,  of  every 
shower  which  shall  fall  during  the  ensuing  year,  with  as  much  certainty  and 
precision  as  we  are  able  to  foretell  the  rising,  setting,  and  southing,  of  the  sun 
and  moon,  or  the  rise  and  fall  of  the  tides  of  the  ocean.  When  it  is  said,  there- 

11 


fore,  that  drought  or  rain  is  expected  to  predominate,  the  uncertainty  implied  by 
the  term  expected  must  be  understood  to  belong  to  the  knowledge,  or  rather 
ignorance,  of  him  who  makes  the  prediction,  and  not  to  the  event,  which,  as 
we  have  shown,  is  necessary,  and  not  contingent. 

But  the  most  absurd  of  these  explanations  is  that  of  the  word  changeable, 
which  is  here  used  in  a  most  novel  sense.  Changeable  weather,  in  the  ordi- 
nary use  of  the  word,  is  applied  to  weather  which  changes  frequently  and  sud- 
denly at  short  intervals,  from  fair  and  clear  to  cloudy  and  wet.  But  the  weather- 
almanac  sense  of  this  term  is,  weather  in  which  it  is  uncertain  whether  drought 
or  rain  will  predominate.  Now,  as  we  have  already  shown  that  no  uncertainty 
can  attend  the  weather  itself,  but  that  the  uncertainty  belongs  only  to  the  mind  of 
the  author  of  the  weather  almanac,  it  will  be  necessary  to  remember  that  change- 
cable  weather  is  weather  about  which  the  said  author  confesses  that  he  has  no 
foreknowledge  ;  thus,  though  for  a  week  the  face  of  the  heaArens  continue  clear 
and  cloudless,  the  temperature  of  the  air  mild  and  uniform,  and  the  atmosphere 
calm  and  still,  yet  the  weather  during  such  week  might  be  changeable,  accord- 
ing to  the  weather  almanac,  and  its  author  would  claim  the  credit  of  a  predic- 
tion fulfilled.  In  fact,  every  day  in  the  year  to  which  he  has  annexed  the 
word  changeable,  must  fulfil  his  prediction,  whatever  be  the  state  of  the 
weather ;  since,  happen  what  will,  no  one  can  doubt  the  uncertainty  of  the 
author's  own  mind  as  to  the  event,  when  that  uncertainty  is  itself  the  essence 
of  his  prediction. 

The  author  states,  that  by  wind  he  means  a  gale,  excluding'  from  this  term  ( 
light  winds  ;  also,  that  by  storm  he  means  a  more  violent  gale ;  and  that  tJiun- 
der  and  storm  are  to  be  considered  to  a  certain  extent  synonymous,  it  being  not 
always  possible  to  decide  in  which  way  these  phenomena  will  develop  them- 
selves. 

To  these  explanations  we  have  nothing  to  object,  and  have  only  to  say,  that 
it  were  better  for  the  author's  reputation  for  honesty  or  sanity,  if  he  had  car- 
ried his  indecision  to  a  much  greater  extent.  We  are  told  in  the  preface,  that — 

"  When  it  is  taken  into  account  that,  as  connected  with  the  principles  and 
laws  of  movement,  of  temperature,  &c.,  in  the  sun  and  planets — a  totally  new 
class  of  proofs — never,  perhaps,  so  much  as  supposed  to  exist  by  the  immortal 
Newton,  nor  by  any  other,  is  proposed  ly  the  present  work ;  and  which,  if  found, 
to  a  certain  extent,  correct,  will  have  the  effect  of  placing  these  departments 
of  science  a  century  in  advance ;  it  will  be  allowed  that,  independent  of  its 
utility  in  other  respects,  this  should  be  sufficient  to  secure  it  a  favorable  recep- 
tion from  an  enlightened  public. 

"  In  regard  to  the  principles  themselves  on  which  the  calculations  of  the 
weather  are  founded,  it  will  be  sufficient  to  say  that,  as,  according  to  any  prin- 
ciples hitherto  known  or  recognised,  calculations  of  the  kind  could  not  be 
mad;,,  the  circumstances  necessarily  presupposes  the  discovery  of  others  ;  and 
as  snowing  the  connexion  of  the  latter  with,  it  may  be  said  every  department 
of  the  physical  sciences,  and,  consequently,  with  the  interests  of  every  class 
of  society — a  scientific  notice  is  subjoined  by  the  editor,  in  order  that  such  of 
the  patrons  of  the  almanac  as  may  feel  disposed  to  obtain  information  on  the 
subject,  may  have  the  opportunity  to  consult  his  views." 

( )n  reading  this,  we  turned  with  strong  feelings  of  curiosity  to  the  scientific 
,  in  the  hope  of  being  informed  of  the  "  totally  new  class  of  proofs,  never 
•>sod  to  exist  by  the  immortal  Newton,  nor  by  any  other."     But  alas!  so 
imperfect  was  our  intellectual  vision,  that  we  looked  in  vain,  and  we  forced  our- 
selves with  those  others  who,  in  common  with  "the  immortal  Newton,"  not  only 
never  supposed  such  proofs  to  exist,  but  cannot  persuade  ourselves  even  now 
of  their  existence.     In  truth,  were  it  not  for  the  high  scientific  reputation  of 


WEATHER  ALMANACS.  163 


Mr.  Murphy,  and  the  respect  we  entertain  for  the  discrimination  of  the  mem- 
bers of  the  N society,  who  elected  him  into  their  body,  we  would  pro- 
nounce the  said  scientific  notice  to  be  as  sheer  and  unmitigated  nonsense  as  it 
has  ever  been  our  fortune  to  encounter.  As  matters  stand,  however,  we  must 
ascribe  all  to  the  feebleness  of  our  own  powers  compared  to  those  of  Mr.  Murphy. 

Having  thus  candidly  acknowledged  our  inability  to  comprehend  the  author's 
theory  of  meteoric  action,  the  sublimity  of  which  we  shall  not  be  so  presump- 
tuous as  to  doubt,  much  less  to  dispute,  we  must  be  content  with  the  more 
humble  office  of  comparing  the  predictions  of  the  Weather  Almanac  with  the 
actual  phenomena,  so  far  as  time  has  converted  the  future  into  the  past. 
We  have  the  less  hesitation  in  adopting  this  test  of  the  validity  of  the  author's 
principles,  as  it  is  one  which  he  has  himself  courted. 

"The  event  in  reference  to  these  predictions  being  thus  admitted  to  be  in 
some  decree  contingent,  it  may  be  asked — If  certainty  cannot  be  attached  to 
the  prediction,  of  what  use  can  it  be  ?  To  this  we  answer,  that  the  exceptions 
in  reference  to  the  predictions  as  marked  in  the  tables,  will,  it  is  anticipated, 
be  found  to  bear  but  a  small  proportion  to  the  remainder;  and  in  our  turn  we 
shall  demand,  if,  in  nine  cases  out  of  ten,  the  event  be  found  to  correspond 
»  with  the  prediction,  does  it  follow,  because  one  of  the  anticipated  effects,  as 
set  down  in  the  table,  does  not  take  place,  that  the  public  is  to  remain  ignorant 
of  the  remaining  nine  ?  For  if  an  objection  such  as  this  were  valid,  it  were 
the  same  to  say,  because  the  quadrature  of  the  circle  cannot  be  found,  that  the 
practical  parts  of  mathematics  should  be  abandoned  :  such  exceptions,  as  in 
other  cases,  serve  but  the  more  fully  to  prove  the  rule,  as  to  the  correctness 
of  the  principles  of  calculation  on  which  the  predictions  in  the  tables  are 
founded." 

Undoubtedly  nothing  could  be  more  unreasonable  or  unphilosephical ;  nay, 
we  will  go  further,  and  will  admit  that  the  author  must  be  classed  among  the  great 
lights  of  the  age,  if  his  predictions  be  fulfilled  even  in  a  much  smaller  ratio 
than  that  which  he  proposes.  Nothing  can  be  more  true  than  the  observation 
with  which  he  concludes  his  preface  : — 

"  It  may  not,  however,  be  amiss  to  add,  in  regard  to  these  principles  of  cal- 
culation, that,  though  by  chance  the  state  of  the  weather  at  any  particular  time 
might  possibly  be  predicted,  that  it  is  quite  different  as  refers  to  a  number  of 
facts  :  as  to  attempt  to  follow  the  sinuosities  of  the  weather  (as  in  the  present 
almanac)  from  fair  to  rain  and  from  rain  to  fair,  even  for  seven  days  consecu- 
tirply,  without  the  aid  of  correct  principles,  were  about  the  same  as  to  attempt 
a  discourse  in  an  unknown  tongue  ;  the  thing  never  having  been  done  before, 
and  for  a  sufficiently  simple  reason,  because  it  was  utterly  impossible." 

Let  us  see  whether  the  author  has  "followed  the  sinuosities  of  the  weather" 
even  for  three  days  successively. 

We  have  before  us,  on  the  one  hand,  the  predictions  of  the  Weather  Alma- 
nac for  the  first  forty-eight  days  of  the  present  year,  and  on  the  other,  the  Me- 
teorological Journal,  kept  by  order  of  the  council  of  the  Royal  Society  during 
that  time.  We  shall  resolve  these  forty-eight  days  into  three  classes:  Is'., 
Those  on  which  the  weather  fulfilled  the  prediction  ;  2d,  Those  on  which  the 
weather  did  not  fulfil  the  prediction;  and,  3d,  Those  for  which  no  prediction 
was  made,  which,  as  we  have  already  shown,  is  the  case  of  all  those  days  to 
which  changeable  is  annexed. 

In  deciding  whether  the  prediction  has  been  fulfilled  or  not,  we  have  been 
careful  to  follow  those  explanations  of  his  terms  which  the  author  has  very 
properly  given  in  his  preface  ;  and  when  the  character  of  the  day,  as  recorded 
in  the  journal  of  the  Royal  Society,  has  been  doubtful,  as  compared  with  the 
prediction,  we  have  given  the  author  the  benefit  of  it : — 


164  WEATHER  ALMANACS. 


)    164 


Prediction  fulfilled— Jan.  7,  8   12,  13,  19,  20,  26,  27,  28  ;  Feb.  1,  6,  9,  10, 
13.     Number  of  days,  14. 

Prediction  not  fulfilled— Jan.  1,  2,  3,  9,  10,  11,  15,  16,  17,  18,  24,  25,  30, 
31  ;  Feb.  3,  8,  12,  14,  16,  17.     Number  of  days,  20. 

No  prediction  made— Jan.  4,  5,  6,  14,  21,  22,  23,  29;  Feb.  2,  4,  5,  7,  11, 
15.     Number  of  days,  14. 

?  Thus  it  appears  that,  of  forty-eight  days,  the  weather  corresponded  with  the 
$  prediction  on  fourteen ;  it  did  not  correspond  with  it  on  twenty ;  and  on  the 
^  fourteen  remaining  days  no  prediction  was  made. 

)  Now,  we  will  ask,  if  any  person  of  common  observation  acquainted  with  the 
f  climate  of  the  country,  were  to  annex  to  each  of  the  first  forty-eight  succes- 

>  sive  days  of  the  year  at  hazard,  the  characters  of  weather  generally  found  to 
^  prevail  at  that  season,  whether  he  would  not,  according  to  all  probability,  be 

>  right  in  a  greater  number  of  cases  than  fourteen  in  forty-eight,  that  is,  one  case 
'  in  three  and  a  half? 

The  predictions  of  the  Weather  Almanac,  then,  fail  in    seventeen  cases 
)  out  of  twenty-four!  yet  this  is  the  production  which  the  public  bought,  at  a 

>  high  price,  by  the  hundred  thousand !     This  is  the  production  for  which  the 
/  demand  was  so  urgent,  and  for  which  the  public  impatience  was  so  irrepressi- 
)  ble,  that  the  shop  of  the  bookseller,  like  those  of  bakers  in  a  famine,  was 
?  obliged  to  be  protected  by  the  police,  so  violent  was  the  demand  of  the  thou- 
)  sands  who  flocked  to  obtain  it ! 

J       By  reference  to  the  above  table  it  will  be  seen,  that  there  is  no  case  in  which 
i  the  predictions  have  been  fulfilled,  even  for  three  successive  days,  except  from 
'  the  26th  to  the  28th  of  January  inclusive.     Even  in  that  case,  the  prediction 
)  for  the  26th  agrees  but  imperfectly  with  the  event;  the  prediction  being  fair, 
without  mention  of  wind  or  frost,  while  the  Meteorological  Journal  says  over- 
cast ;  brisk  wind  the  whole  day  ;  sharp  frost.     Much  of  the  attention  this  pub- 
lication received  has  been   ascribed  to  the  supposed  fulfilment  of  the    pre- 
diction for  the  20th  of  January,  which  is  marked  in  the  Weather  Almanac  as 
the  lowest  winter  temperature.     This  was  a  fortuitous  coincidence,  such  as 
)  happens  frequently  in  other  cases,  as  in  the  fulfilment  of  dreams,  &c.     We 
,  shall  not  insist  here  on  the  fact,  that  the  20th  was  not  the  day  of  the  greatest 
)  cold  by  the  diary  of  the  Royal  Society,  since  the  thermometer  fell  a  little  lower 
I  on  the  16th,  because  we  think  it  really  unimportant.* 

;  But  it  may  be  said,  that,  although  the  prediction  has  failed  as  to  the  exact 
j  time  at  which  the  several  changes  took  place,  yet,  in  the  main,  the  changes 

>  predicted  did  take  place,  and  that  the  prediction  "  followed  the  sinuosities  of 
I  the  weather." 

Let  us,  then,  see  how  far  the  predictions  in  the  Weather  Almanac  will  bear 

)  a  comparison  with  the  actual  succession  of  changes. 

i 

'  Actual  succession  of  changes.  Succession  of  changes  predicted. 

Number  of  days.  Number  of  days. 

6  Mild  and  warm.  3  Frost. 

14  Frost.  3  Changeable. 

3  Thaw.  7  Frost. 

4  Frost.  1  Changeable. 
4  Thaw.  6  Frost. 

6  Frost.  3  Changeable. 

3  Thaw.  2  Rain. 

•  The  thermometer  at  the  Horticultural  Society  is  said  to  have  been  four  degrees  below  zero  on  ( 
tbe  night  of  the  19th  and  20th.  This  is  BO  much  at  variance  with  the  journal  of  the  Koy:il  fck-ri.  iv  ; 
that  \V3  doubt  the  accuracy  of  the  observation. 


WEATHER  ALMANACS. 


165 


8  Frost. 
48 


Frost. 

Changeable. 

Rain. 

Frost. 

Changeable. 

Rain. 


2  Changeable. 


1 
1 

2 
1 

1 
1 
1 
1 
1 
1 
1 

48 


Fair. 

Changeable. 

Rain. 

Fair. 

Changeable. 

Rain  and  wind. 

Fair. 

Rain  and  wind. 

Changeable. 

Rain. 

Fair  and  frost. 


We  shall  leave  it  to  the  skill  of  our  readers  to  discover  where  the  correspond- 
ence lies  between  "  the  sinuosities  of  the  weather,"  and  the  sinuosities  of  Mr. 
Murphy's  predictions.  Dismissing  this  very  absurd  publication,  to  which  we 
have  given  more  space  than  it  deserves,  we  shall  merely  add,  that  it  is  not  the 
only  production  of  the  kind  which  public  credulity  fostered  into  life.  Be- 
sides the  eternal  Francis  Moore,  physician,  we  had  also  the  Meteorological 
Almanac,  and  Farmers'  and  Shipmasters'  Guide,  containing  the  general  charac- 
ter of  the  weather  all  through  the  year  1838,  by  Lieutenant  Morrison,  R.N., 
Member  of  the  London  Meteorological  Society,  and  numerous  others. 

Without  further  discussing  the  prognostications  of  such  persons,  or  compar- 
ing them  with  facts,  we  shall  merely  ask  those  who  appear  to  afford  them  so 
i  easy  faith,  to  consider  the  nature  of  the  physical  questions  pretended  to  be 
[  solved,  and  the  qualifications  of  those  who  profess  to  have  solved  them.  The 
investigation  of  the  causes  which  affect  the  atmosphere  and  produce  the  vicis- 
situdes of  temperature  and  of  drought,  is  a  problem  of  transcendent  difficulty, 
to  the  solution  of  which  even  the  most  extensive  powers  of  modern  science  are 
inadequate.  It  is  a  problem  to  which,  hitherto,  scarcely  an  approximation  has 
been  made,  even  by  the  most  eminent  natural  philosophers  ;  and,  as  it  is  one  of 
the  details  of  which  the  public  in  general  cannot  be  expected  to  understand, 
they  can  only  regulate  the  confidence  which  they  will  place  in  its  pretended 
solutions  by  the  reputation  and  authority  of  those  who  propound  them. 

Who,  then,  it  may  be  asked,  are  the  persons  that  put  forth  those  predictions  ; 
and  on  what  grounds  do  they  ask  the  faith  of  the  public  ?  Among  these  prog- 
nosticators,  is  any  name  found  holding  a  respectable  rank  in  the  community  of 
science  ?  What  have  the  labors  and  researches  of  these  persons  contributed 
to  the  actual  advancement  of  our  knowledge  of  nature  ?  What  are  the  works 
on  which  their  reputations  are  founded  ?  Do  these  weather-prophets  possess 
any  of  the  recognised  qualifications,  founded  on  education  and  previous  attain- 
ments' which  would  fit  them  for  encountering  such  a  problem  ?  What  learned 
societies  in  Europe  have  these  pretenders  enriched  by  their  labors  ?  Where 
are  the  transactions  in  which  their  investigations  and  discoveries  have  appeared  I 
These  questions  would  be  answered  by  a  mere  enumeration  of  their  names — 
names  utterly  unknown  in  philosophy  or  letters.  It  would  be  answered  that  among 
them  there  is  found  not  one  individual  whose  presence  would  be  tolerated  in 


f —  — 

<    166  WEATHER  ALMANACS. 


any  scientific  reunion  in  Europe.  Such  are  the  class  of  persons  to  whom  the 
public,  in  the  contemptuous  silence  of  the  great  leaders  and  guides  of  science 
in  every  part  of  the  world,  surrendered  their  faith. 

As  the  subject  of  this  article  has  given  us  occasion  to  notice  the  late  visita- 
tion of  cold,  it  may  be  not  uninteresting  to  compare  the  particulars  of  that  part 
of  the  season  with  similar  events  in  former  years. 

The  weather  in  London,  from  last  Christmas  until  the  seventh  of  January, 
was  remarkably  fine  and  mild.  During  the  first  four  days  of  January,  the  ther- 
mometer was  never  lower  than  40  degrees,  and  ranged  between  that  and  50  de- 
grees. On  the  6th  it  fell  to  32  degrees,  between  which  and  38  degrees  it  ranged 
on  that  day.  On  the  7th  the  severe  frost  commenced,  the  thermometer,  how- 
ever, being  rather  higher  on  that  than  on  the  preceding  day.  But  on  the  fol- 
lowing day  (the  8th)  the  frost  became  rigorous,  the  thermometer  falling  more 
than  four  degrees  below  the  freezing  point.  The  temperature  continued  to  fall 
until  the  16th,  when  it  attained  the  minimum — the  thermometer  then  having 
descended  to  11  -4  degrees,  which  is  twenty  degrees  and  a  half  below  the  freez- 
ing point.  A  very  slight  increase  of  temperature  succeeded  for  the  next  three 
days,  when,  on  the  20th,  the  temperature  again  fell  to  11^  degrees  of  the  ther- 
mometer. On  that  day  the  thermometer  ranged  between  that  temperature  and 
21  degrees  (eleven  degrees  below  the  freezing  point).  This  was  the  day  of 
greatest  average  cold,  though,  strictly  speaking,  it  was  not  the  day  on  which 
the  temperature  was  lowest.  On  the  22d  and  23d,  the  thermometer  rose  to 
above  40  degrees,  and  a  rapid  thaw  ensued  ;  which,  however,  was  succeeded 
by  a  return  of  frost — the  thermometer  again  falling  seven  or  eight  degrees  be- 
low the  freezing  point.  On  the  29th  commenced  a  rapid  thaw,  the  thermome- 
ter rising  to  44  degrees  on  the  30th.  Frost  succeeded  this  on  the  1st  of  Feb- 
ruary, which  continued  until  the  6th,  when  it  was  succeeded  by  a  thaw,  which 
continued  through  the  7th,  8th,  and  9th.  On  the  ]  Oth  the  frost  recommenced, 
and  has  continued  to  the  moment  of  writing  these  observations  (the  17th). 

Thus  between  the  7th  of  January  and  the  17th  of  February,  the  lowest  point 
to  which  the  temperature  fell  was  111  degrees,  which  it  attained  twice — name- 
ly, on  the  16th  and  20th.  The  average  of  the  lowest  daily  temperature 
throughout  this  periods  was  25£  ;  the  average  of  the  highest  daily  temperature 
was  36-J. 

Throughout  this  frost  there  was  so  little  snow  that  the  face  of  the  ground  was 
not  covered  and  protected,  and  vegetables  were,  consequently,  exposed  to  a 
temperature  so  rigorous  as  to  occasion  extensive  destruction  of  the  products  of 
the  garden. 

The  last  severe  frost  with  which  this  can  be  compared  occurred  in  January, 
1826.  On  the  8th  of  that  month  the  thermometer  fell  one  degree  below  the 
freezing  point,  and  on  the  16th  it  stood  at  17  degrees  at  9  in  the  morning — be- 
ing fifteen  degrees  below  the  freezing  point,  the  lowest  temperature  recorded 
since  that  day  to  the  present  time.  The  frost  terminated  on  the  18th,  the  ther- 
mometer then  rising  to  36  degrees. 

This  frost  of  1826  can  only  be  compared  to  the  recent  cold  in  the  extreme 
of  its  temperature,  its  duration  having  been  only  ten  days. 

A  severe  frost  took  place  in  January,  1814,  which  continued  throughout  that 
month,  and  did  not  terminate  until  the  6th  of  February.  The  lowest  tempera- 
ture recorded  during  this  frost  is  17  degrees,  which  was  the  temperature  at  8 
in  the  morning  on  the  ]  Oth.  The  greatest  height  of  the  thermometer  through- 
out the  month  of  January  was  40  degrees,  and  the  mean  temperature  of  the 
month  was  28-08.  This  frost,  therefore,  in  its  duration,  was  less  than  the  late 
f  frost ;  the  lowest  and  mean  temperatures  were  also  lower  in  the  present  year 
than  in  1814 


WEATHER  ALMANACS.  167 


In  January,  1795,  there  occurred  a  frost  which,  for  rigor  and  continuance, 
exceeded  the  present.  The  mean  temperature  during  that  month  was  about  2G 
degrees,  and  on  the  25th  of  the  month  the  thermometer  stood  at  7  degrees — 
being  25  degrees  below  the  freezing  point.  The  mean  temperature  during  the 
frost  was  about  the  same  as  during  the  present,  but  the  extreme  temperature 
was  four  degrees  lower.  Since  1795  till  the  present  time — a  period  of  forty- 
two  years — there  has  been  no  cold  of  intensity  and  duration  equal  to  the  pres- 
ent. 

Since  the  preceding  observations  were  sent  to  press,  we  have  received  a 
journal  of  the  state  of  the  weather  during  the  last  month  in  Paris,  the  particu- 
lars of  which  may  not  be  uninteresting  to  compare  with  the  corresponding  phe- 
nomena in  London.  As  in  London,  the  first  days  of  the  month  were  mild  and 
fair,  the  thermometer  ranging  from  the  first  to  the  sixth  between  33^  degrees 
and  29  degrees.  On  the  seventh,  as  in  London,  the  frost  commenced,  and  the 
thermometer  gradually  fell  until  the  fourteenth,  on  which  day  the  maximum 
temperature  was  13  degrees,  and  the  minimum  4  degrees. 

The  thermometer  rose  on  the  fifteenth,  but  afterward  gradually  fell  until  the 
twentieth,  when  it  attained  the  lowest  temperature  of  the  month.  On  that  day 
the  highest  temperature  was  21  degrees  below  the  freezing  point,  and  the  low- 
est was  34  degrees  below  it. 

The  mean  maximum  temperature  from  the  first  to  the  tenth  was  33^  degrees, 
and  the  mean  minimum  was  27  degrees. 

The  mean  maximum  temperature  from  the  eleventh  to  the  twentieth  was  19 
degrees,  and  the  mean  minimum  temperature  was  8  degrees. 

The  mean  maximum  temperature  from  the  twenty-first  to  the  thirty-first  was 
35  degrees,  and  the  mean  minimum  temperature  was  21  degrees. 

The  mean  maximum  temperature  throughout  the  month  was  35  degrees,  and 
the  mean  minimum  temperature  was  18  degrees. 

The  absolute  mean  temperature  of  the  month  was  a  little  under  24  degrees. 

The  fourth  and  fifth  of  the  month  were  attended  with  a  thick  fog,  followed 
by  a  clouded  sky  on  the  sixth  and  seventh.  Between  the  seventh  and  twelfth 
there  occurred  a  fall  of  snow,  followed  by  almost  continuous  fair  weather 
till  the  twenty-fifth.  The  last  six  days  of  the  month  were  cloudy. 

From  a  comparison  of  these  particulars  with  those  of  the  weather  in  London, 

it  will  be  perceived  that  the  day  of  the  greatest  cold  was  the  twentieth  in  both 

places,  but  that  the  minimum  temperature  was  much  lower  in  Paris.     In  London 

the  thermometer  fell  on  the  twentieth  20  degrees  below  the  freezing  point,  but 

|  in  Paris  it  fell  on  the  same  day  34  degrees  below  it.     In  London,  the  highest 

<  temperature  on  the  twentieth  was  1 1  degrees  below  the  freezing  point ;  in  Paris 

£  the  highest  temperature  on  the  same  day  was  31  degrees  below  it.     In  London 

(  the  mean  temperature  of  the  month  was  1  degree  above  the  freezing  point ;  in 

^  Paris  it  was  8  degrees  below  it. 

It  will  be  perceived  that  the  severity  of  cold  in  Paris  was  in  every  point  of 
view  greater  than  that  in  London. 

It  is  remarkable,  also,  that  the  frost  not  only  commenced  on  the  same  day  in 
Paris  as  in  London,  but  the  cold  varied  in  very  nearly  the  same  manner,  though 
in  different  degrees.     The  increase  of  temperature  perceptible  in  London  on 
the  sixteenth,  commenced  in  Paris  on  the  fifteenth,  and  was  of  the  same  dura- 
tion.    On  the  twenty-second  and  twenty-third  in  London,  the  thermometer 
)  rose  to  above  40  degrees ;  and  on  the  same  day  in  Paris  it  likewise  rose  to 
j  above  40  degrees.     This  increase  of  temperature  was  in  like  manner  followed 
;  by  a  return  of  frost,  which  continued  till  the  twenty-ninth,  when  the  thermom- 
eter rose  to  44  degrees  in  both  places. 


1G8 


WEATHER  ALMANACS. 


The  subject  of  the  weather,  and  the  influences  which  are  supposed  to 
affect  it,  will  be  noticed  on  another  occasion,  when  I  shall  examine  in  all  the 
necessary  detail  the  question  of  the  supposed  influence  exerted  by  the 
phases  of  the  moon  upon  tb.3  changes  of  the  weather. 


•    *"X_-^*-* 


II ALLEY'S    COMET. 


Predictions  of  Science. — Structure  of  tho  Solar  System. — Motion  of  Comets. — How  to  identify  them. — 
Intervals  of  their  Appearance. — Halley's  Comet. — Its  History. — Newton's  Conjectures. — S.-<t.-:icity 
of  Voltaire. — Halley's  Researches. — Foretells  the  Reappearance  of  the  Comet  in  1759. — Principle  of 
Gravitation  applied  to  its  Motion  by  Clairaut. — Researches  of  that  Mathematician. — Anecdotes  of 
Lalunde  and  Madame  Lepaute. — Minute  and  circumstantial  Krediction  of  the  Reappearance  of 
Halley's  Comet.— Discovery  of  the  Planet  Herschel  anticipated  by  Clairaut. — Reappearance  of 
the  Comet  at  the  predicted  Time. — Second  Prediction  of  its  Return  in  1830. — Prediction  fulfill- 
ed.— Observations  on  its  Appearance  in  1835. 


HALLEY'S    COMET. 


171 


HALLEY'S    COMET. 


FOR  the  civil  and  political  historian  the  past  alone  has  existence — the  pres- 
ent he  rarely  apprehends,  the  future  never.  To  the  historian  of  science  it  is 
permitted,  however,  to  penetrate  the  depths  of  past  and  future  with  equal  clear- 
ness and  certainty ;  facts  to  come  are  to  him  as  present,  and  not  unfrequently 
more  assured  than  facts  which  are  passed.  Although  this  clear  perception  of 
causes  and  consequences  characterizes  the  whole  domain  of  physical  science, 
and  clothes  the  natural  philosopher  with  powers  denied  to  the  political  and 
moral  inquirer,  yet  foreknowledge  is  eminently  the  privilege  of  the  astronomer. 
Nature  has  raised  the  curtain  of  futurity,  and  displayed  before  him  the  succes- 
sion of  her  decrees,  so  far  as  they  effect  the  physical  universe,  for  countless 
ages  to  come  ;  and' the  revelations  of  which  she  has  made  him  the  instrument, 
are  supported  and  verified  by  a  never-ceasing  train  of  predictions  fulfilled.  He 
"  shows  us  the  things  which  will  be  hereafter,"  not  obscurely  shadowed  out  in 
figures  and  i-n  parables,  as  must  necessarily  be  the  case  with  other  revelations,  but 
attended  with  the  most  minute  precision  of  time,  place,  and  circumstance.  He 
converts  the  hours  as  they  roll  into  an  ever-present  miracle,  in  attestation 
of  those  laws  which  his  Creator  through  him  has  unfolded ;  the  sun  cannot 
rise — the  moon  cannot  wane — a  star  cannot  twinkle  in  the  firmament,  without 
bearing  witness  to  the  truth  of  his  prophetic  records.  It  has  pleased  the 
"  Lord  and  Governor"  of  the  world,  in  his  inscrutable  wisdom,  to  baffle  our 
inquiries  into  the  nature  and  proximate  cause  of  that  wonderful  faculty  of  intel- 
lect— that  image  of  his  own  essence  which  he  has  conferred  upon  us  ;  nay, 
the  springs  and  wheelwork  of  animal  and  vegetable  vitality  are  concealed  from 
our  view  by  an  impenetrable  veil,  and  the  pride  of  philosophy  is  humbled  by 
the  spectacle  of  the  physiologist  bending  in  fruitless  ardor  over  the  dissection 

NOTE.— A  portion  of  the  matter  which  forms  my  lectures  on  Comets,  was  formerly  contributed 

by  me,  on  various  occasions,  to  the  Edinburgh  Review,  and  other  leading  periodicals  in  England  ;  ( 

and  a  part  was  included  among  the  additions  to  the  late  edition  of  Arago's  Lectures,  edited  by  me  J 

in  America  j 


of  the  human  brain,  and  peering  in  equally  unproductive  inquiry  over  the  gam- 
bols of  an  animalcule.     But  how  nobly  is  the  darkness  which  envelopes  meta- 
physical inquiries  compensated  by  the  flood  of  light  which  is  shed  upon  the 
physical  creation!     There  all  is  harmony,  and  order,  and  majesty,  and  beauty. 
From  the  chaos  of  social  and  political  phenomena  exhibited  in  human  records —  ( 
phenomena  unconnected  to  our  imperfect  vision  by  any  discoverable  law,  a  war  ; 
of  passions  and  prejudices,  governed  by  no  apparent  purpose,  tending  to  no  ap-  ( 
parent  end,  and  setting  all  intelligible  order  at  defiance — how  soothing  and  yet  j 
how  elevating  it  is  to  turn  to  the  splendid  spectacle  which  offers  itself  to  the  ( 
habitual  contemplation  of  the  astronomer  !     How  favorable  to  the  development 
of  all  the  best  and  highest  feelings  of  the  soul  are  such  objects !     The  only 
passion  they  inspire  being  the  love  of  truth,  and  the  chiefest  pleasure  of  their 
votaries  arising  from  excursions  through  the  imposing  scenery  of  the  universe — 
scenery  on  a  scale  of  grandeur  and  magnificence,  compared  with  which  whatever 
we  are  accustomed  to  call  sublimity  on  our  planet,  dwindles  into  ridiculous  insig- 
nificancy.    Most  justly  has  it  been  said,  that  nature  has  implanted  in  our  bosoms 
a  craving  after  the  discovery  of  truth,  and  assuredly  that  glorious  instinct  is 
never  more  irresistibly  awakened  then  when  our  notice  is  directed  to  what  is 
going  on  in  the  heavens.     "  Quoniam  eadem  Natura  cupiditatem  ingenuit  homi- 
nibus  veri  inveniendi,  quod  facillime  apparet,  cum  vacui  curis,  etiam  quid  in  } 
cffilo  fiat,  scire  avemus  ;  his  initiis  indued  omnia  vera  diligimus  ;  id  est,  fidelia,  <. 
simplicia,  constantia ;  turn  vana,  falsa,  fallentia  odimus."* 

Among  the  multitude  of  appearances  which  succeed  each  other  in  their  ap-  •, 
pointed  order,  and  of  the  times  and  manner  of  which  the  perfect  knowledge  ^ 
of  the  astronomer  enables  him  to  advertise  us,  there  are  some  which  mor;  I 
powerfully  seize  upon  the  popular  mind,  as  well  by  reason  of  their  infrequenc/  ) 
and  the  extraordinary  circumstances  which  attend  them,  as  by  the  imaginary  > 
consequences  with  which  ignorance  and  superstition  have,  in  times  past  and.  ' 
present,  invested  them.  Among  these,  Solar  Eclipses  had  a  prominent  place  ;  , 
but  a  still  more  interesting  position  must  be  assigned  to  Comets. 

It  is  well  known  that  the  solar  system,  of  which  our  planet  forms  a  part,  con-  ^ 
sists  of  a  number  of  smaller  bodies  revolving  in  paths,  which  are  very  nearly  ) 
circular,  round  the  great  mass  of  the  sun  placed  in  the  centre.  Thes3  paths,  > 
or  orbits,  are  very  nearly  in  the  same  plane ;  that  is  to  say,  if  the  earth,  for  / 
example,  be  conceived  to  be  moving  on  a  flat  surface,  extended  as  well  beyond  ( 
its  orbit  as  within  it,  then  the  other  planets  never  depart  much  above  or  below  > 
this  plane.  A  spectator  placed  upon  the  earth  keeps  within  his  view  each  of  <* 
the  other  planets  of  the  system  throughout  nearly  the  whole  of  its  course.  In- 
deed,  there  is  no  part  of  the  orbit  of  any  planet  in  which,  at  some  time  or  other,  ', 
it  may  not  be  seen  from  the  earth.  Every  point  of  the  path  of  each  planet  , 
can  therefore  be  observed ;  and  although  without  waiting  for  such  observation  > 
its  course  might  be  determined,  yet  it  is  material  here  to  attend  to  the  fact,  that  > 
the  whole  orbit  may  be  submitted  to  direct  observation.  The  different  planets  , 
also  present  peculiar  features  by  which  each  may  be  distinguished.  Thus  they  > 
are  observed  to  be  spherical  bodies  of  various  'magnitudes.  The  surfaces  of  J 
some  are  marked  by  peculiar  modes  of  light  and  shade,  which,  although  varia-  > 
ble  and  shifting,  still,  in  each  case,  possess  some  prevailing  and  permanent  \ 
characters  by  which  the  identity  of  the  object  may  be  established,  even  were  \ 
there  no  other  means  of  determining  it.  The  sun  is  the  common  centre  of  at-  < 
traction,  the  physical  bond  by  which  this  planetary  family  are  united,  and  pre-  / 
vented  from  wandering  independently  through  the  abyss  of  space.  Fach  planet  » 
thus  revolving  in  a  circle,  has  the  same  tendency  to  fly  from  the  centre  that  a  < 

•  Cic.  de  Fin.  Bon.  et  Mai.  ii.  14. 


HALLEY'S  COMET. 


173 


stone  has  when  whirled  in  a  sling.  Why,  then,  it  will  be  asked,  do  not  the 
planets  yield  to  this  natural  tendency  ?  What  enables  them  to  resist  it  ?  To 
this  question  no  satisfactory  answer  can  be  given  ;  but  the  fact  that  the  tendency 
is  resisted,  being  certain,  the  existence  of  some  physical  principle  in  which 
the  means  of  such  resistance  resides,  is  proved.  As  the  tendency  to  fly  off  is 
directed  from  the  centre  of  the  sun,  the  opposing  physical  influence  must  con- 
sequently be  directed  toward  that  centre.  This  central  influence  is  what  has 
been  called  gravitation.  Although  we  are  still  ignorant  of  the  nature  or  proxi- 
mate cause  of  this  force,  and  of  its  modus  operandi,  we  have  obtained  a  per- 
fect knowledge  of  the  laws  by  which  it  acts ;  and  this  is  all  that  is  necessary 
or  material  to  enable  us  to  follow  out  its  consequences.  By  virtue  of  this  force 
of  gravitation,  then,  the  planetary  masses  receive  a  tendency  to  drop  toward 
the  sun,  which  tendency  equilibrates  with  the  opposite  tendency  to  fly  away, 
produced  by  their  orbitual  motion.  On  the  exact  equilibrium  of  these  two  op- 
posite physical  principles,  depends  the  stability  of  the  system.  If  the  centrif- 
ugal tendency  proceeding  from  the  orbitual  motion  were  in  excess,  the  planets 
would  fall  off  from  the  central  body,  and  depart  for  ever  into  the  depths  of  space ;  ; 
if,  on  the  other  hand,  the  central  influence,  or  gravitation  toward  the  sun,  ex-  < 
isted  in  excess,  these  bodies  would  gradually  approach  that  luminary,  and  finally 
coalesce  with  his  mass. 

Besides  these  bodies,  the  greater  part  of  which  have  been  long  known,  and 
the  motions  of  most  of  which  have  been  in  some  degree  understood,  even  from 
remote  antiquity,  there  is  a  still  more  numerous  class  of  objects,  whose  appear- 
ances in  the  system  were  of  such  a  nature  as  to  defy  the  powers  of  philosophi- 
cal inquiry,  until  these  powers  received  that  prodigious  accession  of  force 
which  was  conferred  upon  them  by  the  discoveries  of  Newton.  Unlike  planets, 
comets  do  not  present  to  us  those  individual  characters  above  mentioned,  by 
which  their  identity  may  be  determined.  None  of  them  have  been  satisfacto- 
rily ascertained  to  be  spherical  bodies,  nor  indeed  to  have  any  definite  shape. 
Ic  is  certain  that  many  of  them  possess  no  solid  matter,  but  are  masses  con- 
sisting entirely  of  aeriform  or  vaporous  substances  ;  others  are  so  surrounded 
with  this  vaporous  matter,  that  it  is  impossible,  by  any  means  of  observation 
which  we  possess,  to  discover  whether  this  vapor  enshrouds  within  it  any  solid 
mass.  The  same  vapor  which  thus  envelopes  the  body  (if  such  there  be  with- 
in it),  also  conceals  from  us  its  features  and  individual  character.  Even  the 
limits  of  the  vapor  itself  are  subject  to  great  change  in  each  individual  comet. 
Within  a  few  days  they  are  sometimes  observed  to  increase  or  diminish  some 
hundred  fold.  A  comet  appearing  at  distant  intervals,  presents,  therefore,  no 
very  obvious  means  of  recognition.  A  like  extent  of  surrounding  vapor  would 
evidently  be  a  fallible  test  of  identity ;  and  not  less  inconclusive  would  it  be  to 
infer  diversity  from  a  different  extent  of  nebulosity. 

If  a  comet,  like  a  planet,  revolved  round  the  sun  in  an  orbit  nearly  circular, 
it  might  be  seen  in  every  part  of  its  path,  and  its  identity  might  thus  be  estab- 
lished independently  of  any  peculiar  characters  in  its  appearance.  But  such 
is  not  the  course  which  comets  are  observed  to  take.  These  bodies  usually 
are  observed  to  rush  into  our  system  suddenly  and  unexpectedly,  from  some 
particular  quarter  of  the  universe.  They  first  follow  in  a  straight  line,  or  nearly 
so,  the  course  by  which  they  entered  ;  and  this  course  is  commonly  directed 
to  some  point  not  far  removed  from  the  sun.  As  they  approach  that  luminary, 
their  path  becomes  curved;  at  first  slightly,  but  afterward  more  and  more  ;  the 
curve  being  concave  toward  the  sun.  Having  arrived  at  a  certain  least  dis- 
tance from  the  centre  of  our  system,  they  again  begin  to  recede  from  the  sun, 
and  as  their  distance  increases,  their  path  becomes  less  and  less  curved  ;  until 
at  length  they  shoot  off  in  a  straight  course,  and  make  their  exit  from  our  sys- 


tern  toward  some  quarter  of  the  universe  wholly  different  from  that  from  which 
they  came. 

We  have  stated  that  none  of  the  planets  depart  much  above  or  below  the 
plane  of  the  earth's  orbit ;  it  is  quite  otherwise  with  comets,  which  follow  no 
certain  law  in  this  respect.  Some  of  them  sweep  the  system  nearly  in  the 
plane  in  which  the  planets  move  ;  others  rush  through  it  in  curves,  oblique  in 
various  degrees  to  this  plane  ;  while  some  move  in  planes  perpendicular  to  it. 
The  planets  also  move  round  the  sun  all  in  one  direction.  Comets,  on  ths  other 
hand,  rebel  against  this  law,  and  move,  some  in  one  direction  and  some  in 
another. 

So  far  then  as  observation  informs  us,  we  are  left  to  decide  between  two 
suppositions  :  1.  That  the  comet  has  entered  the  system  for  the  first  time  ;  and 
that  having  swept  behind  the  sun,  it  has  emerged  in  a  different  direction,  never 
to  return :  2.  That  it  moves  in  a  large  orbit,  of  which  the  sun  is  not  the  cen- 
tre, but.  on  the  contrary,  is  placed  very  near  the  path  of  the  body  itself;  that 
the  comet  is  visible  only  in  that  part  of  its  orbit  which  is  in  the  immediate 
neighborhood  of  the  sun,  but  is  invisible  throughout  that  large  part,  which  per- 
haps extends,  through  depths  of  space,  far  beyond  our  most  remote  planet.  If 
the  latter  supposition  be  adopted,  it  would  follow  that  the  same  comet,  after 
emerging  from  our  system,  would,  after  the  lapse  of  a  certain  time,  return  to  it, 
arid  pursue  the  same  path,  or  nearly  the  same  path,  round  the  sun  as  before. 
If  then  we  find,  after  the  lapse  of  a  certain  time,  a  comet  following  the  same 
path  wrhile  visible,  as  a  former  comet  was  observed  to  follow,  we  infer  that 
they  also  followed  the  same  path  during  that  much  longer  period  in  which  they 
were  beyond  the  sphere  of  our  observation,  and  consequently  we  infer,  with  a 
high  degree  of  probability,  that  the  comets  which  thus  follow  precisely  the 
same  track,  must  be  the  same  comet.  We  say  with  probability,  because  there 
is  a  possibility,  although  it  be  a  bare  possibility,  that  two  different  comets 
should  move  precisely  in  the  same  orbit. 

Now,  let  us  suppose  that,  during  the  appearance  of  a  comet,  its  path  from 
day  to  day,  or  perhaps  from  hour  to  hour,  is  so   carefully  observed,  that  we 
could  delineate  it  with  a  corresponding  degree  of  accuracy  in   any  plan   or 
model  of  the  system.     This  path  would,  as  we  have  seen,  form  a  very  small 
fragment  of  its   entire  orbit ;  but  if  the  nature  of  that  orbit  were  known,  the 
principles  of  geometry  would  also  enable  us  to  complete  the  curve.     Thus,  if 
a  small  arc  of  a  large  circle  be  traced  upon  paper,  every  one  conversant  with 
geometry  will  be  able  to  complete  the  circle,  even  though  he  be  not  told  with 
what  centre  the   small  arc  was  described,  or  with  what  length  of  radius.     It 
is  the  same  with  other  curves.     Newton  has  proved  that  every  mass  of  matter 
which  is  moved  through  the  system,  under  the  attracting  influence  of  the  sun. 
must,  by  its  motion,  trace  one  or  other  of  those  curves  called   conic  sections: 
and  that  the  curve  must  be  so  placed,  that  the  centre  of  the  sun  shall  be  in  that 
point  which  is  called  its  focus.     Now,  conic  sections  are  of  three  kinds  ;  the 
ellipse,  which  is  a  curve  of  oval  form,  such  that  a  point  moving  on  it  would  re- 
trace the  same  course  every  revolution.     But  the  other  two  species  (called  the 
parabola  and  hyperbola),  consist  of  two  branches  diverging  from  their  point  of 
connexion  in  two  different  directions,  and  proceeding  in  those  directions  with- 
out ever  again  reuniting.     If  a  body  (as  it  might  do  by  the  established  law  of 
!  gravitation)  entered  our  system  by  one  branch  of  such  a  curve,  it  would,  after 
|  sweeping  behind  the  sun,  emerge  by  the  other  branch  never  to  return.     Thus  it 
i  appears,  that  either  of  the  two  suppositions  which  we  have  just  made,  would  be 
|  compatible  with  the  law  of  gravitation  ;  and  it  is  possible  that  some  comets  misrht 
i  mov'e  in  ellipses,  returning  continually  over  the  same  path  at  stated  intervals, 
[  while  others,  moving  in  parabolas,  or  hyperbolas,  entering  our  system  for  the  first 


J 


HALLE Y'S  COMET. 


175 


and  only  time,  would  emerge  from  it  in  another  direction,  and  quit  it  for  ever. 
It  will  perhaps  be  asked,  if  the  orbits  must  be  conic  sections,  with  the  sun  in  the 
focus,  how  is  it  that  the  planetary  orbits  are  considered  as  circles  ?  The  fact 
is,  the  planetary  orbits  are  not.  strictly  circular,  though  very  nearly  so  ;  they 
are  ellipses,  which  are  so  slightly  oval,  that,  when  exhibited  in  a  drawing,  they 
would  not  be  perceived  to  be  so,  unless  their  length  and  breadth  were  ac- 
curately measured.  The  centre  of  the  sun,  also,  is  in  their  focus,  and  not  in 
their  centre  ;  but  owing  to  their  slightly  oval  form,  the  distance  of  the  focus 
from  the  centre  is  very  inconsiderable  compared  with  their  whole  magnitude. 

To  obtain  a  correct  notion  of  the  form  of  an  ellipse,  let  a  flexible  string  be 
attached  to  two  points,  such  as  A  and  B,  and  let  a  pencil  be  looped  in  it  so 
that  when  the  string  is  stretched  the  pencil  will  be  at  D  ;  the  string  extending 
from  A  to  D,  and  from  D  to  B.  Let  the  pencil  be  moved,  carrying  the  loop 


with  it.  It  will  pass  successively  to  the  points  C,  E,  M,  &c.,  &c.,  describing 
the  oval  curve,  D,  C,  E,  M,  L.  This  curve  is  called  an  ellipse.  The  points 
A  and  B  are  called  its  foci,  and  the  point  0,  at  the  middle  of  the  distance  A 
B,  is  called  its  centre.  The  ellipse  will  be  more  or  less  oval  as  the  string  is 
less  or  greater  than  the  distance  A  B. 

Such  is  the  general  form  of  the  curves  in  which  the  comets  move.  If  the 
entire  ellipse  except  the  part  D,  L,  G,  were  blotted  out,  it  would  be  very  dif- 
ficult to  distinguish  the  arc  D,  L,  G,  from  that  of  a  parabola  or  hyperbola. 

On  the  appearance  of  a  comet  then,  the  first  question  which  presents  itself 
to  the  astronomical  inquirer  is,  whether  the  same  comet  has  ever  appeared  be- 
fore ?  and  the  only  means  which  he  possesses  of  answering  this  inquiry  is,  by 
ascertaining,  from  such  observations  as  may  be  made  during  its  appearance, 
the  exact  "path  it  follows  ;  and  this  being  known,  to  discover,  by  the  principles 
i  of  geometry,  the  nature  of  its  orbit.  If  the  orbit  be  found  to  be  an  ellipse,  then 
the  return  of  the  comet  would  be  certain,  and  the  time  of  the  return  would  be 
known  by  the  magnitude  of  the  ellipse.  If  the  path,  on  the  other  hand,  should 
appear  to  be  either  a  parabola  or  hyperbola,  then  it  would  be  equally  certain  that 
the  comet  had  never  been  before  in  our  system,  and  would  never  return  to  it. 


176  HALLEY'S  COMET. 


But  a  difficulty  of  a  peculiar  nature  obstructs  the  solution  of  this  question. 
It  so  happens  that  the  only  part  of  the  course  of  a  comet  which  ever  can  be 
visible,  is  a  portion  such  as  D,  L,  G,  throughout  which  the  ellipse,  the  para- 
bola, and  hyperbola  so  closely  resemble  one  another,  that  no  observations  can 
be  obtained  with  sufficient  accuracy  to  enable  us  to  distinguish  one  from  the 
other.  In  fact,  the  observed  path  of  any  comet,  while  visible,  may  indiffer- 
ently belong  to  an  ellipse,  parabola,  or  hyperbola. 

There  is,  nevertheless,  a  certain  degree  of  definiteness  in  the  observed 
course  of  the  body,  which,  although  insufficient  to  enable  us  to  say  what  the 
nature  of  the  entire  orbit  may  be,  is  still  sufficiently  exact  to  enable  us  to  rec- 
ognise any  other  comet,  which  moves,  while  visible,  nearly  in  the  same  course. 
If  then,  after  the  lapse  of  a  certain  time,  a  comet  should  be  found  following 
that  course,  there  would  be  a  strong  presumption  that  it  is  the  same  comet  re- 
turning again  to  our  system,  after  having  traversed  the  invisible  part  of  its 
orbit.  This  probability  would  be  strengthened,  if,  on  the  two  occasions,  the 
body  should  present  a  similar  appearance  ;  although  this  is  not  essential  to  the 
identity,  since,  as  has  been  stated,  the  same  comet  is  observed  to  undergo  con- 
siderable changes,  even  during  a  single  appearance. 

The  time  between  the  appearances  of  comets  following  nearly  the  same  path 
being  noted,  the  interval — the  identity  of  the  bodies  being  assumed — must 
either  consist  of  a  single  period,  or  of  two  or  more  complete  periods.  The 
epoch  which  is  usually  taken  as  the  commencement  of  a  new  revolution  being 
the  instant  of  time  at  which  the  comet  is  at  its  least  distance  from  the  sun,  the 
place  of  the  comet  at  that  moment  is  called  its  perihelion.  The  period  of  a 
comet  may,  therefore,  be  defined  to  be  the  interval  of  time  between  two  suc- 
cessive arrivals  at  its  perihelion. 

Having  succeeded  in  identifying  the  path  of  any  two  comets,  we  are  then 
in  a  condition  to  predict  with  some  degree  of  probability  the  time  at  which  the 
next  appearance  may  be  expected.  It  is  certain — providing  that  it  be  the  same 
comet — that  it  will  arrive  at  its  perihelion  after  the  same  interval  nearly ;  also 
that  it  may  arrive  at  half  the  interval,  or  a  third  of  the  interval,  or  any  other 
fraction  corresponding  to  the  possible  number  of  unobserved  appearances  which 
may  have  taken  place  in  the  interval  between  those  appearances  from  which 
its  return  has  been  predicted.  The  times,  therefore,  at  which  the  comet  .may 
be  looked  for  with  a  probability  of  rinding  it,  may  without  difficulty  be  predicted  ; 
and  if  it  has  been  a  conspicuous  body  in  the  heavens  on  the  occasion  of  its 
former  appearances,  there  is  a  probability  that  the  whole  interval  between  these 
appearances  constituted  but  one  period,  and  that  no  return  of  the  comet  had 
escaped  observation. 

Such  are  the  circumstances  which  may  have  been  conceived  to  have  pre- 
sented themselves  when  the  idea  first  occurred  of  attempting  to  ascertain  the 
identity  of  former  comets,  and  to  discover  the  means  of  predicting  their  future 
return.  The  Principia  of  Newton,  which  laid  the  foundation  of  all  sound  as- 
tronomical science,  had  appeared  soon  after  tie  middle  of  the  seventeenth 
century ;  and  Halley,  the  contemporary  and  friend  of  Newton,  had  his  atten- 
tion naturally  directed  to  the  physical  inquiries  which  that  immortal  book  sug- 
gested. 

One  of  the  most  curious  and  interesting  of  these  questions  was  that  to  which 
we  now  allude.  Halley,  referring  to  the  records  of  all  former  observers, 
with  a  view  to  obtain  means  of  determining,  so  far  as  possible,  the  course 
of  former  comets,  succeeded  in  identifying  one  which  he  had  himself  ob- 
served in  1682,  with  comets  which  had  appeared  on  several  former  occa- 
sions ,  and  found,  that  the  interval  between  its  successive  returns  was  from 
75  to  76  years.  This  discovery  has  since  been  fully  confirmed,  and  the  comet 


HALLEY'S  COMET.  ]77 


has  received  the  name  of  Halley's  comet.     We  now  propose  to  lay  before  the 
reader  the  history  of  this  celebrated  comet. 

In  retracing  the  history  of  a  body  of  this  nature  so  far  as  we  can  collect  it 
from  ancient  chroniclers  and  historians,  it  is  necessary  to  bear  in  mind  that  ? 
the  terror  which  the  appearance  of  comets  inspired,  had  a  tendency  to  produce 
an  exaggeration  of  their  effects.  The  propensity  to  ascribe  to  supernatural 
causes,  effects  which  the  understanding  fails  to  account  for,  has  rendered 
comets  peculiarly  objects  of  superstitious  terror.  Thev  have  been  accordingly 
regarded  in  past  ages  as  the  harbingers  of  war,  pestilence,  and  famine,  and  of 
all  the  greatest  scourges  which  have  visited  the  human  race.  But  more  es- 
pecially they  have  presided  at  the  birth  and  death  of  the  most  celebrated 
heroes.  Thus,  a  conspicuous  body  of  this  kind  appeared  for  several  days  suc- 
ceeding 'be  death  of  Julius  Caesar,  and  was  regarded  as  the  soul  of  that  illus- 
trious f  •*«.  './transferred  to  the  heavens.  Another  was  seen  at  Constantinople 
in  the  year  o»f  the  birth  of  Mohammed.  It  is  obvious,  that  under  the  influence 
of  such  powerful  prejudices,  the  circumstances  attending  these  appearances 
would  naturally  ba  amplified  and  exaggerated  ;  and  the  probability  of  exag- 
geration is  inci eased  by  the  fact  that  since  science  has  shed  its  light  upon  the 
civilized  world,  these  terrible  objects  have,  in  a  great  degree,  disappeared,  and 
comets  have  dwindled  for  the  most  part  into  very  insignificant  appearances. 
One  of  the  ill  consequences  of  this  exaggeration  is,  that  it  greatly  increases 
the  difficulty  of  identifying  the  bodies  which  have  been  described  with  those 
which  have  appeared  in  more  recent  times.  In  fact,  we  have  little  more  to 
guide  us  than  the  epochs  of  the  respective  appearances ;  and,  antecedently  to 
the  fifteenth  century,  we  possess  absolutely  no  other  evidence  of  the  identity 
of  these  bodies  except  the  record  of  their  appearance  at  the  times  at  which  we 
know,  from  their  ascertained  periods,  they  ought  to  have  appeared.  Adopting 
this  test  of  identity,  it  would  seem  at  least  probable  that  the  first  recorded  ap- 
pearance of  Halley's  comet  was  that  which  was  supposed  to  signalize  the 
birth  of  Christ.  It  is  said  to  have  appeared  for  twenty-four  days  ;  its  light  is 
described  to  have  surpassed  that  of  the  sun ;  its  magnitude  to  have  extended 
over  a  fourth  part  of  the  firmament ;  and  it  is  stated  to  have  occupied  conse- 
quently about  four  hours  in  rising  and  setting. 

In  the  year  323,  a  comet  appeared  in  the  sign  Virgo.  Another,  according 
to  the  historians  of  the  Lower  Empire,  appeared  in  the  year  399,  seventy  five 
years  after  the  last ;  this  last  interval  being  the  period  of  Halley's  comet. 

The  interval  between  the  birth  of  Mithridates  and  the  year  323  was  four 
hundred  and  fifty-three  years,  which  would  be  equivalent  to  six  periods  of  sev- 
enty-five and  a  half  years.  Thus,  it  would  seem,  that  in  the  interim  there  were 
five  returns  of  this  comet  unobserved,  or  at  least  unrecorded.  The  appearance 
in  the  year  399  was  attended  with  extraordinary  circumstances.  In  the  T/ie- 
atrum  Cumetarum  of  Lobienietski,  it  is  described  as  cometa  prodigiosa  magni- 
tudinis,  hornbilis  aspectu,  comam,  ad  terrain  usque  demittere  visus.  The  next 
recorded  appearance  of  a  comet  agreeing  with  the  ascertained  period,  marks 
the  taking  of  Rome  by  Totila  in  the  year  550 ;  an  interval  of  one  hundred  and 
fifty-one  years,  or  two  periods  of  seventy-five  and  a  half  years,  having  elapsed. 
One  unrecorded  term  must,  therefore,  have  taken  place  in  this  interim.  The 
next  appearance  of  a  comet  coinciding  with  the  assigned  period  is  three  hun- 
dred and  eighty  years  afterward,  viz.,  in  the  year  930,  five  revolutions  having 
been  completed  in  the  interval.  The  next  appearance  is  recorded  in  the  year 
1005,  after  an  interval  of  a  single  period  of  seventy-five  years.  Three  revo- 
lutions would  now  seem  to  have  passed  unrecorded,  when  the  comet  again 
makes  its  appearance  in  1230.  In  this,  as  well  as  in  former  appearances,  it  is 
to  state  once  more,  that  the  sole  test  of  identity  of  these  comets  with  that 


L, 


of  Halley,  is  the  coincidence  of  the  times  of  their  appearances,  as  nearly  as 
historical  records  enable  us  to  ascertain,  with  the  epochs  at  which  the  comet 
of  Halley  might  have  been  expected  to  appear.  That  such  evidence,  however, 
must  needs  be  imperfect  will  be  evident,  if  the  frequency  of  cometary  appear- 
ances be  considered  ;  and  if  it  be  remembered  that  hitherto  we  find  no  recorded 
observations  which  could  enable  us  to  trace  even  with  the  rudest  degree  of 
approximation  the  paths  of  those  comets,  the  times  of  whose  appearances  raise 
a  presumption  of  their  identity  with  that  of  Halley.  We  now,  however,  de- 
scend to  times  in  which  more  satisfactory  evidence  may  be  expected. 

In  the  year  1305,  one  of  those  in  which  the  comet  of  Halley  may  have  been 
expected,  a  comet  is  recorded  of  remarkable  appearance  :  Cometa  horrendce 
masnitudinis  visus  est  circa  ferias  Pasckatis,  quern  secuta  est  pestileniia  maxima. 
Had  the  horrid  appearance  of  this  body  alone  been  recorded,  this  description 
might  have  passed  without  the  charge  of  great  exaggeration  ;  butVhen  we  find 
the  Great  Plague  connected  with  it  as  a  consequence,  it  is  impossible  not  to  con- 
clude that  the  comet  was  seen  by  its  historians  through  the  magnifying  medium 
of  the  calamity  which  followed  it.  Another  appearance  is  recorded  in  the  year 
1380,  unaccompanied  by  any  other  circumstance  than  its' mere  date.  This, 
however,  is  in  strict  accordance  with  the  ascertained  period  of  Halley's 
comet. 

We  now  arrive  at  the  first  appearance  at  which  observations  were  taken, 
possessing  sufficient  accuracy  to  enable  subsequent  investigators  to  determine 
the  path  of  the  comet :  and  this  is  accordingly  the  first  comet,  the  identity  of 
which  with  the  comet  of  Halley  can  be  said  to  be  conclusively  established. 
In  the  year  1456,  a  comet  is  stated  to  have  appeared,  of  "  unheard-of  magni- 
tude ;"  it  was  accompanied  by  a  tail  of  extraordinary  length,  which  extended 
over  sixty  degrees  (a  third  of  the  heavens),  and  continued  to  be  seen  during 
the  whole  of  the  month  of  June.  The  influence  which  was  attributed  to  this 
appearance  renders  it  probable  that  in  the  record  there  exists  more  or  less  of 
exaggeration.  It  was  considered  as  the  celestial  indication  of  the  rapid  sue-  \ 
cess  of  Mohammed  II.,  who  had  taken  Constantinople,  and  struck  terror  into  \ 
the  whole  Christian  world.  Pope  Calixtus  II.  levelled  the  thunders  of  the  ) 
church  against  the  enemies  of  his  faith,  terrestrial  and  celestial,  and  in  the 
same  bull  exorcised  the  Turks  and  the  comets  ;  and  in  order  that  the  memory 
of  this  manifestation  of  his  power  should  be  for  ever  preserved,  he  ordained 
that  the  bells  of  all  the  churches  should  be  rung  at  midday — a  custom  which  is 
preserved  in  those  countries  to  our  times.  It  must  be  admitted  that,  notwith- 
standing the  terrors  of  the  church,  the  comet  pursued  its  course  with  as  much 
ease  and  security  as  those  with  which  Mohammed  converted  the  church  of  St. 
Sophia  into  his  principal  mosque. 

The  extraordinary  length  and  brilliancy  which  was  ascribed  to  the  tail  upon 
this  occasion,  have  led  astronomers  to  investigate  the  circumstances  under 
which  its  brightness  and  magnitude  would  be  the  greatest  possible  ;  and,  upon 
tracing  back  the  motion  of  the  comet  to  the  year  1456,  it  has  been  found  that  it 
was  then  actually  under  the  circumstances  of  position  with  respect  to  the 
earth  and  sun  most  favorable  to  magnitude  and  splendor.  So  far,  therefore 
the  results  of  astronomical  calculation  corroborate  the  records  of  history. 

The  next  return  took  place  in  the  year  1531.  Pierre  Appian,  who  first  as- 
certained the  fact  that  the  tails  of  comets  are  usually  turned  from  the  sun,  ex- 
amined this  comet,  with  a  view  to  verify  his  statement,  and  to  ascertain  the 
true  direction  of  its  tail.  He  made  accordingly  numerous  observations  upon 
its  position,  which,  though,  compared  with  the  present  standard  of  accuracy, 
they  must  be  regarded  as  of  a  rude  nature,  were  still  sufficiently  exact  to  enable 
Halley  to  identify  this  comet  with  that  observed  by  himself  in  1682. 


The  next  return  took  place  in  1607,  when  the  comet  was  observed  by  the 
celebrated  Kepler.  This  astronomer,  on  his  return  from  a  convivial  party,  first 
saw  it  on  the  evening  of  the  26th  of  September  ;  it  had  the  appearance  of  a 
star  of  the  first  magnitude,  and,  to  his  vision,  was  without  a  tail ;  but  the  friends 
who  accompanied  him,  having  better  sight,  distinguished  the  tail.  Before 
three  o'clock  the  following  morning,  the  tail  had  become  clearly  visible,  and 
had  acquired  great  magnitude.  Two  days  afterward  the  comet  was  observed 
by  Longomontanus  ;  he  describes  its  appearance,  to  the  naked  eye,  to  be  like 
Jupiter,  but  of  a  paler  and  more  obscure  light ;  that  its  tail  was  of  considerable 
length,  of  a  paler  light  than  that  of  the  head,  and  more  dense  than  the  tails  of 
ordinary  comets.  He  states  that  on  the  24th  of  September  following,  the  comet 
was  not  apparent ;  that  on  the  24th  of  October  it  was  seen  obscurely,  and  some 
days  afterward  disappeared  altogether. 

The  next  appearance,  and  that  which  was  observed  by  Halley  himself,  took 
place  in  1682,  a  little  before  the  publication  of  the  Principia.  A  comet  of 
frightful  magnitude  had  appeared  in  1680,  and  had  so  terrified  all  Europe,  that 
the  subject  of  our  present  inquiry,  though  of  such  immense  astronomical  im- 
portance, excited  comparatively  little  popular  notice.  In  the  interval,  however, 
between  1607  and  1682,  practical  astronomy  had  made  great  advances  ;  instru- 
ments of  observations  had  been  brought  to  a  state  of  comparative  perfection ; 
numerous  observatories  had  been  established,  and  the  management  of  them  had 
been  confided  to  the  most,  eminent  astronomers  of  Europe.  In  1682,  the  sci- 
entific world  was,  therefore,  prepared  to  examine  this  visiter  of  our  system 
with  a  degree  of  care  and  accuracy  before  unknown.  It  was  observed  at  Paris 
by  Lahire,  Picard,  and  Dominique  Cassini ;  at  Dantzic,by  Hevelius  ;  at  Padua. 
by  Alontonari ;  and  in  England,  by  Halley  and  Flamstead. 

In  1686,  about  four  years  afterward,  Newton  published  his  Principia,  in 
which  ho  applied  to  the  comet  of  1680  the  general  principles  of  physical  in- 
vestigation first  promulgated  in  that  work.  He  explained  the  means  of  deter- 
mining, by  geometrical  construction,  the  visible  portion  of  the  path  of  a  body 
of  this  kina,  and  invited  astronomers  to  apply  these  principles  to  the  various 
recorded  comets — to  discover  whether  some  among  them  might  not  have  ap- 
peared at  different  epochs,  the  future  returns  of  which  might  consequently  be 
predicted.  Such  was  the  effect  of  the  force  of  analogy  upon  the  mind  of 
Newton,  that,  without  awaiting  the  discovery  of  a  periodic  comet,  he  boldly 
assumed  these  bodies  to  be  analogous  to  planets  in  their  revolution  round  the 
sun. 

In  the  ifrird  book  of  his  Principia,  he  calls  them  a  species  of  planets  re- 
volving in  elliptic  orbits,  of  a  very  oval  form,  and  even  remarks  an  analogy 
observable  between  the  order  of  their  magnitudes  and  those  of  the  planets.  He 
says,  "  As  among  planets  without  tails,  those  which  revolve  in  less  orbits,  and 
nearer  to  the  sun,  are  of  less  magnitude,  so  comets  which  in  their  perihelia 
approach  still  nearer  to  the  sun  than  the  planets,  are  much  less  than  the  plan- 
ets, that  their  attraction  may  not  act  too  strongly  on  the  sun.  But,"  he  con- 
tinues, '•  I  leave  to  be  determined  by  others  the  transverse  diameters  and 
periods,  by  comparing  comets  which  return  after  long  intervals  of  time  to  the 
same  orbits." 

It  is  interesting  to  observe  the  avidity  Avith  which  minds  of  a  certain  order 
snatch  at  generalizations,  even  when  but  slenderly  founded  upon  facts.  These 
conjectures  of  Newton  were  soon  after  adopted  by  Voltaire  :  "  II  y  a  quelque 
apparence,"  says  he,  in  an  essay  on  comets,  "  qu'on  connaitra  un  jour  un  cer- 
tain nombre  de  ces  autres  planetes  qui  sous  le  nom  de  cometes  tournent  comme 
nous  autour  du  soleil,  mais  il  ne  faut  pas  esperer  qu'on  les  connaissent  toutes." 

And  again,  elsewhere,  on  the  same  subject : — 


'•  Cometes,  que  Ton  craint  a  1'egal  du  tonnere, 
Cessez  d'epouvanter  les  peuples  de  la  terre ; 
Dans  une  ellipse  immense  achevez  votre  cours, 
Itemoutez,  descendez  pres  de  1'astre  des  jours." 

Extraordinary  as  these  conjectures  must  have  appeared  at  the  time,  they 
were  soon  strictly  realized.  Halley  undertook  the  labor  of  examining  the  cir- 
cumstances attending  all  the  comets  previously  recorded,  with  a  view  to  dis- 
cover whether  an\c  and  which  of  them,  appeared  to  follow  the  same  path. 
Antecedently  to  the  year  1700,  four  hundred  and  twenty-five  of  these  bodies 
had  been  recorded  in"  history  ;  but  those  which  had  appeared  before  the  four- 
teenth century  had  not  been  submitted  to  any  observations  by  which  their  paths 
could  be  ascertained — at  least  not  with  a  sufficient  degree  of  precision  to  afford 
any  hope  of  identifying  them  with  those  of  other  comets.  Subsequently  to  the 
year  1300,  however,  Halley  found  twenty-four  comets  on  which  observations 
had  been  made  and  recorded,  with  a  degree  of  precision  sufficient  to  enable 
him  to  calculate  the  actual  paths  which  these  bodies  followed  while  they  were 
visible.  He  examined  with  the  most  elaborate  care  the  courses  of  each  of 
these  twenty-four  bodies  ;  he  found  the  exact  points  at  which  each  of  them 
penetrated  the  plane  of  the  earth's  orbit ;  also  the  angle  which  the  direction  of 
their  motion  made  with  that  plane  ;  he  also  calculated  the  nearest  distance  at 
which  each  of  them  approached  the  sun,  and  the  exact  place  of  the  body  when 
at  that  nearest  distance.  In  a  word,  he  determined  all  the  circumstances 
which  were  necessary  to  enable  him  to  lay  down,  with  sufficient  precision, 
the  path  which  these  comets  must  have  followed  while  they  continued  to  be 
visible. 

On  comparing  their  paths,  Halley  found  that  one  which  appeared  in  1661, 
followed  nearly  the  same  path  as  one  which  had  appeared  in  1532.  Suppo- 
sing, then,  these  to  be  two  successive  appearances  of  the  same  comet,  it  would 
follow  that  its  period  would  be  one  hundred  and  twenty-nine  years ;  and 
Halley  accordingly  conjectured  that  its  next  appearance  might  be  expected 
after  the  lapse  of  one  hundred  and  twenty-nine  years,  reckoning  from  1661. 
Had  this  conjecture  been  well  founded,  the  comet  must  have  appeared  about 
the  year  1790.  No  comet,  however,  appeared  at  or  near  that  time  following  a 
similar  path. 

In  his  second  conjecture,  Halley  was  more  fortunate,  as  indeed  might  be 
expected,  since  it  was  formed  upon  more  conclusive  grounds.  He  found  that 
the  paths  of  comets  which  had  appeared  in  1531  and  1606,  were  very  nearly 
identical,  and  that  they  were  in  fact  the  same  as  the  path  followed  by  the 
comet  observed  by  himself  in  1682.  He  suspected,  therefore,  that  the  appear- 
ances at  these  three  epochs  were  produced  by  three  successive  returns  of  the 
same  comet,  and  that  consequently  its  period  in  its  orbit  must  be  about  seventy- 
five  and  a  half  years. 

So  little  was  the  scientific  world  at  this  time  prepared  for  such  an  announce- 
ment, that  Halley  himself  only  ventured  at  first  to  express  his  opinion  in  the 
form  of  conjecture  ;  but  after  some  further  investigation  of  the  circumstances 
of  the  recorded  comets,  he  found  three  others  which  at  least  in  point  of  time 
agreed  with  the  period  assigned  to  the  comet  of  1682,  viz.,  those  of  1305, 
1380,  and  1456.*  Collecting  confidence  from  these  circumstances,  he  an- 
nounced his  discovery  as  the  result  of  combined  observation  and  calculation, 
and  entitled  to  as  much  confidence  as  any  other  consequence  of  an  established 
physical  law. 

There  were  nevertheless  two  circumstances,  which  to  the  fastidious  skeptic 

*  The  path  of  the  comet  of  1456  was  afterward  fully  identified  with  that  of  1682. 


might  be  supposed  to  offer  some  difficulty.  These  were,  first,  that  the  inter- 
(  vals  between  the  supposed  successive  returns  to  perihelion  were  not  precisely 
equal ;  and,  secondly,  that  the  inclination  of  the  comet's  path  to  the  plane  of  the 
earth's  orbit  was  not  exactly  the  same  in  each  case.  Halley,  however,  with  a 
degree  of  sagacity  which,  considering  the  state  of  knowledge  at  the  time,  can- 
not fail  to  excite  unqualified  admiration,  observed  that  it  was  natural  to  suppose 
that  the  same  causes  which  disturbed  the  planetary  motions  must  likewise  act 
upon  comets  ;  and  that  their  influence  would  be  so  much  the  more  sensible  upon 
these  bodies  because  of  their  great  distances  from  the  sun.  Thus,  as  the  at- 
traction of  Jupiter  upon  Saturn  was  known  to  affect  the  velocity  of  the  latter 
planet,  sometimes  retarding  and  sometimes  accelerating  it,  according  to  their 
relative  position,  so  as  to  affect  its  period  to  the  extent  of  thirteen  days,  it 
might  well  be  supposed  that  the  comet  might  suffer  by  a  similar  attraction,  an 
effect  sufficiently  great  to  account  for  the  inequality  observed  in  the  interval 
between  its  successive  returns  ;  and  also  for  the  variation  to  which  the  direc- 
tion of  its  path  upon  the  plane  of  the  eclipti,c  was  found  to  be  subject.  He 
observed,  in  fine,  that  as  in  the  interval  between  1607  and  1682  the  comet 
passed  so  near  Jupiter  that  its  velocity  must  have  been  augmented,  and  conse- 
quently  its  period  shortened  by  the  action  of  that  planet,  this  period,  therefore, 
having  been  only  seventy-five  years,  he  inferred  that  the  following  period  would 
<  probably  be  seventy-six  years  or  upward ;  and  consequently  that  the  comet 
>  ought  not  to  be  expected  to  appear  until  the  end  of  1758,  or  the  beginning  of 
$  1759.  It  is  impossible  to  imagine  any  quality  of  mind  more  enviable  than  that 
which,  in  the  existing  state  of  mathematical  physics,  could  have  led  to  sucli  a 
prediction.  The  imperfect  state  of  mathematical  science  rendered  it  impossible 
for  Halley  to  offer  to  the  world  a  demonstration  of  the  event  which  he  foretold. 
"  He  therefore,"  says  M.  de  Pontecoulant,  "  could  only  announce  these  felicitous 
conceptions  of  a  sagacious  mind  as  mere  intuitive  perceptions,  which  must  be 
received  as  uncertain  by  the  world,  however  he  might  have  felt  them  himself, 
until  they  cculd  le  verified  by  the  process  of  a  rigorous  analysis." 

The  theory  of  gravitation,  which  was  in  its  cradle  at  the  time  of  Halley's 
investigations,  had  grown  to  comparative  maturity  before  the  period  at  which 
his  prediction  could  be  fulfilled.  The  exigencies  of  that  theory  gave  birth  to 
new  and  more  powerful  instruments  of  mathematical  inquiry :  the  differential 
and  integral  calculus  was  its  first  and  greatest  offspring.  This  branch  of  sci- 
ence was  cultivated  with  an  ardor  and  success  by  which  it  was  enabled  to  an- 
swer all  the  demands  of  physics,  and  consequently  mechanical  science  ad- 
vanced, pari  passu.  Newton's  discoveries  having  obtained  reception  throughout 
the  scientific  world,  his  inquiries  and  his  theories  were  followed  up ;  and  the 
consequences  of  the  great  principle  of  universal  gravitation  were  rapidly  de- 
veloped. Among  these  inquiries  one  problem  was  eminently  conspicuous  for 
the  order  of  minds  whose  powers  vrere  brought  to  bear  upon  it.  One  of  the 
first  and  simplest  results  of  the  theory  of  gravitation  was,  that  if  a  single  planet 
attended  the  sun  (its  mass  being  insignificant  compared  with  that  of  the  sun), 
that  planet  must  revolve  in  an  "ellipse,  the  focus  of  which  must  be  occupied  by 
the  centre  of  the  sun  ;  but,  if  a  second  planet  be  admitted  into  the  system,  then 
the  elliptic  form  of  their  paths  round  the  sun  can  be  preserved  only  by  the  sup- 
position that  the  two  planets  have  no  attraction  for  each  other,  and  that,  no 
physical  influence  is  in  operation,  except  the  attraction  of  the  solar  mass  for 
each  of  them.  But  the  law  of  universal  gravitation  is  founded  upon  the  prin- 
ciple that  every  body  in  nature  must  attract  and  be  attracted  by  every  othr.r  body. 
Thus,  the  elliptic  character  of  the  orbit  is  effaced  the  moment  a  second  planet 
is  introduced.  But  let  us  remember  that  in  this  case  each  of  the  two  supposed 
planets  are  in  mass  absolutely  insignificant  co.npared  with  the  sun.  The 


f 


182 


HALLEY'S  COMET. 


amount  of  attraction  depending  on  the  greatness  of  the  attracting  body,  the  in- 
tensity of  the  solar  attraction  of  each  of  the  planets  must  predominate  enormously 
over  the  comparatively  feeble  influence  of  their  diminutive  masses  on  each  other. 
The  tendency  of  the  solar  attraction  to  impress  the  elliptic  form  on  the  paths 
of  those  planets,  must  therefore  prevail  in  the  main  ;  a4id  although  their  mutual 
attraction,  however  feeble,  cannot  be  wholly  ineffective,  their  orbits  will,  in 
obedience  to  the  solar  mandate,  preserve  a  general  elliptic  form,  subject  to 
those  very  slight  deviations,  or  disturbances,  due  to  their  reciprocal  attraction. 
The  problem  to  discover  the  nature  and  amount  of  these  disturbances  is  one  of 
paramount  importance  in  astronomy,  and  has  been  called  the  "  problem  of 
three  bodies  ;''  and  its  extension  embraces  the  effects  of  the  mutual  gravitation 
of  all  the  planets  of  the  system  upon  each  other.  This  celebrated  problem 
presented  enormous  mathematical  difficulties.  A  particular  case  of  it,  which, 
from  the  comparative  smallness  of  the  third  body  considered,  was  attended 
with  greater  facility,  was  solved  by  Euler,  D'Alembert,  and  Clairaut.  This 
was  the  case  in  which  the  single  planet,  revolving  round  the  sun,  was  the 
earth,  and  the  third  body  the  moon. 

Clairaut  undertook  the  difficult  application  of  this  problem  to  the  case  of  the 
comet  of  1682,  with  a  view  to  calculate  the  effects  which  would  be  produced 
upon  it  by  the  attraction  of  the  different  planets  of  the  system;  and  by  such 
means  to  convert  the  conjecture  of  Halley  into  a  distinct  astronomical  predic- 
tion, attended  with  all  the  circumstances  of  time  and  place.  The  exact  verifi- 
cation of  the  prediction  would,  it  was  obvious,  furnish  the  most  complete  dem- 
onstration of  the  principle  of  universal  gravitation  ;  which,  though  generally  re- 
ceived, was  not  yet  considered  so  completely  demonstrated  as  to  be  independ- 
ent of  so  remarkable  a  body  of  evidence  as  the  fulfilment  of  such  a  calculation 
would  afford. 

To  attain  completely  the  end  proposed,  it  was  necessary  to  solve  two  very 
different  classes  of  problems,  requiring  different  powers  of  mind,  and  different 
habits  of  thought  and  application.  The  mathematical  part  of  the  inquiry, 
strictly  speaking,  consisted  in  the  discovery  of  certain  general  analytical  for- 
mulae, applicable  to  the  case  in  question ;  which,  when  translated  into  ordinary 
language,  would  become  a  set  of  rules  expressing  certain  arithmetical  proces- 
ses, to  be  effected  upon  certain  give*  numbers ;  and  when  so  effected  would 
give  as  the  final  results,  numbers  wnich  would  determine  the  place  of  the 
comet,  under  all  the  circumstances  influencing  it  from  hour  to  hour.  The  ac- 
tual place  of  the  body  being  thus  determined,  it  became  a  simple  question  of 
practical  astronomy  to  ascertain  its  apparent  place  in  the  firmament,  at  corre- 
sponding times.  A  table  exhibiting  its  apparent  place  thus  determined  in  the 
firmament  for  stated  intervals  of  time,  is  called  its  Ephemeris ;  it  is  the  final 
result  to  which  the  whole  investigation  must  tend,  and  is  that  whose  verifica- 
tion by  observation  would  ultimately  decide  the  validity  of  the  reasoning,  and 
the  accuracy  of  the  computations.  Clairaut,  a  mathematician  and  natural  phi- 
losopher, was  eminently  qualified  to  conduct  such  an  investigation,  as  far  as 
the  attainment  of  those  general  analytical  forntulae  which  embodied  the  rules 
by  which  the  practical  astronomer  and  arithmetician  might  woxk  out  the  final 
results  ;  but  beyond  this  point  neither  his  habits  nor  his  powers  would  conduct  { 
aim.  Lalande,  a  practical  astronomer,  no  less  eminent  in  his  own  department, 
and  who,  indeed,  first  urged  Clairaut  to  this  inquiry,  accordingly  undertook  the 
management  of  the  astronomical  and  arithmetical  part  of  the  calculation.  In 
this  prodigious  labor  (for  it  was  one  of  most  appalling  magnitude)  he  was  as- 
sisted by  the  -wile  of  an  eminent  watchmaker  in  Paris,  named  Lepaute,  whose 
exertions  on  this  occasion  have  deservedly  registered  her  name  in  astronom- 
ical history. 


HALLEY'S  COMET. 


183 


It  is  difficult  to  convey  to  one  who  is  not  conversant  with  such  investiga- 
tions, an  adequate  notion  of  the  labor  which  such  an  inquiry  involved,  'ihe 
calculation  of  the  influence  of  any  one  planet  of  the  system  upon  any  other,  is 
itself  a  problem  of  some  complexity  and  difficulty ;  but  still,  one  general  com- 
putation, depending  upon  the  calculation  of  the  terms  of  a  certain  series,  is 
sufficient  for  its  solution.  This  comparative  simplicity  arises  entirely  from  two 
circumstances  which  characterize  the  planetary  orbits.  These  are,  that  though 
they  are  ellipses,  they  differ  very  slightly  from  circles  ;  and  though  the  plan- 
ets do  not  move  in  the  plane  of  the  ecliptic,  yet  none  of  them  deviate  consider- 
ably from  that  plane.  But  these  characters  do  not,  as  we  have  already  stated, 
belong  to  the  orbits  of  comets,  which,  on  the  contrary,  are  highly  eccentric, 
and  depart  from  the  ecliptic  at  all  possible  angles.  The  consequence  of  this 
is,  that  the  calculation  of  the  disturbances  produced  in  the  cometary  orbit  by 
the  action  of  the  planets,  must  be  conducted,  not  like  the  planets,  in  one  gen- 
eral calculation  applicable  to  the  whole  orbit,  but  in  a  vast  number  of  separate 
calculations,  in  which  the  oifeit  is  considered,  as  it  were,  bit  by  bit,  each  bit 
requiring  a  calculation  similar  to  that  of  the  whole  orbit  of  the  planet.  In 
fact,  for  a  very  small  part  of  its  course,  we  treat  the  comet  as  we  would  a 
planet ;  making  our  calculations,  and  completing  them,  nearly  in  the  same 
manner ;  but  for  the  next  part  we  are  obliged  to  enter  upon  a  new  calculation, 
starting  with  a  different  set  of  numbers,  but  performing  over  again  similar 
arithmetical  operations  upon  them.  When  it  i-s  considered  that  the  period  of 
Halh-y's  cornet  is  about  seventy-five  years,  and  that  every  portion  of  its  course, 
for  two  successive  periods,  was  necessary  to  be  calculated  separately  in  this 
way,  some  notion  may  be  formed  of  the  labor  encountered  by  Lalande  and 
Madame  Lepaute.  "  During  six  months,"  says  Lalande,  "  we  calculated  from 
morr.ing  till  night,  sometimes  even  at  meals,  the  consequence  of  which  was, 
that  I  contracted  an  illness  which  changed  my  constitution  for  the  remainder 
of  my  life.  The  assistance  rendered  by  Madame  Lepaute  was  such,  that  with- 
out her  we  never  could  have  dared  to  undertake  this  enormous  labor,  in  which 
it  was  necessary  to  calculate  the  distance  of  each  of  the  two  planets,  Jupiter 
and  Saturn,  from  the  comet,  and  their  attraction  upon  that  body,  separately,  for 
every  successive  degree,  and  for  150  years."* 

These  elaborate  calculations  having  been  completed,  Clairaut,  fearing  that  the 
comet  would  anticipate  his  announcement,  presented  his  first  memoir  to  the 
Academy  on  the  14th  of  November,  1758.  In  this  memoir  he  was  compelled  to 
adopt  the  path  of  the  comet  upon  its  former  appearance,  as  determined  by  the 
observations  of  Appian.  These,  however,  were  made  at  a  time  when  little  at- 
tention was  paid  to  comets  ;  and  were  made,  moreover,  without  that  conscious- 
ness on  the  part  of  the  observer  of  their  future  importance,  which  would  doubt- 
less have  produced  greater  accuracy.  In  calculating  the  effect  of  the  attrac- 
tion of  Jupiter  and  Saturn  upon  the  comet,  in  its  two  periods  between  1707 
and  1682,  and  between  the  latter  period  and  the  expected  return,  Clairaut  pro- 
ceeded upon  the  supposition  that  the  masses  of  these  planets  were  each  what 
they  were  ther.  supposed  to  be.  It  has,  Eowever,  since  appeared,  that  the  es- 
timates  cf  these  masses  were  incorrect,  more  especially  that  of  Saturn.  The 
planet  Hcrschel  being  then  unknown,  its  influence  upon  the  comet  was,  of 

*  The  name  of  Madame  Lepante  does  not  appear  in  Clairaut' s  memoir ;  a  suppression  which  La- 
lande attributes  to  the  influence  exercised  by  another  lady  to  whom  Clairaut  was  attached.  La- 
lande, however,  quotes  letters  of  Clairaut,  in  which  he  speaks  in  terms  of  high  admiration  of  "  la 
savante  calculatrice."  The  labors  of  this  lady  in  the  work  of  calculation  (for  she  also  assisted  La- 
lande in  constructing  his  Ephemeridcs)  at  length  so  weakened  her  sight,  that  she  was  compelled  to 
desist.  She  died  in  1788,  while  attending  on  her  husband,  who  had  become  insane.  See  the  arti- 
cles on  comets,  written  with  considerable  ability,  in  the  Companion  to  the  British  Almanac  for  the 
rear  1833.  They  are  understood  to  be  the  production  of  Mr.  De  Morgan,  secretary  of  the  Astro- 
aomical  Society. 


184 


HALLEY'S  COMET. 


course,  wholly  omitted.  Neither  did  Clairaut  take  into  account  the  action  of 
the  earth.  Encumbered  with  the  disadvantages  of  precision  in  his  data,  he 
predicted,  in  his  first  memoir,  that  the  comet  would  arrive  at  its  nearest  point 
to  the  sun  on  the  18th  of  April,  1759  ;  but  he  stated  at  the  same  tirre  that  the 
imperfection  of  some  of  the  methods  of  calculation  he  was  compelled  to  adopt, 
was  such  as  to  leave  a  possibility  of  his  prediction  being  erroneous  to  the  ex- 
tent of  a  month.  After  presenting  this  memoir  he  resumed  his  calculations, 
and  completed  some  which  he  had  not  time  to  execute  previously.  He  then 
announced  that  the  4th  of  April  would  be  the  day  of  the  comet's  arrival  at  the 
nearest  distance  to  the  sun. 

This  wonderful  astronomical  prediction  was  accompanied  by  a  circumstance 
still  more  remarkable  and  interesting  than  that  which  we  have  noticed  in  the 
conjectures  of  Halley  as  to  the  disturbing  effects  of  the  planets  upon  the  com- 
et's period.  Clairaut  stated  that  there  might-  be  very  many  circumstances 
which,  independently  of  any  error  either  in  the  methods  or  process  of  calcula- 
tion, might  cause  the  event  to  deviate  more  ordess  from  its  predicted  occur- 
rence ;  one  of  which  was  the  probability  of  an  undiscovered  planet  of  our  ays- 
tern  revolving  beyond  the  orbit  of  Saturn,  and  acting  by  its  gravitation  upon  the 
comet.  In  twenty-two  years  after  this  time  this  conjecture  was  accurately 
fuVilled  by  the  discovery  of  the  planet  Herschel,by  the  late  Sir  William  Her- 
schel,  revolving  round  the  sun  one  thousand  millions  of  miles  beyond  the  orbit 
of  Saturn ! 

In  the  successive  appearances  of  the  comet  subsequent  to  1456,  it  was  found 
to  have  gradually  decreased  in  magnitude  and  splendor.  While  in  1456  it 
occupied  two  thirds  of  the  firmament,  and  spread  terror  over  Europe,  in  1607 
its  appearance,  when  observed  by  Kepler  and  Longomontanus,  was  that  of  a 
star  of  the  first  magnitude ;  and  so  trifling  was  its  tail,  that  Kepler  himself, 
when  he  first  saw  it,  doubted  if  it  had  any.  In  1682  it  excited  little  attention 
except  among  astronomers.  Supposing  this  decrease  of  magnitude  and  bril- 
liancy to  be  progressive,  Lalande  entertained  serious  apprehensions  that  on 
its  expected  return  it  might  escape  the  observation  even  of  astronomers  ;  and 
thus  that  this  splendid  example  of  the  power  of  science,  and  unanswerable 
proof  of  the  principle  of  gravitation,  would  be  lost  to  the  world.  It  is  not  un- 
interesting to  observe  the  misgivings  of  this  distinguished  astronomer  with  re- 
spect to  the  appearance  of  the  body,  mixed  up  with  his  unshaken  faith  in  the 
rce.ult  of  the  astronomical  inquiry.  "  We  cannot  doubt,"  says  he,  "  that  it  will 
return  ;  and  even  if  astronomers  cannot  see  it,  they  will  not  therefore  be  the 
less  convinced  of  its  presence  ;  they  know  that  the  faintness  of  its  light,  its  great 
distance,  and  perhaps  even  bad  weather,  may  keep  it  from  our  view  ;  but  the 
world  will  find  it  difficult  to  believe  us ;  they  will  place  this  discovery,  which 
has  done  so  much  honor  to  modern  philosophy,  among  the  number  of  chance 
predictions.  We  shall  see  discussions  spring  up  again  in  the  colleges,  con- 
tempt among  the  ignorant,  terror  among  the  people,  and  seventy-six  years  will 
roll  away  before  there  will  be  another  opportunity  of  removing 'all  doubt." 

Fortunately  for  science,  the  arrival  of  the  expected  visiter  did  not  take  place 
under  such  untoward  circumstances.  As  the  commencement  of  the  year  1759 
approached,  "  Les  Astronomes,"  says  Voltaire,  "  ne  se  coucherent  pas." 

The  honor,  however,  of  the  first  glimpse  of  the  stranger  was  not  reserved 
for  the  possessors  of  scientific  rank,  nor  the  members  of  academies  or  univer- 
sities. On  the  night  of  Christmas  day,  1758,  George  Palitzch  of  Prolitz,  near 
Dresden,  "  a  peasant,"  says  Sir  John  Herschel,  "  by  station,  an  astronomer  by 
nature,"  first  saw  the  comet.  He  possessed  an  eight-foot  telescope,  with 
which  he  made  the  discovery ;  and  the  next  day  communicated  the  fact  to  Dr. 
Hoffman,  who  immediately  went  to  his  cottage,  and  saw  the  comet  on  the  even- 


HALLEY'S  COMET.  185 


1 


ings  of  the  27th  and  28th  of  December.  An  astronomer  of  Leipzic  observed  s 
it  immediately  afterward;  "  but,"  says  M.  de  Pontecoulant,  "jealous  of  his  / 
discovery,  as  a  lover  of  his  mistress,  or  a  miser  of  his  treasure,  he  would  not 
share  it,  and  gave  himself  up  to  the  solitary  pleasure  of  following  the  body  in  its 
course  from  day  to  day,  while  his  contemporaries  throughout  Europe  were  vainly 
directing  their  anxious  search  after  it  to  other  quarters  of  the  heavens."  A*t 
this  time  Delisle,  a  French  astronomer,  and  his  assistant,  Messier,  who,  from 
his  unwearied  assiduity  in  the  pursuit  of  comets,  received  from  Louis  the  Fif- 
teenth the  appellation  of  La  Fttret  de  Comctes(tho  comet-ferret),  had  been  con- 
stantly engaged  for  eighteen  months  in  watching  for  the  return  of  Halley's 
comet.  It  would  seem  that  La  Caille,  and  other  French  astronomers  at  that 
time,  considering  that  Delisle  and  Messier,  from  the  attention  which  they  had 
given  to  such  objects,  and  more  especially  from  the  ardor  and  indefatigable 
perseverance  of  the  latter,  could  not  fail  to  detect  the  expected  body  the  mo- 
ment it  came  within  view,  did  not  occupy  themselves  in  looking  for  it.  Delisle 
computed  an  Ephemeris,  and  made  a  chart  of  its  supposed  course  in  the  heav- 
ens, and  placed  it  in  the  hands  of  Meisser  to  guide  him  in  his  search.  This 
chart  was  erroneou»and  diverted  the  attention  of  Meisser  to  a  quarter  of  the 
firmament  through  wnich  the  comet  did  not  pass,  and  thus,  most  probably,  de- 
prived that  zealous  and  assiduous  observer  of  the  honor  of  first  discovering  its 
return  to  our  system.  He  succeeded,  nevertheless,  in  observing  it  on  the  21st 
of  January,  1759;  nearly  a  month  after  it  had  been  seen  by  Palitzch  and 
Hoffman,  but  without  knowing  that  it  had  been  already  observed.*  The  comet 
was  now  observed  in  various  places.  It  continued  to  be  seen  at  Dresden,  also 
at  Leipzic,  Boulogne,  Brussels,  Lisbon,  Cadiz,  &c.  Its  course  being  observed, 
it  was  found  that  it  arrived  at  its  perihelion,  or  at  its  nearest  point  to  the  sun, 
on  the  13th  of  March,  between  three  and  four  o'clock  in  the  morning ;  exactly 
thirty-seven  days  before  the  epoch  first  assigned  by  Clairaut,  but  only  twenty- 
three  days  previous  to  his  corrected  prediction.  The  comet  on  this  occasion 
appeared  very  round,  with  a  brilliant  nucleus,  well  distinguished  from  the  sur- 
rounding nebulosity.  It  had,  however,  no  appearance  of  a  tail.  About  the 
middle  of  the  latter  month,  it  became  lost  in  the  rays  of  the  sun  while  ap- 
proaching its  perihelion ;  it  afterward  emerged  from  them  on  its  departure 
from  the  sun,  and  was  visible  before  sunrise  in  the  morning  on  the  1st  of  April. 
On  this  day  it  was  observed  by  Messier,  who  states  that  he  was  able  to  dis- 
tinguish the  tail  by  his  telescope.  It  was  again  observed  by  him  on  the  3d, 
15th,  and  17th  of  May.  Lalande,  however,  who  observed  it  on  the  same  oc- 
casions, was  not  able  to  discover  any  trace  of  the  tail. 

Although  it  is  certain  that  the  splendor  and  magnitude  of  the  comet  in  1759 
were  considerably  less  than  those  with  which  it  had  previously  appeared,  yet 
we  must  not  lay  too  much  stress  upon  the  probability  of  its  really  diminished 
magnitude.  In  1759  it  was  seen  under  the  most  disadvantageous  circumstan- 
ces— it  was  almost  always  obscured  by  the  effect  of  twilight,  and  was  in  situ- 
ations the  most  unfavorable  possible  for  European  observers.  It  had  been 
observed,  however,  in  the  southern  hemisphere  at  Pondicherry  by  Pere  Coeur- 
Doux,  and  at  the  isle  of  Bourbon  by  La  Caille,  under  more  favorable  circum- 

*  An  interesting  memoir  of  Messier  may  be  found  in  the  Histoire  de  V  Astronomic  an  dixhuitilme 
Silcle,  by  Delambre.  La  Harpe  (Correspondence  Litteraire,  Paris,  1801,  torn,  i.,  p.  97)  says,  that 
"he  passed  his  life  in  search  of  comets.  The  ne  plus  ultra  of  his  ambition  was  to  be  made  a  mem- 
ber of  the  Academy  of  Petersburg!!.  He  was  an  excellent  man,  but  had  the  simplicity  of  a  child. 
At  a  time  when  he  was  in  expectation  of  discovering  a  comet,  his  wife  took  ill  and  died.  While 
attending  upon  her,  being  withdrawn  from  his  observatory,  Montagne  de  Limoges  anticipated  him 
by  discovering  the  comet.  Messier  was  in  despair.  A  friend  visiting  him  began  to  offer  some  con- 
solation for  the  recent  affliction  he  had  suffered :  Messier,  thinking  only  of  his  comet  exclaimed  :  '/ 
hud  discovered  twelve.  Alas,  that  I  should  be  robbed  of  Ike  thirteenth  by  Montnzne  !'  and  his  eyes 
filled  with  tears.  Then,  remembering  that  it  was  necessary  to  mourn  tor  his  wife,  whose  remains 
were  still  in  the  house,  he  exclaimed, '  Ah  I  cette  pauvre  fernine,'  and  again  wept  for  his  comet." 


186  HALLE Y'S  COMET. 

stances  ;  and  both  of  these  astronomers  agree  in  stating  that  the  tail  was  dis- 
tinctly visible  by  the  naked  eye,  and  varied  in  length  at  different  periods  from 
ten  degrees  to  forty-seven  degrees.*  These  circumstances  are  obviously  in 
perfect  accordance  with  the  former  appearances  of  the  same  be  Jy. 

On  its  departure  from  the  sun  it  continued  to  be  observed  until  the  middle 
of  April,  when  its  southern  position  caused  the  time  of  its  rising  to  follow  that 
of  the  sun  ;  consequently  it  ceased  to  be  visible  in  the  morning.  By  a  further 
change  in  its  position,  however,  it  again  appeared  after  sunset  on  the  29th,  and 
Messier  then  describes  it  as  having  the  appearance  of  a  star  of  the  first  mag- 
nitude. But  here  again  unfortunately  another  circumstance  interposed  a  dif- 
ficulty— the  light  of  the  moon  was  at  that  time  so  strong  as  in  a  great  degree 
to  overcome  the  effect  of  the  comet.  The  body  disappeared  altogether  in  the 
beginning  of  June. 

The  comet  had  now  commenced  a  new  period  under  circumstances  far  more 
favorable  than  had  ever  before  occurred.  An  interval  of  seventy-six  years 
would  throw  its  return  into  the  year  1835.  But  during  that  interval,  the 
science  of  analysis,  more  especially  in  its  application  to  physical  astronomy, 
has  made  prodigious  advances.  The  methods  of  investigation  have  acquired 
greater  simplicity,  and  have  likewise  become  more  general  and  comprehensive  ; 
and  mechanical  science,  in  the  large  sense  of  that  term,  now  embraces  in  its 
formularies  the  most  complicated  motions  and  the  most  minute  effects  of  the 
mutual  influences  of  the  various  members  of  our  system.  These  formulre  ex- 
hibit to  the  eye  of  the  mathematician  a  tableau  of  all  the  evolutions  of  these 
bodies  in  ages  past,  and  of  all  the  changes  they  must  undergo  (the  laws  of  na- 
ture remaining  unchanged)  in  ages  to  come.  Such  has  been  the  result  of  the 
combination  of  transcendent  mathematical  genius  and  unexampled  labor  and 
perseverance  for  the  last  century.  The  learned  societies  established  in  the 
various  centres  of  civilization,  have  more  especially  directed  their  attention  to 
the  advancement  of  physical  astronomy :  and  have  stimulated  the  spirit  of  in- 
quiry by  a  succession  of  prizes  offered  for  the  solution  of  problems  arising  out 
of  the  difficulties  which  were  progressively  developed  by  the  advancement  of 
astronomical  knowledge.  Among  these  questions  the  determination  of  the  re- 
turn of  comets,  and  the  disturbances  which  they  experience  in  their  course,  by 
the  action  of  the  planets  near  which  they  happen  to  pass,  hold  a  prominent 
place.  The  French  Academy  of  Sciences,  in  the  year  1778,  offered  a  high 
mathematical  prize  for  an  essay  on  this  subject,  which  was  the  means  of  call- 
ing forth  the  splendid  Memoir  of  Lagrange,  which  formed  at  once  a  complete 
solution  and  a  model  for  all  future  investigations  of  the  same  kind.  Lagrange's 
•investigation  was,  however,  of  a  general  nature,  and  it  remained  to  apply  it  to 
the  particular  case  of  Halley's  comet,  the  only  one  then  known  to  be  periodic. 
In  1820,  the  Academy  of  Sciences  at  Turin  offered  a  prize  for  this  application 
of  Lagrange's  formula,  which  was  awarded  to  M.  Damoiseau.  In  1826,  the 
French  Institute  proposed  a  similar  prize,  having  twice  before  offered  it  with- 
out calling  forth  any  claimant.  On  this  occasion  M.  de  Pontecoulant  aspired 
to  the  honor.  "  After  calculations,"  says  he,  "  of  which  those  alone  who  have 
engaged  in  such  researches  can  estimate  the  extent  and  appreciate  the  fatigue- 
ing  monotony,  I  arrived  at  a  result  which  satisfied  all  the  conditions  proposed 
by  the  Institute.  I  determined  the  perturbations  of  Halley's  comet  by  taking 
into  account  the  simultaneous  actions  of  Jupiter,  Saturn,  Uranus  (Herschel), 
and  the  earth;  the  comet  having  passed  in  1759  sufficiently  near  our  planet  to 
produce  in  it  (the  comet)  sensible  disturbances  ;  and  I  then  fixed  its  return  to 
its  nearest  point  to  the  sun  for  the  7th  of  November,  1835."  Subsequently  to 
this,  however,  M.  de  Pontecoulant  made  some  further  researches,  which  have 

*  Mcmoires  de  1' Academic  des  Sciences,  1760. 


HALLEY'S  COMET. 


187 


led  him  to  correct  the  former  result ;  and  he  has  since  announced  that  the 
time  of  its  arrival  at  its  nearest  point  to  the  sun  will  be  on  the  morning  of  the 
14th  November. 

The  comet  appeared  in  the  heavens  in  August,  1835,  exactly  in  the  position 
it  was  predicted  to  have,  and  passed  its  perihelion,  on  the  16th  November, 
within  48  hours  of  the  predicted  epoch. 

The  drawing  of  this  comet  usually  given  is  here  subjoined. 


One  of  the  circumstances,  not  the  least  surprising,  connected  with  this 
comet,  is  the  magnitude  of  its  orbit.  It  is  a  very  oblong  oval,  the  total  length 
of  which  is  about  thirty-six  times  the  earth's  distance  from  the  sun  ;  and  the 
greatest  breadth  about  ten  times  that  distance.  The  nearer  extremity  of  the 
oval  is  at  a  distance  from  the  sun  equal  to  about  half  the  earth's  distance ;  and 
the  more  remote  extremity  at  a  distance  equal  to  thirty-five  and  a  half  times  the 
earth's  distance  from  the  sun.  The  earth's  distance  from  the  sun,  is,  in  round 
numbers,  one  hundred  millions  of  miles ;  the  comet's  least  distance  then  will 
be  fifty  millions  of  miles,  and  its  greatest  distance  three  thousand  five  hundred 
and  fifty  millions  of  miles.  Also,  since  the  heat  and  light  supplied  by  the  sun 
to  bodies  which  surround  it,  diminish  in  the  same  proportion  as  the  square  of 
the  distance  increases,  it  follows,  that  at  the  nearest  distance  of  the  comet,  the 
heat  and  light  of  the  sun  will  be  four  times  the  heat  and  light  at  the  earth,  and 
at  the  greatest  distance  they  will  be  about  twelve  hundred  times  less.  Also 
the  heat  and  light  at  the  more  remote  extremity  of  the  orbit,  will  be  nearly  live 
thousand  times  less  than  at  the  nearer  extremity  ;  so  that  while  the  sun 


HALLEY'S  COMET. 


from  the  comet  will  appear  four  times  as  large  as  it  appears  at  the  earth  at  the 
nearer  extremity,  it  will  be  reduced  to  the  magnitude  of  a  star  at  the  more 
remote  extremity.  The  vicissitudes  of  temperature,  not  to  mention  those  of 
light,  consequent  upon  this  change  of  position,  will  be  sufficiently  obvious.  If 
the  earth  were  transported  to  the  more  remote  extremity  of  the  comet's  orbit, 
every  liquid  substance  would  become  solid  by  congelation  ;  and  it  is  extremely 
probable  that  atmospheric  air  and  other  permanant  gases  might  become  liquids. 
)  If  the  earth  was,  on  the  other  hand,  transferred  to  the  nearer  extremity  of  the 
;  comet's  orbit,  all  the  liquids  upon  it  would  be  converted  into  vapor,  would  form 
permanent  gases,  and  would  either  by  their  mixture  constitute  atmospheric  air, 
or  would  arrange  themselves  into  strata,  one  above  the  other,  according  to  their 
specific  gravities.  All  the  less  refractory  solids  would  be  fused,  and  ^ould 
form  in  the  cavities  of  the  nucleus,  oceans  of  liquid  metal. 

The  following  observations  of  Dick  on  this  comet  will  be  read  with 
interest : — 

"  Soon  after  the  middle  of  September,  as  I  was  taking  a  sweep  with  a  two- 
feet  telescope  over  the  northeastern  quarter  of  the  heavens,  near  the  poiat 
where  I  expected  its  appearance,  I  happened  to  fix  my  eye  on  this  long-expected 
visiter,  which  appeared  very  small  and  obscure.  I  immediately  directed  an 
excellent  three  and  a  half  feet  achromatic  telescope,  with  a  diagonal  eye  piece, 
magnifying  about  thirty-four  times,  to  the  comet,  when  it  was  distinctly  seen, 
and  appeared  of  a  considerable  diameter,  but  still  somewhat  hazy  and  obscure. 
I  afterward  applied  a  power  of  forty-five,  and  another  of  ninety-five  ;  but  it  was 
seen  most  distinctly  with  the  lower  power.  With  ninety-five  it  appeared 
extremely  obscure,  and  nearly  of  the  apparent  size  of  the  moon.* 

"  There  appeared  at  this  time  nothing  like  a  tail,  but  the  centra]  part  was 
much  more  luminous  than  the  other  portions  of  the  comet,  and  presented  some- 
thing like  the  appearance  of  a  star  of  the  third  or  fourth  magnitude,  surrounded 
with  a  haze.  In  some  of  the  views  I  took  of  this  object,  the  luminous  part,  or 
nucleus,  appeared  to  be  considerably  nearer  one  side  than  another.  At  this 
period,  and  for  a  week  or  ten  days  afterward,  the  comet  was  Altogether  invisible 
to  the  naked  eye.  Many  subsequent  observations  were  made  and  published  in 
the  provincial  newspapers,  but  which  my  present  limits  prevent  me  from 
inserting. 

"  After  the  comet  became  visible  to  the  naked  eye,  the  tail  began  to  appear, 
and  increased  in  length  as  it  approached  its  perihelion,  and  at  its  utmost  extent 
was  estimated  to  be  above  thirty  degrees  in  length.  On  the  13th  of  October, 
according  to  the  observations  of  Arago,  a  luminous  sector  was  visible  in  its 
head ;  on  the  day  following,  this  sector  had  disappeared,  and  a  more  brilliant 
one,  and  of  greater  longitudinal  extent,  was  formed  in  another  place.  This 
second  sector  was  observed  on  the  17th,  when  it  appeared  less  bright ;  and  on 
the  1  8th  its  weakness  had  decidedly  increased.  This  comet  was  concealed 
till  the  21st,  but  on  that  day  three  distinct  sectors  were  visible  in  the  nebulosity. 
On  the  23d,  all  traces  of  these  sectors  had  disappeared,  the  nucleus,  which 
had  previously  been  brilliant  and  well  defined,  having  become  so  large  and 
diffuse  that  the  observer  could  scarcely  believe  in  the  reality  of  such  a  sudden  / 
and  important  alteration,  till  he  satisfied  himself  that  the  appearance  was  not  < 
occasioned  by  moisture  on  the  glasses  of  hifc  instrument.  It  appears,  likewise,  ' 
that  one  of  these  uiminous  fans  or  sectors  was  observed  by  Sir  J.  Herschel,  at 
the  Cape  of  Good  Hope,  after  the  comet  had  passed  its  perihelion.  The  nebu- 
losity of  this  comet  appears  to  have  increased  in  magnitude  as  it  approached 

*  In  viewing  comets,  telescopes  with  large  apertures,  and  comparatively  low  magnifying  power?, 
should  generally  be  used,  as  the  faint  li-lit  emitted  by  comets,  whether  it  be  inherent  or  reflected, 
will  not  permit  the  use  of  to  high  magnifying  powers  as  may  be  applied  to  the  planets. 


j  HALLEY'S  COMET. 

;  the  sun,  but  its  changes  were  sometimes  unaccountably  rapid  :  on  one  occasion 
it  was  observed  to  become  obscure  and  enlarged  in  the  course  of  a  few  hours, 
though  a  little  before,  its  nucleus  was  clear  and  well  defined.  On  the  llth  of 
October,  the  Rev.  T.  W.  Webb  and  two  other  observers  remarked  corusca- 
tions in  the  tail.  On  that  evening,  at  seven  hours  and  thirty  minutes,  the  tail 
was  very  conspicuous,  extending  between  x  and  y  Draconis,  and  evidently 
fluctuated,  or  rather  coruscated,  in  length,  being  occasionally  short,  and  then 
stretching  in  the  twinkling  of  an  eye  to  its  full  extent,  which  was  at  least  equal 
to  ten  degrees.  Its  changes  were  extremely  similar  to  the  kindling  and  fading 
of  a  very  faint  streamer  of  the  auroia  borealis. 

"  The  influence  of  the  ethereal  medium  on  the  motion  of  Halley's  comet  will 
be  known  after  another  revolution,  and  future  astronomers  will  learn,  by  the 
accuracy  of  its  returns,  whether  it  has  met  with  any  unknown  cause  of  distur- 
bance in  its  distant  journey.  Undiscovered  planets  beyond  the  visible  boundary 
of  our  system  may  change  its  path  and  the  period  of  its  revolution,  and  thus 
may  indirectly  reveal  to  us  their  existence,  and  even  their  physical  nature  and 
orbit.  The  secrets  of  the  yet  more  distant  heavens  may  be  disclosed  to  future 
generations  by  comets  which  penetrate  still  further  into  space,  such  as  that  of 
1763,  which,  if  any  faith  may  be  placed  in  the  computation,  goes  nearly  forty- 
three  times  further  from  the  sun  than  Halley's  does,  and  shows  that  the  sun's 
attraction  is  powerful  enough  at  the  distance  of  144,600,000,000  of  miles  to 
recall  the  comet  to  its  perihelion.  The  periods  of  some  comets  are  said  to  be 
many  thousand  years,  and  even  the  average  time  of  the  revolution  of  comets 
generally  is  about  a  thousand  years ;  which  proves  that  the  sun's  gravitating 
force  extends  very  far.  La  Place  estimates  that  the  solar  attraction  is  felt 
throughout  a  sphere  whose  radius  is  a  hundred  millions  of  times  greater  than 
the  distance  of  the  earth  from  the  sun." 

"  The  orbit  of  Halley's  comet  is  four  times  longer  than  it  is  broad  ;  its  length 
is  about  three  thousand  four  hundred  and  twenty  millions  of  miles — about 
thirty-six  times  the  mean  distance  of  the  earth  from  the  sun.  At  this  perihe- 
lion it  comes  within  fifty-seven  millions  of  miles  of  the  sun,  and  at  its  aphelion 
it  is  sixty  times  more  distant.  On  account  of  this  extensive  range,  it  must 
experience  three  thousand  six  hundred  times  more  light  when  nearest  to  the 
sun  than  in  the  most  remote  point  of  its  orbit.  In  the  one  position  the  sun  will 
seem  to  be  four  times  larger  than  he  appears  to  us,  and  at  the  other  he  will 
not  be  apparently  larger  than  a  star."  * 

The  appearance  of  this  comet  so  near  the  time  predicted  by  astronomers, 
and  in  positions  so  nearly  agreeing  with  those  which  were  previously  calcu- 
lated, is  a  clear  proof  of  the  astonishing  accuracy  which  has  been  introduced 
into  astronomical  calculations,  and  of  the  soundness  of  those  principles  on 
which  the  astronomy  of  comets  is  founded.  It  likewise  shows  that  comets  in 
general  are  -permanent  bodies  connected  with  the  solar  system,  and  that  no  very 
considerable  change  in  their  constitution  takes  place  while  traversing  the 
distant  parts  of  their  orbits.f 

*  Mrs.  Somerville's  "  Connexion  of  the  Physical  Sciences,"  a  work  which,  though  written  in  a 
popular  style,  would  do  honor  to  the  first  philosophers  of  Europe.  Of  this  lady's  profound  mathe- 
matical work  on  the  "Mechanism  of  the  Heavens,"  the  Edinburgh  Reviewers  remark:  "It  is 
unquestionably  one  of  the  most  remarkable  works  that  female  intellect  ever  produced  in  any  age  or 
country ;  and  with  respect  to  the  present  day,  we  hazard  little  in  saying,  that  Mrs.  Somerville  is  the  \ 
only  individual  of  her  sex  in  the  world  who  could  have  written  it"  } 

t  The  most  particular  observations  on  Halley's  comet,  during  its  appearance  in  1835,  which  I 
have  seen,  are  those  which  were  made  by  the  Rer.  T.  W.  Webb,  of  Tretire,  near  Ross,  an  account 
of  which,  with  deductions  and  remarks,  was  read  to  the  Worcestershire  Natural  History  Society. 
The  observations  were  made  with  an  excellent  achromatic  telescope,  by  Tnlley,  of  5  feet  6  inches 
focal  length,  and  37-10  inches  aperture.  Through  the  kindness  of  this  gentleman,  I  was  favored 
wii'a  a  manuscript  copy  of  these  observations,  and  would  have  availed  myself  of  many  ol  his 
judicious  remarks,  had  my  limits  permitted. 


190  HALLEY'S  COMET. 


Among  the  circumstances   attached  to  the  comet,  of  Halley,  which  will  at- 
tract attention,  is  the  fact  of  its  gradually  decreasing  brightness.     \Ye  have  seen  J 
that  oh  some  occasions  of  its  recorded  visits  at  remote  periods,  it  pre-sented  an  I 
appearance  which  filled  the  people  with  terror.     Every  one  knows  how  insig-  £ 
nifieant  an  object  it  was  on  its  return  in  1835.     If  it  be  true  that  comets  thus 
waste  themselves  away,  new  data  will  be  afforded  to  aid  in  forming  a  physi- 
cal theory  for  their  explanatioa. 

1 


I 

L 


THE    ATMOSPHERE. 


.Atmospheric  Air  is  Material. — Its  Color. — Cause  of  the  Blue  Sky. — Cause  of  the  Green  Sea. — Air 
lius  Weight. — Experimental  Proofs. — Air  has  Inertia. — Examples  of  its  Resistance. — It  acquires 
Moving  Force. —  Examples  of  its  Impact. — Air  is  Impenetrable. — Experimental  Proofs. — Elastic 
and  compressing  Forces  equal. — Limited  Height  of  the  Atmosphere. — Elasticity  proportioned  to 
the  Density. — Experimental  Proofs. — Internal  and  External  Pressure  on  close  Vessels  containing 
Air. 


THE  ATMOSPHERE. 


193 


THE  ATMOSPHERE. 


THE  Atmosphere  is  the  thin  transparent  fluid  which  surrounds  the  earth  to 
a  considerable  height  above  its  surface  and  which,  in  virtue  of  one  of  its  con- 
stituent elements,  supports  animal  life  by  respiration,  and  is  necessary,  also, 
to  the  due  exercise  of  the  vegetable  functions.  This  substance  is  generally, 
but  erroneously  regarded  as  invisible.  That  it  is  not  invisible  may  be  proved 
by  turning  our  view  to  the  firmament :  that,  in  the  presence  of  light,  appears  a 
vault  of  an  azure  or  blue  color.  This  color  belongs  not  to  anything  which 
occupies  the  space  in  which  the  stars  and  other  celestial  objects  are  placed, 
but  to  the  mass  of  air  through  which  these  bodies  are  seen.  It  may  probably 
be  asked,  if  the  air  be  an  azure-colored  body,  why  is  not  that  which  immedi- 
ately surrounds  us  perceived  to  have  this  azure  color,  in  the  same  manner  as  a 
blue  liquid  contained  in  a  bottle  exhibits  its  proper  hue  ?  The  question  is 
easily  answered. 

There  are  certain  bodies  which  reflect  color  so  faintly,  that  when  they  exist 
in  limited  quantities,  the  portion  of  colored  light  which  they  reflect  to  the 
eye  is  insufficient  to  produce  sensation ;  that  is,  to  excite  in  the  mind  a  per- 
ception of  the  color.  Almost  all  semi-transparent  bodies  are  examples  of  this. 
Let  a  champagne  glass  be  filled  with  sherry,  or  other  wine  of  that  color.  At  the 
thickest  part,  near  the  top  of  the  glass,  the  wine  will  strongly  exhibit  its  pecu- 
liar color,  but  as  the  glass  tapers,  and  its  thickness  is  diminished,  this  color 
will  become  more  faint  and,  at  the  lowest  point,  it  will  almost  disappear,  seem- 
ing nearly  as  transparent  as  water. 

Now  let  a  glass  tube,  of  very  small  bore,  be  dipped  in  the  same  wine,  and 
the  finger  being  applied  to  the  upper  end,  let  it  be  raised  from  the  liquid,  the 
wine  will  remain  suspended  in  the  tube,  and  if  it  be  looked  at  through  the  tube 
it  will  be  found  to  have  all  the  appearance  of  water  and  to  be  colorless.  In 
this  case  there  can  be  no  doubt  that  the  wine  in  the  tube  has  actually  the  same 
color  as  the  liquid  of  which  it  originally  formed  a  part,  but  existing  only  in  a 

13 


small  quantity,  that  color  is  transmitted  to  the  eye  so  faintly  as  to  be  inefficient  j 
in  producing  perception. 

The  water  of  the   sea  exhibits  another  remarkable  example  of  this  effect,  i 
If  we  look  into  the  sea  where  the  water  has  considerable  depth,  we  find  that  ] 
its   color  is  a  peculiar  tint  of  green ;  but  if  we  take  up  a  glass  of  the  water  i 
which  thus  appears  green,  we  shall  find  it  perfectly  limpid  and  cold-less.  The  ) 
reason  is,  that  the  quantity  of  the  color  is  too  small  to  be  perceivable  ;  while  the 
great  mass  of  water,  viewed  when  we  look  into  the  deep  sea,  throws  up  the 
color  in  such  abundance  as  to  produce  a  strong  and  decided  perception  of  it. 

The  atmosphere  is  in  the  same  circumstances  ;  the  color,  from  even  a  con- 
siderable portion  of  it,  is  too  faint  to  be  perceptible.  Hence  the  air  which 
fills  an  apartment,  or  which  immediately  surrounds  us  when  abroad  appears 
colorless  and  transparent.  But  when  we  behold  the  immense  mass  of  atmo- 
sphere through  which  we  view  the  firmament,  the  color  is  reflected  with  suffi- 
cient force  to  produce  distinct  perception.  But  it  is  not  necessary  for  this  that 
so  great  an  extent  of  air  should  be  exhibited  to  us  as  that  which  forms  the 
whole  depth  or  thickness  of  the  atmosphere.  Distant  mountains  appear  blue, 
not  because  that  is  their  color,  but  because  it  is  the  color  of  the  medium  through 
which  they  are  seen. 

Although  the  preceding  observations  belong  more  properly  to  optics  than  to 
mir  present  subject,  yet  still,  since  the  exhibition  of  color  is  one  of  the  mani- 
festations of  the  presence  of  body,  they  may  not  be  considered  as  foreign  to 
an  investigation  of  the  mechanical  properties  of  atmospheric  air.  The  mind  un- 
accustomed to  physical  inquiries  finds  it  difficult  to  admit  that  a  thing  so  light, 
attenuated,  impalpable,  and  apparently  spiritual  as  air,  should  be  composed  of 
parts  whose  leading  properties  are  identical  with  those  of  the  most  solid  and  ada- 
mantine masses.  The  knowledge  that  we  see  the  air  must,  at  least,  prepare 
the  mind  for  the  admission  of  the  truth  of  this  proposition  that  "  air  is  a  body." 

WEIGHT    OF    AIR. 

Among  the  properties  which  are  observed  to  appertain  to  natter,  and  which 
as  far  as  we  know  are  inseparable  from  it,  in  whatever  form,  and  under  what- 
ever circumstances  it  exists,  weight  and  inertia  hold  a  conspicuous  p-lace.  To 
be  convinced,  therefore,  that  air  is  material,  we  ought  to  ascertain  whether  it 
possesses  those  properties.  We  shall  have  frequent  and  numerous  proofs  of 
this  ;  but  it  will  at  present  be  convenient  to  demonstrate  it  in  such  a  manner 
that  we  shall  be  warranted  in  assuming  it  in  some  of  the  explanations  which 
we  shall  have  to  offer. 

The  most  direct  proof  that  air  has  weight,  is  the  fact  that  if  a  quantity  of  it 
be  suspended  from  one  arm  of  a  balance,  it  will  require  a  definite  weight  to 
counterpoise  it  in  the  opposite  scale.  By  the  aid  of  certain  pneumatical  en- 
gines, the  nature  of  which  will  be  explained  hereafter,  but  the  operation  and 
effects  of  which  will  for  the  present  be  assumed,  this  may  be  experimentally 
established. 

Let  a  vessel  containing  about  two  quarts,  be  formed  of  thin  copper,  with  a 
narrow  neck,  in  which  is  placed  a  stop-cock,  by  turning  which  the  vessel  may 
be  opened  or  closed  at  pleasure.  Let  two  instruments  be  provided  called  syr- 
inges ;  one,  the  exhausting  syringe,  and  the  other  the  condensing  syringe. 
Let  the  exhausting  syringe  be  screwed  upon  the  neck  of  the  vessel  and  let  the 
stop-cock  be  opened  so  that  the  interior  of  the  vessel  shall  have  free  communica- 
tion with  the  bottom  of  the  syringe  ;  if  the  syringe  be  now  worked,  a  large  portion 
of  the  air  contained  in  the  vessel  may  be  withdrawn  from  it.  When  this  has 
been  done,  let  the  stop-cock  be  closed  to  prevent  the  re-admission  of  air,  and  lot 


THE  ATMOSPHERE. 


^•N^VX~*ta^-^ 

195  / 


the  vessel  be  detached  from  the  syringe.  Let  it  then  be  placed  in  the  dish  of 
a  well-constructed  balance  and  accurately  counterpoised  by  weights  in  the  op- 
posite s,cale.  The  weight  which  is  thus  counterpoised  is  that  of  the  vessel, 
and  the  small  portion  of  air  which  remains  in  it,  if  the  latter  have  any  weight. 
Let  the  stop-cock  be  now  opened  and  the  external  air  will  be  immediately  heard 
rushing  into  the  vessel. 

When  a  small  quantity  has  been  thus  admitted  let  the  stop-cock  be  again 
closed.  It  will  be  found  that  the  copper  vessel  is  now  heavier,  in  a  small  de- 
gree, than  it  was  before  the  air  was  admitted,  for  the  arm  of  the  balance  from 
which  it  is  suspended  will  be  observed  to  preponderate.  Let  such  additional 
weights  be  placed  in  the  opposite  scale  as  will  restore  equilibrium,  the  stop- 
cock being  now  once  more  opened,  the  air  will  be  observed  to  rush  in  as  be- 
fore, and  will  continue  to  do  so  until  as  much  has  passed  into  the  vessel  as  it 
contained  before  the  exhausting  syringe  was  applied.  The  weight  of  the  ves- 
sel will  now  be  observed  to  be  further  increased,  the  end  of  the  beam  from 
which  it  is  suspended  preponderating. 

These  facts  are,  perhaps,  sufficient  proofs  that  air  has  weight ;  but  the  ex- 
periment may  be  carried  further.  Let  the  condensing  syringe  be  now  attached 
to  the  neck  of  the  vessel,  and  let  the  stop-cock  in  the  neck  be  opened  so  as  to 
leave  a  free  communication  between  the  vessel  and  the  bottom  of  the  syringe. 
The  construction  of  this  instrument  is  such  that  by  working  it  an  increased 
quantity  of  air  may  be  forced  into  the  vessel  to  any  extent  which  the  strength 
of  the  vessel  is  capable  of  bearing.  A  considerably  increased  quantity  of  air 
being  thus  deposited  in  the  vessel,  let  the  stop-cock  be  closed  so  as  to  pre- 
vent its  escape.  The  vessel  being  detached  from  the  syringe,  is  restored  to 
the  dish  of  the  balance :  the  weights  which  counterpoised  it  before  the  in- 
creased quantity  of  air  was  forced  in  still  remaining  unchanged  in  the  opposite 
scale.  The  vessel  will  now  no  longer  remain  counterpoised,  but  will  prepon- 
derate, and  will  require  an  increased  weight  in  the  opposite  scale  to  restore 
it  to  equilibrium. 

In  this  experiment,  we  see  that  every  increase  which  is  given  to  the  quan- 
tity of  air  contained  in  a  vessel  produces  a  corresponding  increase  in  its 
weight,  and  that  every  diminution  of  the  quantity  of  air  it  contains  produces  a 
corresponding  diminution  in  its  weight.  Hence  we  infer  that  the  air  which  is 
introduced  into  or  withdrawn  from  the  vessel  has  weight,  and  that  it  is  by  the 
amount  of  its  weight  that  the  weight  of  the  vessel  is  increased  or  diminished. 

We  shall  hereafter  have  many  other  instances  of  the  gravitation  of  atmo- 
spheric air,  but  we  shall  for  the  present  assume  the  principle  that  air  has 
weight,  founded  on  the  experimental  proof  just  given. 

INERTIA    OF    AIR. 

That  air,  in  common  with  all  other  bodies,  possesses  the  qualify  of  inertia, 
numerous  familiar  effects  make  manifest.  Among  the  effects  which  betray  this 
quality  in  solid  bodies,  is  the  fact  that  when  one  solid  body  puts  another  in 
motion,  the  former  loses  as  much  force  as  the  latter  receives.  This  loss  of 
force  is  called  resistance,  and  is  attributed  to  the  quality  of  inertia,  or  inability 
in  either  the  striking  or  struck  body  to  call  into  existence  more  force  in  a  given  \ 
direction  than  previously  existed.  When  the  atmosphere  is  calm  and  free  from 
wind,  the  particles  of  air  maintain  their  position,  and  are  in  a  state  of  rest.  If 
a  solid  body,  presenting  a  broad  surface,  be  moved  through  the  air  in  this 
state,  it  must,  as  it  moves,  drive  before  it  and  put  in  motion  those  parts  of  the 
air  which  lie  in  the  space  through  which  it  passes.  Now,  if  the  air  had  no 
inertia,  it  would  require  no  force  to  impart  this  motion  to  them,  and  to  drive 


196  THE  ATMOSPHERE. 


them  before  the  moving  solid  ;  and  as  no  force  would  in  that  case  be  imparted 
to  the  air,  so  no  force  would  be  lost  by  the  solid  ;  in  other  words,  the  solic 
would  suffer  no  resistance  to  its  motion. 

But  every  one's  experience  proves  this  not  to  be  the  case.  Open  an  um- 
brella and  attempt  to  carry  it  along  swiftly  with  its  concave  side  presented  for- 
ward— it  wi'l  immediately  be  felt  to  be  opposed  by  a  very  considerable  re- 
sistance, and  to  require  a  great  force  to  draw  it  along.  Yet  this  force  is  noth- 
ing more  than  what  is  necessary  to  push  the  air  before  the  umbrella. 

On  the  deck  of  a  steamboat  propelled  with  any  considerable  speed,  we  fee 
on  the  calmest  day  a  breeze  directed  from  the  stem  to  the  stern.  This  arises 
from  the  sensation  produced  by  our  body  displacing  the  air  as  we  are  carriei 
through  it. 

It  is  the  inertia  of  the  atmosphere  which  gives  effect  to  the  wings  of  birds 
Were  it  possible  for  a  bird  to  live  without  respiration,  and  in  a  space  void  oi 
air,  it  would  no  longer  have  the  power  of  flight.  The  plumage  of  the  wings 
being  spread,  beating  with  a  broad  surface  on  the  atmosphere  beneath  them 
is  resisted  by  the  inertia  of  the  atmosphere,  so  that  the  air  forms  a  fulcrum,  as 
it  were,  on  which  the  bird  rises  by  the  leverage  of  its  wings. 

As  a  body  at  rest  manifests  its  inertia  by  the  resistance  which  it  offers  when 
put  in  motion,  so  a  body  in  motion  exhibits  the  same  quality  by  the  force  wit] 
which  it  strikes  a  body  at  rest.  We  have  seen  examples  of  the  resistance 
which  the  atmosphere  at  rest  offers  to  a  body  in  motion  ;  but  the  force  with 
which  the  atmosphere  in  motion  acts  upon  a  body  at  rest  is  exhibited  by  ex- 
amples far  more  numerous  and  striking.  Wind  is  nothing  more  than  moving 
air  ;  and  its  force,  like  that  of  every  other  body,  depends  on  the  quantity  moved, 
and  the  speed  of  the  motion.  Every  example,  therefore,  of  the  effects  of  the 
power  of  wind,  is  an  example  of  the  inertia  of  atmospheric  air.  In  a  wind- 
mill, the  moving  force  of  all  the  heavy  parts  of  the  machinery  is  derived  from 
the  moving  force  of  the  wind  acting  upon  the  sails,  and  the  resistance  of  the 
work  to  which  the  mill  is  applied  is  oveicome  by  the  same  power.  A  ship 
is  propelled  through  the  deep,  and  the  deep  itself  is  agitated  and  raised  in 
waves  by  the  inertia  of  the  atmosphere  in  motion.  As  the  velocity  increases, 
the  force  becomes  more  irresistible,  and  we  find  buildings  totter,  trees  torn 
from  the  roots,  and  even  the  solid  earth  itself  yield  before  the  force  of  the  hur- 
ricane. 

IMPENETRABILITY    OF    AIR. 

Since  air  may  be  seen  and  felt — since  it  has  color  and  weight — and  since 
it  opposes  resistance  when -acted  upon,  and  strikes  with  a  force  proportionate 
to  the  speed  of  its  motion — we  can  scarcely  hesitate  to  admit  that  it  has  quali- 
ties which  entitle  it  to  be  classed  among  material  substances ;  but  one  other 
quality  still  remains  to  be  noticed,  which  perhaps  decides  its  title  to  materiality 
more  unanswerably  than  any  of  the  others.  Air  is  impenetrable  ;  it  enjoys 
that  peculiar  property  of  matter  by  which  it  refuses  admission  to  any  other 
body  to  fhe  space  it  occupies,  until  it  quit  that  space.  This  property  air  pos- 
sesses as  positively  as  adamant.  The  difficulty  which  is  commonly  felt  in 
conceiving  the  impel  etrability  of  substances  of  this  nature  arises  partly  from 
confounding  the  quality  of  impenetrability  with  that  of  hardness,  and  partly 
from  not  attending  to  the  fact  that,  when  a  body  moves  through  the  air,  it  ( 
drives  the  air  before  it  in  the  same  manner  as  a  vessel  moving  through  the  » 
water  propels  the  fluid. 

Let  a  bladder  be  filled  with  air,  and  tied  at  the  mouth  :  we  shall  then  be  abie  ? 
to  feel  the  air  it  contains  as  distinctly  as  if  the  bladder  were  filled  with  a  solid  \ 

*J 


body.  We  shall  find  it  impossible,  so  long  as  the  air  is  prevented  from  es- 
caping, to  press  the  sides  of  the  bladder  together ;  and  if  the  bladder  be  sul/- 
mitted  to  such  severe  pressure  as  may  be  produced  by  mechanical  means,  it 
will  burst  before  the  air  will  allow  it  to  collapse. 

That  air  will  not  allow  the  entrance  of  another  body  into  the  space  where 
it  is  present,  may  also  be  proved  by  the  following  experiment : — 

Let  A  B,  fig.  1,  be  a  glass  vessel  open  at  the  end  A,  and  having  a  short  tube 
from  the  bottom,  furnished  with  a  stopcock  C.  Let  D  E,  fig.  2,  be  another 
glass  vessel  containing  water.  On  the  surface  of  this  water  let  a  small  piece 
of  cork  F  float.  Let  the  vessel  A  B,  having  the  stopcock  C  closed,  bo  now 
inverted  ;  let  its  mouth  A  be  placed  over  the  cork  F,  and  let  it  thus  be  pressed 
to  any  depth  in  the  reservoir  D  E.  If  the  air  in  A  B  were  capable  of  permit- 
ting the  entrance  of  another  body  into  the  space  in  which  it  is  present,  the 
water  in  the  reservoir  D  E  would  now  enter  at  the  mouth  of  the  vessel  A,  and 
rising  in  it,  would  stand  at  the  same  level  within  the  vessel  A  B  as  that  which 
it  has  without  it.  But  this  is  not  found  to  be  the  case.  When  the  vessel  A 
B  is  pressed  into  the  reservoir,  the  surface  of  the  water  within  A  B  will  be 
observed  still  near  the  mouth  A,  as  will  be  indicated  by  the  position  of  the 
cork  which  floats  upon  it,  and  as  is  represented  in  fig.  3.  It  appears,  there- 


Fig. 


fore,  manifestly,  that  whatever  be  the  cause,  the  water  is  excluded  from  the 
vessel  A  B.  That  this  cause  is  the  presence  of  the  air  included  in  the  ves- 
sel, is  proved  by  opening  the  stopcock  C,  and  allowing  the  air  to  escape.  By 
the  established  principles  of  hydrostatics,  the  surface  of  the  water  within  the 
vessel  A  B  exerts  an  upward  pressure  proportionate  to  the  depth  of  that  sur- 
face below  the  surface  of  the  water  exterior  to  the  vessel  A  B.  This  pressure 
acting  upon  the  air  enclosed  in  the  vessel  A  B,  forces  it  out  the  moment  the 
stopcock  C  is  opened,  and  immediately  the  surface  of  the  water  within  A  B 
rises  to  the  level  of  the  surface  without  it. 

We  have  stated  that  the  surface  of  the  water  within  A  B  remains  nearly  at 
the  mouth  of  that  vessel  when  it  is  plunged  in  the  reservoir.  It  would  remain 
exactly  at  the  mouth  if  air  were  incompressible  ;  but,  on  the  contrary,  this  fluid 
is  highly  compressible,  allowing  itself  to  be  forced  into  reduced  dimensions  by 
the  application  of  adequate  mechanical  force.  It  is  necessary,  however,  not 
to  confound  compressibility  with  penetrability.  So  far  from  these  qualities 
being  identical,  the  one  implies  the  absence  of  the  other.  A  body  is  compres- 
sible when  the  forcible  intrusion  of  another  body  into  the  space  within  which 
it  is  confined  causes  its  particles  to  retreat  and  to  accommodate  their  ar- 
rangement to  the  more  limited  space  within  which  they  are  compelled  to 
exist.  , 

The  very  fact  of  their  thus  retreating  before  the  intruding  body  is  a  distinct 
manifestation  of  their  impenetrability.  If  they  were  penetrable,  the  body 


198 


THE  ATMOSPHERE. 


would  enter  the  space  in  which  they  were  confined,  without  driving  them  be- 
fore it,  or  otherwise  disturbing  their  arrangement. 

ELASTICITY    AND    COMPRESSIBILITY    OF    AIR. 

It  will  be  evident,  upon  the  slightest  reflection,  that  the  elasticity  of  air  must 
be  equal  to  the  force  which  is  necessary  to  confine  it  within  the  space  it  oc- 
cupies. Let  us  suppose  that  A  B,  fig.  4,  is  a  cylinder,  having  a  piston  P  fitting 


D 


air-tight  at  the  top  ;  and  let  us  imagine  that  this  piston  P  is  not  acted  upon  by 
any  external  force  having  a  tendency  to  keep  it  in  its  place.  If  the  cylinder 
below  the  piston  be  filled  with  air,  this  air  will  have  a  tendency,  by  virtue  of 
its  elasticity,  to  expand  into  a  wider  space,  and  this  tendency  will  be  mani- 
fested by  a  pressure  exerted  by  the  air  on  all  parts  of  the  surfaces  which  con- 
fine it.  The  piston  P  will  therefore  be  subject  to  a  force  tending  to  displace 
it  and  drive  it  from  the  cylinder,  the  amount  of  which  will  be  the  measure  of 
the  elasticity  of  the  air  beneath  it.  Now,  if  this  piston  be  not  subject  to  the 
action  of  a  force  directed  inward,  exactly  equal  in  amount  to  the  pressure  thus 
exerted  by  the  elastic  force  of  the  air,  it  cannot  maintain  its  position.  If  it  be 
subject  to  an  inward  force  of  less  amount  than  the  elastic  pressure,  then  the 
latter  will  prevail,  and  the  piston  be  forced  out.  If  it  be  subject  to  an  inward 
force  greater  in  amount  than  the  elastic  pressure,  then  the  former  will  prevail, 
and  the  piston  will  be  forced  in,  the  air  being  compelled  to  retreat  within  a 
more  confined  space.  In  no  case,  therefore,  can  the  piston  maintain  its  posi- 
tion, except  when  it  is  subject  to  an  inward  pressure  exactly  equal  to  the  elastic 
force  of  the  air  enclosed  in  the  cylinder. 

The  property  of  elasticity  renders  it  necessary  that,  in  whatever  state  air 
exist,  it  shall  be  restrained  by  adequate  forces  of  some  definite  amount,  and 
which  serve  as  antagonist  principles  to  the  unlimited  power  of  dilatation  which 
the  elastic  property  implies.  In  all  cases  which  fall  under  common  obser- 
vation, air  is  either  restrained  by  the  resistance  of  solid  surfaces,  or  it  is  pressed 
by  the  incumbent  weight  of  the  mass  of  atmosphere  placed  above  it.  It  may 
be  asked,  however,  whether  it  will  not  follow  from  this,  that  the  extent  of  our 
atmosphere  is  infinite  :  for  that,  as  we  ascend  in  it,  the  weight  of  the  superior 
mass  of  air  must  be  gradually  and  unceasingly  lessened,  and  therefore  the 
force  which  resists  the  expansive  principle  being  removed  by  degrees,  the  fluid 
will  spread  through  dimensions  which  are  subject  to  no  limitation.  Although 
it  is  undoubtedly  true  that  these  considerations  lead  us  justly  to  conclude  that 
our  atmosphere  extends  to  a  great  distance  from  the  surface,  and  that  tho 
higher  strata  of  it  are  attenuated  to  a  degree  which  not  only  exceeds  the  pow- 
ers of  art  to  imitate,  but  even  outstrips  the  powers  of  imagination  to  con- 
ceive ;  yet  still  the  understanding  can  suggest  a  definite  limit  to  this  expansion. 
Numerous  physical  analogies  favor  the  conclusion  that  the  divisibility  of  matter 


THE  ATMOSPHERE. 


199 


?  Ins  a  limit,  or  that  all  material  substances  consist  of  ultimate  constituent  par- 
ticles or  atoms,  which  admit  of  no  further  subdivision,  and  on  the  mutual 
relations  of  which  the  form  and  properties  of  the  various  species  of  bodies 
depend. 

Now  those  ultimate  particles  of  the  air  are  endued  with  a  certain  definite 
weight,  because  it  is  the  aggregate  of  their  weights  which  form  the  weight  of 
any  mass  of  air.  It  is  a  fact,  established  by  experiment,  that  in  proportion  as 
air  expands,  its  elastic  force  is  diminished  ;  and  therefore,  if  it  continue  to 
expand,  it  will  at  length  attain  a  state  of  attenuation  in  which  the  disposition 
of  its  constituent  particles  to  separate  by  their  elasticity  is  so  far  diminished 
as  not  to  exceed  the  gravity  of  those  constituent  particles  themselves.  In  this 
state  the  two  forces  will  be  in  equilibrium,  and  the  elastic  force  being  neutral- 
ized, the  particles  will  no  longer  be  dilated. 

In  these  observations  we  have  assumed  a  principle  which  is  of  the  last 
importance  in  pneumatics,  and  which,  indeed,  may  be  regarded  as  forming 
the  basis  of  this  part  of  physical  science,  in  the  same  manner  as  the  power  of 
transmitting  pressure  is  the  fundamental  principle  of  hydrostatics.  This  latter 
principle,  indeed,  also  extends  to  elastic  fluids  ;  and  all  the  consequences  of 
the  free  transmission  of  pressure  which  do  not  also  involve  the  supposition  of 
incompressibility,  are  applicable  to  elastic  fluids  with  as  much  truth  as  to 
liquids.  But  the  principle  to  which  we  now  more  especially  refer,  and  which 
may  be  looked  upon  as  the  chief  characteristic  of  this  form  of  body,  and  neces- 
sary to  render  deh'nite  the  notion  of  their  elasticity,  may  be  announced  as  fol- 
lows : — 

"  The  elastic  force  of  any  given  portion  of  air  is  augmented  in  exactly 
the  same  proportion  as  the  space  within  which  it  is  enclosed  is  diminished  ; 
and  its  elastic  force  is  diminished  in  exactly  the  same  proportion  as  the  space 
through  which  it  is  allowed  to  expand  is  augmented." 


Fig.  5. 


Fig.  6. 


D 


Lie 


iLc 


To  explain  this,  let  A  B  C  D,  fig.  5,  be  conceived  to  be  a  cylinder,  in  which  a 
piston,  A  B,  moves  air  tight,  and  without  friction,  and  let  us  suppose  the  distance 
of  the  lower  surface,  A  B,  of  the  piston,  from  the  bottom,  D  C,  of  the  cylinder,  to 
be  12  inches.  Let  air  be  imagined  to  be  enclosed  below  the  piston,  and  let  us  sup- 
pose that  the  elastic  force  of  this  air  is  such  as  to  press  the  piston  with  a  force 
of  16  ozs.  From  what  has  already  been  stated,  it  is  clear  that,  to  maintain  the 
piston  in  its  place,  it  is  necessary  that  it  should  be  pressed  downward  with  an 
equivalent  force  of  16  ozs.  Now  let  the  force  upon  the  piston  be  doubled,  or  let 
the  piston  be  loaded  with  a  pressure  of  32  ounces.  The  inward  pressure  pre- 
vailing over  the  elasticity,  the  piston  will  immediately  be  forced  toward  D  C, 
but.  will  cease  to  move  at  a  certain  distance,  A  B,  fig.  6,  from  the  bottom.  Now, 
if  this  distance  A  D  be  measured,  it  will  be  found  to  be  exactly  6  inches.  The 
air  has,  therefore,  contracted  itself  into  half  its  former  dimensions. 


200 


THE  ATMOSPHERE. 


Since  the  piston  is  sustained  in  the  position  represented  in  fig.  6,  it  follows 
that  the  elasticity  of  the  air  beneath  it  is  equivalent  to  the  weight  of  the  piston, 
A  B  ;  and,  therefore,  that  the  air  included  in  the  cylinder  acquires  double  its 
original  elasticity  when  it  is  compressed  into  half  its  original  bulk. 

Let  the  piston  be  now  loaded  with  three  times  its  original  weight,  or  48 
ounces  ;  it  will  be  observed  to  descend  into  the  cylinder,  and  further  to  com- 
press the  air,  until  its  distance  from  the  bottom  is  reduced  to  4  inches.  At 
that  distance  it  will  rest,  being  balanced  by  the  increased  elasticity  of  the  air : 
this  air  is  now  compressed  into  one  third  of  its  original  bulk,  and  it  has  three 
times  its  original  elastic  force. 

In  the  same  manner,  in  whatever  proportion  the  weight  of  the  piston  be 
augmented,  in  the  same  proportion  will  the  distance  from  the  bottom  at  which 
it  will  rest  in  equilibrium  be  diminished,  and,  consequently,  the  elastic  force  of 
the  air  is  increased  in  the  same  proportion  as  the  space  into  which  it  is  com- 
pressed is  diminished. 

Let  us,  again,  suppose  the  piston  to  be  loaded  with  sixteen  ounces,  and  to 
be  balanced,  as  in  fig.  5,  by  the  resistance  of  the  air  at  12  inches  from  the  bot- 
tom of  the  cylinder.  But  let  us  also  suppose  the  cylinder  continued  upward 
to  a  height  exceeding  24  inches  ;  let  the  weight  upon  the  piston  be  now  re- 
duced to  eight  ounces.  Since  the  elasticity  of  the  air  beneath  the  piston  was 
capable  of  supporting  sixteen  ounces,  it  will  now  prevail  against  the  dimin- 
ished pressure  of  eight  ounces.  The  piston  will  continue  to  rise  in  the  cylin- 
der until  the  elasticity  of  the  air  is  so  far  diminished  by  expansion  that  it  is 
capable  of  supporting  no  more  than  eight  ounces  ;  the  piston  will  then  remain 
in  equilibrium.  If  the  height  of  the  piston  above  the  bottom  be  now  measured, 
it  will  be  found  to  be  24  inches,  that  is,  double  its  former  height ;  the  air  has, 
therefore,  expanded  to  double  its  former  dimensions,  and  is  reduced  to  half  its 
former  elasticity. 

In  like  manner  it  may  be  shown  that  if  the  weight  upon  the  piston  were  re- 
duced to  four  ounces,  or  a  fourth  of  its  original  amount,  the  piston  Avould  rise  to  four 
times  its  original  height,  or  48  inches,  before  it  would  be  capable  of  balancing 
the  reduced  elasticity  of  the  air.  Thus,  by  expanding  to  four  times  its  primi- 
tive dimensions,  the  elasticity  of  the  air  is  reduced  to  one  fourth  of  its  primitive 
amount. 

By  like  experiments,  it  is  easy  to  see  how  the  general  law  may  be  estab- 
lished. In  whatever  proportion  the  weight  of  the  piston  may  be  increased  or 
diminished,  in  the  same  proportion  exactly  will  the  space  filled  by  the  air 
which  balances  it  be  diminished  or  increased. 

The  preceding  illustration  has  been  selected  with  a  view  rather  to  make  the  ) 
property  itself  intelligible,  than  as  a  practical  experimental  proof  of  it.  The  { 
use  of  pistons  moveable  in  cylinders  is  attended  with  inconvenience  in  cases  ) 
of  this  kind,  arising  from  the  effects  of  friction,  and  the  difficulties  of  making  \ 
due  allowance  for  them.  There  is,  however,  another  method  of  bringing  the  ) 
law  to  the  test  of  experiment,  which  is  not  less  direct,  and  is  more  satisfactory.  { 

Let  A  B  C  D,  fig.  7,  be  a  glass  tube  curved  at  one  end,  B  C,  and  having  ( 
the  short  leg,  C  D,  furnished  with  a  stop-cock  at  its  extremity  ;  let  the  leg  B  A  ^ 
be  more  than  60  inches  in  length.     The  stop-cock  D  being  opened  so  as  to  ? 
allow  a  free  communication  with  the  air,  and  the  mouth  A  of  the  longer  leg 
being  also  open,  let  as  much  mercury  be  poured  into  the  tube  as  will  till  the 
curved  part  B  C,  and  rise  to  a  small  height  in  each  leg.     By  the  principles  of 
hydrostatics,  the  surfaces  of  the  mercury  E  and  F  will  stand  at  the  same  level. 
Let  the  stop-cock  D  be  now  closed,  the  levels  E  F  will  still  remain  undis- 
turbed.    When  the  stop-cock  D  was  opened,  the  surface  F  sustained  a  pres- 
sure equal  to  the  weight  of  a  column  of  air  continued  from  F  upward  as  far  as 


THE  ATMOSPHERE. 


201 


the  atmosphere  extends.  But  the  stop-cock  D  being  closed,  the  effect  of  the 
weight  of  all  the  air  above  that  point  is  intercepted ;  and,  consequently,  the 
surface  F  can  sustain  no  pressure  arising  from  weight,  except  the  amount  of 
the  weight  of  the  small  quantity  of  air  included  between  F  and  D,  which  is 
altogether  insignificant.  But  the  air  thus  included  presses  on  the  surface  F 
by  its  elasticity ;  and  the  amount  of  this  pressure  is  equal  to  the  force  which 
confined  the  air  within  the  space  F  D  before  the  stop-cock  was  closed  :  but 
this  force  was  the  weight  of  the  column  of  atmosphere  above  D ;  and  hence  it 
appears,  that  the  elastic  force  of  the  air  confined  in  the  space  D  F  is  equal  to 
the  atmospheric  pressure. 

Now  the  other  surface,  E,  the  end  A  of  the  tube  being  open,  is  subject  to 
the  atmospheric  pressure.  Thus  the  two  surfaces,  F  and  E,  of  the  mercury, 
are  each  subject  to  a  pressure  arising  from  a  different  quality  of  atmosphere  ; 
the  one  F,  being  pressed  by  its  elasticity,  and  the  other,  E,  being  pressed  by 
its  weight.  These  pressures  being  equal,  the  surfaces  F  and  E  continue  at  the 
same  level. 


Fig.  7. 


Fig.  8. 


The  method  of  ascertaining  experimentally,  the  pressure  arising  from  the 
weight  of  the  atmosphere,  will  be  fully  explained  hereafter ;  meanwhile,  it  is 
necessary  for  our  present  purpose  to  assume  this  pressure  as  known. 

Let  us  suppose,  then,  that  the  atmospheric  pressure  acting  upon  the  surface 
E  is  the  same  as  would  be  produced  by  a  column  of  mercury  30  inches  in 
height  resting  on  the  surface  E :  the  force  with  which  the  elasticity  of  the  air 
confined  in  D  F  presses  on  the  surface  F  is  therefore  equal  to  the  weight  of  a 
a  column  of  thirty  inches  of  mercury.  The  pressure  of  the  atmosphere  acting 
on  the  surface  E  is  transmitted  by  the  mercury  to  the  surface  F  and  balances 
the  elastic  force  just  mentioned.  Let  the  position  of  the  surface  F  be  marked 
upon  the  tube,  and  let  mercury  be  poured  into  the  longer  leg  at  A.  The  in- 
creased pressure  produced  by  the  weight  of  this  mercury  will  be  transmitted 
to  the  surface  F,  and  will  prevail  over  the  elasticity  of  the  confined  air  ;  this 
surface  will  therefore  rise  toward  D,  compressing  the  air  into  a  smaller  space. 
Let  the  mercury  continue  to  be  poured  in  at  A,  until  the  surface  F  rise  to  F', 
fig.  8,  the  middle  point  between  the  end  D  of  the  tube,  and  its  first  position 
F.  The  air  included  is  thus  compressed  into  half  its  former  dimensions,  and 
its  elasticity  will  be  measured  by  the  amount  of  the  force  with  which  the  sur- 


face  A  is  pressed  upward  against  it :  this  force  is  the  weight  of  the  column  of 
mercury  in  the  leg  B  A  above  the  level  of  F  together  with  the  height  of  the 
atmosphere  pressing  on  the  top  G  of  the  column.  Let  a  horizontal  line  he 
drawn  from  the  surface  F',  to  the  leg  B  A,  and  let  the  column  G  H  be  meas- 
ured ;  its  height  will  be  found  to  be  accurately  30  inches,  and  its  weight  is, 
therefore,  equal  to  the  atmospheric  pressure.  The  force  with  which  F  is 
pressed  upward  is,  therefore,  equal  to  twice  the  atmospheric  pressure,  or  to 
double  the  force  with  which  F,  in  fig.  7,  was  pressed  upward.  Hence  it  np- 
pears  that  the  elasticity  of  the  air  confined  in  the  space  D  F,  fig.  8,  is  double 
its  former  elasticity  when  filling  the  space  D  F',  fig.  7.  Thus,  when  the  air  is 
compressed  into  half  its  volume  its  elasticity  is  doubled. 

In  like  manner,  if  mercury  be  poured  into  the  tube  A  until  the  air  included 
in  the  shorter  leg  is  reduced  to  a  third  of  its  bulk,  the  compressing  force  will 
be  found  to  be  three  times  the  atmospheric  pressure,  and  so  on. 

That  the  elasticity  of  the  air  which  surrounds  us  is  equal  to  the  weight  of 
the  incumbent  atmosphere,  has  been  proved  incidentally  in  the  preceding  ex- 
periment. Indeed,  this  is  a  proposition  the  truth  of  which  must  appear  evi- 
dent upon  the  slightest  consideration,  and  which  is  manifested  by  innumerable 
familiar  effects.  If  the  elastic  force  of  the  air  around  us  were  less  than  the 
weight  of  the  incumbent  atmosphere,  it  would  yield  and  suffer  itself  to  be  com- 
pressed until  it  acquired  an  elastic  force  equal  to  that  weight.  If  it  were 
greater  in  amount  than  the  weight  of  the  incumbent  atmosphere,  it  would  over- 
come that  weight,  and  would  press  the  atmosphere  upward  until,  by  expand- 
ing, its  elasticity  were  reduced  to  equality  with  the  weight  of  the  atmosphere, 
and  these  effects  are  continually  going  forward. 

The  incumbent  atmosphere  is  subject  to  continual  fluctuations  in  weight,  as 
will  hereafter  be  proved,  and  the  lowest  stratum  of  air  which  surrounds  us  is 
continually  undergoing  corresponding  contractions  and  expansions,  ever  ac- 
commodating its  elasticity  to  the  pressure  which  it  sustains.  Also  this  stra- 
tum of  air  is  itself  subject  to  changes  of  elasticity  from  vicissitudes  of  tempera- 
ture proceeding  from  the  earth  to  which  it  is  contiguous.  These  changes  pro- 
duce a  necessity  for  expansion  and  contraction  in  it,  even  while  the  weight  of 
the  incumbent  atmosphere  remains  unchanged ;  but  the  full  development  of 
this  last  consideration  belongs  to  the  theory  of  heat  rather  than  to  our  present 
subject. 

An  open  vessel  which  is  commonly  said  to  be  empty,  is,  in  fact,  filled  with 
air ;  and  when  any  solid  or  liquid  is  placed  in  it,  so  much  of  the  air  is  ex- 
pelled as  occupied  the  space  into  which  the  solid  or  liquid  entered.  If  such  a 
vessel  be  closed  by  a  lid  or  stopper,  the  pressure  of  the  external  atmosphere 
will  act  upon  every  part  of  the  exterior  surface  with  an  intensity  proportionate 
to  its  weight.  The  air  which  is  enclosed  in  the  vessel  will,  however,  act  on 
the  interior  surface  with  an  intensity  proportionate  to  its  elasticity.  Accord- 
ing to  what  has  already  been  explained,  this  elasticity  is  equal  to  the  pressure  ; 
and,  therefore,  there  is  a  force  tending  to  press  the  sides  of  the  vessel  outward 
exactly  equal  to  the  pressure  acting  on  the  exterior  surface,  and  tending  to 
press  them  inward.  These  two  forces  neutralize  each  other,  and  the  ve.«?t  is 
circumstanced  exactly  as  if  neither  of  them  acted  upon  it. 

When  the  operation  and  properties  of  some  pneumatical  instruments  have 
been  explained,  we  shall  have  occasion  to  notice  many  other  effects  of  the 
elasticity  of  air. 


r 


THE    IE¥   PLAIETS. 


Indications  of  a  Gap  in  the  Solar  System. — Bode's  Analogy. —  Prediction  founded  npon  it. — Piuzzi 
discovers  Ceres. — Dr.  Olbers  discovers  Pallas. — Harding  discovers  Juno. — Dr.  Others  discovers 
Vesta. — Indications  afforded  by  these  Bodies  of  the  Truth  of  Bode's  Predictions. — Fragments 
of  a  Broken  Planet. — Others  probably  still  undiscovered. — Their  Ultra-Zodiacal  Motions. — Their 
Eccentricities. — They  are  probably  not  Globular. — Other  Singularities  of  their  Appearance. 


•~ 
THE  NEW  PLANETS.  205 


THE    O¥   PLANETS. 


AT  a  very  early  period  of  astronomical  inquiry  it  was  observed  that  the 
spaces  which  intervene  in  the  solar  system  between  planet  and  planet  aug- 
ment in  a  double  proportion  as  the  planets  recede  from  the  sun.  Thus  the 
space  between  Mercury  and  Venus  is  only  half  that  which  intervenes  between 
Venus  and  the  earth.  The  latter,  again,  is  only  half  that  which  separates  this 
planet  from  Mars.  In  like  manner,  the  space  between  Jupiter  and  Saturn  is 
only  half  the  space  between  Saturn  and  Herschel.  To  this  remarkable  law, 
however,  a  conspicuous  exception  was  noticed  by  Kepler,  and  was  more  em- 
phatically insisted  upon  and  more  strictly  demonstrated  in  the  latter  part 
of  the  last  century,  by  Bode  of  Berlin.  While  the  spaces  which  successively 
intervene  between  the  planets  Mercury,  Venus,  the  earth,  and  Mars,  are  con- 
tinually in  the  proportion  of  one  to  two,  that  which  intervenes  between  Mars 
and  Jupiter,  instead  of  being  as  it  ought  to  be,  in  accordance  with  the  law  thus 
indicated — double  the  space  between  Mars  and  the  earth — is,  in  fact,  nearly 
six  times  that  space.  A  planet,  therefore,  which  would  move  between  Mars 
and  Jupiter,  at  a  distance  beyond  Mars  equal  to  twice  the  distance  of  Mars 
from  the  earth,  would  complete  the  system  ;  for  then  there  would  be  between 
such  a  planet  and  Jupiter  twice  the  space  which  would  intervene  between  it 
and  Mars.  The  presence  of  such  a  planet  would  then  remove  all  exception  in 
the  system  to  this  law  of  increasing  distance.  Professor  Bode  ventured  to 
predict  that  a  planet  would  at  some  future  period  be  discovered  revolving  in 
that  position  ;  and  even  if  no  such  planet  were  discovered,  he  maintained  that 
we  should  be  justified  in  the  inference  that,  at  some  former  epoch,  a  planet  did 
exist  in  such  a  position. 

There  is  an  instinctive  faith  in  the  harmony  and  universality  of  nature's 
laws ;  and  when  we  behold  in  any  of  them  a  glaring  exception,  we  are  led  at 
once  to  anticipate  that  such  exception  is  only  apparent,  and  that  by  increased 
knowledge  we  shall  discover  that  the  law  is  in  reality  universal. 

This  remarkable  prediction,  as  may  be  easily  imagined,  attracted  the  atten- 
tion of  astronomers  to  those  quarters  of  the  firmament  where  the  suspected  ( 


planets  ought  to  be  seen  ;  and  on  the  first  day  of  the  present  century,  PIAZZI, 
an  Italian  astronomer,  had  his  attention  engaged  by  a  small  star  of  the  fifth 
magnitude,  which  he  thought  presented  peculiar  appearances.  He  observed 
it  accordingly  from  night  to  night,  and  soon  found  that  it  had  a  motion  among 
the  fixed  stars,  which  was  incompatible  with  the  supposition  that  it  could  be  a 
body  of  that  class.  In  short,  he  soon  discovered  that  this  object  was  a  true 
planet,  and  afterward  applying  to  the  observations  made  upon  it  the  usual 
methods  of  calculation,  he  found  that  it  moved  in  the  solar  system  round  the 
sun  in  the  space  between  Mars  and  Jupiter,  in  such  a  position  that  its  distance 
from  the  latter  was  double  its  distance  from  the  former.  In  short,  it  appeared 
that  this  planet  filled  the  vacant  place. 

On  the  28th  of  March,  in  the  following  year,  Dr.  OLBERS,  of  Bremen,  dis- 
covered the  planet  PALLAS,  moving  nearly  at  the  same  distance.  In  Septem- 
ber, 1803,  HARDING,  also  at  BREMEN,  discovered  JUNO  ;  and  finally,  on  the 
29th  of  March,  1807,  Dr.  OLBERS  discovered  the  fourth  new  planet,  VESTA. 
Thus  within  the  first  five  years  of  the  present  century,  four  new  members  of 
the  solar  system  were  discovered,  presenting,  among  other  anomalous  circum- 
stances, the  spectacle  of  four  planets  equidistant  from  the  sun,  and  therefore 
all  equally  filling  the  vacant  place  declared  to  exist  in  the  system  by  Kepler 
and  Bode.  As  these  four  planets  move  nearly  at  the  same  distance  from  the 
sun,  they  also  have  nearly  equal  periods. 

The  analogy  prevailing  between  the  distances  of  the  planets,  indicated  by 
Bode  and  Kepler,  justified  the  expectation  of  the  discovery  of  a  single  planet : 
how,  then,  are  we  to  reconcile  the  principle  indicated  by  this  analogy  with  the 
known  existence  of  four  such  bodies  1  This  difficulty  has  been  attempted  to 
be  removed  by  the  hypothesis  that  the  four  new  planets  are,  in  fact,  fragments 
of  a  single  planet  which  has  been  broken  !  But  how,  it  may  be  asked,  could 
such  a  catastrophe  as  the  fracture  of  a  planet  be  brought  about  ?  To  this  it  is 
answered  that  there  are  two  causes — the  possibility  and  reality  of  which  are 
not  disputed — either  of  which  might  produce  such  an  effect.  The  volcanic 
phenomena  developed  on  our  own  globe  indicate  to  us  the  existence  of  internal 
causes  which  may  easily  be  supposed  to  acquire  sulficient  energy  to  cause  the 
explosion  of  the  planet.  The  intersection,  on  the  other  hand,  of  the  solar  sys- 
tem, by  innumerable  comets  rushing  among  the  planets  constantly  and  in  every 
direction,  renders  the  collision  of  such  a  body  with  a  planet  a  possible  occur- 
rence. Either  of  these  causes,  then,  being  sufficient  to  produce  the  supposed 
catastrophe,  and  both  being  possible,  the  next  question  to  be  settled  is,  whether 
the  circumstances  attending  the  appearance,  condition,  and  motion,  of  the  new 
planets,  are  such  as  would  attend  the  fragments  of  a  single  planet  exploded  or 
broken  by  either  of  these  causes. 

In  't'e  first  place,  then,  it  is  evident  that  the  magnitude  of  these  four  bodies 
recently  discovered  afford  a  strong  presumption  in  favor  of  such  a  supposition. 
Their  magnitudes  are  so  minute,  that  astronomical  observers  as  yet  have  been 
unable  to  agree  as  to  their  dimensions  ;  but  it  seems  certain  that  their  diameters 
do  not  amount  to  more  than  a  few  hundred  miles.     They  are  therefore  not  only 
incomparably  smaller  than  any  of  the  other  planets,  but  even  smaller  than  the 
satellites.     It  is  estimated  that  the  bulk  of  VESTA  does  not  exceed  the  twenty- 
five  thousandth  part  of  the  earth.     HERSCHEL  states  that  the  diameter  of  CERES 
)  cannot  much  exceed  a  hundred  and  fifty  miles,  and  that  that  of  JUNO  is  under 
(  one  hundred  miles.     It  is  calculated  that  the  aggregate  of  the  volumes  of  all 
)  these  four  planets  united  would  not  exceed  the  twenty-fifth  pnrt  of  the  bulk  of 
\  our  globe. 

This  minuteness  of  size  is  evidently  a  circumstance  that  might  naturally  be 
\  expected  in  the  fragments  of  a  single  planet ;  and  as  from  their  smallness  it  is 


THE  NEW  PLANETS.  207 


difficult  to  observe  these  planets  even  by  the  aid  of  telescopes,  it  seems  proba- 
ble there  may  be  other  fragments  revolving  round  the  sun  too  minute  to  be 
discovered. 

If  a  planet  were  broken  into  fragments,  whether  by  external  collision  or  by 
internal  explosion,  it  is  demonstrable  that  the  fragments  into  which  it  would 
be  resolved  would  severally  revolve  round  the  sun  as  independent  planets. 
Their  orbits  would  be  all  nearly  at  the  same  distance  from  the  sun  as  the  orbit 
of  the  original  planet.  These  orbits,  however,  would  be  likely  to  differ  from 
that  of  the  original  planet  in  some  respects.  It  is  consistent  with  mechanical 
laws  that  these  orbits  should  some  of  them  be  inclined  at  a  considerable  angle  to 
the  general  plane  of  the  solar  system.  It  is  also  probable  that  these  orbits  or 
some  of  them,  might  be  more  eccentric  in  their  elliptical  character  than  the 
planetary  orbits  generally  are.  Now  we  find  on  examining  the  orbits  of  the 
four  new  planets,  that  they  partake  of  these  characters.  They  are  inclined  to 
the  ecliptic  at  angles  so  considerable  that  they  are  the  only  planets  which  tran- 
scend the  limits  of  the  zodiac,  and  are  thence  called  ultra-zodiacal  planets.  The 
eccentricities  of  some  of  their  orbits  are  three  or  four  times  greater  than  those 
of  the  planets  generally. 

It  is  also  demonstrable  that  if  a  planet  were  broken  by  any  cause  the  orbits  of 
its  fragments  which  would  form  independent  planets  would  all  pass  through  a 
common  point.  Now  this  is  a  character  which  is  also  found  to  attach  to  the 
four  new  planets  generally. 

These  circumstances  would  themselves  afford  a  presumption  so  strong  in 
support  of  the  supposition  that  the  new  planets  are  in  fact  fragments  of  a  sin- 
gle planet  that  has  been  broken  as  to  amount  almost  to  a  moral  certainty 
— but  they  are  not  the  only  ones  that  favor  this  hypothesis. 

Appearances  have  been  observed  upon  these  planets  which  render  it  ex- 
tremely probable  if  not  certain  that  they  are  not  like  the  other  bodies  of  the  sys- 
tem globular  but  that  they  are  irregular  in  their  form,  having  corners  and  angu- 
lar extremities.  This  fact  has  been  indicated  by  the  sudden  diminution  of 
their  light  when  the  angular  points  pass  the  line  of  vision. 

It  is  remarkable  that  VESTA,  which  is  the  smallest  of  the  four  in  its  absolute 
magnitude,  appears,  nevertheless,  the  most  brilliant,  having  the  lustre  of  a  star 
of  the  fifth  or  sixth  magnitude.  SCHROTER,  for  this  reason,  was  led  to  the 
supposition  that  VESTA  was  a  self-luminous  body.  The  three  other  planets, 
which  are  greater  in  magnitude  than  Vesta,  have  the  appearance,  nevertheless, 
of  stars  of  the  ninth  and  tenth  magnitude.  CERES  would  seem  to  be  extremely 
irregular  in  its  shape,  since  its  light  is  very  variable  ;  sometimes  it  is  reddish 
and  vivid,  sometimes  whitish  and  pale. 

The  atmospheric  circumstances  attending  these  bodies  are  very  remarkable. 
CERES  and  PALLAS,  especially,  seem  to  be  enveloped  in  very  dense  atmo- 
spheres, which  'extend  to  a  height  from  their  surface  from  twelve  to  fifteen 
times  greater  than  ours. 

The  light  of  Vesta  is  more  intense  and  white  than  that  of  any  other  of  the 
new  planets.  It  also  differs  from  them  in  not  being  surrounded  by  any  nebu- 
losity. Schroter  affirms  that  he  saw  it  several  times  with  the  naked  eye,  a 
circumstance  which  must  have  arisen  from  the  brilliant  light  reflected  from  its 
surface  not  being  obscured  by  any  nebulous  envelope. 

The  planet  Juno  subtends  to  the  eye,  when  nearest  to  the  earth,  an  angle 
of  three  seconds.    It  is  of  a  reddish  color ;  and  Schroter  discovered  around  it 
an  atmosphere  which  he  considered  to  be  more  dense  than  any  of  the  atmo- 
spheres of  the  old  planets.     Remarkable  and  sudden  changes  were  observed 
in  the  light  of  this  planet,  which   Schroter  first  attributed  to  atmospheric  phe- 
|  nomena  upon  it,  but  which  have  been  since  ascribed  to  great  irregularity  in 
I 


208 


THE  NEW  PLANETS. 


its  form.  He  imagined  also  that  its  appearance  afforded  indications  of  a  diur- 
nal rotation  in  twenty-seven  hours  :  this,  however,  has  not  been  confirmed  by 
subsequent  observation. 

The  apparent  magnitude  of  Ceres  is  about  six  seconds :  it  is  an  object  of  a 
ruddy  color,  appears  about  the  size  of  a  star  of  the  eighth  or  ninth  magnitude,  and 
is  invisible  to  the  naked  eye.  It  is  surrounded  with  a  dense  atmosphere,  and 
shows  an  ill-defined  disk.  Schroter  found,  by  a  great  number  of  observations, 
that  the  height  of  its  atmosphere  amounted  to  nearly  seven  hundred  miles — that 
it  was  very  dense  near  the  surface  ®f  the  planet,  and  more  attenuated  at  greater 
heights — and  that  it  was  subject  to  changes  which  produced  great  variations  in 
the  apparent  size  of  the  planet. 

Sir  William  Herschel,  about  the  year  1802,  immediately  after  the  discovery 
of  Ceres  and  Pallas,  undertook  a  series  of  observations  with  his  powerful  re- 
flecting telescopes,  with  a  view  of  ascertaining  whether  either  of  these  planets 
were  attended  by  satellites.  Many  minute  stars  appeared  near  the  disk  of 
Ceres,  but  none  exhibited  that  change  of  position  which  could  be  supposed  to 
belong  to  a  satellite.  His  observations  fully  corroborated  those  of  Schroter. 
He  says  that  when  viewed  with  a  power  of  550,  Ceres  is  surrounded  with  a 
strong  haziness  ;  the  breadth  of  the  coma  beyond  the  disk  may  amount  to  the 
extent  of  a  diameter  of  the  disk,  which  is  not  very  sharply  defined.  Were  the 
whole  coma  and  star  taken  together,  they  would  be  at  least  three  times  as 
large  as  the  star.  The  coma  was  very  dense  near  the  nucleus,  but  lost  itself 
pretty  abruptly  on  the  outside,  though  a  gradual  diminution  was  still  very  per- 
ceptible. 

The  planet  Pallas  has  a  ruddy  appearance,  but  not  so  much  so  as  Ceres. 
It  is  surrounded  also  by  a  nebulosity,  but  not  so  extensive.  The  height  of  its 
atmosphere,  according  to  Schroter,  is  about  450  miles,  being  two  thirds  of  that 
of  Ceres.  The  light  of  the  planet  is  eminently  subject  to  those  sudden  varia- 
tions which  have  been  taken  to  indicate  irregularity  of  form. 

Sir  William  Herschel  says,  in  speaking  of  Pallas  :  "  I  cannot,  with  the  ut- 
most attention,  and  under  the  most  favorable  circumstances,  perceive  any  sharp 
termination  which  might  denote  a  disk ;  it  is,  rather,  what  I  would  call  a  nu- 
cleus. The  appearance  of  Pallas  is  cometary,  the  disk,  if  it  has  any,  being 
ill-defined.  When  I  see  it  to  the  best  advantage,  it  appears  like  a  much-com- 
pressed, extremely-small,  but  ill-defined,  planetary  nebula.  With  a  twenty- 
foot  reflector,  power  477,  I  see  Pallas  well.  I  perceive  a  very  small  disk, 
with  a  coma  of  some  extent  about  it,  the  diameter  of  which  may  amount  to  six 
or  seven  times  that  of  the  disk  alone."  These  observations  were  made  in  1802. 

Great  diversity  of  opinion  has  prevailed  respecting  the  actual  diameter  of 
the  new  planets,  Herschel  estimating  all  of  them  to  be  considerably  under  200  < 
miles,  while  Schroter  maintains  that  some  of  them  are  as  large  as  our  moon.  \ 
This  diversity  is  doubtless  produced  by  the  extreme  smallness  of  the  planets,  ' 
their  great  distance,  and  the  undefined  appearance  they  have,  owing  to  the  ] 
nebulosity  which  surrounds  them. 

We  shall  have  occasion  again  to  notice  the  theory  which  explains  them  by  ] 
the  supposition  that  they  are  fragments  of  a  broken  planet,  when  we  shall  refer  < 
to  the  subject  of  meteoric  stones. 


r 


THE     TIDES. 


Correspondence  between  the  Tides  and  Phases  of  the  Moon  shown  by  Kepler. — Erroneous  popular 
Notion  of  the  Moon's  Influence. — Actual  Manner  in  which  the  Moon  operates. — Influence  of  the 
Sun. — Combined  Action  of  the  Sun  and  Moon. — Spring  Tides. — Counter-action  of  the  Sun  and 
Moon. — Neap  Tides. — Priming  and  Lagging  of  the  Tides. — Discussions  at  the  British  Associa- 
tion.— Whewell's  Researches. — Effect  of  Continents  and  Islands  on  the  Tides. — General  Progress 
of  the  Great  Tidal  Wave.— Velocity  of  the  Tidal  Wave.— Eange  of  the  Tide. 


THE    TIDES. 


THE  phenomena  of  the  tides  of  the  ocean  are  too  remarkable  and  important 
to  the  social  and  commercial  interests  of  mankind,  not  to  have  attracted  notice 
at  an  early  period  in  the  progress  of  knowledge.  The  intervals  between  the 
epochs  of  high  and  low  water  everywhere  corresponding  with  the  intervals  be- 
tween the  passage  of  the  moon  over  the  meridian  above  and  below  the  horizon, 
suggested  naturally  the  physical  connexion  between  these  two  effects,  and  in- 
dicated the  probability  of  the  cause  of  the  tides  being  found  in  the  motion  of 
the  moon. 

KEPLER  developed  this  idea,  and  demonstrated  the  close  connexion  of  these 
phenomena ;  but  it  was  not  until  the  theory  of  GRAVITATION  was  established 
by  Newton,  and  its  laws  fully  developed,  that  all  the  circumstances  of  the  tides 
were  clearly  explained,  and  shown  incontestably  to  depend  on  the  influence  of 
the  sun  and  moon. 

There  are  few  subjects  in  physical  science  about  which  there  prevail  moie 
erroneous  notions  among  those  who  are  but  a  little  informed,  than  with  re- 
spect to  the  tides.  A  common  idea  is,  that  the  attraction  of  the  moon  draws 
the  waters  of  the  earth  toward  that  side  of  the  globe  on  which  the  moon  hap- 
pens to  be  placed,  and  that  consequently  they  are  heaped  up  on  that  side,  so 
that  the  oceans  and  seas  acquire  there  a  greater  depth  than  elsewhere  ;  and 
thus  it  is  attempted  to  be  established  that  high  water  will  take  place  under,  or 
nearly  under,  the  moon.  But  this  neither  corresponds  with  the  fact,  nor,  if  it 
did,  would  it  explain  it.  High  water  is  not  produced  merely  under  the  moon, 
but  is  equally  produced  upon  those  parts  most  removed  from  the  moon.  Sup- 
pose a  meridian  of  the  earth  so  selected,  that,  if  it  were  continued  beyond  the 
earth,  its  plane  would  pass  through  the  moon ;  then  we  find  that,  subject  to  cer- 
tain modifications,  a  great  tidal  wave,  or  what  is  called  htgh  water,  will  be  formed 
on  both  sides  of  this  meridian  ;  that  is  to  say,  on  the  side  next  the  moon,  and 
on  the  side  remote  from  the  moon.  As  the  moon  mores  in  her  monthly  course 
round  the  earth,  these  two  great  tidal  waves  follow  her.  They  are,  of 


212  THE  TIDES. 


course,  separated  from  each  other  by  half  the  circumference  of  the  globe. 
As  the  globe  revolves  with  its  diurnal  motion  upon  its  axis,  every  part  of  its 
surface  passes  successively  under  these  tidal  waves  ;  and  at  all  such  parts  as 
they  pass  under  them,  there  is  the  phenomenon  of  high  water.  Hence  it  is 
that  in  all  places  there  are  two  tides  daily,  having  an  interval  of  about  twelve 
hours  between  them.  Now  if  the  common  notion  of  the  cause  of  the  tides 
were  well  founded,  there  would  be  only  one  tide  daily ;  viz.,  that  which  would 
take  place  when  the  moon  is  at  or  near  the  meridian. 

That  the  moon's  attraction  upon  the  earth  simply  considered  would  not  ex- 
plain the  tides,  is  easily  shown.  Let  us  suppose  that  the  whole  mass  of  mat- 
ter on  the  earth,  including  the  waters  which  partially  cover  it,  were  attracted 
equally  by  the  moon  ;  they  would  then  be  equally  drawn  toward  that  body,  and 
no  reason  would  exist  why  they  should  be  heaped  up  under  the  moon  ;  for  if  they 
were  drawn  with  the  same  force  as  that  with  which  the  solid  globe  of  the  earth 
under  them  is  drawn,  there  would  be  no  reason  for  supposing  that  the  waters 
would  have  a  greater  tendency  to  collect  toward  the  moon  than  the  solid  bot- 
tom of  the  ocean  on  which  they  rest.  In  short,  the  whole  mass  of  the  earth, 
solid  and  fluid,  being  drawn  with  the  same  force,  would  equally  tend  toward 
the  moon ;  and  its  parts,  whether  solid  or  fluid,  would  preserve  among  them- 
selves the  same  relative  position  as  if  they  were  not  attracted  at  all. 

When  we  dbserve,  however,  in  a  mass  composed  of  various  particles  of  mat- 
ter, that  the  relative  arrangement  of  these  particles  is  disturbed,  some  being 
driven  in  certain  directions  more  than  others,  the  inference  is,  that  the  compo- 
nent parts  of  such  a  mass  must  be  placed  under  the  operation  of  different 
forces  ;  those  which  tend  more  than  others  in  a  certain  direction  being  driven 
with  a  proportionally  greater  force.  Such  is,  in  fact,  the  case  with  the  earth, 
placed  under  the  attraction  of  the  moon.  NEWTON  showed  that  the  law  of 
gravitation  is  such,  that  its  attraction  increases  as  the  distance  of  the  attracted 
object  diminishes,  and  diminishes  as  the  distance  of  the  attracted  object  in- 
creases. The  exact  proportion  of  this  change  of  energy  of  the  attractive 
force,  is  technically  expressed  by  stating  that  it  is  the  inverse  proportion  of  the 
square  of  the  distance  ;  the  meaning  of  which  is,  that  the  attraction  which  any 
body  like  the  moon  would  exercise  at  any  proposed  distance,  is  four  times  that 
which  it  would  exercise  at  twice  the  distance ;  nine  times  that  which  it  would 
exert  at  three  times  the  distance  ;  one  fourth  of  that  which  it  would  exercise 
at  half  the  distance,  and  one  ninth  of  that  which  it  would  exercise  at  one  third 
the  distance,  and  so  on.  Thus  we  have  an  arithmetical  rule,  by  which  we  can 
with  certainty  and  precision  say  how  the  attraction  of  the  moon  will  vary  with  any 
change  of  its  distance  from  the  attracted  object.  Let  us  see  how  this  will  be 
brought  to  bear  upon  the  explanation  of  the  effect  of  the  moon's  attraction  upon 
the  earth. 

Let  A,  B,  C,  D,  E,  F,  G,  H,  represent  the  globe  of  the  earth,  and,  to  simplify 
the  explanation,  let  us  first  suppose  the  entire  surface  of  the  globe  to  be  covered 
with  water.  Let  M,  the  moon,  be  placed  at  the  distance  K  L  from  the  nearest  point 
of  the  surface  of  the  earth.  Now  it  will  be  very  apparent  that  the  various  points 
of  the  earth's  surface  are  at  different  distances  from  the  moon,  M.  A  and  G  are 
more  remote  than  H ;  B  F  still  more  remote  ;  C  and  E  more  distant  again, 
and  D  more  remote  than  all.  The  attraction  which  the  moon  exercises  at  H 
is,  therefore,  greater  than  that  which  it  exercises  at  A  and  G,  and  still  greater 
than  that  which  it  produces  Rt  B  and  F ;  and  the  attraction  which  it  exercises 
at  D  is  least  of  all.  Now  this  attraction  equally  afffects  matter  in  every  state 
and  condition.  It  affects  the  particles  of  fluid  as  well  as  solid  matter,  but  there 
is  this  difference  between  these  effects  ;  that  where  it  acts  upon  solid  matter, 
the  component  parts  of  which  are  at  different  distances  from  it,  and  therefore  I 


subject  to  different  attractions,  it  will  not  disturb  the  relative  arrangement  of 
these  particles,  since  such  disturbances  or  disarrangements  are  prevented  by 
the  cohesion  which  characterizes  a  solid  body ;  but  this  is  not  the  case  with 
fluid,  the  particles  of  which  are  mobile,  and  which,  when  solicited  by  different 
forces,  will  have  their  relative  arrangements  disturbed  in  a  corresponding 
manner. 

The  attraction  which  the  moon  exercises  upon  the  shell  of  water  which  is 
collected  immediately  under  it  near  the  point  Z,  is  greater  than  that  which  it 
exercises  upon  the  solid  mass  of  the  globe  at  H  and  D  ;  consequently  there 
will  be  a  greater  tendency  of  this  attraction  to  draw  the  fluid  which  rests  upon 
the  surface  at  H  toward  the  moon,  than  to  draw  the  solid  mass  of  the  earth 
which  is  more  distant. 

As  the  fluid,  by  its  nature,  is  free  to  obey  this  excess  of  attraction,  it  will 
necessarily  heap  itself  up  in  a  pile  or  wave  at  H,  forming  a  more  convex  pro- 
tuberance, as  represented  in  the  figure  between  R  and  I.  Thus  high  water 
will  take  place  at  H,  immediately  under  the  moon.  The  water  which  thus 
collects  at  H,  will  necessarily  flow  from  the  regions  B  and  F,  where,  there- 
fore, there  will  be  a  diminished  quantity  of  water  in  the  same  proportion. 

But  let  us  now  consider  what  happens  to  that  part  of  the  earth,  D,  most  re- 
mote from  the  moon.  Here  the  waters  being  more  remote  from  the  moon  than 
the  solid  mass  of  the  earth  under  them,  will  be  less  attracted  ;  and  consequent- 
ly will  have  a  less  tendency  to  gravitate  toward  the  moon.  The  solid  mass  of 
the  earth,  D  H,  will,  as  it  were,  recede  from  the  waters  at  N,  in  virtue  of  the 
excess  of  attraction,  leaving  these  waters  behind  it,  which  will  thus  be  heaped 
up  at  N,  so  as  to  form  a  convex  protuberance  between  L  and  K,  similar,  ex- 
actly to  that  which  we  have  already  described  between  R  and  I.  As  the  dif- 
ference between  the  attraction  of  the  moon  on  the  waters  at  Z  and  the  solid 
earth  under  the  waters,  is  nearly  the  same  as  the  difference  between  its  attrac- 
tion on  the  latter  and  upon  the  waters  at  N,  it  follows  that  the  height  of  the 
fluid  protuberances  at  Z  and  N  are  equal.  In  other  words,  the  height  of  the 
tides  on  opposite  sides  of  the  earth,  the  one  being  under  the  moon  and  the  other 
most  remote  from  it,  are  equal. 

Now  from  this  explanation,  it  will,  we  trust,  be  apparent,  that  the  cause  of 
the  tides,  so  far  as  the  action  of  the  moon  is  concerned,  is  not,  as  is  vulgarly 
supposed,  due  to  the  mere  attraction  of  the  earth  ;  since,  if  that  attraction 
were  equal  in  all  the  component  parts  of  the  earth,  there  would  assuredly  be 
no  tides.  We  are  to  look  for  the  cause,  then,  not  in  the  attraction  of  the  moon, 
but  in  the  inequality  of  its  attraction  on  different  parts  of  the  earth.  The  greater 
this  inequality  is,  the  greater  will  be  the  tides.  Hence,  as  the  moon  is  sub- 
ject to  a  slight  variation  of  distance  from  the  earth,  it  will  follow,  that  when  it 
is  at  its  least  distance,  or  at  the  point  called  perigee,  the  tides  will  be  greatest ;  and 


214 


THE  TIDES. 


when  it  is  the  greatest  distance,  or  at  the  point  called  apogee,  the  tides  will  be  least ; 
not  because  the  entire  attraction  of  the  moon  in  the  former  case  is  greater  than 
in  the  latter,  but  because  the  diameter  of  the  globe  bearing  a  greater  proportion 
to  the  lesser  distance  than  the  greater,  there  will  be  a  greater  inequality  of  at- 
traction. 

It  will  doubtless  occur  to  those  who  bestow  on  these  observations  a  little 
reflection,  that  all  which  we  have  stated  in  reference  to  the  effect  produced  by 
the  attraction  of  the  moon  upon  the  earth,  will  also  be  applicable  to  the  attrac- 
tion of  the  sun.  This  is  undoubtedly  true ;  but  in  the  case  of  the  sun  the 
effects  are  modified,  in  some  very  important  respects,  as  will  readily  be  seen. 
The  sun  is  at  four  hundred  times  a  greater  distance  than  the  moon,  and  the 
actual  amount  of  its  attraction  on  the  earth  would,  on  that  account,  be  one  hun- 
dred and  sixty  thousand  times  less  than  that  of  the  moon ;  but  the  mass  of  the 
sun  exceeds  that  of  the  moon  in  a  much  greater  ratio  than  that  of  one  hundred 
and  sixty  thousand  to  one.  It  therefore  possesses  a  much  greater  attracting 
power  in  virtue  of  its  mass,  compared  with  the  moon,  than  it  loses  by  its  in- 
creased distance.  The  effect  is,  that  it  exercises  upon  the  earth  an  attraction 
enormously  greater  than  the  moon  exercises.  Now,  if  the  simple  amount  of  its 
attraction  were,  as  is  commonly  supposed,  the  cause  of  the  tides,  the  sun  ought 
to  produce  a  vastly  greater  tide  than  the  moon.  The  reverse  is,  however,  the 
case,  and  the  cause  is  easily  explained.  Let  it  be  remembered  that  the  tides 
are  due  solely  to  the  inequality  of  the  attraction  on  different  sides  of  the  earth, 
and  the  greater  that  inequality  is,  the  greater  will  be  the  tides,  and  the  less  that 
inequality  is,  the  less  will  be  the  tides. 

Now  in  the  case  of  the  sun,  its  total  distance  from  the  earth  is  one  hundred 
millions  of  miles,  and  the  difference  between  its  distance  from  one  side  of  the 
earth,  and  from  the  other,  is  only  eight  thousand  miles,  or  about  one  hundred 
and  twenty  thousandth  part  of  the  whole  distance.  The  inequality  of  the  at- 
traction of  the  sun,  therefore,  on  different  sides  of  the  earth  will  be  in  the  pro- 
portion of  the  square  of  the  numbers  one  hundred  and  twenty  thousand  and  one 
hundred  and  twenty  thousand  and  one  to  each  other,  a  proportion  which  it  will 
be  evident,  is  extremely  small.  But  in  the  case  of  the  moon,  the  distance 
of  that  object  being  about  two  hundred  and  forty  thousand  miles,  or  thirty 
diameters  of  the  earth,  the  difference  between  its  distance  from  one  side  to 


THE  TIDES. 


215 


the  other  will  be  in  the  proportion  of  thirty  to  thirty-one  ;  and  the  difference  I 
of  the  attraction  will  be  in  the  proportion  of  the  squares  of  those  numbers.  * 
In  the  case,  therefore,  of  the  sun,  the  difference  of  the  distances  to  the  whole, 
then,  is  in  proportion  of  one  to  one  hundred  and  twenty  thousand  ;  whereas, 
in  the  case  of  the  moon  it  is  in  the  proportion  of  one  to  thirty. 

Still,  although  the  difference  of  the  attractions  of  the  sun  on  different  sides 
of  the  earth  is  infinitely  less  than  those  of  the  moon,  it  is  not  imperceptible ; 
and  the  sun  does  actually  produce  sensible  tides  on  opposite  sides  of  the  earth, 
as  the  moon  does.  When  the  sun  and  moon,  therefore,  are  either  on  the  same 
side  of  the  earth,  or  on  the  opposite  sides  of  the  earth ;  in  other  words,  when 
it  is  new  or  full  moon,  then  their  effects  in  producing  tides  are  combined,  and 
the  spring  tide  is  produced  ;  the  height  of  which  is  equal  to  the  solar  and  lunar 
tides  taken  together.  These  positions  are  represented  in  the  preceding  dia- 
gram, where  S  is  the  sun,  A  the  moon  when  new,  and  B  the  moon  when  full. 
Hence  it  is  that,  at  the  epochs  of  new  and  full  moons,  we  have  tides  of  ex- 
traordinary elevation,  called  spring  tides. 

On  the  other  hand,  when  the  sun  and  moon  are  separated  from  each  other 
by  a  distance  of  one  fourth  of  the  heavens,  that  is,  when  the  moon  is  in  the 
quarters,  the  effect  of  the  solar  tide  has  a  tendency  to  diminish  that  of  the  lunar 
tide.  This  position  is  represented  in  the  annexed  diagram. 


If  Q  and  R  represent  positions  of  the  moon,  and  5  that  of  the  sun  at  the 
epochs  of  the  quarters,  then  the  lunar  tides  would  cause  the  waters  to  be  col- 
lected at  Z  and  N;  whereas  the  solar  tides  would  take  place  at  B  and  F.  The 
tendency,  therefore,  of  the  sun,  would  be  to  draw  the  water  from  Z  and  N 
toward  B  and  F;  and  to  the  same  extent  would  diminish  the  effect  of  the 
moon's  attraction.  The  lunar  tides  would  be  less,  under  these  circumstances, 
than  in  other  positions  of  the  moon.  These  have,  therefore,  been  called  the 
neap  tides. 

If  physical  effects  followed  immediately,  without  any  appreciable  interval 
of  time,  the  operation  of  their  causes,  then  the  tidal  wave  produced  by  the 
moon  would  be  on  the  meridian  of  the  earth  directly  under  and  opposite  to 
that  luminary  ;  and  the  same  would  be  true  of  the  solar  tides.  But  the  waters 
of  the  globe  have,  in  common  with  all  other  matter,  the  property  of  inertia,  and 
it  takes  a  certain  interval  of  time  to  impress  upon  them  a  certain  change  of 
position.  Hence  it  follows  that  the  tidal  wave  produced  by  the  moon  is  not 
formed  immediately  under  that  body  but  follows  it  at  a  certain  distance.  In 
consequence  of  this,  the  tide  raised  by  the  moon  does  not  take  place  for  2  or  3 
hours  after  the  moon  passes  the  meridian ;  and  as  the  action  of  the  sun  is  still 


216 


THE  TIDES. 


*o*"^-^» 


more  feeble,  there  is  a  still  greater  interval  between  the  transit  of  the  sun  and 
occurrence  of  the  solar  tide. 

But  besides  these  circumstances,  the  tide  is  affected  by  other  causes.  It  is 
not  the  separate  effect  of  either  of  these  bodies,  but  to  the  combined  effect  of 
both,  and  at  every  period  of  the  month,  the  time  of  actual  high  water  is  either 
accelerated  or  retarded  by  the  sun.  In  the  first  and  third  quarters  of  the  moon, 
the  solar  tide  is  westward  of  the  lunar  one  ;  and,  consequently,  the  actual  high 
water  which  is  the  result  of  the  combination  of  the  two  waves  will  be  to  the 
westward  of  the  place  it  would  have  if  the  moon  acted  alone,  and  the  time  of 
high  water  will  therefore  be  accelerated.  In  the  second  and  fourth  quarters 
the  general  effect  of  the  sun  is,  for  a  similar  reason,  to  produce  a  retardation 
in  the  time  of  high  water.  This  effect  produced  by  the  sun  and  moon  com- 
bined, is  what  is  commonly  called  the  priming  and  lagging  of  the  tides. 

The  highest  spring  tides  occur  when  the  moon  passes  the  meridian  about 
an  hour  after  the  sun  ;  for  then  the  maximum  effect  of  the  two  bodies  coincides. 

The  subject  of  the  tides  has  of  late  years  received  much  attention  from  sev- 
eral scientific  investigators  in  Europe.  The  discussions  held  at  the  annual 
meetings  of  the  British  association  for  the  advancement  of  science,  on  this  sub- 
ject, have  led  to  the  development  of  much  useful  information.  The  labors  of 
Professor  Whewell  have  been  especially  valuable  on  these  questions.  Sir 
John  Lubbock  has  also  published  a  valuable  treatise  upon  it.  To  trace  the  re- 
sults of  these  investigations  in  all  the  details  which  would  render  them  clear  and 
intelligible,  would  greatly  transcend  the  necessary  limits  of  this  discourse.  We 
shall,  however,  briefly  advert  to  a  few  of  the  most  remarkable  points  connected 
with  these  questions. 

The  apparent  time  of  high  water  at  any  port  in  the  afternoon  of  the  day  of 
new  or  full  moon,  is  what  is  usually  called  the  establishment  of  the  port.  Pro- 
fessor Whewell  calls  this  the  vulgar  establishment,  and  he  calls  the  corrected  es- 
tablishment the  mean  of  all  the  intervals  of  the  tides  and  transit  of  half  a  month. 
This  corrected  establishment  is  consequently  the  luni-tidal  interval  correspond- 
ing to  the  day  on  which  the  moon  passes  the  meridian  at  noon  or  midnight. 

The  two  tides  immediately  following  another,  or  the  tides  of  the  day  and 
night,  vary,  both  in  height  and  time  of  high  water,  at  any  particular  place  with 
the  distance  of  the  sun  and  moon  from  the  equator.  As  the  vertex  of  the  tide 
wave  always  tends  to  place  itself  vertically  under  the  luminary  which  produ- 
ces it,  it  is  evident  that  of  two  consecutive  tides  that  which  happens  when  the 
moon  is  nearest  the  zenith  or  nadir  will  be  greater  than  the  other  ;  and,  conse- 
quently, when  the  moon's  declination  is  of  the  same  denomination  as  the  lati- 
tude of  the  place,  the  tide  which  corresponds  to  the  upper  transit  will  be 
greater  than  the  opposite  one,  and  vice  versa,  the  differences  being  greatest 
when  the  sun  and  moon  are  in  opposition,  and  in  opposite  tropics.  This  is 
called  the  diurnal  inequality,  because  its  cycle  is  one  day ;  but  it  varies  greatly 
at  different  places,  and  its  laws,  which  appear  to  be  governed  by  local  circum- 
stances, are  very  imperfectly  known. 

We  have  now  described  the  principal  phenomena  that  would  take  place 
were  the  earth  a  sphere,  and  covered  entirely  with  a  fluid  of  uniform  depth. 
But  the  actual  phenomena  of  the  tides  are  infinitely  more  complicated.  From 
the  interruption  of  the  land,  and  the  irregular  form  and  depth  of  the  ocean, 
combined  with  many  other  disturbing  circumstances,  among  which  are  the  in- 
ertia of  the  waters,  the  friction  on  the  bottom  and  sides,  the  narrowness  and 
length  of  the  channels,  the  action  of  the  wind,  currents,  difference  of  atmo- 
spheric pressure,  &c.,  &c.,  great  variation  takes  place  in  the  mean  times 
and  heights  of  high  water  at  places  differently  situated ;  and  the  inequali- 
ties above  alluded  to,  as  depending  on  the  parallax  of  the  moon,  her  posi- 


THE  TIDES. 


217 


tion  with  respect  to  the  sun,  and  the  declination  of  the  two  bodies,  are  in  many 
cases  altogether  obliterated  by  the  effects  of  the  disturbing  influences,  or  can 
only  be  detected  by  the  calculation  and  comparison  of  long  series  of  observa- 
tions. 

By  reason  of  these  disturbing  causes,  it  becomes  a  matter  of  great  difficulty  to 
trace  the  propagation  of  the  tide  wave,  and  the  connexion  of  the  tides  in  different 
parts  of  the  world.  In  the  Philosophical  Transactions  for  1832,  Sir  John  Lub- 
bock  published  a  map  of  the  world,  in  which  he  inserted  the  times  of  high 
water  at  new  and  full  moon  at  a  great  number  of  places  on  the  globe,  collected 
from  various  sources,  as  works  on  navigation,  voyages,  sailing  directions,  &c., 
and  in  order  that  the  march  of  the  tide  wave  might  be  'traced  more  readily,  the 
times  were  expressed  in  Greenwich  time,  as  well  as  the  time  of  the  place.  In 
the  same  Transactions  for  1833,  Mr.  Whewell  prosecuted  this  subject  at 
greater  length,  and  availing  himself  of  a-priori  considerations,  as  well  as  of  a 
mass  of  information  collected  in  the  hydrographer's  office  at  the  admiralty,  in- 
serted in  the  map  a  series  of  cotidal  lines,  or  lines  along  which  high  water 
takes  place  at  the  same  instant  of  time.  But  these  cotidal  lines,  as  Sir  John 
Lubbock  remarks,  are  entirely  hypothetical ;  for  we  have  few  opportunities  of 
determining  the  time  of  high  water  at  a  distance  from  the  coast,  though  this  is 
sometimes  possible  by  means  of  a  solitary  island,  such  as  St.  Helena. — Lub- 
bocft's  Elementary  Treatise  on  the  Tides,  1839. 

According  to  Mr.  Whewell's  deduction,  the  general  progress  of  the  great 
tide  wave  may  be  thus  described  ;  it  is  only  in  the  Southern  ocean,  between 
the  latitudes  of  30°  and  70°,  that  a  zone  of  water  exists  of  sufficient  extent  to 
allow  of  the  tide-wave  being  formed.  Suppose,  then,  a  line  of  contemporary 
tides,  or  cotidal  line,  to  be  formed  in  the  Indian  ocean,  as  the  theory  supposes, 
that  is  to  say,  in  the  direction  of  the  meridian,  and  at  a  certain  distance  to  the 
eastward  of  the  meridian  in  which  the  moon  is.  As  this  tide-wave  passes  the 
Cape  of  Good  Hope,  it  sends  off  a  derivative  undulation,  which  advances 
northward  up  the  Atlantic  ocean,  preserving  always  a  certain  proportion  of  its 
original  magnitude  and  velocity.  In  travelling  along  this  ocean  the  wave  assumes 
a  curved  form,  the  convex  part  keeping  near  the  middle  of  the  ocean,  and  ahead 
of  the  branches,  which,  owing  to  the  shallower  waters,  lag  behind  on  the  Amer- 
ican and  African  coasts,  so  that  the  cotidal  lines  have  always  a  tendency  to  make 
very  oblique  angles  with  the  shore,  and,  in  fact,  run  parallel  to  it  for  great  dis- 
tances. The  main  tide,  Mr.  Whewell  conceives,  after  reaching  the  Orkneys, 
will  move  forward  in  the  sea  bounded  by  the  shores  of  Norway  and  Sibe- 
ria on  one  side  and  those  of  Greenland  and  America  on  the  other,  will  pass 
the  pole  of  the  earth  and  finally  end  its  course  on  the  shores  in  the  neighbor- 
hood of  Behring's  straits.  It  may  even  propagate  its  influence  through  the 
straits,  and  modify  the  tides  of  the  North  Pacific.  But  a  branch  tide  is  sent 
off  from  this  main  tide  into  the  German  ocean  ;  and  this,  entering  between 
the  Orkneys  and  the  coast  of  Norway,  brings  the  tide  to  the  east  coast  of  Eng- 
land and  to  the  coasts  of  Holland,  Denmark,  and  Germany.  Continuing  its 
course,  part  of  it  passes  through  the  strait  of  Dover  and  meets  in  the  British 
channel  the  tide  from  the  Atlantic,  which  arrives  on  the  coast  of  Europe 
twelve  hours  later  ;  but  in  passing  along  the  English  coast,  another  part  of  it 
is  reflected  from  the  projecting  land  of  Norfolk  upon  the  north  coast  of  Ger- 
many, and  again  meets  the  tide  ..wave  on  the  shores  of  Denmark.  Owing  to 
this  interference  of  different  tide-waves,  the  tides  are  almost  entirely  oblitera- 
ted on  the  coast  of  Jutland,  where  their  place  is  supplied  by  continual  high 
water. 

In  the  Pacific  ocean  the  tides  are  very  small;  but  there  are  not  sufficient, 
observations  to  determine  the  forms  and  progress  of  the  cotidal  lines.  Off  Cape 


218  THE  TIDES. 


Horn,  and  round  the  whole  shore  of  Terra-del-Fuego,  from  the  western  ex- 
tremity of  Magellan's  strait  to  Staten  Island,  it  is  very  remarkable  that  the 
tidal  wave,  instead  of  following  the  moon  in  its  diurnal  course,  travels  to  the 
eastward.  This,  however,  is  a  partial  phenomenon  ;  and  a  little  farther  to  the 
north  of  the  last-named  places,  the  tides  set  to  the  north  and  west.  In  the 
Mediterranean  and  Baltic  seas  the  tides  are  inconsiderable,  but  exhibit  irregu- 
larities for  which  it  is  difficult  to  account.  The  Indian  ocean  appears  to  have 
high  water  on  all  sides  at  once,  though  not  in  the  central  parts  at  the  same 
time. 

Since  the  tide*  on  our  coast  are  derived  from  the  oscillations  produced  under 
the  direct  agency  of  the  sun  and  moon  in  the  Southern  ocean,  and  require  a 
certain  interval  of  time  for  their  transfer,  it  follows  that,  in  general,  the  tide  is 
not  due  to  the  moon's  transit  immediately  preceding,  but  is  regulated  by  the 
position  which  the  sun  and  the  moon  had  when  they  determined  the  primary 
tide.  The  time  elapsed  between  the  original  formation  of  the  tide  and  its  ap- 
pearance at  any  place  is  called  the  age  of  the  tide,  and  sometimes,  after  Ber- 
noulli, the  retard.  On  the  shores  of  Spain  and  North  America,  the  tide  is  a 
day  and  a  half  old  ;  in  the  port  of  London,  it  appears  to  be  two  days  and  a  half 
old  when  it  arrives. 

VELOCITY    OF    THE    TIDE    WAVES. 

In  the  open  ocean  the  crest  of  tide  travels  with  enormous  velocity.  If  the 
whole  surface  were  uniformly  covered  with  water,  the  summit  of  the  tide  wave, 
being  mainly  governed  by  the  moon,  would  everywhere  follow  the  moon's 
transit  at  the  same  interval  of  time,  and  consequently  travel  round  the  earth 
in  a  little  more  than  twenty-four  hours.  But  the  circumference  of  the  earth 
at  the  equator  being  about  25,000  miles,  the  velocity  of  propagation  would 
therefore  be  about  1,000  miles  per  hour.  The  actual  velocity  is,  perhaps,  no- 
where equal  to  this  and  is  very  different  at  different  places.  In  latitude  60° 
south,  where  there  is  no  interruption  from  land  (excepting  the  narrow  promonto- 
ry of  Patagonia),  the  tide  wave  will  complete  a  revolution  in  a  lunar  day,  and 
consequently  travel  at  the  rate  of  670  miles  an  hour.  On  examining  Mr. 
Whewell's  map  of  cotidal  lines,  it  will  be  seen  that  the  great  tide  wave  from 
the  Southern  ocean  travels  from  the  Cape  of  Good  Hope  to  the  Azores  in 
about  twelve  hours,  and  from  the  Azores  to  the  southernmost  part  of  Ireland  in 
about  three  hours  more.  In  the  Atlantic,  the  hourly  velocity  in  some  cases  ap- 
pears to  be  10°  latitude,  or  near  700  miles,  which  is  almost  equal  to  the  velocity 
of  sound  through  the  air.  From  the  south  point  of  Ireland  to  the  north  point 
of  Scotland,  the  time  is  eight  hours,  and  the  velocity  about  160  miles  an  hour 
along  the  shore.  On  the  eastern  coast  of  Britain,  and  in  shallower  water,  the 
velocity  is  less.  From  Buchanness  to  Sunderland  it  is  about  sixty  miles  an 
hour  ;  from  Scarborough  to  Cromer,  thirty-five  miles  ;  from  the  north  Foreland 
to  London,  thirty  miles  ;  from  London  to  Richmond,  thirteen  miles  an  hour  in 
that  part  of  the  river.  (Whewell,  Phil.  Trans.  1833  and  1836.)  It  is  scarce- 
ly necessary  to  remind  the  reader  that  the  above  velocities  refer  to  the  trans- 
mission of  the  undulation,  and  are  entirely  different  from  the  velocity  of  the 
current  to  which  the  tide  gives  rise  in  shallow  water. 

RANGE    OF    THE    TIDE. 


The  difference  of  level  between  high  and  low  water  is  affected  by  various  ( 

causes,  but  chiefly  by  the  configuration  of  the  land,  and  is  very  different  at  dif-  ( 

rent  places.     In  deep  inbends  of  the  shore,  open  in  the  direction  of  the  tide  < 

'J 


THE  TIDES.  219 


wave  and  gradually  contracting  like  a  funnel,  the  convergence  of  water  causes 
a  very  great  increase  of  the  range.  Hence  the  very  high  tides  in  the  Bristol 
channel,  the  bay  of  St.  Malo,  and  the  bay  of  Fundy,  where  the  tide  is  said  to 
rise  sometimes  to  the  height  of  one  hundred  feet.  Promontories,  under  certain 
circumstances,  exert  an  opposite  influence,  and  diminish  the  magnitude  of  the 
tide.  The  observed  ranges  are  also  very  anomalous.  At  certain  places  on  the 
southeast  coast  of  Ireland,  the  range  is  not  more  than  three  feet,  while  at  a 
little  distance  on  each  side  it  becomes  twelve  or  thirteen  feet ;  and  it  is  re- 
markable that  these  low  tides  occur  directly  opposite  the  Bristol  channel,  where 
(at  Chepstow)  the  difference  between  high  and  low  water  amounts  to  sixty  feet. 
In  the  middle  of  the  Pacific  it  amounts  to  only  two  or  three  feet.  At  the  Lon- 
don docks,  the  average  range  is  about  22  feet ;  at  Liverpool,  15.5  feet ;  at 
Portsmouth,  12.5  feet ;  at  Plymouth,  also  12.5  feet ;  at  Bristol,  33  feet. 

A  great  number  of  observations  of  the  tides  at  the  port  of  Brest  during  the 
last  century  were  discussed  by  Laplace  in  the  Mecanique  Celeste  ;  but  in  order 
to  determine  the  motion  of  the  tide  wave,  and  separate  the  general  laws  of  the 
phenomena  from  local  irregularities,  it  is  necessary  to  have  regular  series  of 
observations  made  at  different  parts  of  the  ocean.  Until  very  recently, 
theory  may  be  said  to  have  been  in  advance  of  observation  ;  but  of  late  years 
the  subject  has  received  great  attention,  and  at  the  present  time  a  more  per- 
fect theory  of  hydrodynamics  appears  to  be  necessary  for  the  physical  ex- 
planation of  the  phenomena.  In  1829,  Sir  John  Lubbock  undertook  the  dis- 
cussion of  the  tide  observations  which  are  made  at  the  London  docks,  with  the 
view  of  obtaining  correct  tables  for  predicting  the  time  and  height  of  the  tides 
for  the  British  Almanac.  The  results,  which  were  published  in  the  Philo- 
sophical Transactions  for  1831,  are  deduced  from  a  series  of  upward  of  thirteen 
thousand  observations  during  a  period  of  nineteen  years,  and  are  of  great  im- 
portance, both  as  affording  materials  for  the  construction  of  tide-tables,  and  as 
pointing  out  the  defects  of  the  equilibrium  theory,  with  which  they  were  accu- 
rately compared.  'In  some  of  the  subsequent  volumes  of  the  Transactions  the 
author  has  continued  his  investigations,  and  has  also  published  separately  an 
account  of  Bernoulli's  Traite  sur  le  Flux  et  Reflux,  and  an  elementary  trea- 
tise which  appeared  in  1839.  In  the  Philosophical  Transactions  for  1833,  Mr. 
Whewell  gave  an  Essay  toward  a  first  Approximation  to  a  Map  of  Cotidal  Lines, 
which  has  been  followed  by  a  series  of  interesting  papers  in  the  subsequent 
volumes.  Mr  Whe well's  researches  have  been  chiefly  directed  to  the  deter- 
mination of  the  following  points  :  First,  the  motion  of  the  tide  wave  at  differ- 
ent parts  of  the  ocean  ;  secondly,  the  comparison  of  the  observed  laws  at 
different  places  with  the  theory ;  and  lastly,  the  laws  of  diurnal  inequality.  In 

1834  the  British  Association  procured  an  extensive  series  of  observations  to 
be  made  on  the  coasts  of  Britain  and  Ireland  at  five  hundred  and  thirty-nine  sta- 
tions of  the  coast  guard.     These  were  repeated  at  the  same  places  in  June, 

1835  ;  and  at  the  request  of  the  British  government,  simultaneous  observations 
were  made  by  the  other  maritime  powers  of  Europe  and  the  United  States.  ' 
The  number  of  stations  in  America  was  twenty-  eight,  extending  from  the  mouth  \ 
of  the  Mississippi  to  Nova  Scotia  ;  and  the  number  on  the  continent  of  Europe  ( 
one  hundred  and  one,  between  the  straits  of  Gibraltar  and  the  North  cape  of  , 
Norway.     The  results  of  these  observations  reduced  under  Mr.  Whewell's  su-  J 
perintendence  were  published  in  the  Philosophical  Transactions  for  1836  ;  and  , 
they  are  of  great  importance,  not  only  as  affording  a  far  more  precise  determi-  j 
nation  of  the  progress  of  the  tide  wave  and  the  forms  of  the  cotidal  line  on  the  < 
coasts  of  Europe  and  North  America  than  previously  existed,  but  as  furnishing  j 
more  correct  data  for  the  construction  of  the  tide-tables. 

Besides  the  numerous  causes  of  irregularity  depending  on  the  local  circum-  j 


220 


THE  TIDES. 


stances,  the  tides  are  also  affected  by  the  state  of  the  atmosphere.  At  Brest, 
the  height  of  high  water  varies  inversely,  as  the  height  of  the  barometer,  and 
rises  more  than  eight  inches  for  a  fall  of  about  half  an  inch  of  the  barometer. 
At  Liverpool,  a  fall  of  one  tenth  of  an  inch  in  the  barometer  corresponds  to  a 
rise  in  the  river  Mersey  of  about  an  inch ;  and  at  the  London  docks,  a  fall  of 
one  tenth  of  an  inch  corresponds  to  a  rise  in  the  Thames  of  about  seven  tenths 
of  an  inch.  With  a  low  barometer,  therefore,  the  tide  may  be  expected  to  be 
high,  and  vice  versa.  The  tide  is  also  liable  to  be  disturbed  by  winds.  Sir 
John  Lubbock  states,  that,  in  the  violent  hurricane  of  January  8,  1839,  there 
was  no  tide  at  Gainsborough,  which  is  twenty-five  miles  up  the  Trent — a  cir- 
cumstance unknown  before.  At  Saltmarsh,  only  five  miles  up  the  Ouse  from 
the  [lumber,  the  tide  went  on  ebbing,  and  never  flowed  until  the  river  was  dry 
in  some  places  ;  while  at  Ostend,  toward  which  the  wind  was  blowing,  con- 
trary effects  were  observed.  During  strong  northwesterly  gales  the  tide  marks 
high  water  earlier  in  the  Thames  than  otherwise,  and  does  not  give  so  much 
water,  while  the  ebb  tide  runs  out  late,  and  marks  lower  ;  but  upon  the  gales 
abating  and  weather  moderating,  the  tides  put  in  and  rise  much  higher,  while 
they  also  run  longer  before  high  water  is  marked,  and  with  more  velocity  of 
current :  nor  do  they  run  out  so  long  or  so  low. 


LIGHT. 


Structure  of  the  Eye. — Manner  in  which  distant  Objects  become  Visible. — Corpuscular  Theory. — 
Undulatory  Theory. — Its  general  Reception. — Velocity  of  Light. — Account  of  its  Discovery  by 
Hoemer. — Measurement  of  the  Waves  of  Light  by  Newton. — Color  produced  by  Waves  of  dif- 
ferent Magnitudes. — Magnitudes  of  Waves  of  different  Color.—  Summary  View  of  the  Corpus- 
cular Theory. — Summary  View  of  the  Undulatory  Theory. — These  Theories  compared. — Discov- 
eries of  Dr.  Young. — Discoveries  of  Mains,  Arago,  Poisson,  Herschel,  and  Airy. — Relations  of 
Light  and  Heat. 


LIGHT. 


223 


LIGHT. 


AMONG  the  many  marvellous  results  of  the  labors  of  the  human  mind  directed 
to  the  discovery  of  the  laws  of  the  physical  creation,  there  is  perhaps  none 
which  strike  us  with  more  astonishment  than  the  knowledge  which  has  been 
obtained  relating  to  the  qualities  and  laws  of  LIGHT.  The  principles  which 
govern  its  reflection  from  opaque  surfaces,  and  its  transmission  through  trans- 
parent bodies,  we  shall  examine  on  another  occasion.  I  propose  for  the  pres- 
ent to  bring  before  you  the  facts  which  have  been  disclosed  regarding  its 
physical  nature  and  its  motion  through  space,  as  well  as  the  manner  in  which 
it  affects  the  organ  of  vision,  so  as  to  produce  the  perception  of  external  and 
distinct  objects. 

Between  the  eye  and  any  distant  object,  there  intervenes  a  space  of  greater 
or  less  extent,  and  often,  as  in  the  instance  of  the  stars,  so  great  as  to  be 
scarcely  capable  of  being  clearly  and  adequately  expressed  by  any  standard  or 
modolus  of  magnitude  with  which  we  are  familiar.  Yet  objects,  at  these  im- 
mense distances,  are  rendered  visible  to  us  by  some  physical  effects  which 
they  are  capable  of  producing  and  which  in  fact  they  do  produce  upon  our 
organs  of  vision. 

We  shall  see  that  the  interior  of  the  eye-ball  is  lined  with  a  membrane 
highly  susceptible  of  mechanical  vibration  and  connected  by  a  continuity  of 
nerves  with  the  brain ;  and  to  this  membrane  admission  is  given  for  light  by 
an  opening  in  front  of  the  eye  called  the  pupil.  The  light  then  proceeding 
from  any  distant  object  must  be  supposed  to  pass  over  the  space  intervening 
between  the  object  »nd  the  eye,  to  enter  the  pupil  and  to  produce  upon  the 
membrane  within  the  eye  a  specific  mechanical  effect,  which  being  propagated 
to  the  brain,  is  the  means  of  producing  in  the  mind  a  perception  of  the  distant 
object. 

How  then  are  we  to  conceive  that  an  object  placed  at  any  distance,  for  ex- 
ample, say  one  hundred  millions  of  miles,  from  the  eye,  can  transmit  over  and 
through  that  space  a  mechanical  effect  which  shall  be  impressed  on  the  eye  ? 


224 


LIGHT. 


We  answer  that  there  are  two  and  only  two  ways   in  which  it  is  possible  to 
conceive  such  an  action  to  take  place.     These  two  are  the  following : — 

First. — The  distant  object  thus  visible  to  us,  may  emit  particles  of  matter 
from  its  surface,  which  particles  of  matter  may  pass  over  the  intervening 
space,  may  enter  the  pupil  of  the  eye,  may  strike  upon  the  nervous  mem- 
brane, arid  so  affect  it  as  to  produce  vision. 

Secondly. — There  may  be  in  the  space  between  the  distant  visible  object 
and  the  eye,  a  medium  possessing  elasticity,  so  as  to  be  capable  of  receiving 
and  transmitting  pulsations  or  undulations  like  those  imparted  to  the  air  by  a 
sounding  body.  If  this  be  admitted,  the  distant  visible  object  may,  without 
emitting  any  particles  of  matter  from  its  surface  affect  such  a  medium  sur- 
rounding it  with  pulsations  or  undulations,  in  the  same  manner  as  a  bell 
affects  the  air  around  it.  These  pulsations  or  undulations  may  pass  along  the 
space  intervening  between  the  visible  object  and  the  eye,  in  the  same  manner 
as  the  pulsations  or  undulations  produced  by  a  bell  pass  along  the  air  between 
the  bell  and  the  ear.  In  this  manner  the  pulsations  transmitted  from  the 
visible  object,  and  propagated  by  the  medium,  we  have  referred  to,  may  reach 
the  eye  and  affect  the  membrane  which  lines  it,  in  the  same  manner  ex- 
actly as  the  pulsations  in  the  air  affect  the  tympanum  of  the  ear. 

These  are  the  two,  and  the  only  two  modes,  in  which  any  human  mind  ever 
yet  conceived  that  a  distant  object  could  become  visible  to  the  eye. 

In  the  first,  there  is  an  analogy  between  the  eye  and  the  organs  of  smelling. 
Odorous  objects  do  actually  emit  material  effluvia,  which  must  be  supposed  to 
form  part  of  their  own  substance.  These  effluvia  reach  the  organ  of  smell- 
ing, and  produce  upon  it  a  specific  effect,  which  impresses  the  mind  with  a 
corresponding  perception.  According  to  the  first  supposition,  a  visible  object 
at  any  distance  would  act  in  the  same  way,  and  would  eject  continual  parti- 
cles of  light,  which  particles  of  light  would  move  to  the  eye  and  produce 
vision,  acting  mechanically  on  its  membrane  in  the  same  manner  as  the  effluvia 
of  a  rose  produce  a  physical  effect  upon  the  organs  of  smelling. 

The  second  method  places  the  eye  in  analogy  with  the  ear.  So  close 
is  this  analogy  that  all  the  mathematical  formulae  by  which  the  effects 
of  sound  are  expressed  in  acoustics,  will,  with  very  slight  changes,  be  capa- 
ble of  expressing  the  effects  of  vision,  according  to  the  latter  hypothesis.  It 
is  evident,  however,  that  as  the  first  hypothesis  requires  us  to  admit  that  dis- 
tant visible  objects  are  continually  ejecting  matter  from  their  surfaces  to  pro- 
duce vision ;  so  the  second  hypothesis  as  peremptorily  requires  the  admission 
of  the  existence  of  some  physical  medium  pervading  the  universe, — some  subtle 
ethereal  fluid  endowed  with  a  property  of  propagating  the  pulsations  or  undu- 
lations of  distant  visible  objects  and  transmitting  them  to  the  eye.  This  hy- 
pothetical fluid  has  been  called  the  luminiferous  ether.  The  first  of  these 
two  celebrated  theories  of  light  has  been  called  the  CORPUSCULAR  THEORY,  and 
the  second  the  UNDULATORY  THEORY. 

Newton,  although  he  did  not  identify  his  investigations  in  optics  with  any 
hypothesis,  but  in  the  spirit  of  the  inductive  philosophy  founded  by  Bacon,  ) 
based  his  conclusions  on  experiments  and  observations  only,  adopted  never-  > 
theless  the  nomenclature  and  language  of  the  corpuscular  theory,  and,  probably,  / 
from  veneration  for  his  authority,  English  philosophers,  until  recently,  have  ( 
very  generally  given  the  preference,  to  that  theory. 

The  undulating  theory,  on  the  other  hand,  was  adopted  by  Huygens,  and  ( 
after  him  by  most  continental  philosophers. 

The  researches  in  the  phenomena  of  optics  within  the  last  hundred  years  have  s 
been  marked  by  singular  diligence  and  success.  A  vast  variety  of  phenomena  ) 
previously  unknown,  have  been  accurately  investigated,  new  laws  have  been  \ 


LIGHT. 


225 


developed,  and  the  general  result  has  been  that  the  undulatory  theory  has  pre- 
vailed over  the  corpuscular.     It  is  perhaps   not  an  unfair  statement  of  the  ac- 
tual condition  of  these  two  celebrated  hypotheses,  to  say  that  while  the  cor-  / 
puscular  system  is  found  sufficient  to  explain  most  of  the  common  and  obvious  I 
phenomena  of  optics,  it  totally  fails  in  explaining  many  of  the  most  remarkable  * 
effects  brought  to  light  by  modern  observations  and  experiments.     On  the 
other  hand,  the  undulatory  theory  in  general  offers  a  satisfactory  explanation 
for  all.     This  circumstance  has  very  properly  and  legitimately  enlisted  under 
that  hypothesis  almost  all  the  leading  scientific  men  of  the  present  day. 

Although  the  principal  facts  which  we  shall  have  now  to  explain  are  in  fact 

a- pendent  of  either  of  these  two  hypotheses,  and  incontestably  true,  which- 
ever may  be  adopted,  yet  in  their  exposition,  it  will  be  necessary  to  adopt  the 
language  of  one  or  the  other  of  these  theories.  We  shall,  for  the  reason  just 
sta-ted,  use  the  nomenclature  of  the  undulatory  theory. 

We  are  then  to  imagine  light  to  consist  of  undulations  propagated  through 
the  universal  ether,  in  the  same  manner  as  the  waves  or  undulations  of  sound 
are  propagated  through  the  air. 

The  first  question  then  that  arises  is,  what  is  the  velocity  with  which  these 
waves  move  ?  At  what  rate  does  light  come  from  a  distant  star  to  the  eye  ? 
Is  it  propagated  instantaneously  ?  Would  a  fire  suddenly  lighted  at  a  point 
one  hundred  millions  of  miles  from  the  eye  be  seen  at  the  moment  the  light 
was  produced  ? — or  would  an  interval  of  time  be  necessary  to  allow  the  light 
to  reach  the  eye  ?  and  if  so,  what  would  be  the  interval  of  time  in  relation  to 
the  distance  of  the  luminous  object? 

In  tracing  the  progress  of  human  knowledge,  we  frequently  have  occasions 
to  behold  with  surprise,  and  not  without  a  due  sense  of  humility,  the  important 
part  which  accident  plays  in  the  advancement  of  science.  Often  are  we  with 
diligent  zeal  in  search  of  things,  which,  if  found,  would  be  of  trifling  or  no 
value,  when  we  stumble  on  inestimable  treasures  of  truth.  The  frequency  of 
this,  strongly  impresses  the  mind  with  the  persuasion  that  there  is  in  secret 
operation  a  power  whose  will  it  is  that  knowledge  and  the  human  mind  should 
be  constantly  progressive.  It  is  in  physics  as  in  morals.  We  ignorantly  seek 
that  which  is  worthless  and  often  find  what  is  inestimable. 

In  the  pursuit  of  knowledge  we  might  well  say  that  which  we  are  taught  to 
express  in  the  pursuit  of  what  is  moral  and  good.  We  might  say  that  the 
power  which  governs  its  progress  knows  better  than  what  we  do,  "  our  neces- 
sities before  we  ask,  and  our  ignorance  in  asking."  We  shall  see  a  striking 
example  of  this  in  the  narrative  which  I  shall  now  offer  of  the  celebrated  dis- 
covery of  the  motion  of  light. 

Soon  after  the  invention  of  the  telescope,  and  the  consequent  discovery  of 
Jupiter's  satellites,  Roemer,  an  eminent  Danish  astronomer,  engaged  in  a  series 
of  observations,  the  object  of  which  was  the  discovery  of  the  exact  time  of 
the  revolution  of  one  of  these  bodies  around  Jupiter.  The  mode  in  which  he 
proposed  to  investigate  this,  was  by  observing  the  successive  eclipses  of  the 
satellite,  and  noticing  the  time  between  them. 

Let  S  represent  the  sun  and  ABCDEFGH  the  successive  positions 
of  the  earth.  Let  J  be  Jupiter  projecting  behind  him  his  conical  shadow,  and 
let  M  N  0.  represent  the  orbit  of  one  of  his  satellites.  After  each  revolution 
the  satellite  will  enter  the  shadow  at  M,  and  emerge  from  it  at  N. 

Now  if  it  were  possible  to  observe  accurately  the  moment  at  which  the  sat- 
ellite would,  after  each  revolution,  either  enter  the  shadow,  or  emerge  from  it, 
the  interval  of  time  between  these  events  would  enable  us  to  calculate  exactly 
the  velocity  and  motion  of  the  satellite.  But  by  attentively  watching  the  sat- 
ellite we  can  note  the  time  it  enters  the  shadow,  for  at  that  moment  it  is  de- 


226 


LIGHT. 


prived  of  the  sun's  light  and  becomes  invisible.  We  can  also  note  the  moment 
of  its  emergence,  because  then  escaping  from  the  edge  of  the  shadow  it  comes 
into  the  sun's  light  and  becomes  visible.  It  was,  then,  in  this  manner  that  Roe- 
mer proposed  to  ascertain  the  motion  of  the  satellite.  But  in  order  to  obtain 
this  estimate  with  the  greatest  possible  precision,  he  proposed  to  continue  his 
observations  for  several  months. 

Let  us,  then,  suppose  that  we  have  observed  the  time  which  has  elapsed 
between  two  successive  eclipses,  and  that  this  time  is,  for  example,  forty-three 
hours.  We  ought  to  expect  that  the  eclipse  would  recur  after  the  lapse  of  every 
successive  period  of  forty-three  hours. 

Imagine,  then,  a  table  to  be  computed  in  which  we  shall  calculate  and  reg- 
ister before  hand  the  moment  at  which  every  successive  eclipse  of  the  satellite 
for  twelve  months  to  come  shall  occur,  and  let  us  conceive  that  the  earth  is  at  A, 
at  the  commencement  of  our  observations,  we  shall  then,  as  Roemer  did,  ob- 
serve the  moments  at  which  the  eclipses  occur  and  compare  them  with  the  mo- 
ments registered  in  the  table. 

Let  the  earth,  be  supposed  at  A,  at  the  commencement  of  these  obser- 
vations, where  it  is  nearest  to  Jupiter.  When  the  earth  has  moved  to  B,  which 
it  will  do  in  about  six  weeks,  it  will  be  found  that  the  occurrence  of  the  eclipse 
is  a  little  later  than  the  time  registered  in  the  table.  When  the  earth  arrives 
at  C,  which  it  will  do  at  the  end  of  three  months,  they  will  occur  still  later 
tlmn  the  registered  time.  In  fact  at  C,the  eclipses  will  occur  about  eight  min- 
utes later  than  the  registered  time.  At  D  they  will  be  twelve  minutes  later, 
and  at  E  sixteen  minutes  later. 

By  observations  such  as  these  Roemer  was  struck  with  the  fact  that  his  pre- 
dictions of  the  eclipses  proved  in  every  case  to  be  wrong.  It  would  at  first 
occur  to  him  that  this  discrepancy  might  arise  from  some  errors  of  his  obser- 
vations, but  if  such  were  the  case,  it  might  be  expected  that  the  result  would 
betray  that  kind  of  irregularity  which  is  always  the  character  of  such  errors. 
Thus  it  would  be  expected  that  the  predicted  time  would  sometimes  be  later, 
and  sometimes  earlier  than  the  observed  time,  and  that  it  would  be  later  and 
earlier  to  an  irregular  extent.  On  the  contrary,  it  was  observed  during  the  six 
mouths  which  the  earth  took  to  move  from  A  to  E,  that  the  observed  time  was 
continually  later  than  the  predicted  time,  and  moreover,  that  the  interval  by 
which  it  was  later  continually  and  regularly  increased.  This  was  an  effect, 
then,  too  regular  and  consistent  to  be  supposed  to  arise  from  the  casual  errors 
of  observation  ;  it  must  have  its  origin  in  some  physical  cause  of  a  regdal 
kind. 

The  attention  of  Roemer  being  thus  attracted  to  the  question,  he  deteimined 
to  pursue  the  investigation  by  continuing  to  observe  the  eclipses  for  another 
half  year.  Time  accordingly  rolled  on,  and  the  earth  transporting  the  astrono- 
mer with  it,  moved  from  E  to  F.  On  arriving  at  F  and  comparing  the  observ- 
ed with  the  predicted  eclipse,  it  was  found  that  the  observed  time  was  now 
only  twelve  minutes  later  than  the  predicted  time.  At  the  end  of  the  ninth 
month  when  the  earth  arrived  at  G,  the  observed  time  was  found  to  be  only 
eight  minutes  later  ;  at  H  it  was  only  four  minutes  later,  and  finally,  when  the 


r 


LIGHT.  227 

earth  returned  to  the  same  relative  position  with  the  planet,  the   observed  time 
corresponded  precisely  with  the  predicted  time.* 

From  this  course  of  observation  and  inquiry  it  became  apparent  that  the 
lateness  of  the  eclipse  depended  altogether  on  the  increased  distance  of  the 
earth  from  Jupiter.  The  greater  that  distance,  the  later  was  the  occurrence 
of  the  eclipse  as  apparent  to  the  observers,  and  on  calculating  the  change  of 
distance,  it  was  found  that  the  delay  of  the  eclipse  was  exactly  proportional 
to  the  increase  of  the  earth's  distance  from  the  place  where  the  eclipse  occur- 
red. Thus  when  the  earth  was  at  E,  the  eclipse  was  observed  16  min- 
utes, or  about  1,000  seconds  later  than  when  the  earth  was  at  A.  The  diame- 
ter of  the  orbit  of  the  earth,  A  E,  measuring  about  two  hundred  millions  of 
miles,  it  appeared  that  that  distance  produced  a  delay  of  a  thousand  seconds, 
which  was  at  the  rate  of  two  hundred  thousand  miles  per  second.  It  appear- 
ed, then,  that  for  every  two  hundred  thousand  miles  that  the  earth's  distance 
from  Jupiter  was  increased,  the  observation  of  the  eclipse  was  delayed  oiie 
second. 

Such  were  the  facts  which  presented  themselves  to  Roemer.  How  were 
they  to  be  explained  ?  It  would  be  absurd  to  suppose  that  the  actual  occur- 
rence of  the  eclipses  was  delayed  by  the  increased  distance  of  the  earth  from 
Jupiter.  These  phenomena  depend  only  on  the  motion  of  the  satellite  and  the 
position  of  Jupiter's  shadow,  and  have  nothing  to  do  with,  and  can  have  no  de- 
pendance  on  the  position  or  motion  of  the  earth,  yet  unquestionably  the  time 
they  appear  to  occur  to  an  observer  upon  the  earth,  has  a  dependance  on  the 
distance  of  the  earth  from  Jupiter. 

To  solve  this  difficulty,  the  happy  idea  occurred  to  Roemer  that  the  moment 
at  which  we  see  the  extinction  of  the  satellite  by  its  entrance  into  the  shadow 
is  not,  in  any  case,  the  very  moment  at  which  that  event  takes  place,  but  some- 
time afterward,  viz.:  such  an  interval  as  is  sufficient  for  the  light  which  left 
the  satellite  just  before  its  extinction  to  reach  the  eye.  Viewing  the  matter 
thus,  it  will  be  apparent  that  the  more  distant  the  earth  is  from  the  satellite, 
the  longer  will  be  the  interval  between  the  extinction  of  the  satellite  and  the 
arrival  of  the  last  portion  of  light  which  left  it,  at  the  earth ;  but  the  moment 
of  the  extinction  of  the  satellite  is  that  of  the  commencement  of  the  eclipse, 
and  the  moment  of  the  arrival  of  the  light  at  the  earth  is  the  moment  the  com- 
mencement of  the  eclipsed  is  observed. 

Thus  Roemer  with  the  greatest  facility  and  success  explained  the  discrep- 
ancy between  the  calculated  and  the  observed  times  of  the  eclipses  ;  but  he 
saw  that  these  circumstances  placed  a  great  discovery  at  his  hand.  In  short,  it 
was  apparent  that  light  is  propagated  through  space  with  a  certain  definite 
speed,  and  that  the  circumstances  we  have  just  explained  supply  the  means  of 
measuring  that  velocity. 

We  have  shown  that  the  eclipse  of  the  satellite  is  delayed  one  second  more 
for  every  two  hundred  thousand  miles  that  the  earth's  distance  from  Jupiter  is 
increased,  the  reason  of  which,  obviously  is,  that  light  takes  ane  second  to 
move  over  that  space  ;  hence  it  is  apparent  that  the  velocity  of  light  is  at  the 
rate,  in  round  numbers,  of  two  hundred  thousand  miles  per  second. 

Such  was  the  discovery  which  has  conferred  immortality  upon  the  name  of 
Roemer  ;  a  discovery  to  which,  as  we  have  shown,  he  was  accidentally  led 
when  seeking  to  determine  the  velocity  of  one  of  the  moons  of  Jupiter.  The 
velocity  thus  determined  would,  in  the  corpuscular  theory,  be  regarded  as  that 
with  which  the  particles  of  light  issuing  from  the  surface  of  a  visible  object  move 

*  Strictly  speaking  ie  interval  is  longer  than  twelve  months,  bat  the  circumstance  is  not  iinpor- 
ant  here. 


228 


LIGHT. 


through  space.  In  the  undulatory  theory,  however,  which  is  more  generally 
received,  this  velocity  must  be  regarded  as  that  with  which  the  waves  or  un- 
dulations of  light  are  propagated  through  space  in  the  same  sense  as  waves 
appear  to  move  on  the  surface  of  water  if  a  pebble  be  dropped  in  to  form  a 
centre  round  which  they  are  propagated.  It  is  necessary  to  remember  when 
considering  any  system  of  undulations,  no  matter  through  what  medium  they 
may  be  propagated,  that  the  progressive  motion  which  belongs  to  them  is  a 
motion  of  form  merely,  and  not  of  matter.  The  waves  which  are  propagated 
round  a  centre  when  a  pebble  is  dropped  into  calm  water,  present  an  appear- 
ance to  the  eye  as  though  the  water  which  formed  the  wave  really  moved  out- 
ward from  the  centre  of  the  undulations.  Such  is,  however,  not  the  case.  No 
particle  of  the  fluid  has  any  progressive  motion  whatever,  of  which  many 
proofs  may  be  offered.  If  any  floating  body  be  placed  on  the  surface  of  the 
water,  it  will  not  be  carried  along  by  the  waves,  and  if  similar  waves  be  form- 
ed, as  they  might  be,  by  giving  a  peculiar  motion  to  a  sheet  or  cloth,  they 
would  have  the  same  appearance  of  progressive  motion,  although  the  parts  of 
the  sheet  or  cloth,  as  is  evident,  would  have  no  other  motion  than  the  up-and- 
down  motion  that  would  form  the  apparent  undulations.  We  are  then  to 
remember  that  when  light  is  propagated  through  space  with  the  astonishing 
velocity  of  two  hundred  thousand  miles  per  second,  there  is  no  material  sub- 
stance which  really  has  this  progressive  velocity ;  it  belongs  merely  to  the 
form  of  the  pulsations,  or  undulations.  The  same  observations,  exactly,  are 
applicable  to  the  transmission  of  the  waves  of  sound  through  the  air. 

In  order  to  submit  the  phenomena  of  light  to  a  strict  physical  analysis,  it  is 
not  enough  to  measure  the  motion  of  its  waves.  We  require  also  to  know  the 
amplitude  or  breadth  of  these  waves,  just  as  in  the  case  of  the  waves  of  the 
sea  Ave  should  require  to  know  not  only  the  rate  at  which  they  are  propagated 
over  the  surface  of  the  water,  but  also  the  space  which  intervenes  between  the 
hollow  "or  crest  of  each  successive  wave  and  the  hollow  or  crest  of  the  suc- 
ceeding one. 

For  the  solution  of  this  refined  problem  in  the  analysis  of  light,  we  are  in- 
debted to  Newton  himself.  To  render  clearly  intelligible  the  mode  in  which 
he  solved  it,  let  us  imagine  a  flat  plate  of  glass,  such  as  A  B,  placed  upon  a 
convex  lens  of  glass,  such  as  C  D,  but  let  it  be  imagined  that  the  degree  of 
convexity  is  much  less  than  that  represented  in  the  figure. 


The  under  surface  of  the  flat  plate  will  touch  the  vertext  of  tbe  convexity 
at  V,  and  the  further  any  point  on  the  under  surface  is  from  V,  the  greater  will 
be  the  distance  between  the  surfaces  of  the  two  glasses.  Thus  the  distance 
between  them  at  1  is  less  than  at  2,  and  the  distance  at  2  is  less  than  at 
O,  and  so  on.  The  distance  at  the  surfaces  gradually  increasing,  in  fact, 
from  V  outward. 

If  looking  down  on  the  plate  A  B,  we  consider  the  point  V  as  a  centre,  and 
a  circle  be  described  round  it,  at  all  points  of  that  circle  the  surfaces  of  the 
glasses  will  have  the  same  distances  between  them,  and  the  greater  that  circle 
is,  the  greater  will  be  the  distances  between  the  surfaces  of  glass. 

Having  the  glasses  thus  arranged,  Newton  let  a  beam  of  light  of  some  par- 
ticular color,  produced  by  a  prism,  as  red,  for  example,  fall  on  the  surface  of 


229 

the  glass,  A  B.  He  found  that  the  effect  produced  was  that  a  black  spot  ap- 
peared at  the  centre,  V,  where  the  glasses  touched  ;  that  immediately  around 
this  spot  there  appeared  a  circle  of  red  light ;  that  beyond  that  circle  appeared 
a  dark  ring;  that  outside  of  that  dark  ring  there  was  another  circle 
of  red  light,  still  having  the  point  V  as  its  centre.  Outside  this  second  circle 
appeared  another  dark  ring,  beyond  which  there  was  another  circle  of  red 
light,  and  so  on,  a  series  of  circles  of  red  light,  alternated  with  dark  rings  be- 
ing formed,  all  having  the  point  V  as  their  common  centre. 

The  distances  between  the  surfaces  of  glass  at  which  the  successive  circles 
of  red  light  were  found,  were  too  minute  to  be  directly  measured,  but  they 
were  easily  calculated  by  measuring  the  diameters  of  the  circles  of  light ;  and 
knowing  the  diameters  of  the  convex  surface  C  V  D,  this  was  a  simple  problem 
of  geometry,  easily  solved,  and  admitting  the  greatest  accuracy. 

On  making  these  calculations,  Newton  found  that  the  distance  between  the 
glass  surfaces  where  the  second  red  circle  was  formed  was  double  the  distance 
corresponding  to  the  first ;  that  at  the  third  red  circle  the  distance  was  triple  that 
of  the  first,  and  so  on.*  It  followed,  of  course,  that  wherever  the  dark  rings 
were  formed,  the  distance  between  the  glass  surfaces  were  not  an  exact  num- 
ber of  times  the  space  corresponding  to  the  first  red  circle. 

Thus  if  we  express  the  space  between  the  glasses  at  the  first  red  circle  by  1, 
the  space  between  them  within  that  circle,  toward  the  centre  V,  would  be  a 
fraction.  The  space  corresponding  to  the  first  dark  ring  outside  the  first  red 
circle,  would  be  expressed  by  1  and  a  fraction  ;  the  space  at  the  second  red 
circle  would  be  expressed  by  2  ;  the  space  at  the  second  dark  ring  would  be 
expressed  by  2  and  a  fraction,  and  so  on. 

Newton  was  not  slow  to  see  that  these  phenomena  were  the  direct  manifes- 
tation of  those  effects  which,  in  the  corpuscular  theory  whose  nomenclature  he 
used,  corresponded  to  the  amplitude  of  the  waves  of  light  in  the  undulatory  ) 
theory.  The  space  between  the  surfaces  of  glass  at  the  first  red  ring  was  the  s 
amplitude  of  a  single  wave,  the  space  at  the  second  red  circle  the  amplitude 
of  two  waves,  and  so  on.  Within  the  first  red  circle,  the  space  between  the 
glasses  being  less  than  the  amplitude  of  a  wave,  the  propagation  of  the  undu- 
lation was  stopped,  and  darkness  ensued ;  in  like  manner,  in  the  space  corre- 
sponding to  the  second  dark  ring,  the  distance  between  the  glasses  being  greater 
than  the  amplitude  of  one  wave,  but  less  than  the  amplitude  of  two,  the  propa- 
gation was  again  stopped,  and  darkness  produced.  But  at  the  second  red 
circle,  the  space  being  equal  to  the  amplitude  of  two  waves,  the  undulations 
were  reflected  and  the  red  ring  produced,  and  so  on. 

It  was  evident,  then,  that  to  measure  the  amplitude  of  the  luminous  waves, 
it  was  only  necessary  to  calculate  the  distance  between  the  glasses  at  the  first 
red  ring. 

When  light  of  other  colors  was  thrown  upon  the  glass,  a  similar  system  of 
luminous  rings  was  produced,  but  it  was  found  in  each  case  that  the  first  ring 
varied  in  its  diameter  according  to  the  color  of  the  light,  and  consequently  that 
the  amplitude,  of  the  waves  of  lights  of  different  colors  are  different.  It  ap- 
peared that  the  waves  of  red  light  were  the  largest ;  orange  came  next  to 
them ;  then  yellow,  green,  blue,  indigo,  and  violet,  succeeded  each  other,  the 
waves  of  each  being  less  than  those  of  the  preceding.  But  the  most  astonish- 
ing part  of  this  most  celebrated  investigation  was  the  minuteness  of  these 
waves.  It  appeared  that  the  waves  of  red  light  were  so  minute,  that  forty 
thousand  of  them  would  be  comprised  within  an  inch,  while  the  waves  of  violet 
light,  forming  the  other  extreme  of  the  series,  were  so  small,  that  sixty  thou- 
sand spread  over  an  inch,  and  the  waves  of  light  of  other  colors  were  of  inter- 
mediate magnitudes. 


Thus  was  discovered  the  physical  cause  of  the  splendor  and  variety  of  colors, 
and  a  singular  and  mysterious  alliance  was  developed  between  color  and  sound. 
T.icrhts  are  °f  various  hues,  according  to  the  magnitude  of  the  pulsations  that 
prom:  t  them,  exactly  as  musical  sounds  vary  their  tone  and  pitch  according 
to  the  magnitude  of  the  aerial  pulsations  from  which  they  result. 

But  this  is  not  all.  The  alliance  between  sound  and  light  does  not  termi- 
nate here.  We  have  only  spoken  of  the  amplitude  of  the  luminous  waves,  and 
have  shown  that  it  determines  the  tints  of  colors.  What  are  we  to  say  for  the 
altitudes  of  the  waves  1  Here,  again,  is  another  link  of  kindred  between  the 
eye  and  the  ear.  As  the  altitude  of  sonorous  waves  determines  the  loudness 
of  the  sounds,  so  the  altitude  of  luminous  waves  determines  the  intensity  or 
brightness  of  the  color. 

There  is  one  step  more  in  the  series  of  wondrous  results  which  these  mem- 
orable investigations  have  unfolded.  As  the  perception  of  sound  is  produced 
by  the  tympanum  of  the  ear  vibrating  in  sympathetic  accordance  with  the  pul- 
sations of  the  air  produced  by  the  sounding  body,  so  the  perception  of  light  and 
color  is  produced  by  similar  pulsations  of  the  membrane  of  the  eye  vibrating 
in  accordance  with  ethereal  pulsations  propagated  from  the  visible  object.  As 
in  the  case  of  the  ear,  the  rigor  of  scientific  investigation  requires  us  to  estimate 
the  rate  of  the  pulsation  of  the  tympanum  corresponding  to  each  particular  note, 
so  in  the  case  of  light  are  we  required  to  count  the  vibrations  of  the  retina  of 
the  eye  corresponding  to  every  tint  and  color.  It  may  well  be  asked,  in  some 
spirit  of  incredulity,  how  the  solution  of  such  a  problem  could  be  hoped  for ; 
yet,  as  we  shall  now  see,  nothing  can  be  more  simple  and  obvious. 

Let  us  suppose  an  object  of  any  particular  color,  as  a  red  star,  for  example, 
placed  at  a  distance  and  seen  by  the  eye.  From  the  star  to  the  eye  there  pro- 
ceeds a  continuous  line  of  waves  ;  these  waves  enter  the  pupil  and  impinge 
upon  the  retina ;  for  each  wave  which  thus  strikes  the  retina,  there  will  be  a 
separate  pulsation  of  that  membrane.  Its  rate  of  pulsation,  or  the  number  of 
vibrations  which  it  makes  per  second,  will  therefore  be  known,  if  we  can  as- 
certain how  many  luminous  waves  enter  the  eye  per  second. 

It  has  been  already  shown  that  light  moves  at  the  rate  of  about  two  hundred 
thousand  miles  per  second  ;  it  follows,  therefore,  that  a  length  of  ray  amount- 
ing to  two  hundred  thousand  miles  must  enter  the  pupil  each  second  ;  the  num- 
ber of  times,  therefore,  per  second,  which  the  retina  will  vibrate,  will  be  the 
same  as  the  number  of  the  luminous  waves  contained  in  a  ray  two  hundred 
thousand  miles  long. 

Let  us  take  the  case  of  red  light.  In  two  hundred  thousand  miles  there  are 
in  round  numbers  a  thousand  millions  of  feet,  and  therefore  twelve  thousand 
millions  of  inches.  In  each  of  these  twelve  thousand  millions  of  inches  there 
are  forty  thousand  waves  of  red  light.  In  the  whole  length  of  the  ray,  therefore, 
there  are  four  hundred  and  eighty  millions  of  millions  of  waves.  Since  this 
ray,  however,  enters  the  eye  in  one  second,  the  retina  must  pulsate  once  for 
each  of  these  waves  ;  and  thus  we  arrive  at  the  astounding  conclusion,  that 
when  we  behold  a  red  object,  the  membrane  of  the  eye  trembles  at  the  rate  of 
four  hundred  and  eighty  millions  of  millions  of  times  between  every  two  ticks 
of  a  common  clock  ! 

In  the  same  manner,  the  rate  of  pulsation  of  the  retina  corresponding  to  other 

'  tints  of  colors  is  determined  ;  and  it  is  found  that  when  violet  light  is  perceived, 

|  it  trembles  at  the  rate  of  seven  hundred  and  twenty  millions  of  millions  of  times 

>  per  second. 

In  the  annexed  table  are  given  the  magnitudes  of  the  luminous  waves  of  each 

»  color,  the  number  of  them  which  measure  an  inch,  and  the  number  of  undula-  / 

[  tions  per  second  which  strike  the  eye : — 


LIGHT. 


Colors. 

Length  of  undulation  in 
part*  of  an  inrli. 

Number  of  undula- 
tion* in  nn  inch. 

NumLer  of  troJnlatioiu  per      r 
second.                       1 

0-0000266 
0-0000256 
0-0000240 
0-0000227 
0-0000211 
0-0000196 
0-0000185 
0-0000174 
0-0000167 

37640 
39180 
41610 
44000 
47460 
51110 
54070 
57490 
59750 

458,000000,000000 
477,000000^000000 
506,000000,000000 
535,000000.000000 
577,000000,000000 
622,000000.000000 
658,OOOOOOJCOOOOO 
699,000000,000000 
727,000000,000000  ' 

Red  

Orange  .  .  . 

Yellow  

Blue  

Violet  

Extreme  Violet  

The  preceding  calculations  are,  as  will  be  easily  perceived,  made  onlv  in 
round  numbers,  with  a  view  of  rendering  the  principles  of  the  investiga'tion 
intelligible.  In  the  table  the  exact  results  of  the  physical  investigations  which 
have  been  carried  on,  on  this  subject,  are  given. 

In  considering  the  two  theories  of  light,  each  of  which  has  been  rendered 
memorable  by  the  eminent  philosophers  who  have  favored  them  respectively, 
it  is  necessary  that  we  should  distinguish  in  each  of  them  that  which  is  purely 
hypothetical,  and  which  remains  yet  to  be  established  as  a  matter  of  fact,  from 
that  which  expresses  real  and  ascertained  phenomena. 

In  explaining  these  points,  we  cannot  do  better  than  adopt  the  clear  and 
candid  language  and  reasoning  of  Sir  John  Herschel.  In  explaining  gener- 
ally the  postulates  of  these  theories,  he  says  that  in  the  corpuscular  hypothesis 
the  following  assumptions  are  made. 

1.  That  light  consists  of  particles  of  matter  possessed  of  inertia,  and  endued 
with  attractive  and  repulsive  forces,  and  projected  or  emitted  from  all  luminous 
bodies  with  nearly  the  same  velocity,  of  about  two  hundred  thousand  miles  per 
second. 

2.  That  these  particles  differ  from  each  other  by  the  intensity  of  the  attrac- 
tive and  repulsive  forces  which  reside  in  them,  and  in  their  relations  to  the 
material  world,  and  also  ID  their  actual  masses,  or  inertia. 

3.  That  these  particles,  impinging  on  the  retina,  stimulate  and  excite  vision  ; 
the  particles  whose  inertia  is  greatest  producing  the  sensation  of  red,  those 
of  the  least  inertia,  violet,  and  those  in  which  it  is  intermediate,  the  interme- 
diate colors. 

4.  That  the  molecules  of  material  bodies  and  those  of  light  exert  a  mutual 
action  on  each  other,  which  consists  in  attraction  and  repulsion,  according  to 
some  law  or  function  of  the  distance  between  them  ;  that  this  law  is  such  as  to 
admit  perhaps  of  several  alternations  or  changes  from  repulsive  to  attractive 
force,  but  that  when  the  distance   is  below  a  certain  very  small   limit,  it  is 
always  attracted  up  to  actual  contact ;   and  that  beyond  this  limit  resides  at 
least  one  sphere  of  repulsion.     This  repulsive  force  is  that  which  causes  the 
reflection  of  light  at  the  external  surfaces  of  dense  media,  and  the  interior  at- 
traction that  which  produces  the  refraction  and  interior  reflection  of  light. 

5.  That  these  forces  havfc  different  absolute  values  or  intensities,  not  only 
for  all  different  material  bodies,  but  for  every  different  species  of  the  luminous 
molecules,  being  of  a  nature  analogous  to  chemical  affinities  or  elective  attrac- 
tions ;  and  that  hence  arises  the  different  refrangibilities  of  the  rays  of  light. 

6.  That  the  motion  of  a  particle  of  light,  under  the  influence  of  these  forces 
and  its  own  velocity,  is  regulated  by  the  same  mechanical  laws  which  govern 
the  motions  of  ordinary  matter  ;  and  that  therefore  each  particle  describes  a 
trajectory,  capable  of  strict  calculation,  as  soon  as  the  forces  which  act  on  it 
are  assigned. 

7.  That  the  distance  between  the  molecules  of  material  bodies  is  exceed- 
ingly small  in  comparison  with  the  extent  of  their  spheres  of  attraction  and 
repulsion  on  the  particles  of  light. 


232 


LIGHT. 


8.  That  the  forces  which  produce  the  reflection  arid  refraction  of  light  are, 
nevertheless,  absolutely  insensible  at  all  measurable  or  appreciable  distances 
from  the  molecules  which  exert  them. 

9  That  every  luminous  molecule,  during  the  whole  of  its  progress  through 
space,  is  continually  passing  through  certain  periodically  recurring  states,  called 
by  Newton  fits  of  easy  reflection  and  easy  transmission,  in  virtue  of  which 
they  are  more  disposed,  when  in  the  former  states  or  phases  of  their  periods, 
to  obey  the  influence  of  the  repulsive  or  reflective  forces  of  the  molecules  of  a 
medium ;  and  when  in  the  latter,  of  the  attractive. 

Such  are  the  principles  necessary  to  be  admitted  in  the  corpuscular  theory. 
Herschel  states  those  of  the  undulatory  theory  as  follows  : — 

1.  That  an  excessively  rare,  subtle,  and  elastic  medium,  or  ether,  fills  all 
space,  and  pervades  all  material  bodies,  occupying  the  intervals  between  their 
molecules  ;  and  either  by  passing  freely  among  them,  or  by  its  extreme  rarity, 
offering  no  resistance  to  the  motion  of  the  earth,  the  planets,  or  comets,  in  their 
orbits,  appreciable  by  the  most  delicate  astronomical  observations  ;  and  having 
inertia,  but  not  gravity. 

2.  That  the  molecules  of  the  ether  are  susceptible  of  being  set  in  motion  by 
the  agitation  of  the  particles  of  ponderable  matter  ;  that  when  any  one  is  thus 
set  in  motion,  it  communicates  a  similar  motion  to  those  adjacent  to  it :  and 
that  the  motion  is  propagated  farther  and  farther  in  all  directions,  according  to 
the  same  mechanical  laws  which  regulate  the  propagation  of  undulations  in 
other  elastic  media,  as  air,  water,  or  solids,  according  to  their  respective  con- 
stitutions. 

3.  That  in  the  interior  of  refracting  media  the  ether  exists  in  a  state  of  less 
elasticity,  compared  with  its  density,  than  in  vacuo  (that  is,  space  empty  of  all 
other  matter) ;  and  that  the  more  refractive  the  medium,  the  greater,  relatively 
speaking,  is  the  elasticity  of  the  ether  in  its  interior. 

4.  That  vibrations  jominanicated  to  the  ether  in  free  space  are  propagated 
through  refractive  media  by  means  of  the  ether  in  their  interior,  but  with  a  ve- 
locity corresponding  to  its  inferior  degree  of  elasticity. 

5.  That  when  regular  vibratory  motions  of  a  proper  kind  are  propagated 
through  the  ether,  and,  passing  through  our  eyes,  reach  and  agitate  the  nerves 
of  our  retina,  they  produce  in  us  the  sensation  of  light,  in  a  manner  bearing  a 
more  or  less  close  analogy  to  that  in  which  the  vibrations  of  the  air  affect  our 
auditory  nerves  with  that  of  sound. 

6.  That  as,  in  the  doctrine  of  sound,  the  frequency  of  the  aerial  pulscc,  or 
the  number  of  excursions  to  and  fro  from  the  point  of  rest  made  by  each  mole- 
cule of  the  air,  determines  the  pitch  or  note  ;  so,  in  the  theory  of  light,  the 
frequency  of  the  pulses,  or  number  of  impulses  made  on  our  nerves  in  a  given 
dine  by  the  ethereal  molecules  next  in  contact  with  them,  determines  the  color 
of  the  light ;  and  that  as  the  absolute  extent  of  the  motion  to  and  fro  of  the  par- 
ades of  air,  determines  the  loudness  of  the  sound,  so  the  amplitude  or  extent  of 
•he  excursions  of  the  ethereal  molecules  from  their  points  of  rest  determines 
'•he  brightness  or  intensity  of  the  light. 

Whichever  theory  we  adopt  to  explain  the  phenomena  of  light,  we  are  led  to 
conclusions  that  strike  the  mind  with  astonishment.  According  to  the  corpus- 
cular theory,  the  molecules  of  light  are  supposed  to  be  endowed  with  attractive 
and  repulsive  forces,  to  have  poles  to  balance  themselves  about  their  centres 
of  gravity,  and  to  possess  other  physical  properties  which  we  can  only  ascribe 
to  ponderable  matter.  In  speaking  of  these  properties,  it  is  difficult  to  divest 
oneself  of  the  idea  of  sensible  magnitude,  or  by  any  strain  of  the  imagination 
to  conceive  that  particles  to  which  they  belong  can  be  so  amazingly  small  as 
those  of  light  demonstrably  are.  If  a  molecule  of  light  weighed  a  single  grain, 


LIGHT. 


233 


its  momentum  (by  reason  of  the  enormous  velocity  with  which  it  moves)  would 
be  such  that  its  effect  would  be  equal  to  that  of  a  cannon-ball  of  one  hundred 
and  fifty  pounds,  projected  with  a  velocity  of  one  thousand  feet  per  second. 
How  inconceivably  small  must  they  therefore  be,  when  millions  of  molecules, 
collected  by  lenses  or  mirrors,  have  never  been  found  to  produce  the  slightest 
effect  on  the  most  delicate  apparatus  contrived  expressly  for  the  purpose  of 
rendering  their  materiality  sensible  ! 

If  the  corpuscular  theory  astonishes  us  by  the  extreme  minuteness  and  pro- 
digious velocity  of  the  luminous  molecules,  the  numerical  results  deduced  from 
the  undulatory  theory  are  not  less  overwhelming.  The  extreme  smallness  of 
the  amplitude  of  the  vibrations,  and  the  almost  inconceivable  but  still  measu- 
rable rapidity  with  which  they  succeed  each  other,  were  computed  by  Doctor 
Young,  and  are  exhibited  in  the  table  previously  shown. 

On  a  cursory  view,  it  must  appear  singular  that  two  hypotheses,  founded  on 
assumptions  so  essentially  different,  should  concur  in  affording  the  means  of 
{  explaining  so  great  a  number  of  facts  with  equal  precision  and  almost  equal 
;  facility.  This,  however,  is  the  case  with  respect  to  the  corpuscular  and  uudu- 
'  latory  theories  of  light,  from  both  of  which  the  mathematical  laws  to  which  the 
phenomena  are  subject  may  be  deduced,  though  not  in  all  cases  with  the  same 
degree  of  facility.  So  far  as  the  corpuscular  doctrine  is  available  for  the  pur- 
poses of  deductive  explanation,  it  possesses  all  the  characteristics  of  a  good 
theory.  It  supposes  the  operation  of  a  force  with  which  we  are  in  some 
measure  familiar.  We  are  accustomed  to  contemplate  the  effects  of  attraction 
in  the  grand  phenomena  of  astronomy  ;  we  perceive  them  at  every  instant  in 
the  downward  tendency  of  all  heavy  bodies  ;  and,  though  they  disappear  in  the 
small  bodies  of  nature,  they  are  reproduced  in  the  phenomena  of  electricity, 
magnetism,  capillary  attraction,  and  various  chemical  actions,  where  they  can 
be  not  only  distinctly  traced,  but  reduced  to  mathematical  formulae,  and  sub- 
mitted to  accurate  calculation.  The  undulatory  hypothesis  is  not  seized  by  the 
mind  with  th*1  name  facility  ;  yet  it  also  possesses  some  of  the  least  equivocal 
characteristics  of  philosophical  truth.  No  phenomenon  has  yet  been  discovered 
decidedly  at  variance  with  any  of  its  principles.  On  the  contrary,  most  of  the 
phenomena  follow  from  those  principles  with  remarkable  ease  ;  and  in  numer- 
ous instances,  consequences  deduced  from  the  theory  by  a  long  and  intricate 
analysis,  and  where  no  sagacity  could  possibly  have  divined  the  result,  have 
been  found  to  be  accurately  true  when  brought  to  the  test  of  experiment.  Hence 
this  hypothesis  begins  to  be  generally  adopted  by  philosophers,  and,  in  recent 
times,  by  far  the  most  illustrious  names  in  the  annals  of  optical  discovery  are 
included  in  the  list  of  its  supporters. 

That  the  sensation  of  light  is  produced  by  the  vibrations  of  an  extremely 
rare  and  subtle  fluid,  is  an  idea  that  was  maintained  by  Descartes,  Hooke,  and 
some  others  ;  but  it  is  to  Huygens  that  the  honor  solely  belongs  of  having  re- 
duced the  hypothesis  to  a  definite  shape,  and  rendered  it  available  to  the  pur- 
poses of  mechanical  explanation.  Owing  to  the  great  success  of  Newton  in 
applying  the  corpuscular  theory  to  his  splendid  discoveries,  the  speculations 
of  Huygens  were  long  neglected  ;  indeed,  the  theory  remained  in  the  same 
state  in  which  it  was  left  by  him  till  it  was  taken  up  by  our  countryman,  the 
late  Dr.  Young.  By  a  train  of  mechanical  reasoning,  which  in  point  of  inge- 
nuity has  seldom  been  equalled,  Dr.  Young  was  conducted  to  some  very  re- 
markable numerical  relations  among  some  of  the  apparently  most  dissimilar 
phenomena  of  optics  to  the  general  laws  of  diffraction,  and  to  the  two  princi- 
ples of  coloration  of  crystallized  substances.  Malus,  so  late  as  1810,  made 
the  important  discovery  of  the  polarization  of  light  by  reflection,  and  success- 
fully explained  the  phenomenon  by  the  hypothesis  of  an  undulatory  propaga- 


234  LIGHT. 

tion.  The  theory  subsequently  received  a  great  extension  from  the  ingenious 
labors  of  Fresnel  ;  and  the  still  more  recent  researches  of  Arago,  Poisson, 
Herschel.  Airy,  and  others,  have  conferred  on  it  so  great  a  degree  of  proba- 
bility, that  it  may  almost  be  regarded  as  ranking  in  the  class  of  demonstrated 
truths.  "  It  is  a  theory,"  says  Herschel,  "  which,  if  not  founded  in  nature,  is 
certainly  one  of  the  happiest  fictions  that  the  genius  of  man  has  yet  invented 
to  group  together  natural  phenomena,  as  well  as  the  most,  fortunate  in  the  sup- 
port it  has  received  from  whole  classes  of  new  phenomena,  which  at  their 
discovery  Deemed  in  irreconcilable  opposition  to  it.  It  is,  in  fact,  in  all  its 
applications  and  details,  one  succession  of  felicities ;  inasmuch  as  that  we  may 
almost  be  induced  to  say,  if  it  be  not  true,  it  deserves  to  be." 

Light  and  heat  are  so  intimately  related  to  each  other,  that  philosophers 
have  doubted  whether  they  are  identical  principles,  or  merely  coexistent  in 
the  luminous  rays.  They  possess  numerous  properties  in  common  :  being 
reflected,  refracted,  and  polarized,  according  to  the  same  optical  laws,  and  even 
exhibit  the  same  phenomena  of  interference.  Most  substances  during  combus- 
tion give  out  both  light  and  heat ;  and  all  bodies,  except  the  gases,  when  heated 
to  a  high  temperature,  become  incandescent.  Nevertheless,  there  are  many 
circumstances  in  which  they  appear  to  differ. 

A  thin  plate  of  transparent  glass  interposed  between  the  face  and  a  blazing 
fire  intercepts  no  sensible  portion  of  the  light,  but  most  sensibly  diminishes 
the  heat.  Light  and  heat  are  therefore  not  intercepted  alike  by  the  same  sub- 
stances. Heat  is  also  combined  in  different  degrees  with  the  different  rays  of 
the  solar  spectrum.  A  very  remarkable  discovery  on  this  subject  was  made 
by  Sir  William  Herschel,  which  would  seem  to  establish  the  independence  of 
the  heating  and  illuminating  effects  of  the  solar  rays.  Having  placed  ther- 
mometers in  the  several  prismatic  colors  of  the  solar  spectrum,  he  found  the 
heating  power  of  the  rays  gradually  increased  from  the  violet  (where  it  was 
least)  to  the  extreme  red,  and  that  the  maximum  temperature  existed  sonu  dis- 
tance beyond  the  red,  out  of  the  visible  pail  of  the  spectrum.  The  experiment 
was  soon  after  repeated  with  great  care  by  Berard,  who  confirmed  Herschel's 
conclusions  relative  to  the  augmentation  of  the  calorific  power  from  the  violet 
to  the  red,  and  not  beyond  the  spectrum.  This  discovery  of  the  inequality  of 
the  heating  power  of  the  different  rays  led  to  the  inquiry  whether  the  chemical 
action  produced  by  light  upon  certain  bodies  was  merely  the  effect  of  the  heat 
accompanying  it,  or  owing  to  some  other  cause.  By  a  series  of  delicate  ex- 
periments, Berard  found  that  this  action  is  not  only  independent  of  the  heating 
power,  but  follows  entirely  a  different  law :  its  intensity  being  greater  in  the 
violet  ray,  where  the  heating  power  is  the  least,  and  least  in  the  red  ray,  where 
the  heating  power  is  the  greatest.  We  are  thus  led  to  the  conclusion  that  the 
solar  rays  possess  at  least  three  distinct  powers — those  of  heating,  illumina- 
ting, and  effecting  chemical  combinations  and  decompositions  ;  and  these  pow- 
ers are  distributed  among  the  different  refrangible  rays  in  such  a  manner  as  to 
show  their  complete  independence  of  each  other. 

I  shall  dismiss  this  subject,  however,  for  the  present,  as  I  shall  have  another 
opportunity  of  more  fully  developing  the  relations  of  heat  and  light. 


THE    MAJOR    PLANETS. 


3pr.c2  between  MARS  and  JUPITER. — Jupiter's  Distance  and  Period. — His  Magnitude  and  Weight. — 
His  Velocity. — Appearance  of  his  Disk. — Day  and  Night  on  Jupiter. — Position  of  his  Axi*. — Ab- 
sence of  Seasons. — His  Telescopic  Appearance. — His  Belts. — Causes  of  his  Belts. — Currents  in 
his  Atmosphere. — Madler's  Telescopic  Views  of  Jupiter. — Appearance  of  the  Sun  as  seen  from 
Jupiter. — His  Satellites. — The  Variety  of  his  Months. — Magnificent  Appearance  of  the  Moons  as 
seen  from  Jupiter. — Their  Eclipses. — SATURN. — His  diurnal  Rotation. — Appearance  of  the  Sun 
as  seen  from  him. — His  Atmosphere. — His  Rings. — Their  Dimensions. — Biot's  Explanation  of 
their  Stability. — Herschel's  Theory  of  the  same. — Appearances  and  Disappearances  of  the 
Rings. — Various  Phases  of  the  Rings.— Saturn's  Satellites. — HERSCHEL  or  URANUS. — His  Dis- 
tance and  Magnitude. — His  Moons. — Reason*  why  there  is  no  Planet  beyond  his  Orbit. 


THE  MAJOR  PLANETS. 


237 


THE  MAJOR  PLANETS. 


PASSING  across  the  wide  space  which  intervenes  between  the  minor  planets 
which,  with  the  earth,  circulate  under  the  immediate  wing  of  the  sun,  in  the 
midst  of  which  space  we  encounter  the  strange  spectacle  of  the  ruins  of  a  shat- 
tered world,  we  arrive  at  the  region  of  the  system  in  which  roll  in  silent  maj- 
esty the  stupendous  orbs  of  JUPITER,  SATURN,  and  HERSCHEL,  accompanied  by 
their  gorgeous  apparatus  of  multiplied  moons,  rings,  and  belts.  The  mind  is  pre- 
pared to  expect  here  another  order  of  worlds,  and  it  is  not  disappointed.  The 
first  of  these  sublime  globes  which  attracts  our  attention  is  that  of  JUPITER, 
whose  diameter  is  eighty-eight  thousand  miles,  and  whose  bulk  is  fifteen  hun- 
dred times  that  of  our  own  globe.  The  distance  of  this  planet  from  the  sun  is 
nearly  five  hundred  millions  of  miles,  and  when  our  globe  is  nearest  to  it,  it  is 
nearly  four  times  more  distant  from  us  than  the  sun.  Nevertheless,  such  is  its 
stupendous  size  that  it  subtends  to  the  eye  an  angle  of  forty-five  seconds,  and 
is,  next  to  the  sun  and  moon,  the  most  brilliant  object  in  the  heavens.  It  has 
in  this  respect  the  advantage  over  VENUS,  that  when  nearest  to  us  its  illumi- 
nated hemisphere  is  presented  directly  to  the  line  of  vision,  and  it  is  seen  in 
the  meridian  at  midnight,  when  the  entire  absence  of  the  sun's  light  so  much 
favors  its  apparent  splendor.  The  orbit  of  the  earth,  which  is  included  in  that 
of  Jupiter,  is  so  small,  compared  with  that  of  the  planet,  that  its  illuminated 
hemisphere,  which  is  presented  precisely  to  the  sun,  is  always  presented  very 
nearly  to  the  earth.  Jupiter,  therefore,  does  not  appear  sensibly  gibbous,  and, 
consequently,  is  always  seen  with  a  full  face. 

The  time  which  Jupiter  takes  to  make  his  complete  revolution  round  the 
sun,  is  4,333  days,  being  something  less  than  twelve  years.  Such  is  the 
length  of  the  year  of  Jupiter. 

The  weight  or  mass  of  the  planet  Jupiter  is*316  times  greater  than  that  of  the 
earth ;  but  its  bulk,  being  greater  than  that  of  the  earth,  in  the  higher  propor- 
tion of  about  fifteen  hundred  to  one,  it  follows  that  its  density  is  about  four 
times  less  than  that  of  the  earth ;  being  nearly  equal  to  the  density  of  the  sun. 


238 


THE  MAJOR  PLANETS. 


The  globe  of  Jupiter  is  therefore  about  as  heavy  as  if  it  was  composed  of 
water  from  its  surface  to  its  centre. 

There  is  nothing  connected  with  the  motion  of  the  planets  more  surprising 
than  their  enormous  velocities,  which,  to  our  observation,  are  nevertheless 
scarcely  perceptible,  owing  to  the  fact  that  their  distances  from  us  are  propor- 
tionally great.  Jupiter,  when  nearest  to  us,  is  at  a  distance  of  four  hundred 
millions  of  miles.  A  cannon-ball  which  moves  at  the  rate  of  five  hundred 
miles  an  hour,  would  require  nearly  a  hundred  years  to  come  from  Jupiter  to 
us,  and  if  a  steam-engine  on  a  railway,  moving  at  twenty  miles  an  hour,  were 
to  take  its  departure  for  Jupiter,  it  would  not  arrive  at  its  destination  until  the 
expiration  of  two  thousand  three  hundred  years. 

Taking  the  diameter  of  Jupiter's  orbit  at  a  thousand  millions  of  miles,  its 
circumference  is  more  than  three  thousand  millions  of  miles,  which  is  traversed 
in  less  than  twelve  years.  The  space  moved  over  annually  by  Jupiter  is,  then, 
two  hundred  and  fifty  millions  of  miles  ;  and  the  space  moved  over  monthly 
about  twenty  millions  of  miles  ;  and  the  space  moved  over  daily  about  seven 
hundred  thousand  miles  ;  and  the  space  moved  over  hourly  about  thirty  thou- 
sand miles  ;  being  at  the  rate  of  about  five  hundred  miles  a  minute  ;  a  velocity 
sixty  times  greater  than  that  of  a  cannon-ball. 

DIURNAL    ROTATION    OF    JUPITER. 

Although  the  varieties  of  light  and  shade  which  characterize  the  disk  of 
Jupiter  are  «uh)ect  to  variations  which  show,  as  will  be  seen  hereafter,  that 
they  are  principally  produced  by  clouds  in  his  atmosphere,  yet  permanent 
marks  weie  discovered  upon  it  at  an  early  epoch,  by  which  the  fact  was  estab- 
<  lished  that  the  planet  has  a  diurnal  rotation.  In  the  years  1664-'5,  Hook  and 
Cassmi  observed  a  spot  on  one  of  the  belts  which  was  permanent  in  its  pt>si- 
uon,  and  was  observed  to  move  across  the  disk  of  the  planet.  It  contracted 
in  us  breadth  as  it  approached  the  edge  of  the  disk ;  a  circumstance  which  ob- 
viously  arose  from  its  being  fore-shortened  by  the  position  in  which  it  was 
there  presented  to  the  eye,  that  portion  of  the  surface  of  the  planet  being  seen 
very  obliquely,  the  spot  disappeared  at  one  side,  and  after  being  invisible  for 
a  time  reappeared  at  the  other.  This  spot  continued  to  be  seen  for  more  than 
a  year,  and  fully  proved  the  fact  that -Jupiter  completes  his  rotation  on  an  axis 
very  slightly  inclined  to  his  orbit  in  nine  hours  and  fifty-six  minutes. 

The  alternations  of  light  and  darkness  on  Jupiter  are  therefore  regulated  by 
intervals  much  shorter  than  those  which  govern  the  days  and  nights  of  the 
minor  planets,  and  we  shall  presently  see  that  this  is  a  character  which  prob- 
ably  prevails  among  all  the  major  planets.  The  average  interval  of  the  days 
and  nights  must  be  a  little  under  five  terrestrial  hours. 

This  rapid  motion,  considered  with  reference  to  the  great  magnitude  of  Ju- 
piter,  leads  to  the  inference  that  the  velocity  of  that  part  of  his  surface  which 
is  near  his  equator  must  be  exceedingly  great.  The  circumference  of  Jupiter 
at  his  equator  must  be  about  two  hundred  and  seventy  thousand  miles,  and  as 
this  revolves  in  ten  hours,  the  motion  of  any  point  upon  it  must  be  at  the 
enormous  rate  of  twenty-seven  thousand  miles  an  hour,  or  a  little  less  than  five 
hundred  miles  a  minute.  Thus  it  appears  that  the  velocity  which  the  equa- 
torial  regions  have,  in  virtue  of  the  diurnal  motion,  is  very  little  less  than  the 
orbitual  motion  of  the  planet  round  the  sun. 

This  rapid  diurnal  rotation  would  produce  a  considerable  variation  in  the 
weights  of  bodies  at  different  latitudes  on  the  surface  of  Jupiter,  since  the  cen- 
trifugal  force  near  the  equator  would  counteract  the  weight  in  a  very  sensible 
manner,  while  toward  the  poles  its  effects  would  cease  to  be  perceptible. 


THE  MAJOR  PLANETS. 


1    /-'^•S^-*^^^ 

239  ! 


The  great  length  of  Jupiter's  year  compared  with  its  rapid  diurnal  rotation, 
will  resolve  the  year  into  a  much  greater  number  of  days  than  its  proportional 
length  compared  with  the  terrestrial  year  would  infer.  While  Jupiter  makes 
one  complete  revolution  round  the  sun,  it  will  make  ten  thousand  four  hundred 
and  seventy  revolutions  on  its  axis.  Such,  therefore,  is  the  number  of  days 
in  Jupiter's  year. 

The  axis  of  Jupiter  is  inclined  to  its  orbit  at  an  angle  of  about  three  degrees, 
and  as  this  inclination  determines  the  limits  of  the  seasons,  it  follows  that  there 
can  be  scarcely  any  perceptible  change  of  season  upon  the  planet  during  one 
half  of  his  year.  The  sun  will,  during  one  half  year,  gradually  pass  to  three 
degrees  north  of  his  equator,  and  during  the  other  half  year  to  three  degrees 
south  of  it.  The  extreme  change  of  the  sun's  meridional  altitude  would  there- 
fore not  exceed  six  degrees.  This  perhaps  might  be  sufficient  for  the  purposes 
of  chronology,  but  could  scarcely  produce  any  effects  on  the  organized  world, 
nor  would  the  temperature  of  the  seasons  undergo  any  observable  change.  The 
range  of  the  tropics  would  be  three  degrees  on  each  side  of  the  equator  of  the 
planet,  and  within  these  regions  the  sun  would  pass  near  the  zenith  daily. 

The  sun  would  rise  and  set  daily  throughout  the  year,  to  every  part  of  the 
planet  except  a  small  circle  extending  three  degrees  round  the  poles. 

The  diameter  of  Jupiter  being  eleven  times  that  of  the  earth,  his  surface  will 
be  greater  than  that  of  our  planet  in  the  proportion  of  a  hundred  and  twenty  to 
one,  and  if  the  distribution  of  land  and  water  be  similar,  it  will  afford  accom- 
modation for  a  population  a  hundred  and  twenty  times  more  numerous. 

The  actual  bulk  of  the  globe  of  Jupiter,  which  is  the  largest  body  of  the 
system  next  to  the  sun,  is  fourteen  hundred  times  greater  than  that  of  the  earth. 
In  other  words,  to  make  a  globe  equal  to  that  of  Jupiter,  we  should  roll  into 
one  fourteen  hundred  globes  like  that  of  the  earth. 


TELESCOPIC    APPEARANCE    OF    JUPITER. 

The  spectacle  presented  to  the  observer  who  enjoys  the  use  of  a  powerful 
telescope  by  the  planet  Jupiter,  is  magnificent  indeed.  The  surface  of  the 
planet  appears  as  large  and  distinct  as  the  full  moon  to  the  naked  eye.  His 
disk  is  marked  with  certain  features  of  light  and  shadow,  which  are  in  general 
variable.  They  are,  therefore,  produced  by  clouds  floating  in  his  atmosphere, 
the  presence  of  which  is  indeed  rendered  quite  evident  by  the  telescope.  Al- 
though these  lights  and  shadows  in  general  are  variable,  yet  they  are  found 
to  be  characterized  by  a  certain  regularity  of  arrangement.  Their  streaks 
are  generally  parallel,  as  in  the  annexed  figures,  which  exhibit  views  of  Jupiter 
seen  on  different  occasions. 

These  streaks,  which  are  called  the  belts  of  Jupiter,  were  observed  before 
the  middle  of  the  17th  century,  and  are  visible  to  telescopes  of  no  very  con- 
siderable power.  They  are  variable  not  only  in  their  breadth  and  form,  btit 
in  their  number.  Sometimes  not  more  than  one  can  be  discovered ;  at  other 
times  two  or  more,  and  sometimes  as  many  as  eight.  Sometimes  they  have 
continued  without  sensible  variation  for  nearly  three  months,  and  sometimes  a 
new  belt  has  appeared  in  an  hour  or  two.  The  annexed  diagrams  have  been 
given  by  different  authors  as  representing  the  appearances  of  these  belts  at 
different  times.  They  have,  sometimes,  though  rarely,  been  see-n  broken  ujp 
and  distributed  over  the  whole  surface  of  the  planet  as  represented  in  fig.  D. 
Fig.  B  gives  a  view  taken  at  an  early  period  by  Dr.  Hook.  Fig.  A  is  a  view 
taken  in  the  year  1832.  Fig.  C  is  in  1837.  It  is,  however,  extremely  dif- 
ficult to  obtain  sketches  of  this  kind  executed  with  tolerable  fidelity. 

Mr.  Thomas  Dick  states  that  he  has  had  frequently  an  opportunity  of  view- 


ing  Jupiter  with  good  telescopes,  both  reflecting  and  refracting,  for  twenty  or 
thirty  years  past ;  and  among  several  hundreds  of  observations,  has  never 
seen  above  four  or  five  belts  at  one  time.  The  most -common  appearance  ob- 
served, is  that  of  two  belts  distinctly  marked,  one  on  each  side  01  the  planet's 
equator,  and  one  at  each  pole,  generally  broader,  but  much  fainter  than  the 
others.  He  has  never  perceived  much  change  in  the  form  or  position  of  the 
belts  during  the  same  season,  but  in  successive  years  a  slight  degree  of  change 
has  been  perceptible,  some  of  the  belts  having  either  disappeared,  or  turned 
much  fainter  than  they  were  before,  or  shifted  somewhat  their  relative  posi- 
tions, but  has  never  seen  Jupiter  without  at  least  two  or  three  beks.  Some 
of  the  largest  of  these  belts  being  at  least  the  one  eighth  part  of  the  diameter 
of  the  planet  in  breadth,  must  occupy  a  space  at  least  11,000  miles  broad,  and 
270,000  miles  in  circumference  ;  for  they  run  along  the  whole  circumference 
of  the  planet,  and  appear  of  the  same  shape  during  every  period  of  its  rotation. 
It  is  probable  that  the  smallest  belts  we  can  distinctly  perceive  by  our  tele- 
scopes are  not  much  less  than  a  thousand  miles  in  breadth. 

CAUSES    OF    THE    BELTS. 

It  is  well  known  that  the  diurnal  motion  of  the  earth,  combined  with  the 
heat  of  the  sun  acting  directly  on  the  intertropical  regions,  produces  those  at- 
mospheric currents  which  blow  with  a  constancy  and  regularity  so  singular 
from  east  to  west  in  the  lower  la-titudes  of  both  hemispheres.  These  currents 
are  attended  with  others  in  a  contrary  direction,  wkick  e«n«titute  their  reac- 
tion, blowing  almost  as  constantly  and  regularly  from  west  to  east  in  the 
higher  latitudes.  Thus  the  atmosphere  covering  th«  surface  of  the  earth  is 
continually  swept  by  systems  of  currents  blowing  in  either  direction  parallel  to 
the  line — and  these  currents  will  have  a  tendency,  in  proportion  to  their  force 
and  regularity,  to  produce  corresponding  arrangements  parallel  to  the  line,  in 


MAJOR  PLANETS. 


241 


the  clouds  which  float  upon  our  atmosphere.     It  is  evident  that  such  an  effect 
Would  be  more  strongly  marked  in  proportion  as  the  energy  of  the  causes  pro-  > 
ducino  it  would  be  increased. 

In  the  case  of  the  earth,  the  surface  at  the  equator  is  moved  by  the  diurnal 
motion  at  the  rate  of  about  a  thousand  miles  an  hour ;  and  the  sun,  at  different 
seasons  of  the  year,  departs  from  the  equator  on  either  side  to  a  distance  of  ' 
twenty-three  and  a  half  degrees.     If  the  velocity  of  the  surface  of  the  equator 
were  to  become  ten  or  twenty  times  greater,  and  the  sun,  instead  of  departing  '! 
from  it  twenty-three  degrees,  were  constantly  vertical  to  it,  then  we  might  ex-  if 
pect  to  have  atmospheric  currents  parallel  to  the  line  much  more  energetic,  s 
constant,  and  regular. 

But  in  the  case  of  JUPITER,  it  will  be  easily  seen  that  the  causes  producing 
such  currents  are  far  more  energetic  than  on  the  earth.  Instead  of  revolving 
in  twenty-four  hours,  Jupiter  revolves  in  ten  hours.  If,  then,  the  globe  of  Ju- 
piter were  equal  to  that  of  the  earth,  the  velocity  of  his  surface  at  the  line 
would  be  greater  than  in  the  case  of  the  earth  in  the  proportion  of  two  and  a 
half  to  one.  The  velocity  of  his  surface  would,  in  fact,  be  about  two  thousand 
five  hundred  miles  an  hour.  But  the  diameter /of  Jupiter,  and  therefore  also 
the  circumference,  is  eleven  times  greater  than  that  of  the  earth ;  and  there- 
fore, on  that  account  alone,  even  though  he  revolved  in  the  same  time,  the  ve- 
locity of  his  surface  would  be  eleven  times  greater  than  that  of  the  earth. 
From  these  two  causes  combined,  it  follows  that  the  velocity  of  the  surface  of 
Jupiter  at  the  equator  is  about  twenty-seven  and  a  half  times  greater  than  that 
of  the  earth,  and  is,  in  fact,  twenty-seven  thousand  five  hundred  miles  an  hour. 

It  is  evident,  then,  that  the  velocity  of  the  surface  of  Jupiter  produced  by  his 
diurnal  revolution  being  nearly  twenty-eight  times  greater  than  that  of  the  earth, 
and  the  sun  appearing  always  vertical  to  his  equator,  or  nearly  so,  the  causes 
which  produce  a  system  of  atmospheric  currents  parallel  to  his  equator,  act 
with  infinitely  more  energy  than  upon  the  earth.  We  accordingly  see  the 
effects  of  such  currents  exhibited  in  the  decided  arrangements  of  the  strata  of 
his  clouds  parallel  to  his  equator.  Thus  we  see  that  there  prevail  in  Jupiter 
atmospheric  currents  similar  to  those  which  prevail  on  the  earth,  blowing 
constantly  from  east  to  west  in  some  latitudes,  and  from  west  to  east  in  others. 
As  we  cannot  doubt  that  they  were  intended  to  fulfil  that  purpose  in  the  social 
intercourse  of  the  people  of  the  globe  which  they  actually  do  fulfill,  we  are 
supplied  with  one  analogy  more  to  support  the  conclusion  that  the  planets  are 
inhabited  globes  like  the  earth. 

Annexed  are  two  views  of  Jupiter,  showing  the  appearance  of  the  belts, 
taken  irom  original  drawings  by  Madler,  made  from  observations  taken  so  re- 
cently as  1841. 


16 


APPEARANCE    OF    THE    SUN    AT    JUPITER. 


If  E  in  the  annexed  figure  represent  the  appearance  of  the  sun  to  the  m- 
habitants  of  the  earth,  J  will  represent  its  appearance  to  those  of  Jupiter. 
The  distance  of  Jupiter  from  the  sun  being  nearly  five  times  that  of  the 


THE  MAJOR  PLANETS.  243 


earth,  the  apparent  diameter  of  the  sun  as  seen  from  Jupiter  will  be  one  fifth 
of  its  apparent  diameter  from  the  earth.  It  will,  therefore,  measure  about  six 
minutes,  since  the  diameter  of  the  earth  measures  about  thirty  minutes.  The 
apparent  magnitude  of  the  sun  as  we  see  it,  is  very  nearly  that  which  a  cent 
piece  would  have  if  seen  at  the  distance  of  one  hundred  and  twenty  feet  from 
the  eye.  The  apparent  magnitude  of  the  sun  as  seen  from  Jupiter  would  then 
be  the  same,  or  nearly  so,  as  that  of  a  cent  piece  seen  at  six  hundred  feet  dis- 
tance. 

.       It  is  proved  in  those  branches  of  physics  in  which  the  laws  of  heat  and 
)  light  are  developed,  that  the  density  of  these  principles  is  diminished  in  pro- 
portion as  the  square  of  the  distance  from  the  body  from  which  they  emanate 
is  increased.     It  follows,  therefore,  that  the  heat  and  light  of  the  sun  at  Jupiter 
will  be  about  twenty-five  times  less  than  at  the  earth. 

JUPITER'S  SATELLITES. 

When  Galileo  directed  the  first  telescope  to  the  examination  of  Jupiter,  he 
observed  four  minute  stars,  which  appeared  in  the  line  of  the  equator  of  the 
planet.  He  took  these  at  first  to  be  fixed  stars ;  but  he  was  soon  undeceived. 
He  saw  them  alternately  approach  and  recede  from  the  planet.  He  observed 
them  pass  behind  it  and  before  it ;  and,  in  fact,  to  oscillate,  as  it  were,  to  the 
right  and  the  left  of  the  planet,  to  certain  limited  distances ;  each  of  the  four 
stars  receding  to  equal  distances  east  and  west  of  the  planet.  He  soon  arrived 
at  the  obvious  conclusion  that  these  objects  were  not  fixed  stars,  but  that  they 
were  bodies  which  revolved  round  Jupiter  in  circular  orbits,  at  limited  dis- 
tances ;  and  that  each  successive  body  included  the  orbit  of  the  others  within 
it.  In  short,  that  they  formed  a  miniature  of  the  solar  system,  in  which,  how- 
ever, Jupiter  himself  played  the  part  of  the  sun.  As  the  telescope  improved, 
it  became  apparent  that  these  bodies  were  small  globes,  related  to  Jupiter  in 
the  same  manner  exactly  as  the  moon  is  related  to  the  earth  ;  that,  in  fine,  they 
were  a  cortege  of  four  moons,  attending  Jupiter  round  the  sun  in  the  same 
manner,  and  subserving  the  same  purpose,  as  our  moon  does  in  reference  to 
the  earth.  v 

Thus,  then,  it  seems  that  the  population  of  Jupiter  are  favored  by  four  moons 
in  their  firmament.  Since  the  examination  of  the  motion  of  these  bodies  has 
been  carried  to  a  greater  extent  of  accuracy,  it  has  been  found  that  there  is  a 
singular  law  prevailing  among  their  motions,  in  virtue  of  which  it  is  impossible 
that  the  four  satellites  can  ever  be  at  the  same  time  on  the  same  side  of  Jupiter  ; 
one,  at  least,  must  be  on  the  contrary  side  from  the  other  three.  Thus  it  fol- 
lows that  there  must  always  be  one  moon  full,  or  nearly  so  ;  for  if  three  of  the 
four  satellites  be  on  the  same  side  of  Jupiter  with  the  sun,  and  therefore  in 
the  condition  of  new  or  waning  moons,  the  fourth  must  be  on  the  opposite  side, 
and  therefore  nearly  a  full  moon. 

But,  connected  with  these  appendages  to  Jupiter,  there  is  perhaps  nothing 
more  remarkable  than  the  period  of  their  revolutions  round  him.  That  moon 
which  is  nearest  to  Jupiter  completes  its  revolution  in  forty-two  hours.  In  that 
brief  space  of  time  it  goes  through  all  its  various  phases  ;  it  is  a  thin  crescent ; 
it  is  halved,  gibbous,  and  full.  It  must  be  remembered,  however,  that  the  day 
of  Jupiter,  instead  of  being  twenty-four  hours,  is  about  ten  nours.  This  moon, 
therefore,  has  a  month  equal  to  a  little  more  than  four  of  Jupiter's  days.  In 
each  day  it  passes  through  one  complete  quarter ;  thus  the  first  day  of  the 
month  it  passes  from  the  thinnest  crescent  to  the  half  moon  ;  in  the  second  day, 
from  the  half  moon  to  the  full  moon  ;  on  the  third  day,  from  the  full  moon  to 
the  last  quarter ;  and  on  the  fourth  day  returns  to  conjunction  with  the  sun. 


THE  MAJOR  PLANETS. 


So  rapid  are  these  changes  that  we  can  conceive  the  gradual  changes  of  the 
phases  of  the  moon  to  be  actually  visible  as  they  proceed.  The  next  satellite 
makes  its  complete  revolution  in  about  eighty-five  hours,  or  in  about  eight  of  Ju- 
piter's days  and  a  half.  Such  is  the  month  of  the  second  satellite.  The  third 
satellite  completes  his  revolution  in  one  hundred  and  seventy  hours,  or  in  about 
seventeen  days  of  Jupiter.  The  fourth  and  most  distant  satellite,  requires  about 
four  hundred  hours,  to  complete  its  revolution,  and  therefore  has  a  month  of 
about  forty  of  Jupiter's  days. 

It  appears,  then,  that  upon  Jupiter  there  are  four  different  months,  correspond- 
ing to  the  four  different  moons  ;  one  of  about  four  days'  duration,  another  about 
eight  days,  a  third  about  seventeen  days,  and  the  fourth  about  forty  days.  What 
a  complicated  system  of  reckoning  time  is  thus  supplied ! 

The  magnitude  of  the  nearest  of  Jupiter's  moons  is  about  a  quarter  greater 
than  that  of  our  own  ;  that  of  the  second  is  equal  to  ours  ;  the  diameter  of  the 
third,  however,  is  nearly  double  to  that  of  our  moon,  and  it  is  nearly  equal  to  the 
planet  Mercury ;  the  diameter  of  the  fourth  satellite  is  about  one  half  greater 
than  that  of  our  moon. 

The  distance  of  the  nearest  moon  from  the  surface  of  Jupiter  is  somewhat 
less  than  the  distance  of  ours  from  the  surface  of  the  earth.  Its  apparent  mag- 
nitude, therefore,  seen  from  Jupiter,  will  be  greater  than  ours.  The  distance 
of  the  second  moon  from  Jupiter  is  about  one  half  greater  than  the  distance  of 
our  moon,  and  as  its  diameter  is  nearly  equal  to  that  of  our  moon,  its  apparent 
magnitude  will  be  proportionally  less.  The  distance  of  the  third  moon  is 
more  than  double  the  distance  of  ours,  but  as  its  magnitude  is  a  little  less  than 
double,  its  appearance  to  the  inhabitants  of  Jupiter  will  be  nearly  the  same  as 
that  of  ours.  The  appearance  of  the  fourth  moon  will  be  somewhat  less. 

Thus  it  appears  that  the  four  moons  which  attend  Jupiter  vary  very  little 
in  the  apparent  magnitude  they  present  to  its  inhabitants  from  that  which  ours 
presents  to  the  inhabitants  of  the  earth. 

One  of  the  peculiarities  in  the  motion  of  our  moon  which  distinguishes  it 
in  a  remarkable  manner  from  the  planets,  is  its  revolution  upon  its  axis.  It 
will  be  remembered,  that  the  planets  generally  rotate  on  their  axes  in  times 
somewhat  analogous  to  that  of  the  earth.  Now,  on  the  contrary,  the  moon  re- 
volves on  its  axis  in  the  same  time  that  it  takes  to  revolve  round  the  earth ;  in 
consequence  of  which  adjustment  of  its  motions  it  turns  the  same  hemisphere 
continually  toward  the  earth.  It  would  seem  that  this  is  a  general  character- 
istic of  all  satellites ;  for  the  observations  of  Sir  William  Herschel  on  those 
of  Jupiter,  show  that  the  same  motion  prevails  among  them  ;  that  they,  as 
they  revolve  round  their  primary,  turn  constantly  the  same  hemisphere  towajd 
Jupiter. 

The  globe  of  Jupiter,  though  of  considerable  magnitude,  is  small  compared 
with  that  of  the  sun.  In  consequence  of  this  it  throws  in  the  direction  oppo- 
site to  that  of  the  sun  a  conical  shadow  of  Considerable  length,  the  thickness  of 
which,  at  Jupiter,  is  equal  to  the  diameter  of  the  planet,  but  which  diminishes  until 
it  is  reduced  to  a  point  in  receding  from  Jupiter.  As  the  satellites  move  round 
Jupiter,  in  the  plane  of  his  equator,  and  as  the  plane  of  his  equator  is  very  { 
nearly  coincident  with  that  of  his  orbit  round  the  sun,  it  follows  that  the  satel- 
lites, every  revolution,  as  they  pass  behind  him,  must  move  through  his  shadow. 
The  only  exception  to  this  is  presented  by  the  fourth,  or  most  distant  satellite, 
which,  owing  to  its  great  distance  from  the  planet,  and  the  obliquity  of  its 
orbit,  sometimes,  in  passing  behind  the  planet,  goes  above  or  below  its  shadow 
When  the  satellites  get  into  the  shadow  of  Jupiter  they  become  invisible  to  us  ; 
and  hence  we  know  that  they  are  opaque  bodies,  which  shine,  like  the  moon 
by  the  reflected  light  of  the  sun.  All  the  circumstances  connected  with  their 


THE  MAJOR  PLANETS.  245 


eclipses  are  visible  to  us.  We  see  them  enter  the  shadow  and  leave' it,  and  \vr 
can  estimate  the  duration  of  each  eclipse,  and  observe  exactly  its  beginning  sn-l 
ending.  These  eclipses,  as  we  shall  show  on  another  occasion,  have  been  in- 
strumental, not  only  to  useful  purposes  in  art,  but  also  to  great  discoveries  in 
sciftnce.  It  is  by  them,  among  other  means,  that  the  longitude  of  places  on 
the  surface  of  the  earth  is  determined  ;  but  by  far  the  most  important  discovery 
connected  with  these  bodies,  is  that  of  the  motion  and  velocity  of  light.  How 
this  was  accomplished  we  shall  also  explain  on  another  occasion.  It  was 
shown,  however,  by  these  means,  that  the  velocity  of  reflected  light  was  the 
same  as  that  of  direct  light. 

SATURN. 

Beyond  the  orbit  of  Jupiter,  a  space  equal  in  extent  to  the  distance  of  Jupi- 
ter from  the  sun,  is  unoccupied  by  any  planetary  body.  At  a  distance  little 
short  of  a  thousand  millions  of  miles  from  the  sun,  the  SATURNIAN  SYSTEM 
revolves,  in  a  period  of  twenty-nine  years  and  a  half,  consisting  of  a  globe  little 
less  than  Jupiter,  begirt  with  two  (and  probably  more)  stupendous  rings,  and  a 
cortege  of  no  less  than  seven  moons. 

The  diameter  of  SATURN  is  eighty  thousand  miles,  and  its  bulk  is,  conse- 
quently, a  thousand  times  greater  than  that  of  the  earth. 


DIURNAL    ROTATION    OF     SATURN. 

The  distance  of  Saturn  is  so  great  that  it  requires  the  most  powerful  tele- 
scopes to  render  the  marks  on  his  disk  visible,  so  as  to  discover  his  diurnal 
motion.  From  purely  theoretical  views,  Laplace  conjectured  that  it  was  per- 
formed in  about  ten  hours.  Sir  William  Herschel,  by  the  aid  of  the  large  in- 
struments constructed  by  him,  inferred  that  it  revolves  in  ten  hours,  sixteen 
minutes,  and  nineteen  seconds.  Sir  John  Herschel  estimates  the  time  of  its 
rotation  to  be  ten  hours,  twenty-nine  minutes,  and  seventeen  seconds. 

The  axis  on  which  it  turns  is,  like  that  of  Jupiter,  at  right  angles  to  the  di- 
rection of  the  belts,  but  unlike  Jupiter,  Saturn  inclines  his  axis  to  the  plane  of 
his  orbit  in  a  manner  similar  to  the  earth  and  Mars.  The  consequence  of  this 
'!  arrangement  is  that  the  year  of  Saturn  is  varied  by  the  same  succession  of 
/  seasons  subject  to  the  same  range  of  temperature  as  those  which  prevail  on  our 
globe. 

The  alternation  of  light  and  darkness  is  the  same  as  upon  Jupiter.  This 
rapid  return  of  day,  after  an  interval  of  five  hours  night,  seems  to  assume  the 
character  of  a  law  among  the  major  planets,  as  the  interval  of  twelve  hours  cer- 
tainly does  among  the  minor  planets. 

The  year  of  Saturn  is  equal  in  duration  to  10,759  terrestrial  days,  or  to 
258,192  hours.  But  as  the  rotation  of  the  planet  is  completed  in  less  than  ten 
hours  and  a  half,  the  number  of  Saturnian  days  in  the  planet's  year  must  be 
24,592. 

The  distance  of  Saturn  from  the  sun  being  above  nine  times  that  of  the 
earth,  the  sun's  apparent  diameter  at  that  planet  will  be  less  than  at  the  earth 
in  a  like  j  roportion.  If  in  the  annexed  figure  E  represent  the  appearance  of 
the  sun  at  the  earth,  S  will  exhibit  its  appearance  at  Saturn. 


ATMOSPHERE    OF    SATURN. 


The  planet  Saturn  has  been  found  to  be  invested  with  an  atmosphere  similar 
to  that  of  Jupiter,  and  attended  in  all  respects  with  the  same  phenomena.  The 
belts  are  effects  of  the  same  kind,  and  produced  by  the  same  causes,  and  all 
that  we  have  said  regarding  the  atmospheric  currents,  clouds,  and  other  me- 
teorological phenomena,  in  JUPITER,  will  be  equally  applicable  in  SATURN. 

RINGS    OF    SATURN. 

At  a  very  early  epoch  in  the  history  of  the  telescope,  the  application  of  thai 
instrument  to  the  examination  of  SATURN  led  to  the  supposition  that  the  planet  5 
was  not  globular,  but  oval.  Further  observation  created  the  impression  that 
ears  or  handles  were  attached  to  each  side  of  the  disk.  But  as  the  means 
oi  observation  were  farther  improved,  the  astonishing  discovery  was  made  that 
Saturn  is  surrounded  by  a  stupendous  ring  of  solid  matter  lying  in  the  plane  of 
his  equator,  the  inner  edge  being  at  a  distance  from  his  surface  of  about  twenty 


THE  MAJOR  PLANETS. 


217 


thousand  miles.  More  recent  observations  made  by  Sir  William  Herschcl 
establish  the  fact  that  this  ring  is  not,  as  was  first  supposed,  a  single  anmilnr 
plate  of  matter,  but  has  a  division  by  which  it  is  separated  into  two  indepen- 
dent rings,  one  outside  the  other,  which  have  no  mutual  point  of  contact  or 
connexion.  This  separation  appeared  at  first,  as  a  dark  streak  upon  tin:  surface 
of  the  ring  running  parallel  to  its  edges.  Sir  William  Herschcl,  however, 
succeeded  in  seeing  stars  which  were  behind  the  ring  through  this  apparent 
streak,  and  consequently  arrived  at  the  conclusion  that  it  was  an  opening  »r 
separation  between  two  independent  rings.  It  was  found  also  that  trie  sur- 
face of  the  ring  was  marked  by  parallel  streaks  or  bands,  like  the  bens  of  the 
planet. 

Very  recent,  observations  made  at  Rome  upon  this  planet,  appear  10  counten- 
ance the  supposition  that  the  ring,  instead  of  being  double,  is  qtum.iple,  and 
that  there  are  four  divisions  instead  of  one,  as  supposed  by  Sir  Winiam  Her- 
schel.  It  is  even  said  that  six  divisions  have  been  observed,  and  therefore 
'there  are  seven  independent  rings,  one  within  another,  all  being  concentric 
with  the  planet  and  in  the  plane  of  its  equator. 

One  of  the  most  striking  discoveries  of  Sir  William  Herscuel  respecting 
Saturn,  was  the  revolution  of  the  rings  around  the  planet.  He  found  that  they 
revolve  round  their  own  centre  and  that  of  the  planet  in  their  own  plane,  and 
that  they  complete  a  revolution  in  the  same  time  that  a  satellite  would  revolve- 
in,  at  the  same  distance.  Their  motion,  therefore,  is  conformable  to  the  laws 
of  gravitation  which  would  govern  that  of  satellites  or  moons.  Tne  dimension!? 
of  the  rings,  as  observed  by  Sir  William  Herschel,  are  as  follows  : — 

Miles. 

Exterior  diameter  of  exterior  ring 176,418 

Interior  diameter  of  exterior  ring 155,272 

Breadth  of  exterior  ring . .    10,573 

Exterior  diameter  of  interior  ring 151,690 

Interior  diameter  of  interior  ring 1 17,339 

Breadth  of  the  interior  ring 17,175 

Equatorial  diameter  of  the  planet 79, 1 60 

Interval  between  the  planet  and  the  interior  ring 19,090 

Interval  of  the  rings 1,791 

Thickness  of  the  rings  not  exceeding 100 

It  appears  then  that  the  thickness  of  the  rings  is  incomparably  smaller  than 
thcirbreadth ;  the  thickness  being  not  more  than  the  three  hundredth  part  of 
the  breadth. 

One  of  the  circumstances  attending  the  contemplation  of  the  planet  Saturn 
which  excites  most  surprise,  is  the  fact  that  the  planet  and  the  two  rings  should 
be  capable  of  maintaining  their  relative  position  with  the  prodigious  velocity 
with  which  they  move  round  the  sun,  without  either  overtaking  the  other  or 
any  collision  taking  place.  Let  it  be  remembered  that  the  circumference  of 
Saturn's  orbit  round  the  sun  measures  about  six  thousand  millions  of  miles,  and 
that  the  planet  completes  this  circuit  in  less  than  thirty  years,  so  that  he  moves 
at  the  rate  of  about  seven  millions  and  three  quarter  miles  per  day,  or  three 
hundred  and  twenty-five  thousand  miles  an  hour.  This  is  a  velocity  six  hun- 
dred ;md  fifty  times  greater  than  that  of  a  cannon-ball.  Yet  with  this  prodigious 
celerity  of  motion  continued  for  countless  ages,  neither  of  the  rings  has  ever 
overtaken  the  planet  or  the  planet  overtaken  them,  and  still  more  wonderful, 
the  two  rings,  separated  only  by  a  space  of  about  eighteen  hundred  miles, 
which  they  would  move  over  with  their  orbitual  motion  in  about  three  minutes, 
have  never  overtaken  each  other.  This  astonishing  precision  of  movement 
would  become  still  more  surprising  if  it  be  true,  as' it  is  suspected  to  be,  that 
there  are  five  or  more  independent  rings,  one  included  within  the  other. 


248  THE  MAJOR  PLANETS. 


This  apparent  mystery  has  however  been  most  clearly  and  beautifully  ex- 
plained by  Biot,  to  whom  the  happy  idea  occurred  that  the  rings  could  be 
brought  under  the  same  laws  of  motion  as  moons.  To  make  this  explanation 
clearly  understood,  let  us  first  imagine  a  globe  like  the  moon  moving  period- 
ically round  the  planet  like  the  earth.  The  manner  in  which  the  attraction  of 
gravitation  combined  with  centrifugal  force  causes  it  to  keep  revolving  round 
the  earth  without  falling  down  upon  it  by  its  gravity  on  the  one  hand,  or 
receding  indefinitely  from  it  by  the  centrifugal  force  on  the  other  is  well 
understood.  In  virtue  of  the  equality  of  these  forces,  the  moon  keeps  con- 
tinually at  the  same  distance  from  the  earth  while  it  accompanies  the  earth 
round  the  sun.  Now  it  would  be  easy  to  suppose  another  moon  revolving  by 
the  same  law  of  attraction  at  the  same  distance  from  the  earth.  It  would  re- 
volve in  the  same  time,  and  with  the  same  velocity,  as  the  first.  We  may  ex- 
tend the  supposition  with  equal  facility  to  three,  four,  or  a  hundred  moons,  at 
the  same  distance.  Nay,  we  may  suppose  as  many  moons  placed  at  the  same 
distance  round  the  earth  as  would  complete  the  circle,  so  as  to  form  a  ring  of 
moons  touching  each  other.  They  would  still  move  in  the  mame  manner  and 
with  the  same  velocity  as  the  single  moon.  Nor  will  the  circumstances  be 
altered  if  this  ring  of  moons  be  supposed  to  be  beaten  out  into  a  thin  flat  ring 
like  those  of  Saturn.  It  is  plain,  then,  that  if  the  ring  of  Saturn  revolve  in  its 
own  plane  round  the  planet  in  the  same  time  as  that  in  *hich  a  single  satellite 
placed  at  the  same  distance  would  revolve,  the  stability  of  the  ring  with  refer- 
ence to  the  planet  is  explicable  exactly  upon  the  same  principles  as  those  by 
which  we  explain  the  motion  of  a  satellite.  But  Sir  William  Herschel,  as  has 
been  already  stated,  discovered  the  important  fact  that  the  rings  do  move  round 
their  own  centre  and  that  of  the  planet  in  the  same  time  that  a  satellite  placed 
at  the  same  distance  would  do.  Biot,  therefore,  has,  with  a  happy  adroitness, 
adopted  this  as  the  key  to  the  explanation  of  the  stability  of  the  ring. 

The  following  observations  of  Sir  John  Herschel  on  the  rings  indicated 
another  cause  of  their  stability  : — 

Although  the  rings  are,  as  we  have  said,  very  nearly  concentric  with  the 
body  of  Saturn,  yet  recent  micrometical  measurements  of  extreme  delicacy  have 
demonstrated  that  the  coincidence  is  not  mathematically  exact,  but  that  the 
centre  of  gravity  of  the  rings  oscillates  round  that  of  the  body  describing  a 
very  minute  orbit,  probably  under  laws  of  much  complexity.  Trifling  as  this 
remark  may  appear,  it  is  of  the  utmost  importance  to  the  stability  of  the  sys- 
tem of  the  rings.  Supposing  them  mathematically  perfect  in  their  circular 
form,  and  exactly  concentric  with  the  planet,  it  is  demonstrable  that  they  would 
form  (in  spite  of  their  centrifugal  force)  a  system  in  a  state  of  unstable  equilib- 
rium, which  the  slightest  external  power  would  subvert — not  by  causing  a  rup- 
ture in  the  substance  of  the  rings — but  by  precipitating  them,  unbroken,  on  the 
surface  of  the  planet.  For  the  attraction  of  such  a  ring  or  rings  cm  a  point  or 
sphere  eccentrically  situate  within  them,  is  not  the  same  in  ill  directions,  but 
tends  to  draw  the  point  or  sphere  toward  the  nearest  part  of  .e  ring,  or  away 
from  the  centre.  Hence,  supposing  the  body  to  become,  from  any  cause,  ever  / 
so  litt.e  eccentric  to  the  ring,  the  tendency  of  their  mutual  gravity  is,  not  to  > 
correc;  but  to  increase  this  eccentricity,  and  to  bring  the  nearest  parts  of  them 
together.  Now,  external  powers,  capable  of  producing  such  eccentricity,  exist 
in  the  attractions  of  the  satellites  ;  and  in  order  that  the  system  may  be  stable, 
and  possess  within  itself  a  power  of  resisting  the  first  inroads  of  such  a  ten- 
dency, while  yet  nascent  and  feeble,  and  opposing  them  by  an  opposite  or 
maintaining  power,  it  has  been  shown  that  it  is  sufficient  to  admit  the  rings  to 
be  loaded  in  some  part  of  their  circumference,  either  by  some  minute  inequality 
of  thickness,  or  by  some  portions  being  denser  than  others.  Such  a  load 


THE  MAJOR  PLANETS.  249 


would  give  to  the  whole  ring  to  which  it  was  attached  somewhat  of  the  charac- 
ter of  a  heavy  and  sluggish  satellite,  maintaining  itself  in  an  orbit  with  a  cer- 
tain energy  sufficient  to  overcome  minute  causes  of  disturbance,  and  establish 
an  average  bearing  on  its  centre.  But  even  without  supposing  the  existence 
of  any  such  load — of  which,  after  all,  we  have  no  proof — and  granting,  there- 
fore, in  its  full  extent,  the  general  instability  of  the  equilibrium,  we  think  we 
perceive,  in  the  periodicity  of  all  the  causes  of  disturbance,  a  sufficient  guar- 
antee of  its  preservation.  However  homely  be  the  illustration,  we  can  con- 
ceive nothing  more  apt  in  every  way  to  give  a  general  conception  of  this  main- 
tenance of  equilibrium  under  a  constant  tendency  to  subversion,  than  the  mode 
in  which  a  practised  hand  will  sustain  a  long  pole  in  a  perpendicular  position 
resting  on  the  finger,  by  a  continual  and  almost  imperceptible  variation  of  the 
point  of  support.  Be  that,  however,  as  it  may,  the  observed  oscillation  of  the 
centres  of  the  rings  about  that  of  the  planet  is  in  itself  the  evidence  of  a  per- 
petual contest  between  conservative  and  destructive  powers — both  extremely 
feeble,  but  so  antagonizing  one  another,  as  to  prevent  the  latter  from  ever  ac- 
quiring an  uncontrollable  ascendency,  and  rushing  to  a  catastrophe. 

Since  the  plane  of  the  rings  coincides  with  that  of  Saturn's  equator,  and  since 
the  sun  is  during  one  half  of  Saturn's  year  north,  and  during  the  other  half  south 
of  his  equator,  it  follows  that  the  northern  side  of  the  ring  is  illuminated,  and  the 
southern  side  dark,  during  the  summer  half  year  of  his  northern  hemisphere, 
and  that  the  southern  side  is  illuminated  and  the  northern  side  dark  during  the 
winter  half  year  of  his  northern  hemisphere.  At  his  equinoxes  the  edge  of 
the  ring  is  presented  to  the  sun,  and  neither  side  of  it  is  illuminated.  Since 
the  half  year  of  Saturn  is  equal  to  fifteen  terrestrial  years,  it  follows  that  the 
northern  and  southern  sides  of  the  rings  are  alternately  illuminated  by  the  sun 
during  intervals  of  fifteen  years. 

It  is  evident  that  the  rings  can  only  be  seen  from  the  earth  when  the 
sun  and  earth  are  at  the  same  side  of  Saturn's  equator.  From  the  great 
magnitude  of  Saturn's  orbit,  compared  with  that  of  the  earth,  this  must  be 
generally  the  case.  In  order  that  the  sun  and  earth  should  be  at  opposite 
sides  of  the  plane  of  the  ring,  that  plane  must  be  so  placed  that  its  edge  is  di- 
rected to  some  point  between  the  sun  and  earth.  This  will  be  the  case  for  a 
short  time  before  and  after  it  is  directed  to  the  sun,  that  is  to  say,  a  little  be- 
fore and  after  Saturn's  equinox. 

If  we  suppose  two  lines  touching  the  earth's  annual  orbit,  and  parallel  to  the 
line  of  nodes  of  Saturn's  ring,  to  be  drawn  and  continued  in  both  directions  to 
Saturn,  it  will  be  only  when  Saturn  is  between  these  lines  that  the  earth  and 
sun  can  be  at  different  sides  of  the  ring.     These  lines  will  include  a  length  of 
(  the  orbit  of  Saturn  equal  to  the  diameter  of  the  orbit  of  .the  earth,  and  since 
)  Saturn  will  move  over  such  a  space  in  his  periodical  course  round  the  sun  in 
(!  one  year,  it  follows  that  the  sun  and  earth  must  be   always  at  the  same  side 
(  of  Saturn's  ring,  except  for  six  months   before  and  six  months  after  each  of 
Saturn's  equinoxes,  at  which  times  it  may  happen  that  the  sun  and  earth  may 
be  on  opposite  sides  of  the  rings. 

Saturn's  rings  may  become  invisible  from  the  earth  by  any  of  three  causes. 

1 .  When  the  edge  of  the  rings  be  presented  to  the  sun,  the  edge  being  then 
the  only  illuminated  part,  and  being  too  thin  to  be   seen  even  by  telescopes  at 
so  great  a  distance,  the  ring  is  invisible.     This  will  happen  once  every  h'iteen 
years. 

2.  When  the  edge  of  the  ring  is  presented  to  the  earth,  it  is  invisible  be- 
cause of  its  minuteness  and  distance.     This  will  happen  once  every  fifteen  years. 

3.  When  the  sun  and  earth  are  on  opposite  sides  of  the   ring.     This  will 
)  also  happen  once  every  fifteen  years. 


THE  MAJOR  PLANETS. 


Except  therefore  for  an  interval  of  a  few  months  every  fifteen  years,  the 
rings  of  Saturn  are  always  in  a  position  to  be  seen  from  the  earth.  Thr.se  cir- 
cumstances occur  when  the  planet  passes  through  the  twentieth  degrees  of  tin- 
signs  Virgo  and  Pisces.  They  took  place  in  the  year  1832-'33,  and  will  recur 
again  in  1847-'48. 

The  angle  at  which  the  plane  of  the  rings  is  inclined  to  that  of  the  ecliptic 
being  about  30°,  the  rings  must  always  be  seen  obliquely  from  the  earth,  more 
or  less  so,  as  the  earth  is  more  or  less  distant  from  the  plane  of  the  rings,  but 
the  obliquity  of.  the  view  can  never  be  less  than  30°.  Now,  since  a  circle 
seen  obliquely  is  always  foreshortened  into  an  oval,  the  appearance  of 
the  rings,  even  in  the  most  favorable  position  must  be  elliptical.  If  a  circle  be 
viewed  at  an  angle  of  30°,  it  will  be  seen  as  an  ellipse  whose  lesser  axis  is 
half  its  greater.  Such  is  the  form  of  the  ring  as  seen  at  intervals  of  seven 
years  and  a  half  from  Saturn's  equinoxes,  or  when  the  planet  is  in  the  siuns 
Scorpio  and  Gemini,  which  takes  place  at  the  middle  of  the  intervals  of  the 
disappearances  of  the  rings.  This  occurred  last  in  1839-'40,  and  will  occur 
again  in  1854-'55.  Between  the  epochs  at  which  the  ring  is  in  its  most 
open  state,  and  the  times  of  its  disappearances  it  undergoes  all  the  intermedi- 
ate phases. 

In  the  annexed  figures  the  appearances  it  presented  between  1832  and  1840 
are  given  from  the  observations  of  William  Dick. 

In  October,  November,  and  December,  1832,  the  ring  appeared  as  in  fig.  1. 
In  the  beginning  of  January,  it  appeared  like  a  pure  thread  of  light  on  each 
side  of  the  planet  as  in  fig.  2.  It  began  to  appear  a  little  larger  during  die 
months  of  January,  February,  and  March,  1833  ;  but  in  April  it  again  disap- 
peared as  the  earth  was  then  in  the  plane  of  the  ring,  and  it  continued  invisible 
till  near  the  end  of  June  ;  after  which  it  again  appeared  as  represented  in  fig. 
2.  In  about  a  year  after  its  second  disappearance,  it  appeared  as  in  fig.  3,  and 
a  year  and  a  half  afterward  was  seen  as  in  fig.  4.  In  1837  it  appeared  as 
in  fig.  5,  and  finally  assumed  its  most  open  form,  as  represented  in  fig.  6. 

From  1838  to  1847,  the  ring  gradually  passes  through  similar  phases  in  a 
contrary  order. 

SATELLITES    OF    SATURN 

On  examining  Saturn  with  powerful  telescopes,  it  is  found  to  be  attended  by 
objects  revolving  round  it  similar  in  all  respects  to  the  satellites  of  Jupiter,  but 
amounting  to  seven  in  number.  These  revolve  nearly  in  the  plane  of  the  ring 
and  beyond  that  body.  The  times  of  revolution  are  such  as  to  present  various 
and  interesting  appearances  to  the  inhabitants  of  the  planet.  The  nearest 
satellite,  makes  its  complete  revolution  in  22^  hours,  which  is  equivalent  to 
about  two  of  Saturn's  days.  This  moon,  therefore,  exhibits  all  its  various 
changes  within  that  time.  It  passes  from  the  crescent  to  the  first  quarter 
in  half  of  one  of  Saturn's  days  ;  from  the  first  quarter  to  the  full  moon  in  an- 
other half  day.  and  from  the  full  to  the  new  moon  in  another  half  day  ;  so  rapid 
is  the  succession  of  its  phases.  The  next  in  the  order  of  distance,  makes  its 
revolutions  in  thirty-three  hours,  or  in  about  three  of  Saturn's  days,  which 
constitutes  another  sort  of  month ;  within  which  it  passes  through  all  its  vari- 
ous phases.  The  third  revolves  in  forty-five  hours,  or  abouv  four  of  Saturn's 
days ;  the  fourth  in  seventy-five  hours,  or  about  seven  and  a  half  of  Saturn's 
days  ;  the  fifth  in  one  hundred  and  eight  hours,  or  nearly  eleven  of  Saturn's 
days ;  the  sixth  in  about  three  hundred  and  eighty  hours,  or  in  about  thirty- 
eight  of  Saturn's  days ;  the  seventh  in  about  nineteen  hundred  hours,  or  one 


252 


THE  MAJOR  PLANETS. 


hundred  and  eighty  of  Saturn's  days.  Such  are  the  seven  different  months 
prevalent  upon  SATURN. 

The  magnitudes  of  the  satellites  of  Saturn  have  not  been  certainly  ascer- 
tained ;  their  distances  from  the  earth  are  too  great  to  enable  us  hitherto,  ac- 
tually to  measure  their  diameters. 

Sir  John  Herschel  estimates  the  diameter  of  the  most  remote  satellite  to  be 
little  less  than  that  of  Mars,  which  is  4,200  miles.  The  next  to  it  cannot  be 
much  less,  being  the  most  conspicuous  in  its  appearance.  As  to  the  magni- 
tudes of  the  four  minor  satellites,  we  are  left  to  conjecture. 

It  is  usual  to  designate  these  bodies  in  the  order  of  their  discovery,  and  not 
in  the  order  of  their  distances  from  Saturn.  If  the  following  figures  represent 
the  succession  of  their  distances,  the  order  of  their  discovery  is  that  expressed 
above  the  figures  respectively  : — 

Seventh,       Sixth,       First,       Second,       Third,       Fourth,       Fifth. 
1  234567 

The  distance  of  the  nearest  satellite  from  the  surface  of  Saturn  does  not  ex- 
ceed 80,000  miles,  a  space  equal  to  one  diameter  of  the  planet.  Its  distance 
beyond  the  edge  of  the  ring  is  only  18,000  miles. 

This  moon  completes  its  revolution  round  Saturn  in  22^  hours,  or  a  little 
more  than  two  Saturniah  days.  In  one  of  the  planet's  days  it  passes  therefore 
from  new  to  full  moon,  and  in  the  next  from  full  to  new  moon.  Its  change  of 
phase  from  hour  to  hour  must,  be  distinctly  perceivable. 

It  is  probable,  from  analogy,  that  its  magnitude  is  greater  than  that  of  our 
moon,  and  since  its  distance  from  the  surface  of  Saturn  is  three  times  less  than 
that,  of  our  moon,  its  apparent  diameter  at  Saturn  must  be  more  than  three 
times  greater.  It  will  therefore  appear  with  a  disk  at  least  ten  times  as  great 
as  that  of  our  moon. 

The  next  moon  is  at  a  distance  of  160,000  miles  from  the  centre,  and  120,000 
miles  from  the  surface  of  Saturn,  which  being  half  the  distance  of  our  moon 
from  the  earth,  shows  that  if,  as  is  probable,  this  satellite  be  equal  in  magnitude 
to  our  moon,  it  will  appear  with  a  disk  four  times  as  great.  It  completes  its 
revolution  in  three  of  Saturn's  days,  within  which  time  it  exhibits  all  its  phases. 

The  moon  next  in  order  is  at  a  distance  of  200,000  miles  from  the  centre 
and  160,000  from  the  surface  of  the  planet.  It  appears  a  little  less  than  four 
times  larger  than  our  moon  and  goes  through  all  its  phases  in  less  than  five 
of  Saturn's  days. 

The  next  satellite  is  at  a  distance  of  260,000  miles  from  the  centre  and 
220,000  miles  from  the  surface  of  Saturn,  and  therefore  appears  larger  at  Sat- 
urn than  our  moon  does  at  the  earth.  It  passes  through  all  its  phases  in  six 
and  a  half  of  Saturn's  days. 

Thus' it  appears  that  Saturn  is  supplied  with  four  moons,  all  moving  nearer 
to  his  surface  than  ours  is  to  the  earth,  and  appearing  from  twice  to  ten  times  as 
large,  and  passing  through  all  their  phases  in  from  two  to  seven  of  Saturn's  days. 

The  fifth  moon  from  Saturn,  completing  its  month  in  eleven  and  a  half  of 
Saturn's  days,  is  at  a  distance  a  little  greater  than  that  of  our  moon,  and  prob- 
ably appears  of  the  same  magnitude  seen  from  Saturn.  The  sixth  moon,  com- 
pleting its  month  in  forty  of  Saturn's  days,  is  at  more  than  three  times  the  dis- 
tance of  our  moon,  but  is  twice  its  diameter.  It  appears  from  Saturn  but  little 
less  than  ours.  The  most  remote  of  this  system  of  moons  completes  its  rev- 
olution in  two  hundred  Satumian  days,  and  its  distance  from  Saturn  is  ten 
times  that  of  our  moon  from  the  earth.  This  is  the  largest  moon  of  the  sys- 
tem, but  still,  owing  to  its  great  distance,  must  appear  smaller  at  Saturn  than 
ours  does  at  the  earth. 


THE  MAJOR  PLANETS. 


253 


The  orbits  of  the  six  inner  satellites  are  nearly  in  the  plane  of  the  ring,  but 
that  of  the  most  remote  one  is  inclined  to  it  at  the  rather  large  angle  of  30°. 

Owing  to  the  great  obliquity  of  the  orbits  of  the  satellites  to  that  of  Saturn, 
they  are  seldom  eclipsed.  The  frequency  of  the  eclipses  of  the  satellites  of 
Jupiter,  is  a  consequence  of  the  fact  that  their  orbits  are  nearly  in  the  plane  of 
that  of  the  planet. 

The  most  remote  of  Saturn's  moons  (commonly  called  the  fifth  satellite) 
exhibits  variations  of  brilliancy  which  have  given  ground  for  the  conjecture 
that  those  moons,  like  our  own  and  those  of  Jupiter,  revolve  on  their  axes  in 
the  time  they  take  to  revolve  in  their  orbits. 

The  two  innermost  satellites  were  the  latest  discovered,  and  are  by  far  the 
most  difficult  to  be  seen.  It  is  only  by  means  of  telescopes  of  the  most  power- 
ful kind,  and  under  circumstances  most  favorable  to  observation,  that  they  can 
be  detected  at  all.  Those  who  have  been  so  fortunate  as  to  possess  instru- 
ments capable  of  observing  them,  say  that  at  the  equinoxes  of  Saturn,  when 
his  ring  becomes  invisible,  they  have  been  seen  threading  like  beads  the  al- 
most infinitely  thin  filament  of  light  to  which  the  ring  is  then  reduced,  and  for 
a  short  time  moving  off  it  at  either  end,  speedily  to  return,  and  hastening  again 
to  their  habitual  concealment. 

OF    HERSCHEL,    OR    URANUS. 

The  planet  of  the  solar  system  which  is  the  most  remote  from  the  sun,  and 
which,  there  are  strong  reasons  for  believing  to  be  the  extreme  limit  of  the 
system,  is  called  Uranus,  and  sometimes,  from  its  distinguished  discover- 
er, Herschel.  This  body  is  a  globe  35,000  miles  in  diameter,  the  bulk  of 
which  is  about  eighty  times  that  of  the  earth ;  and  it  revolves  at  a  distance 
from  the  sun  of  eighteen  hundred  millions  of  miles  ;  being  double  the  dis- 
tance of  Saturn.  The  great  distance  of  this  object  from  the  earth  and  the 
consequent  minuteness  of  its  appearance,  has  rendered  our  knowledge  of  its 
physical  condition  much  less  distinct  and  satisfactory  than  those  of  the  nearer 
planets. 

It  has  been  hitherto  unascertained  whether  it  has  a  diurnal  rotation ;  but 
analogy  favors  the  conjecture  that  it  revolves  rapidly  upon  its  axis  like  the 
cognate  planets,  Jupiter  and  Saturn.  The  disk  has  not  been  seen  with  suffi- 
cient distinctness  to  detect  upon  it  those  indications  which  would  decide  the 
question,  whether  it  is  invested  with  an  atmosphere. 

The  period  for  this  planet  going  round  the  sun  is  eighty-four  terrestrial  years, 
and  as  the  date  of  its  discovery  was  1781,  it  has  not  yet  made  a  complete  rev- 
olution since  astronomical  observation  was  first  directed  to  it.  It  is  a  striking 
example  of  the  power  of  science,  that  we  are  nevertheless  as  certainly  assured 
of  its  periodical  path  round  the  sun,  as  if  it  had  been  observed  for  a  long  suc- 
cession of  its  periods  like  other  planets. 

Being  nearly  twenty  times  farther  from  the  sun  than  the  earth,  the  diameter 
of  the  sun  will  appear  to  it  proportionally  less  ;  and  as  the  sun's  apparent  diameter 
at  the  earth  is  thirty  minutes,  it  will  subtend  at  Herschel  at  an  angle  of  only  a 
minute  and  a  half.  "We  subjoin  here  a  diagram  in  which,  if  we  suppose  the 
larger  circle  E,  to  represent  the  appearance  of  the  sun  as  seen  from  the  earth ; 
the  smaller  one  H,  will  represent  its  appearance  as  seen  from  Herschel. 

As  the  intensity  of  solar  light  diminishes  in  the  same   proportion  as  the  su- 
perficial magnitude  of  the  sun's  disk  diminishes,  it  will  follow  that  the  bright- 
ness of  day  at  the  planet  Herschel  must  be  between  three  and  four  hundred 
times  less  than  at  the  earth !     We   might  be  led,  however,  from  such  a  numer-  i 
ical  estimate  to  form  a  very  incorrect  estimate  of  what  the  solar  light  under  ] 


254 


THE  MAJOR  PLANETS. 


such  circumstances  must  really  be.  The  light  of  the  full  moon  is  about  three 
hundred  thousand  times  less  than  that  of  the  sun  ;  consequently  it  follows  that 
the  light  of  day  at  Herschel  will  be  equal  to  the  light  of  more  than  one  thou- 
sand full  moons. 

Independent  of  this  consideration,  however  it  will  be  remembered,  as  we 

have  urged  on  another  occasion,  that  the  perception  of  the  brightness  of  light, 

does  not  depend  only  upon  the  density  of  the  light  itself;  but  also,  upon  the 

magnitude  of  the  pupil  of  the  eye  and  the  sensibility  of  the  retina.     Nothing 

^  can  be  more  easy  to  imagine  than  a  very  small  alteration  of  the  proportions  of 

?  the  eye,  without  even   the  necessity  of  admitting  any  in  its  structure,  which 

7  would  render  the  light  of  the  sun  at  Herschel   as   efficient  for  the  purpose  of 

>  vision  as  at  the  earth. 

It  has  been,  in  various  popular  works,  and  even  in  some  strictly  scientific 
treatises,  urged  that  the  cold  which  prevails  at  this  and  other  remote  planets, 
must  be  so  intense  that  the  liquids  of  our  globe  could  not  exist  there  ;  and,  on 
the  other  hand,  that  at  the  piauet  Mercury,  a  degree  of  heat  must  exist  equally 


THE    MAJOR    PLANETS.  255 

i  incompatible  with  the  existence  of  physical  arrangements  similar  to  those 
(i  which  prevail  upon  the  earth  ;  such  inferences  are,  as  we  conceive,  premature 
and  unfounded.  They  are  based  upon  the  supposition  that  the  temperature 
depends  solely  upon  the  uensity  of  the  solar  rays.  Now  we  have  noticed  else- 
where the  fact,  that  other  agencies  are  concerned  in  the  production  of  tempera- 
ture, and  have  given  as  an  example  all  the  varieties  of  temperature  which  pre- 
vail between  the  tropics  at  different  elevations. 

In  the  valleys  and  planes  of  these  regions,  we  find  their  proper  climate;  as- 
cending the  tropical  ranges,  at  great  elevations  we  encounter  all  the  vegetable 
phenomena  of  temperate  climates,  and  at  still  greater  elevations  we  arrive  at  a 
temperature  as  rigorous  as  that  at  the  poles.  How  easy  is  it,  then,  to  conceive 
atmospheres  and  geographical  arrangements  provided  on  other  planets,  which, 
combined  with  the  peculiar  intensity  of  solar  light  and  heat,  shall  produce 
a  result  which  will  fix  the  general  temperature  of  any  of  the  planets  within  the 
same  limits  that  restrain  it  on  the  surface  of  the  earth. 


NEPTUNE. 

This  is  the  most  remote  and  the  latest  discovered  of  all  the  large  planets. 
The  extraordinary  circumstances  attending  its  discovery  have  given  to  this  planet 
a  special  interest.  After  the  discovery  of  Uranus,  efforts  were  made  to  reduce 
its  motions  to  the  known  laws  of  gravitation,  but  they  were  found  to  be  very 
irregular,  and  seemed  to  be  under  some  unknown  influence.  Many  were  dis- 
posed to  attribute  these  irregularities  to  a  relaxation  of  the  rigorous  laws  of  gra- 
vitation in  those  distant  regions  of  space,  while  others  conceived  the  possible 
existence  of  a  remote  undiscovered  planet,  whose  attraction  drew  Uranus  out 
of  its  regular  orbit. 

Leverrier,  an  astronomer  of  Paris,  determined  to  investigate  these  irregular- 
ities, and,  if  possible,  discover  the  unknown  planet  which  caused  them.  He 
first  calculated  the  disturbing  influence  of  all  the  known  bodies  in  the  solar 
system.  This  did  not  account  for  all  the  deviations  of  Uranus.  He  therefore 
pursued  his  investigations ;  calculated  the  distance,  mass,  inclination  and  revo- 
lution of  the  unseen  planet,  and  on  the  31st  of  August,  1846,  read  a  memoir 
of  the  results  before  the  Academy  of  Sciences ;  even  pointing  out  the  place  in 
the  heavens  where  the  strange  planet  would  probably  appear.  This  wonderful 
account  excited  the  greatest  interest  among  astronomers,  yet,  such  was  the 
difficulty  of  the  problem,  that  few  could  believe  the  prediction  These  misgiv- 
ings were  soon  dissipated.  On  the  1st  of  September,  Leverrier  wrote  to  Dr. 
Galle  of  Berlin,  asking  him  to  direct  his  telescope  to  that  point  in  the  heavens 
where  he  supposed  it  to  be.  On  the  very  first  evening  of  examination,  Dr. 
Galle  discovered  the  long-sought  planet  within  one  degree  of  the  place  predicted. 

Mr.  Adams,  of  Cambridge,  England,  had  discussed  the  same  problem,  and 
had  reached  results  very  near  those  of  Leverrier.  The  new  planet  was  watched 
by  astronomers  to  determine  if  its  elements  agreed  with  the  prediction.  As  it 
moved  extremely  slow,  this  would  have  required  a  long  series  of  years,  but  for 
a  remarkable  discovery  by  Mr.  S.  C.  Walker,  at  Washington,  D.  C.  He 
traced  its  orbit  backward,  and  found  the  planet  marked  twice  as  a  star  in  the 
catalogue  of  Lelande,  as  far  back  as  1795,  which  gave  sufficient  data  to  com- 
pute its  entire  orbit.  . 

The  mean  distance  of  Neptune  from  the.  Sun  is  2,862,457,000  miles.     The 
eccentricity    of  its    orbit   is   comparatively   small,   49,940,000.     It  revolves 
around  the  Sun  in  60,126f  days.     Its  orbi't  is  inclined  to  the  ecliptic.  1°  47'.   < 
Neptune  is  31,000  miles  "in  diameter.     A  satellite  has  been  discovered,  and   J 


256  THE   MAJOR   PLANETS. 


there  may  be  several  in  attendance  upon  it.  From  irregularities  in  its  telesco- 
pic appearance,  some  astronomers  have  supposed  it  to  be  surrounded  by  a  ring 
similar  to  Saturn's.  Owing  to  the  immense  distance  of  this  new  world  many 
of  its  peculiarities  must  remain  unknown  to  us.  if  is  invisible  to  the  naked  . 
eye,  and  has  only  a  small  diameter  seen  through  the  largest  telescope,  being  * 
equal  in  brightness  to  a  star  of  the  eighth  magnitude.  Four  or  five  generations 
of  mankind  pass  away  during  the  long  period  of  its  revolution,  equal  to  nearly 
165  of  our  years.  The  distance  of  Neptune  being  about  thirty  times  greater 
than  that  of  the  Earth  from  the  Sun,  it  follows  that  the  apparent  diameter  of 
the  Sun,  seen  from  that  remote  world,  is  only  ^  of  the  diameter  seen  by  us, 
or,  as  the  Sun  appears  30'  wide  to  us,  it  must  appear  only  1'  wide  from  Nep- 
tune, and  consequently  the  amount  of  light  and  heat  must  be  about  nine-hun- 
dred times  less  than  to  us.  Suppose  the  smaller  of  the  two  circles  representing 
the  Sun  on  page  254  to  be  reduced  one-third  in  diameter,  which  would  make 
it  2 ^  times  less  in  area ;  then  its  contrast  with  the  larger  circle  will  show  the 
comparative  degree  of  light  and  heat  at  Neptune  and  the  Earth.  Its  light  is, 
nevertheless,  equal  to  that  of  more  than  three  hundred  and  thirty  full  moons, 
and  the  physical  arrangements  of  the  planet  may  be  calculated  to  greatly 
modify  it. 


^s^^r*^*^* 


REFLECTION    OF    LIGHT. 


Ray  of  Light. — Pencil  of  Light. — Reflection. — Irregular  Reflection. — Regular  Reflection. — Different 
Powers  of  Reflection  in  different  Bodies. — Reflection  at  plane  Surfaces. — Its  Laws. — Image  of  an 
Object  in  a  plane  Reflector. — Rejection  of  curved  Surfaces. — Concave  Reflectors. — Convex  Re- 
flectors.— Images  in  spherical  Reflectors. — Illusion  of  the  air-drawn  Dagger. — Effects  of  common- 
Looking-Glasses  analyzed. — A  flattering  Glass  explained. — Metallic  Specula. — Reflection  in  Li- 
quids.— Image  of  the  Banks  of  a  Lake  or  River. 


IT 


REFLECTION  OF  LIGHT. 


REFLECTION   OF    LIGHT. 


THE  physical  theories  by  which  the  phenomena  connected  with  the  propa- 
gation of  light  are  explained,  have  been  given  with  some  details  on  another 
occasion.  We  shall  now  notice  some  of  the  more  simple  and  elementary  laws 
of  optics,  which  must  stand  undisturbed,  whatever  theory  of  light  may  be  adopted. 

Whether  light  consists  of  undulations,  or  of  corpuscles  of  matter,  sui  generis, 
it  is  invariably  propagated  in  straight  lines  so  long  as  it  passes  through  the 
same  medium  ;  the  straight  line  along  which  the  light  holds  its  course  is  called 
a  ray  of  light,  and  any  collection  of  such  lines  of  definite  thickness  is  called  a 
pencil  of  light. 

If  the  rays  composing  the  pencil  be  parallel  to  each  other,  the  pencil  is 
called  a  parallel  pencil ;  if  the  rays  intersect  each  other  at  a  point,  the  pencil 
is  said  to  diverge  from  or  converge  to  that  point  according  to  the  direction  in 
which  the  light  is  conceived  to  move,  and  the  pencil  is  accordingly  called  a 
converging  or  diverging  pencil. 

If  rays  of  light,  after  passing  in  straight  lines  through  any  uniform  medium,  en- 
counter the  boundary  or  surface  of  another  medium  of  a  different  kind,  they  will 
either  turn  back  and  take  other  directions  in  the  medium  from  which  they  came, 
or  they  will  enter  the  new  medium,  and  will  in  general  take  new  directions  in 
it.  In  the  former  case  the  second  medium  is  said  to  be  opaque,  and  the  rays 
are  said  to  be  reflected  from  its  surface  ;  in  the  latter  case  it  is  said  to  be 
transparent,  and  the  rays  are  sakjl  to  be  refracted  by  it. 

Reflection  and  refraction  are  then  two  very  important  effects  to  which  light 
•is  subject,  and  it  will  be  both  interesting  and  profitable  briefly  to  notice  the  lead- 
ing principles  that  govern  these  phenomena. 

REFLECTION    OF    LIGHT. 

The  surfaces  of  opaque  bodies  reflect  the  light  incident  upon  them  in 
various  ways,  and  produce  a  corresponding  variety  of  effects  thereby  on  the 
sense  of  sight. 


REFLECTION  OF  LIGHT. 


All  ordinary  surfaces  are  more  or  less  rough.  The  light  which  falls  upon 
them  is  irregularly  reflected  by  them  ;  each  point  upon  them  being  illuminated, 
disperses  the  light  which  strikes  upon  it  in  every  direction  around  it,  and  it 
is  thus  that  the  point  itself  becomes  visible  to  an  eye  placed  anywhere  within 
view  of  it.  The  surfaces  of  bodies  in  general  are  by  this  means  seen  from 
every  quarter  around. 

But  as  the  light  of  the  sun  is  of  one  uniform  color  and  quality,  it  will  be 
asked  how  it  happens  that  the  surfaces  of  different  bodies  and  different  parts 
of  the  surface  of  the  same  body  produce  different  effects  upon  vision,  appear- 
ing to  have  a  variety  of  colors  and  tints  of  colors.  If  they  reflect  to  the  eye 
no  light  except  that  which  falls  upon  them,  and  if  that  which  falls  upon  them 
be  all  of  a  uniform  quality,  how,  it  may  be  asked,  does  it  happen  that  the 
light  reflected  by  different  surfaces  impresses  the  eye  with  the  perception  of 
different  colors  ?  In  answer  to  this  it  is  necessary  to  explain  that  although 
the  light  of  the  sun  is,  in  a  certain  sense,  of  a  uniform  quality  and  color,  it  is 
nevertheless  not  simple  and  homogeneous  ;  it  is,  in  fact,  a  compound  principle, 
produced  by  the  mixture  of  lights  of  different  colors  in  different  proportions. 
It  is  this  mixture  which  produces  the  white  light  of  the  sun. 

Now,  the  surfaces  of  opaque  bodies  are  endowed  with  various  properties  of 
reflecting  light.  Some  possess  the  virtue  of  reflecting  light,  of  one  color,  while 
they  absorb  or  extinguish  light  of  another.  One,  for  example,  will  have  a 
strong  power  of  reflecting  red  light,  but  will  be  altogether  incapable  of  reflect- 
ing blue  light ;  in  short,  various  surfaces  have  infinitely  various  powers  of 
reflecting  lights  of  different  colors. 

Why,  then,  does  one  opaque  object  appear  to  the  eye  red,  while  another 
appears  blue  ?  Because  in  the  compound  light  of  the  sun,  which  equally  falls 
on  both  of  these  objects,  there  is  contained  both  red  and  blue  light ;  the  sur- 
face of  the  object  which  appears  red  absorbs  or  extinguishes  all  the  elements 
of  the  solar  light  except  the  red  rays  which  it  reflects  ;  and  the  object,  which 
appears  blue,  on  the  other  hand,  absorbs  all  the  elements  of  the  solar  light  ex- 
cept the  blue  rays,  which  alone  are  reflected  by  it. 

Thus  it  appears  that  all  objects,  Avhether  natural  or  artificial,  derive  their 
peculiar  tints  of  color  from  the  property  which  they  possess  of  decomposing 
solar  light.  Such  elementary  colors  as  they  have  the  power  of  reflecting  blend- 
ed together  produce  the  peculiar  tints  which  characterize  them,  the  other  con- 
stituents of  the  solar  light  being  stopped. 

But  besides  the  colors  presented  by  visible  objects,  they  exhibit  various  de- 
grees of  illumination,  or,  what  is  familiarly  called,  various  degrees  of  light  and 
shade.  Ttois  arises  from  the  more  or  less  favorable  position  which  different 
parts  of  their  surfaces  have  with  respect  to  the  light  which  falls  upon  them,  and 
it  is  by  this  means  that  the  form  and  shape  of  bodies  are  perceivable  by  the 
eye.  ' 

Buf  if  the  surface  of  an  opaque  body,  instead  of  being  more  or  less  rough, 
so  as  to  render  each  of  its  points  separately  a  centre  of  reflected  light,  could 
be  rendered  perfectly  smooth  and  polished,  then  the  light  would  not  be  re- 
flected from  it  in  the  manner  now  described.  The  various  points  upon  it  would 
not  then  become  centres  from  which  light  would  be  dispersed  in  every  direc- 
tion ;  on  the  contrary,  the  rays  of  light  falling  on  such  a  surface  would  be  re- 
flected by  peculiar  laws. 

REFLECTION  AT  PLANE  SURFACES. 

Let  us  suppose  that  A  B,fig.  l,is  such  a  surface,  and  that  a  ray  of  light  proceed- 
ing from  the  sun  at  S  illuminates  a  point  I,  placed  upon  this  surface.  In  the 


REFLECTION  OF  LIGHT. 


former  case,  the  light  striking  at  I  or  a  part  of  it,  would  be  dispersed  in  every 
direction  above  the  surface  A  B,  so  as  to  render  the  point  I  visible  to  an  eye 
placed  anywhere  in  the  space  above  A  B.  But  such  is  not  the  case  when  the 
surface  A  B  is  perfectly  smooth  and  polished.  In  that  case,  the  light  proceed- 
ing from  S  and  striking  on  I,  will  be  reflected  only  in  one  direction,  viz.,  as  if 
jt  came  from  a  point  D  as  far  behind  A  B  as  S  is  before  it.  Thus  if  we  draw 
S  A  at  right  angles  to  A  B,  and  continue  it  until  A  D  is  equal  to  A  S,  then  the 
light  will  be  reflected  along  I  O  as  if  it  came  from  D. 

As  a  consequence  of  this,  it  follows  that  the  incident  light  S  I  and  the  re- 
flected light  I  O  make  equal  angles  with  the  reflecting  surface  A  B. 

This  is  a  universal  and  very  important  law  of  optics,  and  is  usually  ex- 
pressed thus  : — 

When  a  ray  of  light  falls  on  a  perfectly  polished,  reflecting  surface,  it  is 
so  reflected  that  the  angle  of  reflection  shall  be  equal  to  the  angle  of  incidence. 
In  the  diagram,  A  I  S  is  the  angle  of  incidence,  and  0  I  B  is  the  angle  of  re- 
flection. 

But  if  a  surface  such  as  A  B,  fig.  2,  be  exposed  to  a  source  of  light,  it  is  not  one 

Fig.  2. 


point,  but  every  point  of  it.  that  will  be  illuminated.  Rays  in  fact  will  diverge 
from  S,  and  will  strike  upon  all  points  of  A  B.  From  what  has  been  already 
stated,  it  will  be  apparent  that,  after  reflection,  they  will  each  of  them  proceed 
as  if  they  had  originally  diverged  from  D.  The  effect,  therefore,  ol  the  re- 
flecting surface  A  B  will  be  to  convert  a  pencil  of  rays,  which  diverges  from  ' 


262 


REFLECTION  OF  LIGHT. 


Such  is  the  simple  explanation  of  the  effects  of  common  plane  mirrors. 
If  we  stand  before  a  mirror,  each  point  of  our  persons  emits  light  of  a  peculiar 
color,  which,  diverging,  falls  on  the  surface  of  the  mirror,  and  is  reflected  by 
that  surface  as  if  it  came  from  a  person  exactly  resembling  ourselves  in  form 
and  color,  facing  us,  and  standing  at  the  same  distance  behind  the  mirror  that 
we  are  before  it 

The  form  of  an  object  thus  rendered  optically  visible  by  a  mirror  is  techni- 
cally called  its  image. 

It  is  evident,  from  what  has  been  stated,  that  if  I  stand  before  a  mirror  and 
see  my  person  in  it,  the  image  of  my  right  arm  being  immediately  opposite  to 
that  arm  and  behind  the  mirror,  will  be  the  left  arm  of  the  image ;  and  in  like 
manner,  the  image  of  my  left  arm  will  be  the  right  arm  of  the  image.  It  is  the 
same  with  the  images  of  all  objects  formed  by  plane  reflectors  :  right  becomes 
left,  and  left  right ;  in  other  words,  the  image  is  reversed  laterally. 

In  some  cases,  as  will  be  seen  hereafter,  optical  images  are  not  merely  re- 
versed laterally,  but  inverted  vertically,  so  as  to  be  seen  upside  down.  This 
is,  however,  not  the  case  with  plane  mirrors ;  for  the  head  and  the  feet  of  the 
image  being  on  the  other  side  of  the  mirror  merely  at  the  same  distance  be- 
hind it  as  the  head  and  the  feet  of  the  object  are  before  it,  the  head  will  be  at 
the  top  and  the  feet  at  the  bottom  of  the  image.  Objects  are  therefore  seen 
erect  in  plane  mirrors. 

In  cases  where  the  arrangement  from  right  to  left  is  essential,  the  images 
produced  by  plane  mirrors  become  defective  for  the  ordinary  purposes  of  exhi- 


the  point  F,  into  another  which  will  have  the  effect  of  diverging  from  the 
point  D. 

Now  let  us  suppose  a  visible  object,  such  as  S  S',  fig.  3,  placed  in  front  of  a 
plane  mirror,  such  as  A  B.     Each  point  of  that  object  will  be  a  separate  source 
of  light  of  the  peculiar  tint  which  may  characterize  the  object.    The  light  which  I 
proceeds  from  each  of  these  points  falling  on  the  surface  A  B,  will  be  reflected  £ 
as  if  it  came  from  a  corresponding  point  behind  the  mirror ;  and  an  eye  placed  ( 
anywhere  before  the  mirror,  as  at  O,  will  receive  that  light  exactly  as  it  would 
receive  it  if  the  body  which  is  at  S  S'  were  really  at  D  D'.     Consequently, 
the  eye  will  see  an  object  at  D  D'  exactly  similar  to  S  S'. 


Fig.  3. 


REFLECTION  OF  LIGHT. 


bition.     Thus  a  printed  word,  or  an  inscription,  when  held  before  a  mirror,  will 
be  altogether  deranged  ;  it  will  have  the  same  appearance  to  the  eye  as  the  ! 
types  have  from  which  it  is  printed. 

REFLECTION  AT  CURVED  SURFACES. 

I 

Whatever  be  the  form  of  a  curved  surface,  it  may  be  conceived  to  consist  of 
separate  parts  of  such  small  dimensions  that  each  of  them  may  be  considered 
as  a  portion  of  a  sphere  or  globe  ;  and  therefore  if  the  principles  which  regu- 
late the  reflection  of  light  from  a  spherical  surface  be  known,  the  effects  of 
curved  surfaces  of  other  forms  maybe  easily  investigated.  We  shall  therefore 
confine  our  observations  here  to  the  reflection  of  light  from  perfectly  smooth 
spherical  surfaces. 

CONCAVE    REFLECTORS. 

LetM  A  M',  fig.  4,  represent  a  portion  of  a  concave  spherical  reflecting  surface, 
and  let  S  represent  a  point  from  which  light  diverges  ;  let  C  be  the  centre  of 
the  spherical  surface.  A  ray  of  light  falling  from  S  upon  the  point  I,  will  be 
reflected  in  the  direction  I  R,  so  as  to  make  the  angle  RIG  equal  to  the  angle 
SIC.  If  the  point  S  be  very  near  to  or  in  the  line*  A  C,  and  at  a  very  great 
distance  from  the  reflector,  then  the  point  R  will  be  at  the  middle  of  the  dis- 
tance C  A,  so  that  it  will  divide  the  radius  C  A  into  two  equal  parts. 

Fig.  4. 


If  the  point  S  be  in  any  object,  the  corresponding  point  R  will  be  its  image, 
and  in  like  manner  the  images  of  all  the  other  points  will  be  formed. 

When  a  concave  speculum  is  presented  to  a  very  distant  object,  an  image  of 
that  object  will  be  formed  in  front  of  the  speculum,  and  at  a  distance  from  it 
equal  to  half  its  radius.  This  image,  however,  will  be  inverted. 

If  the  object  be  not  at  a  very  great  distance  from  the  reflector,  its  image 
will  be  formed  at  a  point  farther  from  the  surface  than  half  the  radius,  and  will 
still  be  inverted. 

In  a  convex  reflecting  surface,  the  image  of  an  object  placed  in  front  will  be 
formed  behind  the  reflecting  surface  ;  as  in  the  case  of  a  plane  mirror,  it  will 
be  erect  and  smaller  than  the  object. 

The  positions  assumed  by  the  images  of  objects  formed  by  concave  and 
convex  reflectors,  have  rendered  this  species  of  mirrors  amusing  means  of  oc- 
casional optical  exhibition. 

If  an  object  be  placed  in  front  of  a  convex  mirror,  its  image  will  be  formed 
behind  the  mirror  at  a  distance  something  less  than  half  the  radius  of  the  con- 
vexity. This  image  will  be  always  erect,  but  will  be  smaller  than  the  object ; 
and  the  more  distant  the  object  is  from  the  mirror,  the  smaller  will  be  the 
image. 

Whatever  be  the  form  of  the  object,  the  image  will  have  a  tendency  to  a 
convex  form,  and  consequently  such  mirrors  always  produce  distortion. 


REFLECTION  OF  LIGHT. 


If  an  object  be  placed  before  a  concave  mirror  at  a  distance  from  it  greater 
than  that  of  the  geometric  centre  of  its  curvature,  an  image  wiA  be  formed  of 
this  object  in  front  of  the  mirror  at  a  distance  from  its  surface  greater  than  half 
its  radius. 

This  image  will  be  inverted,  and  will  be  less  than  the  object ;  as  the  object 
approaches  the  centre  of  the  curvature  of  the  mirror,  the  image  will  also  ap- 
proach that  point,  and  thus  the  object  and  image  will  approach  each  other ;  the 
image  will  at  the  same  time  be  increased  in  magnitude.  If  the  object  be 
placed  within  the  centre  of  curvature  of  the  mirror,  but  farther  from  its  surface 
than  half  its  radius,  a  magnified  image  will  be  formed  at  a  distance  more  or 
less  considerable  in  front  of  the  mirror.  Thus,  let  us  suppose  that  a  mirror 
formed  with  a  curvature  having  a  radius  of  four  feet,  has  an  object  in  front  of  it 
at  a  distance  of  three  feet  from  its  surface  •  an  image  of  that  object  will  be 
formed  at  six  feet  in  front  of  the  mirror,  and  this  image  will  be  double  the 
height  or  length  of  the  object. 

In  this  mariner,  a  mirror  placed  out  of  sight  of  a  person  may  be  made  to 
throw  the  image  of  an  object  close  to  him  ;  thus  a  dagger  may  be  presented 
to  one's  bosom,  which,  however,  is  literally  an  air-drawn  dagger. 

The  only  form  of  reflecting  surface  which  presents  an  object  in  its  natural 
position  and  proportions  is  the  plane  mimfr  commonly  used  for  domestic  pur- 
poses ;  and  even  this,  as  already  explained,  reverses  the  object  laterally — ma- 
king right  left,  and  left  right.  For  the  purposes,  however,  to  which  it  is  usually 
applied,  this  derangement  does  not  impair  its  utility. 

The  perfection  with  which  a  mirror  presents  the  image  of  an  object  placed 
before  it  depends  upon  its  form  and  material.  It  is,  above  all  things,  essential 
that  its  surface  should  be  perfectly  plain  and  even  ;  any  deficiency  in  this  qual 
ity  will  produce  a  corresponding  distortion  of  the  image.  Cheap  looking- 
glasses  are  often  striated  and  streaked  with  inequalities  and  ridges,  which  render 
them  nearly  useless.  Whatever  be  the  substance  used  to  form  a  mirror,  apart 
only  of  the  light  which  falls  upon  it  will  be  instrumental  in  forming  the  image. 
The  entire  quantity  of  light  which  falls  on  the  mirror  may  be  accounted  for  as 
follows  : — 

1 .  A  part  will  be  regularly  reflected  according  to  the  laws  above  explained 
and  it  is  by  this  part  the  image  will  be  formed. 

2.  Another  part  will  be  irregularly  reflected — that  is  to  say,  it  will  be  scat- 
tered in  every  direction  around  from  every  part  of  the  surface.     It  is  this  por- 
tion of  the  light  which  renders  the  surface  of  the  mirror  visible. 

3.  A  part  will  be  absorbed  upon  the  reflecting  surface  and  lost. 

The  more  highly  polished  and  even  the  reflecting  surface  is,  the  less  will 
be  the  part  irregularly  reflected,  and  the  brighter  will  be  the  image.  The  part 
of  the  light  absorbed  or  stopped  will  depend  on  the  physical  quality  of  the 
matter  of  which  the  reflector  is  formed. 

Since  art  cannot  produce  a  perfect  reflecting  surface,  there  will  always 
be  a  portion  of  the  incident  light  irregularly  reflected  and  absorbed.  It  follows, 
therefore,  that  light  is  always  lost  in  reflection ;  and  in  the  case  of  plane  mir- 
rors, where  the  magnitude  of  the  image  is  equal  to  that  of  the  object,  the  bright- 
ness of  the  image  must  always  be  less  than  that  of  the  object. 

There  is  no  substance  which  reflects  with  equal  facility  all  tints  of  color. 
It  generally  happens  that  lights  of  one  tint  are  more  absorbed  than  the  lights 
of  another.  Mirrors,  therefore,  will  produce  a  change  more  or  less  according 
to  their  degree  of  imperfection  in  the  tints  which  characterize  the  object  before 
them  ;  in  other  words,  the  color  or  tints  of  the  image  will  not  correspond  ex-  i 
actly  with  those  of  the  object. 

It  is  therefore  a  fact  true   in  science,  although  sometimes  ridiculed,  that  < 


REFLECTION  OP  LIGHT. 


2G5 


different  looking-glasses  will  present  a  more  or  less  agreeable  representation 
of  the  person  who  uses  them,  according  to  the  colors  which  they  may  happen 
to  absorb.  Thus,  if  a  mirror  has  a  tendency  to  absorb  the  red  tints,  it 
will  give  a  pallid  tint  to  the  complexion  ;  whereas,  if  it  absorb  the  blue  tints, 
it  will  throw  a  blush  over  the  appearance,  and  may  be  called  a  flattering 
glass. 

Glass  is  the  most  convenient  material  for  mirrors  intended  for  domestic  use, 
because  it  is  the  cheapest  and  most  durable  ;  but  it  is  far  from  being  the  best. 
Its  defects  will  become  apparent  by  considering  the  mode  in  which  its  effects 
are  produced.  A  coating  of  metallic  foil  is  attached  to  the  hinder  surface  of 
the  glass,  and  by  the  mode  of  its  adhesion  a  smooth  metallic  surface  is  thus 
formed  under  or  behind  the  glass.  It  is  this  surface,  and  not  the  front  of  the 
glass,  which  is  the  real  mirror :  it  is  by  it  that  the  images  of  objects  in  front 
of  the  looking-glass  are  produced.  The  light  has  to  pass  through  the 
thickness  of  the  glass  to  reach  this  surface,  and  after  being  reflected  by  it, 
has  again  to  pass  through  its  thickness  in  order  to  reach  the  eye  and  pro- 
duce a  perception  of  the  image.  There  are  here  three  successive  stages  in 
which  light  is  lost.  A  part  only  of  the  light  which  strikes  upon  the  front 
surface  of  the  glass  penetrates  it,  and  a  part  of  what  does  penetrate  it  is  lost 
upon  the  hinder  surface  ;  and  again,  after  reflection,  in  issuing  through  the 
front  surface,  another  portion  is  lost. 

But  the  loss  of  light  is  not  the  only  defect :  in  passing  through  the  glass, 
partial  absorption  of  color  takes  place  ;  and  hence,  as  has  been  already  stated, 
the  tints  of  the  image  will  beVdifferent  from  those  of  the  object. 

A  portion  of  the  light  which  falls  on  the  front  surface  of  the  glass  is  regu- 
larly reflected,  and  produces  a  faint  image  of  the  object,  which,  by  careful 
observation,  may  be  easily  distinguished  a  little  in  front  of  the  stronger  image 
produced  by  the  silvered  surface.  The  distance  of  this  faint  image  in  front  of 
the  other  will  be  equal  to  the  thickness  of  the  glass. 

It  is  evident,  from  what  has  been  just  observed,  that  the  thinner  the  glass  is, 
the  better  will  be  the  mirror. 

The  defects  which  have  been  just  explained  have  rendered  glass  reflectors 
inapplicable  to  telescopes  or  any  of  the  class  of  superior  optical  instruments 
used  for  scientific  purposes.  In  these  instruments  metallic  reflectors  alone 
are  used.  An  alloy  of  metals  is  selected  for  this  purpose  as  white  as  possible 
in  color,  and  susceptible  of  a  high  polish.  A  very  accurate  figure  is  imparted 
to  it  and  a  very  perfect  polish  by  various  processes  known  in  the  arts.  Al- 
though with  such  reflectors  incomparably  less  light  is  lost  than  in  common 
looking-glasses,  still  a  much  greater  loss  of  light  takes  place  than  in  trans- 
mission through  transparent  media  ;  hence  the  received  maxim  in  optics,  that 
more  light  is  lost  in  reflection  than  in  refraction.  Liquid  surfaces  afford  in 
general,  when  at  rest,  good  plane  reflectors.  If  the  liquid  be  opaque,  the 
reflection  is  very  perfect.  This  will  be  rendered  apparent  by  pouring  some 
clear  quicksilver  on  a  plate  ;  to  exhibit  this  effect,  the  quicksilver  should  be 
strained  through  a  piece  of  chamois  leather :  it  would  otherwise  have  a 
film  upon  it  composed  of  foreign  matter,  which  would  destroy  its  reflecting 
power. 

The  objects  on  the  banks  of  a  calm  river  or  a  tranquil  lake  will  be  seen 
reflected  in  its  surface  ;  but  it  is  worthy  of  notice  that  the  observer  can  only 
see  this  reflection  when  he  looks  very  obliquely  at  the  surface  of  the  water : 
the  reason  of  which  is,  that  the  rays  which  strike  nearly  at  right  angl<-s  to 
the  water  penetrate  it  in  virtue  of  its  transparency.  It  is  only  those  which 
glance  obliquely  on  it  that  are  reflected  ;  just  as  a  stone  which,  thrown  per- 
pendicularly on  the  water,  would  immediately  sink,  will,  if  projected  at  a 


REFLECTION  OF  LIGHT. 


small  angle  with  the  surface,  be  reflected  from  the  water,  leaping  from  point 
to  point  of  the  surface,  and  affording  the  sport  which  boys  call  "  duck  and 
drake." 

The  laws  which  govern  the  refraction  of  light  through  transparent  media 
show  that  when  a  ray  strikes  the  transparent  surface  of  a  medium  more  rare 
than  that  through  which  it  has  passed,  it  cannot  penetrate  that  surface,  but  will 
be  reflected,  unless  its  angle  of  obliquity  exceed  a  certain  magnitude.  This 
mode  of  reflection  is  the  most  perfect  with  which  we  are  acquainted,  and  is 
resorted  to  with  great  advantage  in  some  optical  instruments. 


PROSPECTS   OP   STEAM   NAVIGATION.  267 


PKOSPECTS  OF  STEAM-IAYIGATIOK 


IN  navigating  the  ocean  a  steam-vessel  of  side-wheel  construction  is  exposed 
to  many  inevitable  disadvantages.  Scarcely  an  hour  throughout  its  entire 
voyage  can  the  impelling  power  work  with  full  and  unimpaired  efficacy.  The 
swell  of  the  ocean  is  incessant,  nor  does  it  even  cease  in  the  intervals  of  the 
abatement  of  the  winds.  The  principles  of  this  reasoning  appear  so  evident, 
that  it  would  be  a  slight  upon  the  understanding  to  enlarge  upon  them.  It 
will  be  easily  perceived  that  the  conclusion  is  inevitable,  that  when  steam- 
vessels  of  the  present  form  are  applied  to  ocean-voyages,  a  large  proportion  of 
the  moving  power  must  be  lost. 

Among  persons  who  have  not  devoted  much  time  to  the  investigation  of  this 
question,  it  is  a  favorite  argument  to  urge  the  immense  speed  obtained  by  the 
steam-vessels  working  with  these  propelling-wheels  upon  the  extensive  inland 
waters  of  this  great  continent.  But  there  is  no  analogy  whatever  between  the 
cases.  Let  it  be  remembered  that  the  condition  upon  which  this  extraordinary  * 
efficiency  depends  can  never  be  fulfilled  in  sea-going  steamers.  That  efficien- 
cy depends  essentially  on  the  smooth  and  unruffled  surface  of  the  water  on 
which  the  vessel  moves,  and  the  power  of  the  vessel  to  maintain  itself  in  a 
constantly  perpendicular  position. 

When  these  observations  are  duly  considered,  it  will  be  readily  admitted  that 
the  attainment  of  perfect  efficiency  in  ocean-steamers  with  the  present  propel- 
ling apparatus  is  hopeless. 

But  the  form,  magnitude,  and  position,  of  the  propelling  machinery,  is  far 
from  being  the  only  obstacle  to  the  full  success  of  the  present  steam-vessels 
when  directed  to  the  general  purposes  of  commerce.  The  engines  themselves, 
and  the  boilers,  from  which  the  moving  power  proceeds,  and  the  fuel  by  which 
they  are  worked,  occupy  the  very  centre  of  the  vessel,  and  engross  the  most 
valuable  part  of  the  tonnage.  The  chimney,  which  gives  efficacy  to  ^the  fur- 
naces, is  also  an  unsightly  excrescence,  and  no  inconsiderable  obstruction. 

If  the  present  form  and  structure  of  steam-vessels  be  obnoxious  to  these  many 
serious  objections  when  considered  with  reference  to  the  purposes  of  general 


268 


PROSPECTS  OF  STEAM  NAVIGATION. 


)  commerce,  they  are  still  more  exceptionable  when  considered  with  reference  'o 
the  purposes  of  national  defence.  It  is  undoubtedly  a  great  power  with  whii 
to  invest  a  vessel-of-war,  to  confer  noon  it  the  faculty  of  proceeding  at  will  anu 
immediately,  in  spite  of  the  opposition  of  wind  or  tide,  in  any  direction  which 
may  seem  most  fit  to  its  commander.  Such  a  power  would  surpass  the  wild- 
est dreams  of  the  most  romantic  and  imaginative  naval  commander  of  the  last 
century.  To  confer  upon  the  vessels  of  a  fleet  the  power  immediately  at  the 
bidding  of  the  commander  to  take  any  position  that  may  be  assigned  to  them 
relatively  to  the  enemy,  or  to  run  in  and  out  of  a  hostile  port  at  pleasure,  or  fly 
with  the  rapidity  of  the  wind  past  the  guns  of  formidable  forts  before  giving 
them  time  to  take  effect  upon  them — are  capabilities  which  must  totally  revo- 
lutionize all  the  established  principles  of  naval  tactics.  But  these  powers  at 
present  are  not  conferred  upon  steamships  without  important  qualifications  and 
serious  drawbacks.  The  instruments  and  machinery  from  which  these  pouvrs 
are  immediately  derived  are  unfortunately  exposed  in  such  a  manner  as  to  ren- 
der the  exercise  of  the  powers  themselves  hazardous  in  the  extreme.  It  needs 
no  profound  engineering  knowledge  to  perceive  thnt  the  paddle-wheels  are 
eminently  exposed  to  shot,  which,  taking  effect,  wo  Id  altogether  disable  the 
vessel,  and  leave  her  at  the  mercy  of  the  enemy  ;  >nd  the  chimney  is  even 
more  exposed,  the  destruction  of  which  would  render  .he  vessel  a  prey  to  the 
enemy  within  itself  in  the  shape  of  fire.  Bui  besides  these  most  obvious 
sources  of  exposure  in  vessels  of  the  present  form  intended  as  a  national  de- 
fence, the  engines  and  boilers  themselves,  being  more  or  less  above  the  water- 
line,  are  exposed  so  as  to  be  liable  to  be  disabled  by  shot. 

Such  are  a  few  of  the  many  defects  incidental  to  the  present  form  of  steam- 
ships as  applied  to  the  purposes  of  national  defence. 

When  long  ocean-voyages  are  contemplated,  such  as  those  between  New 
York  and  the  ports  of  England,  there  is  another  serious  obstacle,  which  is  es- 
pecially felt  in  the  westward  trip,  because  of  the  prevalence  of  adverse  winds. 
When  the  vessel  starts  on  its  lo;ig  voyage,  it  is  necessarily  laden  with  a  large 
stock  of  fuel,  which  is  calculated  to  meet,  not  merely  the  average  exigencies 
of  the  voyage,  but  the  utmost  extremity  of  adverse  circumstances  of  wind  and 
weather  to  which  it  can  by  possibility  be  exposed.  This  fuel  is  gradually 
consumed  upon  the  voyage  ;  the  vessel  is  proportionally  lightened,  and  its  im- 
mersion diminished.  If  its  trim  be  so  regulated  that  the  immersion  of  its 

(  wheels  at  starting  be  such  as  to  give  them  complete  efficiency,  they  may,  be- 

)  fore  the  end  of  the  voyage,  be  almost  if  not  altogether  raised  out  of  the  water. 

\  If,  on  the  other  hand,  the  efficiency  of  propulsion  in  the  latter  part  of  the 
voyage  be  aimed  at,  they  must  have  such  a  depth  at  its  commencement,  as  to 
impair  in  a  serious  degree  their  propelling  effect,  and  to  rob  the  vessel  of  its 
proper  speed.  Under  such  circumstances,  there  is  no  expedient  left  but  com- 
promise. The  vessel  must  start  with  too  great  and  arrive  with  too  little  im- 
mersion. There  is  no  alternative,  save  to  abandon  altogether  the  form  and 
structure  of  the  present  machinery',  and  to  awaken  the  inventive  genius  of  the 
age  to  supply  other  mechanical  expedients,  which  shall  not  be  obnoxious  to 
these  objections. 

Although  no  one  who  has  lived  as  long  and  witnessed  so  many  disappointed 
hopes  and  fallacious   anticipations  in  the  progress  of  improvement  as  I  i. 
will  be  very  forward  to  commit  themselves  as  to  the  results  of  projects  which 
still  exist  in    a  state  but  partially  tested  by  experience,  I  cannot  refrain  from 
giving  expression  to  a  strong  hope  and  confident  anticipation  that  the  cpc 
at  hand  which  will  witness   a  great   advance   in   ocean-navigation,  and  a 
conferred  by  science  upon  the  arts  not  equalled  since  the   invention  o: 
steamboat  and  the  safety-lamp. 


PROSPECTS  OF  STEAM-NAVIGATION. 


269 


It  is  generally  known  that  within  the  last  seven  years  a  form  of  sub-aqueous 
propeller  placed  at  the  stern  of  the  vessel  as  a  substitute  for  the  paddle-wheels, 
lias  been  invented  and  patented  by  Captain  Ericsson.  This  contrivance  has 
now  been  in  practical  operation  for  so  long  a  time,  and  in  so  great  a  number 
and  variety  of  vessels,  that  we  must  cease  to  regard  it  as  an  experiment.  Its- 
efficiency  has  been  tested  on  an  extensive  scale.  The  propelling-wheel  is 
fixed  upon  an  axis  which  is  placed  parallel  to  the  keel,  and  which  issues  from  / 
the  stern  of  the  vessel ;  the  wheel  therefore  revolves  with  its  face  stern  ward.  > 
In  wheels  of  this  form  and  construction,  the  principle  of  action  is  in  general  ' 
similar  to  that  of  the  common  smoke-jack.  The  propelling  surfaces  have  been 
usually  placed  at  an  oblique  angle  to  the  course  of  the  vessel,  and  have  ex- 
tended from  the  axle  or  nave  to  the  outer  edge  of  the  wheel.  Now,  it  will  be 
apparent,  even  to  those  who  are  least  familiar  with  mechanical  inquiries,  that, 
those  parts  of  the  blades  which  are  near  to  the  nave  moving  with  the  least  ve- 
locity, are  the  most  inefficient  for  propulsion  ;  arid  were  it  worth  while,  it  would 
be  no  very  difficult  matter  to  demonstrate  that  they  are  often  an  absolute  ob- 
struction. The  outer  ends  of  the  blades,  moving  with  greater  velocity,  act 
with  proportionately  greater  efficiency. 

These  circumstances  led  Captain  Ericsson  to  construct  his  wheel  in  such  a 
manner  as  to  remove  altogether  those  parts  of  the  blades  nearest  to  the  nave, 
and  which  were  inefficient  for  propulsion,  retaining  only  those  which  were  most 
remote  and  most  effective.  This  he  accomplished  by  forming  a  hoop  of  metal 
concentric  with  the  nave,  and  connected  with  it  by  two  or  more  spokes,  to 
enable  which  to  pass  through  the  water  with  the  le,ast  possible  resistance,  he 
gave  them  a  twisted  or  spiral  form,  regulated  with  such  mathematical  precis- 
ion, that,  by  the  progressive  motion  of  the  vessel,  combined  with  their  own 
rotation,  they  must  always  encounter  the  water  edgewise. 

Drawings  of  this  propeller,  as  applied  to  the  Princeton,  are  given  in  figs. 
1,  2,  and  3.  A  section  parallel  to  the  face  of  the  wheel  is  given  in  fig.  1  ;  a 
horizontal  view  is  shown  in  fig.  2  ;  and  a  section  of  the  axle  and  hoop  in  fig.  3. 
The  nave  in  which  the  axle  is  inserted  is  at  N,  from  which  proceed  six  twist- 
ed spokes  R  R,  attached  to  and  supporting  the  hoop  H  H  H,  bolted  on  to  which 
are  six  spiral  propelling  surfaces  P  P,  &c.  The  axis  inserted  in  the  nave  is 
represented  at  A,  fig.  2,  where  the  obliquity  and  spiral  form  of  the  surfaces 
are  also  shown,  as  well  as  the  manner  in  which  they  are  bolted  on  the  hoop. 

In  order  to  give  to  this  wheel  all  the  possible  strength,  six  spiral  spokes 
were  supplied,  one  for  each  propelling  blade.  The  material  of  the  wheel  is 
composition-metal,  which  resists  oxydation. 

A  propeller  has  been  also  supplied  by  Captain  Ericsson  for  the  United  States 
revenue-cutters  Legare  and  Jefferson,  represented  in  figs.  4, 5,  and  6.  The  corre- 
sponding parts  are  represented  in  the  same  manner  as  in  the  former  diagrams, 
and  are  marked  by  the  same  letters.  In  this  wheel,  the  same  strength  not 
being  necessary,  there  are  only  four  twisted  arms  supporting  the  hoop,  and  the 
material  of  the  propeller  is  wrought  iron. 

Stern-propellers  have  been  invented  and  patented  of  very  various  forms, 
which,  however,  all  agree  in  certain  properties.  When  they  are  totally  sub- 
merged, with  the  face  of  the  wheel  presented  backward,  their  revolution  causes 
a  current  of  water  to  be  projected  backward  from  the  stern,  the  reaction  of 
which  is  in  fact  the  moving  power.  This  effect  is  produced  in  all  of  them  by 
placing  the  surfaces  of  the  radiating  arms  or  plates  i^a  position  inclined  to 
the  course  of  the  vessel.  If  these  surfaces  were  placed  at  right  angles  to  the 
keei,  the  revolution  of  the  wheel  would  make  them  cut  the  water  edgewise,  and 
I-M  reaction  wou:a  be  obtained.  If,  on  the  contrary,  they  were  parallel  to  the  ( 
keel,  with  taeir  edges  in  the  direction  of  the  vessel's  course,  they  would  drive  } 


PROSPECTS  OF  STEAM-NAVIGATION 


PROSPECTS  OF  STEAM-NAVIGATION. 


PROSPECTS  OF  STEAM-NAVIGATION. 


the  water  everywhere  at  right  angles  to  that  course,  and  no  backward  ci.rrent 
would  be  produced  ;  but  by  giving  them  a  position  between  these  two  extn 
— that  is  to  say,  inclined  at  some  oblique  angle  to  the  course  of  the  vessel — 
the  revolution  of  the  wheel  will  cause  them  to  exert  a  certain  portion  of  their 
force  on  the  water  in  producing  a  backward  current :  and  that  particular  obli- 
quity should  be  given  to  them  which  will  make  that  backward  current  most 
effective. 

The  calculation  of  this  obliquity  requires  the  application  of  the  prim-: 
of  mathematical  science,  and  admits  of  a  clear  and  definite  solution,  it  is 
found,  however,  that  the  most  effective  obliquity  for  the  propelling  surface  is 
not  the  same  for  all  distances  from  the  centre  of  the  wheel,  and  consequently 
if  the  best  possible  form  be  given  to  the  propelling  blades,  they  must  be  shaped 
according  to  a  certain  spiral  to  be  determined  by  conditions  depending  upon  a 
variety  of  circumstances  connected  with  the  propeller  and  the  vessel  itself. 

Some  projectors,  ignorant  of  these  scientific  principles,  have  .constructed 
these  propellers  with  plane  surfaces,  without  the  spiral  form.  Such  is  the 
patented  contrivance  called  Loper's  propeller.  They  are  consequently  and 
most  obviously  inefficient. 

But  besides  the  proper  adjustment  of  the  obliquity  of  the  propelling  surfaces, 
the  experience  of  Captain  Ericsson  soon  proved  that  the  parts  of  the  blades 
near  the  centre  of  the  wheel  were  not  only  inefficient  for  propulsion,  but  formed 
an  impediment  to  the  progress  of  the  vessel.  It  was  for  this  reason,  among 
others,  that  he  cut  away  those  parts  of  the  blades  near  the  centre,  retaining 
only  the  more  remote  portions,  and  supported  these  by  bolting  them  on  to  the 
hoop  already  described. 

Such  being  the  general  character  of  this  propelling  instrument,  it  will  be  ap- 
parent that  in  every  position  which  it  can  assume  in  the  water,  it  must  pro- 
duce nearly  the  same  propelling  effect.  However  the  ship  may  pitch  or  roll, 
or  however  unequal  the  surface  of  the  sea  may  be,  it  will  always  produce  the 
backward  current,  without  any  great  variation  of  effect. 

The  circumstances  which  prevent  the  co-operation  of  the  power  of  steam 
with  that  of  the  sails  in  the  steam-vessels  now  in  use,  will  not  operate  with  a 
propeller  of  this  form,  inasmuch  as  its  efficacy  will  be  altogether  independent 
of  the  careening  of  the  ship  ;  but  although  this  defect  is  removed,  the  sub- 
merged stern-propellers  are  still  subject  to  objections  from  which  even  the 
common  paddle-wheels  are  free.  Being  permanently  submerged  and  'liable  to 
accidental  fracture  and  derangement  from  various  causes,  they  are  inacces- 
sible, and  cannot  be  repaired  at  sea ;  but  besides  this,  when  the  object  in  view 
is  to  take  full  advantage  of  the  power  of  the  sails,  that  of  the  machinery  being 
suspended,  the  submerged  propeller  becomes  an  obstruction,  more  or  less  con- 
siderable, to  the  progress  of  the  vessel. 

An  invention,  however,  recently  patented  by  Captain  Ericsson,  has  finally 
removed  this  difficulty,  and  placed  it  in  the  power  of  the  commander  at  any  time 
within  the  space  of  five  minutes  to  raise  the  propeller  out  of  the  water,  or  to 
submerge  it,  so  as  to  convert  for  all  intents  and  purposes  a  steamer  into  a  sail- 
ing-vessel, or  a  sailing-vessel  into  a  steamer,  as  he  may  see  fit. 

The  shaft  on  which  the  propelling-wheel  is  fixed  is  provided  with  a  simple 
mechanism  within  the  vessel  by  which  it  may  be  easily  at  any  time  drawn  out 
of  the  nave  of  the  wheel.  Tne  wheel  itself  is  sustained  by  a  powerful  vertical  arm, 
the  upper  end  of  which  is  attached  to  a  strong  axis,  which  enters  the  vessel 
parallel  to  the  main  axis  of  the  wheel  and  above  the  summit  of  the  wheel.  To 
this  axis  within  the  ve  ssel  is  attached  a  piece  of  mechanism  by  which  it  may 
be  turned  through  hah  a  revolution  by  the  power  of  two  men  with  such  force 
that  the  propeller  will  be  made  to  perform  half  a  revolution  round  the  upper 


PROSPECTS  OF  STEAM-NAVIGATION. 


273 


end  of  the  vertical  arm  which  supports  it,  by  which  that  arm  will  be  presented 
upward  instead  of  downward.  The  wheel,  therefore,  instead  of  being  sub- 
merged, will  be  supported  at  the  stern  of  the  vessel  at  the  place  where  a  boat 
is  usually  suspended. 

The  vessel  will  thus  be  free  from  all  uosiruction  in  passing  through  the 
water,  and  will  acquire  all  the  efficiency  which  any  mere  sailing-vessel  can 
have,  besides  which  the  propeller  is  placed  in  such  a  situation  that  it  may  be 
repaired  if  necessary. 

The  main  shaft  which  drives  the  propeller  when  submerged  is  at  a  depth 
of  seven  or  eight  feet  under  the  lower  deck.  The  cylinders  by  which  it  is 
impelled  are  supported  in  a  slanting  position  on  the  timbers  of  the  vessel, 
their  piston-rods  being  presented  toward  the  crank  on  the  shaft,  which  they 
drive  in  the  usual  manner  by  connecting-rods.  The  boilers  and  the  fuel  occu- 
py the  space  immediately  forward  of  the  cylinders.  The  entire  machinery, 
including  the  boilers  and  fuel,  are  below  the  second  deck  of  the  vessel. 

Such  are  the  general  features  of  the  arrangements  projected  by  Captain 
Ericsson,*  and  proposed  to  be  adopted  in  a  line  of  steam  packet-ships  to  ply 
between  New  York  and  Liverpool.  The  first  of  these  vessels  is  now  in  an 
advanced  state  at  Boston,  and  the  machinery  is  in  progress  in  New  York. 
It  is  expected  that  this  ship  will  make  her  first  voyage  in  August,  1845. 

The  fuel  to  be  used  is  hard  coal,  and  the  furnaces  will  be  ventilated  by 
blowers,  worked  by  the  engine.  There  will  be  no  smoke,  nor  any  need  of 
the  draught  produced  by  a  chimney,  and  therefore  that  appendage  will  have 
no  other  use  than  as  an  exit  for  the  gases  evolved  in  the  combustion.  A 
square  tunnel  designed  for  this  purpose  is  carried  from  the  machinery  upward 
through  the  two  decks,  terminating  on  the  poop-deck,  where  a  sliding  tube, 
having  a  motion  like  a  telescope-joint,  by  which  a  short  discharge-pipe  for 
the  hot  air  and  offensive  gases  can  be  elevated  when  the  machinery  is  worked, 
and  which  can  be  lowered  when  the  vessel  is  under  sail. 

Such  a  vessel,  then,  presents  none  of  the  appearances,  internal  or  external, 
of  a  steamer.  There  is  no  visible  machinery,  no  noise,  heat,  smoke,  or  per- 
ceptible vibration.  The  main-deck,  clear  of  machinery  from  stem  to  stern,  is 
occupied  by  the  cabins,  saloons,  library,  state-room,  and  the  various  other  ac- 

*  The  triumphs  of  genius,  like  all  sublunary  pleasures,  are  not  unattended  with  alloy.  The  moment 
that  any  invention  proves  to  be  successful  in  practice,  a  swarm  of  vermin  are  fostered  into  being  to 
devour  the  legitimate  profits  of  the  inventor,  and  to  rob  geniusof  its  fair  reward.  Captain  ERICSSOX, 
so  long  as  his  submerged  propeller  retained  the  character  of  a  mere  experiment,  was  left  in  undis- 
turbed possession  of  it ;  but  when  it  had  forced  its  way  into  extensive  practical  use — when  it  was 
adopted  in  the  United  States  navy,  and  in  the  revenue  service — when  the  coast  of  this  country  wit- 
nested  its  application  in  numerous  commercial  vessels — when  it  was  known  that  in  France  and 
England  its  adoption  was  decided  upon — then  the  discovery  was  made  for  the  first  time  that  this 
invention  of  Captain  Ericsson's  was  no  invention  at  all — that  it  had  been  applied  since  the  earliest 
dates  in  steam  navigation.  Old  patents,  some  of  which  had  been  stillborn,  and  others  which  had 
been  for  years  dead  and  buried,  were  dug  from  their  graves,  and  their  dust  brought  into  courts  of 
law,  to  overturn  this  invention,  and  wrest  from  Captain  Ericsson  his  justly-earned  reward.  But 
this  was  not  all :  every  mechanical  expedient  has  about  it  accidents  and  essentials.  It  is  tlie  same 
with  genius  and  art.  Imitators,  incapable  of  realizing  the  spirit  or  producing  the  essentials,  are 
nevertheless  capable  of  copying  the  accidents  and  mere  forms.  The  success  of  Ericsson's  inven- 
tions produced  the  usual  swarm  of  imitators  of  this  kind :  and  the  smoke  jack  •was  accordingly  pat- 
ented by  a  so-called  inventor  at  Philadelphia,  in  which,  with  a  sintrnlar  obliquity  of  ingenuity,  he 
stripped  Ericsson's  contrivance  of  everything  that  was  good  about  it,  and  carefully  combined  all  the 
bad  features  which  could  possibly  attach  to  the  common  wheel  of  oblique  action. 

It  is  painful  to  be  compelled  to  state  that  these  base  and  contemptible  proceedings  have  not  failed 
in  some  instances  to  obtain  countenance  in  high  quarters.  Will  it  be  believed  that  the  steamship 
Princeton,  the  performance  of  whose  machinery  was  attended  with  complete  success,  has  had  its 
propeller  removed,  and  another  substituted  which  is  in  fact  a  feeble  and  inefficient  copy  of  the 
original — omitting,  however,  one  or  two  of  its  best  features  ?  It  is  pretended,  also — erroneously,  as 
will  be  proved — that  this  inferior  instrument  has  been  more  elHcicnt  in  operation  than  the  original 
wheel.  No  engineer  or  machinist,  properly  informed,  can  examine  the  wheel  which  has  been  thus 
substituted,  without  being  convinced  that  the  change  mnst  have  been  prompted  by  motives  entirely 
unconnected  with  those  of  the  improvement  of  the  vessel. 

18 


274  PROSPECTS  OF  STEAM  NAVIGATION. 


commodations  for  passengers.  Under  that,  the  second  or  freight  deck,  also 
clear  of  machinery  from  stem  to  stern,  is  occupied  by  the  cargo  ;  and  beneath 
this  again,  buried  in  the  very  bottom  of  the  vessel,  is  the  mechanical  power  of 
propulsion — occupying,  however,  only  about  one  fifth  of  the  space  below  the 
freight-deck.  The  square  tunnel  we  have  referred  to  for  the  discharge  of 
the  gases,  and  the  ventilation  of  (lie  engine-room,  is  carried  up  through  the 
decks  and  stands  in  one  of  the  saloons,  but  presents  no  other  appearance  to 
the  eye  than  that  of  a  pillar  five  feet  square,  handsomely  empannelled  and 
decorated,  and  adorned  with  mirrors.  The  freight-deck  being  interposed  be- 
tween the  cabins  and  the  machinery,  intercepts  all  noise  and  vibration. 

When  this  mode  of  propulsion  is  applied  to  vessels-of-vvar,  as  in  the  case  of 
the  Princeton,  there  is  still  another  object  to  be  accomplished.  It  is  desirable 
that  the  whole  of  the  machinery  should  be  below  the  water-line,  so  as  to  be 
effectually  protected  from  shot.  This  is  accomplished  by  engines  of  a  peculiar 
construction,  invented  and  patented  by  Captain  Ericsson,  which  have  been 
worked  with  complete  success  in  the  Princeton.  A  representation  of  these, 
in  transverse  vertical  section,  is  given  in  fig.  7.  It  consists  of  two  semi-cylin- 
ders, presenting  their  semicircular  sides  downward,  and  being  flat  at  the  top. 
They  are  placed  beside  each  other  above  the  main  shaft,  having  their  axes 
parallel  to  it  and  to  the  keel.  The  ends  of  the  axes  are  represented  at  A  B. 
To  these  axes  are  attached  vibrating  rectangular  planes,  which  move  alter- 
nately from  left  to  right,  and  right  to  left,  within  the  semi-cylinders,  and  in 
steam-tight  contact  with  them.  These  planes  are  attached  to  the  axes  of  the 
cylinders,  the  ends  of  which  appear  at  A  and  B,  so  that  the  vibrating  motion 
of  the  planes  will  impart  a  corresponding  motion  to  the  arms  A  E  and  B  F, 
attached  to  the  ends  of  the  axes  A  and  B.  The  ends  of  these  arms  E  and  F 
are  attached  to  two  connecting-rods,  E  D  and  F  D,  which  are  both  attached  to 
the  crank  S  D,  which  drives  the  main  shaft. 

The  steam  is  admitted  a>  ernalely  to  each  side  of  the  vibrating  planes  with- 
in the  semi-cylinders,  being  at  the  same  time  withdrawn  from  the  other  side 
by  a  condenser. 

The  action  of  the  connecting-rods  on  the  crank  will  be  best  understood  by 
following  them  successively  through  their  various  positions.  In  fig.  8,  the 
rod  F  D  is  in  the  position  in  which  it  has  no  power  on  the  crank  ;  but  the 
rod  E  D,  being  at  right  angles  with  the  crank,  has  full  effect  upon  it.  The 
crank  therefore  moves  from  the  position  represented  in  fig.  8,  to  the  position 
represented  in  fig.  9,  where  the  rod  E  D  becomes  powerless.  The  crank  is 
then  driven  to  the  position  represented  in  fig.  10,  where  the  rod  D  F  becomes 
again  powerless,  and  E  D  is  effective.  The  crank  is  then  moved  to  the  posi- 
tion represented  in  fig.  11,  where  E  D  is  powerless  and  F  D  effective,  and 
so  on. 

Thus  it  appears  by  this  arrangement  that  the  relative  positions  of  the  crank 
and  connecting-rods  are  such  as  to  exercise  a  uniform  action  on  the  main  shaft.  I 

The  space  occupied  by  the  machinery  in  the  lower  part  of  the  stern  of  the  ) 
vessel,  is  surrounded  by  fuel,  as  represented  in  figure  7,  and  the  whole  is  •, 
considerably  below  the  water-line  W. 

This  machinery  is  designed  only  for  war-vessels.  Its  construction  and  op- 
eration are  somewhat  too  expensive  to  be  used  for  the  mere  purposes  of  com- 
merce, where  the  advantages  of  its  being  placed  below  the  water-line  are  of  no 
account. 

The  steam  packet-ships  to  which  we  have  referred  are  calculated  to  make 
an  average  speed  of  nine  statute  miles  per  hour  when  in  full  operation.  It  is 
computed  that  they  can  maintain  the  communication  between  New  York  and 
Liverpool  with  regularity  and  despatch — the  average  western  passage  being 


PROSPECTS  OF  STEAM-NAVIGATION. 


276 


PROSPECTS  OF  STEAM-NAVIGATION. 


about  twenty  and  tLe  eastern  sixteen  days — their  steam-machinery  working  S 
for  about  one  third  the  time  of  the  voyage. 

On  comparing  these  vessels  with  the  Great  Western,  it  is  to  be  considered 
that,  in  order  to  enable  the  latter  vessel  to  make  an  average  speed  of  ten  miles, 
she  is  provided  with  four-hundred-hors.e  power  ;  while  the  power  proposed  to 
be  given  to  the  ship  now  in  preparation  being  only  that  of  one  hundred  and 
seventy-three  horses,  would  give  a  speed  of  seven  and  a  half  knots  per  hour, 
which  is  equivalent  to  nine  statute  miles.  Such  is  the  result  of  a  calculation 
made  on  the  ordinary  and  admitted  principles  of  mechanics.  It  appears,  then, 
that  by  the  small  sacrifice  of  twenty-five  per  cent,  of  the  speed,  the  power  of 
the  machinery  is  reduced  in  the  proportion  of  forty  to  seventeen  ;  and  the  con- 
sumption of  fuel,  and  the  space  occupied  by  it  and  by  the  machinery,  are  di- 
minished in  a  greater  ratio  than  six  to  one.* 

Let  us  consider  for  a  moment  the  effect  which  the  successful  establishment 
of  such  a  line  of  steamships  would  have  upon  the  intercourse  between  this  con- 
tinent and  Europe.  The  average  passage  of  the  Great  Western  to  New  York 
has  been  fifteen  days  and  nineteen  hours.  That  of  the  Cunard  ships  to  Boston 
has  been  thirteen  days.  It  appears,  therefore,  that  these  vessels  at  present 
bring  occasional  intelligence  to  New  York,  the  one  in  sixteen  and  the  other  in 
fourteen  days.  The  proposed  line  of  steamships  will  accomplish  the  same 
passage  in  twenty  days  ;  but  as  they  must,  if  successful  at  all,  be  as  numerous 
as  the  present  London  and  Liverpool  liners,  they  will  be  continually  dropping 
into  this  port,  keeping  up  a  never-ceasing  streajn  of  intelligence,  not  more  than 
twenty  days  later  from  Europe.  Instead,  therefore,  of  the  present  mail-steam- 
ers, bringing,  as  they  do  now,  intelligence  in  winter  often  thirty  days  later, 
and  in  summer  fifteen  days  later,  their  functions  will  be  limited  to  the  convey- 
ance of  news  occasionally  five  or  six  days  later.  In  a  word,  it  is  evident 
that  the  line  of  packet-ships  now  contemplated  will  to  a  great  extent  strip  the 
present  mail-steamers  of  their  great  importance,  not  merely  as  respects  intelli- 
gence, but  also  correspondence.  A  great  epoch  is  indubitably  at  hand. 

One  of  the  numerous  advantages  attending  these  arrangements  is,  that  the 
machinery  is  capable  of  being  applied  to  any  of  the  present  packet-ships  with- 
out any  serious  suspension  of  their  operation,  or  any  injurious  expenditure. 
If  the  experiment  about  to  be  made  shall  therefore  be  attended  with  that  suc- 
cess which  we  confidently  anticipate,  a  brief  period  will  be  sufficient  to  con- 
vert the  entire  fleet  of  packet-ships  between  New  York  and  Britain  into  steam- 
liners — uniting  the  expedition,  certainty,  and  regularity,  with  all  their  present 
capabilities  for  commerce  and  cargo. 

*  This  great  reduction  of  bulk  of  fuel  is  realized  chiefly  by  using  the  expansive  principle  to  a 
considerable  extent 


THE    BAROMETER. 


Maxim  of  the  Ancients. — Abhorrence  of  a  Vacuum. — Suction. — Galileo's  Investigations. — Torricelti 
discovers  the  Atmospheric  Pressure. — The  Barometer. — Pascal's  Experiment. — Requisites  for  a 
good  Barometer. — Means  of  securing  them. — Diagonal  Barometer. — Wheel  Barometer — Ver- 
nier.— Uses  of  the  Barometer. — Variation  of  Atmospheric  Pressure. — Weather-Glass. — Rules  in 
common  Use  absurd. — Correct  Rules. — Measurement  of  Heights. — Pressure  on  Bodies. — Why 
net  apparent. — Effect  of  a  Leather  Sucker. — How  Flies  adhere  to  Ceilings  and  Fishes  to  Rocks. — 
Breathing.— Common  Bellows.— Forge  Bellows.— Vent  Peg.— Tea-Pot.— Kettle.— Ink  Bottles.— 
Pneumatic  Trough. — Gurgling  Noise  in  decanting  Wine. 


THE  BAROMETER. 


THE    BAROMETER. 


IM  the  history  of  human  discovery,  there  are  few  more  impressive  lessons 
of  humility  than  that  which  is  to  be  collected  from  the  records  of  the  progress 
by  which  the  pressure  of  the  atmosphere  which  surrounds  us,  and  the  manner 
in  which  it  is  instrumental  in  producing  some  most  ordinary  phenomena, 
became  known.  Looking  back  from  the  point  to  which  we  have  now  attained, 
and  observing  the  numerous  and  obvious  indication?  of  this  effect  which  pre- 
sent themselves  at  all  times,  and  on  all  occasions,  nature  seems  almost  to  have 
courted  the  philosopher  to  the  discovery.  With  every  allowance  for  the  feeble- 
ness of  the  human  understanding,  and  for  the  disadvantages  which  the  ancients 
labored  under,  as  compared  with  more  recent  investigators,  still  one  is  inclined  to 
attribute  the  lateness  of  the  discovery  of  the  atmospheric  pressure  and  its  effects, 
not  altogether  to  the  weakness  and  inadequacy  of  the  mental  powers  applied 
to  the  investigation.  There  seems  to  be  something  of  wilful  perverseness  and 
obstinacy  instigating  men  to  step  aside  from  that  course,  and  to  turn  their  minds 
from  those  instances  which  nature  herself  continually  forces  upon  them. 

The  ancient  philosophers  observed  that,  in  the  instances  which  commonly 
fell  under  their  notice,  space  was  always  filled  by  a  material  substance.  The 
moment  a  solid  or  a  liquid  was  by  any  means  removed,  immediately  the  sur- 
rounding air  rushed  in  and  filled  the  place  which  it  deserted ;  hence  they 
adopted  the  physical  dogma  that  nature  abhors  a  vacuum.  Such  a  proposition 
must  be  regarded  as  a  figurative  or  poetical  expression  of  a  supposed  law  of 
physics,  declaring  it  to  be  impossible  that  space  could  exist  unoccupied  by  matter. 

Probably  one  of  the  first  ways  in  which  the  atmospheric  pressure  presented 
itself  was  by  the  effect  of  suction  with  the  mouth.  One  end  of  a  tube  being 
immersed  in  a  liquid,  and  the  other  placed  between  the  lips,  the  air  was  drawn 
from  the  tube  by  the  ordinary  process  of  inhaling ;  the  water  was  immediately 
observed  to  fill  the  tube  as  the  air  retreated.  This  phenomenon  was  accounted 
for  by  declaring,  that  "  nature  abhorred  a  vacuum,"and  that  she,  therefore,  com- 
pelled the  water  to  fill  the  space  deserted  by  the  air. 


280 


THE  BAROMETER, 


Tho  eftecvs  of  suction  by  the  mouth  led,  by  a  natural  analogy,  to  suction  by 
artificial  means.  If  a  cylinder  be  open  at  both  ends,  and  a  piston  playing  in  it 
air-tight  be  moved  to  the  lower  end,  upon  immersing  this  lower  end  in  water, 
and  then  drawing  up  the  piston,  an  unoccupied  space  would  remain  between 
the  piston  and  the  water.  "  But  nature  abhors  such  a  space,"  said  the  ancients, 
"  and  therefore  the  water  will  not  allow  such  a  space  to  remain  unoccupied  :  we 
find,  accordingly,  that  as  the  piston  rises  the  water  follows  it."  By  such  poetical 
reasoning  pumps  of  various  kinds  were  constructed. 

The  antipathy  entertained  by  nature  against  an  empty  space  served  the  pur- 
poses of  philosophy  for  a  couple  of  thousand  years,  when  it  so  happened  that 
some  engineers  employed  at  Florence  in  sinking  pumps,  had  occasion  to  con- 
struct one  to  raise  water  from  an  unusually  great  depth.  Upon  working  it,  they 
found  that  the  water  would  rise  no  higher  than  about  thirty-two  feet  above  the 
well.  Galileo,  the  most  celebrated  philosopher  of  that  day,  was  consulted  in 
this  difficulty,  and  it  is  said  that  his  answer  was,  that  "  nature's  abhorrence  of  a 
vacuum  extended  only  to  the  height  of  thirty-two  feet,  but  that  beyond  this  her 
disinclination  to  an  empty  space  did  not  extend."  Some  writers  deny  the  fact 
of  his  having  given  this  answer  ;  others  admit  it,  but  take  it  to  have  been  iron- 
ical. It  has  been  more  generally  taken  as  a  solution  seriously  intended.  It 
appears,  however,  that  Galileo,  having  his  attention  thus  directed  to  the  point, 
soon  saw  the  absurdity  of  the  maxim  that  "  nature  abhors  a  vacuum,"  and  sought 
to  account  for  the  phenomenon  in  other  ways. 

He  attributed  the  elevation  of  the  water  to  an  attraction  exerted  upon  that 
liquid  by  the  piston.  This  attraction  he  conceived  to  have  a  determinate  inten- 
sity, and  when  such  a  column  of  water  was  raised  as  was  equal  in  weight  to 
the  whole  amount  of  the  attraction,  then  any  farther  elevation  of  the  water  by 
the  piston  became  impossible. 

At  a  very  remote  period  air  was  known  to  possess  the  quality  of  weight. 
Aristotle  and  other  ancient  philosophers  expressly  speak  of  the  weight  of  air. 
The  process  of  respiration  is  attributed  by  an  ancient  writer  to  the  pressure  of 
the  atmosphere  forcing  air  into  the  lungs.  Galileo  was  therefore  fully  aware  that 
the  atmosphere  possessed  this  property,  and  it  is  not  a  little  surprising  that 
when  his  attention  was  so  immediately  directed  to  one  of  the  most  striking 
effects  of  it,  he  was  unable  to  perceive  the  connexion. 

Some  writers  affirm,  we  know  not  upon  what  authority,  that  Galileo,  at  the 
time  he  was  interrogated  respecting  the  limited  elevation  of  water  in  a  common 
pump,  was  aware  of  the  true  cause  of  the  effect ;  but  that,  not  having  thoroughly 
investigated  the  subject,  he  evaded  the  question  of  the  engineers,  with  a  view 
to  conceal  his  knowledge  of  the  principle  until  he  had  carried  his  inquiry  to  a 
more  satisfactory  result.  It  does  not,  however,  appear  that  he  published  his 
solution  of  the  problem.  After  his  death,  Torricelli,  his  pupil,  directed  his  at- 
tention to  the  same  problem.  He  argued  that  whatever  be  the  cause  which 
sustained  a  column  of  water  in  a  common  pump,  the  measure  and  the  energy 
of  that  power  must  be  the  weight  of  the  column  of  water  ;  and,  consequently, 
if  another  liquid  be  used,  heavier  or  lighter,  bulk  for  bulk,  than  water,  then 
the  same  force  must  sustain  a  lesser  or  greater  column  of  such  liquid.  By 
using  a  much  heavier  liquid,  the  column  sustained  would  necessarily  be  much 
shorter,  and  the  experiment  in  every  way  more  manageable. 

He  therefore  selected  for  the  experiment  mercury,  the  heaviest  known  liquid. 
The  weight  of  mercury,  bulk  for  bulk,  being  about  13^  times  that  of  water,  it 
follows  that  the  height  of  a  column  of  that  liquid  which  would  be  sustained  by 
a  vacuum  must  be  13^  times  less  that  the  height  of  a  column  of  water  thus 
sustained. 

Hence  he  computed  that  the  height  of  the  column  of  mercury  would  be 


THE  BAROMETER. 


-*^V^-v—*^ 

281  j 


about  28  inches.  He  procured  a  glass  tube,  A  B  (fig.  1).  more  than  30  inches 
in  length,  open  at  one  end,  A,  and  closed  at  the  other  end,  B.  Placing  this 
tube  in  an  upright  position,  with  the  open  end  upward,  he  filled  it  with° mer- 
cury, and  applying  his  finger  to  the  end  A,  so  as  to  prevent  the  escape  of  the 
mercury,  he  inverted  the  tube,  plunging  the  end  A  into  a  cistern,  C  D  (fig.  2), 
containing  mercury,  the  open  end  A  being  below  the  surface  F  of  the  mer- 
cury in  the  cistern,  and  no  air  having  been  allowed  to  communicate  with  it. 

Fig.  1. 

A 


Upon  removing  the  finger,  therefore,  the  mercury  in  the  cistern  came  in  imme- 
diate contact  with  the  mercury  in  the  tube.  Immediately  the  mercury  was 
observed  to  subside  from  the  top  of  the  tube,  and  its  surface  gradually  to  de- 
scend to  the  level  E,  about  28  inches  above  the  mercury  in  the  cistern.  This 
result  was  what  Torricelli  anticipated,  and  clearly  showed  the  absurdity  of  the 
supposition  that  nature's  abhorrence  of  a  vacuum  extended  to  the  height  of  32 
feet.  Torricelli  soon  perceived  the  true  cause  of  this  phenomenon.  The  at- 
mospheric pressure  acting  upon  the  surface  F,  while  the  surface  E  was  pro- 
tected from  this  pressure  by  the  closed  end  B,  of  the  tube,  supported  the  weight 
of  the  column  E  F.  This  pressure  was  transmitted  by  the  liquid  mercury  in 
the  cistern  from  the  external  surface  F,  to  the  base  of  the  column  contained  in 
the  tube. 

This  experiment  and  its  explanation  soon  became  known  to  philosophers  in 
every  part  of  Europe,  and,  among  others,  it  attracted  the  notice  of  the  cele- 
brated Pascal.  In  order  to  subject  the  explanation  of  Galileo  to  the  most  se- 
vere test,  Pascal  proposed  to  transport  a  tube  of  this  kind  to  a  great  elevation 
upon  a  mountain,  and  argued  that,  if  the  cause  which  sustained  the  column  in 
the  tube  were  the  weight  of  the  atmosphere  acting  upon  the  external  surface 
of  the  mercury  in  the  cistern,  then  it  must  be  expected  that  if  the  tube  was 
elevated,  having  a  less  and  a  less  quantity  of  atmosphere  above  it,  the  column 
sustained  by  the  weight  of  this  incumbent  atmosphere  must  suffer  a  correspond- 
ing diminution  in  height.  He  accordingly  directed  a  friend  residing  in  the 
neighborhood  of  a  mountain  called  Pays  de  Dome,  near  Auvergne,  to  ascend 
that  mountain,  carrying  with  him  the  apparatus  already  described.  This  was 
accordingly  done,  and  the  height  of  the  column  noted  during  the  ascent.  Con- 


282 


THE  BAROMETER. 


formably  to  the  principle  explained  by  Torricelli,  the  column  was  observed 
gradually  to  diminish  in  height,  as  the  elevation  of  the  apparatus  was  increased. 
The  same  experiment  was  repeated  by  Pascal  himself,  with  similar  success, 
upon  a  high  tower  in  the  city  of  Paris. 

Meanwhile  other  effects  were  manifested  which  not  less  unequivocally 
proved  the  truth  of  Torricelli's  solution.  The  apparatus  being  kept  for  a  length 
of  time  in  a  fixed  position,  the  height  of  the  column  was  observed  to  fluctuate 
from  day  to  day  between  certain  small  limits.  This  effect  was,  of  course,  to 
be  attributed  to  the  variation  of  the  weight  of  the  incumbent  atmosphere,  ari- 
sing from  various  meteorological  causes. 

The  apparatus  which  we  have  just  described  is,  in  fact,  the  common  barom- 
eter. By  the  principles  of  hydrostatics  it  appears  that  the  height  of  the  col- 
umn E  F,  sustained  by  the  atmospheric  pressure,  will  be  the  same,  whatever 
be  the  magnitude  of  the  bore  of  the  tube.  If  we  suppose  the  section  of  the 
bore  to  be  equal  to  a  square  inch,  then  the  column  E  F  will  be  pressed  up- 
ward, and  held  in  equilibrium  by  the  weight  of  a  column  of  atmosphere  pres- 
sing upon  a  square  inch  of  the  external  surface  F  ;  consequently  the  weight  of 
the  column  E  F,  must  be  equal  to  the  weight  of  a  column  of  the  atmosphere 
whose  base  is  a  square  inch,  and  which  extends  from  the  surface  of  the  mer- 
cury in  the  cistern  to  the  top  of  the  atmosphere.  If  there  be  another  tube 
whose  bore  is  only  half  a  square  inch,  then  the  pressure  which  will  support 
the  column  in  it  will  be  that  of  a  similar  column  of  atmosphere,  whose  base  is 
half  a  square  inch ;  such  pressure,  then,  will  only  be  half  the  amount  of  the 
former,  and  therefore  will  only  sustain  half  the  weight  of  mercury.  But  a 
column  of  mercury  of  half  the  weight,  having  a  base  of  half  the  magnitude, 
must  necessarily  have  the  same  height.  Hence  it  appears  that  so  long  as  the 
atmosphere  presses  upon  a  given  magnitude  of  the  surface  F,  with  the  same 
intensity,  the  column  of  mercury  sustained  in  the  tube  will  have  the  same 
height,  whatever  be  the  magnitude  of  its  bore. 

In  adapting  such  an  apparatus  as  this  to  indicate  minute  changes  in  the  pres- 
sure of  the  atmosphere,  there  are  many  circumstances  to  be  attended  to,  which 
I  propose  to  explain,  so  far  as  they  are  necessary  to  render  intelligible  the 
general  principles  and  use  of  the  barometer. 

It  is,  in  the  first  place,  necessary  to  have  the  means  of  measuring  exactly 
the  height  of  the  column  E  F,  fig.  2.  If  the  surface  F  were  fixed,  and  the 
tube  B  A  maintained  in  its  position,  it  would  be  sufficient  to  mark  a  graduated 
scale  upon  the  tube,  indicating  the  number  of  inches  and  fractions  of  an  inch 
of  any  part  upon  it,  from  the  surface  F.  But  it  is  obvious  that  this  will  not  be  the 
case  when  the  pressure  of  the  atmosphere  is  increased,  as  an  additional  quan- 
tity of  mercury  is  forced  into  the  tube,  and  consequently  an  equal  quantity  is 
forced  out  of  the  cistern.  While  the  surface  E  rises  toward  B,  the  surface 
F  therefore  descends,  and  the  distance  of  E  from  that  surface  is  increased  by 
both  causes. 

A  graduated  scale  marked  upon  the  tube  would  then  only  indicate  the  change 
in  the  position  of  the  surface  E,  but  would  not  show  the  change  in  the  length 
of  the  column  E  F,  so  far  as  that  change  is  affected  by  the  fall  of  the  surface 
F.  There  are  several  ways  in  which  this  defect  may  be  remedied. 

If  the  instrument  be  not  required  to  give  extremely  accurate  indications,  it 
will  be  sufficient  to  use  a  tube  the  bore  of  which  is  small  compared  with  the 
magnitude  of  the  cistern.  In  this  case,  a  small  change  in  the  height  of  the 
column  will  make  but  a  very  inconsiderable  change  in  the  whole  quantity  of 
mercury  in  the  cistern,  and  therefore  will  produce  a  very  minute  effect  upon 
the  position  of  the  surface  F.  If  such  a  change  in  the  level  F,  be  so  small  as 
to  affect  the  indications  of  the  instruments  in  a  degree  which  is  unimportant 


THE  BAROMETER. 


283 


for  the  purposes  to  which  it  is  intended  to  be  applied,  the  surface  F  may  be 
regarded  as  fixed,  and  the  whole  change  in  the  height  of  the  column  may  be 
taken  to  be  represented  by  the  change  in  the  position  of  the  level  E.  All  or- 
dinary barometers  are  constructed  in  this  manner.  But  it  is  not  difficult  to  ad- 
just a  scale  upon  a  tube  which  will  give  with  accuracy  the  actual  variation  in 
the  length  of  the  column  by  means  of  the  change  in  the  level  of  the  surface 
E.  Let  us  suppose  that  the  cistern  P  D  has  a  flat,  horizontal  bottom  and  per- 
pendicular sides,  and  that  the  magnitude  of  the  bottom  bears  a  certain  known 
proportion  to  the  bore  of  the  tube.  Suppose  this  proportion  to  be  that  of  a 
hundred  to  one.  If  the  pressure  of  the  atmosphere  increase,  so  as  to  cause  the 
column  of  mercury  sustained  in  the  tube  to  be  increased  in  height  by  one  inch, 
then  as  much  mercury  as  fills  one  inch  of  the  tube  will  be  withdrawn  from  the 
cistern ;  but  as  the  base  of  the  cistern  is  one  hundred  times  greater  than  the 
bore  of  the  tube,  it  is  evident  that  this  inch  of  mercury  in  the  tube  would  only 
cause  a  fall  of  the  hundredth  of  an  inch  in  depth  of  the  mercury  in  the  vessel. 
Consequently  it  follows  that  the  increased  elevation  of  an  inch  in  the  column 
produces  a  depression  of  a  hundredth  of  an  inch  in  the  surface  F.  Thus  it 
appears  that  the  increased  length  of  the  column  E  F,  is  produced  by  the  sur- 
face F,  falling  through  the  one  hundredth  of  an  inch,  while  the  surface  E  rises 
through  ninety-nine  hundredths  parts  of  an  inch.  The  same  will  be  true 
whatever  change  takes  place  in  the  height  of  the  column.  We  may  therefore 
infer  generally,  that  whatever  variation  may  be  produced  in  the  surface  E,  the 
consequent  variation  produced  in  the  height  of  the  column  is  greater  by  a 
ninety-ninth  part. 

If,  then,  the  top  be  so  graduated  that  a  portion  of  it,  the  length  of  which  is 
one  hundredth  part  less  than  an  inch,  be  marked  as  an  inch,  and  all  other  di- 
visions and  subdivisions  marked  according  to  the  same  proportion,  then  the 
indications  will  be  as  accurate  as  if  the  surface  F  were  fixed,  the  tube  being 
divided  accurately  into  inches  and  parts  of  an  inch. 

Fig.  3. 


The  barometer  is  represented  mounted  and  furnished  with  a  scale,  in  fig.  3 
The  glass  tube  is  surrounded  by  one  of  brass  in  which  there  is  an  aperture  cut 


at  D  E,  of  such  a  length  and  at  such  a  height  above  the  cistern,  as  to  include 
all  that  space  through  which  the  level  of  the  mercury  in  the  tube  usually  va- 
ries in  the  place  in  which  the  barometer  is  intended  to  be  used.  In  these 
countries  the  level  of  the  mercury  never  falls  below  twenty-eight  inches,  nor 
rises  above  thirty-one  inches;  consequently  a  space  somewhat  exceeding  these 
limits  will  be  sufficient  for  the  opening  D  E.  The  tube  is  permanently  con- 
nected with  the  cistern  A  B,  and  a  scale  is  engraved  upon  the  brass  tube,  near 
the  aperture  D  E,  to  indicate  the  fractions  of  the  height  of  the  mercury  in 
the  tube. 

There  is  another  method  of  avoiding  the  difficulty  arising  from  the  change 
in  the  level  of  the  surface  of  the  mercury  in  the  cistern,  used  in  the  barometer 
hero  represented.  The  bottom  of  the  cistern  moves  within  it  in  such  a  man- 
ner as  to  prevent  the  mercury  from  escaping,  and  a  screw  is  inserted  at 
B,  by  turning  which  the  bottom  of  the  cylinder  is  slowly  elevated  or 
depressed.  An  ivory  index  is  attached  to  the  top  of  the  cylinder,  which  is 
presented  downward  and  brought  to  a  fine  point,  so  as  to  mark  a  fixed  level. 
When  an  observation  is  made  with  the  barometer,  the  screw  V  is  turned  until 
the  surface  is  brought  accurately  to  the  point  of  the  index,  by  raising  or  low- 
ering the  bottom  according  as  the  surface  is  below  or  above  that  point.  It  fol- 
lows, therefore,  that  whenever  an  observation  is  made  with  this  instrument,  the 
surface  of  the  mercury  always  stands  at  the  same  level,  and  therefore  the  di- 
visions upon  the  scale  C  F,  represent  the  actual  change  of  height  in  the  bar- 
ometric column. 

Since  the  column  of  mercury  sustained  in  the  barometric  tube  is  taken  to 
represent  the  pressure  of  the  atmosphere,  it  is  clear  that  no  air  or  other  elastic 
fluid  should  occupy  the  part  of  the  tube  above  the  mercury.  To  avoid  such  a 
cause  of  error  is  not  so  easy  or  obvious  as  may  at  first  appear.  Mercury,  as  it 
exists  in  the  ordinary  state,  frequently  contains  air  or  other  elastic  fluids  com- 
bined with  it,  and  which  art  maintained  in  it  by  the  atmospheric  pressure,  to 
which  it  is  usually  subject. 

When  it  has  subsided,  however,  in  the  barometric  tube,  it  is  relieved  from 
that  pressure,  and  the  elastic  force  of  such  air  as  may  be  lodged  in  the  mercu- 
ry, being  relieved  from  the  pressure  which  confined  it  there,  it  will  make  its 
escape  and  rise  to  the  surface,  finally  occupying  the  upper  part,  of  the  tube,  and 
exerting  a  pressure  upon  the  surface  of  the  column  by  means  of  its  elasticity. 
Such  a  pressure  will,  then,  assist  the  weight  of  the  column  of  mercury  in  bal- 
ancing the  atmospheric  pressure,  and  consequently  a  column  of  less  height 
will  balance  the  atmosphere  than  if  the  upper  part  of  the  tube  were  free  from 
air.  To  remove  this  cause  of  error  it  is  necessary  to  adopt  means  of  purify- 
ing the  mercury  used  in  the  barometer  from  all  elastic  fluids  which  may  be 
combined  with  it. 

The  fact  that  the  application  of  heat  gives  energy  to  the  elastic  force  of  gas 
'  es,  enables  us  easily  *u  accomplish  this.     For  if  the  mercury  be  heated,  the  £ 
|  particles  of  air  or  other  elastic  fluids  which  are  combined  with  it  acquire  sucU 
1  a  degree  of  elasticity  that  they  dilate  and  rise  to  the  surface,  and  there  escape  / 
|  in  bubbles.     The  same  process  of  heating  serves  to  expel  any  liquid  impurities  I 
i  with  which  the  mercury  may  be  combined.     These  are  converted  into  vapor  > 
[  and  escape  at  the  surface. 

The  presence  of  an  elastic  fluid  at  the  top  of  the  tube  is  thus  removed  so  far   j 
|  as  such  fluid  can  proceed  from  the  mercury.   But  it  is  also  found  that  small  par-  ( 

>  tides  of  air  and  moisture  are  liable  to   adhere   to  the  interior  surface  of  the  > 
I  glass  ;  and  when  the  mercury  is  introduced,  and  a  vacuum  produced  at  the  top  c 

>  of  the  tube,  these  particles  of  air  dilate,  and  rising,  lodge  at  the  top  and  vitiate   / 
|  the  vacuum  which  ought  to  be  there  ;  the  particles  of  moisture  also  evac  rate   ;' 


THE  BAROMETER.  285 


and  rise  likewise,  both  producing  an  aeriform  fluid  in  the  chamber  above  the 
surface  of  the  mercury,  which  presses  upon  that  surface  with  an  elastic  force 
ind  produces  a  corresponding  diminution  in  the  height  of  the  column  of  quick- 
silver, sustained  by  the  atmosphere  as  already  explained.  This  imperfection 
may  be  avoided  by  previously  heating  the  tube.  The  particles  of  air  which 
adhere  to  its  inner  surface  being  thus  expanded  by  heat,  will  fly  off  by  their 
elastic  force,  and  the  particles  of  moisture  will  be  converted  into  vapor,  and 
likewise  disengaged  from  the  surface. 

All  the  effects  now  explained  may  be  produced  by  filling  the  tube  with  mer- 
cury in  the  first  instance  and  then  boiling  the  liquid  in  it,  which  may  be  easily 
accomplished.  The  heat  will  not  only  expel  all  liquid  and  gaseous  impurities 
from  the  mercury  itself,  but  also  will  disengage  them  from  the  inner  surface  of 
the  tube.  These  precautions  being  taken,  the  column  of  mercury  sustained  in 
the  tube  will  indicate  by  its  weight  the  true  amount  of  the  atmospheric  pressure. 
But  in  order  to  be  able  to  compare  the  result  of  any  one  barometer  with  any 
other,  it  is  necessary  that  the  weights  of  equal  bulks  of  the  liquid  mercury 
used  in  both  cases  should  be  the  same  ;  and  for  this  purpose  we  must  be  as- 
sured that  the  mercury  used  is  pure,  and  not  combined  with  other  substances. 

We  have  just  seen  how  all  substances  in  the  liquid  or  gaseous  form  may 
be  extracted  from  it.  Impurities  may  still,  however,  be  suspended  in  it  in  a 
solid  form. 

To  remove  these  it  is  only  necessary  to  enclose  the  mercury  in  a  small  bag 
of  chamois  leather :  upon  pressing  this  bag  the  quicksilver  will  pass  freely 
through  its  pores,  and  any  minute  solid  impurities  which  may  be  contained  in 
the  mercury  will  remain  in  the  bag.  Pure  and  homogeneous  mercury  being 
thus  obtained,  we  have  advanced  another  step  toward  the  certainty  that  the  in- 
dications of  different  barometers  may  correspond  ;  but  there  is  still  one  other 
cause  of  discordancy  to  be  attended  to.  Suppose  a  barometer  to  be  used  in 
Paris,  and  another  in  London,  at  a  time  when  the  pressure  of  the  atmosphere 
in  both  places  is  the  same,  but  the  temperature  of  the  air  at  Paris  is  higher 
than  the  temperature  of  London.  The  mercury  in  the  one  barometer  will  have 
a  higher  temperature  than  the  mercury  in  the  other.  Now  it  is  well  known 
that  when  mercury  or  any  other  body  is  heated,  its  dimensions  increase.  In 
other  words,  bulk  for  bulk,  it  becomes  slighter.  Consequently,  if  two  columns 
be  equal  in  weight,  that  which  has  the  higher  temperature  will  have  the  greater 
altitude.  Hence  it  appears,  that  under  the  circumstances  supposed,  at  a  time 
when  the  atmospheric  pressure  is  the  same  in  London  as  at  Paris,  the  barom- 
eter at  the  latter  place  will  be  higher  than  at  the  former.  To  guard  against 
this  source  of  error,  it  is  necessary,  in  making  barometric  observations,  to  note 
at  the  same  time  the  contemporaneous  indications  of  the  thermometer.  Tables 
are  computed,  showing  the  changes  in  the  height  of  the  mercury  correspond- 
ing to  given  differences  of  temperature.  It  is  evident  that  in  comparing  the  ( 
results  of  the  same  barometer  observed  at  different  times,  it  is  equally  neces- 
sary to  note  the  difference  of  temperature,  and  to  allow  for  its  effects.  This, 
however,  is  a  refinement  of  accuracy  which  is  not  attended  to,  except  in  ob- 
servations made  for  philosophical  purposes. 

One  of  the  difficulties  attending  barometri ;  observations  arises  from  the  very 
minute  changes  produced  in  the  height  of  the  column  by  slight  variations  in 
the  atmospheric  pressure.  The  whole  play  of  the  upper  surface  of  the  column, 
in  the  most  extreme  cases,  does  not  exceed  three  or  four  inches  in  a  given 
place  ;  and  mercury  being  a  very  heavy  fluid,  a  variation  in  the  pressure  of  the 
atmosphere,  of  sensible  amount,  may  produce  scarcely  any  perceptible  change 
in  the" height  of  the  column.  One  of  the  most  obvious  remedies,  at  first  viiw, 
would  seem  to  be  the  use  of  a  fluid  lighter  than  mercury.  In  the  same  psopor-  . 

•  -s^X^-Xy-* 


THE  BAROMETER, 


tion  as  the  fluid  is  lighter,  will  the  change  in  the  height  of  the  column,  by  a 
given  change  in  the  pressure  of  the  atmosphere,  be  greater ;  but  there  are  diffi- 
culties of  a  different  kind  which  altogether  preclude  the  use  of  other  fluids. 
The  lighter  liquids  are  much  more  susceptible  of  evaporation,  and  the  surface 
of  the  liquid  in  the  tube  being  relieved  from  the  atmospheric  pressure,  offers  no 
resistance  to  the  process  of  evaporation.  The  consequence  is,  that  any  liquid, 
except  mercury,  would  produce  a  vapor,  which,  occupying  the  top  of  the  tube, 
would  press  by  its  elastic  force  upon  the  surface,  and  co-operate  with  the 
weight  of  the  suspended  column  in  balancing  the  atmospheric  pressure.  Even 
from  mercury  we  have  reason  to  know  that  a  vapor  rises,  which  is  present  in 
the  upper  part  of  the  tube ;  but  this  pressure  exerts  no  power  which  can  intro- 
duce inaccuracy  to  any  sensible  extent  into  our  conclusions. 

A  form  is  sometimes  adopted  called  the  diagonal  barometer,  for  the  purpose 
of  increasing  the  range  of  the  mercury  in  the  tube.  This  is  represented  in 
fig.  4,  where  A  C  B  represents  the  barometer  tube. 

C  is  a  point  at  a  distance  above  the  surface  of  the  mercury  in  the  cylinder 
less  than  the  height  of  twenty-eight  inches.  The  space  C  D  includes  the  range 
which  the  mercury  would  have  if  the  tube  were  vertical ;  but  at  C  the  tube  is 
bent  obliquely  in  the  direction  C  B,  having  a  sufficient  length  to  bring  the  ex- 
tremity B  to  the  same  level  as  D.  The  mercury,  which,  had  the  tube  been 
vertical,  would  range  between  C  and  D.,  will  now  have  its  play  extended  through 
the  greater  space  C  B  ;  consequently  the  magnitude  of  any  part,  however 
small,  will  be  increased  in  the  proportion  of  the  line  C  D  to  the  line  C  B. 
Thus,  if  C  D  be  four  inches,  and  C  B  twelve  inches,  then  every  change  in  the 
position  of  the  surface  of  the  mercury  produced  by  a  change  in  the  atmospheric 
pressure,  will  be  three  times  as  great  in  the  diagonal  barometer  as  it  would  be 
in  the  vertical  one. 


Fig.  4. 


Fig.  5. 


32_ 


Another  contrivance  fcr  enlarging  the  scale,  which  is  more  frequently  used, 
and  for  common  domestic  purposes  attended  with  some  convenience,  is  repre- 
sented in  fig   5.     This  is  called  the  wheel  barometer.     The  barometric  tube  is 
here  bent  at  its  lower  extremity  B,  and  turned  upward  toward  C.     The  atmo-  ' 
spheric  pressure  acts  upon  the  surface  F,  and  sustains  a  column  of  mercury  in  | 

— — * 


r 


THE  BAROMETER. 


the  tube  B  A,  which  is  above  the  level  of  F.  The  bore  of  the  tube  being  in 
this  case  equal  in  every  part  of  its  length,  it  is  clear  that,  through  whatever 
space  the  surface  E  falls,  the  surface  F  will  rise,  and  vice  versa.  Hence  it  is 
obvious  that  the  variation  in  the  height  of  the  barometric  column  will  alwavs 
fat  double  the  change  in  the  height  of  either  surface  E  or  F  ;  for  if  the  surface 
F  fall,  the  surface  E  must  rise  through  the  same  space.  They  are  thus  rece- 
ding from  each  other  at  the  same  rate,  and  therefore  their  mutual  distance  will 
be  increased  by  the  space  through  which  each  moves,  or  by  double  the  space 
through  which  one  of  them  moves. 

In  the  same  manner,  if  F  rise,  E  must  fall,  the  two  points  mutually  approach- 
ing each  other  at  the  same  rate  ;  so  that  the  distance  between  them  will  be  dimin- 
ished by  the  space  through  which  each  moves,  or  by  double  the  space  throngh 
which  one  of  them  moves.  The  change,  therefore,  in  the  height  of  the 
barometric  column  will  always  be  double  the  change  in  the  position  of  the 
level  F. 

Upon  the  surface  at  F  floats  a  small  ball  of  iron,  suspended  by  a  string, 
which  is  carried  over  a  pulley  or  small  wheel  at  P,  and  counterpoised  by  the 
weight  at  W,  less  in  amount  than  the  weight  of  the  iron  ball.  When  the  sur- 
face F  rises,  the  iron  ball  being  buoyant,  will  be  raised  with  it,  and  the  coun- 
terpoise W  will  fall ;  arid  when  the  surface  F  falls,  the  weight  of  the  iron  ball 
being  greater  than  the  weight  of  the  counterpoise  W,  will  cause  it  to  descend 
with  the  descending  surface,  and  to  draw  the  counterpoise  W  up.  It  is  evi- 
dent that,  through  whatever  space  the  iron  ball  thus  moves  in  ascending  or 
descending,  an  equal  length  of  the  string  will  pass  over  the  wheel  P.  Now 
this  string  rests  in  a  groove  of  the  wheel  in  such  a  manner  that  by  its  friction 
it  causes  the  wheel  to  revolve,  and  consequently  the  revolution  of  this  wheel  indi- 
cates the  length  of  string  which  passes  over  its  groove,  which  length  is  equal  to 
the  change  in  the  level  of  the  surface  F.  Upon  the  centre  of  this  wheel  P  an 
index  H  is  placed,  which,  like  the  hand  of  a  watch,  plays  upon  a  graduated  cir- 
cular plate.  Let  us  suppose  that  the  circumference  of  the  wheel  P  is  two 
inches  :  then  one  complete  revolution  of  the  wheel  will  correspond  to  a  change 
of  two  inches  in  the  level  F,  and  therefore  to  a  change  of  four  inches  in  the 
barometric  column.  But  in  one  revolution  of  the  wheel  P,  the  hand  or  index 
H  moves  completely  round  the  circle  ;  hence  the  circumference  of  this  circle 
corresponds  to  a  change  of  four  inches  in  the  barometric  column.  Now,  the 
circular  plate  may  easily  be  made  so  that  its  circumference  shall  measure  forty 
inches  ;  consequently  ten  inches  of  this  circumference  will  correspond  to  one 
inch  of  the  column,  and  one  inch  of  the  circumference  will  correspond  to  the 
tenth  of  an  inch  of  the  column.  In  this  way  variations  in  the  height  of  the 
column  amountiag  to  the  tenth  of  an  inch  are  indicated  by  a  motion  of  the  hand 
H  over  one  inch  of  the  circumference  of  the  plate.  By  further  subdivision,  a 
still  greater  accuracy  may  be  obtained. 

In  this  form  of  the  barometer  it  is  evident  that  the  preponderance  of  the  iron 
ball  assists  the  atmospheric  pressure  in  sustaining  the  column.  This  cause  of 
error,  however,  may  be  diminished  almost  indefinitely  by  making  the  prepon- 
derance of  the  ball  over  the  counterpoise  W  barely  sufficient  to  overcome  the 
friction  of  the  wheel  P. 

Again,  when  the  atmosphere  is  diminished  in  weight,  and  when  the  surface 
F  has  a  tendency  to  rise,  it  is  compelled  to  raise  the  ball ;  and  there  is  this 
obvious  limit  to  the  indications  of  the  instrument,  namely,  that  a  change  so 
slight  that  the  difference  ef  pressure  will  not  exceed  the  force  necessary  to 
elevate  the  ball,  will  fail  to  be  indicated. 

For  scientific  purposes,  the  vertical  barometer  is  preferable  to  every  other 
form  of  that  instrument.  In  the  oblique  barometer  the  termination  of  the  mer- 


ctiri-il  column  is  subject  to  some  uncertainty,  arising  from  the  level  of  the  mer- 
cury not  being  perpendicular  to  the  direction  of  the  tube.  In  the  wheel  ba- 
rometer there  are  several  sources  of  error,  which,  though  so  small  in  amount 
as  not  to  injure  it  for  domestic  or  popular  use,  yet  are  such  as  to  render  it  alto- 
aether  unfit  for  scientific  inquiry. 

A  contrivance  called  a  vernier,  for  no'ing  extremely  small  changes,  is  usu- 
ally applied  to  the  vertical  baromete  and  supplies  the  place  of  an  enlarged 
scale.  It  consists  of  a  small  pr5  ^aied  plate,  which  is  moveable  by  a  screw 
or  otherwise,  and  which  slid"  on  the  divided  scale  of  the  barometer.  By 
means  of  this  subsidiary  .oaie,  we  are  enabled  to  estimate  magnitudes  on  the 
principal  scale  amoun'iug  to  very  small  fractions  of  its  smallest  divisions. 

The  principle  of  oie  vernier  is  easily  explained.     Let  B  A,  fig.  G,  represent 


Fig.  6. 

A 

31 

9- 

8- 

7- 

- 

6- 

5- 

4- 

3- 

2- 

1- 

30- 



9- 

8- 

7- 

6- 

5- 

4- 

3_ 

2_ 

1 

D  A 

29  — 

—  •— 

-1 

9~ 

-_. 

-2 

8- 

-3 

7- 

-4 

6- 

5— 

- 

-5 

4- 

--6 

3- 

-7 

2- 

_- 

-8 

1- 

-9 
_i 

B     C 


Fig.  7. 

A 

31 

9- 

8- 

7- 

6- 

5  — 

— 

4- 

3- 

- 

2- 

1- 

Ort 

OU 

9- 

8- 

7- 

6- 

*    D 

n 

'      - 

4- 

V 

1 

3- 

~- 

-2 

2- 

3 

1- 

-_ 

-4 

29  — 

9- 

-6 

8- 

- 

— 

-7 

7- 

G- 

-" 

-8 

^ 

— 

-9 

9 

4- 

•       : 

ylO 

3- 

- 

2- 

1- 

28— 

THE  BAROMETER. 


the  scale  of  the  barometer,  extending  through  three  inches,  and  divided  to 
tenths  of  an  inch.  Let  C  D  be  the  sliding  scale  of  the  vernier,  equal  in  length 
to  eleven  divisions  of  the  principal  scale,  and  divided  into  ten  equal  parts. 

Thus  each  division  of  the  vernier  will  be  the  tenth  of  eleven  divisions  of  the 
instrument  •  that  is,  it  will  be  the  tenth  part  of  1 1  tenths  of  an  inch,  but  1 1 
tenths  of  an  n.^  'he  same  as  110  hundredths,  and  the  tenth  part  of  this  is 
1 1  hundredths.  THUS  it  appears  that  one  division  on  the  vernier  is  in  this 
case  the  1 1  hundredth  part  of  an  inch.  Now,  one  division  on  the  instrument 
being  a  tenth  of  an  inch,  or  10  hundredths  of  an  inch,  it  is  evident  that  a  di- 
vision on  the  vernier  will  exceed  a  division  on  the  instrument  by  the  hundredth 
part  of  an  inch  ;  for  if  we  take  10  hundredths  from  11  hundredths,  the  remain- 
der will  be  1  hundredth.  Let  us  suppose  that  the  vernier  is  placed  so  that  its 
lowest  division,  marked  10,  shall  coincide  with  the  lowest  division  on  the  in- 
strument, marked  28  ;  then  the  first  division  of  the  vernier,  marked  0,  will 
coincide  with  the  division  of  the  instrument  next  above  the  29th.  The  divis- 
ion marked  1  on  the  vernier  will  then  be  a  little  below  the  division  marked  29 
on  the  scale,  and  the  distance  between  these  will  be  the  hundredth  of  an 
inch,  as  already  explained.  The  division  marked  2  of  the  vernier  will  be  a 
little  below  the  division  marked  9  on  the  scale,  and  the  distance  below  it  will 
be  2  hundredth  parts  of  an  inch,  because  two  divisions  of  the  vernier  exceed 
two  divisions  of  the  scale  by  that  amount.  In  like  manner,  the  division  marked 
3  on  the  vernier  will  be  below  the  division  marked  8  on  the  scale  by  3  hun- 
dredths of  an  inch,  and  so  on. 

Let  us  suppose  that  the  mercury  is  observed  to  stand  at  a  height  greater 
than  29  inches  and  5  tenths,  but  less  than  29  inches  and  6  tenths.  Its  level 
being  expressed  by  the  line  M,  figure  7,  let  the  vernier  now  be  moved  on 
.vile  until  its  highest  division  0  exactly  coincides  with  the  level  of  the 
..»  •  ..ry.  On  comparing  the  several  divisions  of  the  vernier  with  those  of  the 
.nstrument,  let  us  suppose  that  we  find  that  the  division  marked  4  on  the  ver- 
nier coincides  with  that  marked  1  on  the  instrument ;  then  the  distance  from 
the  level  of  the  mercury  M  to  the  next  division  below  it,  marked  5,  will  be  4 
hundredth  parts  of  an  inch,  for  the  distance  of  the  division  marked  3  on  the 
vernier  above  the  division  marked  2  on  the  instrument  is  1  hundredth  of  an 
inch,  because  it  is  the  difference  between  a  division  of  the  vernier  and  a  divis- 
ion of  the  instrument.  Again,  the  distance  of  the  division  of  the  vernier 
marked  2,  above  the  division  cf  the  instrument  marked  3,  is  2  hundredths  of 
an  inch,  and  the  distance  of  the  division  of  the  vernier  marked  1,  above  the 
division  of  the  instrument  marked  4,  is  3  hundredths  of  an  inch.  In  like  man- 
ner, the  division  of  the  vernier  marked  0  is  distant  from  the  division  of  the  in- 
strument marked  5  by  4  hundredths  of  an  inch.  This  will  be  manifest  by 
considering  what  has  already  been  explained.  In  general,  we  are  to  observe 
what  division  of  the  vernier  coincides  most  nearly  with  any  division  of  the  in- 
strument, and  the  figure  which  marks  that  division,  of  the  vernier  will  express 
the  number  of  hundredths  of  an  inch  in  the  distance  of  the  level  of  the  mercury 
from  the  next  division  of  the  instrument  below  it. 

The  most  immediate  use  of  the  barometer  for  scientific  purposes  is  to  indi- 
cate the  amount  and  variation  of  the  atmospheric  pressure.  These  variations 
being  compared  with  other  meteorological  phenomena,  form  the  scientific  data 
from  which  various  atmospheric  appearances  and  effects  are  to  be  deduced. 

The  fluctuations  in  ths  pressure  of  the  atmosphere  being  observed  in  con- 
nexion with  changes  in  the  state  of  the  weather,  a  general  correspondence  is 
supposed  to  prevail  between  these  effects.  Hence  the  baiometer  has  been 
called  a  weath.:r-f,lass  Rules  are  attempted  to  be  established,  by  which,  from 
the  height  of  the  mercury,  the  coming  state  of  the  weather  may  be  predicted ; 

19 


290 


s 


THE  BAROMETER. 


and  we  accordingly  find  the  words  "  rain,"  "  fair,"  "  changeable,"  "  frost,"  &c., 
engraved  on  the  scale  attached  to  common  domestic  barometers,  as  if,  when 
the  mercury  stands  at  the  height  marked  by  these  words,  the  weather  is  always 
subject  to  the  vicissitudes  expressed  by  them.  These  marks  are,  however, 
entitled  to  no  attention  ;  and  it  is  only  surprising  to  find  their  use  continued  in 
the  present  times,  when  knowledge  is  so  widely  diffused.  They  are.  in. fact, 
to  be  ranked  scarcely  above  the  vox  stellarum,  or  astrological  almanac. 

It  has  been  already  explained,  that  in  the  same  state  of  the  atmosphere  the 
height  of  the  mercury  in  the  barometer  will  be  different,  according  to  the  eleva- 
tion of  the  place  in  which  the  barometer  is  situated.  Thus  two  barometers, 
one  near  the  level  of  the  Hudson  and  the  other  on  the  heights  of  West 
Point,  will  differ  by  half  an  inch  ;  the  latter  being  half  an  inch  lower  than  the 
former.  If  the  words,  therefore,  engraved  upon  the  plates,  are  to  be  relied 
upon,  similar  changes  of  weather  could  never  happen  at  these  two  situations.  But 
what  is  even  more  absurd,  such  a  scale  would  inform  us  that  the  weather  at  the 
top  of  a  high  building,  such  as  Trinity  church,  New  York,  must  always  be 
different  from  the  weather  in  Wall  street,  at  its  foot. 

The  variation  in  the  altitude  of  the  barometer  in  a  given  place,  together  with, 
the  corresponding  vicissitudes  of  the  weather,  have  been  regularly  recorded 
for  very  long  periods.  It  is  only  by  the  exact  comparison  of  such  results  that 
any  general  rule  can  be  found.  The  rules  best  established  by  such  observations 
are  far  from  being  either  general  or  certain.  It  is  observed  that  the  changes 
of  weather  are  indicated,  not  by  the  actual  height  of  the  mercury,  but  by  its 
change  of  height.  One  of  the  most  general,  though  not  absolutely  invariable 
rules  is,  that  when  the  mercury  is  very  low,  and  therefore  the  atmosphere  very 
light,  high  winds  and  storms  may  be  expected. 

The  following  rules  may  generally  be  relied  upon,  at  least  to  a  certain  ex- 
tent : — 

1 .  Generally  the  rising  of  the  mercury  indicates  the  approach  of  fair  weather : 
the  falling  of  it  shows  the  approach  of  foul  weather. 

2.  In  sultry  weather  the  fall  of  the  mercury  indicates  coming  thunder.     In 
winter  the  rise  of  the  mercury  indicates  frost.     In  frost  its  fall  indicates  thaw ; 
and  its  rise  indicates  snow. 

3.  Whatever  change  of  weather  suddenly  follows  a  change  in  the  barome- 
ter may  be  expected  to  last  but  a  short  time.     Thus,  if  fair  weather  follow  im- 
mediately the  rise  of  the  mercury,  there  will  be  very  little  of  it ;  and  in  the 
same  way,  if  foul  weather  follow  the  fall  of  mercury  it  will  last  but  a  short 
time. 

4.  If  fair  weather  continue  for  several  days,  during  which  the  mercury  con- 
tinually falls,  a  long  succession  of  foul  weather  will   probably  ensue  ;  and 
again,  if  foul  weather  continue  for  several  days,  while  the  mercury  continually 
rises,  a  long  succession  of  fair  weather  will  probably  succeed. 

5.  A   fluctuating   and   unsettled  state   in  the   mercurial   column   indicates 
changeable  weather. 

The  domestic  barometer  would  become  a  much  more  useful  instrument  if 
instead  of  the  words  usually  engraved  on  the  plate,  a  short  list  of  the  best  es- 
tablished rules,  such  as  the  above,  accompanied  it,  which  might  be  either  en- 
graved on  the  plate,  or  printed  on  a  card.  It  would  be  right,  however,  to  ex- 
press the  rules  only  with  that  degree  of  probability  which  observation  of  past 
phenomena  has  justified.  There  is  no  rule  respecting  these  effects  which  will 
hold  good  with  perfect  certainty  in  every  case. 

One  of  the  most  important  scientific  uses  to  which  the  barometer  has  been 
applied,  is  the  measuring  of  heights.  If  the  atmosphere,  like  a  liquid,  were 
incompressible,  this  problem  would  be  very  simple.  The  pressure  on  the  met- 


cury  in  the  cistern  would  be  equally  diminished  in  ascending  through  equal 
heights.  Thus,  if  the  pressure  produced  by  an  ascent  of  10  feet  were  equiva- 
lent to  the  weight  of  one  inch  of  mercury,  then  the  column  would  fall  one  inch 
in  ascending  that  height.  It  would  fall  two  inches  in  ascending  20  feet,  three 
in  ascending  30  feet,  and  so  on.  To  find,  therefore,  the  perpendicular  height 
of  ths  barometer  at  any  time  above  its  .position,  at  any  other  time,  it  would  be 
only  necessary  to  observe  the  difference  between  the  altitude  of  the  mercury 
in  both  cases,  and  to  allow  10  feet  for  every  inch  of  mercury  in  that  difference  ; 
and  a  similar  process  would  be  applicable  if  an  inch  of  mercury  corresponded 
to  any  other  number  of  feet. 

But  this  explanation  proceeds  on  the  supposition  that  in  ascending  through 
equal  heights,  the  barometer  leaves  equal  weights  of  air  below  it.  Suppose 
in  ascending  10  feet  the  mercury  is  observed  to  fall  the  hundredth  of  an  inch, 
then  it  follows,  that  the  air  left  below  the  barometer  in  such  an  ascent  has  a 
weight  equal  to  the  one  hundredth  of  an  inch  of  mercury.  Now.  in  ascending 
the  next  ten  feet,  the  air  which  occupies  that  space  having  a  less  weight  above 
it  will  be  less  compressed,  and,  consequently,  within  that  height  of  10  feet 
there  will  be  contained  a  less  quantity  of  air  than  was  contained  in  the  first  10 
feet  immediately  below  it.  In  this  second  ascent  the  mercury  will,  therefore, 
fall,  not  the  hundredth  of  an  inch,  but  a  quantity  as  much  less  than  the  hun- 
dredth of  an  inch  as  the  quantity  of  air  contained  in  the  second  10  feet  of 
height  is  less  than  the  quantity  of  air  that  is  contained  in  the  first  10  feet  of 
height.  In  like  manner,  in  ascending  the  next  ten  feet  a  still  less  quantity  of 
air  will  be  left  below  the  instrument,  and  the  mercury  will  fall  in  a  proportion- 
ally less  degree.  If  the  only  cause  affecting  density  of  the  air  were  com- 
pression produced  by  the  weight  of  the  incumbent  atmosphere,  it  would  be 
easy  to  find  the  rule  by  which  a  change  of  altitude  might  be  inferred  from  an 
observed  change  of  pressure.  Such  a  rule  has  been  determined,  and  is  capa- 
ble of  being  expressed  in  the  language  of  mathematics,  although  it  is  not  of  a 
nature  which  admits  of  explanation  in  a  more  elementary  and  popular  form. 
But  there  are  other  causes  affecting  the  relation  of  the  pressure  to  the  altitude 
which  must  be  taken  into  account.  The  density  of  any  stratum  of  air  is  not  only 
affected  by  the  weight  of  the  incumbent  atmosphere,  but  also  by  the  temperature 
of  the  stratum  itself.  If  any  cause  increase  this  temperature  the  stratum  will 
expand,  and,  with  a  less  density,  will  support  the  same  incumbent  pressure.  If, 
on  the  contrary,  any  cause  produce,  a  diminution  of  temperature,  the  stratum 
will  contract,  and  acquire  a  greater  density  under  the  same  pressure.  In  the 
one  case,  therefore,  a  change  of  elevation  which  would  be  necessary  to  pro- 
duce a  given  change  in  the  height  of  the  barometer,  would  be  greater  than 
that  computed  on  theoretical  principles,  and  in  the  other  case  the  change  would 
be  less.  The  temperature,  therefore,  forms  an  essential  clement  in  the  calcu- 
lation of  heights  by  the  barometer. 

A  rule  or  formulary  has  been  deduced,  partly  from  established  theory,  and 
partly  from  observed  effects,  by  which  the  change  of  elevation  may  be  deduced 
from  observations  made  on  the  barometer  and  thermometer.  To  apply  that 
rule,  it  is  necessary  to  know.  1st,  the  latitude  of  the  places  of  observation  ;  2d, 
the  height  of  the  barometer  and  thermometer  at  the  higher  station.  By  arith- 
metical computation  the  difference  of  the  levels  of  the  two  stations  may  then 
be  calculated.  The  formulary  does  not  admit  of  being  explained  without  the 
use  of  mathematical  language. 

It  has  been  already  stated,  that  the  atmospheric  pressure  at  the  surface  of 
the  earth  is  capable  of  supporting  a  column  of  water  34  feet  in  height.  It  fol- 
lows, therefore,  that  if  our  atmosphere  were  condensed  to  such  a  degree  that 
its  specific  gravity  would  be  equal  to  that  of  water,  its  height  would  be  34 


292  THE  BAROMETER. 


feet.  Now  the  specific  gravity  of  a  stratum  of  atmosphere  contiguous  to  the 
surface  is  about  840  times  less  than  the  specific  gravity  of  water ;  that  is,  a 
cubic  inch  of  water  weighs  840  times  more  than  a  cubic  inch  of  air.  If  as 
we  ascend  in  the  atmosphere  it  continued  to  have  the  same  density,  then  its 
height  would  be  evidently  840  times  the  height  of  34  feet,  which  would  amount 
to  28,560  feet,  or  5  miles  and  a  quarter.  It  is  obvious,  therefore,  that  since 
v  even  at  a  small  elevation  the  density  of  the  atmosphere  is  reduced  to  half  its 
density  at  the  surface,  the  whole  height  must  be  many  times  greater  than  this. 
The  barometer  in  the  balloon  in  which  De  Luc  ascended,  fell  to  the  height  of 
12  inches.  Supposing  the  barometer  at  the  surface  to  have  stood  at  that  time 
at  30  inches,  it  follows  that  he  must  have  left  three  fifths  of  the  whole  atmo- 
sphere below  him.  His  elevation  was  upward  of  20,000  feet. 

A  column  of  pure  mercury,  whose  base  is  a  square  inch,  and  whose  height 
is  30  inches,  weighs  about  15  Ibs.  avoirdupois.  It  follows,  therefore,  that 
when  the  barometer  stands  at  30  inches  the  atmosphere  exerts  a  pressure  on 
each  square  inch  of  the  surface  of  the  mercury  on  the  cistern,  amounting  to 
15  Ibs.  Now  it  is  the  nature  of  a  fluid  to  transmit  pressure  equally  in  every 
direction,  and  if  the  surface  on  which  the  atmosphere  acts  were  presented  to 
it  laterally,  obliquely,  or  downward,  still  the  pressure  will  be  the  same.  Ta- 
king, therefore,  the  medium  height  of  the  barometric  column  at  30  inches,  it 
follows  that  the  pressure  sustained  by  all  bodies  which  exist  at  the  surface  of 
the  earth,  exposed  to  our  atmosphere,  are  continually  under  this  pressure,  and 
that  every  square  inch  on  their  surface  constantly  sustains  a  force  of  about  15 
pounds.  Thus  the  body  of  a  man  the  surface  of  which  amounts  to  2,000 
square  inches,  will  sustain  a  pressure  from  the  surrounding  air  to  the  enor- 
mous amount  of  30,000  pounds. 

It  might  at  first  view  be  expected  that  this  great  force  to  which  all  bodies 
are  subject,  would  produce  manifest  effects,  so  as  to  crush,  compress,  or  break 
them,  whereas  we  find  bodies  of  most  delicate  texture  unaffected  by  it.  Thus 
a  close  bag,  made  of  the  finest  silver  paper,  and  partially  filled  with  air,  is  ap- 
parently subject  to  no  external  force.  Its  sides  do  not  collapse.  This  arises 
partly  from  the  circumstance  of  the  pressure  on  every  side  and  in  every  direction 
being  equal,  and,  therefore,  producing  mechanical  equilibrium.  It  is  obvious 
that  a  body  which  is  driven  in  every  possible  direction,  upward  and  downward, 
laterally  and  obliquely,  with  equal  forces,  will  not  move  in  any  one  direction, 
for  to  suppose  such  a  motion  would  be  to  assume  that  the  quantity  of  pressure 
in  that  direction  exceeds  the  quantity  of  pressure  in  other  directions.  But 
still,  though  a  body  may  not  be  driven  in  any  direction  by  the  atmospheric 
pressure,  it  may  happen  that  its  parts  are  crushed  and  compressed. 

We  do  not,  however,  find  this  to  happen.  This  arises  from  the  fact, 'that  the 
elastic  force  of  the  air  is  equal  to  its  pressure  ;  and  since  the  internal  cauties 
of  a  body,  such  as  the  thin  bag  above-mentioned,  are  filled  with  air,  whi<  h  is 
confined  within  them,  that  air  has  precisely  the  same  tendency  to  swell  the 
bag,  and  to  keep  the  parts  asunder,  as  the  external  pressure  of  the  atmosphere 
has  to  make  them  collapse. 

In  the  same  manner  we  may  account  for  the  fact  that  animals  move  freely  in 
the  air  without  being  sensible  of  the  enormous  pressure  to  which  their  bodies 
are  subjected.  The  internal  parts  of  their  bodies  are  filled  with  fluids,  both  in 
the  liquid  and  gaseous  states,  which  offer  a  pressure  from  within  exactly  equiv- 
alent to  the  external  pressure  of  the  air.  This  may  be  easily  rendered  mani- 
fest by  applying  to  the  skin  the  mouth  of  a  close  vessel  to  which  an  exhausting 
syringe  is  attached.  By  this  instrument,  which  will  be  described  hereafter, 
the  air  may  be  rarefied  in  the  vessel,  and  the  atmospheric  pressure  conse- 
quently partially  removed  from  the  skin.  Immediately  the  force  of  the  fluid 


293 


from  within  will  swell  the  skin  and  cause  it  to  be  sucked  into  the  glass.  This 
experiment  may  be  performed  by  the  mouth  on  the  flesh  of  the  hand  or  arm. 
If  the  lips  be  applied  to  the  flesh,  and  the  breath  drawn  in  so  as  to  produce  a 
partial  vacuum  in  the  mouth,  the  skin  will  be  drawn  or  sucked  into  the  mouth. 
This  effect  is  owing,  not  to  any  force  resident  in  the  lips  or  the  mouth  drawing 
the  skin  in,  but  to  the  fact  that  the  usual  external  pressure  is  removed,  and 
that  the  piessure  from  within  is  suffered  to  prevail. 

All  cases  of  that  class  of  effects  which  are  commonly  expressed  by  the 
word  suction  are  accounted  for  in  the  same  manner. 

If  a  flat  piece  of  moist  leather  be  put  in  close  contact  with  a  heavy  body,  as 
a  stone,  it  will  be  found  to  adhere  to  it  with  considerable  force,  and  if  a  cord 
of  sufficient  length  be  attached  to  the  centre  of  the  leather,  the  stone  may  be 
raised  by  the  cord.  This  effect  arises  from  the  exclusion  of  the  air  between 
the  leather  and  the  stone.  The  weight  of  the  atmosphere  presses  their  sur- 
faces together  with  a  force  amounting  to  fifteen  pounds  on  every  square  inch 
of  those  surfaces  in  contact.  If  the  weight  of  the  stone  be  less  than  the  num- 
ber of  pounds  which  would  be  expressed  by  multiplying  the  number  of  square 
inches  on  the  surfaces  of  contact  by  fifteen,  then  the  stone  may  be  raised  by 
the  leather  ;  but  if  the  stone  exceed  this  weight,  it  will  not  suffer  itself  to  be  el- 
evated by  these  means. 

'.^he  power  of  flies  and  other  insects  to  walk  on  ceilings  and  surfaces  pre- 
sented downward,  or  upon  smooth  panes  of  glass  in  an  upright  position,  is  said 
to  depend  on  the  formation  of  their  feet.  This  is  such  that  they  act  in  the 
manner  above  described  respecting  the  leather  attached  to  a  stone  ;  the  feet,  in 
fact,  act  as  suckers,  excluding  the  air  between  them  and  the  surface  with  which 
they  are  in  contact,  and  the  atmospheric  pressure  keeps  the  animal  in  its  po- 
sition. In  the  same  manner  the  hydrostatic  pressure  attaches  fishes  to  rocks. 

The  pressure  and  elasticity  of  the  air  are  both  exercised  in  the  act  of 
breathing.  When  we  draw  in  the  breath  we  first  make  an  enlarged  space  in 
the  chest.  The  pressure  of  the  external  atmosphere  then  forces  air  into  this 
space  so  as  to  fill  it.  By  a  muscular  action  the  lungs  are  next  compressed  so 
as  to  give  this  air  a  greater  elasticity  than  the  pressure  of  the  external  atmo- 
sphere. By  the  excess  of  this  elasticity  it  is  propelled,  and  escapes  by  the 
mouth  and  nose.  It  is  obvious,  therefore,  that  the  air  enters  the  lungs  not  by 
any  direct  act  of  these  upon  it,  but  by  the  weight  of  the  atmosphere  forcing  it 
into  an  empty  space,  and  that  it  is  expired  by  the  action  of  the  lungs  in  com- 
pressing it. 

The  action  of  common  bellows  is  precisely  similar,  except  that  the  aperture 
at  which  the  air  is  drawn  in  is  different  from  that  at  which  it  is  expelled.  In  the 
lower  board  of  the  bellows  is  a  hole  covered  by  a  valve,  consisting  of  a  flat 
piece  of  stiff  leather,  moveable  on  a  hinge,  and  which  lies  on  the  hole,  but  is 
capable  of  being  raised  by  a  slight  pressure.  When  the  upper  board  of  the 
bellows  is  raised,  the  internal  cavity  is  suddenly  enlarged,  arid  the  air  contained 
in  it  is  considerably  rarefied.  The  pressure  of  the  atmosphere  forces  in  air  at 
the  nozzle,  but  this  being  too  small  to  allow  its  admission  with  sufficient  ease 
and  speed,  the  valve  covering  the  hole  is  acted  upon  by  the  atmosphere  and 
raised,  aud  air  rushes  in  through  the  large  aperture  under  it.  When  the  space 
between  the  boards  is  filled  with  air  in  its  common  state,  the  upper  board  is 
depressed,  and  the  air  confined  in  the  bellows  is  suddenly  condensed.  The 
valve  covering  the  hole  is  thus  kept  firmly  closed,  and  the  air  has  no  escape 
except  through  the  nozzle,  from  which  it  issues  with  a  force  proportioned  to  the 
pressure  exerted  on  the  upper  board.  A  bellows,  such  as  that  in  common  do- 
mestic use,  thus  simply  constructed,  has  an  intermitting  action  and  blows  by 
fits,  its  action  being  suspended  while  the  upper  board  is  being  raised.  T" 


In 


294  THE  BAROMETEE. 


forces  and  large  factories  in  which  fires  are  extensively  used,  it  is  found  neces- 
sary to  command  a  constant  and  unremitting  stream  of  air,  which  may  be  con- 
ducted through  the  fuel  so  as  to  keep  it  in  vivid  combustion.  This  is  effected 
by  bellows  with  three  boards,  the  centre  board  being  fixed  and  furnished  with 
a  valve  opening  upward,  the  lower  board  being  moveable  with  a  valve  also  open- 
ing upward,  and  the  upper  board  being  under  a  continual  pressure  by  weights 
acting  upon  it.  When  the  lower  board  is  let  down,  so  that  the  chamber  be- 
tween it  and  the  middle  board  is  enlarged,  the  air  included  between  these 
boards  being  rarefied,  the  external  pressure  of  the  atmosphere  will  open  the 
valve  in  the  lower  board,  and  the  chamber  between  the  lower  and  middle  boards 
will  be  filled  with  air  in  its  common  state.  The  lower  board  is  now  raised  by 
the  power  which  works  the  bellows,  and  the  air  between  it  and  the  middle 
board  is  condensed.  It  cannot  escape  through  the  lower  valve,  because  it 
opens  upward.  It  acts,  therefore,  with  a  pressure  proportional  to  the  working 
power  on  the  valve  in  the  middle  board,  and  it  forces  open  this  valve,  which 
opens  upward.  The  air  is  driven  from  between  the  lower  and  middle  boards 
into  the  chamber  between  the  middle  and  upper  boards.  It  cannot  return  from 
this  chamber,  because  the  valve  in  the  middle  board  opens  upward.  The  up- 
per board  being  loaded  with  weights,  it  will  be  condensed  while  included  in 
this  chamber,  and  will  issue  from  the  nozzle  with  a  force  proportionate  to  the 
weights.  While  the  air  is  thus  rushing  from  the  nozzle  the  lower  board  is  let 
down  and  again  drawn  up,  and  a  fresh  supply  of  air  is  brought  into  the  cham- 
ber between  the  upper  and  middle  board.  This  air  is  introduced  between  the 
middle  and  upper  boards  before  the  former  supply  has  been  exhausted,  and  by 
working  the  bellows  with  sufficient  speed,  a  large  quantity  of  air  will  be  col- 
lected in  the  upper  chamber,  so  that  the  weights  on  the  upper  board  will  force 
a  continual  stream  of  air  through  the  nozzle. 

The  effect  produced  by  a  vent-peg  in  a  cask  of  liquid  depends  on  the  atmo- 
spheric pressure.  If  the  vent-peg  stop  the  hole  in  the  top  while  the  liquid  is 
discharged  by  the  cock  below,  a  space  will  remain  at  the  top  of  the  barrel  in 
which  the  air  originally  confined  is  allowed  to  expand  and  become  rarefied ; 
its  pressure  on  the  surface  of  the  liquid  above  will,  therefore,  be  less  than  the 
atmospheric  pressure  resisting  the  escape  of  the  liquid  at  the  cock ;  but  still 
the  weight  of  the  liquid  itself,  pressing  downward  toward  the  cock,  will  cause 
the  discharge  to  continue  until  the  rarefaction  of  the  air  becomes  so  great,  that 
the  excess  of  the  atmospheric  pressure  is  more  than  sufficient  to  resist  the  es- 
cape of  the  liquid  ;  the  flow  from  the  cock  will  therefore  be  stopped.  If  the 
vent-peg  be  now  removed  from  the  hole,  air  will  be  heard  to  rush  in  with  con- 
siderable force  and  fill  the  space  above  the  liquid.  The  atmospheric  pressure 
on  the  surface  above  and  on  the  mouth  of  the  cock  being  now  equal,  the  liquid 
will  escape  from  the  cock  by  the  effect  of  the  pressure  of  the  superior  column, 
according  to  the  principles  established  in  hydrostatics.  If  the  vent-plug  be 
again  placed  in  the  hole,  the  flow  from  the  cock  will  be  gradually  diminished, 
and  will  at  length  cease.  Upon  the  removal  of  the  vent-peg,  the  same  effect 
will  be  observed  as  before. 

If  the  lid  of  a  teapot  be  perfectly  close,  and  fit  the  mouth  air  tight,  or  if  the 
interstices,  as  frequently  happens,  be  stopped  by  the  liquid  which  lies  round 
the  edge  of  the  mouth,  then  all  communication  between  the  surface  of  the  li- 
quid in  the  vessel  and  the  external  air  is  cut  off.  If  we  now  attempt  to  pour 
liquid  from  the  teapot  it  will  flow  at  first,  but  will  immediately  cease.  In  this 
case  the  air  under  the  lid  becomes  rarefied,  and  the  pressure  on  the  surface  of 
the  liquid  in  the  teapot  is  so  far  diminished,  that  the  atmospheric  pressure  re- 
sists its  discharge  at  the  spout. 

To  remedy  this  inconvenience,  it  is  usual  to  make  a  small  hole  somewhere 


THE  BAROMETER. 


295 


in  the  lid  of  the  teapot  for  the  admission  of  air ;  this  hole  serves  the  same 
purpose  as  the  hole  for  the  vent-peg  in  the  cask. 

Although  it  is  not  usually  practised,  a  small  hole  should  he  made  in  the  lid 
of  a  kettle,  but  for  a  different  reason.  If  the  lid  of  a  kettle  fit  it  closely,  so  as 
stop  all  communication  between  the  external  air  and  the  interior  of  the  vessel, 
when  the  water  contained  in  it  becomes  heated,  steam  will  rise  from  its  surface, 
3rd  the  air  enclosed  in  the  space  between  the  surface  and  the  lid  being  heated, 
•wfll  acquire  an  increased  elastic  force.  From  these  causes,  the  pressure 
which  acts  on  the  surface  of  the  water  in  the  kettle  will  continually  increase 
go  long  as  the  lid  maintains  its  position  ;  this  pressure,  transmitted  by  the  wa- 
ter in  the  kettle,  will  overcome  the  pressure  of  the  atmosphere  acting  on  the 
water  in  the  spout,  and  the  effect  will  be  that  the  water  will  be  raised  in  the 
spout,  and  flow  from  it,  or,  if  the  lid  be  not  firmly  enough  fixed  to  withstand 
the  pressure  of  the  steam,  it  will  be  blown  off  the  kettle.  Such  effects  fall 
within  every  one's  experience.  If  a  small  hole  were  made  in  the  lid  these 
effects  would  be  prevented. 

Ink-bottles  constructed  so  as  to  prevent  the  inconvenience  of  the  ink  thicken- 
ing and  drying,  owe  their  efficacy  to  the  atmospheric  pressure.  The  quantity 
of  evaporation  which  takes  place  in  the  liquid,  other  circumstances  being  the 
same,  is  proportional  to  the  quantity  of  surface  exposed  to  the  external  air.  To 
diminish  this  quantity  of  surface  without  inconveniently  diminishing  the  quan- 
tity of  ink  in  the  bottle,  bottles  have  been  constructed  of  the  shape  represented 
in  figure  8. 


A  B  is  a  close  glass  vessel,  from  the  bottom  of  which  a  short  tube,  B,  pro- 
ceeds, from  which  another  short  tube  rises  perpendicularly.  The  depth  of  the 
tube  C  is  such  as  will  be  sufficient  for  the  immersion  of  the  pen.  When  ink 
is  poured  in  at  C,  the  bottle,  being  placed  in  an  inclined  position,  is  gradually 
filled  up  to  the  knob  A  :  if  the  bottle  be  now  placed  in  the  position  represented 
in  the  figure,  the  chamber  A  B  being  filled  with  the  liquid,  the  air  will  be  ex- 
cluded from  it,  and  the  pressure  tending  to  force  the  ink  upward  in  the  short 
tube  C,  will  be  equal  to  the  weight  of  the  column  of  ink,  the  height  of  which 
is  equal  to  the  depth  of  the  ink  in  the  bottle  A  B,  and  the  base  of  which  is 
equal  to  the  section  of  the  tube  C.  This  will  be  manifest  from  the  proper- 
ties of  hydrostatic  pressure,  established  in  hydrostatics.  Now,  the  atmo- 
spheric pressure  acts  on  the  surface  C  with  a  force  which  would  be  capable 
of  sustaining  a  column  of  ink  many  times  the  height  of  the  bottle  A  B  ;  conse- 
sequently,  thrs  pressure  will  effectually  resist  the  escape  of  the  ink  from  the 
mouth  C,  and  will  keep  it  suspended  in  the  bottle  A  B.  In  this  case  the 
whole  surface  which  is  exposed  to  the  effect  of  evaporation,  is  the  surface  of 
liquid  in  the  tube  C,  and,  consequently,  an  ink  bottle  of  this  kind  may  be  left 
many  months  in  a  warm  room  and  no  perceptible  diminution  in  the  quantity  of 
ink  or  change  in  its  quality  will  take  place.  As  the  ink  in  the  short  tube  C  is 
consumed  by  use,  its  surface  will  fall  to  a  level  with  the  tube  B.  A  small 


296  THE  BAROMETER. 


bubble  of  air  will  then  insinuate  itself  through  the  tube  r,  and  will  rise  to  the  ( 
top  of  the  bottle  A  B  ;  there  it  will  exert  an  elastic  pressure,  which  will  cause 
the  surface  in  C  to  rise  a  little  higher,  and  this  effect  will  be  continually  re-  ) 
peated  until  all  the  ink  in  the  bottle  has  been  used. 

The  only  inconvenience  which  has  been  attributed  to  these  ink-bottles  arises  f 
from  sudden  changes  in  the  temperature  to  which  they  are  exposed.  When  ; 
the  external  air,  having  been  previously  warm,  becomes  suddenly  cold,  the  \ 
small  quantity  of  air  which  is  included  in  the  bottle  A  not  being  cooled  so  fast  / 
as  the  external  air,  will  exert  an  elastic  pressure  which  will  cause  the  ink  to  I 
flow  at  C.  This  is  an  effect,  however,  which  we  have  never  observed,  al-  £ 
though  we  have  seen  these  bottles  much  used.  < 

If  such  an  ink-bottle  be  placed  upon  a  marble  chimney-piece,  or  any  other 
surface  heated  beyond  the  temperature  of  the  air  in  the  room,  the  air  confined 
in  the  bottle  will  then  become  heated,  and  acquire  increased  elastic  force,  and 
in  this  case  the  ink  will  overflow. 

The  fountains  for  supplying  water  to  bird-cages  are  constructed  upon  the 
same  principle. 

The  pneumatic  trough  used  in  the  chemical  laboratories,  and  the  gas-hold- 
ers or  gasometers  used  in  gas  works,  depend  on  the  atmospheric  pressure.  A 
vessel  having  its  mouth  upward,  is  completely  filled  with  a  liquid.  The  mouth 
is  then  stopped,  a  flat  piece  of  glass,  or  a  smooth  plate  of  metal,  pressed 
against  it,  and  the  vessel  is  inverted,  the  mouth  being  plunged  in  a  cistern 
filled  with  the  same  liquid.  If  the  height  of  the  vessel  in  this  case  be  less 
than  the  height  of  the  column  of  the  liquid  which  the  atmospheric  pressure 
would  support,  the  vessel  will  continue  to  be  completely  filled  with  the  liquid, 
even  after  the  plate  is  removed  from  its  mouth ;  for  the  atmospheric  pressure, 
acting  on  the  surface  of  the  liquid  in  the  cistern,  will  prevent  the  liquid  con- 
tained in  the  vessel  from  falling  out  of  it.  Any  one  may  satisfy  himself  of  this 
fact.  Take  a  wine-glass  and.  fill  it  with  water,  and  then,  having  applied  a 
piece  of  card  to  its  mouth  so  as  to  prevent  the  water  from  escaping,  invert  it, 
and  plunge  the  mouth  downward  in  a  basin  of  water.  Let  the  card  be  then 
removed,  and  let  the  glass  be  raised  above  the  surface,  still,  however,  keeping 
the  edge  of  its  mouth  below  the  surface.  It  will  be  observed  that  the  glass 
will  still  remain  completely  filled  with  water.  Take  a  small  quill,  or  a  hol- 
low piece  of  straw,  and  insert  one  end  in  the  water,  so  that  it  will  be  im- 
mediately below  the  mouth  of  the  glass,  and  at  the  same  time  blow  gently 


»  through  the  other  end,  so  as  to  introduce  air  in  small  quantities  into  the  water 
)  immediately  under  the  mouth  of  the  glass.     This  air  will  ascend  in  bubbles, 
and  will  find  its  way  to  the  highest  part  of  the  glass,  and,  remaining  there, 
will  expel  the  water  from  it ;  and  this  will  continue  so  long  as  air  is  supplied, 
until  all  the  water  contained  in  the  glass  is  expelled  from  it,  and  the  glass  is 
7  filled  with  air.      If  the   process  be  further  continued,  the  air  will  begin  to 
(  escape  under  the  edge  of  the  glass,  and  rise  in  bubbles  to  the  surface. 
''       The   pneumatic  trough  is  a  large  cistern  filled  with  mercury,  in  which  is 
placed,  below  the  surface  of  the  liquid,  a  shelf  to  support  a  receiver.     By 
plunging  any  vessel  in  the  deeper  part  of  the  trough,  it  may  be  filled  with  mer- 
cury, and  if  it  be  slowly  raised,  keeping  its  mouth  still  below  the  surface  of 
the  liquid,  it  will  still  remain  filled  with  mercury  by  the  pressure  of  the  atmo- 
sphere acting  on  the  surface  of  the  mercury  in  the  trough.     The  mouth  of  the 
vessel  may  then  be  placed  on  the  shelf,  while  the  vessel  itself  is  above  the 
surface  of  the  mercury. 

The  trough  is  represented  in  fig.  9,  at  A  B.  The  shelf  is  placed  in  it  at  C  ; 
a  receiver,  R,  is  placed  on  the  shelf,  with  its  mouth  downward,  over  an  aper- 
ture, D,  which  communicates  with  a  tube,  by  which  gas  may  be  introduced. 


THE  BAROMETER. 


297 


The  gas,  passing  through  the  tube,  rises  in  bubbles  through  the  mercury  in  the 
receiver,  and  lodges  at  the  top,  and,  by  continuing  this  process,  the  whole  of 
the  mercury  will  at  length  be  expelled  from  the  receiver,  and  its  place  filled 
with  the  gas.  In  this  manner  gases  of  various  kinds  may  be  preserved  out  of 
contact  with  the  atmosphere,  and  the  same  shelf  may  be  furnished  with  several 
holes,  and  may  support  a  number  of  different  jars. 

The  gasometer  used  in  gas-works  is  constructed  on  the  same  principle,  only 
on  a  different  scale.  When  used  for  great  supplies  of  gas,  such  as  are  neces- 
sary for  the  illumination  of  towns,  these  vessels  are  constructed  of  a  very  large 
size,  and  are  immersed  in  pits  lined  with  cast-iron,  and  filled  with  water.  It 
is  clear  that  all  which  has  been  just  explained  will  be  equally  applicable,  what- 
ever be  the  liquid  used  in  the  cistern,  'and  for  different  gases  it  is  necessary  to 
use  different  liquids,  since  the  contact  with  particular  liquids  will  frequently 
affect  the  quality  of  the  gas.  The  peculiar  gurgling  noise  which  is  produced 
in  decanting  wine  arises  from  the  pressure  of  the  atmosphere  forcing  air  into 
the  interior  of  the  bottle.  In  the  first  instance,  the  neck  of  the  bottle  is  com- 
pletely filled  with  liquid,  so  as  to  stop  the  admission  of  air.  When  a  part  of 
the  wine  has  flowed  out,  and  an  empty  space  is  formed  within  the  bottle,  the 
atmospheric  pressure  forces  in  a  bubble  of  air  through  the  liquid  in  the  neck, 
which,  by  rushing  suddenly  into  the  interior  of  the  bottle,  produces  the  sound 
alluded  to.  This  effect  is  continually  repeated  so  long  as  the  neck  of  the  bot- 
tle continues  to  be  choked  with  the  liquid.  But  as  the  contents  of  the  bottle 
are  discharged,  the  liquid,  in  flowing  out,  only  partially  fills  the  neck ;  and 
while  a  stream  of  wine  passes  out  through  the  lower  half  of  the  neck,  a  stream 
of  air  passes  in  through  the  upper  part.  The  flow  in  this  case  being  continual 
and  uninterrupted,  no  sound  takes  place. 

The  atmospheric  pressure,  acting  on  the  surface  of  liquids,  maintains  air 
combined  with  them  in  a  greater  or  lesser  quantity,  according  to  the  nature  of 
the  liquid.  If  an  open  vessel,  containing  a  liquid,  be  placed  under  a  receiver, 
and  the  air  be  exhausted,  the  air  combined  with  the  liquid  will  be  immediately 
set  free,  and  will  be  observed  to  rise  in  bubbles  to  the  top ;  this  effect  will  be 
very  perceptible  if  water  be  used,  but  still  more  so  in  the  case  of  beer  or  ale. 

When  liquor  is  bottled,  the  air  confined  under  the  cork  is  condensed,  and 
exerts  upon  the  surface  a  pressure  greater  than  that  of  the  atmosphere.  This 
has  the  effect  of  holding  in  combination  with  the  liquor  air  which,  under  the 
atmospheric  pressure  only,  would  escape.  If  any  air  rise  from  the  liquor  after 
being  bottled,  it  causes  a  still  greater  condensation,  and  an  increased  pressure 
above  its  surface. 


298 


THE  BAROMETER. 


If  the  nature  of  the  liquor  be  such  as  to  produce  air  in  considerable  quan- 
tity, this  condensation  will  at  length  become  so  great  as  to  force  out  the  cork; 
or,  failing  to  do  that,  break  the  bottle.  This  is  found  to  happen  frequently 
with  beer,  ale,  or  porter.  The  corks  in  such  cases  are  tied  down  by  cord  or 
wire. 

When  the  cork  is  drawn  from  a  bottle  containing  liquor  of  this  kind,  the 
fixed  air  being  released  from  the  pressure  of  the  air  which  was  condensed  un- 
der the  cork,  instantly  makes  its  escape,  and,  rising  in  bubbles,  produces  effer- 
vescence and  froth.  Hence  the  bead  observed  on  porter  and  similar  liquors 
and  the  sparkling  of  champagne  or  cider. 


I 


THE     MOON. 


Popular  Interest  attached  to  the  Moon. — Its  Distance. — Its  Rotation. — Same  Face  always  tow  ard 
the  Earth. — Its  Phases — Its  changes  of  Position  with  regard  to  the  Sun. — Has  it  an  Atmoxphcre  ?— 
Optical  Test  to  determine  it. — Physical  dualities  of  Moonlight. — Is  Moonlight  Warm  or  Cold  ? — 
Does  Water  exist  on  the  Moon  ? — Does  the  Moon  influence  the  Weather? — Mode  of  determining 
this. — Physical  condition  of  the  Lunar  Surface. — Absence  of  Air  and  Gases. — A  bsencc  of  Liquids. — 
Appearance  of  the  Earth  as  seen  from  the  Moon. — Prevalence  of  Mountains  upon  it. — Their  gen- 
eral Volcanic  Character. — Appearance  of  the  Mountain  Tycho. — Heights  of  Lunar  Mountains  aud 
Depths  of  Ravines. — Telescopic  Views  of  the  Moon  by  Beer  and  Madler. — Detached  Views  of 
the  Lunar  Surface. — Condition  of  a  Lunar  Crater  deducud  from  Analogy. 


THE  MOON. 


301 


THE  MOON. 


ALTHOUGH  it  be  in  mere  magnitude,  physically  considered,  one  of  the  most 
insignificant  bodies  of  the  solar  system,  yet  for  various  reasor  s  the  MOON  has 
always  been  regarded  by  mankind  with  feelings  of  profound  interest,  and  has 
been  invested  by  the  popular  mind  with  various  influences,  affecting  not  only 
the  physical  condition  of  the  globe,  but  also  connected  with  the  phenomena  of 
the  organized  world.  It  has  been  as  much  an  object  of  popular  superstition  as 
of  scientific  observation.  These  circumstances  doubtless  are  in  some  degree 
owing  to  its  striking  appearance  in  the  firmament,  to  the  various  changes  of 
form  to  which  it  is  subject,  and  above  all  to  its  proximity  to  the  earth,  and  to 
the  close  alliance  existing  between  it  and  our  planet.  It  will  not  be  uninter- 
esting on  the  present  occasion  to  collect  and  present  in  an  intelligible  form,  the 
results  of  scientific  research  concerning  this  body. 


THE    DISTANCE    OF    THE    MOON. 

The  distances  of  all  objects  in  the  heavens  are  ascertained  by  the  same 
general  principles  as  that  by  which  the  common  surveyor  determines  the  dis- 
tance of  inaccessible  objects  upon  the  earth.  It  need  scarcely  be  said  that 
a  very  small  proportion  of  the  terrestrial  distances  with  which  we  are  con- 
versant are  ascertained  by  the  actual  admeasurement  of  the  space  intervening 
between  their  extreme  points.  Other  more  easy  and  accurate  methods  are  avail- 
able, by  which  we  can  accurately  measure  the  distance  of  objects  inaccessible 
to  us,  by  ascertaining  the  proportion  between  these  distances  and  other  spaces 
which  are  accessible  and  measurable  by  us.  In  this  way  it  has  been  ascer- 
tained that  the  distance  of  the  MOON  is  equal  to  about  thirty  times  the  diameter 
of  our  globe,  or  in  round  numbers  a  quarter  of  a  million  of  miles. 


302 


THE  MOON. 


MAGNITUDE    OF    THE    MOON. 

When  the  distance  of  a  visible  object  is  determined,  its  magnitude  may 
easily  be  ascertained  by  comparing  it  directly  with  another  object  of  known 
magnitude  and  a  known  distance.  To  illustrate  this  by  its  application  to  the 
MOON,  let  us  take,  for  example,  a  cent-piece,  which  measures  about  an  inch  in 
diameter,  and  let  it  be  placed  between  the  eye  and  the  moon  at  any  distance 
from  the  eye.  It  will  be  found  on  the  first  trial  that  the  coin  will  appear  larger 
than  the  moon ;  it  will,  in  fact,  completely  conceal  the  moon  from  the  eye  and 
produce  what  may  be  termed  a  total  eclipse  of  that  luminary.  Let  the  coin  be 
moved  however  further  from  the  eye,  and  it  will  then  appear  smaller,  and  will 
apparently  diminish  in  size  as  the  distance  from  the  eye  is  increased.  Let  it 
be  removed  until  it  becomes  equal  in  apparent  magnitude  to  the  moon,  so  that 
it  will  exactly  cover  the  disk  of  the  moon,  and  neither  more  nor  less.  If  its 
distance  from  the  eye  be  then  measured,  it  will  be  found  to  be  about  ten  feet, 
or  one  hundred  and  twenty  inches,  or  what  is  the  same,  two  hundred  and  forty 
half  inches.  But  it  is  known  that  the  distance  of  the  moon  is  about  two  hun- 
dred and  forty  thousand  miles,  and  consequently  it  follows  in  this  case,  that 
one  thousand  miles  in  the  moon's  distance  is  exactly  what  half  an  inch  is  in 
the  coin's  distance.  Now  under  the  circumstances  here  supposed,  the  coin 
and  the  moon  are  similar  objects  of  equal  apparent  magnitude.  In  fact  the 
coin  is  another  moon  on  a  smaller  scale,  and  we  may  use  the  coin  to  measure 
the  moon's  distance,  provided  we  know  the  scale,  exactly  as  we  use  the  space 
upon  a  map  of  any  known  scale  to  measure  a  country.  But  it  has  been  just 
stated  that  the  scale  is  in  this  case  half  an  inch  to  one  thousand  miles ;  since, 
then,  the  coin  measures  two  half  inches  in  diameter,  the  moon  must  measure  i 
two  thousand  miles  in  diameter.  The  moon  is  then  a  globe  whose  diameter  ' 
is  about  one  fourth  of  that  of  the  earth.  Its  bulk  is  about  one  fiftieth  of  that  of  ( 
our  globe,  its  weight  a  little  less  than  one  fiftieth,  and  its  density  something  < 
less  than  three  fourths  of  the  density  of  the  earth. 

ROTATION    OF   THE    MOON. 

While  the  moon  moves  around  the  earth  in  its  monthly  course,  we  find  by 
observations  of  its  appearance,  made  even  without  the  aid  of  telescopes,  that 
the  same  hemisphere  is  always  turned  toward  us.  We  recognise  this  fact  by 
observing  that  the  same  marks  always  remain  in  the  same  place  upon  it.  Now, 
in  order  that  a  globe  which  revolves  in  a  circle  around  a  centre  should  turn 
continually  the  same  hemisphere  toward  that  centre,  it  is  necessary  that  it 
should  make  one  revolution  upon  its  axis  in  the  time  it  takes  so  to  revolve. 
For  let  us  suppose  that  the  globe,  in  any  one  position,  has  the  centre  round 
which  it  revolves  north  of  it,  the  hemisphere  turned  toward  the  centre  is  turned 
toward  the  north.  After  it  makes  a  quarter  of  a  revolution,  the  centre  is  to  die 
east  of  it,  and  the  hemisphere  which  was  previously  turned  to  the  north  must 
now  be  turned  to  the  east.  After  it  has  made  another  quarter  of  a  revolution 
the  centre  will  be  south  of  it,  and  it  must  be  now  turned  to  the  south.  In 
the  same  manner,  after  another  quarter  of  a  revolution,  it  must  be  turned  to  the 
west  As  the  same  hemisphere  is  successively  turned  to  all  the  points  of  the 
compass  in  one  revolution,  it  is  evident  that  the  globe  itself  must  make  a  single 
revolution  on  its  axis  in  that  time. 

It  appears,  then,  that  the  rotaiion  of  the  moon  upon  its  axis  being  equal  to 
that  of  its  revolution  in  its  orbit,  is  27  days,  7  hours,  and  44  minutes.  The  in- 
tervals of  light  darkness  to  the  inhabitants  of  the  moon,  if  there  were  any, 
would  then  be  altogether  different  from  those  provided  in  the  planets  ;  there 


would  be  about  13  days  of  continued  light  alternately  with  13  days  of  con- 
tinued darkness  ;  the  analogy,  then,  which  prevails  among  the  planets  with 
regard  to  days  and  nights,  and  which  forms  a  main  argument  in  favor  of  the 
conclusion  that  they  are  inhabited  globes  like  the  earth,  does  not  ho'i  good  in 
the  case  of  the  moon. 

Although  as  a  general  proposition  it  be  true  that  the  same  hemisphere  of  the  i 
moon  is  always  turned  toward  the  earth,  yet  there  are  small  variations  at  the  / 
edge  called  librations,  which  it  is  necessary  to  notice.  The  axis  of  the  moon  is  < 
not  exactly  perpendicular  to  its  orbit,  but  is  inclined  at  a  small  angle.  By  rea-  \ 
son  of  this  inclination,  the  northern  and  southern  poles  of  the  moon  lean  al- 
ternately in  a  slight  -degree  to  and  from  the  earth. 

When  the  north  pole  leans  toward  the  earth,  we  see  a  little  more  of  that  re- 
gion, and  a  little  less  when  it  leans  the  contrary  way.  This  variation  in  the 
northern  and  southern  regions  of  the  moon  visible  to  us,  is  called  the  libration 
in  latitude. 

In  order  that  in  a  strict  sense  the  same  hemisphere  should  be  continually 
turned  toward  the  earth,  the  time  of  rotation  of  the  earth  upon  its  axis  must  not 
only  be  equal  the  time  of  rotation  in  its  orbit,  which  in  fact  it  is,  but  its 
angular  velocity  on  its  axis  in  every  part  of  its  course,  must  be  exactly  equal  to 
its  angular  velocity  on  its  orbit.  Now  it  happens  that  while  its  angular  ve- 
locity on  its  axis  is  rigorously  uniform  throughout  the  month,  its  angular  ve- 
locity in  its  orbit  is  subject  to  a  slight  variation  ;  the  consequence  of  this  is 
that  a  little  more  of  its  eastern  or  western  edge  is  seen  at  one  time  than  at 
another.  This  is  called  the  libration  in  longitude. 

By  the  diurnal  motion  of  the  earth,  we  are  carried  with  it  round  its  axis  ;  the 
stations  from  which  we  view  the  moon  in  the  morning  and  the  evening,  or  rather 
when  it  rises,  and  when  it  sets,  are  then  different  according  to  the  latitude  of 
the  earth  in  which  we  are  placed.  By  thus  viewing  it  from  different  places, 
we  see  it  under  slightly  different  aspects.  This  is  another  cause  of  a  variation, 
which  we  see  in  its  eastern  and  western  edges  ;  this  is  called  the  diurnal 
libration. 


PHASES    OF    THE    MOON. 

While  the  moon  revolves  round  the  earth,  its  illuminated  hemisphere  is  al- 
ways presented  to  the  sun  ;  it  therefore  takes  various  positions  in  reference  to 
the  earth.  In  the  annexed  diagram  the  effects  of  this  are  exhibited.  Let  S  repre- 
sent the  sun,  and  T  the  earth  ;  when  the  moon  is  at  A,  between  the  sun  and  the 
earth,  its  illuminated  hemisphere  being  turned  toward  the  sun,  its  dark  hemi- 
sphere will  be  presented  toward  the  earth  ;  it  will  therefore  be  invisible.  In 
this  position  the  moon  is  said  to  be  in  conjunction.  When  it  moves  to  the  po- 
sition B,  the  enlightened  hemisphere  being  still  presented  to  the  sun,  a  small 
portion  of  it  only  is  turned  to  the  earth,  and  it  appears  as  a  thin  crescent,  as 
represented  at  b.  When  the  moon  takes  the  position  of  C.  at  right  angles  to 
the  sun,  it  is  said  to  be  in  quadrature  ;  one  half  of  the  enlightened  hemisphere 
only  is  then  presented  to  the  earth,  and  the  moon  appears  halved,  as  represented 
at  c.  When  it  arrives  at  the  position  D,  the  greater  part  of  the  enlightened 
portion  is  turned  to  the  earth,  and  it  is  gibbous,  appearing  as  represented  at  d. 
When  the  moon  comes  in  opposition  to  the  sun,  as  seen  at  E,  the  enlightened 
hemisphere  is  turned  full  toward  the  earth,  and  the  moon  will  appear  full,  un- 
less it  be  obscured  by  the  earth's  shadow,  which  rarely  happens.  In  the  same 
manner  it  is  shown  that  at  F  it  is  again  gibbous  ;  at  G  it  is  halved,  and  at  H 
it  is  a  crescent. 

When  the  moon  is  full,  being  in  opposition  to  the  sun,  it  will  necessarily  be 


304 


THE  MOON. 


in  the  meridian  at  midnight,  and  will  rise  as  the  sun  sets,  and  set  as  the  sun 
rises';  and  thus,  whenever  the  enlightened  hemisphere  of  the  moon  is  turned 
toward  us,  and  when,  therefore,  it  is  the  most  capable  of  benefiting  us,  it  is 
up  in  the  firmament  all  night ;  whereas,  when  it  is  in  conjunction,  as  at  A,  and 
the  dark  hemisphere  is  turned  toward  us,  it  would  then  be  of  no  use  to  us,  and 
is  accordingly  up  during  the  day.  The  position  at  C  is  called  the  "  first  quarter," 
and  at  G  the  "  last  quarter."  The  position  at  B  is  called  the  first  octant ;  D 
the  second  octant ;  F  the  third  octant ;  and  H  the  fourth  octant.  At  the  first 
and  fourth  octants  it  is  a  crescent,  and  at  the  second  and  third  octants  it  is  gib- 
bous. 

Fig.  i. 


The  apparent  motion  of  the  moon  in  the  heavens  is  much  more  rapid  than 
that  of  the  sun ;  for  while  the  sun  makes  a  complete  circuit  of  the  ecliptic  in 
365  days,  and  therefore  moves  over  it  at  about  1°  per  day,  the  moon  makes 
the  same  circuit  in  little  more  than  27  days,  and  consequently  must  move  at 
the  rate  of  a  little  less  than  14°  per  day.  As  the  sun  and  moon  appear  to 
move  in  the  same  direction  in  the  firmament,  both  proceeding  from  west  to 
east,  the  moon  will,  after  conjunction,  depart  from  the  sun  toward  the  east  at 
the  rate  of  about  13°  per  day.  If,  then,  the  moon  be  in  conjunction  with  the 
sun  on  any  given  day,  it  will  be  13°  east  of  it  at  the  same  time  on  the  follow- 
ing day ;  26°  east  of  it  after  two  days,  and  so  on.  If,  then,  the  sun  set  with 
the  moon  on  any  evening,  it  will,  at  the  moment  of  sunset  on  the  following 
evening,  be  13°  east  of  it,  and  at  sunset  will  appear  as  a  thin  crescent,  at  a 
considerable  altitude  ;  on  the  succeeding  day  it  will  be  26°  east  of  the  sun, 
and  will  be  at  a  still  greater  altitude  at  sunset,  and  will  be  a  broader  crescent. 
After  seven  days,  the  moon  will  be  removed  90°  from  the  sun  ;  it  will  be  at  or 
near  the  meridian  at  sunset.  It  will  remain  in  the  heavens  for  about  six  hours 
after  sunset,  and  will  be  seen  in  the  west  as  the  half-moon.  Each  successive 
evening  increasing  its  distance  from  the  sun,  and  also  increasing  its  breadth,  it 
will  be  visible  in  the  meridian  at  a  later  hour,  and  will  consequently  be  longer 
apparent  in  the  firmament  during  the  night — it  will  then  be  gibbous.  After 
about  fourteen  days,  it  will  be  180°  removed  from  the  sun,  and  will  be  full,  and 
consequently  will  rise  when  the  sun  sets,  and  set  when  the  sun  rises — being 
visible  the  entire  night.  After  the  elapse  of  three  weeks,  the  distance  of  the 
moon  from  the  sun  being  about  270°,  it  will  not  reach  the  meridian  until  nearly 
the  hour  of  sunrise  ;  it  will  then  be  visible  during  the  last  six  hours  of  the 
night  only.  The  moon  will  then  be  waning,  and  toward  the  close  of  the 
month  will  only  be  seen  in  the  morning  before  sunrise,  and  will  appear  as  a 
crescent. 


THE  MOON. 


305 


HAS    THE    MOON    AN    ATMOSPHERE  ? 

In  order  to  determine  whether  or  not  the  globe  of  the  moon  is  surrounded  \ 
with  any  gaseous  envelope  like  the  atmosphere  of  the  earth,  it  is  necessary  ; 
first  to  consider  what  appearances  such  an  appendage  would  present,  seen  at 
the  moon's  distance,  and  whether  any  such  appearances  are  discoverable  upon 
the  moon. 

According  to  ordinary  and  popular  notions,  it  is  difficult  to  separate  the  idea 
of  an  atmosphere  from  the  existence  of  clouds ;  yet  to  produce  clouds  some- 
thing more  is  necessary  than  air.  The  presence  of  water  on  the  surface  is 
indispensable,  and  if  it  be  assumed  that  no  water  exist,  then  certainly  the  ab- 
sence of  clouds  is  no  proof  of  the  absence  of  an  atmosphere.  Be  this  as  it 
may,  however,  it  is  certain  that  there  are  no  clouds  upon  the  moon,  for  if  there 
were,  we  should  immediately  discover  them,  by  the  variable  lights  and  shadows 
they  would  produce.  If  there  is,  then,  an  atmosphere  upon  the  moon,  it  is  one 
entirely  unaccompanied  by  clouds. 

One  of  the  effects  produced  by  a  distant  view  of  an  atmosphere  surrounding 
a  globe,  one  hemisphere  of  which  is  illuminated  by  the  sun,  is,  that  the  bounda- 
ry, or  line  of  separation  between  the  hemisphere  enlightened  by  the  sun  and 
the  dark  hemisphere,  is  not  sudden  and  sharply  defined,  but  is  gradual — the 
light  fading  away  by  slow  degrees  into  the  darkness.  This  is  an  effect  pro- 
duced by  a  portion  of  the  atmosphere  which  extends  over  the  dark  hemisphere 
being  illuminated  by  the  sun.  Let  A  B  (fig.  2)  be  a  diameter  of  the  moon 
separating  the  enlightened  hemisphere  A  M  B  from  the  dark  hemisphere  A  N 
B.  Let  C  E  D  F  be  the  upper  surface  of  the  atmosphere.  Let  S  T  be  rays 
from  the  sun  touching  the  moon  at  A  B.  It  is  evident  that  the  portion  of  the 
atmosphere  included  between  A  T  and  C  T,  and  that  between  B  T  and  D  T, 

Fig.  2. 


will  be  illuminated  by  the  sun  ;  and  if  the  moon  be  viewed  from  a  distant  point 
G,  then  these  latter  portions  of  the  atmosphere  will  be  seen  throwing  a  faint 
light  on  a  portion  of  the  dark  hemisphere,  which  light  will  become  gradually 
fainter  till  it  dies  away.  This  is  the  effect  which  on  the  earth  is  the  cause  of 
the  morning  and  evening  twilight. 

Now,  if  such  an  effect  as  this  were  produced  upon  the  moon,  it  would  be 
discoverable  by  us  with  the  naked  eye,  and  still  more  certainly  with  the  tele- 
scope. When  the  moon  is  a  crescent,  its  concave  edge  is  the  boundary  which 
separates  the  enlightened  from  the  dark  hemisphere.  When  it  is  in  the  quar- 

ao 


ters,  the  diameter  of  the  semi-circle  is  also  that  boundary.  In  neither  of  these 
cases,  however,  do  we  ever  discover  the  slightest  indication  of  any  such  ap- 
pearance as  that  which  has  just  been  described.  There  is  no  gradual  fading 
away  of  the  light  into  the  darkness  ;  on  the  contrary,  the  boundary,  though 
serrated  and  irregular,  is  nevertheless  perfectly  well-defined  and  sudden. 

All  these  circumstances  conspire  to  raise  a  presumption  that  there  does  not 
exist  upon  the  moon  any  atmosphere  capable  of  reflecting  light  in  any  sensible 
degree. 

But  it  may  be  contended  that  an  atmosphere  may  still  exist,  though  too  atten- 
uated to  produce  a  sensible  twilight.  Astronomers,  however,  have  resorted  to 
another  test  of  a  much  more  decisive  and  delicate  kind,  the  nature  of  which 
will  be  understood  by  explaining  a  simple  principle  of  optics. 

When  a  ray  of  light  passes  through  a  transparent  medium,  such  as  air,  water, 
or  glass,  it  is  generally  deflected  from  its  rectilinear  course,  so  as  to  form  an 
angle.  A  simple  and  easily-executed  experiment  will  render  this  intelligible. 
Let  a  visible  object,  such  as  a  cent-piece,  be  placed  at  C,  in  the  bottom  of  a 
bucket.  Let  the  eye  be  placed  at  E,  so  that  the  side  of  the  bucket,  when 
empty,  shall  just  conceal  the  coin  from  the  eye,  and  so  that  the  nearest  point  to 
the  coin  visible  to  the  eye  shall  be  at  A,  in  the  direction  of  the  line  E  B  A. 
Let  the  bucket  be  now  filled  with  water,  and  the  coin  will  become  immediate- 
ly visible  ;  the  reason  of  which  is,  that  the  ray  of  light  C  B  proceeding  from  the 
coin  is  bent  at  an  angle  in  passing  from  the  water  into  the  air,  and  reaches  the  eye 
by  the  angular  course  C  B  E.  Thus  it  appears  that  the  coin  will  be  visible 
to  the  eye,  notwithstanding  the  interposition  of  the  opaque  side  of  the  bucket. 

Fig.  3. 


Let  us  see  how  this  principle  can  be  applied  to  the  case  of  the  moon's  atmo- 
sphere, if  such  there  be.  Let  MN  (fig.  4)  represent  the  disk  of  the  moon.  Let  AB 
represent  the  atmosphere  which  surrounds  it.  Let  C  D  and  E  F  represent  two 
lines  touching  the  moon  at  M  and  N,  and  proceeding  toward  the  earth.  Let 
S  T  be  two  stars  seen  in  the  direction  of  these  lines.  If  the  moon  had  no  at- 
mosphere, these  stars  would  appear  to  touch  the  edge  of  the  moon  at  M  and 
N,  because  the  rays  of  light  from  them  would  pass  directly  along  the  lines 
S  M  D  and  T  N  F  toward  the  earth ;  but  if  the  moon  have  an  atmosphere,  then 
that  atmosphere  will  possess  the  property  which  is  common  to  all  transparent 
media  of  refracting  light,  and,  in  virtue  of  such  property,  stars  in  such  positions  as 
Q  and  R,  behind  the  edge  of  the  moon,  would  be  visible  at  the  earth,  for  the  ray 
Q  M,  in  passing  through  the  atmosphere,  would  be  bent  at  an  angle  in  the  direction 
Q  M  P,  and  in  like  manner  the  ray  R  N  would  be  bent  at  the  angle  R  N  0 — so  that 
the  stars  Q  and  R  would  be  visible  at  P  and  O,  notwithstanding  the  interposi- 
tion of  the  edges  of  the  moon.  This  effect  is  precisely  the  same  as  that  in  the 
example  of  the  coin  in  the  bucket ;  the  ray  from  the  star  is  bent  over  the  edge 
of  the  moon  so  as  to  render  the  star  visible  notwithstanding  the  interposition  of 


THE  MOON. 


307 


that  edge  just  for  the  same  reason  and  in  the  same  manner  as  the  ray  from  the 
coin  is  bent  over  the  side  of  the  bucket  so  as  to  render  the  coin  visible  not-  i 
withstanding  the  opacity  of  that  side. 

Fig.  4. 


This  reasoning  leads  to  the  conclusion  that  as  the  moon  moves  over  the  face 
of  the  firmament,  stars  will  be  continually  visible  at  its  edge  which  are  really 
behind  it  if  it  have  an  atmosphere,  and  the  extent  to  which  this  effect  will  take 
place  will  be  in  proportion  to  the  density  of  the  atmosphere. 

The  magnitude  and  motion  of  the  moon  and  the  relative  positions  of  the  stars 
are  so  accurately  known  that  nothing  is  more  easy,  certain,  and  precise,  than 
the  observations  which  may  be  made  with  the  view  of  ascertaining  whether 
any  stars  are  ever  seen  which  are  sensibly  behind  the  edge  of  the  moon.  Such 
observations  have  been  made  by  the  most  skilful  astronomers,  and  no  such  ef- 
fect has  ever  been  detected.  This  species  of  observation  is  susceptible  of 
such  extreme  accuracy,  that  it  is  certain  that  if  an  atmosphere  existed  upon 
the  moon  a  thousand  times  less  dense  than  our  own,  its  presence  must  have 
been  detected. 

But  what  is  an  atmosphere  a  thousand  times  less  dense  than  ours  ?  Our  at- 
mosphere supports  by  its  pressure  a  column  of  thirty  inches  of  mercury  in  the 
barometer.  One  a  thousand  times  less  dense  would  not  support  so  much  as 
the  thirtieth  of  an  inch  ;  in  short,  it  may  be  considered  as  proved  that  there 
does  not  exist  upon  the  moon  an  atmosphere  as  dense  as  is  found  under  the  re- 
ceiver of  the  most  perfect  air-pump  after  that  instrument  has  withdrawn  from 
it  the  air  to  the  utmost  extent  of  its  power.  In  fine,  it  may  be  considered  as 
demonstrated  that  there  is  no  air  upon  the  moon. 

THE    PHYSICAL    QUALITIES    OF    MOONLIGHT. 

It  has  long  been  an  object  oft  inquiry  among  philosophers  whether  the  light 
of  the  moon  has  any  heat,  but  the  most  delicate  experiments  and  observations 
have  failed  to  detect  this  property  in  it. 

A  thermometer  of  extreme  sensibility,  called  a  differential  thermometer,  was 
the  instrument  applied  to  this  inquiry.  Let  E  and  F  be  two  thin  glass  bulbs 
connected  by  a  rectangular  glass  tube  E  A  B  F  partially  filled  with  a  liquid  to 
the  level.  Let  the  bulbs  E  and  F  contain  air.  If  the  bulb  F  be  exposed  to 
any  source  of  heat  or  cold  different  from  E,  the  air  within  it  will  expand  or 
contract.,  and  the  liquid  in  F  B  will  fall  or  rise.  This  instrument  has  such  ex- 
treme sensibility  that  it  is  capable  of  rendering  manifest  a  change  of  tempera- 
ture amounting  to  the  five  hundreth  part  of  a  degree.  The  light  of  the  moon 
was  collected  into  the  focus  of  a  concave  mirror  of  such  magnitude  as  would  <[ 
have  been  sufficient,  if  exposed  to  the  sun's  light,  to  evaporate  gold  or  platinum.  ]» 
The  bulb  of  the  differential  thermometer  was  placed  in  its  focus  so  as  to  re-  i[ 


308 


3                                                                 THE  MOON. 

Fig.  5. 

Pi 

: 

l                         l 

>E 

B  * 

Lk 

ceive  upon  it  the  concentrated  rays  of  the  moon.  Yet  no  sensible  effect  was 
produced  upon  the  thermometer.  We  must  therefore  conclude  that  the  light  of 
the  moon  does  not  possess  the  calorific  property  in  any  sensible  degree. 

This  result  will  create  less  surprise  when  the  comparative  density  of  sun- 
light and  moonlight  are  considered.  It  may  be  assumed  without  sensible  er- 
ror that  the  intensity  of  the  sun's  light  on  the  surface  of  the  moon  and  on  the 
earth  is  the  same,  it  follows  from  this,  that  supposing  no  light  whatever  to  be 
absorbed  by  the  moon,  but  the  entire  light  of  the  sun  to  be  reflected  from  its 
surface  undiminished,  the  intensity  of  moonlight  at  the  earth  would  bear  to  the 
intensity  of  sunlight  the  same  proportion  as  the  magnitude  of  the  moon  bears 
to  the  magnitude  of  the  entire  firmament,  that  is,  the  proportion  very  nearly  of  one 
to  three  hundred  thousand  ;  but  there  is  no  reflecting  surface  however  perfect 
which  does  not  absorb  the  light  incident  upon  it  in  a  very  considerable  degree, 
and  the  rugged  surface  of  the  moon  must  be  a  most  imperfect  reflector.  It  may 
then  be  considered  as  demonstrated  that  the  intensity  of  moonlight  is  much  more 
than  three  hundred  thousand  times  more  feeble  than  that  of  sunlight.  We 
shall  not,  then,  be  surprised  at  the  absence  of  its  heating  power. 

But  if  the  rays  of  the  moon  be  not  warm,  the  vulgar  impression  that  they 
are  cold  is  equally  erroneous.  We  have  seen  that  they  produce  no  effect  either 
way  on  the  thermometer. 

DOES  WATER  EXIST  ON  THE  MOON  ? 

We  shall  presently  see  that  telescopic  observation  proves  the  non-existence 
of  oceans,  seas,  or  any  other  large  reservoirs  of  water,  on  the  surface  of  our 
satellite.  This  is  not  sufficient,  however,  to  establish  the  total  absence  of  wa- 
ter upon  it,  for  besides  its  possible  existence  in  the  form  of  rivers  and  small 
lakes  too  minute  to  be  discovered  by  the  telescope,  it  might  exist  in  the  pores 
of  organized  and  unorganized  matter. 

If,  however,  water,  or  any  other  liquid,  existed  upon  the  moon,  it  \vould  be 
subject  to  the  common  process  of  evaporation,  which  would  take   place  the 
more  freely  because  of  the  absence  of  an  atmosphere.     It  is  evident,  then,  that 
the  existence  of  liquids  on  the  moon  would  necessarily  be  attended  with  the 
existence   of  an  atmosphere  surrounding  the  moon   composed  of  the  vapor 
of  those  liquids.     It  is  difficult  to  imagine  how  such  an  atmosphere  could  ex- 
ist without  clouds,  but  its  non-existence  is  conclusively  proved  by  the  fact  that 
its  presence  cannot  be  detected  by  the  optical  test  above-mentioned,  by  which 
the  absence  of  an  atmosphere  is  proved — an  atmosphere  of  vapor,  having  in  > 
common  with  air  and  other  transparent  media  the  property  of  refraction,  its  ef-  ? 
feet  on  the  stars  will  be  similar,  and  consequently  the  same  test  which  proves  l 
the  absence  of  an  atmosphere  of  air  equally  proves  the  absence  of  an  atmo-  < 
sphere  of  vapor. 


THE  MOON. 


309 


DOES    THE    MOON    INFLUENCE    THE    WEATHER? 

Among  the  many  influences  which  the  moon  is  supposed,  by  the  world  in 
general,  to  exercise  upon  our  globe,  one  of  those  which  have  been  most  uni- 
versally believed,  in  all  ages  and  in  all  countries,  is  that  which  it  is  presumed 
to  exert  upon  the  changes  of  the  weather.  Although  the  particular  details  of 
this  influence  are  sometimes  pretended  to  be  described,  the  only  general  prin- 
ciple, or  rule,  which  prevails  with  the  world  in  general  is,  that  a  change  of 
weather  may  be  looked  for  at  the  epochs  of  new  and  full  moon  :  that  is  to  say, 
if  the  weather  be  previously  fair  it  will  become  foul,  and  if  foul  will  become 
fair.  Similar  changes  are  also,  sometimes,  though  not  so  confidently  looked 
for,  at  the  epochs  of  the  quarters. 

A  question  of  this  kind  may  be  regarded  either  as  a  question  of  science,  or 
a  question  of  fact. 

If  it  be  regarded  as  a  question  of  science,  we  are  called  upon  to  explain 
how  and  by  what  property  of  matter,  or  what  law  of  nature  or  attraction  the 
moon,  at  a  distance  of  a  quarter  of  a  million  of  miles,  combining  its  effects 
with  the  sun,  at  four  hundred  times  that  distance,  can  produce  those  alleged 
changes  ?  To  this  it  may  be  readily  answered  that  no  known  law  or  principle 
has  hitherto  explained  any  such  phenomena.  The  moon  and  sun  must,  doubt- 
less, affect  the  ocean  of  air  which  surrounds  the  globe,  as  they  affect  the  ocean 
of  water — producing  effects  analogous  to  tides  ;  but  when  the  quantity  of  such 
an  effect  is  estimated,  it  is  proved  to  be  utterly  inappreciable,  and  such  as  could 
by  no  means  account  for  the  meteorological  changes  here  adverted  to. 

But  in  conducting  investigations  of  this  kind  we  proceed  altogether  in  the  wrong 
direction,  and  begin  at  the  wrong  end  when  we  commence  with  the  investiga- 
tion of  the  physical  cause  of  the  supposed  phenomena.  That  method  of  con- 
ducting physical  inquiries,  which  was  bequeathed  to  us  by  the  illustrious  Ba- 
con, and  which  has  led  to  such  an  immense  extension  of  our  knowledge  of 
the  universe,  imperiously  requires  that  before  we  begin  to  seek  for  the  causes 
of  any  phenomena,  we  must  first  prove  beyond  the  possibility  of  doubt,  the 
reality  of  these  phenomena,  and  ascertain  with  the  utmost  precision,  all  the 
circumstances  attending  them.  In  other  words,  we  are  required  to  consider  all 
inquiries  of  the  kind  now  adverted  to,  as  mere  questions  of  fact,  before  we 
take  them  as  questions  of  science. 

What,  then,  let  us  see,  is  the  present  question  ?  It  is  asserted  that  the  moon 
produces  such  an  influence  on  the  weather  as  to  cause  it  to  change  at  the  new 
and  full  moon,  and  at  the  quarters.  But  in  this  mode  of  stating  the  proposi- 
tion, there  are  implicitly  included  two  very  distinct  points,  one  of  which  is  a 
simple  matter  of  fact,  and  the  other  a  point  of  physical  science. 

First. — It  is  asserted  that  at  the  epochs  of  a  new  and  full  moon,  and  at  the 
quarters,  there  is  generally  a  change  of  weather.  This  is  a  mere  statement 
of  alleged  fact. 

Second. — It  is  asserted  that  the  phases  of  the  moon,  or  in  other  words,  the 
relative  position  of  the  moon  and  sun  in  regard  to  the  earth  is  the  cause  of 
these  changes. 

Now  it  is  evidently  necessary  to  settle  the  first  question  before  we  trouble 
ourselves  with  the  second,  for  if  it  should  so  happen  that  the  first  statement 
should  prove  to  be  destitute  of  foundation  the  second  falls  to  the  ground. 

The  question  of  fact,  here  before  us,  is  one  most  easily  settled.  In  many 
meteorological  observations  throughout  Europe,  a  register  of  the  weather  in 
all  respects,  has  been  kept  for  a  long  period  of  time.  Thus  the  height  of  the 
barometer,  the  condition  of  the  thermometer,  the  hydrometer,  and  the  rain 
gauge  ;  the  form  and  character  of  the  clouds,  the  times  of  the  falling  of  rain, 


<  310 


THE  MOON. 


hail,  and  snow,  and  in  short,  every  particular  respecting  the  weather  has  been 
duly  registered,  from  day  to  day,  and  often  from  hour  to  hour. 

The  period  of  the  lunar  phases,  it  is  needless  to  say,  has  also  been  reg- 
istered, and  it  is,  therefore,  possible  to  compare  one  set  of  changes  with  the 
other. 

This,  in  fine,  has  been  done.  We  can  imagine,  placed  in  two  parallel  col- 
umns, in  juxtaposition,  the  series  of  epochs  of  the  new  and  full  moons,  and 
the  quarters,  and -the  corresponding  conditions  of  the  weather  at  these  times, 
for  fifty  or  one  hundred  years  back,  so  that  we  may  be  enabled  to  examine, 
as  a  mere  matter  of  fact,  the  conditions  of  tho  weather  for  one  thousand  or 
twelve  hundred  full  and  new  moons  and  quarters.  The  result  of  such  an  exami- 
nation has  been,  that  no  correspondence  whatever  has  been  found  to  exist  be- 
tween the  two  phenomena.  Thus  let  us  suppose  that  one  hundred  and  twenty- 
five  full  moons  be  taken  at  random  from  the  table  :  if  the  condition  of  the 
weather  at  these  several  epochs  be  examined  it  will  be  found,  probably,  that  in 
sixty-three  cases  there  was  a  change  of  weather,  and  in  sixty-two  there  was 
not,  so  that  under  such  circumstances  the  odd  moon  in  this  division  of  one  hun- 
dred and  twenty-five  would  favor  the  popular  opinion  ;  but  if  another  random 
collection  of  one  hundred  and  twenty-five  full  moons  be  taken,  and  similarly 
examined,  it  will  probably  be  found  that  sixty-three  are  not  attended  by  chan- 
ges of  weather,  while  sixty-two  are.  With  its  characteristic  caprice  the  moon 
on  this  occasion  opposes  the  popular  opinion  ;  in  short,  a  full  examination  of 
the  table  shows  that  the  condition  of  the  weather  as  to  change,  or  in  any  other 
respect,  has,  as  a  matter  of  fact,  no  correspondence  whatsoever  with  the  lunar 
phases. 

Such,  then,  being  the  case,  it  would  be  idle  to  attempt  to  seek  for  a  physical 
cause  of  an  effect  which  is  destitute  of  proof. 

PHYSICAL    CONDITION    OF    THE    LUNAR    SURFACE. 

Curiosity  will  doubtless  be  awakened  in  a  very  lively  manner  regarding  the 
physical  condition  of  our  moon  :  what  part  has  the  Maker  of  the  solar  system 
destined  this  body  to  play  in  the  economy  of  his  creation  1  Is  it  a  globe  teem- 
ing with  life  and  organization  like  the  earth  ?  Is  that  orb,  which  rolls  in  silent, 
serene  majesty  in  her  silent  course  through  the  midnight  firmament,  the  abode 
of  life  and  intelligence  ?  The  beauty  of  her  appearance,  and  the  interest  insep- 
arable from  this,  naturally  lead  the  mind  to  conjectures  of  this  kind.  Yet  the 
circumstances  which  I  have  unfolded  regarding  the  total  absence  of  air  and  wa- 
ter, appear  to  exclude  the  possibility  of  any  such  supposition.  How,  may  it  be 
asked,  can  it  be  conceived  that  a  globe  can  have  upon  it  an  organized  world 
which  is  destitute  of  fluid  matter  in  every  form  ?  How  can  growth,  which  im- 
plies gradual  change,  increase,  and  diminution,  and  all  the  various  effects  in  which 
fluidity  is  an  agent,  go  on  there  ?  How  can  they  proceed  upon  such  a  solid, 
arid,  unchangeable,  crude  mass  1  Let  it  be  remembered  what  a  multitude 
of  purposes  in  our  natural  and  social  economy  are  subserved  by  the  combina- 
tion of  the  water  and  the  atmosphere  of  our  globe.  None  of  these  purposes 
can  be  fulfiiled  upon  the  moon.  Perhaps,  however,  our  notions  on  such  ques- 
tions may  be  cleared  up  to  some  extent  by  a  careful  examination  of  the  facts 
that  scientific  research  have  collected  together  respecting  the  physical  condition 
of  the  surface  of  our  satellite. 

If  we  examine  the  moon  carefully,  even  without  the  aid  of  a  telescope,  we 
shall  discover  upon  it  distinct  and  definite  lineaments  of  light  and  shadow. 
These  features  never  change  ;  there  they  remain,  always  in  the  same  position 
upon  the  visible  orb  of  the  moon.  Thus  the  features  that  occupy  its  centre 


e*s**r^r-  . 


THE  MOON. 


now,  have  occupied  the  same  position  throughout  all  human  record.  We  have 
already  stated  that  the  first  and  most  obvious  inference  which  this  fact  suggests, 
is  that  the  same  hemisphere  of  the  moon  is  always  presented  toward  the  earth, 
and  consequently,  the  other  hemisphere  is  never  seen,  nor  can  we  ever  see  it. 
This  singular  characteristic  which  attaches  to  the  motion  of  the  moon  round  j 
the  earth,  seems  to  be  a  general  characteristic  of  all  other  moons  in  the 
system.  Sir  William  Herschel,  by  the  aid  of  his  powerful  telescopes,  as- 
certained that  the  moons  of  Jupiter  revolve  in  the  same  manner,  each  pre- 
senting continually  the  same  hemisphere  to  the  planet.  The  cause  of  this  pe- 
culiar motion  has  been  attempted  to  be  explained  by  the  hypothesis  that  the 
hemisphere  of  the  satellite  which  is  turned  toward  the  planet,  is  very  elonga- 
ted and  protuberant,  and  it  is  the  excess  of  its  weight  which  makes  it  tend  to 
direct  itself  always  toward  the  primary,  in  obedience  to  the  universal  principle 
of  attraction.  Be  this  as  it  may,  the  effect  is  in  the  case  of  the  moon,  that 
our  geographical  knowledge  is  necessarily  limited  to  that  hemisphere  which 
is  turned  toward  us. 

If  the  moon  were  inhabited,  observers  upon  it  would  have  an  extraordinary 
spectacle  presented  to  them  by  the  earth.  In  their  firmament  the  earth  is  an 
object  with  a  diameter  four  times,  and  a  disk  sixteen  times,  greater  than  that 
which  the  moon  presents  to  us.  A  spectator  placed  on  the  centre  of  the  hemi- 
sphere of  the  moon  which  is  toward  us,  would  see  the  orb  of  the  earth  pre- 
senting the  appearance  of  a  gorgeous  moon  of  immense  magnitude,  always  in 
his  zenith  :  it  would  never  rise,  nor  set,  nor  change  its  position  at  all  in  the 
firmament ;  it  would,  however,  undergo  all  the  varieties  of  phases  of  the  moon 
— when  the  moon  appears  to  us  full,  it  would  be  new,  and  when  the  moon  ap- 
pears new,  it  would  be  full ;  when  the  moon  appears  to  us  a  crescent,  it  would 
be  gibbous,  and  vice  versa. 

But  what  is  the  condition  and  character  of  the  surface  of  the  moon  ?  What 
are  the  lineaments  of  light  and  shade  which  we  see  upon  it  ?  There  is  no  ob- 
ject outside  the  earth  with  which  the  telescope  has  afforded  us  such  minute 
and  satisfactory  information. 

If,  when  the  moon  is  a  crescent,  we  examine  with  a  telescope,  even 
of  moderate  power,  the  concave  boundary  which,  is  that  part  of  the  lunar 
surface  where  the  enlightened  hemisphere  ends  and  the  dark  hemisphere 
begins,  we  shall  find  that  this  boundary  is  not  an  even  and  regular  curve,  which 
it  undoubtedly  would  be  if  the  surface  of  the  globe  of  the  moon  were  smooth 
and  regular,  or  nearly  so.  If,  for  example,  the  lunar  surface,  resembled  in  its  < 
general  characteristics  that  of  our  globe  ;  granting  the  total  absence  of  wa-  j 
ter,  and  that  the  entire  surface  is  land,  that  land  had  the  general  character- 
istics of  the  continents  of  the  globe  of  the  earth ;  then  I  say,  that  the  inner 
boundary  of  the  lunar  crescent  would  still  be  a  regular  curve,  broken  or  inter- 
rupted only  at  particular  points.  Where  great  mountain  ranges,  like  those 
of  the  Alps,  the  Andes,  or  the  Himalaya,  might  chance  to  cross  it,  in  such  pla- 
ces these  lofty  peaks  would  project  vastly-elongated  shadows  along  the  adja- 
cent plain  ;  for  it  will  be  remembered,  that,  being  situated  at  the  moment  in 
question,  at  the  boundary  of  the  enlightened  and  darkened  hemispheres,  the 
shadows  would  be  those  of  evening  and  morning  ;  which  are  prodigiously  lon- 
ger than  the  objects  themselves.  The  effects  of  these  would  be  to  cause  gaps 
or  irregularities  in  the  general  outline  of  the  inner  boundary  of  the  crescent; 
with  these  rare  exceptions,  the  inner  boundary  of  the  crescent  produced  by  a 
globe  like  the  earth  would  be  an  even  and  regular  curve. 

Such,  however,  is  not  the  case  with  the  inner  boundary  of  the  lunar  cres- 
cent, even  when  viewed  by  the  naked  eye,  and  still  less  so  when  magniried 
by  a  telescope. 


THE   MOON. 


It  is  found,  on  the  other  hand,  that  this  boundary  is  everywhere  rugged 
and  serrated,  and  brilliantly-illuminated  points  are  seen  in  the  dark  parts  of  the 
moon,  at  some  distance  from  the  general  boundary  of  the  illuminated  part,  while 
dark  shadows  of  considerable  length  appear  to  break  into  the  illuminated  sur- 
face. In  short,  there  is  a  continued  irregularity  throughout  the  whole  extent 
of  the  inner  boundary  of  the  lunar  crescent.  The  inequalities  thus  apparent 
indicate  singular  geographical  and  geological  characteristics  of  the  lunar  sur- 
face. Each  of  the  bright  points  which  are  seen  within  the  dark  hemisphere 
are  the  peaks  of  lofty  mountains  tinted  with  the  sun's  light.  They  are  in  the 
condition  with  which  all  travellers  on  Alpine  points  are  familiar ;  after  the  sun 
has  set,  and  darkness  has  set  in  over  the  valleys  at  the  foot  of  the  chain,  the 
sun's  light  still  continues  to  illuminate  the  lofty  peaks  above.  The  dark  streaks 
which  break  into  the  illuminated  hemisphere  of  the  moon  are  those  of  lofty 
mountains  within  that  hemisphere  which  project  their  shadows  toward  the 
dark  hemisphere. 

It  appears,  then,  that  the  surface  of  the  moon  is  a  continuity  of  mountainous 
regions.  If  we  examine  by  means  of  a  powerful  telescope  the  full  moon,  we  find 
those  features  rendered  larger  and  more  conspicuous,  and  greatly  multiplied  in 
number.  What,  it  may  be  asked  then,  are  those  peculiar  phenomena  thus  dis- 
covered upon  the  full  moon  ?  What  is  signified  by  the  dark  and  what  by  the 
lighter  parts  ?  Elaborate  telescopic  research  has  shown  us  that  the  dark  parts  are 
generally  cavities  into  which  the  light  of  the  sun  penetrates  imperfectly,  while 
the  bright  parts  are  eminences  that  catch  the  sun's  light  with  great  intensity. 
Toward  the  sides  of  the  full  moon,  also,  the  dark  portions  are  caused  by 
the  shadows  of  mountain  peaks  and  ridges,  which  are  more  arid  more 
elongated  the  farther  these  points  are  removed  from  the  centre  of  the  full 
moon. 

Within  a  recent  period  the  moon  has  been  subjected  to  extremely-elaborate 
telescopic  examinations  by  Beer  and  Madler,  who  have  published  some  very 
magnificent  telescopic  views  of  it.  The  telescopic  map  of  the  moon's  surface, 
published  by  these  eminent  observers,  measures  three  feet  in  diameter,  and  may 
truly  be  said  to  exceed  in  accuracy  any  chart  of  the  globe  of  the  earth. 

The  lunar  mountains  are  of  various  formations  and  arrangements :  peaks 
such  as  that  of  Teneriffe  are  common.  Mountain  ranges  following  straight 
or  nearly  straight  courses  are  also  discoverable  ;  but  the  most  frequent  forma- 
tion of  the  lunar  mountains  is  that  which  resembles  the  crater  of  our  volcano. 
It  is  estimated  that  three  fifths  of  the  portion  of  the  moon  visible  to  us  is  cov- 
ered with  caverns  penetrating  to  a  great  depth,  and  surrounded  by  a  circular 
wall  of  rock  of  a  rugged  and  irregular  character.  These  crater-formed  cavities 
are  very  various  in  diameter,  varying  from  50  or  60  miles  to  a  few  hundred  feet, 
and  the  number  of  them  increases  as  the  magnitude  diminishes.  The  ridge 
surrounding  these  craters  is  generally  precipitous  and  nearly  vertical  on  the 
inside,  but  sloping  more  gradually  on  the  outside.  On  descending  to  the  bot- 
tom, it  is  often  found  to  arrange  itself  in  steps  or  terraces.  "  The  bottom  of  the 
crater,"  says  Professor  Nichol,  who  has  examined  in  detail  the  labors  of  Beer 
and  Madler,  "  is  very  often  convex,  and  low  ridges  of  mountains  run  through 
it.  We  also  find  in  it  isolated  conical  peaks  and  smaller  craters,  whose  heights, 
however,  seldom  reach  the  level  of  the  base  of  the  exterior  wall.  These  curi- 
ous objects  are  on  some  parts  of  the  moon  so  crowded  that  they  seem  to  have 
pressed  on  each  other,  and  disturbed  and  even  broken  down  each  other's 
boundaries,  so  that  through  the  mutual  interference  the  most  oddly-shaped  cav- 
erns have  arisen.  It  has  often  been  observed  that  smaller  craters  are  found  on 
the  walls  of  the  crater,  and  in  many  instances  we  can  discern  that  the  wall 
has  been  shaken  by  force. 


THE  MOON. 


Among  the  singular  remarkable  appearances  upon  the  moon,  is  that  of  a 
system  of  rays  which  appear  to  diverge  from  the  crater-shaped  ridges.  One  of 
the  most  remarkable  of  these  is  exhibited  in  the  appearance  of  the  mountain 
called  Tycho.  At  the  time  of  full  moon,  these  appearances  generally  cast 
very  broad,  brilliant  bands,  issuing  from  all  sides  of  the  crater,  and  stretching 
to  a  greater  or  less  distance,  sometimes  extending  over  a  space  of  several  hun- 
dred miles.  Two  characteristics  of  these  singular  bands  necessarily  attract  no- 
tice. First,  the  light  they  throw  is  exactly  of  the  same  kind  as  that  reflected 
from  the  edge  of  the  crater  itself,  and  from  the  lowest  part  of  the  chasm ;  so 
that  we  must  suppose  that  the  matter  forming  them  had  the  same  origin  and 
source  as  the  other  portion  of  these  mountainous  formations.  Secondly,  it  will 
be  observed  that  they  hold  their  course  without  being  interrupted  by  other  for- 
mations on  the  lunar  surface.  If,  instead  of  a  general  rugged  surface,  the 
face  of  the  moon  had  been  one  unbroken  plane,  the  course  of  these  radiating 
lines  could  not  have  been  less  disturbed,  except  that  they  accommodate  them- 
selves to  the  contour  of  the  surface  ;  if  they  meet  a  valley,  they  bend  with  it ; 
if  a  precipitous  mountain,  they  rise  with  it  precipitously ;  and  then  pursue 
their  previous  path. 

Before  we  dismiss  the  mountainous  character  of  the  moon's  surface,  it  may 
be  well  to  state  that  the  heights  of  these  mountains,  and  the  depths,  in  many 
cases,  of  their  cavities,  have  been  pretty  accurately  ascertained  by  the  meas- 
urement of  their  shadows.  It  is  generally  stated  that  they  are  higher 
than  the  mountain  ranges  of  the  earth.  This,  in  a  literal  sense,  is  not  true. 
The  lunar  mountains  do  not  attain  to  the  actual  height  of  some  of  the  highest 
of  the  terrestrial  ranges ;  but,  considering  that  the  moon  is  a  globe  on  a  scale 
one  fourth  that  of  the  earth,  it  may  be  truly  stated  that,  according  to  the  relative 
sizes  of  the  globes,  the  lunar  mountains  are  considerably  higher  than  those  of 
the  earth. 

It  is  not  the  mere  height  of  these  mountains  that  so  forcibly  commands  at- 
tention ;  it  is  their  universal  prevalence. 

At  the  early  epochs  of  telescopic  discoveries,  when  the  moon  was  examined- 
by  telescopes  of  inferior  power,  extensive  regions  were  observed  upon  it,  which 
seemed  to  be  level  surfaces,  and  which  were  therefore  mistaken  for  seas.  These 
regions  in  the  lunar  surface  have  received  names,  every  conspicuous  moun- 
tain being  designated  by  a  peculiar  title,  names  were  also  given  to  those  ap- 
parent level  portions,  such  as  the  Mare  Imbrium,  &c.  As  the  power  of  the 
telescope  was  improved,  it  soon  became  apparent  that  regions  supposed  to  be 
seas,  were  covered  with  asperities  and  inequalities,  less  indeed  in  elevation 
than  other  parts  of  the  moon,  but  still  considerable.  Every  augmentation  of 
power  which  the  telescope  received,  only  adds  fresh  proof  that  there  is  no  por- 
tion of  the  moon  absolutely  level,  and  consequently  that  there  does  not  exist 
upon  it,  at  least  on  the  visible  hemisphere,  a  collection  of  water. 

The  celebrated  telescopic  view  of  the  moon  produced  by  the  labors  of  Beer 
and  Madler,  to  which  I  have  more  than  once  referred,  is  exhibited  on  a  re- 
duced scale  in  the  frontispiece  of  this  volume.  The  mere  inspection  of  that 
drawing  will  afford  abundant  evidence  to  corroborate  the  statements  which 
have  been  here  made ;  more  especially,  if  it  be  remembered  that  minute  por- 
tions of  that  view,  where  no  inequalities  are  exhibited,  will  show  innumerable 
inequalities  if  submitted  to  an  examination  with  a  still  higher  magnifying 
power 

I  annex  here  two  highly-magnified  views  of  detached  portions  of  the  lunar 
surface,  supplied  by  the  observations  of  Madler.  In  these  the  prevalence  of 
the  crater  form  is  especially  conspicuous.  The  names  of  the  more  remarkable 
mountains  are  here  inserted. 


j  314 


THE  MOON 


Fig.  6. 


Pig.  7. 


Astronomers  have  occasionally  extended  their  speculations  beyond  the  im- 
mediate and  rigorous  limits  of  observation,  and  had  endeavored  by  analogy  to 
afford  us  some  idea  of  the  actual  condition  of  lunar  surface.  I  annex  here  a 
drawing  of  a  lunar  crater,  from  the  design  of  a  French  observer. 


THE    ORBIT    OF    THE    MOON. 

Although  in  its  general  form  and  character  the  path  of  the  moon  round  the 
earth  is,  like  that  of  the  orbits  of  the  planets  and  satellites  in  general,  circular, 
yet,  when  it  is  submitted,  to  accurate  observation  we  find  that  it  is  strictly 
an  ellipse  or  oval,  the  centre  of  the  earth  occupying  one  of  its  foci.  This 
fact  can  be  ascertained  by  immediate  observation  upon  the  apparent  magnitude 
of  the  moon.  It  will  be  easily  comprehended  that  any  change  which  the  apparent 
magnitude  of  the  moon  as  seen  from  the  earth  undergoes,  must  arise  from 
corresponding  changes  in  the  moon's  distance  from  us.  Thus,  if  at  one  time 
the  disk  of  the  moon  appears  larger  than  at  another  time,  as  it  cannot  be  sup- 
posed that  the  actual  size  of  the  moon  itself  could  be  changed,  we  can  only 
ascribe  the  increase  of  the  apparent  magnitude  to  the  diminution  of  its  dis 


THE  MOON. 


tance.  Now  we  find  by  observation  that  such  apparent  changes  are  actually 
observed  in  its  monthly  course  around  the  earth.  The  moon  is  subject  to  a 
continual  and  small,  though  perceptible  change  of  apparent  size.  We  find  that 
it  diminishes  until  it  reaches  a  minimum,  and  then  gradually  increases  until  it 
reaches  a  maximum. 

When  the  apparent  magnitude  of  the  moon  is  least,  it  is  at  its  greatest  dis- 
tance from  us,  and  when  its  apparent  magnitude  is  greatest,  it  is  at  its  least 
distance  from  us.  The  positions  in  which  these  distances  lie,  are  directly  op- 
posite. Between  these  two  positions  the  apparent  size  of  the  moon  undergoes 
a  regular  and  gradual  change,  increasing  continually  from  its  minimum  to  its 
maximum,  and  consequently  between  these  positions,  its  distances  must  on 
the  other  hand  gradually  diminish  from  its  maximum  to  its  minimum.  If  we 
lay  down  on  a  chart  or  plan  a  delineation  of  the  course  or  path  thus  determined, 
we  shall  find  that  it  will  represent  an  oval  which  differs  however  very  little 
from  a  circle ;  the  place  of  the  earth  being  nearer  to  one  end  of  the  oval  than 
the  other. 

The  point  of  the  moon's  path  in  the  heavens  at  which  its  magnitude  appears 
the  greatest,  and  when,  therefore  it  is  nearest  the  earth,  is  called  its  perigee  ; 
and  the  point  where  its  apparent  size  is  least,  and  where,  therefore,  its  distance 
i'rom  the  earth  is  greatest,  is  called  its  apogee.  These  two  points  are  called  the 
moon's  apsides. 

If  the  positions  of  these  points  in  the  heavens  be  observed  accurately  for  a 
length  of  time,  it  will  be  found  that  they  are  subject  to  a  regular  change  ;  that 
is  to  say,  the  place  where  the  moon  appears  smallest,  will  every  month  shift 
its  position ;  and  a  corresponding  change  will  take  place  in  the  point  where 
it  appears  largest.  The  movement  of  these  points  in  the  heavens  is  found  to 
be  in  the  same  direction  as  the  general  movement  of  the  planets ;  that  is, 
from  west  to  east,  or  progressive.  This  effect  is  called  the  progression  of  the 
moon  apsides. 

THE    MOON'S    NODES. 

If  the  position  of  the  moon's  centre  in  the  heavens  be  observed  from  day  to 
day,  it  will  be  found  that  its  path  is  a  great  circle,  making  an  angle  of  about 
5°  with  the  ecliptic.  This  path  consequently  crosses  the  ecliptic  at  two 
points  in  opposite  quarters  of  the  heavens.  These  points  are  called  the 
moon's  nodes.  Their  positions  are  ascertained  by  observing  from  time  to  time 
the  distance  of  the  moon's  centre  from  the  ecliptic,  which  is  called  the  moon's 
latitude  ;  by  watching  its  gradual  diminution,  and  finding  the  point  at  which  it 
becomes  nothing ;  the  moon's  centre  is  then  in  the  ecliptic  and  its  position  is 
the  node.  TJhe  node  at  which  the  moon  passes  from  the  south  to  the  north  of 
the  ecliptic  is  called  the  ascending  node,  and  that  at  which  it  passes  from  the 
north  to  the  south  is  called  the  descending  node. 

If  the  positions  of  these  nodes  be  observed  from  time  to  time,  it  will  be 
found  that  they  are  not  fixed  ;  but  that  they  change  their  positions  in  the  eclip- 
tic, moving  upon  that  line  in  a  direction  contrary  to  that  of  the  planets,  or  from 
east  to  west.  This  effect  is  called  the  retrogression  of  the  moon's  nodes. 


HEAT. 


Heat  as  a  Branch  of  elementary  Physics  neglected. — Has  as  strong  Claims  as  Light,  Electricity,  or 
Magnetism. — Is  a  universal  Agent  in  Nature. — In  Art. — In  Science. — Astronomy. — Chemistry. — 
In  every  Situation  of  Life. — Applications  of  it  in  Clothing  and  artificial  Warming  and  Cooling. — 
Lighting. — Admits  of  easy  Explanation. — Dilatation. — Examples. — Thermometer. — Melting  and 
Boiling  Points. — Evaporation. — Specific  Heat. — Heat  produced  by  Compression. — Radiation. — 
Conduction. — Incandescence. 


HEAT. 


319 


IEAT. 


WHILE  almost  every  other  branch  of  physical  science  has  been  made  the 
subject  of  systematic  treatises  without  number,  and  some  have  been,  as  it  were, 
set  apart  from  the  general  mass  of  natural  philosophy,  and  raised  to  the  rank 
of  distinct  sciences  by  the  badge  of  some  characteristic  title,  Heat  alone  has 
been  left  to  form  a  chapter  of  chemistry,  or  to  receive  a  passing  notice  in  trea- 
tises on  general  physics.  Light  has  long  enjoyed  the  exclusive  attention  of 
philosophers,  and  has  been  elevated  to  the  dignity  of  a  science,  under  the  name 
of  Optics.  Electricity  and  Magnetism  have  also  been  thought  worthy  subjects 
for  separate  treatises,  yet,  can  any  one  who  has  observed  the  part  played  by 
heat  on  the  theatre  of  nature,  doubt  that  its  claims  to  attention  are  equal  to  those 
of  light,  and  superior  to  those  of  electricity  and  magnetism.  It  is  possible  for 
organized  matter  to  exist  without  light.  Innumerable  operations  of  nature  pro- 
ceed as  regularly  and  as  effectually  in  its  absence  as  when  it  is  present.  The 
want  of  that  sense  which  it  is  designed  to  affect  in  the  animal  economy,  in  no 
degree  impairs  the  other  powers  of  the  body,  nor  in  man  does  such  a  defect 
interfere  in  any  way  with  the  faculties  of  the  mind.  Light  is,  so  to  speak,  an 
object  rather  of  luxury  than  of  positive  necessity.  Nature  supplies  it,  there- 
fore, not  in  unlimited  abundance,  nor  at  all  times  and  places,  but  rather  with 
that  thrift  and  economy  which  she  is  wont  to  observe  in  dispensing  the  objects 
of  our  pleasures,  compared  with  those  which  are  necessary  to  our  being.  But 
heat,  on  the  contrary,  she  has  yielded  in  the  most  unbounded  plenteousness. 
Heat  is  everywhere  present.  Every  body  that  exists  contains  it  in  quantity 
without  known  limit.  The  most  inert  and  rude  masses  are  pregnant  with  it. 
Whatever  we  see,  hear,  smell,  taste,  or  feel,  is  full  of  it.  To  its  influence  is 
due  that  endless  variety  of  forms  which  are  spread  over  and  beautify  the  sur- 
face of  the  globe.  Land,  water,  air,  could  not  for  a  single  instant  exist  as  they 
do,  in  its  absence ;  all  would  suddenly  fall  into  one  rude  formless  mass — solid 
and  impenetrable.  The  air  of  heaven  hardening  into  a  crust  would  envelope 
the  globe,  and  crush  within  an  everlasting  tomb  all  that  it  contains.  Heat  is 


320 


HEAT. 


the  parent  and  the  nurse  of  the  endless  beauties  of  organization.  The  mine- 
ral,  the  vegetable,  the  animal  kingdoms,  are  its  offspring.  Erery  natural  strue- 
ture  is  either  immediately  produced  by  its  agency,  maintained  by  its  influence, 
or  intimately  dependant  on  it.  Withdraw  heat,  and  instantly  all  life,  motie», 
form,  and  beauty,  will  cease  to  exist,  and  it  may  be  literally  said,  "  Chats  has 
come  again." 

Nor  is  heat  less  instrumental  in  the  processes  of  art,  than  in  the  operations 
of  nature.  All  that  art  can  effect  on  the  productions  of  nature  is  to  chaage 
their  form  or  arrangement — to  separate  or  to  combine  them.  Bodies  are  moulded 
to  forms  which  our  wants  or  our  tastes  demand ;  compounds  are  decomposed, 
and  their  obnoxious  or  useless  elements  expelled,  in  obedience  to  our  wiihos. 
In  all  such  processes  heat  is  the  agent.  At  its  bidding  the  most  obdurate  masses 
soften  like  wax,  and  are  fashioned  to  suit  our  most  wayward  caprices.  Ele- 
ments of  bodies  knit  together  by  the  most  stubborn  affinities — by  forces  which 
might  well  be  deemed  invincible — are  torn  asunder  by  this  omnipotent  solvent, 
and  separately  presented  for  the  uc  3  or  the  pleasure  of  man,  the  great  Master 
of  Art. 

If  we  turn  from  art  to  science,  we  find  heat  assisting  or  obstructing,  as  the 
case  may  be,  but  always  modifying  the  objects  of  our  inquiry.  The  common 
spectator,  who,  on  a  clear  night,  beholds  the  firmament,  thinks  he  obtains  a  just 
notion  of  the  position  and  arrangement  of  the  brilliant  objects  with  which  it  is 
so  richly  furnished.  The  more  exact  vision  of  the  astronomer  discovers,  how- 
ever, that  he  beholds  this  starry  vault  through  a  distorting  medium ;  that,  in 
fact,  he  views  it  through  a  great  lens  of  air,  by  which  every  object  is  removed 
from  its  proper  place  ;  nay,  more,  that  this  distortion  varies  from  night  to  night, 
and  from  hour  to  hour — varies  with  the  varying  heat  of  the  atmosphere  which 
produces  it.  Such  distortion,  and  the  variations  to  which  it  is  subject,  must 
then  be  accurately  sustained,  before  any  inferences  can  be  made  respecting  the 
motion,  position,  magnitude,  or  distance  of  any  object  in  the  heavens  ;  and  as- 
certained it  cannot  be,  unless  the  laws  that  govern  the  phenomena  of  heat  be 
known. 

But  the  very  instruments  which  the  same  astronomer  uses  to  assist  his  vis- 
ion, and  to  note  and  measure  the  positions  and  mutual  distances  of  the  objects 
of  his  inquiry,  are  themselves  eminently  subject  to  the  same  distorting  influence. 
The  metal  of  which  they  are  formed  swells  and  contracts  with  every  fluctuation 
in  the  heat  to  which  it  is  exposed.  A  sunbeam,  a  blast  of  cold  air — nay,  the 
very  heat  of  the  astronomer's  own  body — must  produce  effects  on  the  figure  of 
the  brazen  arch  by  whose  divided  surface  his  measurements  and  his  observations 
are  effected.  Such  effects  must  therefore  be  known,  and  taken  into  account, 
ere  he  can  hope  to  attain  that  accuracy  which  the  delicacy  of  his  investigations 
renders  indispensably  necessary. 

The  chemist,  in  all  his  proceedings,  is  beset  with  the  effects  of  heat,  aiding 
or  impeding  his  researches.  Now  it  promotes  the  disunion  of  combined  ele- 
ments, now  fuses  into  one  uniform  mass  the  most  heterogeneous  materials. 
At  one  time  he  resorts  to  it  as  the  means  of  arousing  dormant  affinities  ;  at  an- 
other he  applies  its  powers  to  dissolve  the  strongest  bonds  of  chemical  attrac- 
tion. Composition  and  decomposition  are  equally  attended  by  its  evolution  and 
absorption ;  and  often  to  such  an  extent  as  to  produce  tremendous  explosions 
on  the  one  hand,  or  cold,  exceeding  the  rigors  of  the  most  severe  polar  winter, 
on  the  other. 

But  why  repair  to  the  observatory  of  the  astronomer  or  to  the  laboratory  of 
the  chemist,  for  examples  of  a  principle  which  is  in  never-ceasing  operation 
around  us !  Sleeping  or  waking,  at  home  or  abroad,  by  night  or  by  day,  at 
rest  or  in  motion,  in  the  country  or  in  the  town,  traversing  the  burning  limits  of 


HEAT. 


321 


the  tropics,  or  exploring  the  rigors  of  the  poles,  we  are  ever  under  its  influence. 
We  are  at  once  its  slaves  and  its  masters. 

We  are  its  slaves  : — Without  it  we  cannot  for  a  moment  live.  Without  its 
well-regulated  quantity  we  cannot  for  a  moment  enjoy  life.  It  rules  our  pleas- 
ures and  our  pains  ;  it  lays  us  on  the  sick  bed,  and  raises  us  from  it.  It  is  our 
disease  and  our  physician.  In  the  ardor  of  summer  we  languish  under  its  ex- 
cess, and  in  the  rigor  of  winter  we  shiver  under  its  defect.  Does  it  accumu- 
late around  us  in  undue  quantity,  we  burn  with  fever;  does  it  depart  from 
us  with  unwonted  rapidity,  we  shake  with  ague  ;  or  writhe  under  the  pains  of 
rheumatism,  and  the  tribe  of  maladies  which  it  leaves  behind  when  it  quits  us. 

We  are  its  masters  : — We  subdue  it  to  our  will  and  dispose  it  to  our  pur- 
poses. Amid  arctic  snows  we  confine  it  around  our  persons,  and  prevent  its 
escape  by  a  clothing*  impervious  to  it.  Under  a  tropical  sun  we  exclude  it  by 
like  means.  We  extort  it  from  water  to  obtain  the  luxury  of  ice  in  hot  seasons, 
and  we  force  it  into  water  to  warm  our  apartmentsf  in  cold  ones.  Do  we  trav- 
erse the  seas — it  lends  wings  to  the  ship,  and  bids  defiance  to  the  natural  op- 
ponents, the  winds  and  the  tides.  Do  we  traverse  the  land — it  is  harnessed  to 
the  chariot,  and  we  outstrip  the  flight  of  the  swiftest  bird,  and  equal  the  fury 
of  the  tempest.;}; 

If  we  sleep,  our  chamber  and  our  couch  are  furnished  with  contrivances  for 
its  due  regulation.  If  we  eat,  our  food  owes  its  savor  and  its  nutrition  to  heat. 
From  this  the  fruit  receives  its  ripeness,  and  by  this  the  viands  of  the  table 
are  fitted  for  our  use.  The  grateful  infusion  which  forms  our  morning  repast 
might  remain  for  ever  hidden  in  the  leaf  |[  of  the  tree,  the  berry§  of  the  plant,  or 
the  kernel^!  of  the  nut,  if  heat  did  not  lend  its  power  to  extract  them.  The 
beverage  that  warms  and  cheers  us,  when  relaxed  by  labor  or  overcome  by  fa- 
tigue, is  distilled,  brewed,  or  fermented,  by  the  agency  of  heat.  The  produc- 
tions of  nature  give  up  their  sanative  principles  to  this  all-powerful  agent ;  and 
hence  the  decoction  or  the  pill  is  produced  to  restore  health  to  the  sinking 
patient. 

When  the  sun  hides  his  face  and  the  heavens  are  veiled  in  darkness,  whence 
do  we  obtain  light  ?  Heat  confers  light  upon  air,  and  the  taper  burns  and  the 
lamp  blazes,**  producing  artificial  day ;  guiding  us  in  the  pursuits  of  business  or 
of  pleasure,  and  thus  adding  to  the  sum  of  life,  by  rendering  hours  pleasant 
and  useful  which  must  otherwise  have  been  lost  in  torpor  or  in  sleep. 

These,  and  a  thousand  other  circumstances,  prove  how  important  a  physical 
agent  is  that  to  the  explication  of  whose  effects  the  pages  of  the  present  dis- 
course are  devoted.  But  it  is  not  alone  the  intrinsic  importance  of  the  sub- 
ject, nor  its  connexion  with  every  natural  appearance  that  can  attract  observa- 
tion or  excite  inquiry,  which  has  induced  us  to  examine  it.  It  presents  other 
advantages  which  merit  peculiar  consideration,  with  a  view  to  popular  instruc- 
tion. 

The  phenomena  all  admit  of  being  explained  without  the  aid  of  abstruse 
reasoning,  technical  language,  or  mathematical  symbols.  The  subject  abounds 

*  Clothing,  in  general,  is  composed  of  non-conducting  substances,  which  in  cold  weather  prevents 
the  heat  produced  by  the  body  from  escaping,  and  preserves  its  temperature;  and  in  hot  weather 
excludes  the  heat  from  the  body,  so  as  to  prevent  unrtue  warmth. 

t  Buildings  are  warmed  by  hot  water  carried  through  the  apartments  in  pipes. 

i  The  swiftest  flight  of  a  carrier  pigeon  does  not  exceed  the  rate  of  twenty-six  miles  an  hour.  It  is 
calculated  that  the  velocity  of  a  high  wind  is  at  the  rate  of  about  thirty  to  thirty-five  miles  an  hour. 
The  steam-carriages  on  the  Manchester  and  Liverpool  Railway  have  been  known  to  travel  about 
six-and-thirty  miles  an  hour ;  and  it  is  slated,  in  the  evidence  before  a  committee  of  the  House  of 
Commons,  that  steam-carriages  have  run  on  common  roads  at  a  speed  exceeding  forty  miles  an  hoar. 

|j  The  tea-tree. 

$  Coffee. 

IF  Chocolate. 

"**  Flame  is  gas,  or  air,  rendered  so  hot  as  to  become  luminous. 

21 


in  examples  of  the  most  felicitous  processes  of  induction,  from  which  the  gen- 
eral reader  may  obtain  a  view  of  that  beautiful  logic,  the  light  of  which  Bacon 
first  let  in  on  the  obscurity  in  which  he  found  physics  involved.  And,  finally, 
the  whole  range  of  our  domestic  experience  presents  a  series  of  familiar  and 
pointed  illustrations  of  the  principles  to  which  it  leads. 

The  first  and  most  common  effect  of  heat  is  to  increase  the  size  of  the  body 
to  which  it  is  imparted.  This  effect  is  called  dilatation,  or  expansion ;  and  the 
body  so  affected  is  said  to  expand,  or  be  dilated.  If  heat  be  abstracted  from  a 
body,  the  contrary  effect  is  produced,  and  the  body  contracts.  These  effects 
are  produced  in  different  degrees,  and  estimated  by  different  methods,  according 
as  the  bodies  which  suffer  them  are  solids,  liquids,  or  airs. 

The  dilatation  of  solids  is  very  minute,  even  by  considerable  additions  of  heat ; 
that  of  liquids  is  greater,  but  that  of  air  is  greatest  of  all. 

The  force  with  which  a  solid  dilates  is  equal  to  that  with  which  it  would 
resist  compression ;  and  the  force  with  which  it  contracts  is  equal  to  that  with 
which  it  would  resist  extension.  Such  forces  are,  therefore,  proportional  to 
the  strength  of  the  solid,  estimated  with  reference  to  the  power  with  which 
they  would  resist  compression  or  extension. 

The  force  with  which  liquids  dilate  is  equivalent  to  that  with  which  they 
would  resist  compression ;  as  liquids  are  nearly  incompressible,  this  force  is 
very  considerable. 

As  air  is  capable  of  being  compressed  with  facility,  its  dilatation  by  heat  is 
easily  resisted.  If  such  dilatation  be  opposed  by  confining  air  within  fixed 
bounds,  then  the  effect  of  heat,  instead  of  enlarging  its  dimensions,  will  be  to 
increase  its  pressure  on  the  surface  by  which  it  is  confined. 

The  works  of  clocks  and  watches  swell  and  contract  with  the  vicissitudes 
of  heat  and  cold  to  which  they  are  exposed,  When  the  pendulum  of  a  clock 
or  balance-wheel  of  a  watch  is  thus  enlarged  by  heat,  it  swings  more  slowly, 
and  the  rate  is  diminished.  On  the  other  hand,  when  it  contracts  by  cold,  its 
vibration  is  accelerated,  and  the  rate  is  increased.  Various  contrivances  have 
been  resorted  to  to  counteract  these  effects.  When  boiling  water  is  poured 
into  a  thick  glass,  the  unequal  expansion  of  the  glass  will  tear  one  part  from 
another,  and  produce  fracture.  The  same  vessel  contains  a  greater  quantity  of 
cold  than  of  hot  water. 

If  a  kettle,  completely  filled  with  cold  water,  be  placed  on  a  fire,  the  water, 
when  it  begins  to  get  warm,  will  swell,  and  spontaneously  flow  from  the  spout 
of  the  kettle  until  it  ceases  to  expand. 

If  a  bottle  well  corked  be  placed  before  the  fire,  especially  if  it  contain  fer- 
mented liquor  in  which  air  is  fixed,  the  air  confined  in  it  will  acquire  increased 
pressure  by  the  heat  imparted  to  it,  and  its  effort  to  expand  will  at  length  be  so 
great  that  the  cork  will  shoot  from  the  bottle,  or  the  bottle  itself  will  burst. 

Thus  we  perceive  that  the  magnitude  of  a  body  depends  on  the  quantity  of 
heat  which  has  been  imparted  to  it,  or  abstracted  from  it ;  and  as  it  must  be  in 
a  state  of  continual  variation,  with  respect  to  the  heat  which  it  contains,  it  fol- 
lows that  it  must  be  in  a  state  of  continual  variation  with  respect  to  its  magni- 
tude. We  can,  therefore,  never  pronounce  on  the  magnitude  of  any  body  with 
exactness,  unless  we  are  at  the  same  time  informed  of  its  situation  with  respect 
to  heat.  Every  hour  the  bodies  around  us  are  swelling  and  contracting,  and 
never  for  one  moment  retain  the  same  dimensions ;  neither  are  these  effects 
confined  to  their  exterior  dimensions,  but  extend  to  their  most  intimate  com- 
ponent particles.  These  are  in  a  constant  state  of  motion,  alternately  ap- 
(  proaching  to  and  receding  from  one  another,  and  changing  their  relative  posi- 
tions and  distances.  Thus,  the  particles  of  matter,  sluggish  and  inert  as  they 
appear,  are  in  a  state  of  constant  motion  and  apparent  activity. 


HEAT.  303 

Since  the  magnitude  of  any  body  changes  with  the  heat  to  which  it  is  ex- 
posed, and  since,  when  subject  to  the  same  calorific  influence,  it  alwa\s  has 
the  same  magnitude,  these  dilatations  and  contractions,  which  are  the  constant 
effects  of  heat,  may  be  taken  as  the  measure  of  the  physical  cause  which  pro- 
duced them.  The  changes  in  magnitude  which  a  body  suffers  by  changes  in 
the  heat  to  which  it  is  exposed,  are  called  changes  of  temperature ;  and  the  ac- 
tual state  of  a  body  at  any  moment,  determined  by  a  comparison  of  its  magni- 
tude with  the  heat  to  which  it  is  exposed,  is  called  its  temperature.  At  the 
same  temperature  the  same  body  always  has  the  same  magnitude  ;  and  when  its 
magnitude  increases,  by  being  exposed  to  heat,  its  temperature  is  said  to  rise  ; 
and,  on  the  contrary,  when  its  magnitude  is  diminished,  its  temperature  is  said 
to  fall.  The  variation  of  magnitude  of  any  body  is  therefore  taken  as  a  meas- 
ure of  temperature  ;  but  as  it  would  be  inconvenient,  in  practice,  to  adopt  dif- 
ferent measures  of  temperature,  one  body  is  selected  by  the  dilatation  and  con- 
traction of  which  those  of  all  other  bodies  are  measured,  and  with  this  body  a 
thermometer,  or  measure  of  temperature,  is  formed. 

The  substance  most  commonly  used  for  this  purpose  is  a  liquid  metal  called 
mercury  or  quicksilver.  Let  a  glass  tube  of  very  small  bore,  and  terminating  in 
a  spherical  bulb,  be  provided,  and  let  the  bulb  and  a  part  of  the  tube  be  filled 
with  mercury.  If  the  bulb  be  exposed  to  any  source  of  heat,  the  liquid  metal 
contained  in  it  will  expand,  and,  the  bulb  being  no  longer  sufficiently  capacious 
for  it,  the  column  in  the  tube  will  be  pressed  upward  to  afford  room  for  the  in- 
creased volume  of  the  mercury.  On  the  other  hand,  if  the  bulb  be  exposed  to 
cold  the  mercury  will  contract,  and  the  column  in  the  tube  will  fall. 

If  we  take  another  similar  instrument,  having  a  bulb  of  the  same  magnitude 
but  a  smaller  tube,  the  same  change  of  temperature  will  cause  the  mercury  in 
the  tube  to  rise  through  a  certain  space,  and  this  space  will  be  greater  than  in 
the  former,  in  the  same  proportion  as  the  bore  of  the  tube  is  smaller,  because 
in  this  case  the  actual  dilatation  of  the  mercury  in  both  tubes  is  the  same ;  but  this 
dilatation  will  fill  a  more  extensive  space  in  the  smaller  tube.  When  the  bulb, 
therefore,  has  the  same  magnitude,  the  thermometer  will  be  more  sensible  the 
smaller  the  tube ;  or,  in  general,  the  less  the  magnitude  of  the  tube,  com- 
pared with  that  of  the  bulb,  the  greater  will  be  the  sensibility  of  the  instru- 
ment. 

It  is  evident,  therefore,  that  the  same  change  of  temperature  would  produce 
very  different  effects  on  these  two  instruments,  and  the  indications  of  the  one 
could  not  be  compared  with  those  of  the  other.  To  render  them  comparable, 
it  will  be  necessary  to  determine  the  effects  which  the  same  temperature  will 
produce  on  both.  Let  the  two  instruments  be  immersed  in  pure  snow  in  a 
melting  state.  The  mercury  will  be  observed  to  stop  in  each  at  a  certain 
height ;  let  these  heights  be  marked  on  the  scales  attached  to  the  tubes  re- 
spectively. Now  it  will  happen  that  at  whatever  time  or  place  the  instruments 
may  be  immersed  in  melting  snow,  the  mercury  will  always  fix  itself  at  the 
points  here  marked.  This,  therefore,  constitutes  one  of  the  fixed  points  of  the 
thermometer,  arid  is  called  the  freezing  point.  Let  the  two  instruments  be  now 
immersed  in  pure  water  in  a  boiling  state,  the  height  of  the  barometer  being 
thirty  inches  at  the  time  of  the  experiment.  The  mercury  will  rise  in  each  to 
a  certain  point.  Let  this  point  be  marked  on  the  scale  of  each.  It  will  be 
found  that  at  whatever  time  or  place  the  instruments  are  immersed  in  pure 
water,  when  boiling,  provided  the  barometer  stand  at  the  same  height  of  thirty 
inches,  the  mercury  will  rise  in  each  to  the  point  thus  marked.  This,  there- 
fore, forms  another  fixed  point  on  the  thermometric  scale,  and  is  called  the 
boiling  point. 

The  distance  between  these  two  points  on  the  two  thermometers  in  ques- 


L 


324 


HEAT. 


tion,  will  be  observed  to  be  different.  In  the  thermometer  which  has  a  tube  P 
with  a  smaller  bore  in  proportion  to  its  bulb,  the  distance  will  be  greater  than 
in  the  other,  because  the  same  volume  of  mercury  which  forms  the  dilatation 
of  that  liquid  from  the  freezing  to  the  boiling  point  fills  a  greater  length  of  the 
smaller  than  of  the  large  tube.  It  is  plain,  therefore,  that  since  this  given  dif- 
ference of  temperature  causes  the  column  of  mercury  to  rise  through  a  greater 
space  in  the  one  than  in  the  other,  the  one  instrument  is  properly  said  to  pos- 
sess a  greater  sensibility  than  the  other. 

Let  the  intervals  on  the  scale  between  the  freezing  and  boiling  points  be 
now  divided  into  180  equal  parts  ;  and  let  this  division  be  similarly  continued 
below  the  freezing  point  to  the  place  0  ;  and  let  each  division  upward  from  that 
be  marked  with  the  successive  numbers,  1,  2,  3,  &c.  The  freezing  point  will 
now  be  the  32d  division,  and  the  boiling  point  will  be  the  212th  division. 
These  divisions  are  called  degrees,  and  the  freezing  point  is,  therefore,  32°, 
and  the  boiling  temperature  212°. 

It  is  evident,  that  although  the  degrees  on  these  two  instruments  are  differ- 
ent in  magnitude,  still  the  same  temperature  is  marked  by  the  same  degree  on 
each,  and  therefore  their  indications  will  correspond. 

The  manner  of  dividing  and  numbering  the  scale  here  described,  is  that 
which  is  commonly  adopted  in  England,  and  is  called  Fahrenheit's  scale. 
Other  methods  have  been  adopted  in  France  and  elsewhere,  which  will  hereaf- 
ter be  described. 

Let  a  mass  of  snow  at  the  temperature  of  0°,  having  a  thermometer  im- 
mersed in  it,  be  exposed  to  an  atmosphere  of  the  temperature  of  80°.  As  the 
snow  gradually  receives  heat  from  the  surrounding  air,  the  thermometer  im- 
mersed in  it  will  be  observed  to  rise  until  it  attain  the  temperature  of  32°. 
The  snow  will  then  immediately  begin  to  be  converted  into  water,  and  the 
thermometer  will  become  stationary.  During  the  process  of  liquefaction,  and 
while  the  snow  constantly  receives  heat  from  the  surrounding  air,  the  ther- 
mometer will  still  be  fixed,  nor  will  it  begin  to  rise  until  the  process  of  lique- 
faction is  completed.  Then,  however,  the  thermometer  will  again  begin  to 
rise,  and  will  continue  to  rise  until  it  attain  the  same  temperature  as  the  sur- 
rounding air. 

Heat,  therefore,  when  supplied  to  the  snow  in  a  sufficient  quantity,  has  the 
effect  of  causing  it  to  pass  from  the  solid  to  the  liquid  state,  and  while  so  em- 
ployed, becomes  incapable  of  affecting  the  thermometer.  The  heat  thus  con- 
sumed or  absorbed  in  the  process  of  liquefaction,  is  said  to  become  latent,  the 
meaning  of  which  is,  that  it  is  in  a  state  incapable  of  affecting  the  ther- 
mometer. 

The  property  here  described,  with  respect  to  snow  is  common  to  all  solids. 
Every  body  in  the  solid  state,  if  heat  be  imparted  to  it,  will  at  length  attain  a 
temperature  at  which  it  will  pass  into  the  liquid  state.  This  temperature  is  called 
its  point  of  fusion,  its  melting  point  or  its  fusing  point ;  and  in  passing  into  the 
liquid  state,  the  thermometer  will  be  maintained  at  the  fixed  temperature  of 
fusion,  and  will  not  be  affected  by  that  heat  which  the  body  receives  while  un- 
dergoing the  transition  from  the  solid  to  the  liquid  state. 

If  water,  at  the  temperature  of  60°,  be  placed  in  a  vessel  on  a  fire  having  a 
thermometer  immersed  in  it,  the  thermometer  will  be  observed  gradually  to 
rise,  and  the  water  will  become  hotter,  until  the  thermometer  arrives  at  the 
temperature  of  212°. 

Other  liquids  are  found  to  undergo  a  like  effect.  If  exposed  to  heat,  their 
temperatures  will  constantly  rise,  until  they  attain  a  certain  limit,  which  is  dif- 
ferent in  different  liquid ;  but  having  attained  this  limit  they  will  enter  into  a 
state  of  ebullition,  and  no  addition  of  heat  can  impart  to  them  a  higher  temper- 


HEAT. 


325 


ature.  The  temperature  at  which  different  liquids  thus  boil  is  called  their 
boiling  point. 

The  melting  or  freezing  point  and  the  boiling  point  constitute  important 
physical  characters,  by  which  different  substances  are  distinguished  from  each 
other. 

When  heat  continues  to  be  supplied  to  a  liquid  which  is  in  the  state  of  ebul- 
lition the  liquid  is  gradually  converted  into  vapor  or  steam,  which  is  a  form  of 
body  possessing  the  same  physical  characters  as  atmospheric  air.  The  steam 
or  vapor  thus  produced  has  the  same  temperature  as  the  water  from  which  it 
was  raised,  notwithstanding  the  great  quantity  of  heat  imparted  to  the  water  in 
its  transition  from  the  one  state  to  the  other.  This  quantity  of  heat  is  therefore 
latent. 

The  abstraction  of  heat  produces  a  series  of  effects  contrary  to  those  just 
described.  If  heat  be  withdrawn  from  a  liquid,  its  temperature  will  first  be 
gradually  lowered  until  it  attain  a  certain  point,  at  which  it  will  pass  into  the 
solid  state.  This  point  is  the  same  as  that  at  which,  being  solid,  it  would  pass 
into  the  liquid  state.  Thus  water,  gradually  cooled  from  sixty  degrees  down- 
ward, will  fall  in  its  temperature  until  it  attains  the  limit  of  thirty-two  degrees ; 
there  it  passes  into  the  solid  state  and  forms  ice  ;  and  during  this  transition  a 
large  quantity  of  heat  is  dismissed,  while  the  temperature  is  maintained  at 
thirty-two  degrees. 

In  like  manner,  if  heat  be  withdrawn  from  steam  or  vapor,  it  no  longer  re- 
mains in  the  aeriform  state,  but  resumes  the  liquid  form.  In  this  case  it  un- 
dergoes a  very  great  diminution  of  bulk,  a  large  volume  of  steam  forming  only 
a  few  drops  of  liquid.  Hence  the  process  by  which  vapor  passes  from  the 
aeriform  to  the  liquid  state  has  been  called  condensation. 

When  a  liquid  boils  vapor  is  generated  in  every  part  of  its  dimensions,  and 
more  abundantly  in  those  parts  which  are  nearest  the  source  of  heat ;  but  li- 
quids generate  vapor  from  their  surfaces  at  all  temperatures.  Thus,  a  vessel 
of  water  at  the  temperature  of  eighty  degrees  will  dismiss  from  its  surface  a 
quantity  of  vapor,  and  if  its  temperature  be  retained  at  eighty  degrees,  it  will 
continue  to  dismiss  vapor  from  its  surface  at  the  same  rate,  until  all  the  water 
in  the  vessel  has  disappeared.  This  process,  by  which  vapor  is  produced  at 
the  surface  of  liquids  at  temperatures  below  their  boiling  point,  is  called  vapor- 
ization. 

The  process  of  vaporization  is  generally  going  on  at  the  surface  of  all  collec- 
tions of  water,  great  or  small,  on  every  part  of  the  globe  ;  but  it  is  in  still  more 
powerful  operation  when  liquid  juices  are  distributed  through  the  pores,  fibres, 
and  interstices  of  animal  and  vegetable  structures.  In  all  these  cases,  the  rate 
at  which  the  liquid  is  converted  into  vapor  is  greatly  modified  by  the  pres- 
sure of  the  atmosphere.  The  pressure  of  that  fluid  retards  vaporization,  if  its 
effects  be  compared  with  that  which  would  take  place  in  a  vacuum ;  but, 
on  the  other  hand,  the  current  of  air,  continually  carrying  away  the  vapor,  as 
fast  as  it  is  formed,  in  the  space  above  the  surface,  gives  room  for  the  formation 
of  fresh  vapor,  and  accelerates  the  transition  of  the  liquids  to  the  vaporous  state. 
The  process  of  vaporization,  thus  modified  by  the  atmosphere  and  its  currents, 
so  far  as  it  affects  the  collections  of  water  and  liquids  generally  in  various  parts 
of  the  earth,  is  denominated  evaporation. 

The  condensation  of  the  vapor,  thus  drawn  up  and  suspended  in  the  atmo- 
sphere by  various  causes,  tending  to  extricate  the  latent  heat  which  gives  to  it 
the  form  of  air,  produces  all  the  phenomena  of  dew,  rain,  hail,  snow,  &c.,  &c. 
A  slight  degree  of  cold  converts  the  vapor  suspended  in  the  atmosphere  into  a 
liquid,  and  by  the  natural  cohesion  of  its  molecules  it  collects  into  spherules  or 
drops,  and  falls  in  the  form  of  rain.  A  greater  degree  of  cold  solidifies  or  con- 


f 


326 


HEAT. 


geals  its  minute  particles,  and  they  descend  to  the  earth  in  flakes  of  snow.  If, 
however,  they  are  first  formed  into  liquid  spherules,  and  then  solidified,  hail  is 
produced. 

Thus  there  is  a  constant  interchange  of  matter  between  the  earth  and  its  at- 
mosphere— the  atmosphere  continually  drawing  up  water  in  the  form  of  vapor, 
and,  when  the  heat  which  accomplishes  this  is  diminished,  precipitating  it  in 
the  form  of  dew,  rain,  snow,  or  hail. 

Different  bodies  are  differently  susceptible  of  the  effects  of  heat.  To  pro- 
duce a  given  change  of  temperature  in  some  requires  a  greater  supply  of  heat 
than  in  others.  Thus,  to  raise  water  from  the  temperature  of  50°  to  the  tem- 
perature of  60°  will  require  a  fire  of  given  intensity  to  act  upon  it  about  thirty 
times  as  long  as  to  raise  the  same  weight  of  mercury  through  the  same  range 
of  temperature.  In  the  same  manner,  if  various  other  bodies  be  submitted  to 
a  like  experiment,  it  will  be  found  that  to  produce  the  same  change  of  temper- 
ature on  the  same  weights  of  each  will  require  the  action  of  the  same  fire  for  a 
different  length  of  time. 

The  quantities  of  heat  necessary  to  produce  the  same  change  of  temperature 
in  equal  weights  of  different  bodies  are  therefore  called  the  specific  heats  of 
these  bodies.  If  1,000  express  the  specific  heat  of  pure  water,  or  the  quantity 
of  heat  necessary  to  raise  a  given  weight  of  pure  water  through  1°,  then  33 
will  express  the  specific  heat  of  mercury,  or  the  quantity  of  heat  necessary  to 
raise  the  same  weight  of  mercury  through  1°  ;  70  will  express  the  specific 
heat  of  tin,  80  of  silver,  110  of  iron,  and  so  on.  The  specific  heat  furnishes 
another  physical  character  by  which  bodies,  whether  simple  or  compound,  of 
different  kinds  may  be  distinguished. 

The  specific  heat  of  the  same  body  is  changeable  with  its  density.  In  gen- 
eral, as  the  density  is  increased,  the  specific  heat  is  diminished.  Now,  if  the 
specific  heat  of  a  body  be  diminished,  since  a  less  quantity  of  heat  will  then 
raise  it  through  1°  of  temperature,  the  quantity  of  heat  which  it  actually  con- 
tains will  make  it  hotter  when  it  is  rendered  more  dense,  and  colder  when  it 
is  rendered  more  rare. 

Hence  we  find  that,  when  certain  metals  are  hammered,  so  as  to  increase 
their  density,  they  become  hotter,  and  sometimes  become  red  hot. 

If  air  be  squeezed  into  a  small  compass,  it  becomes  so  hot  as  to  ignite  tin- 
der ;  and  the  discharge  of  an  air-gun  is  said  to  be  accompanied  by  a  flash  of 
light  in  the  dark. 

On  the  other  hand,  if  air  expand  into  an  enlarged  space,  it  becomes  colder. 
Hence,  in  the  upper  regions  of  the  atmosphere,  where  the  air  is  not  compressed, 
its  temperature  is  much  reduced,  and  the  cold  becomes  so  great  as  to  cause,  on 
high  mountains,  perpetual  snow. 

The  specific  heats  of  compounds  frequently  differ  much  from  those  of  the 
components.  If  the  specific  heat  of  bodies  be  greatly  diminished  by  their  com- 
bination, then  the  quantity  of  heat  which  they  contain  will  render  the  compound 
much  hotter  than  the  components  before  the  combination  took  place.  If,  on  the 
other  hand,  the  specific  heat  of  the  compound  be  greater  than  that  of  the  com- 
ponents, then  the  compound  will  be  colder,  because  the  heat  which  it  contains 
will  be  insufficient  to  sustain  the  same  temperature. 

Hence  we  invariably  find  that  chemical  combination  produces  a  change  of 
temperature.  In  some  cases  cold  is  produced,  but  in  most  cases  a  considera- 
ble increase  of  temperature  is  the  result. 

Heat  is  propagated  through  space  in  two  ways  :  First  by  radiation,  which 
is  apparently  independent  of  the  presence  of  matter,  and,  secondly,  by  conduc- 
tion, a  word  which  expresses  the  passage  of  heat  from  particle  to  particle  of  a 
mass  of  matter. 


HEAT. 


327 


The  principal  properties  of  heat  are  so  nearly  identical  with  those  of  light, 
that  the  supposition  that  heat  is  obscure  light  is  countenanced  by  strong  proba- 
bilities. Heat  proceeds  in  straight  lines  from  the  point  whence  it  emanates, 
diverging  in  every  direction.  These  lines  are  called  rays  of  heat,  and  the 
process  is  called  radiation.  Heat  radiates  through  certain  bodies  which  are 
transparent  to  it,  as  glass  is  to  light.  It  passes  freely  through  air  or  gas  ;  it 
also  passes  through  a  vacuum,  and  therefore  its  propagation  by  radiation  does 
not  depend  on  the  presence  of  matter.  Indeed,  the  great  velocity  with  which 
it  is  propagated  by  radiation  proves  that  it  does  not  proceed  by  transmission 
from  particle  to  particle. 

The  rays  of  heat  are  reflected  and  refracted  according  to  the  same  laws  as 
those  of  light.  They  are  collected  in  foci  by  concave  mirrors  and  convex 
lenses.  These  undergo  polarization,  both  Tjy  reflection  and  refraction,  in  the 
same  manner  as  rays  of  light.  They  are  subject  to  all  the  complicated  phe- 
nomena of  double  refraction  by  certain  crystals,  in  the  same  manner  exactly  as 
rays  of  light. 

Certain  bodies  possess  imperfect  transparency  to  heat :  such  bodies  transmit 
a  portion  of  the  heat  which  impinges  on  them,  and  absorb  the  remainder,  the 
portions  which  they  absorb  raising  their  temperature. 

Surfaces  also  possess  the  power  of  reflecting  heat  in  different  degrees.  They 
reflect  a  greater  or  less  portion  of  the  heat  incident  on  them,  absorbing  the  re- 
mainder. The  power  of  transmission,  absorption,  and  reflection,  vary  accord- 
ing to  the  nature  of  the  body  and  state  of  its  surface,  with  respect  to  smooth- 
ness, roughness,  and  color. 

Rays  of  heat,  like  those  of  light,  are  differently  refrangible,  and  the  average 
refrangibility  of  calorific  rays  is  less  than  that  of  luminous  rays. 

When  a  body  at  a  high  temperature,  as  the  flame  of  a  lamp  or  fire,  is  placed 
in  contact  with  the  surface  of  a  solid,  the  particles  immediately  in  contact  with 
the  source  of  heat  receive  an  elevated  temperature.  These  communicate  heat 
to  the  contiguous  particles,  and  these  again  to  particles  more  remote.  Thus 
the  increased  temperature  is  gradually  transmitted  through  the  dimensions  of 
the  body,  until  the  whole  mass  in  contact  with  the  source  of  heat  has  attained 
the  temperature  of  the  body  in  contact  with  it. 

Different  substances  exhibit  different  degrees  of  facility  in  transmitting  heat 
through  their  dimensions  in  this  manner.  In  some  the  temperature  spreads 
with  rapidity,  and  an  equilibrium  is  soon  established  between  the  body  receiv- 
ing heat  and  the  body  imparting  it.  Such  substances  are  said  to  be  good  con- 
ductors of  heat.  Metals  in  general  are  instances  of  this  ;  earths  and  woods  are 
bad  conductors  ;  and  soft,  porous,  or  spongy  substances  still  worse. 

When  the  temperature  of  a  body  has  been  raised  to  a  certain  extent  by  the 
application  of  any  source  of  heat,  it  is  observed  to  become  luminous,  so  as  to 
be  visible  in  the  absence  of  other  light,  and  to  render  objects  around  it  visible. 
Thus,  a  piece  of  iron,  by  the  application  of  heat,  will  at  first  emit  a  dull,  red 
light,  and  will  become  more  luminous  as  the  temperature  is  raised,  until  the 
red  light  is  converted  to  a  clear,  white  one,  and  the  iron  is  said  to  be  white  hot. 
This  process,  by  which  a  body  becomes  luminous  by  the  increase  of  its  tem- 
perature, is  called  incandescence.  There  is  reason  to  believe  that  all  solid 
bodies  begin  to  be  luminous  when  heated  at  the  same  temperature. 

The  degree  of  heat  of  incandescent  bodies  is  distinguished  by  their  color ; 
the  lowest  incandescent  heat  is  a  red  heat,  next  the  orange  neat,  the  yellow 
heat,  and  the  greatest  a  white  heat. 

The  heating  powers  of  rays  of  light  vary  with  their  color,  in  general  those 
of  the  lightest  color  having  the  most  heating  power.  Thus  yellow  light  has  a 
greater  calorific  power  then  green,  and  green  than  blue. 


328 


Hence  the  absorption  of  heat  from  the  same  light  depends  on  the  color  of 
the  absorbing  bodies.  Those  of  a  dark  color  absorb  more  heat  than  those  of  a 
light  color,  because  the  former  reflect  the  least  calorific  rays,  while  the  latter 
reflect  the  most. 

There  are  several  substances  which,  when  heated  to  a  certain  temperature,  C 
acquire  a  strong  affinity  for  oxygen  gas ;  and  when  the  elevation  of  tempera-  > 
ture  takes  place  in  an  atmosphere  of  oxygen,  or  in  ordinary  atmospheric  air,  < 
the  oxygen  rapidly  combines  with  the  heated  body,  and  in  the  combination  so  J 
great  a  quantity  of  heat  is  evolved  that  light  and  flame  are  produced.  This  ( 
process  is  called  combustion.  Combustion  is,  therefore,  a  sudden  chemical  com-  \ 
bination  of  some  substance  with  oxygen,  attended  by  the  evolution  of  heat  and  j 
light. 

The  flame  of  a  candle  or  lamp  is  an  instance  of  this.    The  substance  in  the  J 

wick,  having  its  temperature  raised  in  the  first  instance  by  the  application  of  / 

heat,  forms  a  rapid  combination  with  the  oxygen  of  the  atmosphere,  and   this  ( 

combination  is  attended  with  the  evolution  of  heat,  which  sustains  the  process  \ 

.of  combustion. 

Flame  is,  therefore,  gaseous  matter,  rendered  so  hot  as  to  be  luminous.  \ 
There  are  a  few  other  substances  besides  oxygen  by  combination  with  which 
light  and  heat  may  be  evolved,  and  which  may  therefore  produce  combustion. 
These  are  the  substances  called,  in  chemistry,  chlorine,  iodine,  and  bromine  ; 
but,  as  they  are  not  of  common  occurrence,  the  phenomenon  of  combustion  at- 
tending them  may  be  regarded  rather  as  a  subject  of  scientific  inquiry  than  of 
practical  occurrence.  All  ordinary  cases  of  combustion  are  examples  of  the 
combination  of  oxygen  with  a  combustible. 

I  have  thus,  in  a  succinct  and  clear  manner,  laid  before  you  the  principal 
phenomena,  and  explained  the  most  ordinary  terms,  which  I  shall  have  occa- 
sion to  use  in  the  discourses  I  intend  to  deliver  on  the  subject  of  heat.  These 
explanations  will,  I  trust,  greatly  facilitate  the  comprehension  of  the  la\vs  and 
the  narrative  of  the  discoveries  which  I  shall  unfold  to  you. 


GALYAIISM. 


}  Origin  of  the  Discovery. — Galvani  Professor  at  Bologna. — Accidental  Effect  on  Frogs. — Igno. 
ranee  of  Galvani. — His  Experiments  on  tlie  Frog. — Accidental  Discovery  of  the  Effect  of  Metal- 
lic Contact. — Animal  Electricity. — Galvani  Opposed  hy  Volta. — Volta's  Theory  of  Contact  Pre- 
vails.— Fabroni's  Experiments. — Invention  of  the  Voltaic  Pile. — La  Couronne  de  Tasses. — Na- 
poleon's Invitation  to  Volta. — Physiological  Effects  of  the  Pile. — Anecdote  of  Napoleon. — Decom- 
position of  Water. — Cruickshank's  Experiments. — Davy  commences  his  Researches. — Effect  of 
Chemical  Action  discovered. — Hitters  Secondary  Pile. — Calorific  Effectsof  the  Pile. — Hypothesis 
of  Grotthns. — Davy's  celebrated  Bakerie.n  Lecture. — Prize  awarded  him  hy  the  French  Acad- 
emy.— His  Discovery  of  the  Transferring  Power  of  the  Pile  in  Chemical  Action. — His  Electro- 
chemical Theory. — Decomposition  of  Potash  and  Soda. — New  Metals,  Potassium  and  Sodium. — 
Discovery  of  Barium. — Strontium,  Calcium,  and  Magnesium. — Rapid  Discover}' of  the  other  new 
Metals. — Dry  Piles. 


I   '*^u~*s*S 


GALVANISM. 


331 


GALVANISM. 


THE  investigation  of  the  mechanical  phenomena  of  material  substances  has 
been,  in  modern  works,  conducted  by  resolving  these  effects  into  two  principal 
divisions ;  those  in  which  the  bodies  exhibiting  them  are  at  rest,  and  those  in 
which  they  are  in  motion.  As  applied  to  solid  bodies,  these  divisions  have 
been  respectively  denominated  STATICS  and  DYNAMICS  ;*  and,  as  applied  to 
fluids,  HYDROSTATICS  and  HYDRODYNAMICS.  Electricity  being  assumed  to  be 
a  physical  agent,  having  the  properties  of  an  elastic  fluid,  and  capable,  like 
the  grosser  solids  and  fluids,  of  being  maintained  in  a  state  of  equilibrium  by 
the  mutual  action  and  reaction  of  antagonist  forces,  or  of  moving  in  definitfe  di- 
rections, and  forming  currents  of  greater  or  less  intensity,  the  analysis  of  its 
effects  would  naturally  be  conducted  by  means  of  the  same  classification  ; 
and,  accordingly,  that  division  of  the  science  in  which  the  electric  fluid  is  con- 
sidered in  a  state  of  equilibrium  or  repose,  and  in  which  the  physical  conditions 
on  which  such  equilibrium  depends  are  investigated,  would  be  denominated 
ELECTRO-STATICS,  while  that  in  which  the  effects  of  currents  of  electricity  are 
considered,  would  be  called  ELECTRO-DYNAMICS. 

REST  being  in  its  nature  more  simple  than  MOTION,  and  the  cases  of  forces 
mutually  destructive  of  each  other's  influence,  and  therefore  productive  of  equi- 
librium, being  more  simple  than  those  in  which  motion  ensues  from  the  com- 
bined action  of  forces  differing  from  each  other  in  various  respects,  it  was  nat- 
ural that,  in  every  part  of  physics,  the  principles  of  statics  should  be  first  es- 
tablished and  understood.  Such  has  been  accordingly  the  course  which  the 
progress  of  discovery  has  taken  in  other  branches  of  natural  philosophy,  and 
electricity  is  not  an  exception  to  it.  All  the  phenomena  which  have  been  hith- 
erto adverted  to  in  this  notice  belong  properly  to  ELECTRO-STATICS.  In  all  of 
them  the  electric  fluid  is  contemplated  in  a  state  of  equilibrium ;  or  if  its  mo- 
tion be  occasionally  considered,  it  is  only  in  sudden  and  momentary  changes 
from  one  state  of  equilibrium  to  another.  Thus,  when  a  Leyden  jar  is  char- 

*  The  terms  STEREO-STATICS  and  STEREO-DYNAMICS  would  be  preferable. 


332  GALVANISM. 

\ ~ 

^  ged,  the  positive  electricity  accumulated  on  the  inner  surface  of  the  glass  is 
maintained  there,  in  spite  of  the  tendency  it  has  to  escape  in  virtue  of  its  self- 
expansive  property,  by  the  attraction  of  the  negative  electricity  accumulated 
on  the  external  surface.  When  a  communication  is  made  between  the  inter- 
nal and  external  surfaces  by  a  metallic  wire,  this  state  of  equilibrium  ceases ; 
the  positive  fluid  of  the  inner  surface  runs  along  the  wire  in  one  direction,  and 
the  negative  fluid  of  the  external  surface  runs  along  it  in  the  other  direction, 
until  each  neutralizes  the  other,  and  a  new  state  of  equilibrium  is  established 
by  the  actual  combination  of  the  two  fluids.  If  this  change  occupied  a  sensi- 
ble interval  of  time,  and  it  were  required  to  investigate  the  effects  which  would 
be  produced  during  that  interval  either  on  the  jar  and  wire,  or  on  any  bodies 
which  might  be  within  their  influence,  the  question  would  properly  belong  to 
ELECTRO-DYNAMICS  ;  but  in  fact  the  discharge,  as  it  is  called,  or  the  transition 
from  the  one  state  of  equilibrium  to  the  other,  is  instantaneous,  and  the  same 
may  be  said  of  all  the  phenomena  which  form  the  subject  of  the  preceding 
pages. 

In  the  commencement  of  this  notice,  the  frequent  influence  of  circumstances, 
apparently  fortuitous,  on  the  progress  of  discovery  in  the  sciences,  has  been 
mentioned.  It  would  be  difficult,  either  in  the  history  of  the  sciences  or  of  the 
political  growth  of  states,  to  find  a  more  signal  example  of  this  than  was  offered 
by  the  discovery  of  that  powerful  instrument  of  physical  investigation,  the 
VOLTAIC  PILE.  "It  may  be  proved,"  says  M.  Arago,  "that  this  immortal  dis- 
covery arose  in  the  most  immediate  and  direct  manner  from  a  slight  cold  with 
which  a  Bolognese  lady  was  attacked  in  1790,  for  which  her  physician  pre- 
scribed the  use  of  frog-broth." 

Galvani  was  professor  of  anatomy  at  Bologna.  At  the  period  just  mentioned, 
it  happened  that  several  frogs,  divested  of  their  skins,  and  prepared  for  cook- 
ing the  broth  prescribed  for  Madame  Galvani,  lay  upon  a  table  in  the  laboratory 
of  the  professor,  near  which  at  the  moment  stood  an  electrical  machine.  One 
of  the  professor's  assistants,  being  employed  in  some  process  in  which  the  ma- 
chine was  necessary,  took  sparks  occasionally  from  the  conductor,  when  Mad- 
ame Galvani  was  astonished  to  see  the  limbs  of  the  dead  frogs  convulsed  with 
movements  resembling  vital  action.  She  called  the  attention  of  her  husband 
to  the  fact,  who  repeated  the  experiment,  and  found  the  motions  reproduced  as 
often  as  a  spark  was  taken  from  the  conductor.  This  was  the  first,  but  not  the 
only  or  chief  part  played  by  chance  in  this  great  discovery. 

Galvani  was  not  familiar  with  electricity.  Had  he  been  so,  he  would  have 
seen  in  the  convulsions  of  the  frog  evidence  of  nothing  more  than  a  high  elec- 
troscopic  sensibility  in  the  nerves  of  that  animal,  and  an  interesting  example 
of  the  known  principle  of  electrical  induction.  But  luckily  for  the  progress  of 
science,  he  was  more  an  anatomist  than  an  electrician,  and  beheld  with  senti- 
ments of  unmixed  wonder  the  manifestation  of  what  he  believed  to  be  a  new 
principle  in  the  animal  economy,  and,  fired  with  the  notion  of  bringing  to  light 
the  proximate  cause  of  vitality,  engaged  with  ardent  enthusiasm  in  a  course  of 
experiments  on  the  effects  of  electricity  on  the  animal  system.  It  is  rarely 
that  an  example  is  found  of  the  progress  of  science  being  favored  by  the  igno- 
rance of  its  professors. 

Chance  now  again  came  upon  the  stage.  In  the  course  of  his  researches  he 
had  occasion  to  separate  the  legs,  thighs,  and  lower  part  of  the  body  of  the 
frog  from  the  remainder,  so  as  to  lay  bare  the  lumbar  nerves.  Having  the 
members  of  several  frogs  thus  dissected,  he  passed  copper  hooks  through  part 
of  the  dorsal  column  which  remained  above  the  junction  of  the  thighs,  for  the 
convenience  of  hanging  them  up  till  they  might  be  required  for  the  purposes  of 
experiment.  In  this  manner  he  happened  to  suspend  several  upon  the  iron 


GALVANISM. 


333 


balcony  in  front  of  his  laboratory,  when,  to  his  inexpressible  astonishment,  the 
limbs  were  thrown  into  strong  convulsions.  No  electrical  machine  was  now 
present  to  exert  any  influence. 

If  the  supply  of  capital  facts  be  occasionally  due  to  chance,  or  to  the  Being 
by  whom  what  is  miscalled  chance  is  directed,  it  is  to  the  operation  of  the  fac- 
ulties of  exalted  minds  that  the  development  of  the  laws  of  nature  is  due  :  if 
rude  lumps  of  the  natural  ore  of  science  be  now  and  then  thrown  under  the  feet 
of  philosophy,  the  discovery  of  the  vein  itself,  its  depth  and  direction,  its  qual- 
ity and  value,  the  separation  of  the  precious  metal  it  contains  from  its  baser 
elements,  the  demonstration  of  its  connexion  with  the  phenomena  of  nature, 
and  its  adaptation  to  the  uses  of  life,  are  all  and  severally  the  work  of  that 
noble  faculty  of  intellect,  that  image  of  his  own  essence,  which  the  Creator 
of  the  universe  has  impressed  upon  man,  and  which  is  never  more  worthily 
exercised  than  in  the  investigation  of  those  laws  of  the  material  world,  in  all 
of  which,  whether  they  affect  the  vast  bodies  of  the  universe,  or  the  imper- 
ceptible molecules  of  those  around  us,  there  is  ever  conspicuous  a  provident 
care  for  the  wellbeing  of  his  creatures. 

In  the  convulsions  of  the  frog,  suspended  by  a  copper  wire  on  an  iron  rail, 
Galvani  saw  a  new  fact,  and  soon  discovered  that  the  circumstance  on  which  it 
depended  was  the  simultaneous  contact  of  the  metals  with  the  nerves  and  mus- 
cles of  the  animal.  He  found  that  the  effects  were  reproduced  whenever  the 
muscles  touched  the  iron  while  the  nerves  touched  the  copper,  but  that  contact 
with  the  copper  alone  did  not  produce  them.  He  next  placed  the  body  of  the 
animal  upon  a  plate  of  iron,  and  touching  the  plate  with  one  end  of  a  copper 
wire,  brought  the  other  end  into  contact  with  the  lumbar  nerves.  The  convul- 
sions followed  as  before.  Galvani  inferred  from  these  and  other  similar  exper- 
iments and  observations,  that  the  conditions  under  which  the  phenomenon  was 
produced  were,  that  a  connexion  should  be  made  between  the  nerves  of  the 
animal  and  the  muscles  with  which  those  nerves  were  united  by  a  continued 
line  or  circuit  composed  of  two  different  metals ;  and  he  explained  this  singu- 
lar effect  by  assuming,  hypothetically,  that,  in  the  animal  economy,  there  exists 
a  natural  source  of  electricity  ;  that,  at  the  junction  of  the  nerves  and  muscles, 
the  natural  electricity  is  decomposed  ;  that  the  positive  fluid  goes  to  the  nerve, 
and  the  negative  to  the  muscle ;  that  the  nerve  and  muscle  are  therefore  anal- 
ogous to  the  internal  and  external  coating  of  a  charged  Leyden  jar ;  that  the 
metallic  connexion  made  between  the  nerve  and  the  muscle  in  the  experiments 
above-mentioned  serves  as  a  conductor  between  these  opposite  electricities ; 
and  that,  on  making  the  connexion,  the  same  discharge  takes  place  as  in  the 
Leyden  experiment. 

This  theory  fascinated  for  a  time  the  physiologists.  The  phenomena  of  animal 
life  had  been  ascribed  to  an  hypothetical  agent,  which  passed  under  the  name  of 
the  "  nervous  fluid."  The  Galvanic  theory  consigned  this  term  to  the  obsolete  list ; 
and  electricity  was  now  the  great  vital  principle,  by  which  the  decrees  of  the 
understanding,  and  the  dictates  of  the  will,  were  conveyed  from  the  organs  of 
the  brain  to  the  obedient  members  of  the  body.  Those  who  know  how  pas- 
sionate is  the  love  of  a  theory  which  appears  to  give  a  satisfactory  account  of 
effects  otherwise  mysterious,  and  how  much  more  gratifying  to  the  amour- 
propre  it  is  to  be  able  to  connect  effects  with  supposed  causes,  than  to  be 
compelled  to  view  the  former  as  the  real  limits  of  our  knowledge,  will  under- 
stand the  reluctance  with  which  the  Bolognese  school  and  its  distinguished 
leader  would  surrender  a  theory  so  dazzling  as  animal  electricity  ;  nevertheless 
it  was  doomed  soon  to  fall  under  the  irresistible  assaults  of  physical  truth  di- 
rected against  it  by  a  giant  intellect,  which,  though  located  in  a  little  village  of 
the  Milanese,  belonged  to  mankind. 


234 


GALVANISM. 


Volta.  professor  of  natural  philosophy  at  Como,  and  subsequently  at  Pavia, 
had  been  already  known  for  his  researches  in  different  parts  of  physics,  but 
more  especially  in  electricity.  The  Bolognese  experiments  naturally  engaged 
his  attention)  and  it  was  not  long  before  his  superior  sagacity  enabled  him  to 
perceive  that  the  theory  of  Galvani  was  destitute  of  any  sound  foundation, 
j  ndeed,  a  single  experiment  was  sufficient  to  overturn  it,  though  not  to  carry 
conviction  of  its  futility  to  the  minds  of  its  partisans.  Volta  applied  the  met- 
als in  contact  with  each  other  to  the  muscle  alone,  without  touching  the  nerves, 
and  the  convulsions  nevertheless  ensued.  The  analogy  of  the  muscle  and 
nerve  to  the  Leyden  phial  was  no  longer  tenable.  Volta  transferred  this  anal- 
ogy to  the  two  metals,  and  contended  that  the  mutual  contact  of  two  dissimilar 
metals  must  be  regarded  as  the  source  of  the  electricity ;  that  by  the  contact 
the  natural  electricity  was  decomposed,  and  the  positive  fluid  passed  to  one 
metal,  and  the  negative  one  to  the  other ;  and  that  the  muscle  merely  played 
the  part  of  a  conductor  in  carrying  off  one  of  the  fluids  thus  developed. 

To  this  Galvani  replied  by  showing  that,  when  a  single  metal  was  used  to 
connect  the  nerves  and  muscles  the  convulsions  ensued,  and  that  therefore  the 
contact  of  dissimilar  metals  could  not  be  the  source  of  the  electricity.  Volta 
rejoined,  that  it  was  impossible  to  be  assured  of  the  perfect  homogeneity  of  the 
metal,  and  that  any  the  least  heterogeneous  matter  contained  in  it  would  be 
sufficient  for^his  hypothesis.  Also,  that  when  a  single  metal  was  used,  the 
convulsions  were  uncertain,  and  never  produced,  except  in  cases  where  the 
organs  were  in  the  highest  state  of  excitability  ;  whereas,  on  the  contrary,  they 
happened  invariably,  and  were  long  continued,  when  the  connexion  was  made 
by  two  dissimilar  metals. 

Tenacious  of  this  cherished  theory  to  the  last,  Doctor  Valli,  a  partisan  of 
Galvani,  confounded  the  advocates  of  the  school  of  Pavia,  by  showing  that,  by 
merely  bringing  the  muscles  themselves  into  contact  with  the  nerves,  without 
the  intervention  of  any  metal  whatever,  the  convulsions  ensued.  To  this — the 
expiring  effort  of  the  Bolognese  party — Volta  readily  and  triumphantly  replied, 
that  the  success  of  the  experiments  of  Valli  required  two  conditions  :  first,  that 
the  parts  of  the  animal  brought  into  contact  should  be  as  heterogeneous  as  pos- 
sible ;  and,  secondly,  the  interposition  of  a  third  substance  between  these 
organs.  This,  so  far  from  overturning  the  theory  of  Volta,  only  gave  it  in- 
creased generality,  showing,  as  it  did,  that  electricity  was  developed,  not  alone 
by  the  contact  of  two  dissimilar  metals,  but  also  by  the  contact  of  dissimilar 
substances  not  metallic. 

From  this  time,  the  partisans  of  animal  electricity  gradually  diminished,  and 
no  effort  worth  recording  to  revive  Galvani's  theory  was  made.  Meanwhile, 
the  hypothesis  of  Volta  was,  as  yet,  regarded  only  as  the  conjecture  of  a  pow- 
erful and  sagacious  mind,  requiring  nevertheless  much  more  cogent  and  direct 
experimental  verification.  This  experimental  proof  he  soon  supplied. 

The  first  analogy  which  Volta  produced  in  support  of  his  theory  of  contact 
was  derived  from  the  well-known  experiment  of  Sulzer.  If  two  pieces  of  dis- 
similar metal,  such  as  lead  and  silver,  be  placed  one  above  and  the  other  below 
the  tongue,  no  particular  effect  will  be  perceived  so  long  as  they  are  not  in 
contact  with  each  other ;  but  if  their  outer  edges  be  brought  to  touch  each 
other,  a  peculiar  taste  will  be  felt.  If  the  metals  be  applied  in  one  order,  the 
taste  will  be  acidulous  ;  if  the  order  be  inverted,  it  will  be  alkaline.  Now,  if  the 
tongue  be  applied  to  the  conductor  of  a  common  electrical  machine,  an  acidu- 
lous or  alkaline  taste  will  be  pe  "ceived,  according  as  the  conductor  is  electri- 
fied positively  or  negatively.  \  olta  contended,  therefore,  that  the  identity  of 
the  cause  should  be  inferred  from  the  identity  of  the  effects  ;  that,  as  positive 
electricity  produced  an  acid  savor,  and  negative  electricity  an  alkaline,  on  the 


GALVANISM.  335 


conductor  of  the  machine,  the  same  effects  on  the  organs  of  taste  produced  by 
the  metals  ought  to  be  ascribed  to  the  same  cause. 

However  sufficient  this  analogy  might  seem  to  the  understanding  of  Volta, 
it  was  insufficient  for  the  rigid  canons  of  the  logic  of  modern  physics,  and  he 
accordingly  sought  and  obtained  more  direct  and  unequivocal  proof  of  his  hy- 
pothesis. Two  disks,  one  of  copper  and  the  other  of  Sine,  were  attached  to 
insulating  handles,  by  means  of  which  they  were  carefully  brought  into  con- 
tact, and  suddenly  separated  without  friction.  They  were  then  presented  sev- 
erally to  a  powerful  condensing  electroscope.  The  usual  indications  of  elec- 
tricity were  obtained,  and  it  was  shown  that  this  electricity  was  positive  on  the 
zinc,  and  negative  on  the  copper.  By  repeating  the  contact,  and  collecting 
the  electricity  by  means  of  the  condenser,  sparks  were  produced,  and  the  dem- 
onstration was  complete. 

That  the  contact  of  dissimilar  metals  was  followed  by  the  evolution  of  elec- 
tricity, could  therefore  no  longer  be  doubted.  It  will,  however,  hereafter 
appear  that  philosophers  are  not  even  yet  agreed  that  the  contact  is  the  imme- 
diate or  the  only  cause  of  the  disengagement  of  electricity  in  such  cases. 
Chemical  agency  is  now  known  to  be  one  of  the  sources  of  electricity  ;  and  its 
operation  is  so  subtle,  often  so  imperceptible,  and  generally  so  inevitable,  when 
heterogeneous  molecules  come  into  contact,  that  doubts  have  been  entertained 
whether,  in  every  case  where  electricity  seems  to  proceed  from  contact,  it  has 
not  really  its  origin  in  feeble  and  imperceptible  chemical  action. 

Although  the  complete  development  of  this  last-mentioned  idea  belongs  to  a 
much  more  recent  epoch  in  the  progress  of  electrical  discovery,  yet  the  chemi- 
cal origin  of  electricity  did  not  altogether  escape  notice  even  at  the  period  to 
which  we  now  refer. 

Of  the  numerous  philosophers  in  every  part  of  Europe  who  took  part  in  the 
discussions,  and  varied  and  repeated  the  experiments  connected  with  these 
questions,  one  of  those  to  whom  attention  is  more  especially  due  was  Fabroni, 
who,  in  the  year  1792,*  two  years  after  the  discovery  of  Galvani,  communi- 
cated his  researches  to  the  Florentine  Academy.  In  this  paper  is  found  the 
first  suggestion  of  the  chemical  origin  of  Galvanic  electricity. 

Fabroni  observes  that  in  the  mutual  contact  of  heterogeneous  metals  there 
is  a  reciprocal  action  which  favors  chemical  change  ;  that  to  this  action  must 
be  ascribed  many  well-known  phenomena,  such  as  the  more  rapid  oxydation 
of  certain  metals  when  combined,  or  in  mere  contact  with  other  metals.  Ac- 
cording to  him,  a  metal,  like  all  chemical  reagents,  has  a  tendency  to  combina- 
tion with  another  metal  when  they  are  brought  into  contact ;  that  this  effect  is 
only  prevented  by  the  superior  force  of  cohesion  which  prevails  among  the 
particles  of  each.  This  cohesive  force  will,  however,  be  lessened  in  its  en- 
ergy by  the  antagonism  of  the  attraction  of  the  molecules  of  the  two  metals 
toward  each  other,  just  in  the  same  manner  as  it  would  be  lessened  by  the 
action  of  heat.  Being  thus  lessened,  its  opposition  to  the  tendency  which  the 
particles  of  either  metal  have  to  combine  with  oxygen,  taken  either  from  the 
atmosphere,  or  obtained  from  the  decomposition  of  water,  would  be  proportion- 
ally diminished,  and  such  oxydation  would  accordingly  be  promoted.  In  this 
way  Fabroni  accounted  for  the  tendency  of  certain  alloys  of  metal  to  oxydation, 
and  for  the  well-known  fact  that  iron  nails,  then  used  in  attaching  the  copper 
sheathing  to  vessels,  were  rendered  so  liable  to  rust  by  their  contact  with  the 
copper,  that  they  became  soon  too  small  for  the  holes  in  which  they  were  in- 
serted. He  supposed,  therefore,  that  in  the  experiments  of  Galvani  and  Volta, 
in  which  the  convulsions  of  the  limbs  of  animals  were  produced,  a  chemical 

*  The  date  of  the  researches  of  thin  philosopher  is  generally,  but  erroneously,  assigned  to  the 
year  1799. 


change  was  made  by  the  contact  of  one  of  these  metals  with  the  liquid  matter 
always  found  on  the  parts  of  the  animal  body ;  and  that  the  immediate  cause 
of  the  convulsions  was  not,  as  supposed  by  Galvani,  due  to  animal  electricity, 
nor,  as  assumed  by  Volta,  to  a  current  of  electricity  emanating  from  the  sur- 
face of  contact  of  the  two  metals,  but  to  the  decomposition  of  the  fluid  upon 
the  animal  substance,  and  the  transition  of  oxygen  from  a  state  of  combination 
with  it  to  combination  with  the  metal.  The  electricity  produced  in  the  experi- 
ments Fabroni  ascribed  entirely  to  the  chemical  changes,  it  being  then  known 
that  chemical  processes  were  generally  attended  with  sensible  signs  of  elec- 
tricity. He  maintained  that  the  convulsions  were  chiefly  due  to  the  chemical 
changes,  and  not  to  the  electricity  incidental  to  them,  which,  if  it  operated  at 
all,  he  considered  to  do  so  in  a  secondary  way. 

The  necessary  limits  of  this  notice  will  not  allow  of  a  further  analysis  of  the 
researches  of  this  philosopher  ;  but  if  his  original  papers  be  referred  to,  it  will 
be  seen  that  he  is  entitled  to  the  credit  of  having  first  distinctly  demonstrated 
the  chemical  origin  of  Voltaic  electricity. 

In  the  year  1800,  the  attention  of  the  scientific  world  was  withdrawn  from 
the  controversy  respecting  the  origin  of  Galvanic  electricity,  and  all  trther 
matters  of  minor  importance,  and  engrossed  by  one  of  those  vast  discoveries 
which  constitute  an  epoch  in  the  progress  of  knowledge,  and  give  a  new  di- 
rection to  the  sciences.  On  the  20th  of  March,  1800,  Volta  addressed  a  letter 
to  Sir  Joseph  Banks,  then  president  of  the  Royal  Society,  in  which  he  an- 
nounced to  him  the  discovery  of  the  VOLTAIC  PILE,  one  of  the  most  powerful 
instruments  for  the  investigation  of  the  laws  of  nature,  as  exhibited  in  the  mu- 
tual relations  of  the  constituent  parts  of  matter,  which  ever  did  honor  to  the 
science  of  any  age,  or  any  nation. 

In  order  to  complete  the  experimental  analysis  of  the  effects  of  Galvanic 
electricity,  Volta  felt  the  necessity  of  collecting  it  in  much  greater  quantities 
than  could  be  obtained  in  the  processes  which  had  then  been  adopted.  Ac- 
cording to  his  theory,  when  two  plates  of  metal,  zinc  and  copper  for  example, 
were  brought  into  contact,  two  currents  of  electric  fluid  originated  at  their 
common  surface,  and  moved  from  that  point  in  opposite  directions.  The  posi- 
tive fluid  passed  along  the  zinc,  and  the  negative  along  the  copper.  If  the 
extremities  of  the  two  metals  most  remote  from  their  mutual  contact  were  con- 
nected by  an  arc  of  conducting  matter,  these  contrary  currents  would  flow 
along  this  arc,  the  positive  fluid  moving  from  the  zinc  toward  the  copper,  and 
the  negative  from  the  copper  toward  the  zinc  ;  but  the  intensity  of  these  cur- 
rents was  supposed  to  be  so  feeble  that  no  ordinary  electroscope,  whatever 
might  be  its  sensibility,  would  be  affected  by  it.  In  order  to  bring  into  opera- 
tion in  this  question  those  instruments  which  had  been  applied  to  common 
electricity,  he  therefore  sought  some  expedient  by  which  he  could  combine, 
and,  as  it  were,  superpose  two  or  more  currents,  and  thus  multiply  the  intensity, 
until  it  should  attain  such  an  augmentation  as  to  produce  effects  analogous  to 
those  which  had  been  obtained  by  ordinary  electricity. 

With  this  object,  he  conceived  the  idea  of  placing  alternately,  one  over  the 
other,  disks  of  different  metals,  such  as  zinc  and  copper.  Let  us  suppose  the 
lowest  disk  to  be  copper,  having  a  disk  of  zinc  upon  it.  On  this  disk  of  zinc 
let  a  second  copper  disk  be  placed,  and  over  that  a  second  disk  of  zinc,  and  so 
on.  According  to  Volta's  theory,  currents  of  electricity  would  be  established 
at  each  surface  of  contact  of  the  two  metals,  the  positive  current  running  along 
the  zinc,  and  the  negative  along  the  copper.  With  the  arrangement  above 
described,  there  would  proceed  from  the  first  surface  a  negative  downward,  and 
a  positive  upward  current ;  from  the  second  a  positive  downward,  and  a  nega- 
tive upward  current ;  from  the  third  a  negative  downward,  and  a  positive  up- 


GALVANISM. 


wur.l  current,  and  so  on  :   the  downward  current  being  negative,  and  the  up- 
ward  positive  from   the   upper   surface   of  each  copper  disk,  and  the  upp<>r 
current  being  negative  and  the  downward  positive  from  the  lower  surface  of 
such  disk.     It  is  evident,  therefore,  that  the  downward  currents  would  he  al- 
ternately  positive  and  negative;   and  the  same  would  be  the  case  with,  the 
upward  currents.     Now,  since  the  surfaces  of  contact  of  the  metals  would  be 
equal,  these  currents  would  have  equal  intensities,  and  accordingly  each  posi- 
.   live  current  would  neutralize  each  negative  current  having  the  same  direction. 
(  The  result  would  be,  that  if  the  lowest  and  highest  disk  of  the  pile  were  of  the 
I  same  metal,  all  the  currents  neutralizing  each  other,  the  pile  would  evolve  no 
electricity  whatever;   and  if  they  were  of  different  metals,  all  the  downward 
currents,  except  one,  would  neutralize  each  other,  and  that  one  would  be  posi- 
live.     The  effect  of  the  pile  would  therefore  be  the  same  as  if  it  consisted  of   ; 
only  two  disks,  one  of  copper,  and  the  other  of  zinc. 

Volta  therefore  saw  the  necessity  of  adopting  some  expedient  by  which  all  ' 
the  currents  in  the  same  direction  should  be  of  the  same  kind ;  so  that,  for  ex-  2 
ample,  all  the  descending  currents  should  be  negative,  and  all  the  ascending  , 
currents  positive.  If  this  could  be  accomplished,  the  current  issuing  from  the  ; 
bottom  of  the  pile  would  be  a  negative  current  as  many  times  more  intense  £ 
than  one  proceeding  from  a  single  pair  of  disks  as  there  were  surfaces  of  con-  { 
tact  supplying  currents,  and  the  same  would  be  true  of  the  positive  current  '; 
issuing  from  the  top  of  the  pile.  ( 

To  effect  this,  it  was  necessary  to  destroy  the  Galvanic  action  at  all  those  sur-  | 
faces  from  which  descending  positive  and  ascending  negative  currents  would  pro-  < 
ceed  ;  that  is,  the  lower  surfaces  of  the  copper  disks  and  the  upper  surfaces  of  the  J 
zinc  disks.     But  while  this  was  effected,  it  was  also  essential  thai  the  progress  t 
of  the  descending  negative  an.d  ascending  positive  currents  should  still  be  un- 
interrupted.    The  interposition  of  any  substance  which  would  have  no  sensible 
Galvanic  action  on  either  of  the  metals  between  each  disk  of  copper  and  the 
disk  of  zinc   immediately  below  it  would  attain  one  of  these  ends,  since  the 
action  of  all  the  surfaces  in  which  ascending  negative  or  descending  positive 
currents  could  originate  would  thus  be  prevented.     But  in  order  to  allow  the   • 
free  progress  of  the  remaining  currents  in  each  direction,  such  substance  must 
be  a  suiliciently  free  conductor  of  electricity.     Volta  selected,  as  the  fittest 
means  of  fulfilling  these  conditions,  disks  of  wet  cloth.     They  would  be  free 
from  any  sensible  Galvanic  action  on  the  metal,  and  their  moisture  would  give   j 
them  sufficient  conducting  power. 

Having  discovered  the  principles  by  which  this  species  of  electricity  can  be  \ 
accumulated  in  quantity  and  strong  currents  obtained,  he  varied  its  form,  and  J 
contrived  the  apparatus  which  is  known  by  the  name  of  La  Couronne  de  Tosses. 
This  arrangement,  which  Volta  himself  most  commonly  used  in  his  experi- 
ments, consisted  of  a  circle  of  cups  filled  with  warm  water,  or  a  solution  of 
sea-salt.  He  immersed  in  each  cup  a  plate  of  zinc  and  one  of  silver,  not  in 
contact,  and  then  established  a  metallic  communication  by  means  of  wire  be- 
tween the  zinc  of  one  cup  and  the  silver  of  the  adjacent  one.  The  positive 
fluid  was  found  to  proceed  from  the  extreme  zinc  plate,  and  the  negative  from 
the  extreme  silver  one,  and  a  continuous  current  was  obtained  by  connecting 
these  by  any  conductors  of  electricity. 

Profoundly  impressed  with  the  importance  of  the  results  likely  to  arise  from 
.  the  application  of  the  powers  of  the  pile  in  physical  inquiries,  and  doubtless 
\  animated  by  the  desire  for  which  he  was  honorably  distinguished  to  extend  all 
(  possible  encouragement  and  advantage  to  those  engaged  in  the  natural  sciences, 
(  Napoleon,  then  first  consul,  and  surrounded  by  the  splendor  of  his  southern 
>  triumphs,  invited  Volta  to  visit  Paris  ;  and  there,  at  the  Institute,  before  the 


elite  of  European  philosophers,  to  explain  personally  his  great  invention,  av.d 
expound  his  views  as  to  its  probable  uses  and  powers  as  an  instrument  of  sci- 
entific research.  Volta  accepted  the  proffered  honor,  and,  in  1801,  attended  at 
three  meetings  of  the  Academy  of  Sciences,  at  which  he  explained  his  theory 
of  contact,  and  developed  his  views  respecting  the  Voltaic,  or,  as  he  called  it> 
electro-motive,  action  of  different  metals  upon  each  other.  Among  the  audience 
at  these  memorable  meetings  was  NAPOLEON  himself,  and  none  present  ap- 
peared to  appreciate  more  justly  the  vastness  of  the  power  which  was  on  that 
occasion  placed  in  the  hands  of  the  experimental  philosopher. 

When  the  report  of  the  committee  on  the  subject  was  read,  the  FIRST  CONSUL 
proposed  that  the  rules  of  the  Academy,  which  produced  some  delay  in  con- 
ferring its  honors,  be  suspended,  and  that  the  gold  medal  be  immediately 
awarded  to  Volta,  as  a  testimony  of  the  gratitude  of  the  philosophers  of  France 
for  his  discovery.  This  proposition  being  carried  by  acclamation,  the  hero  of 
a  hundred  fields,  who  never  did  things  by  halves,  and  who  was  filled  with  a 
prophetic  enthusiasm  as  to  the  powers  of  the  pile,  ordered  two  thousand  crowns 
to  be  sent  to  Volta  the  same  day  from  the  public  treasury,  to  defray  the  ex- 
penses of  his  journey.*  He  also  founded  an  annual  medal,  of  the  value  of 
three  thousand  francs,  for  the  best  experiment  on  the  electric  fluid,  and  a  prize 
of  sixty  thousand  francs  to  him  who  should  give  electricity  or  magnetism,  by 
his  researches,  an  impulse  comparable  to  that  which  it  received  from  the  dis- 
coveries of  Franklin  and  Volta. 

The  relation  in  which  the  Voltaic  pile  stood  in  reference  to  the  Leyden  jar 
and  electrical  machines  now  began  to  be  perceived.  In  the  latter  apparatus  a 
great  quantity  of  electricity  is  accumulated  on  the  surfaces  of  the  jar,  and  held 
there  in  equilibrium,  the  positive  fluid  on  one  side  of  the  glass,  and  the  nega- 
tive on  the  other.  When  the  communication  is  made  between  the  two  surfaces, 
a  torrent  of  the  fluid  precipitates  itself  instantaneously  along  the  line  of  com- 
munication, and  the  electrical  equilibrium  is  re-established  in  an  interval  of  time 
so  short  as  to  be  inappreciable.  A  sudden,  instantaneous,  and  violent  effect  is 
produced  on  whatever  bodies  may  be  exposed  to  the  transit  of  this  electric  fluid. 
On  the  other  hand,  the  Voltaic  pile  is  a  generator  of  electricity,  which  supplies 
to  its  opposite  poles  the  two  fluids,  the  positive  and  the  negative  electricity,  in  a 
continued,  gentle,  and  regulated  current.  It  discharges  it  not  suddenly  or  in- 
stantaneously, or  with  uncontrollable  and  irresistible  violence,  but  with  gentle, 
moderate,  continued,  and  regulated  action.  What  takes  place  in  the  Leyden 
jar  in  an  interval  so  brief  as  to  render  observation  of  its  progress,  or  examina- 
tion of  its  successive  effects,  impossible,  is  with  the  pile  spread  over  as  long 
an  interval  as  the  observer  may  desire.  Besides  this,  the  effects  themselves 
consequent  on  the  two  modes  of  action  are  different.  That  which  in  mechan- 
ical phenomena  is  effected  by  a  violent  blow  or  concussion,  is  not  more  differ- 
ent from  the  effects  of  a  long-continued  action  of  a  uniform  accelerating  force 
or  a  constant  pressure,  than  are  the  effects  of  the  common  electrical  discharge 
from  those  of  the  currents  of  electricity  propagated  between  the  poles  of  the 
pile. 

The  physiological  effects  of  electricity  exhibited  under  these  different  forms, 
differ  in  a  manner  which  might  be  anticipated  from  these  modifications  in  the 
transmission  of  the  electric  fluid.  If  the  wires  proceeding  from  the  opposite 
poles,  and  conducting  the  contrary  currents  of  fluid,  be  taken  in  the  hands,  the 
sudden  and  violent  shock  of  the  Leyden  jar  is  no  longer  felt.  It  is  replaced  by 
a  continued  convulsion  in  the  arms  and  shoulders,  which  does  not  cease  so  long 
as  the  wires  are  held. 

•  Arago,  Eloge  de  Volta,  p.  42. 


If  a  metallic  plate,  in  connexion  with  the  positive  pole,  be  applied  to  the 
i  tongue,  and  another  connected  with  the  negative  pole  to  any  other  part,  a  strong 
'  acidulous  savor  is  perceived.  If  the  plate  applied  to  the  tongue  be  connected 
}  with  the  negative  pole,  a  strong  alkaline  savor  is  felt. 

It  is  not  the  organs  of  taste  only  which  are  sensible  to  the  influence  of  this 
j  instrument.     The  sense  of  sight  is  susceptible  of  its  operation  in  a  manner  even 
|  more  wonderful.     Let  a  metallic  surface  connected  with  one  of  the  poles  be 
\  applied  to  the  forehead,  the  cheek,  the  nose,  the  chin,  or  the  throat;  and,  at 
j  the  same  time,  let  the  patient  take  in  his  hand  the  wire  connected  with  the  I 
'  other  pole.     Immediately  a  light  will  be  perceived,  even  though  the  eyes  be 
closed,  the  splendor  and  appearance  of  which  will  vary  with  the  part  of  the 
face  in  contact  with  the  metallic  plate.     By  similar  means,  the  perception  of 
sound  will  be  perceived  in  the  ears. 

The  action  of  the  pile  on  the  animal  body  after  the  vital  principle  is  de- 
stroyed is  so  well  known,  that  it  is  scarcely  necessary  to  mention  it  here. 
The  trunk  of  a  decapitated  body  will  rise  from  its  recumbent  posture  ;  the  arms 
will  move  and  strike  objects  near  them ;  the  legs  will  elevate  themselves  with 
a  force  sufficient  to  raise  considerable  weights ;  the  breast  will  heave  as  if 
respiration  were  restored  ;  and.  in  fine,  all  the  vital  actions  will  be  manifested 
with  terrific  and  revolting  precision. 

In  the  hands  of  the  entomologist,  the  pile  affords  results  not  less  interesting. 
The  glow-worm,  submitted  to  the  electric  current,  shines  with  increased  splen- 
dor ;  the  grasshopper  chirps,  as  if  under  the  action  of  a  stimulant.* 

The  physiological  action  of  the  pile  was  strongly  suggestive  of  a  mysterious 
connexion  between  the  electric  fluid  and  the  proximate  principle  of  vitality. 
When  some  of  these  effects  were  exhibited  to  Napoleon,  the  emperor  turned 
to  Corvisart,  his  physician,  and  said,  "  Docteur,  voila  1'image  de  la  vie  :  la 
colonne  vertebrale  est  la  pile  ;  le  foie,  le  pole  negatif ;  la  vessie,  le  pole  posi- 
tif."  f 

The  invention  of  the  pile  had  been  scarcely  more  than  hinted  at,  when  that 
course  of  electro-chemical  investigations  began  which  soon  led  to  the  magnifi- 
cent discoveries  of  Davy,  and  the  series  of  experimental  researches  which 
have  been  continued  to  the  present  time  with  results  so  remarkable  by  those 
who  succeeded  him.     The  first  four  pages  only  of  the  letter  of  Volta  to  Sir 
Joseph  Banks  were   despatched  on  the  20th  of  March,  1800  ;  and  as  these 
were  not  produced  in  public  till  the  receipt  of  the  remainder,  the  letter  was 
not  read  at  the  Royal  Society,  or  published,  until  the  26th  of  June  following. 
The  first  portion  of  the  letter,  in  which  was  described  generally  the  formation 
of  the  pile,  was  shown  in  the  latter  end  of  April  by  Sir  Joseph  Banks  to  some 
scientific  men,  and  among  others  to  Sir  Anthony  (then  Mr.)  Carlisle,  who  was 
\  engaged  at  the  time  in  certain  physiological  inquiries.     Mr.  W.  Nicholson,  the 
<  conductor  of  the  scientific  journal  known  as  Nicholson's  Journal,  and  Carlisle, 
i  constructed  a  pile  of  seventeen  silver  half-crown  pieces  alternated  with  equal 
j  disks  of  copper  and  cloth  soaked  in  a  weak  solution  of  common  salt,  with  which 
I  on  the  30th  of  April  they  commenced  their  experiments.     It  happened  that  a 

*  Eloge,  p.  33. 

)       t  This  anecdote  was  told  by  Chaptel,  who  was  present  on  the  occasion,  to  Bequerel  ;  and  1 
(   ter  relates  it  in  the  lirst  volume  of  hia  work  on  electricity,  published  in  1834.     The  idea 
/   tricity  is  the  immediate  principle  of  vitality  has  occurred  to  other  minds.     Sir  John  Herschel,  in  Ins   ( 
(   Preliminary  Discourse  published  iu  the  Cabinet  Cyclopedia  in  1830,  without  any  knowled 
(   above  anecdote,  says  (p.  343),  ••  If  the  brain  be  an  electric  pile  constantly  in  action,  it  may  b< 
(   ceived  to  discharge  itself  at  regular  intervals,  when  the  tension  of  the  electricity  developed  n 
(    a  certain  point,  along  the  nerves  which  communicate  with  the  heart,  and  thus  to  excite  the  pupation   ( 
(   of  that  organ.     This  idea  is  forcibly  suggested  by  the  view  of  that  elegant  apparatus,  the  dry  pi 
\   DC  Lur,  in  which  the  successive  accumulations  of  electricity  are  earned  ott  by  a  suspended 
which  is  kept  by  the  discharge  in  a  state  of  regular  pulsation  for  any  length  of  time.       A  su 
;   idea  occurred  to  Dr.  Artiott,  and  is  mentioned  in  hia  Physics. 


/ 


340 


GALVANISM. 


drop  of  water  was  used  to  make  good  the  contact  of  the  conducting  wire  with 
a  plate  to  which  the  electricity  was  to  he  transmitted  ;  Carlisle  observed  a  dis- 
engagement of  gas  in  this  water,  and  Nicholson  recognised  the  odor  ot'  hydro- 
gen proceeding  from  it.  In  order  to  observe  this  effect  with  more  advantage, 
a  small  glass  tube,  open  at  both  ends,  was  stopped  at  one  end  by  a  cork,  and 
being  then  filled  with  water  was  similarly  slopped  at  the  other  end.  Through 
both  corks  pieces  of  brass  wire  were  inserted,  the  points  of  which  were  ad- 
justed at  a  distance  of  an  inch  and  three  quarters  asunder  in  the  water.  When 
these  wires  were  put  in  communication  with  the  opposite  ends  of  the  pile, 
bubbles  of  gas  were  evolved  from  the  point  of  the  negative  wire,  and  the  end 
of  the  positive  wire  became  tarnished.  The  gas  evolved  appeared  on  examina- 
tion to  be  hydrogen,  and  the  tarnish  was  found  to  proceed  from  the  oxydalion 
of  the  positive  wire.  It  was  inferred  that  the  process  in  which  these  effects 
were  produced  was  the  decomposition  of  water.  This  took  place  on  the  2d 
of  May,  shortly  after  the  receipt  of  the  first  portion  of  Volta's  letter. 

To  ascertain  whether  the  oxydation  of  the  positive  wire  was  an  effect  inci- 
dental to  the  experiment,  or  had  an  influence  in  producing  the  decomposition, 
Nicholson  determined  to  try  the  effect  of  wires  formed  of  metal  more  difficult 
of  oxydation.  Wires  of  platinum  were  accordingly  inserted  through  the  corks, 
and  the  experiment  repeated.  Bubbles  of  gas  were  now  evolved  from  both 
wires.  Two  platinum  wires  were  next  inserted  at  the  closed  ends  of  two 
separate  tubes,  which,  being  open  at  the  other  ends  and  filled  with  water,  -vere 
inserted  in  the  same  vessel  of  water.  Being  placed  side  by  side  close  together, 
and  the  wires  being  continued  to  the  lower  ends  of  the  tubes,  so  that  the  dis- 
tance between  their  points  was  not  more  than  two  inches,  their  upper  extremi- 
ties were  put  in  connexion  with  the  ends  of  the  pile.  Gas  was  evolved  from 
the  points  of  both  wires,  and,  ascending  through  the  water,  was  collected  sep- 
arately in  the  two  tubes.  These  gases  being  examined,  proved  to  be  hydrogen 
from  the  negative,  and  oxygen  from  the  positive  wire,  nearly  in  the  proportion 
known  to  constitute  water.* 

Thus  was  the  decomposing  power  of  the  pile  established  within  a  few  weeks 
after  the  first  intimation  of  the  invention  of  that  instrument  had  been  received 
in  England,  and  before  any  description  of  it  had  been  published.  It  seemed 
proper  to  give  these  details  here,  not  only  on  account  of  the  great  importance 
of  the  discovery,  but  because  it  has  been  sought  to  depreciate  the  merit  of  it 
by  ascribing  it  altogether  to  chance.  It  is  probably  impossible  to  exclude 
chance  altogether  from  such  investigations,  but  in  this  there  Avas  as  little  as  is 
generally  found. 

When  these  experiments  became  known,  Mr.  W.  Cruickshank,  of  Woohvich, 
repeated  them,  and  obtained  similar  results  ;  but  observed  that  when  the  dis- 
tilled water  was  tinged  with  litmus,  the  effects  of  an  acid  Avere  produced  at  the 
positive,  and  those  of  an  alkali  at  the  negative  wire.  Led  by  this  indication, 
he  tried  the  effects  of  the  wires  on  solutions  of  acetate  of  lead,  sulphate  of 
copper,  and  nitrate  of  silver.  In  each  case  he  found  the  metallic  base  depos- 
ited at  the  negative  pole,  and  the  acid  manifested  at  the  positive  pole.  Muri- 
ate of  ammonia  and  nitrate  of  magnesia  Avere  next  decomposed,  the  acid  as  be- 
fore going  to  the  positive,  and  the  alkali  to  the  negative  pole.  These  experi- 
ments of  Mr.  Cruickshank  Avere  made  as  early  as  June,  ISOO.f 

In  the  September  following,  Mr.  Cruickshank  published  the  continuation  of 
his  researches,^  in  Avhich  he  corroborated  the  results  of  his  former  experiments, 
showing  more  generally  the  tendency  of  oxygen  and  the  acids  in  Voltaic  de- 
composition to  collect  round  the  positive  wire,  and  hydrogen,  metals,  alkalies. 
&c.,  round  the  negative  pole. 

*  Nicholson's  Journal,  vol.  iv.,  p.  179.     1800.  t  Ibid.,  p.  137.  t  Ibid.,  p.  251. 


GALVANISM. 

• __ '> 

The  investigations  of  which  the  pile  became  the  instrument  now  began  to 
')  assume  an  importance  which  rendered  it  necessary  to  give  it  considerably  aug- 

>  mented  power,  either  by  increasing  its  height  or  enlarging  its  component  plates. 
;  In  either  case,  inconveniences  were  encountered  which  imposed  a  practical 
?  limit  on  the  increase  of  its  power.     When  the  number  or  magnitude  of  the 
)  metallic  disks  was  considerable,  the  incumbent  pressure  discharged  the  liquid 
/  from  the  intermediate  disks  of  cloth  or  card.     The  trouble  of  refilling  it  when- 
ever its  use  was  required,  and  of  wetting  the  cloth  or  card,  was  very  great. 
Mr.  Cruickshank,  adopting  the  principle  of  Volta's  couronne  des  lasses,  pro- 
posed, as  a  more  convenient  form  for  the  apparatus,  an  arrangement  consisting 
of  a  trough  of  baked  wood,  which  is  a  non-conductor  of  electricity,  divided  by 
parallel  partitions  into  a  series  of  cells.     Into  these  cells  the  liquid  to  be  in- 
terposed between  the  successive  pairs  of  metallic  plates  was  poured.     A  se- 
ries of  rectangular  plates  of  metal,  alternately  zinc  and  copper,  were  arranged 
so  as  to  be  parallel  to  each  other,  and  at  such  a  distance  as  to  allow  the  pnrti- 

>  tions  of  the  trough  to  pass  between  each  pair  of  plates.     This  modification 
'  rendered  the  Voltaic  apparatus  capable  of  having  its  power  increased  without 

practical  limit. 

While  these  investigations  were  proceeding,  Ritter,  afterward  so  distin- 
guished for  his  experimental  researches,  but  then  young  and  unknown,  made 
various  experiments  at  Jena  on  the  effects  of  the  pile  ;  and,  apparently  with- 
out knowing  what  had  been  done  in  England,  discovered  this  property  of  de- 
composing water  and  saline  compounds,  and  of  collecting  oxygen  and  the  acids 
at  the  positive,  and  hydrogen  and  the  bases  at  the  negative  pole.  He  also 
showed  that  the  decomposing  power  in  the  case  of  water  could  be  transmitted 
through  sulphuric  acid,  the  oxygen  being  evolved  from  a  portion  of  water  on 
one  side  of  the  acid,  while  the  hydrogen  was  produced  from  another  separate 
portion  of  water  on  the  other  side  of  it.* 

When  the  chemical  powers  of  the  pile  became  known  in  England,  Sir 
Humphry  (then  Mr.)  Davy  was  commencing  those  labors  in  chemical  science 
which  subsequently  surrounded  his  name  with  so  much  lustre,  and  left  traces 
of  his  genius  in  the  history  of  scientific  discovery  which  must  remain  as  long 
as  the  knowledge  of  the  laws  of  nature  is  valued  by  mankind.  The  circum- 
stance attending  the  decompositions  effected  between  the  poles  of  the  pile 
which  caused  the  greatest  surprise,  was  the  production  of  one  element  of  the 
compound  at  one  pole,  and  the  other  element  at  the  other  pole,  without  any 
discoverable  transfer  of  either  of  the  disengaged  elements  between  the  wires. 
If  the  decomposition  was  conceived  to  take  place  at  the  positive  wire,  the  con- 
stituent appearing  at  the  negative  wire  must  be  presumed  to  travel  through  the 
fluid  in  the  separated  state  from  the  positive  to  the  negative  point ;  and  if  it 
was  conceived  to  take  place  at  the  negative  wire,  a  similar  transfer  must  be 
imagined  in  the  opposite  direction.  Thus,  if  water  be  decomposed,  and  the 
decomposition  be  conceived  to  proceed  at  the  positive  wire  where  the  oxygen 
is  visibly  evolved,  the  hydrogen  from  which  that  oxygen  is  separated  must  be 
supposed  to  travel  through  the  water  to  the  negative  wire,  and  only  to  become 
visible  when  it  meets  the  point  of  that  wire  ;  and  if,  on  the  other  hand,  the  de- 
composition be  imagined  to  take  place  at  the  negative  wire  where  the  hydro- 
gen is  visibly  evolved,  the  oxygen  must  be  supposed  to  pass  invisibly  through 
the  water  to  the  point  of  the  positive  wire,  and  there  become  visible.  But 
what  appeared  still  more  unaccountable  was,  that  in  the  experiment  of  Ritter 
;  ;;  would  seem  that  one  or  other  of  the  elements  of  the  water  must  have  passed 
through  the  intervening  sulphuric  acid.  So  impossible  did  such  an  invisible 

*  Nicholson's  Journal,  vol.  iv.,  p-  511. 


342  GALVANISM. 


!  transfer  appear  to  Ritter,  that  at  that  time  he  regarded  his  experiment  as  pro- 
j  ving  that  one  portion  of  the  water  acted  on  was  wholly  converted  into  oxygen, 
>  and  the  other  portion  into  hydrogen.* 

This  point  was  the  first  to  attract  the  attention  of  Davy,  and  it  occurred  to 
him  to  try  if  decomposition  could  be  produced  in  quantities  of  water  contained 
in  separate  vessels  united  by  a  conducting  substance,  placing  the  positive  wire 
in  one  vessel  and  the  negative  in  the  other.  For  this  purpose,  the  positive  and 
negative  wires  were  immersed  in  two  separate  glasses  of  pure  water.  So  long 
as  the  glasses  remained  unconnected,  no  effect  was  produced  ;  but  when  Davy 
put  a  finger  of  the  right  hand  in  one  glass  and  of  the  left  hand  in  the  other, 
decomposition  was  immediately  manifested.  The  same  experiment  was  after- 
ward repeated,  making  the  communication  between  the  two  glasses  by  a  chain 
of  three  persons.  If  any  material  principle  passed  between  the  wires  in  these 
cases,  it  must  have  been  transmitted  through  the  bodies  of  the  persons  forming 
the  line  of  communication  between  the  glasses. 

The  use  of  the  living  animal  body  as  a  line  of  communication  being  incon- 
venient where  experiments  of  long  continuance  were  desired,  Davy  substituted 
fresh  muscular  animal  fibre,  the  conducting  power  of  which,  though  inferior  to 
that  of  the  living  animal,  was  sufficient.  When  the  two  glasses  were  con- 
nected by  this  substance,  decomposition  accordingly  went  on  as  before,  but 
more  slowly. 

To  ascertain  whether  metallic  communication  between  the  liquid  decompo- 
sed and  the  pile  was  essential,  -he  now  placed  lines  of  muscular  fibre  between 
the  ends  of  the  pile  and  the  glasses  of  water  respectively,  and  at  the  same 
time  connected  the  two  glasses  with  each  other  by  means  of  a  metallic  wire. 
He  was  surprised  to  find  oxygen  evolved  in  the  negative,  and  hydrogen  in  the 
positive  glass,  contrary  to  what  had  occurred  when  the  pile  was  connected 
with  the  glasses  by  wires.  In  none  of  these  cases  did  he  observe  the  disen- 
gagement of  gas  either  from  the  muscular  fibre  or  from  the  living  hand  immer- 
sed in  the  water. 

In  October,  1800,  after  many  experiments  on  the  chemical  effects  of  the 
pile,  Davy  commenced  an  investigation  of  the  relation  which  its  power  had  to 
the  chemical  action  of  the  liquid  conductor  on  the  more  oxydable  (*"  its  metal- 
lic elements.  The  influence  of  chemical  decomposition  in  evolving  the  Voltaic 
electricity  originally  maintained  by  Fabroni,  was  again  brought  under  inquiry 
by  Colonel  Haldane.  Davy  showed  that  at  common  temperatures  zinc,  con- 
nected with  silver,  suffers  no  oxydation  in  water  which  is  well  purged  of  air 
and  free  from  acids  ;  and  that  with  such  water  as  a  liquid  conductor,  the  pile 
is  incapable  of  evolving  any  quantity  of  electricity  which  can  be  rendered  sen- 
sible either  by  the  shock  or  by  the  decomposition  of  water ;  but  that  if  the 
water  used  as  a  liquid  conductor  hold  in  combination  oxygen  or  acid,  then  oxy- 
dation of  the  zinc  takes  place,  and  electricity  is  sensibly  evolved.  In  fine,  he 
concluded  that  the  power  of  the  pile  appeared  to  be,  in  great  measure,  propor- 
tional to  the  power  of  the  liquid  between  the  plates  to  oxydate  the  zinc.f 

He  inferred  from  these  results  that  although  the  exact  i»ode  of  operation 
could  not  be  accounted  for,  the  oxydation  of  the  zinc  in  the  pile,  and  the  chem- 
ical changes  connected  with  it,  were  somehow  the  cause  of  its  electrical  effects. 

To  ascertain  whether  a  liquid  solution  capable  of  conducting  the  electric  cur 
rent  between  the  positive  and  negative  wires  of  a  Voltaic  pile,  but  not  capable 
of  producing  any  chemical  action  on  its  metallic  elements,  would,  when  used 
between  its  plates,  evolve  electricity,  Davy  constructed  a  pile  in  which  the  li- 
quid was  a  solution  of  sulphuret  of  strontia.  When  the  current  from  an  active 
pile  was  transmitted  through  the  liquid,  the  shock  was  as  sensible  as  if  the 

*  Nicholson's  Journal,  vol.  iv.,  p.  512.  t  Nicholson's  Journal,  vol.  iv.,  p.  337. 


343 


communication  had  been  made  through  water ;  but,  on  the  other  hand,  solu- 
tions of  the  sulphurets  were  incapable  of  acting  chemically  on  the  zinc.  If, 
therefore,  chemical  action  on  the  zinc  be  a  necessary  condition  to  ensure  trie 
activity  of  the  pile,  such  an  arrangement  must  be  inactive.  Twenty-five  pairs 
of  silver  and  zinc  plates,  erected  with  cloths  moistened  in  solution  of  sulphuret 
of  strontia,  produced  no  sensible  action,  though  the  moment  the  sides  of  the 
pile  were  moistened  with  nitrous  acid,  the  ends  gave  shocks  as  powerful  as 
those  of  a  similar  pile  constructed  in  the  usual  manner. 

The  next  question  brought  to  the  test  of  experiment  was,  whether  the  chem- 
ical action  which  takes  place  between  the  liquid  and  the  plates  of  the  pile  is 
of  the  same  kind  as  that  which  is  manifested  when  water  is  decomposed  by 
its  extreme  wires  ;  that  is,  whether,  when  the  oxygen  is  freed  upon  the  surface  of 
the  zinc,  the  remaining  constituent  of  the  solution  decomposed  is  also  liberated  at 
the  surface  of  the  zinc,  as  in  ordinary  oxydation ;  or  is  transmitted  invisibly  through 
the  fluid  to  the  surface  of  the  silver,  and  there  deposited,  or  otherwise  liberated, 
as  in  the  decomposition  between  the  positive  and  negative  wires.  An  arrange- 
ment of  zinc  and  copper  plates,  in  the  form  of  the  couronne  des  tosses,  was 
formed,  and  charged  with  spring  water.  The  general  result  of  these  experi- 
ments showed  that  the  hydrogen  liberated  by  the  zinc  was  manifested  not  at  the 
zinc,  but  at  the  silver  surface ;  and,  therefore,  that  the  action  in  the  cells  is 
similar  to  the  decomposition  of  water  at  the  extreme  wires  of  the  pile.  The 
phenomena  were,  however,  rendered  less  decisive  of  the  question  by  the  mod- 
ifications produced  by  the  azote  of  the  common  air  combined  with  the  water, 
and  also  by  saline  matter  which  it  held  in  solution,  effects  which  were  then 
imperfectly  understood. 

The  inventor  of  the  pile  maintained  that,  among  the  metals,  those  which 
held  the  extreme  places  in  the  scale  of  electro-motive  power  were  silver  and 
zinc  ;  and  that,  consequently,  these  metals,  paired  in  a  pile,  would  be  more 
powerful,  coEteris  paribus,  than  any  other.  But  as  he  also  showed  that  pure 
charcoal  was  a  good  conductor  of  the  electric  current,  and  that  the  electro- 
motive virtue  depended  on  the  different  conducting  powers  of  the  metallic  ele- 
ments, it  was  consistent  with  analogy  that  charcoal,  combined  with  another 
substance  of  different  conducting  power,  would  produce  Voltaic  action.  Dr. 
Wells  accordingly  showed  that  a  combination  of  charcoal  and  zinc  produced 
sensible  convulsions  in  the  frog  ;  and  Davy,  adopting  this  principle,  constructed 
a  couronne  des  tasses,  consisting  of  a  series  of  eight  glasses,  with  small  pieces 
of  well-burned  charcoal  connected  with  zinc  by  pieces  of  silver  wire,  using  a 
solution  of  red  sulphate  of  iron  as  the  liquid  conductor.  This  series  gave 
sensible  shocks,  and  rapidly  decomposed  water.  Compared  with  an  equal  and 
similar  series  of  silver  and  zinc,  its  effects  were  much  stronger.  Hence  he 
inferred  that  charcoal  and  zinc  formed  a  combination  equal,  if  not  superior,  to 
any  of  the  metals. 

Volta  was  understood  to  refer  the  electro-motive  power  of  the  metallic  ele- 
ments of  the  pile  to  the  difference  of  their  powers  as  conductors  of  electricity. 
The  experiments  of  Davy  induced  him  to  connect  the  electro-motive  power 
with  the  amount  of  chemical  action  on  the  more  oxydable  metal.  These  two 
prii.dples  might,  nevertheless,  be  compatible,  if  it  could  be  shown  that  the 
oxydaiion  was  dependant  on,  and  proportional  to,  the  difference  of  conducting 
power  of  the  metals.  To  test  this,  it  was  only  necessary  to  construct  a  pile 
with  metals  of  nearly  equal  conducting  power.  With  this  view,  Davy  con- 
structed a  pile  with  gold  and  silver  plates,  these  metals  being  supposed  to  dif- 
fer very  little  in  their  power  of  conducting  electricity,  interposing  disks  of  cloth 
moistened  with  dilute  nitric  acid.  Voltaic  action  was  produced.  A  similar 
pile,  formed  of  plates  of  silver  and  copper,  and  a  solution  of  nitrate  of  mercury, 


344 


GALVANISM. 


acted  powerfully.  The  conducting  powers  of  these  several  metals  were  then 
considered  as  nearly  equa-L* 

•  In  considering  the  various  arrangements  and  combinations  in  which  Voltaic 
action  had  been  manifested,  Davy  observed,  as  a  common  character,  that,  in 
every  case,  one  of  the  two  metallic  elements  was  oxydated,  and  the  other  not. 
Did  the  production  of  the  electric  current,  then,  depend  merely  on  the  pres- 
ence of  two  metallic  surfaces,  one  undergoing  oxydation,  separated  by  a  con- 
ductor of  electricity  ]  and,  if  so,  might  not  a  Voltaic  arrangement  be  made  by 
one  metal  only,  if  its  opposite  surfaces  were  placed  in  contact  with  two  differ- 
ent liquids,  one  of  which  would  oxydate  it,  and  the  other  transmit  electricity 
without  producing  oxydation  ?  To  reduce  this  to  the  test  of  experiment  with 
a.  single  metallic  plate  would  have  been  easy ;  but  in  constituting  a  series  of 
pile,  the  two  liquids,  the  oxydating  and  the  non-oxydating,  must  be  in  contact, 
and  subject  to  intermixture.  To  overcome  this  difficulty,  different  expedients 
were  resorted  to,  with  more  or  less  success  ;  but  the  most  convenient  and 
effectual  method  of  attaining  the  desired  end  was  suggested  to  Davy  by  Count 
Rumford.  Let  an  oblong  trough  be  formed,  similar  to  that  suggested  by 
Cruickshank,  as  a  substitute  for  the  pile  ;  and  let  grooves  be  made  in  it  such 
as  to  allow  of  the  insertion  of  a  number  of  plates,  by  which  the  trough  may  be 
divided  into  a  series  of  water-tight  cells.  Let  plates  of  the  metal  of  which 
the  apparatus  is  to  be  constructed  be  made  to  fit  these  grooves ;  and  let  as 
many  plates  of  glass  or  other  non-conducting  material,  of  the  same  form  and 
magnitude,  be  provided.  Let  the  metallic  plates  be  inserted  in  alternate 
grooves  of  the  trough,  and  the  glass  plates  in  the  intermediate  grooves,  so  as 
to  divide  the  trough  into  a  succession  of  separate  cells,  each  cell  having  on 
one  side  metal,  and  on  the  other  glass.  Let  such  an  arrangement  be  repre- 
sented in  fig.  1,  where  the  metallic  plates  are  represented  at  M,  the  interme- 

Fig.  i. 


d'iate  plates  being  glass.  Let  the  alternate  cells  0  be  filled  with  the  oxyda- 
ting liquid,  and  the  intermediate  cells  L  with  the  liquid  which  conducts 
without  oxydating.  Let  slips  of  moistened  cloth  be  hung  over  the  edge  of 
each  of  the  glass  tubes,  so  that  its  ends  shall  dip  into  the  liquids  in  the  ad- 
jacent cells.  This  cloth,  or  rather  the  liquid  it  imbibes,  will  conduct  the  elec- 
tric current  from  cell  to  cell,  without  permitting  the  intermixture  of  the  liquids. 

In  the  first  arrangements  made  on  this  principle,  the  most  oxydable  metals, 
such  as  zinc,  tin,  and  some  others,  were  tried.  The  oxydating  liquid  O  was 
dilute  nitric  acid,  and  the  liquid  L  was  water.  In  a  combination  consisting  of 
twenty  plates  of  metal,  sensible  but  weak  effects  were  produced  on  the  organs 
of  sense,  and  water  was  decomposed  slowly  by  wires  from  the  extremities. 
The  wire  from  the  end  toward  which  the  oxydating  surfaces  were  directed 
evolved  hydrogen,  and  the  other  oxygen. 

To  determine  whether  the  evolution  of  the  electric  current  was  dependant 
on  the  production  of  oxydation,  or  would  attend  other  chemical  effects  produci- 
ble by  the  action  of  substances  in  solution  upon  metal,  the  oxydating  liquid 
was  now  replaced  by  solutious  of  the  sulphurets,  and  metallic  plates  were  se- 
lected on  which  these  solutions  would  exert  a  chemical  action.  Silver,  copp'.-r, 
and  lead,  were  tried  in  this  way.  Solution  01  suiphuret  of  potash  was  used  in 


The  relative  conducting  power  of  the  metals  has  not  even  yet  been  satisfactorily  established. 


GALVANISM. 


the  cells  O,  and  pure  water  in  L.  A  series  of  eight  metallic  plates  produced 
•sensible  effects.  Copper  was  the  most  active  of  the  metals  tried,  and  lead  the 
least  so.  In  thJse  cases,  the  terminal  wires  produced,  in  the  usual  manner, 
the  d<?compositi»i  of  water,  the  wire  from  which  hydrogen  was  evolved  being 
that  which  was  connected  with  the  end  of  the  series  to  which  the  surfaces  of 
ihe  metal  not  chemically  acted  on  were  presented. 

It  will  be  observed  that  in  this  case  the  direction  of  the  electric  current 
relatively  to  the  surfaces  of  the  metallic  plates  was  the  reverse  of  the  former. 
When  oxydation  was  produced,  the  oxydating  sides  of  the  plates  looked  toward 
the  negative  end  of  the  series.  Comparing  these  two  effects,  Davy  was  led 
by  analogy  to  suspect  that  if  the  cells  O  were  filled  with  an  oxydating  solu- 
tion, while  the  cells  L  were  filled  with  a  solution  of  sulphuret,  or  any  other 
which  would  produce  a  like  chemical  action,  the  combined  effect  of  the  cur- 
rents proceeding  from  the  two  distinct  chemical  processes  would  be  obtained. 
This  was  accordingly  tried,  and  the  results  were  as  foreseen.  The  acid  solu- 
tion was  placed  in  the  cells  O,  and  the  sulphuret  in  the  cells  L.  A  series, 
consisting  of  three  plates  of  copper  or  silver,  arranged  in  this  way,  produced 
sensible  effects ;  and  twelve  or  thirteen  decomposed  water  rapidly.  The 
oxydating  sides  of  the  metal  looked  to  the  negative  end  of  the  series. 

As  it  appeared  from  former  experiments  the  charcoal  possessed,  as  a  Voltaic 
agent,  the  same  properties  as  the  metals,  the  next  step  in  this  course  of  ex- 
periments was  naturally  to  try  whether  a  Voltaic  arrangement  could  not  be 
constructed  without  any  metallic  element,  by  substituting  charcoal  for  the  me- 
tallic*plates  in  the  series  above  described.  This  was  accomplished  by  means 
of  an  arrangement  in  the  form  of  the  couronne  des  tosses.  Pieces  of  charcoal, 
made  from  very  dense  wood,  were  formed  into  arcs ;  and  the  liquids  O  and  L 
were  arranged  in  alternate  glasses,  as  represented  in  fig.  2.  The  charcoal 


arcs  C  were  placed  so  as  to  have  one  end  immersed  in  each  liquid,  the  inter- 
mediate glasses  being  connected  by  slips  of  bibulous  paper  P.  When  the 
liquid  O  was  dilute  acid,  and  L  water,  a  series  consisting  of  twenty  pieces  of 
charcoal  gave  sensible  shocks,  and  decomposed  water.  This  arrangement 
also  acted,  and  with  increased  intensity,  when  the  liquid  O  was  sulphuric  acid,  } 
and  L  was  solution  of  sulphuret  of  potash. 

The  connexion  of  chemical  change  with  the  production  of  electricity  in  the 
pile,  was  too  obvious  not  to  attract  the  attention  of  other  philosophers.     Pepys 
in  England,  and  MM.  Biot  and  Frederic  Cuvier  in   France,  investigated   the 
effect  produced  by  the  pile  on  the  atmosphere  in  which  it  was  placed.     The 
former  placed  the  pile   in  an  atmosphere  of  oxygen,  and  found  that  in  the 
course  of  a  night  200  cubic   inches  of  the  gas  had   been  absorbed.     In  an  at- 
mosphere of  azote  the  pile  had  no  action.     MM.  Biot  and  Cuvier  also  observed 
5  the   quantity  of  oxygen   absorbed,  and   inferred   from  their  experiments   that 
I  "  although,  strictly  speaking,  the   evolution  of  electricity  in  the  pile  was  pro- 
)  duced  by  oxydation,  the  share  which  this  had   in  producing  the  effects  of  the 
\  instrument  bore  no  comparison  with  that  which  was  due  to  the  contact  of  the 
metals,  the  extremity  of  the  series  being  in  communication  with  the  ground." 

Dr.  Wollaston  and  Gautherot,  on  the  other  hand,  reproduced  the  principle 
)  advanced  by  Fabroni  and  Creve.  WoUaston  maintained  that  chemical  action 
{ 


346  GALVANISM. 


was  not  only  the  source  of  the  electricity  of  the  pile,  but  also  of  the  common 
electrical  machine.  He  showed  that  by  conveying  the  electricity  of  the  ma- 
chine to  gold  wires  terminated  in  extremely  fine  points  the  decomposition  of 
water  could  be  effected,  and  that  the  phenomenon  was  the  same  as  when  the 
decomposition  was  effected  by  Voltaic  wires.  He  maintained  that  the  friction 
of  the  rubber  was  attended  with  oxydation,  and  showed  that  the  machine  waj 
ineffective  in  an  atmosphere  of  dry  hydrogen,  or  any  ether  gas  in  which  chem- 
ical action  was  not  produced. 

If  an  oblono;  slip  of  wet  paper  have  its  extremities  in  contact  with  the  poles 
of  a  Voltaic  pile,  each  half  of  the  slip  will  be  electrified ;  that  which  is  in  con- 
tact with  the  positive  pole  will  be  positively  electrified,  and  that  which  is  in 
contact  with  the  negative  pole  will  be  negatively  electrified.  If  it  be  removed 
from  contact  with  the  pile  by  a  rod  of  glass,  or  other  non-conductor,  its  electric 
state  will  continue.  This  means  of  producing  electrical  polarity  was  observed 
by  Volta,  and  about  the  same  time  by  Erhman. 

This  fact  suggested  to  Ritter  the  idea  of  his  secondary  pile,  which  consisted 
of  a  series  of  disks  of  a  single  metal  alternated  with  cloth  or  card,  moistenea 
in  a  liquid  by  which  the  metal  would  not  be  affected  chemically.  If  such  &, 
pile  have  its  extremities  put  in  connexion  by  conducting  substances  with  tht 
poles  of  an  insulated  Voltaic  pile,  it  will  receive  a  charge  of  electricity  in  u 
manner  similar  to  the  band  of  wet  paper,  one  half  taking  a  positive  and  tht, 
other  a  negative  charge ;  and  after  its  connexion  with  the  primary  pile  hd*. 
been  broken,  it  will  retain  the  charge  it  has  thus  received.  The  secondary 
pile,  while  it  retains  its  charge,  produces  the  same  physiological  and  cheftnica* 
effects  as  the  Voltaic  apparatus. 

The  polarity  which  the  band  of  wet  paper  and  the  secondary  pile  acquit 
by  their  temporary  contact  with  the  ends  of  a  Voltaic  apparatus,  is  a  coiibe 
quence  of  their  imperfect  conducting  power.  The  electricity  of  each  specie* 
appears  to  force  its  way  through  the  imperfect  conductor  till  the  two  opposite 
currents  meet  in  the  centre. 

At  the  time  of  the  discovery  of  the  secondary  piles,  it  was  known  that  a  piect- 
of  metallic  wire,  the  ends  of  which  had  been  placed  in  contact  with  the  poles 
of  a  Voltaic  pile,  does  not  instantly  recover  its  natural  state  when  its  contact 
with  the  pile  is  broken. 

From  the  experiments  of  Davy  and  others,  it  appeared  that  if  a  communica- 
tion was  made  between  the  poles  of  an  insulated  pile  and  two  glasses  of  water, 
so  that  the  water  in  the  one  would  be  charged  with  positive,  and  the  other 
with  negative  electricity,  a  metallic  wire  connecting  the  two  portions  of  water 
would  evolve  oxygen  gas  at  one  point,  and  hydrogen  at  the  other.  If,  under 
such  circumstances,  the  connexion  of  the  glasses  with  the  pile  be  suddenly 
broken,  the  action  of  the  wire  will  nevertheless  continue  for  some  time,  but  its 
effects  will  be  reversed ;  the  point  which  before  disengaged  hydrogen  will 
now  disengage  oxygen,  and  vice  versa.  It  appears,  therefore,  that,  the  sudden 
suspension  of  the  action  of  the  pile  has  the  effect  of  reversing  the  direction  of 
the  electric  current  which  passes  through  the  wire.* 

The  continuance  of  the  electric  state  of  a  wire  which  had  been  used  to  con-  / 
nect  the  poles  of  a  pile  after  its  separation  from  the  pile  was  also  demonstrated  \ 
by  Oersted,  who  showed  its  effect  on  the  organs  of  a  frog.f     The  same  effect  , 
was  produced  by  a  wire   through  which  the   current  of  a  powerful  electrical 
machine  had  been  transmitted 

From  the  chemical  effects  of  the  pile,  Davy  turned  his  attention  to  its  calor-  [ 
ific  powers.  The  means  of  experimental  investigation  placed  at  his  disposal  j 

) 

*  Histoire  de  Galvanism  de  Sue,  torn,  iii.,  p.  341. 
t  Journ.  de  Opim.  de  Van-Mons,  No.  iv.,  p.  &8. 


GALVANISM.  347 


were  enlarged  by  the  apparatus  of  the  laboratory  of  the  Royal  Institution, 
which  was  now  under  his  direction.  The  Voltaic  apparatus  consisted  of 
>  a  scries  of  150  pairs  of  four-inch  plates  of  zinc  and  copper,  and  a  series 
of  50  pairs  of  zinc  and  silver  of  the  same  magnitude.  The  plates  were 
cemented  into  four  troughs  of  wood,  according  to  the  method  proposed  by 
Cruickshank.  Another  apparatus  was  provided,  consisting  of  a  series  of 
twenty  pairs  of  thirteen-inch  plates  of  zinc  and  copper. 

With  the  batteries  of  the  smaller  plates  he  repeated  some  of  the  experiments 
on  the  production  of  the  spark,  and  the  combustion  of  the  metals  which  had 
already  been  made.  When  the  poles  consisted  of  two  knobs  of  brass,  the 
spark  which  attended  the  discharge  was  of  dazzling  brightness,  and  one  eighth 
of  an  inch  in  apparent  diameter.  Between  pieces  of  charcoal  it  had  a  vivid 
whiteness,  and  the  charcoal  remained  red-hot  for  some  time  after  the  contact 
was  broken,  and  threw  off  bright  coruscations.  The  current  passing  through 
steel  wire  jyotn  of  an  inch  in  diameter,  rendered  it  white-hot,  and  caused  it 
to  burn  with  great  splendor.  Gold,  silver,  copper,  tin,  lead,  and  zinc,  were 
also  burnt.  Platinum  in  thin  slips  was  rendered  white-hot  and  fused. 

Fourcroy,  Vauquefin,  and  Thenard,  had  investigated  the  different  effects  pro- 
duced by  enlarging  the  plates  of  a  battery,  and  by  increasing  their  number. 
They  demonstrated  that  the  power  of  the  apparatus  to  heat  and  ignite  metallic 
substances  was  augmented  by  enlarging  the  plates,  without  increasing  their 
number ;  but  that  no  increase  of  power  to  decompose  water,  or  to  produce  the 
shock,  ensued.  The  calorific  power,  therefore,  appeared  to  depend,  cattris  pa- 
ribus,  on  the  magnitude  of  the  plates,  while  the  chemical  and  physiological 
power  depended  on  their  number. 

The  battery  of  thirteen-inch  plates  was  tried  successively  with  pure  water, 
a  solution  of  common  salt,  and  dilute  nitric  acid.  With  water  its  effects  were 
feeble,  with  the  solution  of  salt  they  were  much  more  considerable,  and  were 
still  more  energetic  with  nitric  acid.  With  the  last,  three  inches  of  iron  wire, 
yi^th  of  an  inch  in  diameter,  were  rendered  white  hot,  and  two  inches  of  the 
same  wire  were  fused.  The  action  of  the  water,  feeble  as  it  was,  was  as- 
cribed to  the  air  and  saline  matter  it  held  in  solution  ;  and  it  was  judged  from 
analogy  that  water  perfectly  purged  of  air  and  free  from  all  saline  substances, 
would  have  no  Voltaic  action.  A  pile  of  thirty-six  pairs  of  five-inch  plates 
lost  its  activity  in  an  atmosphere  of  azote  and  hydrogen  in  about  two  days ; 
and  its  power  was  constantly  restored  by  common  air,  and  rendered  more  in- 
tense by  oxygen  gas. 

When  two  pieces  of  well-burnt  charcoal,  or  a  piece  of  charcoal  and  a  me- 
tallic wire,  are  connected  with  the  apparatus  an',  immersed  in  water,  on  com- 
pleting the  circuit,  gas  was  abundantly  evolved,  and  the  points  of  the  charcoal 
appeared  red  hot  for  some  time  after  the  contact  was  made.  Sparks  were  also 
produced  by  means  of  charcoal  points  immersed  in  concentrated  nitre  and  sul- 
phuric acids.  When  two  charcoal  points  acted  in  water,  the  gaseous  products 
consisted  of  one  eighth  carbonic  acid,  one  eighth  oxygen,  and  one  eighth  in- 
flammable gas,  apparently  hydrogen.  The  gases  produced  by  a  similar  process 
/  from  alcohol,  ether,  and  dilute  sulphuric  acid,  were  also  a  mixture  of  oxygen 
and  hydrogen.  In  all  these  cases  it  appeared  that  the  gases  proceeded 
chiefly  from  the  decomposition  of  the  water  contained  in  the  several  solutions. 

The  effects  of  the  ignition  of  charcoal  in  muriatic  acid  confined  over  mer- 
cury, were  next  tried.     The  charcoal  being  kept  white  hot  for  nearly  two  hours, 
the  gas  was  very  little  reduced  in  volume,  and  the  charcoal  was  not  sensibly  [ 
consumed.     When  the  gas  was  examined,  three  fourths  of  it  were  absorbed  by 
water,  and  the  remainder  was  inflammable.* 


*  Davy's  Works,  vol.  ii.(  p.  214.     London,  1839. 

U 


J  348  GALVANISM. 


$  Of  fhe  theories  proposed  at  this  early  period  of  the  experimental  inquiry  to 
/  explain  chemical  decomposition  by  the  Voltaic  apparatus,  that  of  Grotthus  was 
the  earliest  and  most  plausible.  To  simplify  the  view  of  this  theory,  we  shall 
take  as  an  example  of  its  application  the  decomposition  of  water.  Each  mo- 
lecule of  water  being  composed  of  a  molecule  of  oxygen  and  a  molecule  of 
hydrogen,  their  natural  electricities  are  in  equilibrium  when  not  exposed  to  any 
disturbing  force,  each  possessing  equal  quantities  of  the  positive  and  negative 
fluids.  The  electricity  of  the  positive  wire  acting  by  induction,  on  the  natural 
electricities  of  the  contiguous  molecule  of  water,  attracts  the  negative  and  re- 
pels the  positive  fluid.  It  is  further  assumed  in  this  theory,  that  oxygen  has  a 
natural  attraction  for  negative,  and  hydrogen  for  positive  electricity  ;  therefore 
the  positive  wire  in  attracting  the  negative  fluid  of  the  contiguous  molecule  of 
water,  and  repelling  its  positive  fluid,  attracts  its  constituent  molecule  of  oxy- 
gen, and  repels  its  molecule  of  hydrogen.  The  particle  of  water,  therefore, 
places  itself  with  its  oxygen  next  the  positive  wire,  and  its  hydrogen  on  the 
opposite  side.  The  positive  electricity  of  the  first  particle  of  water  thus  accu- 
mulated on  its  hydrogen  molecule,  produces  the  same  action  on  the  succeeding 
molecule  of  water  as  the  wire  did  upon  the  first  molecule  ;  and  a  similar  ar- 
rangement of  the  second  molecule  of  water  is  effected.  This  second  molecule 
acts  in  like  manner  on  the  third,  and  so  on.  All  the  particles  of  water  between 
the  positive  and  negative  wires  thus  assume  a  polar  arrangement,  and  have 
their  natural  electricities  decomposed  ;  the  negative  poles  and  oxygen  molecules 
looking  toward  the  positive  wire,  and  the  positive  poles  and  hydrogen  mole- 
cules looking  toward  the  negative  wire.  The  attraction  of  the  positive  wire 
now  separates  the  oxygen  molecule  of  the  contiguous  particle  of  water  from 
its  hydrogen  molecule,  neutralizes  its  negative  electricity,  and  either  dismisses 
it  in  the  gaseous  form,  or  combines  with  it,  according  to  the  degree  of  the  af- 
finity of  the  metal  of  the  wire  for  oxygen.  The  hydrogen  molecule  thus  liber- 
ated effects  in  like  manner  the  decomposition  of  the  second  particle  of  water, 
combining  with  its  oxygen,  and  thus  again  forming  water  and  dismissing  its 
hydrogen.  The  latter  acts  in  the  same  manner  on  the  next  particle  of  water, 
and  so  on.  Thus,  a  series  of  decompositions  and  recompositions  are  supposed 
to  be  carried  on  through  the  fluid,  until  the  process  reaches  the  particle  of  wa- 
ter contiguous  to  the  negative  wire,  and  the  molecule  of  hydrogen  there  disen- 
gaged gives  up  its  positive  electricity,  by  which  an  equal  portion  of  negative 
electricity  proceeding  from  the  wire  is  neutralized,  and  the  molecule  of  hydro- 
gen escapes  in  the  gaseous  form.  It  is  equally  compatible  with  this  theory  to 
suppose  the  series  of  decompositions  and  recompositions  to  commence  at  the 
negative  and  terminate  at  the  positive  wire,  or  to  commence  simultaneously  at 
both,  and  terminate  at  any  intermediate  point  by  the  union  of  the  last  molecule 
of  oxygen  disengaged  in  the  one  series  with  the  last  molecule  of  hydrogen 
disengaged  in  the  other. 

Grotthus  illustrated  this  ingenious  hypothesis  by  comparing  the  supposed 
phenomena  with  the  mechanical  effects  produced  when  a  number  of  elastic 
balls — ivory  balls  for  example — being  suspended  so  that  their  centres  shall  be  in 
the  same  straight  line,  and  their  surfaces  mutually  touch,  either  cf  the  extreme 
balls  of  the  series  being  raised  and  let  fall  against  the  adjacent  ons.  the  effect 
is  propagated  through  the  series,  and  the  last  ball  alone  recoils  in  consequence 
of  the  impact ;  and  although  the  action  and  reaction  are  suffered  by  each  ball 
of  the  series,  and  each  is  ins'rumental  in  transmitting  the  effect,  no  visible 
change  takes  place  in  any  ball  except  the  last,  and  the  effect  is  continued  by 
the  alternate  action  of  the  extreme  balls  until  the  motion  is  gradually  stopped 
by  the  resistance  of  the  air,  and  other  external  causes. 

The  experiments  of  Davy,  which  have  been  a»'eriy  r?;nticned,  •ve'-e 


GALVANISM.  349 


the  prelude  to  a  brilliant  series  of  discoveries,  the  commencement  of  which 
burst  upon  the  scientific  world  in  his  Bakerian  Lecture  for  the  year  1800.  As 
soon  as  the  spendid  results  detailed  in  that  paper  became  known  in  Franco, 
the  members  of  the  Institute,  rising  superior  to  the  feelings  of  naiional  ani- 
mosity which  at  that  time  unhappily  prevailed,  unanimously  conferred  upon  its 
distinguished  author  the  prize  which  had  been  established  by  Napoleon  for  the 
best  experiments  on  Voltaic  electricity.* 

The  genius,  address,  and  perseverance  of  him  whose  vocation  is  to  investi- 
gate the  laws  of  nature,  are  not  always  confined  to  the  grateful  labor  of  devel- 
oping truths.  The  extirpation  of  error  is  a  task  which,  while  it  demands  the 
exercise  of  equally  exalted  powers,  is  never  rewarded  by  that  eclat  which  sur- 
rounds the  discovery  of  natural  harmonies  before  unobserved  and  unsuspected. 
In  the  commencement  of  the  series  of  researches  now  referred  to,  Davy  found 
it  necessary  to  clear  from  his  path  certain  difficulties,  juid,  as  he  rightly  con- 
ceived, errors,  by  which  his  progress  was  obstructed. 

When  the  decomposing  powers  of  -the  pile  were  first  exhibited,  the  excite- 
ment attending  a  discovery  so  unlocked  for  prevented  the  details  of  the  experi- 
ments from  receiving  all  the  attention  to  which  they  were  entitled.  When  the 
circumstances  attending  the  decomposition  of  water  by  the  Voltaic  wires  were 
submitted  to  closer  examination,  it  was  found  that  indications  of  the  presence 
of  an  acid  always  existed  at  the  pole  where  oxygen  was  evolved,  and  those  of 
an  alkali  at  the  other  pole.  In  cases  where  the  water  submitted  to  decomposi- 
tion might  be  supposed  to  hold  saline  matter  in  solution,  such  effects  would 
create  no  surprise;  but  they  were  unequivocally  manifested  when  the  water 
used  was  distilled,  and  when  there  was  every  reason  to  think  it  chemically 
pure.  Mr.  Cruickshank  explained  this,  by  supposing  the  acid  to  be  nitrous 
acid,  proceeding  from  the  combination  of  the  azote  of  the  common  air  held  in 
solution  by  the  water  with  the  oxygen  evolved  at  the  positive  wire  ;  and  the 
alkali  to  be  ammonia,  proceeding  from  the  combination  of  the  same  principle 
with  the  hydrogen  evolved  at  the  negative  wire.  Desormes  maintained  that 
the  acid  was  muriatic  ;  and  Brugnatelli  that  it  was  an  acid  sui  generis,  produ- 
ced by  the  combination  of  positive  electricity  with  one  of  the  constituents  of 
water,  and  called  it  electric  acid.  Some  maintained  that  the  constituents  of  the 
acid  and  alkali  came  over  from  the  liquid  used  in  the  Voltaic  apparatus  in  some 
undiscovered  manner  along  the  wires,  and  was  thus  deposited  in  the  water ; 
and  others  held  that  it  was  generated  out  of  the  elements  of  the  water  by  Vol- 
taic action.  An  article  was  published  in  the  "  Philosophical  Magazine,"  f  by 
f 

/  *  It  is  stated  in  the  Memoirs  of  Davy  by  Dr.  Paris  (p.  168),  that  the  prize  given  to  Davy  was  the 
^  annual  medal,  worth  3,000  francs,  which  was  designed  as  a  reward  for  the  best  experiments  in  elec- 
/  tricky  which  should  be  made  in  each  year.  The  same  statement  is  made  in  a  note  by  the  editor  in 
(  the  fifth  volume  of  Davy's  Works  (p.  56),  edited  by  his  brother,  Dr.  John  Davy :  "  The  minor  prize 
(  af  3.000  francs,  founded  by  Napoleon  when  first  consul,  for  the  most  important  result  in  electrical 
(  research  during  each  year,  •was  awarded  by  the  Institute  to  the  author  for  this  paper:  the  principal 
<  prize  of  60,000  francs,  of  which  the  preceding  was  only  the  interest,  in  the  opinion  of  the  best 

judges  was  rather  due  to  him,  as  it  was  proposed  to  be  given  '  a  celui,  qui  parses  experiences  et 
}  ses  decouvertes,  fera  i  faire  a  1'electricile  et  au  galvanisme  un  pas  comparable  a  cela  qu'ont  fait 

faire  a  ces  sciences  Franklin  et  Volta.'  Thus  the  writer  in  the  Quarterly  Review  already  referred 
'•  .'j  remarks.  It  was  only  questioned  by  those  who  were  capable  of  appreciating  its  importance, 
•  \vl.euier  thej  acted  with  strict  impartiality  in  assigning  to  him  the  annual  interest  only,  when  he 
£  appeared  to  have  a  fair  claim  to  the  principal.' '' 

6a  the  other  hand,  the  French  writers  on  electricity  claim  the  merit  of  having  given  Davy  the 


:.    ete  promis  par  Napol&on  a  1'auteur  des  plus  grandes  decouvertes  en  felectricite,  comparables  a  celle 
(  de  Volta  et  de   Galvani."     Whether  Davy   received  the  bigherorthe  lower  prize  (we  believe  it 

was  the  lo\ver),  ii  is  evident  that  the  French  scientific  authorities  now  think  he  was  entitled  to  the   , 

former. 

t  Vol.  xxi.,  p.  279. 


350 


GALVANISM. 


a  Mr.  Peel,  of  Cambridge,  containing  an  account  of  an  experiment  in  which  > 
the  water  that  remained,  after  a  large  portion  had  been  decomposed  by  the  pile,  I 
yielded  on  evaporation  muriate  of  soda,  although  the  water  used  in  the  experi- 
ment had  been  distilled  with  every  precaution  necessary  to  free  it  from  impu- 
rities. On  inquiry  being  made  at  Cambridge,  no  person  corresponding  with 
the  name  and  address  of  the  professed  author  cou>d  be  found  ;  and  the  state- 
ment was  concluded  to  be  a  mere  attempt  to  practise  on  the  credulity  of  the 
scientific  world,  when  the  surprise  was  revived  by  the  publication  of  experi- 
ments actually  made  by  Professor  Pacchiomf  of  Pisa,  in  which  the  same  re- 
sult was  attained  as  was  stated  in  the  pretended  Cambridge  experiment.  Syl- 
vester being  led  to  the  same  conclusion,  ascribed  the  supposed  effects,  in 
common  with  Pacchioni,  to  the  oxydation  of  hydrogen,  on  the  one  hand  in  a 
higher,  and  on  the  other  in  a  lower  degree  than  that  which  forms  water. 

Such  were  the  confusion  and  obscurity  in  which  the  community  of  science 
was  involved  on  the  subject  of  the  Voltaic  decomposition  of  water,  when  the 
question  was  taken  up  by  Davy.  In  common  with  others,  he  had  observed  at 
an  early  period  the  presence  of  an  acid  and  alkali  in  water  under  the  process 
of  decomposition  ;  but  states,  that,  so  early  as  1800,  he  concluded  from  his  ex- 
periments that  the  acid  proceeded  from  the  animal  and  vegetable  substances 
which  he  employed,  and  that  the  alkali  arose  from  the  corrosion  of  the  glass 
vessels  in  which  the  experiment  was  conducted.  Similar  inferences  were 
made  by  the  Galvanic  Society  of  Paris,  by  MM.  Biot  and  Thenard,  and  by  Dr. 
Wollaston  ;  the  last  of  whom  removed  one  of  the  sources  of  these  disturbing 
elements  by  the  happy  expedient  of  connecting  the  positive  and  negative  por- 
tions of  water  by  a  piece  of  well-washed  asbestos. 

The  investigation  now  undertaken  by  Davy  was  commenced  by  decompo- 
sing distilled  water  in  two  small  cups  of  agate,  P  N  (fig.  3),  connected  by  a 

Pig.  3. 


piece  of  white  transparent  amianthus,  A.  The  wires  of  the  Voltaic  battery  of 
160  pairs  of  four-inch  plates  were  connected  with  the  water,  the  positive  wiie 
being  immersed  in  the  cup  P,  and  the  negative  wire  in  N.  After  the  process 
had  been  continued  for  forty-eight  hours,  the  water  in  the  cup  P  was  found  to 
redden  litmus  paper,  and  turmeric  paper  was  affected  by  the  water  in  N.  It 
appeared,  therefore,  and  further  experiment  confirmed  the  indication,  that  acid 
was  present  in  the  positive  water,  and  alkali  in  the  negative. 


t  Vol.  xxii.,  p.  179. 


J 


GALVANISM.  35] 


This  result,  after  all  the  precautions  which  had  been  taken,  was  quite  unex- 
pected, and,  as  may  be  imagined,  gave  not  a  little  surprise  to  the  experimenter. 
Still  he  did  not  for  a  moment  entertain  any  of  the  speculations  of  the  genera- 
tion of  these  substances  in  the  water.  His  next  step  was  to  repeat  the  exper- 
iment with  glass  instead  of  agate  cups,  using  the  same  quantities  of  the  same 
water,  and  exposing  them  for  the  same  time  to  the  action  of  the  same  battery. 
He  argued,  that  if  the  cause  lay  in  the  water,  the  effects  would  be  the  same ; 
but  that  if  the  cup.*  had  any  share  in  producing  them,  they  might  be  expected 
to  be  different.  The  result  confirmed  his  anticipation.  The  alkali  was  pro- 
duced in  the  cup  N  in  quantity  twenty  times  as  great  as  with  the  agate  cups, 
but  there  was  no  trace  of  the  acid.  The  experiments  were  then  repeated  sev- 
eral times  with  the  agate  cups,  when  the  acid  and  alkali  reappeared  in  quanti- 
ties, which,  when  compared  with  each  other  and  with  the  result  of  the  experi- 
ment with  glass  cu[.s,  left  no  doubt  that  the  agate  cups  themselves  had  been 
the  chief  if  not  the  only  source  of  the  acid,  and,  in  a  considerable  degree,  of 
the  alkali  also.  Still  it  was  impossible  to  ascribe  the  effects  altogether  to  the 
material  of  the  cups ;  and  he  was  impressed  with  the  suspicion  that  the  writer 
itself,  notwithstanding  its  careful  distillation,  must  have  held  more  or  less  alka- 
line matter  in  solution.  It  was  known  that  the  usual  tests  would  fail  to  indi- 
cate the  presence  of  alkaline  impurities  when  their  proportion  in  water  was 
under  a  certain  limit ;  and  the  New  river  water,  which  he  used,  contained  an- 
imal and  vegetable  impurities,  which  might  furnish  neutral  salts  capable  of  be- 
»ng  carried  over  in  the  process  of  distillation. 

The  agate  cups  were  now  replaced  by  two  conical  cups  of  pure  gold  (fig.  4), 

Fig.  4. 


each  containing  about  twenty-five  grains  of  water.     Distilled  water  in  these 
was  exposed  to  the  action  of  a  battery  of  100  pairs  of  six-inch  plate%     Ih  ten 
minutes  indications  of  acid  and  alkali  were  formed  in  the  cups  D  and  N  re- 
spectively.    The  process  was  continued  for  fourteen  hours,  during  the  whole 
of  which  time  the  acid  increased  in  the  cup  D.     The  same  increase  was  not, 
however,  observed  in  the  alkali  in  the  cup  N ;  on  the  contrary,  it  reached  its 
maximum  state  in  a  short  time,  and  continued  without  increase  afterward.    On 
heating  the  cup  N,  the  alkali  diminished,  but  could  not  be  altogether  dismissed. 
These  experiments  being  repeated  with  similar  results,  it  became  apparent 
that  the  source  of  the  acid  and  alkali  must  exist  in  the  water  itself,  and  must 
either  have  arisen  from  saline  matter  remaining  in  solution  in  the  water  after 
distillation,  or  have  been  produced  by  the  azote,  which  exists  in  minute  por- 
?  lions  in  all  water  exposed  to  the  air.     The  latter  supposition  would  not  be  in- 
S  compatible  with  the  circumstance  of  the  alkali  speedily  attaining  &  maximum, 
?  since  the  continued  absorption  of  azote  from  the  atmosphere  by  the  water  would 
i  be  stopped  when  the  latter  would  become  charged  with  hydrogen. 

The  former  supposition  was  adopted,  and  it  was  determined  to  submit  the 
)  water  which  had  been  used  in  the  last  experiments  to  slow  redistillation.     A 


)   352  GALVANISM. 


quart  of  this  water  was  accordingly  evaporated  in  a  silver  still  at  a  terr. .  :  -•-.-  ' 
ture  below  140°,  and  a  saline  residuum  teas  obtained  weighing  seven  tenth  c  \ 
a  grain. 

The  gold  cups  were  now  again  filled  with  the  water  thus  purified,  ai.d  ?,; 
posed  to  the  Voltaic  action.     After  two  hours  the  cup  N  failed  to  show  tri  : 
alkaline  efi'ect  on   turmeric  paper.     By  very  minute  observation,  its  effec4:  c  -\  •{ 
tin-  more  delicate  test  of  litmus  was  perceivable  ;  but  this  disappeared  by  the 
application  of  heat,  and  was,  therefore,  ascribed  to  ammonia  produced  by  the 
combination  of  the  small  quantity  of  azote  contained  in  the  water  with  the 
nascent  hydrogen. 

Finally,  in  order  to  insulate  the  results  from  the  disturbing  effects  of  the  sur- 
rounding atmosphere,  the  gold  cups  containing  the  purified  water  were  placed 
under  the  receiver  of  an  air-pump,  which  was  exhausted  until  the  gauge  stood 
at  half  an  inch.  Hydrogen  gas  was  then  introduced  under  the  receiver,  which, 
mixed  with  the  very  minute  portion  of  atmospheric  air  which  had  remained, 
was  again  withdrawn  by  the  pump.  Pure  hydrogen  gas  was  now  once  more 
introduced  around  the  cups,  which  being  placed  in  connexion  with  the  Voltaic 
apparatus,  were  suffered  to  remain  under  its  action  for  twenty-four  hours,  at  the 
end  of  which  time  neither  of  the  portions  of  the  water  altered  in  the  slightest 
degree  the  tint  of  litmus. 

Thus  were  dispelled  the  speculations  on  the  power  of  electricity  to  generate 
new  principles  in  water ;  and  by  eliminating  the  disturbing  action  of  other 
causes,  the  decomposing- power  of  the  pile  upon  a  binary  compound  was  pre- 
sented in  a  manner  fitted  for  theoretical  investigation. 

If  chance  occasionally  deprives  the  philosopher  of  the  merit  of  discovery  by 
throwing  facts  under  his  feet,  an  ample  field  for  the  exercise  of  his  sagacity 
remains  in  the  due  appreciation  of  the  innumerable  effects  which  are  incidental 
to  his  experimental  researches  ;  to  seize  which  as  they  arise,  to  pursue  them 
through  their  consequences,  to  strip  them  of  the  Protean  disguises  which  they 
borrow  from  other  phenomena  with  which  they  become  related,  to  expand  them 
by  comparison  and  generalization  into  comprehensive  natural  laws,  is  the  prov- 
ince of  the  highest  powers  of  philosophical  inquiry.  Never  was  this  felicitous 
instinct  more  conspicuous  than  in  the  mind  of  Davy.  No  effect,  however  mi- 
nute or  accidental  it  might  apparently  be,  presenting  itself  in  his  experiments, 
escaped  his  vigilance,  if  it  offered  the  least  clue  to  further  discovery.  In  the 
course  of  the  experiments  just  noticed,  he  found  himself  embarrassed  by  the 
disturbing  action  of  the  Voltaic  wires  on  the  material  of  the  vessels  containing 
the  liquid,  which  was  the  immediate  object  of  his  attention.  One  material 
after  another  was  put  aside  to  get  rid  of  this  effect ;  but  ihe  fact  was  not  over- 
looked or  forgotten.  It  proved  the  germ  of  a  vast  discovery. 

The  negative  wire  effected  a  partial  decomposition  of  the  glass  and  agate 
cups,  and  brought  a  portion  of  their  constituents  into  solution  in  the  water  con- 
tained in  them.  Might  not  a  power,  which  thus  subdued  affinities  so  stubborn 
as  those  which  produce  the  aggregation  of  substances  so  insoluble  as  agate  and 
glass,  be  brought  to  bear  on  other  similar  bodies,  and  perchance  resolve  into 
their  components  substances  now  considered  simple  and  elementary?  As  a 
first  trial  of  the  decomposition  of  insoluble  or  difficultly-soluble  bodies,  cups 
were  formed  of  wax,  resin,  marble,  argillaceous  schist  from  Cornwall,  serpen- 
tine from  the  Lizard,  and  graywacke.  Being  filled  with  purified  water*  in  the 
same  manner  as  in  the  experiments  above  described,  decomposition  was  in  all 
cases  effected  and  saline  matter  evolved. 

Pursuing  this  investigation,  he  successively  decomposed  by  the  same  pro- 

*  By  purified  water  in  all  the  following  experiments  is  to  be  understood  water  rendered  chemi-  ) 
cally  pure  by  the  processes  above  described. 


f 


GALVANISM.  353 


the  substance  to  be  submitted  to  Voltaic  action.  Let  them  each  be  filled  with 
purified  water,  and  connected  by  asbestos.  If  A  be  connected  with  the  posi- 
tive and  S  with  the  negative  wire,  it  was  expected  that  any  acid  constituent 
which  may  be  in  the  substance  of  which  S  is  formed  would  pass  into  A,  which 
would  become  an  acid  solution,  and  appear  by  the  application  of  the  usual 
tests.  If,  on  the  other  hand,  A  be  connected  with  the  negative  and  S  with  the 
positive  wire,  any  alkali  which  may  be  in  the  substance  of  which  S  is  formed 
was  expected  to  pass  into  A,  and  to  be  manifested  there  by  the  common  alka- 
line tests. 

In  the  first  case  in  which  his  method  was  tried,  the  cup  S  was  formed  of 
sulphate  of  lime.  The  cup  A  was  connected  with  the  negative  and  S  with  the 
positive  wire.  With  a  battery  of  100  pair  of  plates,  the  water  in  A  was  in 
about  four  hours  converted  into  a  strong  solution  of  lime,  and  the  liquid  in  S 
was  converted  into  sulphuric  acid.  When  the  cup  A  received  the  positive  and 
S  the  negative  wire,  the  effects  were  reversed.  In  that  case,  the  water  in  A 
became  sulphuric  acid,  and  a  solution  of  lime  was  found  in  S. 

Other  saline  cups  were  submitted  to  the  same  process  with  like  results  ;  the 
water  in  the  positive  cup  always  receiving  acid,  and  that  in  the  negative  cup 
alkali. 

Two  cups  of  glass  were  connected  with  the  poles  of  the  battery.  One  was 
filled  with  distilled  water,  and  the  other  with  a  saline  solution.  In  every  case 

as 


cess  sulphate  of  lime,  sulphate  of  strontia,  fluate  of  lime,  sulphate  of  baryta, 
and  other  insoluble  salts,  and  in  each  case  obtained  the  acid  in  the  positive 
ami  the  base  in  the  negative  cup.  Certain  mineral  substances,  such  as  basalt, 
zeolite,  and  vitreous  lava  from  ./Etna,  were  examined ;  and  although  the  saline 
ingredients  in  some  cases  prevailed  in  extremely  minute  proportions,  their 
presence  was,  nevertheless,  distinctly  manifested.  The  soluble  compounds, 
such  as  sulphate  and  nitrate  of  potash,  sulphate  and  phosphate  of  soda,  were 
easily  decomposed,  and  the  results  were  the  same. 

The  metallic  salts  deposited  their  metallic  elements  in  crystals  on  the  nega- 
tive wire,  round  which  the  oxide  was  also  deposited,  while  the  acid  was  col- 
lected in  the  positive  cup. 

These,  however,  were  only  the  first  and  least  important  of  the  consequences 
of  the  idea  of  extending  the  principle  in  virtue  of  which  the  Voltaic  wire  cor- 
roded the  glass.  We  shall  dismiss  this  for  the  present,  to  consider  the  next 
series  of  experiments  in  these  researches,  but  shall  resume  the  subject. 

From  many  of  his  own  experiments,  and  some  described  by  Gautherot, 
Hisinger,  Berzelius,  and  Ritter,  it  was  apparent  that  the  Voltaic  influence  was 
capable  not  only  of  decomposing  compound  bodies,  but  also  of  transferring,  or, 
if  the  term  may  be  permitted,  decanting  their  constituents  from  one  vessel  to 
another.  The  series  of  experiments  which  follows  next  in  order  in  these  re- 
searches was  directed  to  the  examination  of  the  limits  of  that  power,  and  the 
effects  attending  it  under  conditions  not  before  tried. 

When  the  substance  to  be  decomposed  was  insoluble,  it  was  formed  into  a 
cup,  as  in  the  preceding  experiments,  and  water  contained  in  it  was  exposed  to  < 
the  Voltaic  action.     Thus  let  A,  fig.  5,  be  aji  agate  cup,  and  S  a  cup  made  of  \ 

Fig.  5. 


354 


GALVANISM. 


the  salt  was  decomposed,  the  base  passing  into  or  remaining  in  the  negative, 
and  the  acid  in  the  positive  cup. 

The  time  required  for  these  transmissions  appeared  to  increase,  c&lens  pari- 
bus,  as  the  space  through  which  the  decomposed  elements  were  to  be  trans- 
mitted increased. 

To  determine  whether  the  action  of  the  metallic  wires  proceeding  from  the 
Voltaic  battery  was  immediately  engaged  in  the  production  of  these  decompo- 
sitions, the  next  experiments  were  arranged  so  that  the  electric  current  should 
be  transmitted  to  the  solution  to  be  decomposed  through  liquid  conductors. 
For  this  purpose,  three  cups  (P,  I,  and  N,  fig.  6)  were  provided  ;  the  extreme 

Fig.  6. 


ones  P  and  N  receiving  the  positive  and  negative  wires  from  the  battery,  and 
the  cup  I  connected  with  each  of  them  by  amianthus.  The  cups  P  and  N 
were  filled  with  purified  water,  and  the  solution  to  be  decomposed  was  put  into 
the  intermediate  cup  I.  In  every  case  the  acid  constituent  of  the  solution  was 
decanted  into  P,  and  the  alkaline  into  N.  Lest  the  amianthus  siphons  should 
have  any  mechanical  effect  on  the  transference  of  the  solution  between  the 
cups,  the  cups  P  and  N  were  so  filled  that  the  surfaces  of  the  water  in  them 
were  above  that  of  the  solution  in  I. 

As  it  was  how  abundantly  apparent  that  the  elements  of  the  decomposed 
substance  were  drawn  from  cup  N  through  the  interstices  of  the  siphons,  it  Avas 
determined  to  try  how  far  this  decanting  power  could  be  carried  by  breaking 
the  continuity  of  the  siphons,  and  rendering  it  impossible  for  the  decomposed 
element  to  reach  its  destination  without  passing  through  an  intermediate  liquid. 
For  this  purpose,  the  three  cups  being  arranged  as  before,  two  of  them,  P  and 
I,  were  filled  with  distilled  water,  the  water  in  I  being  tinged  with  litmus  ;  and 
the  negative  cup  N  was  filled  with  a  solution  of  the  sulphate  of  potash.  If  the 
energy  of  the  attraction  of  the  positive  wire  for  the  acid  constituent  of  the  salt 
were  sufficiently  strong  to  cause  it  to  pass  from  N  to  P,  through  the  liquid  in 
I,  it  was  naturally  expected  that,  on  its  route,  its  presence  in  I  would  be  rendered 
manifest  by  tiie  usual  effect  of  reddening  the  litmus.  The  acid  passed  from 
N  to"  P  through  I,  but  without  being  manifested  in  I  by  any  redness  of  the  so- 
lution. 

When  the  saline  solution  was  put  in  the  positive  cup  P,  and  the  purified  water 
in  the  negative  cup  N,  the  water  in  I  being  tinged  with  turmeric,  the  alkali 
passed  in  like  manner  from  P  to  N  without  producing  any  effect  on  the  color 
of  the  liquid  I. 

As  the  transmission  of  acid  or  alkali  by  means  of  the  electric  current  through 
water  tinged  with  vegetable  colors  was  effected  without  producing  any  sensible 
change  in  these  delicate  tests  of  their  presence,  it  was  conjectured  that,  while 
in  this  state  of  transition,  or  electrical  progression,  the  chemical  elements  were 


GALVANISM. 


355 


deprived  of  their  wonted  properties,  and  that  therefore  they  would  equally  pass 
through  solutions  of  substances  for  which,  under  ordinary  circumstances,  they  ) 
exhibit  a  strong  affinity,  that  affinity  being  rendered  dormant,  or  counteracted, 
by  the  predominating  influence  of  the  electrical  attraction.  To  reduce  this 
conjecture  to  the  test  of  experiment,  the  water  tinged  with  vegetable  colors  in 
the  intermediate  cup  I  was  replaced  by  a  weak  solution  of  ammonia,  purified 
water  was  put  into  the  cup  P,  and  a  solution  of  the  sulphate  of  potash  in  the 
cup  N.  The  sulphuric  acid,  attracted  by  the  positive  wire,  could  only  reach 
the  cup  P  by  passing  through  the  solution  of  ammonia.  With  a  battery  of  1 50 
pairs,  the  presence  of  the  acid  in  P  was  manifested  in  five  minutes  by  litmus 
paper.  In  half  an  hour,  the  solution  in  P  became  sour  to  the  taste,  and  pre- 
cipitated solution  of  nitrate  of  baryta.  Thus  the  sulphuric  acid  passed  through 
the  solution  of  ammonia  in  I  without  producing  upon  it  any  chemical  change. 
Solutions  of  lime,  potash,  and  soda,  were  successively  substituted,  with  similar 
results. 

Muriate  of  soda  and  nitrate  of  potash  were  successively  placed  in  the  cup  N, 
and  the  muriatic  and  nitric  acids  made  to  pass  through  concentrated  alkaline 
menstrua  in  1  without  any  chemical  effects. 

The  neutral  salts  of  lime,  potash,  soda,  ammonia,  and  magnesia  in  solution, 
were  successively  placed  in  the  cup  P,  distilled  water  in  N,  and  sulphuric,  ni- 
tric, and  muriatic  acids,  successively  in  the  intermediate  cup  I.  The  alkaline 
elements  of  the  salts  were  thus  drawn  through  the  acids,  and  decanted  into  N, 
|  without  undergoing  any  change  themselves,  or  causing  any  change  in  the 
acids. 

Strontia  and  baryta  passed  freely  by  a  similar  process  through  muriatic  and 
nitric  acids,  and  reciprocally  these  acids  passed  with  equal  facility  through  so- 
lutions of  strontia  and  baryta.  The  uniformity  of  this  series  of  phenomena 
was,  however,  broken  when  it  was  attempted  to  transmit  the  same  alkalies 
through  sulphuric  acid,  or  to  pass  sulphuric  acid  through  them.  A  new  order 
of  effects  was  here  evolved. 

A  solution  of  sulphate  of  potash  was  placed  in  the  cup  N,  distilled  water  in 
P,  and  a  solution  of  baryta  in  I.  The  sulphate  of  potash  was  decomposed  as 
before,  and  sulphuric  acid  passed  from  the  negative  cup  on  its  route  toward  the 
positive  wire  ;  but  its  progress  was  arrested  in  the  intermediate  cup,  where  it 
was  seized  by  the  baryta  and  precipitated.  It  appeared,  however,  that  this 
obstruction  to  the  progress  of  the  acid  was  not  absolutely  complete  ;  for  when 
the  process  was  continued  for  several  days,  traces  of  acid  were  found  in  the 
positive  cup.  When  a  solution  of  strontia  was  substituted  for  the  baryta  in  the 
intermediate  cup,  the  effects  were  similar. 

When  the  muriate  of  baryta  was  put  in  the  positive  cup,  sulphuric  acid  in  the 
intermediate  cup  I,  and  water  in  the  negative  cup  N,  no  alkali  passed  to  the 
cup  N,  all  being  arrested  in  I,  where  the  sulphate  of  baryta  was  manifest,  and 
muriatic  acid  remained  in  the  cup  P. 

It  appeared,  therefore,  that  the  exception  to  the  transmission  of  the  elements 
of  bodus  through  menstrua  for  which  they  have  an  affinity,  includes  the  cases 
in  whiJu  the  result  of  that  affinity  would  be  an  insoluble  compound.  The  sul- 
phates of  strontia  arid  baryta  are  insoluble  in  water  ;  and  sulphuric  acid  cannot 
be  transmitted,  by  the  electric  current,  through  strontia  or  baryta,  nor  the  latter 
through  the  former. 

The  operation  of  these  principles  was  very  beautifully  illustrated  by  the  fol- 
lowing experiment :  The  cups  P  and  N  were  filled  with  solution  of  muriate  of 
soda,  and  the  cup  I  with  solution  of  sulphate  of  silver.  The  cup  P  was  con- 
nected with  I  by  a  slip  of  wet  turmeric  paper,  and  the  cup  N  was  connected 
with  I  by  a  slip  of  wet  litmus  paper.  When  the  operation  of  the  battery  com- 


356 


GALVANISM. 


menced,  the  presence  of  soda  in  a  free  state  was  manifested  in  the  cup  N,  and 
muriatic  aud  in  the  cup  P.     The  muriatic  acid  drawn  from  the  cup  N,  through  | 
the  litmus  paper,  was  seen  to  form  a  dense  precipitate  in  the  cup  I,  and  the  t 
soda  passing  through  the  turmeric  paper  from  the  cup  P  was  observed  in  the 
cup  I,  forming  a  more  diffused  and  lighter  precipitate.     But  neither  the  acid  in 
passing  through  the  litmus  paper,  nor  the  alkali  in  passing  through  the  turmeric 
paper,  produced  any  change  in  the  color  of  these  tests. 

When  salts  having  metallic  oxides  as  bases  were  placed  in  the  cup  P,  acid 
solutions  being  put  in  I,  the  oxides  passed  through  the  acids  ;  but  their  prog- 
ress was  much  slower  than  that  of  the  alkalies.  When  a  solution  of  the  green 
sulphate  of  iron  was  placed  in  P,  and  muriatic  acid  in  I,  the  green  oxide  of 
iron  began  to  appear  in  about  ten  hours  on  the  amianthus  connecting  N  and  I  ; 
and  it  took  three  days  to  collect  any  considerable  quantity  of  it  in  the  cup  N. 
The  results  were  similar  when  solutions  of  sulphate  of  copper,  nitrate  of  lead, 
and  nitro-muriate  of  tin,  were  placed  in  the  cup  P. 

The  transmission  of  the  constituents  of  salts  through  solutions  of  the  neutral 
salts  was  next  tried,  and  the  results  were  what  was  anticipated.  Saline  solu- 
tions being  placed  in  N  and  I,  and  purified  water  in  P,  the  alkali  of  I  first 
began  to  pass  into  N  :  then  the  alkali  of  P,  after  passing  through  I,  reached 
N,  and  at  the  same  time  the  acid  of  I  passed  into  P.  Ultimately  the  two  acids 
were  collected  in  P,  and  the  two  alkalies  in  N.  As  an  example  of  this,  the 
cup  N  was  filled  with  a  solution  of  the  muriate  of  baryta,  the  cup  I  with  sul- 
phate of  potash,  and  the  cup  P  with  pure  water.  A  battery  of  150  pairs 
brought  sulphuric  acid  in  five  minutes,  and  muriatic  acid  in  two  hours,  into  P. 

When  the  cup  P  was  filled  with  a  solution  of  sulphate  of  potash,  I  with  mu- 
riate of  baryta,  and  N  with  distilled  water,  the  baryta  appeared  in  the  water  in 
a  few  minutes  ;  after  an  hour,  the  potash  became  sensible  in  it. 

When  the  muriate  of  baryta  was  in  P,  the  sulphate  of  potash  in  I,  and  water 
in  N,  the  potash  soon  appeared  in  the  water  ;  but  the  baryta  was  arrested  in 
the  intermediate  cup  by  the  sulphuric  acid,  and  sulphate  of  baryta  was  abun- 
dantly precipitated.  In  like  manner,  when  sulphate  of  silver  was  placed  in 
the  cup  I,  muriate  of  baryta  being  in  N,  and  water  in  P,  sulphuric  acid  alone 
passed  into  P,  and  a  precipitation  took  place  in  I. 

The  effects  of  the  electric  current  on  the  principles  of  vegetable  and  animal 
substances  was  next  tried.  The  fresh  stalk  of  a  polyanthus-leaf  was  used  in- 
stead of  the  siphon  of  amianthus,  to  connect  the  two  cups  P  and  N  (fig.  7),  the 

Fig.  7. 


cup  I  being  omitted.  The  cup  P  was  filled  with  a  solution  of  nitrate  of  stron- 
tia,  and  the  cup  N  with  purified  water.  The  water  soon  became  green,  and 
showed  the  presence  of  alkali  ;  and  the  solution  in  the  cup  P  indicated  the 
presence  of  free  nitric  acid.  After  ten  minutes,  the  alkaline  matter  in  N  being 


GALVANISM. 


357 


examined,  proved  to  be  potash  and  lime,  but  no  strontia  had  yet  arrived  in  the 
cup.     In  half  an  hour,  however,  strontia  appeared,  and  in  four  hours  was 
/  abundant. 

A  piece  of  the  muscular  flesh  of  beef  was  used  in  like  manner  as  a  siphon 
'  connecting  the  two  cups,  P  containing  a  solution  of  muriate  of  baryta,  and  N 
distilled  water.     Soda,  ammonia,  and  lime,  appeared  first  in  the  water,  an.l 
after  about  an  hour  and  a  quarter  the  baryta  began  to  arrive.     Muriatic  acid  \\  . 
abundantly  liberated  in  the  cup  P. 

It  is  nothing  more  than  a  general  expression  of  the  phenomena  which  have 
been  just  detailed  to  say,  that  hydrogen,  alkaline  matter,  metals,  and  certain 
metallic  oxides,  are  attracted  toward  the  negative,  and  repelled  from  the  posi- 
tive pole  of  a  Voltaic  apparatus  ;  and  that  oxygen  and  acid  substances  are 
affected  with  a  similar  attraction  and  repulsion  in  the  contrary  direction. 

As  to  the  actual  process  by  which  the  transfer  of  the  element  decomposed 
takes  place,  either  between  the  positive  and  negative  wires  in  the  solution  un- 
der decomposition,  or  through  the  intermediate  solution,  no  distinct  opinion  was 
expressed  in  the  paper  now  noticed.  Davy  showed  that  it  is  natural  to  sup- 
pose that  the  repellent  and  attractive  energies  are  conveyed  from  one  particle 
to  another  of  the  same  kind,  and  that  locomotion  (of  these  particles)  takes  place 
in  consequence.  He  considered  this  to  be  proved  by  many  facts.  Thus  when 
an  acid  was  drawn  from  the  negative  to  the  positive  cup  through  an  alkaline 
solution  contained  in  the  intermediate  cup,  if  the  Voltaic  action  was  for  a  mo- 
ment suspended  before  the  transfer  of  all  the  acid  in  the  negative  cup  had  been 
effected,  traces  of  acid  were  always  discoverable  in  the  intermediate  cup.  It 
appears  from  this  that  the  series  of  acid  molecules,  while  moving  between  the 
ends  of 'the  amianthus  siphons  in  the  intermediate  cup,  do  not  enter  into  com- 
bination with  the  alkali ;  but  if  the  motion  be  for  a  moment  suspended,  com- 
bination instantly  takes  place.  In  this  case,  therefore,  it  would  not  appear  that 
any  supposition  of  transmission  by  a  series  of  decompositions  and  recomposi- 
tions  is  compatible  with  the  phenomena. 

In  the  cases,  however,  of  the  decomposition  of  water  (where  the  whole  men- 
struum between  the  decomposing  wires  is  water),  and  of  solution  of  neutral 
salts  (where  also  the  menstruum  is  altogether  composed  of  the  same  solution), 
he  admits  that  there  may  possibly  be  a  succession  of  decompositions  and  re- 
compositions  throughout  the  fluid.  He  admits,  also,  that  the  impossibility  of 
transmitting  through  an  acid  or  alkali  any  element  which  forms  with  it  an  in- 
soluble compound,  although  the  transmission  is  perfect  when  the  compound  is 
soluble,  supports  the  hypothesis  of  a  succession  of  compositions  and  decompo- 
sitions taking  place  in  every  case.  He  maintains,  that  although  in  some  cases 
insoluble  substances  are  transmitted,  the  transmission  is  effected  in  a  manner 
totally  different  from  that  which  takes  place  in  the  more  general  case.  The 
insoluble  matter  was,  in  these  cases,  carried  over  mechanically,  either  through 
the  interstices  of  the  siphons,  or  by  means  of  "  a  thin  stratum  of  pure  water, 
where  the  solution  had  been  decomposed  at  the  surface  by  carbonic  acid." 

It  appears  from  the  tenor  of  the  observations  in  this  paper,  "  on  the  mode 
of  decomposition  and  transition,"  that  the  mind  of  the  author  had  not  yet  ar- 
rived at  any  opinion  satisfactory  to  himself  on  this  subject. 

By  the  experiments  of  Volta  it  had  been  shown  that  different  metals  brought 
into  contact  were  oppositely  electrified  after  separation.  Davy  found  that  an 
acid  and  a  metal  being  in  contact,  the  former  became  negative,  and  the  latter 
positive  ;  but  that  when  an  alkali  and  a  metal  were  in  contact,  the  electrical 
|  effects  were  reversed.  As  a  general  fact  it  appeared,  therefore,  that  positive 
electricity  has  a  tendency  to  pass  from  acids  to  metals,  and  from  metals  to  al- 
kalies, and  negative  electricity  to  flow  in  the  opposite  direction.  Diffeient 


i 


p 

358  GALVANISM. 

bodies  were,  therefore,  regarded  by  Davy  as  having  with  relation  to  each  other 
specific  electrical  energies.  Acids  have  a  negative  and  alkalies  a  positive  enei- 
gy,  with  relation  to  metals  ;  while  metals  have  a  positive  energy  with  relation 
to  acids,  and  a  negative  energy  with  relation  to  alkalies. 

Various  experiments  of  a  delicate  kind  were  made  to  establish  this  general 
principle.  To  avoid  the  disturbing  effects  which  would  be  introduced  by 
chemical  action,  the  substances  of  each  kind  selected  for  experimental  exami- 
nation were  in  the  solid  and  dry  form.  When  oxalic,  succinic,  benzoic,  or  bo- 
racic  acid,  perfectly  dry,  either  in  powder  or  crystals,  was  touched  upon  a 
large  surface  with  a  disk  of  copper,  zinc,  or  tin.  insulated,  the  metal  became 
positive,  and  the  acid  negative.  Phosphoric  acid  and  zinc  gave  a  like  result. 

Metallic  plates  being  brought  in  like  manner  in  contact  with  lime,  strontia, 
magnesia,  or  soda,  became  negative,  the  earths  being  positive.  The  attraction 
of  potash  for  water  was  too  strong  to  allow  that  alkali  to  be  submitted  to  trial. 
Sulphur  became  positive  after  contact  with  a  metallic  plate,  and  the  supposed 
exception  to  this  in  the  case  of  lead  was  removed  by  showing  that  the  sub- 
stance rubbed  against  newly  polished  lead  always  became  positive. 

All  these  facts  went  to  support  the  position,  that  the  electrical  relation  of 
different  substances,  as  shown  by  mere  contact,  was  in  harmony  with  the  law 
according  to  which  electricity  was  developed  in  the  Voltaic  apparatus,  and  with 
the  phenomena  of  decomposition.  To  complete  the  experimental  proof  of  this 
analogy,  it  would  have  been  necessary  to  show  that  oxygen  has  a  negative  and 
hydrogen  a  positive  electrical  energy  in  relation  to  the  metals.  Not  being  able 
to  accomplish  this,  recourse  was  had  to  the  compounds  of  these  substances. 
Sulphuretted  hydrogen  in  water,  used  in  the  Voltaic  arrangement  of  single 
metallic  plates,  plays  the  part  of  an  alkali.  To  support  by  a  like  analogy  the 
negative  character  of  oxygen,  he  showed  that  oxymuriatic  acid*  (chlorine) 
was  more  powerfully  negative  in  relation  to  metal  than  muriatic  acid,  even  in  a 
higher  degree  of  concentration. 

He  assumed  as  a  principle  suggested  by  analogy  and  supported  by  experi- 
ment, that  two  bodies  which  have  contrary  electrical  energies  in  relation  to  a  third 
body  have  contrary  electrical  energies  in  relation  to  each  other ;  that  is  to  say, 
two  bodies,  A  and  B,  being  successively  brought  into  contact  with  a  third  C  ; 
if  A  is  found  to  be  positive  after  separation  and  B  negative,  then  it  follows  that 
if  A  and  B  be  brought  into  mutual  contact,  A  will  be  positive  after  separation 
and  B  negative.  Lime  and  oxalic  acid  in  a  dry  and  solid  state,  the  former 
being  positive  and  the  latter  negative  in  relation  to  metals,  were  brought  into 
contact,  and  the  electricity  collected  after  repeated  contacts  by  a  condensing 
electrometer.  The  lime  was  found  to  be  positive  and  the  acid  negative. 

Guided  by  the  analogies  suggested  by  such  facts,  Davy  maintained,  as  a 
general  principle,  that  oxygen  and  acid  substances  have  a  negative  electrical 
energy  in  relation  to  hydrogen  and  alkaline  substances  ;  and  that  in  the  de- 
compositions and  changes  presented  by  the  effects  of  electricity,  the  different 
bodies  naturally  possessed  of  chemical  affinities  appear  to  be  incapable  of  en- 
tering into  combination  or  of  remaining  in  combination  by  virtue  of  these 
affinities  when  they  are  placed  in  a  state  of  electricity,  contrary  to  the  natural 
relation  of  their  electrical  energies.  Thus  the  acids  in  the  positive  part  of  the 
circuit  separate  themselves  from  the  alkalies,  oxygen  from  hydrogen,  and  so  on  ; 
and  metals  on  the  negative  side  do  not  unite  with  oxygen,  and  acids  do  not  re- 
main in  union  with  their  oxides  ;  and  in  this  way  the  attractive  and  repellant 
agencies  seem  to  be  communicated  from  the  metallic  surfaces  (the  poles  of  the 
ile)  throughout  the  whole  of  the  menstruum. 

*  This  substance  was  then  supposed  to  be  a  compound. 


r 


GALVANISM.  359 


In  all  cases  in  which  bodies  combine  chemically,  they  are  found  to  have 
contrary  electrical  energies.  Examples  are  numerous.  "The  bodies  in  the 
first  of  the  following  columns  are  all  negative  with  respect  to  those  which  are 
opposite  to  them  in  the  second :  — 


Oxygen  Zinc. 

Oxygen  Silver. 

Copper  Zinc. 


Gold  Mercury. 

Metals  Sulphur. 

Acids  Alkalies. 


The  constituent  particles  of  each  of  these  substances  when  brought  into 
contact,  being  naturally  in  opposite  states  of  electricity,  will,  according  to  thn 
common  laws  of  electricity,  attract  each  other.  If  they  be  solid  bodies,  the 
force  of  aggregation  of  these  particles,  which  constitutes  the  character  of  their 
solidity,  will  resist  their  separation ;  but  if  the  constituent  particles  be  free  to 
move  and  intermingle  among  each  other,  then  the  attraction  due  to  their  proper 
electricity  will  take  effect,  combination  will  ensue,  the  conditions  of  equilibri- 
um of  the  electrical  forces  will  be  satisfied,  and  all  signs  of  free  electricity 
will  cease. 

In  support  of  this  hypothesis  it  is  argued,  that  when,  by  artificial  means,  the 
elements  of  any  compound  are  invested  with  electricity  contrary  to  that  which 
naturally  belongs  to  them,  such  electricity  exerting  a  force  contrary  to  that 
which  produces  or  maintains,  or  tends  to  produce  or  maintain  their  combina- 
tion, that  combination,  if  it  exist,  is  dissolved,  and  if  it  tend  to  be  effected,  is 
prevented. 

Thus  zinc  is  one  of  the  metals  which  have  the  strongest  natural  tendency  to 
combine  with  oxygen.  Let  it  be  charged  with  negative  electricity,  and  its  ox- 
ydation  becomes  impossible,  because,  according  to  Davy's  hypothesis,  the  pos- 
itive electricity  naturally  belonging  to  its  molecules  is  neutralized  by  the  nega- 
tive electricity  artificially  imparted  to  it.  Again,  silver  is  one  of  the  metals 
which  have  the  least  tendency  to  unite  with  oxygen  ;  but  let  silver  be  charged 
with  positive  electricity,  and  it  oxydates  easily.  The  positive  electricity  sup- 
plied artificially  gives  increased  power  to  that  which  the  particles  possess,  so 
as  to  augment  their  attraction  for  the  negative  particles  of  the  oxygen. 

The  cases  of  bodies  which  have  contrary  electrical  energies,  either  in  rela- 
tion to  a  third  body  or  in  relation  to  each  other,  are  therefore  simple,  and  easily 
apprehended.  But  two  bodies  may  have  electrical  energies  with  respect  to  a 
third,  the  same  in  kind,  but  unequal  in  degree.  Thus  all  acids  are  negative  in 
relation  to  metals,  but  any  two  of  them  will  be  unequally  so  ;  and  in  like  man- 
ner all  alkalies  are  positive,  but  unequally  positive  in  relation  to  metals.  Sul- 
phuric acid  is  more  negative  than  muriatic  acid  in  relation  to  lead,  and  potash  is 
mure  positive  than  soda  in  relation  to  tin.  Such  bodies  compared  with  each 
other  may  have  the  same  or  contrary  electrical  energies,  or  they  may  be  neu- 
tral. Sulphur  and  the  alkalies  are  positive  in  relation  to  the  metals,  but  their 
electrical  energies  with  respect  to  each  other  are  contrary. 

The  evolution  of  heat  and  light,  which  commonly  attends  the  restoration  of 
electrical  equilibrium  between  two  bodies  strongly  charged  with  electricity  by 
artificial  means,  is  brought  by  Davy  in  further  support  of  his  theory.  It  is  well 
known  that  heat  and  light  also  result  from  intense  chemical  action.  When  the 
electric  current  passes  through  bodies,  the  electricity  being  then  incomparably 
more  feeble  in  intensity  than  that  which  proceeds  from  the  common  machine, 
heat  is  evolved  without  light,  and  the  degree  of  this  heat  is,  catcris  paribns, 
augmented  as  the  intensity  of  the  electricity  is  increased.  In  the  same  man- 
ner in  slow  chemical  combinations  there  is  an  increase  of  temperature  without  ! 
luminous  appearance. 

Heat,  by  producing  fusion,  and  liberating  the  constituent  particles  of  bodies 
from  their  natural  aggregation,  has  been  regarded  as  being  conducive  to  their  < 


360  GALVANISM. 


chemical  combination.  In  the  theory  proposed  by  Davy  it  is,  moreover,  viewed 
as  being  otherwise  instrumental  in  giving  play  to  the  affinities.  That  heat  is 
one  of  the  means  of  exalting  the  electrical  energy  of  bodies,  is  apparent  from 
its  known  effects  on  glass  and  tourmaline.  But  in  the  experiments  now  noticed, 
more  distinct  and  specific  evidence  is  adduced  of  its  direct  electric  agency. 
A  plate  of  sulphur  was  placed  on  an  insulated  plate  of  copper,  and  the  temper- 
ature of  the  bodies  being  gradually  elevated,  their  electrical  state  was  examined 
at  different  stages  of  the  experiment.  At  56°  the  electricity  was  scarcely 
sensible  to  a  condensing  electrometer;  at  100°  it  affected  the  gold  leaves 
without  the  condenser,  and  increased  in  a  still  higher  degree  as  the  sulphur 
approached  its  point  of  fusion. 

Since  heat,  therefore,  increases  the  natural  electrical  energy  of  the  com- 
ponent particles  of  bodies,  it  gives  them,  according  to  the  theory  of  Davy, 
an  increased  tendency  to  combine  chemically,  if  those  energies  be  con- 
trary. 

Hence,  when  a  spark,  or  other  sufficient  source  of  heat,  is  introduced  into  a 
mixture  of  oxygen  and  hydrogen,  it  renders  the  contiguous  molecules  of  oxy- 
gen more  strongly  negative,  and  those  of  hydrogen  more  strongly  positive.  In 
virtue  of  their  increased  mutual  attraction  they  combine,  and  in  combining  heat 
is  evolved,  which  affecting  other  contiguous  molecules  causes  further  combina- 
tion, and  so  on  until  the  combination  is  complete. 

According  to  this  hypothesis,  combination  should  be  rapid,  heat  and  light 
intense,  and  the  compound  neutral  in  its  properties,  whenever  the  electrical 
energies  of  the  two  constituents  are  strong  and  perfectly  equal.  But  when 
they  are  very  unequal,  the  effects  would  be  less  vivid,  and  the  compound  would 
have  the  acid  or  alkaline  character,  according  as  the  energy  of  the  negative  or 
positive  constituent  is  in  excess. 

The  production  of  water  from  the  combination  of  oxygen  and  hydrogen,  and 
the  formation  of  the  metallic  salts,  are  adduced  as  examples  of  strong  and 
equal  energies.  Like  examples  are  afforded  by  the  nitrate,  sulphate,  and  chlo- 
rate of  potash  and  muriate  of  lime,  which  severally,  when  touched  upon  a 
large  surface  by  plates  of  copper  and  zinc,  gave  no  electrical  signs.  Subcar- 
bonate  of  soda  and  borax,  on  the  contrary,  gave  a  slight  negative  charge,  and 
alum  and  superphosphate  of  lime  a  feeble  positive  charge. 

The  next  section  of  this  remarkable  paper  professes  to  explain  the  author's 
views  of  the  "  mode  of  action"  of  the  Voltaic  pile.  The  absence  of  that  per- 
spicuous style  of  expression  which  so  generally  characterizes  his  writings,  in 
this  case  justifies  the  supposition  that  his  own  perceptions  on  the  subject  of  the 
theory  he  proposes  were  not  at  the  time  very  clear  or  well  defined.  It  must 
be  recollected  that  Volta  maintained  that  the  source  of  electricity  in  the  pile 
was  the  contact  of  the  dissimilar  metals,  and  that  the  intervening  fluid  merely 
acted  the  part  of  a  conductor  to  carry  away,  in  a  continued  stream,  the  positive 
electricity  from  each  zinc  surface,  and  the  negative  electricity  from  each  cop- 
per surface.  Fabroni  and  Creve,  and  afterward  Wollaston  and  others,  main- 
tained that  the  source  of  tht  electricity  was  the  chemical  action  between  the 
zinc  and  the  fluid,  and  that  the  intervening  copper  acted  as  a  conductor  to  carry 
away,  in  a  continued  stream,  the  positive  electricity  from  one  side  of  the  fluid, 
and  the  negative  electricity  from  the  other.  Davy  professed  to  reconcile  these 
conflicting  hypotheses  by  admitting,  with  Volta,  that  the  opposite  currents  were 
propagated  from  the  surface  of  contact  of  the  zinc  and  copper ;  but  that  the 
liquid  separating  the  pairs  of  plates  did  not,  and  could  not,  carry  forward  the 
currents,  as  Volta  maintained,  by  their  conducting  power,  but  that  they  effected 
that  object  by  the  chemical  action  which  took  place  between  them  and  the  j 
zinc.  This  is  our  view  of  the  theory  proposed  by  Davy  in  the  paper  now  re-  / 

-X-v^-V/W 


• 

1 


GALVANISM.  361 


ferred  to  ;  but,  as  has  been  already  stated,  the  expressions  are  not  so  clear  as 
to  remove  all  doubt  of  his  exact  meaning. 

Davy  uses  the  term  "  electrical  energy"  apparently  to  express  the  same  phe- 
nomenon which  Volta  called  "  electro-motive  action,"  and  which  had  been  also 
called  "Voltaic  action."  This  term  denotes  the  quantity  of  electricity  evolved 
upon  the  two  metals  on  either  side  of  their  common  surface,  according  to  Vol- 
ia's  theory  of  contact.  The  act  of  conveying  forward  through  the  series  in 
each  direction  the  electricity,  positive  and  negative,  thus  propagated  at  the 
common  surface,  is  called  by  Davy  the  "  restoration  of  the  electrical  equilib- 
rium which  was  disturbed  by  the  electrical  energy  of  the  metals."  Strictly 
speaking,  there  is  no  restoration  whatever  of  electrical  equilibrium  during  the 
action  of  the  pile.  The  electric  fluids  are  never  in  a  state  of  repose.  Two 
currents  run  in  uninterrupted  streams  in  opposite  directions.  When  therefore 
Davy  says  that  "  the  chemical  changes"  produced  by  the  liquid  interposed  be- 
tween the  metallic  elements  of  the  pile  are  "the  causes  that  tend  to  restore 
the  equilibrium,"  he  must,  as  we  conceive,  be  understood  to  mean  that  these 
changes  are  "  the  causes  by  which  the  electric  currents  are  propagated  toward 
the  poles  of  the  pile." 

Having  premised  these  explanations,  let  us  now  consider  the  reasoning  and 
the  facts  on  which  this  theory  of  Davy  has  been  based.  He  denies  that  the 
liquid  elements  of  the  pile  can  act  as  an  ordinary  conductor  of  electricity>  the 
term  conductor  being  used  in  the  same  sense  as  when  applied  to  the  metals 
and  other  solid  conductors,  because,  with  regard  to  electricities  of  such  very 
low  intensity,  water  (as  well  as  liquids  in  general)  is  an  insulating  body.  Be- 
sides, there  is  every  reason  to  believe  that,  "  if  the  fluid  medium  were  a  sub- 
stance incapable  of  decomposition  (by  the  metallic  elements),  the  motion  of  the 
electricity  would  cease."  When  the  liquid  in  a  Voltaic  arrangement  of  zinc 
and  copper  is  a  solution  of  muriate  of  soda,  decomposition  ensues.  The  oxy- 
gen and  muriatic  acid  pass  through  the  fluid  from  the  copper  toward  the  zinc, 
transporting  or  transported  by  the  negative  current ;  and  the  hydrogen  and  soda 
pass  from  the  zinc  toward  the  copper,  transporting  or  transported  by  the  posi- 
tive current.  Whether  the  author  considered  that  the  transfer  of  the  electricity 
is  effected  by  the  locomotion  of  the  decomposed  elements  through  the  fluid,  or 
by  a  series  of  decompositions  and  recompositions,  in  which  there  is  no  motion 
of  translation  imparted  to  any  of  the  elements  resulting  from  the  decomposi- 
tion, and  in  which  the  electricities  themselves  are  not  transferred  through  the 
fluid,  but  rendered  alternately  free  and  latent  as  the  successive  decompositions 
and  recompositions  are  effected,  does  not  appear  from  the  developments  con- 
tained in  this  paper. 

A  pile  of  twenty-four  pairs,  in  which  the  connecting  fluid  was  water  free 
from  air,  had  no  Voltaic  power.  To  determine  whether  another  liquid  with 
superior  conducting  power,  but  still  incapable  of  chemical  action,  would  be  af- 
fected, concentrated  sulphuric  acid  was  tried.  No  permanent  current  was  pro- 
duced. Solutions  of  neutral  salts  render  the  pile  active  at  first ;  but  when,  by 
continued  decomposition,  the  solution  in  contact  with  the  zinc  becomes  acid, 
and  that  in  contact  with  the  copper  alkali,  the  action  ceases.  Dilute  acids 
>3ing  themselves  easily  decomposed,  and  promoting  the  decomposition  of  the 
water,  dissolving  the  oxide  of  zinc  as  fast  as  it  is  formed,  and  evolving  gases 
only  on  the  copper  side,  are  the  most  powerful  and  durable  fluid  elements  for 
a  pile.  All  these  facts  supply  converging  evidence  upon  the  position  that 
chemical  action  is  essential  to  the  vitality  of  the  Voltaic  apparatus. 

Against  the  hypothesis  that  chemical  change  is  the  primary  source  of  the 
action  of  the  pile,  it  is  contended  that  in  a  combination  of  zinc  and  copper 
plates  with  dilute  nitrous  acid,  the  side  of  the  zinc  exposed  to  the  acid  is  posi- 


362 


GALVANISM. 


tive  ;  but  in  a  Voltaic  combination  of  zinc  water  and  dilute  nitric  acid,  the  side 
of  the  zinc  exposed  to  the  acid  is  negative.  The  chemical  action  of  the  acid 
on  the  zinc  being  in  both  cases  the  feame,  it  is  argued  that  if  the  electric  cur- 
rents originated  at  the  common  surface  of  the  zinc  and  acid,  which  they  would 
do  if  chemical  change  were  their  primary  source,  the  direction  of  the  currents 
would  be  the  same,  instead  of  being  contrary  in  the  two  cases. 

As  a  further  argument  against  the  chemical  theory  of  the  pile,  Davy  main- 
tained that  in  mere  cases  of  chemical  change,  electricity  is  never  exhibited; 
and  endeavored  to  support  this  position  by  the  examples  of  iron  burned  in  oxy- 
gen, the  deflagration  of  nitre  and  charcoal,  the  combination  of  solid  potash  and 
sulphuric,  acid,  and  other  chemical  actions.  Subsequent  investigation,  how- 
ever, has  shown  that  this  principle  is  not  tenable,  and  that  chemical  change  is 
attended  with  the  evolution  of  electricity. 

With  Davy,  as  with  Franklin,  application  ever  trod  closely  on  the  heels  of 
discovery.  The  same  memoir  which  disclosed  the  brilliant  series  of  discov- 
eries of  which  we  have  here  attempted  to  give  a  brief  analysis,  also  indicated 
the  vast  applications  of  which  they  were  susceptible,  in  the  further  investiga- 
tions of  the  laws  of  nature,  and  in  arts  conducive  to  the  economy  of  life.  The 
detection  of  acid  and  alkaline  matter  in  mineral,  animal,  and  vegetable  sub- 
stances, and  their  separation  from  them,  was  sufficiently  obvious.  A  piece  of 
muscular  fibre,  through  which  the  electric  current  was  transmitted  for  five  days, 
was  rendered  dry  and  hard.  Potash,  soda,  ammonia,  lime,  and  oxide  of  iron, 
were  carried  from  it  by  the  negative  current ;  and  the  three  mineral  acids,  with 
phosphoric  acid,  passed  off  with  the  positive  current.  From  a  laurel  leaf  the 
negative  current  carried  green  coloring  matter,  resin,  alkali,  and  lime,  and  the 
positive  current  took  vegetable  prussic  acid.  Mint  gave  potash  and  lime  with 
the  negative,  and  an  acid  matter  with  the  positive  current.  The  flesh  of  the 
living  hand,  carefully  washed  in  pure  water,  gave  a  mixture  of  muriatic,  sul- 
phuric, and  phosphoric  acids  with  the  positive  current,  and  fixed  alkaline  mat- 
ter with  the  negative  current.  This  fact  accounts  for  the  acid  and  alkaline 
tastes  first  observed  by  Sulzer  given  by  metals  in  contact. 

By  converting  the  processes,  the  Voltaic  currents  may  be  made  the  means 
of  introducing  acids  and  alkaline  or  metallic  principles,  into  the  animal  and 
vegetable  system.  This  idea  has  since  been  realized  in  medical  practice  by 
some  physicians. 

In  the  experiments  hitherto  made,  the  acids  and  alkalies  themselves  were 
not  decomposed.  The  history  of  scientific  discovery  affords  no  more  remark- 
able example  of  that  instinctive  foresight  which  enables  the  philosopher  to 
suspect  the  direction  in  which  truth  lies,  and  prompts  him  in  the  selection  of 
subjects  of  inquiry,  than  is  apparent  in  comparing  Davy's  present  guesses  with 
the  result  of  his  subsequent  researches.  "  These  facts,"  says  he,  "  induce  us 
to  hope  that  this  new  mode  of  analysis  may  lead  to  the  discovery  of  the  true 
elements  of  bodies,  if  the  materials  acted  on  be  employed  in  a  certain  state  of 
concentration,  and  the  electricity  be  sufficiently  exalted.  For  if  chemical 
union  be  of  the  nature  which  I  have  ventured  to  suppose,  however  strong  the 
natural  electrical  energies  of  the  elements  of  bodies  may  be,  there  is  yet  every 
probability  of  a  limit  to  their  strength :  whereas  the  powers  of  our  artificial 
instruments  seem  capable  of  indefinite  increase." 

How  soon  he  led  the  way  toward  the  realization  of  this  magnificent  conjec- 
ture will  presently  appear. 

Sudden   and   violent  derangements  of  the  electrical  equilibrium  of  the  ele- 
ments of  our  system  are  manifested  in  other  cases  besides  the  glaring  intt 
offered  by  atmospheric  phenomena.     The  electrical  appearances  which  pre- 
cede and  attend  earthquakes  and  volcanic  eruptions  admit  of  easy  explanation 


GALVANISM.  353 


on  the  electro-chemical  theory.  The  slow  and  gradual  changes  observed  by 
the  geologist  are  indications  of  the  more  tranquil  and  incessant  operations  of 
electrical  agency.  Where  strata  of  pyrites  and  coalblende  occur  ;  where  the 
pure  metals  or  the  sulphurets  are  found  in  contact  with  each  other,  or  with  any 
conducting  substances  ;  and  where  different  strata  contain  different  saline  men- 
strua, electricity  must  be  evolved,  and  by  its  agency  mineral  formations  would 
probably  be  influenced  or  produced. 

These  views,  which  have  been  recently  confirmed  by  the  observations  of 
Mr.  Fox  on  the  electrical  condition  of  the  metallic  veins  in  Cornwall,  were  il- 
lustrated by  experiment.  A  mixed  solution  of  muriates  of  iron,  copper,  tin, 
and  cobalt,  was  placed  in  the  positive  cup  P,  and  distilled  water  in  the  nega- 
tive cup  N,  the  cups  being  connected  by  asbestos.  The  four  oxides  passed 
through  the  asbestos  to  the  cup  N ;  a  yellow  metallic  crust  was  formed  on  the 
negative  wire,  round  the  base  of  which  the  oxides  collected  in  a  mixed  state. 
In  another  experiment  the  carbonate  of  copper  was  diffused  in  minute  subdi- 
vision through  water,  and  a  negative  wire  placed  in  a  small  perforated  cube  of 
zeolite  in  the  liquid.  Green  crystals  collected  upon  the  cube  and  adhered  to 
it,  the  particles  being  incapable  of  penetrating  it.  By  the  multiplication  of 
such  instances,  Davy  conceived  that  the  electrical  power  of  decomposition  and 
transference  would  afford  a  satisfactory  explanation  of  some  of  the  principal 
facts  in  geology,  and  his  anticipations  have  since  been  to  a  considerable  extent 
realized  by  the  researches  of  Becquerel  and  others.  "  Natural  electricity," 
says  Davy  in  the  conclusion  of  this  memorable  paper,  "  has  hitherto  been  little 
investigated,  except  in  the  case  of  its  evident  and  powerful  concentration  in 
the  atmosphere.  Its  slow  and  silent  operations  in  every  part  of  the  surface 
will  probably  be  found  more  immediately  and  importantly  connected  with  the 
order  and  economy  of  nature  ;  and  investigation  on  this  subject  can  hardly  fail 
to  enlighten  our  philosophical  systems  of  the  earth,  and  may  possibly  place 
new  powers  within  our  reach."* 

His  theoretical  ideas  on  the  application  of  electrical  decomposition  to  the 
splution  of  the  phenomena  of  geology  were  seized  with  ardor  by  Guyton  Mor- 
veau.  That  eminent  chemist,  like  Davy,  endeavored  to  exhibit  on  a  small 
scale,  by  direct  experiments,  the  processes  which  are  continually  going  on  in 
the  crust  of  the  earth.  The  native  oxide  of  antimony  he  regarded  as  an  ex- 
ample of  slow  transition  from  the  state  of  a  sulphuret  to  that  of  a  pure  oxide, 
by  means  of  the  decomposition  of  water  by  subterranean  electricity.  By  care- 
ful examination  of  a  specimen  of  this  mineral,  he  observed  that  it  still  retained 
the  structure  of  the  crystallized  sulphuret  of  antimony,  and  even  preserved  par- 
tially its  metallic  lustre,  and  inferred  that  a  slow  Voltaic  action  had  changed 
its  composition  without  disturbing  the  arrangement  of  its  constituent  parts.  To 
support  those  ideas  suggested  to  him  in  Davy's  celebrated  paper  by  direct  experi- 
ment, he  submitted  a  piece  of  sulphuret  of  antimony  to  the  action  of  a  power- 
ful voltaic  apparatus.  An  odor  of  sulphuretted  hydrogen  was  soon  perceiva- 
ble ;  the  liquid  asumed  a  yellow  color,  and  the  sulphuret  appeared  of  a  darker 
tint,  and  iridescent,  indicating  incipient  decomposition.  The  negative  plato 
became  black;  and  the  positive  one  was  coated  with  a  light  yellow  incrusta- 
tion, which  proved  to  be  the  oxide  of  antimony.  Thus  it  appeared  that  the 
sulphuret  of  antimony  was  capable  of  being  transferred  immediately  into  the 
oxide  by  the  mere  operation  of  the  Voltaic  forces.  Other  native  sulphurets 
were  tried  in  like  manner,  and  gave  similar  results.f 

During  the  twelve  months  next  succeeding  the  date  of  the  memoir  above 
noticed,  Davy  devoted  his  labors,  and  directed  all  the  powers  of  his  genius,  to 
the  development  of  the  consequences  of  the  theoretical  principles  which  he 

*  Philosophical  Transactions,  1807.  t  Annales  de  Chimie,  torn,  liii.,  p.  113. 


GALVANISM. 


had  propounded,  and  to  the  realization  of  the  ideas  he  had  ventured  to  throw 
out  respecting  the  resolution  of  natural  substances,  before  regarded  as  simple, 
into  their  constituents.  Never  before  did  theory  more  surely  lead  to  discov- 
ery ;  never  was  the  prophetic  instinct  of  a  philosopher  more  speedily  or  more 
magnificently  satisfied.  His  foreknowledge  of  the  facts  to  be  disclosed  and 
the  instruments  for  their  disclosure,  of  the  end  to  be  attained  and  the  means 
of  attaining  it,  of  the  route  to  be  followed  and  the  goal  to  be  reached,  was  dis- 
tinctly expressed ;  and  with  the  confidence  inspired  by  clear  perceptions  and 
conscious  power,  he  immediately  advanced  in  the  course  he  described,  ar,J 
attained  the  end  he  foresaw.  The  resolution  of  the  alkalies  and  earths  into 
their  elements  was  the  splendid  result  of  his  labors  during  the  year  1807,  and 
was  consigned  to  the  Bakerian  lecture  read  before  the  Royal  Society  on  the 
19th  of  November  in  that  year. 

His  first  efforts  were  directed  to  potash,  which  was  submitted  in  a  state  of 
solution  to  the  electric  current.  The  water  only  was  decomposed,  the  alkali 
refusing  to  yield.  In  its  dry  state  it  would  not  transmit  the  current.  In  order 
to  give  it.  a  conducting  power,  and  at  the  same  time  exclude  water,  on  which 
by  preference  the  current  appeared  to  act,  the  alkali  was  now  placed  in  a  pla- 
tinum spoon,  and  exposed  to  the  flame  of  a  lamp  directed  upon  it  by  a  blast  of 
oxygen.  When  reduced  to  the  fluid  state  by  such  means,  the  potash  transmit- 
ted the  Voltaic  current.  When  the  metal  of  the  spoon  was  positive,  and  the 
point  of  a  platinum  wire  inserted  in  the  fluid  alkali  negative,  combustion  at- 
tended by  intense  splendor  was  exhibited  at  the  wire,  and  a  column  of  flame 
arose  from  the  point  of  contact  of  the  wire  with  the  alkali.  When  the  spoon 
was  negative,  and  the  wire  positive,  a  vivid  light  appeared  on  the  former ; 
aeriform  globules  rose  through  the  liquid  potash,  which  inflamed  as  soon  as 
they  escaped  into  the  air. 

It  was  conjectured  that  the  constituent  of  the  potash,  attracted  by  the  n^ Da- 
tive pole,  was  the  matter  which  in  these  cases  escaped  in  bubbles ;  and  that 
its  affinity  for  oxygen  was  so  strong,  that  the  moment  it  came  in  contact  with  the 
atmosphere  it  recombined  with  oxygen  and  produced  combustion.  The  question 
therefore,  now  was,  how  to  arrest  that  element,  and  submit  it  to  examination. 

As  the  liquefaction  of  the  alkali  by  heat  appeared  to  entail,  as  a  conse- 
quence the  immediate  recombination  of  its  separated  constituent,  it  was  nov 
attempted  to  give  the  necessary  conducting  power  to  the  potash,  by  allowing 
it  to  imbibe  from  the  atmosphere  as  much  moisture  as  would  give  a  conducting 
power  to  its  surface.  The  alkali  in  this  state  was  placed  on  a  platinum  disk, 
which  was  connected  with  the  negative  pole,  while  a  wire  connected  with  the 
positive  pole  was  applied  to  its  upper  surface.  At  the  upper  surface,  there  was 
a  disengagement  of  gas ;  at  the  lower  surface  small  metallic  globules  appeared, 
like  mercury,  in  their  visible  character.  Some  of  these  burnt  by  contact  wi?.h 
the  air.  Others  had  their  metallic  lustre  tarnished,  and  finally  covered  with  a 
white  film,  which  defended  them  from  the  atmosphere,  and  preserved  them  in 
their  metallic  state. 

The  gas  disengaged  at  the  positive  wire  was  oxygen,  and  the  metal  depos- 
ited was  the  base  of  the  alkali,  afterward  called  POTASSIUM. 

Soda,  when  submitted  to  a  like  process,  gave  a  similar  result,  and  the  metal 
educed  from  it  was  that  which  is  now  called  SODIUM. 

This  capital  discovery  was  made  in  October,  1807.  Potassium  was  dis- 
covered on  the  6th  of  that  month,  and  sodium  a  few  days  after. 

Sensitive  friends  of  the  great  British  chemist  have  been  moved  to  vindicate  J 
the  glory  of  this  discovery  from  those  who  would  tarnish  it  by  ascribing  to  the  j 
accidental  possession  of  the  laboratory  and  apparatus  of  the  Royal  Institution  j 
of  Great  Britain  a  share  in  producing  it  These  generous  survivors  may  tran-  j 


GALVANISM.  355 


quillize  their  fears.     Possibly  such  vindication  may  be  called  for  by  a  portion 
of  the  present  generation  having  pretensions  sufficient  to  raise  them  to  the 
.level  of  envy,  but  wanting  those  better  qualities  which  would  elevate  them  ] 
above  it.     Certainly  no  such  apology  will  be  needful  with  posterity. 

The  strong  affinities  of  these  new  metals  for  one  or  other  of  the  constituents  < 
of  almost  every  body  with  which  they  were  brought  in  contact,  and  of  every  i 
menstruum  or  atmosphere  with  which  they  could  be  surrounded,  was  very  em-  \ 
barrassing,  and  rendered  the  examination  of  their  physical  properties  extremely 
difficult.     It  was  found  most  convenient,  either  to  preserve  them  in  a  tube  pro- 
tected from  the  contact  of  the  air  above  recently  distilled  naphtha,  or  to  allow 
them  to  combine  with  mercury  so  as  to  form  an  amalgam,  and  in  that  state  to 
preserve  them,  separating  them  by  heat  when  the  pure  metal  was  required. 

The  analogy  suggested  by  the  decomposition  of  the  fixed  alkalies  naturally 
led  to  a  like  inquiry  with  respect  to  the  earths  which  enjoy  with  the  former 
common  properties,  and  those  which  seemed  most  analogous  to  the  alkalies. 
Baryta,  strontia,  lime,  and  magnesia,  were  tried  by  like  methods,  but  without 
any  satisfactory  result.  Being  slightly  moistened  at  their  surfaces,  they  were 
exposed  to  the  electric  current  transmitted  by  iron  wire  under  naphtha.  At 
the  negative  pole  they  assumed  a  darker  color,  and  small  particles  appeared 
there,  showing  metallic  lustre,  and  which  gradually  whitened  by  exposure  to 
air.  In  the  experiments  on  potassium  it  was  found  that  when  a  mixture  of 
potash  and  the  oxide  of  mercury,  tin,  or  lead,  was  exposed  to  the  Voltaic  cur- 
rent, decomposition  ensued,  and  an  amalgam  of  potassium  was  produced.  The 
same  method  was  accordingly  tried  with  the  alkaline  earths.  Mixtures  of 
these  substances  with  oxides  of  tin,  lead,  silver,  and  mercury,  were  exposed 
to  the  current.  In  these  cases,  a  small  quantity  of  a  substance  having  the 
whiteness  of  silver  was  deposited  at  the  negative  pole,  which  was  found  to  be 
an  amalgam.  Still  the  results  were  not  conclusive  or  satisfactory. 

The  labors  of  Davy  had  attained  this  point  when,  in  June,  1808,  he  re- 
ceived a  letter  from  M.  Berzilius,  informing  him  that,  assisted  by  Dr.  Pontin, 
that  chemist  had  succeeded  in  decomposing  baryta  and  lime,  by  exposing  them 
in  contact  with  mercury  to  the  current.     Davy  immediately  repeated  the  ex- 
J  periment,  and  obtained  the  amalgam  of  the  metallic  base  of  baryta  at  the  neg- 
)  ative  pole.    This  was  accomplished  by  a  battery  of  500  pairs,  weakly  charged, 
]  acting  on  a  surface  of  slightly  moistened  baryta  through  the  medium  of  a  glob- 
ule of  mercury.     The  mercury  gradually  became  less  fluid,  and,  after  a  few 
minutes,  was  found  covered  with  a  white  film  of  baryta ;  and  when  the  amal- 
gam was  thrown  into  water,  the  latter  was  decomposed,  hydrogen  was  dis- 
missed, mercury  precipitated,  and  a  solution  of  baryta  formed.     A  like  process 
gave  a  similar  result  with  lime. 

Having  thus  verified  the  results  obtained  by  Berzelius,  Davy  extended  the 
same  method  to  strontia  and  magnesia.  The  former  readily  yielded ;  the  lat- 
ter was  more  intractable.  By  continuing  the  process,  however,  for  a  longer 
time,  and  keeping  the  earth  continually  moist,  at  last  a  combination  of  the  basis 
with  mercury  was  obtained,  which  slowly  produced  magnesia  by  absorption  of 
oxygen  from  the  air,  or  by  decomposing  water. 

Thus  were  discovered  BARIUM,  STRONTIUM,  CALCIUM,  and  MAGNESIUM,  as 
an  immediate  consequence  of  the  first  great  step  made  in  this  course  of  investi- 
gation by  the  discovery  of  potassium  and  sodium. 

The  next  group  of  earths  brought  to  trial  consisted  of  alumina,  silica,  zirco- 
nia,  and  glucinia,  which  proved  more  refractory  than  any  of  the  former.     Driven 
in  search  of  other  methods  of  experimenting,  he  considered  minutely  their 
qualities  in  relation  to  other  bodies,  with  a  view  to  the  discovery  of  analogies  / 
by  which  his  researches  might  be  conducted.     From  the  absence  of  any  ten-  j 


clency  in  alumina  and  silica  to  yield  to  the  attraction  of  the  electric  current  in 
the  direction  of  either  pole,  he  inferred  the  probability  of  their  partaking  of  the 
.nature  of  nutro-saiine  substances,  and  attempted  their  decomposition  by  pro- 
cesses suggested  by  that  supposition.  Failing  in  these,  and  observing  that 
alumina  and  silica  have  both  a  strong  affinity  for  potash  and  soda,  and  consid- 
ering that  such  affinity  could  not  proceed  from  the  oxygen  which  might  be  one 
of  their  constituents,  he  inferred  that  it  must  be  a  quality  of  their  metallic  bases, 
and  that  it  would,  in  that  case,  be  probable  that,  if  mixed  with  soda  or  potash, 
and  exposed  to  the  electric  current,  the  base  might  be  made  to  separate,  and  to 
attach  itself  to  the  base  of  the  alkali.  A  mixture  of  silica  and  potash,  in  the 
proportion  of  one  to  six,  was  accordingly  put  in  a  platinum  crucible,  and  re- 
duced to  a  fluid  state  over  a  charcoal  fire.  The  crucible  was  put  in  connexion 
with  the  positive  pole  of  a  battery  of  five  hundred  pairs,  and  a  rod  of  platinum 
connected  with  the  negative  pole  was  brought  in  contact  with  the  alkaline 
menstruum.  The  moment  the  end  of  the  negative  rod  touched  the  liquid,  glob- 
ules rose  through  it  to  the  surface,  on  which  they  swam  about  in  a  state  of 
brilliant  combustion.  When  the  mixture  cooled,  the  platinum  bar  was  removed, 
and  the  alkali  and  salex  which  adhered  to  it  detached  ;  there  remained  upon  it 
brilliant  metallic  scales,  which,  immediately  on  exposure,  became  covered  with 
a  white  crust,  and  some  of  which  burnt  spontaneously.  Being  plunged  in  wa- 
ter, the  end  of  the  platinum  produced  effervescence,  and  an  alkaline  solution 
was  formed,  which,  upon  examination,  was  proved  to  contain  silica.  The  same 
process  applied  to  alumni  gave  a  like  result. 

It  was  now  determined  to  try  the  effect  of  the  Voltaic  current  upon  the  earths, 
in  contact  with  potassium  itself.  An  amalgam  of  potassium,  in  contact  with 
silica,  was  negatively  electrified  under  naphtha.  After  being  acted  on  for  an 
hour,  the  amalgam  was  made  to  decompose  water,  and  the  alkali  thus  obtained 
was  neutralized  by  acetous  acid.  A  white  precipitate  was  obtained  having  all 
the  characters  of  silica. 

The  same  process  was  applied,  with  the  same  results,  to  alumina,  glucinia, 
and  zirconia.  It  was  inferred,  therefore,  that  these  earths  were  oxides  of  met- 
als, to  which  respectively  the  names  of  SILICIUM,  ALUMINIUM,  GLUCINIUM,  and 
ZIRCONIUM,  were  given. 

Having  established,  by  direct  experiments,  the  fact  that  so  many  of  the  al- 
kaline and  earthy  substances  were  oxides  with  metallic  bases,  it  was  consistent 
with  sound  physical  logic  to  assume,  as  a  general  law,  that  "  the  alkalies  and 
earths  are  oxides  of  metals." 

The  question,  how  far  the  volatile  alkali,  ammonia,  was  to  be  regarded  in 
relation  to  such  a  law,  naturally  presented  itself.  Without  reference  to  this> 
analogy,  or  offering  any  hypothesis  to  explain  the  fact,  Seebeck  had  already 
shown  that  an  amalgam  could  be  obtained  by  the  action  of  ammonia  on  mercu- 
ry. This  fact  was  reproduced  by  Berzelius  and  Pontin,  and  communicated  by 
mem,  with  various  circumstances  attending  it,  to  Davy.  Berzelius  maintained 
that  ammonia  came  within  the  scope  of  the  general  law,  and  that  an  idea  which 
had  been  previously  thrown  out  by  Davy  was  justified  by  the  phenomena  which 
showed  that  ammonia  was  a  binary  metallic  base.  This  question  was  theft 
taken  up  by  Davy,  and  the  experiments  of  Berzelius  repeated,  but  without  ar 
riving  at  any  certain  or  clear  result.  Gay-Lussac  and  Thenard  opposed  the 
views  of  Davy  and  Berzelius ;  and  a  contest  arose,  for  which,  as  it  has  little 
connexion  with  the  progress  of  electrical  science,  we  shall  merely  refer  to  the 
scientific  periodical  works  in  which  it  was  carried  on.* 

It  has  been  already  observed,  that  the  character  of  Davy's  mind  was  to  pass 


AnnalesdeC'iimie,  torn.  Ixxii.,  p.  193.,  Ixxv.,  256-291.;  Biblioth.  Brit.,  June,  1809,  p.  122, 


GALVANISM.  367 


directly  from  discovery  to  application.  In  the  same  memoir  which  contained 
the  announcement  of  the  subjugation  of  the  alkalies  and  earths  by  the  powers 
of  the  pile,  is  found  his  brilliant  hypothesis  to  explain  the  phenomena  of  vol- 
canoes and  aerolites.  The  metallic  bases  of  the  alkalies  and  earths  cannot 
exist  at  the  surface  of  the  earth  in  their  simple  or  uncombined  form,  nor  even 
alloyed  with  the  more  perfect  metals,  because  of  the  intensity  of  their  affinity 
for  oxygen.  But  the  same  cause  does  not  prevent  their  existence  in  the  inte- 
rior parts  of  the  globe.  Let  the  possibility  of  the  existence  of  potassium,  so- 
dium, calcium,  or  any  other  metals  of  the  same  class  in  the  inferior  strata  of 
the  earth,  either  in  a  separate  state  or  in  combination  with  other  metallic  sub- 
stnnces,  be  admitted  ;  and  it  is  only  necessary  to  imagine  their  occasional  ex- 
posure to  the  action  of  air  or  water,  to  obtain  a  satisfactory  solution  for  volcanic 
eruptions.  These  highly  combustible  metallic  principles,  combining  with  ox- 
ygen, attended  by  violent  combustion,  are  ejected  from  the  bowels  of  the  earth, 
and  form  the  craters  of  volcanoes,  the  combination  being  an  earthy  matter  ex- 
hibited after  its  ejection  as  lava.  The  formation  of  aerolites  might  proceed 
from  the  same  causes,  their  luminous  appearance  and  detonation  being  produced 
by  the  combustion  attending  the  combination  of  the  metals  with  oxygen  as  they 
enter  the  atmosphere. 

With  a  view  to  test  the  validity  of  these  ingenious  hypotheses,  Davy  inves- 
tigated carefully  the  phenomena  of  active  volcanoes  ;  and,  not  finding  them  to 
be  in  sufficient  accordance  with  these,  he  relinquished  his  theory,  without  any 
of  that  regret  which  attends  the  failure  of  a  favorite  hypothesis,  when  the  dis- 
covery of  truth  is  an  object  secondary  to  the  attainment  of  personal  distinc- 
tion. 

The  powers  of  decomposition  and  transfer  by  Voltaic  electricity,  so  stri- 
kingly exhibited  in  the  researches  of  Davy,  directed  the  attention  of  physiolo- 
gists and  others  once  more  to  the  investigation  of  the  agency  of  electricity  in 
the  vegetable  and  animal  economy.  The  experiments  which  had  been  made 
to  show  that  the  alkaline  and  earthy  elements  found  in  organized  vegetable  sub- 
stances were  evolved,  by  the  process  of  vegetation,  from  air  and  water,  had 
always  been  inconclusive  and  unsatisfactory  ;  and  Davy's  experiments,  in 
which  it  was  shown  that  even  in  water  carefully  distilled  there  is  still  held  in 
solution  a  portion  of  saline  or  metallic  matter,  together  with  the  known  fact, 
that  air  almost  always  holds  in  mechanical  suspension  solid  matter  of  various 
kinds,  finally  overturned  such  hypotheses.  All  the  substances  developed  in  or- 
ganized nature  may  be  produced,  by  ordinary  processes,  from  combination  of 
known  constituents.  The  compounds  of  iron,  alkalies,  and  earthy  bodies  with 
mineral  acids,  abound  in  vegetable  soil.  The  decomposition  of  basaltic,  gran- 
iti^,  and  other  rocks,  affords  a  constant  supply  of  earthy,  alkaline,  and  ferru- 
ginous matter  to  the  superficial  part  of  the  earth.  In  the  seeds  of  all  plants  , 
which  have  been  examined,  nutro-saline  compounds,  containing  potash,  soda,  A 
or  iron,  have  been  found.  It  is  easy  to  imagine  that  these  principles  pass  from  ( 
vegetables  to  animals. 

The  same  analogies  suggested  to  Dr.  Wollasjon  the  idea,  that  something 
<  like  the  decomposing  and  transmitting  powers  of  the  pile  is  the  agent  to  which 
?  the  animal  secretions  are  due,  especially  as  the  existence  of  such  agency  in  a 

>  considerable  degree  of  intensity,  in  certain  animals,  was  proved  by  the  effects 

>  of  the  torpedo  and  Gymnotus  electricus ;  and  he  considered  that  the  universal 
prevalence  of  the  same  power,  lower  only  in  degree  in  other  animals,  was  ren- 

|  dered  highly  probable  by  the  extreme  suddenness  with  which  the  nervous  in- 
,  fluence  is  propagated  from  one  part  of  the  living  system  to  another.     Although 
'  the  electric  power  of  decomposition  and  transfer  has  been  experimentally  dem- 
onstrated only  in  cases  of  comparatively  high  intensity  of  action,  yet  analog)' 


368 


GALVANISM. 


countenanced  the  idea  that  very  feeble  electric  energies  would  produce  like 
effects  more  slowly,  in  proportion  to  their  weakness.  To  illustrate  this  by  im- 
mediate experiment,  he  tied  a  piece  of  clean  bladder  over  one  end  of  a  glass 
tube  three  quarters  of  an  inch  in  diameter,  and  two  inches  long,  and  filled  it 
with  water  holding  -3^  of  its  weight  of  salt  in  solution.  Placing  it  on  a  shil- 
ling, he  connected  the  silver  with  the  surface  of  the  water  by  a  wire  of  zinc, 
and  found  that  alkali  was  transmitted  through  the  bladder  to  the  silver  by  the 
attraction  of  the  negative  electricity.  Decisive  indications  of  this  were  ob- 
tained in  five  minutes.  The  efficacy  of  a  power  so  feeble  confirms  the  con- 
jecture that  similar  agents  may  be  instrumental  in  various  animal  secretions. 
The  blood,  which  is  alkaline,  supplies  the  bladder  with  matter  in  which  acid 
is  strongly  manifested  ;  while  an  excess  of  alkali,  above  that  contained  in  the 
blood,  is  manifested  in  bile.  These  effects  would  be  explained  by  admitting  a 
permanent  state  of  positive  electricity  in  the  kidneys,  and  negative  electricity 
in  the  liver.  The  coincidence  of  this  view  with  the  guesses  of  Napoleon,  al- 
ready mentioned,  is  curious  and  interesting.* 

The  last  great  discovery  of  Davy  directed  the  attention  of  the  philosophers 
of  the  continent  to  the  same  field  of  inquiry  :  and,  much  as  had  been  expected 
from  the  powers  of  the  pile  when  its  illustrious  inventor  expounded  its  nature 
and  properties  to  the  assembled  members  of  the  Institute  in  1801,  it  was  now, 
from  day  to  day,  rendered  more  evident  that  these  powers  were  inadequately 
estimated,  and  imperfectly  understood,  and  that  it  was  still  destined  to  enrich 
every  branch  of  physical  science  by  the  development  of  new  and  unlooked-for 
phenomena.  Napoleon,  in  the  magnificent  spirit  with  which  his  encouragement 
of  the  sciences  was  always  manifested,  had  presented  to  the  laboratory  of  the 
Polytechnic  School  a  Voltaic  apparatus  of  immense  magnitude  and  power. 
With  this  instrument  MM.  Gay-Lussac  and  Thenard  undertook  an  experimental 
investigation  of  the  powers  of  the  pile,  with  the  view  of  determining  more 
especially  the  influence  which  the  number  of  the  metallic  elements,  and  the 
nature  of  the  liquid  used  to  charge  the  pile,  have  on  its  chemical  action.  As- 
suming, as  a  modulus  of  the  chemical  energy  of  the  pile,  the  quantity  of  gas 
evolved  in  the  process  of  decomposition  in  a  given  time,  they  arrived  at  the 
following  conclusions:  1.  The  decomposing  energy  depends  conjointly  on 
the  conducting  power  of  the  liquid  under  decomposition,  and  on  the  nature  of 
that  which  is  used  to  charge  the  pile.  2.  It  is  greater  when  the  pile  is  charged 
with  a  mixture  of  acid  and  salt,  than  with  salt  alone.  3.  The  chemical 
effects  are  proportional  to  the  force  of  the  acids  by  which  it  is  put  in  action : 
and,  4.  They  do  not  augment  in  the  same  ratio  as  the  number  of  pairs  of  plates, 
but  very  nearly  in  the  ratio  of  the  cube  root  of  that  number. 

That  part  of  the  electro-chemical   theory  of  Davy  in  which  the  negative 
character  natural  to  certain  physical  elements,  and  the  positive  to  others,  is  as- 
sumed, was  implicitly,  if  not  expressly,  included  in  the  hypothesis  of  Grotthus. 
Without  such  a  supposition,  the  series  of  decompositions  and  recompositions 
imagined  by  that  philosopher  could  scarcely  be  admitted.     The  probable  con- 
nexion of  chemical  attractions  with  electric  forces  had  been  also  conjectured 
by  Hube  it  his  Traite  de  Physique,  and  Ritter  obscurely  expressed  some  ideas 
of  the    same    kind.     Immediately  before    the   commencement  of  Davy's  re- 
searches, Oersted,  since  so  celebrated  for  his  discoveries  in  electro-magnetism,  ( 
promulgated  a  theory,!  in  which  he  maintained  that  all  the  phenomena  of  chem-  j 
istry  might  be  regarded  as  the  result  of  two  general  forces  common  to  all  mat-  i 
ter,  and  that  the  same  forces  produced  those  effects  which  were  rendered  sen- 


*  See  Philosophical  Magaziue,  vol.  xxxiii.,  p.  1088. 
t  Recherchea  sur  1'Identite  des  Forces   Chimiqo 
1813. 


miques  et  Electriqucs.      Traduit  de  PAllemand. 


GALVANISM. 


sible  in  electric  attractions  and  repulsions.  This  work,  however,  was  exclu- 
sively of  a  speculative  kind,  unsupported  by  any  experiments  which  could  give 
force  or  validity  to  the  theory  it  proposed. 

The  electro-chemical  theory  of  Davy  was  the  first  which  had  ever  professed 
to  be  based  on  clear  and  wuii-ascertained  facts.  It  was  laid  down  as  a  funda- 
mental  principle  in  this  theory,  that  when  two  bodies,  the  particles  of  which 
are  in  opposite  electrical  states,  and  sufficiently  exalted  to  enable  their  electric 
attraction  to  overcome  the  force  of  aggregation  of  their  particles,  ar*  brought 
into  contact,  they  will  unite,  and  heat  and  light  will  be  developed  by  »n«  com- 
bination  of  the  two  electric  fluids.  When  the  combination  is  effected,  all  signs 
of  electricity  cease,  as  would  necessarily  ensue  from  the  union  of  the  two 
fluids,  but  by  what  power  the  aggregation  of  the  new  compound  was  main- 
tained  was  not  explained. 

Berzelius  and  Ampere,  who,  of  all  the  philosophers  of  the  continent,  evinced 
most  justice  and  candor  in  their  appreciation  of  Davy's  merit,  took  up  the 
electro-chemical  theory,  which  was  not  pursued  through  its  consequences  by 
its  author,  owing  probably  to  the  natural  disposition  of  his  mind  to  investigate 
new  facts  rather  than  discuss  the  merits  of  hypotheses.  Berzelius  assumed 
that  the  constituent  atoms  of  bodies  were  not  only  naturally  electrical,  as  Davy 
had  maintained,  but  that  they  possessed  electric  polarity,  and  that  the  intensi- 
fies of  their  poles  are  unequal.  He  investigated,  in  the  tirst  place,  the  two  > 
'  uestions,  How  electricity  exists  in  bodies?  and,  How  it  is  that  some  bodies 
are  naturally  negative,  and  others  sometimes  positive  and  sometimes  negative  ? 

A  body  never  becomes  electric,  without  manifesting  the  two  opposite  electric 
principles,  either  in  different  parts  of  it,  or  in  the  sphere  of  its  action ;  when 
the  two  electricities  appear  separately  in  a  continuous  body,  they  are  always 
found  on  opposite  sides.  The  tourmaline  and  some  other  crystals  offer  an  ex- 
ample of  this.  But,  since  the  parts  of  a  body  possess  the  same  properties  as 
the  body  itself,  it  is  necessary  to  admit  that  bodies  are  composed  of  atoms, 
each  of  which  has  an  electric  polarity,  and  its  poles  have  unequal  intensities. 
On  this  polarity  depend  the  chemical  phenomena,  and  its  unequal  intensity  is 
the  cause  of  the  different  force  exercised  by  their  affinities.  Bodies  are  ac- 
cordingly electro-positive  or  electro-negative  in  combining,  according  as  the 
influence  of  the  one  or  other  of  their  atomic  poles  predominates. 

The  degree  of  polarity  in  this  theory  is  influenced  by  the  temperature. 
Thus  many  substances  at  common  temperatures  manifest  but  feeble  electric 
polarity,  which,  at  a  red-heat,  show  a  very  strong  one. 

No  combination  can  be  effected  unless  thr  polarized  molecules  of  one  or 
both  of  the  combining  bodies  have  free  mobility  among  each  other,  each  being 
at  liberty  to  turn  on  its  own  centre  in  any  direction,  so  that  the  particles  may 
present  toward  each  other  their  contrary  poles  in  obedience  to  their  electric 
attraction.  This  condition  renders  it  necessary  that  one  or  both  of  the  com- 
bining bodies  be  in  the  fluid  state. 

The  vulnerable  point  of  this  theory  was  found  in  the  phenomena  of  aggre- 
gation. In  what  manner  can  the  electric  forces  which  it  assumes  produce  the 
hardness,  brittleness,  ductility,  and  tenacity,  of  different  species  of  solids,  the 
viscidity  of  liquids,  or  the  elasticity  of  gases  ? 

Berzelius  admits  that  these  effects  are  not  explicable  by  this  hypothesis. 
M.  Ampere  attempted  to  solve  this  question,*  by  assuming  that  the  atoms  of 
bodies  possessing  each  its  proper  electricity,  in  virtue  of  which  they  are  ur.ited 
in  combinations  in  the  same  manner  as  two  leaves  of  paper  oppositely  electri- 
fied adhere  to  each  other,  also  act  by  their  electricity  on  the  electricity  of  the 

'Journal  de  Physique,  1821. 


370 

medium  in  which  they  exist,  attracting  the  fluid  of  the  contrary  name,  and  re- 
pelling the  fluid  of  the  same  name.  The  atoms  are  therefore  considered  as 
strictly  analogous  to  the  Leyden  jar ;  the  internal  charge  representing  the 
natural  electricity  of  the  atom,  and  the  external  that  which  is  drawn  from  the 
surrounding  medium.  If  a  combination  is  formed  between  an  electro-positive 
and  an  electro-negative  body,  a  discharge  takes  place  ;  the  atoms  dismiss  their 
external  charge,  and  rush  into  union  in  virtue  of  the  reciprocal  attraction  of 
their  opposite  natural  electricities.  The  atmospheres  of  the  atoms,  as  well  as 
the  atoms  themselves,  are  combined ;  but,  as  the  atoms  cannot  emerge  from 
(  them,  their  electricities  act  on  those  of  their  atmospheres,  exerting  attractions 
and  repulsions,  so  as  to  produce  electrical  phenomena  the  reverse  of  those 
which  attended  their  combination. 

The  zinc  plates  of  a  Voltaic  apparatus,  being  subject  to  continual  oxydation, 
are  at  length  so  reduced  in  thickness,  as  to  render  it  necessary  to  replace 
them  by  new  ones.  This  gradual  wear  of  the  pile  by  use  rendered  it  desira- 
ble to  seek  for  means  of  constructing  a  pile  composed  of  solid  elements  only ; 
a  project,  however,  which  could  only  be  entertained  by  those  who  conceived 
that  chemical  action  was  merely  incidental,  and  not  essential,  to  the  develop- 
ment of  Voltaic  electricity.  Although  the  high  probability,  if  not  the  certainty, 
that  chemical  action  is  indispensable,  must  render  abortive  all  attempts  at  the 
discovery  of  a  dry  pile,  such  researches  have  nevertheless  been  attended  with 
some  advantage. 

The  term  dry  pile  was  intended  originally  to  express  a  Voltaic  pile,  of  which 
all  the  elements  were  solid ;  and  the  advantages  of  such  an  instrument,  if  it 
could  be  discovered,  were  so  apparent,  that  the  attention  of  electricians  was  di- 
rected to  it  at  an  early  period  in  the  history  of  Voltaic  discovery.  If  a  pile 
composed  of  solid  elements  (thought  they)  could  but  be  discovered,  neither 
evaporation  nor  chemical  action  could  take  place ;  the  electricity  due  to  the 
contact  of  heterogeneous  bodies,  according  to  Volta's  theory,  would  be  contin- 
ually evolved  ;  and  as  the  bodies  evolving  it  would  suffer  no  change,  the  quan- 
tity and  intensity  of  the  electricity  supplied  by  the  instrument  would  be  abso- 
lutely uniform  and  invariable.  In  1803,  MM.  Hachette  and  Desormes  substi- 
tuted starch  for  the  liquid  in  the  common  pile  ;  and,  in  1809,  De  Luc  invented 
a  pile  apparently  free  from  any  liquid  element.  This  apparatus  consisted  of  a 
column  formed  of  alternate  disks  of  zinc  and  paper  gilt  on  one  side,  the  gilt 
sides  of  the  paper  disks  being  all  turned  in  one  direction.  This  was  in  reality 
not  a  dry  pile ;  the  paper  imbibed  and  retained  moisture  enough  to  give  a  feeble 
activity  to  the  apparatus. 

De  Luc's  pile  was  improved  by  Zamboni  in  1812.  He  rejected  the  disks 
of  zinc,  and  composed  the  pile  of  disks  of  paper  only,  one  surface  being  tinned, 
and  the  other  coated  thinly  with  the  peroxide  of  manganese,  brushed  with  a 
mixture  of  flour  and  milk  ;  or  gilt  or  silver  paper  may  be  used,  the  metallic 
surface  being  wetted  with  a  saturated  solution  of  the  sulphate  of  zinc,  on 
which,  when  dry,  the  peroxide  of  manganese  in  powder,  may  be  spread. 
Several  leaves  of  paper  thus  prepared  are  placed  one  upon  the  other,  and  cut 
into  the  required  form  by  a  circular  cutter.  As  many  disks  are  thus  formed  by 
one  operation  as  there  are  leaves  of  paper  superposed ;  and  these  being  after- 
ward laid  one  upon  the  other,  the  pile  is  formed.  Thi's  pile  is  usually  placed  j 
in  a  hollow  cylinder,  of  the  same  internal  diameter.  The  paper  disks  are  forced  ) 
into  close  contact  by  pressure  produced  by  screws. 

Although,  by  the  aid  of  a  condenser,  the  electricity  evolved  in  these  piles 
may  be  rendered  sensible,  and  sparks  may  even  be  obtained,  the  power  is  in- 
comparably more  feeble  than  that  of  the  common  pile,  even  ia  its  most  ineffi- 
cient state.  It  is  found  that  by  increasing  beyond  a  certain  limit  the  number  j 


GALVANISM.  371 

of  disks  composing  these,  their  power  is  diminished.  Their  effects  have  been 
generally  limited  to  those  produced  on  the  condenser;  but,  by  diminishing  con- 
siderably the  number  of  disks,  M.  Pelletier  has  succeeded  in  decomposing 
water  by  these  instruments.  Their  action,  however,  ceases  after  the  lapse  of 
a  certain  period,  when  the  paper  has  lost  all  its  humidity. 

The  sources  of  the  disengagement  of  electricity  in  this  pile  are  various  and 
complicated.  Besides  what  may  arise  from  the  contact  of  heterogeneous  sub- 
stances, chemical  action  intervenes  in  several  ways.  The  organic  matter  acts 
upon  the  zinc  as  well  as  upon  the  peroxide  of  manganese,  reducing  the  latter 
to  a  lower  state  of  oxydation. 

Zamboni  examined  the  effects  produced  on  the  electricity  of  the  pile  by 
soaking  the  paper  to  which  the  tin  leaf  was  pasted  in  different  liquids,  and 
found  that,  according  as  the  state  of  the  other  side  of  the  paper  was  changed, 
the  poles  of  the  pile  were  thrown  to  different  ends.  If  the  paper  be  soaked 
in  oil,  the  poles  are  in  a  direction  contrary  to  that  which  they  assume  when  a 
coating  of  manganese  is  used.  On  the  other  hand,  when  the  paper  is  soaked 
in  honey,  in  an  alkaline  solution,  a  solution  of  the  sulphate  of  zinc,  or  half 
curdled  milk,  the  poles  have  the  same  position  as  when  they  arc  coated  with 
manganese. 

No  sensible  shock  is  received  from  a  pile  of  two  thousand  pairs,  although 
the  tension  at  the  poles  is  sufficient  to  produce  a  sensible  effect  on  the  proof 
plane,  and  a  condenser  applied  to  one  of  the  poles  will,  in  a  few  moments,  give 
sparks  an  inch  in  length,  and  a  Leyden  battery  may  receive  from  it  a  charge. 

The  conducting  power  of  the  vapor  suspended  in  the  atmosphere,  carrying 
away  a  portion  of  the  electricity  of  these  piles  from  their  poles,  produces  a  con- 
(  tinual  variation  in  the  tension  of  the  electricity  at  these  points. 

Zamboni  found  that  the  energy  of  the  pile  was  greater  in  summer  than  in 
winter,  whether  measured  by  the  tension  of  the  electricity  at  the  poles,  or  the 
rate  at  which  the  fluids  were  produced  and  propagated.  M.  Doune  compared 
the  tension  with  the  height  of  the  barometer,  but  could  discover  no  relation  be- 
tween them.  He  found  the  tension  the  same  in  a  vacuum  as  under  the  pressure 
of  the  atmosphere. 

It  is  known  that  electricity  may  be  developed  on  a  plate  of  a  single  metal, 
by  causing  one  surface  of  the  plate  to  be  acted  on  chemically,  in  a  degree  or 
manner  different  from  the  other  surface.     This  may  be  effected  by  merely  render- 
ing one  surface  smooth  and  the  other  rough.     This  expedient  is  said  to  have  been 
resorted  to  in  the  construction  of  a  Voltaic  battery  with  one  metal,  without  any 
liquid  element.     From  sixty  to  eighty  plates  of  zinc,  of  four  square  inches  of 
surface,  are  made  clean  and  polished  on  one  side,  the  other  remaining  rough  as 
it  comes  from  the  mould.     These  are  fixed  in  a  wooden  trough  parallel  to  each 
other,  their  polished  surfaces  all  turned  toward  the  same  end  of  the  trough,  and  ? 
with  an  open   space  between  the  successive  plates  of  from  the  tenth  to  the  | 
twentieth  part  of  an  inch.     These  intermediate  spaces  are  filled  by  thin  plates  < 
of  atmospheric  air.     If  one  extremity  of  this  apparatus  be  put  in  communica-  J 
tion  with  the  ground,  and  the  other  with  an  electroscope,  the  latter  will  receive  ( 
a  very  sensible  charge. 

We  can  regard  the  dry  pile  in  no  other  light  than  as  an  extended  \  oltaic  ( 
series.     The  moisture,  which  is  essential  to  its  activity,  is  in  the  condition  of   ( 
anything  but  freedom  of  motion  ;  so  that  the  renewal  of  contact  by  the  pres- 
ence of  fresh  particles,  which  seems  essential  in  all  developments  of  electrici- 
ty, exists  in  the  lowest  degree  ;  and  then  again  the  feeble  chemical  actions  ex- 
isting between  elements  under  circumstances  so  unfavorable,  all  conspire  in 
producing  the  small  quantity  of  electricity  for  which  these  instruments  are  re- 
)  markable  ;  while  the  great  length  of  series  produces  the  high  tension  of  the  ( 


372 


GALVANISM. 


poles.  It  is  only  recently  that  chemical  decomposition  has  been  obtained  by 
the  dry  pile.  Mr.  Gassiot  prepaired  10,000  Zamboni's  disks  ;  and  by  carefully 
directing  the  electricity  through  hydriodate  of  potassium  on  a  slip  of  glass,  he 
obtained  the  development  of  iodine  on  the  wire  connected  with  the  oxide  of 
manganese  end  of  the  series.  He  could  not  obtain  heating  effects  on  Harris's 
thermo-electroscope,  unless  he  allowed  the  charge  to  pass  in  sparks. 

The  only  uses  to  which  dry  piles  have  been  hitherto  applied  are — 1.  To 
produce  a  continued  motion,  by  an  electrical  pendulum  suspended  between  the 
contrary  poles  of  two  such  piles  placed  side  by  side,  so  that  the  positive  pole 
of  one  and  the  negative  pole  of  the  other  shall  be  at  the  summit.  This  motion 
will  be  continued  as  long  as  sufficient  moisture  is  retained  by  the  elements  of 
the  piles  to  sustain  their  activity ;  but  it  will  not  be  regular,  since  the  develop- 
ment of  electricity  will  be  affected  by  variable  atmospheric  causes.  2.  In  con- 
densing electrometers,  to  detect  the  presence  of  very  small  quantities  of  elec- 
tricity on  the  inferior  plate  of  the  condenser.* 

I  shall  conclude  this  notice  of  the  progressive  advancement  of  Voltaic  elec- 
tricity here.  The  phenomena  and  laws  whose  development  followed  the  ex- 
perimental researches  which  have  been  explained,  will  probably  be  noticed  on  a 
future  occasion,  when  I  shall  offer  a  view  of  the  actual  state  of  Voltaic  elec- 
tricity, its  relations  with  magnetism  and  heat. 

*  Becquerel,  TraitS  de  1'Electricitg.  torn,  i.,  p.  166. 


I 


THE  MOON  AID  THE  WEATHER, 


Ancient  Prognosticsof  Aristotle,  Theophrastug,  Aratns,  Theon,  Pliny,  Virgil. — Recent  Prediction*. — 
Theory  of  Lunar  Attraction  not  in  accordance  with  popular  Opinion. — Changes  of  Weather  com- 
pared with  Changes  of  the  Moon. — Prevalence  of  llain  compared  with  Lunar  Phases. — Direction 
of  the  Wind. — Height  of  Barometer  compared  with  Lunar  Phases. — Erroneous  Notions  of  Cyclei 
of  nineteen  and  nine  Years. — Cycle  of  four  and  eight  Years  mentioned  by  Pliny. 


- 


r 


THE  MOON  AND  THE  WEATHER. 


375 


THE  MOON  AND  THE  WEATHER, 


THE  physical  laws  which  govern  the  phenomena  of  our  atmosphere,  and  reg- 
ulate the  changes  of  the  weather,  have  always  been  a  favorite  topic  of  specu- 
lation. As  the  principles  of  astronomical  science  supplied  means  of  predicting-, 
with  the  highest  possible  degree  of  certainty  and  precision,  the  motions  and 
appearances  of  the  heavenly  bodies,  it  was  not  unnaturally  expected  that  at- 
mospherical phenomena  might  be  brought  under  equally  clear  and  certain  rules. 
The  connexion  of  the  lunar  motions  with  the  tides  was  apparent,  long  before 
the  mechanical  influence  by  which  the  moon  produced  the  rise  and  fall  of  the 
waters  of  the  ocean  was  explained  ;  and  this  gave  countenance,  at  a  very  early 
period,  to  the  idea  that  that  body  had  an  influence  on  the  atmosphere,  if  not  as 
certain  and  regular  as  on  the  waters,  still  sufficiently  so  to  furnish  probable 
grounds  for  conjecture  as  to  certain  periodical  changes. 

But  even  before  analogies  of  this  kind  could  have  furnished  much  ground  for 
reasoning,  and  when  the  heavenly  bodies  must  have  been  regarded  more  as 
signs  than  causes,  meteorological  phenomena  were  connected  with  them  by 
popular  observation.  The  influence  of  climate  on  all  the  interests  of  a  people 
in  a  pastoral,  and  subsequently  in  an  agricultural  state,  is  obvious  ;  and  accord- 
ingly we  find  weather  prognostics  coming  down  by  tradition  from  the  most  re- 
mote antiquity.  By  a  course,  however,  contrary  to  most  other  subjects  of  ob- 
servation and  inquiry,  this  was  corrupted  rather  than  improved  with  the  progress 
of  knowledge  and  civilization ;  and  what  was  once  a  mere  system  of  signs  of 
a  certain  present  state  of  the  atmosphere,  indicating  certain  approaching  changes, 
was,  by  the  craving  of  philosophy  after  the  relations  of  cause  and  effect,  con- 
verted into  the  most  absurd  system  of  rules,  having  no  foundation  in  nature, 
never  fulfilled  by  the  phenomena  except  fortuitously,  and  maintaining  their  as- 
cendency by  the  unbounded  credulity  of  mankind. 

In  the  writings  of  Aristotle,  and,  after  him,  in  those  of  Theophrastus,  Aratus, 
Theon,  and  others,  although  meteorology  is  treated  as  a  part  of  astronomy,  or 
astrology,  it  is  easy  to  trace  the  simple  views  of  the  more  ancient  and  less  phi- 


THE  MOON  AND  THE  WEATHER. 


losophical  observers,  and  to  perceive  that  the  appearances  referred  to  were  by 
them  regarded  merely  as  signs,  prognosticating  (whether  truly  or  not  we  shall 

\  see  presently)  approaching  changes,  and  not  at  all  as  physical  causes  effecting 

c  these  changes. 

;       We  shall  limit  ourselves  to  a  few  of  the  more  remarkable  and  generally  re- 

<  reived  ancient  meteorological  maxims,  as  examples  of  the  whole. 

f'  In  the  work  of  Aratus,  entitled  Aioff^fAS/a  (prognostics'),  and  in  the  Scho- 
lia of  Theon,  and  elsewhere,  the  appearances  of  the  moon  in  different  phases 
are  described  as  prognosticating  the  weather  for  a  certain  time  to  come  : — 


5'  oDr1  «£'  flrarfiv  sic'  ij/jiao'i  TTavra  rirvxrau. 

"AX.X'   Otfa    /JLSV    TPITOCTTJ  TETparaiTJ    T6 

ys  (xsv 

£X  <5yf 

5s  oi  aur 

M^voj  owroiyo/ASvou.  APAT 

Sin  ortu  quarto  (namque  is  certissimus  auctor), 
Pura,  neque  obtusis  per  crelum  cornibus  ibit, 
Totus  et  ille  dies,  et  qui  nascentur  ab  illo, 
Exactum  ad  mensem  pluvia  ventisque  carebunt. 

VIRGIL,  Georg.,  Lib.  I.,  1.  432. 

If  the  horns  of  the  lunar  crescent  on  the  third  day  after  new  moon  are  sharply 
and  clearly  defined,  the  weather  may  be.  expected  to  be  fair  during  the  ensuing 
month. 

Let  us  see  how  far  this  prognostic  will  stand  the  test  of  rational  examina- 

ion.  The  lunar  crescent  is  produced  by  a  peculiar  relation  of  position  which 
subsists  between  the  aspects  of  the  moon  presented  to  the  sun  and  earth.  If 
only  half  the  hemisphere  which  receives  the  sun's  light  be  presented  toward 
the  earth,  the  moon  is  exactly  halved  ;  if  a  quarter  of  the  hemisphere  be  turned 
to  the  earth,  the  moon  is  crescent,  and  its  age  is  then  nearly  four  days.  When 
its  age  is  less  than  two  days,  therefore,  less  that  one  eighth  of  its  illuminated 

lemisphere  is  presented  to  our  planet,  and  consequently  it  appears  a  very  thin 
crescent.  It  is  evident  that  these  effects,  if  seen  through  perfectly  transpa- 
rent space,  could  not  alter  with  circumstances,  and  that,  in  the  same  position 
of  the  moon  with  respect  to  the  earth  and  sun,  the  crescent  must  be  at  all  times 
equally  sharp  and  distinct.  But  when  the  moon  is  viewed  (as  it  is  by  us) 
through  an  atmosphere  that  is  from  thirty  to  forty  miles  high — that  atmosphere 

jeing  liable  to  be  more  or  less  loaded  with  imperfectly  transparent  vapors — it 
will  be  seen  with  more  or  less  distinctness,  according  to  the  varying  transpa- 
rency of  the  medium  through  which  it  is  viewed.  The  fact,  therefore,  of  the 
crescent  appearing  distinct  and  well  defined,  or  obscurely,  with  the  points  of 
the  horns  blunted,  is  merely  in  consequence  of  our  atmosphere  being  at  one 
time  more  pure,  clear,  and  transparent,  than  at  another. 

When  the  moon  is  under  three  days  old,  it  is  only  visible  for  a  short  time 
after  sunset,  and  therefore  the  phenomenon  in  question  can  only  be  observed  in 
the  evening,  a  little  above  the  western  horizon.  This  prognostic  of  Aratus 
may  be  thus  translated  :  "  When  the  atmosphere  above  the  western  horizon 

'  soon  after  sunset  on  the  third  day  of  the  moon  is  serene,  the  weather  will  be 

'  fair  for  the  remainder  of  the.  month  ;  but  if  it  be  loaded  with  vapors,  the  con- 

'  trary  event  will  ensue." 

All  the  world,  says  Arago,  will  doubtless  reject  the  prognostic  when  thus 
stated  ;  nevertheless,  the  words  only  in  which  it  is  expressed  are  changed,  the 
meaning  being  absolutely  the  same. 


But  what  shall  be  the  import  of  this  prognostic,  if  (as  must  frequently  hap- 
pen) the  horns  of  the  crescent,  during  the  same  evening,  be  at  one  time  well, 
and  at  another  ill  defined  ;  at  one  time  sharp  and  distinct,  at  another  time  blunl 
and  confused  ?  Are  we  then  to  infer  contradictory  propositions  ?  Shall  the 
prognostic  be  true  for  both  or  false  for  both  ?  Another  prognostic  of  Aratus  is, 
that  if  on  the  fourth  day  the  moon  project  no  shadow,  we  are  to  expect  bad 
weather  during  the  month. 

As  we  have  already  observed,  the  light  of  the  moon,  or  rather  the  light  of 
the  sun  reflected  from  the  moon,  must  in  reality  be  the  same,  and  would,  in  > 
fact,  always  appear  the  same  in  like  positions  to  an  eye  placed  beyond  the  ^ 
limits  of  our  atmosphere.     The  presence  or  absence  of  shadow  is  merely  an  ; 
indication  of  a  certain  intensity  of  light,  having  reference  to  the  sensibility  of  • 
the  human  eye.     That  the  moon  in  a  certain  phase  should  at  one  time  produce,  / 
and  at  another  time  not  produce  a  shadow,  is,  therefore,  merely  an  indication  that  < 
the  atmosphere  through  which  her  light  has  passed  is  at  one  time  more  trans- 
parent than  another.     Now  as  the  pure  atmosphere  has  always  the  same  de- 
gree of  transparency,  these  varying  effects  can  only  proceed  from  the  vapors 
which  are  mixed  with  it ;  and  thus,  as  before,  the  moon  in  this  case  is  only  a 
sign  of  a  certain  state  of  the  air  at  a  particular  time,  and  in  a  particular  direc- 
tion.    The  fourth  day  of  the  moon  is  selected,  because  on  that  day,  if  the  at- 
mosphere be  very  free  from  vapors,  the  light  of  the  crescent  is  just  sufficient 
to  produce  a  shadow ;  but  if  any  considerable  quantity  of  vapors  be  present  in 
the  atmosphere,  even  though  they  should  not  constitute  what  is  called  a  cloud, 
they  may  impair  its  transparency  so  much  as  to  deprive  the  faint  light  of  the 
lunar  crescent  of  the  power  of  producing  a  shadow.     Thus,  as  in  the  former 
case,  the  moon  is  here  used  as  a  meteorological  instrument  to  ascertain  the  hu- 
)  midity  of  the  air,  and  that  only  in  the  western  direction,  at  or  after  sunset ;  so 
|  that  when  translated  into  its  true  meteorological  language,  this  prognostic  is 
equivalent  to  that  to  which  we  have  just  adverted. 

Varro,  as  quoted  by  Pliny,  gives  the  following  meteorological  maxim  : — Nas- 
cens  Luna  si  cornua  superior  obatro  surget,  pluvias  decrescent  dal>it ;  si  infenore, 
ante  plenilunium ;  si  in  media  nigritia  illafuent,  imbrem  in  plena. 

"  If  the  new  moon  have  its  upper  horn  darkened,  the  declining  moon  will  be 
attended  with  rain ;  if  the  new  moon  have  its  inferior  horn  darkened,  there 
will  be  rain  before  the  full  moon  ;  and  if  the  middle  of  the  crescent  be  dark- 
ened, there  will  be  rain  at  the  full  moon." 

The  obscurity  here  mentioned  must,  like  those  already  alluded  to,  be  produced 
by  the  atmospheric  vapors,  rendering  the  medium  through  which  the  crescent  is 
beheld  imperfectly  transparent.  If  two  lines  be  conceived  to  be  drawn  from 
the  eye  of  the  observer  in  the  direction  of  the  points  of  the  horns,  and  an  inter- 
mediate line  toward  the  middle  of  the  crescent,  it  will  be  evident  that  these  lines 
will  diverge  from  one  another  very  slightly.  Now  the  obscurity  of  either  the 
upper  or  lower  horn,  or  of  the  middle,  the  other  parts  being  clear,  would  only 
indicate  the  presence  of  imperfectly  transparent  vapor  in  the  direction  of  one 
of  these  lines,  from  which  the  others  are  free.  To  what,  then,  will  this  prog- 
)  nostic  amount  ?  That  if  the  highest  of  these  lines  happen  to  encounter,  at  any 
point  of  the  space  which  it  traverses,  a  sufficient  quantity  of  vaporous  matter 
to  render  the  superior  horn  indistinct,  rain  may  be  expected  toward  the  de- 
cline of  the  moon  ;  if  a  like  portion  of  vapor  be  found  in  the  direction  of  the 
middle  Kne,  from  which  the  other  two  lines  are  free,  rain  may  be  expected  at 
the  full  of  the  moon  ;  and  if  the  obscure  vapor  be  in  the  direction  of  the  line 
to  the  lower  horn,  rain  may  be  expected  in  the  increase  of  the  moon  !  It  is 
presumed  that  the  absurdity  of  all  this  is  sufficiently  glaring,  but  it  will  be  ren- 
dered more  so  if  it  be  considered  that,  by  the  spectator  changing  his  position 


378  THE  MOON  AND  THE  WEATHER. 

through  a  distance  of  a  few  hundred  yards,  he  may  so  place  himself  that  the 
vapor  which  obscures  the  upper  horn  in  one  position,  will  obscure  the  middle 
in  another,  and  the  lower  horn  in  the  third.  What  then  becomes  of  the  pre- 
diction ?  Are  we  to  infer  that  the  same  little  portion  of  vapor  suspended  in  ) 
the  air  will  produce  rain  at  three  different  times  in  the  month,  at  three  places 
situated  a  short  distance  asunder  ? 

The  truth  is,  that  the  ancient  prognostics,  whether  derived  from  the  moon, 
from  the  sun,  or  from  the  stars,  were,  in  the  first  instance,  used  legitimately  as 
mere  indications  of  the  state  of  the  atmosphere  by  persons  too  simple-minded 
and  uneducated  to  trouble  themselves  much  with  the  philosophy  of  cause  and 
effect ;  but  when  these  appearances  came  into  the  hands  of  philosophers,  they 
were  at  once  elevated  to  the  rank  of  physical  causes,  and  their  dominion  ex- 
tended in  proportion  to  the  dignity  and  importance  thus  conferred  upon  them. 
Such  notions  were  in  keeping  with  a  philosophy  which  made  the  moon  the 
boundary  between  corruption,  change,  and  passiveness,  on  the  one  hand,  and 
the  active  powers  of  nature  on  the  other.  "  Thus,"  says  Horsley,  "  the  uncer- 
tain conclusions  of  an  ill-conducted  analogy,  and  false  metaphysics,  were  mix- 
ed with  a  few  simple  precepts,  derived  from  observation,  which  probably  made 
the  whole  of  the  science  of  the  prognostication  in  its  earliest  and  purest  state." 

Although  from  age  to  age,  the  particular  circumstances  and  appearances 
connected  with  the  moon,  by  which  the  atmospheric  vicissitudes  were  prog- 
nosticated, were  changed,  still  the  faith  of  mankind  in  general  in  her  influence 
on  the  weather  has  never  been  shaken  ;  and  even  the  present  day,  when 
knowledge  is  so  widely  diffused,  and  physical  science  brought,  as  it  were,  to 
the  doors  of  all  who  have  the  slightest  pretension  to  education,  this  belief  is 
almost  universal.  Many,  it  is  true,  may  discard  predictions  which  affect  to 
define,  from  day  to  day,  the  state  of  the  weather.  There  are  few,  however, 
who  do  not.  look  for  a  change  of  the  weather  with  a  change  of  the  moon.  It  is 
a  belief  nearly  universal,  that  the  epochs  of  a  new  and  full  moon  are  in  the 
great  majority  of  instances  attended  by  a  change  of  weather,  and  that  the  quar- 
ters, though  not  so  certain,  are  still  epochs  when  a  change  may  be  probably  ex- 
pected. Those  who  have  least  faith  in  the  meteorological  influence  of  the  moon, 
extend  their  belief  thus  far. 

There  are  two  ways  in  which  this  question  may  be  considered.  It  may  be 
asked  whether,  by  the  known  principles  of  physics,  the  moon  can  have  any, 
and  if  any,  what  influence  on  our  atmosphere  1  And  whether  that  influence  be 
such  as  would  cause  a  change  of  weather  at  the  epochs  of  the  principal  pha- 
ses ?  Or,  on  the  other  hand,  we  may  limit  the  inquiry  to  the  m-are  matter  of 
fact,  and  ask  whether,  by  immediate  observation,  it  has  been  found  that  the 
epochs  of  the  chief  lunar  phases  have  been,  in  the  majority  of  instances,  at- 
tended  by  changes  of  weather  ?  or,  to  put  the  question  more  generally,  wheth- 
er any  periodicity  of  atmospheric  phenomena  is  actually  observed  to  correspond 
with  the  moon's  phases. 

It  would  seem  at  first  view  that  neither  of  these  inquiries  could  be  attended 
with  any  doubt  or  difficulty ;  yet  the  case  is  quite  otherwise.  The  former,  in- 
volving as  it  does  the  whole  theory  of  the  moon's  attraction  on  our  atmosphere,  < 
modified  by  a  multitude  of  disturbing  causes,  is  a  physical  problem  as  difficult  i 
and  complicated  as  could  well  be  propounded.  Indeed,  it  is  one,  taken  in  its 
most  comprehensive  form,  which  does  not  admit  of  solution  in  the  present 
state  of  physical  science.  The  latter  being  merely  a  question  of  fact  and  ob- 
servation, is  not  attended,  properly  speaking,  with  ultimate  difficulty,  but  it  is 
one  which  would  require  a  course  of  observation  carefully  and  accurately  con- 
duct-ad, continued  for  a  series  of  years.  Such  observations  when  skilfully  ex- 
amined and  discussed,  would  furnish  grounds  for  safe  and  certain  conclusion. 


THE  MOON  AND  THE  WEATHEE.  379 

But  such  observations  have  not  been  carried  to  the  necessary  extent.  If  the  ques- 
tion of  fact  were,  whether  there  be  any  obvious  and  glaring  correspondence  of 
periodicity  between  the  lunar  phases  and  the  atmospheric  vicissitudes,  'it  would 
be  instantly  answered  in  the  negative.  For  although  we  do  not  possess  sufficient- 
ly accurate  and  long-continued  series  of  observations  to  decide  the  question  wheth- 
er the  moon  has  any  atmospheric  influence,  however  small,  we  possess  a  sufficient 
bcdy  of  ascertained  facts  to  justify  the  conclusion  that  her  influence  is  certain- 
ly not  considerable,  and  that,  whatever  be  its  amount,  it  is  probably  in  a  great 
degree  obliterated  by  the  vast  number  of  modifying  and  disturbing  causes 
which  are  constantly  in  action. 

Let  us  consider  for  a  moment  the  theoretical  question.  If  the  moon  can  act 
upon  our  atmosphere  by  attraction,  as  she  acts  upon  the  waters  of  the  ocean, 
she  will  produce  atmospheric  tides,  similar  to  those  of  the  waters.  The  great- 
er mobility  of  air  will  cause  those  tides  to  be  formed  more  rapidly  than  the 
water  tides ;  and  it  may  be,  perhaps,  assumed  that  the  tides  of  the  atmosphere 
will  always  be  placed,  either  exactly,  or  very  nearly  under  the  moon.  Thus, 
is  there  is  high  water  twice  daily,  so  would  there  be  high  air  twice  daily  ;  and 
'he  times  of  this  air  tide  would  correspond  with  the  moments  of  the  transit  of 
•lie  moon  over  the  meridian  above  and  below  the  horizon.  , 

The  same  causes,  also,  which  at  new  and  full  moon,  produce  spring  tides, 
wid  at  the  quarters,  neap  tides,  would  produce  spring  and  neap  atmospheric 
'.ides  at  the  same  epochs.  At  new  and  full  moon,  therefore,  the  air  ought  to 
be  higher,  daily,  at  noon  and  midnight  than  at  any  other  times  during  the 
month  ;  and,  on  the  other  hand,  at  the  quarters  it  ought  to  be  lower. 

If,  then,  the  barometer  be  observed  twice  daily,  viz.,  at  the  times  of 
the  moon's  transit  over  the  meridian,  above  and  below  the  horizon,  it  ought 
(so  far  as  it  will  be  affected  by  the  sun  and  moon)  to  be  the  highest  at  new 
and  full  moon,  and  lowest  at  the  quarters.  Now  as  the  rise  of  the  barometer 
generally  indicates  fair  weather,  and  its  fall  foul  weather,  the  conclusion  to 
which  this  would  lead,  would  be,  that  the  epochs  of  new  and  full  moon  should 
be  generally  fair,  while  at  the  quarters  bad  weather  would  generally  prevail. 
This,  however,  is  not  the  popular  opinion.  The  traditional  maxim  is  that  a 
change  may  be  looked  for  at  new  and  full  moon ;  that  is,  if  the  weather  be 
previously  fair,  it  will  become  foul ;  if  previously  foul,  fair. 

M.  Arago  has  made  an  ingenious  attempt  at  the  evaluation  of  the  very  mi- 
nute effect  of  what  we  have  called  atmospheric  tides.     To  comprehend  his  rea- 
soning it  will  only  be    necessary  to   consider  that,  at  a  new   and   full  moon, 
the  sun  and  moon  pass  the  meridian  above   and  below  the  horizon  together  ; 
and  therefore,  that  high  air,  or  atmospheric  tides,  must  at  these  times  take  place 
at  noon  and  midnight ;  low  air  would  therefore  occur  about  six,  A.  M.,  and  six. 
P.  M.     Thus  so  far  as  the  attraction  of  the  moon  affects  the  atmosphere,  the 
barometer,  which  rises  and  falls  as  the  atmosphere  rises  and  falls,  would  be 
affected  by  an  ascending  movement  for  six  hours  before  noon  and  midnight, 
I  and  for  six  hours  after  these  times.     But,  when   the   moon  is  in  the  quarters, 
)  being  then  one  fourth  of  the  heavens  removed,  before  or  behind  the  sun,  it  will 
J  pass  the  meridian,  whether  above  or  below  the  horizon,  about  six  hours  later 
1  or  earlier  than  the  sun.     At  the  quarters,  therefore,  the  atmospheric  tides  would 
J  occur  about  six,  A.  M.,  and  six,   P.  M.     Thus  at  the  quarters  the  barometric 
'  column,  so  far  as  it  is  influenced  by  the  moon's  attraction,  would  be  affected 
!  with  a  descending  motion  for  about  six  hours  after  these  times.     It  will  be  ev- 
|  ident,  that  if  we  were  in  a  condition  to  estimate  the  amount  of  these  baromet- 
!  ric  movements,   we  should  be  at  once  in  a  condition  to  declare  the  amount  of 
'  the  lunar  attraction  on  our  atmosphere. 

But  these  effects,  if  appreciable  at  all,  are  modified  by  at  least  one  other  in- 


THE  MOON  AND  THE  WEATHER. 


fluence,  which  has  been  the  subject  of  certain  and  satisfactory  observation. 
There  is  a  daily  fluctuation  in  the  barometric  column,  called  the  diurnal  vari- 
ation, which  has  an  obvious  relation  to  the  apparent  diurnal  motion  of  the  sun, 
and  which  probably  is  caused  by  solar  heat.  It  is  observed  that  the  baromet- 
ric column  falls  daily,  from  nine  in  the  morning  till  noon.  In  Europe,  this 
effect  is  frequently  obliterated  by  other  disturbing  causes  ;  but  ii  is  always  ob- 
servable when  a  mean  is  taken  of  observations,  continued  for  any  considerable 
number  of  days.  This  diurnal  variation  will  be  combined  with  the  effect  of  the 
lunar  attraction  in  the  results  of  the  observations.  Now  at  a  new  and  full 
moon  these  causes  produce  contrary  effects  on  the  barometric  column.  Du- 
ring the  three  hours  preceding  noon,  the  lunar  attraction  has  a  tendency  to  im- 
part to  it  an  ascending  movement ;  while,  by  reason  of  the  diurnal  variation,  it 
would  have  at  the  same  time  a  descending  movement ;  the  result  would  con- 
sequently be  the  difftrcn.ee  of  the  two  effects.  If  the  diurnal  variations  were 
equal  to  the  effects  of  the  moon's  attraction,  the  motions  would  neutralize  each 
other,  and  the  column  would  be  stationary  ;  but  if  they  be  unequal,  the  column 
will  ascend  or  descend  by  their  difference.  At  the  quarters  these  two  effects 
will  conspire  in  producing  a  descending  movement  of  the  barometric  col- 
umn during  those  hours  before  noon,  and  the  result  of  observation  will  be  a 
descent  equal  to  the  sum  of  the  two  effects. 

Observations,  therefore,  made  at  and  before  noon  at  the  times  of  new  and 
full  moon,  and  at  the  quarters,  ought  to  supply  estimates  of  the  sum  and  the 
difference  of  these  two  physical  effects  ;  and  if  such  observations  be  continued 
for  a  sufficient  length  of  time,  a  mean  estimate  may  be  obtained  from  which  the 
effects  of  disturbing  causes  will  be  eliminated.  M.  Arago  has  applied  this 
method  of  investigation  to  a  series  of  observations  conducted  for  twelve  years 
in  Paris,  and  he  has  found  that  the  effect  of  the  lunar  attraction  on  the  barom- 
eters produced  between  the  high  and  low  states  of  the  atmosphere,  correspond- 
ing to  high  and  low  water,  cannot  exceed  the  six  hundredth  part  of  an  inch — 
a  quantity  too  small  to  be  appreciated  by  any  meteorological  instruments,  and, 
certainly  such  as  could  produce  no  sensible  effect  on  the  atmosphere. 

It  is  evident,  then,  that  if  the  moon  has  any  influence  on  our  atmosphere, 
it  does  not  proceed  from  any  cause  analogous  to  that  which  produces  the  tides  of 
the  ocean  ;  and  therefore,  that  the  fact,  that  the  moon  does  produce  such  tides 
can  afford  no  countenance  to  her  imputed  meteorological  influence. 

But  it  may  be  said  that  although  the  moon  may  not  affect  the  atmosphere  by 
her  gravitation,  yet  she  may  influence  it  by  her  light,  or  by  electrical  or  mag- 
netical  emanations,  or,  in  fine,  by  some  occult  physical  causes  not  yet  discover- 
ed by  astronomers.  This  is  an  objection  that,  from  its  vagueness  and  indefi- 
niteness,  is  difficult  to  be  rebutted  by  any  means  which  theory  can  furnish.  It 
is  known  that  the  light  of  the  moon  concentrated  in  a  point  by  the  most  pow- 
erful burning  lenses,  is  incapable  of  producing  the  slightest  sensible  effect  on 
the  most  susceptible  thermometer,  neither  is  it  found  to  produce  any  effects 
of  an  electrical  or  magnetical  kind.  It  may  be  assumed  generally,  ihat 
the  effects  commonly  imputed  to  the  moon,  in  producing  change  of  weatlur  at 
her  principal  phases,  are  so  contradictory  that  it  is  impossible  to  imagine  any 
physical  causes  which  could  account  for  them.  If  the  new  and  full  moon  ami 
the  quarters  are  attended  by  changes  of  the  weather,  the  cause  producing 
this  effect,  under  the  same  circumstances,  has  incompatible  influences  :  it'  lair 
weather  precede  the  phase,  the  supposed  physical  cause  must  be  such  as  to  be 
capable  of  converting  it  into  foul  weather;  and  if  foul  weather  precede  the 
phase,  the  same  cause  must  convert  it  into  fair  weather.  It  will  be  admitted 
that  it  is  hard  to  imagine  any  physical  agent  whatever,  which,  under  precisely 
the  same  circumstances,  shall  produce  upon  the  same  body  effects  so  opposite. 


381 


But  let  us  dismiss  the  theoretical  view  of  the  question,  and  inquire  as  to  the 
facts.  Has  it  been  found,  as  a  matter  of  fact,  that  the  epochs  which  mark  the 
principal  phases  of  the  moon  have  been,  in  the  majority  of  cases,  attended  with 
a  change  of  weather?  Before  this  question  can  satisfactorily  be  answered,  it 
will  be  indispensable  that  the  meaning  of  the  phrase,  change  "of  weather,  be  dis- 
tinctly understood.  An  observer  who  is  predisposed  to  a  belief  in  the  influ- 
ence of  the  lunar  phases,  will  consider  himself  warranted  in  classing  as  a 
change  of  weather,  every  transition  from  a  calm  to  a  wind,  whether  feeble  or 
forcible — every  change  from  a  clear  and  serene  firmament  to  one  ever  so  little 
clouded — from  a  firmament  a  little  clouded  to  one  quite  covered  over.  He 
will  consider  the  change  from  a  day  absolutely  free  from  rain  to  one  in  which 
a  few  drops  may  chance  to  fall,  as  well  entitled  to  be  recorded  as  a  change  of 
weather  as  if  the  transition  had  been  from  a  day  absolutely  fair  to  one  of  in- 
cessant rain.  On  the  other  hand,  a  disbeliever  in  the  lunar  influences  will 
class  all  very  slight  changes  as  settled  weather,  and  will  only  register  as  chan- 
ges those  of  a  very  decisive  character.  These  are  difficulties  hard  to  remove, 
but  unless  they  be  removed  how  is  it  possible  to  compare  together,  with  any 
probability  of  arriving  at  the  truth,  the  records  of  different  observers  ?  What 
value  or  importance  are  we  to  attach  to  the  results  of  any  such  observations, 
unless  the  prejudices  of  the  observer  are  admitted  into  our  estimate  ? 

Toaldo  has  given  the  result  of  a  comparison  of  observations  continued  for 
forty-five  years  at  Padua,  in  which  changes  of  weather  are  recorded  in  juxta- 
position with  the  lunar  phases.  Without  detailing  the  particulars  of  these 
calculations,  we  may  state  at  once  the  following  results  of  them.  He  found 
that  for  every  seven  new  moons  the  weather  changed  at  six  and  was  settled  only 
at  one  ;  for  every  six  full  moons  the  weather  changed  at  five  and  was  settled 
at  one  ;  for  every  three  epochs  of  the  quarters  there  were  two  changes  of 
weather. 

He  also  examined  the  state  of  the  weather  in  reference  to  the  moon's  dis- 
tance from  the  earth,  which  is  subject  to  some  variation.  The  position 
of  the  moon  when  most  distant  from  the  earth  is  called  apogee,  and  her  posi- 
tion when  nearest  is  called  perigee.  He  found  that  of  every  six  passages  of 
the  moon  through  pe.rigee  there  were  five  changes  of  weather  ;  and  of  every  five 
through  apogee  there  were  four  changes  of  weather.  It  is  clear  that  if  those 
results  would  bear  the  test  of  rigid  examination,  they  would  be  decisive  in  fa- 
vor of  the  popular  notion  of  the  influence  of  the  lunar  phases.  But  let  us  see 
in  what  manner  Toaldo  conducted  his  inquiry. 

He  was  himself  an  avowed  believer  in  the  lunar  influence,  not  merely  upon 
the  atmosphere,  but  even  on  the  state  of  organized  matter.  In  his  memoir  he 
has  not  informed  us  what  atmospherical  changes  he  has  taken  as  changes 
of  weather ;  and  it  is  fair  to  presume  that  the  bias  of  his  mind  would  lead  him 
to  class  the  slightest  vicissitudes  under  this  head.  But,  further,  Toaldo,  in 
recording  the  changes  of  weather  coinciding  with  the  epochs  of  the  phases, 
did  not  confine  himself  to  changes  which  took  place  upon  the  particular  day 
of  the  phase.  On  the  pretext  that  time  must  be  allowed  for  the  physi- 
cal cause  to  produce  its  effect,  he  took  the  results-  of  several  days.  At  the 
new  and  full  moon  he  included  in  his  enumeration  all  changes  which  took 
place  two  or  three  days  before  or  two  or  three  days  after  the  day  of  new  or 
full  moon ;  while  for  the  quarters  he  only  included  the  day  preceding  and  the 
day  following  the  phases  ;  and  for  epochs  not  coincident  with  the  innar  pha- 
ses he  only  counted  the  changes  of  weather  which  took  place  on  the  particular 
day  in  question. 

It  appears,  then,  that  by  the  changes  coinciding  with  a  new  and  full  moon 
recorded  by  Toaldo  are  understood  any  changes  occurring  within  the  space  of 


382  THE  MOON  AND  THE  WEATHER. 

from  three  to  five  days ;  for  the  changes  recorded  at  the  quarters  are  to  be  un-  \> 
derstood  those  which  occurred  within  the  space  of  two  or  three  days ;  and  for  I 
those  not  coinciding  with  the  phases  the  changes  which  occurred  on  a  particu-  > 
lar  day.     It  will  not,  we  presume,  require  much  mathematical  sagacity  to  per-  ' 
ceive  that  the  results  of  such  an  inquiry  must  have  been  just  what  Toaldo  found 
them  to  be  ;  and  that  if  instead  of  taking  the  epochs  of  the  lunar  phases  he  had 
taken  any  other  periods  whatsoever,  and  tried  them  by  the  same  test,  he  would 
have  arrived  at  the  same  results.     Five  days  at  the  new  and  full  moon  would  \ 
include  rather  more  than  a  third  of  the  entire  lunar  month ;  and  thus  a  third  of 
all  the  changes  of  weather  which  occurred  in  that  period  were  ascribed  by  To- 
aldo to  the  lunar  influence  at  these  epochs. 

Professor  Pilgrim  has  examined  a  series  of  observations  on  the  lunar  phases  j 
as  connected  with  the  changes  of  weather,  made  at  Vienna,  and  continued  from  ( 
1763  to  1787 — a  period  of  25  years — and  he  has  found  that,  of  every  him-  ( 
dred  cases  of  the  phases,  the  proportion  of  the  occurrence  of  changes  to  that 
of  the  settled  state  of  the  weather  was  as  follows  : — 

Changes.         Settled  Weather. 

New  moon 58 42 

Full  moon 63 37 

Quarter 63 37 

Perigee 72 28 

Apogee 64 36 

New  moon  at  perigee 80 20 

New  moon  at  apogee 64 36 

Full  moon  at  perigee 81 19 

Full  moon  at  apogee 68 32 

Admitting  these  results,  it  would  follow,  contrary  to  popular  belief  and  to  the 
observations  of  Toaldo,  that  the  new  moon  is  the  least  active  of  the  phases  ;  and 
that  the  full  moon  and  quarters  are  equally  active ;  also  that  the  influence  of 
perigee,  or  the  nearest  position  of  the  moon,  is  greater  than  than  that  of  any  of 
the  phases,  while  the  influence  of  apogee,  or  its  greatest  distance,  is  equal  to 
that  of  the  quarters  and  full  moon,  and  greater  than  that  of  the  new  moon. 

But  Pilgrim's  calculations  are  liable  to  objections  similar  to  those  to  which 
Toaldo's  are  obnoxious.  Like  Toaldo,  he  included  in  his  enumerations  of 
changes,  corresponding  to  the  phases,  changes  which  occurred  the  days  pre- 
ceding and  following  the  phases :  this  being  the  case,  the  only  wonder  is  that 
the  proportion  which  he  has  found,  especially  for  the  new  moon,  is  not  more 
favorable  to  his  hypothesis.  But  independently  of  this,  Pilgrim's  results  are 
not  entitled  to  any  confidence  :  they  bear  internal  evidence  of  their  inaccuracy; 
and  besides,  the  observations  were  not  continued  for  a  sufficient  length  of  time 
to  give  a  safe  and  certain  conclusion. 

In  the  years  1774  and  1775,  Dr.  Horsley  directed  his  attention  to  the 
question,  and  published  two  papers  in  the  Phi/.osophical  Transactions  (to  which 
we  have  already  adverted),  with  a  view  to  dispel  the  popular  prejudice  on  the 
subject  of  lunar  influences.  Horsley's  observations,  however,  were  confined  to  so 
short  a  period  of  time  (two  years)  that  they  could  not  be  expected  to  afibrd  any 
satisfactory  results.  He.  found  that  in  the  year  1774  there  were  only  two 
changes  of  weather  which  corresponded  with  the  new  moon,  and  none  with 
the  full  moon ;  and  that  in  the  year  1775  there  were  only  four  changes  which 
corresponded  with  the  new  moon,  and  three  with  the  full  moori. 

Dismissing,  then,  this  popular  notion  of  the  correspondence  of  changes  of 
the  weather  with  the  lunar  phases,  let  us  consider  the  question  of  lunar  influ- 
ences in  a  more  general  point  of  view,  and  see  whether  observation  has  sup- 
plied any  ground  for  the  supposition  of  any  relation  of  periodicity  between  the 
moon  and  the  weather.  M.  Schubler  examined  this  question  with  considera- 


THE  MOON  AND  THE  WEATHER. 


ble  care  so  recently  as  1830,  and  published  the  results  of  his  observations, 
which,  shortly  after,  were  re-examined  by  M.  Arago. 

Schubler's  calculations  were  founded  on  meteorological  observations  made 
at  Munich,  Stutgard,  and  Augsburg,  for  twenty-eight  years.*  His  object  \\;is 
to  ascertain  whether  any  correspondence  existed  between  the  lunar  phases  and 
the  quantity  of  rain  which  fell  in  different  parts  of  the  month.  He  defined  a 
rainy  day  to  be  one  in  which  a  fall  of  rain  or  snow  was  recorded  in  the  mete- 
orological journals,  provided  it  affected  the  rain  gauge  to  an  extent  exceeding 
the  six  hundredth  part  of  an  inch.  The  following  are  the  results  of  his  obser- 
vations of  the  number  of  wet  days  which  occurred  in  each  quarter  ef  the 
month,  and  in  each  half  of  the  month. 


From  the  new  moon  to  the  first  quarter..  . 
Fron.  the  first  quarter  to  the  full  moon..  .  . 
From  the  full  moon  to  the  last  quarter.  .  .  . 
From  the  last  quarter  to  the  new  moon.  .  . 

Number  of  wet  Days. 

Wtihin 
20 
Years. 

From 
1809 
to 
1812. 

From 
'   1813 
to 
1816. 

From 
1817 
to 
1820. 

From 
1831 
to 
1824. 

From      ( 
18*5       ( 
to        ( 

1828.      ( 

764 
845 
761 
696 

132 
145 
124 
110 

142 
169 
145 
139 

145 
173 
162 
135 

179 
180 
166 
153 

166 
178 
164 
159 

1609 
1457 

277 
237 

311 

284 

318 
297 

359 
319 

344       $ 
323       I 

152 

43 

27 

21 

40 

21       j 

M.  Schiibler  also  calculated  the  number  of  rainy  days  which  happened  upon  ( 
the  days  of  the  principal  phases,  including  not  merely  days  of  new  and  full 
moons,  and  the  quarters,  but  also  the  days  of  the  octants  intermediate  between 
these.     The  following  table  includes  the  results  at  which  he  arrived  ;  first  for 
twenty  years'  observation  and  then  for  the  whole  period  of  twenty-eight  years.  He 
took  at  each  phase  the  mean  of  two  consecutive  days,  with  a  view  to  obliterate 
the  effect  of  disturbing  causes,  and  obtain  a  more  regular  series  of  numbers  :  — 

( 

> 

Number  of  wet  Days. 

During  20  Years.   |i  During  28  Yeara- 

On 

the 

Mean       1  1        On 
of  4                   U,« 
Daya.       1  1        Day. 

Mean 
oft 
Dayt. 

105 
113 

109     1!     i48 
1(        |!    148 

148 

119 
115 

117     ll     152 
117     ||     148 

150 

On  the  succeeding  day  

111 
113 

112     II     156 
II    151 

153 

124 
128 

126     li     164 
126     ||    167 

165 

116 
113 

115     II     m 
II     161 

161 



255 

On  the  succeeding  day  

125 
109 

117    ||   m 

On  the  succeeding  day  

92 
96 

II     130 
94     I]     140 

m 

,  On  the  succeeding  day  

100 

88 

94 

II     I38 
II     139 

133    > 

On  the  succeeding  day  

1 

I 

*  At  Munich,  from  1781  to  1788  inclusive;  at  Stutgard,  from  1809  to  1812  inclusive  ;  and  at  Autr.i- 
burg,  from  1813  to  1S28  iitclusive. 


These  tables  agree  in  indicating,  with  tolerable  clearness,  an  increase  of  the 
number  of  rainy  days  from  the  new  moon  to  the  second  octant,  that  is,  from 
the   day  of  the  new  moon  to  the  eleventh  day  of  the  moon's   age  ;   after-  i 
ward  there  is  a  gradual  decrease,  the  minimum  occurring  between   the  last 
quarter  and  the  fourth  octant. 

So  far  as  these  observations  may  be  relied  upon,  it  would  follow,  that  in  the 
places  where  they  were  made,  out  of  every  10,000  rainy  days  the  following  are 
the  number  of  those  days  which  would  happen  at  the  different  lunar  phases  :--  • 

New  moon 306 

First  octant 306 

First  quarter 325 

Second  octant 341 

Full  moon 337 

Third  octant 313 

Last  quarter 284 

Fourth  octant 290 

Now  as  there  are  twenty-nine  days  and  a  half  in  the  lunar  month,  if  we  sup- 
pose the  fall  of  rain  to  be  distributed  equally  through  every  part  of  the  month,  the 
total  number  of  these  10,000  days  which  should  happen  on  the  eight  days  of  t.he 
phases,  would  be  found  by  a  simple  proportion  ;  since  it  would  bear  to  1 0,000 
the  same  proportion  that  8  bears  to  29^  :  the  number  would  therefore  be  27.12. 
Whereas,  it  appears  from  the  above  table,  that  the  actual  number  which  fell 
upon  these  days  were  25.02  :  it  appears,  therefore,  that  less  than  the  propor- 
tional c mount  occurred  upon  them. 

Pilgrim   had  already,  in    1788,  attempted  to  ascertain  the  influence  of  the 
lunar  phases  on   the   fall   of  rain  ;  and  he  found  that  in  every  hundred  cases  i 
there  were  29  days  of  rain  on  the  full  moon,  26   at  the  new  moon,  and  25  at  t 
the  quarters. 

The  preceding  observations  refer  only  to  the  number  of  wet  days.  Schubler, 
however,  also  directed  his  inquiries  to  the  influence  of  the  lunar  phases,  on 
the  quantity  of  rain  and  on  the  clearness  of  the  atmosphere.  From  observa- 
tions continued  for  sixteen  years  at  Augsburg,  including  199  lunations,  he  ob- 
tained the  following  results  : — 


Epochs. 

Number  of  clear  days 
in  16  years. 

Number  of  overcast  days 
in  16  years. 

Quantity  of  rain  in  16 
years  in  inches. 

New  moon  

31 

61 

26-551 

38 

57 

24-597 

Second  octant  

25 

65 

26-728 

26 

61 

24-686 

Last  quarter.  .  , 

41 

53 

19-536 

In  this  table,  by  a  clear  day,  is  such  days  as  exhibited  a  cloudless  sky  at 
s-even  in  the  morning,  and  at  two  and  nine  o'clock  in  the  afternoon ;  those  that 
were  not  clear  at  these  hours,  were  counted  as  cioudy  days.  These  results 
are  in  accordance  with  the  former.  It  appears  that  the  number  of  clear  days 
is  more  frequent  in  the  last  quarter,  which  is  an  epoch  at  which,  by  the  former 
method  of  inquiry,  the  number  of  rainy  days  was  least ;  also  the  number  of 
cloudy  days  is  greatest  at  the  second  octant,  which  is  a  period  at  which  the 
number  of  rainy  days  were  found  to  be  greatest ;  also  the  depth  of  rain  agrees 
with  this,  being  the  greatest  about  the  second  octant,  and  least  at  the  last  quar- 
ter. Schubler  extended  his  inquiries  to  the  influence  of  the  moon's  distance 
on  rain  ;  and  he  found  that,  on  examining  371  passages  of  the  moon  through 
the  positions  of  her  extreme  limits  of  distance,  during  the  seven  days  nearest 


THE  MOON  AND  THE  WEATHER. 


3S  5 


to  perigee  it  rained  1,169  times;  and  during  the  seven  days  nearest  apogee  it 
rained  1,096  times.  Thus,  cateris  paribus,  the  nearer  is  the  moon  to  the  earth 
the  greater  would  be  the  chances  for  rain. 

From  observations  of  Pilgrim  at  Vienna  (which,  however,  are  much  less  to 
be  depended  on),  it  appears  that  the  proportion  of  the  prevalence  of  rain  be- 
tween perigee  and  apogee  is  that  of  nine  to  five — an  improbable  result. 

From  all  that  has  been  stated,  it  can  scarcely  be  denied  that  there  exists 
some  permanent  and  regular  correspondence  between  the  prevalence  of  rain 
and  the  phases  of  the  moon.  What  that  exact  correspondence  is,  remains  for 
more  extended  and  accurate  observations  to  inform  us  ;  meanwhile,  that  rain 
falls  more  frequently  about  four  days  before  full  moon,  and  less  frequently  about 
four  or  five  days  before  new  moon  than  at  other  parts  of  the  month,  seems  to 
be  a  conclusion  attended,  to  say  the  least  with  some  degree  of  probability. 

Schubler  also  examined  the  question  of  a  correspondence  between  the  di- 
rection of  the  wind  and  the  lunar  phases,  and  found  that  winds  from  the  south 
and  southwest,  became  more  and  more  frequent  at  those  periods  of  the  month 
at  which  rain  was  also  observed  to  increase,  and  that  such  winds  were  more 
and  more  rare,  while  winds  in  the  contrary  direction  occurred  oftener  toward 
those  epochs  of  the  month  when  least  rain  was  observed  to  prevail.  These 
results,  it  will  be  seen,  are  quite  in  accordance,  and  the  question  respecting 
the  mode  of  action  by  which  the  periods  of  rain  are  produced,  would  be  re-  < 
duced  to  the  question  t»f  the  physical  action  by  which  the  moon  affects  the 
currents  of  the  atmosphere. 

The  connexion  of  barometric  indications  with  atmospheric  phenomena  is  so 
obvious,  that  the  inquiry  as  to  a  correspondence  between  the  lunar  phases  and 
the  variations  of  the  barometer,  could  scarcely  escape  the  attention  of  meteo- 
rologists. M.  Flaugergues  accordingly  made  a  series  of  observations  at  Viviers 
(in  the  department  of  Ardeche),  in  France,  which  were  continued  from  1808 
to  1828,  a  period  of  twenty  years,  on  the  heights  of  the  barometer  in  relation 
to  the  lunar  phases  :  that  the  influence  of  the  sun  might  be  always  the  same, 
the  observations  were  made  at  noon,  and  the  heights  of  the  barometer  were 
reduced  to  what  they  would  be  at  the  temperature  of  melting  ice.  The  fol- 
lowing are  the  mean  heights  of  the  barometer,  deduced  from  these  observa- 
tions : — 

New  moon 29*743 

First  octant 29-761 

First  quarter 29-740 

Second  octant 29-716 

Full  moon 29-736 

Third  octant 29-751 

Last  quarter 29-772 

Fourth  octant 29-744 

Hence  it  appears  that  the  height  of  the  barometer  is  least  about  four  days 
before  full  moon,  and  greatest  six  or  seven  before  new  moon.  Now  these  are 
about  the  times  at  which  the  investigations  of  Schubler  give  the  greatest  and 
least  quantity  of  rain  :  and,  since  the  fall  of  the  barometer  generally  indicates 
i  a  tendency  to  rain,  these  results  are  in  accordance.  Although  it  must  be  ad- 
'  milted  that  the  variation  of  the  barometer  is  in  this  case  so  minute,  that  a  sen- 
sible effect  could  hardly  be  expected  from  it,  still,  though  minute,  it  is  quite 
listinct  and  decided. 

M.  Flaugergues  also  observed  the  mean  height  of  the  barometer  when  the 
t  noon  was  at  her  greatest  and  least  distance  from  the  earth,  and  found  that  at 
perigee  it  was  29-713,  and  at  apogee  29-753. 

So,  far,  therefore,  as  this  small  difference  can  be  supposed  to  indicate  any- 

35 


thing,  it  would  indicate  a  prevalence  to  rain  at  perigee  and  at  apogee,  which  is 
in  accordance  with  the  observations  of  Schubler. 

"  In  spite,  therefore,"  says  M.  Arago, "  of  the  distance  which  separates  Stutgard 
from  Viviers,  and  ia  spite  of  the  different  methods  pursued,  and  the  difference 
of  instruments  used,  MM.  Flaugergues  and  Schubler  have  arrived  at  analogous 
results."  It  seems  very  difficult,  therefore,  at  present,  not  to  admit  that  the 
moon  exercises  upon  our  atmosphere  an  action  very  small,  it  is  true,  but  which 
is  nevertheless  appreciable  even  with  the  instruments  which  meteorologists 
commonly  use. 

We  have  shown  that  the  theory  of  the  moon's  attraction,  applied  to  explain 
atmospheric  tides  similar  to  those  of  the  ocean,  would  lead  to  the  conclusion 
that  the  height  of  the  barometer  observed  at  noon,  when  the  moon  is  in  her 
quarters,  would  be  less  than  its  height  at  noon  at  new  and  full  moon.  Obser- 
vation, however,  shows  the  very  reverse  as  a  matter  of  fact.  The  observation 
of  M.  Flaugergues  gives  the  mean  height  at  the  barometer  quadratures  29-756, 
and  at  new  and  full  moon  29- 739 ;  the  height  quadratures  being  in  excess  to 
the  amount  of  0'017.  This  result  has  been  further  confirmed  by  the  more  recent 
observations  of  M.  Bouvard,  at  the  Paris  observatory :  he  has  found  the  mean 
height  of  the  barometer  at  the  quarters  29-786,  and  at  new  and  full  moon 
29-759  ;  the  excess  at  the  quarters  being  0*027. 

Although,  therefore,  it  cannot  be  denied  that  there  exists  a  relation  between 
the  barometric  column  and  the  lunar  phases,  yet  it  is  not  the  relation  which 
the  theory  of  atmospheric  tides  would  indicate ;  and  by  whatever  physical  in- 
fluence the  effect  may  be  produced,  it  is  certainly  not  the  gravitation  of  the 
moon  affecting  our  atmosphere  in  a  manner  analogous  to  that  by  which  she  af- 
fects the  waters  of  the  ocean.  Any  physical  effects  which  depend  on  the  rel- 
ative positions  of  the  sun  and  moon,  as  seen  from  the  earth,  would  necessarily 
occur  in  the  same  order  throughout  the  year,  when  these  two  luminaries  them- 
selves have  corresponding  positions  in  the  heavens  on  the  same  days  of  the 
year.  At  a  very  early  period  in  the  history  of  astronomical  discovery,  it  was 
known  that,  after  the  lapse  of  nineteen  years,  the  sun  and  moon  assume  on  suc- 
cessive days  of  the  year  relative  positions. 

Thus,  for  example,  if  the  moon  were  90°  behind  the  sun  on  a  certain 
day  of  a  certain  month  in  the  year  1800,  it  would  be  90°  behind  the  sun  on 
the  same  day  of  the  same  month  in  the  year  1819,  and  again  in  the  year  1838, 
and  so  on  ;  but  on  the  same  day  of  the  same  month  in  any  intermediate  year 
it  would  have  a  different  relative  position  with  respect  to  the  sun.  This  cycle 
of  nineteen  years  was  known  to  the  Greeks,  and  was  called  the  Mctonic  cycle, 
from  Melon,  its  reputed  discoverer ;  and  it  has  always  been  used  as  a  conve- 
nient method  of  calculating  eclipses  and  other  phenomena  depending  on  the 
relative  positions  of  the  sun  and  moon.  In  a  solar  eclipse,  the  sun  and  moon 
must  occupy  nearly  the  same  position  in  the  heavens ;  and  in  a  lunar  eclipse, 
nearly  opposite  positions :  it  is  evident,  therefore,  that  if  an  eclipse  occur  on 
any  day  in  any  given  year,  an  eclipse  of  the  same  kind  must  occur  on  the  cor- 
responding day  in  every  nineteenth  succeeding  year.  The  tides,  depending  as 
they  do  on  the  relative  positions  of  the  sun  and  moon,  would  be  calculated 
with  facility  by  means  of  the  same  cycle  ;  and  meteorologists  who  hold  the 
doctrine  that  atmospheric  vicissitudes  depend  solely  or  chiefly  upon  the  rela- 
tive aspects  of  the  sun  and  moon,  have  favored  the  doctrines,  that  there  is  a 
general  cycle  of  weather,  the  period  of  which  corresponds  with  that  which  we 
have  noticed.  Thus  they  hold,  that  the  general  changes  of  weather  succeed 
each  other  in  the  same,  or  almost  the  same  order,  throughout  every  successive 
period  of  nineteen  years. 

We  shall  not  here  object,  on  theoretical  grounds,  to  the  doctrine  that  the  true 


THE  MOON  AND  THE  WEATHER, 


387 


amount  of  the  Metonic  cycle  is  not  precisely  nineteen  years.  But  it  is  sub- 
ject to  a  stronger  objection  founded  on  the  principles  which  its  supporters 
themselves  rely  upon.  The  attraction  of  bodies  in  virtue  of  their  gravitation, 
increases  in  the  same  proportion  as  the  square  of  the  distance  diminishes  ;  and 
as  we  have  already  stated  that  the  moon's  distance  from  the  earth  is  variable  to 
an  extent  not  inconsiderable,  it  is  evident,  that  her  influence  on  the  atmosphere 
ought  to  be  expected  to  depend  much  more  on  that  variation  of  distance,  than 
on  her  relative  position  with  respect  to  the  sun.  Now,  although  the  cycle  of 
nineteen  years  corresponds  with  the  changes  of  her  relative  position  to  the  sun 
as  seen  from  the  earth,  yet  it  has  no  correspondence  whatever  with  the  varia- 
tion of  her  distance  ;  and  although,  on  each  day  of  each  succeeding  period  of 
'nineteen  years,  she  will  have  the  same  apparent  position  relatively  to  the  sun, 
she  will  not  have  the  same  distance  from  the  earth,  and,  therefore,  will  not  ex- 
ert the  same  attraction  on  our  atmosphere.  Seeing,  then,  that  the  theory  of  the 
moon's  attraction  does  not  lend  its  unqualified  support  to  this  assumed  period 
of  nineteen  years  as  a  cycle  of  weather,  let  us  see  how  far  fact  and  ob- 
servation countenance  such  a  meteorological  period.  M.  Arago  (to  whom 
we  are  indebted  for  the  most  complete  investigation  of  this  question,  and  for 
the  collection  of  the  labors  of  others  upon  it)  has  successfully  shown  that 
observation  affords  no  countenance  or  confirmation  whatever  to  this  hypothe- 
sis. 

It  has  been  said  that  the  years  1701,  1720,  1739,  and  1758,  being  cor- 
responding years  in  successive  intervals  of  nineteen  years,  show  in  the  differ- 
ent months  the  same  characters  of  weather.  Now  to  try  this  fact,  it  will  be 
necessary  to  adopt  some  distinct  test  of  the  characters  of  the  seasons  which  has 
nothing  in  it  arbitrary,  and  about  which  two  observers  cannot  differ.  For  this 
purpose  we  shall  take  the  highest  and  lowest  temperature  observed  in  each 
ol  the  years,  and  the  annual  quantity  of  rain  which  fell  in  them  respectively  : — 


Dates. 

1701 

1720. 

1739 , 

1758 , 


Temp.  Max. 

...90-5 

...89-5 

...92-7 

...93-9 


Temp.  Min. 

27-5... 

29-3... 

28-6... 

27-3 


Rain,  inches. 

22-7 

18-3 

, 20-4 


Such  is  the  kind  of  congruity  on  which  the  advocates  for  the  Metonic  cycle 
rely.  If  any  four  years  were  taken  indiscriminately  at  any  given  places,  the 
extremes  of  temperature  and  quantities  of  rain  could  scarcely  be  expected  to 
exhibit  greater  differences.  M.  Arago  had  extended  the  comparison  to  other 
seasons  separated  by  the  same  interval  of  nineteen  years,  or  by  multiples 
of  nineteen  years. 


Years. 

Max.  Temp. 

Min.  Temp. 

Annual  quantity 
of  rain  in  inches. 

1725 
1782 

88-2 
90-5 

24-6 
7-2 

18-6 
23-5 

1709 
1728 

87-1 
87-1 

5-8 
16-9 

23-2 
17-2 

1710 
1748 

83-1 
98-4 

7-3 
9-3 

16-9 
18-4 

1711 
1730 

85-3 
'    88-2 

14-9 
19-6 

26-8 
17-0 

1733 
1771 

90-5 
92.7 

28-2 
9-1 

19-6 
19-2 

1734 
1753 

89-4 
100-6 

23-0 
11.3 

18-7 
18-9 

388 


THE  MOON  AND  THE  WEATHER. 


There  are  here  no  traces  of  correspondence  in  the  extremes  of  temperature, 
or  the  quantities  of  rain.  It  is  manifest  that  any  season  taker;  at  hazard  would 
not  present  greater  discordances  than  are  found  in  the  above  table. 

The  variation  of  the  moon's  distance  from  the  earth  (to  which  we  have  more 
than  once  adverted)  is  occasioned  by  the  fact  that  her  path  round  the  earth  is 
not  circular,  but  oval — the  position  of  the  earth  being  nearer  to  the  one  end 
than  the  other.  As  the  moon,  therefore,  approaches  the  furthermost  extremity 
of  her  oval  orbit,  her  distance  from  the  earth  continually  increases  until,  arri- 
ving at  that  point,  it  becomes  greatest ;  as  she  moves  from  that  extremity  of  the 
orbit  to  the  other  end  of  the  oval,  her  distance  continually  diminishes  until  ar- 
riving at  the  other  end,  it  becomes  least.  These  variations  of  distance  are 
produced  every  revolution  of  the  moon  round  the  earth.  Now,  owing  to  a* 
certain  change  of  position,  to  which  the  moon's  orbit  is  subject,  the  points  which 
mark  her  greatest  and  least  distances  are  subject  to  a  slow,  gradual,  and  regu- 
lar change. ;  so  that  the  points  in  the  heavens  at  which  she  reaches  her  great- 
est and  least  distances  are  different  every  revolution.  After  the  lapse,  how- 
ever, of  eight  years  and  ten  months,  these  points  having  traversed  the  whole 
circumference  of  the  heavens,  resume  their  former  position  very  nearly ;  so 
that  the  actual  times  at  which  the  moon  is  observed  at  the  same  distances  from 
the  earth,  and  also  at  the  same  points  in  the  heavens,  recur  in  a  cycle,  the 
length  of  which  is  about  eight  years  and  ten  months. 

So  far,  therefore,  as  the  vicissitudes  of  the  weather  can  be  supposed  to  be 
influenced  by  this  cause,  their  period  should  be  such  that,  after  the  lapse  of 
nine  years,  the  corresponding  states  of  the  weather  would  be,  as  it  were,  two 
mo;iths  in  advance  :  thus  the  effect  produced  in  December,  1800,  would  again 
be  produced  in  October,  1809,  in  August,  1818,  and  so  on. 

If  the  purpose  be  to  determine  the  cycle  in  which  the  lunar  influence,  so  far 
as  it  depends  on  distance,  would  produce  the  same  effects  upon  the  same  days 
of  the  year,  the  duration  of  the  cycle  would  be  six  times  eight  years  and  ten 
months  :  for  in  six  successive  intervals  of  that  period,  there  are  exactly  fifty- 
three  years ;  but  any  less  number  of  periods  of  eight  years  and  ten  months  do 
not  make  a  complete  number  of  years.  Therefore  after  a  cycle  of  fifty-three 
years,  the  moon  being  on  the  same  day  of  each  successive  year  at  the  same 
distance  from  the  earth,  her  influence,  so  far  as«depends  on  distances,  will  be 
ti\3  same,  and  will  produce  the  same  effect  upon  the  weather. 

Now  we  cannot  better  illustrate  the  loose  and  inaccurate  manner  in  which  sci- 
entific principles  are  applied  by  some  meteorologists  than  by  stating  that  this  cy- 
cle of  eight  years  and  ten  months  has  formed  the  theoretical  grounds  for  a  re- 
puted meteorological  period  of  nine  years.  It  has  been  maintained  that, 
through  every  successive  interval  of  nine  years,  the  changes  of  weather  have 
a  general  correspondence :  thus,  if  the  state  of  the  weather  throughout  the 
year  1800  be  examined,  it  has  been  said  to  correspond  with  the  weather 
throughout  the  years  1809,  and  1818,  &c. 

That  the  changes  in  the  positions  of  the  points  of  the  moon's  greatest  and 
least  distance  are  insufficient  in  theory  to  account  for  such  meteorological  cy- 
cle as  we  have  explained.  But  let  us  see  how  the  fact  stands. 

Toaldo,  whose  meteorological  researches  we  have  adverted  to,  has  stated, 
that  at  Padua,  by  resolving  a  long  interval  of  time  into  successive  periods  of 
nine  years,  the  quantities  of  rain  collected  in  each  of  these  periods  were  equal, 
but  he  adds  this  equality  would  disappear  if  the  whole  interval  were  resolved 
into  groups  of  eight  years,  or  into  successive  intervals  of  any  other  number  of 
years.  M.  Arago,  taking  the  Italian  meteorologist  at  his  word,  and  accepting 
without  question,  his  own  tables  and  data,  has  given  the  following  estimate  of 
the  quantity  of  rain  which  had  fallen  in  successive  intervals  of  nine  years  : — 


THE  MOON  AND  THE  WEATHER. 


389 


In  the  nine  years 
commencing  in 
1725  

...  to  .... 

And  ending 
inclusively  in 
1733  

Rain  whirh  had 
fallen  at  Padua. 

inches. 
ii 

H 

inches. 

<c 

(C 

From  1734  

...  to  .  ... 

1742  

From  1743  

...   to  .... 

1751  

320         " 

From  1752  

1760  

333         « 

From  1761  

...  to  .... 

1769  

320        " 

From  1699  

...   to   .... 

1707  

Paris  gives 

1716  

166        " 

From  1717 

.  .  .  .  to  .... 

1725  

...  to  .  .  .  . 

1734  

]  25         " 

.  .  .  .  to 

1743  

139         " 

From  1744.. 

.   to  . 

..1752.., 

.  .  160         " 

The  confidence  to  which  Toaldo's  reasoning  and  calculations  are  entitled, 
may  be  estimated  by  comparing  the  quantities  of  rain  which  fall  in  any  other 
intervals,  from  which  it  will  be  seen  that  it  is  not  subject  to  greater,  variation 
than  that  which  exists  among  the  above  results. 

M.  Arago  gives  some  amusing  examples  of  the  kind  of  speculation  and  rea- 
soning in  which  meteorologists  sometimes  indulge.  Some,  he  says,  found  the 
assumed  cycle  of  nine  years  on  the  passage  of  Pliny,  where  he  says  that  every 
fourth,  and,  more  especially,  every  eighth  year,  the  seasons  undergo  a  kind  of 
effervescence  by  the  revolution  of  the  hundredth  moon.  Admitting  Pliny's 
maxim  to  be  true,  and  supposing  by  the  word  effervescence  we  are  to  under- 
stand a  regular  recurrence  every  eight  years  of  the  changes  of  the  weather 
which  took  place  in  the  preceding  eight  years,  what  are  we  to  conclude  1  Is 
not  the  question  here,  whether  the  vicissitudes  of  weather  recur  at  intervals 
of  nine  years  ?  and  the  celebrated  Roman  naturalist  speaks  of  a  period  of  only 
eight  years. 

From  all  that  has  been  stated,  it  follows,  then,  conclusively,  that  the  popular 
notions  concerning  the  influence  of  the  lunar  phases  on  the  weather  have  no 
foundation  in  the  theory,  and  no  correspondence  with  observed  facts.  That 
the  moon,  by  her  gravitation,  exerts  an  attraction  on  our  atmosphere  cannot  be 
doubted  ;  but  the  effects  which  that  attraction  would  produce  upon  the  weather 
are  not  in  accordance  with  observed  phenomena ;  and,  therefore,  these  effects 
are  either  too  small  in  amount  to  be  appreciable  in  the  actual  state  of  meteor- 
ological instruments,  or  they  are  obliterated  by  other  more  powerful  causes, 
from  which  hitherto  they  have  not  been  eliminated.  It  appears,  however,  by 
some  series  of  observations,  not  yet  confirmed  or  continued  through  a  sufficient 
period  of  time,  that  a  slight  correspondence  may  be  discovered  between  the 
periods  of  rain  and  the  phases  of  the  moon,  indicating  a  very  feeble  influence, 
depending  on  the  relative  position  of  that  luminary  to  the  sun,  but  having  no  ( 
discoverable  relation  to  the  lunar  attraction.  This  is  not  without  interest  as  a  ( 
subject  of  scientific  inquiry,  and  is  entitled  to  the  attention  of  meteorologists ; 
but  its  influence  is  so  feeble  that  it  is  altogether  destitute  of  popular  interest  as 
a  weather  prognostic.  It  may,  therefore,  be  stated  that,  as  far  as  observation 
combined  with  theory  has  afforded  any  means  of  knowledge,  there  are  no 
grounds  for  the  prognostications  of  weather  erroneously  supposed  to  be  derived 
from  the  influence  of  the  sun  and  moon. 

Those  who  are  impressed  with  the  feeling  that  an  opinion  so  universally  en- 
tertained even  in  countries  remote  from  each  other,  as  that  which  presumes  an 
influence  of  the  moon  over  the  changes  of  the  weather,  will  do  well  to  remem- 
ber that  against  that  opinion  we  have  not  here  opposed  mere  theory.  Nay.  we 
have  abandoned  for  the  occasion  the  support  that  science  might  afford,  and  the 
light  it  might  shed  on  the  negative  of  this  question,  and  have  dealt  with  it  as  a 
mere  question  of  fact.  It  matters  little,  so  far  as  this  question  is  concerned,  i 


390 


THE  MOON  AND  THE  WEATHER. 


in  what  manner  the  moon  and  sun  may  produce  an  effect  on  the  weather,  nor 
even  whether  they  he  active  causes  in  producing  such  effect  at  all.  The  point, 
and  the  only  point  of  importance  is,  whether,  regarded  as  a  mere  matter  of  fact, 
any  correspondence  between  the  changes  of  the  moon  and  those  of  the  weather 
exists?  And  a  short  examination  of  the  recorded  facts  proves  that  IT  DOES 
XOT. 


PERIODIC    COMETS. 


Enckc's  Comet. — It?  Period  arid  Orbit. — How  its  Motion  shows  the  Existence  of  a  resistii  g  Me- 
dium.— This  Result  corroborated  by  the  Theory  of  Light. — Newton's  Conjectures  resj  ecting 
Comet*. — Bit-la's  Comet. — Its  Period  and  Orbit. — Lexell's  Comet. — Causes  of  its  Appearance  and 
DisapiMjarance. — Whiston's  Comet. — His  Theory. — Did  this  Comet  produce  the  Deluge? — Orbit 
of  this  Comet.  * 


PERIODIC  COMETS.  393 


PERIODIC    COMETS. 


O.v  another  occasion,  I  gave  at  some  length  the  history  of  Halley's  comet, 
by  far  the  most  interesting  of  all  the  periodic  comets  yet  discovered.  I  shall 
now  bring  under  your  notice  the  remaining  bodies  of  this  class. 

A  periodic  comet,  as  the  name  implies,  is  one  which  is  known  to  return  at 
regular  intervals  to  our  system,  and  whose  reappearance  in  the  heavens  can 
therefore  be  predicted.  The  paths  of  these  bodies  round  the  sun  are  eccentric 
ellipses,  having  the  centre  of  the  sun  in  one  of  their  foci.  / 

ENCKE'S  COMET. 

In  the  year  1818,  a  comet  was  observed  at  Marseilles,  on  the  26th  of  No- 
vember, by  M.  Pons.  In  the  following  January,  its  path  being  calculated,  M. 
Arago  immediately  recognised  it  as  identical  with  one  which  had  appeared  in 
1805.  Subsequently,  M.  Encke  of  Berlin  succeeded  in  calculating  its  entire 
orbit — inferring  the  invisible  from  the  visible  part — and  found  that  its  period 
round  the  sun  was  about  twelve  hundred  days.  This  calculation  was  verified 
by  the  fact  of  its  return  in  1822,  since  which  time  the  comet  has  gone  by  the 
name  of  Encke's  comet,  and  returned  regularly. 

This  comet  exhibited  the  appearance  of  a  mass  of  nebulous  vapor,  so  trans- 
parent, even  at  its  centre,  that  stars  can  be  seen  through  it.  It  is  round,  or 
rather  oval,  in  its  form,  and  is  too  attenuated  and  feeble  in  its  light  to  be  dis- 
covered without  the  aid  of  a  telescope.  The  annexed  figure,  1,  is  that  which  is 
usually  given  as  a  representation  of  its  telescopic  appearance. 

The  orbit  of  Encke's  comet  is  an  oval,  whose  length  is  about  double  its 
breadth.  At  its  nearest  approach  to  the  sun,  the  distance  of  the  comet  is  about 
thirty-four  millions  of  miles,  which  is  about  the  distance  of  the  planet  Mercury 
When  most  remote  from  the  sun,  its  distance  is  about  four  hundred  and  forty- 
three  millions  of  miles,  which  is  nearly  four  and  a  half  times  the  earth's  dis- 
tance, and  is  little  less  than  the  distance  of  Jupiter.  The  orbit  is  inclined  to 


that  of  the  earth  at  nearly  thirteen  degrees.  This  comet  may  be  considered 
as  a  planet,  revolving  within  the  orbit  of  Jupiter,  and  nearly  in  the  common 
plane  of  the  solar  system.  Its  motion  is  in  the  same  direction  as  that  of  the 
planets. 

In  the  calculations  of  Encke  for  the  determination  of  the  movement  of  this 
comet,  the  most  scrupulous  account  was  taken  of  the  effects  which  the  planets 
must  produce  upon  it.  Nevertheless,  a  small  discrepancy  was  found  to  exist 
between  its  observed  and  computed  returns  ;  and  what  was  sull  more  remark- 
able, this  discrepancy  was  of  the  same  nature  in  every  case,  so  that  it  is  im- 
possible to  suppose  that  it  could  have  arisen  from  any  casual  error  of  compu- 
tation or  of  observation ;  since,  had  it  so  occurred,  it  would  have  affected  the 
result  irregularly.  We  must  therefore  conclude  that  this  comet  does  not  pre- 
cisely retrace  its  course  each  revolution.  It  is  found,  however,  that  this  irregu- 
larity, from  whatever  cause  it  may  proceed,  does  not  disturb  the  plane  of  the 
comet's  p^ath.  It  is,  in  fact,  according  to  the  observations  and  reasonings  of 
Professor  Encke,  precisely  the  effect  which  would  be  produced  if  the  space 
through  which  the  comet  moves  was  filled  by  a  subtle  fluid,  offering  a  small 
resistance  to  the  motion  of  the  comet :  just  as  our  atmosphere  resists  the  motion 
of  any  light  body  through  it. 

The  existence  of  an  extremely  subtle  ethereal  fluid  which  fills  the  infini- 
tude of  space,  has  been  adopted  hypothetically  to  explain  the  phenomena  of 
optics.  In  fact,  light  itself  is,  according  to  the  undulatory  theory,  supposed  to 
consist  in  vibrations  transmitted  through  such  a  fluid,  just  as  sound  is  known 
to  consist  in  similar  undulations  transmitted  through  the  atmosphere.  Hith- 
erto this  assumed  cause  for  light  has  been  justly  regarded  as  an  ingenious  hy- 
pothesis not  proved,  but  which  accounts  for  the  various  phenomena  more  fully 
and  satisfactorily  than  the  corpuscular  theory,  which,  being  open  to  the  same 
objection,  completely  fails  when  applied  to  some  phenomena  of  light  which 
recent  investigations  have  developed.  If  an  effect  similar  to  that  which  has 
been  observed  in  Encke's  comet  should  be  discovered  on  the  approaching  re- 
turn of  Halley's  comet,  and  still  more,  if  it  be  observed  on  the  next  return  of 
Biela's  comet,  the  undulatory  hypothesis  will  begin  to  assume  the  character  of 
a  vera  causa  ;  and  that  theory  of  light  must,  under  such  circumstances,  be  con- 
sidered as  established. 

The  effect  on  the  return  of  a  comet  produced  by  this  resistance,  contrary  to 
what  might  at  lirst  be  expected,  is  to  accelerate  it,  or  to  make  the  actual  re- 
turn anticipate  the  return  as  computed  on  the  supposition  that  the  comet  moves 


in  an  unresisting  medium.  This  difficulty  will,  however,  be  removed,  if  it  be 
remembered  that  a  resisting  medium,  by  diminishing  the  velocity  of  the  body 
in  its  orbit,  diminishes  the  influence  of  the  centrifugal  force  to  resist  solar  at- 
traction. The  body,  therefore,  follows  a  path  consiantly  nearer  to  the  sun  ;  in 
other  words,  the  orbit  is  in  a  progressive  state  of  diminution.  Now,  the  less 
the  orbit  is,  the  less  time  necessary  to  describe  it ;  and  consequently  the  shorter 
the  period  of  the  successive  returns  of  the  body  to  the  same  position. 

If  the  successive  returns  of  the  periodic  comets  should  establish  satisfacto- 
rily the  existence  of  the  luminous  ether,  it  will  follow  that  after  the  lapse  of  a 
certain  time  every  comet  will  ultimately  fall  into  the  sun.  In  every  succeed- 
ing revolution  of  the  same  comet,  its  path  would  fall  a  little  within  its  former 
course,  and  it  woujd  describe  a  spiral  line  round  the  sun,  continually  approach- 
ing that  body,  until  at  length  it  would  arrive  close  to  its  surface  ;  before  this 
could  happen,  it  would  doubtless  be  wholly  converted  into  a  light  gas  by  his 
heat,  which  would  probably  mingle  with  the  solar  atmosphere. 

In  the  efforts  by  which  the  human  mind  labors  after  truth,  it  is  curious  to 
observe  how  often  that  desired  object  is  stumbled  upon  by  accident,  or  arrived 
at  by  reasoning  which  is  false.  One  of  Newton's  conjectures  respecting  com- 
ets was,  that  they  are  "  the  aliment  by  which  suns  are  sustained  ;"  and  he 
therefore  concluded  that  these  bodies  were  in  a  state  of  progressive  decline 
upon  the  suns,  round  which  they  respectively  swept ;  and  that  into  these  suns 
they  from  time  to  time  fell.  This  opinion  appears  to  have  been  cherished  by 
Newton  to  the  latest  hours  of  his  life  :  he  not  only  consigned  it  to  his  immor- 
tal writings,  but,  at  the  age  of  eighty-three,  a  conversation  took  place  between 
him  and  his  nephew  on  this  subject,  which  has  come  down  to  us.  "  I  cannot 
say,"  said  Newton,  "  when  the  comet  of  1680  will  fall  into  the  sun  :  possibly 
after  five  or  six  revolutions ;  but  whenever  that  time  shall  arrive,  the  heat  of 
the  sun  will  be  raised  by  it  to  such  a  point,  that  our  globe  will  be  burnt,  and 
all  the  animals  upon  it  will  perish.  The  new  stars  observed  by  Hipparchus, 
Tycho,  and  Kepler,  must  have  proceeded  from  such  a  cause,  for  it  is  impossi- 
ble otherwise  to  explain  their  sudden  splendor."  His  nephew  then  asked  him, 
"  why,  when  he  stated  in  his  writings  that  comets  would  fall  into  the  sun,  did 
he  not  also  slate  those  vast  fires  they  must  produce,  as  he  supposed  they  had 
done  in  the  stars  ?" — "  Because,"  replied  the  old  man,  "  the  conflagrations  of 
the  sun  concern  us  a  little  more  directly.  I  have  said,  however,"  added  he, 
smiling,  "  enough  to  enable  the  world  to  collect  my  opinion." 

It  may  be  asked,  if  the  existence  of  a  resisting  medium  be  admitted,  whether 
the  same  ultimate  fate  must  not  await  the  planets  ?  To  this  inquiry  it  may  be 
answered  that,  within  the  limits  of  past  astronomical  record,  the  ethereal  me- 
dium, if  it  exist,  has  had  no  sensible  effect  on  the  motion  of  any  planet.  That 
it  might  have  a  perceptible  effect  upon  comets,  and  yet  not  upon  planets,  will 
not  be  surprising,  if  the  extreme  lightness  of  the  comets  compared  with  their 
bulk  be  considered.  The  effect  in  the  two  cases  may  be  compared  to  that  of 
the  atmosphere  upon  a  piece  of  swan's  down  and  upon  a  leaden  bullet  moving 
through  it.  It  is  certain  that  whatever  may  be  the  nature  of  this  resisting  me- 
dium, it  will  not,  for  many  hundred  years  to  come,  produce  the  slightest  per- 
ceptible effect  upon  the  motions  of  the  planets. 

BIELA'S  COMET. 

On  February  28,  1826,  M.  Biela,  an  Austrian  officer,  observed  in  Bohemia 
a  comet,  which  was  seen  at  Marseilles  about  the  same  time  by  M.  Gambart. 
The  path  which  it  pursued  was  observed  to  be  similar  to  that  of  comets  which 
had  appeared  in  1772  and  1806.  Finally,  it  was  found  that  this  body  moved 


96 


PERIODIC  COMETS. 


und  the  sun  in  an  oval  orbit,  and  that  the  time  of  its  revolution  was  about  six 
ears  and  eight  months.  It  has  since  returned  at  its  predicted  times  ;  and  has 
>een  adopted  as  a  member  of  our  system,  under  the  name  of  Biela's  comet. 

The  annexed  diagram,  fig.  2,  exhibits  the  form  and  position  of  the  orbit  of  this 
omet  in  relation  to  those  of  the  principal  planets,  giving  the  successive  posi- 
ons  it  assumed  during  its  appearance  in  1832. 

Fig.  2. 


Biela's  comet  moves  in  an  orbit  whose  plane  is  nearly  the  same  with  those 
of  the  planets.  It  is  but  slightly  oval,  the  length  being  to  the  breadth  in  the 
proportion  of  about  three  to  two.  When  nearest  to  the  sun,  its  distance  is 
nearly  equal  to  that  of  the  earth  ;  and  when  most  remote  from  the  sun,  its  dis- 
tance soiyewhat  exceeds  that  of  Jupiter.  Thus  it  ranges  through  the  solar 
system,  between  the  orbits  of  Jupiter  and  the  earth. 

Notwithstanding  the  discovery  of  the  periodic  comets  of  Encke  and  Biela, 
still  the  comet  of  Halley  maintains  a  paramount  astronomical  interest,  and  may 
be  considered  to  stand  alone  in  exhibiting  those  physical  phenomena  which 
seem  to  be  the  exclusive  characteristics  of  the  class  to  which  it  belongs.  Al- 
though the  cornets  of  Encke  and  Biela  are  unquestionably  objects  of  interest  to 
the  geometer  and  astronomer,  yet  their  short  periods,  the  limited  space  within 


which  they  are  circumscribed  in  their  motion,  the  small  obliquity  and  eccen- 
tricity of  their  orbits,  and  consequently  the  very  slight  disturbance  which  they 
sustain  from  the  attraction  of  the  planets,  render  them,  for  all  physical  purposes, 
nothing  more  than  new  planets  of  inappreciable  mass  belonging  to  our  system. 
Unlike  other  known  comets,  they  do  not  rush  from  the  invisible  and  inacessible 
depths  of  space,  and,  after  sweeping  our  system,  depart  to  distances  under 
the  conception  of  which  the  imagination  itself  is  confounded ;  they  possess 
uone  of  that  grandeur  which  is  connected  with  whatever  appears  to  break 
through  the  fixed  order  of  the  universe.  It  is  still  reserved  for  the  comet  of 
Halley  alone  to  exhibit  a  phenomenon,  so  far  as  we  know,  unique  ;  to  afford  a 
splendid  result  of  those  powers  of  calculation  by  which  we  are  enabled  to  follow 
it  through  the  depths  of  space  two  thousand  millions  of  miles  beyond  the  ex- 
treme verge  of  the  solar  system  ;  and,  notwithstanding  disturbances  which 
render  each  succeeding  period  of  its  return  different  from  the  last,  to  foretell 
that  return  with  precision. 

LEXELL'S  COMET. 

In  the  month  of  June,  1770,  Messier  observed  a  comet,  which  was  after- 
ward sufficiently  observed  to  render  its  course  through  the  system  calculable. 
It  was  found  not  to  correspond  with  that  of  any  comet  previously  known.  It 
remained  visible  for  an  unusual  length  of  time  ;  and  continued  observations  on 
it  proved  that  it  moved,  not  as  comets  were  then  generally  found  to  move,  in  a 
parabola,  or  very  elongated  ellipse,  but  in  an  oval  of  very  small  dimensions. 

Its  orbit  was  calculated  by  the  celebrated  Lexell,  and  found  to  be  an  ellipse, 
of  which  the  greater  axis  was  only  equal  to  three  times  the  diameter  of  the 
earth's  orbit,  which  showed  that  its  periodical  revolution  round  the  sun  would 
be  completed  in  jive  years  and  a  half, 

With  so  short  a  period,  the  comet  ought  frequently  to  be  seen.  But  here 
springs  up  a  difficulty.  This  comet  was  never  seen  before,  and  has  never 
been  seen  since  !  What,  then,  has  become  of  it  ?  and  where  and  how  did  it 
exist  before  its  discovery  by  Messier  ?  Its  appearance  was  too  conspicuous 
and  its  light  too  vivid  to  allow  of  the  supposition  that  it  could  have  been  pres- 
ent, yet  not  observed. 

The  law  of  gravitation  discovered  by  Newton,  and  fully  developed  by  his 
illustrious  successors,  enables  us  fully  to  explain  this  difficulty.  We  shall 
adopt  the  words  of  Arago  : — 

Why  has  not  the  comet  been  seen  every  Jive  years  and  a  half  before  177P  ? 
Because  the  orbit  was  then  totally  different  from  that  it  has  since  pursued. 

Why  has  not  the  comet  been  seen  since  1770  ?  For  the  reason  that  its  pas- 
sage to  the  point  of  perihelion  in  1776  took  place  by  day  ;  and  before  the  fol- 
lowing return,  the  form  of  the  orbit  was  so  altered,  that  had  the  comet  been 
visible  from  the  earth  it  would  not  have  been  recognised. 

Lexell  had  already  remarked,  according  to  his  elements  of  1770,  that  the 
comet  ought  to  pass  in  the  vicinity  of  Jupiter  in  1767,  less  than  the  fifty-eighth 
part  of  his  distance  from  the  sun  ;  that  in  1779,  when  it  returned  to  us,  it  would 
be,  near  the  end  of  August,  about  five  hundred  times  nearer  that  same  planet 
than  to  the  sun  ;  so  that  then,  notwithstanding  the  immense  size  of  the  solar 
globe,  its  attractive  power  on  the  comet  was  not  the  two  hundredth  part  that 
of  Jupiter.  Thus  it  could  not  be  doubted  that  the  comet  had  experienced  con- 
siderable perturbations  in  1767  and  1779  ;  but  it  is  yet  necessary  to  establish 
that  these  perturbations  were  numerically  strong  enough  to  explain  the  total 
want  of  observations,  as  well  before  as  after  the  year  1770. 

The  formularies  in  the  fourth  volume  of  the  Mecanique  Celeste  give  the  ana- 


398  PERIODIC  COMETS. 


lyrical  solution  of  this  problem  :  the  actual  elliptic  orbit  of  a  comet  being  known 
what  was  its  previous  orbit  ?  What  will  it  be  hereafter,  taking  into  accoun 
in  both  cases  the  perturbating  effects  caused  by  the  planets  of  our  system  ? 

Well,  then,  by  putting  these  formularies  into  numbers — by  substituting,  fo 
its  component  indeterminate  letters,  the  particular  elements  of  the  comet  ol 
1770 — it  will  first  be  found  that  in  1767,  previous  to  the  approach  of  that  body 
to  Jupiter,  the  elliptic  orbit  which  it  described  corresponds,  not  to  five  but  to 
fifty  years  of  revolution  round  the  sun  ;  afterward,  that  in  1779,  on  its  depar 
turc  out  of  the  attraction  of  the  same  planet,  the  orbit  of  the  comet  could  not  be 
completed  in  less  than  twenty  years.  From  the  same  researches  it  results  that 
before  1767,  during  the  whole  progress  of  its  revolutions,  the  shortest  distance 
of  the  comet  from  the  sun  was  one  hundred  and  ninety-nine  millions  of  leagues 
(five  hundred  and  ninety-seven  millions  of  miles),  and  that  after  1779  the  mini- 
mum of  distance  became  one  hundred  and  thirty-one  millions  of  leagues  (three 
hundred  and  ninety-three  millions  of  miles).  This  was  still  too  far  removed 
for  the  comet  to  be  perceptible  from  the  earth. 

However  singular  it  may  appear,  we  are,  then,  fully  authorized  to  say  of  the 
comet  of  1770,  that  the  action  of  Jupiter  brought  it  to  us  in  1767,  and  that 
the  same  action,  producing  an  inverse  effect,  removed  it  from  us  in  the  year 
1779. 

WHISTON'S  COMET. 

A  remarkable  comet  appeared  in  the  year  1680,  which  has  been  rendered 
$  memorable  by  the  attempt  of  Whiston  to  prove  that  it  was   periodic,  and  that 
}  on  one  of  its  former  visits  it  was  the  proximate  cause  of  the  Mosaic  deluge.  < 
$  Arago,  in  his  essay  on  comets,  has  discussed  fully  the   question  raised  by  $ 
I  Whiston. 

£  Whiston,  says  he,  proposed  to  show  not  only  in  what  manner  a  comet  might 
)  have  occasioned  the  deluge  of  Noah,  but  was  desirous,  moreover,  that  his  ex- 
^  planation  should  agree  minutely  with  all  the  circumstances  of  that  great  catas- 
<J  trophe  as  related  in  Genesis.  Let  us  see  how  he  has  succeeded  in  his  object. 
The  biblical  deluge  happened  in  the  year  2349  before  the  Christian  era,  ac- 
cording to  the  modern  Hebrew  text ;  or  the  year  2926,  after  the  Samaritan 
text,  the  Septuagint,  and  Josephus.  Is  there,  then,  reason  to  suppose  that  at 
either  of  those  periods  a  great  comet  had  appeared  ? 

Among  the  comets  observed  by  modern  astronomers,  that  of  1680  may,  from 
its  brilliancy,  without  hesitation  be  placed  in  the  first  rank. 
$       A  great  many  historians,  both  native  and  foreign,  mention  a  very  large  comet, 
j  in  similitude  to  the  blaze  of  the  sun,  having  an  immense  train,  which  appeared  in 
)  the  year  1106.     In  ascending  still  higher,  we  find  a  very  large  and  terrific 
comet  designated  by  the  Byzantine  writers  by  the  name  of  Lampadias,  because 
it  resembled  a  burning  lamp,  the  appearance  of  which  may  be  fixed  in  the  year 
531.     All  the  world  knows,  in   fine,  that  a  comet  appeared  in  the  month  of 
September,  in  the  year  of  the  death  of  Caesar,  during  the  games  given  by  the 
emperor  Augustus  to  the  Roman  people.     That  comet  was  very  brilliant,  as  it 
became  visible  from  the  eleventh  hour  of  the  day,  that  is,  about  five  o'clock  ir 
the  evening,  or  brfore  sunset.     Its  date  is  in  the  year  43  before  our  era. 

Since  we  have  not  any  exact  observation  of  the  comets  of — 43,  or  531,  or 
of  1106  ;  since  we  cannot  calculate  their  parabolic  orbits  ;  since  we  want  th? 
only  criterion  which  would  enable  us  to   decide  with  perfect  certainty  eith.-r 
the  identity  or  dissemblance  of  two  comets,  let  us  at  least  remember  that  those  £ 
of  1680,  of  1106,  of  531,  and  of  — 43,  were  very  brilliant,  and  let  us  compare  ? 
with  each  other  the  dates  of  these  apparitions  : — 


From  1106  to  1680  we  find 574  years. 

"       531  «  1106    "     «    575    " 

"     —43  "    531    "     "    575     " 

As  we  have  not  reckoned  the  months  or  portions  of  years,  these  periods  may 
be  regarded  as  equal  to  each  other,  and  thence  it  becomes  probable  enough 
that  the  comet  of  the  death  of  Caesar,  of  531,  of  1106,  and  of  1680,  have  been 
only  the  reappearances  of  one  and  the  same  comet,  which,  after  having 
run  through  its  orbit — after  having  made  its  complete  revolution  in  about  five 
hundred  and  seventy-five  years — became  again  visible  from  the  earth.  Then 
if  the  period  of  five  hundred  and  seventy-five  years  is  multiplied  by  four,  we 
have  twenty-three  hundred,  which,  added  to  43,  the  date  of  Csesar's  comet, 
gives,  with  the  difference  of  only  six  years,  the  epoch  of  the  deluge,  resulting 
from  the  modern  Hebrew  text.  In  multiplying  by  five,  the  date  of  the  Septua- 
gint  is  found  within  eight  years. 

If  we  recollect  the  marked  differences  of  the  comet  of  1759  in  the  period 
of  its  revolution  round  the  sun,  we  shall  acknowledge  that  Whiston  might  le- 
gitimately have  felt  authorized  to  suppose  that  the  great  comet  of  1680,  or  of 
the  death  of  Caesar,  was  near  the  earth  at  the  period  of  Noah's  deluge,  and  that 
it  had  some  part  in  that  great  phenomenon. 

I  shall  not  stop  to  explain  minutely  the  series  of  transformations  by  which 
the  earth,  which,  according  to  Whiston,  was  originally  a  comet,  became  the 
globe  we  now  inhabit.  I  shall  content  myself  by  saying  that  he  considers 
the  nucleus  of  the  earth  as  a  hard  and  compact  substance,  which  was  the 
ancient  nucleus  of  the  comet ;  that  the  matters  of  various  natures  confusedly 
mixed,  which  composed  the  nebulosity,  subsided  more  or  less  quickly,  accord- 
ing to  their  specific  gravities  ;  that  then  the  solid  nucleus  was  at  first  surround- 
ed by  a  dense  and  thick  fluid ;  that  the  earthy  matters  precipitated  themselves 
afterward,  and  formed  a  covering  over  the  dense  fluid — a  kind  of  crust,  which 
may  be  compared  to  the  shell  of  an  egg  ;  that  the  water,  in  its  turn,  came  to 
cover  this  solid  crust ;  that  in  a  considerable  degree  it  became  filtered  through 
the  fissures,  and  spread  itself  over  the  thick  fluid  ;  that,  in  fine,  the  gaseous 
matters  remaining  suspended,  purified  themselves  gradually,  and  constituted  our 
atmosphere. 

Thus  in  this  system  the  great  biblical  abyss  is  supposed  to  consist  of  a  solid 
nucleus  and  of  two  concentric  orbs.  Of  these  orbs,  that  nearest  to  the  centre 
is  formed  of  a  heavy  fluid  which  first  precipitated  itself ;  the  second  is  of  water  ; 
it  is,  then,  properly  speaking,  upon  the  last  of  these  fluids  that  the  exterior  and 
solid  crust  of  the  earth  reposes. 

It  is  proper  now  to  examine  how,  after  that  constitution  of  the  globe  to  which 
at  least  many  geologists  could  oppose  more  than  one  difficulty,  Whiston  ex- 
plains the  two  principal  events  of  the  deluge  described  by  Moses. 

In  the  six  hundredth  year  of  Noah's  life,  says  the  book  of  Genesis,  on  the 
seventeenth  day  of  the  second  month,  the  same  day  were  all  the  fountains  of 
the  great  deep  broken  up,  and  the  windows  of  heaven  were  opened. 

At  the  period  of  the  deluge,  the  comet  of  1680,  says  Whiston,  was  only  nine 
or  ten  thousand  miles  from  the  earth :  it  attracted,  therefore,  the  water  from 
the  great  deep,  as  the  moon  at  present  attracts  the  waters  of  the  ocean.  Its 
action,  on  account  of  that  great  proximity,  must  have  tended  to  produce  an 
'  immense  tide.  The  terrestrial  shell  could  not  resist  the  impetuosity  of  the 
inundation ;  it  broke  in  at  a  great  number  of  points,  and  the  waters,  then  free, 
spread  themselves  over  the  continents.  The  reader  will  here  recognise  the 
rupture  of  the  fountains  of  the  great  deep. 

The  ordinary  rains  of  our  days,  even  continued  for  forty  days,  would  have 
produced  but  a  small  accumulation.  In  taking  for  daily  rain  that  which  falls 


400  PERIODIC  COMETS. 


at  Paris  annually,  the  produce  of  six  weeks,  far  from  covering  the  highest 
mountains,  would  scarcely  have  formed  a  depth  of  eighty  feet.  It  is  therefore 
necessary  to  refer  to  other  sources  than  the  cataracts  of  heaven.  Whiston  has 
found  them  in  the  nebulosity  and  tail  of  the  comet. 

According  to  him,  the  nebulosity  reached  the  earth  near  the  Gordian 
(Ararat)  mountains  Those  mountains  intercepted  the  entire  tail.  The  ter- 
restrial atmosphere,  thus  charged  with  an  immense  quantity  of  aqueous  parti- 
cles, was  sufficient  to  produce  forty  days'  rain  of  such  violence  as  the  ordinary 
state  of  the  globe  can  give  us  no  idea. 

Notwithstanding  all  its  strangeness,  I  have  exposed  the  theory  of  Whiston 
in  detail,  both  on  account  of  the  celebrity  which  it  has  so  long  enjoyed,  as  well 
as  because  it  appeared  improper  to  treat  with  contempt  the  productions  of 
the  man  whom  Newton  himself  designed  as  his  successor  in  the  university  of 
Cambridge  ;  yet  the  following  are  objections  which  it  seems  his  theory  cannot 
resist. 

Whiston  having  required  an  immense  tide  to  explain  the  mystery  of  the  bib- 
lical phenomena  of  the  great  deep,  was  not  content  to  pass  his  comet  extremely 
near  the  earth  at  the  moment  of  the  deluge :  he  has,  moreover,  given  it  a  very 
great  magnitude,  in  supposing  it  six  times  greater  than  the  moon. 

Such  a  supposition  is  completely  gratuitous,  and  this  is  also  its  least  fault ; 
for  it.  is  not  sufficient  to  account  for  the  phenomena.  If  the  moon  really  pro- 
duces a  tide  on  the  waters  of  the  ocean,  it  is  because  its  angular  diurnal 
motion  is  not  very  considerable  ;  that  in  the  space  of  some  hours  its  distance 
from  the  earth  scarcely  varies  ;  during  a  considerable  time  it  remains  vertically 
over  almost  the  same  points  of  the  globe  ;  the  fluid  which  it  attracts  has  there- 
fore always  time  to  yield  to  its  action  before  it  moves  to  a  region  where  the 
force  which  emanates  from  it  will  be  otherwise  directed.  But  it  was  not  the 
same  with  the  comet  of  1680.  Near  to  the  earth,  its  apparent  angular  motion 
must  have  been  extremely  rapid  ;  in  a  few  minutes  it  corresponded  with  a  nu- 
merous series  of  points  situated  on  terrestrial  meridians  very  distant  from  each 
other.  As  to  its  rectilinear  distance  from  the  earth,  it  might,  without  doubt, 
have  been  very  small,  but  only  during  a  few  instants.  The  union  of  these  cir- 
cumstances, it  must  be  observed,  was  but  little  favorable  to  the  production  of 
a  great  tide. 

I  am  very  well  aware  that,  to  diminish  these  difficulties,  it  is  sufficient  to  in- 
crease the  comet — to  make  its  mass  not  only  six  times  the  size  of  the  moon, 
but  thirty  or  forty  times  larger ;  but  I  reply  that  the  comet  of  1680  does  not 
afford  that  latitude.  On  the  1st  of  November  in  that  year  it  passed  very  near 
to  the  earth.  (See  figure  3,  in  which  the  orbit  of  this  comet  is  represented.) 
It  is  shown  that  at  the  period  of  the  deluge  its  distance  was  not  less ;  then,  as 
in  1680,  it  produced  neither  celestial. cataracts,  nor  terrestrial  tides,  nor  ruptures 
of  the  great  deep  ;  as,  moreover,  its  train  nor  its  hair  did  not  inundate  us,  we 
may  in  all  confidence  say  that  Whiston's  theory  is  a  mere  romance,  unless,  in 
abandoning  the  comet  of  16<80,  we  venture  to  attribute  the  same  effect  to 
another  much  more  considerable  star  of  the  same  description. 

Whiston,  as  we  have  just  seen,  proposed  to  attribute  to  physical  causes  not 
only  some  deluge,  but  that  of  Moses,  with  all  the  circumstance  related  in  the 
book  of  Genesis.  His  celebrated  countryman,  Halley,  had  viewed  the  prob- 
lem in  a  less  special  manner. 

There  exists,  says  he,  far  from  the  sea,  marine  productions,  even  upon  the 
highest  mountains,  which  regions  have  been  formerly  under  the  sea.  From 
what  impulse  has  the  ocean  abandoned  the  limits  in  which,  in  our  days,  it  with 
very  slight  oscillations  remains  constantly  bounded  ?  It  is  here  that  Halley 
calls  to  his  aid,  not  like  Whiston,  a  comet  passing  in  our  vicinity  and  causing 


PERIODIC  COMETS. 


40! 


Fig.  3. 


a  \eryhigh  tide,  but  a  star  of  the  same  description,  which,  in  its  elliptic  course 
about  the  sun,  directly  struck  the  earth.  Let  us  examine  closely  what  would 
be  the  effect  of  such  an  event. 

Let  us  conceive  a  solid  body  proceeding  in  a  straight  line  with  a  certain  ra- 
pidity, and  upon  which  from  the  outset  another  much  smaller  body  had  b<t-n 
mertty  placed.  These  two  bodies,  although  not  fastened  together,  will  not  sep- 
arate in  their  progress,  because  the  force  which  moves  them  will  have  gradu- 
ally and  from  the  commencement  imparted  equal  velocities  to  them.  But  let 
us  suppose  that  an  insurmountable  obstacle  suddenly  presents  itself  in  the  way 
of  the  first  bod)  and  stops  it  instantly  ;  the  fore  part  of  the  surface,  the  parts 
struck,  are,  strictly  speaking,  the  only  parts  whose  velocity  is  directly  destroyed 
by  the  obstacle  ;  but  as  all  the  other  parts  are  intimately  attached  to  the  first — 
as,  from  our  hypothesis,  ihe  hotly  is  solid — the  whole  of  that  body  will  stop. 

It  will  not  be  so  with  the  small  body  which  we  have  yitnply  laid  upon  the 
first.  This  we  may  stop  without  the  other,  to  which  nothing  attaches  it.  un- 
less it  may  be  a  slight  degree  of  friction  ;  and  it  will  experience  no  effect  — 
lose  none  of  its  celerity.  By  virtue  of  this  acquired  and  undiminished  velocity, 
the  small  body  will  separate  itself  from  the  large  one,  and  will  continue  to 
move  in  the  original  direction  until  the  moment  when  its  own  weight  shall 

M 


r" 
402  PERIODIC  COMETS. 


bring  it  to  the  earth.  Hence  it  will  be  understood  how  a  person  is  thrown 
forward  when  his  horse,  in  falling  down,  suddenly  stops  ;  in  what  manner 
travellers  seated  on  the  imperial  of  a  steam-carriage,  moving  with  great  ve- 
locity over  an  iron  railroad,  are  launched  into  the  air  like  so  many  pro- 
jectiles when  an  accident  instantaneously  stops  the  motion  of  these  ingenious 
contrivances.  But  is  our  earth  anything  else  than  a  carriage,  which,  in  its 
progress  through  regions  of  space,  requires  neither  wheels  nor  railways  ?  All 
we  said,  therefore,  is  directly  applicable  to  it. 

Our  velocity  round  the  sun  is  about  twenty  miles  per  second.  If  a  comet 
of  a  sufficient  mass  in  meeting  the  globe  should,  by  a  single  shock,  instanta- 
neously stop  its  motion,  the  bodies  placed  upon  its  surface,  such  as  animated 
beings,  our  carriages,  furniture,  utensils,  all  objects  in  short  not  implanted  di- 
rectly or  indirectly  in  the  soil,  would  fly  off  to  the  point  of  the  earth  shocked 
by  the  comet  with  the  velocity  with  which  they  were  in  common  originally 
endued — a  velocity  of  twenty  miles  per  second.  The  effects  of  such  an  event 
may  be  better  conceived  if  I  here  remark  that  a  twenty-four-pound  shot  has 
not  even  on  its  discharge  from  the  gun  a  velocity  of  more  than  twelve  hun- 
dred feet  per  second.  All  animated  nature  would  certainly  be  destroyed  in  an 
instant. 

As  for  the  waters  of  the  ocean — since  they  are  moveable — as  nothing  fastens 
them  to  the  solid  portion  of  the  earth — they  would  also  be  projected  in  mass 
toward  the  point  of  percussion.  This  terrific  liquid  mass  would  in  its  im- 
petuous course  overthrow  every  obstacle  in  its  way.  It  would  pass  the 
summits  of  the  highest  mountains,  and  in  its  reflux  would  produce  ravages 
scarcely  less  tremendous.  The  disorder  which  is  occasionally  observed  in 
the  strata  of  the  different  sorts  of  earth  forming  the  crust  of  the  globe  is,  it  may 
be  said,  but  a  microscopic  accident  compared  with  the  frightful  chaos  that 
would  inevitably  occur  on  a  shock  of  a  comet  sufficiently  powerful  to  stop  the 
earth. 

It  is  only  necessary  to  diminish  in  some  degree  these  prodigious  effects  to 
find  what  results  would  be  experienced  from  the  shock  of  a  comet,  which, 
without  stopping  our  globe,  should  sensibly  decrease  its  velocity.  Certain  it 
is,  however,  that  the  globe  has  never  been  stopped  completely  ;  for  in  such 
case,  the  central  force  not  being  counterbalanced,  it  must  have  fallen  in  a  di- 
rect line  toward  the  sun,  where  it  would  have  arrived  sixty-four  and  a  half 
days  after  the  shock. 

The  velocity  of  the  earth  and  the  magnitude  of  its  orbit  are  so  nearly  con-  < 
nected,  that  one  cannot  change  without  at  the  same  time  producing  a  variation  ; 
in  the  other.     It  is  unknown  whether  the  dimensions  of  the  orbit  have  remained  ( 
constant ;  nothing,  then,  proves  that  the  velocity  of  the  globe  in  the  course  of   \ 
ages  has  not  been  more  or  less   altered  by  a  cometary  concussion.     At  all  \ 
events,  it  is  incontestable  that  the  inundations  which  would  be  produced  by 
such  an  event  do  not  explain  the  effects  which  the  variations  of  the  earth  has 
undergone,  now  so  well  described  by  geologists. 

A  few  words,  again,  before  quitting  this  subject,  on  the  consequences  of  ( 
cometary  shock  as  respects  its  influence  on  the  rotary  movement  of  the  earth.  > 

The  earth  turns  upon  itself  in  twenty-hours  from  the  west  to  the  east.  The  < 
axis  of  rotation  is  called  the  axis  of  the  world ;  its  extremities,  the  poles  ;  and  | 
the  circle,  equally  distant  from  the  two  poles,  the  equator.  The  circle  of  the  •» 
equator  is  about  25,000  miles  in  circumference. 

Twenty-five  thousand  miles  are  in  consequence  the  space  through  which  a 
point  on  the  equatorial  region,  solid  or  fluid,  passes  every  twenty-four  hours  by  ^ 
the  rotation  of  the  globe.     An  observer  situated  above  the  earth  and  its  atmo-  •: 
sphere,  would  not  be  drawn  into  this  movement,  but  would  see  all  the  parts  of   , 


PERIODIC  COMETS. 


403 


the  equator  pass  below  him  with  a  velocity  of  about  a  thousand  miles  an  hour. 
At  the  polos  themselves  this  kind  of  movement  does  not  exist ;  at  intermediate 
latitudes  it  is  less  than  at  the  equator. 

The  waters  of  the  ocean,  although  they  partake  of  this  rapid  motion,  do  not 
invade  the  surrounding  country,  for  in  every  place  the  shore  has  precisely  the 
same  velocity  as  the  water,  and  under  all  latitudes  the  continents  and  the  seas 
that  bathe  them  are  in  a  relative  repose.  If  this  state  of  things  were  to  change  ; 
if  the  waves  at  any  given  point  were  to  continue  their  original  velocity,  while 
that  of  the  adjacent  land  was  suddenly  to  diminish,  the  ocean  would  at  the  same 
time  overflow  its  limits.  . 

In  order  to  fix  our  ideas,  let  us  imagine  the  oblique  shock  of  a  comet  instan-  * 
taneously  to  turn  the  whole  solid  part  of  the  earth  round  its  diameter  at  the 
point  of  Brest.  That  city  having  become  the  pole,  the  whole  peninsula  of 
Brittany  would  be  in  an  almost  perfect  repose  ;  but  the  ocean  which  washes 
its  shores  on  the  west  would  not  be  so  ;  for  as  we  have  before  observed  on 
the  occasion  of  the  movement  of  translation,  it  would  be  only  resting  on  the 
solid  base  of  which  its  bed  is  formed.  The  waters  would  precipitate  them- 
selves in  mass  upon  a  shore  which  would  no  longer  run  before  them  with  the 
former  velocity  of  the  parallel  of  Brest. 

Behold,  then,  extensive  parts  of  the  continent  inundated,  lofty  regions  buried 
under  the  waves  by  cometary  influence.  But  have  the  marine  deposites  which 
are  actually  discovered  on  the  mountains  been  conveyed  in  this  manner  ?  By 
no  means.  These  deposites  are  frequently  horizontal,  of  great  breadth,  very 
thick,  and  very  regular.  The  varied  and  often  very  small  shells  which,  com- 
pose them  have  preserved  their  crests,  their  most  delicate  points,  their  most 
brittle  parts,  unbroken.  Every  circumstance,  then,  dissipates  the  idea  of  a 
violent  transposition  ;  everything  shows  the  deposites  to  have  been  formed  on 
the  spot.  What  now  remains  to  complete  the'  explanation  without  having  re- 
course to  an  eruption  of  the  sea  ?  It  must  be  admitted  that  the  mountains  and 
undulating  grounds  upon  which  they  are  based  have  risen  up  from  below, 
like  mushrooms ;  that  they  have  grown  up  through  the  bosom  of  the  waters. 
In  1694,  Halley  already  cited  this  hypothesis  as  a  possible  explanation  of  the 
presence  of  marine  productions  upon  the  sides  and  on  the  summits  of  the 
highest  mountains.  This  explanation  was  the  true  one  ;  it  is  at  present  al- 
most generally  admitted.  A  comet  which  should  perceptibly  alter  either  the 
movement  of  rotation  or  the  progress  of  translation  of  the  earth  would,  without 
any  doubt,  occasion  terrific  convulsions  in  the  shell  of  the  globe  ;  but,  it  must 
be  repeated,  these  physical  revolutions  would  differ  in  a  thousand  circumstances  « 
from  those  which  are  at  present  the  objects  of  geological  research. 

The  first  glance  of  the  matter  of  the  present  discourse  may  perhaps  raise  a 
question  with  some  whether  all  comets  must  not  be  periodic  ;  the  difference 
among  them  being  only  that  the  periods  of  a  few  of  them  have  been  discovered, 
and  those  of  the  others  still  remain  unascertained.  It  does  not,  however,  fol- 
low at  all  that  the  comets  move  periodically  round  the  sun.  Newton  showed 
that  the  law  of  gravitation  would  allow  a  body  to  move  under  the  sun's  attrac- 
tion in  any  of  those  species  of  curves  called  conic  sections  ;  and  that  the  par- 
ticular species  in  which  any  body  might  happen  to  move  would  depend  alto- 
gether on  the  velocity  and  direction  in  \Vhich  such  body  might  have  originally 
been  projected.  There  are  three  species  of  conic  sections  :  the  ellipse,  the 
parabola,  and  the  hyperbola.  Now  it  is  only  the  ellipse  which  would  cause 
a  periodical  revolution  round  the  sun.  A  body  moving  in  either  of  the  other 
curves  would  enter  the  system  in  some  determinate  direction,  and  leave  it  in 
another — never  to  return  to  it. 

Although  it  is  not  certainly  ascertained  that  any  comets  have  moved  in 


404  PERIODIC  COMETS. 


parabolas  or  hyperbolas,  it  seems  probable,  nevertheless,  that  such  has  been 
the  case  ;  and  we  may  therefore  consider  with  propriety  comets  to  consist  of 
two  classes  :  first,  those  which  revolve  round  the  sun  in  regular  periods, 
reappearing  in  the  system  after  equal  intervals  of  time  ;  and  secondly,  those 
which  enter  it  once,  and  depart  from  it,  never  to  return. 


Radiation  a  Property  of  Heat. — Prismatic  Spectrum. — Invisible  Rays. — Two  Hypotheses. — Invisi- 
ble Rays  alike  in  their  Properties  to  luminous  Rays. — Discoveries  of  Leslie. — Differential  Ther- 
aoineter. — Radiation,  Reflection,  and  Absorption. — Effect  of  Screens. — Supposed  Rays  of  Cold.— 
Common  Phenomenon  explained. — Theory  of  Dew. 


RADIATION  OF  HEAT. 


407 


RADIATION  OF  HEAT. 


WHEN  any  physical  effect  is  progressively  transmitted  or  propagated  in  ' 
straight  lines,  especially  if  those  lines  proceed  in  various  directions  round  the 
point  whence  the  effect  originates,  the   phenomenon  is  called  radiation.     The 
effect  is  said  to  be   radiated,  and  the  lines  along  which  it  is  transmitted  are 
called  rays. 

Several  natural  phenomena  present  examples  of  this,  of  which  light  is  by 
far  the  most  remarkable.  Every  point  of  a  visible  object  emits  rays  of  light 
which  diverge  in  all  possible  directions  from  that  point,  and  it  is  by  these  rays 
of  light  that  the  point  itself  becomes  visible.  These  rays  of  light,  in  like  man- 
ner, when  they  proceed  from  a  luminous  object,  such  as  the  sun,  or  the  flame 
of  a  lamp,  falling  on  other  objects,  illuminate  them,  and  making  the  points  of 
their  surfaces  become  new  centres  of  radiation,  render  them  visible. 

The  secondary  rays  which  they  thus  radiate  by  reflection  meeting  the  eye, 
produce  a  corresponding  sensation,  which  excites  a  consciousness  of  the  pres- 
ence of  the  object.  Radiation  is  likewise  a  property  of  heat.  A  hot  body, 
such  as  a  ball  of  iron,  raised  to  the  temperature  of  400°,  placed  in  the  middle 
of  a  chamber,  will  transmit  heat  in  every  direction  round  it.  Now  this  heat 
may  easily  be  proved  not  to  be  transmitted  merely  by  means  of  the  surrounding 
air,  for  in  this  case  the  effect  would  be  an  upward  current  of  hot  air,  which 
would  ascend  by  reason  of  its  comparative  lightness ;  on  the  other  hand,  the 
heat  which  proceeds  from  the  ball  is  found  to  be  transmitted  downward,  hor- 
izontally, and  obliquely,  and  in  every  possible  direction.  It  is  likewise  trans- 
mitted almost  instantaneously,  at  least  the  time  of  its  transmission  is  utterly  in- 
appreciable. A  delicate  thermometer,  placed  at  any  distance  below  the  ball, 
will  be  immediately  affected  by  it,  and  the  proof  that  this  is  true  radiation,  is 
found  in  the  fac  that  the  ray»  may  be  intercepted  by  a  screen  composed  of  a 
material  not  pervious  to  heat.  The  rays  may  be  proved  to  be  transmitted  in 
straight  lines  in  exactly  the  same  manner,  and  by  the  same  reasoning,  as  is  ap- 
plied to  rays  o/  light. 


408 


RADIATION  OF  HEAT. 


But  the  radiation  of  heat,  independently  of  any  power  of  transmission  which 
may  reside  in  the  air,  is  put  beyond  dispute  by  the  fact  that  a  thermometer  sus- 
pended in  the  receiver  of  an  air-pump,  when  it  is  exhausted,  is  affected  by  the 
solar  rays  direited  upon  it. 

The  effects  of  the  radiation  of  hot  bodies  prove  that  rays  of  heat  exist  unac- 
companied by  light.  On  the  other  hand,  the  calorific  property  which  con- 
stantly accompanies  the  solar  rays,  as  well  as  the  rays  proceeding  from  flame, 
would  indicate  that  heat  is  a  necessary  concomitant  or  property  of  light.  It  is 
ascertained  also  that  the  calorific  principle  exists  with  different  degrees  of 
energy  in  lights  of  different  colors.  Sir  William  Herschel,  being  engaged  in 
telescopic  observations  on  the  sun,  found  that  the  colored  glasses  which  he 
used  to  mitigate  the  brilliancy  of  that  luminary,  in  order  to  enable  the  eye  to 
bear  its  splendor,  were  cracked  and  broken  in  pieces  by  the  heat  which  they 
absorbed  from  the  light  which  acted  on  them.  This  led  him  to  investigate  the 
calorific  properties  of  the  different  component  parts  of  solar  light ;  and  the  ex- 
periments which  he  instituted  led  to  an  important  extension  of  the  analysis  of 
light  originally  discovered  by  Newton. 

Let  A,  B,  C,  fig.  1,  be  a  section  of  a  glass  prism  cut  at  right  angles  to  its 
length,  and  let  S,  S,  be  a  ray  of  light  admitted  through  a  small  aperture  in  a 
window-shutter,  and  striking  the  surface  of  the  glass  at  S.  It  is  a  property  of 
glass,  which  is  explained  in  optics,  that  when  light  enters  it  in  this  manner, 
the  ray  is  bent  from  its  course,  and  instead  of  proceeding  in  the  direction  S,  S', 
as  it  would  do.  if  it  did  not  encounter  the  glass,  it  is  deflected  upward  in  an- 


other  direction,  forming  an  angle  with  its  original  course.  Now  it  is  found 
that  the  ray  thus  bent  upward  does  not  continue  to  form  one  line  of  white 
light  as  before,  but  it  spreads  or  diverges,  and  if  received  on  the  screen,  instead 
of  illuminating  a  single  spot,  as  it  would  do  if  it  were  not  intercepted  by  the 
prism,  it  covers  an  extended  line  on  the  screen  from  V  to  R,  and  the  length 
of  this  line  increases  if  the  screen  be  moved  from  the  prism,  and  decreases  if 
the  screen  be  moved  toward  the  prism ;  a  necessary  consequence  of  the  di- 
vergence of  the  rays  issuing  from  the  prism.  It  is  also  observed  that  this  line 
of  light  thus  produced  on  the  screen,  is  not  a  uniform  white  light  like  the  spot 
which  would  be  jjroduced  on  a  screen  held  between  A,  B,  C,  and  the  window- 
shutter.  On  the  other  hand,  an  appearance  is  produced  of  a  regular  succession 
of  brilliant  colors,  the  highest  color,  V,  being  violet,  the  next  below  this,  indigo, 
which  is  succeeded  by  blue,  green,  yellow,  orange,  and  finally  red,  in  regular 
succession,  each  color  occupying  a  certain  space  on  the  line  of  light.  This 
effect  is  commonly  called  the  prismatic  spectrum,  and  it  depends  upon  two  facts 
which  are  ascertained  in  optics,  namely :  first,  that  the  ray  of  light,  S,  S,  is 
compounded  of  several  distinct  rays,  which  differ  from  each  other  in  color ; 
secondly,  that  the  glass  of  the  prism  A,  B,  C,  is  capable  of  refracting  or  bending 


n 

09   ( 


RADIATION  OF  HEAT.  409 

out  of  their  course  these  different-colored  lights  in  different  degrees.  Thus  it 
is  capable  of  deflecting  the  violet  light  more  than  the  indigo,  the  indigo  more 
than  the  blue,  and  so  on,  each  color  in  succession  being  more  refrangible  by  the 
prism  than  that  which  occupies  a  lower  place,  and  red  being  therefore  the 
least  refrangible  component  part  of  the  solar  beam. 

Let  us  now  suppose  that  the  bulbs  of  a  series  of  thermometers  are  placed  in 
the  different  colored  lights,  from  the  violet  to  the  red,  in  regular  succession. 
The  relative  heating-powers  of  these  different  colors  will  be  indicated  by  the 
effect  which  they  produce  on  the  several  thermometers,  the  most  powerful 
being  that  which  raises  the  thermometer  exposed  to  its  influence  highest. 
It  is  found  that  the  thermometer  whose  bulb  is  covered  with  the  violet  light  is 
less  elevated  than  that  which  is  exposed  to  the  indigo.  This  again  is  less 
raised  than  that  which  is  exposed  to  the  blue,  and  the  elevation  of  the  several 
thermometers  go  on,  thus  regularly  increasing ;  that  which  is  acted  upon  by 
the  red  light  standing  at  a  greater  elevation  than  any  of  the  others.  Hence  we 
infer  that  the  calorific  power  of  the  red  light  is  greater  than  that  of  any  other 
component  part  of  the  solar  beam.  It  might  at  first  view  be  supposed  that  the 
calorific  power  had  some  dependance  on  or  connexion  with  the  illuminating 
power  of  light,  and  that  the  light  which  was  most  brilliant  would  likewise  be 
most  hot.  This,  however,  is  not  the  fact ;  for  the  most  brilliant  part  of  the 
prismatic  spectrum  is  found  in  the  position  of  the  yellow  light,  and  the  bril- 
liancy gradually  diminishes  toward  the  extremity  of  the  red,  where  the  heat 
is  found  to  be  greatest. 

It  occurred  to  Sir  William  Herschel,  that  as  hot  bodies  emit  calorific  rays 
which  are  not  luminous,  it  was  possibla  that  non-luminous  calorific  rays  might 
exist  in  solar  light  itself.  To  determine  this  point,  he  placed  a  thermometer  in 
the  space  immediately  below  R,  the  red  eSufemity  of  the  spectrum.  He  accord- 
ingly found,  as  he  had  anticipated,  that  the  thermometer  still  continued  to  be 
affected,  and  consequently  that  the  presence  of  calorific  rays,  invisible  and 
non-luminous,  was  manifested ;  but  what  was  more  singular,  he  found  that  the 
calorific  power  of  these  invisible  rays  was  even  greater  than  that  of  the  lumin- 
ous red  rays,  in  fact,  the  maximum  effect  of  the  calorific  rays  was  found  at  a 
point  H,  a  little  below  R.  From  that  point  downward  the  calorific  influence  rap- 
idly diminished,  until  it  altogether  disappeared.  There  are,  therefore,  a  num- 
ber of  invisible  rays  proceeding  from  the  prism,  and  occupying  the  space  H, 
below  R.  These  rays  are  refracted  by  the  prism  in  the  same  manner  as  the 
luminous  rays,  but  the  refraction  is  less  in  quantity.  These  invisible  rays  also 
differ  from  each  other  in  refrangibility,  in  the  same  manner  as  the  luminous 
rays  do,  since  they  occupy  a  space  of  some  extent  below  R.  Those  whose 
position  is  lowest  being  less  refrangible  than  those  nearer  to  the  luminous  rays. 

Soon  after  these  experiments  of  Sir  William  Herschel,  the  attention  of 
several  distinguished  philosophers  was  attracted  to  the  investigation  of  the 
properties  of  the  prismatic  spectrum,  and  among  others  the  late  Dr.  Wollaston, 
Ritter,  and  Beckmann.  It  had  been  long  known  that  the  solar  light  pro- 
duced an  influence  on  certain  chemical  processes.  Thus  the  chloride  of 
silver,  exposed  to  the  direct  rays  of  the  sun,  was  known  to  acquire  a  black 
color.  Chemical  effects  were  also  produced  on  the  oxides  of  certain  metals. 
It  was  shown  by  Scheele  and  others  that  these  effects  were  produced  by  the 
rays  of  light  which  occupy  the  upper  part  of  the  spectrum,  and  not  at  all  by  the 
red  rays.  A  feeble  effect  was  produced  by  the  green  ray,  and  the  chemical 
energy  was  increased  by  ascending  toward  the  violet  ray.  The  circumstance 
of  Herschel  having  discovered  invisible  calorific  rays  under  the  lower  extremity 
of  the  spectrum,  and  even  finding  the  point  of  extreme  energy  in  that  space, 
suggested  to  these  philosophers  the  inquiry,  whether  the  chemical  influence 


410 


RADIATION  OF  HEAT. 


which  was  observed  to  increase  in  ascending  toward  the  upper  extremity, 
might  not  exist  in  the  space  above  that  point,  where  no  luminous  rays  were 
apparent.  They  accordingly  found,  on  exposing  substances  highly  susceptible 
of  this  chemical  influence  in  the  several  spaces  occupying  the  upper  part  of  the 
spectrum,  and  also  in  the  space  immediately  above  V,  that  the  chemical  action 
was  continued,  as  they  had  anticipated,  beyond  the  luminous  rays ;  and  as  the 
maximum  heating-power,  was  found  below  R,  so  the  maximum  chemical  influ- 
ence was  found  to  be  in  the  space  above  V,  in  ascending  beyond  that  point  the 
chemical  influence  rapidly  diminished  until  it  disappeared.  It  follows,  there- 
fore, that  there  are  a  number  of  chemical  rays  proceeding  from  the  prism  more 
refrangible  than  any  luminous  rays,  and  falling  on  the  screen  above  the  point 
V,  in  the  space  C.  These  chemical  rays  are  found  to  be  altogether  destitute 
of  the  heating  principle,  or  at  least,  their  effects  on  a  thermometer  were  inap- 
preciable. 

The  experiments  of  Herschel  were  repeated  by  several  other  philosophers, 
with  various  success,  some  being  unable  to  detect  any  calorific  rays  beyond 
luminous  spectrum,  others  detecting  their  existence,  but  fixing  the  maximum 
calorific  influence  in  the  red  rays,  and  others  again  agreeing  in  all  respects 
with  Herschel.  Of  these,  the  most  valuable  were  experiments  instituted  by 
Berard,  in  the  laboratory  of  Berthollet  at  Paris.  This  philosopher  used  a 
heliostat,  which  is  an  instrument  constructed  for  the  purpose  of  reflecting  a 
ray  of  the  sun  constantly  in  one  direction,  notwithstanding  the  change  of  posi- 
tion of  the  sun  by  its  diurnal  motion.--  He  thus  obtained  a  perfectly  steady  and 
immoveable  spectrum ;  and  he  repeated  the  experiment  under  much  more  fa- 
vorable circumstances  than  those  in  which  Herschel's  investigations  were  con- 
ducted. 

These  experiments  fully  corroborated  the  results  of  former  investigations, 
and  put  beyond  all  question  the  presence  of  invisible  rays  beyond  both 
extremities  of  the  spectrum,  the  one  possessing  the  chemical,  the  other  the 
calorific  property.  Berard,  however,  found  the  maximum  calorific  influence 
exactly  at  the  extremity  of  the  luminous  spectrum,  where  the  bulb  of  a 
thermometer  was  completely  covered  with  red  light.  The  only  difference  then 
which  remained  to  be  accounted  for  in  the  results  of  different  experiments,  was 
the  point  of  maximum  calorific  power,  and  it  was  conjectured  by  Biot  that  this 
apparent  discordance  might  be  accounted  for  by  the  different  materials  of 
which  the  prisms  were  composed.  This  conjecture  was  subsequently  verified 
by  Seebeck,  who  proved  that  the  position  of  greatest  calorific  intensity  de- 
pended on  the  nature  of  the  prism  by  which  the  rays  are  refracted.  He  found 
that  a  hollow  prism,  filled  with  water  or  alcohol,  fixed  the  point  of  greatest 
calorific  intensity  in  the  yellow  rays.  If  filled  with  a  solution  of  corrosive 
sublimate,  or  with  sulphuric  acid,  this  point  was  found  in  the  orange  ray. 
When  a  prism  of  crown-glass  was  used,  it  was  situated  in  the  red  ray,  but 
when  a  prism  of  flint-glass  was  used,  the  point  of  greatest  calorific  intensity 
took  the  position  which  Herschel  assigned  to  it,  in  the  non-luminous  space 
below  the  red  ray.  Thus  all  the  apparent  discordances  in  the  experiment 
were  satisfactorily  accounted  for.  The  results  of  these  experiments  have 
given  rise  to  two  distinct  hypotheses  respecting  the  constitution  of  solar  light. 

In  one  it  is  supposed  that  the  solar  ray,  S,  S,  is  composed  of  three  distinct 
physical  principles  :  the  chemical,  the  luminous,  and  the  calorific.  Let  us 
imagine  a  screen,  M,  N,  fig.  2,  placed  between  the  prism  and  window-shutter, 
which  is  capable  of  intercepting  the  luminous  and  the  calorific  principle,  but 
which  allows  the  chemical  rays  to  be  transmitted.  In  that  case,  the  prism 
will  refract  the  chemical  rays,  and  cause  them  to  diverge  and  occupy  a  space 
on  the  screen  between  the  point  C,  and  C',  corresponding  to  the  highest  point 


r 


RADIATION  OF  HEAT. 


411 


above  the  luminous  spectrum,  where  the  chemical  influence  is  found,  and  C', 
the  lowest  point  in  the  green  light,  where  its  presence  is  discoverable.     Let 

Fig.  2. 


us  next  suppose  the  screen  M,  N,  to  allow  the  luminous  rays  to  be  likewise 
transmitted,  these  will  be  refracted  by  the  prism,  and  will  occupy  the  space  L, 
L/,  corresponding  to  that  already  described  as  limited  by  the  violet  and  red 
lights.  Finally,  if  the  screen  M,  N,  be  removed,  and  all  the  rays  allowed  to 
pass  through  the  prism,  the  calorific  rays  will  occupy  the  space  from  H,  to  H', 
these  being  the  points  where  the  thermometer,  in  ascending  and  descending, 
ceased  to  be  affected.  Thus,  according  to  this  supposition,  three  distinct 
spectra,  if  they  may  be  so  called,  are  formed :  the  chemical  spectrum,  the  lu- 
minous spectrum,  and  tb.3  calorific  spectrum.  These  spectra,  to  a  certain  ex- 
tent, are  superposed,  or  laid  one  upon  another ;  but  the  chemical  spectrum  ex- 
tends beyond  the  luminous,  at  the  upper  part,  while  the  calorific  spectrum  ex- 
tends beyond  the  luminous,  at  the  lower  end.  Each  spectrum  consists  of  rays 
differently  refrangible  by  the  prism ;  and  if  the  middle  ray  be  considered  as 
representing  its  mean  refrangibility,  it  will  follow  that  the  mean  refrangibility 
of  the  chemical  rays  is  greater  than  that  of  the  luminous  rays,  and  the  mean 
refrangibility  of  the  luminous  rays  greater  than  that  of  the  calorific  rays.  If 
prisms  of  different  materials  be  used,  the  relative  degree  of  mean  refrangibility 
will  be  subject  to  change  ;  thus,  the  liquid  prism  above-mentioned,  will  cause 
the  mean  refrangibility  of  the  calorific  rays  to  be  more  nearly  equal  to  that  of 
the  luminous  rays  than  the  glass  prism. 

According  to  the  other  hypothesis,  the  solar  beam  consists  of  a  number  of 
rays,  which  differ  from  each  other  in  their  capability  of  being  deflected  by  any 
refracting  medium.  When  transmitted  through  a  prism  and  received  on  a 
screen,  the  most  refrangible  passes  to  the  highest  point,  and  the  least  refrangi- 
ble to  the  lowest  point,  those  of  intermediate  degrees  of  refrangibility  taking 
intermediate  places.  It  is  assumed  that  the  rays  which  thus  differ  in  refran- 
gibility, have,  also,  different  properties  and  qualities,  and  that  they  possess  the 
same  quality  in  different  degrees.  Thus  rays  of  different  refrangibility  have 
different  illuminating  powers,  and  they  possess  the  chemical  agency  with  dif- 
ferent degrees  of  energy.  So  far  as  the  sensibility  of  thermometers  enable  us 
to  discover  the  existence  of  the  calorific  principle,  it  extends  from  a  certain 
point  below  R,  to  a  certain  point  in  the  violet  light,  but  the  diminution  of  its 
temperature  is  observed  to  be  gradual  in  approaching  its  limit,  and  it  is  consis- 
tent with  analogy  that  it  should  exist,  in  a  degree  not  discoverable  by  thermom- 
eters, beyond  these  points.  Although,  therefore,  the  thermometer  does  not  in- 
dicate the  calorific  principle  in  the  invisible  chemical  rays  at  the  top  of  the 
spectrum,  yet  we  cannot  infer  that  these  rays  are  altogether  destitute  of  that 
principle,  without  assuming  that  the  sensibility  of  thermometers  has  no  limiis. 
In  like  manner  the  chemical  influence,  so  far  as  experiment  determines  its 
presence,  ends  somewhere  in  the  green  light,  about  the  middle  of  the  luminous 


412 


RADIATION  OF  HEAT. 


spectrum,  but  the  diminution  of  its  influence  to  this   point,  is  gradual ;  and  it 
cannot  be  inferred  with  certainty,  that  it  might  not  exist  in  less   degree  in  the  ( 
rays  below  this  limit,  and  even  in  those  invisible  rays   which   are   beyond  the 
red  ray,  unless  we   assume  (hat  there  are  no  tests  of  chemical   influence  of 
greater  sensibility  than  those  which  have  been  used  by  the  philosophers  who  ,' 
instituted  experiments  on  this  subject. 

The  presence  of  the  luminous  quality  is  determined  by  its  effect  on  tho  hu- 
man eye,  and  the  discovery  of  it  must,  therefore,  be  limited  to  the  sensibility 
of  that  organ.  To  pronounce  that  there  are  no  luminous  rays  beyond  the  lim- 
its of  the  visible  spectrum,  is  to  declare  that  the  sensibility  of  the  human  eye 
is  infinite.  Now,  it  is  notorious,  not  only  that  the  sensibility  of  sight  in  dif- 
ferent individuals  is  different,  but  even  that  the  sensibility  of  the  eye  of  the  ( 
same  person  at  different  times,  is  susceptible  of  variation.  If  a  person  pass 
suddenly  from  a  strongly-illuminated  apartment  into  a  chamber,  the  windows 
of  which  are  closed,  he  will  be  immediately  impressed  with  a  sensation  of  ut- 
ter darkness,  and  will  be  totally  unable  to  discover  any  object  in  the  room  ;  but 
when  he  has  remained  some  time  in  the  darkened  room,  he  will  begin  to  be 
sensible  of  the  presence  of  light,  and  will,  at  length,  even  discern  distinct  ob- 
jects. In  this  case,  the  eye,  while  exposed  to  the  intense  light  of  the  first 
chamber,  accommodated  its  powers  to  the  quantity  of  light  to  which  it  was  ex- 
posed, and,  by  a  provision  of  nature,  limited  its  sensibility  in  proportion  as  the 
light  was  abundant.  Passing  suddenly  into  the  darkened  chamber,  where  a 
very  small  quantity  of  light  was  admitted  through  the  crevices  of  the  windows, 
the  eye  was  incapable,  in  its  actual  state,  of  any  perception  of  light,  notwith- 
standing the  undoubted  presence  of  that  physical  principle  ;  but  when  time 
was  allowed  for  the  organ  to  adapt  itself  to  the  new  circumstances  in  which  it 
was  placed,  its  sensibility  was  increased,  and  a  distinct  perception  of  light  ob- 
tained. 

It  is,  therefore,  perfectly  certain,  that  the  sensibility  of  the  eye  is  variable 
in  the  same  individual,  and  even  changeable  at  will.  It  is  likewise  perfectly 
certain,  that  different  individuals  have  different  sensibilities  of  sight,  one  indi- 
vidual being  capable  of  perceiving  light  which  is  not  visible  to  another.  Cir- 
cumstances render  it  highly  probable  that  many  inferior  animals  have  a  sensa- 
tion of  light,  under  circumstances  in  which  the  human  eye  has  no  perception 
of  it ;  and  it  is,  therefore,  consistent  with  analogy  to  admit,  at  least,  the  possi- 
bility, if  not  the  probability,  that  the  invisible  rays  which  fall  on  the  space  be- 
yond each  extremity  of  the  luminous  spectrum,  may  be  of  the  same  nature  as 
the  other  rays  of  light,  although  they  are  incapable  of  exciting  the  retina  of 
the  human  eye  in  a  sufficient  degree  to  produce  sensation.  This,  probably, 
will  receive  still  further  support  and  confirmation,  if  we  can  show  that  these 
nvisible  rays  enjoy  all  the  optical  properties,  save  and  except  that  of  affecting 
he  sight,  which  other  luminous  rays  possess. 

It  has  already  appeared  that  the  non-luminous  calorific  rays,  H,  fig.  2,  are  re- 
racted  by  transparent  media  in  different  degrees  ;  this  refraction  is  also  proved 
o  be  subject  to  the  same  laws  as  the  refraction  of  luminous  rays.  Thus  the 
sine  of  the  angle  of  incidence  bears  a  constant  ratio  to  the  sine  of  the  angle 
of  refraction,  when  the  refracting  medium  is  given,  and  refracting  media  of  dif- 
erent  kinds  refract  these  rays  in  different  degrees. 

If  the  invisible  calorific  rays  at  H,  fig.  3,  be  allowed  to  pass  through  a  hole 
n  the  screen,  and  be  received  on  the  plane  reflector  M,  they  will  be  reflected  in 
he  direction  M  H,  in  the  same  manner  as  a  ray  of  light  would  be  under  the 
same  circumstances ;  that  is,  the  rays  M  H'  and  M  H  will  be  equally  inclined 
o  the  plane  of  the  reflector.  If  rays  of  heat  be  received  on  a  concave  reikc 
or,  they  will  be  reflected  to  a  focus  in  exactly  the  same  manner  as  rays  of 


RADIATION  OF  HEAT. 


413 


light ;  and  in  a  word,  all  the  phenomena  explained  in  optics,  concerning  the 
reflection  of  light  by  surfaces,  whether  plane  or  curved,  are  found  to  accompa- 
ny the  reflection  of  the  non-luminous  calorific  rays.  This  is  actually  found  to 
take  place,  whether  the  non  luminous  rays  be  those  which  are  obtained  by  re- 
flecting the  solar  light  by  the  prism,  or  produced  from  a  heated  body 

Fig.  3. 


In  the  experiments  of  Berard,  the  question  of  the  identity  of  the  calorific 
and  luminous  rays  was  submitted  to  tests  even  more  severe.  There  are  certain 
crystallized  bodies  called  double  refracting  crystals,  which  produce  peculiar 
effects  on  the  rays  of  light  transmitted  through  them.  Let  A  B,  fig.  4,  be  the 
surface  of  a  piece  of  Iceland  spar,  or  carbonate  of  lime,  which  is  one  of  this 
class  of  bodies,  and  let  L  L'  be  a  ray  of  light  striking  obliquely  on  the  surface 
of  this  crystal ;  if  the  crystal  were  common  glass  this  ray  would  be  bent  out 
of  its  course,  and  would  pass  through  it  in  another  direction  ;  but,  in  the  case 
of  Iceland  spar  it  is  observed  that  the  ray  L  L'  is  divided  into  two  distinct 
rays,  which  proceed  in  two  different  directions,  L/  M,  L'  M',  through  the 
crystal.  Let  a  non-luminous  calorific  ray,  taken  from  the  lower  end  of  the 
spectrum,  be  in  like  manner  transmitted  to  the  surface  of  such  a  crystal,  it 
will  be  found,  that,  in  penetrating  the  crystal,  it  will  be  divided  into  two  rays, 
and  that  these  two  rays  will  be  deflected  according  to  the  same  laws,  exactly 
as  a  luminous  ray  is  under  the  same  circumstances. 


Fig.  4. 


A  luminous  ray  thus,  after  its  transmission  through  a  double  refracting  crys- 
tal, is  observed  to  have  received  a  peculiar  physical  modification,  which  is 


414 


RADIATION  OF  HEAT. 


called  polarization.  In  fact,  a  mirror,  placed  in  a  certain  inclined  position, 
above  or  below  one  of  these  two  rays,  is  capable  of  reflecting  them  in  the  or- 
dinary  way  ;  but  if  placed  in  the  same  oblique  position,  on  either  side  of  them, 
it  becomes  utterly  incapable  of  reflecting  them.  The  other  ray  possesses  a 
similar  quality,  but  the  position  of  the  non-reflecting  side  is  reversed.  Now, 
the  two  rays  into  which  a  non-luminous  calorific  ray,  transmitted  through  such 
a  crystal,  is  resolved,  are  found  to  possess  precisely  the  same  property — 
they  are  polarized. 

A  ray  of  light  falling  on  a  reflecting  surface  at  a  certain  angle,  the  magni- 
tude of  which  will  depend  on  the  nature  of  the  surface,  is  found,  when  reflect- 
ed in  the  ordinary  way,  to  be  polarized  or  put  into  the  physical  state  just  now 
mentioned,  to  result  from  the  double  refraction  of  a  crystal.  It  is  capable  of 
being  reflected  by  an  oblique  mirror  placed  above  or  below  it,  but  it  is  incapa- 
ble of  being  reflected  by  the  same  mirror,  similarly  placed,  on  either  side.  A 
non-luminous  calorific  ray,  whether  proceeding  from  the  prism,  or  from  a  hot 
body  reflected,  is  found  to  undergo  the  same  effect,  and  to  be  also  polarized. 

In  the  experimental  investigation  of  the  phenomena  attending  thp  radiation 
of  heat,  it  is  necessary  to  distinguish  the  effect  of  radiated  heat  from  the  cas- 
ual variation  of  the  temperature  of  the  air  in  the  apartment  in  which  the  exper- 
iment may  be  conducted.  The  use  of  the  thermometer  would,  in  this  case,  be 
attended  with  material  inconvenience,  inasmuch  as  it  would  be  extremely  diffi- 
cult to  distinguish  the  effect  of  the  heat  radiated,  from  the  casual  change  of 
temperature  of  the  medium  in  which  the  thermometer  is  placed.  A  second 
thermometer,  it  is  true,  might  be  used  in  such  experiments,  the  variations  of 
which  would  show  the  change  of  temperature  of  the  medium ;  but  this  second 
thermometer  could  never  be  placed  exactly  in  the  same  position  as  the  ther- 
mometer affected  by  the  radiant  heat :  and  it  would  not  follow  that  the  changes 
of  temperature  of  two  different  parts  of  the  same  chamber  would,  necessarily, 
be  exactly  alike.  An  instrument,  therefore,  which  is  not  affected  by  any 
change  of  temperature  in  the  medium  in  which  it  is  placed  would  be  capable 
of  giving  much  more  accurate  indications  for  such  a  purpose.  Such  an  in- 
strument was  invented  and  applied  by  Sir  John  Leslie,  in  his  experiments  on 
radiant  heat,  the  results  of  which  have,  so  justly,  placed  that  distinguished 
philosopher  in  the  first  rank  of  modern  discoverers  in  physics. 

The  differential  thermometer  of  Leslie  consists  of  a  small  glass  tube,  fig.  5, 
at  each  extremity  of  which  is  placed  two  thin  hollow  bulbs,  F  E,  of  glass,  and 
the  tube  is  bent  into  the  rectangular  form,  E  A  B  F,  and  supported  on  a  stand 
S,  the  bulbs  being  presented  upward.  This  tube  contains  a  small  quantity  of 
sulphuric  acid,  tinged  red  with  carmine,  to  render  it  easily  visible,  filling  the 
greater  part  of  the  legs  and  horizontal  branch.  To  one  of  the  legs,  F  B,  a 


RADIATION  OF  HEAT. 


415 


scale  is  attached,  divided  into  100°,  and  the  liquid  contained  in  the  tube  is  so 
disposed,  that  it  stands  in  the  graduated  leg  opposite  that  point  of  the  scale 
which  is  marked  0°,  when  both  bulbs  are  exposed  to  the  same  temperature. 
The  glass  ball  attached  on  the  leg  of  the  instrument  which  bears  the  scale,  is 
called  the  focal  ball.  Dry  air  is  contained  in  the  balls  above  the  sulphuric 
acid,  which,  not  being  vaporizable,  does  not  affect  the  pressure  of  the  air  above 
it  by  its  vapor. 

If  this  instrument  be  brought  into  a  warm  room,  the  air  contained  in  both 
bulbs  is  equally  affected  by  the  increase  of  temperature,  and  tkerefore  no  change 
takes  place  in  the  position  of  the  liquid ;  and  whatever  changes  the  tempera- 
ture of  the  apartment  may  undergo,  for  the  same  reason,  produce  no  effect  on 
the  instrument.  Suppose,  however,  that  the  focal  ball  F  is  submitted  to  the 
effect  of  heat,  from  which  the  ball  E  is  free ;  then  the  air  in  F  will  acquire  a 
greater  degree  of  elasticity,  while  the  air  in  E  maintains  its  former  pressure ; 
the  liquid  in  the  leg  F  B  will,  therefore,  be  pressed  downward,  until  the  in- 
creased space  obtained  by  the  air  in  F,  and  the  diminished  space  into  which 
the  air  in  E  is  pressed  by  the  ascent  of  the  liquid  in  A  E  is  such,  that  the  pres- 
sure of  the  air  in  the  two  balls,  by  diminishing  that  of  the  air  in  F  and  increas- 
ing that  of  the  air  in  E,  acquires  a  difference  which  is  equal  to  the  weight  of 
the  column  by  which  the  height  of  the  liquid  in  A  E  exceeds  the  height  of  the 
liquid  in  B  F.  In  fact,  the  least  attention  to  the  instrument  will  show,  that  the 
difference  of  the  heights  of  the  columns  of  liquid  in  the  two  vertical  tubes, 
will  represent  the  difference  between  their  pressures  of  the  air  contained  in  the 
two  bulbs.  It  is  from  this  property  of  indicating,  not  the  absolute  temperatures, 
but  the  difference  of  the  two  adjacent  points,  that  the  instrument  has  received 
its  name. 

Let  M  M',  fig.  6,  be  two  concave  mirrors,  placed  face  to  face,  at  the  distance 
of  ten  or  twelve  feet,  having  a  certain  form  called  parabolic,  the  property  of 
which  we  shall  now  describe : — If  the  flame  of  a  candle,  or  any  other  source 
of  light,  be  placed  at  a  point/,  called  the  focus  of  the  mirror  M,  the  rays  of 
light  which  proceed  from  it  in  every  direction,  and  strike  on  the  concave  surface 
of  the  mirror  M,  will  be  reflected  in  parallel  lines  toward  the  mirror  M'.  .  When 
these  parallel  rays  encounter  the  surface  of  the  reflector  M',  they  will  be  again 
reflected  by  it,  in  lines  which  all  converge  to  the  same  point/',  which  is  the 
focus  of  M'.  Now,  instead  of  a  luminous  flame,  let  amadou,  gunpowder,  or 
other  matter  easily  inflammable,  be  placed  in  the  focus/  and  place  a  red-hot 
metallic  ball  in  the  other  focus /'.  In  a  few  minutes  the  amadou  or  gunpow- 
der will  be  inflamed  or  exploded  by  the  heat  radiated  by  the  ball  and  collected 
at  (he  point  fby  the  reflectors  M  M'. 

Fig.  6. 


Bui  to  prove  that  the  rays  of  non-luminous  heat  are  similarly  reflected,  let 
the  red-hot  ball  be  removed,  and  a  hollow  ball  of  metal,  filled  with  boiling  wa- 
ter, be  substituted  for  it  at/' ;  let  the  focal  ball  of  a  differenti^  thermometer  be 


• 


placed  at/*—  instantly  the  liquid  will  be  depressed  in  the  leg  of  the  thermome- 
ter, and  the  presence  of  the  source  of  heat  greater  than  that  of  the  surrounding 
nit  ilium  will  be  thus  indicated.  That  this  source  of  heat  is  derived  from  the 
vessel  of  hot  water  in  the  focus  f  may  be  easily  proved.  Let  this  vessel  be 
removed,  and  immediately  the  liquid  in  the  thermometer  will  rise  to  its  ordina- 
ry level ;  but  it  may  be  said  that  the  effect  is  produced  on  the  thermometer  by 
the  heat  transmitted  direct  from/7  toy.  This,  however,  may  be  .proved  not  to 
be  the  case ;  for  let  the  hot  water  be  placed  as  before  at/7,  and  let  the  mirror 
M  be  removed,  the  effect  produced  on  the  thermometer  will  immediately  erase. 

The  rapidity  with  which  the  heat  thus  radiated  from/'  and  reflected  by  ihe 
mirrors  to/is  propagated,  may  be  shown  by  interposing  between/*and  f  a  screen, 
composed  of  any  substance  not  pervious  to  calorific  rays.  When  the  screen  is 
thus  interposed,  the  liquid  in  the  thermometer  will  recover  its  ordinary  level ; 
but  the  moment  the  screen  is  again  withdrawn,  the  liquid  instantly  fails  in  the 
focal  leg ;  and  this  takes  place  by  whatever  distance  the  two  mirrors  may  be 
separated. 

Of  the  two  hypotheses  already  mentioned,  which  have  been  proposed  for  the 
explanation  of  the  phenomena  observed  in  the  prismatic  spectruin,  that  which 
supposes  light  to  consist  of  three  distinct  principles  seems  to  be  attended  with 
a  variety  of  circumstances  which  throw  improbability  upon  it.  The  three 
principles  thus  distinguished  enjoy  the  same  leading  properties.  They  all  obey, 
with  the  most  minute  precision,  the  ordinary  laws  of  optics,  and,  in  fact,  pos- 
sess every  property  of  light  except  the  most  prominent  and  obvious  one  of 
affecting  the  sight.  The  other  hypothesis,  on  the  contrary,  has  the  advantage 
of  great  simplicity ;  in  it  light  is  considered  as  compounded  of  a  number  of 
rays  unequally  refrangible,  and  possessing,  consequently,  different  influences 
on  other  bodies,  and  on  vision.  The  calorific  and  chemical  properties  which 
disappear  alternately  at  the  extremities  of  the  spectrum,  are  cqnsidered  as  de- 
pending on,  or  connected  with,  the  difference  of  refrangibility,  and  as  becom- 
ing insensible  under  different  variations  in  that  property;  it  is  very  conceivable 
that  the  calorific  power  of  rays  may  vary  in  some  inverse  proportion  with  re- 
spect to  their  refrungibility,  while  the  energy  of  the  chemical  power  may  change 
in  a  contrary  direction.  In  a  word,  since  all  the  rays  refracted  by  the  prism 
agree  in  by  for  the  greater  number  of  their  properties,  and  disagree  only  in 
some  peculiar  effects ;  and  since  even  this  disagreement  may  be  considered 
more  as  apparent  than  real,  and  may  arise  from  the  want  of  sufficient  sensibili- 
ty in  the  tests  by  which  these  effects  may  be  practically  ascertained,  it  seems 
more  philosophical  to  regard  all  the  rays  as  of  one  species,  than  to  adopt  an 
hypothesis  which  classes  things  alike  in  all  their  leading  qualities,  under  differ- 
ent denominations.  It  is  not,  however,  necessary  to  assume  either  supposition, 
nor  to  adopt  it  as  the  basis  of  reasoning.  Experiment  is  the  sure  and  only 
guide  in  physics ;  and  whether  heat  be  obscure  and  imperceptible  light,  or  a 
distinct  physical  agent,  we  shall  regard  it  as  a  principle  attended  with  certain 
sensible  effects,  capable  of  being  ascertained  by  experiment  or  observation,  and 
from  such  effects  arid  such  only,  can  legitimate  inferences  be  drawn. 

The  heat  which  passes  from  a  body  by  radiation  has  a  tendency  to  cause  its 
temperature  to  fall ;  and,  the   rate  of  this  process  of  cooling,  is  propomoiiate  * 
to  the  difference  between  the  temperature  of  the  body  and  that  of  the  surround- 
ing medium,  when  this  difference  is  not  of  very  great  amount.     It  follows,  the,:,  > 
that  a  hot  body  at  first,  when  its  temperature  greatly  exceeds  that  of  the  sur- 
rounding air,  cools  rapidly  ;  but  as  its  temperature  falls,  and  approaches  ncaier  ) 
to  equality  with  the  temperature  of  the  medium  in  which  it  is  placed,  the  rate  i 
at  which  it  cools  gradually  diminishes.     This  law  of  bodies  cooling  was  !ir>*  .• 
observed  by  Newton,  and  reduced  to  an  exact  mathematical  expression,  i>y  < 


RADIATION  OF  HEAT. 


417 


which  the  rates  of  the  cooling  of  bodies  under  given  circumstances  might  be 
calculated  with  precision.  Numerous  experiments  have  been  made  on  the 
rates  at  which  bodies  cool  in  media  of  lower  temperatures,  and  become  hot  in 
media  of  higher  temperatures  ;  and  the  results  of  observation  have  been  found 
to  have1  a  very  exact  conformity  with  those  which  are  calculated  on  the  New- 
tonian law,  provided  the  difference  of  the  temperature  does  not  exceed  a  cer- 
tain limit. 

As  radiation  takes  place  altogether  from  the  points  of  a  body  which  are  on  or 
very  near  its  surface,  it  may  naturally  be  expected  that  the  radiating  power  of 
bodies  will  mainly  depend  on  the  nature  of  their  surfaces.  This  idea  suggested 
to  Sir  John  Leslie  a  series  of  experiments  which  led  to  some  of  the  most  re- 
markable discoveries  ever  made  respecting  the  radiation  of  heat.  In  these  ex- 
periments, cubical  vessels,  or  canisters,  of  tin  were  employed,  the  side  of  which 
varied  from  three  inches  to  ten.  These  vessels  were  filled  with  hot  water  arid 
placed  before  a  tin  reflector,  M,  fig.  7,  like  those  already  described,  in  the  focus 
y"of  which  was  placed  the  focal  bnll  of  a  differential  thermometer.  The  face 
of  the  canister  c  containing  water  being  presented  to  the  reflector,  rays  of  heat 
proceeded  directly  from  it,  and  striking  on  the  reflector  M  were  collected  into 
the  focus /"on  the  ball  of  the  thermometer.  The  depression  of  the  liquid  in  the 
thermometer  furnished  a  measure  of  the  intensity  of  the  heat  radiated. 


The  first  consequence  of  these  experiments  was  a  verification  of  the  law  al- 
eady  mentioned,  that,  other  things  being  the  same,  the  intensity  of  the  radia- 
tion was  always  proportional  to  the  difference  between  the  temperature  of  the 
water  and  the  temperature  of  the  air.     Thus  suppose,  the  temperature  of  the 
air  being  50°,  that  of  the  water  100°,  that  the  thermometer  fall  20°  ;  then  if 

y  the  temperature  of  the  air  were  the  same,  and  the  temperature  of  the  water  at 
150°,  the  thermometer  would  fall  40°  ;  and  again,  if  the  temperature  of  the 
water  were  200°,  the  thermometer  would  fall  60°,  and  so  on. 

If,  while  the  temperature  of  the  water  remains  the  same,  the  canister  is 
placed  successively  at  different  distances  from  the  reflector,  it  is  found  that  the 
thermometer  is  differently  affected  ;  and  that,  as  the  distance  of  the  radiating 
svrface  from  the  reflector  is  increased,  the  intensity  of  its  effect  is  in  the  same 
proportion  diminished.  It  was  likewise  ascertained,  that  if  the  magnitude  of 

(  the  radiating  surface  were  increased,  the  distance  remaining  the  same,  the  in- 
tensity of  the  radiation  would  be  in  the  direct  proportion  of  the  magnitude  of 
the  radiating  surface.  From  this  it  necessarily  follows,  that  if  the  magnitude 
of  the  radiating  surface  be  increased  in  the  same  proportion  as  the  distance  is 
increased,  the  intensity  of  the  radiation  will  remain  the  same  ;  for  as  much  is 
gained  by  the  increased  magnitude  of  the  radiating  surface,  as  is  lost  by  the 
increased  distance  ;  and  accordingly  it  was  found  that  the  thermometer  was 
equally  affected  by  a  surface  of  double  magnitude  at  a  double  distance,  and  of 
triple  magnitude  at  a  triple  distance. 

We  have  hitherto  supposed  that  the  face  of  the  canister  is  placed  parallel  to 


418 


RADIATION  OF  HEAT. 


the  reflector,  so  that  the  rays  of  heat  take  a  direction  perpendicular  to  the  ra- 
diating surface  ;  but  if  each  point  of  the  surface  radiates  heat  in  all  possible 
directions,  it  will  follow  that  the  surface,  when  presented  obliquely  to  the  mir- 
ror, will  still  affect  the  thermometer.  When  the  surface  of  the  canister  was 
presented  thus  obliquely,  the  effect  produced  on  a  thermometer  was  found  to 
be  the  same  as  would  be  produced  by  a  surface  of  less  magnitude,  in  the  pro- 
portion of  the  actual  magnitude  of  the  radiating  surface  to  that  of  its  projection. 
It  follows,  therefore,  that  the  more  inclined  the  radiating  surface  is  to  the  di- 
rection of  the  radiation,  the  less  will  be  the  intensity  of  the  radiation  ;  but  in 
general  this  intensity  will  be  diminished,  in  the  proportion  of  the  actual  magni- 
tude of  the  radiating  surface  and  the  magnitude  of  its  orthographical  projection 
on  the  mirror. 

We  have  hitherto  supposed  the  nature  of  the  radiating  surface  to  remain  un- 
altered. The  effect  of  any  change  in  this,  however,  may  be  easily  ascertained 
by  covering  the  side  of  the  canister  with  the  different  substances  the  effect  of 
which  is  required.  Thus,  let  the  four  sides  of  the  canister  be  coated  with  dif- 
ferent substances — one  with  lampblack,  another  with  isinglass,  another  with 
china  ink,  and  a  fourth  left  uncovered,  and  therefore  presenting  a  surface  of 
polished  tin.  The  vessel  being  now  filled  with  hot  water,  all  the  surfaces  will 
acquire  the  same  temperature,  and  may  be  successively  presented  to  the  re- 
flector at  the  same  distance  ;  they  will  be  observed  to  produce  different  effects 
on  the  thermometer.  If  the  lampblack  depresses  the  liquid  100°,  the  china 
ink  will  depress  it  88°,  the  isinglass  80°,  and  the  tin  12°.  The  great  differ- 
ence in  the  radiating  power  produced  by  the  different  nature  of  the  surfaces 
will  be  hence  very  apparent. 

The  inquiries  of  Professor  Leslie  were  directed  to  this  point  with  great  ef- 
fect, and  he  found  that  various  substances  possessed  very  different  radiating 
powers.  In  general,  metallic  bodies  proved  to  be  the  most  feeble  radiators. 
The  following  table  exhibits  the  relative  power  of  radiation  of  different  sub- 
stances, as  exhibited  in  these  experiments  : — 


Lampblack 100 

Water,  by  estimate ' 100 

"Writing-paper 98 

Rosin  " 96 

Sealing-wax 95 

Crown  glass 90 

China  ink 88 

Ice 85 

Minium ...  80 


Isinglass 80 

Plumbago 75 

Tarnished  lead 45 

Mercury 20 

Clean  lead 19 

Iron  polished 15 

Tin-plate 12 

Gold,  silver,  copper 12 


When  the  substance  forming  the  radiating  surface  remains  of  the  same  na- 
ture, its  radiating  power  is  subject  to  considerable  elevation,  according  to  its 
state  with  respect  to  smoothness,  or  roughness.  In  general,  the  more  polished 
and  smooth  a  surface  is,  the  more  feeble  will  be  its  power  of  radiation.  Any- 
thing which  tarnishes  the  surface  of  metal  also  increases  its  radiating  power. 
In  the  preceding  table,  tarnished  lead  radiated  45°,  while  clean  lead  radioed 
only  19°.  If  the  surface  of  a  body  be  rendered  rough  by  mechanical  means, 
such  as  scratching  with  a  file,  or  with  sand-paper,  the  radiating  power  is  in- 
creased. 

Leslie  also  proved  that  the  particles  forming  the  surface  of  a  body  are  not 
the  only  ones  which  radiate,  but  that  radiation  proceeds  from  particles  at  a  cer- 
tain small  depth  within  the  surface.  He  determined  this  curious  point  by  cov- 
ering one  side  of  a  vessel  containing  hot  water  with  a  thin  coating  of  jelly,  and 
pulling  on  another  side  four  times  the  quantity.  In  each  case,  when  dried,  the 


EADIATION  OF  HEAT. 


419 


:  jelly  formed  an  extremely  thin  film  on  the  surface.     Now,  although  the  nature 
'.  of  these  two  surfaces  was  precisely  the  same  with  respect  to  material  and 
>  smoothness,  they  were  found  to  radiate  very  differently  ;  the  thinner  film  de- 
i  pressed  the  thermometer  38°,  while  the  thicker  depressed  it  54°.     The  in- 
creased radiation  must  in  this  case  be  attributed  to  the  increased  quantity  of 
radiating  material.     The  increase  of  radiation  was  found  to  continue  until  the 
coating  amounted  to  the  thickness  of  about  1000th  part  of  an  inch,  after  which 
no  further  increase  took  place.    It  might,  therefore,  be  inferred  that,  in  the  case 
of  the  surface  of  jelly,such  as  that  here  submitted  to  experiment,  the  particles 
radiate  heat  from  a  depth  below  the  surface  equal  to  the  1000th  part  of  an  inch. 
A  similar  effect  was  found  with  other  substances.    In  the  case  of  metals,  no 
increase  was  observed  when  leaf  metal  of  gold,  silver,  and  copper,  was  used  ; 
but  on  using  glass,  enamelled  with  gold,  a  slight  increase  of  radiating  power 
was  produced,  as  compared  with  the  ordinary  radiating  power. 

In  these  experiments  the  heat  radiated  undergoes  three  distinct  physical  ef- 
fects :  1.  The  radiation  from  the  surface  of  the  canister;  2.  The  reflection 
from  the  surface  of  the  reflector  ;  3.  Absorption  by  the  glass  surface  of  the 
focal  ball,  for  without  such  absorption  the  air  included  could  not  be  affected. 
Now,  of  these  three  effects,  we  have  hitherto  examined  but  one,  viz.,  the  radi- 
ating power.  Let  us  consider  what  circumstances  affect  the  power  of  reflect- 
ing heat,  and  the  power  of  absorbing  it. 

The  reflector  used  in  the  experiments  already  described  was  formed  of  pol- 
ished tin.  If,  instead  of  this,  a  reflector  of  glass  be  used,  it  will  be  found  that 
the  thermometer  will  be  affected  in  a  much  less  degree,  whence  we  infer  that 
glass  is  a  worse  reflector  than  metal.  If  the  surface  of  the  reflector  be  coated 
with  lampblack,  all  reflection  whatever  is  destroyed,  and  no  effect  is  produced 
on  the  thermometer.  Thus  it  appears  that,  as  different  surfaces  have  different 
radiating  powers,  so  also  they  have  different  reflecting  powers  ;  but  to  deter- 
mine the  reflecting  power  of  different  surfaces  with  great  exactness,  Leslie  re- 
ceived the  rays  proceeding  from  the  reflector  M,  fig.  8,  on  a  flat  reflecting  sur- 

Fig.  8. 


face,  S,  before  they  came  to  a  focus ;  and  by  the  laws  of  reflection  they  were 
reflected  to  another  focus,/,  as  far  before  the  reflecting  surface  S  as  the  focus 
/,  to  which  they  would  have  proceeded  is  behind  it.  The  reflecting  power  of 
the  surface  S  will,  therefore,  be  determined  by  the  intensity  of  the  heat  in  the 
focus/',  compared  with  the  intensity  which  it  would  have  had  in  the  focus/, 
had  the  rays  been  allowed  to  converge  to  that  point.  By  experiments  con- 
ducted in  this  way,  and  exposing  the  surfaces  of  different  substances  to  receive 


the  rays,  as  at  S,  Leslie  determined  the  reflecting  powers  of  several  bodies  as 
follow : — 


Brass 100 

Silver 90 

Tin-foil 85 

Block  tin , 80 

Steel 70 


Lead 60 

Tin-foil,  softened  with  mercury 10 

Glass    10 

Glass,  coated  with  wax  or  oil 5 


If  these  results  be  compared  with,  the  table  of  radiating  powers  in  page  476, 
it  will  be  found  that,  generally,  those  substances  which  are  the  best  radiators 
are  the  worst  reflectors,  and  vice  versa.  In  fact,  in  proportion  as  the  radiating 
power  is  increased,  the  reflecting  power  is  diminished.  This  analogy  is  fur- 
ther confirmed  by  the  fact,  that  the  reflecting  power  is  increased  by  every  in- 
crease in  smoothness  or  polish  of  the  reflecting  surface  ;  while,  on  the  contra- 
ry, this  cause,  as  we  have  seen,  diminishes  its  radiating  power.  The  effect 
of  coating  the  reflector  with  a  thin  film  of  jelly  or  other  substance  has,  in  con- 
formity with  the  same  analogy,  exactly  a  contrary  effect  to  that  which  such  a 
coating  produced  on  radiation.  It  was  found  that,  as  the  thickness  of  the  coating 
increased  to  a  certain  limit,  the  intensity  of  the  radiation  was  likewise  increased. 
On  the  other  hand,  in  the  case  of  reflection,  the  intensity  of  the  reflection  is 
diminished  in  proportion  as  the  thickness  of  the  coating  is  increased. 

Let  us  now  consider  the  effect  produced  on  the  focal  ball,  which  will  lead 
us  to  determine  the  different  powers  of  absorption  which  different  bodies  pos- 
sess. In  all  the  experiments  to  which  we  have  hitherto  alluded,  the  focal  ball 
has  presented  a  polished  surface  of  glass,  and  the  effect  produced  on  a  ther- 
mometer, other  things  being  the  same,  has  depended  on  the  absorptive  power 
of  the  glass  over  the  heat  incident  upon  it.  When  radiant  heat  strikes  on  the 
surface  of  different  substances,  we  have  seen  that  a  portion  of  it  is  reflected, 
and  that  this  portion  varies  according  to  the  nature  of  the  substance  and  ac- 
cording to  the  state  of  the  surface.  It  is  clear  that  all  that  portion  of  the  inci- 
dent heat  which  is  not  reflected  must  be  absorbed  ;  and  we  are  led,  therefore, 
by  analogy  to  the  inference  that,  in  proportion  as  the  reflecting  power  of  a  sur- 
face is  great,  its  absorptive  power  is  small,  and  vice  versa. 

To  bring  this  inference  to  the  test  of  experiment,  let  the  ball  of  a  thermome- 
ter be  coated  with  tin-foil,  which  was  found  to  be  one  of  the  best  reflectors. 
If  the  side  of  the  vessel  coated  with  lampblack,  while  the  focal  ball  is  covered 
with  tin-foil,  be  now  presented  to  the  reflector,  the  thermometer  will  only  indi- 
cate 20°,  whereas  it  indicates  100°  when  the  surface  of  the  ball  was  uncovered. 
If  the  bright  side  of  a  canister  be  presented  to  the  reflector  when  the  focal  ball 
is  uncovered,  the  thermometer  indicates  12°  ;  but,  if  the  focal  ball  be  covered 
with  tin-foil,  it  will  indicate  only  2^-°.  Thus  we  see  that  the  anticipation  of 
theory  is  confirmed.  If  the  surface  of  the  tin-foil  be  rubbed  with  sand-paper, 
so  as  to  render  it  rough,  and  therefore  to  diminish  its  reflecting  power,  its  ab- 
sorbing power  will  be  increased,  and  the  effects  on  the  thermometer  will  be 
likewise  augmented.  Like  experiments  performed  on  other  bodies  lead  to  the 
general  conclusion,  that  the  absorptive  power  of  bodies  increases  as  the  reflect- 
ing power  decreases. 

Since  the  radiating  power  of  a  surface  is  inversely  as  its  reflecting  power, 
it  follows,  also,  that  the  power  of  absorption  is  always  in  the  same  proportion 
as  the  power  of  radiation.  In  reference  to  their  power  of  transmitting  light, 
bodies  are  denominated  transparent  or  opaque.  A  body  which  is  pervious  to 
;light  is  said  to  be  transparent,  and  one  which  does  not  allow  light  to  pass 
through  it  is  said  to  be  opaque.  Transparency  is  also  a  quality  which  bodies 
possess  in  different  degrees  :  some,  such  as  glass,  water,  or  air,  being  almost 


RADIATION  OP  HEAT. 


421 


perfectly  transparent,  while  others,  such  as  paper,  horn,  &c.,  are  imperfectly 
so.  Analogy  leads  us  to  inquire  whether  bodies  are  also  pervious  to  heat. 

In  the  preceding  experiments,  rays  of  heat  passed  through  the  atmosphere, 
which  is  therefore,  transparent  to  heat.  It  appears  from  the  experiments  of 
Leslie  and  others,  which  have  been  since  instituted,  that  all  gases  are  pervi- 
ous to  the  rays  of  heat,  and  equally  so ;  for  the  radiation  of  a  given  surface  is 
the  same  in  whatever  gas  it  takes  place. 

Gases,  therefore,  as  they  have  perfect  or  nearly  perfect  transparencies  for 
the  rays  of  light,  have  the  same  quality  in  reference  to  the  rays  of  heat.  A  hot 
body  placed  behind  a  solid  or  a  liquid  is  found,  however,  not  to  radiate  sensibly 
through  them.  But  the  most  direct  method  of  determining  the  transparency  of 
bodies  for  the  rays  of  heat,  is  to  interpose  a  screen  between  the  radiating  body 
and  the  reflector,  in  the  experiment  already  described,  and  to  observe  the 
effect  produced  on  the  thermometer  by  this  circumstance.  Leslie's  investiga- 
tion respecting  the  property  of  transparency  to  heat  of  different  bodies,  form  a 
very  remarkable  part  of  that  philosopher's  discoveries. 

Different  substances  are  pervious  by  heat  in  different  degrees.  A  screen 
of  thin  deal  board,  placed  between  the  canister,  c,  and  the  focal  ball,/,  figure 
7,  produced  a  diminution  in  the  effect  on  the  thermometer,  but  did  not  destroy 
that  effect  altogether.  The  heat  transmitted  through  the  board  varied  with  its 
thickness,  slowly  diminishing  as  its  thickness  increased.  The  radiation  of 
the  surface  of  the  lampblack,  which,  while  unobstructed,  produced  an  effect  of 
100°  on  the  thermometer,  produced  an  effect  of  20°  when  a  deal  board  the 
eighth  of  an  inch  thick  was  interposed.  It  produced  an  effect  of  15°  when 
the  thickness  was  three  eighths  of  an  inch,  and  an  effect  of  9°  when  the  board 
was  an  inch  thick.  A  plane  of  glass  interposed  reduced  the  effect  of  the  radi- 
ation by  the  surface  of  lampblack  from  100°  to  20°. 

The  distance  of  the  screen  from  the  canister  was  also  found  to  produce  a 
considerable  effect  on  its  transparency.  When  placed  near  the  canister,  a  con- 
siderable quantity  of  heat  was  transmitted  ;  but  if  the  distance  was  increased, 
the  quantity  of  heat  transmitted  diminished.  A  pane  of  glass  at  the  distance 
of  two  inches  reduced  the  effect  of  radiation  from  100°  to  20°.  As  its  dis- 
tance from  the  radiating  surface  was  slowly  increased,  the  effect  on  the  ther- 
mometer was  gradually  diminished ;  and  at  the  distance  of  one  foot  from  the 
radiating  surface  all  effect  of  radiation  was  destroyed. 

It  appeared  that  the  metals,  even  when  reduced  to  an  extreme  degree  of 
tenuity,  were  absolutely  opaque  to  heat.  A  screen  of  tinfoil  absolutely  inter- 
cepted all  radiation.  The  thinnest  gold  leaves,  300,000  of  which,  piled  one 
upon  another  would  not  measure  an  inch,  also  absolutely  stopped  the  rays  of 
heat.  White  paper  is  partially  opaque. 

It  appears,  generally,  that  the  bodies  which  intercept  heat  most  effectually 
are  those  which  radiate  heat  worst,  and  vice  versa.  This,  indeed,  might  easily 
have  been  anticipated  from  what  has  been  already  proved  of  reflection.  The 
screens  which  are  the  best  reflectors  are  the  worst  radiators,  and  must  evi- 
dently be  also  most  powerful  in  intercepting  heat ;  for  if  they  reflect  much  they 
can  transmit  but  little.  Some  other  effects,  which  Leslie  observed  in  his  ex- 
periments with  screens,  may  also  be  accounted  for  by  the  same  circumstance. 
He  took  two  panes  of  glass  and  coated  one  side  of  each  with  tinfoil.  He  then 
placed  their  uncovered  sides  in  close  contact,  so  as  to  form  one  double  pane, 
both  surfaces  of  which  were  covered  with  tinfoil.  When  this  was  interposed 
as  a  screen  before  the  radiating  surface,  all  effect  on  the  thermometer  was  de- 
stroyed, and  all  the  radiant  heat  intercepted.  This  is  easily  accounted  for  by 
the  perfect  power  of  reflection  which  the  coating  of  tinfoil  possesses.  The  heat 
incident  on  the  surface  of  tinfoil  is  nearly  all  reflected ;  and,  consequently,  no 


422 

sensible  quantity  is  transmitted.  He  next  placed  the  two  panes  with  the-ir 
coated  surfaces  in  contact,  the  uncovered  surfaces  being  outside.  A  part  of 
the  radiant  heat  was  now  transmitted,  and  the  effect  on  the  thermometer  was 
observed  to  be  18°.  Thus  about  one  fifth  of  the  radiant  heat  incident  on  the 
screen  was  transmitted.  In  fact,  nearly  as  much  heat  was  thus  transmitted  by 
the  two  panes  of  glass  with  the  tinfoil  between  them,  as  would  have  been 
transmitted  by  a  pane  of  uncovered  glass.  From  this  result  it  would  appear 
that  the  tinfoil  loses  its  power  of  reflecting  heat  when  the  rays  of  he<xt  have 
previously  passed  through  a  medium  of  glass  instead  of  a  medium  of  air ;  and  ) 
that,  instead  of  reflecting  them,  it  transmits  them. 

The  idea  of  investigating  the  effects  which  different  temperatures  in  a  radi- 
ant body  produce  on  the  power  of  the  radiated  heat  to  penetrate  screens  of  dif- 
ferent substances,  does  not  seem  to  have  suggested  itself  to  Sir  John  Leslie. 
Later  experiments,  instituted  by  M.  de  la  Roche,  prove  that  the  power  of  cal- 
orific rays  to  penetrate  bodies  increases  with  the  temperature  of  the  radiator. 
This  heat  radiating  from  a  surface  at  a  certain  temperature,  fails  to  penetrate 
glass,  except  in  a  very  limited  degree ;  but  if  the  radiating  body  be  considera- 
bly elevated  in  its  temperature,  then  the  rays  penetrate  the  glass  in  much 
greater  quantities.  In  fact,  the  degree  of  transparency  of  glass  relatively  to 
the  rays  of  heat  would  seem  to  depend  on  the  temperature  of  the  radiating 
body,  and  to  increase  with  that  temperature. 

'l^he  results  of  the  preceding  experiments,  and,  indeed,  all  the  phenomena 
connected  with  the  radiation  of  heat,  are  satisfactorily  explained  by  the  theory 
of  exchanges,  first  proposed  by  Prevost  of  Geneva.  According  to  this  theory, 
every  point  at  and  near  the  surfaces  of  bodies  is  regarded  as  a  centre  from 
which  rays  of  heat  diverge  in  all  directions.  The  surfaces  also  reflect  rays  of 
heat  incident  upon  them,  in  a  greater  or  less  degree,  rays  of  heat  striking  on  a 
body,  arid  reflected  or  radiated  by  the  other  bodies  around.  Thus  every  body, 
so  far  as  regards  heat,  is  constantly  under  the  operation  of  three  distinct  pro- 
cesses— it  radiates,  reflects,  and  absorbs:  it  follows,  from  this,  that  betrveeii 
bodies  which  are  placed  in  each  other's  neighborhood,  there  must  be  a  coijscant 
interchange  of  heat.  The  heat  which  is  radiated  by  one  body  strikes  on  oth- 
ers ;  part  of  it  is  absorbed  by  them,  and  is  retained  within  their  dimensions,  so 
as  to  raise  their  temperature,  while  another  part  is  reflected,  and  scrikes  on 
other  bodies,  where  it  is  subject  to  like  effects.  The  body  which  radiates 
heat  in  this  manner  is,  at  the  same  time,  receiving  on  its  surface  rays  of  heat 
which  proceed  from  other  bodies  in  its  neighborhood  ;  and  these  rays  of  heat 
are  subject  to  the  same  effects  on  its  surface  as  the  rays  which,  proceeding 
from  it,  encounter  on  the  surface  of  other  bodies — they  are  partly  absorbed  and 
partly  reflected. 

If  a  body  raised  to  a  high  temperature  be  placed  in  the  neighborhood  of  other 
bodies  at  a  lower  temperature,  it  will  radiate  a  greater  quantity  of  heat  than  the 
bodies  which  surround  it ;  consequently  the  heat  which  it  receives  from  them 
will  be  less  than  the  heat  which  it  transmits  to  them.  They  will  receive  more 
heat  than  they  give,  and  it  will  give  more  heat  than  it  receives  ;  the  temperature, 
therefore,  of  the  hot  body,  will  gradually  fall,  while  the  temperature  of  the  sur- 
rounding bodies  will  gradually  rise.  This  will  continue  until  the  temperatures 
of  the  bodies  are  equalized.  Then  the  heat  radiated  by  each  of  ihem  will  be 
exactly  equal  to  the  heat  absorbed,  and  the  temperature  will  remain  stationary. 

It  has  appeared  from  the  result  of  direct  experiments,  that  the  bodies  which 
are  the  best  radiators  are  also  the  best  absorbers  of  heat.  This  would  follow 
as  a  necessary  consequence  of  the  theory  which  has  been  just  explained.  If 
a  body  which  is  a  powerful  radiator  were  at  the  same  time  a  bad  absorber,  the 
consequence  would  be  that  it  would  radiate  heat  faster  than  it  would  absorb  it ; 


RADIATION  OF  HEAT.  423 


consequently  its  temperature  would  continually  fall,  and  this  depression  of 
temperature  would  continue  without  any  limit.  Now  this  is  not  supported  by 
observation.  It  therefore  follows,  as  a  necessary  consequence,  that  the  power 
of  radiation  in  every  body  must  be  equal  to  its  power  of  absorption. 

It  has  likewise  appeared  that  the  best  reflectors  are  the  worst  radiators. 
This  effect  might  likewise  be  foreseen  on  the  principle  of  the  theory  just  ox- 
plained.  A  good  reflector  is  a  body  which  reflects  the  principal  part  of  the 
rays  of  heat  which  strike  upon  it.  Now  the  heat  which  is  incident  on  a  body 
must  be  either  reflected  or  absorbed,  and  whatever  portion  of  it  is  not  reflected 
must  be  absorbed.  If,  therefore,  a  great  part  be  reflected,  a  proportionally 
small  part  remains  to  be  absorbed  ;  consequently  it  follows,  that  in  the  same 
proportion  as  a  body  is  a  good  reflector  it  must  be  a  bad  absorber  ;  and,  vice  v>:rsa, 
if  it  be  a  bad  reflector,  it  must  in  proportion  be  a  good  absorber.  But  it  neces- 
sarily follows,  if  a  body  be  a  powerful  absorber  of  heat,  that  it  must  also  be  a 
powerful  radiator  of  heat,  for  otherwise  its  temperature  would  rise  infinitely  by 
the  heat  which  it  absorbs  accumulating  in  it,  and  not  being  carried  off  by  radi- 
ation. A  good  reflector,  therefore,  will  be  a  bad  radiator,  and  vice  versa.  In 
the  experiments  of  Leslie  with  the  concave  reflector,  our  attention  was  only 
directed  to  the  radiation  of  the  hot  surface,  and  we  considered  only  the  ray* 
which,  proceeding  from  it,  were  collected  on  the  bulb  of  a  thermometer  by  the 
concave  reflector.  It  might  appear  to  follow,  from  an  extension  of  this  experi- 
ment, that  bodies  radiate  cold  as  well  as  heat.  Let  one  of  the  cubical  vessels 
used  by  Leslie  in  his  experiment  be  filled  with  snow,  and  placed  before  a  re- 
flector. Immediately  the  focal  ball  of  the  differential  thermometer  placed  in 
the  focus  will  exhibit  a  rapid  depression  of  temperature.  Are  we,  therefore,  to 
suppose  in  this  case  that  rays  of  cold  proceed  from  sides  of  the  vessel,  and  are 
collected  on  the  ball  of  the  thermometer  ?  On  the  contrary,  it  has  appeared 
from  previous  investigation,  that  no  body  is  perfectly  destitute  of  heat,  and  that 
snow  itself,  as  well  as  mixtures  much  colder  than  it,  are  capable  of  imparting 
heat  to  other  bodies,  and  therefore  possess  heat  in  them.  The  surface,  there- 
fore, of  a  vessel  containing  snow,  in  this  case  radiates  heat,  and  these  rays  of 
heat  are  collected  on  the  bulb  of  the  thermometer  in  the  same  manner  as  when 
that  vessel  was  filled  with  boiling  water.  The  bulb  of  the  thermometer,  how- 
ever, itself,  like  all  other  bodies,  radiates  heat,  and  this  heat  is  reflected  by  the 
concave  reflector  toward  the  surface  of  the  vessel  containing  the  snow.  The 
two  bodies,  therefore,  are  radiating  heat  toward  each  other  ;  but  the  bulb  of  the 
thermometer  having  the  higher  temperature,  radiates  more  heat  than  it  re- 
ceives, while  the  surface  of  the  vessel  containing  the  snow  receives  more  heat 
than  it  radiates.  The  thermometer,  therefore,  gradually  falls  in  its  tempera- 
ture, while  the  vessel  containing  the  snow  gradually  rises. 

In  the  experiment  with  the  concave  reflector  already  described,  the  hot 
body  placed  in  one  focus,  and  the  bulb  of  the  thermometer  placed  in  the  other, 
are  both  radiators  and  absorbers  of  heat ;  the  hot  body  radiates  heat  to  the 
bulb,  and  the  bulb  radiates  heat  to  it.  The  hot  body  absorbs  the  heat  which 
is  radiated  by  the  bulb,  and  the  bulb  absorbs  the  heat  radiated  by  the  hot  body. 
But  the  hot  body,  radiating  more  heat  than  the  bulb,  necessarily  absorbs  less, 
consequently  the  temperature  of  this  body  gradually  falls,  while  that  of  the 
bulb  of  the  thermometer  rises.  Let  us  now  suppose  that  instead  of  a  hot  body, 
a  globe  of  snow  be  placed  in  the  focus  of  the  reflector,  the  bulb  of  the  thermom- 
eter having  a  higher  temperature,  will  radiate  more  heat  than  it  receives  from 
the  snow,  and  it  will  become  a  hot  body  relatively  to  the  snow.  Since,  there- 
fore, it  radiates  more  heat  than  it  absorbs,  its  temperature  will  fall  until  it  be- 
comes equal  to  that  of  the  snow ;  the  interchange  of  heat  being  then  equal,  no 
further  alteration  in  temperature  will  take  place. 


424  RADIATION  OF  HEAT. 


Numerous  facts  of  ordinary  occurrence,  and  many  interesting  natural  phe- 
nomena, admit  of  easy  and  satisfactory  explanation  on  the  principle  of  the 
above  theory  of  radiation. 

Vessels  intended  to  contain  a  liquid  at  a  higher  temperature  than  the  sur- 
rounding medium,  and  to  keep  that  liquid  as  long  as  possible  at  the  higher 
temperature  should  be  constructed  of  materials  which  are  the  worst  radiators 
of  heat.  Thus,  tea-urns  and  tea-pots  are  not  adapted  for  their  purpose  when 
constructed  of  black  porcelain.  A  black  porcelain  tea-pot  is  the  worst  con- 
ceivable material  for  that  vessel,  for  both  its  material  and  color  are  good  ra- 
diators of  heat,  and  the  liquid  contained  in  it  cools  with  the  greatest  possible 
rapidity.  On  the  other  hand,  a  bright  metal  tea-pot  is  best  adapted  for  the 
purpose,  because  it  is  the  worst  radiator  of  heat,  and  therefore  cools  as  slowly 
as  possible.  A  polished  silver  or  brass  tea-urn  is  better  adapted  to  retain  the 
heat  of  the  water  than  one  of  a  dull  brown  color,  such  as  is  most  commonly 
used  in  England. 

A  tin  kettle  retains  the  heat  of  water  boiled  in  it  more  effectually  if  it  is 
kept  clean  and  polished,  than  if  it  be  allowed  to  collect  the  smoke  and  soot,  to 
which  it  is  exposed  from  the  action  of  the  fire.  When  coated  with  this,  its 
surface  becomes  rough  and  black,  and  is  a  powerful  radiator  of  heat. 

A  set  of  polished  fire-irons  may  remain  for  a  long  time  in  front  of  a  hot  fire 
without  receiving  from  it  any  increase  of  temperature  beyond  that  of  the  cham- 
ber, because  the  heat  radiated  by  the  fire  is  all  reflected  by  the  polished  sur- 
face of  the  irons,  and  none  of  it  is  absorbed  ;  but  if  a  set  of  rough,  unpolished 
irons,  were  similarly  placed,  they  would  speedily  become  hot,  so  that  they 
could  not  be  used  without  inconvenience.  The  polish  of  fire-irons  is;  there- 
fore, not  merely  a  matter  of  ornament,  but  of  use  and  convenience.  The  rough, 
unpolished  poker,  sometimes  used  in  a  kitchen,  soon  becomes  so  hot  that  it 
cannot  be  held  without  pain. 

A  close  stove,  intended  to  warm  an  apartment,  should  not  have  a  polished 
surface,  for  in  that  case  it  is  one  of  the  worst  radiators  of  heat,  and  nothing 
could  be  contrived  more  unfit  for  the  purpose  to  which  it  is  applied.  On  the 
other  hand,  a  rough  unpolished  surface  of  cast-iron  is  favorable  to  radiation, 
and  a  fire  in  such  a  stove  will  always  produce  a  more  powerful  effect. 

A  metal  helmet  and  cuiras,  worn  by  some  regiments  of  cavalry,  is  a 
cooler  dress  than  might  be  at  first  imagined.  The  polished  metal  being  a 
nearly  perfect  reflector  of  heat,  throws  off  the  rays  of  the  sun,  and  is  incapable 
of  being  raised  to  an  inconvenient  temperature.  Its  temperature  is  much  less 
increased  by  the  influence  of  the  sun  than  that  of  common  clothing. 

The  polished  surfaces  of  different  parts  of  the  steam-engine,  especially  of  the 
cylinder,  is  not  matter  of  mere  ornament,  but  of  essential  utility.  A  rough 
metal  surface  would  be  a  much  better  radiator  of  heat  than  the  polished  sur- 
face, and  if  rust  were  collected  on  it,  its  radiating  power  would  be  still  further 
increased,  and  the  steam  contained  in  it  would  be  more  exposed  to  condensa- 
tion by  loss  of  heat. 

It  may  be  frequently  observed  that  a  deposition  of  moisture  has  taken  place 
on  the  interior  surface  of  the  panes  of  glass  of  a  chamber-window,  on  a  morn- 
ing which  succeeds  a  cold  night.  The  temperature  of  the  external  air  during 
the  night  being  colder  than  the  atmosphere  of  the  chamber,  it  communicates 
its  temperature  to  the  external  surface  of  the  glass,  and  this  is  transmitted  to 
the  interior  surface,  which  is  exposed  to  the  atmosphere  of  the  room.  This 
atmosphere  is  always  more  or  less  charged  with  vapor,  and  the  cold  of  the  ex- 
ternal surface  of  the  glass  acting  on  the  air  in  contact  with  it,  reduces  its  tem- 
perature below  the  point  of  saturation,  arid  a  condensation  of  vapor  takes  place 
on  the  surface  of  the  panes,  which  is  observed  by  a  copious  deposition  of 


RADIATION  OF  HRAT. 


425 


moisture  in  the  morning.     If  the  temperature  of  the  external  air  be  at  or  be-  j 
low  the  freezrng  point,  this  deposition  will  form  a  rough  coating  of  ice  on  the  < 
pane.   ,  Let  a  small  piece  of  tin-foil  be  fixed  on  a  part  of  the  exterior  surface  j 
of  one  pane  of  the  window  in  the  evening,  and  let  another  piece  of  tin-foil  be  ( 
iixed  on  a  part  of  the  interior  surface  of  another  pane.     In  the  morning  it  will  j 
be  found  that  that  part  of  the  interior  surface  which  is  opposite  to  the  external  i 
foil  will  be  nearly  free  from  ice,  while  every  other  part  of  the  same  pane  will  ] 
be  thickly  covered  with  it.     On  the  contrary,  it  will  be  found  that  the  surface  i 
of  the  internal  tin-foil  will  be  more  thickly  covered  with  ice  than  any  other  part  ' 
of  the  glass.     These  effects  are  easily  explained  by  the  principle  of  radiation.  < 
When  the  tin-foil  is  placed  on  the  exterior  surface,  it  reflects  the  heat  which 
strikes  on  the  exterior  surface,  and  protects  that  part  of  the   glass  which  is 
covered   from  its  action.     The  heat  radiated  from  the  objects   in  the  room 
striking  on  the  surface  of  the  glass,  penetrates  it,  and  encountering  the  tin-foil 
attached  to  the  exterior  surface,  is  reflected  by  it  through  the  dimensions  of 
the  glass,  and  its  escape  into  the  exterior  atmosphere  is  intercepted  ;  the  por- 
tion  of  the  glass,  therefore,  covered  by  the  tin-foil,  is  in  this  case  subject  to 
the  action  of  the  heat  radiated  from  the  chamber,  but  protected  from  the  action 
of  the  external  heat.     The  temperature  of  that  part  of  the  glass  is   therefore 
less  depressed  by  the  effects  of  the  external  atmosphere  than  the  temperature 
of  those  parts  which  are  not  covered  by  the  tin-foil.     Now,  glass  being,  as 
will  appear  hereafter,  a  bad  conductor  of  heat,  the  temperature  of  that  part  op- 
posite to  the  tin-foil  does  not  immediately  affect  the  remainder  of  the   pane, 
and   consequently  we  find  that  while  the  remainder  of  the  interior  surface  of 
the  pane  is  thickly  covered  with  ice,  the  portion  opposite  the  tin-foil  is  com- 
paratively free  from  it.     On  the   contrary,  when  the   tin-foil  is  placed  on  the 
internal  surface,  it  reflects   powerfully  the  heat  radiated  from  the  objects  in 
the  room,  while  it  admits  through  the  dimensions   of  the  glass,  the  heat  pro- 
ceeding from  the  external  atmosphere.     The  portion  of  the   glass,  therefore, 
covered  by  the  tin-foil,  becomes  colder  than  any  other  part  of  the  pane,  and 
the  tin-foil  itself  receives  the  same  temperature,  which  is  not  reduced  by  the 
effect  of  the  radiation  of  objects  in  the  room,  because  the  tin-foil  itself  is  a  good 
reflector  of  heat,  and  a  bad  absorber.     Hence  the  tin-foil  presents  a  colder  sur- 
face to  the  atmosphere  of  the  room  than  any  other  part  of  the  surface  of  the 
pane,  and  consequently  receives  a  more  abundant  deposition  of  ice. 

If  a  body,  which  is  a  good  radiator  of  heat,  be  exposed  in  a  situation  where 
other  good  radiators  are  not  present,  it  will  have  a  tendency  to  fall  in  its  tempe- 
rature below  the  temperature  of  the  surrounding  medium  ;  because,  in  this  case, 
while  it  loses  heat  by  its  own  radiation,  its  absorbing  power  is  not  satisfied  by 
a  corresponding  supply  of  heat  from  other  objects.  A  clear  sky,  in  the  ab- 
sence of  the  sun,  has  scarcely  any  sensible  radiation  of  heat ;  if,  therefore,  a 
good  radiator  be  exposed  to  the  aspect  of  an  unclouded  firmament  at  night,  it 
will  lose  heat  considerably  by  its  own  radiation,  and  will  receive  no  corres- 
ponding portion  from  the  radiation  of  the  firmament  to  repair  this  loss,  and  its 
temperature  consequently  will  fall. 

A  curious  experiment  made  by  Dufay  affords  a  striking  illustration  of  this 
fact.  He  exposed  a  glass  cup,  placed  in  a  silver  basin,  to  the  atmosphere  du- 
ring a  cold  night,  and  he  found  in  the  morning  a  copious  deposition  of  moisture 
on  the  glass,  while  the  silver  vessel  remained  perfectly  dry.  He  next  reversed 
the  experiment,  and  exposed  a  silver  cup  in  a  glass  basin.  The  result  was  the 
same  :  the  glass  was  still  covered  with  moisture,  and  the  metal  free  from  it. 
Now  metal  is  a  bad  radiator  of  heat,  and  consequently  has  a  tendency  to  pre- 
serve its  temperature.  Glass  is  a  much  better  radiator,  and  has  therefore  a 
|  tendency  to  lose  its  temperature.  These  vessels  being  exposed  to  the  aspect 


RADIATION  OF  HEAT. 


of  a  clear  sky,  received  no  considerable  rays  of  heat  to.  supply  the  loss  sus- 
tained by  their  radiation.  This  loss  in  the  metal  was  inconsiderable,  and 
therefore  it  maintained  its  temperature  nearly  or  altogether  equal  to  that  of  the 
air  ;  the  glass,  however,  radiating  more  abundantly,  and  absorbing  little,  suf- 
fers a  depression  of  temperature.  The  glass,  therefore,  presented  a  cold  sur- 
face to  the  air  contiguous  to  it,  and  reduced  the  temperature  of  that  air,  until  it 
attained  that  temperature  at  which  it  was  below  a  state  of  saturation  with  re-, 
spect  to  the  vapor  with  which  it  was  charged  ;  a  deposition  of  vapor,  therefore, 
took  place  on  the  glass. 

This  discovery  of  Dufay  remained  a  barren  fact  until  the  attention  of  Dr. 
Wells  was  directed  to  the  subject.  The  result  of  his  inquiries  was  the  dis- 
covery of  the  cause  of  the  phenomena  of  dew,  and  affords  one  of  the  most 
beautiful  instances  of  inductive  reasoning  which  any  part  of  the  history  of  phys- 
ical discovery  has  presented.  Dr.  Wells  argued  that,  as  a  clear  and  cloudless 
sky  radiates  little  or  no  heat  toward  the  surface  of  the  earth,  all  objects  placed 
on  the  surface  which  are  good  radiators  must  necessarily  fall  in  temperature 
during  the  night,  if  they  be  in  a  situation  in  which  they  are  not  exposed  to  the 
radiation  of  other  objects  in  their  neighborhood.  Grass  and  other  products  of 
vegetation  are,  in  general,  good  radiators  of  heat.  The  vegetation  which  cov- 
ers the  surface  of  the  ground  in  an  open,  champaign  country,  on  a  clear  night, 
will  therefore  undergo  a  depression  of  temperature,  because  it  will  absorb  less 
heat  than  it  radiates.  This  fact  was  ascertained  by  direct  experiment,  both  by 
Dr.  AVells  and  Mr.  Six.  A  thermometer,  laid  on  a  grass  plot  on  a  cleai  night, 
was  observed  to  sink  even  so  much  as  20°  below  another  thermometer  sus- 
pended at  some  height  above  the  ground.  The  vegetables,  which  thus  acquire 
a  lower  temperature  than  the  atmosphere,  reduce  the  .air  immediately  contigu- 
ous to  them  to  a  temperature  below  saturation,  and  a  proportionally  copious 
condensation  of  vapor  takes  place,  and  a  deposition  of  dew  is  formed  on  the 
leaves  and  flowers  of  all  vegetables.  In  fact,  every  object,  in  proportion  as  it 
is  a  good  radiator,  receives  a  deposition  of  moisture.  On  the  other  hand,  ob- 
jects which  are  bad  radiators  are  observed  to  be  free  from  it.  Blades  of  grass 
sustain  large,  pellucid  dew-drops,  while  the  naked  soil  in  their  neighborhood  is 
free  from  them. 

In  the  close  and  sheltered  streets  of  cities  the  deposition  of  dew  is  very  rare- 
ly observed,  because  there  the  objects  are  necessarily  exposed  to  each  other's 
radiation,  and  an  interchange  of  heat  takes  place  which  maintains  them  at  a 
temperature  uniform  with  that  of  the  air.  A  deposition  of  dew,  in  this  case, 
can  only  take  place  when  the  natural  temperature  of  the  air  falls  below  its  point 
of  saturation. 

In  an  obscure,  cloudy  night  no  deposition  of  dew  takes  place,  because  in  this 
case,  although  the  vegetable  productions  radiate  heat  as  powerfully  as  before, 
yet  the  clouds  are  also  radiators,  and  they  transmit  heat,  which,  being  absorbed 
by  the  vegetables,  their  temperature  is  prevented  from  sinking  much  below  that 
of  the  atmosphere. 


I 


METEORIC  STONES  &  SHOOTING  STABS. 


(  Inductive  Method. — Appearances  accompanying  Meteorites. — Theories  to  explain  them.— Kxnmina 
lion  of  these  Theories. — Shooting  Stars. — November  and  Auarast  Meteors. — O  rbits  and  Distances.— 
Heights. — Ch  adni's  Hypothesis. 


METEORIC  STONES  AND  SHOOTING  STARS. 


429 


METEORIC  STONES  &  SHOOTING  STARS. 


WHEN  we  reflect  upon  the  length  of  time  which  has  elapsed  since  just 
methods  of  investigating  nature  were  first  formally  taught  by  BACON*,  \ve  can 
not  fail  to  be  struck  with  surprise  at  witnessing  the  frequency  with  which  chose 
inestimable  precepts  are  neglected  and  overlooked.  There  appears  to  be  a  dis- 
position inherent  in  the  mind — springing  probably  from  that  arrogance  and  vanity, 
which  are  invariably  the  offspring  of  ignorance — that  induces  a  disposition,  in 
every  case,  precipitately  to  rush  to  the  formation  of  theories  and  the  assump- 
tion of  causes,  omitting,  or  postponing,  the  far  more  important  though  less  ambi- 
tious duty  of  analyzing  phenomena.  It  is  true  that  these  observations  are  less 
applicable  to  that  order  of  minds  which  have  been  disciplined  in  the  severe 
schools  of  the  old  and  long-established  universities,  where  the  works  of  BACON, 
and  the  mathematical  classics  of  NEWTON  and  LAPLACE,  are  studied  with 
a  zeal  and  perseverance  which  do  not  fail  to  infuse  their  spirit  into  the  minds 
of  their  aspiring  successors.  But  in  the  much  larger  class  of  half-disciplined 
or  self-taught  aspirants  to  scientific  rank,  the  disposition  we  refer  to  frequently 
exists,  and  to  a  proportionate  extent  retards  their  progress,  and  impairs  the 
value  of  their  labors. 

The  public  teacher  should,  therefore,  omit  no  proper  opportunity  of  incul- 
cating the  true  spirit  of  the  inductive  philosophy,  which,  in  our  day,  has  afforded 
so  rich  a  harvest  of  discovery.  I  shall  avail  myself  of  the  opportunity  which 
the  consideration  of  aerolites  offers,  to  afford  you  an  example  of  the  rigorous 
observance  of  the  canons  of  Bacon's  philosophy  in  the  investigation  of  nature. 

Every  one  possessed  of  the  smallest  amount  of  the  current  information  of 
the  day,  imagines  that  he  knows  what  meteoric  stones  are.  He  knows  that 
they  fall  from  the  air,  and  that  they  are  accompanied  by  fire  and  noise.  With 
this  amount  of  information  he  unhesitatingly  sets  about  to  conjecture  their  origin, 
and  to  get  up  a  theory  to  explain  them.  As  might  be  expected,  the  theory  pro- 
duced under  such  circumstances  is  always  crude  and  absurd,  and  falls  to  pieces 
upon  the  slightest  comparison  with  the  phenomena. 


When  any  new  and  unexplained  phenomenon  offers  itself  to  our  inquiry,  the 
first  duty  of  the  investigator  is  to  inform  himself,  with  the  most  scrupulous  ac- 
curacy, of  all  the  circumstances,  however  minute,  which  accompany  it ;  and  if 
past  observation  can  not  answer  all  circumstantial  inquiries  which  his  under- 
standing may  suggest  as  necessary,  he  must  patiently  wait  the  recurrence  of  a 
like  phenomenon,  and  diligently  observe  it.  When  he  shall  have  thus  collect- 
ed all  the  circumstances  that  can  be  imagined  to  throw  light  on  its  origin,  he 
will  then,  and  not  until  then,  be  in  a  condition  to  justify  an  inquiry  into  its 
cause. 

Let  us  see,  then,  what  circumstances  attending  the  appearance  of  meteorites 
past  observation  has  supplied. 

It  is  agreed  by  all  observers,  in  every  part  of  the  earth,  that  these  meteors 
manifest  themselves  by  the  appearance  of  a  stream  of  light,  passing  with  great 
velocity  through  the  firmament ;  after  which  an  explosion  usually  takes  place, 
so  loud  that  windows  and  doors,  and  even  buildings  themselves,  are  some- 
times shaken  as  if  by  an  earthquake. 

The  phenomenon  is  sometimes  called  ball-lightning,  a  term  which  is  liable 
to  the  objection  that  it  implies  an  analogy,  or  identity  of  origin,  between  these 
meteors  and  common  lightning  ;  which  not  only  is  not  proved,  but  is  attended 
with  no  probability. 

The  luminous  appearance  and  subsequent  explosion  attending  these  meteors 
was  long  known  ;  the  fact,  however,  that  heavy  substances,  now  called  mete- 
oric stones,  were  projected  upon  the  surface  of  the  earth  at  the  same  tine,  was 
not  clearly  proved  or  generally  admitted  until  the  present  century.  Abundant 
evidence,  however,  has  been  supplied,  by  the  vigilance  and  zeal  of  contempo- 
raneous philosophers,  of  the  reality  of  these  deposites.  Chladni,  in  his  work 
on  this  subject,  has  supplied  an  extensive  chronological  catalogue  of  the  mete- 
oric stones  whose  falls  have  been  recorded  in  different  parts  of  the  earth,  which 
supplies  examples  of  these  phenomena  occurring  in  various  parts  of  the  world 
several  times  in  each  year  of  the  present  century. 

The  fact,  then,  may  be  regarded  as  conclusively  established,  that  masses  of 
stony  matter,  of  various  size  and  magnitude,  and  often  of  very  considerable 
weight,  are  frequently  seen  passing  athwart  the  heavens,  with  great  apparent 
velocity,  which  are  afterward  precipitated  upon  the  earth  with  extraordinary 
force. 

The  second  circumstance  I  shall  mention  as  worthy  of  attention  is,  that  these 
bodies  rarely  strike  the  surface  of  the  earth  in  a  direction  either  vertical  or 
nearly  so.  They  generally,  on  the  other  hand,  come  in  a  direction  very  ob- 
lique to  the  plane  of  the  horizon.  It  may  be  asked,  how  the  direction  in  which 
they  strike  the  earth  can  be  ascertained  unless  they  are  seen,  which  rarely 
happens  at  the  moment  of  their  fall.  To  this  I  answer,  that  their  direction  is 
rendered  manifest  by  the  manner  in  which  they  penetrate  the  surface  of  the 
ground — which  they  always  do,  and  to  a  depth  more  or  less  considerable. 

The  velocity  of  their  motion  when  they  encounter  the  earth,  is  another  cir- 
cumstance of  much  importance.  This  velocity  is  discoverable  by  observa- 
tion on  their  movement  while  visible,  as  well  as  by  inferring  the  force  with 
which  they  struck  the  ground  from  the  depth  to  which  they  penetrated. 

It  is  accordingly  found  by  means  of  such  observations,  that  the  velocities  of 
these  bodies  belong  to  the  kind  of  motions  which  characterize  the  bodies  of 
the  solar  system,  and  such  as  are  never  witnessed  upon  the  surface  of  the  earth. 
They  are  velocities  which  could  not  be  imagined  to  be  imparted  by  the  earth's 
gravitation  to  any  masses  attracted  from  points  within  the  limits  of  the  atmo- 
sphere. 

On  examining  the  physical  condition,  and  analyzing  the  constituents  of  the 


METEORIC  STONES  AND  SHOOTING  STARS. 


431 


masses  thus  precipitated,  several  circumstances  worthy  to  be  noted  are  pre- 
sented. It  is  found  that  their  surfaces  are  generally  black,  having  a  burnt  ap- 
pearance ;  but  the  most  remarkable  circumstance  attending  them  is,  that  at  what- 
ever time,  or  in  whatever  part  of  the  earth  they  may  have  fallen,  they  generally 
consist  of  the  same  constituent  parts,  and  always  very  nearly  in  the  same  pro- 
portion. Their  ingredients  are  silex,  magnesia,  sulphur,  iron,  nickel,  and  chro- 
mium. There  is  occasionally,  but  not  invariably,  a  trace  of  charcoal. 

It  is  important  to  observe  here,  that  the  iron  and  nickel  found  in  these  bodies 
are  always  in  the  metallic  form — a  state  in  which  they  are  never  known  to  ex- 
ist naturally  on  the  surface  of  the  earth.  These  metals,  when  found  in  the 
earth,  are  invariably  combined  with  oxygen,  and  it  is  their  oxides  only  which 
have  a  place  among  natural  terrestrial  substances.  The  iron  and  nickel  used 
in  the  arts  are  obtaiaed  by  the  decomposition  of  the  ores  in  the  process  of  met- 
allurgy. 

The  distances  from  the  earth  at  which  these  meteors  pass  when  they  are 
visible  has  been  ascertained  with  a  tolerable  degree  of  approximation,  by  ob- 
serving the  length  and  position  of  their  visible  course  at  the  same  time  from  two 
distant  places.  It  has  been  found  by  these  means  that  they  are  frequently  visi- 
ble at  the  height  of  from  30  to  40  miles.  This  is  generally  considered  as  the 
limit  of  the  height  of  the  atmosphere. 

Such  are  the  circumstances  attending  the  exhibition  of  these  meteors,  which 
have  been  collected  from  careful  and   accurate  information.     Let  us  now  turn 
our  attention  to  the  different  methods  by  which  it  has  been  attempted  to  explain 
them.     Three  different  hypotheses,  or  theories,  have  been  proposed  for  this  ) 
purpose. 

First. — It  is  supposed  that  the  matter  composing  them  has  been  drawn  up 
from  the  surface  of  the  earth  in  a  state  of  infinitely  minute  subdivision,  as  va- 
por is  drawn  from  liquids  ;  that,  being  collected  in  clouds  in  the  higher  regions 
of  the  atmosphere,  it  is  there  agglomerated  and  consolidated  in  masses,  and 
falls  by  its  gravity  to  the  surface  of  the  earth ;  being  occasionally  drawn  from 
the  vertical  direction  which  would  be  imparted  to  it  by  gravity,  by  the  effect  of 
atmospheric  currents,  and  thus  occasionally  striking  the  earth  obliquely.  We 
shall  call  this  the  atmospheric  hypothesis. 

Secondly. — It  is  supposed  that  meteoric  stones  are  ejected  from  volcanoes, 
with  sufficient  force  to  carry  them  to  great  elevations  in  the  atmosphere,  in 
falling  from  which  they  acquire  the  velocity  and  force  with  which  they  strike 
the  earth.  The  oblique  direction  with  which  they  strike  the  ground  is  ex- 
plained by  the  supposition  that  they  may  be  projected  from  the  volcanoes  at 
corresponding  obliquities,  and  that,  by  the  principles  of  projectiles,  they  must 
strike  the  earth  at  nearly  the  same  inclination  as  that  with  which  they  have 
been  ejected.  This  we  shall  call  the  volcanic  hypothesis. 

Thirdly. — It  has  been  supposed  that  these  bodies  are  not  either  terrestrial  or 
atmospheric,  but  belonging  to  the  solar  system ;  and  that  their  origin  is  the 
same  as  that  which  has  produced  the  small  planets  which  have  been  discovered 
moving  between  the  orbits  of  Mars  and  Jupiter. 

This  theory  supposes  that,  at  some  former  epoch,  the  solar  system  possessed 
a  planet  which  revolved  round  the  sun  at  the  distance  of  two  hundred  and  fifty 
millions  of  miles  ;  a  supposition  which  is  rendered  highly  probable,  if  not  mor- 
ally certain,  by  reasons  which  are  fully  detailed  in  my  discourse  on  the  new 
planets.  The  catastrophe  by  which  this  former  planet  was  broken  into  pieces 
is  supposed  to  have  been  produced,  either  by  internal  explosion  (from  some 
cause  similar  to  that  which  produces  on  the  earth  volcanoes  and  earthquakes), 
or  by  the  collision  of  a  comet.  It  is  supposed  that  the  new  planets  are  not  the 
only  fragments  which  resulted,  but  that  innumerable  smaller  pieces  may  have 


METEORIC  STONES  AND  SHOOTING  STARS. 


been  scattered  about  the  system,  which,  owing  to  their  extreme  minuteness, 
may  have  been  subject  to  disturbing  causes  that  have  occasionally  brought  them 
so  near  the  earth,  that  they  have  been  drawn  by  its  attraction  within  the  limits 
of  the  atmosphere,  and  have  ultimately,  by  the  resistance  of  that  fluid,  fallen 
apon  the  earth.  We  shall  call  this  the  planetary  hypothesis. 

Fourthly. — It  has  been  suggested  by  LAPLACE,  that  meteoric  stones  may  be 
substances  ejected  from  lunar  volcanoes,  either  now  or  formerly  in  active  opera- 
tion. He  has  proved  that  no  very  improbable  amount  of  mechanical  force  would 
be  sufficient  to  produce  such  an  effect,  since  there  is  no  atmosphere  around  the 
moon,  or,  at  least,  none  that  could  be  sufficient  to  offer  a  sensible  resistance  to 
the  motion  of  a  solid  body.  The  force,  therefore,  that  would  be  required  is 
only  that  which  would  be  sufficient  to  overcome  the  moon's  attraction,  which  is 
found  by  calculation  to  be  about  four  times  the  force  with  which  a  ball  is  ex- 
pelled from  a  cannon  with  the  ordinary  charge  of  gunpowder.  A  body  pro- 
jected toward  the  earth,  with  the  velocity  of  about  eight  thousand  feet  per  sec- 
ond from  the  lunar  surface,  would  rise  to  such  a  height  that  it  would  arrire  at 
a  point  between  the  earth  and  moon  where  the  attraction  of  the  earth  would 
predominate  and  prevent  its  return.  It  would,  consequently,  continue  to  move 
toward  the  earth  with  accelerated  speed,  and,  arriving  within  the  limits  of  the 
atmosphere,  would  necessarily  reach  the  surface.  We  shall  call  this  the  lunar 
hypothesis. 

Fifthly. — It  has  been  supposed  that  meteoric  stones,  showers  of  dust,  and 
other  similar  meteorological  phenomena,  proceed  from  chaotic  matter  which 
prevails  in  the  spaces  within  which  the  planets  move,  and  which  is  generally 
but  irregularly  diffused  throughout  the  universe,  producing  in  the  heavens  the 
appearances  called  nebulae.  This  matter  is  supposed  to  lie  irregularly  in  the 
space  through  which  the  earth  annually  passes  and  its  neighborhood  ;  that  it 
is  occasionally  brought  by  the  attraction  of  the  earth  within  the  limits  of  the 
atmosphere,  and  thus  descends  to  the  surface.  This  we  shall  call  the  nebular 
hypothesis. 

Such  are  the  various  theories  which  have  been  offered  to  explain  the  phe- 
nomena attending  meteoric  stones.  The  evolution  of  light  which  attends  their 
rapid  progress  through  space  has  been  accounted  for  in  all  of  them  in  the  same 
manner.  It  is  supposed  that,  in  the  rapid  motion  with  which  the  body  pro- 
ceeds, the  air  which  lies  in  its  path  is  so  extremely  condensed,  as  either  to  be- 
come itself  luminous,  or  to  acquire  so  intense  a  heat  as  to  render  the  stone  in- 
candescent, or,  perhaps,  to  produce  upon  it  even- a  superficial  combustion,  the 
signs  of  which  are  exhibited  in  the  blackness  which  marks  the  surface  of  these 
bodies.  This  reasoning  is  attempted  to  be  supported  by  the  well-known  ex- 
periment of  the  fire-syringe.  In  that  instrument  a  solid  piston  is  fitted  in  a 
cylinder,  so  as  to  be  air-tight,  carrying  a  piece  of  amadou  or  other  easily  com- 
bustible matter,  at  its  end.  When  the  piston  is  suddenly  forced  down,  so  as  to 
produce  an  instantaneous  and  severe  compression  of  the  air  under  it,  tru"1  ama- 
dou takes  fire,  and,  if  the  cylinder  be  glass,  a  flash  of  light  is  visible  through 
it.  It  has  therefore  been  contended,  that  in  this  experiment  the  air  under 
the  piston  has  acquired,  by  compression,  such  a  temperature  as  renders  it  lu- 
minous. 

More  recent  experiments,  however,  made  in  France  (an  account  of  which 
has  fallen  in  my  way),  throw  doubt  upon  the  validity  of  this  inference.  It 
is  said  that  the  unctuous  matter  commonly  used  to  lubricate  the  piston  in  the 
fire-syringe  is,  in  fact,  the  source  of  the  ignition ;  for  that,  when  experiments 
were  made  with  pistons  not  so  lubricated,  the  flash  of  light  was  not  produced. 
It  is,  therefore,  considered  not  to  be  satisfactorily  proved,  that  air  by  mere  me- 
chanical compression  can  ever  become  luminous.  Still, however,  it  might  be  con- 


L 


METEORIC  STONES  AND  SHOOTING  STABS. 


tended  that,  even  though  the  air  were  not  to  become  luminous,  it  might,  never- 
theless, be  raised  to  such  a  temperature  by  compression  as,  by  contact  with 
the  meteorite,  might  render  the  latter  luminous  ;  but,  even  admitting  the  possi- 
bility of  this  supposition,  as  applied  to  the  air  contiguous  to  the  earth,  or  even 
at  any  moderate  elevation,  an  almost  insuperable  difficulty  arises  from  the  vavt 
height  at  which  meteorites  have  been  visible.  By  barometric  experiments  and 
observations  made  on  the  duration  of  the  morning  and  evening  twilight,  it  may 
be  considered  as  proved,  that  beyond  the  elevation  of  thirty  miles  there  exists 
no  atmosphere  possessing  any  sensible  mechanical  properties.  We  may  safely 
conclude  that  at  such  elevations  the  air,  if  any  really  exists  there,  must  be  so 
infinitely  attenuated  as  to  be  divested  of  all  sensible  resistance  or  inertia.  The 
space  there  must,  for  example,  be  a  more  absolute  vacuum  than  any  which 
could  be  produced  under  the  receiver  of  the  most  perfect  philosophical  air- 
pump  ;  how,  then,  can  we  imagine  such  a  compression  of  that  fluid  to  be  pro- 
duced, as  would  be  necessary  to  evolve  the  enormous  temperature  requisite  to 
render  luminous  the  matter  composing  meteoric  stones  ?  still  less  to  become  lu- 
minous itself. 

In  short,  it  must  be  admitted  that  none  of  these  theories  afford  a  satisfactory 
explanation  of  the  luminous  appearances  which  accompany  these  meteors.  Let 
us,  however,  examine  these  theories  respectively,  and  see  how  far  they  \vill 
bear  a  further  comparison  with  the  actual  circumstances  of  the  phenomena. 

The  atmospheric  hypothesis  is  subject  to  objections  so  unanswerable,  that  it 
may  be  considered  as  altogether  set  aside.    In  order  to  suppose  it  probable  that 
aerolites  could  be  formed  in  the  atmosphere,  we  must  show  that  their  constituent 
elements  can  exist  there.      We  know  that  hail  and  snow  can  be  formed  in  the 
air,  because  it  can  be  proved  that  aqueous  vapor  is  suspended  there,,  ajid  that  a 
temperature  is  sometimes  produced  there  so  low  as  to  convert  tha,t  vapor,  first, 
into  a  liquid,  and  then  into  the  solid  form  of  snow  or  hail.    But  th,e  most  rigor- 
ous analysis  has  never  detected  in  the  atmosphere  any  of  the  constituents  of 
meteoric  stones,  nor  is  there  any  proof  that  the  constituent  principles  of  the  air 
could  dissolve,  evaporate,  or  sublimate  such  substances.     Nor  can  it  be  said 
that,  although  the  atmosphere  which  immediately  surrounds  us  may  not  have 
such  properties,  yet,  that  at  the  great  elevations  in  which  meteorites  are  formed, 
the  air  may  consist  of  different  constituents,  for,  besides  the  fact  that  it  has  been  } 
ascertained  by  direct  analysis  that  the  atmosphere,  at  all  elevations  to  which  * 
man  has  ever  yet  attained,  consists  of  exactly  the  same  constituents,  in  exactly  <• 
the  same  proportions,  there  is  a  general  law,  which  prevails  among  all  gaseous  j 
substances,  that  when  different  gases  are  superposed  they  will,  notwithstanding  { 
their  different  degrees  of  levity,  ultimately  mingle  so  as  to  forrn  a  uniform  (. 
mass  ;  thus,  if  we  could  imagine  for  a  moment  a  stratum  of  air  to  exist  near  the 
top  of  the  atmosphere,  having  constituents  different  from  those  around  us,  such. 
stratum  would  gradually  intermingle  with  the  strata  below  it,  until  the  whole- 
would  acquire  a  uniform  quality.     It  is,  therefore,  physically  impossible  thav 
there  can  exist  in  any  elevated  region  of  the  air  any  substances  capable  of  dis- 
solving or  sublimating  the  matter  of  meteoric  stones. 

To  these  objections  we  may  add  others.  Although  it  may  be  admitted,  as  Ala- 
go  argues,  that  the  constituent  principles  of  aerolites  should  really  exist  in  the 
atmosphere  at  all  heights,  and  that  they  only  escape  analysis  because  of  ikeir 
extreme  minuteness,  it  would  still  be  necessary  to  explain  with  such  feeble  and 
such  dispersed  elements  a  sudden  precipitation,  yielding  stones  of  several  hun- 
dred weight,  such  as  those  preserved  at  Ensenheim,  in  Alsace,  or  3,000  or  4,000 
stones  of  various  dimensions,  like  those  which  were  separated  and  shot  off  by 
the  Laigle  meteor.  It  would  be  necessary  to  assign  the  cause  that  combines 
the  scattered  molecules,  and  forms  them  into  a  single  mass.  It  is  not  affinity, 

as 


434 


METEORIC  STONES  AND  SHOOTING  STARS. 


for  the  elements  composing  aerolites  are  not  in  a  state  of  combination,  but  sim- 
ply agglomerated  and  held  together  in  juxtaposition.  And  yet,  if  they  are  not 
subjected  to  any  force,  these  little  globuL-s  ought  to  fall  separately  as  they  are 
formed.  It  is  in  vain  to  object  that  they  might  be  suspended,  for  more  or  less 
time,  by  a  cause  analogous  to  that  which,  according  to  the  ingenious  opinion 
of  Volta,  balances  the  particles  of  hail  between  two  clouds,  so  as  to  give  them 
time  to  enlarge  by  the  addition  of  new  layers  of  ice.  The  fact  still  remains, 
that  these  latter  have  never  been  seen  to  amount  to  several  hundred  weight, 
though  the  elements  that  form  hail  are  much  more  abundant  in  the  air  than  those 
of  aerolites  are  supposed  to  be.  Besides,  in  Volta's  theory,  the  suspension  of 
hail  in  the  atmosphere  is  attributed  to  the  reciprocal  action  of  electric  clouds, 
a  cause  which  can  not  be  in  like  manner  adapted  to  the  formation  of  aerolites, 
since  the  meteors  that  carry  them  sometimes  burst  in  the  clearest  weather. 

But  even  granting  all  this,  and  admitting  the  formation  of  aerolites  in  the  at- 
mosphere by  some  unknown  agency,  how  shall  we  account  for  the  circumstan- 
ces attending  their  collision  with  the  surface  of  the  earth  1  According  to  this 
theory,  they  would  move  to  the  surface  of  the  earth  by  the  operation  of  terres- 
trial gravity  alone,  and  would  meet  the  earth  with  a  velocity  due  to  the  height 
from  which  they  fell.  Now  the  actual  velocities  with  which  they  are  known 
to  strike  the  earth  could  never  be  acquired  under  the  mere  agency  of  terrestrial 
gravity,  through  any  height  within  the  ordinary  limits  of  the  air. 

But,  if  the  velocity  of  the  meteorites  be  incompatible  with  this  theory,  their 
direction  is  still  more  so.  Their  obliquity  could  never  be  produced  by  any  con- 
ceivable atmospheric  current. 

We  may,  therefore,  safely  pronounce  the  atmospheric  theory  to  be  incom- 
patible with  the  ascertained  circumstances  of  the  phenomena,  and  to  require 
admissions  inconsistent  with  the  established  principles  of  physics. 

The  volcanic  theory  is  subject  to  objections  as  decisive  as  that  we  have  first 
examined.  The  nature  of  the  substances  ejected  from  terrestrial  volcanoes  is 
well  known,  and  we  do  not  find  among  them  the  substances  which  form  the 
constituents  of  meteorites ;  besides  this,  it  is  found  that  meteoric  stones  fall  on 
parts  of  the  earth  so  remote  from  volcanoes,  and  at  times  so  distant  from  any 
known  extensive  eruptions,  that  it  is  impossible  to  admit  the  supposition  that 
they  have  proceeded  from  this  cause.  For  these  and  other  reasons,  needless 
to  dwell  on,  the  volcanic  hypothesis  is  set  aside. 

The  planetary  hypothesis  is  subject  to  less  difficulty,  and  is  much  more  in 
harmony  with  the  phenomena.  The  velocity  and  direction  of  meteoric  stones 
when  they  strike  the  earth  are  quite  in  accordance  with  this  theory,  and  the 
existence  in  them  of  constituents  like  metallic  iron  and  nickel,  which  have  no 
natural  existence  on  the  earth,  is  also  explicable ;  but  these  circumstances  are 
equally  accounted  for  by  all  the  extra  terrestrial  theories,  and  afford,  therefore, 
no  more  countenance  to  the  planetary  than  to  the  lunar  or  nebular  hypothesis. 
On  the  other  hand,  a  serious  difficulty  is  presented  in  the  uniform  analysis  of 
the  meteorites.  How  can  it  be  supposed  that  all  the  various  fragments  of  a 
broken  planet  should  consist  of  the  same  constituents  in  the  very  same  propor- 
tion ?  If  the  earth  were  split  in  pieces  by  any  cause  internal  or  external,  would 
its  fragments  be  so  uniform  in  its  constituents  ?  Assuredly  not.  We  should 
find  fragments  of  very  heterogeneous  character.  One  would  consist  of  a  mass 
of  sandstone,  another  a  lump  of  granite  ;  here  would  be  an  agglomerate  of  one 
kind,  there  of  another.  It  is,  therefore,  in  the  highest  degree  improbable  that 
the  fragments  of  another  planet  should  be  uniform  in  their  constituents,  and  this 
improbability  is  rendered  greater  by  the  fact  that  the  meteorites  are  composed 
of  heterogeneous  materials,  mechanically  agglomerated,  and  not  of  a  uniform 
substance,  composed  of  different  elements,  united  like  those  of  water  or  air. 


METEORIC  STONES  AND  SHOOTING  STARS. 


435 


Until,  therefore,  the  advocates  of  the  planetary  hypothesis  can  remove  these 
difficulties,  that  theory  cannot  be  admitted. 

The  lunar  hypothesis  appears  to  be  compatible,  generally,  with  the  circum- 
stances of  aerolites.  It  explains  satisfactorily  enough  the  force  and  direction 
of  their  collision  with  the  earth.  If  it  be  admitted  that  they  proceed  from  the 
same  lunar  volcano,  or  that  all  lunar  volcanoes  eject  the  same  kind  of  substan- 
ces, the  similarity  of  their  constituents  will  be  explained  ;  in  short,  all  that  is 
>  necessary  to  raise  the  lunar  hypothesis  to  the  rank  of  a  theory  is  to  prove  the 
fact  that  there  really  do  exist  volcanoes  in  the  moon.  Now  although  observa- 
tion has  supplied  circumstances  which  give  some  probability  to  that  idea,  yet 
it  is  still  very  far  from  being  clearly  established.  Telescopic  examination  of 
the  lunar  surface,  has  certainly  and  clearly  established  the  fact  that  it  is  covered 
in  every  part  that  is  visible  with  mountains,  having  all  the  external  forms  and  char- 
acters of  terrestrial  volcanoes.  The  craters  are  not  only  distinctly  visible,  but 
we  have  been  enabled  to  ascertain  the  existence  of  the  cones  within  them. 
Sir  John  Herschel,  who  has  had  the  advantage  of  observing  with  the  most  \ 
powerful  reflecting  telescopes,  has  declared  that  the  generality  of  the  lunar 
mountains  present  a  striking  uniformity  and  singularity  of  aspect.  They  are 
wonderfully  numerous,  occupying  by  far  the  larger  portion  of  the  surface,  and 
almost  universally  of  an  exactly  circular  or  cup-shaped  form,  foreshortened, 
however,  into  ellipses  toward  the  limb  ;  but  the  larger  have  for  the  most  part 
flat  bottoms  within,  from  which  rises  centrally  a  small,  steep,  conical  hill. 
They  offer,  in  short,  in  its  highest  perfection,  the  true  volcanic  character,  as  it 
may  be  seen  in  the  crater  of  Vesuvius,  and  in  a  map  of  the  volcanic  districts 
of  the  Campi  Phlegraei  or  the  Puy  de  Dome.  And  in  some  of  the  principal 
ones,  decisive  marks  of  volcanic  stratification,  arising  from  successive  depos- 
ites  of  ejected  matter,  may  be  clearly  traced  with  powerful  telescopes.  What 
is,  moreover,  extremely  singular  in  the  geology  of  the  moon  is,  that  although 
nothing  having  the  character  of  seas  can  be  traced  (for  the  dusky  spots  which 
are  commonly  called  ceas,  when  closely  examined,  present  appearances  incom- 
patible with  the  supposition  of  deep  water),  yet  there  are  large  regions  per- 
fectly level,  and  apparently  of  a  decidedly  alluvial  character. 

But  this  condition  of  things  may  have  resulted  from  volcanic  action,  which 
took  place  at  an  epoch  long  antecedent  to  the  commencement  of  the  present 
condition  of  our  globe,  and  it  may  be  required  to  establish  the  fact  of  the  pres- 
ent existence  of  active  volcanoes  on  the  moon. 

To  this  it  may  be  answered,  first,  that  if  active  volcanoes  existed  at  any  re- 
mote period,  the  substances  ejected  from  them  may  have  been  ever  since  re- 
volving in  the  space  around  the  earth,  and  that  they  may  now,  from  time  to 
time,  become  entangled  in  the  earth's  atmosphere  and  descend  to  the  surface. 

Secondly,  it  may  be  replied  that  we  do  possess  indications  of  the  present 
existence  of  lunar  volcanoes,  inasmuch  as  bright,  luminous  spots  have  been 
detected  by  various  astronomers  at  different  times  and  places,  on  the  occasion 
of  total  eclipses  of  the  sun,  on  the  surface  of  the  moon,  then  dark,  and  that  it  is 
impossible,  on  the  one  hand,  to  deny  the  existence  of  what  has  been  witnessed 
by  so  many  competent  observers,  and  that  no  other  supposition  has  been  offer- 
ed to  explain  such  luminous  spots,  except  one,  which  from  its  extreme  improb- 
ability cannot  be  seriously  entertained,  namely,  that  which  supposes  the  sun 
to  have  been  rendered  visible  by  holes  through  the  moon. 

Thus,  then,  stands  the  lunar  theory  of  meteorites.  It  is  exempt  from  most 
of  the  difficulties  and  objections  that  attend  the  other  hypotheses,  but  neverthe- 
less, until  it  be  actually  established  beyond  all  question  that  there  are,  or  have 
been,  active  volcanoes  on  the  moon,  and  that  substances  ejected  from  these 
have  actually  fallen  upon  the  earth,  the  luna^theory  of  meteorites  cannot  be 


436 


METEORIC  STONES  AND  SHOOTING  STARS. 


pronounced   to  be   established  according  to  the  rigid  rules   of  inductive  phi- 
losophy. 

The  nebular  hypothesis  can  scarcely  be  regarded  in  a  more  definite  point  of 
view  than  as  a  conjecture.  We  have  no  observation  to  prove  what  the  nature 
of  the  nebulous  matter  is,  nor  whether  it  is  solid,  liquid,  or  gaseous.  We  know 
that  as  it  exists  in  the  stellar  regions  it  is  self-luminous ;  but  there  is  no  indi- 
cation of  such  a  quality  in  any  matter  existing  in  the  solar  system.  It 
may  also  be  contended  that  if  it  exist  within  the  solar  system  in  the  quantity- 
contemplated  in  this  hypothesis,  we  might  expect  it  to  be  visible,  if  not  by  its 
own  light,  at  least  by  the  reflected  light  of  the  sun. 

From  the  exposition  I  have  here  given  it  will  be  perceived  that  the  origin 
of  meteoric  stones  is  still  involved  in  much  obscurity.  We  may,  perhaps,  pro- 
nounce with  some  degree  of  confidence  that  they  are  not  of  terrestrial  origin, 
nor  generated  in  the  atmosphere,  and  that  strictly  speaking  they  are  cosmical. 

But  we  are  not  yet  in  possession  of  all  the  information  which  observa- 
tion may  supply  respecting  them.  It  is  not  yet  clearly  ascertained  whether 
they  are  identical  with  the  appearances  so  often  exhibited  in  the  heavens,  call- 
ed shooting  stars,  nor  has  the  cause  of  this  latter  meteor  been  explained.  A 
great  impediment  to  the  correct  information  of  these  phenomena,  arises  from 
the  fact  that  their  exhibition  in  the  heavens  is  not  preceded  by  any  circumstance 
which  can  prepare  the  observer  for  them,  and  their  continuance  is  seldom  long 
enough  to  afford  opportunity  for  correct  observations.  We  are,  therefore,  com- 
pelled to  collect  from  scattered  sources,  and  loose  records,  much  of  ths  infor- 
mation which  is  available  respecting  them. 

One  of  the  most  interesting  narratives  of  this  kind  on  record  is  that  of  a 
meteor  which  appeared  in  America,  on  the  13th  of  November,  1833.  It  was 
published  in  the  American  Journal  of  Science,  and  is  entitled  to  especial  notice. 
The  following  is  an  abstract  of  this  narrative  : — 

The  meteors  began  to  attract  notice  by  their  frequency  as  early  as  9  o'clock 
on  the  preceding  evening  (November  12)  ;  the  exhibition  became  strikingly 
brilliant  about  11  o'clock,  but  most  splendid  of  all  about  4  o'clock,  and  continued 
with  but  little  intermission  until  darkness  merged  in  the  light  of  day.  A  few  large 
fire-balls  were  seen  even  after  the  sun  had  risen.  The  entire  extent  of  the 
exhibition  is  not  ascertained,  but  it  covered  no  inconsiderable  portion  of  the 
earth's  surface.  It  has  been  traced  from  the  longitude  of  61°  in  the  Atlantic 
ocean,  to  longitude  of  100°  in  central  Mexico,  and  from  the  North  American 
lakes  to  the  southern  side  of  the  island  of  Jamaica.  Everywhere  within  these 
limits,  the  first  appearance  was  that  of  fire-works  of  the  most  imposing  gran- 
deur, covering  the  entire  vault  of  heaven  with  myriads  of  fire-balls  resembling 
sky-rockets.  On  more  attentive  inspection,  it  was  seen  that  the  meteors  ex- 
hibited three  distinct  varieties  ;  the  first  consisting  of  phosphoric  lines,  appa- 
rently described  by  a  point :  the  second  of  large  fire-balls,  that  at  intervals 
darted  along  the  sky,  leaving  numerous  trains,  which  occasionally  remained  in 
view  for  a  number  of  minutes,  and  in  some  cases  for  half  an  hour  or  more ; 
the  third,  of  undefined,  luminous  bodies,  which  remained  nearly  stationary  for 
a  long  time. 

One  of  the  most  remarkable  circumstances  attending  this  display  was,  that 
the  meteors  all  seemed  to  emanate  from  one  and  the  same  point.  They  set  out 
at  different  distances  from  this  point,  and  proceeded  with  immense  velocity, 
describing,  in  some  instances,  an  arc  of  30°  or  40°  in  less  than  four  seconds. 
At  Poland,  on  the  Ohio,  a  meteor  (of  the  third  variety)  was  distinctly  visible  in 
the  northeast  for  more  than  an  hour.  At  Charleston,  South  Carolina,  another 
of  extraordinary  size  was  seen  to  course  the  heavens  for  a  great  length  of 
time,  and  then  was  heard  to  explode  with  the  noise  of  a  cannon.  The  point 


METEORIC  STONES  AND  SHOOTING  STARS.  437 

from  which  the  meteors  seemed  to  emanate,  was  observed  by  those  who 
fixed  its  position  among  the  stars  to  be  in  the  constellation  Leo  ;  and  what 
is  very  remarkable,  this  point  was  stationary  among  the  stars  during  the 
whole  period  of  observation ;  that  is  to  say,  it  did  not  move  along  with 
the  earth  in  its  diurnal  rotation  eastward,  but  accompanied  the  stars  in  their 
apparent  progress  westward.  It  is  not  certain  whether  the  meteors  were, 
in  general,  accompanied  by  any  peculiar  sound.  A  few  observers  reported  that 
they  heard  a  hissing  noise,  like  the  rushing  of  a  sky-rocket,  and  slight  explo- 
sions, like  the  bursting  of  the  same  bodies.  Nor  does  it  appear  that  any  sub- 
stance reached  the  ground  which  could  be  clearly  established  to  be  a  residu- 
um or  deposite  from  the  meteors.  A  remarkable  change  of  weather  from 
warm  to  cold,  accompanied  the  meteoric  shower,  or  immediately  followed  it,  in 
all  parts  of  the  United  States. 

From  these  circumstances  and  other  particulars  recorded,  it  has  been  infer- 
red that  had  these  meteors  appeared  to  emanate  from  a  point  not  in  the  direction 
of  the  earth's  rotation,  they  had  not  their  origin  in  the  atmosphere.  By  com- 
paring observations  made  upon  them  in  different  latitudes,  it  was  calculated 
that  their  distance  from  the  surface  of  the  earth  must  have  been  above  2,000 
miles.  Assuming  this  result,  which  is,  however,  only  an  approximation,  the 
velocity  with  which  they  would  enter  the  atmosphere  may  be  computed. 

A  body  falling  from  the  height  of  2,000  miles  would  acquire  by  the  attrac- 
tion of  gravity,  at  50  miles  from  the  earth,  where  it  might  be  supposed  to  en- 
ter the  atmosphere,  a  velocity  of  four  miles  per  second,  being  ten  times  the 
velocity  of  a  cannon-ball.  It  is  contended,  therefore,  that  on  entering  the  at- 
mosphere they  would  produce  a  sudden  compression  of  air,  and  corresponding 
evolution  of  heat.  That  the  heat  thus  produced  would  render  the  bodies  in- 
candescent, and  if  they  were  combustible,  would  set  them  on  fire.  It  is  argued 
that  the  quantity  of  heat  which  would  be  extricated  from  the  air  by  such  com- 
pression would  exceed  that  of  the  hottest  furnace  ;  but  that  if  the  velocity 
arising  from  the  earth's  motion  were  added  to  the  proper  velocity  of  the  body 
itself,  which  it  must  be,  if  these  motions  are  contrary,  there  would  then  be  an 
effective  velocity  of  fourteen,  instead  of  four  miles  per  second,  and  a  still 
greater  amount  of  heat  would  be  produced.  It  is  argued  that  these  meteors 
must  have  been  constituted  of  very  light  materials  ;  for  if  their  quantity  of 
matter  had  been  considerable,  with  so  great  a  velocity  they  would  have  had 
sufficient  momentum  to  reach  the  earth,  and  the  most  disastrous  consequences 
might  have  ensued.  From  the  apparent  magnitude  of  many  of  the  meteors, 
and  their  probable  distance,  it  was  conjectured  that  they  were  bodies  of  a  very 
large  size,  although  it  was  impossible  to  ascertain  their  magnitude  with  any 
certainty.  It  was  supposed  that  they  were  only  stopped  in  the  atmosphere,  and 
prevented  from  reaching  the  earth  by  transferring  their  motion  to  columns  of  air, 
large  volumes  of  which  they  would  suddenly  and  violently  displace.  It  was  con- 
sidered remarkable  that  the  state  of  the  weather,  and  the  condition  of  the  seasons 
following  this  meteoric  shower,  were  just  such  as  might  have  been  anticipated 
from  these  disturbing  circumstances  of  the  atmospheric  equilibrium.  Such 
were  the  speculations  to  which  this  remarkable  phenomenon  gave  rise. 

Whatever  be  the  origin  of  the  phenomena  of  shooting  stars,  it  cannot  fail  to 
be  interesting  to  learn  the  principal  circumstances  which  observation  has  col- 
lected respecting  them. 

Their  apparent  magnitudes  are  very  various.  Sometimes  they  are  not  bright- 
er or  larger  than  the  smallest  star  visible  to  the  naked  eye,  and  at  other  times 
they  surpass  in  splendor  the  most  brilliant  of  the  planets.  Sometimes  the  glob- 
ular form  can  be  distinctly  recognised  upon  them,  and  they  are  not  distinguish- 
able from  the  meteors  called  fire-balls. 


438  METEORIC  STONES  AND  SHOOTING  STARS. 

Shooting  stars  seem  to  prevail  equally  in  every  climate  and  in  every  state  of 
the  weather.     They  are  occasionally  seen  at  all  seasons  of  the  year,  but  more  c 
frequently  in  summer  or  at  the  end  of  the  autumn.     They  appear  usually  to  be  ] 
followed  by  a  luminous  train  of  intensely  white  light. 

A  question  will  immediately  arise,  whether  this  be  a  real  continued  physical  | 
line  of  light,  or  whether  it  must  not  rather  be  ascribed  to  the  same  cause  which, 
makes  us  see  a  complete  circle  of  light  when  a  lighted  stick  revolves  rapidly  in 
a  circle.  In  that  case  the  circle  of  light  is  not  real,  the  effect  being  an  optical 
illusion.  The  membrane  of  the  eye  which  is  affected  by  light  has  been  ascer- 
tained to  preserve  the  impression  made  upon  it  for  about  one  tenth  of  a  second 
after  the  cause  which  produced  that  impression  has  ceased  to  act.  We,  conse- 
quently, continue  to  see  a  visible  object  in  any  position  for  a  tenth  of  a  sec- 
ond after  it  has  left  that  position.  If,  then,  a  luminous  object  move  over  a  cer- 
tain space  in  one  tenth  of  a  second,  the  eye  will  see  it  at  the  same  time  in 
every  part  of  that  space,  and  consequently,  that  space  will  appear  one  contin- 
uous line  of  light. 

If,  therefore,  the  luminous  train  which  is  visible  after  a  shooting  star,  extends 
through  a  space  over  which  the  star  moved  in  one  tenth  of  a  second,  it  is  then 
possible  that  such  luminous  train  may  be  illusory,  being  a  mere  optical  effect 
of  the  rapid  motion  of  the  star.  But  if  it  be  longer  than  this,  or  if  it  be  visible 
in  one  place  for  more  than  the  tenth  of  a  second  after  the  star  has  moved  from 
that  place,  then  it  cannot  be  explained  on  this  principle  and  must  be  admitted 
to  be  an  actual  train  of  light.  Now  it  is  stated  by  observers  of  these  meteors, 
that  the  trains  are  sometimes  seen  for  several  minutes.  In  the  case  of  actual 
fire-balls,  Dr.  Olbers  observed  trains  which  continued  visible  for  six  or  seven 
minutes,  and  Brandes  in  one  instance  estimated  that  fifteen  minutes  elapsed 
between  the  extinction  of  the  fire-ball  and  the  disappearance  of  the  luminous 
train.  In  general  the  trains  have  the  same  hollow,  cylindrical  appearance  as 
the  tails  of  comets,  their  inner  part  appearing  to  be  void  of  luminous  matter, 
and  a  further  resemblance  to  comets  is  exhibited  in  the  curved  form,  which 
they  sometimes  assume. 

Various  and  discordant  have  been  the  explanations  offered  of  these  luminous 
trains.  Some  have  ascribed  them  to  an  oily  sulphurous  vapor  existing  in  the 
atmosphere,  which,  being  disposed  in  thin  layers  and  becoming  inflamed 
would  exhibit  the  appearance  of  a  brilliant  spark  passing  rapidly  from  point  to 
point.  Beccaria  and  Vassali  considered  them  to  be  lines  of  electrical  sparks, 
an  hypothesis,  however,  which  has  been  abandoned.  Lavoisier,  Volta,  and 
others,  explain  these  meteors  by  supposing  that  hydrogen  gas  accumulated,  by 
its  lightness,  in  the  higher  regions  of  the  atmosphere,  was  inflamed.  But  the 
general  law  of  gases,  which  gives  them  a  tendency  to  mingle,  notwithstanding 
the  effect  of  their  specific  gravities,  puts  aside  this  hypothesis. 

In  the  year  1798  an  investigation  of  the  heights  of  shooting  stars  was  un- 
dertaken by  Brandes,  at  Leipsig,  and  Benzenberg,  at  Dasseldorf.  Having  se- 
lected a  base  line  (about  nine  miles  in  length),  they  placed  themselves  at  its  ex- 
tremities, on  appointed  nights,  and  observed  all  the  shooting  stars  which  ap- 
peared, tracing  their  courses  through  the  heavens  on  a  celestial  map,  and 
noting  the  instants  of  their  appearances  and  extinctions  by  chronometers  pre-  ) 
viously  compared.  The  difference  of  the  paths  traced  on  the  heavens  afforded 
data  for  the  determination  of  the  parallaxes,  and  consequently  the  heights  and 
the  lengths  of  the  orbits.  On  six  evenings,  between  September  and  Novem- 
ber, the  whole  number  of  shooting  stars  seen  by  both  observers  was  402  :  of 
these,  22  were  identified  as  having  been  observed  by  each  in  such  a  manner 
that  the  altitude  of  the  meteor  above  the  ground  at  the  instant  of  extinction 
could  be  computed.  The  least  of  the  altitudes  was  about  6  English  miles.  Of 


METEORIC  STONES  AND  SHOOTING  STARS.  439 

the  whole,  there  were  7  under  45  miles  ;  9  between  45  and  90 ;  6  above  90  ; 
and  the  highest  was  above  140  miles.  There  were  only  two  observed  so  com- 
pletely as  to  afford  data  for  determining  the  velocity.  The  first  gave  25  miles, 
and  the  second  from  17  to  21  miles,  in  a  second.  The  most  remarkable  result 
was,  that  one  of  them,  certainly,  was  observed  not  to  fall,  but  to  move  in  a 
direction  away  from  the  earth. 

By  these  observations  a  precise  idea  was  first  obtained  of  the  altitudes,  dis- 
tances, and  velocities,  of  these  singular  meteors.  A  similar  but  more  extended 
plan  of  observation  was  organized  by  Brandes,  in  1823,  and  carried  into  effect 
at  Breslau  and  the  neighboring  towns,  by  a  considerable  number  of  persons, 
observing  at  the  same  time  on  concerted  nights.  Between  April  and  October 
about  1800  shooting  stars  were  noted  at  the  different  places — out  of  which 
number  62  were  found  which  had  been  observed  simultaneously  at  more  than 
one  station,  in  such  a  manner  that  their  respective  altitudes  could  be  deter- 
mined, and  36  others  of  which  the  observations  furnished  data  for  estimating 
the  entire  orbits.  Of  these  98,  the  heights  (at  the  time  of  extinction)  of  4  were 
computed  to  be  under  15  English  miles  ;  of  13,  between  15  and  30  miles ;  of 
22,  between  30  and  45  ;  of  33,  between  45  and  70  ;  of  13,  between  70  and  90  ; 
and  of  11,  above  90  miles.  Of  these  last,  two  had  an  altitude  of  about  140 
miles,  one  of  220  miles,  one  of  280,  and  there  was  one  of  which  the  height 
was  estimated  to  exceed  460  miles. 

On  the  36  computed  orbits,  in  26  instances  the  motion  was  downward,  in 
one  case  horizontal,  and  in  the  remaining  nine  more  or  less  upward.  The 
velocities  were  between  18  and  36  miles  in  a  second.  The  trajectories  were 
frequently  not  straight  lines,  but  incurvated,  sometimes  in  the  horizontal  and 
sometimes  in  the  vertical  direction,  and  sometimes  they  were  of  a  serpentine 
form.  The  predominating  direction  of  the  motion  of  the  meteors  from  north- 
east to  southwest,  contrary  to  that  of  the  earth  in  its  orbit,  was  very  remarka- 
ble, and  is  important  in  reference  to  their  physical  theory. 

A  similar  set  of  observations  was  made  in  Belgium,  in  1824,  under  the  di- 
rection of  M.  Quetelet,  the  results  of  which  are  published  in  the  Annnaire  de 
Bruxdles  for  1837.  M.  Quetelet  was  chiefly  solicitous  to  determine  the  velocity 
of  the  meteors.  He  obtained  six  corresponding  observations,  from  which  this 
element  could  be  deduced,  and  the  result  varied  from  10  to  25  English  miles 
in  a  second.  The  mean  of  the  six  results  gave  a  velocity  of  nearly  17  miles 
per  second,  a  little  less  than  that  of  the  earth  in  its  orbit. 

Another  set  of  corresponding  observations  was  made  in  Switzerland,  on  the 
10th  of  August,  1838,  a  circumstantial  account  of  which  is  given  by  M.  Wart- 
manri  in  QueleleCs  Correspondence  Mathematique  for  July,  1839.  M.  Wart- 
mann  and  five  other  observers,  provided  with  celestial  charts,  stationed  them- 
selves at  the  observatory  of  Geneva,  and  the  corresponding  observations  were 
made  at  Planchettes,  a  village  about  sixty  miles  to  the  northeast  of  that  city. 

In  the  space  of  seven  and  a  half  hours  the  number  of  meteors  observed  by  the 
six  observers  at  Geneva  was  381,  and  during  five  and  a  half  hours  the  number 
observed  at  Planchettes  by  two  observers,  was  104.  All  the  circumstances  of 
the  phenomena — the  place  of  the  apparition  and  disappearance  of  each  meteor, 
the  time  it  continued  visible,  its  brightness  relatively  to  the  fixed  stars,  whether 
accompanied  with  a  train,  &c. — were  carefully  noted,  and  the  trajectories  de- 
scribed by  the  meteors,  were  very  different,  varying  from  8°  to  70°  of  angular 
space.  The  velocities  appeared  also  to  differ  considerably ;  but  the  average 
velocity  was  supposed  by  M.  Wartmann  to  be  25°  per  second.  It  was  found, 
from  the  comparison  of  the  simultaneous  observations,  that  the  average  height 
above  the  ground  was  about  550  miles  ;  and  hence  the  relative  velocity  was 
computed  to  be  about  240  miles  in  a  second.  But  as  the  greater  number 


METEORIC  STONES  AND  SHOOTING  STARS. 


moved  in  a  direction  opposite  to  that  of  the  earth  in  its  orbit,  the  relative  ve- 
locity must  be  diminished  by  the  earth's  velocity  (about  19  miles  in  a  second), 
this  still  leaves  upward  of  220  miles  per  second  for  the  absolute  velocity  of  the 
meteor,  which  is  more  than  1 1  times  the  orbitual  velocity  of  the  earth,  seven 
and  a  half  times  that  of  the  planet  Mercury,  and  probably  greater  than  that  of 
many  of  the  comets  at  their  perihelion. 

Such  are  the  principal  facts  which  have  yet  been  established  respecting  the 
heights,  velocities,  and  orbits,  of  the  shooting  stars  :  and  it  is  from  these, 
chiefly,  that  we  are  enabled  to  form  any  probable  conjectures  respecting  their 
origin.  And  since  it  is  now  established  that  no  difference  is  observable  be- 
tween the  larger  shooting  stars  and  small  fire-balls,  both  having  similar  altitudes 
and  velocities,  and  presenting  absolutely  the  same  appearances,  we  may  as- 
sume them  to  be  of  the  same  nature,  and  that  whatever  has  been  proved  re- 
specting fire-balls  will  apply  equally  to  the  larger  shooting  stars.  Whether 
the  meteoric  appearances  to  which  the  latter  term  is  applied  may  not  include 
objects  of  totally  different  natures,  is  a  question  admitting  a  doubt.  It  is  possi- 
ble that  among  the  shooting  stars  there  may  be  objects  which  are  merely  electric 
sparks,  or  which  have  their  origin  in  spontaneously-inflammable  gases,  known 
or  unknown,  existing  in  the  atmosphere  ;  but  the  greater  part  of  them  must  be 
considered  as  identical  with  fire-balls. 

The  lunar  hypothesis  advanced  by  Laplace,  Berzelius,  and  others,  to  ex- 
plain meteoric  stones,  appears  to  be  attended  with  serious  difficulties,  if,  in- 
deed, it  be  not  altogether  incompatible  with  the  phenomena  of  shooting  stars.  In 
order  to  enter  our  atmosphere  with  a  velocity  of  20  miles  in  a  second,  it  may 
be  shown  that,  if  they  come  from  the  moon,  they  must  have  been  projected  from 
the  lunar  surface  with  a  velocity  of  about  120,000  feet,  in  a  second,  which  may 
be  regarded  as  almost  impossible. 

It  thus  appears  that  those  shooting  stars  and  fire-balls  which  have  the  plane- 
tary velocity  of  from  20  to  40  miles  in  a  second,  cannot,  with  any  probability, 
be  regarded  as  having  their  origin  in  the  moon.  Whether  any  individual  bod- 
ies, moving  with  a  smaller  velocity,  may  have  a  lunar  origin,  is  a  question 
(  which  cannot  be  decisively  answered.  "To  me,"  says  Dr.  Olbers,  "it  does 
)  not  appear  at  all  probable ;  and  I  regard  the  moon,  in  its  present  circumstan- 
i  ces,  as  an  extremely  peaceable  neighbor,  which,  ftom  its  want  of  water  and 
y  atmosphere,  is  no  longer  capable  of  any  strong  explosions." 
I  The  hypothesis  first  suggested  by  Chladni  is  that  which  appears  to  have 
)  met  with  most  favor,  having  been  adopted  by  Arago  and  other  eminent  astrono- 
(  mers  of  the  present  clay  to  explain  the  November  phenomena.  It  consists  in 
;  supposing  that,  independently  of  the  great  planets,  there  exist  in  the  planetary 
(  regions  myriads  of  small  bodies  which  circulate  about  the  sun,  generally  in 
)  groups  of  zones,  and  that  some  of  these  zones  intersect  the  ecliptic,  and  are, 
{  consequently,  encountered  by  the  earth  in  its  annual  revolution.  The  princi- 
)  pal  difficulties  attending  this  theory  are  the  following : — 

J  First,  that  bodies  moving  in  groups  in  the  circumstances  supposed,  must 
necessarily  move  in  the  same  direction,  and  consequently  they  become  visi- 
ble from  one  point  and  move  toward  the  opposite.  Now  although  the  observa- 
tions seem  to  show  that  the  predominating  direction  is  from  northeast  to  south- 
west, yet  shooting  stars  are  observed  on  the  same  nights  to  emanate  from  all 
points  of  the  heavens,  and  to  move  in  all  possible  directions.  Secondly,  their 
average  velocity  (especially  as  determined  by  Wartmann),  greatly  exceeds  that 
which  any  body  circulating  about  the  sun  can  have  at  the  distance  of  the  earth. 
Thirdly,  from  their  appearance,  and  the  luminous  train  which  they  generally 
leave  behind  them,  and  which  often  remains  visible  for  several  seconds,  some- 
times for  whole  minutes,  and  also  from  their  being  situated  within  the  earth's 


shadow,  and  at  heights  far  exceeding  those  at  which  the  atmosphere  can  be 
supposed  capable  of  supporting  combustion,  it  is  manifest  that  their  light  is  not 
reflected  from  the  sun,  they  must  therefore  be  self-luminous,  which  is  contrary 
to  every  analogy  of  the  solar  system.  Fourthly,  if  masses  of  solid  matter  ap- 
proached so  near  the  earth  as  many  of  the  shooting-stars  do,  some  of  them 
would  inevitably  be  attracted  to  it,  but  of  the  thousands  of  shooting-stars  which 
have  been  observed,  there  is  no  authenticated  instance  of  any  one  having  ac- 
tually reached  the  earth.  Fifthly,  instead  of  the  meteors  being  attracted  to 
the  earth,  some  of  them  are  observed  actually  to  rise  upward  and  to  describe 
orbits  which  are  convex  toward  the  earth,  a  circumstance  of  which,  on  the 
present  hypothesis,  it  seems  difficult  to  give  any  rational  explanation. 

From  the  difficulties  attending  every  hypothesis  which  has  hitherto  been 
proposed,  it  may  be  inferred  how  very  little  real  knowledge  has  yet  been  ob- 
tained respecting  the  nature  of  the  shooting-stars.  It  is  certain  that  they  ap- 
pear at  great  altitudes  above  the  earth,  and  that  they  move  with  prodigious 
velocity,  but  everything  else  respecting  them  is  involved  in  profound  mystery. 
From  the  whole  of  the  facts,  M.  Wartmann  thinks  that  the  most  rational  con- 
clusion we  can  adopt  is,  that  the  meteors  probably  owe  their  origin  to  the  dis- 
engagement of  electricity,  or  of  some  analogous  matter,  which  takes  place  in 
the  celestial  regions  on  every  occasion  in  which  the  conditions  necessary  for 
the  production  of  the  phenomena  are  renewed. 

The  presumption  in  favor  of  the  cosmical  origin  of  the  shooting  stars  are 
chiefly  founded  on  their  periodical  recurrence  at  certain  epochs  of  the  year, 
and  the  extraordinary  displays  of  the  phenomena  in  various  years  on  the  nights 
of  the  12th  or  13th  of  November. 

We  shall  here  merely-state  the  principal  circumstances  accompanying  those 
of  1799,  which  put  the  notion  of  a  lunar  origin  entirely  out  of  the  question. 

On  the  morning  of  the  12th  of  November,  1799,  before  sunrise,  Humboldt 
and  Bonpland,  then  on  the  coast  of  Mexico,  were  witnesses  to  a  remarkable 
exhibition  of  shooting  stars  arid  fire-balls.  They  filled  the  part  of  the  heavens 
extending  from  due  east  to  about  30°  toward  the  north  and  south.  They  rose 
from  the  horizon  between  the  east  and  northeast  points,  described  arcs  of  un- 
equal magnitude,  and  fell  toward  the  south ;  some  of  them  rose  to  the  height 
of  40°,  all  above  25°  or  30°.  Many  of  them  appeared  to  explode,  but  the 
larger  number  disappeared  without  emitting  sparks  ;  some  had  a  nucleus  ap- 
parently equal  to  Jupiter.  This  most  remarkable  spectacle  was  seen  at  the 
same  time  in  Camana,  on  the  borders  of  Brazil,  in  French  Guiana,  in  the  chan- 
nel of  Bahama,  on  the  continent  of  North  America,  in  Labrador,  and  in  Green- 
land, and  even  at  Carlsruhe,  Halle,  and  other  places  in  Germany,  many  shoot- 
ing stars  were  seen  on  the  same  day.  At  Nain  and  HorTenthal  in  Labrador, 
and  at  Neuhernhut  and  Lichtenau  in  Greenland,  the  meteors  seem  to  have  ap- 
peared the  nearest  to  the  earth.  At  Nain  they  fell  toward  all  points  of  the 
horizon,  and  some  of  them  had  a  diameter  which  the  spectators  estimated  at 
half  an  ell.  (See  Humboldt's  Recueil  des  Voyages,  &c.,  Vol.  II.) 

A  not  less  stupendous  exhibition  took  place  in  North  America  on  the  night 
of  the  12th  of  November,  1833.  In  1834  similar  phenomena  occurred  on  the 
night  of  the  13th  of  November;  but  on  this  occasion  the  meteors  were  of  a 
smaller  size.  In  1835,  1836,  and  1838,  shooting  stars  were  observed  on  the 
night  of  November  13,  in  different  parts  of  the  world,  but  though  diligently 
looked  for  on  the  same  nights  in  1839  and  1840,  they  do  not  appear  to-  have 
been  more  numerous  than  on  other  nights  about  the  same  season  of  the  year. 

The   second   great  meteoric   epoch  is  the  10th  of  August,  first  pointed  out 

by  M.  Quetelet,  and   although  no  displays  similar  to  those  of  the  November 

\  period  have  been  witnessed  on  this  night,  there  are  more  instances  of  the  re- 

V^^y^X^^^^^^ ' ^^X^^^X^^^^>*^N.^N^^^>^^V^^X^^^^^^^X-^r^x**-^»^»^^^* 


442 


METEORIC  STONES  AND  SHOOTING  STARS. 


currence  of  the  phenomena.  In  the  last  three  years  (1838,  1839,  1840), 
shooting  stars  were  observed  in  great  numbers  both  on  the  9th  and  10th  ;  but 
they  appear  in  general  to  be  unusually  abundant  during  the  first  two  weeks  of 
August.  The  other  periods  which  have  been  remarked,  are  the  18th  of  Octo- 
ber, the  23d  or  24th  of  April,  the  6th  and  7th  of  December,  the  nights  from 
the  loth  to  the  20th  of  June,  and  the  2d  of  January. 

Halley  first  suggested  the  idea  that  the  shooting  stars  may  be  observed  as 
signals  for  determining  differences  of  latitude  by  simultaneous  observations, 
and  Maskelyiie  in  1783  published  a  paper  on  the  subject,  in  which  he  c;tlls 
the  attention  of  astronomers  to  the  phenomena,  and  distinctly  points  out  this 
application.  The  idea  was  revived  by  Benzenberg  in  1802,  but  so  long  as 
they  were  regarded  merely  as  casual  phenomena,  it  could  scarcely  be  hoped 
that  they  would  be  of  much  use  in  this  respect  to  practical  astronomy.  As 
soon,  however,  as  their  periodicity  became  probable,  the  phenomena  acquired 
a  new  interest,  and  some  recent  attempts  to  determine  longitudes  in  this  man- 
ner have  proved  that  the  method  is  not  to  be  disregarded. 

The  probability  of  the  conjecture  that  the  causes  of  the  meteoric  phenomena 
observed  in  the  months  of  August  and  November  is  to  be  found  in  the  fact  that 
tfie  particular  regions  of  the  solar  system  through  which  the  earth  passes  at 
these  seasons,  are  the  seats  of  an  unusual  quantity  of  the  matter  composing 
these  meteors,  must  in  a  great  degree  depend  on  the  extent  to  which  it  can  be 
proved  by  observation  that  such  meteors  do  really  prevail  at  each  of  those 
periods  of  the  year. 

With  a  view  of  testing  this,  I  have  collected  together,  from  various  sources, 
the  dates  of  the  most  remarkable  atmospheric  appearances  of  this  class  from 
the  eighth  century  to  the  present  time.  In  the  following  table,  the  day  of  the 
month  when  it  has  been  recorded,  is  placed  in  the  column  under  the  month, 
and  in  the  line  with  the  year  of  the  occurrence.  Where  an  asterisk  occurs 
under  the  month,  the  particular  night  has  not  been  recorded,  but  the  appear- 
ance has  merely  been  mentioned  as  having  occurred. 


; 

METEORIC  STONES  AND  SHOOTING  STARS.                             443 

Years. 

Jan. 

Feb. 

March. 

April. 

May. 

June. 

July. 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

763 
902 
1029 
1092 
1202 
1741 
1777 
1779 
1781 
1784 
1785 
1798 
1799 
1803 
1805 
1806 
1811 
1812 
1813 
1815 
1818 
1819 
1819 
1820 
1822 
1823 
1823 
1824 
1826 
1826 
1827 
1828 
1829 
1830 
1831 
1832 
1833 
1834 
1835 
1836 
1836 
1837 

• 

• 
19 

25 

7 

• 

25 

17 

9 

8 
9 

27 
27 

9 

11 
? 

8 
19 

12 
12 

6 

12 
13 
13 
13 
13 
13 
13 

22 

23 

• 

10 
10 

18 

11 
10 
14 
6 
13 
9 

2 
10 



15 
10 
14 
14 
10 
14 
10 
14 

10 
10 
10 
8 
10 
10 

There  are  here  recorded  fifty-two  nights  on  which  these  appearances  pre- 
vail to  such  a  degree  as  to  attract  particular  notice.     Of  these,  twenty-six  oc- 
curred between  the  8th  and  15th  of  August,  and  thirteen  the  6th  and  19th  of 
November.     Thus  three  fourths  of  the  nights  recorded  correspond  to  the  epochs 
to  which  we  have  referred. 
We  have  not   seen   any  sufficiently  precise  account  of  the  number  of  these 
phenomena  which  were  observed  in  November,    1837,  and  in  July,   1838. 
Fewer  were  noticed  in  Paris  in  November,  1837,  than  were  expected;  but  on 
the  night,  between  the    15th   and  16th,  seventeen  were  seen  at  that  place  by 
M.  Arago,  within  a  minute  and  a  half.     At  Jamble,  in  the  department  of  the 
Seine  and  Loire,  thirty-nine  were  observed  on  the  night  between  the  14th  and 
15th  ;  and  ten  were  observed  at  Marseilles  on  the  night  between  the  12th  and 
13th  ;  six  were  observed  on  the  same  night  at  Geneva,  and  four  at  Montpellier. 
Some  disappointment  was  produced  in  1837,  by  the  circumstance  of  an  un- 
usually small  number  being  seen  on  the   night  between  the   12th  and  13th,   > 
arising  from  an  erroneous  impression  that  that  was  the  night  on  which  their   , 
periodical  return  should  be  expected.     It  will  be  seen,  however,  from  the  pre-    | 

444  METEORIC  STONES  AND  SHOOTING  STARS. 

ceding  table,  that  these  appearances  have  not  at  all  been  confined  to  the  night 
of  the  12th  ;  but  independently  of  this,  the  night  of  the  12th  at  Paris  was  so 
bright,  that  stars  of  the  second  magnitude  were  not  visible,  and  consequently 
meteors — even  supposing  them  to  have  existed  of  similar  or  of  inferior  bright- 
ness— could  not  have  been  observed.  It  should  also  be  considered,  that  their 
non-appearance  at  any  particular  place,  is  no  proof  of  their  non-existence  in 
our  atmosphere.  They  may  be  produced  during  the  day,  or  they  may  be  pro- 
duced in  a  part  of  the  atmosphere  not  visible  from  the  place  in  question.  Thus, 
in  1833,  when  they  were  a  general  object  of  terror  to  the  people  of  America, 
they  attracted  but  little  attention  in  Europe.  On  the  other  hand,  they  some- 
times appear  contemporaneously  in  the  atmosphere  on  opposite  sides  of  the 
globe.  In  1837,  they  were  observed  from  the  French  ship  Bonite,  on  the 
other  side  of  the  globe,  while  on  the  same  day  in  Europe,  a  vast  number  ap- 
peared. 

On  the  night  of  the  12th  of  November,  1836,  Sir  John  Herschel  observed 
these  phenomena  at  the  cape  of  Good  Hope.  Their  number  was  not  very 
considerable,  but  their  motion  had  a  marked  regularity  ;  they  appeared  to 
diverge  from  a  centre  or  focus,  which  preserved  a  fixed  position  with  respect 
to  the  horizon,  but  had  no  such  fixed  relation  to  the  objects  on  the  firmament. 
This  point,  or  centre,  to  which  their  common  directions  converged,  was  a  point 
of  about  thirty  degrees  above  the  horizon,  and  sixty  degrees  west  of  north. 

On  the  night  of  the  9th  of  August,  1837,  M.  Warlmann  observed  these  phe- 
nomena at  Geneva ;  between  nine  o'clock,  P.  M.,  and  midnight,  eighty-two 
were  seen  in  different  parts  of  the  heavens.  They  were  most  frequent  about 
ten  o'clock,  and  then  appeared  to  emanate  from  a  centre  or  focus  situated  be- 
tween the  star  B,  in  the  constellation  of  Bootes,  and  the  star  A,  in  the  con- 
stellation of  the  Dragon.  At  a  quarter  past  ten,  twenty-seven  were  seen,  and 
were  remarkable  for  their  bright  bluish  light.  Other  observers  in  the  same 
neighborhood  and  on  the  same  night,  counted  one  hundred  and  forty-nine  in 
one  part  of  the  heavens,  between  a  quarter  before  nine  and  half  past  eleven 
o'clock. 

Of  these  hundred  and  forty-nine  meteors,  three  had  the  appearance  of  round 
disks,  or  globes,  of  a  ruddy  red  color,  measuring  from  4  to  5  minutes  in  di- 
ameter, being  about  one  sixth  part  of  the  moon's  diameter.  Twenty-six  were 
more  brilliant  than  the  planet  Venus,  and  of  resplendent  whiteness ;  the  re- 
mainder had  the  appearance  of  stars  from  the  first  to  the  third  magnitude,  their 
colors  varying  between  blue,  yellow,  and  orange. 

On  the  night  of  the  llth  of  November,  1832,  M.  Tharand,  a  retired  officer 
at  Limoges,  stated  that  workmen  who  were  employed  in  laying  the  foundation  of 
the  bridge  over  the  river  Vienne,  observed  the  firmament  brilliant,  with  meteors, 
which  at  first  only  amused  them,  but  after  some  hours  the  number  and  splen- 
dor of  these  luminous  appearances  were  so  greatly  augmented,  that  the  people 
were  seized  with  panics,  and  so  great  was  their  terror,  that  they  abandoned 
their  labor  and  flew  to  their  families,  exclaiming  that  the  end  of  the  world 
had  arrived.  On  the  next  day  these  people  were  interrogated  on  the  subject, 
and  their  accounts  varied  according  to  the  different  impressions  which  had 
been  produced  on  their  imaginations.  Some  declared  that  they  saw  streams 
of  blue  fire ;  others  that  they  beheld  bars  of  red  iron  crossing  each  other  in 
all  directions  ;  others  that  they  beheld  an  immense  quantity  of  flying  rockets. 
All  agreed  that  the  phenomena  were  diffused  over  every  part  of  the  firmament ; 
that  they  commenced  at  eleven  o'clock,  and  continued  till  four  the  next 
morning. 


THE.    EARTH. 


A  difficult  Subject  of  Investigation. — Form  of  the  Earth. — How  proved  Globular. — Its  Magnitude. — 
Its  annual  Motion. — Elliptic  Form  of  its  Orbit. — Proofs  of  its  annual  Motion  from  the  Theory  of 
Gravitation. — From  the  Motion  of  Light. — The  Earth's  diurnal  Motion. — Inequalities  of  Day  and 
Night.-«Weight  of  the  Earth. — Maskelyne's  Experiment. — Cavendish's  Experiment. — Their  Ac- 
cordance.— Density  of  the  Earth. — The  Season*. — Calorific  Effect  of  the  Snn's  Rays. — Why  the 
longest  is  not  the  hottest  Day. — Why  the  shortest  Day  is  not  the  coldest. — The  hottest  Season  takes 
place  when  the  Sun  is  farthest  from  the  Earth. — Proofs  of  the  diurnal  Rotation. — Spheroidal  Form 
of  the  Earth  proved  by  Theory  and  by  Observation. 


THE  EARTH. 


THE    EARTH. 


LOCKE  somewhere  observes,  with  his  usual  felicity  of  illustration,  that  the 
"  mind,  like  the  eye,  while  it  makes  us  see  and  perceive  all  other  things,  can 
never  turn  its  view  with  advantage  upon  itself."  We  encounter  something 
similar  to  this  in  our  researches  through  the  universe  ;  for  of  all  the  objects 
which  compose  it,  one  of  the  most  difficult  with  which  to  obtain  a  complete 
and  accurate  knowledge  is  the  planet  which  we  inhabit.  The  cause  of  this 
is  our  proximity  to  it,  and  intimate  connexion  with  it.  We  are  confined  upon 
its  surface,  from  which  we  cannot  separate  ourselves.  We  cannot  obtain  a 
bird's-eye  view  of  it,  nor  at  any  one  time  behold  more  than  an  insignificant 
portion  of  its  surface.  We  have  the  same  difficulty  in  obtaining  an  acquaint- 
ance with  it  that  a  microscopic  animalcule  would  have  in  acquiring  a  perfect 
knowledge  of  the  form  and  dimensions  of  ,a  terrestrial  globe  twelve  inches  in 
diameter,  on  the  surface  of  which  it  creeps. 

Still,  by  a  variety  of  indirect  methods  supplied  by  the  ingenuity  of  scientific 
research,  we  have  been  enabled  to  ascertain  its  form,  and  dimensions,  and 
physical  constitution,  with  a  considerable  degree  of  accuracy. 

FORM    OF    THE    EARTH. 

The  first  impression  produced  upon  the  eye  of  an  observer,  who  has  not 
carried  his  inquiries  further,  is,  that  the  surface  of  the  earth  is  a  flat  plane,  in- 
terrupted only  by  the  inequalities  of  the  land.  A  little  careful  observation, 
however,  upon  the  many  phenomena  which  are  easily  accessible  to  every 
observer,  will  correct  this  erroneous  impression. 

1.  It  is  well  known  that  if  a  voyage  were  made  upon  the  earth,  continually 
preserving  one  and  the  same  direction,  or  doing  so  as  nearly  as  circumstances 
will  permit,  we  should  at  length  arrive  at  the  place  Horn  which  we  departed. 
If  the  earth  were  an  indefinite  plane,  this  could  not  happen.  It  is  evident, 
then,  that  whatever  be  the  exact  form  of  the  earth,  it  is  a  body  which  is  on 


448 


THE  EARTH. 


every  side  limited,  and  one  which  niust  therefore  have  such  a  surface  that  a 
traveller  or  navigator  can  completely  surround  it  in  one  continuous  course. 

Let  us  see,  however,  whether  we  may  not  obtain  evidence  more  distinct  as 
to  its  form.  If  we  stand  on  the  deck  of  a  ship  at  sea,  and  out  of  sight  of  land, 
the  view  being  bounded  only  by  sea  and  sky,  and  look  at  the  horizon  when  a 
ship  approaches,  we  shall  at  first  see  its  topmast  rising  out  of  the  water  like  a 
pole.  As  it  gradually  comes  nearer  to  us,  more  of  the  mast  will  become  visi- 
ble, and  the  sails  will  be  seen — cut  off,  however,  horizontally,  by  the  line  at 
which  the  water  and  sky  unite.  Upon  the  nearer  approach  of  the  ship,  the 
hull  will  at  length  become  visible.  Now,  since  this  takes  place  on  all  sides 
around  us,  it  will  follow  that  when  the  ship  is  at  a  distance,  there  must  be 
something  interposed  between  the  eye  and  it  which  intercepts  the  view  of  it ; 
but  as  the  surface  of  the  water  is  generally  uniform,  and  not  subject  to  sudden 
and  occasional  inequalities  like  that  of  the  land,  we  can  only  imagine  its  gen- 
eral form  to  be  convex,  and  that  its  convexity  is  interposed  between  the  eye 
and  the  object  so  as  to  intercept  the  view. 

Since  the  same  effects  are  observed  from  whatever  direction  the  ship  may 
approach,  it  will  follow  that  the  same  convexity  must  prevail  on  every  side. 

If  we  admit  the  earth  to  be  globular,  or  nearly  so,  and  the  surface  of  the 
water  to  partake  of  this  figure,  1,  the  manner  in  which  a  ship  becomes  visible 
on  approaching  the  eye  will  be  easily  and  simply  explained. 


Fig.  i. 


In  the  position  c,  in  the  annexed  figure,  the  convexity  of  the  globe  being 
between  the  ship  and  the  eye,  the  view  of  it  is  intercepted ;  but  as  the  ship 
approaches  toward  b,  the  masts  first  and  then  the  sails  and  rigging  rise  above 
the  line  of  sight  and  come  into  view,  and  lastly  the  hull  will  be  seen. 

If,  on  the  contrary,  the  surface  extending  from  the  eye  to  the  ship  were  a 
plane,  the  ship  would  be  rendered  invisible  only  by  reason  of  its  distance  ; 
whereas  it  is  ascertained  that  a  ship  frequently  is  invisible  at  a  distance  at 
which  it  must  be  seen  but  for  the  interposition  of  some  other  object ;  this  may 
be  tested,  and  in  fact  is  frequently  tested  at  sea  by  mounting  to  the  masthead, 
whence  the  seaman  being  enabled  to  overlook  the  convexity,  sees  vessels  which 
are  invisible  from  the  deck,  athough,  strictly  speaking,  he  is  nearrr  to  those 
vessels  on  the  deck  than  at  the  masthead. 

When  the  mariner,  after  completing  a  long  voyage,  discovers  by  his  obser- 
vations and  reckonings  that  he  is  approaching  the  desired  coast,  be  ascends  to 
the  topmast  and  looks  out  for  the  appearance  of  mountains  or  other  elevated 
land,  and  he  invariably  sees  them  from  that  point  long  before  they  are  visible 
from  the  deck.  He  afterward  sees  them  from  the  deck  long  before  the  gen- 
eral level  of  the  country  will  be  observed  by  him.  All  these  are  natural  and 
necessary  consequences  of  the  convexity  of  the  surface  of  the  ocean.  The 
same  effects  would  be  seen  ia  any  part  of  a  continent  which  is  sufficiently  free 
from  mountains  and  otler  inequalities. 

But  we  have  a  still  more  conclusive  and  convincing  proof  of  the  general 
form  of  the  earth  even  than  those  which  have  been  explained.  "When  the 


THE  EARTH. 


moon  passes  directly  behind  the  earth,  so  that  the  shadotv  winch  th«  -earth  pro* 
jects  behind  it  in  the  directkm  opposite  to  the  sun  shall  full  upon  the  moon, 
we  invariably  find  that -shadow-  to  be,  not  as  is  commonly  said,  circular,  but 
such  exactly  as  one  globe  would  project  upon  the  surface  of  another  globe. 
Now,  as  this  takes  place  always*  ift  whatever  position  the  earth  may  be,  arnl 
while  the  earth  is  revolving  rapidly  with  its  diurnal  motion  upon  its  axis,  it. 
follows  that  the  earth  must  either  be  an  exact  globe  or  so  little  different  from 
a  globe  Unit  its  deviation  from  that  figure  is  undiscoverable  in  its  shadow. 

We  may,  then,  consider  it  demonstrated  that  the  earth  may  be  practically 
regarded  u4  globular  in  its  form.  We  shall  hereafter  see  that  it  sligbtly 'de- 
parts from  the  spherical  figure,  but  our  present  purpose  will  be  best  hnswerei 
by  regarding  it  as  a  globe. 

-  The. .objection  will  doubtless  occur  to  many  minds  that  the  inequality  which 
.exists  on  the  surface  of  that  portion  of  the  globe  that  is  covered  by  land,  espe- 
cially the  loftier  ridges  of  mountains,  such  as  the  Andes,  the  Alps,  the  Hima- 
laya, and  others,  are  incompatible  witli  the  idea  of  a  globular  figure.  If  the 
term  globular  figure  were  used  in  the  strictest  geometrical  sense,  this  objection 
doubtless,  would  have  great  force.  But  Jet  us  see  the  real  extent  of  this  pre- 
sumed deviation  from  the  globular  form.  The  highest  mountain  on  the  surfaceof 
the  globe  does  not  exceed  five  miles  above  the  general  level  of  the  sea.  The 
entire  diameter  of  the  globe,  as  we  shall  presently  see,  is  eight  thousand  miles. 
The  proportion,  then,  which  the  highest  summit  of  the  loftiest  mountains  bears 
to  the  entire,  diameter  of  the  gloi>o  will  be  that  of  fife  to  eight  thousand,  or  one 
to  sixteen  hundred.  If  we  take  an  ordinary  terrestrial  globe  of  sixteen  inches 
in  diameter,  each,  inch  upon  the  globe  will  correspond  to  five  hundred  miles 
upon  the  earth,  and  the  sixteen  hundredth  part  of  its  diameter,  or  the  hundredth 
part  of  an  inch,  will  correspond  to  five  miles.  If,  then,  we  take  a  narrow  atrip 
of  paper,  so  thin  that  it  would  take  one  hundred  leaves  to  make  an  inch  in 
thickness,  and  paste  such  a  strip  on  the  surface  of  the  globe,  the  thickness  of 
the  strip  would  represent  upon  the  sixteen-inch  globe  the  height  of  the  loftiest 
mountain  on  the  earth.  We  are  then  to  consider  that  the  highest  mountain- 
ranges  on  the  earth  deprive  it  of  its  globular  figure  only  in  the  same  degree 
and  to  the  same  extent  as  a  sixteen-inch  globe  would  be  deprived  of  its  globu- 
lar figure  by  a  strip  of  paper  pasted  upon  it  the  hundredth  part  of  an  inch 
thick. 

It  is  supposed  that  the  greatest  depth  of  the  ocean  which  covers  any  portion 
of  the  globe  does  not  exceed  the  greatest  height  of  the  mountains  upon  the 
land.  If  this  be  true,  the  ocean  upon  the  earth  might  be  represented  by  a  film 
of  liquid  laid  with  a  camel's-hair  pencil  upon  the  surface  of  a  sixteen-inch 
globe. 

It  is  apparent,  therefore,  that  depths  and  heights  which  appear  to  the  com- 
mon observer  to  be  stupendous,  are  nothing  when  considered  with  reference 
to  the  magnitude  of  the  earth;  and  that,  so  far  as  they  are  concerned,  we  inay 
practically  regard  the  earth  as  a  true  globe. 
.(i 

THE  MAGNITUDE  OF  THE  EARTH. 

Having  ascertained  satisfactorily  the  figure  of  the  earth,  our  next  inqtriry 
must  be  us  to  its  magnitude  ;  and  since  it  is  a  globe,  all  that  vre  are  required  to 
know  is  the  length  of  its  diameter. 

If  a  line  were  described  surrounding  the  globe,  so  as  to  form  a  circle  upon 
it,  the  centre  of  which  should  be  at  the  centre  of  the  globe,  such  a"  circle  is 
called  a  gr£at  circl-e  of  the  earth.  Now  if  we  know  the  length  of  the  circum- 
ference of  such  a  circle,  we  could  easily  calculate  the  length  of  its  diameter, 

39 


450  THE  EARTH. 


for  the  proportion  of  the  circumference  to  the  diameter  is  exactly  known.  But 
we  could  calculate  the  circumference  if  we  knew  the  length  of  one  degree 
upon  it,  since  we  know  that  the  circumference  consists  of  three  hundred  and 
sixty  degrees ;  we  should  therefore  only  have  to  multiply  the  length  of  one 
degree  by  three  hundred  and  sixty  to  obtain  the  circumference,  and  should 
thence  calculate  the  diameter. 

On  another  occasion,  in  our  discourse  upon  latitudes  and  longitudes,  it  was 
shown  how  the  latitude  of  a  place  can  be  ascertained.  Now,  let  us  suppose 
two  places  selected  which  are  upon  the  same  meridian  of  the  earth,  and  there- 
fore have  the  same  longitude,  and  which  are  not  very  far  removed  from  each 
other.  Let  them,  moreover,  be  selected  so  that  the  distance  between  them  can 
be  easily  and  accurately  measured.  Now  let  the  latitude  of  these  two  places  be 
exactly  determined,  and  let  us  suppose  that  the  difference  between  these  two 
latitudes  is  found  to  be  one  degree  and  a  half;  and  suppose  als-0  that  on  meas- 
uring the  distance  between  them,  that  distance  is  found  to  be  one  hund-red  and 
four  miles  and  thirty-five  hundredths.  We  should  thence  infer  that  such  must 
be  the  length  of  one  degree  and  a  half  of  the  earth's  surface,  and  that  conse- 
quently the  length  of  one  degree  would  be  two  thirds  of  this,  or  sixty-nine  and 
a  half  miles.  Having  thus  found  the  length  of  a  degree,  AVC  should  have  to 
multiply  it  by  three  hundred  and  sixty,  by  which  we  should  obtain  the  circum- 
ference of  the  earth.  This  would  give  twenty-five  thousand  and  twenty  miles, 
and  we  should  then  find  by  the  usual  mode  of  calculation  the  diameter  of  the 
earth,  which  would  prove  to  be  a  little  under  eight  thousand  miles. 

We  have  made  these  calculations  chiefly  with  a  view  of  rendering  the  prin- 
ciples of  the  investigation  intelligible.  The  more  exact  dimensions  of  the 
earth  will  be  explained  hereafter. 

We  conclude,  then,  that  the  earth  is  a  globe  eight  thousand  miles  in  di- 
ameter. 

ANNUAL    MOTION    OF    THE    EARTH. 

We  have  on  other  occasions  shown  that  the  distance  of  the  earth  from  the 
sun  may  be  expressed  in  round  numbers  by  one  hundred  millions  of  miles.  It 
is  more  exactly  ninety-five  millions  of  miles. 

We  have  also  considered  in  general  the  path  of  the  earth  in  its  annual  course 
round  the  sun  to  be  a  circle,  in  the  centre  of  which  the  centre  of  the  sun  is 
placed.  This  is  nearly  but  not  exactly  true.  That  the  path  of  the  earth  is 
not  a  circle  with  the  sun  in  its  centre,  has  been  ascertained  by  the  following 
observations. 

In  astronomical  telescopes  there  are  placed  by  a  particular  arrangemer.t,  « 
within  the  eye-pieces,  certain  A'ery  fine  threads  or  wires,  which  are  extended 
parallel  to  each  other  across  the  field  of  view.     These  wires  are  so  constructed  { 
that,  by  a  simple  mechanical  contrivance,  they  may  be  moved  toward  each  other, 
preserving,  however,  their  parallelism.      The    mechanism   which  so  moves 
them  is  made  to  measure  exactly  the  distance  between  them. 

When  such  a  telescope  is  presented  to  the  sun  or  moon,  the  wires  may  al- 
Avays  be  so  adjusted,  by  turning  a  screw,  that  one  of  them  shall  touch  the  upper 
and  the  other  the  lower  limb  of  the  disk,  as  represented  in  the  annexed  dia- 
gram, fig.  2,  where  S  represents  the  disk  of  the  sun,  and  A  B  and  C  D  the 
wires. 

Now  let  us  suppose  that  a  telescope  is  pointed  to  the  sun,  and  the  Avires  so 
adjusted  that  they  shall  exactly  touch  the  upper  and  lower  limbs.     Let  the  ob- 
server then  watch  from  day  to  day  the  appearance  of  the  sun  and  the  position  ) 
of  the  wires  :   he  will  find  that,  after  a  certain  time,  the  wires  will  no  longer  ( 

.•w> 


Fig.  2. 


-£X. 

touch  the  sun,  but  will  perhaps  fall  a  little  within  it,  as  represented  in  the  an- 
nexed figure,  3. 

Fig.  3. 


And  after  a  further  lapse  of  time,  he  will  find,  on  the  other  hand,  that  they 
fall  a  little  without  it,  as  in  the  following  figure,  4. 

Fig.  4. 


Now,  as  the  wires  throughout  such  a  series  of  observations  are  maintained  s 
always  in  the  same  position,  it  follows  that  the  disk  of  the  sun  must  appear  ? 
smaller  at  one  time,  and  larger  at  another — that,  in  fact,  the  apparent  magni-  s 
tude  of  the  sun  must  be  variable.  It  is  true  that  this  variation  is  confined  within  ) 
very  small  limits,  but  still  it  is  distinctly  perceptible.  What,  then,  it  may  be 
asked,  must  be  its  cause  ?  Is  it  possible  to  imagine  that  the  sun  really  under- 
goes a  change  in  its  size  ?  This  idea  would,  under  any  circumstances,  be  ab- 
surd ;  but  when  we  have  ascertained,  as  we  may  do,  that  the  change  of  apparent 
magnitude  of  the  sun  is  regular  and  periodical — that  for  one  half  of  the  year 
it  continually  diminishes  until  it  attains  a  minimum,  and  then  for  the  next  .'lalf 
year  it  increases  until  it  attains  a  maximum — such  a  supposition  as  that  of  a 
real  periodical  change  in  the  globe  of  the  sun,  becomes  altogether  incredible. 

If,  then,  an  actual  change  in  the  magnitude  of  the  sun  be  impossible,  there 
is  but  one  other  conceivable  cause  for  the  change  in  its  apparent  magnitude — 
which  is,  a  corresponding  change  in  the  earth's  distance  from  it.  If  the  earth 
at  one  time  be  more  remote  than  at  another,  the  sun  will  appear  proportionally 
smaller.  This  is  an  easy  and  obvious  explanation  of  the  changes  of  appear- 


452 


THE  EARTH. 


ance  that  are  observed,  and  it  has  been  demonstrated  accordingly  to  be  the 
true  one. 

On  examining  the  change  of  the  apparent  diameter  of  the  sun,  it  is  found 
that  it  is  least  on  the  1st  of  July,  and  greatest  on  the  31st  of  December  ;  that 
from  December  to  July,  it  regularly  decreases  ;  and  from  July  to  December,  it 
regularly  increases.  By  observing  the  rate  of  its  variation  through  these  inter- 
vals, it  will  be  found  that  the  path  of  the  earth  around  the  sun  is  an  ellipse, 
having  the  sun  in  one  of  the  fuci. 

In  the  annexed  figure,  5,  if  S  represent  the  place  of  the  sun,  A  will  represent 
that  of  the  earth  when  at  that  place  called  perihelion,  or  that  point  where  it  is 
nearest  the  sun ;  and  B  its  position  at  aphelion,  or  the  point  where  it  is  most 
distant  from  the  sun.  The  elliptic  path  of  the  earth  is  represented  by  the  figure 
A  D  B  O  ;  C  being  its  centre,  and  S  its  focus. 

Fig.  5. 


It  is  proper  to  observe  here  that  the  earth's  orbit  departs  infinitely  less  from  the 
circular  shape  than  the  oval  exhibited  in  the  annexed  diagram.  In  fact,  the  real 
figure  of  the  orbit  of  the  earth  is  so  slightly  oval,  and  so  little  different  from  a 
circle,  that  if  it  were  delineated  on  paper  in  its  true  proportions,  the  eye  conld 
not  discover  its  difference  from  a  circle  ;  actual  instrumental  measure  alone 
could  detect  it.  If  the  greatest  distance  of  the  earth  from  the  sun  were  ex- 
pressed by  1,000,  its  least  distance  would  be  expressed  by  983. 

It  i&  worthy  of  observation  that  the  earth  is  most  remote  from  the  sun  at 
midsummer,  and  nearest  to  it  at  midwinter. 

It  was  not  until  the  date  of  the  revival  of  letters  that  the  annual  motion  of 
^  the  earth  was  admitted.  The  apparent  stability  of  our  globe  was,  until  that 
epoch,  generally  maintained  ;  and  even  now,  when  so  universal  is  the  assent 
.jriVen  to  this  fundamental  principle  of  astronomy,  it  may  still  perhaps  be  useful 
briefly  to  state  the  leading  arguments  by  which  it  is  established. 

When  the  sun  is  observed  in  the  firmament,  it  appears  to  move  among  the 
siurs  from  west  to  east,  following  a  course  in  the  heavens  which  has  been  called 
the  KC.UPTIC  ;  and  at  the  end  of  a  year  its  centre  returns,  after  a  complete  cir- 
cuit of  the  heavens,  to  the  point  from  which  it  set  out.  This  is  an  effect  which 
uo'ild  lie  produced  by  a  reai  motion  of  the  sun  round  the  earth  in  a  year.  Bv 


THE  EARTH.  453 


such  a  motion,  being  placed  in  the  centre  and  at  rest,  we  should  see  the  sun 
(  progressively  moving  round  us  ;  we  should  project  his  disk  among  the  stars, 
5  and  the  apparent  motion  would  be  to  us  what  it  is.  But  it  is  most  necessary 
to  reflect  that  the  very  same  effect  would  be  produced  without  a  single  change 
of  circumstance,  if,  instead  of  the  earth  being  at  rest  and  the  sun  moving  round 
it,  the  sun  were  at  rest,  and  the  earth  were  carried  annually  round  it.  Such  a 
motion  of  the  earth  would  cause  the  sun  to  be  successively  seen  at  all  points 
of  the  ecliptic  at  which  it  is  seen  throughout  the  year ;  and,  in  short,  would 
give  to  the  sun  exactly  the  same  apparent  motion  which  it  appears  to  us  to 
have.  It  is  therefore  evident  that  the  annual  motion  of  the  sun  will  be  exr 
plained  with  equal  clearness,  and  would  be  equally  produced  or  caused,  either 
by  its  own  motion  round  the  earth,  or  by  the  annual  motion  of  the  earth  round 
it,  There  is  nothing  in  the  appearance  of  the  sun  itself  which  would  give  a 
preference  or  confer  a  greater  probability  on  either  of  these  suppositions  rather 
than  the  other.  If  we  are  to  choose  between  them,  we  must  therefore  seek 
the  grounds  of  choice  in  some  other  circumstances. 

But  the  long-continued  and  deeply-rooted  opinion  that  the  sun  and  not  the 
earth  moves,  must  have  had  some  natural  and  intelligible  grounds.  These 
grounds  undoubtedly  arose  from  impressions  that  if  the  earth  moved,  we  should 
in  some  way  or  other  be  sensible  of  its  motion  ;  more  especially  if  that  motion 
had  the  enormous  velocity  which  must  be  imputed  to  the  earth  if  it  be  granted 
that  it  moves  round  the  sun  at  all. 

But,  on  the  other  hand,  it  must  be  considered  that  we  are  conscious  of  mo- 
tion through  the  senses  only  by  observing  the  relative  change  of  position  of  some 
external  sensible  objects.  We  see  the  mutual  distance  and  relative  position 
of  two  or  more  visible  objects  change,  and  we  infer  immediately  that  some  one 
or  other  of  them  must  have  moved.  We  can  be  rendered  sensible  of  the  mo- 
tion of  the  room  we  occupy,  or  of  the  ground  upon  which  we  stand,  only  by 
some  derangement  of  the  position  of  these  relative  to  our  own  body.  But 
li  we  could  conceive  all  the  objects  that  surround  us  moving  with  perfect  uni- 
formity in  a  fixed  direction,  and  that  our  own  bodies  should  participate  in  the 
motion,  we  should  thea  have  no  evidence  by  which  we  could  ascertain  the  ex- 
istence of  that  motion  at  all.  This  will  be  clear  to  every  one  by  considering 
the  effect  produced  when  we  are  in  the  cabin  of  a  boat  which  is  drawn  uni- 
formly on  smooth  water.  If  we  cannot  look  at  the  banks  of  the  river  or  canal, 
we  then  shall  be  entirely  unconscious  that  the  boat  is  moving;  but  if  we 
are  enabled  to  look  out  from  a  window  from  which  we  can  see  the  banks,  the 
first  impression  will  be  that  the  banks  are  moving  in  the  contrary  direction  to 
the  boat,  and  it  is  only  by  reason  and  reflection  that  this  impression  will  be 
corrected.  If  we  are  in  the  cabin  of  a  steamboat  from  which  we  cannot  look 
abroad,  the  only  motion  of  which  we  are  conscious  is  the  tremulous  motion 
produced  by  the  working  of  the  machinery,  and  we  are  only  conscious  of  this 
because  it  changes  in  a  slight  degree,  and  momentarily,  the  relative  position 
of  the  frame  of  the  boat  and  our  own  bodies.  But  we  are  even  then  uncon-  / 
scious  of  the  progressive  motion  of  the  boat.  :*  «i 

It  will,  then,  be  easily  conceived  that  the  motion  of  the  globe  of  the  earth 
through  space  being  perfectly  smooth  and  uniform,  we  can  have  no  sensible 
means  of  knowing  it,  except  the  same  which  -we  possess  in  the  case  of  a  boat 
moving  smoothly  along  a  river  :  that  is,  by  looking  abroad  at  some  external 
objects  which  do  not  participate  in  the  motion  imputed  to  the  earth.  Now, 
when  we  do  look  abroad  at  such  objects,  we  find  that  they  appear  to  more — 
exactly  as  stationary  objects  would  appear  to  move,  seen  from  a  moveable  sta- 
tion like  the  earth.  It  is  plain,  then,  even  if  it  be  true  that,  the  earth  really 
has  the  annual  motion  round  the  sun  which  is  contended  for,  that  we  cannot 


154 


THE  EARTH. 


expect  to  be  conscious  of  this  motion  from  anything  which  can  be  observed  on 
our  own  bodies  or  those  which  surround  us  on  the  surface  of  the  earth  :  we 
must  look  for  it  elsewhere. 

But  it  will  be  contended  that  the  apparent  motion  of  the  sun,  even  upon  the 
argument  just  stated,  may  equally  be  explained  by  the  motion  of  the  earth 
round  the  sun,  or  the  motion  of  the  sun  round  the  earth  ;  and  that  therefore  this 
appearance  can  still  prove  nothing  positively  on  this  question.  We  have,  how- 
ever, other  proofs,  of  a  very  decisive  character. 

Newton  showed  that  it  was  a  general  law  of  nature,  and  part,  in  fact,  of  the 
principle  of  gravitation,  that  any  two  globes  placed  at  a  distance  from  each 
other,  if  they  are  in  the  first  instance  quiescent,  and  free,  must  move  with  an 
accelerated  motion  to  their  common  centre  of  gravity,  where  they  will  meet 
and  coalesce  ;  but  if  they  be  projected  in  a  direction  not  passing  through  this 
centre  of  gravity,  they  will  both  of  them  revolve  in  orbits  around  that  point 
periodically.  And  in  fact  the  same  will  be  the  case  with  any  number  of 
globes  whatsoever,  and  consequently  would  be  applicable  to  the  solar  system 
itself. 

Now,  the  centre  of  gravity  of  the  solar  system,  owing  to  the  immense  pre- 
dominance of  the  mass  of  the  sun  over  all  the  rest  of  the  bodies  composing  it 
put  together,  is  situated  within  the  sun,  and  near  its  centre.  All  the  bodies 
of  the  system,  and  the  earth  among  them,  must  therefore,  according  to  this  law, 
revolve  periodically  round  that  point. 

But  as  the  principle  of  gravitation  itself  may  by  some  be  considered  as  based 
upon  some  previous  admission  of  the  motion  of  the  planets,  it  may  be  desirable 
to  obtain  a  still  more  direct  and  positive  manifestation  of  the  annual  motion  of 
the  earth.  Fortunately,  the  discovery  which  has  been  developed  by  the  labors 
of  astronomical  observers  have  put  us  in  possession  of  a  decisive  test,  which 
has  been  considered  as  setting  at  rest  for  ever  the  question  of  the  earth's  an- 
nual motion.  If  the  earth  were  moved  round  the  sun — as  it  certainly  must  be 
if  the  sun  is  not  moved  round  it — an  effect  would  be  produced  upon  the  apparent 
position  of  the  fixed  stars,  owing  to  the  combination  of  the  motion  of  light  with 
the  motion  of  the  globe.  Light  is  propagated  from  the  stars  in  straight  lines 
with  a  velocity  of  about  two  hundred  thousand  miles  per  second.  The  earth, 
if  it  moves  at  all,  moves  with  a  velocity  of  about  twenty  miles  per  second  ; 
and  with  this  velocity,  the  eye  of  the  observer  upon  the  earth  strikes  the  light 
in  the  direction  of  the  earth  s  motion,  while  the  light  itself  comes  in  another 
direction.  The  direction  in  which  the  observer  will  see  the  star  will  be  de- 
termined by  the  combined  effect  of  the  velocity  of  light  and  the  velocity  of  the 
earth,  inasmuch  as  the  impact  of  the  light  upon  the  eye  will  be  the  result  of 
these  two  motions  ;  thus,  if  the  earth  moved  with  a  velocity  equal  to  that  of 
light,  the  star  would  be  seen  forty-five  degrees  in  advance  of  its  real  position. 
If  the  earth  moved  with  a  less  velocity,  it  would  be  seen  less  in  advance  of 
its  true  position  in  proportion  to  the  relative  velocity  of  the  earth  and  light. 

Now,  the  velocity  of  the  earth  being  incomparably  smaller  than  that  of  light, 
the  star  ought  to  be  seen  in  advance  of  its  true  position  to  an  extent  which  is 
proportionate  to  this  small  ratio,  and  the  deviation  of  the  star  or  planet's  true 
position  should  also  be  in  the  direction  of  the  earth's  motion.  This  effecl, 
moreover,  should  be  found  to  be  produced  upon  all  stars  and  planets  visible  in 
the  firmament ;  modified,  however,  in  a  certain  complicated  manner,  according 
to  their  position  with  respect  to  the  orbit  of  the  earth. 

The  observations  of  Bradley  and  subsequent  astronomers  detected  these 
effects  ;  and  as  they  are  everywhere  produced  upon  the  countless  myriads  of 
objects  that  glitter  upon  the  firmament,  and  everywhere  produced  in  the  manner 
and  degree  exactly  in  which  they  ought  to  be  produced  by  the  earth's  an- 


niral  motion,  an  unanswerable  demonstration  is  obtained  of  the  reality  of  that 
motion. 

We  have  seen  that  the  observation  of  the  sun  establishes  demonstratively 

his  alternative — either  that  the  earth  revolves  round  the  sun  annually,  or  that 

he  sun  revolves  round  the  earth  annually.  There  is  no  other  motion  which 
would  be  consistent  with  the  phenomena.  Now,  the  effect  on  the  stars  called 

he  aberration  of  light,  just  explained,  proves  that  of  the  sides  of  this  alterna- 

ive,  that  which  must  be  adopted  is  the  motion  of  the  earth. 

There  is  an  instinct  of  the  human  mind  which  leads  it  to  anticipate  discov- 
eries. The  grounds  on  which  the  annual  motion  of  the  earth  and  the  stationa- 
ry position  of  the  sun  are  demonstrated,  are  modern.  The  theory  of  gravita- 

ion  dates  only  from  the  era  illustrated  by  Newton.  The  discovery  of  the  ab- 
erration of  light  is  still  more  recent ;  and  yet  the  first  suggestion  of  the  annual 
motion  of  the  earth,  and  the  stationary  position  of  the  sun,  dates  as  far  back  as 

he  era  of  Pythagoras.  It  is  true  that  this  hypothesis  did  not  obtain  general 
assent  until  it  was  urged  by  the  sagacity  of  Copernicus,  and  reinforced  by  the 
eloquence  and  talents  of  Galileo  and  Kepler.  But  still  it  affords  an  example 
of  one  of  those  wonderful  anticipations  of  human  intellect  which  leads  us  irre- 
sistibly back  to  the  impression  that  the  mind  is  itself  an  emanation  of  the  Divine 
spirit  which  was  breathed  into  our  nostrils  when  He  who  created  us  gave  us 

he  breath  of  life,  and  made  us  a  living  soul. 

THE  EARTH'S  DIURNAL  MOTION. 

WThile  the  earth  revolves  annually  round  the  sun,  it  has  a  motion  of  rotation 
at  the  same  time  upon  a  certain  diameter  as  an  axis  which  is  inclined  from  the 
perpendicular  to  its  orbit  at  an  angle  of  23°,  28'.  During  the  annual  motion 
of  the  earth  this  diameter  keeps  continually  parallel  to  the  same  direction,  and 
the  earth  completes  its  revolution  upon  it  in  twenty-three  hours  and  fifty-six 
minutes.  In  consequence  of  the  combination  of  this  motion  of  rotation  of  the 
earth  upon  its  axis  with  its  annual  motion  round  the  sun,  we  are  supplied  with 
the  alternations  of  day  and  night,  and  the  succession  of  seasons. 

When  the  globe  of  the  earth  is  in  such  a  position  that  its  north  pole  leans 
toward  the  sun,  the  greater  portion  of  its  northern  hemisphere  is  enlightened, 
and  the  greater  portion  of  the  southern  hemisphere  is  dark.  This  position  is 
represented  in  the  annexed  figure,  6,  where  C  is  the  north  pole,  and  D  the 


fc 


& 


south  pole.  As  the  earth  revolves  upon  its  axis,  the  parallels  of  the  equator 
are  unequally  divided  by  the  circles  of  light  and  darkness  :  the  greater  segment 
of  each  of  them  being  illuminated,  and  the  lesser  segment  dark.  The  days 


are  therefore  longer  than  the  nights  in  the  northern  hemisphere.  The  reverse 
is  the  case  with  the  southern  hemisphere,  for  there  the  greater  segments  of 
the  parallels  are  dark,  and  the  lesser  segments  enlightened  ;  the  days  are  there- 
fore shorter  than  the  nights.  Upon  the  equator,  however,  at  B,  the  circle  of 
the  earth,  is  equally  divided,  and  the  days  and  nights  are  equal.  When  the 
south  pole  leans  toward  the  sun,  which  it  does  exactly  at  the  opposite  point  of 
the  earth's  annual  orbit,  circumstances  are  reversed :  then  the  days  are  longer 
than  the  nights  in  the  southern  hemisphere,  and  the  nights  are  longer  than 
the  days  in  the  northern  hemisphere.  At  the  intermediate  point  of  the  earth's 
annual  path,  figure  7,  when  the  axis  assumes  a  position  perpendicular  to  the 

Fig.  7. 


• 


- 


• 


direction  of  the  sun,  then  the  circle  of  light  and  darkness  passes  through  the 
poles  ;  all  parallels  in  every  part  of  the  earth  are  equally  divided,  and  there  is 
consequently  equal  day  and  night  all  over  the  globe. 

In  the  annexed  perspective  diagram,  fig.  8,  these  four  positions  of  the  earth 
are  exhibited  in  such  a  manner  as  to  be  clearly  intelligible. 

On  the  day  of  the  21st  of  June,  the  north  pole  is  turned  in  the  direction  of 
the  sun  ;  on  the  21st  of  December,  the  south  pole  is  turned  in  that,  direction. 
On  the  days  of  the  equinoxes,  the  axis  of  the  earth  is  at  right  angles  to  the 
direction  of  the  sun,  and  it  is  equal  day  and  night  everywhere  on  the  earth. 

The  annual  variation  of  the  position  of  the  sun  with  reference  to  the  equa- 
tor, or  the  changes  of  its  declination,  are  explained  by  these  motions.  The 
summer  solstice — the  time  when  the  sun's  distance  from  the  equator  is  the 
greatest — takes  place  when  the  north  pole  leans  toward  the  sun  ;  and  the  win- 
ter solstice — or  the  time  when  the  sun's  distance  south  of  the  equator  is  great- 
est— takes  place  when  the  south  pole  leans  toward  the  sun. 

In  virtue  of  these  motions,  it  follows  that  the  sun  is  twice  a  year  vertical  at 
all  places  between  the  tropics  j.arid  at  the  tropics  themselves  it  is  vertical  once 
a  year.  In  all  higher  latitudes  the  point  at  which  the  sun  passes  the  meridian 
daily  alternately  approaches  to  and  recedes  from  the  zenith.  From  the  21st 
of  December  until  the  2 1st  of  June,  the  point  continually  approaches  the  zenith. 
It  comes  nearest  to  the  zenith  on  the  21st  of  June  ;  and  from  that  day  until  the 
21st  of  December,  it  continually  recedes  from  the  zenith,  and  attains  its  lowest 
position  on  the  latter  day.  The  difference,  therefore,  between  the  meridional 
altitudes  of  the  sun  on  the  days  of  the  summer  and  winter  solstices  at  all  places 
will  be  twice  twenty-three  degrees  and  twenty-eight  minutes,  or  forty-six  de- 
grees and  fifty-six  minutes.  In  all  places  beyond  the  tropics  in  the  northern 
hemisphere,  therefore,  the  sun  rises  at  noon  on  the  21st  of  June,  forty-six  de- 
grees and  fifty-six  minutes  higher  than  it  rises  on  the  21st  of  December. 
These  are  the  limits  of  meridional  altitude  which  determine  the  influence  of 
the  sun  in  different  places. 


THE  EARTH. 


457 


Fig.  8. 

^i^SSfek 
K     ft 

jMBBHHmiMMK 


• 

• 

• 

"" 
• 


WEIGHT    OF    THE    EARTH. 


• 


It  was  at  a  recent  epoch  in  the  progress  of  knowledge  that  the  problem  to 
ascertain  the  weight  of  the  globe  of  the  earth,  or  the  actual  quantity  of  matter 
it  contains  relative  to  some  known  standard,  was  solved. 

The  researches  of  Newton  had  established  the  general  fact  that  the  weights 
of  bodies  were  the  exponents  of  their  masses  or  quantities  of  matter,  and  that 
the  weights  themselves  were  nothing  more  than  the  attractions  which  the 
bodies  in  question  suffered  from  other  bodies  near  them. 

Toward  the  end  of  the  last  century,  two  philosophers  of  great  eminence  in 
England— the  celebrated  Cavendish  and  Dr.  Maskelyne— achieved  the  solu- 
tion of  this  problem  by  different  methods  ;  and  the  accordance  of  the  results 
which  they  obtained  is  the  best  test  of  their  accuracy  and  truth. 

The  method  of  Dr.  Maskelyne  consisted  in  comparing  the  attraction  which 


458 


THE  EARTH. 


the  entire  globe  of  the  earth  would  exert  in  a  body  near  it.  with  that  which 
a  mass  of  matter  of  known  weight,  such  as  a  mountain,  would  exert  upon  the 
same  body.  The  mode  of  executing  that  memorable  experiment  was  as 
follows  :  Let  A  B,  fig.  9,  represent  a  small  portion  of  the  earth's  surface, 


which  may  be  regarded  as  a  plane  ;  lt:t  C  1)  represent  a  mountain,  and  let  0 
be  supposed  to  be  its  centre  of  gravity.  The  entire  attraction  of  the  mass  of 
the  mountain  will  then  be  exerted  as  if  it  were  concentrated  on  the  point  0. 
The  direction  of  the  earth's  attraction  will  be  perpendicular  to  the  plane  A  B. 
Now,  let  L  be  a  weight  suspended  from  any  point ;  M  L  forming  what  is  called 
a  plumb-line.  If  the  weight  L  were  solicited  by  no  force  except  the  earth's 
attraction,  the  string  by  which  it  is  suspended  would  take  a  position  at  right 
angles  to  the  plane  A  B  ;  but  as  this  plumb-line  is  suspended  near  the  mount- 
ain C  D,  it  will  be  attracted  by  the  gravitation  of  the  mass  of  the  mountain, 
which  will  be  exerted  in  the  direction  M  O  toward  the  centre  of  gravity  of  the 
mountain.  If  we  could  imagine  the  globe  of  the  earth  on  which  the  mountain 
rests  removed,  and  the  mountain  alone  to  remain  near  the  plumb-line,  then  the 
weight  L  would  be  drawn  in  the  direction  M  0,  and  the  string  M  L  suspend- 
ing it  would  take  that  direction  ;  for  in  that  case,  the  only  force  by  which  L 
would  be  attracted  would  be  the  gravitation  of  the  mountain,  which  takes  place 
in  the  direction  M  O.  If,  on  the  other  hand,  the  mountain  were  removed,  and 
the  earth  alone  left  to  affect  the  plumb-line,  it  would  take  the  usual  direction, 
M  L,  perpendicular  to  A  B  ;  but  in  the  case  actually  supposed,  the  weight  L  is 
solicited  at  the  same  time  by  both  attractions — by  the  attraction  of  the  globe 
of  the  earth  drawing  it  perpendicularly  to  A  B,  and  by  the  attraction  of  the 
mountain  drawing  it  in  the  direction  M  0.  By  the  common  principles  of  me- 
chanics, the  weight  L  will  in  this  case  take  a  direction  M  L',  intermediate 
between  M  L  and  M  O,  leaning  toward  the  mountain  but  very  slightly,  in- 
asmuch as  the  attraction  of  the  mountain  is  incomparably  less  than  that  of  the 
earth. 

Now,  if  we  could  exactly  ascertain  the  degree  in  which  the  plumb-line  is 
deflected  from  its  true  vertical  position  by  the  attraction  of  the  mountain,  that 
deviation  or  deflection  will  enable  us  immediately  to  estimate  the  proportion 
which  the  attraction  of  the  mountain  bears  to  the  whole  attraction  of  the  earth, 
and  that  proportion  would  be  the  same  as  that  which  the  weight  of  the  moun- 
tain or  the  mass  of  matter  contained  in  it  bears  to  the  mass  of  matter  contained 
in  the  globe  of  the  earth.  But  where  the  deviation  of  the  plumb-line  is  so 
small,  and  \vh£re  any  ordinary  test  of  its  deviation  would  be  affected  by  the 
same  cause  as  the  plumb-line  itself,  there  would  be  a  difficulty  in  determin- 
ing it. 

If  the  plumb-line  were  undisturbed  by  the  mountain,  its  direction  ought  to 
point  to  a  star  in  the  zenith  of  the  place  of  the  observer  ;  but  being  dis- 
turbed by  the  attraction  of  the  mountain,  it  will  point  to  a  star  at  one  side  of 
the  zenith — say,  for  example,  to  the  east  of  it. 


THE  EARTH. 


4-59 


Let  us  suppose  now  that  another  plumb-line  is  suspended  similarly  on  the 
opposite  side  of  the  mountain,  to  m  I :  it  is  evident  that  the  attraction  of  the 
mountain  will  draw  the  plumb-line  in  this  case  in  a  direction  opposite  to  that 
in  which  it  draws  the  former.  Both  plummets  will  be  drawn  toward  the 
mountain ;  and  if  the  string  suspending  one  be  made  to  point  a  little  to  the 
eastward  of  the  zenith,  the  string  suspending  the  other  will  be  made  to  point  a 
little  to  the  westward  of  it. 

By  due  attention  to  this  circumstance,  we  shall  easily  find  the  real  deviation 
of  the,  plumb-line  from  the  zenith.  Let  the  points  in  the  heavens  to  which  the 
two  plumb-lines  are  respectively  directed  be  accurately  observed  :  one  of  these 
points  will  be  as  much  to  the  eastward  as  the  other  will  be  westward  of  the 
true  zenith.  If  we  take  half  the  space  between  them,  that  will  be  the  devia- 
tion of  the  direction  of  the  plumb-line  from  the  zenith,  or,  in  other  words,  it 
will  be  the  actual  deviation  of  the  plumb-line  from  the  true  vertical  direction. 

We  have  then  the  amount  of  the  deflection,  and  can  therefore  calculate  the 
proportion  which  the  mass  of  the  earth  bears  to  the  mass  of  the  mountain.  If, 
then,  we  knew  the  mass  of  the  mountain,  we  should  necessarily  know  the  mass 
of  the  earth. 

The  mountain  on  which  Dr.  Maskelyne  tried  this  celebrated  experiment 
was  Schehallien,  in  Wales.  The  geological  structure  of  this  mountain  was 
known,  and  the  magnitude  and  nature  of  its  stratification  had  been  ascertained. 
The  weight,  therefore,  of  the  materials  that  composed  it  was  easily  calculated, 
and  thus  the  weight  of  the  mountain  obtained. 

By  computing  thence,  by  means  of  the  experiments  just  described,  the 
weight  of  the  earth,  it  was  found  to  be  about  five  times  the  weight  of  its  own 
bulk  of  water. 

The  method  adopted  by  Cavendish  for  solving  this  problem  depended  on  a 
different  mechanical  principle.  It  is  well  known  that  the  vibrations  of  the 
common  pendulum,  used  as  a  measure  of  time  in  clocks,  are  produced  by  the 
attraction  of  the  globe  of  the  earth  on  the  matter  composing  the  ball  or  disk. 
If  that  attraction  were  greater,  its  vibration  would  be  more  rapid  ;  if  it  were 
less,  it  would  be  slower  ;  in  short,  the  rate  of  vibration  of  the  pendulum  is  the 
exponent  of  the  energy  of  force  by  which  it  is  moved. 

If  we  suppose,  then,  two  globes,  containing  different  quantities  of  attractive 
matter,  and  near  these  globes  two  pendulums  to  be  placed,  each  pendulum 
being  kept  in  a  state  of  vibration  by  their  attraction  :  by  noting  the  rates 
of  vibration  of  these  two  pendulums,  we  should  be  enabled  to  compare  the 
relative  quantities  of  matter  in  the  two  globes.  In  making  this  comparison, 
however,  there  are  several  circumstances  which  should  be  attended  to,  which 
need  not  be  particularly  adverted  to  here.  Cavendish  adopted  this  principle 
as  the  basis  of  his  method  for  determining  the  weight  of  the  earth.  He  took 
a  large  globe  of  metal,  of  known  weight,  and  suspended  near  it  in  a  horizontal 
position  a  fine  vertical  needle,  the  point  of  suspension  corresponding  with  its 
centre  of  gravity.  The  effect  of  the  earth's  attraction  was  thus  neutralized.  Its 
susceptibility  of  vibration  in  a  horizontal  plane  depended  upon  the  torsion  of 
the  filament  by  which  it  was  suspended.  The  ball  of  this  pendulum  was  then 
directed  to  the  centre  of  the  metallic  globe,  and  the  pendulum  was  put  in  vi- 
bration near  it,  subject  to  the  same  mechanical  condition  as  those  by  which  a 
common  pendulum  is  affected  near  the  surface  of  the  earth.  By  observing  the 
rate  of  vibration  of  this  horizontal  pendulum,  and  comparing  it  with  the  rate  of 
vibration  of  the  ordinary  pendulum  subject  to  the  earth's  attraction,  Cavendish 
was  enabled  to  obtain  the  numerical  proportion  which  the  earth's  attraction 
bore  to  the  attraction  of  the  metallic  globe  which  he  used  in  his  experiments. 
Having  computed  thence  the  weight  of  the  earth,  he  arrived  at  *  conclusion 


THE  EARTH. 


nearly  the  same  as  that  to  which  Dr.  Maskelyne  had  previously  arrived  by  a 
different  method.  It  was  thus  finally  established  that  the  weight  of  the  globe 
of  the  earth  is  about  five  an<?  a  half  times  greater  than  the  weight  of  its  own 
bulk  of  water. 

It-follows  from  this  that  the  mean  density  of  the  earth  is  five  and  a  half  times 

greater  than  the  density  of  water.     We  are,  however,  carefully  to  remember 

that  this  conclusion  affects  the  mean  density  of  the  earth  only.     Now,  as  the 

,'  density  immediately  at  its  surface  is  not  nearly  so  great  as  this,  it  follows  that 

'I  the  density  of  those  parts  nearer  to  its  centre  must  be  much  greater. 

THE    SEASONS 

' 

The  succession  of  spring,  summer,  autumn,  and  winter,  and  the  variations 
of  temperature  of  the  seasons — so  far  as  these  variations  depend  on  the  posi- 
tion of  the  sun — will  now  require  to  be  explained. 

The  influence  of  the  sun  in  heating  a  portion  of  the  earth's  surface,  will  de- 
pend partly  on  its  altitude  above  the  horizon.  The  greater  that  altitude  is,  the 
more  perpendicular  the  rays  will  fall,  and  the  greater  will  be  their  calorific 
effect 

To  explain  this,  let  us  suppose  ABC  and  D,  fig.  10,  to  represent  a  beam  of 

Fig.  10. 
F          D  -•!• 


E 


- 


B 


the  solar  light ;  let  C  D  represent  a  portion  of  the  earth's  surface,  upon  which 
the  beam  would  fall  perpendicularly  ;  and  let  C  E  represent  that  portion  on  ivhich 
it  would  fall  obliquely  ;  the  same  number  of  rays  will  strike  the  surfaces  C  D 
and  G  E  ;  but  the  surface  C  E  being  obviously  greater  than  C  D,  the  rays  will 
necessarily  fall  more  densely  on  the  latter:  and  as  the  heating  power  must  be 
in  proportion  to  the  density  of  the  rays,  it  follows  that  C  D  will  be  heated  more 
than  C  E  in  just  the  same  proportion  as  C  E  is  greater  than  C  D.  But  if  we 
would  compare  two  surfaces  on  neither  of  which  the  sun's  rays  fall  perpendic- 
ularly, let  us  take  C  E  and  C  F.  They  fall  on  C  E  with  more  obliquity  than 
on  C  F  ;  but  C  E  is  evidently  greater  than  C  F,  and  therefore  the  rays  being 
diffused  over  a  larger  surface,  are  less  dense,  and  therefore  less  effective  in 
heating. 

The  calorific  effect,  of  the  sun's  rays  on  a  surface  more  oblique  to  their  di- 
rection than  another,  will  then  be  proportionably  less. 

If  the  sun  be  in  the  zenith,  its  rays  will  strike  the  surface  perpendicularly, 
and  the  heating  effect  will  therefore  be  greater  than  when  the  sun  is  in  any 
other  position. 

The  greater  the  altitude  to  which  the  sun  rises,  the  less  obliquely  will  be 
the  direction  in  which  its  rays  will  strike  the  surface  at  noon,  and  the  more 
effective  will  be  their  heating  power.     So  far,  then,  as  the  heating  power  de- 
pends on  the  altitude  of  the  sun,  it  will  be  increased  with  every  increase  of  its  \ 
meridian  altitude. 


Hence  it  is  that  the  heat  of  summer  increases  as  we  approach  the  equator. 
The  lower  the  latitude  is,  the  greater  will  be  the  height  to  which  the  sun  will 
rise.  The  maridian  altitude  of  the  sun  at  the  summer  solstice  being  every- 
where forty-six  degrees  and  fifty-six  minutes  more  than  at  the  winter  solstice, 
the  heating  effect  will  be  proportionately  greater. 

But  this  is  not  the  only  cause  which  produces  the  greatly  superior  heat  of 
summer  as  compared  with  winter,  especially  in  the  higher  latitudes.  The 
heating  effect  of  the  sun  depends  not  alone  on  its  altitude  at  midday;  it  also 
depends  on  the  length  of  time  which  it  is  above  the  horizon  and  belo\v  it. 
While  the  sun  is  above  the  horizon,  it  is  continually  imparting  heat  to  the  air 
and  to  the  surface  of  the  earth  ;  and  while  it  is  below  the  horizon,  the  heat  is 
continually  being  dissipated.  The  longer,  therefore — other  things  being  the 
same — the  sun  is  above  the  horizon,  and  the  shorter  time  it  is  below  it,  the 
greater  will  be  the  amount  of  heat  imparted  to  the  earth  every  twenty-four 
hours.  Let  us  suppose  that  between  sunrise  and  sunset,  the  sun,  by  its  cal- 
orific effect,  imparts  a  certain  amount  of  heat  to  the  atmosphere  and  the  sur- 
face of  the  earth,  and  that  from  sunset  to  sunrise  a  certain  amount  of  this  heat  j 
is  lost:  the  result  of  the  action  of  the  sun  will  be  found  by  deducting  the  latter 
from  the  former. 

Thus,  then,  it  appears  that  the  influence  of  the  sun  upon  the  seasons  de- 
pends as  much  upon  the  length  of  the  days  and  nights  as  upon  its  altitude  ;  but 
it  so  happens  that  one  of  these  circumstances  depends  upon  the  other.  The 
greater  the  sun's  meridional  altitude  is,  the  longer  will  be  the  days,  and  the 
shorter  the  nights ;  and  the  less  it  is,  the  longer  will  be  the  nights,  and  the 
shorter  the  days.  Thus  both  circumstances  always  conspire  in  producing 
the  increased  temperature  of  summer,  and  the  diminished  temperature  of 
winter. 

A  difficulty  is  sometimes  felt  when  the  operation  of  these  causes  is  consid- 
ered, in  understanding  how  it  happens  that,  notwithstanding  what  has  been 
stated,  the  21st  of  June — when  the  sun  rises  the  highest,  when  the  days  are 
longest  and  the  nights  shortest — is  not  the  hottest  day,  but  that  on  the  contrary, 
the  dog-days,  as  they  are  called,  which  comprise  the  hottest  weather  of  the  year, 
occur  in.  August ;  and  in  the  same  manner,  the  2lst  of  December-^— when  the 
height  to  which  the  sun  rises  is  least,  the  days  shortest,  and  the  nights  longest 
— is  not  usually  the  coldest  day,  but  that,  on  the  other  hand,  the  most  inclem 
ent  weather  occurs  at  a  later  period. 

To  explain  this,  so  far  as  it  depends  on  the  position  of  the  sun  and  the 
length  of  the  days  and  nights,  we  are  to  consider  the  folio  wing  circum- 
stances : — 

As  midsummer  approaches,  the  gradual  increase  of  the  temperature  of  the 
weather  has  been  explained  thus  :  The  days  being  considerably  longer  than 
the  nights,  the  quantity  of  heat  imparted  by  the  sun  during  the  day  is  greater 
than  the  quantity  lost  during  the  night ;  and  the  entire  result  during  the  twenty 
four  hours  gives  an  increase  of  heat.  As  this  augmentation  takes  place  alter 
each  successive  day  and  night,  the  general  temperature  continues  to  increase. 
On  the  21st  of  June,  when  the  day  is  longest,  and  the  night  is  shortest,  and  the 
sun  rises  highest,  this  augmentation  reaches  its  maximum  ;  but  the  temperature 
of  the  weather  does  not  therefore  cease  to  increase.  After  the  21st  of  June, 
there  continues  to  be  still  a  daily  augmentation  of  heat,  for  the  sun  still  con- 
tinues to  impart  more  heat  during  the  day  than  is  lost  during  the  night.  The 
temperature  of  the  weather  will  therefore  only  cease  to  increase  when,  by  th»> 
diminished  length  of  the  day,  the  increased  length  of  thu  night,  and  the  dimin- 
ished meridional  altitude  of  the  sun,  the  heat  imparted  during  the  day  is  just 
balanced  by  the  heat  lost  during  the  night.  There  will  be,  then,  no  further 


462 


THE  EARTH. 


increase  of  temperature,  and  the  heat  of  the  weather  will  have  attained  its 
maximum. 

But  it  might  occur  to  a  superficial  observer  that  this  reasoning  would  lead 
to  the  conclusion  that  the  weather  would  continue  to  increase  in  its  tempera- 
ture until  the  length  of  the  days  would  become  equal  to  the  length  of  the 
nights,  and  such  would  be  the  case  if  the  loss  of  heat  per  hour  during  the 
night  were  equal  to  that  gain  of  heat  per  hour  during  the  day.  But  such  is 
not  the  case  ;  the  loss  is  more  rapid  than  the  gain,  and  the  consequence  is  that 
the  hottest  dayvusually  comes  within  the  month  of  July,  but  always  long  before 
the  day  of  the  autumnal  equinox. 

The  same  reasoning  will  explain  why  the  coldest  weather  does  not  usually 
occur  on  the  21st  of  December,  when  the  day  is  shortest  and  the  night  longest, 
and  when  the  sun  attains  the  lowest  meridional  altitude.  The  decrease  of  the 
temperature  of  the  weather  depends  upon  the  loss  of  heat  during  the  night 
being  greater  than  the  gain  during  the  day ;  and  until,  by  the  increased  length 
of  the  day  and  the  diminished  length  of  the  night,  these  effects  are  balanced, 
the  coldest  weather  will  not  be  attained. 

These  observations  must  be  understood  as  applying  only  so  far  as  the  tem- 
perature of  the  weather  is  affected  by  the  sun,  and  by  the  length  of  the  days 
and  nights.  There  are  a  variety  of  other  local  and  geographical  causes  which 
interfere  with  these  effects,  and  vary  them  at  different  times  and  places. 

On  referring  to  the  annual  motion  of  the  earth  round  the  sun,  it  appears  that 
the  position  of  the  sun  within  the  elliptic  orbit  of  the  earth  is  such  that  the 
earth  is  nearest  to  the  sun  about  the  1st  of  January,  and  most  distant  from  it 
about  the  1st  of  July.  As  the  calorific  power  of  the  sun's  rays  increases  as 
the  distance  from  the  earth  diminishes,  in  even  a  higher  proportion  than  the 
change  of  distances,  it  might  be  expected  that  the  effect  of  the  sun  in  heating 
the  earth  on  the  1st  of  January  would  be  considerably  greater  than  on  the  1st 
of  July.  If  this  were  admitted,  it  would  follow  that  the  annual  motion  of  the 
earth  in  its  elliptic  orbit  would  have  a  tendency  to  diminish  the  cold  of  the 
winter  in  the  northern  hemisphere,  and  mitigate  the  heat  of  summer,  so  as  to  a 
certain  extent  to  equalize  the  seasons ;  and  on  the  contrary,  in  the  southern 
hemisphere,  where  the  1st  of  January  is  in  the  middle  of  summer  and  the  1st 
of  July  the  middle  of  winter,  its  effects  would  be  to  aggravate  the  cold  in  winter 
and  the  heat  in  summer.  The  investigations,  however,  which  have  been  made 
in  the  physics  of  heat,  have  shown  that  that  principle  is  governed  by  laws 
which  counteract  such  effects.  Like  the  operation  of  all  other  physical  agen- 
cies, the  sun's  calorific  power  requires  a  definite  time  to  produce  a  given  effect, 
and  the  heat  received  by  the  earth  at  any  part  of  its  orbit  will  depend  con- 
jointly on  its  distance  from  the  sun  and  the  length  of  time  it  takes  to  traverse 
that  portion  of  its  orbit.  In  fact,  it  has  been  ascertained  that  the  heating  power 
depends  as  much  on  the  rate  at  which  the  sun  changes  its  longitude  as  upon 
the  earth's  distance  from  it.  Now  it  happens  that  in  consequence  of  the  laws 
of  the  planetary  motions,  discovered  by  Kepler,  and  explained  by  Newton, 
when  the  earth  is  most  remote  from  the  sun,  its  velocity  is  least,  and  conse- 
quently the  hourly  changes  of  longitude  of  the  sun  will  be  proportionally  less. 
Thus  it  appears  that  what  the  heating  power  loses  by  augmented  distance,  it 
gains  by  diminished  velocity ;  and  again,  when  the  earth  is  nearest  to  the  sun, 
what  it  gains  by  diminished  distance,  it  loses  by  increased  speed.  There  is 
thus  a  complete  compensation  produced  in  the  heating  effect  of  the  sun  by 
the  diminished  velocity  of  the  earth  which  accompanies  its  increased  dis- 
tance. 

The  place  of  aphelion,  or  the  point  where  the  earth  is  most  distant  from  the 
sun,  and  the  place  of  perihelion,  or  the  point  where  it  is  nearest  to  the  sun,  are 


THE  EARTH. 


463 


ascertained  by  observing  when  and  where  the  sun's  diameter  is  least  and 
greatest. 

The  diurnal  rotation  of  the  earth  on  its  axis  is  a  fact  which  all  the  world 
are  now  so  habituated  to  admit,  and  are  taught  so  early,  that  few  even  think  of  the 
necessity  of  asking  for  any  demonstration  of  it ;  and  yet  for  thousands  of  years 
this  fundamental  fact  of  astronomy  was  not  only  not  admitted,  but  any  one  who 
would  have  had  the  temerity  to  have  asserted  it,  would  have  been  deemed  a  fit 
candidate  for  an  asylum  for  insane  persons.  Such  is  the  wonderful  force  of 
habit. 

Let  us,  however,  suppose  ourselves  ignorant  of  this  fact,  and  that  for  the 
first  time  we  should  be  told  that  the  place  we  dwell  on  and  the  ground  on 
which  we  walk  is  carried  round  the  diameter  passing  through  the  poles  of  the 
globe  once  in  twenty-four  hours  ;  that  if  we  happen  to  be  on  or  near  the  equa- 
tor, we  are  thus  whirled  round  at  the  rate  of  a  thousand  miles  an  hour,  and  that 
at  the  latitude  of  forty  to  fifty  degrees  we  should  be  transported  at  about  half 
that  speed  :  it  is  surely  conceivable  that  such  an  assertion  heard  for  the  first 
time  would  excite  very  naturally  astonishment  and  incredulity ;  and  although 
habit  has  taught  us  to  assent  to  it,  reason  must  still  suggest  the  question, 
"  What  arguments  have  induced  mankind  to  instil  into  the  minds  of  the  young 
this  principle  as  an  indubitable  fact  ?" 

We  direct  our  view  to  the  firmament,  and  we  see  all  the  objects  upon  it  rise 
in  the  east  and  set  in  the  west,  the  sun  among  the  number.  The  stars  pre- 
serve their  relative  positions  ;  and,  in  short,  all  objects  which  appear  in  the 
firmament  move  as  though  the  motions  did  not  belong  to  them,  but  as  if  the 
whole  firmament  was  carried  round  the  earth  every  twenty-four  hours  with 
a  common  motion,  carrying  all  the  bodies  which  appear  upon  it  with  that 
motion. 

Now.  there  are  two  suppositions,  either  of  which  will  with  equal  precision 
explain  this  appearance  ;  and  there  is  no  other  possible  way,  save  these  two, 
by  which  it  can  be  explained. 

It  may  either  be  produced — as  at  the  first  view  it  appears  to  be — by  the 
whole  universe  turning  with  a  common  motion  every  twenty-four  hours  round 
the  globe  of  the  earth,  or  by  the  globe  of  the  earth  itself  turning  on  its  axis 
once  every  twenty-four  hours.  How  long  mankind  embraced  by  preference 
the  former  supposition,  will  be  rendered  apparent  by  the  very  etymology  of  the 
term  universe*  itself.  Yet,  to  our  apprehension,  informed  as  we  are  of  the 
magnitudes,  distances,  and  general  structure,  not  of  the  solar  system  only,  but 
of  the  stellar  universe,  how  eminently  absurd  does  not  such  a  supposition  ap- 
pear !  It  would  compel  us  to  admit,  not  only  that  the  stupendous  globe  of  the 
sun,  nearly  a  million  and  a  half  times  greater  than  that  of  the  earth,  revolves 
every  twenty-four  hours  round  the  earth  at  a  distance  of  one  hundred  millions 
of  miles,  but  also  that  the  planets,  including  Jupiter,  fourteen  hundred  times, 
and  Saturn,  one  thousand  times  greater  than  the  earth,  the  one  at  four  hundred 
millions  of  miles  and  the  other  at  nine  hundred  millions  of  miles  from  the  earth, 
have  also  this  inconceivable  motion.  But  this  is  not  all :  we  should  be  forced 
to  admit  not  only  that  the  entire  solar  system  whirls  round  the  earth  once  a 
day,  but  we  should  have  to  impute  the  same  diurnal  rotation  to  the  countless 
myriads  of  stars  placed  in  regions  of  the  universe  so  distant  that  light  takes 
several  hundred  years  to  come  from  them  to  the  earth,  moving  at  the  rate  of 
two  hundred  thousand  miles  per  second  ;  these  stars,  moreover,  being  suns, 
many  of  them  more  stupendous  than  our"Wn  !  It  will  be  readily  admitted  that 
such  suppositions  are  invested  with  a  degree  of  improbability  amounting  to 

*  Universe,  from  UNUS,  one ;  and  TERSUM,  a  THING  TURNED  :  signifying  to  turn  icilh  one  common 
motion. 


THE  EARTH. 


moral  impossibility.  It  may,  however,  be  asked  how  they  could  have  been 
entertained  by  the  world  for  so  long  a  succession  of  ages.  The  answer  is,  that 
so  long  as  the  rotation,  of  the  universe  round  the  earth  was  admitted,  mankind 
was  ignorant  of  its  vast  dimensions  and  of  the  comparative  insignificance  of  ( 
the  earth,  with  which  every  person  of  ordinary  education  is  now  more  or  less  < 
familiar.  The  discovery  of  this  has  been  reserved  for  modern  times,  and  con-  ( 
sequently  the  absurdity  of  the  supposition  that  the  earth  is  at  rest  and  the  • 
universe  revolving  daily  round  it  was  not  apparent,  as  it  now  is. 

The  first  demonstration  which  we  have  to  offer  of  the  motion  of  the  earth 
upon  its  axis,  is  what  is  called,  in  the  language  of  schools,  a  disjunctive  syllo- 
gism. 

1.  Either  the  earth  must  tiirn  diurnally  on  its  axis,  or  the  universe  must  * 
turn  diuraally  round  it. 

2.  But  it  is  absurd  to  suppose  that  the  whole  universe  should  turn  diurnally 
round  the  earth. 

Condus-ion.    The  earth  must  therefore  turn  diurnally  on  its  axis. 
Although  this  negative  demonstration  be  sufficiently  conclusive  to   satisfy 
the  understanding,  it  has  always  been   considered   desirable  that  we   should 
obtain  some  positive  and  direct  evidence  that  the  earth  really  has  this  diurnal 
motion.     Now,  an  experiment  has  been  suggested  and  actually  executed,  by 
which  a  mechanical  effect  produced  by  the  diurnal  motion  is  actually  exhib- 
ited.    Let  us  suppose  a  lofty  tower  erected  on   the  surface  of  the  earth  ;  the 
top  of  the  tower  would,  of  course,  be  more  distant  than  its  base  from  the  centre 
of  the  earth  ;  consequently  it  is  evident  that  if  the  earth  had  a  diurnal  motion, 
the  top  of  the  tower,  in  virtue  of  that  motion,  would  describe  a  greater  circle 
than  the  bottom,  and  consequently  would  move  from  west  to  east  with  a  greater 
velocity.     Let  us  suppose,  then,  a  heavy  body,  such  as  a  leaden  bullet,  held 
on  the  top  of  the  tower ;  that  body  would  participate  in  the  velocity  from  west 
to  east  which  the  top  of  the  tower  has  by  the  earth's  diurnal  motion.     If  the 
bullet  were  then  disengaged  and  allowed  to  fall  to  the  base  of  the  tower,  it 
i  would  still  retain  the  velocity  which  it  had  at  the  top  of  the  tower,  and  in  fact 
|  it  would  have  a  downward  motion  and  an  eastward  motion  at  the  same  time. 
.  In  virtue  of  the  downward  motion,  it  would  fall  to  the  ground  at  the  base  of 
|  the  tower  ;  but  in  virtue  of  the  eastward  motion,  it  would  fall  as  far  to  the  east- 
i  ward  as  the  top  of  the  tower  would  have  moved  more  than  the  bottom  in  the 
\  time  of  its  fall. 

i  Now  it  must  be  remembered  that  the  motion  of  the  base  of  the  tower  east- 
\  ward  by  the  diurnal  motion  of  the  earth  is  less  than  that  of  the  top  of  the  tower, 
1  and  consequently  in  the  time  the  ball  would  take  to  fall  from  the  top  of  the 
|  tower  to  the  ground,  the  base  of  the  tower  would  not  be  as  far  eastward  as  the 
1  top  would  move  ;  and  consequently  the  ball  ought  to  be  expected  to  fall  east- 
!  ward  of  the  foot  of  the  tower  at  a  distance  equal  to  the  difference  between  the 
1  space  through  which  the  top  and  the  base  would  have  moved  in  the  time  of 
the  fall. 

But  if  the  tower  and  the  earth  on  which  it  was  built  had  not  this  diurnal 
\  motion,  but  were  at  rest,  then  the  ball  ought  to  fall  exactly  at  the  foot  of  the 
1  tower,  or  vertically  under  the  point  from  which  it  was  disengaged.  Thus, 
I  then,  we  have  a  positive  experiment,  the  result  of  which,  if  rightly  executed 
1  and  accurately  observed,  must  discover  to  us  the  fact  of  the  earth's  motion,  if 
1  such  motion  existed. 

The  experiment  has  been  inaae  ;  ihe  question  has  been  asked  ;  nature  has 
I  been  submitted  to  cross-examination  by  science  :  and  the  secret  has  been 
|  extorted  from  her.  The  ball  has  fallen,  not  at  the  point  vert^'illy  un- 
,  er  the  place  where  it  was  disengaged,  but  eastward  of  that  place  to  the  ex- 


THE  EARTH. 


460 


tent  and  in  the  degree  which  it  ought  to  do  in  virtue  of  the  earth's  diurnal 
motion. 

SPHEROIDAL    FORM    OF   THE    EARTH. 

Although  the  earth  be  said  to  be  a  globe  in  the  ordinary  sense  of  the  term, 
and  when  extreme  accuracy  is  not  sought,  yet,  strictly  speaking,  it  deviates 
from  the  globular  form.  It  has  been  ascertained  that  its  figure  is  that  which 
in  geometry  is  called  an  oblate  spheroid.  To  acquire  a  notion  of  this  form,  we 
have  only  to  imagine  an  oval,  such  as  A  B  C  D,  fig.  11,  to  revolve  upon  its 
short  axis  B  D.  The  figure  it  would  produce  by  such  a  revolution  would  be 
an  oblate  spheroid.  It  will  differ  from  that  of  a  sphere,  inasmuch  as  the  polar 
diameter  B  D  will  be  shorter  than  the  equatorial  diameter  A  C. 

Fig.  11. 


A  familiar  example  of  this  figure  is  presented  by  a  turnip,  or  in  a  less  ex- 
aggerated form  by  an  orange. 

The  degree  in  which  the  earth  has  this  peculiar  form  is,  however,  so  very 
slight,  that  if  we  made  a  model  of  it  in  a  lathe,  the  eye  could  not  discover  that 
it  was  not  a  true  globe.  Its  oblateness  could  only  be  detected  by  accurate 
measurement,  or  by  causing  it  to  revolve  in  different  positions  in  the  lathe,  and 
applying  to  it  a  tool  fixed  on  a  rest.  In  fact,  the  equatorial  diameter  of  the 
earth  is  to  the  polar  diameter  in  the  proportion  of  three  hundred  and  one  to 
three  hundred  ;  or,  in  other  words,  the  diameter  of  the  equator  exceeds  the 
length  of  the  polar  axis  by  one  part  in  three  hundred.  If,  then,  we  take  in 
round  numbers  the  polar  diameter  to  be  eight  thousand  miles,  we  shall  find  the 
equatorial  diameter  to  be  eight  thousand  and  twenty-six  miles  ;  thus  the  parts 
of  tha  earth's  surface  at  the  equator  are  twenty-six  miles  further  from  the  centre 
of  the  earth  than  the  parts  near  the  poles. 

Such  being  understood  to  be  the  real  figure  of  our  globe,  it  will  be  asked 
how  it  has  been  ascertained  to  be  so.  This  question  may  be  examined  in 
either  of  two  ways — either  as  one  of  theory  or  one  of  fact.  We  may  show, 
that,  from  the  known  laws  of  mechanics,  a  globe  like  the  earth  revolving  on  an 
axis  in  twenty-four  hours,  must  become  an  oblate  spheroid  of  the  above  dimen- 
sions ;  or  we  may  show  by  measurements  made  on  different  parts  of  the  earth's 
surface,  that  it  is,  in  fact,  such  a  spheroid,  whatever  cause  may  have  imparted 
that  figure  to  it. 

It  is  well  known  that  when  any  particle  of  matter  revolves  in  a  circle,  it  has 
a  tendency  to  recede  from  the  centre  of  the  circle,  in  virtue  of  what  is  called 
centrifugal  force.  Now  all  points  on  the  surface  of  the  earth  revolve  very 
rapidly  in  circles  by  reason  of  the  diurnal  motion  of  the  globe.  Any  point,  for 
example,  on  the  equator,  revolves  in  a  circumference  of  twenty-five  thousand 
miles  in  twenty-four  hours.  A  point  at  a  higher  latitude  revolves  in  the  same 
time  in  a  less  circle  ;  arid  the  circles  of  diurnal  revolution  become  gradually 

30 


THE  EARTH. 


less  and  less  as  we  approach  the  poles.  Since,  then,  the  centrifugal  force 
depends  conjointly  on  the  magnitude  of  the  circle  of  revolution  and  the  velocity 
of  the  motion,  it  fellows  that  it  will  be  less  and  less  as  we  approach  the  poles, 
and  greater  and  greater  as  we  approach  the  equator. 

This  force,  however,  exists  at  all  latitudes,  in  a  greater  or  less  degree  of 
energy,  and  it  is  everywhere  directed  from  the  centre  of  the  circle  of  di- 
urnal rotation.  Let  N  0  S,  figure  12,  be  the  earth,  and  E  Q  the  equator. 

Fig.  12 


Let  P  be  a  point  on  the  surface  of  the  earth  anywhere  between  the  equator 
and  poles.  Since  P  is  carried  by  the  diurnal  motion  round  the  centre  C,  it 
will  have  a  tendency  to  fly  from  the  centre  in  the  direction  P  R.  This  ten- 
dency will  be  partially  counteracted  by  its  gravity,  which  acts  in  the  direc- 
tion P  O.  But  since  P  O  is  not  directed  immediately  against  P  R,  the  result 
will  be  that  a  particle  of  matter  P  thus  acted  on  will  move  toward  Q.  To  coun- 
teract this  tendency,  there  must  be  such  a  protuberance  at  Q  as  will  place  an 
acclivity  before  P  so  steep  as  to  prevent  its  ascent.  Without  such  a  protuber- 
ance, all  the  fluid  and  loose  matter  on  the  globe  would  run  toward  the  line. 

It  appears,  then,  that  the  effect  of  the  earth's  revolution  would  be  to  cause 
all  loose  matter  placed  on  the  surface  of  the  earth  in  either  hemisphere  to 
move  toward  the  equator  ;  and  that  if  the  earth  were  a  perfect  globe,  there 
would  be  no  power  to  resist  this  tendency,  and  the  effect  would  consequently 
be  actually  produced. 

Let  us,  then,  suppose  an  exact  globe,  partially  covered  with  land  and  water, 

revolving  on  an  axis  in  twenty-four  hours  ;  the  land  or  solid  matter  composing 

it  would  be   affected  by  the   centrifugal  force,  like   all  other  matter,  but  the 

cohesive  principle  which  gives  it  solidity  would  prevent  a  derangement  of  its 

structure  or  change  of  position  by  such  a  cause,  and  the  effect  of  the  centrim- 

!  gal  force  would  therefore  be  confined  to  the  fluid  matter,  which,  in  obedience 

J  to  the  tendency  above  described,  would  flow  from  either  hemisphere  toward 

i  the  regions  about  the  equator,  where  it  would  be  gradually  heaped  up  so  as  to 

'  form  a  convex  protuberance  around  the  line  between  the  tropics,  and  to  give  to 

I  the  earth,  so  far  as  the  fluid  matter  upon  it  is  concerned,  the  form  of  an  oblate 

I  spheroid.     But  this  movement  of  the  fluid  would  cease  as  soon  as  the  equato- 

)  rial  protuberance  should  attain  a  certain  limit ;  for  we  may  regard  such  a  pro- 

?  tubenmce  as  a  sort  of  mountain  piled  round  the  equator,  down  the  sides  of  which 

(  there  would  be  a  tendency  to  fall,  in  obedience  to  gravitation,  as  would  be  the 

f  case  down  any  other  declivity. 

5       The  particles  of  fluid  placed  upon  the  side  of  this  protuberance  would  be 
I  affected  by  two  opposite  forces  :  that  which  would  result  from   the   rotation 


THE  EARTH.  467 


would  have  a  tendency  to  move  them  toward  the  line — that  is,  ascending  the 
acclivity — while  their  gravity,  on  the  other  hand,  would  have  a  tendency  to 
make  them  descend,  or  to  move  them  from  the  acclivity.  When  the  protu- 
berance would  attain  the  limit  at  which  these  two  tendencies  would  become 
equal,  so  that  the  descending  force  of  gravity  should  be  equal  to  the  ascending 
force  proceeding  from  the  rotation,  the  particles  of  the  fluid  would  be  at  rest, 
and  would  neither  approach  the  line  nor  recede  from  it.  It  is  within  the  prov- 
ince of  mathematical  physics  to  calculate  what  the  limit  of  this  protuberance 
would  be  which  would  produce  this  state  of  equilibrium,  and  the  result  of  such 
calculations  has  given  us  a  form  which  corresponds  nearly  to  that  which  the 
earth  is  actually  found  to  have. 

But  it  may  be  objected  that  such  reasoning  would  apply  only  to  fluid  matter 
upon  the  earth,  whereas  the  oblate  form  is  known  to  belong  to  its  solid  as  well 
as  its  fluid  surface. 

This  circumstance  has  been  explained  in  two  ways.  1.  It  is  said  that  the 
earth  in  its  original  formation  was  altogether  fluid  ;  that  in  that  fluid  state  it 
received  its  diurnal  rotation,  and  consequently  took  the  form  corresponding 
with  that  rotation  which  we  have  just  explained  ;  that,  by  cooling  down,  the 
fluid  matter  partially  hardened  into  a  solid  matter,  leaving  the  liquid  ocean  cov- 
ering about  two  thirds  of  the  globe. 

But  if  this  original  fluid  state  of  the  globe  be  denied  or  doubted,  and  if  it  be  I 
maintained  that  the  globe  received  its  revolution  upon  its  axis  when  it  was  com- 
posed as  it  is,  partly  of  land  and  partly  of  water,  it  is  nevertheless  contended 
that  its  present  figure  is  explicable.  If  a  true  globe,  diversified  by  land  and  by 
water,  received  a  diurnal  rotation  like  that  of  ours,  the  water  would  in  the  first 
instance  flow  toward  the  equator,  and  the  geographical  condition  of  the  globe 
would  be,  two  polar  continents,  separated  by  an  extensive  equatorial  ocean. 
But  after  the  lapse  of  ages,  the  ocean,  washing  continually  upon  the  shores  of 
the  continents,  would  cause  the  constant  abrasion  of  their  solid  matter,  which, 
in  the  form  of  mud  and  sand,  would  mix  with  the  liquid  of  the  ocean,  and  would 
obey  all  its  tendencies.  In  fact,  in  process  of  time  the  land  by  decadence  and 
abrasion  would  obey  the  same  principles  which  would  affect  a  fluid  ;  and  the 
earth  would  at  length,  though  after  a  long  lapse  of  time,  assume  the  form  of 
fluid  equilibrium.  The  present  distribution  of  land  and  water  which  characterizes 
it  has  arisen  from  causes  belonging  more  properly  to  geology  than  astronomy. 

Such  is  the  theoretical  reasoning  applicable  to  the  form  of  the  earth.  We  are 
still,  however,  required  by  the  rigorous  principles  of  inductive  philosophy  to 
ascertain,  as  a  matter  of  fact,  independent  of  all  theory,  the  actual  figure  of  the 
globe.  This  has  accordingly  been  done. 

The  section  of  an  oblate  spheroid  made  by  a  plane  passing  through  the  poles, 
is  au  oval,  the  longer  axis  of  which  is  in  the  equator.  It  will  be  evident  upon 
mere  inspection  that  the  curvature  of  the  earth  having  such  a  form,  would  in- 
crease as  we  approach  the  equator,  and  diminish  as  we  approach  the  poles  ; 
that  i%  to  say,  a  piece  of  a  meridian  taken  near  the  equator  would  be  part  of  a 
less  circle  than  a  similar  piece  taken  near  the  poles.  This  is  equivalent  to 
stating  that  a  degree  of  latitude  near  the  equator  would  be  shorter  than  a  degree 
of  latitude  near  the  poles. 

Thus,  then,  the  question  of  the  figure  of  the  earth  is  in  fact  resolved  into  the 
measurement  of  a  degree  of  latitude  at  different  parts  of  the  globe. 

Such  measurement  has  accordingly  been  executed  with  great  precision,  and 
it  has  been  found,  as  was  anticipated,  that  the  degrees  of  latitude  become 
shorter  as  we  approach  the  equator,  and  longer  as  we  approach  the  poles.  A 
comparison  of  their  lengths  has  given  the  degree  that  characterizes  the  oblate- 
ness  of  the  earth. 


468 


THE  EARTH. 


But  this  is  not  the  only  test  by  which  the  figure  of  the  earth  has  been  ascer- 
tained. If  the  earth  were  a  true  globe  revolving  on  its  axis  in  twenty-four  hours, 
the  effect  of  its  revolution  would  cause  gravity  to  diminish  on  approaching  the 
equator,  and  increase  on  approaching  the  poles  ;  for  the  centrifugal  force  due 
to  the  rotation  increasing  toward  the  equator  would  cause  a  greater  diminution 
of  gravity  there  than  toward  the  poles,  where  it  lessens.  Now,  it  is  possible 
to  calculate  the  effect  of  such  centrifugal  force  upon  the  earth  if  it  had  the 
figure  of  a  true  globe.  The  effect  of  this  diminution  of  gravity  will  be  ascer- 
tained with  great  exactness  by  observing  the  vibration  of  a  pendulum  in  differ- 
ent parts  of  the  earth.  It  has  been  already  explained  that  the  motion  of  a 
pendulum  is  produced  by  the  gravity  of  the  earth  acting  upon  the  ball  of  the 
pendulous  body,  and  that  the  greater  the  attraction  of  gravity,  the  more  rapid 
will  b@  the  vibration ;  and  vice  versa.  We  carry,  then,  a  pendulum  alternately 
toward  the  equator  and  toward  the  poles,  and  find  invariably  that  its  vibration 
is  slower  when  taken  toward  the  equator,  and  more  rapid  when  taken  toward 
the  poles.  But  we  find  that  this  variation  in  its  vibration  does  not  correspond 
to  that  which  it  ought  to  have  if  the  earth  were  an  exact  globe.  It  is  just  the 
variation  which  ought  to  take  place  if  the  earth  were  an  oblate  spheroid,  of  the 
form  already  described. 

Thus  we  have  two  independent  tests  of  the  figure  of  the  earth,  which  give 
accordant  results. 


LUNAR  INFLUENCES. 


The  Red  Moon. — Supposed  Effect  of  the  Moon  on  the  Movement  of  Sap  in  Plants. — Prejudice  re- 
specting the  time  for  felling  Timber. — Extent  of  this  Prejudice. — Its  Prevalence  among  Trans- 
atlantic People. — Prejudices  respecting  Effects  on  Grain. — On  Wine. — On  the  Complexion. — On 
Putrefaction. — On  Wounds. — On  the  Size  of  Oysters  and  Shellfish. — On  the  Marrow  of  Animals. — 
On  the  Weight  of  the  Human  Body. — On  the  Time  of  Births. — On  the  Hatching  of  Eggs. — On 
Human  Maladies. — On  Insanity. — On  Fevers. — On  Epidemics. — Case  of  Vallisnieri. — Case  of 
Bacon. — On  Cutaneous  Diseases,  Convulsions,  Paralysis,  Epilepsy,  &c. — Observations  of  Dr. 
Gibers. 


LUNAR  INFLUENCES. 


471 


LUNAR   INFLUENCES. 


ON  a  former  occasion  I  examined  the  question  respecting  the  supposed 
influence  of  the  moon  upon  the  weather,  and  demonstrated  that  so  far  as  ac- 
tual observation  has  hitherto  afforded  grounds  for  reasoning,  there  is  no  dis- 
coverable correspondence  between  the  lunar  changes  and  the  vicissitudes  of 
rain  and  drought  which  can  justify  or  in  any  degree  countenance  the  popular 
belief  so  generally  entertained  as  to  dependance  of  change  of  weather  upon 
the  changes  of  the  moon. 

But  meteorological  phenomena  are  not  the  only  effects  imputed  to  our  satel- 
lite ;  that  body,  like  comets,  is  made  responsible  for  a  vast  variety  of  interfe- 
rences with  organized  nature.  The  circulation  of  the  juices  of  vegetables,  the 
qualities  of  grain,  the  fate  of  the  vintage,  are  all  laid"to  its  account;  and 
timber  must  be  felled,  the  harvest  cut  down  and  gathered  in,  and  the  juice  of 
the  grape  expressed,  at  times  and  under  circumstances  regulated  by  the  aspects 
of  the  moon,  if  excellence  be  hoped  for  in  these  products  of  the  soil. 

According  to  popular  belief,  our  satellite  also  presides  over  human  maladies  ; 
and  the  phenomena  of  the  sick  chamber  are  governed  by  the  lunar  phases  ; 
nay,  the  very  marrow  of  our  bones,  and  the  weight  of  our  bodies,  suffer  in- 
crease or  diminution  by  its  influence.  Nor  is  its  imputed  power  confined  to 
physical  or  organic  effects  ;  it  notoriously  governs  mental  derangement. 

If  these  opinions  respecting  lunar  influence  were  limited  to  particular  coun- 
tries, they  would  be  less  entitled  to  serious  consideration  ;  but  it  is  a  curious 
fact  that  many  of  them  prevail  and  have  prevailed  in  quarters  of  the  earth  so 
distant  and  unconnected,  that  it  is  difficult  to  imagine  the  same  error  to  have 
proceeded  from  the  same  source.  At  all  events,  the  extent  of  their  prevalence 
alone  renders  them  a  fit  subject  for  serious  investigation  ;  and  I  propose  at 
present  to  lay  before  you  some  of  the  principal  facts  and  arguments  bearing  on 
these  points,  for  the  collection  of  which  we  are  mainly  indebted  to  the  industry 
and  research  of  M.  Arago. 

A  large  volume  would  be  necessary  to  analyze  all  the  popular  opinions 


472  LUNAR  INFLUENCES. 


which  refer  to  the  supposed  lunar  influences.  We  shall  confine  ourselves 
therefore  to  the  principal  of  them,  and  shortly  examine  how  far  they  can  be 
reconciled  with  the  established  principles  of  astronomy  and  physics. 

The  Red  Moon. — It  is  believed  generally,  especially  in  the  neighborhood  of 
Paris,  that  in  certain  months  of  the  year,  the  moon  exerts  a  great  influence  up- 
on the  phenomena  of  vegetation.  Gardeners  give  the  name  of  Red  Moon  to 
that  moon  which  is  full  between  the  middle  of  April  and  the  close  of  May.  Ac- 
cording to  them  the  light  of  the  moon  at  that  season  exercises  an  injurious  in- 
fluence upon  the  young  shoots  of  plants.  They  say  that  when  the  sky  is 
clear  the  leaves  and  buds  exposed  to  the  lunar  light  redden  and  are  killed  as 
if  by  frost,  at  a  time  when  the  thermometer  exposed  to  the  atmosphere  stands 
at  many  degrees  above  the  freezing  point.  They  say  also  that  if  a  clouded 
sky  intercepts  the  moon's  light  it  prevents  these  injurious  consequences  to  the 
plants,  although  the  circumstances  of  temperature  are  the  same  in  both  cases. 

Any  person  who  is  acquainted  with  the  beautiful  theory  of  dew,  which  we 
owe  to  Dr.  Wells,  will  find  no  difficulty  in  accounting  for  these  effects  errone- 
ously imputed  to  the  moon.  If  the  heavens  be  clear  and  unclouded,  all  sub- 
stances on  the  surface  of  the  earth  which  are  strong  and  powerful  radiators  of 
heat,  lose  temperature  by  radiation,  while  the  unclouded  sky  returns  no  heat  to 
them  to  restore  what  they  have  lost.  Such  bodies,  therefore,  under  these  cir- 
cumstances, become  colder  than  the  surrounding  air,  and  may  even,  if  they  be 
liquid,  be  frozen.  Ice.  in  fact,  is  produced,  in  warm  climates,  by  similar 
means.  But  if  the  firmament  be  enveloped  in  clouds,  the  clouds  havii.g  the 
quality  of  radiating  heat,  will  restore  by  their  radiation,  to  substances  upon  the 
surface  of  the  earth,  as  much  heat  as  such  substances  lose  by  radiation  ;  the 
temperature,  therefore,  of  such  bodies  will  be  maintained  at  a  point  equal  to 
that  of  the  air  surrounding  them. 

Now  the  leaves  and  flowers  of  plants  are  strong  and  powerful  radiators  of 
heat ;  when  the  sky  is  clear  they  therefore  lose  temperature  and  may  be  frozen ; 
if,  on  the  other  hand,  the  sky  be  clouded,  their  temperature  is  maintained  for 
the  reasons  above  stated. 

The  moon,  therefore,  has  no  connexion  whatever  with  this  effect ;  and  it  is 
certain  that  plants  would  suffer  under  the  same  circumstances  whether  the 
moon  is  above  or  below  the  horizon.  It  equally  is  quite  true  that  if  the  moon 
be  above  the  horizon,  the  plants  cannot  suffer  unless  it  be  visible  ;  because  a 
clear  sky  is  indispensable  as  much  to  the  production  of  the  injury  to  the  plants 
as  to  the  visibility  of  the  moon ;  and,  on  the  other  hand,  the  same  clouds 
which  veil  the  moon  and  intercept  her  light  give  back  to  the  plants  that  warmth 
which  prevents  the  injury  here  adverted  to.  The  popular  opinion  is  therefore 
right  as  to  the  effect,  but  wrong  as  to  the  cause ;  and  its  error  will  be  at  once 
discovered  by  showing  that  on  a  clear  night,  when  the  moon  is  new,  and, 
therefore,  not  visible,  the  plants  may  nevertheless  suffer. 

Time  for  felling  Timber. — There  is  an  opinion  generally  entertained  that  tim- 
ber should  be  felled  only  during  the  decline  of  the  moon  ;  for  if  it  be  cut  down 
during  its  increase,  it  will  not  be  of  a  good  or  durable  quality.  This  impression 
prevails  in  various  countries.  It  is  acted  upon  in  England,  and  is  made  the 
ground  of  legislation  irt  France.  The  forest  laws  of  the  latter  country  inter- 
dict the  cutting  of  timber  during  the  increase  of  the  moon.  M.  Auguste  de 
Saint  Hilaire  states,  that  he  found  the  same  opinion  prevalent  in  Brazil. 
(  Signer  Francisco  Pinto,  an  eminent  agriculturist  in  the  province  of  Espirito 

<  Santo,  assured  him  as  the  result  of  his  experience,  that  the  wood  which  was 
;  not  felled  at  the  full  of  the  moon  was  immediately  attacked  by  worms  and  very 

<  soon  rotted. 

In  the  extensive  forests  of  Germany,  the  same  opinion  is  entertained  and  acted 


LUNAR  INFLUENCES.  473 


upon  with  the  most  undoubting  confidence  in  its  truth.  Sauer,  a  superintendent  of 
some  of  these  districts,  assigns  what  he  believes  to  be  its  physical  cause.  Ac- 
cording to  him  the  increase  of  the  moon  causes  the  sap  to  ascend  in  the  lim- 
ber; and,  on  the  other  hand,  the  decrease  of  the  moon  causes  its  descent.  If 
the  timber,  therefore,  be  cut  during  the  decrease  of  the  moon  it  will  be  cut  in 
a  dry  state,  the  sap  having  retired  ;  and  the  wood,  therefore,  will  be  compact, 
solid,  and  durable.  But  if  it  be  cut  during  the  increase  of  the  moon,  it  will  be 
felled  with  the  sap  in  it,  and  will  therefore  be  more  spongy,  more  easily  at- 
tacked by  worms,  more  difficult  to  season,  and  more  readily  split  and  warped 
by  changes  of  temperature. 

Admitting  for  a  moment  the  reality  of  this  supposition  concerning  the  motion 
of  the  sap,  it  would  follow  that  the  proper  time  for  felling  the  timber  would  be 
the  new  moon,  that  being  the  epoch  at  which  the  descent  of  the  sap  would 
have  been  made,  and  the  ascent  not  yet  commenced.  But  can  there  be 
imagined  in  the  whole  range  of  natural  science,  a  physical  relation  more  ex- 
traordinary and  unaccountable  than  this  supposed  correspondence  between  the 
movement  of  the  sap  and  the  phases  of  the  moon  ?  Assuredly  theory  affords 
not  the  slightest  countenance  to  such  a  supposition  ;  but  let  us  inquire  as  to  the 
fact  whether  it  be  really  the  case  that  the  quality  of  timber  depends  upon  the 
state  of  the  moon  at  the  time  it  is  felled. 

M.  Duhamel  Monceau,  a  celebrated  French  agriculturist,  has  made  direct 
and  positive  experiments  for  the  purpose  of  testing  this  question ;  and  has 
clearly  and  conclusively  shown  that  the  qualities  of  timber  felled  in  different 
parts  of  the  lunar  month  are  the  same.  M.  Duhamel  felled  a  great  many  trees 
of  the  same  age,  growing  from  the  same  soil,  and  exposed  to  the  same  aspect, 
and  never  found  any  difference  in  the  quality  of  the  timber  when  he  compared 
those  which  were  felled  in  the  decline  of  the  moon  with  those  which  were 
felled  during  its  increase  ;  in  general  they  have  afforded  timber  of  the  same 
quality.  He  adds,  however,  that  by  a  circumstance,  which  was  doubtless  for- 
tuitous, a  slight  difference  was  manifested  in  favor  of  timber  which  had  been 
felled  between  the  new  and  full  moon — contrary  to  popular  opinion. 

Supposed  Lunar  Influence  on  Vegetables. — It  is  an  aphorism  received  by  all 
gardeners  and  agriculturists  in  Europe,  that  vegetables,  plants,  and  trees, 
which  are  expected  to  flourish  and  grow  with  vigor,  should  be  planted,  grafted, 
and  pruned,  during  the  increase  of  the  moon.  This  opinion  is  altogether  erro- 
neous. The  increase  or  decrease  of  the  moon  has  no  appreciable  influence  on 
the  phenomena  of  vegetation  ;  and  the  experiments  and  observations  of  several 
French  agriculturists,  and  especially  of  M.  Duhamel  du  Monceau  (already  al- 
luded to)  have  clearly  established  this. 

Montanari  has  attempted,  like  M.  Sauer,  to  assign  the  physical  cause  for 

this  imaginary  effect.     During  the  day,  he  says,  the  solar  heat  augments  the 

quantity  of  sap  which  circulates  in  plants  by  increasing  the  magnitude  of  the 

tube  through  which  the  sap  moves ;  while  the  cold  of  the  night  produces  the 

opposite  effect  by  contracting  these  tubes.     Now,  at  the  moment  of  sunset,  if 

the  moon  be  increasing,  it  will  be  above  the  horizon,  and  the  warmth  of  its 

light  would  prolong  the  circulation  of  the  sap  ;  but,  during  its  decline,  it  will  not 

rise  for  a  considerable  time  after  sunset,  and  the  plants  will  be  suddenly  exposed 

)  to  the  unmitigated  cold  of  the  night,  by  which  a  sudden  contraction  of  leaves 

|  and  tubes  will  be  produced,  and  the  circulation  of  the  sap  as  suddenly  obstructed. 

If  we  admit  the  lunar  rays  to  possess  any  sensible  calorific  power,  this  rea- 
soning might  be  allowed  ;  but  it  will  have  very  little  force  when  it  is  consid- 
ered that  the  extreme  change  of  temperature  which  can  be  produced  by  the 
lunar  light,  does  not  amount  to  the  thousandth  part  of  a  degree  of  tho  ther- 
mometer. 


474 


LUNAR  INFLUENCES. 


It  is  a  curious  circumstance  that  this  erroneous  prejudice  prevails  on  the 
American  continent.  M.  Auguste  de  Saint  Hiliare  states,  that  in  Brazil  cul- 
tivators plant  during  the  decline  of  the  moon,  all  vegetable  whose  roots  are 
used  as  food,  and,  on  the  contrary,  they  plant  during  the  increasing  moon, 
the  sugar-cane,  maize,  rice,  beans,  &c.,  and  those  which  bear  the  food  upon 
their  stocks  and  branches.  Experiments,  however,  were  made  and  reported  by 
M.de  Chauvalon,  at  Martinique,  on  vegetables  of  both  kinds  planted  at  different 
limes  in  the  lunar  month,  and  no  appreciable  difference  in  their  qualities  was 
discovered. 

There  are  some  traces  of  a  principle  in  the  rule  adopted  by  the  South 
American  agronornes,  according  to  which  they  treat  the  two  classes  of  plants 
distinguished  by  the  production  of  fruit  on  their  roots  or  on  their  branches  dif- 
ferently ;  but  there  are  none  in  the  European  aphorisms.  The  directions  of 
Pliny  are  still  more  specific :  he  prescribes  the  time  of  the  full  moon  for  sow- 
ing beans,  and  that  of  the  new  moon  for  lentils.  "  Truly,"  says  M.  Arago, 
"  we  have  need  of  a  robust  faith  to  admit  without  proof  that  the  moon,  at  the 
distance  of  240, 000  miles,  shall  in  one  position  act  advantageously  upon  the 
vegetation  of  beans,  and  that  in  the  opposite  position,  and  at  the  same  distance, 
she  shall  be  propitious  to  lentils." 

Supposed  Lunar  Influence  on  Grain. — Pliny  states  that  if  we  would  collect 
grain  for  the  purpose  of  immediate  sale,  we  should  do  so  at  the  full  of  the 
moon  ;  because,  during  the  moon's  increase  the  grain  augments  remarkably  in 
magnitude :  but  if  we  would  collect  the  grain  to  preserve  it,  we  should  choose 
the  new  moon,  or  the  decline  of  the  moon. 

So  far  as  it  is  consistent  with  observation  that  more  rain  falls  during  the  in- 
crease of  the  moon  than  during  its  decline,  there  may  be  some  reason  for  this 
maxim  ;  but  Pliny,  or  those  from  whom  we  receive  the  maxim,  can  barely  have 
credit  for  grounds  so  rational :  besides  which,  the  difference  in  the  quantity  of 
rain  which  falls  during  the  two  periods  is  too  insignificant  to  produce  the 
effects  here  adverted  to. 

Supposed  Lunar  Influence  on  Wine-making. — It  is  a  maxim  of  wine-growers, 
that  wine  which  has  been  made  in  two  moons  is  never  of  a  good  quality,  and 
cannot  be  clear.  Toaldo,  the  celebrated  Italian  meteorologist,  whose  mind  ap- 
pears to  have  been  predisposed  for  the  reception  of  lunar  prejudice,  attempts 
to  justify  this  maxim.  "  The  vinous  fermentation,"  he  says,  "  can  only  be  car- 
ried on  in  two  moons  when  it  begins  immediately  before  the  new  moon  ;  and, 
consequently,  that  this  being  a  time  when  the  enlightened  side  of  the  moon  is 
turned  for  the  most  part  from  the  earth,  our  atmosphere  is  deprived  of  the  heat 
of  the  lunar  rays  ;  that  therefore  the  temperature  of  the  air  is  lowered,  and  the 
fermentation  is  less  active. 

To  this  we  need  only  answer,  that  the  moon's  rays  do  not  affect  the  temper- 
ature of  the  air  to  the  extent  of  one  thousandth  part  of  a  degree  of  the  ther- 
mometer, and  that  the  difference  of  temperatures  of  any  two  neighboring  places 
in  which  the  process  of  making  the  wine  of  the  same  soil  and  vintage  might  be 
conducted,  must  be  a  thousand  times  greater  at  any  given  moment  of  time,  and 
yet  no  one  ever  imagines  that  such  a  circumstance  can  affect  the  quality  of  the 
wine. 

It  is  a  maxim  of  Italian  wine  merchants,  that  wine  ought  never  to  be  trans- 
ferred from  one  vessel  to  another  in  the  month  of  January  or  March,  unless  in 
the  decline  of  the  moon,  under  penalty  of  seeing  it  spoiled. 

Toaldo  has  not  favored  us  with  any  physical  reason  for  this  maxim ;  but  it 
is  remarkable  that  Pliny,  on  the  authority  of  Hyginus,  recommends  precisely 
the  opposite  course.  We  may  presume  that  from  such  contrary  rules,  it  may 
reasonably  be  inferred  that  the  moon  has  no  influence  whatever  in  this  case. 


LUNAR  INFLUENCES.  475 


Among  the  maxims  of  Pliny  we  find  that  grapes  should  be  dried  by  night  at 
new  moon,  and  by  day  at  full  moon. 

When  the  moon  is  new  it  is  below  the  horizon  during  the  night,  and  above 
it  during  the  day  ;  and  when  it  is  full  it  is  above  the  horizon  during  the  night, 
and  below  it  during  the  day.  The  maxim  of  Pliny,  therefore,  is  equivalent  to 
a  condition  requiring  that  the  grapes  should  be  dried  when  the  moon  is  below 
the  horizon.  It  is  evident  that  the  absence  of  the  moon  is  not  required  in  this 
case  in  consequence  of  any  effect  which  her  light  might  produce  if  she  were 
present ;  for  when  the  moon  is  new  she  affords  no  light,  even  when  in  the  fir- 
manent,  the  illuminated  side  being  turned  from  the  earth.  If  the  maxim  be 
founded  upon  any  reason,  it  must,  therefore,  either  be  on  some  influence  which 
the  moon  is  supposed  to  produce  when  present,  independent  of  her  light  (the 
absence  of  which  influence  is  desired),  or  it  may  be  that  she  may  be  supposed 
to  transmit  some  effect  through  the  solid  mass  of  the  earth  when  on  the  other 
side  of  it  which  she  is  incapable  of  producing  without  its  intervention.  The 
maxim  is  probably  as  absurd  and  groundless  as  the  other  effects  imputed  to  the 
moon. 

Supposed  Lunar  Influence  on  the  Complexion. — It  is  a  prevalent  popular  no- 
tion in  some  parts  of  Europe,  that  the  moon's  light  is  attended  with  the  effect 
of  darkening  the  complexion. 

That  light  has  an  effect  upon  the  color  of  material  substances  is  a  fact  well 
known  in  physics  and  in  the  arts.  The  process  of  bleaching  by  exposure  to  the 
sun  is  an  obvious  example  of  this  class  of  facts.  Vegetables  and  flowers  which 
grow  in  a  situation  excluded  from  the  light  of  the  sun  are  different  in  color 
from  those  which  have  been  exposed  to  its  influence.  The  most  striking  in- 
stance, however,  of  the  effect  of  certain  rays  of  solar  light  in  blackening  a  light 
colored  substance,  is  afforded  by  chloride  of  silver,  which  is  a  white  substance, 
but  which  immediately  becomes  black  when  acted  upon  by  the  rays  near  the 
red  extremity  of  the  spectrum.  This  substance,  however,  highly  susceptible 
as  it  is  of  having  its  color  affected  by  light,  is,  nevertheless,  found  not  to  be 
changed  in  any  sensible  degree  when  exposed  to  the  light  of  the  moon,  even 
when  that  light  is  condensed  by  the  most  powerful  burning  lenses.  It  would 
seem,  therefore,  that  as  far  as  any  analogy  can  be  derived  from  the  qualities  of 
this  substance,  the  popular  impression  of  the  influence  of  the  moon's  rays  in 
blackening  the  skin  receives  no  support. 

M.  Arago  (who  generally  inclines  to  favor  rather  than  oppose  prevailing 
popular  opinions),  appears  to  think  it  possible  that  some  effect  may  be  pro- 
duced upon  the  skin  exposed  on  clear  nights,  explicable  on  the  same  principle 
as  that  by  which  we  have  explained  the  effects  erroneously  imputed  to  what  is 
called  the  red  moon.  The  skin  being,  in  common  with  the  leaves  and  flowers 
of  vegetables,  a  good  radiator  of  heat,  will,  when  exposed  on  a  clear  night,  for 
the  same  reasons,  sustain  a  loss  of  temperature.  Although  this  will  be  to  a 
certain  extent  restored  by  the  sources  of  animal  heat,  still  it  may  be  contended 
that  the  cooling  produced  by  radiation  is  not  altogether  without  effect.  It  is 
well  known  that  a  person  who  sleeps  exposed  in  the  open  air  on  a  night  when 
the  dew  falls,  is  liable  to  suffer  from  severe  cold,  although  the  atmosphere  around 
him  never  falls  below  a  moderate  temperature  ;  and  although  no  actual  depo- 
sition of  dew  may  take  place  upon  his  skin.  This  effect  must  arise  from  the 
constant  lowering  of  temperature  of  the  skin  by  radiation.  In  military  cam- 
paigns the  effects  of  bivouacking  at  night  appear  to  be  generally  admitted  to 
darken  the  complexion.* 

*  Le  hale  de  bivouac  is  an  effect  quite  recognised.    Hale  is  a  term  which  expresses  a  state  of  the 
air  which  makes  an  impression  upon  the  complexion,  rendering  tanned  and  burnt. 


There  is  a  proverb  which  is  used  in  certain  parts  of  France  as  a  wnrning  ' 
against  night  promenades  : — 

"  due  Ion  PO!  y  la  sereine 
Fau  gene  la  gent  Mouraine." 

It  is  remarkable  that  this  proverb  is  current  in  places  where  the  red  moon  is 
not  noticed. 

Supposed  Lunar  Influence  on  Putrffaction. — Pliny  and  Plutarch  have  trans- 
milted  it  as  a  maxim,  that  the  light  of  the  moon  facilitates  the  putrefaction  of 
animal  substances,  and  covers  them  with  moisture.  The  same  opinion  pre- 
vails in  the  West  Indies,  and  in  South  America.  An  impression  is  prevalent, 
also,  that  certain  kinds  of  fruit  exposed  to  moonlight  lose  their  flavor  and  be- 
come soft  and  flabby ;  and  that  if  a  wounded  mule  be  exposed  to  the  light  of 
the  moon  during  the  night,  the  wound  will  become  irritated,  and  frequently  be- 
come incurable. 

Such  effects,  if  real,  may  be  explained  upon  the  same  principles  as  those  by 
which  we  have  already  explained  the  effects  imputed  to  the  red  moon.  Ani- 
mal substances  exposed  to  a  clear  sky  at  night,  are  liable  to  receive  a  deposi- 
tion of  dew,  which  humidity  has  a  tendency  to  accelerate  putrefaction.  But 
this  effect  will  be  produced  if  the  sky  be  clear,  whether  the  moon  be  above  the 
horizon  or  not.  The  moon,  therefore,  in  this  case,  is  a  witness  and  not  an 
agent ;  and  we  must  acquit  her  of  the  misdeeds  imputed  to  her. 

Supposed  Lunar  Influence  on  Shell-flsh. — It  is  a  very  ancient  remark,  that 
oysters  and  other  shell-fish  become  larger  during  the  increase  than  during  the 
decline  of  the  "moon.  This  maxim  is  mentioned  by  the  poet  Lucilius,  by  Au- 
lus  Gellius,  and  others ;  and  the  members  of  the  academy  del  Cimrnto  appear 
to  have  tacitly  admitted  it,  since  they  endeavor  to  give  an  explanation  of  it. 
The  fact,  however,  has  been  carefully  examined  by  Rohault,  who  has  com- 
pared shell-fish  taken  at  all  periods  of  the  lunar  month,  and  found  that  they  ex- 
hibit no  difference  of  quality. 

Supposed  Lunar  Influence  on  the  Marrow  of  Animals. — An  opinion  is  preva- 
lent among  butchers  that  the  marrow  found  in  the  bones  of  animals  varies  in 
quantity  according  to  the  phase  of  the  moon  in  which  they  are  slaughtered. 
This  question  has  also  been  examined  by  Rohault,  who  made  a  series  of  ob- 
servations which  were  continued  for  twenty  years  with  a  view  to  test  it ;  and 
the  result  was  that  it  was  proved  completely  destitute  of  foundation. 

Supposed  Lunar  Influence  on  the  Weight  of  the  Human  Body. — Sanctorius, 
whose  name  is  celebrated  in  physics  for  the  invention  of  the  thermometer,  held 
it  as  a  principle  that  a  healthy  man  gained  two  pounds  weight  at  the  begin- 
ning of  every  lunar  month,  which  he  lost  toward  its  completion.  This  opinion 
appears  to  be  founded  on  experiments  made  upon  himself;  and  affords  another 
instance  of  a  fortuitous  coincidence  hastily  generalized.  The  error  would 
have  been  corrected  if  he  had  continued  his  observations  a  sufficient  length  of 
time. 

Supposed  Lunar  Influence  on  Births. — It  is  a  prevalent  opinion  that  births 
occur  more  frequently  in  the  decline  of  the  moon  than  in  her  increase.  This 
opinion  has  been  tested  by  comparing  the  number  of  births  with  the  periods 
of  the  lunar  phases ;  but  the  attention  directed  to  statistics  as  well  in  this 
country  as  abroad,  will  soon  lead  to  the  decision  of  this  question.* 

Supposed  Lunar  Influence  on  Incubation. — It  is  a  maxim  handed  down  by 
Pliny,  that  eggs  should  be  put  to  cover  when  the  moon  is  new.  In  France  it 
is  a  maxim  generally  adopted,  that  the  fowls  are  better  and  more  successfully 
reared  when  they  break  the  shell  at  the  full  of  the  moon.  The  experiments  ai.d 

*  Other  sexual  phenomena,  such  as  the  period  of  eertation,  vulgarly  supposed  to  have  some  relu- 
tion  to  the  lunar  mouth,  have  no  relation  whatever  to  that  period. 


LUNAR  INFLUENCES. 


observations  of  M.  Girou  de  Buzareingues  have  given  countenance  to  this 
opinion.  But  such  observations  require  to  be  multiplied  before  the  maxim  can 
be  considered  as  established.  M.  Girou  inclines  to  the  opinion  that  during 
the  dark  nights  about  new  moon  the  hens  sit  so  undisturbed  that  they  either  kill 
their  young  or  check  their  development  by  too  much  heat ;  while  in  moonlight 
nights,  being  more  restless,  this  effect  is  not  produced. 

Supposed  Lunar  Influence  on  Mental  Derangement  and  other  Human  Maladies. 
— The  influence  on  the  phenomena  of  human  maladies  imputed  to  the  moon  is 
rery  ancient.  Hippocrates  had  so  strong  a  faith  in  the  influence  of  celestial 
objects  upon  animated  beings,  that  he  expressly  recommends  no  physician  to 
be  trusted  who  is  ignorant  of  astronomy.  Galen,  following  Hippocrates,  main- 
tained the  same  opinion,  especially  of  the  influence  of  the  moon.  Hence  in 
diseases  the  lunar  periods  were  said  to  correspond  with  the  succession  of  the 
sufferings  of  the  patients.  The  critical  days  or  crises  (as  they  were  afterward 
called),  were  the  seventh,  fourteenth,  and  twenty-first  of  the  disease,  corres- 
ponding to  the  intervals  between  the  moon's  principal  phases.  While  the 
doctrine  of  alchymists  prevailed,  the  human  body  was  considered  as  a  micro- 
cosm ;  the  heart  representing  the  sun,  the  brain  the  moon.  The  planets  had 
each  its  proper  influence  :  Jupiter  presided  over  the  lungs,  Mars  over  the 
liver,  Saturn  over  the  spleen,  Venus  over  the  kidneys,  and  Mercury  over  the 
organs  of  generation.  Of  these  grotesque  notions  there  is  now  no  relic,  ex- 
cept the  term  lunacy,  which  still  designates  unsoundness  of  mind.  But  even 
this  term  may  in  some  degree  be  said  to  be  banished  from  the  terminology  of 
medicine,  and  it  has  taken  refuge  in  that  receptacle  of  all  antiquated  absurdities 
of  phraseology — the  law.  Lunatic,  we  believe,  is  still  the  term  for  the  subject 
who  is  incapable  of  managing  his  own  affairs. 

Although  the  ancient  faith  in  the  connexion  between  the  phases  of  the  moon 
and  the  phenomena  of  insanity  appears  in  a  great  degree  to  be  abandoned,  yet 
it  is  not  altogether  without  its  votaries ;  nor  have  we  been  able  to  ascertain 
that  any  series  of  observations  conducted  on  scientific  principles,  has  ever 
been  made  on  the  phenomena  of  insanity,  with  a  view  to  disprove  this  con- 
nexion. We  have  even  met  with  intelligent  and  well-educated  physicians  who 
still  maintain  that  the  paroxysms  of  insane  patients  are  more  violent  when  the 
moon  is  full  than  at  other  times. 

Mathiolus  Faber  gives  an  instance  of  a  maniac  who  at  the  very  moment  of 
an  eclipse  of  the  moon,  became  furious,  seized  upon  a  sword,  and  fell  upon 
every  one  around  him.  Ramazzini  relates  that,  in  the  epidemic  fever  which 
spread  over  Italy  in  the  year  1693,  patients  died  in  an  unusual  number  on  the 
21st  of  January,  at  the  moment  of  a  lunar  eclipse. 

Without  disputing  this  fact  (to  ascertain  which,  however,  it  would  be  neces- 
sary to  have  statistical  returns  of  the  daily  deaths),  it  may  be  objected  that  the 
patients  who  thus  died  in  such  numbers  at  the  moment  of  the  eclipse,  might 
have  had  their  imaginations  highly  excited,  and  their  fears  wrought  upon  by 
the  approach  of  that  event,  if  popular  opinion  invested  it  with  danger.  That 
such  an  impression  was  not  unlikely  to  prevail  is  evident  from  the  facts  which 
have  been  recorded. 

At  no  very  distant  period  from  that  time,  in  August,  1654,  it  is  related  that 
patients   in  considerable  numbers  were  by  order  of  the  physicians  shut  up  in 
chambers  well  closed,  warmed,  and  perfumed,  with  a  view  to  escape  the   in- 
jurious influence  of  the  solar  eclipse,  which  happened  at  that  time  ;  and  such 
was  the  consternation  of  persons  of  all  classes,  that  the  numbers  who  flocked  , 
to  confession  were  so  great  that  the  ecclesiastics  found  it  impossible  to  admin-  | 
ister  that  rite.     An  amusing  anecdote  is  related  of  a  village  curate  near  Paris, 
who,  with  a  view  to  ease  the   minds   of  his  flock,  and  to  gain  the  necessary 


478 


LUNAR  INFLUENCES. 


time  to  get  through  his  business,  seriously  assured  them  that  the  eclipse  was  ) 
postponed  for  a  fortnight. 

Two  of  the  most  remarkable  examples  recorded  of  the  supposed  influence 
of  the  moon  on  the  human  body,  are  those  of  Vallisnieri  and  Bacon.  Vallis- 
nieri  declares  that  being  at  Padua  recovering  from  a  tedious  illness,  he  suffered 
on  the  12th  of  May,  1706,  during  the  eclipse  of  the  sun,  unusual  weakness 
and  shivering.  Lunar  eclipses  never  happened  without  making  Bacon  faint ; 
and  he  did  not  recover  his  senses  till  the  moon  recovered  her  light. 

That  these  two  striking  examples  should  be  admitted  in  proof  of  the  ex- 
istence of  lunar  influence,  it  would  be  necessary,  says  M.  Arago,  to  establish 
the  fact  that  feebleness  and  pusillanimity  of  character  are  never  connected 
with  high  qualities  of  mind. 

Menuret  considered  that  cutaneous  maladies  had  a  manifest  connexion  with 
the  lunar  phases.  He  says  that  he  himself  observed  in  the  year  1760,  a  pa- 
tient afflicted  with  a  scald  head  (teigne),  who,  during  the  decline  of  the  moon, 
suffered  from  a  gradual  increase  of  the  malady,  which  continued  until  the 
epoch  of  the  new  moon,  when  it  had  covered  the  face  and  breast,  and  produced 
insufferable  itching.  As  the  moon  increased,  these  symptoms  disappeared  by 
degrees ;  the  face  became  free  from  the  eruption ;  but  the  same  effects  were 
reproduced  after  the  full  of  the  moon.  These  periods  of  the  disease  continued 
for  three  months. 

Menuret  also  stated  that  he  witnessed  a  similar  correspondence  between 
the  lunar  phases  and  the  distemper  of  the  itch;  but  the  circumstances  were 
the  reverse  of  those  in  the  former  case  ;  the  malady  obtaining  its  maximum  at 
the  full  of  the  moon,  and  its  minimum  at  the  new  moon. 

Without  disputing  the  accuracy  of  these  statements,  or  throwing  any  sus- 
picion on  the  good  faith  of  the  physician  who  has  made  them,  we  may  observe 
that  such  facts  prove  nothing  except  the  fortuitous  coincidence.  If  the  rela- 
tion of  cause  and  effect  had  existed  between  the  lunar  phases  and  the  phe- 
nomena of  these  distempers  the  same  cause  would  have  continued  to  produce 
the  same  effect  in  like  circumstances  ;  and  we  should  not  be  left  to  depend  for 
the  proof  of  lunar  influence  on  the  statements  of  isolated  cases,  occurring  under 
the  observation  of  a  physician  who  was  hi-mself  a  believer. 

Maurice  Hoffman  relates  a  case  which  came  under  his  own  practice,  of  a 
young  woman,  the  daughter  of  an  epileptic  patient.  The  abdomen  of  this  girl 
became  inflated  every  month  as  the  moon  increased,  and  regularly  resumed  its 
natural  form  with  the  decline  of  the  moon. 

Now  if  this  statement  of  Hoffman  were  accompanied  by  all  the  necessary 
details,  and  if,  also,  we  were  assured  that  this  strange  effect  continued  to  be 
produced  for  any  considerable  length  of  time,  the  relation  of  cause  and  effect 
between  the  phases  of  the  moon  and  the  malady  of  the  girl  could  not  legiti- 
mately be  denied ;  but  receiving  the  statement  in  so  vague  a  form,  and  not 
being  assured  that  the  effect  continued  to  be  produced  beyond  a  few  months, 
the  legitimate  conclusion  at  which  we  must  arrive  is,  that  this  is  another  ex- 
ample of  fortuitous  coincidence,  and  may  be  classed  with  the  fulfilment  of 
dreams,  prodigies,  &c.,  &c. 

As  may  naturally  be  expected,  nervous  diseases  are  those  which  have  pre- 
sented the  most  frequent  indications  of  a  relation  with  the  lunar  phases.  The 
celebrated  Mead  was  a  strong  believer,  not  only  in  the  lunar  influence,  but  in 
the  influence  of  all  the  heavenly  bodies  on  all  the  human.  He  cites  the  case 
of  a  child  who  always  went  into  convulsions  at  the  moment  of  full  moon. 
Pyson,  another  believer,  cites  another  case  of  a  paralytic  patient  whose  tl 
was  brought  on  by  the  new  moon.  Menuret  records  the  case  of  an  epileptic 
patient  whose  fits  returned  with  the  full  moon.  The  transactions  of  learned  | 


LUNAR  INFLUENCES.  479 


societies  abound  with  examples  of  giddiness,  malignant  fever,  somnambulism, 
&c.,  having  in  the.ir  paroxysms  more  or  less  corresponded  with  the  lunar 
phases.  Gall  states,  as  a  matter  having  fallen  under  his  own  observation,  that 
patients  suffering  under  weakness  of  intellect,  had  two  periods  in  the  month 
of  peculiar  excitement ;  and  in  a  work  published  in  London  so  recently  as 
1829,  we  are  assured  that  these  epochs  are  between  the  new  and  full  moon. 

Against  all  these  instances  of  the  supposed  effect  of  lunar  influence,  we  have 
little  direct  proof  to  offer.  To  establish  a  negative  is  not  easy.  Yet  it  were 
to  be  wished  that  in  some  of  our  great  asylums  for  insane  patients,  a  register 
•should  be  preserved  of  the  exact  times  of  the  access  of  all  the  remarkable 
paroxysms ;  a  subsequent  comparison  of  this  with  the  age  of  the  moon  at  the 
time  of  their  occurrence  would  furnish  the  ground  for  legitimate  and  safe  con- 
elusions.  We  are  not  aware  of  any  scientific  physician  who  has  expressly 
directed  his  attention  to  this  question,  except  Dr.  Olbers  of  Bremen,  celebrated 
for  his  discovery  of  the  planets  Pallas  and  Vesta.  He  states  that  in  the  course 
of  a  long  medical  practice,  he  was  never  able  to  discover  the  slightest  trace  of 
any  connexion  between  the  phenomena  of  disease  and  the  phases  of  the  moon. 
In  the  spirit  of  true  philosophy,  M.  Arago,  nevertheless,  recommends  caution 
in  deciding  against  this  influence.  The  nervous  system,  says  he,  is  in  many 
instances  an  instrument  infinitely  more  delicate  than  the  most  subtle  apparatus 
of  modern  physics.  Who  does  not  know  that  the  olfactory  nerves  inform  us 
of  the  presence  of  odoriferous  matter  in  air,  the  traces  of  which  the  most  re- 
fined physical  analysis  would  fail  to  detect  ?  The  mechanism  of  the  eye  is 
<  highly  affected  by  that  lunar  light  which,  even  condensed  with  all  the  power 
>  of  the  largest  burning  lenses,  fails  to  affect  by  its  heat  the  most  susceptible  ( 
thermometers,  or,  by  its  chemical  influence,  the  chloride  of  silver ;  yet  a  small 
portion  of  this  light  introduced  through  a  pin-hole  will  be  sufficient  to  produce 
an  instantaneous  contraction  of  the  pupil ;  nevertheless  the  integuments  of  this 
membrane,  so  sensible  to  light,  appear  to  be  completely  inert  when  otherwise 
affected.  The  pupil  remains  unmoved,  whether  we  scrape  it  with  the  point  of 
a  needle,  moisten  it  with  liquid  acids,  or  impart  to  its  surface  electric  sparks. 
The  retina  itself,  which  sympathizes  with  the  pupil,  is  insensible  to  the  influ- 
ence of  the  most  active  mechanical  agents.  Phenomena  so  mysterious  should 
teach  us  with  what  reserve  we  should  reason  on  analogies  drawn  from  experi- 
ments made  upon  inanimate  substances,  to  the  far  different  and  more  difficult 
case  of  organized  matter  endowed  with  life. 

In  conclusion,  then,  it  appears  that  of  all  the  various  influences  popularly 
supposed  to  be  exerted  on  the  surface  of  the  earth,  few  have  any  foundation  in 
fact.  The  precession  of  the  equinoxes,  the  accumulated  effect  of  which  ren- 
dered necessary  the  alteration  of  the  calendar,  which  produced  the  distinction  be- 
tween the  old  and  new  style,  is  a  consequence  of  the  moon's  attraction  combined 
with  that  of  the  sun  upon  the  protuberant  matter  around  the  equatorial  parts  of 
the  earth  ;  and  the  nutation  of  the  earth's  axis,  and  the  consequent  periodical 
change  of  the  obliquity  of  the  ecliptic,  is  an  effect  due  to  the  same  cause.  I 
have  on  another  occasion  shown  that  the  tides  of  the  ocean  are  real  effects 
also  arising  from  the  combined  attractions  of  the  moon  and  sun,  but  chiefly  of 
the  former. 

The  precession  of  the  equinoxes  is  a  progressive  annual  change  in  the  posi- 
tion of  those  points  on  the  firmament  where  the  centre  of  the  sun  crosses  the 
(  equator  on  the  21st  of  March  and  the  21st  of  September.     It  has  been  ascer- 
J  tained  by  observation,  and  verified  by  theory,  that  these  points  move  annually 
<,  on  the  ecliptic  with  a  slow  motion  in  a  contrary  direction  to  the  apparent  mo-  I 
*  tion  of  the  sun  ;  in  consequence  of  which  the  sun,  after  each  revolution  of  the  J 
^  ecliptic,  meets  these  points  before  that  revolution  has  been  completed  ;  conse-  ( 


480 


LUNAR  INFLUENCES. 


quently  the  sun's  centre  returns  to  the  same  equinoctial  point  be-fore  it  makes 

one  complete  revolution  of  the  heavens  :  hence  has  arisen  the  distinction  be- 

l  tween  a  sidereal  year,  which  is  the  actual  time  the  earth  takes  to  make  a  com- 

)  plete  revolution  round  the  sun,  and  an  equinoctial  or  civil  year,  which  is  the 

period  between  the  successive  returns  of  the  centre  of  the  sun  to  the  same 

equinoctial  point,  and  is  the  interval  within  which  the  periodical  vicissitudes 

of  the  seasons  are  completed 


PHYSICAL  CONSTITUTION  OF  COMETS. 


Orbitual  Motion  of  Comets. — Their  Number. — Their  Light. — Explanati(Mi  of  thu — Theory  of  Her 
schel. — Constitution  of  Comet*. — Nebulosity.— Nucleus. — Tail — Cornels  of  1811 — 1680 — 1769 — 
1744—1843—1844. 


31 


PHYSICAL  CONSTITUTION  OF  COMET8. 


483 


PHYSICAL  CONSTITUTION  OF  COMETS. 


OF  a.l  the  objects  which  attract  attention  in  the  heavens,  none  have  excited 
feelings  of  greater  awe,  or  awakened  sentiments  of  more  intense  curiosity,  than 
comets.  What  are  these  bodies  ?  or  are  they  bodies  at  all  ?  What  is  their 
character  and  constitution  ?  Whence  do  they  derive  their  light  ?  Do  they  be- 
long to  our  system  ?  Whence  have  they  come,  and  whither  do  they  go  I  Are 
they,  as  was  long  believed,  of  the  same  class  as  the  aurora  borealis  ?  Although 
much  still  remains  to  be  discovered  before  full,  clear,  and  definite  answers  can 
be  given  to  these  and  similar  questions,  yet  much  that  is  interesting  has  been 
ascertained  by  the  labors  chiefly  of  contemporary  astronomers.  We  shall,  on 
"the  present  occasion,  present  what  is  certainly  known  in  as  brief  a  space  as 
possible. 

ORBITUAL    MOTIONS    OF    COMETS. 

Comets  are  attached  to  the  solar  system  by  the  tie  of  gravitation,  and  in  their 
motions  round  the  sun  are  governed  by  the  same  law  of  attraction,  as  that 
which  operates  on  the  planets.  Since  they  are  susceptible  of  gravitation,  they 
must  therefore  be  material. 

In  their  motions,  however,  they  present  circumstances  strikingly  different 
from  those  which  characterize  the  planets.  The  law  of  gravitation  determines 
nothing  regarding  the  orbit  of  a  body  in  moving  round  the  sun,  except  that  it 
be  one  or  other  of  those  curves  called  conic  sections,  and  that  the  place  of  the 
sun  shall  be  ihe  focus  of  the  curve.  Subject  to  this  restriction,  the  orbit  of  a 
revolving  body  may  be  very  various  in  magnitude,  form,  position,  and  direction. 
The  orbits  of  the  planets  are,  nevertheless,  all  very  nearly  of  the  same  form, 
being  all  nearly  circular,  and  all  in  the  same  position,  being  all  very  nearly  in 
the  plane  of  the  ecliptic;  and  they  all  move  in  the  same  direction,  being  that  of 
the  annual  motion  of  the  earth.  The  comets  observe  none  of  these  charac- 
teristics in  their  orbitual  motions.  Their  orbits  vary  indefinitely  in  form.  None 


484 


PHYSICAL  CONSTITUTION  OP  COMETS. 


are  circular,  or  even  nearly  so.  Some  are  ovals  of  various  eccentricity.  Some 
are  either  parabolas,  or  ellipses  of  such  extreme  eccentricity  as  to  be  undistin- 
guishable  from  parabolas  by  any  observations  we  have  been  enabled  to  make 
upon  them.  Others,  again,  seem  to  move  in  hyperbolas. 

The  magnitudes  of  the  planetary  orbits  increase  regularly,  according  to  a 
certain  harmonious  proportion.  No  order  or  regularity  is  discoverable  among 
the  magnitudes  of  the  cometary  orbits. 

The  orbits  of  comets  are  not  confined  to  the  plane  of  the  ecliptic  :  they  are 
found  to  be  at  every  possible  angle  with  it  from  0°  to  90°.  Arago  has  exam- 
ined the  position  of  the  orbits  of  a  great  number  of  comets,  and  has  found  that 
an  equal  number  move  at  every  inclination  with  the  ecliptic. 

Unlike  planets,  comets  do  not  move  in  one  uniform  direction  round  the  sun. 
Some  move  in  the  same  direction  as  the  earth,  and  some  in  the  opposite  direc- 
tion. There  are  about  as  many  retrogade  as  direct. 

Such  are  the  chief  circumstances  which  distinguish  the  motions  of  the  com- 
ets from  those  of  the  planets. 


NUMBER    OF    COMETS. 

The  determination  of  the  number  of  comets  connected  with  our  system  is  a 
question  which,  although  not  admitting  of  a  demonstrative  solution,  may  be 
solved  upon  grounds  of  a  high  degree  of  probability ;  and  it  is  one  of  so  much 
interest,  that  we  are  induced  here  to  lay  before  our  readers  the  views  of  M.  Ara- 
go and  others  on  this  point. 

The  total  number  of  distinct  comets,  whose  paths  during  the  visible  parts  of 
their  course  had  been  ascertained  up  to  the  year  1832,  was  one  hundred  and 
thirty-seven.  In  order  to  discover  whether  bodies  of  this  nature  prevail  more 
in  any  particular  regions  of  space  than  in  others — whether,  like  the  planets, 
they  crowd  into  a  particular  plane,  or  are  distributed  through  the  universe  with- 
out any  preference  of  any  one  region  to  any  other — it  was  necessary  to  exam- 
ine and  compare  the  paths  of  these  hundred  and  thirty-seven  bodies.  After  a 
close  examination  of  the  planes  of  their  orbits  with  respect  to  that  of  the  earth, 
it  appears  that  the  numbers  inclined  at  various  angles,  from  0°  to  90°,  is  pretty 
nearly  the  same.  Thus,  at  angles  between  80°  and  90°  there  are  fifteen  com- 
ets ;  while  at  angles  between  10°  and  20°  there  are  thirteen  ;  and  between  30° 
and  40°  there  are  seventeen.  Again,  the  points  where  they  pass  through  the 
plane  of  the  earth's  orbit  are  found  to  be  uniformly  distributed  in  every  direc- 
tion around  the  sun.  The  points  where  they  pass  nearest  to  the  sun  are  like- 
wise distributed  uniformly  round  that  body.  Their  least  distances  from  the  sun 
also  vary  in  such  a  manner  as  leads  to  the  supposition  of  their  uniform  distri- 
bution through  space.  Thus,  if  we  suppose  a  globe,  of  which  the  sun  is  the 
centre,  to  pass  through  the  orbit  of  Mercury,  so  as  to  enclose  the  space  round 
the  sun,  extending  a  distance  on  every  side  equal  to  the  distance  of  Mercury, 
thirty  of  the  ascertained  comets,  when  at  their  least  distance  from  the  sun,  pass 
within  that  globe.  Between  that  globe  and  a  similar  one  through  the  orbit  of 
Venus,  forty-four  comets  pass  under  like  circumstances.  Between  the  latter 
globe  and  a  like  one  through  the  orbit  of  the  earth,  thirty-four  pass.  Between 
the  globe  through  the  orbit  of  the  earth  and  one  through  the  orbit  of  Mars, 
twenty-three  pass  ;  and  between  the  latter  and  a  globe  through  the  orbit  of  Ju- 
piter, six  pass.  No  comet  has  ever  been  visible  beyond  the  orbit  of  Jupiter. 
It  must  be  here  observed,  that  beyond  the  orbit  of  Mars  it  is  extremely  difficult 
to  discern  comets ;  and  this  may  account  for  the  comparatively  small  number 
of  ascertained  comets  which  do  not  come  nearer  to  the  sun  than  that  limit  A 


PHYSICAL  CONSTITUTION  OP  COMETS. 


SyN^Oa^^v 

485 


comparison  of  the  above  numbers  with  the  spaces  included  between  these  suc- 
cessive imaginary  globes,  and  with  the  relative  facility  or  difficulty  of  discern- 
ing comets  in  the  different  situations  thus  assigned,  leads  to  a  demonstration  ! 
that,  so  far  as  these  hundred  and  thirty-seven  observed  c»mets  can  be  consid- 
ered as  an  indication  of  the  general  distribution  of  comets  through  space,  that 
distribution  ought  to  be  regarded  as  uniform  ;  that  is,  an  equal  number  of  com- 
ets have  their  least  distances  included  in  equal  portions  of  space. 

Adopting,  then,  this  conclusion,  M.  Arago  reasons  in  the  following  manner: 
>  The  number  of  ascertained  comets  which,  at  their  least  distances,  pass  within 
1  the  orbit  of  Mercury  is  thirty.  Now,  our  most  remote  planet,  Herschel,  is 
forty-nine  times  more  distant  from  the  sun  than  Mercury ;  consequently,  a 
globe,  of  which  the  sun  is  the  centre,  and  whose  surface  would  pass  through 
the  orbit  of  Herschel,  would  include  a  space  greater  than  a  similar  globe 
through  the  orbit  of  Mercury,  in  the  proportion  of  the  cube  of  forty-nine  to  one. 
Assuming  the  uniform  distribution  of  comets,  it  will  follow  that,  for  every  com- 
et included  in  a  globe  through  the  orbit  of  Mercury  when  at  its  least  distance, 
there  will  be  a  hundttd  and  seventeen  thousand  six  hundred  and  forty-nine 
comets  similarly  included  within  the  globe  through  the  orbit  of  Herschel.  But 
as  there  are  thirty  ascertained  to  be  within  the  former  globe,  there  will,  there- 
fore, be  three  millions  five  hundred  and  twenty-nine  thousand  four  hundred  arid 
seventy  within  the  orbit  of  Herschel. 

Thus  it  appears  that,  supposing  no  comet  ranging  within  the  limits  of  Mer- 
cury has  escaped  observation,  that  portion  of  space  enclosed  within  the  globe 
through  Herschel  must  be  swept  by  at  least  three  millions  and  a  half  of  comets. 
But  there  can  be  no  doubt  that  many  more  than  thirty  comets  pass  within  the 
globe  through  Mercury ;  for  it  would  be  contrary  to  all  probability  to  assume 
that,  notwithstanding  the  many  causes  obstructing  the  discovery  of  comets,  and 
the  short  time  during  which  we  have  possessed  instruments  adequate  to  such 
an  inquiry,  we  should  have  discovered  all  the  comets  ranging  within  that  limit. 
It  is,  therefore,  more  probable  that  seven  millions  of  comets  are  enclosed  within 
the  known  limits  of  the  system  than  the  lesser  number  !  Such  is  the  astound- 
ing conclusion  to  which  M.  Arago's  reasoning  leads. 

LIGHT    OF    COMETS. 

The  light  of  comets  is  an  effect  of  which  astronomers  have  hitherto  given 
no  satisfactory  account.  If  any  of  these  bodies  had  been  observed  to  have 
exhibited  phases  like  those  of  the  moon  and  the  inferior  planets,  the  fact  of 
their  being  opaque  bodies,  illuminated  by  the  sun,  would  be  at  once  establish- 
ed. But  the  existence  of  such  phases  must  necessarily  depend  upon  the  come* 
itself  being  a  solid  mass.  A  mere  mass  of  cloud  or  vapor,  though  not  self-lu- 
minous, but  rendered  visible  by  borrowed  light,  would  still  exhibit  no  effect  of 
this  kind  :  its  imperfect  opacity  would  allow  the  solar  light  to  affect  its  con- 
stituent parts  throughout  its  entire  depth — so  that,  like  a  thin  fleecy  cloud,  it 
would  appear  not  superficially  illuminated,  but  receiving  and  reflecting  light 
through  all  its  dimensions.  With  respect  to  comets,  therefore,  the  doubt  which 
has  existed  is,  whether  the  light  which  proceeds  from  them,  and  by  which 
they  become  visible,  is  a  light  of  their  own,  or  is  the  light  of  the  sun  shining 
upon  them,  and  reflected  to  our  eyes  like  light  from  a  cloud.  For  a  long  peri- 
od this  question  was  sought  to  be  determined  by  the  discovery  of  phases.  M. 
Arago  then  proceeded  to  apply  to  the  question  a  very  elegant  mode  of  investi- 
gation, depending  on  a  property*  by  which  reflected  light  may  be  distinguished 

*  Polarization. 


486 


PHYSICAL  CONSTITUTION  OF  COMETS. 


from  direct  light,  and  the  existence  of  which  property  there  are  sufficient  opti- 
cal means  of  detecting.  He  has,  however,  more  recently  furnished  us  with, 
as  we  conceive,  much  more  simple  and  satisfactory  means  of  putting  the  ques- 
tion finally  at  rest  ;  jf,  indeed,  it  be  not  already  decided. 

It  is  an  established  property  of  self-shining  bodies,  that  at  all  distances  from 
the  eye  they  have  the  same  apparent  splendor.  Thus  the  sun,  as  seen  from  the 
planet  Herschel,  seems  as  bright  as  when  seen  from  the  earth.  It  is  true  that 
he  is  much  smaller,  but  stiil  equally  bright.  The  smallest  brilliant  may  be  as 
bright  as  the  largest  diamond.  We  must  not  here  be  understood  to  imply  that 
he  affords  the  same  light ;  that  is  quite  another  effect.  W'hat  is  intended  to 
be  conveyed,  will  perhaps  be  best  understood  by  considering  the  effect  of 
viewing  the  sun  through  a  pin-hole  made  in  a  card.  The  card  being  placed  at 
a  small  distance  from  the  eye,  it  is  evident  that  the  eye  will  view  only  a  small 
portion  of  the  sun's  disk,  limited  by  the  magnitude  of  the  pin-hole ;  but  that 
portion,  so  far  as  it  goes,  will  be  as  bright  as  it  would  be  were  the  card  remov- 
ed. Now,  the  effect  here  produced,  by  limiting  the  portion  of  the  sun's  disk 
which  the  eye  is  permitted  to  see,  is  precisely  the  sam^  as  if  the  eye  were 
carried  to  so  great  a  distance  from  the  sun,  that  its  apparent  magnitude  would 
be  reduced  to  equality  with  that  portion  of  its  disk  which  is  seen  through  the 
hole  in  the  card.* 

Now,  applying  this  principle  to  the  question  of  cometary  light,  it  will  follow 
that,  if  a  comet  shines  by  light  of  its  own,  and  not  by  light  received  from  the 
sun,  it  will,  like  all  other  self-luminous  bodies,  have  the  same  apparent  bright- 
ness at  all  distances.  It  will,  therefore,  coase  to  be  visible,  not  from  want  of 
sufficient  apparent  brightness,  but  from  want  of  sufficient  visual  magnitude. 
Now,  it  may  be  shown  that  the  limit  of  visual  magnitude  which  would  cause 
the  disappearance  of  a  self-luminous  body  is  so  extreme,  that  it  would  be  to- 
?  tally  inapplicable  to  this  case.  By  varying  the  magnitude  of  the  object-glass 
)  of  a  telescope  (which  may  be  easily  done),  with  which  such  a  body  is  viewed, 
(  in  proportion  to  the  magnifying  power  of  the  eye-glass,  it  is  always  possible  to 
make  the  image  of  the  same  apparent  brightness  ;  that  is,  supposing  the  object 
itself  to  maintain  a  uniform  splendor.  Consequently,  if  a  body  submitted  to 
this  species  of  observation,  cease  to  be  visible  even  by  a  telescope,  it  will  fol- 
low, that  it  must  disappear  either  by  a  very  extreme  diminution  of  visual  mag- 
nitude, or  by  the  loss  of  its  own  intrinsic  splendor.  Now,  to  apply  this  test  to 
the  question  of  comets.  Let  us  ask  in  what  manner  they  disappear  ?  Is  their 
disappearance  the  consequence  of  an  excessive  diminution  of  visual  magnitude  ? 
or  is  it  to  be  attributed  to  the  diminished  quantity  of  light  which  they  transmit? 
Every  astronomer  will  immediately  reply  that  the  latter  only  can  cause  the 
disappearance.  The  greater  number  of  comets,  including  the  most  brilliant 
and  remarkable  one  of  1680  more  especially,  have  obviously  disappeared  by 
the  gradual  enfeeblement  of  their  light.  They  were,  as  it  were,  extinguished. 
At  the  very  time  they  ceased  to  be  visible,  they  possessed  considerable  visual 
magnitude.  But  such  a  mode  of  disappearance  is  incompatible  with  the  char- 
acter of  a  self-luminous  body,  unless  we  suppose  that,  from  some  physical 
cause,  it  gradually  loses  its  luminosity. 

But  in  answer  to  this  is  adduced  the  observed  fact,  that  the  dimensions  of 
comets  are  enlarged  as  they  recede  from  the  sun  ;  that  the  luminous  matter, 
thus  existing  in  a  less  condensed  state,  will  shine  with  a  proportionally  enfee- 
bled splendor ;  and  that  at  length,  by  the  dilation  of  the  body,  the  light  be- 
comes so  dilute,  that  it  is  incapable  of  affecting  the  retina  so  as  to  produce 
sensation. 

*  This  property  is  demonstrable  by  mathematical  reasoning. 


In  answer  to  this  objection,  M.  Arago  has  submitted  to  examination  the  rate 
at  which  comets  increase  their  dimensions  as  they  recede  from  the  sun,  ao- 
cording  to  Valz ;  and  calculates  the  corresponding  diminution  of  intrinsic 
splendor  which  would  arise  from  such  a  cause.  The  question  then  is,  wheth- 
er, by  such  a  diminution  of  splendor,  the  brightest  comets  would  be  invisible 
beyond  the  orbit  of  Jupiter  ?  This  question  he  proposes  to  decide  by  the  fol- 
lowing experimental  test,  to  be  applied  to  some  future  comet. 

Let  a  telescope  be   selected  having  a  large  opening  and  low  magnifying 
power,  by  the  aid  of  which  the  comet  may  be  observed  in  every  part  of  its 
visible  course.     Let  the  body  be  observed  with  this  instrument  at  some  deter- 
minate distance  from  the  sun,  such  as,   for  example,  the  distance  of  Venus. 
M.  Arago  shows  how,  by  applying  different  magnifying  powers  to  the  teles- 
cope under  these  circumstances,  the  image  of  the  comet  may  be  made  to  as- 
sume different  degrees  of  brightness.     He  shows,  also,  how  the  magnifying 
power  may  be  regulated,  so  as  to  exhibit  the  image  of  the  comet  with  just  that  ( 
degree  of  brightness  with  which  it  would  appear  at  any  given  increased  dis-  < 
tance  to  the  lowest  magnifying  power ;  on  the  supposition  of  its  being  a  self-  5 
shining  body,  losing  brightness  by  reason  of  the  enlargement  of  its  dimensions.  ' 
In  this  way,  he  shows  that  the  actual  brightness  which  the  comet  ought  to  have  \ 
at  any  given  distance  from  the  sun,  when  looked  at  with  any  given  magnifying  j 
power,  may  be  predicted.     He  proposes,  then,  that,  this  observation  being  pre-  ) 
viously  made,  the  comet  should  be  observed  subsequently  at  the  proposed  dis-  * 
tances.     If  it  appear  with  that  degree  of  brightness  which  it  ought  to  have  in  } 
correspondence  with  such  previous  observations,  then  there  will  be  a  presump- 
tion that  it  shines  with  its  own  light.     But  if,  as  is  probable,  and  perhaps  near- 
ly certain,  the  splendor  of  the  comet  at  increased  distances  will  be  greatly  less 
than  it  ought  to  be,  and  that  it  will  be  wholly  invisible  at  distances  at  which  it 
ought  to  be  seen,  then  there  will  be  conclusive  proof  that  it  is  a  body  not  self- 
luminous,  but.  one  which  derives  its  light  from  the  sun ;  and  that  its  disappear- 
ance, when  removed  to  any  considerable  distance  from  that  luminary,  arises 
from  the  extreme  faintness  of  the  light  which  its  attenuated  matter  reflects. 

It  will,  of  course,  be  perceived,  that  the  enlargement  of  the  volume  of  the 
comet  will  produce  a  diluting  effect  upon  its  reflected  light,  as  much  as  it 
would  if  it  shone  with  direct  light ;  and  this  furnishes  an  additional  reason  for 
its  rapid  disappearance  as  it  recedes  from  the  sun. 

It,  will  doubtless  excite  surprise,  that  the  dimensions  of  a  comet  should  be 
enlarged  as  it  recedes  from  the  source  of  heat.  It  has  been  often  observed  in 
astronomical  inquiries,  that  the  effects,  which  at  first  view  soem  most  improba- 
ble, are  nevertheless  those  which  frequently  prove  to  be  true  ;  and  so  it  is  in 
this  case.  It  was  long  believed  that  comets  enlarged  as  they  approached  the 
sun  ;  and  this  supposed  effect  was  naturally  and  probably  ascribed  to  the  heat 
of  the  sun  expanding  their  dimensions.  But  more  recent  and  exact  observa- 
tions have  shown  the  very  reverse  to  be  the  fact.  Comets  increase  their  volume 
as  they  recede  from  the  sun  ;  and  this  is  a  law  to  which  there  appears  to  be  no 
well-ascertained  exception.  This  singular  and  unexpected  phenomenon  has 
been  attempted  to  be  accounted  for  in  several  ways.  Valz  ascribed  it  to  the 
pressure  of  the  solar  atmosphere  acting  upon  the  comet ;  that  atmosphere,  being 
more  dense  near  the  sun,  compressed  the  comet  and  diminished  its  dimensions  ; 
and,  at  a  greater  distance,  being  relieved  from  this  coercion,  the  body  swelled 
to  its  natural  bulk.  A  very  ingenious  train  of  reasoning  was  produced  in  sup- 
port of  this  theory.  The  density  of  the  solar  atmosphere  and  the  elasticity  ot 
the  comet  being  assumed  to  being  such  as  they  might  naturally  be  supposed, 
the  variations  of  the  comet's  bulk  were  deduced  fay  strict  reasoning,  and  showed 
a  surprising  coincidence  with  the  observed  change  in  the  dimensions.  But 


488 


PHYSICAL  CONSTITUTION  OF  COMETS. 


this  theory  is  tainted  by  a  fatal  error.  It  proceeds  upon  the  supposition  that 
the  comet,  in  the  one  hand,  is  formed  of  an  elastic  gas  or  vapor ;  and,  on  the 
other,  that  it  is  impervious  to  the  solar  atmosphere  through  which  it  moves.  / 
To  establish  the  theory,  it  would  be  necessary  to  suppose  that  the  elastic  fluid 
composing  the  comet  should  be  surrounded  by  a  nappe  or  envelope  as  elastic  as 
the  fluid  composing  the  comet,  and  yet  wholly  impenetrable  by  the  solar  at- 
mosphere. 

Several  solutions  of  this  phenomenon  have  been  proposed  by  Sir  John  Her- 
schel  :*  one  is,  that  the  comet  consists  of  a  cloud  of  particles,  which  either 
have  no  mutual  cohesion,  or  none  capable  of  resisting  their  solar  gravita- 
tion ;  that,  therefore,  these  particles  move  round  the  sun  as  separate  and  inde- 
pendent planets,  each  describing  an  ellipsis  or  parabola,  as  the  case  may  be. 
If  this  be  admitted,  it  is  demonstrable  on  geometrical  principles,  and,  indeed, 
it  follows  as  a  necessary  consequence  of  the  principle  of  gravitation,  that  the 
particles  thus  independently  moving,  must  converge  as  they  approach  the  sun, 
so  as  to  occupy  a  more  limited  space,  and  to  become  condensed ;  and  that  on 
receding  from  the  sun,  they  will  again  diverge  and  occupy  increased  dimen- 
sions. 

Herschel  insists  on  this  the  more,  because  he  conceives  it  has  the  character 
of  a  vera  causa.  The  fact  is,  the  hypothetical  part  of  it  consists,  not  in  the 
assumed  effect  of  the  gravitation  of  the  particles  of  the  comet,  but  in  the  as- 
sumption that  the  mutual  cohesion  or  mutual  gravitation  of  these  particles  is  a 
quantity  evanescent  in  comparison  with  their  separate  gravitation  toward  the 
sun.  This  can  scarcely  be  ranked  as  anything  but  a  supposition  assumed  to 
account  for  the  phenomena. 

Another  theory  proposed  by  Sir  John  Herschel,  which  indeed  is  not  al- 
together incompatible  with  the  simultaneous  operation  of  the  former  cause,  iar 
that  the  nebulous  portion  of  the  comet,  or  that  portion  which  reflects  the  sun's 
rays,  is  of  the  nature  of  a  fog,  or  a  collection  of  discrete  particles  of  a  vapor - 
izable  fluid  floating  in  a  transparent  medium  ;  similar,  for  example,  to  the  cloud 
of  vapor  which  appears  at  some  distance  from  the  spout  of  a  boiling  kettle. 
Now,  since  these  molecules,  during  the  comet's  approach  to  the  sun,  absorb  its 
rays  and  become  heated,  a  portion  of  them  will  be  constantly  passing  from  the 
liquid  to  the  gaseous  or  invisible  state.  As  this  change  must  commence  from 
without,  and  must  be  propagated  inward,  the  effect  will  be  a  diminution  of  the 
comet's  visible  bulk.  On  the  other  hand,  as  it  retreats  from  the  sun,  it  will  lose 
by  radiation  the  heat  thus  acquired ;  which,  in  conformity  with  the  general 
analogy  of  radiant  heat,  will  escape  chiefly  from  the  unevaporated  or  nebulous 
mass  within.  The  dimensions  of  this  will  therefore  begin  and  continue  to  in- 
crease by  the  precipitation  immediately  above  it  of  fresh  nebula ;  just  as  we 
see  fogs  in.  cold  and  still  nights  forming  on  the  surface  of  the  earth,  and  grad- 
ually extending  upward  as  the  heat  near  the  surface  is  dissipated.  The  comet 
would  thus  appear  to  enlarge  rapidly  in  its  visible  dimensions,  at  the  moment 
that  its  real  volume  is  in  fact  slowly  shrinking  by  the  general  abstraction  of 
heat  from  the  mass. 

"  This  process,"  says  Sir  John  Herschel,  "  might  go  on  in  the  entire  absence 
of  any  solid  or  fluid  nucleus ;  but  supposing  such  a  nucleus  to  exist,  and  to 
have  acquired  a  considerable  increase  of  temperature  in  the  vicinity  of  t'he  sun, 
evaporation  from  its  surface  would  afford  a  constant  and  copious  supply  of  va- 
por, which,  rising  into  its  atmosphere,  and  condensing  it  at  its  exterior  parts, 
would  tend  yet  more  to  dilate  the  visible  limits  of  the  nebula.  Some  such  pro- 
cess would  naturally  enough  account  for  the  appearances  which  have  been 


*  Memoirs  Royal  Astron.  Soc.,  vol.  vi.,  p.  104. 


PHYSICAL  CONSTITUTION  OF  COMETS. 


489 


|  no'iced  in  the  head  of  certain  comets,  where  a  stratum  void  of  nebula  has  been 

>  observed,  interposed,  as  it  were,  between  the  denser  portion  of  the  head,  or 
[  nucleus,  and  the  coma.     It  is  analogous  to  the  meteorological  phenomenon  of 

>  a  definite  vapor  plane,  so  commonly  observed  ;  and  in  certain  cases,  may  admit 
[  of  two  or  more  alternations  of  nebula  and  clear  atmosphere." 

Sir  John  offers  a  third  supposition  to  account  for  the  effects,  by  attributing 
[  them  to  the  ethereal  medium  surrounding  the  sun. 

"  Fourier,"  says  he,  "  has  rendered  it  not  improbable,  that  the  region  in 
|  which  the,  earth  circulates  has  a  temperature  of  its  own  greatly  superior  to 
i  what  may  be  presumed  to  be  the  absolute  zero,  and  even  to  some  artificial  de- 
|  grees  of  cold.  I  have  shown,  I  think,  satisfactorily,  that  if  this  be  the  case, 
such  temperature  cannot  be  due  simply  to  the  radiation  of  the  stars,  but  must 
arise  from  some  other  cause,  such  as  the  contact  of  an  ether,  possessing  itself 
a  determinate  temperature,  and  tending,  like  all  known  fluids,  to  communicate 
this  temperature  to  bodies  immersed  in  it.  Now  if  we  suppose  the  tempera- 
ture of  the  ether  to  increase  as  we  approach  the  sun,  which  seems  a  natural, 
and  indeed  a  necessary  consequence,  of  regarding  it  as  endued  with  the  ordi- 
nary relations  of  fluids  to  heat,  we  are  furnished  with  an  obvious  explanation 
of  the  phenomenon  in  question.  A  body  of  such  extreme  tenuity  as  a  comet, 
may  be  presumed  to  take  very  readily  the  temperature  of  the  ether  in  which 
it  is  plunged  ;  and  the  vicissitude  of  warmth  and  cold  thus  experienced,  may 
alternately  convert  into  transparent  vapor,  and  reprecipitate  the  nebulous  sub- 
stance, just  as  we  see  an  increase  of  atmospheric  temperature  dissipate  the 
fog,  not  by  abstracting  or  annihilating  its  aqueous  particles,  but  by  causing 
them  to  assume  the  elastic  and  transparent  state  which  they  lose,  and  again 
appear  in  fog  when  the  temperature  sinks." 

CONSTITUTION    OF    THE    COMETS. 

The  word  comet  is  derived  from  a  Greek  word  signifying  hair,  and  hence 
the  name  implies  a  hairy  star.  The  nebulosity,  or  a  sort  of  illuminated  haze 
which  always  appears  around  these  bodies,  is  that  from  which  the  name  was 
probably  taken. 

The  head  of  the  comet  is  the  brightest  part  of  the  centre,  usually  supposed 
to  be  a  nucleus  something  like  that  of  a  planet ;  but  this  is  so  enveloped  in  the 
hair,  or  nebulosity,  that  it  has  never  yet  been  satisfactorily  ascertained  whether 
it  be  solid  matter. 

A  luminous  train,  varying  in  length,  is  frequently,  though  not  always,  attached 
to  these  objects.  It  has  been  generally  called  the  tail.  Sometimes  comets 
have  more  than  one  of  these  appendages. 

THE   NEBULOSITY. 

As  the  brightness  of  the  nebulosity  gradually  fades  away  toward  the  edges, 
there  is  sometimes  a  difficulty  in  measuring  its  bulk.  Its  form  is  generally 
globular,  and  its  light  is  often  so  faint  that  the  comet  can  only  be  discovered  by 
telescopes.  The  diameter  of  the  nebulous  mass  has  been  found  to  vary  from 
6,000  miles  upward.  The  comets  of  1795,  1797,  1798,  and  1804,  were  sur- 
rounded by  a  nebulosity  which  measured  less  than  7,000  miles  in  diameter. 

That  many  comets  have  no  solid  matter  in  the  centre  of  the  nebulosity  is 
proved  by  the  fact  that  the  smallest  stars  are  often  visible  through  them  ;  even 
the  ancients,  without  the  aid  of  the  telescope,  ascertained  this  fact.  Seneca 
reported  that  stars  were  discoverable  through  comets,  although  he  does  not 
distinctly  state  through  what  part  of  the  comet  they  were  seen.  Sir  William 


490 


PHYSICAL  CONSTITUTION  OF  COMETS. 


Herschel,  however,  distinctly  saw  a  star  of  the  16th  magnitude  through  the 
very  centre  of  the  head  of  the  comet  which  appeared  in  the  year  1795.  Prof. 
Struve,  on  the  28th  of  Nov.,  1828,  saw  a  star  of  the  llth  magnitude,  so  small 
as  to  be  invisible  to  the  naked  eye,  through  the  centra  of  Encke's  comet. 

The  parts  of  the  nebulosity  which  immediately  surround  the  nucleus  appear 
to  be  much  less  luminous  than  the  more  distant  parts,  as  if  the  nebulous  atmo- 
sphere became  less  dense  and  more  transparent  near  its  surface.  At  some  dis- 
tance from  its  centre  the  luminous  effect  suddenly  increases  so  as  to  assume  the 
appearance  of  rings  of  light  aromid  the  nucleus  ;  sometimes  two,  three,  or 
more,  such  concentric  rings  have  been  perceived  surrounding  comets,  separated 
by  dark  intervals. 

It  must  be  understood,  that  the  arrangement  which  produces  the  appearance 
of  these  concentric  rings,  is.  in  reality,  a  succession  of  spherical  shells  of  va- 
por or  nebulous  matter,  which  alternately  increases  and  decreases  in  density, 
forming  an  atmosphere  of  various  densities  around  the  comet.  This  has  been 
illustrated  by  Arago  by  comparing  it  to  successive  layers  of  clouds  of  different 
heights  surrounding  our  globe.  To  perfect  the  analogy  we  have  only  to  im- 
agine three  transparent  spherical  shells,  still  retaining  the  peculiar  optical  quality 
which  distinguishes  them  from  the  pure  air  by  which  they  are  separated. 

The  memorable  comet  of  181]  was  enveloped  by  a  nebulosity  the  thickness 
of  which  measured  30,000  miles  above  the  surface  or  nucleus  of  the  comet. 

The  thickness  of  the  nebulosity  of  the  comet  of  1807  was  36,000  miles; 
that  of  1799  was  24,000  miles 

In  comets  which  have  a  tail,  the  rings  we  have  now  adverted  to  are  not  com- 
plete :  they  terminate  at  the  edges  of  the  tail,  and  are  open  through  the  space 
where  the  tail  abuts  upon  the  head. 

THE    NUCLEUS. 

Some  difference  of  opinion  prevails  among  observers  whether  comets  real- 
ly have  nuclei  at  all.     Wrhen,  however,  they  are  supposed  to  have  them,  they 
are  generally  admitted  to  be  small,  and  of  doubtful  magnitude.     The  following  . 
measurements  are  given  by  Arago  as  having  been  ascertained,  or,  at  least,  as-  i 
sumed  : — 

The  comet  of  1798  had  a  nucleus  whose  diameter  was  30  miles  ;  that  of 
1805,  35  miles  ;  the  comet  of  1799,  450  miles  ;  the  comet  of  1807,  650  miles  ; 
and  the  second  comet  of  1811,  about  3,000  miles. 

Those  who  deny  the  existence  of  solid  matter  within  the  nebulosity  of  comets, 
maintain  that  even  the  most  brilliant  and  most  conspicuous  of  those  bodies,  and 
those  which  have  presented  the  strongest  resemblance  to  planets,  are  complete- 
ly transparent.  It  might  be  supposed  that  a  fact  so  simple  as  this,  in  this  age 
of  astronomical  activity,  could  not  remain  doubtful ;  but  it  must  be  considered, 
that  the  combination  of  circumstances  which  alone  would  test  the  truth  of  this 
doctrine,  is  of  rare  occurrence.  It  would  be  necessary  that  the  centre  of  the 
head  of  the  comet,  although  very  small,  should  pass  critically  over  a  star,  in 
order  to  ascertain  whether  such  star  is  visible  through  it.  With  comets  having 
extensive  nebulosity  without  nuclei,  this  has  sometimes  occurred  ;  but.  we  have 
not  had  such  satis>r'ictory  examples  in  the  more  rare  instances  of  those  which 
have  distinct  nuclei.  The  following  examples  are,  however,  adduced  : — 

On  the  23d  of  October,  177  1,  Montaigne,  at  Limoges,  saw  a  star  of  the  6th 
magnitude  through  the  nucleus  of  a  small  comet ;  but,  unfortunately,  he  has 
not  stated  through  what  part,  of  the  nucleus  he  saw  it,  and  the  power  of  the 
telescope  he  used  was  too  limited  to  entitle  his  observations  to  much  consider- 
ation. 

„  '-**S*^***^ 


PHYSICAL  CONSTITUTION  OF  COMETS. 


491 


On  the  1st  of  April,  1796,  Dr.  Olbers,  at  Bremen,  saw  a  star  of  the  sixth  or 
seventh  magnitude,  and  although  it  was  covered  by  a  comet,  he  found  that  its 
light  was  not  perceptibly  diminished.  The  observer  in  this  case  did  not  feel 
sure  that  the  nucleus  was  between  the  eye  and  the  star. 

MESSIER,  when  observing  a  comet  in  1774,  saw  a  small  telescopic  star  be- 
side it,  and  having  looked  at  it  again  after  the  lapse  of  some  hours,  he  ob- 
served a  second  star  near  the  first.  He  explained  this  by  the  supposition  that 
at  the  moment  of  his  first  observation  the  nucleus  of  the  comet  concealed  the 
second  star.  • 

WARTMANN  states  that  on  the  night  of  the  28th  November,  1828,  a  star  of 
the  8th  magnitude  was  completely  eclipsed  by  Encke's  comet.  Here  again, 
however,  it  is  objected  that  Wartmann's  telescope  was  too  feeble  to  be  trusted 
in  such  an  observation. 

In  the  absence  of  a  more  decisive  test  of  the  occultation  of  a  star  by  the 
nucleus,  it  has  been  maintained  that  the  existence  of  a  solid  nucleus  may  be 
fairly  inferred  from  the  great  splendor  which  has  attended  the  appearance  of 
some  comets.  A  mere  mass  of  vapor  could  not,  it  is  contended,  reflect  such 
brilliant  light.  The  following  are  the  examples  adduced  by  Arago : — 

In  the  year  43  before  Christ,  a  comet  appeared  which  was  said  to  be  visible 
to  the  naked  eye  bv  daylight.  It  was  the  comet  which. the  Romans  considered 
to  be  the  soul  of  Caesar  transferred  to  the  heavens  after  his  assassination. 

In  the  year  1402  two  remarkable  comets  were  recorded.  The  first  was  so 
brilliant  that  the  light  of  the  sun  at  noon,  at  the  end  of  March,  did  not  prevent 
its  nucleus,  or  even  its  tail,  from  being  seen.  The  second  appeared  in  the 
month  of  June,  and  was  visible  also  for  a  considerable  time  before  sunset. 

In  the  year  1532,  the  people  of  Milan  were  alarmed  by  the  appearance  of  a 
star  which  was  visible  in  the  broad  daylight.  At  that  time  Venus  was  not  in 
a  position  to  be  visible,  and  consequently  it  is  inferred  that  this  star  must  have 
been  a  comet. 

The  comet  of  1577  was  discovered  on  the  13th  of  November  by  Tycho  Bra- 
che,  from  his  observatory  on  the  isle  of  Huene,  in  the  sound,  before  sunset. 

On  the  1st  of  February,  1744,  Chizeaux  observed  a  comet  more  brilliant 
than  the  brightest  star  in  the  heavens,  which  soon  became  equal  in  splendor  to 
Jupiter,  and  in  the  beginning  of  March  it  was  visible  in  the  presence  of  the 
sun.  By  selecting  a  proper  position  for  observation,  on  the  1st  of  March  it 
was  seen  at  one  o'clock  in  the  afternoon  without  a  telescope. 

Such  is  the  amount  of  evidence  which  observation  has  supplied  respecting 
the  existence  of  a  solid  nucleus  within  the  nebulosity  of  comets.  The  most 
that  can  be  said  of  it  is,  that  it  presents  a  plausible  argument,  giving  some  prob- 
ability, but  no  positive  certainty,  that  comets  have  visited  our  system  which 
have  solid  nuclei,  but,  meanwhile,  this  can  only  be  maintained  with  respect  to 
few;  most  of  those  which  have  been  seen,  and  all  to  whictf  very  accurate  ob- 
servations have  been  directed,  have  afforded  evidence  of  being  mere  masses  of 
semi-transparent  vapor. 

THE    TAIL. 

Although  by  far  the  great  majority  of  comets  are  not  attended  by  tails,  yel 
that  appendage,  in  the  popular  mind,  is  more  inseparable  from  the  idea  of  a 
comet  than  any  other  attribute  of  these  bodies.  This  circumstance  probably 
proceeds  from  its  singular  and  striking  appearance,  and  from  the  fact  that  most 
comets  visible  to  the  naked  eye  have  had  tails.  In  the  year  1531,  on  the  occa- 
sion of  one  of  the  visits  of  Halley's  comet  to  the  solar  system,  Pierre  Apian 
observed  that  the  comet  generally  presented  its  tail  in  the  direction  from  the 


~] 


492  PHYSICAL  CONSTITUTION  OF  COMETS. 

, I 

sun.  This  principle  was  hastily  generalized,  and  is  even  at  present  too  gen-  ? 
erally  adopted.  It  is  true  that  in  most  cases  the  tail  extends  itself  from  that 
part  of  the  comet  which  is  most  remote  from  the  sun  ;  but  its  direction  rarely 
corresponds  with  the  direction  of  a  shadow  of  the  comet.  Sometimes  it  has 
happened  that  the  tail  forms  with  the  line  drawn  to  the  sun  a  considerable  an- 
gle, and  cases  have  occurred  when  it  was  actually  at  right  angles  to  the  direc- 
tion of  the  sun. 

Another  character  which  has  been  observed  to  attach  to  the  tails  of  comets, 
which,  however,  is  not  invariable,  is,  that  they  incline  constantly  toward  the 
region  last  quitted  by  the  comet,  as  if,  in  its  progress  through  space,  it  were 
subject  to  the  action  of  some  resisting  medium,  so  that  the  nebulous  matter  with 
which  it  is  invested,  suffering  more  resistance  than  the  solid  nucleus,  remains 
behind  it  and  forms  the  tail. 

The  tail  sometimes  appears  to  have  a  curved  form.  The  comet  of  1744 
formed  almost  a  quadrant.  It  is  supposed  that  the  convexity  of  the  curve,  if  it 
exists,  is  turned  in  the  direction  from  which  the  comet  moves.  It  is  proper  to 
state,  however,  that  these  circumstances  regarding  the  tail  have  not  been  clearly 
and  satisfactorily  ascertained. 

The  tails  of  comets  are  not  of  uniform  breadth  or  diameter ;  they  appear  to 
diverge  from  the  comet,  enlarging  in  breadth  and  diminishing  in  brightness  as 
their  distance  from  the  comet  increases.  The  middle  of  the  tail  usually  pre- 
sents a  dark  stripe,  which  divides  it  longitudinally  into  two  distinct  parts.  It 
was  long  supposed  that  this  dark  stripe  was  the  shadow  of  the  body  of  the 
comet,  and  this  explanation  might  be  accepted  if  the  tail  was  always  turned 
from  the  sun  ;  but  we  find  the  dark  stripe  equally  exists  when  the  tail,  being 
turned  sideward,  is  exposed  to  the  effect  of  the  sun's  light. 

This  appearance  is  usually  explained  by  the  supposition  that  the  tail  is  a 
hollow,  conical  shell  of  vapor,  the  external  surface  of  which  possesses  a  cer- 
tain thickness.  When  we  view  it,  we  look  through  a  considerable  thickness 
of  vapor  at  the  edges,  and  through  a  comparatively  small  quantity  at  the  mid- 
dle. Thus,  upon  the  supposition  of  a  hollow  cone,  the  greatest  brightness  would 
appear  at  the  sides,  and  the  existence  of  a  dark  space  in  the  middle  would  be 
perfectly  accounted  far. 

The  tails  of  comets  are  not  always  single  ;  some  have  appeared  at  different 
times  with  several  separate  tails.  The  comet  of  1744,  which  appeared  on  the 
7th  or  8th  of  March,  had  six  tails,  each  about  4°  in  breadth,  and  from  30°  to  41° 
in  length.  Their  sides  were  well  defined  and  tolerably  bright,  and  the  spaces 
between  them  were  as  dark  as  the  other  parts  of  the  heavens. 

The  tails  of  comets  have  frequently  appeared,  not  only  of  immense  real 
length,  but  extending  over  considerable  spaces  of  the  heavens.  It  will  be  easi- 
ly understood  that  the  apparent  length  depends  conjointly  upon  the  real  length 
of  the  tail  and  the  position  in  which  it  is  presented  to  the  eye.  If  the  line  of 
vision  be  at  right  angles  to  it,  its  length  will  appear  as  great  as  it  can  do  at  its 
existing  distance  ;  if  it  appear  oblique  to  the  eye,  it  will  be  foreshortened  more 
or  less,  according  to  the  angle  of  obliquity.  The  real  length  of  the  tail  is  easi- 
ly calculated  when  the  apparent  length  is  observed  and  the  angle  of  known  ob- 
liquity. The  following  results  of  actual  observation  and  calculation  have  been 
given  by  Arago. 

The  comet  of  1811  exhibited  a  tail  which  extended  over  23°  of  the  heavens. 
It  was  observed  by  Herschel  and  Schroeter,  the  latter  of  whom  deduced  from 
his  calculations  the  following  results  :  That  the  central  globe  of  light  or  nucleus 
was  50,000  miles  in  diameter,  or  about  six  and  a  half  times  the  diameter  of  the 
earth.  The  nebulosity  was  extremely  rarified  in  comparison  with  nucleus,  re- 
sembling a  faint,  whitish  light,  scattered  in  separate  portions.  It  was  separated 


r 


PHYSICAL  CONSTITUTION  OF  COMETS. 


493 


into  two,  one  immediately  encompassing  the  nucleus,  the  other  of  a  moro  faint 
and  grayish  light,  sweeping  round  it  at  a  distance  and  forming  its  double  tail. 
The  head-veil,  as  he  called  it,  surrounded  the  nucleus  at  a  distance  equal  to  its 
breadth,  and  seemed  as  unconnected  with  the  nucleus  as  the  ring  of  Saturn  is 
with  its  body.  The  diameter  of  this  ring  measured  nearly  a  million  of  milrs, 
being  greater  than  the  diameter  of  the  sun.  Between  the  4th  and  Gth  of  De- 
cember a  great  change  took  place  in  its  appearance,  the  rarefied  nebulous  mat- 
ter, which  had  for  three  months  been  so  unusually  repelled  from  the  nucleus 
on  every  side,  was  again  attracted  to  it. 

The  double  tail  of  this  comet  was  exceedingly  faint  when  compared  with  its 
nucleus.  On  the  16th  of  October  a  small  tail  instantaneously  issued  from  it, 
then  vanished,  and  suddenly  reappeared,  when  its  length  was  nearly  two  mill- 
ions and  a  half  of  miles. 

Herschel's  estimate  of  the  magnitude  of  the  nucleus  is  much  less  than  that 
of  Schrtieter  ;  he  calculates  that,  on  the  15th  of  October,  the  tail  measured  one 
hundred  millions  of  miles,  and  was,  consequently,  greater  than  the  entire  dis- 
tance of  the  sun  from  the  earth.  He  estimated  its  breadth  on  the  12th  of  Octo- 
ber at  fifteen  millions  of  miles. 

Attempts  have  been  made  to  calculate  on  probable  grounds  the  elliptic  orbit 
of  this  cornet.  Bessel  computed  that  its  period  is  three  thousand  three  hundred 
and  eighty-three  years,  and  other  astronomers  make  it  more  than  four  thousand 
years.  A  sketch  of  the  comet  of  1811  is  annexed. 


The  comet  of  1680  exhibited  a  tail  measuring  68°,  of  a  curved  form  ;  of 
which  a  traditional  sketch  is  annexed. 


The  comet  of  1680,  which  was  observed  by  all  the  European  astronomers 
of  that  day,  exhibited  a  tail  which  extended  over  90°  of  the  heavens  at  its  peri- 
helion ;  its  distance  from  the  surface  of  the  sun  was  not  more  than  one  sixth 
of  the  sun's  diameter  ;  and  it  was  calculated  in  that  position  to  have  a  velocity 
of  more  than  120,000  miles  an  hour.  When  the  head  of  this  comet  was  seen 
at  the  zenith,  its  tail  reached  the  horizon.  The  actual  length  of  the  tail  was 
calculated  to  be  one  hundred  and  twenty-three  millions  of  miles  ;  so  that  if  the 
head  of  this  comet  were  at  the  sun,  the  tail  would  extend  thirty  millions  of 
miles  beyond  the  earth's  orbit. 

In  1769  a  comet  appeared,  the  tail  of  which  spread  over  a  space  of  97° 
of  the  heavens,  and  its  actual  length  was  fifty  millions  of  miles.  Difier- 
ent  estimates  have  been  given  of  the  length  of  the  tails  of  the  comet  of  1744. 


494  PHYSICAL  CONSTITUTION  OP  COMETS. 

Arago  assigns  their  length  at  about  ten  millions  of  miles,  others  have  estimated 
it.  at  twenty-three  millions  of  miles.  The  following  description  of  it  is  taken 
from  the  memoirs  of  the  Academy  of  Sciences  for  ]744.  It  was  first  seen  at 
Lausanne,  in  Switzerland,  December  13,  1743:  from  that,  period  it  increased 
in  brightness  and  magnitude  as  it  approached  nearer  the  sun.  On  the  evening 
of  January  23,  1744,  it  appeared  exceedingly  bright  and  distinct,  and  the  di- 
ameter of  its  nucleus  was  nearly  equal  to  that  of  Jupiter.  Its  tail  then  extended 
above  16°  from  its  body,  and  was  supposed  to  be  about  twenty-three  millions  of 
miles  in  length.  On  the  llth  of  February  the  nucleus,  which  had  before  been 
always  round,  appeared  oblong  in  the  direction  of  the  tail,  and  seemed  divided 
into  two  parts  by  a  black  stroke  in  the  middle.  One  of  the  parts  had  a  sort  of 
beard  brighter  than  the  tail :  this  beard  was  surrounded  by  two  unequal  dark 
strokes  that  separated  the  beard  from  the  hair  of  the  comet.  These  odd  phe- 
nomena disappeared  the  next  day,  nnd  nothing  was  seen  but  irregular  obscure 
spaces,  like  smoke,  in  the  middle  of  the  tail,  and  the  head  resumed  its  natural 
form.  On  the  15th  of  February  the  tail  was  divided  into  two  branches — the 
eastern,  about  8°  long,  the  western,  24.°  On  the  23d  the  tail  began  to  be  bent. 
It  showed  no  tail  till  it  was  as  near  the  sun  as  the  orbit  of  Mars,  and  it  in- 
creased in  length  as  it  approached  that  luminary.  At  its  greatest  length  it  was 
computed  to  equal  a  third  part  of  the  distance  of  the  earth  from  the  sun.  This 
was  one  of  the  most  brilliant  comets  that  had  appeared  since  that  of  1680.  Its 
tail  was  visible  for  a  long  time  after  its  body  was  hid  under  the  horizon.  It 
extended  20  or  30  degrees  above  the  horizon  two  hours  before  sunrise. 

In  the  month  of  March,  1843,  a  comet  appeared  in  the  heavens  exhibiting  a 
great  extent  of  tail,  but  very  faintly  luminous.  Its  course  was  calculated  from 
the  observations  made  upon  it.  but  no  satisfactory  grounds  were  obtained  by 
which  it  might  be  identified  with  any  former  body  of  the  same  kind.  The 
form  of  the  tail  was  remarkable,  inasmuch  as  its  edges  were  parallel  and  not 
divergent.  The  length  of  the  tail  was  calculated  from  the  observations,  and 
said  to  amount  to  above  one  hundred  millions  of  miles.  This  comet  was  ren- 
dered memorable  by  the  fact  of  its  having  passed  at  its  perihelion  so  close  to 
the  sun  that  Arago  believed  it  must  have  grazed  its  surface.  A  sketch  of  this 
comet  is  annexed  on  the  opposite  page. 

The  following  observations  of  Professor  Nichol  on  this  comet  will  be  read 
with  interest : — 

"  Early  in  the  year  1843,  an  object  appeared  in  the  heavens  that  must  have 
astonished  many  worlds  besides  ours.  Situated  in  the  region  below  the 
constellation  Orion,  it  had  the  appearance  of  a  long  auroral  streak,  visible 
immediately  after  sunset,  and  evidently  pursuing  a  course  through  our  system. 
Unfavorable  weather  concealed  it  from  me  until  the  25th  of  March,  when  it 
presented  the  dim  and  strange  appearance  I  have  shown  in  the  frontispiece.  The 
beginning  or  head  of  this  streak,  although  never  observed  here,  was  often  seen 
in  southerly  latitudes,  where  it  appeared  like  a  very  small  star  with  an  enor- 
mous misty  envelope ;  beyond  which  that  immense  tail  streamed  through  the 
sky.  There  is  no  reason  to  believe  that  this  nucleus  was  in  reality  a  star,  but 
only  a  denser  portion  of  the  nebulous  substance  of  which  the  whole  object  was 
composed ;  for  with  other  apparitions  of  the  same  kind,  whose  brighter  parts 
looked  like  a  star,  the  application  of  a  very  small  telescopic  power  has  always 
been  enough  to  dissipate  the  illusion,  and  to  resolve  what  seemed  their  solid 
region  into  a  thin  vapor. 

'  This  extraordinary  visiter  was  measured,  and  the   nature  of  its  path  de- 
tected ;  and  certainly  the  results  of  these  inquiries  caused  us  to  look  on  it  with 
/  still  greater  wonder.     The  diameter  or  breadth  of  its  nucleus  was  rather  more 
(  than  a  hundred  thousand  miles  ;  and  the  tail  streaming  from  it,  which  in  some 

•v^*. 


PHYSICAL  CONSTITUTION  OF  COMETS 


496  PHYSICAL  CONSTITUTION  OF  COMETS. 

parts  was  thirty  times  as  broad,  stretched  through  the  celestial  spaces  to  the 
enormous  distance  of  one  hundred  and  seventy  millions  of  miles,  or  about  the 
whole  size  of  the  orbit  of  the  earth.  Nor  were  its  motions  less  singular.  Un- 
like any  globe  connected  with  the  sun.  it  did  not  move  in  a  continuous  curve, 
which,  like  the  circle  or  ellipse,  re-enters  into  itself,  and  thus  constitutes,  to 
the  body  that  has  adopted  it,  a  fixed,  however  eccentric  home ;  but  spying  our 
luminary  afar  off,  as  it  lay  amid  those  outer  abysses,  it  approached  along  the 
arm  of  a  hyperbola,  rushed  across  the  orderly  orbits  of  our  system  into  closest 
neighborhood  with  the  sun,  being  at  that  time  apart  from  him  only  by  a  sev- 
enth part  of  our  distance  from  the  moon,  and,  defying  his  attraction,  by  force  of 
its  own  enormous  velocity,  which  then  was  nothing  less,  in  one  part  of  its 
mass,  than  one  third  of  the  velocity  of  light,  it  entered  on  the  other  divergent 
arm  of  its  course,  and  sped  toward  new  immensities. 

"  It  was  when  retiring  that  this  unexpected  visitant  was  seen  for  a  brief  pe- 
riod in  Europe.  In  the  course  of  its  approach  it  must  have  passed  between  us 
arid  the  sun,  causing  a  cometic  eclipse,  and,  in  so  far,  an  interception  of  his 
heating  rays  ;  but  that  occurred  during  our  night. 

'•  And  now,  what  is  to  be  made  of  this  extraordinary  apparition  ?  what  is  its 
nature  ?  what  its  relations  to  our  system  ?  and  what  new  revelation  does  it 
bring  concerning  the  structure  of  the  universe?  Its  relations  with  our  system 
appear  to  have  been  few  and  transitory ;  and  in  this  it  resembles  the  probable 
millions  of  such  masses,  that  have,  since  observation  began,  crossed  the  plane- 
tary orbits  toward  the  sun,  and,  after  bending  round  him,  gone  in  pursuit  of 
some  other  fixed  star.  No  more  than  three  are  known  to  belong,  properly 
speaking,  to  the  scheme  dependant  on  our  luminary — Encke's,  Biela's,  and 
Halley's  ;  but  though  these  do  revolve  around  him  in  fixed  periods,  the  cir- 
cumstance must  be  regarded  in  the  light  of  an  accident,  their  orbits  being 
wholly  unlike  any  other,  and  having  little  assurance  of  stability ;  for  as  they 
cross  the  planetary  paths,  every  one  of  them  may  yet  undergo  the  fate  of  Lex- 
ell's,  which,  by  the  action  of  Jupiter,  was  first  twisted  from  its  diverging  orbit 
into  a  comparatively  short  ellipse  ;  and  then,  after  making  two  consecutive 
revolutions  around  the  sun,  so  that  it  might  have  begun  to  deem  itself  a  den- 
izen, was,  by  the  same  planet  twisted  back  again,  and  sent  off,  never  to  revisit 
us,  away  to  the  chill  abysses  !  Strange  objects,  with  homes  so  undefined — 
flying  from  star  to  star — twisting  and  winding  through  tortuous  courses,  until, 
perhaps,  no  depth  of  that  infinite  has  been  untraversed  !  What,  then,  is  it  your 
destiny  to  tell  us  ?  To  what  new  page  of  that  infinite  book  are  you  an  index? 
We  missed,  indeed,  only  very  narrowly,  an  opportunity  of  information  which 
might  have  been  not  the  most  convenient;  for  the  earth  escaped  being  involved 
in  the  huge  tail  of  our  recent  visiter,  merely  by  being  fourteen  ilmjs  beiiind  it. 
For  one,  I  should  have  had  no  apprehension  even  in  that  case,  of  the  realization 
of  geological  romances,  viz.,  of  our  equator  being  turned  to  the  pole,  and  the 
pole  to  the  equator — the  ocean,  meanwhile,  leaping  from  its  ancient  bed.  But 
if  that  mist,  thin  though  it  was,  had,  with  its  next  to  inconceivable  swiftness, 
brushed  across  our  globe,  certainly  strange  tumults  must  have  occurred  in  the 
atmosphere  ;  and  probably  no  agreeable  modification  of  ihe  breathing  medium 
of  organic  beings.  Right,  certainly,  to  be  most  curious  about  comers  ;  but  pru- 
dent, withal,  to  inquire  concerning  them  from  a  greater  distance  than  that :  al- 
though one  night  in  November,  1837,  I  cannot  be  persuaded  that  the  earth  did 
not  venture  on  a  similar,  but  comparatively  small  experiment.  It  was  when 
our  globe  passed  from  the  peaceful  vacant  spaces  into  that  mysterious  meteor 
region.  The  sky  became  kiflamed  and  red  as  blood;  coruscations,  like  auro- 
ras, darted  across  it ;  not  as  usual,  streaming  from  one  district,  but  shilling 
constantly,  and  sweeping  the  whole  heavens." 


PHYSICAL  CONSTITUTION  OF  COMETS.  j;i7 

In  the  year  1844*  two  comets  appeared,  the  first  of  whic-h  w  as  scon  in  ?»..- 
month  of  July.     It  was   described  to  have  a  bright  white  color— tint  its  t  ,ii 
was  turned  from  the  earth  so  as  not  to  be  visible  to  us.     Stars  of  very  - 
magnitude  were  visible  through  its  body,  ami  its  light  was  s 
said  to  be  easily  detected  in  the  heavens,  in  Curope,  during  the  bright 
of  July.     A  drawing  of  this  comet,  obtained  ;u  th«  Royal  ObservatoiT, 
is  annexed. 


The  second  comet  of  1844  was  seen  in  ihe  month  of  September.  It  \v:is 
observed  at  Kensington,  by  Sir  James  South,  on  the  evening  of  the  loth.  In 
the  course  of  that  month  a  drawing  was  obtained  of  it  by  the  assistance  of  Sir 

•  The  following  note  is  annexed  to  the  last  edition  of  Nicbol'e  Solar  System,  relative  to  one  of  tLe 
comets  of  1843: — 


"  As  this  volume  is  leaving  the  press,  intelligence  has  been  received  of  11  new  cotr.ot  being 

to  our  system.     Its  orbit  ha/becu  determined  by  the  illustrious  Gauss,  and  its  period  in  nearly  Bevcn  r 
yearn. 

"  The  importance  of  this  fact  cannot  well  be  overrated ;  for.  along  with  EncXe'*  nnd  Biola  s,  it 
must  advance  oar  know'edpe  of  some  of  the  mysterious  points  connected  with  tlie  eaotftoMM  "t 
tho  planetary  scheme.  We  are  yet  ignorant  whether  tliis  body  ha«  merely  i  ot  lw<»>n  (/b-erveil 
till  MOW,  or  whether,  like  Lexell's.  it  has  beeu  construined  into  a  new  orbit  by  die  action  of  0oot« 
I'lant't.-.  ' 


498 


PHYSICAL  CONSTITUTION  OF  COMETS. 


James   South  and  the  use  of  his   instruments.     We  subjoin  a  copy  of  this 
drawing. 


This    comet    appeared    to   have   a  brilliant  and   well-defined  nucleus  five 
seconds  in  diameter,  and  a  broad  luminous  tail  of  about  two  degrees  in  length 


THUNDER-STORMS. 


The  Deficiency  of  our  present  Knowledge. — Of  common  Thunder-Clouds. — Character  and  Electric 
Charge  of  Clouds. — Discharge  between  vicinal  Clouds. — Conditions  for  such  Discharge. — Discharge 
between  the  Clouds  and  the  Earth. — Mutual  Attraction  or  Repulsion  of  Electrized  Clouds. — Char- 
acters of  the  upper  and  of  the  lower  Surface  of  Clouds. — Negative  Testimony,  respecting  Thun- 
der from  an  isolated  Cloud. — Cases  of  Lightning  from  an  isolated  Cloud. — Afresh  Case  related  by  I 
M.  Duperrey. — Obvious  Inferences  from  the  above  Cases. —  Of  Volcanic  Thunder-Clouds. —  ( 
Lightning  from  the  Ashes,  Smoke,  and  Vapor  of  Volcanoes. — Theoretical  Ideas  of  its  Origin. —  < 
Of  the  Height  of  Stormy  Clouds. — Mode  of  Observation. — Ascending  Flashes  of  Lightning. — Mi-  < 
nor  Limits  of  the  Height  of  Storm-Clouds. — Inefficiency  of  many  recorded  Observations. — Table 
of  Observations  as  collected  by  Arago. — Flash  of  Lightning  from  a  Cloud  upward. — Of  Light- 
ning.— Varieties  of  Lightning. — Zigzag  Lightning. — Forked  Lightning. — Deficiency  in  our  Vo- 
cabulary of  Terms. — Sheet  Lightning. — Table  of  Instances  of  Ball-Lightning. — Mr.  Harris's 
Explanation  of  Ball-Lightning. — On  the  Speed  of  Lightning. — Theory  of  Vision  illustrated  by 
a  rotating  Disk. — Wheatstone's  Experiments. — Observations  of  the  Velocity  of  Lightning. — 
Silent  Lightning. — Heat  Lightning. — Thunder  Bursts. — Of  Luminous  Clouds. — Clouds  them- 
selves faintly  Luminous. — Possession  of  the  duality  in  various  Degrees. — Clouds  visibly  Lumin- 
ous.— Various  Observations  of  luminous  Clouds. — Sabine's  Observations. — Of  Thunder. — Rolling 
of  Thunder. — Duration  and  Intensity  of  rolling  Thunder. — Violent  Thunder  from  Ball-Lightning. — 
Interval  between  Lightning  and  Thunder. — A  case  in  which  they  were  almost  simultaneous. — 
Thunder  without  Lightning. — Noise  attendant  on  Earthquakes. — Of  the  Attempts  to  explain  the 
Phenomena  of  Thunder  and  Lightning. — Identity  of  Lightning  and  Electricity. — Whether  pon- 
derable Matter,  or  a  Propagation  of  Undulations. — Difficulties  of  the  Undulatory  Hypothesis. — 
Bull-Lightning  and  the  Inferences  to  which  it  leads. — Bituminous'  Matter  accompanying  a  Case 
of  Lightning  Discharge. — Explanations  of  silent  Lightnings. — Observations  of  silent  Lightnings. — 
Difficulties  in  the  Explanation  of  silent  Lightnings. — Arago's  Suggestion  for  Observations. — Light- 
ning hidden  by  dense  Clouds.— Place  of  the  Sound  of  Thunder.— Greatest  Distance  at  which 
Thunder  is  heard. — Case  of  Distance  beyond  which  it  was  Inaudible. — Distance  at  which  other 
Sounds  have  been  heard. — Effects  of  Heat,  Cold,  Wind,  &c. — On  the  Transmission  of  Sound. — 
Thunder  heard  when  no  Cloud  was  Visible.— Hypothesis  of  the  Cause  of  Thunder  from  the  Cre- 
ation  of  a  Vacuum. — Contractions  and  Dilatations  of  the  Air  assigned  as  the  Cause. — Ponillet'a 
Theory  of  Decompositions  and  Recompositions. — Influence  of  Echo  in  causing  the  Roll. — Dura- 
tion of  an  Echo.— Duration  of  the  Roll  of  Thunder  at  Sea.— Dr.  Robison'a  Explanation  of  the 
Roll. — Application  of  the  Theory  to  Zigzag  Lightning. — Inefficiency  of  the  Theory. — Means  of 
obtaining  a  Minor  Limit  of  the  Length  of  a  Flash. 


THUNDERSTORMS.  601 


THUNDER-STORMS. 


SINCE  the  epoch  of  the  memorable  experiments  of  Franklin,  meteorologists, 
in  all  parts  of  the  world  where  physical  science  is  cultivated,  have  observed 
with  increased  interest  the  phenomena  of  thunder-storms.  Although  a  great 
body  of  facts  have  been,  by  such  means,  accumulated,  the  general  conclu- 
sions deducible  from  them  are  few ;  nor  are  even  these  few  invariably  sus- 
tained by  that  consistency,  and  harmony  of  effects  which  are  necessary,  to 
command  universal  assent.  Indeed,  the  facts  themselves,  on  which,  alone,  any 
safe  and  certain  generalization  could  be  based,  were  isolated,  and  scattered 
through  the  memoirs  of  the  various  scientific  bodies  to  which  their  observers 
had  originally  consigned  them ;  and  many  of  the  most  important  and  valuable 
observations  remained  in  unpublished  memoranda,  or  were  incidentally  men- 
tioned in  the  narratives  of  voyagers  and  travellers,  where  they  were  little  like- 
ly to  attract  the  attention  of  those  who,  alone,  are  capable  of  estimating  their 
value,  until,  by  the  indefatigable  zeal  of  M.  Arago  they  were  collected,  ar- 
ranged, and  compared,  and  presented  to  the  world,  invested  with  all  those 
charms  of  style  which  render  the  productions  of  that  philosopher  so  universal- 
ly .attractive.*  It  is  natural  that  the  impatient  student  should  desire  to  be  supplied 
with  clear  and  comprehensive  principles,  and  be  relieved  from  the  tedious  de- 
tails of  particular  observations  and  experiments ;  that  facts  should  be  laid  before 
him  in  extensive  groups  and  classes,  so  as  to  suggest  easy  and  obvious  gener- 
alizations. It  is  equally  natural  that  the  authors  of  elementary  and  general 
treatises  should  desire,  in  every  case,  to  present  the  scientific  truths  in  concise 
and  general  propositions,  connected  together  by  distinct  logical  relations.  The 
temptation  to  yield  to  this  disposition  by  presenting  all  physical  problems  as 
completely  resolved,  and  all  elementary  questions  as  completely  exhausted — 
of  laying  down  sweeping  conclusions  and  general  principles,  on  matters  which  , 
are,  m  fact,  surrounded  with  difficulty  and  doubt— is  most  hurtful  to  the  progress  ( 

*  See  Notice  eur  le  Tonncrre  dans  1'Annuaire  du  Bureau  dea  Longitudes  pour  1'An  i.838.  v 


of  science,  and  a  great  impediment  to  the  development  of  truth.  To  no  part 
of  physical  science  do  these  observations  apply  with  more  force  than  to  the 
subject  of  the  present  discourse.  That  the  phenomena  of  thunder  and  lightning 
proceed  from  sudden  and  violent  derangements  of  the  electrical  equilibrium  of 
the  atmosphere  or  the  clouds  which  float  in  it,  may  be  regarded  as  certain  ;  and 
that  the  laws  which  are  observed  to  prevail  among  electrical  phenomena  offer 
various  analogies  which  afford  explanations  more  or  less  plausible  and  proba- 
ble, for  some  of  the  facts  observed  in  thunder-storms,  may  be  admitted.  But 
that  any  comprehensive  and  general  principles  have  been  established  from 
which  the  various  atmospheric  phenomena  in  which  thunder  and  lightning  are 
exhibited,  can  be  deduced  in  the  same  manner,  and  with  the  same  clearness 
and  certainty,  as  the  effects  of  common  electricity  have  been  deduced  from  the 
theory  of  Dufaye,  Summer,  and  Poisson,  cannot  be  maintained.  Under  such 
circumstances,  both  author  and  reader  must  patiently  submit  to  the  investiga- 
tion of  facts  separated  from  theory  or  hypothesis  ;  and  when  these  facts  have 
been  clearly  and  fully  stated,  such  general  consequences  as  they  justify  may 
be  easily  deduced  from  them,  and  the  apparent  discordances  which,  by  com- 
parison, may  be  apparent  among  them,  will  afford  grounds  for  further  observa- 
tion and  inquiry  to  those  who  devote  their  labor  to  such  researches. 

COMMON    THUNDER-CLOUDS. 

It  is  generally  agreed  that  the  formation  of  clouds  is  due  to  the  partial  con- 
densation, in  the  upper  regions  of  the  air,  of  the  vapors  which  have  exhaled 
from  the  surface  of  the  earth.  This  condensation  may  be  effected  by  any 
cause  which  produces  a  diminution  of  temperature,  and  is,  probably,  in  most 
cases,  the  consequence  of  the  mixture  of  two  currents  of  air.  charged  with 
vapor,  and  having  different  temperatures.  The  positive  electricity  which  rises 
into  the  atmosphere  with  the  vapor,  and  which  augments  in  intensity,  as  the 
height  increases,  to  the  greatest  elevation  to  which  observation  is  extended,  is 
collected  in  the  clouds  thus  formed ;  and  when  the  globules  or  vesicles  com- 
posing the  cloud  have  collected  together  in  sufficiently  close  proximity,  the 
cloud  takes  the  nature  of  one  continued  conductor  and  the  free  electricity  accu- 
mulates on  its  surface  in  the  same  manner  as  on  the  conductor  of  an  electrical 
machine.  The  existence  of  positively-electrified  clouds  is,  therefore,  easily 
conceived. 

If  the  electroscopic  observations  which  indicate  negatively-electrified  clouds 
be  rightly  interpreted,  and  the  existence  of  such  clouds  be  admitted,  several 
hypotheses  have  been  proposed  to  explain  them. 

If  a  cloud  in  its  natural  state,  or  feebly  charged  with  positive  electricity,  ap- 
proach another  cloud  strongly  charged  with  the  same  electricity,  the  latter  will 
exercise  upon  it  an  inductive  action,  by  which  its  natural  electricities  will  be 
decomposed,  the  positive  electricity  being  repelled  to  the  most  remote  part,  and 
the  negative  fluid  being  accumulated  at  the  nearest  part.  If,  under  these  cir- 
cumstances, the  most  remote  part  be  in  contact  with  the  earth,  as  it  might  be, 
with  the  summit  of  a  mountain,  for  example,  the  positive  electricity  will  es- 
cape to  the  earth,  and  the  cloud  will  remain  charged  with  negative  electricity. 
If  any  cause  disengage  this  cloud  from  contact  with  the  earth,  it  will  float  in 
the  atmosphere  and  afford  an  example  of  a  negatively-electrified  cloud. 

If  two  clouds,  one  or  both  of  which  are  charged  with  electricity,  approach 
each  other,  the  same  phenomena  must  be   evolved  as  when   two  conductors, 
one  or  both  of  which  are  similarly  charged,  come  together.     If  it  happen  (a 
circumstance  against  which  the  chances  are  infinite),  that  the  quantities  of 
electricity  with  which  they  are  charged  have  the  same  relation  as   they  w 


THUNDER-STORM?. 


have  when  the  clouds  are  in  contact,  then  their  approach  and  suh-cquent  con- 
tact will  cause  no  change  in  their  electrical  state  save  what  would  he  din-  to 
inductive  action.  Their  charges  after  contact  will  be  the  same  as  In-fur.-.  u<» 
electricity  passing  from  either  to  the  other.  But  if  their  electrical  cli 
have  not  this  particular  relation,  then  a  new  distribution  of  electricity  wilTbe 
the  consequence  of  their  mutual  approach  ;  that  which  has  less  positive  elec- 
tricity than  the  condition  of  contact  requires  will  receive  the  deficiency  from 
the  other,  and  this  change  will  be  effected  by  an  explosion  before  the"  actual 
contact  of  the  clouds,  in  the  same  manner  as  the  electrical  equilibrium  of  two 
conductors  is  established  by  the  transmission  of  the  spark  before  contact. 
The  distance  at  which  the  explosion'will  take  place,  and  its  force,  will  di-prnd 
on  many  circumstances,  such  as  the  difference  between  the  actual  charges  of 
'}  the  clouds,  and  the  charges  due  to  contact,  the  form  of  the  clouds,  and  the 
state  of  the  intervening  atmosphere. 

It  is  evident,  therefore,  that  an  electrical  explosion  may  take  place  between 
iwo  clouds,  whether  they  are  both  similarly  electrified,  or  oppositely  electrified, 
or  one  be  electrified  and  the  other  in  its  natural  state. 

As  the  ground  is,  in  general,  negatively,  and  the  clouds  positively  elec- 
trified, a  discharge  will  take  place  between  the  clouds  and  the  earth  when  the* 
former  approach  the  earth  within  such  a  distance  that  the  force  of  the  electri- 
city shall  overcome  the  resistance  of  the  surrounding  air. 

Since  free  electricity  accumulates  in  great  intensity  at  prominent  points 
of  a  conducting  body,  the  negative  electricity  of  the  earth  may  be  expected  to 
be  most  intense  at  mountain  summits.  Clouds  being,  in  general,  charged  with 
positive  electricity,  an  attraction  will,  consequently,  be  exerted  upon  them 
which,  conspiring  with  the  attraction  of  gravitation,  will  draw,  them  round  such 
summits. 

The  mutual  approach  of  two  clouds  oppositely  electrified  is  promoted  by  the 
attraction  due  to  their  electricities  :  but  when  two  clouds  are  similarly  electri- 
fied they  will  repel  each  other  and  their  approach  must  be  due  to  contrary  our- 
rents  of  air  passing  through  strata  of  the  atmosphere  at  different  elevations,  by 
which  the  clouds  are  brought  one  under  the  other. 

Beccaria,  who  observed  at  Turin,  in   Piedmont,  in  a  country  eminently  fa- 
vorable for  such  observations,  being  almost  surrounded  by  lofty  ranges,  has  re- 
corded, with  great  precision,  the  appearances  of  the   clouds   precursive  of  a 
storm.     The  observations  of  this  philosopher  being  limited  to  the  lower  sur- 
face of  the  clouds,  M.  Arago  has  obtained  some  accounts  of  the  superior  sur-  < 
face,  from  the  military  engineers  employed  in  the  trigonometrical  survey,  and  < 
who,  being  placed  at  elevated  stations  on  the  Pyrenees,  were  enabled  to  ob-  , 
serve  the  superior  surface  of  the  strata  of  clouds  situated  below  them.     From  \ 
the  reports  of  these  officers,  and  especially  those  of  MM.  Peytier  aad  Hos- 
sard,   it   appears    that   there   is   no  correspondence  between    the  upper   and 
lower  surface  of  a  stratum  of   thunder-clouds  ;  that  when   the  inferior  sur- 
face is  perfectly  even  and  level,    the  superior  surface  wiii  bs  broken  in  o 
ridges  and  protuberances,  rising  upward  to  great  altitudes,  like  '.he  surface  of 
the  earth  in  an  alpine  district.     In  times  of  great  heat,  such  strata  were  ob- 
served suddenly  to  send  upward  lofty  vertical  cones,   which,  stretching  into 
higher  regions  of  the  air,  established,  by  their  conducting  power,  an  electrical 
communication  between  strata  of  the  atmosphere  at  very  different  heights. 
This  appearance  was  generally  observed  to  precede  a  thunder-storm. 

Franklin,  Saussure,  and  most  other  meteorologists,  have  agreed  that  thunder 
never  proceeds  from  a  solitary,  isolated  cloud.  Franklin  states,  that  if  a  thun- 
der-cloud be  at  any  considerable  distance  from  the  zenith  of  the  observer,  so 
as  to  be  viewed  obliquely,  it  will  be  apparent  that  there  are,  in  every  such  case,  > 


504  THUNDER-STORMS. 


a  series  of  two  or  more  clouds,  situate  at  different  elevations,  one  below  the 
other  ;  avid  that  sometimes  the  lowest  of  the  series  is  not  far  removed  from  the 
suriace  of  the  earth. 

Saussure  states  that  he  never  witnessed  lightning  to  proceed  from  a  solitary 
cloud.  In  observations  on  the  Col  de  Geant,  when  a  single  cloud,  however 
dense  and  dark  it  might  be,  was  seen  upon  the  summit,  no  thunder  was  ever 
heard  to  issue  from  it ;  but  whenever  two  strata  of  two  such  clouds  were  formed, 
one  below  the  other,  or  if  clouds  ascended  from  the  plain  and  approached  that 
collected  round  the  summit,  the  encounter  was  attended  by  a  storm  of  thunder, 
hail,  and  rain. 

Such  is  the  negative  testimony  of  Franklin  and  Saussure  against  the  fact  of 
thunder  proceeding  from  solitary  clouds.  Franklin  is  even  more  circumstan- 
tial than  Saussure,  and  maintains  that  thunder  never  proceeds  from  any  save  a 
cloud  of  great  magnitude,  below  which  are  placed  a  series  of  smaller  clouds, 
identical  in  fact,  with  the  adscititious  clouds  of  Beccaria. 

Negative  evidence  is,  however,  not  conclusive  against  a  fact,  unless  the  wit- 
ness be  actually  present  at  the  time  and  place  of  its  alleged  occurrence.  Had 
the  eminent  philosophers  above  mentioned  consulted  the  records  of  science, 
their  persuasion  of  the  impossibility  of  thunder  issuing  from  a  single  cloud 
would  have  been  shaken.  It  is  related  in  a  memoir  of  the  academician 
Marcordle  of  Toulouse,  that  on  the  12th  of  September,  1747,  the  heavens  being 
generally  cloudless,  a  single  small  cloud  was  seen,  from  which  thunder  rolled 
and  lightning  issued,  by  which  a  female  by  name  Bordenare  was  killed. 

In  his  meteorological  observations  made  at  Denainvilliers,  Duhamel  de  Mon- 
ceau  relates  that  on  the  30th  of  July,  1764,  at  half  past  rive,  A.  M.,  in  bright 
sunshine  and  a  clear  sky,  there  appeared  a  small  dark  solitary  cloud,  from 
which  thunder  and  lightning  proceeded,  by  which  an  elm-tree  near  the  chateau 
was  stricken. 

Similar  observations  of  lightning  having  issued,  followed  by  thunder,  from 
solitary  clouds,  have  been  recorded  by  Bergman  and  by  Captain  Hossard,  al- 
ready mentioned. 

M.  Duperrey,  who  commanded  the  French  corvette  Uranie,  relates  that  being 
in  the  straits  of  Bombay,  in  November,  1818,  he  saw  a  small  white  cloud  in  a 
clear  sky,  from  which  lightning  issued  in  all  directions.  It  ascended  slowly 
in  the  heavens  in  a  direction  opposed  to  the  wind,  and  was  at  a  great  distance 
from  all  other  clouds,  which  appeared  to  be  fixed  upon  the  horizon.  This 
cloud  was  round  in  its  form,  and  did  not  exceed  the  apparent  magnitude  of  the 
sun.  Zigzag  lightning  issued  from  it,  followed  by  thunder  which  resembled 
the  irregular  discharge  of  musketry  from  a  battalion  commanded  to  fire  at 
pleasure.  This  phenomenon  lasted  for  about  thirty  seconds,  and  the  cloud 
completely  disappeared  with  the  last  detonations. 

feuch  are  the  evidences  on  the  question  whether  the  presence  and  proximity 
of  a  plurality  of  clouds  be  essential  to  the  development  of  the  phenomena  of 
thunder  and  lightning.     The  analogies  offered  by  common  electricity  favor  the 
supposition  that  two  or  more  clouds  are  essential ;  and  for  this  very  reason  the 
greater  should  be  the  caution  for  receiving  the  testimony  of  observers.     It  is 
difficult  for  those  whose  minds  are  prepossessed  by  theory  to  observe  and  re- 
cord facts  and  appearances  as  they  are  ;  there  is  a  disposition  sometimes — 
perhaps  often — to  see  them  as  it  is  supposed  they  ought  to  be,  and  consequent-  ( 
ly  the  testimony  of  the  ignorant  is  frequently  more  deserving  of  attention  than  \ 
that  of  the  better  informed.     Be  this  as  it  may,  the  subject  is  one  well  worthy  < 
of  attention,  and  all  persons,  who  happen  to  be  located  in  regions  where  these  ( 
?  phenomena  prevail,  will  have  it  in  their  power  to  contribute  to  the  real  advance-  ( 
s  incut  of  science,  by  carefully  and  accurately  noting  down  what  passes  above  < 


them,  more  efl'ectually  than  those  who  with  greater  pretensions  attempt  to  build 
up  theories,  which,  at  best,  can  have  no  other  object  than  as  means  of  classify- 
ing facts  and  guiding  observers  to  the  fittest  objects  of  examination. 

OF    VOLCANIC    THUNDER-CLOUDS. 

The  clouds  of  ashes,  smoke,  and  vapor,  which  issue  from  volcanoes,  exhibit 
i IK-  phenomena  of  thunder  and  lightning.  All  observers,  ancient  and  modern, 
concur  in  their  evidence  on  this  question.  Pliny  the  younger,  in  his  celebrated 
letters  to  Tacitus,  speaks  of  the  lightning  that  issued  from  the  clouds  in  the 
eruption  of  Vesuvius,  in  the  year  79  of  the  Christian  era,  in  which  his  uncle, 
Pliny  the  naturalist,  lost  his  life.  Delia  Torre  gives  the  same  evidence  respect- 
ing the  eruption  of  1182  ;  and  Bracini  states  that  the  column  of  smoke  which 
issued  from  the  same  volcano  in  the  eruption  of  1631,  and  which  spread  in  the 
atmosphere  to  a  distance  of  forty  leagues,  was  attended  by  lightning,  by  which 
many  persons  and  animals  were  killed.  The  lightning  in  all  these  accounts 
is  described  as  being  tortuous  and  serpentine.  The  same  description  is  given 
by  Giovanni  Valetta  of  the  appearance  of  the  eruption  of  1707. 

The  inhabitants  of  the  foot  of  the  mountain  assured  Sir  William  Hamilton 
that,  in  the  eruption  of  1767,  there  were  more  terrified  at  the  lightning  which 
fell  among  them  than  at  the  burning  lava  and  other  fearful  circumstances  at- 
tending the  eruption. 

Sir  William  Hamilton  states,  that  in  the  eruption  of  1779  there  issued  from 
the  crater  of  Vesuvius,  together  with  the  red-hot  fluid  lava,  constant  puffs  of 
black  smoke,  intersected  by  serpentine  lightning,  which  appeared  at  the  mo- 
ment it  escaped  from  the  crater. 

In  1779  the  lightning  was  not  attended  by  audible  thuwder.  It  was  othef- 
wi.se  in  the  eruption  of  the  16th  of  June,  1794,  of  which  an  account  has  been 
supplied  by  the  same  observer.  During  the  latter  eruption,  the  loudest  and 
mu -it-continued  claps  of  thunder  were  heard.  The  lightning  was  in  this  case 
productive  of  the  usual  effects.  Houses  stricken  by  it  were  destroyed,  and  the 
clouds  of  ashes,  from  which  these  lightnings  issued,  were  carried  by  the  wind 
as  i';ir  as  Tarentum,  a  distance  of  one  hundred  leagues  from  Vesuvius,  where 
the  lightning  struck  a  building  and  destroyed  a  part  of  it.  The  ashes  of  which 
this  cloud  was  composed  were  as  fine  as  common  snuff. 

According  to  Seneca,  a  great  eruption  of  Etna,  in  his  own  time,  was  accom- 
panied by  similar  effects,  and  the  same  phenomena  are  recorded  by  the  Abbe 
.Francesco  Ferrara  of  the  eruption  of  1755. 

When  the  island  called  Sabrina,  in  the  neighborhood  of  the  Azores  (which 
has  since  disappeared),  rose  from  the  sea  in  1811,  columns  of  intensely  black 
smoke,  composed  of  dust  and  ashes,  ascended  from  the  bosom  of  the  deep,  and 
wern  intersected  in  their  darkest  and  most  opaque  parts  by  vivid  lightnings. 

The  same  appearances  were  observed  in  *he  small  volcano  which,  in  July, 
1831,  appeared  between  Sicily  and  Pantellaria. 

It  would  be  natural  to  ascribe  the  electricity  of  volcanic  clouds  to  the  aque- 
ous vapor  which  is  ejected,  mixed  with  the  dust,  ashes,  and  lava,  in  great  quan- 
tities from  the  crater ;  but  this  supposition  is  not  so  free  from  difficulties  as  to 
be  admitted  without  some  hesitation.  In  the  eruption  of  Vesuvius,  in  1794,  it 
is  hard  to  conceive  that  the  vapor  should  be  carried  uncondensed  from  Vesu- 
vius to  Tarentum  ;  nor  was  there  anything  in  the  appearances  on  thayjccasion 
which  indicated  the  presence  of  any  other  substance  in  the  cloud  save  a  fine 
dust ;  yet  the  lightning  struck  a  building  at  that  place.  According  to  the  nar- 
rative of  M.  Tellard,  who  witnessed  the  phenomenon,  columns  of  black  smoke 
rose  from  the  ocean  before  the  island  of  Sabrina  was  formed.  In  this  case, 


r 

(.   50^  THUNDER-STORMS. 


any  aqueous  vapor  which  might  have  been  ejected  from  the  submarine  crater 
must,  have  been  condensed  before  the  column  reached  the  surface  of  the  sea, 
and  the  smoke  which  rose  into  the  atmosphere  must  have,  therefore,  been  free 
from  vapor  ;  yet  this  smoke  or  cloud  of  vjlcanic  dust  was  intersected  by  light- 


OF    THE    HEIGHT    OF    STORMY    CLOUDS. 

The  distance  of  the  clouds  from  which  lightning  proceeds  is  estimated  by 
obse'rving  the  interval  of  time  which  elapses  between  the  moment  at  which  the 
flash  is  seen  and  that  at  which  the  thunder  is  heard.  It  has  been  demonstrated 
by  certain  astronomical  observations,  that  light  is  propagated  through  space  at 
tht-  rate  of  about  two  hundred  thousand  miles  in  a  second  of  time.  This  space 
being  greater  in  avast  proportion  than  the  greatest  distance  at -which  any  thun- 
der cloud  can  be  placed  from  the  observer,  it  may  be  assumed  that  the  moment 
at  which  the  lightning  is  seen  is  practically  coincident  with  the  moment  at 
which  it  emanates  from  the  cloud.  It  has,  however,  been  also  proved  that 
sound  is  propagated  through  the  air  at  about  eleven  hundred  feet  per  second. 
This  rate  is  subject  to  some  small  variations,  depending  on  the  temperature  of 
the  air,  but  for  our  present  purpose  it  may  be  taken  at  its  mean  value.  If,  then, 
the  number  of  seconds  be  observed,  which  elapse  between  the  moment  a  flash 
of  lightning  is  seen  and  the  moment  the  thunder  consequent  upon  it  is  heard, 
and  eleven  hundred  feet  be  allowed  for  each  second  in  that  interval,  the  dis- 
tance of  the  place  whence  the  lightning  issues  from  the  observer  will  be  de- 
termined. Thus,  if  five  seconds  elapse,  the  distance  will  be  five  thousand 
five  hundred  feet ;  for  six  seconds,  it  will  be  six  thousand  six  hundred  feet,  and 
so  on. 

If  the  cloud  be  vertically  over  the  observer,  this  distance  will  be  equal  to  its 
actual  height  above  the  level  of  the  observer.  If  it  be  not  vertical,  then  its  an- 
gular elevation  must  be  observed,  and  the  height  above  the  level  of  the  observer 
will  be  obtained  by  multiplying  the  computed  distance  by  the  trigonometrical 
sine  of  the  angular  elevation. 

'»  The  height  of  thunder-clouds  is  also  attempted  to  be  determined,  by  observ- 
ing the  effects  produced  upon  objects  in  elevated  situations  stricken  by  the 
lightning  which  issues  from  them.  If  it  be  admitted  that  lightning  always  de- 
scends from  the  clouds  toward  the  earth,  then  it  may  be  inferred  that  the  place 
where  such  effects  are  manifested  must  be  lower  than  the  position  of  the  cioud 
from  which  the  lightning  proceeds  ;  but,  if  it  shall  appear  that  lightnings  some- 
times dart  upward,  nothing  respecting  the  height  of  the  cloud  can  be  inferred 
from  such  effects.  Among  those  effects  which  lightning  produces  when  it 
strikes  the  earth  is  the  superficial  vitrification  of  rocks.  Such  effects  have 
been  observed  on  the  summits  of  some  of  the  highest  mountains  of  South 
America  by  Humboldt,  on  the  summit  of  Mont-Blanc  by  Saussure,  and  on  the 
Pyrenees  by  Ramond. 

In  cases  where  no  means  have  been  taken  by  those  who  witnessed  thunder- 
storms to  determine  the  height  of  the  clouds  from  which  they  proceed,  the  sit- 
uations of  the  observers  themselves  afford  a  minor  limit  of  the  value  of  that 
height.  Bouguer  and  La  Condamine  were  assailed  by  a  thunder-storm  on  one 
of  the  summits  of  the  Cordilleras,  in  Peru.  Saussure  and  his  son  encountered 
violent  storms  on  the  Col  du  Geant  and  Mont-Bljw.  MM.  Peytier  and  lios- 
sard  witnessed  thunder-storms  on  the  Pic  de  Troumouse,  the  Pic  tie  Bal> 
and  the  Tuc  de  Maitpas,  in  the  Pyrenees. 

Such  are  the  principal  observations  collected  by  M.  Arago,  made  in  mount 
ainous  localities.     The  comparison  of  the  results  of  these  with  the  heights  u! 


THUNDER-STORMS. 


507   1 1 


thunder  clouds,  computed  from  observations  made  in  flat  countries  and  at 
would  supply  means  of  determining  whether  the  development  of  storms  H  af- 
fected by  the  density  of  the  air  in  which  the  clouds  float,  or  by  their  proximity 
to  the  surface  of  the  earth.  Thus,  if  it  should  appear  that,  in  rlouds  at  the 
same  height  above  the  level  of  the  sea,  storms  are  developed  more  frequently 
when  these  clouds  are  in  the  neighborhood  of  mountains,  and  therefore  at  *a 
comparatively  small  distance  from  the  surface  of  the  earth,  it  would  follow,  with 
a  probability  proportionate  to  the  number  and  character  of  the  facts  observed, 
that  the  earth  exerts  an  influence  on  clouds  charged  with  electricity  independ- 
ently of  the  atmosphere  in  which  these  clouds  float. 

The  height  of  thunder-clouds  observed  in  a  flat  country,  or  at  sea,  are  ob- 
tained by  the  method  first  mentioned,  that  is,  by  observing  the  interval  between 
the  flash  and  the  thunder,  and  measuring  or  estimating  the  angular  elevation 
of  the  cloud.  Unfortunately,  the  latter  element  of  the  computation  has  been  very 
frequently  neglected  by  observers,  the  sole  object  having  been  apparently  to 
determine  the  distance  of  the  cloud  from  their  station,  and  not  its  vertical  height. 
In  some  cases  it  appears,  incidentally,  that  the  cloud  from  which  lightning  is- 
sued was  in  the  neighborhood  of  the  zenith,  and  consequently  the  distance  may 
be  taken  as  equivalent  to  the  height.  In  some  few  the  angular  elevation  has 
been  observed  and  recorded,  and  consequently  the  vertical  height  of  the  cloud 
may  be  computed. 

The  following  results  of  the  labors  of  various  observers  have  been  collected 
by  M.  Arago  : — 


Heightof 

Vertical 

tbunder- 

leigutof 

itricken 

bunder- 

Place. 

Observer. 

Date. 

rock 

cloud 

Observation*. 

above  Uie 

bovethe 

level  of 

level  of 

the  sea. 

tbeueo. 

Feet. 

Feet. 

Summit  of  Mont  Blanc 

Saussure  - 

... 

15,777 

Mont  Perdu(Pyrenees) 

Ramond    ... 

- 

11.185 
9  627 

Pinchincha,  (Cord.)     - 

Bouguer  and 

Storm  mentioned  by  Bon- 
guer  in  his  work  on  the  figure 
of  the  earth. 

La  Condamine 

The  thunder  in  this  storm 

ColduGgant 

Saussure  - 

5th  July,  1788  - 

• 

11,382 
14  760 

succeeded  the  lightning  with- 
out any  sensible  interval. 

Pic  de  Troumouse(Pyr) 

Peytier  and  Hossard 

1R9fi 

9,840 
10  496 

Tuc  de  Maupas  (Pyr.) 
Paris    - 

1827  . 

10,824 

De  L'Isle  - 

6th  June,  1712 

25,500 

Tobolsk  (Siberia) 

Chappe     - 

2d  July,  1761 
13th  July,  1761 

10,955 
11,382 

Berlin  - 

Lambert    - 

25th  May,  1773 
17th  June,  1773 

6,232 
5,248 

Pondicherry 
Tobolsk 

Legentil   - 
Chappe 

28th  Oct.,  1769 
1761 

From 

10,824 
700  to 

2,600 

The  height  of  thunder-clouds  determined  by  other  data  being  in  some  cases 
greater  than  the  heights  of  rocks  vitrified  by  lightning,  there  is  nothing  in  the 
comparison  of  the  results  exhibited  in  the  preceding  table,  to  justify  the  suppo- 
sition that  the  vitrifications  observed  by  Humboldt,  Saussure,  and  Ramon,  .Ud 
not  proceed  from  lightning  which  issued  from  clouds  at  a  greater  elevation. 
But,  on  the  other  hand,  facts  are  not  wanting  to  show  that  this  inference  can- 
not be  certainly  made.  There  is  a  church  in  Styria  erected  on  a  summit  of  i 
lofty  peak  called  Mount  Saint  Ursula.  Jean-Baptiste  Werloschnigg,*  medica 
practitioner,  who  happened  to  visit  this  church  on  the  first  of  May,  1700.  ot 
served  a  stratum  of  dense  black  clouds  to  be  formed  below  him  at  about  half  he 
elevation  of  the  place  where  he  stood.  These  clouds  soon  became  the  seat  of 


508  THUNDER-STORMS. 


a  violent  thunder-storm.  Meanwhile  the  heavens  remained  perfectly  clear,  the 
sun  shining  with  unusual  splendor.  No  one  thought  for  a  moment  of  danger; 
nevertheless,  a  flash  of  lightning,  ascending  from  the  cloud,  struck  the  church, 
and  killed  seven  persons  who  were  in  company  with  Werioschnigg. 

It  is,  therefore,  clearly  established  that  lightning  may  issue  upward  from 
thunder-clouds. 

LIGHTNING. 

Lightnings  are  resolved  by  M.  Arago  into  three  classes  :  First,  the  zigzag, 
which  present  the  appearance  of  narrow,  well-defined  threads  or  lines  of  light, 
following  a  course  which  is  clearly  enough  expressed  by  their  name.  In 
color  they  vary,  being  often  white,  sometimes  purple,  blue,  or  violet.  Second, 
those  lightnings  which  appear  diffused  over  extensive  surfaces,  and  which  are 
commonly  called  sheet-lightning.  In  color  these  also  vary,  being  often  an  in- 
tense red,  but  occasionally  white,  blue,  or  violet.  This  lightning  has  an  ap- 
pearance of  a  momentary  light  seen  through  a  plate  of  glass  rendered  semi- 
transparent  by  having  its  surface  ground.  Third,  lightning  which  moves 
through  the  air  at  a  comparatively  slow  rate,  appearing  like  a  luminous  ball  or 
sphere,  or  like  a  globe  of  fire.  Let  us  call  this  ball-lightning. 

The  almost  incredible  velocity,  as  will  hereafter  appear,  of  lightning  of  the 
first  class,  would  hardly  seem  compatible  with  the  sudden  and  extreme  changes 
of  direction  to  which  its  motion  is  subject.  This  frequent  reversion  of  direc- 
tion has  been  more  especially  observed  in  the  lightning  which  traverses  vol- 
canic clouds.  Minute  and  circumstantial  accounts  of  such  appearances  have 
been  supplied  by  Sir  WILLIAM  HAMILTON  and  others,  who  have  observed  the 
eruptions  of^Vesuvius.  In  the  eruption  of  1707,  described  by  SORRENTINO, 
the  lightnings  which  issued  from  the  crater  traversed  the  cloud  of  ashes  as  far 
as  the  cape  Pausillippo,  where  the  cloud  terminated.  After  attaining  that  point 
the  lightning  retraced  its  course,  and  struck  the  summit  of  the  volcano. 

Sir  William  Hamilton  states,  that  in  the  eruption  of  1779  the  lightning  was 
generally  confined  in  its  play  to  the  cloud  of  ashes  which  extended  toward 
Naples  ;  that  in  traversing  that  cloud  from  the  crater  to  its  limits,  it  seemed  to 
menace  the  city  with  destruction  ;  but  it,  nevertheless,  after  reaching  the  limit 
of  the  cloud,  returned  toward  the  crater,  where  it  rejoined  the  ascending  col- 
umn whence  it  originally  issued. 

Zigzag  lightning  seldom  flashes  between  two  clouds.  It  is  generally  mani- 
fested between  a  cloud  and  some  terrestrial  object. 

It  has  been  supposed  that  the  extremity  of  the  lightning  of  the  first  class  has 
a  barbed  form,  like  the  point  of  an  arrow.     Of  this  there  is  no  sufficient  evi- 
dence.    It  is,  however,  sufficiently  ascertained  that  it  is  often  attended  by  the 
effect  which  has  given  it  the  name  of  forked  lightning.     Thus,  when  a  single 
luminous  line  issuing  from  a  cloud  has  traversed  a  certain  distance  it  will 
sometimes  divide  itself  into  two  lines,  which,  diverging  at  an  angle  more  or  \ 
less  considerable,  will  strike  distant  objects.     In  some  cases  it  has  been  seen  ) 
to  separate  into  three  perfectly  distinct  lines.     The  former  may  be  called  bi- 
cuspidated,  and  the  latter  tri-cuspidated  lightning. 

Well-ascertained  examples  of  these  phenomena  are  rare ;  the  occasional 
occurrence  is  not,  however,  the  less  certain.  The  abbe  RICHARD  states  that 
he  witnessed  a  flash  of  lightning  which  left  the  cloud  in  a  single  line  of  light, 
and  at  some  distance  from  the  earth  dividing  into  two,  and  each  part  struck  a 
separate  object. 

NICHOLSON  states,  that,  in  a  storm  which  broke  over  the  west  end  of  London, 
on  the  19th  of  June,  1781,  being  at  Battersea,  he  saw  distinctly  several  flashes 
of  bi-cuspidated  lightning. 


THUNDER-STORMS.  5Q9 


The  Abbe  FERRARA  relates  that  on  the  18th  of  June,  1763,  he  witnessed 
tri-cuspidated  lightnings  in  the  clouds  which  issued  from  the  southern  side  of 
Etna  during  an  eruption. 

The  German  meteorologist,  KAMTZ,  states  that  he  witnessed  on  one  occa- 
sion, and  one  only,  tri-cuspidated  lightning. 

';       If  the  simultaneous  destruction  of  two  or  more  objects  in  the  same  locality 
'{  by  lightning  could  be  taken  as  conclusive  evidence  of  a  corresponding  sub-di- 
vision of  a  single  flash,  numerous  examples  might  be  given  of  multi-cuspidated 
lightning.     Such  grounds  are,  however,  too  conjectural  to  be  admitted  as  the 
basis  of  any  safe  conclusions. 

It  is  a  general  opinion  that  cuspidated  lightnings,  or  lightnings  of  the  first 
class,  are  those  only  by  which  terrestrial  objects  are  stricken.* 

The  lightnings  of  the  second  class,  or  sheet-lightnings,  are  inferior  in  the  in- 
tensity, and  generally  different  in  the  color  of  their  light,  from  those  of  the 
first  class.  These  distinctions  are  very  apparent  whenever  the  space  over  which 
sheet-lightning  is  diffused  is  intersected  by  flashes  of  cuspidated  lightning. 
Sheet-lightning  sometimes  appears  to  illuminate  the  edges  only  of  the  clouds  ; 
occasionally,  however,  it  seems  to  issue  from  the  interior  of  their  mass.  The 
common  expression  that  the  clouds  appear  to  open,  is  strongly  indicative  of  its 
appearance. 

Sheet-lightning  is  that  which  is  the  most  frequent,  and  every  one  is  familiar 
with  its  appearance,  many  having  never  seen,  or  never  noticed  any  other.  In 
common  thunder-storms  it  appears  in  a  thousand  cases  for  one  in  which  cus- 
pidated, or  ball-lightning,  is  exhibited. 

The  flashes  of  sheet-lightning  often  appear  in  very  rapid  succession,  and 
continue,  with  interruptions,  for  many  hours.  In  extreme  heat,  these  flashes 
succeed  each  other  as  rapidly  as  the  flapping  of  the  wings  of  a  small  bird,  and 
present  a  flickering  appearance  in  the  clouds  which  they  illuminate.  The 
thunder  by  which  they  are  accompanied  is  generally  low  and  distant. 

Lightning  of  the  third  class,  or  ball-lightning,  is  still  more  rare  in  its  appear- 
ance than  the  zig-zag,  or  cuspidated  lightning.  The  following  instances  of  this 
meteor  have  been  collected  by  M.  Arago  : — 

*  If  tbe  reader  has  attentively  considered  the  preceding:  paragraphs,  and  what  has  been  elsewhere 
written  on  this  subject,  he  will  be  sensible  of  the  deficiency  in  the  vocabulary  of  the  English  lan- 
guage as  regards  the  effects  necessary  to  be  expressed.  T  here  are  tliree  distinct  terras  in  the  French 
language,  Le  Tonnerre,  L' Eclair,  and  La  Foudre.  The  first  expresses  the  sound  proceeding  from  the 
clouds  which  usually  follows  the  flash  of  light,  and  is  properly  translated  by  thunder.  Tbe  second 
expresses  the  light  which  precedes  the  thunder,  and  the  third  expresses  the  actual  mutter,  the  physi- 
cal sitosfatu-e,  whatever  it  may  be.  which  strikes  terrestrial  objects,  and  produces  those  effects  which 
are  so  well  known.  In  English  there  is,  properly  speaking,  no  term  corresponding  to  La  Foudre. 
The  terms  thunder  and  lightning  are  indifferently  used  to  express  the  same  effect  as  when'we  say 
thunder-struck  and  struck  with,  lightning.  In  French  there  is  also  the  useful  and  necessary  verb 
foudroyer,  of  which  there  is  no  better  English  synouyme  than  to  strike  u-ith  ligltfning.  The  temi 
thunder-bolt  corresponds  to  La  Foudre,  but  it  is  scarcely  admissible  into  the  nomenclature  of  science. 
The  electric  fluid,  which  is  sometimes  used  to  avoid  the  term  thunder-bolt,  is  faulty,  iuaa.imch  as  an 
effect  familiar  to  all  mankind  in  all  ages,  ought  not  to  be  expreise-i  by  a  term  having  imnHxtitUe  refer- 
ence to  modern  physical  science. 


510 


THUNDER-STORMS. 


Place. 

Time. 

Observer. 

Appearances. 

Effects. 

Conesnon,  near 

April    14-15, 

M.  Deslandes. 

Th»ee  globe*  of  lire  three  and 

They  destroyed  a 

Brest. 

1718. 

a  half  teet  in  diameter. 

church. 

Horn. 

March,   1720. 

. 

A  globe  of  fire   struck  the 

After  the  reSionnd 

earth  and  rebounded. 

struck  the  dome 

of  a  tower  and 

set  fire  to  it. 

Northampton- 

July 3d,  1725. 

Rev.  Jos.  Wasse. 

A  trlobe  of  fire  the  apparent 

shire. 

size  of  the  inooii,  accompa- 

nied by  a  hissing  noise. 

Northampton- 

July 3d,  1725. 

. 

A  globe  of  lire  as  large  as  the 

shire. 

head  of  a  man.  which  broke 

into  four  pieces  near  the 

J.-.I..  i  c    i  fxn 

church. 

Dorking,  ourrcy. 

uly  ID,  i  /ou. 

breaking  into  a  prodigious 

A      house      near 
which  they  b'ke 

number  of  fragments,  were 

was  struck   by 

dispersed  in  all  directions. 

them. 

Ludsroan,    Corn- 

Dec. 1752. 

Borlase. 

Several  balls  of  fire  projected 

wall. 

from  the  clouds  to  the  earth. 

Scheranitz,  Hun- 

Jan. 1770. 

. 

A  globe  of  fire  as  large  as  a 

It  struck  the  tow- 

gary. 

barrel  ftonneav). 

er  of  the  church. 

Isle  of  France. 

1770. 

Legentil. 

Three  globes  of  fire  issued 

from  low  clouds,  and  sud- 

denly disappeared  without 

any  explosion. 

Steeple  Aston, 

1772. 

. 

A  globe  of  fire  oscillated  for 

It  destroyed    the 

Wiltshire. 

a  long  time  in  the  air  over 

houses  ou  which 

the  village,  on  which  it  fell 

it  fell. 

vertically. 

Wakefield. 

Mar.  1,  1774. 

Nicholson. 

Meteors  like  falling  stars  fell 

from    the    higher   of   two 

clouds  to  the  lower. 

Eastbourne,  Sus- 

Sept. 1780. 

Jas.  Adair. 

Several  balls  of  fire  fell  from 

Two  serv'ts  were 

sex. 

a  large  black  cloud  into  the 

killed     in     the 

MM. 

house  of  the  ob- 

server   at  .  the 

same  moment. 

Villers     la     Ga- 

Aug.  18,1732.' 

Haller. 

A  globe  of  fire  passed  over 

It  str'ck  the  house 

renne. 

the  village. 

of  Haller. 

Portsmouth. 

Feb.  14,  1809. 

. 

Thiee  successive  balls  of  fire 

They  struck  three 

fell  from  the  clouds  in   a 

times  the   ship 

short  interval  of  time. 

Warren    Hast- 

ings     in      the 

harbor,  passing 

down  the  masts 

each  time. 

Cheltenham. 

April,  1814. 

Howard. 

A  globe  of  fire  fell  from  the 

It    struck  a    mill 

clouds. 

which     it      de- 

stroyed. 

Vesuvius. 

1779  and  1794 

Sir  W.  Hamilton. 

Luminous  globes  appeared  in 

the  volcanic  clouds,  which 

burst  like  shells  from  a  mor- 

tar, projecting   on    every 

side  zig-zag  Hashes. 

Two  balls  of  lire  bounded 

The  roval  palace 

Mad  lid. 

like   elastic    balls   in    the 

was  struck  with 

chapel  and  burst  in  pieces. 

lightning. 

Samford,      Cour- 

Oct.  7,  1711. 

. 

A  voluminous  globe  of  fire 

One  of  the  towers 

tenay,    Devon- 

fell among  persons  assem- 

of   the    church 

shire. 

bled  under  the  porch  of  the 

was    destroyed 

church  during  a  storm.    At 

by    the     light- 

the   same     moment    four 

ning. 

smaller  globes  burst  within 

the  church  and  filled  it  with 

a  sulphureous  smoke. 

Steeple   Aston, 

1772. 

Reverend  Messrs. 

In  the  same  storm,  the  observ- 

P itcairne  was 

Wiltshire. 

Wainhouse  and 

ers  being  in  a  room  in  the 

dangerously 

Pitcairue. 

vestry,  saw  suddenly  ap- 

wounded ;  —  his 

i 

pear  before  them  at  a  foot 

body,     clothes, 

1 

distance,  and   about  their 

shoes,    and    his 

i 

own  height,  a  ball  of  fire 

watch,  showed 

I 

about  the  size  of  a  closed 

the  usual  marks 

i, 

hand,    surrounded    by    a 

of  being  struck 

THUNDER-STORMS. 


511 


PUce. 

Time. 

Observer. 

Appearances. 

Effect*. 

black  smoke.   It  burst  with 

by       lightning. 

a  noise   like  that  of   the 

He  stated   that 

simultaneous  discharge  of 

he      saw      the 

several  pieces  of  ordnance. 

globe  of  fire  in 

A  sulphureous  vupor  •was 

tin;     room    for 

diffused  through  the  house. 

one  or  two  wo- 

Lights  of  various  colors. 

unds    after    he 

and  having  various  oscilla- 

was sensible  of 

tory  motions,  were  seen  to 

having     been 

Petersburg. 

1752. 

Sokoloff. 

play  through  the  room. 
On  the  occasion  of  the  death 

struck. 
Richmann      was 

of  Richmaun,  a  ball  of  fire 

killed. 

passed  from  the  conductor 

to  his  body. 

Newcastle  on 

1809. 

David  Button. 

In  a  thunder-storm  the  light- 

Tyne. 

ning  descended  the  chim- 

ney of  the  house  of  the  ob- 

server, and  after  an  explo- 

sion, several  persons  assem- 

bled in  a  room,  saw  at  the 

door  of  the  room  a  globe  of 

fire,   which,  after  remain- 

ing   sometime  immovable. 

advanced      to    the     mid- 

• 

dle  of  the  room,  •where  it 

burst    into    several    frag- 

ments with  an   explosion 

like  that  of  a  rocket 

At  sea.  35°40'lat 

July  13,  1798. 

A  globe  of  fire  fell  from  the 

It  killed  one  sailor 

8.,  52  Ion.  E. 

clouds  upon  the  ship  Good 
Hope,  which  burst  with  a 

and      severely 
•wounded      an- 

violent explosion. 

other. 

Before  the  concurrent  force  of  this  evidence  all  doubt  as  to  the  reality  of  ball- 
)  lightning  must  disappear. 

But  while  on  the  one  hand  we  are  compelled  to  admit  that  such  phenom- 
;  eria  do  occur,  and  that  they  are  true  electrical  effects,  on  the  other  hand  we 
(  are  no  less  compelled  to  trace  in  them  the  characters  of  a  different  kind  of 
j  electrical  discharge  from  the  ordinary  lightning  flash.  Professor  Faraday  divides 
the  forms  of  discharge  into  the  spark,  the  brush,  and  the  glow.  The  glow  is 
most  readily  obtained  in  the  rarefied  air  of  a  partially  exhausted  receiver  ;  and 
differs  from  the  brush  in  being  due  to  a  constant  renewal  of  discharge  instead 
of  an  intermitting  action.  Now  Mr.  Snow  Harris  suggests  in  his  recent  Trea- 
tise, on  Thunder  Storms,  p.  38,  that  the  ball  discharge  in  question  possesses 
many  features  of  resemblance  to  the  glow  ;  and  in  addition  it  possesses  motion. 
The  latter  fact  is  readily  accounted  for,  inasmuch  as  the  cloud  which  causes  the 
discharge  is  always  progressing.  The  transition  from  the  glow  to  the  spark,  or 
flash,  is  easily  explained ;  for  when  the  cloud  passes  over  any  terrestrial  ob- 
ject by  which  the  resistance  to  discharge  is  reduced  within  the  striking  dis- 
tance, disruptive  discharge  must  take  place  ;  the  glow  remaining  only  so  long 
as  the  resistance  opposed  the  actual  flash.  Such  a  ball  discharge  is  described 
as  having  approached  the  ship  "  Montague,"  and  to  have  exploded  on  the  top- 
mast ;  and  this  is  just  what  Mr.  Harris's  theory  would  lead  us  to  expect.  Am 
there  is  reason  to  believe  that  many  of  the  cases  before  us  are  not  to  be  classed 
among  the  effects  of  lightning.  We  shall  again  advert  to  this. 

ON    THE    SPEED    OF    LIGHTNING. 

The  solution  of  this  problem  is  due  to  Wheatstone,  and  like  oome  other  re 
suits  of  physical  inquiry,  such  as  the  abstraction  of  lightning  from  the  clouds, 
\\  Inch  was  effected  by  a  boy's  kite,  and  the  iridescent  effect  due  to  the  varying 


512 


THUNDER-STORMS. 


of  luminous  undulations,  which  were  derived  from  observations  on 
y<r;i:-lmbbles  blown  from  a  tobacco-pipe,  it  is  found  in  the  plaything  of  a  child. 
Every  one  knows  that  if  the  end  of  a  lighted  stick  be  whirled  rapidly  rounu  in 
a  circle  or  other  curve,  it  will  present  the  appearance  of  a  continued  line  of  j 
liL'iit,  the  lighted  end,  which  occupies,  in  succession,  every  point  of  the  curve, 
appearing  to  the  eye  to  be  continually  present  at  all  its  points. 

Ficr.  1. 


To  develope  the  principle  on  which  this  fact  rests,  let  fig.  1  represent  a 
wheel  with  ten  thin  spokes  or  radii,  dividing  its  circumference  in  ten  eqv.2,1 
parts,  and  of  some  strong  bright  color,  such  as  red.  Let  this  wheel  be  put  in 
communication  with  clock-work,  so  as  to  be  made  to  revolve  uniformly  at  any 
required  rate.  This  wheel,  having  its  face  vertical,  and  turning  on  a  horizon-1 
tal  axis  ;  let  a  screen  be  placed  before  it,  so  as  to  conceal  it  from  view,  and  in 
this  screen  let  an  oblong  opening  be  made,  corresponding  in  magnitude  and 
position  to  that  spoke  of  the  wheel  which  is  in  the  vertical  position  and  pre- 
sented from  the  centre  upward.  Let  the  screen,  with  such  an  aperture,  be 
represented  in  fig.  2. 

Fig.  2. 


As  the  wheel  revolves  its  spokes  pass  the  opening  o,  in  succession,  and  if  ) 
the  motion  of  the  wheel  be  not  very  rapid,  a  person  placed  before  the  screen  ( 
will  perceive  the  spokes  appear  and  disappear  in  regular  and  uniform  succes-  ) 
sion  at  the  opening.  If  the  velocity  of  the  wheel  be  gradually  increased,  the  > 
succession  of  appearances  and  disappearances  will  be  rendered,  by  degrees,  ? 

^o 


THUNDER-STORMS.  513 


indistinct,  until,  at  length,  a  velocity  will  be  attained  which  will  cause  a  spoke 
to  be  continually  seen  at  the  opening  o,  in  the  same  manner  as  if  the  whn-1 
were  at  rest,  and  the  spoke  a  were  placed  behind  the  aperture.  Now,  since 
it  is  certain  that  in  this  case  the  presence  of  the  spokes  at  the  aperture  is  suc- 
cessive, and  that  the  intervals  which  the  spokes  are  absent  bear  to  the  intervals 
of  their  presence,  the  proportion  of  the  breadth  of  the  spokes  to  the  breadth 
of  the  spaces  between  them,  it  necessarily  follows  that  the  eye  perceives  a 
spoke  at  the  aperture  during  the  intervals  when  no  spoke  is  present  there. 

This  circumstance  is  accounted  for  by  considering  the  manner  in  which  vis- 
ion is  effected  by  means  of  the  mechanism  of  the  eye.  The  light  proceeding 
from  a  visible  objeqt,  entering  the  pupil,  strikes  the  retina  and  produces  in  it  a 
certain  vibration,  which  vibration  is  the  immediate  cause  of  the  perception  of 
the  object  from  which  the  light  has  been  transmitted.  After  the  object  has 
ceased  to  transmit  light  to  the  eye,  this  vibration  continues  for  a  certain  time, 
just  as  the  vibration  of  a  musical  string  continues  for  a  certain  interval  after 
the  bow  which  put  it  into  vibration  has  been  withdrawn  ;  and,  as  the  vibration 
of  the  string  continued,  after  the  bow  is  withdrawn,  produces  the  perception  of 
a  proportionately  prolonged  sound,  so  the  vibration  of  the  retina,  after  the  visi- 
ble object  has  been  withdrawn,  produces  a  proportionately  prolonged  perception 
of  its  presence.  In  fact,  there  is  no  damper  in  the  mechanism  of  the  eye  to 
stop  the  effect  of  the  action  of  light  at  the  instant  that  action  ceases.  It  is, 
therefore,  an  interesting  physiological  problem  to  determine  how  long  after  that 
visible  object  is  withdrawn,  and  the  action  of  light  ceases,  the  effect  on  the 
retina  remains,  and  the  object  continues  to  be  seen.  This  problem  is  beauti- 
fully solved  by  the  apparatus  above  described.  The  velocity  of  the  wheel  be- 
ing gradually  augmented  until  the  spoke  appears  to  be  continually  present  at 
the  opening,  it  has  been  found  that  t his  effect  is  produced  when  the  wheel  performs 
one  complete  revolution  in  a  second  of  time.  Since  the  space  round  the  centre 
of  the  wheel  is  equally  divided  by  the  ten  spokes,  it  follows  that  in  this  case 
the  interval  between  the  arrival  of  two  successive  spokes  at  the  opening  is  one 
tenth  of  a  second,  and  this  must,  therefore,  be  the  duration  of  the  impression 
of  an  object  on  the  retina  after  it  has  been  withdrawn.  If  the  duration  were 
less  than  this  the  colored  spoke  would  not  appear  continually  at  the  aperture  o 
when  the  wheel  revolves  in  one  second,  but  would  alternately  appear  and  dis- 
appear. If  it  were  greater,  a  less  velocity  than  one  revolution  per  second 
would  be  sufficient  to  cause  its  continuous  appearance. 

Since  there  is  nothing  in  what  has  been  stated  to  render  it  necessary  that 
the  aperture,  through  which  the  spokes  are  seen,  should  be  in  the  vertical, 
rather  than  any  other  position,  it  follows  that  in  whatever  position,  round  the 
centre,  that  aperture  be  placed,  a  spoke  will  appear  to  be  continually  behind 
it,  so  long  as  the  wheel  revolves  at  a  rate  of  not  less  than  one  revolution  per 
second. 

If,  therefore,  there  be  two  or  more  such  apertures  made  in  the  screen,  a 
spoke  will  appear  constantly  behind  each  of  them.  In  fine,  if  there  be  an  in- 
finite number  of  such  apertures  round  the  centre,  or,  in  other  words,  il  the 
screen  be  altogether  removed,  spokes  will  be  seen  in  every  direction  round  the 
centre  without  any  open  spaces  between  them,  or  what  is  the  same,  the  wheel 
will  appear  as  a  circular  disk  of  uniform  red,  no  spokes  being  distinguishable. 

We  have  here  supposed  that  the  wheel  is  continually  illuminated.  It  is  ne- 
cessary now  to  inquire  how  long  light  must  shine  upon  it  in  order  that,  revolv- 
ing once  per  second,  it  may  appear  as  a  plane  disk  without  spaces  between  the 
spokes.  If  the  light  fall  upon  it  only  for  an  instant,  that  is,  an  infinitely  short 
time,  then  the  wheel  will  be  distinctly  seen,  for  the  tenth  of  a  second,  in  the 
position  which  it  had  when  the  light  fell  upon  it.  The  spokes  will  be  as  distinct- 

33 


514  THUNDER-STORMS. 


ly  visible  as  if  the  wheel  were  at  rest.  But  if  the  light  continue  to  fall  upon 
the  wheel  during  the  tenth  of  a  second,  then  each  spoke  will  continue  to 
be  illuminated  from  the  position  it  has  the  moment  the  light  first  falls  upon  it, 
until  it  arrives  at  the  position  which  the  preceding  spoke  had  at  that  mo- 
ment. Each  spoke  will,  therefore,  act  upon  the  eye  while  it  passes  through 
the  space  between  two  successive  spokes,  and  will,  therefore,  be  seen  at  every 
point  of  that  space  ;  and  as  the  perception  it  causes  at  any  point  will  continue  < 
while  the  spoke  passes  through  the  whole  of  that  space,  it  follows  that  the 
wheel  will  appear  to  the  eye  as  a  flat,  circular  disk  uniformly  illuminated. 

If,  however,  the  light  continue  to  fall  on  the  wheel  during  an  interval  less 
than  the  tenth  of  a  second,  suppose,  for  example,  the  twentieth  of  a  second, 
then  each  spoke  will  be  illuminated  while  passing  through  half  the  interval 
between  two  successive  spokes,  and  the  wheel  will  present  the  appearance  of 
a  circle  divided  into  ten  equal  sectors,  half  of  each  sector  being  visible  and 
half  invisible.  If  the  duration  of  the  light  be  any  other  part  of  the  tenth  of  a 
second,  the  wheel  will,  for  the  same  reason,  present  the  appearance  of  a  circle 
divided  into  ten  equal  sectors,  a  portion  of  each  sector  being  visible,  bearing 
to  the  remaining  portion,  invisible,  the  same  ratio  as  the  duration  of  the 
light  bears  to  the  difference  between  that  duration  and  the  tenth  of  a  second. 
Such  an  instrument  will,  therefore,  serve  as  the  means  of  estimating  the  du- 
ration of  any  light  which  continues  to  illuminate  the  wheel  for  a  period  of  time 
not  exceeding  the  tenth  of  a  second ;  and  it  is  evident  that,  by  varying  the 
number  of  spokes  and  the  velocity  of  the  wheel,  the  duration  of  any  light  may 
be  measured  when  its  amount  is  greater  or  less  than  the  tenth  of  a  second. 

Such  is  the  instrument  which  has  been  applied  by  its  inventor  to  measure 
the  duration  of  a  flash  of  lightning,  and,  also,  of  the  electric  spark.  A  wheel 
consisting  of  a  hundred  spokes,  dividing  the  space  round  the  centre  into  as 
many  equal  sectors,  was  exposed  to  the  light  of  lightning  during  a  thunder- 
storm. By  clock-work,  it  was  made  to  revolve  ten  times  per  second,  making, 
therefore,  one  revolution  in  the  tenth  of  a  second,  and  moving  through  the  in- 
terval between  two  spokes  in  the  thousandth  part  of  a  second.  If  the  duration 
of  the  light  by  which  this  wheel  was  illuminated  amounted  to  the  thousandth 
part  of  a  second,  it  would  appear  as  a  complete  illuminated  disk  without  spokes. 
If  it  amounted  to  half  a  thousandth  of  a  second,  it  would  appear  as  a  circle 
divided  into  a  hundred  equal  sectors,  half  of  each  sector  being  visible  and  half 
invisible.  If  the  duration  of  the  light  were  instantaneous,  it  would  appear  as 
a  wheel  with  a  hundred  spokes  stationary,  in  the  particular  position  it  had  -it 
the  moment  the  light  fell  upon  it. 

Now,  such  a  wheel,  being  thus  exposed  to  the  flashes  of  lightning,  in  a  storm, 
H  was  found  that  when  illuminated  it  always  appeared  stationary,  though  revolv- 
iag  ten  times  in  a  second.  The  spokes  were  seen  distinctly,  with  no  more 
than  their  proper  thickness.  It,  therefore,  follows  that  the  duration  of  the  light 
of  the  flashes  did  not  amount  to  so  great  a  fraction  of  the  thousandth  part  of  a 
second  as  was  capable  of  being  appreciated  by  estimating  the  apparent  width 
of  the  spokes  when  seen  by  the  light  of  the  flashes.  The  duration  of  the 

£  flashes  must  then  have  been  a  very  small  fraction  of  the  thousandth  part  of  a 

(  second. 

But  the  duration  of  a  flash  is  the  time  which  the  lightning  takes  to  move 

)  through  that  part  of  space  which  it  traverses  while  it  is  visible.     Hence  it  fol- 

'  lows,  that  whatever  be  the  extent  of  such  a  distance,  it  is  traversed  in  a  very 

t  minute  'fraction  of  the  thousandth  of  a  second. 

j  This  method  of  observation  has  only  been  applied  to  lightning  of  the  first 
and  second  kind,  no  opportunity  having  yet  been  found  to  apply  it  to  ball-light- 
ning 


THUNDER-STORMS. 


515 


SILENT    LIGHTNING. 

When  the  heavens  are  perfectly  serene  in  hot  weather,  lightnings  ars  f.c- 
quently  observed  to  continue  flashing  in  the  atmosphere  for  many  hours  unac- 
companied by  thunder.  These  have  been  called  heat  lightnings.  Such  appear- 
ances are  not  confined,  as  has  been  supposed,  to  those  parts  of  the  atmosphere 
which  are  near  the  horizon  ;  on  the  contrary,  their  light  extends  frequently  over 
the  whole  visible  firmament. 

Lightning,  unaccompanied  by  thunder,  appears  much  more  rarely  when  the 
heavens  are  clouded.  Sufficient  evidence,  however,  of  this  phenomenon  in  dif- 
ferent parts  of  the  globe  has  been  collected  by  M.  Arago. 

Thibalt  de  Chanvalon,  in  his  meteorological  observations,  records  its  occur- 
rence on  two  days  in  July,  1751,  at  Martinique.  Such  lightning  is  very  com- 
mon at  the  Antilles.  Dorta  mentions  the  same  phenomena  at  Rio  Janeiro,  in 
a  paper  published  in  the  memoirs  of  the  Academy  of  Sciences  of  Lisbon,  in 
the  years  1783,  1784,  1785,  and  1787,  during  which  time  he  witnessed  one 
hundred  and  seventy  days  on  which  lightnings  were  seen  unaccompanied  by 
thunder. 

Lind  witnessed  at  Patna,  in  India,  latitude  N.  25°  37',  in  the  year  1826,  on 
seventy-three  days  lightning  without  thunder  ;  but  neither  Lind  nor  Dorta  state 
whether  the  heavens  were  clear  or  clouded.  The  probability  is,  that  where 
the  occurrence  of  the  phenomenon  was  so  frequent,  they  were  sometimes 
clouded. 

De  Luc,  the  younger,  mentions  a  great  storm  which  took  place  at  Geneva  on 
the  1st  of  August,  1791,  daring  which  very  vivid  lightnings  were  seen  without 
any  audible  thunder.  Some  of  the  flashes  on  this  occasion  were  so  strong  that 
the  loudest  claps  of  thunder  would  have  been  expected  to  follow  them.  In  the 
same  storm,  however,  other  flashes  were  accompanied  by  loud  thunder. 

Dalton  states  that,  in  Kendal,  on  the  15th  of  August,  1791,  at  nine  o'clock  in 
the  evening,  he  witnessed  in  a  storm  vivid  and  continual  flashes  of  lightning, 
but  heard  only  some  thunder  which  was  distant. 

At  Philadelphia,  in  the  month  of  July,  in  the  year  1841,  and  in  New  York, 
in  the  following  month,  I  witnessed  frequent  thunder-bursts  (as  they  are  there 
called),  in  which  in  a  clouded  sky  I  saw  a  constant  succession  of  flashes  of 
lightning,  which  sometimes  continued  for  several  hours,  accompanied  by  very 
short,  occasional  showers  of  rain.  On  these  occasions  thunder  was  sometimes 
not  heard  at  all,  and  sometimes  it  was  only  heard  after  long  intervals  of  silence, 
and  seemed  from  its  sound  to  be  distant.  The  lightnings,  nevertheless,  were 
vivid,  and  illuminated  the  heavens  to  the  zenith.  They  appeared  generally  like 
a  light  behind  the  clouds,  the  edges  of  which  were  strongly  illuminated,  ths 
centres  more  faintly.  These  lightnings  sometimes  succeeded  each  oth',r  so 
rapidly  that  they  had  a  fluttering  appearance,  like  the  motion  of  the  wings  of  a 
small  bird  ;  and  this  fluttering  of  light  would  be  often  continued  for  three  or 
four  seconds.  These  trembling  lightnings  would  succeed  each  other  '-'.  inter- 
vals of  some  minutes. 

OF    LUMINOUS    CLOUDS. 

In  the  darkest  nights  of  winter,  at  the  hour  of  midnight,  when  the  influence 
of  the  solar  light  is  altogether  withdrawn  from  the  atmosphere,  and  in  the  ab- 
sence of  moonlight,  a  sufficient  quantity  of  light  is  always  uffused  to  render 
objects  around  us  faintly  visible,  and  to  enable  us  to  walk  without  hesitation  in 
any  open  country.  If  the  firmament  be  serene  and  cloudlese:  this  light  is  as- 


THUNDER-STORMS. 


cribed  to  the  stars.     But  let  the  heavens  be  overcast,  let  the  stars  be  hidden 
by  an  unbroken  mass  of  the  most  dense  clouds,  and  still  a  sufficiency  of  light 
will  be  diffused  in  the  open  country  to  prevent  any  of  the  difficulty  and  incon- 
venience which  would  attend  any  attempt  to  walk  in  a  dark  cave,  or  in  an 
apartment  with  closed  windows.     It  cannot,  then,  be  doubted  that,  in  the  most 
clouded  nights  of  deep  winter,  light,  proceeding  from  some  source,  is  diffused 
through  the  air.     If  this  light  be  supposed  to  be  that  of  the  stars  penetrating 
the  clouds,  it  is  necessary  to  admit  that  the  light  of  the  stars  in  a  clear  night  is 
greater,  in  the  same  proportion  as  the  splendor  of  the  unclouded  noonday  sun 
•'  sxceeds  the  light  when  the  firmament  is  covered  with  dense  clouds.     No  one 
(  having  the  least  powers  of  observation  can  admit  such  an  assumption ;  and  if 
(  it  be  not  admitted,  there  remains  no  other  explanation  of  the  nocturnal  light  of 
^  a  clouded  sky,  except  in  the  admission  that  the  clouds  themselves  are  faintly  lu- 
minous. 

If  the  supposition  of  the  self-luminous  property  of  clouds  be  entertained,  the 
probability  that,  under  varied  circumstances  of  form,  density,  mutual  position, 
temperature,  and  many  other  conditions,  which  will  easily  suggest  themselves 
to  every  mind,  clouds  may  be  endowed  with  this  quality  in  various  degrees. 
The  probability,  therefore,  of  the  hypothesis  which  we  have  just  proposed  to 
account  for  nocturnal  light,  will  be  strengthened,  if  it  can  be  shown  that,  on 
particular  occasions,  clouds  have  been  observed  unequivocally  and  in  much 
higher  degrees  luminous. 

In  a  memoir  of  Rozier,  dated  15th  of  August,  1781,  that  philosopher  states 
that,  being  at  Bezieres  on  that  day,  in  the  evening,  at  a  quarter  before  eight 
o'clock,  the  sun  having  gone  down,  and  the  firmament  being  overcast,  thunder 
was  heard.  At  five  minutes  past  eight,  it  being  then  complete  night,  the  storm 
having  attained  its  height,  Rozier  observed  a  luminous  point  above  the  brow  of  a 
hill  fronting  his  house,  which  gradually  augmented  in  magnitude  until  it  as- 
sumed the  form  and  appearance  of  a  phosphoric  zone,  subtending  at  his  eye  an 
angle  of  about  sixty  degrees  measured  horizontally,  and  having  the  apparent 
height  of  a  few  feet.  Above  this  luminous  zone  was  a  dark  space  equal  to  its 
own  breadth,  and  over  that  space  appeared  another  horizontal  zone,  of  the  same 
breadth,  and  about  half  the  apparent  length.  The  middle  of  each  of  these  zones 
exhibited  a  uniform  brightness,  but  the  edges  were  irregular.  Lightning  is- 
sued three  times  from  the  edges  of  the  inferior  zone,  but  no  thunder  was  audi- 
ble. The  duration  of  this  extraordinary  phenomenon  was  nearly  a  quarter  of 
an  hour. 

Nicholson  relates  that,  on  the  30th  of  July,  1797,  at  about  five  o'clock  in  the 
morning,  he  observed  the  heavens  covered  with  dense  clouds,  which  moved 
raf idly  to  the  west-southwest.  Lightnings  played  constantly  at  northwest  and 
southwest,  which,  after  an  interval  of  twelve  seconds,  were  succeeded  by  loud 
claps  of  thunder.  The  lower  parts  of  the  clouds,  which  were  undulated  and 
checkered,  exhibited  a  red  light  which  was  very  vivid.  At  one  moment,  houses 
placed  in  front  of  that  which  he  inhabited  had  the  appearance  which  would 
have  been  produced  by  viewing  them  through  a  deep-blue  glass ;  at  that  time, 
on  looking  at  the  clouds,  they  appeared  to  emit  a  blue  light. 

Beccaria  states  that  the  clouds  over  his  observatory  at  Turin  frequently  dif- 
fused in  all  directions  a  strong  reddish  light,  which  was  sometimes  so 
intense  as  to  enable  him  to  read  a  page  printed  in  ordinary  type.  This 
nocturnal  light  was  especially  observed  in  winter,  between  successive  snow- 
showers. 

The  selfsame  luminous  quality  has  been  observed  in  fogs.  The  dry  fog  of 
1783  was  described  by  M.  Verdueil,  a  physician  of  Lausanne,  as  having  dif- 
fused at  night  a  light  sufficiently  strong  to  render  distant  objects  visible,  and 


j  iusea 


517 


this  light  was  equally  spread  in  all  directions.     It  resembled  the  light  of  the 
moon  seen  through  clouds. 

De  Luc  states  that,  returning  home  to  his  lodgings  in  the  neighborhood  of 

London,  on  a  winter  night,  when  the  atmospherewas  clear,  and  not  cold,  he 

saw  a  band  of  clouds  intersecting  the  southern  meridian,  about  thirty  or  forty 

degrees  from  the  zenith,  and  extending  on  either  side  nearly  to  the  eastern  and 

(  western  horizons.    The  brightness  of  this  cloud  resembled  that  of  a  thin  cloutj 

I  concealing  the  moon,  and  was  sufficient  to  render  the  stars  in  its  neighborhood 

(  invisible. 

Dr.  Robinson,  professor  of  Astronomy  at  Armagh,  states,  in  a  letter  to  M. 
Arago,  that,  during  the  voyage  of  Major  Sabine  in  Scotland,  undertaken  to  ob- 
serve the  lines  of  equal  magnetic  intensity,  that  officer,  being  at  anchor  in 
Lough  Scarig,  in  the  Isle  of  Sky,  observed  a  cloud  which  constantly  enveloped 
the  summit  of  one  of  the  naked  and  lofty  mountains  which  surround  that  island. 
This  cloud,  which  resulted  from  the  precipitation  of  the  vapor  brought  by  the 
constant  west  winds  from  the  Atlantic,  was  self-luminous  at  night,  not  occa- 
sionally, but  permanently.  Major  Sabine  saw  frequently  issue  from  it  jets  of 
light  resembling  those  of  the  aurora.  He  rejects,  however,  the  supposition  that 
these  jets  were  produced  by  real  auroras  .near  the  horizon,  and  which  were 
concealed  from  direct  observation  by  the  mountain.  He  regarded  all  these 
phenomena  of  continued  and  intermitting  light  as  originating  in  some  physical 
property  of  the  cloud  itself. 

OF    THUNDER. 

Thunder,  as  every  one  knows,  is  a  certain  noise,  proceeding  apparently  from 
the  clouds,  which  usually  follows,  after  a  greater  or  less  interval,  the  appear- 
ance of  a  flash  of  lightning.  Of  all  natural  phenomena,  those  which  occupy 
the  meteorologist  present  the  greatest  difficulties,  when  it  is  necessary  to  con- 
vey a  precise  notion  of  them  to  those  who  may  not  immediately  have  witnessed 
them.  It  is,  doubtless,  to  this  difficulty  that  we  must  ascribe  the  practice  of 
meteorological  writers  of  resorting  to  similes  and  other  like  illustrations  in  their 
descriptions. 

Thunder  is  described  by  some  as  a  sound  resembling  the  acute  noise  pro- 
duced when  stiff  paper  is  torn,  or  when  a  strong  silk  cloth  is  suddenly  torn,  or 
\vhen  a  heavy  wagon  is  rolled  rapidly  over  a  rough,  stony  road.  It  is  imitated 
with  much  effect  in  theatres  by  shaking  a  piece  of  sheet-iron  about  four  feet 
long  and  two  feet  broad.  This  is  held  in  the  hand  at  one  of  its  corners,  and 
the  varieties  of  thunder  may  be  imitated  by  skilfully  varying  the  movement  of 
the  hand. 

Thunder  is  sometimes  heard  as  a  clear,  single,  distinct  sound,  like  the  report 
of  a  gun,  unattended  by  any  reverberation.  More  frequently  the  sound  is  deep, 
or,  in  a  musical  sense,  grave,  and  consists,  not  of  a  single  sound,  but  of  that 
rapid  succession  of  sounds,  first  increasing  and  afterward  diminishing  in  inten- 
sity, which  has  been  expressed  by  the  term  rolling. 

The  difficulty  of  expressing  and  recording  in  words  the  exact  nature  of  such 
phenomena  has  limited  to  a  small  number  the  observations  on  which  any  safe 
reasoning  can  be  based. 

The  duration  of  the  rolling  of  thunder  was  observed  and  recorded  by  De 
L'Isle,  in  Paris,  in  the  year  1712.  On  one  occasion  it  was  observed  to  endure  J 
for  forty-five  seconds.  On  other  occasions,  during  the  same  storm  (17th  June), 
the  roll  continued  from  thirty-four  to  forty -one  seconds.  On  the  3d,  8th,  and 
28th  of  July,  the  roll  continued  on  different  occasions  from  thirty-five  to  thirty- 
nine  seconds. 


r 

{  518 


THUNDER-STORMS. 


)       De  L'Isle  also  observed  the  varying  intensity  of  the  sound  in  each  roll.     In 
i  some  cases  the  clap  is  loudest  at  the  commencement,  and  afterward  declines 
)  gradually  until  it  ceases  to  be  heard.     Sometimes  it  commences  with  a  low  and 
barely  audible  sound,  which  augments  in  force  until  it  attains  a  maximum  loud- 
ness,  after  which  it  diminishes  gradually  in  intensity  until  it  becomes  inaudible. 
These  changes  were  carefully  observed  and  recorded  on  several  occasions  by 
De   L'Isle.     The   following   examples   will   serve    to   illustrate   the  phenom- 
enon : — 


1712  Seconds. 

17th  of  June,          0  Lightning  flashed. 

3  Thunder  feebly  audible. 

12  Thunder  loudest. 

19  Thunder  became  gradually  inaudible. 

21st  of  July,  0         Lightning  flashed. 

16         Thunder  feebly  heard. 

20  Thunder  loudest. 

32        Thunder  became  gradually  inaudible. 

8th  of  July,  0         Lightning  flashed. 

11  Thunder  feebly  heard. 

12  Thunder  loudest. 

38        Loudest  thunder  began  to  decrease  in  force. 
47        Thunder  became  gradually  inaudible. 

8th  of  July,  0         Lightning  flashed. 

11  Thunder  feebly  heard. 

12  Thunder  became  loudest. 

38  Thunder  began  to  decrease  in  loudness. 
47         Thunder  became  gradually  inaudible. 

8th  of  July,  0         Lightning  flashed. 

10         Thunder  feebly  heard. 

13  Thunder  became  loud. 

20        Thunder  broke  with  redoubled  force. 
35        Thunder  began  to  lose  its  force. 

39  Thunder  became  gradually  inaudible. 

It  appears  from  these  observations  that  the  durations  of  the  loudest  part  of 
each  roll  varied  from  twenty,  to  thirty  seconds. 

The  degree  of  loudness  is  also  very  various.  On  the  2d  of  March,  1769, 
the  tower  of  the  church  at  Buckland  Brewer  was  struck  by  lightning,  followed 
by  a  clap  of  thunder  described  by  an  ear-witness  as  equal  to  the  simultaneous 
report  of  one  hundred  pieces  of  cannon. 

The  most  violent  thunder  sometimes  follows  ball-lightning.  When  the  ship 
Montague  was  struck,  on  the  4th  of  November,  1749,  the  captain  (Chalmers) 
declared  that  the  sound  produced  by  the  explosion  was  equal  to  the  simulta- 
neous discharge  of  several  hundred  pieces  of  ordnance,  but  that  it  did  not  last 
above  half  a  second. 

The  interval  of  time  which  elapses  between  the  flash,  of  lightning  and  the 
thunder  which  succeeds  it  is  an  important  element  in  the  theoretical  investiga- 
tion of  the  atmospheric  conditions  which  produce  these  phenomena.  It  is  es- 
pecially useful  to  ascertain  the  major  and  minor  limits  of  this  interval.  The 
observations  of  this  kind  collected  by  M.  Arago  are  arranged  in  the  following 
table  : — 


THUNDER-STORMS. 


5i; 


Places. 

Time. 

Observer.      j     Interval*. 

Kncnnd*. 

Peters  burgh. 

2d  May,  1712 

De  L'Isle. 

42 

— 

— 

— 

48 

— 

— 

— 

48 

— 

6th  June,  1712 

— 

47 

— 

— 

— 

48 

— 

— 

— 

48 

— 

— 

— 

49 

— 

30th  April,  1712 



72 

Tobolsk. 

2d  July,  1761 

— 

42 

— 

— 

— 

45 

— 

— 



47 

— 

10th  July,  1761 

— 

46 

— 

— 

— 

2 

— 

— 

De  L'Isle. 

3 

— 

— 

— 

4 

— 

— 

— 

5 

M.  Arago  states,  as  the  general  impression  on  his  memory,  that  he  has  often 
observed  the  thunder  follow  the  flash  after  an  interval  so  brief  as  half  a  second. 

In  the  early  part  of  June,  1841,  being  in  the  reading-room  of  the  Alfie- 
iianm  at  Philadelphia,  I  witnessed  a  vivid  flash  of  lightning  which  was  suc- 
ceeded by  the  loudest  clap  of  thunder  I  ever  recollect  to  have  heard.  The  in- 
terval was,  by  my  estimation,  a  very  small  fraction  of  a  second.  An  ordinary 
observer  would  have  said  that  the  flash  and  the  sound  were  simultaneous 

The  occurrence  of  thunder  not  preceded  by  lightning  has  not  been  proved 
by  evidence  as  clear  and  satisfactory  as  that  by  which  the  existence  of  silent 
lightnings  have  been  established.  No  example  is  found  of  it  in  any  of  the  me- 
teorological registers  kept  at  observatories  in  Europe.  Tkibanlt  de  Chanvalun, 
already  quoted,  mentions  in  the  register  of  his  observations  made  at  Martinique, 
that  in  October,  1751,  there  were  two  days  on  which  thunder  was  heard  with- 
out the  appearance  of  lightning ;  and  that  on  one  day  in  November  there  were 
three  loud  claps  of  thunder  without  lightning. 

On  the  19th  of  March,  the  vessel  in  which  Bruce  the  traveller  had  embarked 
on  the  Red  sea,  near  Cosseir  encountered  a  clap  of  thunder  so  violent  as  to 
strike  the  seamen  with  terror.  There  was  no  lightning. 

The  occurrence  of  thunder  when  the  firmament  is  cloudless  has  been  doubted. 
SENKBIER  speaks  of  thunder  on  clear  days  as  a  known  fact,  but  does  not  state 
whether  such  was  the  result  of  his  own  observations.  VOLXEY  states,  that  on 
the  12th  of  July,  1788,  at  six  o'clock  in  the  morning,  the  sky  being  unclouded, 
he  heard  at  Pont  Chartrain,  a  place  four  leagues  from  Versailles,  four  or  five 
claps  of  thunder.  At  a  quarter  past  seven  clouds  began  to  rise  in  the  south- 
west, and  in  some  minutes  the  heavens  were  covered.  Soon  afterward  hail- 
stones fell  as  large  as  a  man's  fist. 

The  noise  which  often  attends  earthquakes  is  similar  to  thunder,  and  by  an 
acoustic  deception  not  yet  clearly  explained,  it  is  heard  as  if  it  proceeded  irom 
the  upper  regions  of  the  air.  Observations,  therefore,  of  supposed  thunder 
with  a  clear  sky,  in  places  subject  to  earthquakes,  cannot  safely  be  received  as 
evidence  of  real  thunder. 

THE    ATTEMPTS   TO   EXPLAIN    THE    PHENOMENA    OF    THUNDER    AND    LIGHTNING. 


Although  the  investigations  of  Franklin  removed  all  doubts  respecting  the  £ 

identity  of  lightning  and  artificial  electricity,  still,  in  the  great  variety  of  atmo-  < 

spheric  phenomena  developed  in  the  disturbances  of  electrical  equilibrium  which  ( 

are  produced  on  so  grand  a  scale  in  the  vast  regions  of  the  air,  much  remained  / 


520  THUNDER-STORMS. 


!  and  still  remains  unexplained.  Succeeding  philosophers  have  accomplished 
'  little  more  than  exhibiting,  by  direct  experiments,  and  by  the  comparison  oi 
numerous  observations,  analogies  which  throw  more  or  less  light  on  the  rela- 
tions between  the  appearances  which  are  exhibited  in  the  atmosphere  and 
those  general  laws  which  have  been  deduced  from  experiments  made  on  arti- 
ficial electricity. 

The  luminous  appearances  which  attend  the  electrical  discharges  in  the  at- 
mosphere, and  which  characterize  the  different  kinds  of  lightning,  must  be  re- 
garded as  explicable  on  the  same  principles  as  those  of  artificial  electricity  ;  and 
the  various  hypotheses  and  conjectures,  more  or  less  plausible,  which  have 
been  proposed  to  account  for  the  one  must  equally  be  brought  to  bear  on  the 
other. 

To  regard  the  principle  which  darts  through  space  with  the  enormous  ve- 
locity which  the  observations  of  Professor  Wheatstone  have  shown  lightning 
to  be  endowed  with,  as  ponderable  matter,  is  extremely  difficult.  If  it  be  pon- 
derable matter  it  must  follow  the  path  of  projectiles,  and,  consequently,  its  course 
must  be  curved  with  a  concavity  turned  toward  the  earth,  except  when  it  fol- 
lows the  vertical  direction.  In  the  zigzag  path  of  cuspidated  lightning  there 
is  nothing  analogous  to  this.  On  the  other  hand,  such  rapid  and  rectilinear 
motions  are  quite  consonant  with  the  supposition  of  a  system  of  undulations 
propagated  through  a  highly  elastic  medium,  and  are  in  all  respects  analogous 
to  the  actual  phenomena  of  light.  The  bi-cuspidated  lightning  finds  its  obvi- 
ous type  in  the  double  refraction  of  crystallized  media,  and  the  heterogeneous 
matter  suspended  in  different  strata  of  the  air  through  which  the  lightning  is 
transmitted  completes  the  parallel. 

The  undulatory  hypothesis  is,  nevertheless,  beset  with  its  own  difficulties. 
How  can  the  pulsations  of  an  imponderable  ether  be  reconciled  with  the  me- 
chanical effects  of  lightning  ?  The  analogy  to  the  phenomena  of  light  fails 
when  it  is  considered  that,  notwithstanding  its  velocity  of  200,000  miles  per 
second,  light  has  never  acquired  in  its  motion,  even  when  condensed  by  the 
largest  burning  reflector,  sufficient  momentum  to  affect  in  any  sensible  degree 
the  lightest  substance  suspended  in  vacuo  by  a  filament  of  spider's  web,  while, 
on  the  contrary,  the  electric  fluid,  issuing  from  the  clouds,  splits  rocks,  over- 
turns the  most  massive  structures,  destroys  gigantic  trees,  and  projects  to  a 
distance  enormous  weights. 

But  of  all  the  forms  under  which  the  results  of  electrical  explosions  in  the 
air  present  themselves,  the  most  inexplicable  is  that  of  ball-lightning.     Obser- 
vation seems  to  countenance  the  supposition  that  these  globes  of  fire  are  real 
agglomerations  of  ponderable  matter  formed  in  the  regions  of  the  air  by  some 
unexplained  process.    Where  such  formations  are  made ;  whence  proceed  their 
$  ponderable  constituents  ;  what  is  their  nature  ;  what  sustains  them  in  the  air ; 
/  and  what  causes  finally  precipitate  them ;  are  questions  before  which  science 
\  is  mute. 

^  The  constituents  of  the  atmosphere  are  oxygen  and  azote,  in  the  proportion 
'  of  four  parts  by  weight  of  the  former  to  fourteen  of  the  latter.  If  the  electric 
(  spark  be  transmitted  through  a  mixture  of  these  two  gases  confined  in  a  glass 
^  tube,  a  portion  of  the  oxygen  will  combine  chemically  with  a  portion  of  the 
J  azote,  and  nitric  acid  will  be  formed.  What  the  electric  spark  does  in  such  a 
S  mixture  the  transmission  of  the  electric  fluid  accomplishes  in  the  atmosphere, 
(  and  nitric  acid  is  formed,  distinct  traces  of  which  are  discoverable  in  the  rain 
which  falls  in  thunder-storms.  If,  then,  this  power  of  determining  the  chemi- 
cal combination  of  these  constituents  of  the  air  be  undeniable  in  this  case,  we 
cannot  reject  the  possibility  of  other  combinations  being  effected  by  the  same 
agency.  Besides  oxygen  and  azote,  the  proper  constituents  of  pure  atrao- 


THUNDER-STORMS. 


52' 


speric  air,  there  are  various  foreign  substances  occasionally  suspended  in  it,  of 
which  the  chief  but  not  the  only  one  is  the  vapor  of  water.  Carbonic  acid  ex- 
ists in  it  in  variable  quantity  but  it  is  nowhere  totally  absent.  SAUSSURE 
found  it  in  air  collected  at  the  top  of  Mont  Blanc.  FUSINIERI  states  That  he 
constantly  found  sulphur,  iron,  and  its  different  oxides,  in  fissures  through 
which  lightning  has  forced  its  way. 

If  such  analogies  be  considered  to  have  any  weight,  it  is  not  impossible  to 
imagine  the  constituents  of  solids  to  be  suspended  in  the  atmosphere  in  a 
vaporous  sublimated  state,  and  to  coalesce  and  enter  into  combination  by  the 
transmission  through  them  by  a  strong  discharge  of  electricity.  But  as  a*  mat- 
ter of  fact  is  it  proved  that  ponderable  masses  in  a  state  of  ignition  have  actu- 
ally fallen  from  the  clouds  ?  The  following  evidence  is  produced  by  M.  Arago 
on  this  question  : — 

Boyle  states  that  in  July,  1681,  the  British  ship  Albemarle  was  struck  with  \ 
lightning  off'  Cape  Cod.  A  mass  of  burning  bituminous  matter  fell  in  tho  boat 
suspended  at  the  stern  of  the  vessel,  which  diffused  an  odor  like  that  of  gun- 
powder. It  was  consumed  in  the  place  where  it  fell,  after  ineffectual  ef- 
forts to  extinguish  it  by  water,  or  to  throw  it  out  of  the  boat  with  rods  of 
wood. 

Silent  lightnings,  whether  they  appear  in  a  clear  or  clouded  sky,  are  usually 
explained  by  the  supposition  that  they  are  the  reflection  of  lightnings  which 
issue  from  clouds  below  the  horizon,  and  so  distant  that  the  thunder  which 
accompanies  them  cannot  be  heard.  It  has  been,  on  the  other  hand,  objected, 
that  the  splendor  of  lightning  is  not  sufficiently  intense  to  cause  a  reflection  so  \ 
bright  as  the  silent  lightnings,  and  that  a  reflection  inferior  in  brightness  to  light- 
ning itself  in  the  same  proportion  as  twilight  is  to  the  brightness  of  the  sun, 
would  not  be  visible.  To  this  objection  M.  Arago  replies  by  the  following 
facts  : — 

CASSINI  and  LACAILLE,  when  engaged  in  making  a  series  of  experiments  on 
the  velocity  of  sound,  in  the  year  1739,  saw  the  light  produced  by  the  dis- 
charge of  a  piece  of  ordnance  placed  at  the  base  of  the  lighthouse  of  Cctle, 
although  at  the  station  they  occupied  both  the  town  and  the  lighthouse  were 
concealed  by  intervening  hills. 

la  1803  M.  ZACH  gave  signals  on  the  Brockcn  (a  mountain  of  the  Harz 
range),  by  exploding  six  or  seven  ounces  of  gunpowder.  The  light  produced 
by  this  was  seen  by  observers  stationed  on  Mount  Kellenberg,  at  a  distance  of 
nearly  three  leagues  from  the  Brocken.  Since  a  direct  view  would  have  been 
rendered  impossible  by  the  convexity  of  the  earth,  the  light  must  have  been  seen 
by  reflection. 

The  flashes  of  artillery  discharged  at  the  base  of  the  Hold  dts  Invalided,  at 
Paris,  are  visible  in  the  gardens  of  the  Luxembourg,  near  the  Rue  d'Enfer, 
although  the  highest  point  of  the  dome  of  the  hotel  is  invisible  from  that  place. 

If,  then,  the  feeble  effect  produced  by  the  explosion  of  a  few  ounces  of  gun- 
powder be  sufficient  to  be  so  apparent  by  reflection,  may  it  not  be  expected 
that  the  more  resplendent  illumination  produced  by  lightning  would  be  infi- 
nitely more  vivid  ? 

That  this  mode  of  explaining  silent  lightning  may  not  take  the  character  of 
mere  conjecture,  it  will  be  necessary  to  show  that  distant  lightnings  are  actually 
visible  when  the  thunder  which  accompanies  them  is  inaudible.  Two  unex- 
ceptionable observations  are  adduced  for  this  purpose. 

On  the  night  between  the  10th  and  llth  of  July,  1783,  the  weather  being 
calm  and  the  sky  unclouded,  Saussure,  stationed  at  theHospice  of  the  Grimsel, 
looking  in  the  direction  of  Geneva,  saw  on  the  horizon  some  streaks  of  clouds 
from  which  lightning  issued,  but  no  thunder  was  heard.  It  was  afterward  as- 


522  THUNDER-STORMS. 


cerfiined  that  at  the  moment  this  occurred  a  storm  broke  over  Geneva  the  most 
terrific  that  the  people  of  that  country  ever  witnessed. 

On  ihe  21st  of  July,  1813,  Mr.  LUKE  HOWARD,  observed  at  Tottenham,  near 
London,  in  a  clear  sky,  lightning,  such  as  is  called  heat-lightning,  appear  tow- 
ard the  southeast.  It  was  afterward  ascertained  that  a  violent  storm  at  tha 
moment  raged  in  France,  which  extended  from  Calms  to  Dunkirk.  This  light- 
ning, above  fifty  leagues  distant,  was  visible  in  the  atmosphere  of  London. 

It  must  then  be  admitted  as  proved,  that  silent  lightnings  may  be  and  yo 
tunes  urr  produced  by  the  reflection  in  the  atmosphere  of  lightning  of  which  the 
thunder  is  too  distant  to  be  heard.  But  it  does  not  therefore  follow  that  sucl 
appearances  must  be  and  always  are  produced  by  that  cause.  On  the  contrary 
heat-lightnings  frequently  present  appearances,  to  explain  which  it  would  be 
almost  impossible  to  admit  the  hypothesis  of  distant  storms.  Thus  it  frequently 
happens  that  when  the  whole  visible  firmament  is  unclouded,  these  lightnings 
will  play  for  entire  nights  on  every  side  of  the  horizon,  and  will  extend  even  to 
the  zenith.  If  distant  storms  were  admitted  to  explain  such  phenomena,  i 
would  be  necessary  to  suppose  that  portion  of  the  atmosphere  visible  from  a 
single  place  clear  and  serene,  yet  surrounded  on  every  side  by  a  ring  of  clouds 
throughout  which  storms  rage.  The  improbability  of  such  an  hypothesis  is 
apparent. 

M.  Arago  proposed  for  the  decision  of  this  question,  the  same  expedien 
which  he  suggested  a  few  years  ago,  in  his  essay  on  comets,  to  determine 
whether  their  tails  were  self-luminous,  or  derived  their  light  from  the  sun. 
There  are  certain  crystals  endowed  with  optical  properties,  in  virtue  of  which, 
objects  viewed  through  them  are  seen  under  different  appearances  according 
as  those  objects  are  self-luminous  or  illuminated  by  light  derived  from  other 
objects.  He  proposes  that  the  silent  lightnings  shall  be  observed  through  such 
crystals,  and  the  question  whether  they  be  actual  lightnings,  unattended  by 
thunder,  or  only  reflections  of  distant  lightnings,  be  thus  decided. 

Thunder  unaccompanied  by  lightning,  is  explained  by  M.  Arago,  by  sup- 
posing two  strata  of  clouds  at  different  heights,  of  which  the  superior  stratum 
is  the  seat  of  the  thunder-storm,  and  of  which  the  inferior  stratum  is  sufficiently 
dense  to  be  impervious  to  the  light  which  precedes  the  thunder.  Nevertheless, 
the  density  of  the  inferior  cloud  will  not  at  all  impede  the  transmission  of  sound 
through  it,  and  the  thundej  will  consequently  be  heard  while  the  lightning  is 
invisible. 

The  method  of  computing  the  distance  of  stormy  clouds  by  observing  the 
interval  which  elapses  between  the  flash  and  the  thunder,  is  based  upon  the 
assumption  that  the  sound  is  produced  in  the  cloud.  It  has  been  however  main- 
tained by  some  persons,  that  when  the  electric  discharge  takes  place  between 
a  cloud  and  the  earth,  the  lightning  issues  from  the  earth  to  the  cloud.  Ac- 
cording to  the  hypothesis  of  a  single  electric  fluid,  this  would  always  be  the 
case  when  the  cloud  is  negatively  electrified.  As  a  test  of  this,  M.  Arago  pro- 
poses to  observe  the  interval  between  the  appearance  of  the  lightning  and  the 
perception  of  the  thunder  under  circumstances  in  which  the  distance  of  the 
cloud  is  known  by  othrr  means  within  a  given  limit.  If  the  distance  obtained 
by  computation  from  observing  the  interval  between  the  light  and  the  sound  be 
manifestly  less  than  the  known  minor  limit  of  the  distance  of  the  cloud,  it  must 
then  follow  that  the  seat  of  the  sound  is  not  the  cloud,  but  is  some  place  in  the 
atmosphere  less  distant,  which  would  necessarily  be  the  case  if  the  lightning 
issued  upward  from  the  earth.  This  method  of  observation  might  be  practised 
in  the  neighborhood  of  any  lofty  tower  or  steeple,  or  near  a  hill,  or  by  means 
of  a  small  balloon  confined  by  a  cord  to  a  given  height.  If  the  cloud  were  ob- 
served to  be  considerably  above  any  such  objects  and  yet  the  computed  distance 


of  the  seat  of  the  sound  considerably  below  them,  the  conclasion  ju»t  stated 
would  he  justified. 

From  the  observations  which  have  been  recorded  of  the  lime  between,  the 
flash  and  the  thunder,  it  appears  that  although  in  one  instance  this  interval 
amounted  to  seventy-two  seconds,  it  usually  does  not  exceed  forty-fi^lr 
ouds.  It  follows,  then,  that  the  greatest  distance  from  which  the  atmospheric 
explosions  which  produce  thunder  are  heard  at  about  ten  miles.  If  the  single 
Accorded  observation  of  an  interval  of  seventy-two  seconds  can  be  relied  on, 
it  would  follow  that  in  that  particular  case  thunder  was  heard  at  the  distance 
of  fifteen  miles. 

Evidence  still  more  direct  and  convincing  can  be  adduced  that  beyond  the 
distance  of  eight  or  ten  miles  thunder  is  inaudible. 

When  the  steeple  of  Lestwithiel  in  Cornwall  was  struck  by  lightning,  on  the 
25th  of  January,  1757,  and  almost  entirely  destroyed,  the  thunder  was  terrihc ; 
yet  Smeaton  the  engineer,  who  was  then  within  thirty  miles  of  the  place, 
heard  no  thunder.  Muschenbroeck  states  that  thunder  at  the  Hague  is  inaudi- 
ble at  Leyden  and  at  Rotterdam,  the  distance  of  the  former  being  ten  and  the 
latter  twelve  miles.  There  are  also  examples  of  violent  storms  breaking  over 
Amsterdam  which  were  inaudible  at  Leyden,  the  distance  being  about  twenty 
miles. 

To  deduce  right  conclusions  from  these  facts  it  will  be  necessary  to  con- 
sider the  distances  at  which  other  sounds,  generally  much  less  intense  than 
thunder,  are  heard.  Cannon  discharged  at  Florence  are  heard  at  Leghorn, 
a  distance  of  fifty  miles ;  at  Leghorn,  are  heard  at  Porto  Ferraio,  the  suine 
distance.  The  cannonade  at  the  siege,  was  audible  at  Leghorn,  a  distance  of 
about  ninety  miles.  It  may  be  added  that  the  great  bell  of  St.  Paul's  cathe- 
dral in  London,  is  said  to  be  audible  at  Windsor,  a  distance  of  about  twenty- 
four  miles. 

The  conditions  of  the  atmosphere,  which  affect  the  transmission  of  sound, 
are  imperfectly  understood,  and  it  is  therefore  the  more  necessary  to  accumulate 
well-ascertained  facts,  to  form  a  safe  basis  for  general  reasoning.  It  is  generally 
believed  that  sounds  are  heard  more  distinctly  and  at  greater  distances  in  win- 
ter, especially  in  frost,  than  in  summer.  This  popular  impression  has  been 
corroborated  in  the  narrative  of  those  who  have  made  voyages  to  the  polar  re- 
gions. Parry  states  that  he  frequently  heard  distinctly  at  the  distance  ot  a 
mile,  men  conversing  in  their  ordinary  voice.  On  the  1 1th  of  February,  iti20, 
he  heard  a  man  singing  to  himself  (and  therefore  probably  in  rather  a  low 
tone),  at  more  than  a  mile  distant. 

Durham  observes  that  new-fallen  snow  impedes  the  transmission  ot  sound, 
and  that  fogs  also  deaden  its  force.  This  latter  effect,  however,  is  not  inva- 
riable. In  a  November  fog,  in  1812,  Mr.  Howard  heard  distinctly  at  live  miles 
from  London,  the  noise  of  the  carriages  rolling  over  the  streets. 

Humboldt  has  proved  that  sounds  are  audible  at  greater  distances  by  night 
than  by  day ;  and  from  the  circumstances  under  which  his  observations  were 
made,  it  would  appear  that  the  silence  of  night  could  not  be  assumed  as  an  ex- 
planation of  this.  • 

It  seems  to  be  established  that  an  adverse  wind  is  an  impediment  to  the 
transmission  of  sound  ;  but  according  to  the  observations  of  M.  F.  Delaroohe, 
a  favorable  wind  does  not  assist  it. 

Volney,  at  Pontchartrain,  heard  four  or  five  claps  of  thunder.  Looking  care- 
fully round  him,  he  could  see  no  clouds  either  in  the  heavens  or  near  the  earth. 
Now  since  thunder  has  never  been  heard  at  a  greater  distance  than  tlitecn 
miles,  and  since  an  object  to  be  invisible  at  that  distance  with  a  well-defined 
horizon  must  have  an  elevation  less  than  about  one  hundred  feet,  it  follows 


24  THUNDER-STORMS. 


either  hat  the  tlmnder  heard  by  Volney  on  that  occasion  was  produced  in  the 
elf  ar  atmosphere,  or  that  it  proceeded  from  a  cloud  not  more  than  thirty-three 
yards  from  the  ground,  at  a  distance  of  about  fifteen  miles  from  the  observer. 

It  has  been  elsewhere  stated  that  the  explanation  proposed  and  universally 
received  as  accounting  for  the  phenomena,  is  a  sudden  displacement  of  the  air. 
produced  by  the  electrical  discharges,  in  which  lightning  is  evolved.  Since 
all  sound  must  proceed  from  an  agitation  of  the  air,  and  since  lightning  and 
electricity  are  identified,  this  explanation  consists  of  little  more  than  a  state-  ( 
merit  of  the  facts.  A  more  rigoro.us  account,  however,  must  be  exacted  from 
those  who  would  propound  an  adequate  theory  of  thunder. 

Some  have  explained  the  origin  of  thunder,  by  supposing  that  the  electric 
fluid,  in  passing  with  great  velocity  through  the  air,  leaves  behind  it  a  vacuum  ; 
that  the  air  rushing  suddenly  into  this  vacuum  produces  a  detonation  like  that 
which  takes  place  in  the  common  experiment  in  which  a  vacuum  being  pro- 
duced under  a  bladder  extended  tigh'ly  over  the  mouth  of  a  receiver,  the  blad- 
der is  broken  by  the  pressure  of  the  external  air.  To  make  this  explanation 
valid,  it  would  be  necessary  to  show  how  the  vacuum  is  produced,  or  that  it  is, 
injact,  produced,  otherwise  the  explanation  is  reduced  to  a  mere  conjecture. 

It  is  also  explained  by  supposing  that  the  electric  fluid  in  passing  through 
the  air,  compresses  successively  the  air  lying  before  it,  whence  there  results 
a  displacement  of  those  masses  of  air  which  are  contiguous,  and  consequently 
a  series  of  contractions  and  dilatations,  which,  extending  to  a  distance,  produce 
long-continued  reverberations. 

M.  Pouillet  rejects  these  hypotheses  as  insufficient  to  explain  the  phe- 
nomenon. He  considers  that  if  such  were  the  cause  of  thunder,  the  passage 
of  a  cannon-ball  through  the  air  ought  to  produce  a  like  effect.  M.  Pouillet 
maintains  that  when  an  electric  discharge  takes  place  between  two  bodies 
charged  with  opposite  electricities,  the  fluid  does  not  actually  pass  from  the  one 
body  to  the  other,  but  that  the  effect  is  produced  by  a  series  of  decompositions 
and  recompositions  of  the  natural  electricities  of  the  molecules  of  the  inter- 
vening medium,  precisely  similar  to  that  which  takes  place  in  a  liquid  solution 
in  which  the  poles  of  the  Voltaic  arrangement  are  immersed.  He  argues  that 
there  must  thence  result  vibrations  more  or  less  violent  in  the  ponderable  mat- 
ter of  that  medium,  which  would  be  sufficient  to  explain  the  sound. 

The  rolling  of  thunder  has  by  some  been  ascribed  to  the  effect  of  echo.  That 
echo  has  in  some  cases  a  share  in  the  production  of  the  phenomena  cannot  be 
doubted  by  any  one  who  has  ever  witnessed  an  Alpine  storm.  A  multitude  of 
causes  affecting  the  loudness,  the  reverberation,  and  the  continuity  of  the  peals, 
are  quite  apparent.  The  question  is  whether  echo  is  the  only  cause  of  the 
rolling  thunder. 

It  has  been  shown  that  the  duration  of  the  thunder-roll  amounts  sometimes 
to  forty-five  seconds.  Whether  the  echoes  of  any  sound  ever  have  such  dura- 
tion, can  only  be  determined  by  observation.  The  example  of  the  often-re- 
iterated echo  at  a  certain  island  on  the  lake  of  Killarney,  is  known  to  all  travel- 
lers. Mr.  Scoresby  observed  on  a  particular  occasion  its  duration,  and  found 
it  about  thirty  seconds.  The  original  sound  is  usually  produced  by  the  dis- 
charge of  a  small  piece  of  cannon. 

It  would  seem  that  on  the  occasion  of  Mr.  Scoresby's  observations,  a  pistol 
was  used.  It  is  argued  by  M.  Arago,  that  if  a  cannon  had  been  used,  the  du- 
ration would  have  been  much  greater,  and  probably  equal  to  the  continuance 
of  the  longest  roll  of  thunder. 

During  the  experiments  made  to  determine  the  velocity  of  sound  in  June, 
1822,  MM.  Humboldt,  Bouvard,  Gay-Lussac,  and  Emile  de  Laplace,  heard 
the  echo  of  a  cannon  discharged  near  them  during  twenty-five  seconds. 


THUNDER-STORMS. 


525 


Mariners  state  that  thunder  heard  at  sea  is  marked  by  rolling  as  loni;  con- 
tinued as  on  land,  although  none  of  those  causes  which  are  generally  supposed 
to  produce  echoes,  such  as  walls,  rocks,  wood,  hills,  or  mountains,  are  present. 
Unless  the  surface  of  the  clouds  reflects  sounds,  no  means  of  producing  an  echo 
can  exist  under  such  circumstances.  Although  it  might  seem  that  the  clouds 
would  be  as  little  capable  of  reflecting  sound  as  the  air  itself,  there  appears  to 
be  some  reason  to  judge  otherwise.  Muschenbroeck  states,  as  the  result  of 
his  own  observations,  that  a  cannon,  which,  being  discharged  when  the  heavens 
are  unclouded,  produced  only  a  single  report,  had  its  sounds  several  times  re- 
verberated when  discharged  in  the  same  place  under  a  clouded  sky.  in  the 
course  of  the  experiments  made  in  1822,  to  determine  the  velocity  of  sound  al- 
ready referred  to,  the  same  observation  was  made. 

In  the  posthumous  works  of  Hooke,  published  in  1706,  an  explanation  was 
proposed  for  the  rolling  of  thunder,  which  was  more  recently  reproduced  with 
more  full  developments  by  Dr.  Robinson  in  the  Encyclopaedia  Britannica,  and 
)  which  seems  more  adequate,  and  open  to  fewer  objections,  than  any  other  hy- 
^  pothesis  yet  suggested.     The  sound  is  supposed  to  be  developed  by  the  light- 
;  ning  in  passing  through  the  air,  and  consequently  separate  sounds  are  pro-  I 
1  duced  at  every  point  through  which  the  lightning  passes.    As  the  object  of  the  / 
hypothesis  is  to  explain  the  rolling  or  succession  of  sounds,  and  not  the  sound  ^ 
itself,  it  is  immaterial  what  the  manner  of  producing  the  sound  may  be. 

Let  us  first  suppose  that  the  lightning  were  to  move  in  a  circle,  of  which  the 
observer  is  the  centre.     The  velocity  of  the  lightning  is  so  extreme  that,  for 
the  purposes  of  this  explanation,  it  may  be  assumed  to  be  at  the  same  moment 
in  every  part  of  the  circle.     Explosions  will,  therefore,  be  produced  simulta- 
neously at  every  point  in  the  circumference  of  the  circle,  and,  as  all  these 
)  sounds  have  the  same  distance  to  traverse  in  coming  to  the  observer,  they  will 
)  arrive  at  his  ear  at  the  same  instant ;  the  effect  would,  therefore,  be  a  single 
\  sound,  having  a  force  due  to  the  combined  effects  of  all  the  sounds  produced 
;  in  the  circumference  of  a  circle.     To  apply  this  reasoning  to  the  actual  case 
of  thunder,  let  it  be  supposed  that  two  small  clouds  oppositely  electrified  are 
situated  near  each  other,  and  at  the  same  height  in  the  zenith  of  the  observer. 
The  clouds  may  be  considered  as  placed  in  the  surface  of  a  sphere,  in  the  cen- 
tre of  which  the  observer  stands.     If  the  electric  discharge  takes  place  between 
the  clouds,  the  thunder  would  be  heard  by  the  observer  as  a  single  clap,  with- 
out any  roll  or  reverberation. 

Let  us  next  suppose  the  lightning  to  move  in  any  line  which  is  not  part  of  a 
circle  or  sphere,  with  the  observer  in  the  centre ;  let  its  course  be  a  straight 
line,  for  example,  such  as  A  B,  the  observer  being  at  O.  From  0,  suppose  a 


Fig.  3. 


perpendicular,  O  L1,  drawn  to  A  B,  and  let  two  lines,  0  L2,  the  lergth  of 
which  shall  exceed  O  L1  by  one  hundred  and  ten  feet,  be  inflected  from  O  on 


526 


THUNDER-STORMS. 


1  *^-^«-^l^ 


A  B,  one  on  each  side  of  0  L1 ;  let  other  two  lines,  O  L3,  exceeding  O  L2 
by  one  hundred  and  ten  feet,  be  also  inflected  on  A  B,  and  in  the  same  manner 
let  a  series 'ot  lines,  such  as  0  L2,  O  L3,  O  L4,  be  successively  inflected  on  A 
B,  each  line  exceeding  that  which  precedes  it  by  one  hundred  and  ten  feet.  If 
we  suppose  sounds  to  be  simultaneously  produced  at  the  points  LJ,  L2,  L3, 
thai  v.hich  is  produced  at  L1  will  be  first  heard  by  the  observer.  Since  sound 
iiK.'Vfs  at  the  rate  of  eleven  hundred  feet  per  second,  it  will  take  the  tenth  of  a 
second  to  move  through  one  hundred  and  ten  feet ;  therefore  the  two  sounds 
emitted  at  L2  will  arrive  together  at  the  ear  of  the  observer  a  tenth  of  a  second 
alter  the  sound  at  L1  has  been  heard.  In  the  same  manner,  the  two  sounds 
eimued  at  L3  will  arrive  after  another  ten-th  of  a  second,  and  so  on.  Thus  ev- 
ery ten  sounds  of  the  series,  though  simultaneously  produced,  would  take  a 
second  in  being  heard,  and  would  be  recognised  by  the  ear  as  a  distinct,  though 
rapid  succession  of  ten  sounds. 

If  it  be  admitted,  then,  that  the  electric  fluid,  in  passing  through  the  air  with 
the  great  velocity  it  is  proved  to  have  by  the  experiments  of  Professor  Wheat- 
stone,  produces  sonorous  vibrations  of  this  kind  in  the  air,  the  rolling  of  thun- 
der vvourd  be  a  necessary  consequence. 

According  to  this  manner  of  viewing  the  phenomena,  the  thunder  would  be 
loudest  which  proceeds  from  L1,  the  nearest  point  to  the  observer,  and  would 
gradually  be  enfeebled  for  points  more  and  more  distant  from  L1.  Therefore 
the  roll  would  always  be  loudest  at  the  commencement,  and  would  gradually 
diminish  in  force  until  it  becomes  inaudible.  This  is  not  in  accordance  with 
the  actual  phenomena. 

But  the  preceding  explanation  proceeds  on  the  supposition  that  the  lightning; 
moves  continually  in  the  same  straight  line.  Let  us  see  what  the  effects  of  a 
zigzag  course  would  be,  such  as  that  represented  by  the  line  A,  B.  Taking 


Fig.  4. 


the  place  of  the  observer,  O,  as  a  common  centre,  let  a  series  of  circular  arcs 
be  drawn  with  radii  increasing  in  magnitude  each  successive  distance  exceed- 
ing the  last  by  one  hundred  and  ten  feet.  These  arcs  will  intersect  the  zigzag 
course  of  the  lightning  in  several  points  more  or  less  in  number,  according  to 
the  position  of  the  directions  of  the  lightning,  and  the  magnitude  of  the  radius 
of  the  circle.  The  first  sound  which  will  reach  the  observer  will  be  that  pro- 
duced at  the  points  where  the  least  of  the  circles  meets  the  lightning,  and  the 
succeeding  sounds  will  correspond  to  those  emitted  at  the  point  of  intersection 
of  the  succeeding  circles  with  the  course  of  the  lightning.  It  is  easy  to  con- 
ceive, that  the  mutual  position  of  the  zigzag  lightning  and  the  observer  may  be 
such  that  the  number  of  points  of  intersection  of  the  circles  with  the  lightning 
may  alternately  augment  and  diminish  in  a  manner  corresponding  to  any  sup- 
posable  variations  in  the  intensity  of  the  rolling  of  the  thunder. 

It  is  evident  that,  independently  of  the  infinite  varieties  of  sound  capable  of 
being  explained  by  this  hypothesis  applied  to  zigzag  lightnings,  the  changes 
are  not  le*,s  various  for  lightning  which  preserves  a  single  course,  the  same 


THUNDER-STORMS.  527 


flash,  according  to  its  direction  with  respect  to  the  observer,  being  susceptible 
of  an  infinite  variety  of  sonorous  effects. 

An  objection  to  this  fascinating  hypothesis  occurs  to  me,  which  appears  to 
have  escaped  the  attention  of  its  advocates,  and  which,  nevertheless,  is  entitled 
to  consideration.  I  have  supposed,  for  the  sake  of  illustration,  in  the  prece- 
ding developments  that  a  succession  of  distinct  sounds  are  emitted  at  points  of 
a  space  the  difference  of  whose  distance  from  the  observer  is  one  hundred  and 
ten  feet,  and  therefore  these  sounds  succeed  each  other  at  intervals  of  a  tenth 
of  a  second.  Any  other  difference  of  distance  would  equally  serre  the  purpo- 
ses of  illustration,  the  interval  between  the  successive  detonations  being  deter- 
mined by  it  according  to  the  known  velocity  of  sound.  But  it  does  not  appear 
to  me  that  there  is  anything  in  the  physical  effects  to  warrant  the  supposition 
of  a  series  of  separate  sounds  emitted  at  points  of  space  more  or  less  distant 
from  each  other.  The  electric  fluid  rushes  through  space,  producing  the  sarru 
efft-cl  at  every  point.  The  analogy  on  which  Dr.  Robinson  bases  the  expla-  S 
nation  (to  a  file  of  soldiers,  placed  at  certain  distances  asunder,  who  discharge  / 
their  muskets  at  the  same  instant,  but  are,  nevertheless,  heard  in  succession)  s 
does  not  seem  to  be  in  accordance  with  the  phenomena.  The  passage  of  the  » 
electric  fluid  through  the  air  would  be  more  aptly  illustrated  by  a  bow  drawn  \ 
over  the  string  of  a  violin,  or  the  current  of  air  driven  by  the  mouth  through  a  ) 
wind  instrument,  or  by  a  bellows  through  an  organ-pipe.  There  would,  ac-  ( 
cording  to  such  analogy,  be  one  sustained  sound,  instead  of  a  succession  or  se-  ) 
ries  of  distinct  sounds.  It  is  true  that,  in  the  gravest  note  on  aa  organ,  and  eve* 
in  those  produced  on  certain  wind  instruments  (the  trombone,  for  example),  and 
on  the  strings  on  the  double  base,  the  vibrations  are  distinguishable ;  but  these 
vibrations  do  not  seem  to  have  any  analogy  to  the  series  of  sounds  which  form 
the  rolling  of  thunder. 

If  this  hypothesis,  nevertheless,  be  admitted  to  explain  the  rolling  of  thunder, 
the  duration  of  the  rolling  will  become  an  important  element  in  determining  the 
minor  limit  of  the  space  through  which  the  lightning  passes.  Supposing  that 
no  line  drawn  from  the  observer  to  the  course  of  the  lightnirtg  is  perpendicular 
to  it,  it  will  follow  that  one  extremity  of  the  course  is  nearer  than  any  other 
point  of  it  to  the  observer,  and  the  other  extremity  more  remote.  The  difler- 
ence  between  the  distance  of  these  extreme  points  would  be  the  length  of  the 
flash,  if  its  direction  was  immediately  toward  or  from  the  observer ;  and  if  it 
have  any  other  direction,  this  difference  will  be  less  than  the  length  of  the 
flash.  The  duration  of  the  roll  of  the  thunder  being  the  time  sound  would  take 
to  move  over  the  difference  between  the  greatest  and  the  least  distance,  this 
difference  may  be  computed,  and  thence  a  minor  limit  of  the  length  of  the  flash 
^  may  be  obtained. 

From  the  observations  of  De  L'Isle,  it  appears  that  the  rolling  of  thunder, 
)  observed  by  him  in  17]  2,  lasted  in  some  instances  forty-five  seconds.  Allow- 
<  ing  eleven  hundred  feet  for  each  second,  this  would  amount  to  forty-nine  thou- 
)  sand  five  hundred  feet,  or  very  near  ten  miles.  The  length  of  the  flash  must, 
I  therefore,  have  exceeded  this  distance. 

I  have,  in  these  explanations,  assumed  that  the  loudest  sound  is  that  which 
I  proceeds  from  the  nearest  focus  of  sound  to  the  observer.     The  loudness  of  a 
S  sound,  however,  depends  partly  on  the  temperature  and  hygrometric  condition 
J  of  the  air  at  the  place  where  the  sound  is  developed.     It  might  happen  that  ] 
these  conditions,  varying  in  different  parts  of  the  air  where  the  sounds  are  < 
produced,  would  render  more  remote  sounds  sometimes  louder  than  nearer 
ones. 

One  of  the  circumstances  in  the  natural  exhibition  of  lightning,  which  seems 
not  so  satisfactorily  explicable  as  most  of  the  others,  is  the  frequent  repetition 


528  THUNDER-STORMS. 


of  the  flashes  from  the  same  cloud,  which  often  follow  each  other  in  rapid  sue- 
cession,  contrary  to  what  takes  place  in  metallic  conductors  in  which  the  elec- 
trie  equilibrium  is  restored  in  a  single  discharge,  or  nearly  so.     The  most  ob- 
vious way  of  explaining  this  is  by  supposing  that  the  vapor  composing  thunder 
clouds  being  a  much  less  perfect  conductor  than  metal,  and  the  cloud  being 
often  of  extensive  magnitude,  possibly  measuring  miles  in  length  or  breadth, 
the  equilibrium  cannot  be  restored,  except  by  successive  discharges,  accord-  j 
ing  as  the  fluid  dispersed  over  or  through  the  cloud  can  collect  at  or  near  the  ) 
striking  point. 


•'••^^•^^•^WS^--'. 


THE  LATITUDES  AND  LONGITUDES. 


Definition  cf  the  Equator  and  Poles — Northern  and  Southern  Hemispheres. — Latitude  of  a  Place. —  ; 

Parallel  of  Latitude. — Meridian  of  a  Place. — Longitude  of  a  Place. — Standard  Meridian. — Mcth-  • 

ods  of  determining  Latitude  and  Longitude  various. — To  find  the  Latitude. — Methods  applicable  ^ 

in  Observatories. — At  Sea. — Hadley's  Sextant — To  determine  the  Longitude. — How  to  find  the  * 

Time  of  Day  at  Land. — At  Sea. — Use  of  Chronometers. — Lunar  Method  of  finding  the  Longi-  / 

tude — Apparatus  provided  at  Greenwich  tor  giving  the  exact  Time  to  Ships  leaving  the  Port  of  C 
London. — Method  of  determining  Longitude  by  Moon-Culminating  Stars. 


THK  L,VT!THjr.«  AM)   U>SU!TI  UK*. 


TltE  LATITUDES  ANT)  LONGITUDES. 


BEFORE  it  is  possible  to  acquire  a  distinct  knowledge  of  the  position  or  dis- 
tances of  any  bodies  in  the  universe  outside  the  surface  of  the  earth,  it  is  first 
indispensable  that  we,  who  have  to  make  these  calculations,  should  distinctly 
ascertain  our  own  position  in  refer3nce  to  the  bodies  we  observe.  But  as  our 
position  is  subject  to  continual  change,  as  well  by  reason  of  the  diurnal  rota- 
tion of  the  earth  upon  its  axis,  o.i  the  surface  of  which  we  are  carried  round, 
us  the  annual  motion  of  the  globe  in  its  orbit  round  the  sun,  we  are  obliged  as 
a  necessary  preliminary  to  analyze  with  accuracy  all  the  circumstances  of 
these  motions.  But  even  befoi  3  we  are  in  a  condition  to  accomplish  this, 
there  is  another  prelimir  ary  e.cp  not  less  indispensable,  which  is  to  ascertain 
our  own  position  on  the  surface  of  the  globe  we  inhabit. 

This  is  not  so  easy  a  mat  er  as  at  the  first  view  it  might  seem  to  be.  The 
earth  we  dwell  on  is  a  ^..o;  i  of  stupendous  magnitude.  The  range  of  our 
vision  around  any  sitratic.i  vhich  we  may  occupy  upon  the  surface  of  this 
globe  is  small.  In  the  rrcc-.  unobstructed  situation  we  can  obtain — that  which 
is  presented  us  at  sea,  when  out  of  sight  of  land,  on  the  clearest  day — our  ob- 
servation is  circumscribed  by  a  radius  of  a  few  miles.  The  portion  of  the 
surface  which  we  see  at  one  and  the  same  time,  forms  in  reality  so  small  a 
patch  of  the  globe  of  the  earth,  that  it  is  only  by  indirect  reasoning  that  we  can 
recognise  upon  it  any  character  save  that  of  a  flat  plane.  How,  then,  are  we 
to  know  in  what  part  of  the  terrestrial  globe  that  small  patch  of  surface  is 
situated  ? 

To  answer  this  question,  it  is  evidently  necessary  first  to  settle  some  fixed 
points  or  lines  to  which  we  may  refer  various  places,  and  by  which  we  may 
express  their  positions.  The  points  which  have  been  usually  selected  for  this 
purpose  are  the  poles  and  the  equator.  The  poles  are  those  points  on  the  sur- 
face of  the  earth  where  the  axis  on  which  it  performs  its  diurnal  rotation  ter- 
minates, and  they  are  distinguished  as  is  well  known  by  the  names  of  the  north 
and  scuth  poles. 


' 


532 


THE  LATITUDES  AND  LONGITUDES. 


"\ 


If  we  imagine  a  circle  surrounding  the  surface  of  the  globe  in  such  a  man- 
ner as  to  divide  it  into  two  hemispheres,  having  in  the  midst  of  one  the  north 
pule,  and  in  the  midst  of  the  other  the  south  pole,  such  a  circle  is  called  the 
cq/iutiir,  and  is  so  called  from  equally  dividing  the  globe.  Every  point  in  this 
circle  will  be  at  the  same  distance  from  the  poles,  and  if  we  imagine  the  globe  to 
!>e  cut  by  a  plane  through  the  poles,  that  plane  will  be  at  right  angles  to  this 
circle,  and  the  section  it  forms  will  be  what  is  called  a  terrestrial  meridian. 
The  arc  of  this  meridian  between  either  pole  and  the  equator  will  be  one  quar- 
ter of  its  entire  circumference,  and  will  therefore  be  90°.  The  equator  is, 
therefore,  everywhere  90°  from  each  of  the  poles. 

The  hemispheres  into  which  the  equator  divides  the  earth  are  called  the 
northern  and  southern  hemispheres.  That  which  includes  the  north  pole,  being 
the  northern,  and  that  which  includes  the  south  pole,  the  southern. 

The  position  of  a  place  in  either  hemisphere  with  reference  to  the  equator 
is  expressed  by  stating  the  number  of  degrees  of  a  terrestrial  meridian  included 
between  the  place  and  the  equator.  This  is  called  the  latitude  of  the  place; 
which  is  the  distance  of  the  place  from  the  equator  expressed  in  degrees  of 
the  meridian.  Thus,  if  a  place  be  midway  between  the  pole  and  the  equator, 
its  latitude  is  45°.  If  it  be  distant  from  the  equator  by  two  thirds  of  the  entire 
distance  from  the  equator  to  the  pole,  its  latitude  will  be  60°  and  so  on. 

The  latitude  is  said  to  be  northern  and  southern,  according  as  the  place  is 
in  the  northern  or  southern  hemisphere. 

But  it  is  evident  that  the  latitude  alone  will  be  insufficient  for  the  determina- 
tion of  the  position  of  a  place.  If  we  state  that  a  certain  place  is  45°  north 
of  the  equator,  it  will  be  impossible  to  ascertain  certainly  the  place  in  question, 
inasmuch  as  there  is  a  circle  of  points  on  the  earth,  all  of  which  are  45°  north 
of  the  equator.  If  we  suppose  a  line  drawn  on  the  surface  of  the  r.orthern 
hemisphere  parallel  to  the  equator,  at  the  distance  from  the  equator  of  45°, 
every  point  of  such  line  or  circle  will  be  equally  characterized  by  the  latitude 
of  45  3  north. 

Such  a  circle  is  called  a  parallel  of  latitude,  and  it  is  therefore  apparent  that 
wherever  such  a  parallel  may  be  drawn  upon  the  earth,  all  the  places  upon  it 
will  have  the  same  latitude. 

The  latitude  is,  then,  insufficient  to  determine  the  position  of  any  place. 
How,  then,  it  may  be  asked,  can  the  exact  position  of  any  place  be  expres- 
sed 1 

Let  us  suppose  that  a  meridian  is  arbitrarily  selected,  passing  through  some 
particular  place,  such  as  the  Capitol  at  Washington.  We  may  conceive  an- 
other meridian  drawn  upon  the  earth  east  or  west  of  that,  so  that  the  two  me- 
ridians shall  include  between  them  an  arc  of  the  equator,  consisting  of  a  defi- 
nite number  of  degrees ;  say,  for  example,  that  it  shall  consist  of  20°  ;  then 
such  a  meridian  will  be  defined  by  stating  that  it  is  20°  east  or  west  of  the 
menuim  of  Washington.  All  that  can  be  settled  by  such  a  statement  is  the 
position  of  the  meridian  in  which  the  place  lies  with  reference  to  the  arbitrarily 
chosen  meridian  of  Washington.  This  relative  position  of  the  two  meridians 
is  called  the  longitude  of  the  place.  As  the  meridian  from  which  the  longitude 
is  measured  is  altogether  arbitrary,  there  being  no  physical  or  geographical 
reason  why  one  meridian  should  be  chosen  rather  than  another,  each  nation 
has  naturally  selected  as  the  zero  of  longitude  the  meridian  of  some  noted 
place  in  its  precincts.  In  England,  the  Royal  Observatory  at  Greenwich  h-;.s 
been  the  place  selected,  and  accordingly  in  all  English  works  on  geography, 
political  and  physical,  longitudes  are  invariably  expressed  in  reference  to  the 
meridian  of  Greenwich.  It  will,  therefore,  be  most  convenient  for  us  here 
chiefly  to  refer  to  that  meridian. 


THE   LATITUDES  AND  LONG  ITU  1; 

When  these  explanations  are  clearly  understood,  we  shall  1*  in  a  condition. 
distinctly  and  definitely,  to  express  the  position  of  a  place  upmi  the  Mirl: 
the  globe  of  the  earth.     If  we  state  its  latitude  and  its  longip,:  „  f,x 

at  once,  and  unequivocally,  the  position  of  a  place.  Thus,  let  us  snppus-  t!r,,t  its 
,  latitude  is  50°  north,  its  longitude  30°  east  of  Greenwich;  its  position  will  l,e 
;  found  by  imagining  aline  parallel  to  the  equator  drawn  upon  the  m.rthern  hem- 
}  isphere  at.  a  distance  of  50°  from  the  equator;  then,  supposing  a  meridian 
)  drawn  through  Greenwich,  intersecting  this  parallel,  and  anothe/drawn  so  as 

*  to  cross  the  equator  at  a  point  30°  east  of  the  former;  the  place   in  i;n 

S  will  be  upon  the  line  parallel  to  tho  equator  first  drawn,  inasmuch  :,s  it  will  be 
I  50°  north  of  the  equator,  arid  it  will  be  aleo  in  the  meridian  last  drawn,  inas- 
much as  it  will  be  30°  east  of  Greenwich.     Since,  then,  it  will  be  :u  the  same 
/  lime  in  both  these  lines,  it  will  necessarily  be  at  the  point  when?  thev 
,   each  other  at  the' east  of  the  standard  meridian  of  Greenwich. 
^       Thus,  then,  we  have  succeeded  at  least  in  establishing  standards  of  position 
)  and  a  nomenclature  by  which  the  exact  position  of  a  place  on  the  surf;. 
I  the  glebe  can  be  expressed.     But  we  have  still  another  much  more  important 
and  difficult  question  to  settle.     How  are  we  to  discover  in  what  part  of  the 
globe  any  place  is  which  we  may  occupy  at  a  given  time  ;  in  other  words,  how 
are  we  to  discover  its  latitude  and  its  longitude?     These  are  question 

•  pecially   the    latter,   attended   with    some    difficulty,    and   which    have 
solved  by  different  methods,  applicable  in  different  cases,  according  to  the  cir- 
cumstances under  which  the  position  of  the  place  is  sought,  and  the  pnrp<>M. 
for  which  such  position  is  to  be  determined. 

At  any  place  on  land  where  the  geographical  position  is  once  determined, 
it  may  be  recorded,  so  as  to  be  permanently  known  for  the  future  without  a 
repetition  of  the  process  for  determining  it ;  but  it  is  otherwise  at  sea.     ( )n  the 
trackless  surface  of  the  deep  all  marks  of  events  and  operations  are  immedi- 
ately obliterated,  and  a  new  investigation  must  be  instituted  in  every  case  when 
the  position  of  any  pcint  is  to  be  determined.     The  mariner  must,  therefore, 
be  supplied  not  only  with  the  means  of  determining  the  position  of  his  ship  at 
all  times,  but  with  means  the  application  of  which  is  practicable  under  the 
(  peculiar  circumstances  in  which   he    is    placed.     The    instruments    he    uses 
)  must  not  only  be  portable,  but  must  be  such  as  may  admit  of  being  manipulated, 
)  subject  to  the  disturbances  and  the  vicissitudes  of  the  sea.     The  object  of  his 
observations  must  be  such  as  are  almost  always  in  his  view.     It  is  evident, 
then,  that,  the  problem,  as  applicable  on  land,  is  wholly  different  in  its  Cir- 
cumstances and  conditions  from  that  which  is  applied  on  the  deep.     Uut  even 
on  land  the  problem  presents  itself  under  various  circumstances  and  conditions. 
?  In  the  fixed  observatory,  where  the  philosopher  is  supplied  with  instruments 
(  of  the  greatest  magnitude,  of  the  most  refined  accuracy,  and  the  m.)>t  absolute  ^ 
•;  stability,  methods  have   been   used  which  are  susceptible  of  the  last  conceiv-  ( 
\  able  degree  of  accuracy,  and  accordingly  the  position  of  those  points  un  the  £ 
globe  where  such  observatories  have  been  erected,  are  usually  determined  with 
the  greatest  degree  of  precision.     Such  points  on  tho  globe  serve,  therefore, 
as  a  sort  of  geographical  landmarks,  relative  to  which  the  position  of  all  sur- 
rounding places  may  be  determined. 

The  circumstances  under  which  the  scientific  traveller  and  geographer  makes 
his  observations,  with  a  view  to  the  general  determination  of  the  points  of  a 
country,  are  less  favorable  to  accuracy  than  those  available  to  the  astronomer, 
but  still  are  more  susceptible  of  precision  than  those  which  can  be  pi  a-.-d  at 
the  disposal  of  the  mariner.  It  is,  however,  the  business  and  the  duty  of  those 
who  devote  their  lives  to  the  advancement  of  the  sciences,  to  supply  to  each  • 
of  observers  those  instruments  and  methods  of  inquiry  which  are  capable,  respeyt- 


THE  LATITUDES  AND  LONGITUDES, 


ively  of  giving  results  which,  in  the  circumstances  of  the  case,  have  the  gieat- 
est  attainable  accuracy. 


TO    FIND    THE    LATITUDE. 

Let  us  suppose  the  globe  of  the  earth  to  be  represented  at  O,  and  let  N  be  its 
north  pole,  and  E  its  equator ;  let  P  be  a  place  upon  it,  whose  latitude,  that  is, 
whose  distance  from  the  equator  is  to  be  determined.  Let  n  Z  e.  represent  the 
firmament  surrounding  the  globe  at  an  indefinite  distance.  The  point  n,  imme- 
diately over  the  north  pole,  and  which  is  in  fact,  the  continuation  of  the  line  0 
N  will  be  the  place  of  the  north  pole  in  the  heavens,  very  near  to  which  is  a 
star,  called  the  Polar  star.  The  point  e,  in  the  continuation  of  the  line  O  E, 
will  be  that  which  is  directly  over  the  equator  and  will  be  that  point  in  the 
heavens,  representing  the  position  of  the  equator  and  the  point  Z,  in  v'ie 
continuation  of  the  line  O  P,  the  point  of  the  heavens  which  is  directly 
over  the  observer  at  the  place  P,  will  be  that  which  is  called  his  zenith.  This 
point  is  that  to  which  a  plumb  line  would  direct  itself. 

Now  the  points  n,  Z,  and  e,  are  the  points  in  the  firmament  which  correspond 
with  the  points  N,  P,  and  E,  upon  the  earth,  and  it  is  evident  that  whatever 
arcs  of  the  terrestrial  meridian  N  P  E  are  included  between  these  points, 
similar  arcs  of  the  celestial  meridian  must  be  included  between  the  points  n 
Z  e.  If,  then,  P  E  were  40°,  Z  e  must  also  be  40°,  just  as  n  e  is  90°,  while 
N  E  is  also  9(P. 

In  short,  the  zenith  of  any  place  in  the  heavens  is  the  point  in  the  firmament 
which  corresponds  with  the  position  of  the  place  on  the  g.obe,  and  the  distance  of 


THE  LATITUDES  AND  LONGITUDES. 


> 


the  zenith  in  the  heavens  of  one  place,  from  the  zenith  of  another  must  necessarily 
be  the  same  in  degrees  as  the  distance  between  two  places  on  earth.  Thog  n  is 
the  zenith  of  P  ;  e  is  the  zenith  of  E  ;  n  is  the  same  number  of  degrees  from  « 
as  P  is  from  E.  This  being  clearly  understood,  it  is  evident  tliat  if  we  can,  by 
any  means  ascertain  by  observations,  the  distance  from  Z  to  n,  we  can  infer  at 
once  the  distance  from  P  to  N,  and  hence,  can.  discover  the  distance  from 
P  to  E,  or  the  latitude  of  the  place. 

It  is  apparent,  then,  if  we  can  observe  the  distance  of  the  zenith  of  any 
place  from  the  celestial  pole,  that  will  give  us  the  distance  in  degrees  of  the  place 
itself  from  the  terrestrial  pole,  and  by  subtracting  that  from  9(P,  we  shall  obtain 
the  distance  of  the  place  itself  from  the  equator,  or  what  is  the  same,  its  latitude. 
As  an  example  of  this,  l£t  us  suppose  that  in  measuring  the  distance  from  Z  to 
n  we  find  it  to  be  50°  ;  "we  infer,  therefore,  that  since  the  distance  of  the  zenith 
from  the  pole  is  50°,  the  distance  of  the  place  from  the  terrestrial  pole  is  also  50°. 

But  since  the  terrestrial  pole  is  90 D  from  the  equator,  it  follows  that  the  dis- 
tance of  the  place  from  the  equator  must  be  40°,  and  it  is  north  or  south,  ac- 
cording as  the  zenith  of  the  place  is  in  the  northern  or  southern  hemisphere  of 
the  firmament. 

Thus,  then,  it  appears  that  the  latitude  of  a  place  can  always  be  found,  provided 
we  can  measure  the  distance  of  its  zenith  from  the  celestial  pole ;  and  this,  of 
course,  can  always  be  done  by  the  use  of  proper  instruments,  provided  that  the 
zenith  and  the  pole  can  be  distinctly  seen.  Now  the  direction  of  the  zenith 
can  always  be  determined  by  the  plumb  line  ;  but  although  the  pole  star  is  very 
near  the  pole,  it  is  not  exactly  at  it ;  there  is,  in  fact,  no  star  exactly  at  the 
pole,  and  there  being  no  visible  object  there,  it  is  impossible  to  measure  direct- 
ly its  distance  from  the  zenith.  This  difficulty  is  eluded  by  measuring  the 
distance  of  the  zenith  from  some  star,  or  other  celestial  object,  whose  distance 
from  the  pole  happens  to  be  known  :  for  example,  suppose  that  there  were  a 
star  directly  between  the  zenith  and  pole,  whose  distance  from  the  pole  was 
known  to  be  10°.  Then  if  we  find  by  observing  the  distance  of  the  zenith 
from  this  star  was  40°,  we  should  immediately  infer  the  distance  of  the  zenith 
frum  the  pole  to  be  50°.  * 

It  is  ia;-fact,  then,  by  this  device  that  the  latitude  is  always  ascertained.  By 
various  observations  made  by  astronomers,  the  positions  of  most  of  the  stars 
and  other  celestial  objects,  with  respect  to  the  poles,  are  known  and  recorded ; 
and  when  we  desire  to  determine  the  latitude  of  any  place,  we  measure  the 
distance  of  the  zenith  of  that  place  from  some  celestial  object  whose  position 
with  respect  to  the  pole  is  known,  and  thence  infer  the  position  of  the 
place  with  respect  to  the  terrestrial  pole ;  and  from  that  deduce  at  once  the 
latitude. 

But  our  purpose  would  be  equally  served  if  we  were  supplied  with  the  po- 
sition of  any  visible  object  with  reference  to  the  celestial  equator.  Thus,  if 
we  know  the  distance  of  the  centre  of  the  sun  from  the  celestial  equator,  we 
shall  readily  be  able  to  find  the  latitude  ;  for  it  would  only  be  necessary  when 
the  sun  is  in,  or  very  near  the  meridian,  that  is,  at  or  near  noon,  to  measure 
the  distance  of  the  zenith  of  the  place  from  the  centre  of  the  sun.  This 
would  be  done  by  measuring  the  distance  of  the  zenith,  first  from  the  upper, 
and  then  from  the  lower  limb  of  the  sun.  The  distance  from  the  centre  would 
(  be  the  mean  between  these. 

Let  us  suppose,  for  example,  that  the  sun  being  between  the  zenith  of  the 
\  equator,  we  find  that  the  distance  from  the  zenith  to  the  centre  of  the 
)  sun  is  20°,  and  that  we  also  ascertain  from  the  table  of  the  position  of  the 
sun,  that  the  distance  of  the  centre  of  the  sun  at  that  time  from  the  equa- 
tor, is  also  20°,  we  should  infer  at  once  that  the  distance  of  the  zenith 


536 


from  the  equator  must  be  40°,  and  that  such,   therefore,  must  be   the  latitude 
of  the  place. 

This  method  of  ascertaining  the  latitude  is,  perhaps,  the  most  easily  practi- 
cable. The  observations  may  be  performed  daily,  at  noon,  when  the  sun  is 
visible  :  and  in  all  almanacs,  the  distance  of  the  centre  of  the  sun  from  the 
equator,  which  is  called  the  sun's  declination,  is  registered.  The  instrument  by 
which  the  observations  are  executed  on  land  are,  usually,  a  quadrant  furnished 
with  a  telescope  moving  upon  its  centre.  One  radius  of  the  quadrant  is  placed 
in  the  direction  of  the  plumb  line,  and  therefore  points  to  the  zenith.  The 
telescope  moves  round  the  centre  until  it  is  directed  to  the  object  whose 
distance  from  the  zenith  is  to  be  observed.  The  angle  between  the  telescope 
and  the  vertical  radius  of  the  quadrant  will  then  be  the  same  as  the  distance 
of  the  object  from  the  zenith. 

In  astronomical  observatories  methods  of  observation  have  been  applied  sus- 
ceptible of  much  greater  accuracy.  Stars  upon  the  meridian  can  thereby  be  used 
with  great  advantage.  The  distance  of  these  stars  from  the  pole  are  accurate- 
ly known,  and  the  astronomer  selects  for  his  observation  those  conspicuous 
stars  which  pass  very  near  to  his  zenith.  He  observes  the  arc  of  the  celes- 
tial meridian  between  his  zenith  and  these  stars.  And  from  the  magnitude  of  the 
arc  and  the  distance  of  the  star  of  the  celestial  pole,  he  discovers  the  dis- 
tance of  the  zenith  from  the  pole  and  thence  the  latitude. 

The  principal  source  of  accuracy  in  this  method  is,  that  the  distance  be- 
tween the  zenith  and  the  star  being  very  small,  is  capable  of  more  exact  meas- 
urement, for  reasons  connected  with  the  structure  of  the  astronomical  instru- 
ment, than  could  be  attained  .in  the  measurement  of  greater  angles. 

In  observations  made  at  sea,  it  is  not  practicable,  however,  to  use  the  plumb 
line,  and  indeed,  even  for  the  purposes  of  geographers  it  is  not  always  con- 
venient. An  admirable  instrument  has  been  invented  equally  applicable  to 
observations  by  land  or  by  water,  called  Hadley's  sextant,  by  means  of  which 
the  observations  can  be  made  with  reference  to  the  horizon,  independent  of  the 
zenith,  and  therefore  independent  of  the  plumb  line. 

It  is  not  our  purpose  here  to  enter  into  a  description  of  the  principles  and 
structure  of  this  celebrated  and  most  useful  instrument.  It  will  be  sufficient 
for  the  present  purpose  to  state  that  it  is  capable  of  being  applied  to  the  meas- 
urement of  the  angular  distances  between  any  two  visible  objects  with  a  very 
great  degree  of  precision,  and  that  it  may  be  used  with  facility,  even  when 
the  position  of  the  observer  is  subject  to  all  the  unsteadiness  incidental  to  the 
condition  of  the  mariner. 

When  this  instrument  is  used,  instead  of  observing  the  distance  of  any  ob- 
ject from  the  zenith,  we  observe  its  distance  from  the  horizon,  which  will  an- 
swer the  same  purpose,  inasmuch  as  that  whenever  the  distance  of  an  object 
from  the  horizon  is  known,  its  distance  from  the  zenith  can  be  found,  since  the 
distance  from  the  zenith  to  the  horizon  being  90°,  if  we  subtract  the  distance 
of  the  object  from  that,  the  remainder  will  be  the  distance  of  the  object  fro'n 
the  zenith. 

At  sea  we  have  generally,  indeed  almost  always,  a  well-defined  horizon. 
If  the  mariner  desires  to  measure  the  altitude  of  an  object,  he  has  only  io 
measure  the  distance  of  the  object  from  the  horizon  in  a  direction  perpendicular 
to  it,  and  this  he  is  enabled  to  do  with  a  little  practice,  with  admirable  facility 
and  precision,  with  Hadley's  sextant. 

Let  us  see,  then,  how  the  mariner  is  thus  enabled  daily  to  determine  the  lati- 
tude of  hi.s  ship. 

i       As  noon   approaches,  the  sky  being  sufficiently  clear  to  render  the  disk  of 
J  the  sun  visible,  he  applies   the   instrument  and  measures  the  altitude  of  the 


THE  LATITUDES  AND  LONGITUDES.  53? 


lower  and  upper  limbs  of  the  sun  from  the  verge  of  the  horizon.  The  mean 
of  these  will  be  the  altitude  of  the  sun's  centre.  If  this  altitud.)  be  taken  from 
9(P,  the  remainder  will  be  the  distance  of  the  sun's  centre  from  the  zenith. 
He  finds  in  his  almanac  the  distance  of  the  centre  of  the  sun  on  that  day  from 
the  equator,  and  hence  he  at  once,  as  already  explained,  obtains  the  distance 
of  his  zenith  from  the  equator ;  that  is,  the  latitude  of  the  ship. 

There  are  several  minute  circumstances  observed  in  the  practice  of  this  prob- 
lem, which  do  not  affect  its  general  spirit,  and  the  introduction  of  which  here 
would  be  unsuitable  to  the  object  of  these  discourses  ;  we  therefore  omit 
them. 

Thus  we  see  that,  whether  by  sea  or  by  land — whether  in  the  observatory 
of  the  astronomer,  traversing  the  sands  of  the  desert,  or  the  forests  of  America, 
or  voyaging  over  the  trackless  and  unimpressible  surface  of  the  oceau — we  are 
in  every  case  by  science  supplied  with  suitable  and  practicable  means  by  which 

/  we  can  ascertain  the  distance  of  the  place  where  we  are,  north  or  south,  east 

£  or  west  on  the  globe. 

)  •  * 

TO  DETERMINE  THE  LONGITUDE. 

In  expiessing  and  determining  the  latitude  of  a  place,  we  have  fixed  points 
*  and  lines  on  the  firmament  to  refer  to — such  as  the  celestial  pole  and  equator ; 
(  and  to  find  it,  nothing  more  is  necessary  than  to  ascertain  the  position  of 
the  zenith  of  the  place  with  reference  to  these.  But  with  respect  to  the 
longitude,  the  case  is  very  different ;  it  is  impossible  even  to  express  the 
longitude  without  involving  a  reference  to  two  places  at  least — that  of  which 
we  wish  to  determine  the  longitude,  and  that  which  is  selected  as  the  starting 
point  Trom  which  all  longitudes  are  to  be  measured.  If  we  could  observe  in 
the  firmament  the  two  points  which  at  the  same  time  form  the  zeniths  df  the ' 
two  places,  then  the  difference  of  their  longitudes  could  be  found  by  noting  the 
times  at  which  these  two  points  would  cross  the  meridian  of  the  place  whose 
I  longitude  is  to  be  determined. 

To  comprehend  fully  the  spirit  of  the  celebrated  problem  of  finding  the  lon- 
gitude, we  must  imagine  the  globe  of  the  earth  turning  on  its  axis,  having  around  < 
it  the  starry  firmament.     Let  us  suppose  A  B  to  be  the  northern  hemisphere  of  ! 
the  globe,  p  being  the  pole,  and  let  F  E  represent  the  firmament.     Let  P  4>e  a  < 
place  whose  zenith  is  the  point  on  the  firmament  marked  by  Z.     If  we  suppose 
the  globe  to  turn  upon  its  axis  in  the  direction  of  Q  P  N,  P  will,  by  its  ro-  i 
tation,  be  carried  to  the  right  of  Z,  and  the  same  point  Z  will  become  succes- 
sively the  zenith  of  the  points  R  Q  ;  and,  in  fact,  every  point  in  the  circumfer- 
ence of  the  earth  will  successively  come  under  the  point  Z,  which  will  be, 
therefore,  in  regular  succession,  their  zenith  points.     In  twenty-four  hours,  or, 
more  accurately,  in  twenty-three  hours  and  fifty-six  minute.s,  the  globe  will 
make  its  complete  revolution  ;  therefore  three  hundred  and  sixty  degrees  of  the 
I  earth  will  successively  pass  under  the  same  point  of  the  firmament. 
<       By  knowing  exactly  the  time  of  rotation  of  the  earth,  and  having  ascertained 
\  that  its  diurnal  motion  is  uniform,  we  can  ascertain  by  simple  arithmetic  what 
;  extent  of  its  surface  will  pass,  in  a  given  time,  under  any  point  of  the  firma- 
|  ment.     Thus  if  we  say  in  round  numbers  that  the  whole  circumference  corre- 
:  sponds  to  twenty-four  hours,  it  will  follow  that  fifteen  degrees  will  move  under 
(  the  point  Z  each  hour,  or  one  degree  in  four  minutes. 

j       If  we  suppose  Z  to  represent  the  place  of  the  sun,  then  it  will  be  noon,  or 
)  twelve  o'clock,  at  the  place  which  is  immediately  under  Z ;  that  is,  at  P.     If 
I  R  be  fifteen  degrees  west  of  P,  then  it  will  arrive  under  Z  one  hour  aft 
)  consequently,  when  it  is  noon  at  P  it  is  eleven  o'clock  at  a  place  fifteen  d< 


538 


:  LATITUDES  AND  LONGITUDES. 


)  to  the  west  of  P  ;  and,  for  the  same  reason,  it  is  ten  o'clock  at  a  place  thir'y 

?  degrees  to  the  west  of  P,  and  so  on. 

Again :  if  O  be  a  place  fifteen  degrees  to  the  east  of  P,  O  must  have  been 
under  Z  an  hour  before  P  reached  it.  It  will  be  noon,  therefore,  at  O,  an  hour 
before  it  is  noon  at  P  ;  therefore,  when  it  is  noon  at  P  it  is  one  o'clock  at  0. 
In  the  same  manner,  and  for  like  reasons,  if  N  be  a  place  thirty  degrees  east  of 
P,  N  will  pass  under  Z  two  hours  before  P  ;  and  therefore  when  P  passes  under 
Z  it  will  be  two  o'clock  at  N. 


It  will  be  apparent  from  these  explanations,  that,  in  general,  the  hour  of  the 
day  at  different  places  upon  the  earth,  at  the  same  time,  will  depend  upon  their 
relative  position  east  or  west  of  each  other.  If  one  place  be  east  of  another, 
the  hour  at  that,  place  will  be  later  with  respect  to  noon  than  the  hour  at  the 
other ;  and  the  extent  to  which  it  is  later  will  depend  on  the  distance  which 
one  place  is  east  of  the  other.  In  calculating  this  difference  of  time  from  the 
difference  of  position  east  or  west,  we  may  take  fifteen  degrees  to  correspond 
with  an  hour,  as  already  explained. 

But  this  distance  of  one  place  east  or  west  of  another,  expressed  in  degrees, 
is,  in  fact,  the  difference  of  their  longitudes  ;  and  if  one  of  the  tv/o  places  in 
question  be  that  from  which  the  longitudes  are  measured,  the  determination  of 
the  longitude  of  a  place  would  resolve  itself  into  the  discovery  of  the  hour 
of  the  day  in  the  place  whose  longitude  we,  want  to  find,  and  also  at  the  place 
from  which  the  longitudes  are  measured. 

Thus,  for  example,  let  us  suppose  that  we  ascertain  the  hour  of  the  day  in 
New  York,  and  find  that  it  is  2  o'clock  in  the  afternoon,  and  that  we  have  a 


THE  LATITUDES  AND  LONGITUDES.  539 

^  """"" 

>  means  by  which  we  can  discover,  at  the  same  time,  what  the  hour  of  the  day 

>  is  at  Greenwich,  and  that  by  these  means  we  know  that  it  is  56  minutes  past 
j  6  o'clock.     We  know,  then,  that  the  time  is  4  hours  5G  minutes  earlier  at  New 
J  York  than  at  Greenwich,  and  consequently  we  infer  that  New  York  must  be 

west  of  Greenwich  by  a  longitude  which  corresponds  to  4  hours  56  minutes. 
]  Now  4  hours  correspond  to  60°,  and  56  minutes  correspond  to  14°  ;  therefore 

>  it  follows,  that  the  longitude  of  New  York  must  be  74°  west  of  Greenwich, 
j  We  can,  then,  always  discover  the  longitude  of  any  place,  provided  we  can 

ascertain,  at  any  moment,  the  hour  of  the  day  at  the  place  in  question,  and 
I  know,  at  the  same  time,  what  the  hour  of  the  day  is  in  that  place  from  which 
;  the  longitude  is  measured.* 

There  are   simple   methods  of  observation  and  calculation  by  which  the 

>  hour  of  the  day  in  the  place  where  we  are  can  be  determined,  with  more  or 
less  accuracy,  according  to  the  circumstances  of  our  position.     If  we  are  on 
land,  and  supplied  with  a  proper  transit  instrument,  we  can,  by  its  means,  ob- 
serve the  moment  at  which  the  centre  of  the  sun's  disk  passes  the  meridian. 
Thus,  as  the  moment  of  noon  arrives,  by  observing  it,  we  can  set  a  good  clock, 
which  will  inform  us  of  every  other  hour  of  the  day.     But  even  in  the  absence 
of  a  clock  we  can  determine  the  hour  of  the  day  at  any  moment  at  which  the 
sun  is  visible,  by  observing  its  altitude,  having  previously  ascertained  the  lati- 
tude of  the  place  at  which  we  are. 

If  we  are  at  sea,  where  we  cannot  command  a  transit  instrument,  nor  use  it  if 
\ve  could,  the  latitude  of  the  place  of  the  ship  is  first  determined,  and  then  the- 
hour  is  found  by  observing  the  altitude  of  the  sun  at  any  convenient  time  in  the 
afternoon  or  forenoon.  The  hour  being  once  found,  the  time  can  be  kept  by  a 
chronometer  for  any  number  of  hours  afterward.  Thus  it  appears,  under  all  cir- 

)  cumstances,  whether  by  sea  or  by  land,  there  is  no  practical  difficulty  in  de- 
termining what  o'clock  it  is  where  we  are.     This  at  once  reduces  the  problem  ( 
of  the  longitude  to  the  simple  discovery  of  the  hour  of  the  day,  at  any  given  * 
time,  at  the  place  from  which  the  longitudes  are  reckoned. 

The  first  and  most  obvious  method  of  accomplishing  this  which  would  occur  j 
to  the  mind,  would  be  to  carry   a  good    chronometer  from  the    place  from  < 
\\hich  the  longitude  is  reckoned.     Supposing  this  chronometer  subject  to  no  {. 
error,  it  will  continue  to  inform  you  of  the  hour  of  the  day  at  that  place.     Thus,  i 
suppose  that  on  leaving  London  the  mariner  takes  with  him  a  chronometer  set  | 
according  to  the  time  at  Greenwich,  and  with  it  makes  his  voyage  to  New  < 
York ;  the  chronometer  will  continue  to  inform  him  what  the  time  is  from  hour  * 
to  hour  at  Greenwich.     When  he  arrives  at  New  York,  he  will  find  that  when 
the  chronometer  points  to  12  o'clock,  or  noon,  it  will  be  early  in  the  morning; 
and  if  he  ascertains  the  hour  exactly,  he  will  find  that  it  will  be  4  minutes  after 
7  o'clock.     He  will  therefore  know  that  the  time  at  New  York  is  4  hours  56 
minutes  earlier  than  at  Greenwich,  and,  consequently,  that  New  York  must  be 
74°  west  of  Greenwich.     It  is  for  these  reasons  that  the  perfection  of  chro- 
nometers has  always  been  considered  so  essential  to  the  progress  of  navigation. 
Every  ship  that  makes  a  long  voyage  ought  to  be  supplied  with  one,  at  least,  of 
these  instruments  ;  but  as  they  are  liable  to  accident,  and  as  even  the  best  of 
them  cannot  be  rendered  perfect,  it  is  usual  with  ships  that  are  well  provided 
for  long  voyages  to  carry.more  than  one  chronometer. 

Although  the  art  of  constructing  time-keepers  has  been  brought  to  a  high  de-  5 
gree  of  perfection  by  the  skill  of  modern  artisans,  these  instruments  are  even  yet, 

'l  and  probably  will  ever  continue  to  be,  too  imperfect  to  be  implicitly  and  exclu- 

*  There  are  several  corrections  to  be  attended  to  in  the  practical  working  of  the  methods  of  deter- 
S  mining  latitude  and  longitude  which  I  have  purposely  omitted,  as  they  do  not  affect  the  ipint  of 
)  the  method,  which  is  all  I  would  here  convey. 

C-V.-X 


540  THE  LATITUDES  AND  LONGITUDES. 

sively  relied  upon.  If  we  only  required  their  indications  for  short  spaces  of 
time,  such  as  a  few  days,  or  even  weeks,  we  might  perhaps  place  a  secure  re- 
liance upon  them  ;  especially  if  the  voyager  were  provided  with  more  than  one 
instrument  of  this  kind.  But  in  voyages  or  journeys  which  occupy  mon.ns, 
we  cannot  rely  on  the  indications  of  these  instruments,  even  when  most  liberally 
provided  and  most  perfectly  constructed. 

In  the  absence,  then,  of  a  chronometer,  how,  it  will  be  asked,  can  the  lon- 
gitude of  a  place  be  ascertained  at  all.  The  first  method  that  will  occur  to  the 
min  1,  will  be  that  of  some  conspicuous  signal  which  can  be  seen  at  the  same 
time  at.  the  two  places,  whose  difference  of  longitude  is  to  be  determined. 
For  this  we  require  two  observers  ;  but  it  is  perhaps  the  method  of  all  others,  sus- 
ceptible of  the  greatest  accuracy.  Let  us  suppose  that  on  some  elevated  posi- 
tion between  two  distant  places,  such  as  New  York  and  Boston,  a  sudden 
and  conspicuous  light  is  produced,  such  as  the  celebrated  Drummond  liijht. 
which  might  be  exhibited  on  the  top  of  a  high  mountain  so  as  to  be  visible  a 
great  distance.  Let  this  signal  be  exhibited  at  any  required  moment,  so  as  to 
render  it  suddenly  visible  at  the  two  places.  Let  the  observers  at  these  places 
note  precisely  the  hour  of  the  day  or  night  at  which  the  light  is  seen.  By 
comparing  afterward,  these  times,  their  difference  will  at  once  give  us  the 
difference  of  the  longitude  at  the  two  places. 

But  this  method  is  evidently  applicable  only  on  a  limited  scale,  and  under  pe- 
culiar  circumstances  ;  it  is  altogether  unavailable   to   the  mariner.     Now  the 
.  astronomer  supplies  him  with   a  chronometer  of  unerring  precision  ;  a  chro- 
nometer which   can   never  go  down,  nor  fall   into  disrepair;  a  chro  :• 
which  is  exempt  from  the  accidents  of  the  deep ;  which  is  undisturbed  by  the 
siion  of  the  vessel;  which  will  at  all  times  be  present  and  available  to  him 
wherever  he   may  wander  over  the   trackless   and  unexplored,  regions  of  the 
ocean.     Such  a  chronometer  has  been  found ;  made  by  an  Artisan  who  cannot 
err,  and  into  whose  works  imperfection  can  never  enter.     Such  a  chronometer 
is  supplied  by  the  firmament  itself.     The  unwearied  labors  of  modern  as 
omers  have  converted  the  face  of  the  heavens  into  a  clock,  and  have    : 
the  'iiariner  to  read  its  complicated  but  infallible  indications.      We  may  ;• 
for  this  purpose  the  firmament  as   the  dial-plate. of  a  chronometer  on   an   im- 
mense scale.     The  constellations  and  the  fixed  stars  upon  it,  which  for  count- 
less ages  are  subject  to  no  change  in  position,  serve  as  the  hour  and  minute- 
marks.     The   sun,  the   moon,   the    planets,  and    the    satellites,    which    move 
continually  over  the  surface  of  this  splendid  piece  of  mechanism,  play.hr 
of  the  hands  of  the  clock.     The  positions  of  these  bodies  from  day  to  d,. 
from  hour  to  hour,  and  every  change  of  their  positions,  are  accurately  foreknown 
and  exactly  registered  in  a  book  published  some  two  or  three  years  in  advance. 

the  "  Nautical  Almanac,''  and  circulated  for  the  benefit  of  ti.arii!ers. 
this  work,  the  navigator  is  told  what  the  hour  is  or  will  be  at  Greenwich  for 
variety  of  position  which  the  sun,  moon,  and  planets,  shall  have  from  time  to  time 
upon  the  heavens.     But  of  all  objects  in  the  heavens,  that  which  is  best  suited 
for  this  species  of  observation  is  the  moon,  and  hence  this  method   of  deter- 
mining the  longitude   at   sea   has   been  distinguished  by  the  appellation  ol 
lunar  method.     By  the  use  of  Hadley's  sextant,  which  we  1, 

to,  it  is  easy,  whenever  the  Leavens  are.  clear,  to  obser.v  the  angular  distance  \ 
of  the  moon  either  from  the  sun  or  from  the  most  conspicuous  stnrs  cr  pi; 
The   motion  of  the  moon  in  the  firmament  is  so  rapid  that  it:'  c  ;;•;:•£(.  oi 
tion  is  perceptible,  even  by  such  observations  as  can  be  mat  <-  IT,  c^r.l  a 
from  hour  to  hour. 

How,  then,  it  may  be  asked,  can  such  observations  be  i>: 
the  discovery  of  the  longitude  of  a  ohip  I      Nothing  can  be  more,  oiiii! 


THE  LATITUDES  AND  LOXUTl 


I 


navigator  requires  only  to  know  what  is  ihe  hour  at  Creenwich  at  tin-  tj.,,,  |l(. 
makes  his  observation.  This  he  discovers  in  the  following  manner-  IJ. 
observes  with  the  sextant  the  distance  of  the  moon  from  the  Snn,  or  from 
some  of  the  most  conspicuous  stars  ;  he  then,  after  certain  preliminary  rai- 
culations  not  necessary  to  detail  here,  examines  in  the  Nautical  Almafac 
where  he  learns  what  the  hour  is  at  Greenwich,  when  it  particular 

distances  from  the  sun  or  the  stars.  Knowing  this,  and  knowing  the  hour 
where  he  is,  the  difference  of  the  longitude  of  a  ship  and  the  observatory  at 
Greenwich  is  known  to  him. 

Although  the  moon  be  of  all  the  celestial  objects  the  best  adapted  for  this 
observation,  it  is  not  the  only  one  which  has  been  resorted  to.  It  may  be  in  a 
position  so  near  the  sun  that  it  cannot  be  conveniently  observed ;  in  its  ab- 
sence, the  navigator  may  resort  to  planets  which  may  happen  to  be  visible. 
These  may  be  used  in  the  same  manner  and  according  to  the  same  principles 
as  the  moon,  but  they  do  not  afford  a  result  susceptible  of  the  same  accuracy, 
inasmuch  as  their  motions  being  slower,  he  cannot  be  so  certain  of  their 
positions. 

The  advantage  which  the  lunar  method  of  determining  the  longitude  has  for 
the  purpose  of  the  mariner  is,  that  it  is  always  available,  when  the  sky  i*  un- 
clouded. There  are.  however,  other  methods  which  are  applicable  occasion- 
ally, both  by  sea  and  by  land,  which  ought  not  to  be  omitted  here ;  union  j 
these  the  most  frequent,  and  consequently  the  most  generally  available,  is  tin; 
eclipses  of  Jupiter's  satellites.  Whenever  that  planet  is  sufficiently  removed 
from  the  sun  to  be  visible  after  night-fall,  his  moons  may  be  seen  with  an  <;ni;- 
nary  telescope  ;  indeed,  they  were  discovered  at  so  early  a  period  in  the  pro- 
gressive improvement  of  the  telescope,  that  they  must  have  been  first  observed 
with  a  very  inferior  instrument  of  that  kind.  The  periodic  time  of  the  first  of 
these  satellites,  or  that  which  is  nearest  to  Jupiter,  being  only  about  42  hours, 
and  its  position  and  motion  being  such  that  it  cannot  pass  beliind  Jupit&r  with- 
out going  through  his  shadow,  its  eclipse  must  regularly  recur  every  42  hours. 
The  times  of  the  eclipses  at  Greenwich  are  registered  in  the  Nautical  Alma- 
nac, and  if  they  are  observed  at  a  distant  place,  the  time  at  which  they  occur 
may  be  compared  with  the  time  at  which  they  would  be  seen  at  Green- 
wich, and  the  longitude  of  the  place  consequently  known.  In  fact  these  eclip- 
ses may  be  regarded  as  signals  which  can  be  seen  at  the  same  time  from  iho 
two  places ;  the  only  difference  between  them  and  common  signals  being  lhat 
their  occurrence  can  be  certainly  and  accurately  predicted.  It  is  proper  how- 
ever to  observe,  that  although  this  method  is  eminently  useful  to  the  geographical 
traveller,  it  can  scarcely  be  said  to  be  available  in  navigation. 

There  are  other  celestial  phenomena  of  occasional  occurrence  which  may 
also  be  used  for  determination  of  longitudes.  Such  are  solar  eclipses,  but  more 
especially  the  occultation  of  stars  by  the  dark  edge  of  the  moon.  This  latter 
phenomena  is  one  which  admits  of  very  great  precision. 

In  connexion,  with  the  subject  of  this  discourse,  it  may  not  be  uninterest- 
ing or  unprofitable  to  explain  the  expedient  by  which  the  British  government 
enable  all  navigators  leaving  the  Thames  to  take  with  them  the  precise  Green- 
wich time,  which,  as  we  have  shown,  is  necessary  for  the  determination  of  the 
longitude  of  the  ship  in  the  absence  of  the  opportunity  or  ability  of  practising 
the  lunar  method.  For  a  great  number  of  years,  the  establishment  of  an  easy 
and  certain  method  of  accomplishing  this  was  regarded  as  an  object  of  great 
national  importance  by  the  English  public.  At  length  the  object  was  accom- 
plished by  the  expedient  now  in  use,  and  which  we  are  about  to  explain. 

The  Royal  Observatory  of  England  is  built  on  the  summit  of  an  elevaied 
rid^e  that  overhangs  the  town  of  Greenwich,  on  the  right  bank  of  the  Thames. 


542 


THE  LATITUDES  AND  LONGITUDES. 


and  forms  a  conspicuous  object  from  the  river.  The  towers  of  the  observatorv 
ar<;  at  all  times  visible  from  ships  sailing  down  the  river.  It  was,  therefore, 
decided  that  a  signal  should  be  given  at  the  instant  of  one  o'clock  in  the  after- 
noon of  each  day  ;  by  observing  which,  navigators  within  view  of  the  observa- 
tory could  correct  their  chronometers.  The  signal  adopted  for  this  purpose 
was  the  sudden  fall  of  a  large  black  ball,  placed  upon  a  pole  raised  from  the 
top  of  one  of  the  towers  of  the  observatory. 

Before  elevating  the  ball,  at  five  minutes  before  one  o'clock,  a  signal  is  made 
of  the  intention  to  do  so  by  raising  it  half-mast  high.  Observers  are  then  in- 
structed to  prepare  their 'chronometers  ;  and  as  the  descent  of  the  ball  occupies 
several  seconds,  they  should  confine  their  attention  to  observing  the  moment 
when  the  ball  leaves  the  top,  as  it  is  that  alone  which  indicates  the  hour. 

The  use  of  this  signal  is  not  merely  confined  to  the  indication  of  the  mean 
time  at  Greenwich  for  navigators  going  down  the  river.  By  observing  the 
drop  of  the  ball,  repeated  day  after  day,  mariners  who  are  in  the  river  will  be 
enabled  to  ascertain  the  daily  rate  of  their  chronometers.  Thus,  if  a  clock 
were  found  to  show  the  time  of  3  min.  5  sec.  after  1  o'clock  at  the  moment  of 
dropping  the  ball  one  day,  it  will  appear  that  the  clock  is  3  min.  5  sec.  faster 
than  the  mean  Greenwich  solar  time.  On  the  following  day,  if  you  again  ob- 
serve the  descent  of  the  ball,  and  find  that  the  clock  shows  3  min.  7  sec.  after 
1  o'clock,  you  find  that  it  gains  2  seconds  per  day.  Thus  you  are  enabled,  not 
only  to  ascertain  the  actual  error  of  the  chronometer,  but  also  predict  the  man- 
ner in  which  that  error  will  be  augmented  or  diminished  for  the  future. 

In  noticing  the  different  methods  which  have  been  proposed  for  determining 
the  longitude,  I  ought  not  to  omit  one  which  has  been  recently  resorted  to  with 
considerable  advantage,  and  which  is  called  the  method  of  determining  the 
longitude  by  moon-culminating  stars.  In  the  practice  of  this  method  a  star  is 
chosen  which  culminates  or  passes  the  meridian  nearly  at  the  same  time  with 
the  moon,  and  which  differs  so  little  in  declination  with  the  moon,  that  it  may 
be  seen  at  the  same  time  in  the  field  of  view  of  the  telescope.  The  transit  of 
the  star  and  that  of  the  moon's  limb,  is  observed  at  both  stations,  and  the  differ- 
ence of  the  time  at  the  two  stations  noted.  This  difference  being  dependant 
on  the  moon's  change  of  position  on  the  firmament,  in  passing  from  the  meridian 
of  one  station  to  the  meridian  of  the  other,  will  enable  the  observers  to  deter- 
mine the  time  which  the  centre  of  the  moon  takes  to  pass  from  the  one  meridian 
to  the  other,  which  will  give  the  difference  of  the  longitudes. 

The  spirit  of  this  method  is  derived  from  the  great  accuracy  of  the  knowl- 
edge we  have  acquired  of  the  moon's  motions,  and  the  precision  with  which 
we  can  observe  its  transits  over  the  meridians.  In  the  practice  of  this  method, 
it  is  indispensable  that  the  moon  and  star  should  differ  so  little  in  declination 
that  the  position  of  the  telescope  will  not  require  to  be  changed  to  observe 
their  respective  transits.  Although  the  method  has  been  called  that  of  moon- 
culminating  stars,  the  only  reason  why  the  moon  and  star  should  be  required 
to  pass  the  meridian  nearly  together  is,  that  the  same  errors  may,  as  far  as 
possible,  affect  both  transits,  and  if  so  no  effect  would  be  produced  on  the  ulti- 
mate result. 


THEORY    OF    COLORS. 


Refraction  of  a  Ray  of  Light. — At  plane  Surfaces. — By  a  Prism. — The  Prismatic  S|w?ctruni. — The 
Decomposition  of  Li^lit. — Newton's  Discoveries. — Colors  of  the  Spectrum. — Brewster'n  Discovery 
of  three  Colors — How  three  Colors  can  produce  the  Spectrum. — Colors  of  natural  Bodie«  —  How 
they  are  produced. 


THEORY  OF  COLORS. 


WHEN  a  ray  of  light  meets  the  surface  of  a  transparent  medium,  such  as 
water  or  glass,  in  a  line  perpendicular  to  that  surface,  it  will  pass  through  \ 
without  changing  its  course  ;  but,  if  it  meet  the  surface  at  any  oblique  angle,  it 
will  be  bent  into  another  direction,  which  will  depend  on  the  direction  of  the 
incident  ray,  and  the  relative  densities  of  the  media,  between  which  the  ray 
passes.  Generally,  when  it  passes  from  a  less  dense  into  a  more  dense  medi- 
um, it  is  bent  toward  the  perpendicular  drawn  to  the  surface  of  the  medium  at 
the  point  of  incidence  of  the  ray.  In  this  deflection  it  does  not  leave  the 
plane  passing  through  the  incident  ray,  and  that  perpendicular. 

Fig.  i. 


To  render  this  more  clear,  let  c,  fig.  1,  be  any  visible  object  placed  on  the  ( 
bottom  of  a  vessel  of  water.     Let  c  n  be  a  ray  of  light  passing  from  that  ob-  < 
ject  to  the  surface  of  the  water,  that  ray  after  leaving  the  surface  of  the  water 
and  passing  into  the  air  will  not  continue  in  the  direction  c  n,  but  will  take 


35 


546 


THEORY  OF  COLORS. 


another  direction,  n  E,  so  that  an  eye   placed  at  E  would  see  the  object  in  the 
direction  E  n. 

This  deflection  which  a  ray  of  light  suffers  in  passing  from  one  transparent 
medium  into  another,  having  a  different  density,  is  called  refraction. 

REFRACTION  AT  PLANE  SURFACES. 

Let  S  S',  fig.  2,  represent  the  surface  which  separates  two  transparent  media. 
P  0  being  less  dense  than  P  0'.  Let  A  P  be  a  ray  of  light  falling  at  P,  and  let 
0  0'  be  perpendicular  to  S  S'.  After  passing  into  the  denser  medium  the  r.iy 
will  follow  the  course  P  A',  making  with  the  perpendicular  P  0,  a  less  angle 
than  A  P  O. 

Fig.  2. 


If,  on  the  other  hand,  the  ray  passed  from  A'  to  P,  it  would  follow  the  course 
P  A  in  the  less  dense  medium.  This  law  of  refraction  is  usually  expressed 
thus  :  when  light  passes  from  a  rare  into  a  dense  medium,  as  from  air  to  water, 
or  from  water  to  glass,  it  is  always  deflected  toward  the  perpendicular  to  the 
reflecting  surface,  and  when  it  passes  from  a  denser  medium  into  a  rarer,  as 
from  glass  to  water,  or  from  water  to  air,  it  is  bent  from  the  perpendicular. 

The  extent  of  this  deflection  has  been  determined  by  a  general  law,  which, 
expressed  in  the  language  of  geometry,  is,  that  the  sine  of  the  angle  of  inci- 
dence bears  to  the  sine  of  the  angle  of  refraction,  a  fixed  ratio  when  the  media 
are  given. 

From  this  it  follows  that  the  deflection  of  light  by  refraction  will  always  be 
increased  with  the  obliquity  of  the  incident  rays. 

It  is  also  found  that  the  degree  of  refraction  will  be  greater  the  greater  the 
difference  of  the  density  of  the  media  is.  Thus  the  refraction  is  greater  when 
a  ray  passes  from  air  into  glass  than  when  it  passes  from  air  into  water ;  it  is, 
also,  greater  when  it  passes  from  glass  into  air  than  from  glass  into  water. 

In  his  celebrated  optical  investigations,  Newton  found  that  the  solar  beam 
was  composed  of  different  kinds  of  light,  which,  besides  differing  in  color,  also 
differ  in  refrangibility,  that  is  to  say,  if  they  fall  at  the  same  angle  on  any  re- 
flecting surface,  they  will  not  pass  in  the  same  direction  through  it,  but  will 
follow  different  directions,  according  to  their  different  susceptibilities  of  being 
refracted. 

The  kind  of  experiment  by  which  this  remarkable  fact  was  ascertained  is  as 
follows  : — 

Suppose  a  beam  of  light  proceeding  from  the  sun  to  enter  a  hole  in  a  win- 
dow-shutter and  to  fall  obliquely  on  the  surface  of  a  triangular  piece  of  glass, 


THEORY  OP  COLORS. 


54? 


called  a  prism,  at  D.  The  parts  of  that  ray  in  passing  through  tho  prism  will 
diverge  from  each  other,  and  falling  upon  the  second  surface  of  the  prism  at 
?,  will  issue  from  it  still  more  divergent.  If  the  prism  had  not  been  murno*- 
ed,  a  circle  of  light  would  be  formed  upon  a  white  screen  at  E  N,  which  would 
correspond  with  the  magnitude  of  the  opening  in  the  window-shutter.  But 
when  the  light  is  made  to  pass  through  the  prism  an  oblong  spectrum  will  be 
formed  on  the  screen,  the  breadth  of  which  will  correspond  with  E  N,  but 
which  will  have  considerable  length.  This  spectrum  will  exhibit  a  series  of 
colors,  the  lowest  of  which  will  be  red,  and  the  highest  violet.  They  will 
succeed  each  other  in  the  following  order,  proceeding  upward  :  red,  orange, 
yellow,  green,  blue,  indigo,  and  violet.  These  colors  will  not,  however,  have 
distinct  boundaries,  but  will  pass  gradually,  by  insensible  tints,  one  into  another, 
so  that  it  will  be  impossible  to  say  exactly  where  the  red  ends  and  the  orange 
begins,  and  so  of  the  others. 

Fig.  3. 


White. 


This  remarkable  phenomenon  was  explained  by  Newton  by  showing  that  the 
solar  light  was  composed  of  a  number  of  different  kinds  of  light,  which  were 
capable  of  being  refracted  in  different  degrees  by  the  prism,  those  lights  which 
were  least  refrangible  passing  to  the  lower  extremity,  and  those  that  were  most 
refrangible  to  the  upper  extremity  of  the  spectrum.  By  inspecting  the  figure 
it  will  be  evident  that  the  red  light  is  less  deflected  from  its  straight  course 
than  the  orange ;  the  orange  less  than  the  yellow  ;  the  yellow  less  than  the 
green,  and  so  on.  Newton,  therefore,  inferred  that  there  were  lights  of  seven 
distinct  kinds,  having  seven  different  degrees  of  refrangibility,  and  seven  dif- 
ferent colors. 

This  conclusion,  however,  has  been  subject  to  much  modification  by  subse- 
quent optical  investigators. 

It  is  found  that  rays  of  light  of  the  same  color  differ  slightly  in  refrangibility, 
and  the  investigations  of  Brewster,  and  others,  appear  to  justify  the  conclusion, 
that  the  solar  light,  instead  of  consisting  of  seven  elementary  colors,  ia  only 
composed  of  three. 

At  so  early  a  period  as  the  year  1775,  it  was  suspected  that  the  conclusion 
of  Newton,  that  the  spectrum  was  divisible  into  seven  different  simple  con- 
stituent lights,  was  fallacious.  Mayer  maintained  that  there  were  but  three 
elementary  colors,  red,  yellow,  and  blue,  and  at  a  later  epoch,  Dr.  Young  sug- 
gested that  all  colors  were  compounded  of  red,  green,  and  violet. 

Let  us,  however,  for  a  moment  contemplate  the  actual  result  of  the  prismatic 
experiment  of  Newton,  and  let  us  separate,  carefully,  that  which  is  matter  of 
observation  in  it,  from  that  which  is,  properly  speaking,  matter  of  hypothesis 
or  theory. 

In  passing  through  the  prism,  and  being,  thereby,  submitted  to  a  considerable 
refracting  action,  a  single  beam  of  light  is  spread  out  into  a  fan  of  rays  as  rep- 


THEORY  OF  COLORS. 


resented  in  fig.  3.  This  fan-like  form  is  produced  by  the  fact  that  some  of  the 
<  rays  which  compose  the  beam  are  more  strongly  refracted  by  the  prism  than 
others,  and  the  divergence  of  the  fan  depends  upon  the  difference  between  the 
extent  of  the  deflection  of  the  most  refrangible,  and  the  least  refrangible  rays. 
The  angle  of  divergence  of  the  fan  has  been  called  the  dispersion  of  the  origi- 
nal beam  by  the  prism. 

When  the  rays,  thus  dispersed,  in  virtue  of  their  different  susceptibility  of 
refraction,  are  received  upon  a  white  screen,  they  exhibit  a  streak  of  surface 
illuminated  by  a  series  of  different  tints  of  color,  which,  in  their  general  char- 
acter, are  conformable  to  the  distinction  assigned  to  them  by  Newton  ;  but  ac- 
curate examination  shows  that  there  are  no  distinguishable  boundaries  between 
the  successive  tints ;  that  throughout  the  limits  of  the  red  the  degree  of  red- 
ness varies,  that  it  insensibly  melts  away  into  the  beginning  of  the  orange, 
which,  increasing  to  a  point  where  its  intensity  is  greatest,  again  gradually 
melts  away  insensibly  into  the  yellow,  and  so  on,  the  successive  colors  and  tints 
of  color  fading  imperceptibly  into  each  other.  Now  there  is  nothing  in  these 
circumstances  to  afford  any  rigid  justification  of  the  seven  elementary  colors 
assigned  by  Newton,  and  when  we  consider,  what  is  not  disputed  by  Newton 
himself,  that  the  commingling  or  blending  together  of  lights  of  different  colors 
will  produce  intermediate  tints,  it  follows  that  there  are  an  infinite  variety  of 
ways  in  which  the  constituent  colors  of  light  might  be  imagined  to  be  arranged 
which  would  equally  produce  the  phenomenon  of  the  prismatic  spectrum. 

This  problem  has,  accordingly,  been  taken  up  in  our  own  times  by  Sir  Da- 
vid Brewster,  with  all  the  advantages  which  the  increased  knowledge  and  experi- 
ence of  the  age,  and  improved  methods  of  inquiry,  could  afford.  He  has  shown, 
by  innumerable  experiments  on  the  transmission  of  light  through  colored  me- 
dia, and  on  artificial  lights,  produced  by  combustion,  of  various  circumstances, 
that  the  pure  and  elementary  simple  lights  are  one  or  other  of  the  three 
colors,  red,  yellow,  and  blue  ;  that  the  light  of  each  of  these  colors,  respect- 
ively, is  composed  of  constituent  rays  which  are  differently  refrangible,  so  that 
if  a  beam  of  any  one  of  these  lights  were  transmitted  through  a  prism,  an  ob- 
long spectrum  would  be  produced,  of  one  uniform  color,  corresponding  to  that 
of  the  light  itself.  .Thus  if  we  suppose  a  beam  of  red  light  transmitted  through 
a  prism  in  the  same  manner  as  the  original  beam  of  white  light,  fig.  3,  was 
transmitted,  then  we  should  obtain  an  oblong  spectrum,  similar  in  form  and 
length  to  that  which  we  originally  obtained,  but  all  of  one  tint.  It  would  be 
all  red,  although  the  redness  would  be  greatest  at  one  particular  point,  and 
would  decrease  from  that  point  toward  each  extremity,  and  gradually  fade 
away.  These  circumstances  may  be  represented  by  the  diagram,  fig.  4. 

Let  L  M  represent  the  screen,  and  let  L  represent  the  lower  and  M  the  up- 
per end  of  the  spectrum ;  let  N  be  the  point  at  which  the  redness  is  most  in- 
tense, it  will  gradually  diminish  from  N  to  M  and  from  N  to  L.  Let  us  sup- 
pose that  we  draw  a  curved  line,  L  P'  P  P"  M,  so  that  the  lines  or  distances 
N  P',  N  P,  N  P",  &c.,  shall,  respectively,  represent  the  intensities  of  the 
light  at  the  several  points  N'  N7',  &c.  Such  a  figure  will  exhibit,  geomet- 
rically, the  gradation  of  tints  from  the  point  N,  where  the  red  is  brightest,  up- 
ward and  downward  to  the  points  where  it  fades  away.  It  is  found  by  ex- 
periment that  the  point  where  it  is  brightest  is  near  the  lower  extremity  of  the 
spectrum. 

In  like  manner,  if  a  beam  of  pure  yellow  light  be  transmitted  through  the 
prism,  a  similar  yellow  spectrum  will  be  produced,  which  may  be  represented 
in  -I  similar  manner,  the  point  of  greatest  brightness,  however,  being  at  a  high- 
er point  in  the  spectrum,  represented  in  figure  5,  by  similar  letters. 

Finally,  let  us  suppose  a  beam  of  blue  light  transmitted  through  the  prism  in  ( 


THEORY  OF  COLORS. 


Fig.  4. 


¥ig.  ft 


(  like  manner.    Its  point  of  maximum  brilliancy  will  be  still  higher  than  that  of 
«  yellow,  as  represented  in  fig.  6. 

Fig.  6. 


550 


THEORY  OF  COLORS. 


In  the  same  manner,  throughout  the  whole  extent  of  the  three  uniform  spec- 
tra thus  intermingled  the  tints  of  color  will  correspond  to  the  intensities  of  the 
spectra  at  the  same  point. 

In  this  manner  the  succession  of  colors  exhibited  by  the  prismatic  spectrum 
is  explained.  The  orange,  for  example,  is  only  the  intermixture  of  a  consid- 
erable quantity  of  red  and  yellow,  qualified  by  a  small  quantity  of  blue.  The 
green,  in  the  same  manner,  is  a  mixture  of  a  considerable  quantity  of  blue  and 
yellow,  qualified  by  a  very  small  quantity  of  red. 

There  is  a  certain  proportion  in  which  these  three  elementary  colors  may 
be  mixed  together  so  as  to  produce  white.  If  any  one  of  them,  the  red,  for 
example,  be  in  excess  above  this  proportion,  the  other  two  observing  it,  the  re- 
sulting color  will  be  a  red  diluted  with  white.  If,  on  the  other  hand,  there  be 
a  deficiency  of  the  proper  proportion  of  red,  the  tint  will  be  green  diluted  with 
white,  produced  by  the  excess  of  blue  and  yellow. 

In  general,  if  we  take  the  actual  proportion  in  which  these  three  elementary 


Now,  if  we  suppose  a  beam  of  white  light,  like  the  natural  light  of  the  sun, 
which  is  composed  of  these  three  constituent  elementary  lights,  to  be  transmit- 
ted through  the  prism,  we  ought  to  expect  these  three  spectra  of  the  element- 
ary colors,  red,  yellow,  and  blue,  to  be  simultaneously  produced,  the  maximum 
of  each  being  at  the  place  already  assigned  to  it.  The  combination  of  these  is  ] 
represented  in  the  diagram,  fig.  7,  and  the  tint  of  color  at  each  point  of  the  } 
spectrum  will  be  that  which  would  result  from  the  corresponding  mixture  of  col- 
ors. Thus  at  R  N,  where  the  red  is  most  intense,  a  portion  of  blue,  represent- 
ed by  N  b,  and  of  yellow,  represented  by  N  a,  are  mixed  with  it,  and  the  re- 
sulting tint  will  be  that  which  will  be  produced  by  the  mixture  ;  in  like  man- 
ner at  Y  N,  where  the  yellow  is  most  intense,  a  portion  of  blue,  represented 
by  N  b'  and  a  portion  of  yellow,  represented  by  N  a',  will  be  mingled 
with  it. 

Fig.  7. 


THEORY  OF  COLORS. 


colors  are  combined,  and  assuming  that  wjhich  is  least  intense  among  them, 
combine  with  it  the  proportion  of  the  other  two,  which  is  necessary  to  produce 
white,  the  resulting  tint  will  be  such  as  would  be  produced  by  the  balance  of 
the  remaining  colors  diluted  by  the  resulting  white. 

By  following  out  this  reasoning,  it  will  be  seen  how  the  infinite  variety  of 
tints  of  color  may  be  produced  by  the  simple  component  colors,  red,  yellow, 
and  blue,  existing  in  different  degrees  of  intensity. 

The  color  called  black  is  produced  by  the  absence  of  all  light,  and  is,  in  fact, 
a  name  for  absolute  darkness.  If  it  were  possible  to  find  a  substance  abso- 
lutely incapable  of  reflecting  any  light  to  the  eye,  or  what  is  the  same,  of  ab- 
sorbing all  the  light  which  falls  upon  it,  such  substance  would  appear  absolute- 
ly black.  But  as  no  substance  in  nature  is,  on  the  one  hand,  capable  of  reflect- 
ing all  the  light  which  falls  upon  it,  so,  on  the  other  hand,  no  substance  in  na- 
ture is  capable  of  absorbing  all  the  light  that  falls  upon  it.  If  we  take  the 
blackest  known  substance  and  throw  upon  it  strongly-condensed  light,  it  will 
become  distinctly  visible  to  the  eye  by  a  small  portion  of  light  which  it  will 
reflect,  which  will  make  it  appear  of  a  gray  color,  or  faint  white.  It  appears, 
then,  that  objects  which  are  popularly  termed  black,  are,  in  fact,  faintly  white. 
A  true  black  would  be  an  object  having  no  color  at  all. 

Experiments  made  on  finely-divided  substances  have  proved  that  there  is  no 
substance  absolutely  opaque.  The  most  dense  substances  known,  and  those 
that  are,  apparently,  most  impervious  to  light,  are  found,  when  cut  into  leaves 
or  filaments  sufficiently  thin,  to  be  transparent ;  but  the  light  which  goes  through 
them  is  always  of  a  tint  contrary  to  that  which  they  reflect.  Thus  if  an  object 
appears  to  the  eye  to  be  of  a  yellow  color,  we  know  that  the  reason  is  that  it 
reflects  to  the  eye  yellow  light.  What,  then,  becomes,  it  may  be  asked,  of  the 
red  and  the  blue  components  of  the  solar  light  which  falls  upon  it  ?  If  we  ob- 
tain a  shaving  of  the  body  sufficiently  thin,  and  look  behind  it,  we  shall  find 
that  it  will  appear  of  a  color  composed  of  the  red  and  blue  ;  that  is,  it  trans- 
mits through  it  the  colors  which  it  fails  to  reflect. 

Hence  it  has  been  inferred  that  the  absorption  of  light  which  takes  place  in 
colored  bodies  is  effected,  not  immediately  on  their  surface,  but  at  some  defi- 
nite depth  within  their  dimensions,  and  that  such  portion  of  the  compound  so- 
lar light  that  falls  upon  it,  as  is  not  reflected,  passes  successively  through  la- 
mina, one  within  another,  each  of  which  absorbs  a  portion  of  it,  until,  at  length, 
it  is  altogether  lost. 

As  heat  is,  by  some  means  not  clearly  known  to  us,  connected  with  light,  we 
have,  in  these  circumstances,  a  clear  explanation  of  the  fact,  that  more  heat  is 
absorbed  by  bodies  of  a  dark  color  than  by  those  of  a  light  color.  In  general 
the  lighter  the  color  the  greater  the  proportion  is  of  the  reflected  light,  and  the 
darker  the  color  the  less  the  proportion  is.  The  greater  the  proportion  of 
light  that  is  absorbed  the  greater  will  be  the  proportion  of  the  heat  which  at- 
tends that  light.  Hence  it  follows  that,  as  dark  colors  absorb  more  heat  than 
light  ones,  and  as  black  absorbs  the  most  of  all,  dark  colors  are,  in  gener- 
al, warm,  and  black  the  most  so.  If  two  pieces  of  cloth  be  thrown  upon 
snow,  one  black  and  the  other  white,  the  black  will  sink  through  it,  melting 
the  snow  under  it,  before  the  other  penetrates  into  it  perceptibly. 

Hence,  dark-colored  cloths  are  most  suitable  in  cold  weather,  and  light-col- 
ored in  warm  weather. 

After  all  that  has  been  explained,  it  will  be  scarcely  necessary  to  say  that 
the  sense  in  which  color  is  commonly  understood  to  be  a  quality  of  bodies,  is 
incorrect,  and,  strictly  speaking,  it  is  true,  although  it  may  sound  paradoxical 
to  say  that  leaves  are  not  green,  and  that  the  sky  is  not  blue.  The  green  and 
the  blue  colors  belong,  properly  speaking,  not  to  the  objects  which  appear  to 


552 


THEORY  OF  COLORS. 


the  eye  to  be  green  or  blue,  but  to  the  light  which  they  reflect  from  their  sur- 
faces. A  red  object  is  one  which  reflects  red  light  and  absorbs  all  other  col- 
ors, a  blue  object  one  which  reflects  blue  light  and  absorbs  other  tints,  and  so 
on.  The  color  of  a  body,  then,  or  more  properly,  the  cause  which  produces 
the  color,  is  the  quality  possessed  by  its  particles  to  reflect  certain  lights  and 
absorb  others. 

That  the  color  which  seems  to  belong  to  a  body  is  not  really  inherent  in  the 
body,  or  inseparable  from  it,  is  proved  by  showing  that  we  can  give  any  color 
that  may  be  desired  to  a  body  by  exposing  it  to  light  of  that  peculiar  tint. 
Thus  if  a  piece  of  blue  cloth  be  illuminated  by  a  beam  of  pure  red  light,  it 
will  appear  red  ;  or,  if  by  yellow  light,  it  will  appear  yellow  ;  but  neither  the 
yellow,  nor  the  red,  will  be  as  vivid  as  the  color  it  would  exhibit  if  illuminated 
by  blue  light. 


f 

I 

( 

I 

THE    VISIBLE    STARS. 


What  occupies  the  Space  beyond  the  Limits  of  the  Solar  System. — Wide  Vacuity  between  thi*  Sys- 
tem and  the  Stars. —  Indications  of  this  observable  in  the  Motions  of  the  Planet* — Iudiciit':>n*  in   \ 
the  Motions  of  the  Comets. — The  immense  Distance  of  the  Stars  proved  by  the  Earth's  uniwa!  ) 
Motion. — Observations  made  at  Greenwich. — Besscl's  Discovery  of  the  Parallax. — The  conse<]:ient  S 
Dist:in.-e  of  tho  Stars. — Illustrations  of  the  Magnitude  of  this  Distai.ce. — The  different  Orders  and 
Magnitudes  of  the  Stars. — How  accounted  for. — Why  those  of  the  lowest  Magnitude  arc  most  Nu- 
merous.— The  real    Magnitude  of  the   Stars. — The    Telescope  unable  to   Magnify   them— Dr. 
WoMastoa's  Investigations  of  the  comparative  Brightness  and  Magnitude  of  the  Stars  in  Relation 
to  the  Sun. — Their  stupendous  Magnitude. — Application  of  this  to  the  Dog-star. 


THE  VISIBLE  STARS. 


5.-,;, 


THE    VISIBLE    STARS. 


ON  former  occasions  we  have  taken  a  survey  of  the  group  of  inhabited  globes 
which,  in  company  with  the  earth,  revolve  around  the  sun.  We  have  examined 
their  motions  and  estimated  their  magnitudes  and  distances.  Passing  succes- 
sively from  planet  to  planet,  the  mind  has  been  oppressed  by  the  stupendous  di- 
mensions offered  to  its  contemplation.  Jupiter,  a  globe  1,400  times  the  bulk  of 
the  earth,  revolving  at  a  distance  of  five  hundred  millions  of  miles  from  the  sun  ; 
the  Saturnian  system,  with  its  globe  a  thousand  times  larger  than  the  earth — its 
system  of  revolving  rings,  and  its  suite  of  seven  moons — sweeping  round  the 
sun  in  a  vast  orbit  at  a  distance  of  a  thousand  millions  of  miles,  and  having  a 
year  thirty  times  the  length  of  ours,  diversified  by  similar  seasons,  but  varied 
by  seven  different  kinds  of  months ;  and,  finally,  having  attained  the  extreme 
limit  of  the  system,  the  planet  Herschel  is  found,  moving  at  such  a  distance 
from  the  sun  that  that  luminary  is  reduced  to  a  star,  with-  moous  too  distant  to 
allow  of  their  number  being  satisfactorily  ascertained,  and  probably  other  illu- 
minating apparatus,  the  discovery  of  which  is  reserved  to  future  observers. 
Such  are  the  objects,  such  the  distances,  and  such  the  motions,  here  presented 
to  us.  But  the  aspirations  of  the  inquisitive  spirit  of  man  rest  not  here  con- 
tented. Taking  its  station  at  this  extreme  verge  of  the  system,  and  throwing 
its  searching  glance  toward  the  interminable  realms  of  space  which  extend  be- 
yond those  limits,  it  still  asks — What  lies  there  ?  Has  the  Infinite  circumscribed 
the  exercise  of  his  creative  power  within  the  precincts  of  the  solar  system — 
and  has  he  left  the  unfathomable  depths  of  space  that  stretch  beyond  it  a  wide 
solitude  ?  Has  He  whose  dwelling  is  immensity,  and  whose  presence  is  every- 
where and  eternal,  remained  inactive  throughout  regions  in  the  universe  com- 
pared with  which  the  solar  system  itself  shrinks  into  a  point f 

Even  though  scientific  research  should  have  left  us  without  definite  informa- 
tion on  these  questions,  the  light  which  has  been  shed  on  the  Divine  character, 
as  well  by  reason  as  by  revelation,  would  have  filled  us  with  the  assurance  that 


556  THE  VISIBLE  STARS. 


there  is  no  region  of  space  however  remote,  which  does  not  teem  with  evi- 
dences of  the  exalted  power,  the  inexhaustible  wisdom,  and  the  untiring  good- 
ness of  the  Most  High. 

But  science  has  not  so  deserted  us.  It  has  not  failed  to  afford  us  much  in- 
teresting and  elevating  information  regarding  those  distant  regions  of  space. 
The  sagacity  and  activity  of  modern  astronomers  have  supplied  us  with  much 
interesting  information  respecting  regions  of  the  universe  the  extent  of  which 
is  so  great  that  even  the  whole  dimensions  of  the  solar  system  supply  no  mod- 
ulus sufficiently  great  to  enable  us  to  express  their  magnitude.  It-will  not,  then, 
be  unprofitable  or  unpleasing,  on  the  present  occasion,  to  carry  our  inquiries 
into  those  realms  of  space  that  stretch  beyond  the  limits  of  our  own  system, 
and  to  inquire  into  the  condition  of  the  physical  creation  there. 

We  are  furnished  with  a  variety  of  evidence,  establishing,  incontestably,  the 
fact,  that  around  our  system  to  avast  distance  on  every  side  there  exists  an  un- 
occupied space ;  that  the  solar  system  stands  alone  in  the  midst  of  a  vast  soli- 
tude. What  are  the  proofs  of  this  ?  Newton  has  demonstrated  in  his  investi- 
gations respecting  the  law  of  gravitation,  that  all  masses  of  matter  exercise 
upon  each  other  mutual  attraction  ;  in  virtue  of  which,  the  presence  of  any 
mass  in  the  neighborhood  of  another  is  betrayed,  even  though  we  should  not 
see  it,  by  the  effects  which  it  produces  on  the  condition  and  motion  of  the  other. 
The  group  of  globes  constituting  the  solar  system,  exercise  upon  each  other 
this  influence  ;  arid  although,  from  the  enormous  preponderance  of  its  mass 
above  all  the  rest,  the  sun  seems  to  annihilate  the  separate  influence  of  the 
planets  and  satellites  upon  each  other,  yet,  by  rigorous  examination  of  the  mo- 
tions of  these  bodies,  we  are  able  to  detect  the  effects  of  their  reciprocal  influ- 
ences. The  motion  of  each  body  of  the  system  is  the  combined  result  of  the 
attraction  of  the  sun  and  the  other  bodies  of  the  system  upon  it.  A  rigorous 
analysis  of  the  motions  of  the  planets  has  exhibited  all  these  effects,  and  in 
these  motions  we  can  distinctly  see  the  gravitating  influences  of  the  various 
bodies  of  the  system.  Now,  if  there  exists  beyond  the  limits  of  the  system, 
and  within  a  distance  not  so  great  as  to  render  the  attraction  of  gravitation  im- 
perceptible, any  mass  of  matter,  such  as  another  sun  like  our  own,  such  a  mass 
would  undoubtedly  exercise  a  gravitating  force  upon  the  various  bodies  of  the 
solar  system.  It  would  cause  each  of  them  to  move  in  a  manner  different  from 
what  it  would  have  moved  if  no  such  body  existed. 

Thus  it  appears  that,  even  though  the  presence  of  a  mass  of  matter  in  our 
neighborhood  should  escape  direct  observation,  its  presence  would  be  invaria- 
bly betrayed  by  the  effects  which  its  gravitation  would  necessarily  produce 
upon  the  planets.  No  such  effects,  however,  are  discoverable.  The  planets 
move  as  they  would  move  if  the  solar  system  were  independent  of  any  external 
disturbing  attraction.  These  motions,  which  are  accurately  observed,  are  such, 
and  such  only,  as  can  be  accounted  for  by  the  attraction  of  the  sun  and  the  re- 
ciprocal attraction  of  the  other  bodies  of  the  system.  The  inevitable  inference 
from  this  is,  that  there  does  not  exist  any  mass  of  matter  in  the  neighborhood 
of  the  solar  system  within  any  distance  which  permits  such  a  mass  to  exercise 
upon  it  any  discoverable  gravitating  influence,  and  that,  if  any  body  analogous 
to  our  sun  exists  in  the  universe,  it  must  be  placed  at  a  distance  from  our  sys- 
tem inconceivably  great — so  great,  indeed,  that  the  whole  magnitude  of  our  sys- 
tem will  shrink  into  a  point  compared  with  it. 

But  we  have  other  indications  of  this  condition  of  things.  The  solar  system 
is  supplied  with  feelers,  which  it.  is  enabled  to  throw  out  into  the  regions  sur- 
rounding it  to  vast  distances,  and  these  are  endowed  with  the  highest  con- 
ceivable susceptibility,  which  would  cause  them  to  betray  to  us  the  presence  in 
these  regions  even  of  masses  of  matter  of  very  limited  dimensions.  These 


THE  VISIBLE  STARS. 


Iff 


feelers  are  the  COMETS,  and  in  particular  that  called  Halley's  comet.  Thia  body 
emerges  from  the  system  periodically,  and  makes  an  excursion  into  the  sur- 
rounding regions  to  a  distance  of  little  less  than  two  thousand  millions  of  miles 
beyond  the  limits  of  our  system,  and  returns  at  regular  intervals  to  the  sun.  It 
is  a  body  of  extreme  levity  and  tenuity  compared  even  with  the  smallest  plan- 
etary masses  ;  it  is,  therefore,  eminently  susceptible  of  the  effects  of  gravitation 
proceeding  from  a  body  external  to  it. 

We  have  shown,  on  another  occasion,  that  when  this  body,  once  in  seventy- 
live  years,  departs  from  our  system  to  make  its  vast  excursion  through  distant 
regions  of  space,  the  eye  of  science  pursues  it  along  its  path,  watches  its  move- 
ments, and  follows  its  course.  That  course  is  calculated  upon  the  supposition 
that  it  is  subject  to  no  attraction  through  the  entire  range  of  its  orbit  except 
those  of  the  sun  and  planets,  and  the  calculations  of  its  return  are  based  upon 
that  supposition.  The  time  and  the  place  of  each  of  its  successive  returns  to 
our  system  have  been  foretold  on  these  suppositions  ;  and  we  have  found  that 
its  returns  have  corresponded  faithfully  with  such  predictions.  It  is  certain, 
then,  that,  in  its  range  through  space,  this  body  has  not  passed  in  the  neigh- 
borhood of  any  mass  of  matter  capable  of  exercising  an  observable  attraction 
upon  it.  In  fact,  it  moves  exactly  as  it  would  move  if  no  material  object  exist- 
ed in  the  creation  save  those  of  the  solar  system  itself.  It  follows,  therefore, 
that  all  other  objects  must  be  too  distant  from  our  system  to  produce  any  dis- 
coverable attraction  even  on  so  light  a  body  as  this. 

Yet  when,  on  any  clear  night,  we  contemplate  the  firmament,  and  behold  the 
countless  multitudes  of  objects  that  sparkle  upon  it,  and  remember  what  a  com- 
paratively small  number  are  comprised  among  those  of  the  solar  system,  and 
even  of  these  how  few  are  visible  at  any  one  time,  we  are  naturally  impel- 
led to  the  inquiry,  Where  in  the  universe  are  these  vast  numbers  of  objects 
placed  ? 

Very  little  reflection  and  reasoning,  applied  to  the  consideration  of  our  own 
position,  and  to  the  appearances  of  the  heavens,  will  convince  us  that  the  ob- 
jects that  chiefly  appear  in  the  firmament  must  be  at  almost  immeasurable  dis- 
tances from  our  system.  The  earth  in  its  annual  course  round  the  sun  moves 
in  a  circle,  the  diameter  of  which  is  about  two  hundred  millions  of  miles.  We, 
who  observe  the  heavens,  are  transported  upon  the  globe  round  that  vast  circle. 
The  station  from  which  we  observe  the  universe  at  one  period  of  the  year  is, 
then,  two  hundred  millions  of  miles  from  the  station  to  which  we  are  transport- 
ed at  another  period  of  the  year.  Thus,  if  we  view  the  heavens  on  the  night 
of  the  1st  of  January  and  note  their  aspect,  and  view  them  again  on  the  night 
of  the  1st  of  July,  we  know  that  the  two  stations  from  which  we  take  these 
two  surveys  are  separated  by  a  space  of  two  hundred  millions  of  miles. 

Now  it  is  a  fact  within  the  familiar  experience  of  every  one,  that  the  relative 
position  of  objects  will  depend  upon  the  point  from  which  they  are  viewed.  If 
we  stand  upon  the  bank  of  a  river,  along  the  margin  of  which  a  multitude  of 
ships  are  stationed,  and  view  the  masts  of  the  vessels,  they  will  have  among 
each  other  a  certain  relative  arrangement.  If  we  change  our  position,  however, 
through  the  space  of  a  few  hundred  yards,  the  relative  position  of  these  masts 
will  not  be  the  same  as  before.  Two  which  before  lay  in  line  will  now  be  seen 
separate,  and  two  which  before  were  separated  are  now  brought  into  line.  Two, 
one  of  which  was  to  the  right  of  the  other,  are  now  reversed ;  that  which  was 
to  the  right,  is  at  the  left,  and  vice  versa ;  nor  are  these  changes  produced  by 
any  change  of  position  of  the  ships  themselves,  for  they  are  moored  in  station- 
ary positions.  The  changes  of  appearance  are  the  result  of  our  own  change  of 
position,  and  the  greater  that  change  of  position  is,  the  greater  will  be  the  rela- 
tive change  of  these  appearances.  Let  us  suppose,  however,  that  we  are  moved 


>  a  much  greater  distance  from  the  shipping ;  a  very  slight  change  in  our  po- 
sition will  produce  much  less  effect  upon  the  relative  position  of  the  masts  ; 
perhaps  it  will  require  a  very  considerable  change  of  position  to  produce  a  per- 
ceivable change  upon  them.  In  fine,  in  proportion  as  our  distance  from  the 
masts  is  increased,  so  in  proportion  will  it  require  a  greater  change  in  our  own 
position  to  produce  the  same  apparent  change  in  the  position  of  the  masts. 

Thus  it  is  with  all  visible  objects.  When  a  multitude  of  stationary  objects 
are  viewed  at  a  distance,  their  relative  position  will  depend  upon  the  position  of 
the  observer,  arid  if  the  station  of  the  observer  be  changed,  a  change  in  the 
relative  position  of  the  objects  must  be  expected  ;  and  if  no  perceptible  change 
is  produced,  it  must  be  inferred  that  the  distance  of  the  object  is  incomparably 
greater  than  the  change  of  position  of  the  observer. 

Let  us  now  apply  these  reflections  to  the  case  of  the  earth  and  the  stars. 
The  stars  are  analogous  to  the  masts  of  the  ships,  and  the  earth  is  the  station  on 
which  the  observer  is  placed,  and  which  is  changeable  in  its  position  by  reason 
of  its  annual  motion.  It  would,  doubtless,  be  expected  that  the  magnitude  of 
the  globe,  being  eight  thousand  miles  in  diameter,  would  produce  a  change  of 
position  of  the  observer  sufficient  to  cause  a  change  in  the  relative  position  of 
the  stars,  but  we  find  that  such  is  not  the  case.  The  stars,  viewed  from  oppo- 
site sides  of  the  globe  of  the  earth,  present  exactly  the  same  appearance  ;  we 
must,  therefore,  infer  that  the  diameter  of  the  globe  of  the  earth  is  absolutely 
nothing  compared  to  their  distance. 

But  the  astronomer  has  still  a  much  larger  modulus  to  fall  back  upon.  He 
reflects,  as  has  been  already  observed,  that  he  is  enabled  to  view  the  stars 
from  two  stations,  separated  from  each  other,  not  by  eight  thousand  miles,  the 
diameter  of  the  earth,  but  by  two  hundred  millions  of  miles,  that  of  the  earth's 
orbit.  He,  therefore,  views  the  heavens  on  the  first  of  January,  and  views  them 
again  on  the  first  of  July,  yet  he  finds,  to  his  amazement,  that  the  aspect  is  the 
same.  He  thinks  that  this  cannot  be — that  so  great  a  change  of  position  in 
himself  cannot  fail  to  make  some  change  in  the  apparent  position  of  the  stars  ; — 
that,  although  their  general  aspect  is  the  same,  yet  when  submitted  to  exact 
examination  a  change  must  assuredly  be  detected.  He  accordingly  resorts  to 
the  use  of  instruments  of  observation  capable  of  measuring  the  relative  posi- 
tions of  the  stars  with  the  last  conceivable  precision,  and  he  is  more  than  ever 
confounded  by  the  fact,  that  still  no  discoverable  change  of  position  is  found. 

For  a  long  period  of  time  this  result  seemed  inexplicable,  and,  accordingly, 
it  formed  the  greatest  difficulty  with  astronomers  in  admitting  the  annual  mo- 
tion of  the  earth.  The  alternative  offered  was  this  :  it  was  necessary,  either 
to  fall  back  upon  the  Ptolemaic  system,  in  which  the  earth  was  stationary,  or 
to  suppose  that  the  immense  change  of  position  of  the  earth  in  the  course  of 
half  a  year,  which  we  have  already  mentioned,  could  produce  no  discoverable 
change  of  appearance  in  the  stars  ;  a  fact  which  involves  the  inference  that  the 
diameter  of  the  earth's  orbit,  which  measures  two  hundred  millions  of  miles, 
must  be  a  mere  point  compared  with  the  distance  of  the  nearest  stars.  Such 
an  idea  appeared  so  preposterous  and  inconceivable,  that  for  a  long  period  of 
time  many  preferred  to  embrace  the  Ptolemaic  hypothesis,  beset  as  it  wus 
with  difficulties  and  contradictions. 

Since,  however,  tlie  annual  motion  of  the  earth  must  now  be  regarded  as  a 
proved  fac^,  we  are  driven  to  the  inference,  deduced  from  the  absence  of  all  change 
of  relative  apparent  position  in  the  stars,  that  the  distances  of  these  objects  from 
our  system  is,  in  the  common  popular  sense  of  the  word,  infinitely  great  com- 
pared with  the  dimensions  of  our  system,  and  this  inference  is  in  perfect  ac- 
cordance with  the  other  indications  of  the  wide  vacuity  that  surrounds  the 
system. 


THE  VISIBLE  STARS.  559  j 


In  such  a  state  of  things,  it  will  easily  be  imagined  that  astronomers  have  ' 
diligently  directed  their  observations  to  the  discovery  of  some  chance  of  appa- 
rent position,  however  small,  produced  upon  the  stars  by  the  earth's  motion  \s 
those  stars  most  likely  to  be  affected  by  the  motion  of  the  earth  are  those  which 
are  nearest  to  the  system,  and  therefore  probably  which  are  brightest  and  lar- 
gest, it  has  been  to  such  chiefly  that  this  kind  of  observation  has  been  directed. 
Since  it  was  certain,  that  if  any  observable  effect  was  produced  by  the  earth's 
motion  at  all  it  must  be  extremely  small,  the  nicest  and  most  difficult  means 
of  observation  were  those  alone  from  which  the  discovery  could  be  exp. 
Among  the  many  expedients  used  for  the  detection  of  such  effects,  we  shall  se- 
lect as  an  example  one  which  was  adopted  at  the  Royal  Observatory  at  Green- 
wich. A  telescope  of  great  length  was  attached  to  the  side  of  a  pier  of  solid 
masonry  erected  upon  a  foundation  of  rock.  This  instrument  was  scrcunl 
into  such  a  position  that  particular  stars  as  they  crossed  the  meridian  would 
necessarily  pass  within  its  field  of  view.  Micrometric  wires  were  in  the  usual 
manner  placed  in  its  eye-piece,  so  that  the  exact  point  at  which  the  stars  passed 
the  meridian  each  night  could  be  observed  and  recorded  with  the  greatest  pre- 
cision. The  instrument  being  thus  fixed  and  immoveable,  the  transits  of  the 
stars  were  noted  each  night,  and  the  exact  places  where  they  passed  the  merid- 
ian recorded.  This  kind  of  observation  was  carried  on  through  the  year,  and 
if  the  earth's  change  of  position,  by  reason  of  its  annual  motion,  should  produce 
any  effect  upon  the  apparent  position  of  the  stars,  it  was  anticipated  that  such 
effect  would  be  discovered  by  the'se  means.  After,  however,  making  all  allow- 
ance  for  the  usual  carses  which  we  knew  to  affect  the  apparent  position  of  the 
stars,  such  as  refraction  or  aberration,  no  change  of  position  was  discovered 
which  could  be  assigned  to  the  earth's  motion. 

Within  the  last  few  years,  however,  Professor  Bessel  has  directed  his  scien- 
tific labors  to  this  inquiry,  and  has  succeeded  in  detecting  a  small  effect  on  one 
of  the  stars  in  the  constellation  of  the  Swan.  In  a  communication,  made  in 
1838  by  that  astronomer  to  Sir  John  Herschel,  he  says  :  "  After  so  many  un- 
successful attempts  to  determine  the  parallax  of  a  fixed  star,  I  thought  it  worth 
while  to  try  what  might  be  accomplished  by  means  of  the  accuracy  which 
my  great  Fraunhoffer  heliometer  gives  to  the  observations.  I  undertook  to 
make  this  investigation  upon  the  star  61  Cygni ;  which,  by  reason  of  its  great 
proper  motion,  is  perhaps  the  best  of  all,  which  affords  the  advantage  of  being 
a  double  star,  and  on  that  account  may  be  observed  with  greater  accuracy,  and 
which  is  so  near  the  pole  that,  with  the  exception  of  a  small  part  of  the  year, 
it  can  always  be  observed  at  night  at  a  sufficient  altitude." 

These  observations  were  continued  for  four  years,  and  the  result  was  the 
discovery  that  the  position  of  the  star  in  question  was  affected  by  the  earth's 
motion  to  the  extent  of  a  little  less  than  one  third  of  a  second.  From  this  may 
be  calculated  the  distance  of  the  star  from  the  solar  system. 

To  render  intelligible  the  spirit  of  the  method  by  which  the  distance  of  the  stars 
may  be  inferred  from  their  discovered  parallax,  let  us  suppose  two  lines,  drawn 
from  a  star  to  opposite  ends  of  a  diameter  of  the  earth's  orbit,  or  to  two  positions 
which  the  earth  occupies  after  an  interval  of  six  months.     The  angle  formed 
by  these  two  lines  is,  in  fact,  the  amount  of  the  apparent  change  of  position  of 
the  star  by  reason  of  the  earth's  motion,  and  it  is  technically  called  the  parallax. 
We  may  in  this  case  consider  the  diameter  of  the  orbit  as  a  portion  of  an  enor- 
mous circle,  the  centre  of  which  is  at  the  star,  and  the  radius  of  which  is  the 
distance  of  the  star  from  the  earth.     It  is  known,  in  geometry,  that  an  arc 
circle  which  measures  one  second  is  in  length  the  206,265th  part  of  the  rnd 
and  if  it  measures  one  third  of  a  second,  it  will,  of  course,  be  the  618,79 
part  of  the  radius. 


560  THE  VISIBLE  STARS. 

. ^ j 

Professor  Bessel  found  that  the  angle  contained  by  those  two  lines,  drawn  ' 
from  the  star  in  question  to  the  opposite  sides  of  the  orbit,  contained  an  angle  , 
amounting  to  two  thirds  of  a  second,  and,  consequently,  that  the  angle  included  ' 
by  the  lines  between  the  sun  and  the  earth  would  form  one  third  of  a  second.  ! 
From  this  it  would  follow,  that  the  distance  from  the  star,  being  the  radius  of  a 
circle,  of  which  the  distance  between  the  earth  and  sun  is  an  arc  of  one  third 
of  a  second,  will  be  618,795  times  the  length  of  the  earth's  distance  from  the  sun. 
Taking  round  numbers,  then,  it  will  follow  from  this  observation  that  the  dis- 
tance of  this  star  is  600,000  times  greater  than  the  distance  of  the  earth  from 
the  sun.    But  the  distance  of  the  earth  from  the  sun  being  100  millions  of  miles, 
it  will  follow  that  the  distance  of  the  star  must  be  sixty  millions  of  millions  of 
miles. 

Such  is  the  nearest  approximation  that  observation  has  supplied  for  the  space 
that  separates  the  solar  system  from  other  bodies  of  the  universe. 

Minds  unaccustomed  to  the  contemplation  of  great  numbers  and  magnitudes 
are  overwhelmed  in  their  efforts  to  conceive  such  distances ;  and  even  astron- 
omers have  been  compelled  to  resort  to  extraordinary  expedients  to  express  and 
conceive  clearly  such  spaces. 

On  another  occasion  we  have  shown  that  light  moves  through  space  at 
the  rate  of  200,000  miles  per  second.  This  motion  of  light  has  accordingly 
been  adopted  as  the  most  convenient  modulus  for  expressing  the  distances  of 
the  stars ;  and  we  are  accustomed  to  express  them  by  saying  how  long  light 
would  take  to  move  over  them.  If,  then,  sixty  millions  of  millions  of 
miles  be  divided  by  200,000  we  shall  obtain  the  number  of  seconds  which 
light  would  take  to  come  from  the  nearest  star  to  the  solar  system ;  and  if  this 
number  of  seconds  be,  in  the  usual  manner,  reduced  to  years,  it  will  be  found 
that  light  would  take  about  ten  years  to  travel  from  the  nearest  star  to  the 
earth.  Such  is,  then,  the  space  that  divides  us  from  them. 

To  conceive  this  prodigious  distance  more  clearly  still,  it  has  been  calcula- 
ted that  a  cannon-ball,  which  moves  with  a  velocity  of  500  miles  an  hour, 
would  take  to  travel  from  the  nearest  star  to  the  earth,  an  interval  of  14,255,418 
years.  Again  :  it  has  been  computed  that  a  steam-carriage  starting  from  the 
earth,  and  moving  toward  the  star  at  the  rate  of  20  miles  an  hour,  would  take 
to  reach  the  star,  356,385,466  years;  a  period  of  time  61,000  times  greater 
than  the  whole  interval  since  the  creation  of  the  world,  according  to  Mosaic 
chronology. 

But  this  is  only  the  interval  that  separates  our  system  from  the  nearest  stars. 
Analogy  and  all  the  grounds  of  probability  lead  to  the  conclusion  that  corre- 
sponding intervals  separate  the  stars  from  each  other.  We  shall  hereafter  see 
that  the  stars  are,  in  fact,  suns  like  our  own,  or,  what  is  the  same,  that  our  sun 
is  a  star ;  and  it  is  consistent  and  natural  to  suppose  our  sun  is  no  farther  re- 
moved from  the  stars  than  the  stars  are  from  each  other. 

Among  the  multitude  of  stars  which  we  behold  in  the  firmament  we  find 
a  great  variety  of  splendor.  Those  which  are  the  brightest  and  largest,  and 
which  are  said  to  be  of  the  first  magnitude,  are  few ;  the  next  in  order  of 
brightness,  which  are  called  of  the  second  magnitude,  are  more  numerous  ;  and 
as  they  decrease  in  brightness  their  number  rapidly  increases. 

The  number  of  stars  of  the  first  magnitude  does  not  exceed  twenty  ;  those  of 
the  second,  fifty ;  those  of  the  third,  two  hundred ;  and  so  on,  the  number  of 
the  smallest  being  incapable  of  estimation. 

The  stars  which  are  capable  of  being  seen  by  the  naked  eye  are  usually  re- 
solved into  seven  orders  of  magnitudes — the  first  being  the  brightest  and  largest, 
while  those  of  the  seventh  magnitude  are  the  smallest  that  the  eye  can  dis- 
tinctly see. 


THE  VISIBLE  STARS.  5QJ 


Are  we  to  suppose,  then,  that  this  relative  brightness  which  we  perv 
really  arises  from  any  difference  of  intrinsic  splendor  between  the  object*  tin-ni- 
sei ves,  or  does  it,  as  it  may  equally  do,  arise  from  their  difference  of  dislan 
Are  the  stars  of  the  seventh  magnitude  so  much  less  bright  and  conspicuous 
than  those  of  the  first  magnitude  because  they  are  really  smaller  orbs  placed 
at  the  same  distance,  or  because,  being  intrinsically  equal  in  splendor,  th< 
tance  of  those  of  the  seventh  magnitude  is  so  much  greater  than  the  disi 
of  those  of  the  first  magnitude  that  they  are  diminished  in  their  apparent 
brightness  ?     We  know  that  by  the  laws  of  optics  the  brightness  of  a  luminous 
object  diminishes  in  a  very  rapid  proportion  as  the  distance  increases.     Thus 
at  double  the  distance  the  brightness  will  be  four  times  less,  at  triple  the  dis- 
tance it  will  be  nine  times  less,  at  a  hundred  times  the  distance  it  will  be  ten 
thousand  times  less,  and  so  on. 

It  is  evident,  then,  that  the  great  variety  of  brightness  which  prevails  among  the 
stars  may  be  indifferently  explained,  either  by  supposing  them  objects  of  differ- 
ent intrinsic  brightness  and  magnitude,  placed  at  the  same  distance,  or  objects 
generally  of  the  same  order  of  magnitude  placed  at  a  great  diversity, of  distances. 

Of  these  two  suppositions,  the  latter  is  infinitely  the  most  probable  and  nat- 
ural ;  it  has,  therefore,  been  usually  adopted  :  and  we  accordingly  consider  the 
stars  to  derive  their  variety  of  brightness  almost  entirely  from  the  positions 
assigned  to  them  in  the  universe  being  at  various  distances  from  us. 

Taking  the  stars  generally  to  be  intrinsically  the  same  in  brightness,  varipus 
theories  have  been  proposed  as  to  the  positions  which  would  explain  their  ap- 
pearances ;  and  the  most  natural  and  probable  is,  that  their  distances  from  each 
other  are  generally  equal,  or  nearly  so,  and  correspond  with  the  distance  of  our 
sun  from  the  nearest  of  them.  In  this  way  the  fact  that  a  small  number  of  stars 
only  appear  of  the  first  magnitude,  and  that  the  number  increases  very  rapidly 
as  the  magnitude  diminishes,  is  easily  rendered  intelligible. 

If  we  imagine  a  person  standing  in  the  midst  of  a  wood,  surrounded  by  trees 
on  every  side  and  at  every  distance,  those  which  immediately  surround  him 
will  be  few  in  number,  and  by  proximity  will  appear  large.  The  trunks  or 
stumps  of  those  which  occupy  a  circuit  beyond  the  former  will  be  more  nu- 
merous, the  circuit  being  wider,  and  will  appear  smaller,  because  their  dis- 
tance is  greater.  Beyond  these  again,  occupying  a  still  wider  circuit,  will  ap- 
pear a  proportionally  augmented  number,  whose  apparent  magnitude  will  again 
be  diminished  by  increased  distance  ;  and  thus  the  trees  which  occupy  wider 
and  wider  circuits  at  greater  and  greater  distances  will  be  more  and  more  nu- 
merous, and  will  appear  continually  smaller.  It  is  the  same  with  the  stars ; 
we  are  placed  in  the  midst  of  an  immense  cluster  of  suns  surrounding  us  on 
every  side  at  inconceivable  distances.  Those  few  which  are  placed  immedi- 
ately about  our  system  appear  bright  and  large,  and  we  call  them  stars  of  the 
first  magnitude.  Those  which  lie  in  the  circuit  beyond,  and  occupying  a 
wider  range,  are  more  numerous  and  less  bright ;  and  we  call  them  stars  of  the 
second  magnitude.  And  there  is  thus  a  progression  increasing  in  number  and 
distance  and  diminishing  in  brightness,  until  we  attain  a  distance  so  great  that 
the  stars  are  barely  visible  to  the  naked  eye.  This  is  the  limit  of  vision, 
is  the  range  of  the  universe  which  the  eye  in  its  natural  condition  is  destined 
to  behold ;  but  an  eye  has  been  given  us  more  potent  still,  and  of  infinitely 
wider  range,— the  eye  of  the  mind.  The  telescope,  a  creature  of  the  under- 
standing, has  conferred  upon  the  bodily  eye  an  infinitely  augmented  range,  and, 
as  we  shall  presently  see,  has  enabled  us  to  penetrate  into  realms  of  thi 
universe,  which,  without  its  aid,  would  never  have  been  known  to  us. 
let  us  pause  for  the  present  and  dwell  for  a  moment  upon  that  range  of  space 
which  comes  within 'the  scope  of  natural  vision. 

38 


THE  VISIBLE  STARS. 


Sir  William  Herschel,  to  whose  researches  we  are  indebted  for  a  large  ror- 
tion  of  the  knowledge  which  we  possess  respecting  the  fixed  stars,  has  inves- 
tigated the  probable  progression  of  distances  which  regulate  the  stars  visible 
to  the  naked  eye,  and  has  shown  reasonable  grounds  for  concluding  that  the 
smallest  visible  star  is  at  a  distance  about  twelve  times  greater  than  stars  of 
the  first  magnitude.  He  supposes  that  the  intermediate  stars  between  the 
smallest  that  can  be  seen  by  the  naked  eye,  and  stars  like  the  dogstar,  which, 
from  their  brightness,  must  be  presumed  to  be  nearest  to  us,  are  ranged  at  in- 
termediate distances.  It  would  therefore  follow  that  if  we  assume  the  distance 
of  the  nearest  star  according  to  the  results  of  Bessel's  observations,  to  be  a 
space  that  light  would  move  over  it  in  10  years,  the  distance  of  the  smallest 
star  perceivable  by  unassisted  vision  must  be  such  that  light  would  take  120 
years  to  move  over !  If,  then,  we  imagine  a  sphere  surrounding  us,  the  radius 
of  which  is  equal  to  the  space  that  light  moves  over  in  120  years,  that  sphere 
is  the  range  of  natural  and  unassisted  vision,  and  is  that  portion  of  the  universe 
which  men  are  privileged  to  contemplate  unaided  by  art. 


MAGNITUDE    OF    THE    STARS. 

The  extent  of  the  stellar  universe  visible  to  the  naked  eye,  and  the  arrange- 
ment of  stars  in  it  and  their  relative  distances,  have  just  been  explained.  But  a 
natural  curiosity  will  be  awakened  to  discover  not  merely  the  position  and  ar- 
rangement of  those  bodies,  but  to  ascertain  what  is  their  nature,  and  what  parts 
they  play  on  the  great  theatre  of  creation  ?  Are  they  analogous  to  our  planets  ? 
Are  they  inhabited  globes,  warmed  and  illuminated  by  neighboring  suns  ?  Or 
on  the  other  hand,  are  they  themselves  suns,  dispensing  light  and  life  to  sys- 
tems of  surrounding  worlds. 

When  a  telescope  is  directed  to  a  star,  the  effect  produced  is  strikingly  dif- 
ferent from  that  which  we  find  when  it  is  applied  to  a  planet.  A  planet,  to 
the  naked  eye,  with  one  or  two  exceptions,  appears  like  a  common  star.  The 
telescope,  however,  immediately  presents  it  to  us  with  a  distinct  circular  disk 
similar  to  that  which  the  moon  offers  to  the  naked  eye,  and  in  the  case  of  some 
of  the  planets  a  powerful  telescope  will  render  them  apparently  even  larger  than 
the  moon.  But  the  effect  is  very  different  indeed  when  the  same  instrument 
is  directed  even  to  the  brightest  star.  We  find  that  instead  of  magnifying,  it 
actually  diminishes.  There  is  an  optical  illusion  produced,  when  we  behold 
a  star,  which  makes  it  appear  to  us  to  be  surrounded  with  a  radiation  which 
causes  it  to  be  represented  when  drawn  on  paper,  by  a  dot  with  rays  diverging 
on  every  side  from  it.  The  effect  of  the  telescope  is  to  cut  off  this  radiation, 
and  present  to  us  the  star  as  a  mere  lucid  point,  having  no  sensible  magnitude  ; 
nor  can  any  augmented  telescopic  power  which  has  yet  been  resorted  to  pro- 
duce any  other  effect.  Telescopic  powers  amounting  to  six  thousand  were 
occasionally  used  by  Sir  William  Herschel,  and  he  stated  that  with  these  the 
apparent  magnitude  of  the  stars  seemed  less,  if  possible,  than  with  lower 
powers. 

We  have  other  proofs  of  the  fact  that  the  stars  have  no  sensible  disks,  among 
which  may  be  mentioned  the  remarkable  effect  called  the  occultation  of  a  star  by 
the  dark  edge  of  the  moon.  When  the  moon  is  a  crescent  or  in  the  quarters,  as 
it  moves  over  the  firmament,  its  dark  edge  successively  approaches  to  or  recedes 
from  the  stars.  And  from  time  to  time  it  happens  that  it  passes  between  the 
stars  and  the  eye.  If  a  star  had  a  sensible  disk  in  this  case,  the  edge  of  the 
moon  would  gradually  cover  it,  and  the  star,  instead  of  being  instantaneously 
extinguished,  would  gradually  disappear.  This  is  found  not  to  be  the  case ; 
the  star  preserves  all  its  lustre  until  the  moment  it  comes  into  contact  with  the 


THE  VISIBLE  STARS. 


663 


dark  edge  of  the  moon's  disk,  and  then  it  is  instantly  extinguished,  without  the 
slightest  appearance  of  diminution  of  its  brightness.  This  effect  also  presents 
a  striking  proof  of  the  non-existence  of  an  atmosphere  round  the  moon. 

It  may  be  asked  then,  if  such  be  the  case,  if  none  of  the  stars,  great  or  small, 
have  any  discoverable  magnitude  at  all ;  with  what  meaning  can  we  speak  of 
stars  of  the  first,  second,  or  other  orders  of  magnitude  ?  The  term  magnitude 
thus  applied,  was  used  before  the  invention  of  the  telescope,  when  the  start, 
j  having  been  observed  only  with  the  naked  eye,  were  really  supposed  to  hare 
different  magnitudes.  We  must  accept  the  term  now  used  to  express  not 
the  comparative  magnitude,  but  the  comparative  brightness  of  the  stars. 
Thus  a  star  of  the  first  magnitude,  means  of  the  greatest  apparent  brightness ; 
a  star  of  the  second  magnitude  means  that  which  is  in  the  next  degree  of 
splendor,  and  so  on.  But  what  are  we  to  infer  from  this  singular  fact,  that  no 
magnifying  power,  however  great,  will  exhibit  to  us  a  star  with  any  sensible 
magnitude  ?  must  we  admit  that  the  optical  instrument  loses  its  magnifying 
power  when  applied  to  the  stars,  while  it  retains  it  with  every  other  visible 
object?  Such  a  consequence  would  be  eminently  absurd.  We  are  therefore 
driven  to  an  inference  regarding  the  magnitude  of  stars  as  astonishing  and  al- 
most as  incredible  as  that  which  was  forced  upon  us  respecting  their  distan- 
ces. We  saw  that  the  entire  magnitude  of  the  annual  orbit  of  the  earth,  stupen- 
dous as  it  is,  was  nothing  compared  to  the  distance  of  one  of  those  bodies,  and 
consequently  if  that  orbit  were  filled  by  a  sun  whose  magnitude  would  there- 
fore be  infinitely  greater  than  that  of  ours,  such  a  sun  would  not  appear  to  an 
observer  at  the  nearest  star  of  greater  magnitude  than  one  third  of  a  second  ; 
consequently  would  have  no  magnitude  sensible  to  the  eye,  and  would  appear 
as  a  mere  lucid  point  to  an  observer  at  the  star !  We  are  then  prepared  for 
the  inference  respecting  the  fixed  stars  which  the  telescopic  observations  al- 
ready mentioned  leads  to.  The  telescope  of  Sir  William  Herschel,  to  which 
he  applied  a  power  of  six  thousand,  did  undoubtedly  magnify  the  stars  six 
thousand  times,  but  even  then  their  apparent  magnitude  was  inappreciable. 
We  are  then  to  infer  that  the  distance  of  these  wonderful  bodies  is  so  enor- 
mous compared  with  their  actual  magnitude,  that  their  apparent  diameter,  seen 
from  our  system,  is  above  six  thousand  times  less  than  any  which  the  eye  is 
capable  of  perceiving. 

Under  such  circumstances  it  might  appear  hopeless  to  attempt  to  discover 
the  probable  magnitude  and  brightness  of  the  stars  as  compared  with  any  stand- 
ard known  to  us.  Yet  this  problem,  however  hopeless  it  may  seem,  has 
yielded  to  the  ardor  of  astronomical  inquiry. 

Dr.  Wollaston  instituted  a  series  of  observations  and  calculations,  which 
terminated  in  an  estimate  of  the  magnitude  and  brightness  of  the  fixed  stars  as 
compared  with  our  sun. 

There  are  optical  instruments  called  photometers,  the  nse  and  application  of 
which  is  to  ascertain  the  comparative  brightness  of  luminous  objects.  By 
such  instruments  we  can  take  any  two  visible  luminous  objects  and  compare 
them  so  as  to  be  enabled  to  say  what  is  the  numerical  ratio  of  the  lights  which 
they  afford.  Thus  a  common  candle  and  a  gas-lamp  may  be  hied,  and  we 
should  be  enabled  immediately  to  say  how  many  candles  would  be  necessary 
to  give  light  equal  to  that  of  the  lamp. 

By  instruments  of  this  species  Dr.  Wollaston  prosecuted  inves 
object  of  which  was  to  ascertain  the  numerical  proportion  between  the  light 
afforded  by  the  sun  and  that  afforded  by  the  stars.  Let  us  take,  for  example, 
the  case  of  Sinus,  or  the  dogstar.  He  found  by  such  means,  that  tho  Hgfat 
received  by  us  from  Sirius  was  20,000,000,000  of  times  less  lhan  that  received 
from  the  sun.  This,  be  it  observed,  was  a  result  not  of  theory  or  speculation, 


but  of  immediate  observation  and  measurement.  Having  ascertained  this,  his 
next  object  was  to  compute  the  distance  to  which  our  sun  would  have  to  be 
removed  in  order  that  it  should  assume  an  appearance  like  that  of  the  dogstar. 
Although  this  might  at  the  first  view  appear  a  difficult  problem,  it  was  by  no 
means  so.  We  know  by  the  principles  of  optics,  that  if  the  sun  were  removed 
to  twice  its  present  distance  it  splendor  would  be  four  times  less  ;  at  three  times 
its  present  distance  it  would  be  nine  times  less ;  at  ten  times  the  distance  it 
would  be  one  hundred  times  less,  and  so  on. 

We  have,  therefore,  a  simple  arithmetical  rule  of  calculation,  by  the  applica- 
tion of  which  we  can  say  in  what  proportion  the  brightness  of  the  sun  would 
be  reduced  by  any  proposed  increase  of  distance,  or  what  increase  of  distance 
would  be  necessary  to  produce  any  proposed  diminution  of  brightness".  If  this 
rule  be  applied  to  determine  how  much  further  the  sun  should  be  removed  from 
us  than  it  now  is,  in  order  that  it  should  be  reduced  to  the  appearance  of  the 
dogstar,  it  will  be  found  that  the  requisite  increase  of  distance  would  be  in 
proportion  of  about  150,000  to  1.  If,  then,  the  sun  were  removed  to  150,000 
times  its  present  distance  it  would  be  seen  by  us  as  a  second  dogstar. 

Now  it  will  be  apparent,  that  if  we  had  reason  to  know  that  the  dogstar  is 
at  a  distance  of  150,000  times  greater  than  that  of  the  sun,  it  would  immedi- 
ately follow  that  the  dogstar  must  be  a  sun  equal  to  our  own,  because  then  it 
would  be  inferred  that  the  sun,  if  placed  where  the  dogstar  is,  would  have  ex- 
actly the  same  splendor  and  magnitude. 

But  if,  on  the  other  hand,  we  had  reason  to  know  that  the  real  distance  of 
the  dogstar  is  greater  than  150,000  times  that  of  the  sun,  then  it  would  follow 
that  the  dogstar  at  a  greater  distance  would  have  the  same  splendor  as  the  sun 
at  a  less  distance  ;  and,  consequently,  the  inevitable  inference  would  be  that 
the  dogstar  must  be  larger  and  more  splendid  than  the  sun. 

The  discovery  of  Bessel  having  led  to  the  conclusion  that  the  distance  of 
the  nearest  stars  is  at  least  600,000  times  greater  than  that  of  the  sun,  it 
follows  that  these  objects,  at  that  distance,  are  as  large  and  bright  as  the  sun 
would  be  at  a  distance  four  times  less.  This  being  admitted,  it  immediately 
follows  that  the  stars,  or  at  least  many  of  them,  must  be  objects  transcendentally 
greater  and  brighter  than  the  sun. 

At  the  time  of  the  observations  of  Dr.  Wollaston  it  was  not  supposed  that  the 
distances  of  the  stars  were  as  great  as  they  are  now  known  to  be ;  and  Dr. 
Wollaston,  adopting  a  much  less  distance  than  the  truth,  felt  himself  warranted 
in  the  inference  that  the  dogstar  must  be  a  sun  equal  at  least  to  fourteen  of 
ours.  Had  he  known  what-has  since  been  inferred  from  the  observations  of  Pro- 
fessor Bessel,  how  much  more  stupendous  would  he  not  have  inferred  the  stars 
to  be! 

But  still,  it  may  be  asked,  what  are  those  wondrous  objects  ?  Are  they  plan- 
ets shining  with  reflected  light  ?  or  are  they  themselves  native  fountains  of 
light,  like  our  sun  ?  It  is  easy  to  perceive  that  no  reflected  light  could  be  in- 
tense enough  to  be  visible  at  distances  so  enormous  ;  independent  of  which, 
the  splendor  of  the  stars  as  seen  through  powerful  telescopes  is  such  as  to  sat- 
isfy us  that  they  must  be  suns.  Sir  William  Herschel  stated  that  when  his 
great  telescope  was  directed  to  the  region  of  the  heavens  through  which  the 
star  Sirius  passed,  the  appearance  exhibited  on  the  approach  of  that  star  was 
like  that  of  the  eastern  firmament  on  the  approach  of  sunrise  ;  and  that  when  the 
glorious  object  itself  entered  the  field  of  view,  although  it  appeared  as  a  mere 
lucid  point,  having  no  sensible  magnitude,  its  light  was  so  overpowering  that 
he  was  compelled  to  protect  his  eye  with  a  colored  glass.  It  is  needless  to 
say  that  such  splendor  could  not  proceed  from  an  opaque  globe  shining  with 
borrowed  light  at  a  distance  of  sixty  millions  of  millions  of  miles. 


THE  VISIBLE  STARS. 


To  persons  not  familiar  with  optical  researches  it  may  appear  incomprehen- 
(  sible  that  a  star  presenting,  even  with  the  telescope,  no' disk  of  sensible  mag- 
nitude, could,  nevertheless,  appear  so  splendid.     There  is,  however,  a  law  of 
light,  clearly  established  in  optics,  which  will  probably  remove  this  difficulty. 
It  is  demonstrated  that  the  apparent  brightness  of  an  object  is  not  diminished 
by  its  removal  from  the  eye,  although  tb*  quantity  cf  light  which  it  p\. 
I  decreased  in  a  high  proportion.     This  statement  may  appear  at  first  paradoxi- 
i  cal ;  let  us,  however  explain  it. 

j       If  the  sun,  for  example,  were  removed  to  twice  its  present  distance  it  would 
,  appear  to  the  eye  with  half  its  present  diameter ;  yet,  in  its  diminished  size, 
j  t'he  apparent  brightness  of  its  surface  would  be  the  same  as  that  with  which 
j  we  behold  it  at  the  lesser  distance.     To  illustrate  this,  let  us  suppose  that  a 
small  circular  opening  is  made  in  a  card,  and  that  the  card  is  presented  to  the 
sun,  so  that  a  portion  of  the  sun's  disk  only  shall  be  seen  through  it,  but  that 
that  portion  shall  be  circular ;  the  opening  will  present  to  the  eye  the  appear- 
ance of  a  sun  of  less  magnitude  than  the  real  one,  but  of  equal  brightness.    Let 
j  the  card  then  be  held  at  such  a  distance  from  the  eye  that  the  circular  portion 
I  of  the  sun's  disk  visible  through  it  shall  have  a  diameter  equal  to  half  of  the 
f  entire  disk.     A  sun  will  thus  be  seen  of  equal  brightness  with  the  true  sun, 
;  but  of  only  half  the  linear  diameter,  and  one  fourth  the  superficial  magnitude. 

From  this  illustration  it  will  be  easily  perceived  that  one  object  may  be 
i  smaller  than  another  in  apparent  magnitude,  and  that  it  may  give  less  light, 
.  but,  nevertheless,  be  equally  bright. 

This  being  clearly  understood,  it  remains  to  be  shown,  that  if  the  sun  were 
j  removed  to  double  its  present  distance  it  would  exhibit  a  surface  to  the  eye  as 
;  bright,  though  only  half  of  the  diameter.  To  comprehend  this,  let  it  be  re- 
1  membered  that  the  light  which  proceeds  from  the  smaller  sun  seen  from  double 

>  the  distance,  issues  from  the  entire  surface  of  the  sun,  while  the  light  which 
|  would  proceed  from  an  equal  portion  of  the  sun's  disk  seen  at  its  present  dis- 
i  tance,  would  only  proceed  from  one  fourth  of  the  entire  area  of  the  disk.    The 
]  actual  quantity  of  light,  therefore,  which  issues  from  the  small  sun,  seen  from  the 
/  larger  distance,  is  greater,  in  the  proportion  of  4  to  1,  than  that  which  proceeds 
!  from  the  small  portion  of  the  larger  sun,  seen  at  the  lesser  distance.     It  fol- 
i  lows,  then,  that  the  actual  quantity  of  light  by  which  the  object  is  rendered  visi- 
|  ble  at  the  greater  distance,  is  four  times  more  than  that  by  which  the  equivalent 

>  part  of  the  nearer  object  is  rendered  visible  at  the  lesser  distance  ;  but  in  con- 
J  sequence  of  the  distance  being  less  in  the  latter  case,  the  intensity  of  the  les- 
)  ser  quantity  of  light  is  four  times  greater.     In  short,  it  follows  that  as  the  ob- 
l  ject  recedes  from  the  eye  the  quantity  of  light  which  proceeds  from  a  given  por- 
[  lion  of  the  visual  area  is  increased  in  the  same  proportion  as  the  square  of  the 
}  distance,  while  the  intensity  of  the  light  is  diminished  in  exactly  the  same  pro- 
)  portion.     What  is,  therefore,  lost  in  intensity  by  increased  distance,  is  gained 

in  quantity  ;  and  the  effect  is,  that  the  splendor  of  the  object  is  not  changed  by 
distance,  but  only  its  apparent  magnitude. 

The  apparent  diameter  of  the  sun  is  very  nearly  2,000  seconds  of  a  degree. 
If  it  were  removed  to  2,000  times  its  present  distance  it  would  present  a  diam- 
eter of  one  second ;  but  it  would  appear  as  bright  as  a  small  portion  of  the 
present  disk  would  appear  having  an  apparent  diameter  2,000  times  less  than  its 


me  appearance  of  such  portiuu  wumu  uc,  ao  uj  uiigum^t»»  v~  *.~.*  —  —  — e — 
tude,  that  which  the  sun  would  have  at  2,000  times  its  present  distance. 

Since,  then,  the  brightness  of  the  stars,  in  the  proper  sense  of  the  term  j 
•iohtness,  is' not  diminished  by  increased  distance,  we  shall  be  the  less  sur- 

I 


566  THE  VISIBLE  STARS. 


}  prised  at  their  being  visible,  notwithstanding  that  they  present  no  sensible  dis' 

\  even  when  magnified  by  the  most  powerful  telescope. 

It  may  again  be  asked  how  it  can  be  said  that  the  brightness  of  a  star  i 
not  diminished  by  distance,  when  it  is  maintained  that  the  splendor  of  the  dog 
star  compared  with  one  of  the  seventh  magnitude,  is  owing  to  the  greatness  o 
the  distance  of  the  latter.  To  this  we  reply,  according  to  the  proper  term 
brightness  the  dogstar  is  not  brighter  than  an  equal  star  of  the  seventh  magni 
tude.  It  is  a  more  splendid  object  as  viewed  by  the  eye,  because  it  transmit 
more  light  to  the  eye,  but  its  intrinsic  splendor  may  be  the  same.  The  sun  a 
seen  from  the  earth  and  as  seen  from  the  planet  Herschel,  has  the  same  in 
tnnsic  brightness,  but  its  apparent  magnitude  at  Herschel  200  times  less. 


\ 

L 


WATERSPOUTS  AND  WHIELWlNDri. 


WATER-SPOUTS  AND  WHIRLWINDS. 


WATER-SPOUTS  apparently  consist  of  dense  masses  of  aqueous  vapor,  pre- 
senting, often  a  gyratory  and  progressive  motion,  and  resembling  in  form  a  con- 
ical cloud,  the  base  of  which  is  presented  upward,  and  the  vertex  of  which 
generally  rests  upon  the  ground,  but  sometimes  assumes  the  contrary  position. 
This  phenomenon  is  attended  with  a  sound  like  that  of  a  wagon  rolling  upou 
a  rousjh  pavement. 

Violent  mechanical  effects  sometimes  attend  these  meteors.  Large  trees  torn 
up  by  the  roots,  stripped  of  their  leaves,  and  exhibiting  all  the  appearances  of 
having  been  struck  by  lightning,  are  projected  to  great  distances.  Houses  are 
often  thrown  down,  unroofed,  and  otherwise  injured  or  destroyed,  when  they 
lie  in  the  course  of  a  water-spout.  Rain,  hail,  and  frequently  globes  of  fire, 
like  the  ball-lightning  already  mentioned,  accompany  these  meteors,  which  are 
manifested  equally  at  sea  and  on  land. 

Although  the  electrical  effects  which  attend  this  meteor  prove  that  it  is  close- 
ly connected  with  atmospheric  electricity,  yet,  as  no  theory  has  hitherto  been 
proposed  which  affords  a  satisfactory  and  adequate  explanation  of  the  phenom- 
ena, it  is  the  more  necessary  to  state,  with  as  much  clearness  and  precision  as 
possible,  independently  of  all  hypotheses,  the  exact  circumstances  which  have 
been  found  to  attend  them  in  the  various  parts  of  the  globe  where  they  have 
been  observed.  They  are  called  water-spouts  or  land-spouts,  according  as  they 
take  place  over  the  surface  of  the  water  or  the  land. 

In  the  history  of  the  Academy  of  Sciences  is  the  following  narrative : — 

"  On  the  2d  of  November,  1729,  about  8  o'clock  in  the  morning,  at  Montpellier, 
a  small  and  very  obscure  cloud  was  seen,  in  a  very  elevated  position,  in  the  di- 
rection of  the  southeast,  whence  the  wind  then  blew.  It  advanced  toward 
the  town  with  a  noise  at  first  low,  "but  which  augmented  as  it  approached : 
it  gradually  descended  toward  the  ground,  and  a  light  was  perceived  to  issae 
from  it,  like  that  which  accompanies  the  smoke  of  a  great  fire.  After  the  pas- 
,  this  cloud,  a  strong  odor  of  sulphur  was  perceived,  like  that  which  is 


568 


WATER-SPOUTS  AND  WHIRLWINDS. 


diffused  in  places  that  have  been  struck  by  lightning.  This  cloud  had  a  very 
rapid  motion,  and  formed  round  it  a  whirlwind,  which  extended  to  a  distance 
of  above  a  hundred  yards  round,  the  force  of  which  was  so  prodigious  that  it 
tore  up  trees  by  the  roots,  carried  away  the  roofs  of  houses,  overturned  build- 
ings, and  scattered  their  ruins  to  a  distance  of  nearly  500  yards.  After  having 
moved  along  half  a  league,  with  a  width  of  above  200  yards,  it  Avas  dissipated, 
followed  by  heavy  rain,  but  not  accompanied  by  thunder  or  lightning." 

In  the  Journal  de  Physique  for  November,  1780,  is  the  following  description 
of  one  of  these  meteors,  which  took  place  at  five  o'clock  in  the  evening,  near 
Carcassonne : — 

"  This  meteor  originated  upon  the  borders  of  the  Aude.  It  commenced  by 
pouring  down  a  great  quantity  of  water  ;  it  then  projected  upward,  to  a  great 
height,  quantities  of  sand.  It  unroofed  eighty  houses,  and  scattered  over  the 
country  the  sheaves  of  corn  which  it  carried  away.  It  tore  up  by  the  roots 
large  oaks,  and  transported  to  a  distance  of  fifty  yards  their  branches,  project- 
ing them  in  a  direction  contrary  to  that  of  its  own  motion.  It  broke  the  doors, 
windows,  and  furniture  of  a  chateau;  it  destroyed  the  pavement  in  the  middle 
of  a  room,  without  deranging  china  cups  which  were  placed  there  ;  it  broke 
the  frame  of  a  looking-glass  which  was  placed  upon  a  chimney-piece,  and  scat- 
tered the  fragments  upon  the  chairs  of  the  room,  leaving  the  glass,  however,  in 
its  place  uninjured." 

In  the  Memoirs  of  the  Academy  of  Toulouse,  vol.  v.,  is  the  following  descrip- 
tion of  a  land-spout,  which,  on  the  15th  of  June,  1785,  devastated  the  neigh- 
borhood of  Esclades,  about  four  leagues  from  Narbonne  : — 

"  The  night  before  this,  terrible  visitation  was  very  fine,  the  sun  rose  unob- 
scured  by  a  single  cloud,  and  the  morning  air  was  calm  and  pure.  At  half-past 
six  o'clock  the  heat  became  very  great,  and  continued  to  increase  till  seven 
o'clock,  when  it  was  excessive.  At  that  time  there  appeared  in  the  west  a  small 
cloud,  which,  gradually  augmenting,  extended  in  an  hour  over  the  whole  hori- 
zon. The  thermometer  of  Reaumur  stood  at  29°,*  and  the  barometer  at  28 
inches.  There  was  a  light  wind  from  the  west.  Such  being  the  state  of  the 
atmosphere  at  two  o'clock  in  the  afternoon,  a  kind  of  smoky  and  blustering 
(Iruyantr]  column  was  formed  in  the  west,  which  passed  between  Esclades  and 
Mont  Brun.  In  its  course  it  swept  away  earth  and  sand,  tore  up  trees,  and 
ravaged  everything  which  came  before  it.  This  lasted  for  about  five  minutes. 
At  about  five  miles  from  Esclades  it  became  stationary  for  about  five  minutes, 
after  which  it  returned  upon  its  steps :  the  noise  which  it  made  resembled  the 
continual  roaring  of  thunder.  It  burst  upon  Esclades  in  a  terrific  shower  of 
hail.  This  hail  was  succeeded  by  a  rain  so  abundant  that  the  country  was  in- 
undated. During  this  shower,  which  lasted  three  quarters  of  an  hour,  lightning 
fell  in  several  places.  The  thermometer  rose  to  32°.f 

"  The  barometer  rose  a  quarter  of  an  inch,  and  the  wind  was  very  violent. 
After  the  meteor  disappeared  the  weather  became  cool,  and  the  barometer  fell 
an  inch  and  a  quarter." 

Humboldt  slates  that,  in  the  Steppes  of  South  America,  the  plain  or  table 
land  presents  an  extraordinary  spectacle,  which  he  describes  as  follows  : — 

"  The  sand  rises  in  the  middle  of  a  rarefied  whirlwind,  probably  charged 
with  electricity,  like  a  vapor,  or  a  cloud  in  the  form  of  a  funnel,  the  point  of 
which  slides  upon  the  ground,  and  resembling  the  blustering  water-spout  so 
much  feared  by  the  experienced  navigator.  On  the  roads  in  Europe,  we  see 
something  which  approaches  the  singular  appearance  of  these  whirlwinds  of 
sand  ;  but  they  are  especially  observed  in  the  sandy  deserts  situate  in  Peru,  be- 
tween Coquimbo  and  Amotape.  It  is  worthy  of  remark,  that  these  partial  cui 

*  Equal  to  100  degrees  Fahr.  t  Equal  to  104  degrees  Fahr. 


WATERSPOUTS  AND  WHIRLWINDS. 


rents  of  air  which  encounter  each  other  are  only  perceived  when  tho  atmo- 
sphere is  entirely  calm— the  ocean  of  air,  therefore,  like  the  ocean  of  v. 
encountering  each  other  only  in  a  dead  calm."* 

The  Courier  of  the  19th  of  September,  1826,  published  the  following  narra-  ' 
tiye  of  a  meteor  which  ravaged  the  arrondissement  of  Carcassonne  on  the  VJf.th 
of  August  preceding  : — 

"  The  wind  was  from  the  south,  and  the  heat  of  the  morning  was  suffoca- 
ting. About  noon,  the  clouds  accumulated  in  the  west,  and  a  violent  wind  :: 
A  thick  black  cloud  appeared,  suspended  over  a  piece  of  land  near  thn  ch 
of  La  Counette.  In  the  direction  of  Fombraise,  the  clouds  were  seen  to  en- 
counter each  other,  and,  after  the  collision,  to  descend  very  low,  as  if  they  wf-re 
attracted  by  the  earth.  The  thunder  grumbled  on  every  side  with  a  dulfrolling 
noise ;  domestic  animals  fled  to  their  sheds.  Suddenly  a  frightful  explosion 
(craquemcnt)  was  heard  in  the  west ;  the  air,  violently  agitated,  was  drawn  with 
extreme  velocity  toward  the  black  cloud  above  mentioned :  the  moment  they 
encountered  was  signalized  by'a  loud  detonation,  and  the  appearance  of  aii 
enormous  column  of  fire,  which,  sweeping  over  the  field,  tore  up  everything  in 
its  way.  A  young  man  of  17  was  carried  away  by  this  whirlwind,  raised  in 
the- air,  and  dashed  against  a  rock,  by  which  his ' head  was  split;  14  sheep 
were  carried  away,  and  fell  senseless. 

"  This  column  of  air  and  fire  overturned  walls,  displaced  enormous  rocks,  tore 
up  by  the  roots  the  largest  trees,  broke  into  the  chateau  by  two  openings,  tore 
up  and  overturned  the  stones  of  the  porte  cuchsre,  broke  the  gate,  twisted  all 
the  iron  work,  broke  through  a  window,  entered  the  saloon  on  the  first  floor, 
broke  through  its  ceiling,  entered  the  second  floor,  passed  to  the  roof,  and,  in 
fine,  reduced  to  ruin  these  three  stories.  The  ladies,  who  were  in  the  saloon 
on  the  first  floor,  saw  a  globe  of  fire  enter  it,  and  owed  their  safety  only  to  an 
enormous  beam  which  formed  an  arch  to  support  the  wood-work.  A  vortex  of 
air,  entering  by  the  window  above  the  kitchen,  broke  through  a  partition,  raised 
the  floor,  broke  the  furniture,  overturned  the  beds,  opened  the  closets  without 
disturbing  their  contents,  penetrated  a  thick  wall  and  projected  its  ruins  to  a 
great  distance,  broke  the  timber-work  of  the  chateau,  tore  up  by  the  roots  an 
enormous  oak  five  feet  in  circumference,  crushed  two  small  houses,  carried 
away  wagons,  which  it  precipitated  into  a  ravine,  uprooted  several  enormous 
walnut-trees,  ravaged  the  vines,  leaving  in  the  earth  deep  trenches,  and  im- 
pregnating the  air  with  a  strong  odor  of  sulphur.  This  meteor  disappeared  in 
the  direction  of  Forcenas,  and  was  succeeded  by  very  heavy  rain.  The  heavens 
then  became  serene,  and  a  wind  arose  from  the  east." 

In  1823,  this  meteor  made  great  ravages  in  the  neighborhood  of  Dreux  and 
Mantes  in  France. 

"  In  the  village  of  Marc/iefroid,  fifty-three  houses  were  destroyed  in  the  space 
of  one  minute,  yet  the  storm  was  scarcely  heard,  and  the  appearance  of  the 
water-spout  was  only  preceded  by  a  little  hail.     A  child  three  years  old,  who 
stood  beside  its  mother  in  a  court-yard,  was  killed  upon  the  spot.     On  exam- 
ining its  body,  no  wounds  were  found  upon  it  except  a  hole  of  a  certain  depth  | 
in  the  neck.     Entire  roofs  were  carried'away  either  in  the  direction  in  which 
the  meteor  moved,  or  in  the  contrary  direction.     The  four  walls  of  a  ^.H,li  n 
were  thrown  down  in  a  regular  manner,  all  tailing  on  the  outside  of  th 
their  fall  was  marked  by  great  regularity.     After  the  meteor  passed  away,  the 
temperature  did  not  seem  changed,  and 'the  sun  immediately  reap, 

On  the  6th  of  July,  1822,  a  land-spout  was  formed  in  the  plain  of  <  ta 
near  the  village  of  that  name,  in  the  department  of  the  Pas  do  Calais. 

•  Tableau  <lc  la  Nature,  torn  i.,  pp.  43  and  177. 


570 


WATER-SPOUTS  AND  WHIRLWINDS. 


Clouds  coming  from  different  directions  and  collecting  over  the  plain,  ulti- 
mately formed  a  single  cloud  which  covered  the  heavens  :  immediately  after- 
ward a  cone  descended  from  this  cloud,  presenting  its  vertex  downward,  and 
having  its  base  in  the  cloud.  This  meteor,  driven  by  the  wind,  beat  down  a 
barn,  tore  and  carried  away  the  tops  of  the  largest  trees,  overturned  twenty- 
five  to  thirty  of  them,  and  strewed  them  in  different  directions,  proving  that 
the  meteor  had  a  revolving  motion.  It  carried  away  and  crushed  other  trees 
from  sixty  to  seventy  feet  high.  Globes  of  fire  and  sulphureous  vapor  were 
seen  from  time  to  time  to  issue  from  its  centre.  This  meteor,  in  its  rapid 
course,  was  attended  with  a  sound  like  that  of  a  heavy  carriage  rolling  on  a 
paved  road. 

It  then  penetrated  into  the  valley  of  Wctternester  and  Lambre  ;  in  the  former 
of  these  villages,  only  eight  habitations  of  forty  were  uninjured :  the  meteor 
left  everywhere  traces  of  its  passage. 

On  the  18th  of  June,  1839,  the  neighborhood  of  Chatenay,  in  the  department 
of  Seine  et  Oise,  was  visited  by  a  meteor,  which  happened  to  be  witnessed  by 
MM.  Peltier,  Bouchard,  and  Becquerel.  The  following  narrative  of  it  is  abridg- 
ed from  the  account  given  of  it  by  M.  Peltier  : — 

In  the  morning,  a  storm  was  formed  to  the  south  of  Chatenay,  and  about  ten 
o'clock  it  took  the  direction  of  the  valley  between  the  hills  of  Ecouen  and  Clia- 
tenay.  The  clouds,  which  were  high,  after  extending  above  the  extremity  of 
the  village,  came  to  a  stand,  the  thunder  muttered,  and  the  first  cloud  followed 
the  ordinary  route,  when,  toward  noon,  a  second  storm  coming  also  from  the 
south,  advanced  toward  the  same  plain  and  the  same  hills.  Arriving  near  the 
extremity  of  the  plain  over  Fontenay,  in  presence  of  the  first  storm  which,  by 
its  elevation,  it  overtopped,  a  pause  took  place,  doubtless  while  the  two  storms 
were  presenting  themselves  to  each  other  by  means  of  their  clouds  charged  with 
the  same  electricity,  and  repelling  each  other. 

To  this  time,  thunder  which  was  heard  proceeded  from  the  second  cloud, 
when  suddenly  one  of  the  inferior  clouds  descending,  fell  into  communication 
with  the  earth,  and  the  thunder  seemed  to  cease.  A  prodigious  attraction  was 
manifested  ;  all  light  bodies  and  all  the  dust  which  cohered  the  surface  of  the 
ground,  was  raised  toward  the  point  of  the  cloud  :  a  continual  rolling  noise  suc- 
ceeded ;  little  clouds  were  fluttering  and  whirling  round  the  inverted  cone,  and 
rising  and  falling  rapidly.  Trees,  placed  to  the  southeast  of  the  meteor,  were 
struck  on  their  northwest  side  which  faced  it,  the  other  side  remaining  in  its 
usual  state.  The  sides  which  were  struck  exhibited  strong  marks  of  the 
meteor,  while  the  other  parts  preserved  their  sap  and  their  vegetable  life.  The 
meteor  descended  the  valley  to  the  extremity  of  Fontenay,  toward  a  row  of 
trees  planted  along  the  bed  of  a  stream  which  was  then  without  water,  though 
still  humid.  After  having  broken  and  uprooted  these,  it  traversed  the  valley, 
and  advanced  toward  other  plantations  which  it  also  destroyed.  There,  having 
arrived  at  the  point  vertically  under  the  limits  of  the  first  cloud,  it  paused,  and 
the  latter,  which  was  hitherto  stationary,  began  to  be  agitated  and  to  retreat 
toward  the  valley  west  of  Chatenay,  and,  overthrowing  all  that  it  encountered  in 
its  way,  it  passed  to  the  park  of  the  chateau  of  Chatenay,  which  it  completely 
desolated.  The  walls  were  overturned,  and  the  roofs  and  chimneys  of  the 
buildings  carried  away.  Trees  were  transported  several  hundred  yards  ;  win- 
dows, rafters,  tiles  were  thrown  to  a  distance  of  upward  of  500  yards. 

The  meteor  having  ravaged  that  place,  descended  a  mountain  toward  the 
north,  and  paused  over  a  fish  pond,  where  it  overthrew  and  parched  the  trees, 
killed  all  the  fish,  and  proceeded  slowly  along  an  alley  of  willows.  Here  it 
lost  a  great  portion  of  its  extent  and  violence.  It  then  proceeded  still  more 
slowly  over  a  neighboring  plain,  and  after  advancing  three  quarters  of  a  mile,  it 


WATERSPOUTS  AND  WHIRLWINDS. 


.'.71 


divided  itself  into  two  portions  near  a  clump  of  trees,  one  part  rising  into  the 
clouds,  while  the  other  part  sunk  into  the  ground  and  disappear. 

All  the  trees  struck  by  this  meteor  had  their  sap  completely  evaporated, 
the  ligneous  part  being  as  much  dried  as  if  it  had  been  exposed  in  a  - 
at  the  temperature  of  300°.  The  immense  quantity  of  vapor  suddenly  form- 
ed by  the  sap,  having  no  means  of  escape  from  the  interstices  of  the  wood, 
split  the  tree  in  the  longitudinal  direction.  All  the  trees  presented  marks  ol 
this  effect. 

By  observing  the  progress  of  this  phenomenon,  the  transformation  of  a  com- 
mon storm  into  a  land-spout  will  be  apparent.  Two  stormy  clouds  inovril 
toward  the  same  vertical  line  in  which  they  settled  at  different  altitudes,  linn» 
charged  with  the  same  electricity,  the  lower  cloud  descends  toward  the  ground, 
and  is  put  in  electrical  communication  with  the  ground  by  whirlwinds  of  dust 
and  by  trees.  This  communication  once  established,  the  noise  of  the  thunder 
immediately  ceases,  the  discharge  taking  place  by  the  continuous  conductor 
formed  by  the  clouds  which  have  descended  and  the  trees  upon  the  plain. 
These  last,  traversed  by  the  electricity,  have  their  sap  dried  up  and  their  trunks 
split ;  finally,  flashes  of  light,  balls  of  fire,  and  sparks  appear,  and  a  sulphure- 
ous odor  remains  in  the  houses  for  several  days,  the  curtains  of  which  are 
everywhere  scorched. 

In  his  voyage  to  the  Pacific  Captain  Beechey  witnessed  water-spouts  off 
Ciermont  Tonnerre,  lat.  19°  south,  long.  137°  west,  of  which  he  has  given 
the  drawings,  from  which  figs.  1  and  2  have  been  taken. 

Fig.  1. 


Colonel  Reid,  in  his  work  on  storms,  has  given  the  following  extract  from  a 
letter  addressed  to  him  by   Captain  Beechey,  containing  a  circumstantial  ac 
count  of  water-spouts,  witnessed  by  him  in  the  same  voyage :  " 
been  very  sultry,  and  in  the  afternoon  a  long  arch  of  heavy  cumuli  and  mm 
rose  slowly  above  the  southern  horizon  ;  while  watching  its  movements  a  w 
ter-spout  began  to  form,  at  a  spot  on  the  under  side  of  the  arch  that  * 
er  than  the  rest  of  the  line.     A  thin  cone  (fig.  3),  first  appeared,  whicl 
ally  became  elongated,  and  was  shortly  joined  by  several  others   whi 
on  increasing  in  length  and  bulk  until  the  columns  had  reached  about  I 
down  to  the  horizon      The  sea  beneath  had  hitherto  been   undistur»,d 
when  the  columns  united  it  became  perceptibly  agitated,  and  almost  iminc 


572 


WATER-SPOUTS  AND  WHIRLWINDS. 


Fig.  2. 


Fig.  3. 


ately  became  whirled  in  the  air  with  a  rapid  gyration  and  formed  a  vast  basin, 
from  the  centre  of  which  the  gradually  lengthening  column  appeared  to  drink 
fresh  supplies  of  water  (fig.  4). 

Fig.  4. 


WATERSPOUTS  AND  WHIHLWINDS. 


673 


"  The  column  had  extended  to  about  two  thirds  of  the  way  toward  the  sea, 
and  nearly  connected  itself  with  the  basin,  when  a  heavy  shower  of  rain  feli 
from  the  right  of  the  arch,  and  shortly  after  another  fell  from  the  opposite 
side.  This  discharge  appeared  to  have  an  effect  on  the  water-spout,  which 
now  began  to  retire. 

"  The  sea,  on  the  contrary,  was  perceptibly  more  agitated,  and  for  several  'i 
minutes  the  basin  continued  to  increase  in  size,  although  the  column  wan  con-  < 
siderably  diminished  (fig.  5). 


Fig.  5. 


"  In  a  few  minutes  more  the  column  had  entirely  disappeared.  The  sea. 
however,  still  continued  agitated,  and  did  not  subside  for  three  minutes  after  all 
the  disturbing  causes  from  above  had  vanished.  The  phenomenon  was  unac- 
companied by  thunder  or  lightning,  although  the  showers  of  rain  which  fell  so 
suddenly  seemed  to  be  occasioned  by  some  such  disturbance." 

M.  Peltier  has  attempted  to  illustrate  the  electrical  origin  of  these  phenom- 
ena by  producing  them  artificially.  With  this  view  he  has  represented  the 
cloud  in  which  the  meteor  originates  by  a  globe  of  metal  kept  constantly  charg- 
ed with  electricity  by  a  machine.  The  inequalities  of  the  cloud  he  represent- 
ed bv  points  raised  on  the  surface  of  a  globe.  By  means  of  the  influence 
which  this  globe  exercised  upon  water,  vapors,  and  dust,  he  was  able  to  pro- 
duce a  depression  of  the  liquid,  and  the  vortical  or  gyratory  motion,  and  some 
other  effects  similar  to  those  observed  in  the  meteor. 

All  these  effects  disappeared  when  the  globe  was  divested  of  points.  In 
this  case,  instead  of  a  depression,  an  elevation  was  produced ;  the  vapors 
rose  under  the  smooth  ball,  but  showed  little  agitation.  When  the  points 
were  restored,  the  vapor  was  increased  in  more  than  a  threefold  proportion. 

The  globules  of  vapor,  being  electrified  at  a  distance  by  the  pointSj  were 
repelled  in  all  directions,  and  made  to  whirl,  more  or  less,  according  to  the  de- 
gree of  the  electric  charge. 

There  are  other  electrical  experiments  made  with  other  views,  which  M. 
Peltier  brings  to  bear  on  the  illustration  of  water-spouts. 

A  plate  of  copper,  not  insulated,  being  placed  under  a  sphere,  a  little  light 
ball  is  placed  between  them.  When  the  sphere  is  electrified,  the  ball  plays 
alternately  upward  and  downward  between  the  sphere  and  the  plate  ;  but  if, 
instead  of  the  ball,  elongated  or  flat  bodies  be  interposed,  so  as  to  present  only 
a  long  and  narrow  strip  of  gold  leaf,  the  alternate  motion  just  described  is 
transformed  into  a  vortical  motion,  which  ultimately  becomes  one  of  rapid  ro- 


WATER-SPOUTS  AND  WHIRLWINDS. 


tstion  between  the  sphere  and  the  plate.    Such  are  the  gyratory  motions  which 
M.  Peltier  conceives  to  arise  from  electrical  radiation. 

The  consequences  which  he  deduces  from  these  and  similar  facts  are  as 
follows  : — 

1.  All  the  immediate  phenomena  observed  in  water-spouts  are  due  to  elec- 
tricity :  they  are  the  results  of  secondary  phenomena,  which  almost  always 
accompany  them.     The  latter  vary  with  the  locality  and  the  state  of  the  atmo- 
sphere. 

2.  Their  general  effects  are  due  either  to  statical  or  dynamical  electricity: 
most  generally  they  proceed  from  both. 

3.  The  statical  effects  are  phenomena  of  attraction  and  repulsion. 

4.  The  attraction  of  an  electrical  cloud  is  accompanied  by  a  rush  of  air  tow- 
ard this  cloud,  whence  result  currents  directed  from  the   exterior  to  the  inte- 
rior, and  proceeding  from  all  surrounding  points.     It  is  manifested  also  by  the 
projection  of  the  vapor  of  water,  of  liquid  water  itself,  and  of  bodies  that  it 
raises  or  tears,  according  to  the  force  with  which  it  acts. 

5.  The  progress  of  its  attractive  power  is  plainly  marked  both  on  sea  and 
land.     On  sea  it  appears  by  the  boiling  of  the  waters,  and  the  smoky  appear- 
ance which  is  raised  from  them,  as  represented  in  figures  1  and  2.     On  land 
its  course  is  rendered  manifest  by  its  effects  upon  the  air,  the  ground,  and  all 
loose  bodies  which  it  encounters. 

6.  The  attraction  of  the  clouds   is  also  manifest  by  the  greatly  increased 
evaporation  of  the  waters,  and  the  consequent  fall  of  their  temperature.     The 
repulsion  is  manifested  by  currents  of  the  air  which  issue  from  the   electric 
cloud,  and  only  exist  in  its  neighborhood.     At  a  little  distance  from  it  a  dead 
calm  prevails.     These  double  currents  undergo  various  modifications,  produced 
by  the  localities  and  various  qualities  of  the  ground. 

7.  The  repulsion  is  also  manifested  by  the  cone  which  is  formed  in  the  sea, 
in  the  very  centre  of  the   smoky  vapors,  an  effect  which  can  be  easily  repro- 
duced experimentally. 

8.  If  an  inductive  action  take  place  between  two  clouds  charged  with  oppo- 
site electricities,  placed  at  a  certain  distance  asunder,  a  portion  of  their  vapor 
will  resume  the  state  of  common  vapor ;  this  will  lower  the  temperature  of  the 
neighboring  parts,  which  may  descend  even  below  the  freezing  point ;  then 
the  vapor  of  water  crystallizes  in  snowy  flakes,  which  act  immediately  after 
their  formation,  like  other  light  bodies.     The  portion  thus  transformed  into 
snow,  and  which  is  charged  with  the  electricity  of  the   inferior  cloud,  is  at- 
tracted by  the  superior  cloud,  then  there  is  a  neutralization  of  electricity,  a  fall 
of  temperature,  and  so  on. 

9.  Finally,  the  electrical  tension  of  the  superior  cloud  facilitates  the  evapo- 
ration of  the  liquid  which  moistens  the  snowy  globule,  or  which  already  covers 
the  ice. 

The  electrified  clouds,  acting  by  induction  upon  the  ground,  are  attracted  to 
it.  The  clouds  thus  approach  the  earth  in  a  greater  or  less  quantity,  depend- 
ing on  the  energy  of  the  attraction,  and  their  specific  gravity. 

When  the  tension  of  the  clouds  and  their  density  differ  little  from  those  of 
the  inferior  strata  of  air,  or  when  superior  clouds,  having  the  same  electricity, 
act  upon  the  inferior  by  repulsion,  the  latter  may  approach  the  earth  suflicit;.t!y 
to  be  discharged  without  explosion  by  the  intervention  of  other  clouds  which 
touch  it. 

It  happens,  often,  that  all  the  bodies  placed  upon  the  surface  of  the  earth  un- 
der these  clouds,  which  have  the  form  of  an  inverted  cone,  serve  as  conductors 
in  various  degrees,  according  to  their  constituent  matter,  their  form,  their  ex- 
tent, and  the  magnitude  of  their  contact  with  the  ground.  Light  and  small 


WATER  SPOUTS  AND  WHIRLWINDS. 


575 


bodies,  oppositely  electrified,  are  attracted  and  raised  toward  the  cloud  ;  when 
their  electricity  is  neutralized  they  fall  again  upon  the  earth,  where,  hcinj 
once  more  charged  with  electricity,  they  reascend,  and  so  on.  It  is  thus  th:it 
an  immense  cloud  of  dust  is  formed  under  the  cone.  If  the  bodies  are  attached 
to  the  earth,  like  trees  or  buildi.igs,  they  are  instantaneously  charged  with  an 
immense  quantity  of  electricity.  The  earth,  which  is  contiguous  to  them,  par- 
takes of  this  electricity,  yields  to  the  attraction  of  the  cloud,  and  the  trees, 
buildings,  or  other  objects  upon  it,  are  torn  up  and  transported  afar.  It  is  in 
this  manner  that  bodies  which  are  strongly  attached  to  the  earth  are  torn  from 
it,  while  others  in  their  immediate  neighborhood  are  undisturbed.  All  those 
effects  are  subject  to  infinite  variation,  according  to  the  conducting  powers  of 
the  bodies,  and  of  the  parts  of  the  earth  to  which  they  are  attached. 

If  the  great  lightness  of  the  clouds  prevents  them  from  falling  sufficiently  low 
to  be  in  electrical  communication  with  the  ground,  then  the  electricity  will  be 
discharged  at  a  distance,  attended  by  the  flash  of  lightning  and  the  roll  of  thun- 
der. The  electric  tension  will  gradually  diminish,  rain  will  ensue,  and  the 
cloud  will  rise. 

The  sound  which  sometimes  accompanies  this  phenomenon  is  attributed, 
by  M.  Peltier,  to  a  number  of  small  partial  explosions,  which  take  place  be- 
tween the  cloud  and  ground.  They  are  louder  in  the  case  of  water-spouts 
which  traverse  the  land,  because  of  the  imperfectness  of  the  conductors  pre- 
sented to  them ;  they  lose  their  intensity  over  the  sea  because  water  is  a  bet- 
ter conductor. 

Considering  the  progress  of  the  air  under  the  different  attractions  and  repul- 
sions to  which  it  is  submitted,  and  the  contrary  and  unequal  currents  encounter- 
ing different  obstacles,  M.  Peltier  endeavors  to  explain  how  the  direct  motion 
impressed  on  the  air  is  changed  into  a  gyratory  motion  more  or  less  decided. 
It  results  from  this,  that  the  same  meteor  may  present  at  different  moments  an 
example  of  direct  and  gyratory  motion. 

When  the  meteor  is  presented  over  water,  its  inductive  action  gives  to  the 
water  near  the  surface  an  opposite  electricity,  and  a  consequent  attraction  en- 
sues. If  the  contrary  fluids  do  not  unite  by  explosion,  the  surface  of  the  water 
will  swell  upward  at  the  several  points  of  attraction,  and  the  moment  a  dis- 
charge takes  place,  and  the  contrary  fluids  unite  by  explosion,  this  elevation 
subsides. 

If,  however,  the  electrified  cloud  is  formed  with  points  or  prominences, 
which  favor  the  escape  of  the  electric  fluid,  the  water  becomes  charged  with 
the  fluid  descending  from  the  cloud,  and,  being  similarly  electrified,  is  repelled  ) 
by  the  cloud,  and  therefore  depressed.     Currents  result  from  this  in  the  water,  ' 
which  soon  acquire  a  vortical  motion. 

On  similar  principles,  M.  Peltier  explains  the  rapid  disappearance  of  pools,  , 
or  small  collections  of  water,  the  entire  mass  being  electrified  by  induction, 
and  raised  like  trees  and  other  objects. 

The  discharge  of  electricity  through  water  may  kill  the  fish  contained  in 
it ;  but  the  mere  transmission  of  an  electric  current  through  the  liquid  without 
explosion  will  not  have  this  effect,  unless  a  considerable  elevation  of  tempera-  > 
ture  takes  place.     An  electric  discharge  passing  near  water,  but  not  through  < 
it,  may  kill  animals  in  it,  by  the  effect  of  the  lateral  shock.     By  these  pnnci-  > 
pies,  many  of  the  observed  effects  of  water-spouts  are  explained. 

When  by  induction  the  electrical  tension  of  the  ground  and  objects  upon  it 
is  elevated,  the  fluid  with  which  it  becomes  charged  will  have  a  tendency  to 
escape  by  all  pointed  conductors,  and  to  issue  upward  toward  the  cloud. 
the  conductor  be  imperfect,  an  elevation  of  temperature  will  attend  these  up- 
ward currents,  the  effects  of  which  will  be  apparent  in  the  conductors  by  which 


576  V/ATER-SPQUTS  AND  WHIRLWINDS. 

they  escape.  Trees,  plants,  and  vegetables,  conducting  the  electric  fluid  im- 
perfectly by  means  of  tLeir  sap,  are  dried  up  by  this  temperature  ;  and  when 
the  elevation  takes  place  suddenly,  the  vapor  into  which  the  sap  is  converted 
splits  the  wood. 

Such  is  a  general  outline  of  the  theory  of  M.  Peltier,  by  which  the  phenom- 
ena attending  water-spouts  and  whirlwinds  are  explained. 


-  '  Ǥ  m  ' 
1     '    1  .  -' 


Q 

171 

L35 

1859 

v.l 

P&ASci 


Lardner,  Dionysius 

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