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THE    MOON 


COXSIDERED   AS 


A    PLANET,    A    AVORLD,    AND    A    SATELLITE. 


Digitized  j0wff\e  dfiternef  Archive 

in  20C>B  with  furiding  from 

JVIicro^bCdonporation 


http://www.archive.org/details/consideredasmoonOOnasmrich 


FRONTISPIECE. 


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THE    MOON 


CONSIDERED    AS 


A  PLANET,  A  WOELD,   AND  A  SATELLITE. 


By    JAMES    NASMYTH,    C.E. 


AND 


JAMES    CAEPENTER,    F.E.A.S. 

LATE  OF  THE   ROYAL   OBSERVATORY,   GREENWICH. 


WITH    TWENTY-SIX    ILLUSTRATIVE    PLATES    OF    LUNAR    OBJECTS,    PHENOMENA, 
AND    SCENERY:     NUMEROUS    WOODCUTS,    &c. 


LONDON: 
JOHN    MUKEAY,     ALBEMARLE     STREET. 

1885. 


V\3 


LONDOX  : 
BRADBURY,   AOXEW,   &  CO,,   PRINTERS,   WHITEFRURS. 


3/J^4^ 


Cic" 


TO 

HIS  GEACE  THE  DUKE  OF  AKGYLL, 

m  RECOGNITION  OF  HIS  LONG  CONTINUED  INTEEEST  IN  THE  SUBJECT 
OF  WHICH  IT  TREATS, 


^fti^  Volume 


IS    MOST    RESPECTFULLY   DEDICATED 
BY 


THE  AUTHORS. 


PEEFACE, 


The  reason  for  this  book's  appearance  may  be  set  forth 
in  a  few  words.  A  long  course  of  careful  scrutiny  of 
the  lunar  surface  with  the  aid  of  telescopes  of  considerable 
power,  and  a  consequent  familiarity  with  the  wonderful 
details  there  presented,  convinced  us  that  there  was  yet 
something  to  be  said  about  the  moon,  that  existing  works 
on  Astronomy  did  not  contain.  Much  valuable  labour  has 
been  bestowed  upon  the  topography  of  the  moon,  and  this 
subject  we  do  not  pretend  to  advance.  Enough  has  also 
been  written  for  the  benefit  of  those  who  desire  an  acquaint- 
ance with  the  intricate  movements  of  the  moon  in  space  ; 
and  accordingly  we  pass  this  subject  without  notice.  But 
very  little  has  been  written  respecting  the  moon's 
physiography,  or  the  causative  phenomena  of  the  features, 
broad  and  detailed,  that  the  surface  of  our  satellite  presents 
for  study.  Our  observations  had  led  us  to  some  con- 
clusions, respecting  the  cause  of  volcanic  energy  and  the 
mode   of  its   action   as  manifested  in  the   characteristic 


viii  PREFACE. 

craters  and  other  eruptive  phenomena  that  abound  upon 
the  moon's  surface.  We  have  endeavoured  to  explain  these 
phenomena  by  reference  to  a  few  natural  laws,  and  to 
connect  them  with  the  general  hypothesis  of  planet  forma- 
tion which  is  now  widely  accepted  by  cosmologists.  The 
principal  aim  of  our  work  is  to  lay  these  proffered  explana- 
tions before  the  students  and  admirers  of  astronomy  and 
science  in  general ;  and  we  trust  that  what  we  have 
deduced  concerning  the  moon  may  be  taken  as  referring  to 
a  certain  extent  to  other  planets. 

Some  reflections  upon  the  moon  considered  as  a  world, 
in  reference  to  questions  of  habitability,  and  to  the  peculiar 
conditions  which  would  attend  a  sojourn  on  the  lunar 
surface,  have  appeared  to  us  not  inappropriate.  These, 
though  instructive,  are  rather  curious  than  important. 
More  worthy  of  respectful  consideration  are  the  few 
remarks  we  have  offered  upon  the  moon  as  a  satellite  and 
a  benefactor  to  the  inhabitants  of  this  Earth. 


In  reference  to  the  Illustrations  accompanying  this  work, 
more  especially  those  which  represent  certain  portions  of 
the  lunar  surface  as  they  are  revealed  by  the  aid  of  powerful 
telescopes,  such  as  those  which  we  employed  in  our  scrutiny, 
it  is  proper  that  we  should  say  a  few  words  here  on  the 
means  by  which  they  have  been  produced. 


PREFACE.  ix 

During  upwards  of  thirty  years  of  assiduous  observation, 
every  favourable  opportunity  has  been  seized  to  educate  the 
eye,  not  only  in  respect  to  comprehending  the  general 
character  of  the  moon's  'surface,  but  also  to  examining 
minutely  its  marvellous  details  under  every  variety  of 
phase,  in  the  hope  of  rightly  understanding  their  true 
nature,  as  well  as  the  causes  which  had  produced  them. 
This  object  was  aided  by  making  careful  drawings  of  each 
portion  or  object  when  it  was  most  favourably  presented 
in  the  telescope.  These  drawings  were  again  and  again 
repeated,  revised,  and  compared  with  the  actual  objects, 
the  eye  thus  advancing  in  correctness  and  power  of 
appreciating  minute  details,  while  the  hand  was  acquiring, 
by  assiduous  practice,  the  art  of  rendering  correct  repre- 
sentations of  the  objects  in  view.  In  order  to  present  these 
Illustrations  with  as  near  an  approach  as  possible  to  the 
absolute  integrity  of  the  original  objects,  the  idea  occurred 
to  us  that  by  translating  the  drawings  into  models  which, 
when  placed  in  the  sun's  rays,  would  faithfully  reproduce 
the  lunar  effects  of  light  and  shadow,  and  then  photo- 
graphing the  models  so  treated,  we  should  produce  most 
faithful  representations  of  the  original.  The  result  was  in 
every  way  highly  satisfactory,  and  has  yielded  pictures 
of  the  details  of  the  lunar  surface  such  as  we  feel  every 
confidence  in  submitting  to  those  of  our  readers  who  have 
made  a  special  study  of  the  subject.  It  is  hoped  that  those 
also  who  have  not  had  opportunity  to  become  intimately 


X  PREFACE. 

acquainted  with  the  details  of  the  lunar  surface,  will  be 
enabled  to  become  so  by  aid  of  these  Illustrations. 

In  conclusion,  we  think  it  desirable  to  add  that  the 
photographic  Illustrations  above  referred  to  are  printed  by- 
well-established  pigment  processes  which  ensure  their  entire 
permanency. 


PEEFACE   TO   THE   THIED   EDITION. 


The  first  and  second  editions  of  this  work,  which  has 
been  so  well  received  by  those  who  are  specially  qualified 
to  judge  of  its  value,  have  been  out  of  print  for  several 
years,  and  as  enquiries  for  copies  continue  to  be  made  we 
have  been  induced  to  bring  out  a  new  edition,  in  a  more 
compact  size  and  at  a  reduced  price.  It  is  hoped  that  these 
qualifications  may  bring  the  book  within  the  reach  of  many 
who  have  hitherto  been  unable  to  obtain  it. 


CONTENTS. 


CHAPTER   I. 

ON  THE  COSMICAL  OKIGIN  OF  THE  PLANETS  OF  THE  SOLAE 

SYSTEM. 


PAGE 


Origination  of  Material  Things — Celestial  Vapours — Nebulae  —  Their  vast 
Numbers — Sir  W.  Herschel's  Observations  and  Classification — Buffon's 
Cosmogony  —  Laplace's  Nebular  Hypothesis  —  Doubts  upon  its 
Validity — Support  from  Spectrum  Analysis 1 


CHAPTER   IT. 

THE    GENERATION    OF    COSMICAL    HEAT. 

Conservation  of  Force — Indestructibility  of  Force — Its  Convertibility  into 
Heat — Dawn  of  the  Doctrine — Mayer's  Deductions — Joule's  Experi- 
ments— Mechanical  Equivalent  of  Heat — Gravitation  the  Source  of  , 
Cosmical  Heat— Calculations  of  Mayer  and  Helmholtz — The  Moon  as 
an  Incandescent  Sphere — Not  necessarily  Burning — ^Loss  of  Heat  by 
Radiation — Cooling  of  External  Crust — Commencement  of  Selenor 
logical  History 13 


CHAPTER   III. 

THE    SUBSEQUENT    COOLING    OF    THE    IGNEOUS    BODY. 

Cooling  commenced  from  Outer  Surface — Contraction  by  Cooling— Expan- 
sion of  Molten  Matter  upon  Solidification — Water  not  exceptional — 
Similar  Behaviour  of  Molten  Iron — Floating  of  Solid  on  Molten  Metal 
— Currents  in  a  Pot  of  Molten  Metal — Bursting  of  Iron  Bottle  by 
Congelation  of  Bismuth  within — Evidence  from  Furnace  Slag — From 
the  Crater  of  Vesuvius — Effects  of  Contraction  of  Moon's  Crust  and 
Expansion  of  Interior — Production  of  Ridges  and  Wrinkles — Theory  of 
Wrinkles — Examples  from  Shrivelled  Apple  and  Hand         ...      21 


xu  CONTENTS. 


CHAPTER    TV. 

THE   FORM,   MAGNITUDE,  WEIGHT,  AND  DENSITY  OF  THE 
LUNAR    GLOBE. 

PAGE 

Form  of  Moon — Not  perfectly  Spherical — Bulged  towards  Earth — Diameter 
— Angular  Measure — Linear  Measure — Parallax  of  Moon — Distance — 
Area  of  Lunar  Sphere — Solid  Contents — Mass  of  Moon — Law  of  Gravi- 
tation— Mass  determined  by  Tides  and  other  Means — Density — How 
obtained — Specific  Gravity  of  Lunar  Matter — Force  of  Gravity  at 
Surface — How  determined — Weights  of  similar  Bodies  on  Earth  and 
Moon — Effects  of  like  Forces  acting  against  Gravity  on  Earth  and 
Moon .       35 


CHAPTER    V. 

ON    THE    EXISTENCE    OR   NON-EXISTENCE    OF    A   LUNAR 
ATMOSPHERE. 

Subject  of  Controversy — Phenomena  of  Terrestrial  Atmosphere — No  Counter- 
parts on  Moon — Negative  Evidence  from  Solar  Eclipses — No  Twilight 
on  Moon — Evidence  from  Spectrum  Analysis — From  Occultations  of 
Stars — Absence  of  Water  or  Moisture — Cryophorus — No  Reddening  of 
Sun's  Rays  by  Vapours  on  Moon — No  Air  or  Water  to  complicate 
Discussions  of  Lunar  Volcanic  Phenomena 44 


CHAPTER    VI. 

THE    GENERAL  ASPECT    OF    THE   LUNAR   SURFACE. 

Pre-Telescopic  Ideas — Human  Countenance — Other  supposed  Resemblances — 
Portrait  of  Full  Moon — Permanence  of  Features — Rotation  of  Moon — 
Solar  Period  and  Solar  Day  on  Moon  —  Libration — Diurnal — In 
Latitude — In  Longitude — Visible  and  Invisible  Hemispheres — Teles- 
copic Scrutiny — Galileo's  Views — Features  Visible  with  Low  Power — 
Low  Powers  on  small  and  large  Telescopes — Salient  Features — Craters 
— Plains — Bright  Streaks— Mountains — Higher  Telescopic  Powers — 
Detail  Scrutiny  of  Features  therewith — Discussion  of  High  Powers — 
Education  of  Eye — Highest  practicable  Power — Size  of  smallest 
Visible  Objects 58 

CHAPTER  VII. 

TOPOGRAPHY    OF    THE    MOON. 

Eeasons  for  Mapping  the  Moon — Early  Maps — ^Labours  of  Langreen — 
Hevelius — Riccioli — Cassini — Schroeter — Modem  Maps  — Lohrman's — 
Beer    and    Maedler's — Excellence    of   the    last  —  Measurement    of 


CONTENTS: 


Mountain  Heights — ^Need  of  a  Picture  Map — Formation  of  our  own — 
Skeleton  Map — Table  of  conspicuous  Objects — Descriptions  of  special 
Objects — Copernicus — Gassendi — Eudoxus  and  Aristotle — Triesnecker 
— Theophilus,  Cyrillus,  and  Catharina — Ptolemy,  Alphons,  and  Arza- 
chael — Thebit — Plato — Valley  of  the  Alps — Pico — Tycho — Wargentin 
— Aristarchus  and  Herodotus — Walter — Archimedes  and  the  Apennines      74 

CHAPTER  YIII. 

ON    LUNAR    CEATEES. 

Use  of  term  Crater  for  Terrestrial  and  Lunar  Formations — Truly  Volcanic 
Nature  of  Lunar  Craters — Terrestrial  and  Lunar  Volcanic  Areas 
compared — Similarity — Difference  only  in  Magnitude — Central  Cone 
— Found  in  great  ndd  small  Lunar  Craters — Formative  Process 
of  Terrestrial  Volcanoes  —  Example  from  Vesuvius  —  Vast  Size  of 
Lunar  Craters — Eeasons  assigned — Origin  of  Moon's  Volcanic  Force 
—  Aqueous  Vapour  Theory  untenable  —  Expansion  upon  Solidifi- 
cation Theory  —Formative  Process  of  a  Lunar  Crater — Volcanic  Vent 
— Commencement  of  Eruption — Erection  of  Eampart — Hollowing  of 
Crater — Formation  of  Central  Cone — Of  Plateau — ^Various  Heights  of 
Plateaux  —  Coneless  Craters — Filled-up  Craters — Multiple  Cones — 
Craters  on  Plateau — Double  Eamparts — Landslip  Terraces  —  Eutted 
Eamparts — Overlapping  and  Superposition  of  Craters — Source-Connec- 
tion of  such — Frothlike  Aggregations  of  Craters — Majestic  Dimensions 
of  Larger  Craters 74 

CHAPTER    IX. 
ON  THE  GEEAT  EING-FOEMATIONS  NOT  MANIFESTLY  VOLCANIC. 

Absence  of  Central  Cones — Vast  Diameters — Difficult  of  Explanation — 
Hooke''s  Idea — Suggested  Cause  of  True  Circularity — Scrope's  Hypo- 
thesis of  Terrestrial  Tumescences — Eozet's  Tourbillonic  Theory — 
Dana's  Ebullition  Theory 133 

CHAPTER    X. 

PEAKS  AND  MOUNTAIN  EANGES. 
Paucity  of  extensive  Mountain  Systems  on  Moon — Contrast  with  Earth —  • 
Lunar  Mountains  found  in  less  disturbed  Eegions — Lunar  Apennines, 
Caucasus,  and  Alps — Valley  of  Alps — "  Crag  and  Tail  "  Contour — 
Isolated  Peaks — How  produced — Analogy  from  Freezing  Fountain — 
Terrestrial  Counterparts  and  their  Explanation  by  Scrope — Blowing 
Cone  on  Teneriffe — Comparative  Gentleness  of  Mountain-forming 
Action — Eelation  between  Mountain  Systems  and  Crater  Systems — 
Wrinkle  Eidges 140 


xiv  CONTENTS. 

CHAPTER   XI. 

CEACKS  AND   RADIATING  STREAKS. 

PAGE 

Description — Divergence  from  Focal  Craters — Experimental  Explanation  of 
their  Cause — Radial  Cracking  of  Crust — Outflow  of  Matter  there- 
from— Analogy  from  "  Starred  "  Ice — No  Shadows  cast  by  Streaks — 
Their  probable  Slight  Elevation— Open  Cracks — Great  Numbers — 
Length — Depth — In-fallen  Fragments — Shrinkage  a  Cause  of  Cracks 
— ^Lateness  of  their  Production 150 


CHAPTER    XII. 

COLOUR  AND  BRIGHTNESS  OF  LUNAR  DETAILS  :  CHRONOLOGY 
OF  FORMATIONS,  AND  FINALITY  OF  EXISTING  FEATURES. 

Absence  of  Conspicuous  Colour — Slight  tints  of  "  Seas  " — Cause — Probable 
Variety  of  Tints  in  small  Patches — Diversity  of  Brightness  of  Details 
— Most  Conspicuous  at  Full  Moon— Classification  of  Shades — Exag- 
gerated Contrasts  in  Photographs — Brightest  Portions  probably  the 
latest  formed — Chronology  of  Formations — Large  Craters  older  than 
Small — Mountains  older  than  Craters — Bright  Streaks  comparatively 
recent — Cracks  most  recent  of  all  Features — Question  of  existing 
Change — Evidence  from  Observation — Paucity  of  such  Evidence — 
Supposed  Case  of  Linne — Theoretical  Discussion — Relative  Cooling 
Tendencies  of  Earth  and  Moon — Earth  nearly  assumed  its  Final 
Condition — Moon  probably  cooled  Ages  upon  Ages  ago — Possible  slight 
Changes  from  Solar  Heating — Disintegrating  Action  .        .        .161 


CHAPTER   XIII. 

THE  MOON  AS  A  WORLD  :  DAY  AND  NIGHT  UPON  ITS  SURFACE. 
Existence  of  Habitants  on  other  Planets — Interest  of  the  Question — Con- 
ditions of  Life — Absence  of  these  from  Moon — No  Air  or  Water  and 
intense  Heat  and  Cold — Possible  Existence  of  Protogerms  of  Life — A 
Day  on  the  Moon  imagined — Instructiveness  of  the  Realization — 
Length  of  Lunar  Day — No  Dawn  or  Twilight — Sudden  Appearance  of 
Light— Slowness  of  Sun  in  Rising— No  Atmospheric  Tints— Blackness 
•  of  Sky  and  Visibility  of  Stars  and  Fainter  Luminosities  at  Noon-Day — 
Appearance  of  the  Earth  as  a  Stationary  Moon — Its  Phases — Eclipse  of 
Sun  by  Earth — Attendant  Phenomena — Lunar  Landscape — Height 
Essential  to  secure  a  Point  of  View — Sunrise  on  a  Crater — Desolation 
of  Scene— No  Vestige  of  Life— Colour  of  Volcanic  Products— No  At- 
mospheric Perspective— Blackness  of  Shadows— Impressions  on  other 
Senses  than  Sight— Heat  of  Sun  untempered— Intense  Cold  in  Shade 
— Dead  Silence— No  Medium  to  conduct  Sound — Lunar  Afternoon 


CONTENTS.  XV 

PAGE 

and  Sunset — Night — The  Earth  a  Moon— Its  Size,  Eotation,  and 
Features — Shadow  of  Moon  upon  it — Lunar  Night-Sky — Constellations 
— Comets  and  Planets — No  Visible  Meteors — Bombardment  by  Dark 
Meteoric  Masses — Lunar  Landscape  by  Night — Intensity  of  Cold        .    175 


CHAPTER   XIV. 

THE  MOON  AS  A  SATELLITE  :   ITS  RELATION  TO  THE  EARTH 

AND  MAN. 

The  Moon  as  a  Luminary— Segoadary  Nature  of  Light-giving  Function- 
Primary  Office  as  a  Sanitary  Agent — Cleansing  Effects  of  the  Tides — 
Tidal  Rivers  and  Transport  thereby — The  Moon  a  "  Tug  " — Available 
Power  of  Tides— Tide-Mills— Transfer  of  Tidal  Power  Inland— The 
Moon  as  a  Navigator's  Guide— Longitude  found  by  the  Moon — Moon's 
Motions — Discovered  by  Observations — Grouped  into  Theories — Repre- 
sented by  Tables — The  Nautical  Almanac — The  Moon  as  a  Long- 
Period  Timekeeper — Reckoning  by  "  Moons  " — Eclipses  the  Starting- 
Points  of  Chronologies — Furnish  indisputable  Dates — Solar  Surround- 
ings revealed  by  Eclipses' when  Moon  screens  the  Sun — Solar  Corona — 
Moon  as  a  Medal  of  Creation,  a  Half -formed  "World — Abuses  of  the 
Moon — Superstitions — Erroneous  Ideas  regarding  Moonlight  betrayed 
by  Artists  and  Authors — The  Moon  and  the  Weather — Errors  and  Facts 
— ^Atmospheric  Tides — ^Warmth  from  Moon — Paradoxical  Effect  in 
cooling  the  Earth 193 


CHAPTER   XV. 
CONCLUDING  SUMMARY 208 


LIST  OF  PLATES, 


( To  face  each  other    33 


\  .  To  face  each  otlicr  101 


♦ 

PLATES.  PAGE 

Gassendi Frontispiece. 

I.    Ceatee  op  Vesuvius,  1864 Tofacexmge    29 

II.    Back  of  Hand 

III.  Sheivelled  Apple 

IV.  Full  Moon Tofaceimge    59 

V.    Pictuee  Map  of  the  Moon .    79 

VI.    Vesuvius  and  Neighbouehood  of  Naples  ) 

VII.      POETION  OF   THE   MoON'S   SUEFACE 

VIII.     COPEENICUS Tofaceimge  110 

IX.    The  Lunae  Apennines,  Aechimedes,  etc 114 

X.    Aeistotle  and  Eudoxus To  face  page  120 

XI.    Teiesneckee 124 

XII.    Theophilus,  Cyeillus,  and  Cathaeina 128 

XIII.  Ptolemy,.  Alphons,  Aezachael,  etc 132 

XIV.  Plato,  the  Valley  of  the  Alps,  Pico,  etc 136 

XV.    Meecatoe  and  Campanus 140 

XVI.    Tycho  and  its  Sueeoundings 144 

XVII.    Ideal  Sketch  of  Pico 148 

XVIII.    Glass  Globe,  Ceacked  by  Inteenal  Peessuee  \ 

\  To  face  each  other  151 
XIX.    Full  Moon ) 

XX.    WAEGENTIN Tofaceimge  154 

XXI.    Aeistaechus  and  Heeodotus 160 

XXII.      OVEELAPPING  CEATEES 166 

XXIII.     NOEMAL  LUNAE  CEATEE 176 

XXIV.    Aspect  op  an  Eclipse  of  the  Sun  as  it  would  appeae  as 

SEEN  FEOM  THE  MOON 184 

XXV.  Geoup  OP  Lunae  Mountains At  end. 


THE     MOON. 


CHAPTER    I. 


ON    THE    COSMICAL    ORIGIN    OF    THE    PLANETS    OF    THE 
SOLAR   SYSTEM. 

In  this  chapter  we  propose  to  treat  briefly  of  the  probable  forma- 
tion of  the  various  members  of  the  solar  system  from  matter  which 
previously  existed  in  space  in  a  condition  different  from  that  in 
which  we  at  present  find  it — i.e.,  in  the  form  of  planets  and 
satellites. 

It  is  almost  impossible  to  conceive  that  our  world  with  its 
satellite,  and  its  fellow  worlds  with  their  satellites,  and  also  the 
great  centre  of  them  all,  have  always,  from  the  commencement  of 
time,  possessed  their  present  form :  all  our  experiences  of  the 
working  of  natural  laws  rebel  against  such  a  supposition.  In 
every  phenomenon  of  nature  upon  this  earth — the  great  field  from 
which  we  must  glean  our  experiences  and  form  our  analogies — we 
see  a  constant  succession  of  changes  going  on,  a  constant  pro- 
gression from  one  stage  of  development  to  another  taking  place,  a 
perpetual  mutation  of  form  and  nature  of  the  same  material 
substance  occurring :  we  see  the  seed  transformed  into  the  plant, 
the  flower  into  the  fruit,  and  the  ovum  into  the  animal.  In  the 
inorganic  world  we  witness  the  operation  of  the  same  principle  ; 
but,  by  reason  of  their  slower  rate  of  progression,  the  changes 

9 


2  THE    MOON.  [chap.  i. 

there  are  manifested  to  us  rather  by  their  resulting  effects  than 
by  their  visible  course  of  operation.  And  when  we  consider,  as  we 
are  obliged  to  do,  that  the  same  laws  work  in  the  greatest  as  well 
as  the  smallest  processes  of  nature,  we  are  compelled  to  believe  in 
an  antecedent  state  of  existence  of  the  matter  that  composes  the 
host  of  heavenly  bodies,  and  amongst  them  the  earth  and  its 
attendant  moon. 

In  the  pursuit  of  this  course  of  argument  we  are  led  to  inquire 
whether  there  exists  in  the  universe  any  matter  from  which 
planetary  bodies  could  be  formed,  and  how  far  their  formation 
from  such  matter  can  be  explained  by  the  operation  of  known 
material  laws. 

Before  the  telescope  revealed  the  hidden  wonders  of  the  skies, 
and  brought  its  rich  fruits  into  our  garner  of  knowledge  concerning 
the  nature  of  the  universe,  the  philosophic  minds  of  some  early 
astronomers,  Kepler  and  Tycho  Brahe  to  wit,  entertained  the  idea 
that  the  sun  and  the  stars — the  suns  of  distant  systems — were 
formed  by  the  condensation  of  celestial  vapours  into  spherical 
bodies ;  Kepler  basing  his  opinion  on  the  phenomena  of  the 
sudden  shining  forth  of  new  stars  on  the  margin  of  the  Milky 
Way.  But  it  was  when  the  telescope  pierced  into  the  depths  of 
celestial  space,  and  brought  to  light  the  host  of  those  marvellous 
objects,  the  nebulae,  that  the  strongest  evidence  was  afforded  of  the 
probable  validity  of  these  suppositions.  The  mention  of  "  nebulous 
stars"  made  by  the  earlier  astronomers  refers  only  to  clusters  of 
telescopic  stars  which  the  naked  eye  perceives  as  small  patches  of 
nebulous  light ;  and  it  does  not  appear  that  even  the  nebula  in 
Andromeda,  although  so  plainly  discernible  as  to  be  often  now-a- 
days  mistaken  by  the  uninitiated  for  a  comet,  was  known,  until  it 
was  discovered  by  means  of  a  telescope,  in  1612,  by  Simon  Marius, 
who  described  it  as  resembling  a  candle  shining  through  semi-trans- 
parent horn,  as  in  a  lantern,  and  without  any  appearance  of  stars. 
Forty  years  after  this  date  Huygens  discovered  the  splendid  nebula 
iu  the  sword  handle  of  Orion,  and  in  1665  another  was  detected 


CHAP.  I.]        C08MICAL    ORIGIN    OF   PLANETARY    SYSTEM.  3 

by  Hevelius.  In  1667  Halley  (afterwards  Astronomer  Royal), 
discovered  a  fourth;  a  fifth  was  found  by  Kirsch  in  1681,  and  a 
sixth  by  Halley  again  in  1714.  Half  a  century  after  this  the 
labours  of  Messier  expanded  the  list  of  known  nebulae  and  clusters 
to  103,  a  catalogue  of  which  appeared  in  the  '*  Connaissance  du 
Temps"  (the  French  **  Nautical  Almanac")  for  the  years  1783 — 
1784.  But  this  branch  of  celestial  discovery  achieved  its  most 
brilliant  results  when  the  rare  penetration,  the  indomitable  per- 
severance, and  the  powerful  instruments  of  the  elder  Herschel 
were  brought  to  bear  upon  it.  In  the  year  1779  this  great 
astronomer  began  to  search  after  nebulse  with  a  seven-inch  reflector, 
which  he  subsequently  superseded  by  the  great  one  of  forty  feet 
focus  and  four  feet  aperture.  In  1786  he  published  his  first  cata- 
logue of  1000  nebulsB ;  three  years  later  he  astonished  the  learned 
world  by  a  second  catalogue  containing  1000  more,  and  in  1802  a 
third  came  forth  comprising  other  500,  making  2500  in  all !  This 
number  has  been  so  far  increased  by  the  labours  of  more  recent 
astronomers  that  the  last  complete  catalogue,  that  of  Sir  John 
Herschel,  published  a  few  years  ago,  contains  the  places  of  5063 
nebulae  and  clusters. 

At  the  earlier  periods  of  Herschel's  observations,  that  illustrious 
observer  appears  to  have  inclined  to  the  belief  that  all  nebulae  were 
but  remote  clusters  of  stars,  so  distant,  so  faint,  and  so  thickly 
agglomerated  as  to  afiectthe  eye  only  by  their  combined  luminosity, 
and  at  this  period  of  the  nebular  history  it  was  supposed  that 
increased  telescopic  power  would  resolve  them  into  their  component 
stars.  But  the  familiarity  which  Herschel  gained  with  the  phases 
of  the  multitudinous  nebulae  that  passed  in  review  before  his  eyes, 
led  him  ultimately  to  adopt  the  opinion,  advanced  by  previous 
philosophers,  that  they  were  composed  of  some  vapoury  or  elemen- 
tary matter  out  of  which,  by  the  process  of  condensation,  the 
heavenly  bodies  were  formed ;  and  this  led  him  to  attempt  a 
classification  of  the  known  nebulae  into  a  cosmical  arrangement,  in 
which,  regarding  a  chaotic  mass  of  vapoury  matter  as  the  primordial 


4  THE    MOON.  [chap.  i. 

state  of  existence,  he  arranged  them  into  a  series  of  stages  of 
progressive  development,  the  individuals  of  one  class  heing  so 
nearly  allied  to  those  in  the  next  that,  to  use  his  own  expression, 
not  so  much  difference  existed  between  them  "  as  there  would  be 
in  an  annual  description  of  the  human  figure  were  it  given  from 
the  birth  of  a  child  till  he  comes  to  be  a  man  in  his  prime." 
{Philosophical  Transactions,  Vol.  CI.,  pp.  271,  et  seq.) 

His  category  comprises  upwards  of  thirty  classes  or  stages  of 
progression,  the  titles  of  a  few  of  which  we  insert  here  to  illustrate 
the  completeness  of  his  scheme. 

Class  1.  Of  extensive  diffused  nebulosity.     (A  table  of  52  patches 
of  such  nebulosity  actually  observed  is  given,  some  of 
which  extend  over  an  area  of  five  or  six  square  degrees, 
and  one  of  which  occupies  nine  square  degrees.) 
6.  Of  milky  nebulosity  with  condensation. 
15.  Of  nebulae  that  are  of  an  irregular  figure. 
17.  Of  round  nebulae. 

20.  Of  nebulae  that  are  gradually  brighter  in  the  middle. 
25.  Of  nebulae  that  have  a  nucleus. 

29.  Of  nebulae  that  draw  progressively  towards  a  period  of 
final  condensation. 

30.  Of  planetary  nebulae. 
33.  Of  stellar  nebulae  nearly  approaching  the  appearance  of 

stars. 

In  a  walk  through  a  forest  we  see  trees  in  every  stage  of  growth, 
from  the  tiny  sapling  to  the  giant  of  the  woods,  and  no  doubt  can 
exist  in  our  minds  that  the  latter  has  sprung  from  the  former. 
We  cannot  at  a  passing  glance  discern  the  process  of  development 
actually  going  on  ;  to  satisfy  ourselves  of  this,  we  must  record  the 
appearance  of  some  single  tree  from  time  to  time  through  a  long 
series  of  years.  And  what  a  walk  through  a  forest  is  to  an 
observer  of  the  growth  of  a  tree,  a  lifetime  is  to  the  observer  of 


CHAP.  I.]         C08MICAL    ORIGIN   OF   PLANETARY   SYSTEM,  5 

changes  in  such  objects  as  the  nebulae.  The  transition  from  one 
state  to  another  of  the  nebulous  development  is  so  slow  that  a  life- 
time is  hardly  sufficient  to  detect  it.  Nor  can  any  precise  evidence 
of  change  be  obtained  by  the  comparison  of  drawings  or  descriptions 
of  nebulae  at  various  epochs,  with  whatever  care  or  skill  such 
drawings  be  made,  for  it  will  be  admitted  that  no  two  draughtsmen 
will  produce  each  a  drawing  of  the  most  simple  object  from  the 
same  point  of  view,  in  which  every  detail  in  the  one  will  coincide 
exactly  with  every  detail  in  the  other.  There  is  abundant  evidence 
of  this  in  the  existing  representations  of  the  great  nebula  in  Orion  ; 
a  comparison  of  the  drawings  that  have  been  lately  made  of  this 
object,  with  the  most  perfect  instruments  and  by  the  most  skilful 
of  astronomical  draughtsmen,  reveals  varieties  of  detail  and  even  of 
general  appearance  such  as  could  hardly  be  imagined  to  occur  in 
similar  delineations  of  one  and  the  same  subject ;  and  any  one  who 
himself  makes  a  perfectly  unbiassed  drawing  at  the  telescope  will 
find  upon  comparison  of  it  with  others  that  it  will  offer  many  points 
of  difference.  The  fact  is  that  the  drawing  of  a  man,  like  his  pen- 
manship, is  a  personal  characteristic,  peculiar  to  himself,  and  the 
drawings  of  two  persons  cannot  be  expected  to  coincide  any  more 
than  their  handwritings.  The  appearance  of  a  nebula  varies  also 
to  a  great  extent  with  the  power  of  the  telescope  used  to  observe  it 
and  the  conditions  under  which  it  is  observed ;  the  drawings  of 
nebulae  made  with  the  inferior  telescopes  of  a  century  or  two 
centuries  ago,  the  only  ones  that,  by  comparison  with  those  made 
in  modern  times,  could  give  satisfactory  evidence  of  changes  of 
form  or  detail,  are  so  rude  and  imperfect  as  to  be  useless  for  the 
purpose,  and  it  is  reasonable  to  suppose  that  those  made  in  the 
present  day  will  be  similarly  useless  a  century  or  two  hence.  Since 
then  we  can  obtain  no  evidence  of  the  changes  we  must  assume 
these  mysterious  objects  to  be  undergoing,  ipso  facto,  by  observa- 
tion of  one  nebula  at  various  periods,  we  must  for  the  present 
accept  the  prima  facie  evidence  offered  (as  in  the  case  of  the  trees 
in  a  forest)  by  the  observation  of  various  nehulce  at  one  period. 


6  THE    MOON.  [chap.  i. 

"  The  total  dissimilitude,"  says  Herschel  at  the  close  of  the 
observations  we  have  alluded  to,  **  between  the  appearance  of  a 
diffusion  of  the  nebulous  matter  and  of  a  star,  is  so  striking,  that 
an  idea  of  the  conversion  of  the  one  into  the  other  can  hardly  occur 
to  any  one  who  has  not  before  him  the  result  of  the  critical 
examination  of  the  nebulous  system  which  has  been  displayed  in 
this  [his]  paper.  The  end  I  have  had  in  view,  by  arranging  my 
observations  in  the  order  in  which  they  have  been  placed,  has  been 
to  show  that  the  above-mentioned  extremes  may  be  connected  by 
such  nearly  allied  intermediate  steps,  as  will  make  it  highly  pro- 
bable that  every  succeeding  state  of  the  nebulous  matter  is  the 
result  of  the  action  of  gravitation  upon  it  while  in  a  foregoing  one, 
and  by  such  steps  the  successive  condensation  of  it  has  been 
brought  up  to  the  planetary  condition.  From  this  the  transit  to 
the  stellar  form,  it  has  been  shown,  requires  but  a  very  small  addi- 
tional compression  of  the  nebulous  matter." 

Where  the  researches  of  Herschel  terminated  those  of  Laplace 
commenced.  Herschel  showed  how  a  mass  of  nebulous  matter  so 
diffused  as  to  be  scarcely  discernible  might  be  and  probably  was,  by 
the  mere  action  of  gravitation,  condensed  into  a  mass  of  compara- 
tively small  dimensions  when  viewed  in  relation  to  the  immensity 
of  its  primordial  condition.  Laplace  demonstrated  how  the  known 
laws  of  gravitation  could  and  probably  did  from  such  a  partially 
condensed  mass  of  matter  produce  an  entire  planetary  system  with 
all  its  subordinate  satellites. 

The  first  physicist  who  ventured  to  account  for  the  formation  of 
the  various  bodies  of  our  solar  system  was  Buffon,  the  celebrated 
French  naturalist.  His  theory,  which  is  fully  detailed  in  his 
renowned  work  on  natural  history,  supposed  that  at  some  period 
of  remote  antiquity  the  sun  existed  without  any  attendant  planets, 
and  that  a  comet  having  dashed  obliquely  against  it,  ploughed  up 
and  drove  off  a  portion  of  its  body  sufficient  in  bulk  to  form  the 
various  planets  of  our  system.  He  suggests  that  the  matter  thus 
carried  off  "  at  first  formed  a  torrent  the  grosser  and  less  dense 


CHAP.  I.]         C08MICAL    ORIGIN    OF   PLANETARY    SYSTEM.  1 

parts  of  which  were  driven  the  farthest,  and  the  densest  parts, 
having  received  only  the  like  impulsion,  were  not  so  remotely 
removed,  the  force  of  the  sun's  attraction  having  retained  them :  " 
that  "  the  earth  and  planets  therefore  at  the  time  of  their  quitting 
the  sun  were  burning  and  in  a  state  of  liquefaction;  "  that  '*  by 
degrees  they  cooled,  and  in  this  state  of  fluidity  they  took  their 
form."  He  goes  on  to  say  that  the  obliquity  of  the  stroke  of  the 
comet  might  have  been  such  as  to  separate  from  the  bodies  of  the 
principal  planets  small  portions  of  matter,  which  would  preserve 
the  same  direction  of  motion  as  the  principal  planets,  and  thus 
would  form  their  attendant  satellites. 

The  hypothesis  of  BufFon,  however,  is  not  sufficient  to  explain  all 
the  phenomena  of  the  planetary  system ;  and  it  is  imperfect,  inas- 
much as  it  begins  by  assuming  the  sun  to  be  already  existing, 
whereas  any  theory  accounting  for  the  primary  formation  of  the 
solar  system  ought  necessarily  to  include  the  origination  of  the 
most  important  body  thereof,  the  sun  itself.  Nevertheless,  it  is 
but  due  to  Buffon  to  mention  his  ideas,  for  the  errors  of  one 
philosophy  serve  a  most  useful  end  by  opening  out  fields  of  inquiry 
for  subsequent  and  more  fortunate  speculators. 

Laplace,  dissatisfied  with  Bufi'on's  theory,  sought  one  more  pro- 
bable, and  thus  was  led  to  the  proposition  of  the  celebrated  nebular 
hypothesis  which  bears  his  name,  and  which,  in  spite  of  its  dis- 
believers, has  never  been  overthrown,  but  remains  the  only  pro- 
bable, and,  with  our  present  knowledge,  the  only  possible  explana- 
tion of  the  cosmical  origin  of  the  planets  of  our  system.  Although 
Laplace  puts  forth  his  conjectures,  to  use  his  own  words,  *'  with 
the  deference  which  ought  to  inspire  everything  that  is  not  a  result 
of  observation  and  calculation,"  yet  the  striking  coincidence  of  all 
the  planetary  phenomena  with  the  conditions  of  his  system  gives 
to  those  conjectures,  again  to  use  his  modest  language,  "  a  pro- 
bability strongly  approaching  certitude." 

Laplace  conceived  the  sun  to  have  been  at  one  period  the  nucleus 
of  a  vast  nebula,  the   attenuated   surrounding  matter  of  which 


8  THE    MOON.  [chap.  i. 

extended  beyond  what  is  now  the  orbit  of  the  remotest  planet  of  the 
system.  He  supposed  that  this  mass  of  matter  in  process  of  con- 
densation possessed  a  rotatory  motion  round  its  centre  of  gravity, 
and  that  the  parts  of  it  that  were  situated  at  the  limits  where 
centrifugal  force  exactly  counterbalanced  the  attractive  force  of  the 
nucleus  were  abandoned  by  the  contracting  mass,  and  thus  were 
formed  successively  a  number  of  rings  of  matter  concentric  with 
and  circulating  around  the  central  nucleus.  As  it  would  be  impro- 
bable that  all  the  conditions  necessary  to  preserve  the  stability  of 
such  rings  of  matter  in  their  annular  form  could  in  all  cases  exist, 
they  would  break  up  into  masses  which  would  be  endued  with  a 
motion  of  rotation,  and  would  in  consequence  assume  a  spheroidal 
form.  These  masses,  which  hence  constituted  the  various  planets, 
in  their  turn  condensing,  after  the  manner  of  the  parent  mass,  and 
abandoning  their  outlying  matter,  would  become  surrounded  by 
similarly  concentric  rings,  which  would  break  up  and  form  the 
satellites  surrounding  the  various  planetary  masses  ;  and,  as  a 
remarkable  exception  to  the  rule  of  the  instability  of  the  rings  and 
their  consequent  breakage,  Laplace  cited  the  case  of  Saturn  sur- 
rounded by  his  rings  as  the  only  instances  of  unbroken  rings  that 
the  whole  system  offers  us ;  unless  indeed  we  include  the  zodiacal 
light,  that  cone  of  hazy  luminosity  that  is  frequently  seen  stream- 
ing from  our  luminary  shortly  before  and  after  sunset,  and  which 
Laplace  supposed  to  be  formed  of  molecules  of  matter,  too  volatile 
to  unite  either  with  themselves  or  with  the  planets,  and  which 
must  hence  circulate  about  the  sun  in  the  form  of  a  nebulous 
ring,  and  with  such  an  appearance  as  the  zodiacal  actually 
presents. 

This  hypothesis,  although  it  could  not  well  be  refuted,  has  been 
by  many  hesitatingly  received,  and  for  a  reason  which  was  at  one 
time  cogent.  In  the  earlier  stages  of  nebular  research  it  was 
clearly  seen,  as  we  have  previously  remarked,  that  many  of  the 
so-called  nebulae,  which  appeared  at  first  to  consist  of  masses  of 
vapouiy   matter,   became,    when    scrutinised    with    telescopes    of 


CHAP.  I.]         COSMIC AL    ORIGIN    OF   PLANETARY    SYSTEM.  9 

higher  power,  resolved  into  clusters  containing  countless  numbers 
of  stars,  so  small  and  so  closely  agglomerated,  that  their  united 
lustre  only  impressed  the  more  feeble  eye  as  a  faint  nebulosity ; 
and  as  it  was  found  that  each  accession  of  telescopic  power 
increased  the  numbers  of  nebulae  that  were  thus  resolved,  it  was 
thought  that  every  nebula  would  at  some  period  succumb  to  the 
greater  penetration  of  more  powerful  instruments ;  and  if  this  were 
the  case,  and  if  no  real  nebulae  were  hence  found  to  exist,  how,  it 
was  argued,  could  the  nebular  hypothesis  be  maintained  ?  One  of 
the  most  important  nebulae  bearing  upon  this  question  was  the 
great  one  in  the  sword  handle  of  Orion,  one  of  the  grandest  and 
most  conspicuous  in  the  whole  heavens.  On  account  of  the  bright- 
ness of  some  portions  of  this  object,  it  seemed  as  though  it  ought 
to  be  readily  resolvable,  supposing  all  nebulae  to  consist  of  stars, 
but  all  attempts  to  resolve  it  were  in  vain,  even  with  the  powerful 
telescopes  of  Sir  John  Herschel  and  the  clear  zenethal  sky  of  the 
Cape  of  Good  Hope.  At  length  the  question  was  thought  to  be 
settled,  for  upon  the  completion  of  Lord  Kosse's  giant  reflector, 
and  upon  examination  of  the  nebula  with  it,  his  lordship  stated 
that  there  could  be  little,  if  any,  doubt  as  to  its  resolvability,  and 
then  it  was  maintained,  by  the  disbelievers  in  the  nebular  theory, 
that  the  last  stronghold  of  that  theory  had  been  broken 
down. 

But  the  truths  of  nature  are  for  ever  playing  at  hide  and  seek 
with  those  who  follow  them : — the  dogmas  of  one  era  are  the 
exploded  errors  of  the  next.  Within  the  past  few  years  a  new 
science  has  arisen  that  furnishes  us  with  fresh  powers  of  penetra- 
tion into  the  vast  and  secret  laboratories  of  the  universe ;  a  new 
eye,  so  to  speak,  has  been  given  us  by  which  we  may  discern,  by 
the  mere  light  that  emanates  from  a  celestial  body,  something  of 
the  chemical  elements  of  which  it  is  composed.  When  Newton 
two  hundred  years  ago  toyed  with  the  prism  he  bought  at  Stour- 
bridge fair,  and  projected  its  pretty  rainbow  tints  upon  the  wall, 
his    great  mind    little   suspected  that  that   phantom   riband  of 


10  THE    MOON.  [chap.  i. 

gorgeous  colours  would  one  day  be  called  upon  to  give  evidence 
upon  the  probable  cosmical  origin  of  worlds.  Yet  such  in  truth 
has  been  the  case.  Every  substance  when  rendered  luminous 
gives  off  light  of  some  colour  or  degree  of  refrangibility  peculiar 
to  itself,  and  although  the  eye  cannot  detect  any  difference  between 
one  character  of  light  and  another,  the  prism  gives  the  means  of 
ascertaining  the  quality  and  degree  of  refrangibility  of  the  light 
emanating  from  any  source  however  distant,  and  hence  of  gaining 
some  knowledge  of  the  nature  of  that  source.  If,  for  instance,  a 
ray  of  light  from  a  solid  body  in  combustion  is  passed  through  a 
prism,  a  spectrum  is  produced  which  exhibits  light  of  all  colours  or 
all  degrees  of  refrangibility ;  if  the  light  from  such  a  body,  before 
passing  through  the  prism,  be  made  to  pass  through  gases  or 
certain  metallic  vapours,  the  resulting  spectrum  is  found  to  be 
crossed  transversely  by  numbers  of  fine  dark  lines,  apparently 
separating  the  various  colours,  or  cutting  the  spectrum  into  bands. 
The  solar  spectrum  is  of  this  class ;  the  once  mysterious  lines  first 
observed  by  WoUaston,  and  subsequently  by  Fraunhofer,  and  known 
as  **  Fraunhofer 's  lines,"  have  now  been  interpreted,  chiefly  by  the 
sagacious  German  chemist  Kirchhoff,  and  identified  as  the  effects 
of  absorption  of  certain  of  the  sun's  rays  by  chemical  vapours  con- 
tained in  his  atmosphere.  The  fixed  stars  yield  spectra  of  the 
same  character,  but  varying  considerably  in  feature,  the  lines 
crossing  the  stella  spectra  differing  in  position  and  number  from 
those  of  the  sun,  and  one  star  from  another,  proving  the  stars  to 
possess  varied  chemical  constitutions.  But  there  is  another  class 
of  spectra,  exhibited  when  light  from  other  sources  is  passed 
through  the  prism.  These  consist,  not  of  a  luminous  riband  of 
light  like  the  solar  spectrum,  but  of  bright  isolated  lines  of  coloured 
light  with  comparatively  wide  dark  spaces  separating  them.  Such 
spectra  are  yielded  only  by  the  light  emitted  from  luminous  gases 
and  metals  or  chemical  elements  in  the  condition  of  incandescent 
vapour.  Every  gas  or  element  in  the  state  of  luminous  vapour 
yields  a  spectrum  peculiar  to  itself,  and  no  two  elements  when 


CHAP.  I.]         C08MICAL    ORIGIN    OF   PLANETARY   SYSTEM.  11 

vaporized  before  the  prism  show  the  same  combinations  of  luminous 
lines. 

Now  iu  the  course  of  some  observations  upon  the  spectra  of  the 
fixed  stars  by  Dr.  Huggins,  it  occurred  to  that  gentleman  to  turn 
his  telescope,  armed  with  a  spectroscope,  upon  some  of  the  brighter 
of  the  nebulae,  and  great  was  his  surprise  to  find  that  instead  of 
yielding  continuous  spectra,  as  they  must  have  done  had  their  light 
been  made  up  of  that  of  a  multitude  of  stars,  they  gave  spectra 
containing  only  two  or  three  isolated  bright  lines ;  such  a  spectrum 
could  only  be  produced  by  some  luminous  gas  or  vapour,  and  of 
this  form  of  matter  we  are  now  justified  in  declaring,  upon  the 
strength  of  numerous  modern  observations,  these  remarkable  bodies 
are  composed ;  and  it  is  a  curious  and  interesting  fact  that  some  of 
the  nebulae  styled  resolvable,  from  the  fact  of  their  exhibiting 
points  of  light  like  stars,  yield  these  gaseous  spectra,  whence  Dr. 
Huggins  concludes  that  the  brighter  points  taken  for  stars  are  in 
reality  nuclei  of  greater  condensation  of  the  nebular  matter :  and  so 
the  fact  of  the  apparent  resolvability  of  a  nebula  affords  no  positive 
proof  of  its  non- nebulous  character. 

These  observations — which  have  been  fully  confirmed  by  Father 
Secchi  of  the  Roman  College — by  destroying  the  evidence  in  favour 
of  nebulae  being  remote  clusters,  add  another  attestation  to  the 
probability  of  the  truth  of  the  nebular  hypothesis,  and  we  have  now 
the  confutation  of  the  luminologist  to  add  to  that  of  the  astronomers 
who,  in  the  person  of  the  illustrious  Arago,  asserted  that  the  ideas 
of  the  great  author  of  the  '*  Mecanique  Celeste  "  **  were  those  only 
which  by  their  grandeur,  their  coherence,  and  their  mathematical 
character  could  be  truly  considered  as  forming  a  physical  cos- 
mogony." 

Confining,  then,  our  attention  to  the  single  object  of  the  universe 
it  is  our  task  to  treat  of — the  Moon — and  without  asserting  as  an 
indisputable  fact  that  which  we  can  never  hope  to  know  otherwise 
than  by  inference  and  analogy,  we  may  assume  that  that  body  once 
existed  in  the  form  of  a  vast  mass  of  diffused  or  attenuated  matter, 


12  THE    MOON.  [chap.  i. 

and  that,  by  the  action  of  gravitation  upon  the  particles  of  that 
matter,  it  was  condensed  into  a  comparatively  small  and  compact 
planetary  body. 

But  while  the  process  of  condensation  or  compaction  was  going 
on,  another  important  law  of  nature — but  recently  unfolded  to  our 
knowledge — was  in  powerful  operation,  the  discussion  of  which  law 
we  reserve  for  a  separate  Chapter. 


CHAPTER    II. 

THE    GENERATION    OF    COSMICAL    HEAT. 

In  the  preceding  Chapter  we  endeavoured  to  show  how  the 
action  of  gravitation  upon  the  particles  of  diffused  primordial  matter 
would  result  in  the  formation,  by  condensation  and  aggregation,  of 
a  spherical  planetary  body.  We  have  now  to  consider  another 
result  of  the  gravitating  action,  and  for  this  we  must  call  to  our  aid 
a  branch  of  scientific  enquiry  and  investigation  unrecogi^ized  as 
such  at  the  period  of  Laplace's  speculations,  and  which  has 
been  developed  almost  entirely  within  the  past  quarter  of  a 
century. 

The  "  great  philosophical  doctrine  of  the  present  era  of  science," 
as  the  subject  about  to  engage  our  attention  has  been  justly  termed, 
bears  the  title  of  the  "  Conservation  of  Force,"  or — as  some 
ambiguity  is  likely  to  attend  the  definition  of  the  term  "  Force  " — 
the  "  Conservation  of  Energy."  The  basis  of  the  doctrine  is  the 
broad  and  comprehensive  natural  law  which  teaches  us  that  the 
quantity  of  force  comprised  by  the  universe,  like  the  quantity  of 
matter  contained  in  it,  is  a  fixed  and  invariable  amount,  which  can 
be  neither  added  to  nor  taken  from,  but  which  is  for  ever  under- 
going change  and  transformation  from  one  form  to  another.  That 
we  cannot  create  force  ought  to  be  as  obvious  a  fact  as  that  we 
cannot  create  matter  ;  and  what  we  cannot  create  we  cannot  destroy. 
As  in  the  universe  we  see  no  new  matter  created,  but  the  same 
matter  constantly  disappearing  from  one  form  and  reappearing  in 


14  THE    MOON.  [chap.  ii. 

another,  so  we  can  find  no  new  force  ever  coming  into  action — no 
description  of  force  that  is  not  to  be  referred  to  some  previous 
manner  of  existence. 

Without  entering  upon  a  metaphysical  discussion  of  the  term 
*'  force,"  it  will  be  sufficient  for  our  purpose  to  consider  it  as  some- 
thing which  produces  or  resists  motion,  and  hence  we  may  argue 
that  the  ultimate  effect  of  force  is  motion.  The  force  of  gravity  on 
the  earth  results  in  the  motion  or  tendency  of  all  bodies  towards  its 
centre,  and  similarly,  the  action  of  gravitation  upon  the  atoms  or 
particles  of  a  primeval  planet  resulted  in  the  motion  of  those 
particles  towards  each  other.  We  cannot  conceive  force  otherwise 
than  by  its  effects,  or  the  motion  it  produces. 

And  force  we  are  taught  is  indestructible ;  therefore  motion 
must  be  indestructible  also.  But  when  a  falling  body  strikes  the 
earth,  or  a  gun-shot  strikes  its  target,  or  a  hammer  delivers  a  blow 
upon  an  anvil,  or  a  brake  is  pressed  against  a  rotating  wheel, 
motion  is  arrested,  and  it  would  seem  natural  to  infer  that  it  is 
destroyed.  But  if  we  say  it  is  indestructible,  what  becomes  of  it  ? 
The  philosophical  answer  to  the  question  is  this — that  the  motion 
of  the  mass  becomes  transferred  to  the  particles  or  molecules 
composing  it,  and  transformed  to  molecular  motion,  and  this 
molecular  motion  manifests  itself  to  us  as  heat.  The  particles  or 
atoms  of  matter  are  held  together  by  cohesion,  or,  in  other  words, 
by  the  action  of  molecular  attraction.  When  heat  is  applied  to 
these  particles,  motion  is  set  up  among  them,  they  are  set  in 
vibration,  and  thus,  requiring  and  making  wider  room,  they  urge 
each  other  apart,  and  the  well-known  expansion  by  heat  is  the 
result.  If  the  heat  be  further  continued  a  more  violent  molecular 
motion  ensues,  every  increase  of  heat  tending  to  urge  the  atoms 
further  apart,  till  at  length  they  overcome  their  cohesive  attraction 
and  move  about  each  other,  and  a  liquid  or  molten  condition  results. 
If  the  heat  be  still  further  increased,  the  atoms  break  away  from 
their  cohesive  fetters  altogether  and  leap  off  the  mass  in  the  form 
of  vapour,  and  the  matter  thus  assumes  the  gaseous  or  vaporous 


CHAP.  II.]  THE    GENERATION    OF    C08MICAL    HEAT.  15 

form.     Thus  we  see  that  the  phenomena  of  heat  are  phenomena  of 
motion,  and  of  motion  only. 

This  mutual  relation  between  heat  and  work  presented  itself  as 
an  embryo  idea  to  the  minds  of  several  of  the  earlier  philosophers, 
by  whom  it  was  maintained  in  opposition  to  the  material  theory 
which  held  heat  to  be  a  kind  of  matter  or  subtle  fluid  stored  up  in 
the  inter-atomic  spaces  of  all  bodies,  capable  of  being  separated 
and  procured  from  them  by  rubbing  them  together,  but  not 
generated  thereby.  Bacon,  in  his  **  Novum  Organum,"  says  that 
*'  heat  itself,  its  essence  and  quiddity,  is  motion  and  nothing  else." 
Locke  defines  heat  as  ''a  very  brisk  agitation  of  the  insensible 
parts  of  an  object,  which  produces  in  us  that  sensation  from  whence 
we  denominate  the  object  hot ;  so  what  in  our  sensation  is  heat,  in 
the  object  is  nothing  but  motion.''  Descartes  and  his  followers 
upheld  a  similar  opinion.  Eichard  Boyle,  two  hundred  years  ago, 
actually  wrote  a  treatise  entitled  *'  The  Mechanical  Theory  of  Heat 
and  Cold,"  and  the  ingenious  Count  Eumford  made  some  highly 
interesting  and  significant  experiments  on  the  subject,  which  are 
described  in  a  paper  read  before  the  Koyal  Society  in  1798,  entitled 
"  An  Inquiry  concerning  the  Source  of  Heat  excited  by  Friction." 
But  the  conceptions  of  these  authors  remained  isolated  and  un- 
fruitful for  more  than  a  century,  and  might  have  passed,  meantime, 
into  the  oblivion  of  barren  speculation,  but  for  the  impulse  which 
this  branch  of  inquiry  has  lately  received.  Now,  however,  they 
stand  forth  as  notable  instances  of  truth  trying  to  force  itself  into 
recognition  while  yet  men's  minds  were  unprepared  or  disinclined 
to  receive  it.  The  key  to  the  beautiful  mechanical  theory  of  heat 
was  found  by  these  searching  minds,  but  the  unclasping  of  the  lock 
that  should  disclose  its  beauty  and  value  was  reserved  for  the 
philosophers  of  the  present  age. 

Simultaneously  and  independently,  and  without  even  the  know- 
ledge of  each  other,  three  men,  far  removed  from  probable  inter- 
course, conceived  the  same  ideas  and  worked  out  nearly  similar 
results  concerning  the   mechanical  theory  of  heat.     Seeing  that 


16  THE    MOON.  [chap.  ii. 

motion  was  convertible  into  heat,  and  heat  into  motion,  it  became 
of  the  utmost  importance  to  determine  the  exact  relation  that 
existed  between  the  two  elements.  The  first  who  raised  the  idea 
to  philosophic  clearness  was  Dr.  Julius  Robert  Mayer,  a  physician 
of  Heilbronn  in  Germany.  In  certain  observations  connected  with 
his  medical  practice  it  occurred  to  him  that  there  must  be  a 
necessary  equivalent  between  work  and  heat,  a  necessary  numerical 
relation  between  them.  "  The  variations  of  the  diiBference  of  colour 
of  arterial  and  venous  blood  directed  his  attention  to  the  theory  of 
respiration.  He  soon  saw  in  the  respiration  of  animals  the  origin 
of  their  motive  powers,  and  the  comparison  of  animals  to  thermic 
machines  afterwards  suggested  to  him  the  important  principle  with 
which  his  name  will  remain  for  ever  connected." 

Next  in  order  of  publication  of  his  results  stands  the  name  of 
Colding,  a  Danish  engineer,  who  about  the  year  1843  presented  a 
series  of  memoirs  on  the  steam-engine  to  the  Royal  Society  of 
Copenhagen,  in  which  he  put  forth  views  almost  identical  with 
those  of  Mayer. 

Last  in  publication  order,  but  foremost  in  the  importance  of  his 
experimental  treatment  of  the  subject,  was  our  own  countryman. 
Dr.  Joule  of  Manchester.  ''  Entirely  independent  of  Mayer,  with 
his  mind  firmly  fixed  upon  a  principle,  and  undismayed  by  the 
coolness  with  which  his  first  labours  appear  to  have  been  received, 
he  persisted  for  years  in  his  attempts  to  prove  the  invariability  of 
the  relation  which  subsists  between  heat  and  ordinary  mechanical 
power."  (We  are  quoting  from  Professor  TyndalFs  valuable  work 
on  "  Heat  considered  as  a  Mode  of  Motion.")  "  He  placed  water 
in  a  suitable  vessel,  agitated  the  water  by  paddles,  and  determined 
both  the  amoimt  of  heat  developed  by  the  stirring  of  the  liquid  and 
the  amount  of  labour  expended  in  its  production.  He  did  the  same 
with  mercury  and  sperm  oil.  He  also  caused  discs  of  cast  iron  to 
rub  against  each  other,  and  measured  the  heat  produced  by  their 
friction,  and  the  force  expended  in  overcoming  it.  He  urged  water 
through  capillary  tubes,   and   determined    the  amount   of   heat 


CHAP,  n.]  THE    GENERATION    OF    COSMICAL    HEAT.  17 

generated  by  the  friction  of  the  liquid  against  the  sides  of  the 
tubes.  And  the  results  of  his  experiments  leave  no  shadow  of 
doubt  upon  the  mind  that,  under  all  circumstances,  the  quantity  of 
heat  generated  by  the  same  amount  of  force  is  fixed  and  invariable. 
A  given  amount  of  force,  in  causing  the  iron  discs  to  rotate  against 
each  other,  produced  precisely  the  same  amount  of  heat  as  when  it 
was  applied  to  agitate  water,  mercury,  or  sperm  oil.  *  *  *  * 
The  absolute  amount  of  heat  generated  by  the  same  expenditure  of 
power,  was  in  all  cases  the  same." 

"  In  this  way  it  was  found  that  the  quantity  of  heat  which  would 
raise  one  pound  of  water  one  degree  Fahrenheit  in  temperature,  is 
exactly  equal  to  what  would  be  generated  if  a  pound  weight,  after 
having  fallen  through  a  height  of  772  feet,  had  its  moving  force 
destroyed  by  collision  with  the  earth.  Conversely,  the  amount  of 
heat  necessary  to  raise  a  pound  of  water  one  degree  in  temperature, 
would,  if  all  applied  mechanically,  be  competent  to  raise  a  pound 
weight  772  feet  high,  or  it  would  raise  772  pounds  one  foot  high. 
The  term  *  foot-pounds '  has  been  introduced  to  express  in  a  con- 
venient way  the  lifting  of  one  pound  to  the  height  of  a  foot.  Thus 
the  quantity  of  heat  necessary  to  raise  the  temperature  of  a  pound 
of  water  one  degree  Fahrenheit  being  taken  as  a  standard,  772 
foot-pounds  constitute  what  is  called  the  mechanical  equivalent  of 
heat." 

By  a  process  entirely  different,  and  by  an  independent  course  of 
reasoning,  Mayer  had,  a  few  months  previous  to  Joule,  determined 
this  equivalent  to  be  771*4  foot-pounds.  Such  a  remarkable  coin- 
cidence arrived  at  by  pursuing  different  routes  gives  this  value  a 
strong  claim  to  accuracy,  and  raises  the  Mechanical  Theory  of  Heat 
to  the  dignity  of  an  exact  science,  and  its  enunciators  to  the 
foremost  place  in  the  ranks  of  physical  philosophers. 

In  linking  together  the  labours  of  the  two  remarkable  men  above 
alluded  to,  Prof.  Tyndall  remarks,  that  "  Mayer's  labours  have  in 
some  measure  the  stamp  of  profound  intuition,  which  rose  however 
to  the   energy  of  undoubting   conviction   in   the   authors   mind. 


18  THE   MOON.  [chap.  ii. 

Joule's  labours,  on  the  contrary,  are  an  experimental  demonstration. 
Mayer  thought  his  theory  out,  and  rose  to  its  grandest  applications. 
Joule  worked  his  theory  out,  and  gave  it  the  solidity  of  natural 
truth.  True  to  the  speculative  instinct  of  his  country,  Mayer  drew 
large  and  mighty  conclusions  from  slender  premises;  while  the 
Englishman   aimed   above  all  things  at  the  firm  establishment  of 

facts To  each  belongs  a  reputation  which  will 

not  quickly  fade,  for  the  share  he  has  had,  not  only  in  establishing 
the  dynamical  theory  of  heat,  but  also  in  leading  the  way  towards  a 
right  appreciation  of  the  general  energies  of  the  universe." 

But  from  these  generalities  we  must  pass  to  the  application  of 
the  mechanical  theory  of  heat  to  our  special  subject.  We  have 
learnt  that  every  form  of  motion  is  convertible  into  heat.  We 
know  that  the  falling  meteor  or  shooting  star,  whose  motion  is 
impeded  by  friction  against  the  earth's  atmosphere,  is  heated 
thereby  to  a  temperature  of  incandescence.  Let  us  then  suppose 
that  myriads  of  such  cosmical  particles  come  into  collision  from  the 
effect  of  their  mutual  attraction,  or  that  the  component  atoms  of  a 
vast  nebulous  mass  violently  converged  under  the  like  influence. 
What  would  follow  ?  Obviously  the  generation  of  an  intense  heat 
by  the  arrest  of  converging  motion,  such  a  heat  as  would  result  in 
the  fusion  of  the  whole  into  one  mass.  Mayer,  in  one  of  his  most 
remarkable  papers  ("  Celestial  Dynamics ")  remarks  that  the 
"  Newtonian  theory  of  gravitation,  whilst  it  enables  us  to  determine 
from  its  present  form,  the  earth's  state  of  aggregation  in  ages  past, 
at  the  same  time  points  out  to  us  a  source  of  heat  powerful  enough 
to  produce  such  a  state  of  aggregation — powerful  enough  to  melt 
worlds  :  it  teaches  us  to  consider  the  molten  state  of  a  planet  as  the 
result  of  the  mechanical  union  of  cosmical  masses,  and  to  derive 
the  radiation  of  the  sun  and  the  heat  in  the  bowels  of  the  earth 
from  a  common  origin." 

And  the  same  laws  that  governed  the  formation  of  the  earth,  gov- 
erned also  the  formation  of  the  moon :  the  variations  of  Nature's  oper- 
ations are  quantitative  only  and  not  qualitative.  The  Divine  Will  that 


CHAP.  II.]  THE    GENERATION    OF    C08MICAL    HEAT.  19 

made  the  earth  made  the  moon  also,  and  the  means  and  mode  of  work- 
ing were  the  same  for  both.  The  geological  phenomena  of  the  earth 
afford  unmistakeable  evidence  of  its  original  fluid  or  molten  condi- 
tion, and  the  appearance  of  the  moon  is  as  unmistakeahly  that  of  a 
body  once  in  an  igneous  or  molten  state.  The  enigma  of  the 
earth's  primary  formation  is  solved  by  the  application  of  the 
dynamical  theory  of  heat.  By  this  theory  the  generation  of  cos- 
mical  heat  is  removed  from  the  quicksands  of  conjecture  and 
established  upon  the  firm  ground  of  direct  calculation :  for  the 
absolute  amount  of  heat  generated  by  the  collision  of  a  given 
amount  of  matter  is  (of  course,  with  some  little  uncertainty) 
deducible  from  a  mathematical  formula.  Mayer  has  computed  the 
amount  of  heat  that  the  matter  of  the  earth  would  have  generated, 
if  it  had  been  formed  originally  of  only  two  parts  drawn  into 
collision  by  their  mutual  attraction,  and  has  found  that  it  would  be 
from  0  to  32,000  or  47,000*  Centigrade  degrees,  according  as  one 
part  was  infinitely  small  as  compared  with  the  other,  or  as  the 
two  parts  were  of  equal  size.  Professor  Helmholtz,  another 
labourer  in  the  same  field  of  science,  has  computed  the  amount  of 
heat  generated  by  the  condensation  of  the  whole  of  the  matter  com- 
posing the  solar  system  :  this  he  finds  would  be  equivalent  to  the 
hes-t  that  would  be  required  to  raise  the  temperature  of  a  mass  of 
water  equal  to  the  sum  of  the  masses  of  all  the  bodies  of  the 
system  to  28,000,000  (twenty-eight  million)  degrees  of  the 
Centigrade  scale. 

These  examples  afford  abundant  evidence  of  sufficient  heat  having 
been  generated  by  the  aggregation  of  the  matter  of  the  moon  to  re- 
duce it  to  a  state  of  fusion,  and  so  to  produce,  from  a  nebulous  chaos 
of  diffused  cosmical  matter,  a  molten  body  of  definite  outline  and  size. 

It  is  requisite  here  to  remark  that  fusion  does  not  necessarily 
imply  combustion.  It  has  been  frequently  asked.  How  can  a 
volcanic  theory  of  the  lunar  phenomena  be  upheld  consistently  with 
the  condition  that  it  possesses  no  atmosphere  to  support  fire  ?     To 

*  The  melting  temperature  of  iron  is  1500°  Centigrade. 

c  2 


20  THE    MOON,  [chap.  ii. 

this  we  would  reply  that  to  produce  a  state  of  incandescence  or  a 
molten  condition  it  is  not  necessary  that  the  hody  he  surrounded  hy 
an  atmosphere.  The  intensely  rapid  motion  of  the  particles  of 
matter  of  hodies,  which  the  dynamical  theory  shows  to  he  the 
origin  of  the  molten  state,  exists  quite  independently  of  such 
external  matter  as  an  atmosphere.  The  complex  mixture  of  gases 
and  vapours  which  we  term  **  air,"  has  nothing  whatever  to  do  with 
the  fusion  of  suhstances,  whatever  it  may  have  to  do  with  their 
combustion.  Combustion  is  a  chemical  phenomenon,  due  to  the 
combination  of  the  oxygen  of  that  air  with  the  heated  particles  of 
the  combustible  matter :  oxygen  is  the  sole  supporter  of  combus- 
tion, and  hence  combustion  is  to  be  regarded  rather  as  a  phenome- 
non of  oxygen  than  as  a  phenomenon  of  the  matter  with  which  that 
oxygen  combines.  The  greatest  intensity  of  heat  may  exist  without 
oxygen,  and  consequently  without  combustion.  In  support  of  this 
argument  it  will  be  sufficient  to  adduce,  upon  the  authority  of  Dr. 
Tyndall,  the  fact  that  a  platinum  wire  can  be  raised  to  a  luminous 
temperature  and  SLctn&Wy  fused  in  a  perfect  vacuum. 

But  while  the  mass  of  condensing  cosmical  matter  was  thus 
accumulating  and  forming  the  globe  of  the  moon,  the  heat  conse- 
quent upon^the  aggregation  of  its  particles  was  suffering  some 
diminution  from  the  effect  of  radiation.  So  long  as  the  radiated 
heat  lost  fell  short  of  the  dynamical  heat  generated,  no  effect  of 
cooling  would  be  manifest ;  but  when  the  vis  viva  of  the  condensing 
matter  was  all  converted  into  its  equivalent  of  heat,  or  when  the 
accession  of  heat  fell  short  of  that  radiated,  a  necessary  cooling  must 
ensue,  and  this  cooling  would  be  accompanied  by  a  soUdification  of 
that  part  of  the  mass  which  was  most  free  to  radiate  its  heat  into 
surrounding  space  :  that  part  would  obviously  be  the  outer  surface. 

With  the  solidification  of  this  external  crust  began  the  "  year 
one  "  of  selenological  history. 

The  phenomena  attendant  upon  the  cooling  of  the  mass  we  will 
consider  in  the  next  Chapter. 


CHAPTER    III. 

THE   SUBSEQUENT   COOLING   OF   THE    IGNEOUS    BODY. 

In  the  foregoing  chapters  we  have  endeavoured  to  show,  by 
the  light  of  modern  science,  first,  how  diffused  cosmical  matter 
was  probably  condensed  into  a  planetary  mass  by  the  mutual  gravita- 
tion of  its  particles,  and  secondly,  how,  the  after  destruction  of  the 
gravitative  force,  by  the  collision  of  the  converging  particles  of 
matter,  resulted  in  the  generation  of  such  sufficient  heat  as  to 
reduce  the  whole  mass  to  a  molten  condition.  Our  present  task 
is  to  consider  the  subsequent  cooling  of  the  mass,  and  the 
phenomena  attendant  upon  or  resulting  therefrom.  This  brief 
chapter  is  important  to  our  subject,  as  we  shall  have  frequent 
occasion  to  refer  to  the  leading  principle  we  shall  endeavour  to 
illustrate  in  it,  in  subsequently  treating  of  the  causes  to  which  the 
special  selenological  features  are  to  be  attributed. 

First,  then,  as  regards  the  cooling  of  the  igneous  mass  that  con- 
stituted the  moon  at  the  inconceivably  remote  period  when  possibly 
that  body  was  really  **  a  lesser  light  "  shining  with  a  luminosity  of 
its  own,  due  to  its  then  incandescent  state,  and  not  simply  a 
reflector,  as  it  is  now,  of  light  which  it  receives  from  the  sun. 
If  we  could  conceive  it  possible  that  the  igneous  mass  in  the  act  of 
cooling  parted  with  its  heat  from  the  central  part  first  and  so 
began  to  solidify  from  its  centre,  or  if  it  had  been  possible  for  the 
mass  to  have  cooled  uniformly  and  simultaneously  throughout  its 
whole  depth,  or  that  each  substratum  had  cooled  before  its  super- 
stratum, we  should  have  had  a  moon  whose  surface  would  have 


22  THE    MOON.  [chap.  hi. 

been  smooth  and  without  any  such  remarkable  asperities  and 
excrescences  as  are  now  presented  to  our  view.  But  these  sup- 
positions are  inadmissible :  on  the  contrary  we  are  compelled  to 
consider  that  the  portion  of  the  igneous  or  molten  body  that  first 
cooled  was  its  exterior  surface,  which,  radiating  its  heat  into 
surrounding  space,  became  solid  and  comparatively  cool  while  the 
interior  retained  its  hot  and  molten  condition.  So  that  at  this 
early  stage  of  the  moon's  history  it  existed  in  the  form  of  a  solid 
shell  inclosing  a  molten  interior. 

Now  at  this  period  of  its  formation,  the  moon's  mass,  partly 
cooled  and  solidified  and  partly  molten,  would  be  subject  ta  the 
influence  of  two  powerful  molecular  forces  :  the  first  of  these 
would  consist  in  the  diminution  of  bulk  or  contraction  of  volume 
which  accompanies  the  cooling  of  solidified  masses  of  previously 
molten  substances;  the  second  would  arise  from  a  phenomenon 
which  we  may  here  observe  is  by  no  means  so  generally  known  as 
from  its  importance  it  deserves  to  be  :  and  as  we  shall  have  fre- 
quent occasion  to  refer  to  it  as  one  of  the  chief  agencies  in  produc- 
ing the  peculiar  structural  characteristics  of  the  moon's  surface,  it 
may  be  well  here  to  give  a  few  examples  of  its  action,  that  our 
reference  to  it  hereafter  may  be  more  clearly  understood. 

The  broad  general  principle  of  the  phenomenon  here  referred  to 
is  this  : — that  fusible  substances  are  (with  a  few  exceptions)  spe- 
cifically heavier  while  in  their  molten  condition  than  in  the 
solidified  state,  or  in  other  words  that  molten  matter  occupies  less 
space,  weight  for  weight,  than  the  same  matter  after  it  has  passed 
from  the  melted  to  the  solid  condition.  It  follows  as  an  obvious 
corollary  that  such  substances  contract  in  bulk  in  fusing  or  melt- 
ing, and  expand  in  becoming  solid.  It  is  this  expansion  upon 
solidification  that  now  concerns  us. 

Water,  as  is  well  known,  increases  in  density  as  it  cools,  till  it 
reaches  the  temperature  of  SQ''  Fahrenheit,  after  which,  upon  a 
further  decrease  of  temperature,  its  density  begins  to  decrease,  or 
in  other  words  its  bulk  expands,  and  hence  the  well-known  fact  of 


CHAP.  III.]     THE  SUBSEQUENT  COOLING  OF  THE  IGNEOUS  BODY.     23 

ice  floating  in  water,  and  the  inconvenient  fact  of  water-pipes  bursting 
in  a  frost.  This  action  in  water  is  of  the  utmost  importance  in 
the  grand  economy  of  nature,  and  it  has  been  accepted  as  a  mar- 
vellous exception  to  the  general  law  of  substances  increasing  in 
density  (or  shrinking)  as  they  decrease  in  temperature.  Water  is, 
however,  by  no  means  the  exceptional  substance  that  it  has  been  so 
generally  considered.  It  is  a  fact  perfectly  familiar  to  iron- 
founders,  that  when  a  mass  of  solid  cast-iron  is  dropped  into  a  pot 
of  molten  iron  of  identical  quality,  the  solid  is  found  to  float 
persistently  upon  the  molten  metal — so  persistently  that  when  it 
is  intentionally  thrust  to  the  bottom  of  the  pot,  it  rises  again  the 
moment  the  submerging  agency  is  withdrawn.  As  regards  the 
amount  of  buoyancy  we  believe  it  may  be  stated  in  round  numbers 
to  be  at  least  two  or  three  per  cent.  It  has  been  suggested  by 
some  who  are  familiar  with  this  phenomenon  that  the  solid  mass 
may  be  kept  up  by  a  spurious  buoyancy  imparted  to  it  by  a  film  of 
adhering  air,  or  that  surface  impurities  upon  the  solid  metal  may 
tend  to  reduce  the  specific  gravity  of  the  mass  and  thereby  prevent 
it  sinking,  and  that  the  fact  of  floatation  is  not  absolutely  a  proof  of 
greater  specific  lightness.  But  in  controversion  of  the  suggestions, 
we  can  state  as  the  result  of  experiment  that  pieces  of  cast-iron 
which  have  had  their  surface  roughness  entirely  removed,  leaving 
the  bright  metal  exposed,  still  float  on  the  molten  metal,  and 
further  that  when,  under  the  influence  of  the  great  heat  of  the 
molten  mass,  the  solid  is  gradually  melted  away,  and  consequently 
any  possible  surface  impurities  or  adhering  air  must  necessarily 
have  been  removed,  the  remaining  portion  continues  to  float  to  the 
last.  The  inevitable  inference  from  this  is  that  in  the  case  of  cast- 
iron  the  solid  is  specifically  lighter  than  the  molten,  and,  therefore, 
that  in  passing  from  the  molten  to  the  solid  condition  this  substance 
undergoes  expansion  in  bulk. 

We  are  able  to  offer  a  confirmation  of  this  inference  in  the  case 
of  cast-iron  by  a  remarkable  phenomenon  well  known  to  iron- 
founders,  but   of  which  we  have   never  met  with  special  notice. 


24 


THE    MOON. 


[chap.  in. 


When  a  ladle  or  pot  of  molten  iron  is  drawn  from  the  melting 
furnace  and  allowed  to  stand  at  rest,  the  surface  presents  a  most 
remarkable  and  suggestive  appearance.  Instead  of  remaining  calm 
and  smooth  it  is  a  scene  of  a  lively  commotion :  the  thin  coat  of 
scoria  or  molten  oxide  which  forms  on  the  otherwise  bright  surface 
of  the  metal  is  seen,  as  fast  as  it  forms  at  the  circumference  of  the 
pot,  to  be  swept  by  active  convergent  currents  towards  the  centre, 


Fig.  1. 


where  it  accumulates  in  a  patch.  "While  this  action  is  proceeding, 
the  entire  upper  surface  of  the  metal  appears  as  if  it  were  covered 
with  animated  vermicules  of  scoria,  springing  into  existence  at  the 
circumference  of  the  pot,  and  from  thence  rapidly  streaming  and 
wriggling  themselves  towards  the  centre. 

Our  illustration  (Fig.  1)  is  intended,  so  far  as  such  means  can  do 
so,  to  convey  some  idea  of  this  remarkable  appearance  at  one  instant 
of  its  continued  occurrence.  To  interpret  our  illustration  rightly  it 
is  necessary  to  imagine  this  vermicular  freckling  to  be  constantly 


CHAP.  III.]     THE  SUBSEQUENT  COOLING  OP  THE  IGNEOUS  BODY.     25 

and  rapidly  streaming  from  all  points  of  the  periphery  of  the  pot 
towards  the  centre,  where,  as  we  have  said,  it  accumulates  in  the 
form  of  a  floating  island.  We  may  observe  that  the  motion  is  most 
rapid  when  the  hot  metal  is  first  put  into  the  cool  ladle :  as  the 
fluid  metal  parts  with  some  of  its  heat  and  the  ladle  gets  hot  by 
absorbing  it,  this  remarkable  surface  disturbance  becomes  less 
energetic. 


Fig.  2. 


Now  if  we  carefully  consider  this  peculiar  action  and  seek  a  cause 
for  the  phenomenon,  we  shall  be  led  to  the  conclusion  that  it  arises 
from  the  expansion  of  that  portion  of  the  molten  mass  which  is  in 
contact  with  or  close  proximity  to  the  comparatively  cool  sides  of 
the  ladle,  which  sides  act  as  the  chief  agent  in  dispersing  the  heat  of 
the  melted  metal.  The  motion  of  the  scoriae  betrays  that  of  the 
fluid  metal  beneath,  and  careful  observation  will  show  that  the 
motion  in  question  is  the  result  of  an  upward  current  of  the  metal 
around  the  circumference  of  the  ladle,  as  indicated  by  the  arrows  A, 
B,  c  in  the  accompanying  sectional  drawing  of  the  ladle  (Fig.  2). 


?6       ;  THE    MOON.  [chap.  hi. 

The  upward  current  of  the  metal  can  actually  be  seen  when  specially 
looked  for,  at  the  rim  of  the  pot,  where  it  is  deflected  into  the  con- 
vergent horizontal  direction  and  where  it  presents  an  elevatory 
appearance  as  shown  in  the  figure.  It  is  difficult  to  assign  to  this 
effect  any  other  cause  than  that  of  an  expansion  and  consequent 
reduction  of  the  specific  gravity  of  the  fluid  metal  in  contact  with 
or  in  close  proximity  to  the  cooler  sides  of  the  pot,  as,  according 
to  the  generally  entertained  idea  that  contraction  universally 
accompanies  cooling,  it  would  be  impossible  for  the  cooler  to  float 
on  the  hotter  metal,  and  the  curious  surface-currents  above  referred 
to  would  be  in  contrary  direction  to  that  which  they  invariably  take, 
i.e.,  they  would  diverge  from  the  centre  instead  of  converging  to  it. 
The  external  arrows  in  the  figure  represent  the  radiation  of  the  heat 
from  the  outer  sides  of  the  pot,  which  is  the  chief  cause  of  cooling. 

Turning  from  cast-iron  to  other  metals  we  find  further  manifesta- 
tions of  this  expansive  solidification.  Bismuth  is  a  notable  example. 
In  his  lectures  on  Heat,  Dr.  Tyndall  exhibited  an  experiment  in 
which  a  stout  iron  bottle  was  filled  with  molten  bismuth,  and  the 
stopper  tightly  closed.  The  whole  was  set  aside  to  cool,  and  as 
the  metal  within  approached  consolidation  the  bottle  was  rent  open 
by  its  expansion,  just  as  would  have  been  the  case  had  the  bottle 
been  filled  with  water  and  exposed  to  freezing  temperature.  Mercury 
affords  another  example.  Thermometers  which  have  to  be  exposed 
to  Arctic  temperatures  are  generally  filled  with  spirit  instead  of 
quicksilver,  because  the  latter  has  been  found  to  burst  the  bulbs 
when  the  cold  reached  the  congealing  point  of  the  metal,  the  burst- 
ing being  a  consequence  of  the  expansion  which  accompanies  the 
act  of  congelation.  Silver  also  expands  in  passing  from  the  fluid  to 
the  solid  state,  for  we  are  informed  by  a  practical  refiner  that  solid 
floats  on  molten  silver  as  ice  floats  on  water ;  it  also,  as  likewise  do 
gold  and  copper,  exhibits  surface  converging  currents  in  the  melting- 
pot  like  those  depicted  above  for  molten  iron. 

It  may,  however,  be  objected  that  metals  are  too  distantly 
related  to  volcanic  substances  to  justify  inferences  being  drawn 


CHAP.  III.]     THE  SUBSEQUENT  COOLING  OF  THE  IGNEOUS  BODY.     27 

from  their  behaviour  in  explanation  of  volcanic  phenomena.  "With 
a  view  therefore  of  testing  the  question  at  issue  with  a  substance 
admitted  as  closely  allied  to  volcanic  material,  we  appealed  to  the 
furnace  slag  of  iron-works.  The  following  are  extracts  from  the 
letters  of  an  iron  manufacturer  of  great  experience  *  to  whom  we 
referred  the  question : — 

"I  beg  to  inform  you  that  cold  slag  floats  in  molten  slag  in  the 
same  way  cold  iron  floats  in  molten  iron. 

"I  filled  a  box  with  hot  molten  felag  run  quickly  from  a  blast 
furnace  ;  the  box  was  about  5  J  feet  square  by  2  feet  deep,  and  I 
dropped  into  the  slag  a  piece  of  cold  slag  weighing  16  lbs.,  when  it 
came  to  the  top  in  a  second.  I  pushed  it  down  to  the  bottom 
several  times  and  it  always  made  its  appearance  at  the  top  :  indeed 
a  small  portion  of  it  remained  above  the  molten  slag." 

Here  then  we  have  a  substance  closely  allied  to  volcanic  material 
which  manifests  the  expansile  principle  in  question  ;  but  we  may  go 
still  further  and  give  evidence  from  the  very  fountain-head  by 
instancing  what  appears  to  be  a  most  cogent  example  of  its  opera- 
ation  which  we  observed  on  the  occasion  of  a  visit  to  the  crater  of 
Vesuvius  in  1865  while  a  modified  eruption  was  in  progress.  On 
this  occasion  we  observed  white-hot  lava  streaming  down  from 
apertures  in  the  sides  of  a  central  cone  within  the  crater  and  form- 
ing a  lake  of  molten  lava  on  the  plateau  or  bottom  of  the  crater ; 
on  the  surface  of  this  molten  lake  vast  cakes  of  the  same  lava  which 
had  become  solidified  were  floating,  exactly  in  the  same  manner  as 
ice  floats  in  water.     The  solidified  lava  had  cracked,  and  divided 

*  Mr.  T.  Heunter,  Manager  of  the  Iron-works  of  James  Murray,  Esq.,  of  Dal- 
mellington,  Ayrshire.  Another  authority  (Mr.  Snelus,  of  the  West  Cumberland 
Iron  Company),  writes  as  follows :  "  I  had  a  hole  dug  on  the  '  cinder-fall,'  and 
allowed  the  running  slag  to  flow  through  it  so  as  to  form  a  tolerably  large  pool  and 
yet  keep  fluid.  Any  crust  that  formed  was  skimmed  off.  A  portion  of  the  same 
slag  was  cooled,  and  the  solid  lump  thrown  into  the  pool.  It  floated  just  at  the 
surface."  Mr.  Snelus  adds,  by  the  way,  that  he  tried  "  Bessemer- Pig  "  in  the  same 
way,  and  that  the  solid  pig  sunk  in  the  molten  for  a  minute  and  then  rose  and 
floated  just  at  the  surface,  with  about  one-twentieth  of  its  bulk  above  the  level  of 
the  fluid. 


THE    MOON. 


[chap.  ni. 


x 


O     03 


cr 


CHAP.  III.]     THE  SUBSEQUENT  COOLING  OF  THE  IGNEOUS  BODY.     29 

into  cakes,  in  consequence  of  its  contraction  and  also  of  the 
uprising  of  the.  accumulating  fluid  lava  on  which  it  floated,  more 
and  more  space  being  thus  afforded  for  it  to  separate,  on  account 
of  the  crater  widening  upwards,  while  through  the  joints  or  fissures 
the  fluid  lava  could  be  seen  beneath.  But  for  the  decrease  in 
density  and  consequent  expansion  in  volume  which  accompanied 
solidification,  this  floating  of  the  solidified  lava  on  the  molten  could 
not  have  occurred.     Reference  to  Fig.  3,  which  represents  a  section 


Fig.  4. 

of  the  crater  of  Vesuvius  on  the  occasion  above  referred  to,  will 
perhaps  assist  the  reader  to  a  more  clear  idea  of  what  we  have 
endeavoured  to  describe.  A  a  are  the  streams  of  white-hot  lava 
issuing  from  openings  in  the  sides  of  the  central  cone,  and 
accumulating  beneath  the  solidified  crust  b  b  in  the  lake  of  molten 
lava  at  c  c  ;  the  solidified  crust  b  b  as  it  was  floated  upwards 
dividing  into  separate  cakes  as  represented  in  Fig.  4.  (See  also 
Plate  I.) 

Let  us  now  consider  what  would  be  the  effect  produced  upon  a 
spherical  mass  of  molten  matter  in  progress  of  cooling,  first  under 
the  action  of  the  above  described  expansion  which  precedes  solidifi- 
cation, and  then  by  the  contraction  which  accompanies  the  cooling 


30    .  THE   MOON.  [chap.  in. 

of  a  solidified  body.  The  first  portion  of  sucli  a  mass  to  part  witli 
its  heat  being  its  external  surface,  this  portion  would  expand,  but 
there  being  no  obstacle  to  resist  the  expansion  there  would  be  no 
other  result  than  a  temporary  slight  enlargement  of  the  sphere. 
;This  external  portion  would  on  cooling  form  a  solid  shell  encompas- 
sing a  more  or  less  fluid  molten  nucleus,  but  as  this  interior  has  in 
its  turn,  on  approaching  the  point  of  solidification,  to  expand  also, 
and  there  being,  so  to  speak,  no  room  for  its  expansion,  by  reason 
of  its  confinement  within  its  solid  casing,  what  would  be  the 
consequence  ? — the  shell  would  be  rent  or  burst  open,  and  a  portion 
of  the  molten  interior  ejected  with  more  or  less  violence  according 
to  circumstances,  and  many  of  the  characteristic  features  of  volcanic 
action  would  be  thus  produced :  the  thickness  of  the  outer  shell, 
the  size  of  the  vent  made  by  the  expanding  matter  for  its  escape, 
and  other  conditions  conspiring  to  modify  the  nature  and  extent  of 
the  eruption.  Thus  there  would  result  vast  floodings  of  the 
exterior  surface  of  the  shell  by  the  so  extruded  molten  matter, 
volcanoes,  extruded  mountains,  and  other  manifestations  of  eruptive 
phenomena.  The  sectional  diagram  (Fig.  5)  will  help  to  convey  a 
clear  idea  of  this  action.  Basing  our  reasoning  on  the  principle  we 
have  thus  enunciated,  namely,  that  molten  telluric  matter  expands 
on  nearing  the  point  of  solidification,  and  which  we  have  en- 
deavoured to  illustrate  by  reference  to  actual  examples  of  its 
operation,  we  consider  we  are  justified  in  assuming  that  such  a. 
course  of  volcanic  phenomena  has  very  probably  occurred  again  and 
again  upon  the  moon ;  that  this  expansion  of  volume  which 
accompanies  the  solidification  of  molten  matter  furnishes  a  key  to 
the  solution  of  the  enigma  of  volcanic  action;  and  that  such 
theories  as  depend  upon  the  agency  of  gases,  vapour,  or  water  are 
at  all  events  untenable  with  regard  to  the  moon,  where  no  gases, 
vapour,  or  water,  appear  to  exist. 

That  an  upheaving  and  ejective  force  has  been  in  action  with 
varying  intensity  beneath  the  whole  of  the  lunar  surface  is  manifest 
from  the  aspect  of  its  structural  details,  and  we  are  impressed  with 


CHAP,  ni.]     THE  SUBSEQUENT  COOLING  OF  THE  IGNEOUS  BODY.     31 

the  conviction  that  the  principle  we  have  set  forth,  namely  the 


11 

I    ^ 


o    o 

rd        IS 


a-^ 


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2  O  Ki 

a  -^  ^ 

g  «  fl 

a  ^  « 

g  °  o 

^  §  a 

'ti  "zi  ■*^ 

O  c8  o 


-g  a  i 
ft  £  ^ 

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o    o 


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paroxysms  of  expansion  which  successively  occurred  as  portions  of 
its   molten   interior  approached   solidification,   supply  us  with   a 


32 


THE    MOON. 


[chap.  III. 


rational  cause  to  wliich  such  vast  ejective  and  upheaving  phenomena 
may  be  assigned.  Many  features  of  terrestrial  geology  likewise 
req^uire  such  an  expansive  force  whereby  to  explain  them ;  we 
therefore  venture  to  recommend  this  source  and  cause  of  ejective 
action  to  the  careful  consideration  of  geologists. 

When  the  molten  substratum  had  burst  its  confines,  ejected  its 
superfluous  matter,  and  produced  the  resulting  volcanic  features,  it 
would,  after  final  solidification,  resume  the  normal  process  of  con- 


FiG.   6. 


Fig.  7. 


traction  upon  cooling,  and  so  retreat  or  shrink  away  from  the 
external  shell.  Let  us  now  consider  what  would  be  the  result  of 
this.  Evidently  the  external  shell  or  crust  would  become  relatively 
too  large  to  remain  at  all  points  in  close  contact  with  the  subjacent 
matter.  The  consequence  of  too  large  a  solid  shell  having  to 
accommodate  itself  to  a  shrunken  body  underneath,  is  that  the 
skin,  so  to  term  the  outer  stratum  of  solid  matter,  becomes 
shrivelled  up  into  alternate  ridges  and  depressions,  or  wrinkles. 
In  its  attempt  to  crush  down  and  follow  the  contracting  substratum 


t^ ;  t ''"  r    I 


.'Nasrryth, 


""WooAburytype" 


K     OF    H  A  N  D   TO  ILLUSTRATE  THE  ORIGIN   OF  CERTAIN   MOUNTAIN  RANGES 
RESU  lTING   FROM    S  H  R  I  N  K  AGE   0  F  THE   i  NT  E  Ri  OR 


PLATE    ill, 


J.Nasmyth. 


'  Wo  o  dbucrytyp  e  " 


SH  RIVELLED     APPLE. 

TO     ILLUSTRATE    THE     ORIGIN    OF    CERTAIN    MOUNTAIN    RANGES 

RESULTING    FROM    SHRINKAGE    OF    THE    INTERIOR 

OF    THE    GLOB  E. 


UNINKUSlTl 


CHAP.  III.]     THE  SUBSEQUENT  COOLING  OF  THE  IGNEOUS  BODY.     33 

it  would  have  to  displace  the  superabundant  or  superfluous  material 
of  its  former  larger  surface  by  thrusting  it  (by  the  action  of  tan- 
gential force)  into  undulating  ridges  as  in  Fig.  6,  or  broken 
elevated  ridges  as  in  Fig.  7,  or  overlappings  of  the  outer  crust  as  in 
Fig.  8,  or  ridges  capped  by  more  or  less  fluid  molten  matter 
extruded  from  beneath,  as  indicated  in  Fig.  9,  a  class  of  action 
which  might  occur  contemporaneously  with  the  elevation  of  the 
ridge  or  subsequently  to  its  formation. 

A  long-kept  shrivelled  apple  affords  an  apt  illustration  of  this 
wrinkle  theory ;  another  example  may  be  observed  in  the  human 


t   \  J ;i__' \_ 


c 

Fig. 


L   /    ^ 


face  and  hand,  when  age  has  caused  the  flesh  to  shrink  and  so 
leave  the  comparatively  unshrinking  skin  relatively  too  large  as  a 
covering  for  it.  We  illustrate  both  of  these  examples  by  actual 
photographs  of  the  respective  objects,  which  are  reproduced  on 
Plates  II.  and  III.  Whenever  an  outer  covering  has  to  accommodate 
and  apply  itself  to  an  interior  body  that  has  become  too  small  for  it, 


34  THE    MOON.  [chap.  hi. 

wrinkles  are  inevitably  produced.  The  same  action  that  shrivels 
the  human  skin  into  creases  and  wrinkles,  has  also  shrivelled 
certain  regions  of  the  igneous  crust  of  the  earth.  A  map  of  a 
mountainous  part  of  our  globe  affords  abundant  evidence  of  such  a 
cause  having  been  in  action  ;  such  maps  are  pictures  of  wrinkles. 
Several  parts  of  the  lunar  surface,  as  we  shall  by-and-by  see,  present 
us  with  the  same  appearances  in  a  modified  degree. 

To  the  few  primary  causes  we  have  set  forth  in  this  chapter — to 
the  alternate  expansion  and  contraction  of  successive  strata  of  the 
lunar  sphere,  when  in  a  state  of  transition  from  an  igneous  and 
molten  to  a  cooled  and  solidified  condition,  we  believe  we  shall  be 
able  to  refer  well-nigh  all  the  remarkable  and  characteristic  features 
of  the  lunar  surface  which  will  come  under  our  notice  in  the  course 
of  our  survey. 


CHAPTER    IV. 

THE    FORM,     MAGNITUDE,     WEIGHT,     AND    DENSITY    OF    THE 
LUNAR    GLOBE. 

We  have  not  hitherto  had  occasion  to  refer  to  what  we  may 
term  the  physical  elements  of  the  moon  :  by  which  we  mean 
the  various  data  concerning  form,  size,  weight,  density,  &c. 
of  that  body,  derived  from  observation  and  calculation.  To 
this  purpose,  therefore,  we  will  now  devote  a  few  pages,  con- 
fining ourselves  to  such  matters  as  specially  bear  upon  the 
requirements  of  our  subject,  omitting  such  as  are  irrelevant  to 
our  purpose,  and  touching  but  lightly  upon  such  as  are  com- 
monly known,  or  are  explained  in  ordinary  elementary  treatises 
on  astronomy. 

First,  then,  as  regards  the  form  of  the  moon.  The  form  of  the 
lunar  disc,  when  fully  illuminated,  we  perceive  to  be  a  perfect 
circle  ;  that  is  to  say,  the  measured  diameters  in  all  directions  are 
equal ;  and  we  are  therefore  led  to  infer  that  the  real  form  of  the 
moon  is  that  of  a  perfect  sphere.  We  know  that  the  earth  and  the 
rest  of  the  planets  of  our  system  are  spheroidal,  or  more  or  less 
flattened  at  the  poles,  and  we  also  know  that  this  flattening  is  a 
consequence  of  axial  rotation ;  the  extent  of  the  flattening,  or  the 
oblateness  of  the  spheroid,  depending  upon  the  speed  of  that  rota- 
tion. But  in  the  case  of  the  moon  the  axial  rotation  is  so  slow  that 
the  flattening  produced  thereby  although  it  must  exist,  is  so 
slight  as  to  be  imperceptible  to  our  observation.  We  might  there- 
fore conclude  that  the  moon  is  a  perfectly  spherical  body,  did  not 

D    2 


36  THE    MOON.  [chap.  iv. 

theory  step  in  to  show  us  that  there  is  another  cause  by  which  its 
form  is  disturbed.  Assuming  the  moon  to  have  been  once  in  a 
fluid  state,  it  is  demonstrable  that  the  attraction  of  the  earth  would 
accumulate  a  mass  of  matter,  like  a  tidal  elevation,  in  the  direction 
of  a  line  joining  the  centres  of  the  two  bodies :  and  as  a  conse- 
quence, the  real  shape  of  the  moon  must  be  an  ellipsoid,  or  some- 
what egg-shaped  body,  the  major  axis  of  which  is  directed  towards 
the  earth.  That  some  such  phenomenon  has  obtained  is  evident 
from  the  coincidence  of  the  times  of  orbital  revolution  and  axial 
rotation  of  the  lunar  sphere.  "  It  would  be  against  all  probability," 
says  Laplace,  "to  suppose  that  these  two  motions  had  been  at  their 
origin  perfectly  equal ;  "  but  it  is  sufficient  that  their  primitive 
difference  was  but  small,  in  which  case  the  constant  attraction  by 
the  earth  of  the  protuberant  part  of  the  moon  would  establish  the 
equality  which  at  present  exists. 

It  is,  however,  sufficient  for  all  purposes  with  which  we  are  con- 
cerned to  regard  the  moon  as  a  sphere,  and  the  next  point  to  be 
considered  is  its  size.     To  determine  this,  two  data  are  necessary 
— its  apparent  or  angular   diameter,  and  its   distance   from    the 
earth.     The  first  of  these  is  obtained  by  measuring  the  angle  com- 
prised between  two  lines  directed  from  the  eye  to  two  opposite 
**  limbs  "  or  edges  of  the  moon.     If,  for  instance,  we  were  to  take  a 
pair  of  compasses  and,  placing  the  joint  at  the  eye,  open  out  the 
legs  till  the  two  points  appear  to  touch  two  opposite  edges  of  the 
moon,  the  two  legs  would  be  inclined  at  an  angle  which  would 
represent  the  diameter  of  the  moon,  and  this  angle  we   could 
measure  by  applying  a  divided  arc  or  protractor  to  the  compasses. 
In  practice   this   measurement   is  made  by  means  of  telescopes 
attached  to  accurately  divided  circles ;  the  difference  between  the 
readings  of  the  circle  when  the  telescope  is  directed  to  opposite 
limbs  of  the  moon  giving  its  angular  diameter  at  the  time  of  the 
observation.     But  from  the  fact  that  the  orbit  of  the  moon  is  an 
ellipse,  it  is  evident  that  she  is  at  some  times  much  nearer  to  us 
than  at  others,  and,  as  a  consequence,  her  apparent  magnitude  is 


CHAP.  IV.]      FORM,   MAGNITUDE,    WEIGHT,   AND   DENSITY.  37 

variable :  there  is  also  a  slight  variation  depending  upon  the 
altitude  of  the  moon  at  the  time  of  the  measurement ;  the  mean 
diameter,  however,  or  the  diameter  at  mean  distance  from  the 
centre  of  the  earth  has,  from  long  course  of  observation,  been 
found  to  be  Sr  9''. 

To  convert  this  apparent  angular  diameter  into  real  linear 
measurement,  it  is  necessary  to  know  either  the  distance  of  the 
moon  from  the  earth,  or  in  astronomical  language  as  leading  to  a 
knowledge  of  that  distance,  what  is  the  amount  of  the  moon's 
parallax.  Parallax,  generally,  is  an  apparent  change  of  position  of 
an  object  arising  from  change  of  the  point  of  view.  The  parallax 
of  a  heavenly  body  is  the  angle  which  the  earth  would  subtend  if  it 
were  seen  from  that  body.  Supposing  an  observer  on  the  moon 
could  measure  the  earth's  angular  diameter,  just  as  we  measure 
that  of  the  moon,  his  measurement  would  represent  what  is  called 
the  parallax  of  the  moon.  But  we  cannot  go  to  the  moon  to  make 
such  a  measurement ;  nevertheless  there  is  a  simple  method, 
explained  in  most  treatises  on  astronomy,  which  consists  in  observ- 
ing the  moon  from  stations  on  the  earth  widely  separated,  and  by 
which  we  can  obtain  a  precisely  similar  result.  Without  detailing 
the  process,  it  is  sufficient  for  us  to  know  that  the  angle  which 
would  be  subtended  by  the  earth  if  seen  from  the  moon,  or  the 
moon's  parallax,  is  according  to  the  latest  determination,  equal  to 
1°  54'  5".  This  value,  however,  varies  considerably  with  the  varia- 
tions of  distance  due  to  the  elliptic  orbit  of  the  moon  :  the  number 
we  have  given  represents  the  mean  parallax,  or  the  parallax  at  mean 
distance. 

But  we  have  to  turn  these  angular  measurements  into  miles.  To 
effect  this  we  have  only  to  work  a  simple  rule-of-three  sum.  .  It 
will  easily  be  understood  that,  as  the  angular  diameter  of  the  earth 
seen  from  the  moon  is  to  the  angular  diameter  of  the  moon  seen 
from  the  earth,  so  is  the  diameter  of  the  earth  in  miles  to  the  dia- 
meter of  the  moon  in  miles.  The  diameter  of  the  earth  we  know 
to  be  7912  nxiles :  putting  this  therefore  in  its   proper   place  in 


88  THE    MOON.  [chap.  iv. 

the  proportion  sum,  and  duly  working  it  out  by  the  schoolboy's 
rule,  we  get : — 

MILES.       MILES. 

V   54'  .  5"   :   31'  .9"   :  :   7912   :   2160 

And  2160  miles  is  therefore  the  diameter  of  the  lunar  globe. 

Knowing  the  diameter,  we  can  easily  obtain  the  other  elements 
of  magnitude.  According  to  the  well-known  relation  of  the  dia- 
meter of  a  sphere  to  its  area,  we  find  the  area  of  the  moon  to  be  f 
14,657,000  square  miles  :  or  half  that  number,  7,328,500  miles,  as 
the  area  of  the  hemisphere  at  any  one  time  presented  to  our  view. 
And  similarly,  from  the  relation  of  the  solidity  of  a  sphere  to  its 
diameter,  we  find  the  solid  contents  of  the  moon  to  be  5276  millions 
of  cubic  miles  of  matter. 

Comparing  these  data  with  corresponding  dimensions  of  the 
earth,  we  find  that  the  diameter  of  the  moon  is  g^j ;  the  area 
^^  ;  and  the  volume  ^g.^,  of  the  respective  elements  of  the  earth. 
Those  who  prefer  a  graphical  to  a  numerical  comparison,  may 
judge  of  the  sizes  of  the  two  bodies  by  the  accompanying  illustra- 
tion (Fig.  10).  To  gain  an  idea  of  their  distance  from  each  other 
it  is  necessary  to  suppose  the  two  discs  in  the  diagram  to  be  five 
feet  apart ;  the  real  distance  of  the  moon  from  the  earth  being 
about  238,790  miles  at  its  mean  position. 

Next,  we  come  to  what  is  technically  termed  the  mass,  but  what 
in  common  language  we  may  call  the  weight  of  the  moon.  It  is 
important  to  know  this,  because  the  weight  of  a  body  taken  in  con- 
nection with  its  size  furnishes  us  with  a  knowledge  of  its  density, 
or  the  specific  gravity  of  the  material  of  which  it  is  composed.  But 
it  is  not  quite  so  easy  to  determine  the  mass  as  the  dimensions  of 
the  moon  :  to  measure  it,  we  have  seen  is  easy  enough ;  to  weigh  it 
is  a  comparatively  difficult  matter.  To  solve  the  problem  we  have 
to  appeal  to  Newton's  law  of  universal  gravitation.  This  law 
teaches  us  that  every  particle  of  matter  in  the  universe  attracts 
every  other  particle  with  a  force  which  is  directly  proportional  to 
the  mass,  and  inversely  proportional  to  the  square  of  the  distance 


CHAP.  IV.]      FORM,   MAGNITUDE,    WEIGHT,  AND  DENSITY. 


of  the  attracting  par- 
ticles. There  are 
several  methods  hy 
which  this  law  is 
applied  to  the  mea- 
surement of  the 
mass  of  the  moon. 
One  of  the  simplest 
is  by  the  agency  of 
the  Tides.  We  know 
that  the  moon,  at- 
tracting the  waters, 
produces  a  certain 
amount  of  elevation 
of  the  aqueous  cover- 
ing of  the  earth ; 
and  we  know  that 
the  sun  produces 
also  a  like  elevation, 
but  to  a  much 
smaller  extent,  by 
reason  of  its  much 
greater  distance. 
Now  measuring  ac- 
curately the  heights 
of  the  solar  and 
lunar  tides,  and 
making  allowance 
for  the  difference  of 
distance  of  the  sun 
and  moon  from  the 
earth,  we  can  com- 
pare directly  the 
effect  that  is  due  to 


40  THE    MOON.  [chap.  iv. 

the  sun  with  the  effect  that  is  due  to  the  moon :  and  since  the 
masses  of  the  two  bodies  are  just  in  proportion  to  the  effects  they 
produce,  it  is  evident  that  we  have  a  comparison  between  the  mass 
of  the  sun  and  that  of  the  moon ;  and  knowing  what  is  the  sun's 
mass  we  can,  by  simple  proportion,  find  that  of  the  moon.  Another 
method  is  as  follows  : — The  moon  is  retained  in  her  orbital  path 
by  the  attraction  of  the  earth  ;  if  it  were  not  for  this  attraction  she 
would  fly  off  from  her  course  in  a  tangential  line.  She  has  thus  a 
constant  tendency  to  quit  her  orbit,  which  the  earth's  attraction  as 
constantly  overcomes.  It  is  evident  from  this  that  the  earth  pulls 
the  moon  towards  itself  by  a  definite  amount  in  every  second  of 
time.  But  while  the  earth  is  pulling  the  moon,  the  moon  is  also 
pulling  the  earth  :  they  are  pulling  each  other  together ;  and 
moreover  each  is  exerting  a  pull  which  is  jproportional  to  its  mass. 
Knowing,  then,  the  mass  of  the  earth,  which  we  do  with  consider- 
able accuracy,  we  can  find  what  share  of  the  whole  pulling  force 
is  due  to  it,  the  residue  being  the  moon's  share  :  the  proportion 
which  this  residue  bears  to  the  earth's  share  gives  us  the  pro- 
portion of  the  moon's  mass  to  that  of  the  earth,  and  hence  the 
mass  of  the  moon. 

There  are  yet  two  other  methods  :  one  depending  upon  the  phe- 
nomena of  nutation,  or  the  attraction  of  the  sun  and  moon  upon  the 
protuberant  matter  of  the  terrestrial  spheroid ;  and  the  other  upon 
a  displacement  of  the  centre  of  gravity  of  the  earth  and  moon, 
which  shows  itself  in  observations  of  the  sun.  By  each  and  all  of 
these  methods  has  the  lunar  mass  been  at  various  times  determined, 
and  it  has  been  found,  as  the  latest  and  best  accepted  value,  that 
the  mass  of  the  moon  is  one-eightieth  that  of  the  earth. 

From  the  known  diameter  of  the  earth  we  ascertain  that  its 
volume  is  259,360  millions  of  cubic  miles  :  and  from  the  various 
experiments  that  have  been  made  to  determine  the  mean  density  of 
the  earth,  it  has  been  found  that  that  mean  density  it  about  5  J 
times  that  of  water ;  that  is  to  say,  the  earth  weighs  5 J  times 
heavier  than  would  a  sphere  of  water  of  ecjual  size.     Now  a  cubic 


CHAP.  IV.]      FORM,  MAGNITUDE,    WEIGHT,   AND  DENSITY.  41 

foot  of  water  weighs  62*3211  pounds,  and  from  this  we  can  find  by 
simple  multiplication  what  is  the  weight  of  a  cubic  mile  of  water, 
and,  similarly,  what  would  be  the  weight  of  259*360  cubic  miles  of 
water,  and  the  last  result  multiplied  by  5  J  will  give  the  weight  of 
the  earth  in  tons  :  The  calculation,  although  extremely  simple,  in- 
volves a  confusing  heap  of  figures ;  but  the  result,  which  is  all  that 
concerns  us,  is,  that  the  weight  of  the  earth  is  5842  trillions  of 
tons :  and  since,  as  we  have  above  stated,  the  mass  of  the  earth  is 
80  times  that  of  the  moon,  it  follows  that  the  weight  of  the  moon 
is  73  trillions  of  tons. 

The  cubical  contents  of  a  body  compared  with  its  weight  gives 
us  its  density.  In  the  moon  we  have  5276  millions  of  cubic  miles 
of  matter,  the  total  weight  of  which  is  73  trillions  of  tons.  Now, 
5276  millions  of  cubic  miles  of  water  would  weigh  about  21J- 
trillions  of  tons  ;  and  as  this  number  is  to  73  as  1  is  to  3*4,  it  is  clear 
that  the  density  of  the  lunar  matter  is  3*4  greater  than  water:  and 
inasmuch  as  the  earth  is  5j  times  denser  than  water,  we  see  that 
the  moon  is  about  0*62  as  dense  as  the  earth,  or  that  the  material 
of  the  moon  is  lighter,  bulk  for  bulk,  than  the  mean  material  of  the 
terraqueous  globe  in  the  proportion  of  62  to  100,  or,  nearly  6  to  10. 
This  specific  gravity  of  the  lunar  material  (3*4)  we  may  remark  is 
about  the  same  as  that  of  flint  glass  or  the  diamond  :  and  curiously 
enough  it  nearly  coincides  with  that  of  some  of  the  aerolites  that 
have  from  time  to  time  fallen  to  the  earth ;  hence  support  has  been 
claimed  for  the  theory  that  these  bodies  were  originally  fragments 
of  lunar  matter,  probably  ejected  at  some  time  from  the  lunar 
volcanoes  with  such  force  as  to  propel  them  so  far  within  the  sphere 
of  the  earth's  attraction  that  they  have  ultimately  been  drawn  to  its 
surface. 

Beverting,  now,  to  the  mass  of  the  moon  :  we  must  bear  in  mind 
that  the  mass  or  weight  of  a  planetary  body  determines  the  weight 
of  all  objects  on  its  surface.  What  we  call  a  pound  on  the  earth, 
would  not  be  a  pound  on  the  moon  ;  for  the  following  reason : — - 
When  we  say  that  such  and  such  an  object  weighs  so  much,  we 


42  THE    MOON.  [chap.  iv. 

really  mean  that  it  is  attracted  towards  the  earth  with  a  certain 
force  depending  upon  its  own  weight.  This  attraction  we  call 
gravity ;  and  the  falling  of  a  weight  to  the  earth  is  an  example  of 
the  action  of  the  law  of  universal  gravitation.  The  earth  and  the 
weight  fall  together — or  are  held  together  if  the  weight  is  in  con- 
tact with  the  earth — with  a  force  which  depends  directly  upon  the 
mass  of  the  two,  and  upon  the  distance  hetween  them.  Newton 
proved  that  the  attraction  of  a  sphere  upon  external  objects  is  pre- 
cisely as  if  the  whole  of  its  matter  were  contained  at  its  centre.  So 
that  the  attractive  force  of  the  earth  upon  a  ton  weight  at  its  surface 
is  the  attraction  which  5842  trillions  of  tons  exert  upon  one  ton 
situated  3956  miles  (the  radius  of  the  earth)  distant.  If  the  weight 
of  the  earth  were  only  half  the  above  quantity,  it  is  clear  that  the 
attraction  would  be  only  half  what  it  is ;  and  hence  the  ton  weight, 
being  pulled  by  only  half  the  force,  would  only  be  equal  to  half  a 
ton ;  that  is  to  say,  only  half  as  much  muscular  force  (or  any  other 
force  but  gravity)  would  be  required  to  lift  it.  It  is  plain,  there- 
fore, that  what  weighs  a  pound  on  the  earth  could  not  weigh  a 
pound  on  the  moon,  which  is  only  -^  of  the  weight  of  the  earth. 
What,  then,  is  the  relation  between  a  pound  on  the  earth  and  the 
same  mass  of  matter  on  the  moon  ?  It  would  seem,  since  the 
moon's  mass  is  -gV  of  the  earth,  that  the  pound  transported  to  the 
moon  ought  to  weigh  the  eightieth  part  of  a  pound  there  ;  and  so 
it  would  if  the  distance  from  the  centre  of  the  moon  to  its  surface 
were  the  same  as  the  distance  of  the  centre  of  the  earth  from  its 
surface.  But  the  radius  of  the  moon  is  only  ^^  that  of  the  earth  ; 
and  the  force  of  gravity  varies  inversely  as  the  square  of  the  distance 
between  the  centres  of  the  gravitating  masses.  So  that  the  attrac- 
tion by  the  moon  of  a  body  at  its  surface,  as  compared  with  that  of 
the  earth,  is  Vo  divided  by  the  square  of  ~  ;  and  this,  worked  out, 
is  equal  to  J.  The  force  of  gravity  upon  the  moon  is,  therefore,  J  of 
that  on  the  earth ;  and  hence  a  pound  upon  the  earth  would  be 
little  more  than  2|  ounces  on  the  moon  ;  and  it  follows  as  a  conse- 
quence that  any  force,  such  as  muscular  exertion,  or  the  energy  of 


CHAP.  IV.]      FORM,  MAGNITUDE,   WEIGHT,   AND  DENSITY.  43 

chemical,  plutonic  or  explosive  forces,  would  be  six  times  more 
effective  upon  the  moon  than  upon  the  earth.  A  man  who  could 
jump  six  feet  from  the  earth,  could  with  the  same  muscular  effort 
jump  thirty-six  feet  from  the  moon;  the  explosive  energy  that 
would  project  a  body  a  mile  above  the  earth  would  project  a  like 
body  six  miles  above  the  surface  of  the  moon. 

It  is  the  practice,  in  elementary  and  popular  treatises  on 
astronomy,  to  state  merely  the  numerical  results  in  giving  data 
such  as  those  embodied  in  the  foregoing  pages ;  and  uninitiated 
readers,  not  knowing  the  means  by  which  the  figures  are  arrived 
at,  are  sometimes  disposed  to  regard  them  with  a  certain  amount 
of  doubt  or  uncertainty.  On  this  account  we  have  thought  it 
advisable  to  give,  in  as  brief  and  concise  a  form  as  possible,  the 
various  steps  by  which  these  seemingly  unattainable  results  are 
obtained. 

The  data  explained  in  the  foregoing  text  are  here  collected  to 
facilitate  reference. 

Diameter  of  Moon  .        .        .2160  miles  .        .        .    .  5:^  that  of  earth. 

Area 14,657,000  square  miles .        .  j^^     „  „ 

Area  of  the  visible  hemisphere      7,328,500  square  miles 

Solid  contents     .        ...     5276  millions  of  cubic  miles  .  ^^     „  „ 

Mass 73  trillions  of  tons  >        .        .  ^       „  „ 

Density 3-39  (water  =  1)     .        .     .  0.62     „  „ 

Force  of  gravity  at  surface i        ?,  » 

Mean  distance  from  earth       .        .        .    238,790  miles. 


CHAPTER    V. 

ON    THE    EXISTENCE    OR    NON-EXISTENCE    OF    A    LUNAR 
ATMOSPHERE. 

At  the  close  of  the  preceding  chapter  we  stated  that  any  force 
acting  in  opposition  to  that  of  gravity  would  be  six  times  more 
effective  on  the  moon  than  on  the  earth.  But,  in  fact,  it  would  in 
many  cases  be  still  more  so;  at  all  events,  so  far  as  projectile 
forces  are  concerned  ;  for  the  reason  that  "  the  powerful  coercer  of 
projectile  range,"  as  the  earth's  atmosphere  has  been  termed,  has 
no  counterpart,  or  at  most  a  very  disproportionate  one,  upon  the 
moon. 

The  existence  of  an  atmosphere  surrounding  the  moon  has  been 
the  subject  of  considerable  controversy,  and  a  great  deal  of  evidence 
on  both  sides  of  the  question  has  been  offered  from  time  to  time, 
and  is  to  be  found  scattered  through  the  records  of  various  classes 
of  observations.  Some  of  the  more  important  items  of  this 
evidence  it  is  our  purpose  to  set  forth  in  the  course  of  the  present 
chapter. 

With  the  phenomena  of  the  terrestrial  atmosphere,  with  the 
effects  that  are  attributable  to  it,  we  are  all  well  familiar,  and  our 
best  course  therefore  is  to  examine,  as  far  as  we  are  able,  whether 
counterparts  of  any  of  these  effects  are  manifested  upon  the  moon. 
For  instance,  the  clouds  that  are  generated  in  and  float  through 
our  air  would,  to  an  observer  on  the  moon,  appear  as  ever-changing 
bright  or  dusky  spots,  obliterating  certain  of  the  permanent  details 
of  the  earth's  surface,  and  probably  skirting  the  terrestrial  disc, 


CHAP,  v.]       NON-EXISTENCE    OF   A    LUNAR    ATMOSPHERE.  45 

like  the  changing  belts  we  perceive  on  the  planet  Jupiter,  or 
diversifying  its  features  with  less  regularity,  after  the  manner 
exhibited  by  the  planet  Mars.  If  such  clouds  existed  on  the  moon 
it  is  evident  that  the  details  of  its  surface  must  be,  from  time  to 
time,  similarly  obscured ;  but  no  trace  of  such  obscuration  has 
ever  been  detected.  When  the  moon  is  observed  with  high 
telescopic  powers,  all  its  details  come  out  sharp  and  clear,  without 
the  least  appearance  of  change  or  the  slightest  symptoms  of 
cloudiness  other  than  the  occasional  want  of  general  definition, 
which  may  be  proved  to  be  the  result  of  unsteadiness  or  want 
of  homogeneity  in  our  own  atmosphere ;  for  we  must  tell  the 
uninitiated  that  nights  of  pure,  good  definition,  such  as  give  the 
astronomer  opportunity  of  examining  with  high  powers  the  minute 
details  of  planetary  features,  are  very  few  and  far  between.  Out  of 
the  three  hundred  and  sixty-five  nights  of  a  year  there  are  probably 
not  a  dozen  that  an  astronomer  can  call  really  fine  :  usually,  even 
on  nights  that  are  to  all  common  appearance  superbly  brilliant, 
some  strata  of  air  of  dift'erent  densities  or  temperatures,  or  in  rapid 
motion,  intervene  between  the  observer  and  the  object  of  his 
observation,  and  through  these,  owing  to  the  ever-changing 
refractions  which  the  rays  of  light  coming  from  the  object  suffer  in 
their  course,  observation  of  the  delicate  markings  of  a  planet  is 
impossible :  all  is  blurred  and  confused,  and  nothing  but  bolder 
features  can  be  recognized.  It  has  in  consequence  sometimes 
happened  that  a  slight  indistinctness  of  some  minute  detail  of  the 
moon  has  been  attributed  to  clouds  or  mists  at  the  lunar  surface, 
whereas  the  real  cause  has  been  only  a  bad  condition  of  our  own 
atmosphere.  It  may  be  confidently  asserted  that  when  all  indis- 
tinctness due  to  terrestrial  causes  is  taken  account  of  or  eliminated, 
there  remain  no  traces  whatever  of  any  clouds  or  mists  upon  the 
surface  of  the  moon. 

This  is  but  one  proof  against  the  existence  of  a  lunar 
atmosphere,  and,  it  may  be  argued,  not  a  very  conclusive  one ; 
because   there   may  still   be   an   atmosphere,   though    it   be   not 


4B  THE   MOON.  [chap.  v. 

sufficiently  aqueous  to  condense  into  clouds  and  not  sufficiently 
dense  to  obscure  the  lunar  details.  The  probable  existence  of  an 
atmosphere  of  such  a  character  used  to  be  inferred  from  a 
phenomenon  seen  during  total  eclipses  of  the  sun.  On  these 
occasions  the  black  body  of  the  moon  is  invariably  surrounded  by  a 
luminous  halo,  or  glory,  to  which  the  name  "corona"  has  been 
applied ;  and,  further,  besides  this  corona,  apparently  floating  in 
it  and  sometimes  seemingly  attached  to  the  black  edge  of  the 
moon,  are  seen  masses  of  cloud-like  matter  of  a  bright  red  colour, 
which,  from  the  form  in  which  they  were  first  seen  and  from  their 
flame-like  tinge,  have  become  universally  known  as  the  ''red- 
flames.*'  It  used  to  be  said  that  this  corona  could  only  be  the 
consequence  of  a  lunar  atmosphere  lit  up  as  it  were  by  the  sun's 
rays  shining  through  it,  after  the  manner  of  a  sunbeam  lighting  up 
the  atmosphere  of  a  dusty  chamber ;  and  the  red  flames  were  held 
by  those  who  first  observed  them  to  be  clouds  of  denser  matter 
floating  in  the  said  atmosphere,  and  refracting  the  red  rays  of  solar 
light  as  our  own  clouds  are  seen  to  do  at  sunrise  and  sunset.  But 
the  evidence  obtained,  both  by  simple  telescopic  observation  and 
by  the  spectroscope,  from  recent  extensively  observed  eclipses  of 
the  sun  has  set  this  question  quite  at  rest ;  for  it  has  been  settled 
finally  and  indisputably  that  both  the  above  appearances  pertain 
to  the  sun,  and  have  nothing  whatever  to  do  with  the  moon. 

The  occurrence  of  a  solar  eclipse  offers  other  means  in  addition 
to  the  foregoing  whereby  a  lunar  atmosphere  would  be  detected. 
We  know  that  all  gases  and  vapours  absorb  some  portion  of  any 
light  which  may  shine  through  them.  If  then  our  satellite  had  an 
atmosphere,  its  black  nucleus  when  seen  projected  against  the 
bright  sun  in  an  eclipse  would  be  surrounded  by  a  sort  of  penumbra, 
or  zone  of  shadow,  in  contact  with  its  edge,  somewhat  like  that  we 
have  shown  in  an  exaggerated  degree  in  the  annexed  cut  (Fig.  11), 
and  the  passage  of  this  penumbra  over  solar  spots  and  other 
features  of  the  solar  photosphere  would  to  some  extent  obscure  the 
more  minute  details  of  such  features.     No  such  dusky  band  has 


CHAP,  v.]       NON-EXISTENCE    OF    A    LUNAR    ATMOSPHERE.  47 

however  been  at  any  time  observed.  On  the  contrary,  a  band 
somewhat  brighter  than  the  general  surface  of  the  sun  has 
frequently  been  seen  in  contact  with  the  black  edge  of  the  moon  : 
this  in  its  turn  was  held  to  indicate  an  atmosphere  about  the  moon; 
but  Sir  George  Airy  has  shown  that  a  lunar  atmosphere,  if  it  really 
did  exist,  could  not  produce  such  an  appearance,  and  that  the  cause 
of  it  must  be  sought  in  other  directions.  If  this  effect  were  really 
due  to  the  passage  of  the  solar  rays  through  a  lunar  atmosphere  a 


Fig.  11. 

similar  effect  ought  to  be  produced  by  the  passage  of  the  sun's  rays 
through  the  terrestrial  atmosphere  :  and  we  might  hence  expect  to  / 
see  the  shadow  of  the  earth  projected  on  the  moon  during  a  lunar  \ 
eclipse  surrounded  by  a  sort  of  bright  zone  or  halo  :  we  need  hardly 
say  such  an  appearance  has  never  manifested  itself.  Similarly  as 
we  stated  that  the  delicate  details  of  solar  spots  would  be  obscured 
by  a  lunar  atmosphere,  small  stars  passing  behind  the  moon  would 
suffer  some  diminution  in  brightness  as  they  approached  apparent 
contact  with  the  moon's  edge :  this  fading  has  been  watched  for  on 
many  occasions,  and  in  a  few  cases  such  an  appearance  has  been 
suspected,  but  in  by  far  the  majority  of  instances  nothing  like  a 
diminution  of  brightness  or  change  of  colour  of  the  stars  has  been 
seen ;  stars  of  the  smallest  magnitude  visible  under  such  circum- 


48 


THE    MOON. 


[chap.  v. 


stances  retain  their  feeble  lustre  unimpaired  up  to  the  moment  of 
their  disappearance  behind  the  moon's  limb. 

Again,  in  a  solar  eclipse,  even  if  there  were  an  atmosphere  about 
the  moon  not  sufficiently  dense  to  form  a  hazy  outline  or  impair 
the  distinctness  of  the  details  of  a  solar  spot,  it  would  still  manifest 
its  existence  in  another  way.  As  the  moon  advances  upon  the 
sun's  disc  the  latter  assumes,  of  course,  a  crescent  form.  Now  if 
air  or  vapour  enveloped  the  moon,  the  exceedingly  delicate  cusps  of 
this  crescent  would  be  distorted  or  turned  out  of  shape.     Instead  of 


Fig.  12. 


remaining  symmetrical,  like  the  lower  one  in  the  annexed  drawing 
(Fig.  12),  they  would  be  bent  or  deformed  after  the  manner  we  have 
shown  in  the  upper  one.  The  slightest  symptom  of  a  distortion 
like  this  could  not  fail  to  obtrude  itself  upon  an  observer's  eye ;  but 
in  no  instance  has  anything  of  the  kind  been  seen. 

Reverfing  to  the  consequences  of  the  terrestrial  atmosphere :  one 
of  the  most  striking  of  these  is  the  phenomenon  of  diffused  day- 
light, which  we  need  hardly  remind  the  reader  is  produced  by  the 
scattering  or  diffusion  of  the  sun's  rays  among  the  minute  particles 
of  vapour  composing  or  contained  in  that  atmosphere.  Were  it  not 
for  this  reflexion  and  diffusion  of  the  sun's  light,  those  parts  of  our 
earth  not  exposed  to  direct  sunshine  would  be  hidden  in  darkness, 


CHAP,  v.]       NON-EXISTENCE    OF    A    LUNAR    ATMOSPHERE.  49 

receiving  no  illumination  beyond  the  feeble  amount  that  might  be 
reflected  from  proximate  terrestrial  objects  actually  illuminated  by 
direct  sunlight.  Twilight  is  a  consequence  of  this  reflexion  of 
light  by  the  atmosphere  when  the  sun  is  below  the  horizon.  If, 
then,  an  atmosphere  enveloped  the  moon,  we  should  see  by  diffused 
light  those  parts  of  the  lunar  details  that  are  not  receiving  the 
direct  solar  beams ;  and  before  the  sun  rose  and  after  it  had  set 
upon  any  region  of  the  moon,  that  region  would  still  be  partially 
illuminated  by  a  twilight.  But,  on  the  contrary,  the  shadowed 
portions  of  a  lunar  landscape  are  pitchy  black,  without  a  trace  of 
diffused-light  illumination,  and  the  effects  that  a  twilight  would 
produce  are  entirely  absent  from  the  moon.  Once,  indeed,  one 
observer,  Schroeter,  noticed  something  which  he  suspected  was  due 
to  an  effect  of  this  kind  :  when  the  moon  exhibited  itself  as  a  very 
slender  crescent,  he  discovered  a  faint  crepuscular  light,  extending 
from  each  of  the  cusps  along  the  circumference  of  the  unenlight- 
ened part  of  the  disc,  and  he  inferred  from  estimates  of  the  length 
and  breadth  of  the  line  of  light  that  there  was  an  atmosphere  about 
the  moon  of  5376  feet  in  height.  This  is  the  only  instance  on 
record,  we  believe,  of  such  an  appearance  being  seen. 

Spectrum  analysis  would  also  betray  the  existence  of  a  lunar 
atmosphere.  The  solar  rays  falling  on  the  moon  are  reflected  from 
its  surface  to  the  earth.  If,  then,  an  atmosphere  existed,  it  is 
plain  that  the  solar  rays  must  first  pass  through  such  atmosphere 
to  reach  the  reflecting  surface,  and  returning  from  thence,  again 
pass  through  it  on  their  way  to  the  earth ;  so  that  they  must  in 
reality  pass  through  virtually  twice  the  thickness  of  any  atmosphere 
that  may  cover  the  moon.  And  if  there  be  any  such  atmosphere, 
the  spectrum  formed  by  the  moon's  light,  that  is,  by  the  sun's  light 
reflected  from  the  moon,  would  be  modified  in  such  a  manner  as  to 
exhibit  absorption-lines  different  from  those  found  in  the  spectrum 
of  the  direct  solar  rays,  just  as  the  absorption-lines  vary  according 
as  the  sun's  rays  have  to  pass  through  a  thinner  or  a  denser 
stratum  of  the  terrestrial  atmosphere.     Guided  by  this  reasoning, 


60  THE    MOON.  [chap.  v. 

Drs.  Huggins  and  Miller  made  numerous  observations  upon  the 
spectrum  of  the  moon's  light,  which  are  detailed  in  the  *'  Philo- 
sophical Transactions  "  for  the  year  1864;  and  their  result,  quoting 
the  words  of  the  report,  was  "  that  the  spectrum  analysis  of  the 
light  reflected  from  the  moon  is  wholly  negative  as  to  the  existence 
of  any  considerable  lunar  atmosphere." 

Upon  another  occasion,  Dr.  Huggins  made  an  analogous  observa- 
tion of  the  spectrum  of  a  star  at  the  moment  of  its  occultation, 
which  observation  he  records  in  the  following  words  : — **  When  an 
observation  is  made  of  the  spectrum  of  a  star  a  little  before,  or  at 
the  moment  of  its  occultation  by  the  dark  limb  of  the  moon,  several 
phenomena  characteristic  of  the  passage  of  the  star's  light  through 
an  atmosphere  might  possibly  present  themselves  to  the  observer. 
If  a  lunar  atmosphere  exist,  which  either  by  the  substances  of 
which  it  is  composed,  or  by  the  vapours  diffused  through  it,  can 
exert  a  selective  absorption  upon  the  star's  light,  this  absorption 
would  be  indicated  to  us  by  the  appearance  in  the  spectrum  of  new 
dark  lines  immediately  before  the  star  is  occulted  by  the  moon." 

*'  If  finely  divided  matter,  aqueous  or  otherwise,  were  present 
about  the  moon,  the  red  rays  of  the  star's  light  would  be  enfeebled 
in  a  smaller  degree  than  the  rays  of  higher  refrangibilities." 

**  If  there  be  about  the  moon  an  atmosphere  free  from  vapour, 
and  possessing  no  absorptive  power,  but  of  some  density,  then  the 
spectrum  would  not  be  extinguished  by  the  moon's  limb  at  the  same 
instant  throughout  its  length.  The  violet  and  blue  rays  would  lie 
behind  the  red  rays." 

"  I  carefully  observed  the  disappearance  of  the  spectrum  of 
€  Piscium  at  its  occultation  of  January  4,  1865,  for  these  pheno- 
mena ;  but  no  signs  of  a  lunar  atmosphere  were  detected." 

But  perhaps  the  strongest  evidence  of  the  non-existence  of  any 

r  Appreciable  lunar  atmosphere  is  afforded  by  the  non-refraction  of 
the  light  of  a  star  passing  behind  the  edge  of  the  lunar  disc. 
Refraction,  we  know,  is  a  bending  of  the  rays  of  light  coming  from 
any  object,  caused  by  their  passage  through  strata  of  transparent 


CHAP,  v.]       NON-EXISTENCE    OF    A    LUNAR    ATMOSPHERE.  51 

matter  of  different  densities  ;  we  have  a  familiar  example  in  the 
apparent  bending  of  a  stick  when  half  plunged  into  water.  There 
is  a  simple  schoolboy's  experiment  which  illustrates  refraction  in  a 
very  cogent  manner,  but  which  we  should,  from  its  very  simplicity, 
hesitate  to  recall  to  the  reader's  mind  did  it  not  very  aptly  represent 
the  actual  case  we  wish  to  exemplify.  A  coin  is  placed  on  the 
bottom  of  an  empty  basin,  and  the  eye  is  brought  into  such  a 
position  that  the  coin  is  just  hidden  behind  the  basin's  rim. 
Water  is  then  poured  into  the  basin  and,  without  the  eye  being 
moved  from  its  former  place,  as  the  depth  of  water  increases,  the 
coin  is  brought  by  degrees  fully  into  view  ;  the  water  refracting  or 
turning  out  of  their  course  the  rays  of  light  coming  from  the  coin, 
and  lifting  them,  as  it  were,  over  the  edge  of  the  basin.  Now  a 
perfectly  similar  phenomenon  takes  place  at  every  sunrise  and 
sunset  on  the  earth.  When  the  sun  is  really  below  the  horizon, 
it  is  nevertheless  still  visible  to  us  because  it  is  brought  up  by  the 
refraction  of  its  light  by  the  dense  stratum  of  atmosphere  through 
which  the  rays  have  to  pass.  The  sun  is,  therefore,  exactly 
represented  by  the  coin  at  the  bottom  of  the  basin  in  the  boy's 
experiment,  the  atmosphere  answers  to  the  water,  and  the  horizon 
to  the  rim  or  edge  of  the  basin.  If  there  were  no  atmosphere 
about  the  earth,  the  sun  would  not  be  so  brought  up  above  the 
horizon,  and,  as  a  consequence,  it  would  set  earlier  and  rise  later 
by  about  a  minute  than  it  really  does.  This,  of  course,  applies 
not  merely  to  the  sun,  but  to  all  celestial  bodies  that  rise  and  set. 
Every  planet  and  every  star  remains  a  shorter  time  below  the 
horizon  than  it  would  if  there  were  no  atmosphere  surrounding 
the  earth. 

To  apply  this  to  the  point  we  are  discussing.  The  moon  in  her 
orbital  course  across  the  heavens  is  continually  passing  before,  or 
occulting,  some  of  the  stars  that  so  thickly  stud  her  apparent  path. 
And  when  we  see  a  star  thus  pass  behind  the  lunar  disc  on  one 
side  and  come  out  again  on  the  other  side,  we  are  virtually 
observing  the  setting  and  rising  of  that  star  upon  the  moon.     If, 

B  3 


52  THE    MOON.  [chap.  v. 

then,  the  moon  had  an  atmosphere,  it  is  clear,  from  analogy  to  the 
case  of  the  earth,  that  the  star  must  disappear  later  and  reappear 
sooner  than  if  it  has  no  atmosphere :  just  as  a  star  remains  too 
short  a  time  below  the  earth's  horizon,  or  behind  the  earth,  in 
consequence  of  the  terrestrial  atmosphere,  so  would  a  star  remain 
too  short  a  time  behind  the  moon  if  an  atmosphere  surrounded 
that  body.  The  point  is  settled  in  this  way : — The  moon's 
apparent  diameter  has  been  measured  over  and  over  again  and  is 
known  with  great  accuracy ;  the  rate  of  her  motion  across  the  sky 
is  also  known  with  perfect  accuracy  :  hence  it  is  easy  to  calculate 
how  long  the  moon  will  take  to  travel  across  a  part  of  the  sky  exactly 
equal  in  length  to  her  own  diameter.  Supposing,  then,  that  we 
observe  a  star  pass  behind  the  moon  and  out  again,  it  is  clear  that, 
if  there  be  no  atmosphere,  the  interval  of  time  during  which  it 
remains  occulted  ought  to  be  exactly  equal  to  the  computed  time 
which  the  moon  would  take  to  pass  over  the  star.  If,  however,  from 
the  existence  of  a  lunar  atmosphere,  the  star  disappears  too  late  and 
reappears  too  soon,  as  we  have  seen  it  would,  these  two  intervals 
will  not  agree ;  the  computed  time  will  be  greater  than  the 
observed  time,  and  the  difiference,  if  any  there  be,  will  represent 
the  amount  of  refraction  the  star's  light  has  sustained  or 
suffered,  and  hence  the  extent  of  atmosphere  it  has  had  to  pass 
through. 

Comparisons  of  these  two  intervals  of  time  have  been  repeatedly 
made,  the  most  recent  and  most  extensive  was  executed  under  the 
direction  of  the  Astronomer-Royal  several  years  ago,  and  it  was 
based  upon  no  less  than  296  occupation  observations.  In  this 
determination  the  measured  or  telescopic  semidiameter  of  the 
moon  was  compared  with  the  semidiameter  deduced  from  the 
occultations,  upon  the  above  principle,  and  it  was  found  that  the 
telescopic  semidiameter  was  greater  than  the  occultation  semi- 
diameter by  two  seconds  of  angular  measurement  or  by  about  a 
thousandth  part  of  the  whole  diameter  of  the  moon.  Sir  George 
Airy,  commenting  on  this  result,  says  that  it  appears  to  him  that 


CHAP,  v.]       NON-EXISTENCE    OF   A    LUNAR    ATMOSPHERE  53 

the  origin  of  this  difference  is  to  he  sought  in  one  of  two  causes. 
"  Either  it  is  due  to  irradiation  *  of  the  telescopic  semi  diameter, 
and  I  do  not  douht  that  a  part  at  least  of  the  two  seconds  is  to  be 
ascribed  to  that  cause  ;  or  it  may  be  due  to  refraction  by  the 
moon's  atmosphere.  If  the  whole  two  seconds  were  caused  by 
atmospheric  refraction  this  would  imply  a  horizontal  refraction  of 
one  second,  which  is  only  ^oVo  part  of  the  earth's  horizontal 
refraction.  It  is  possible  that  an  atmosphere  competent  to  pro- 
duce this  refraction  would  not  make  itself  visible  in  any  other 
way."  This  result  accords  well,  considering  the  relative  accuracy 
of  the  means  employed,  with  that  obtained  a  century  ago  by  the 
French  astronomer  Du  Sejour,  who  made  a  rigorous  examination 
of  the  subject  founded  on  observations  of  the  solar  eclipse  of  1764. 
He  concluded  that  the  horizontal  refraction  produced  by  a  possible 
lunar  atmosphere  amounted  to  1"'5 — a  second  and  a  half — or 
about  14*0  0  of  that  produced  by  the  earth's  atmosphere.  The 
greater  weight  is  of  course  to  be  allowed  to  the  more  recent  deter- 
mination in  consideration  of  the  large  number  of  accurate  obser- 
vations upon  which  it  was  based. 

But  an  atmosphere  2,000  times  rarer  than  our  air  can  scarcely 
be  regarded  as  an  atmosphere  at  all.  The  contents  of  an  air-pump 
receiver  can  seldom  be  rarefied  to  a  greater  extent  than  to  about 
T^oc  of  the  density  of  air  at  the  earth's  surface,  with  the  best  of 
pneumatic  machines;  and  the  lunar  atmosphere,  if  it  exist  at  all, 
is  thus  proved  to  be  twice  as  attenuated  as  what  we  are  accus- 
tomed to  recognise  as  a  vacuum.  In  discussing  the  physical 
phenomena  of  the  lunar  surface,  we  are,  therefore,  perfectly  justified 
in  omitting  all  considerations  of  an  atmosphere,  and  adapting  our 
arguments  to  the  non-existence  of  such  an  appendage. 

*  Irradiation  is  an  ocular  phenomenon  in  virtue  of  iwhich  all  strongly  illu- 
minated objects  appear  to  the  eye  to  be  larger  than  they  really  are.  The 
impression  produced  by  light  upon  the  retina  appears  to  extend  itself  around  the 
focal  image  formed  by  the  lenses  of  the  eye.  It  is  from  the  effect  of  irradiation 
that  a  white  disc  on  a  black  ground  looks  larger  than  a  black  disc  of  the  same  size 
on  a  white  ground. 


54  THE    MOON.  [chap.  v. 

And  if  there  be  no  air  upon  the  moon,  vre  are  almost  forced  to 
conclude  that  there  can  be  no  water  ;  for  if  water  covered  any  part 
of  the  lunar  globe  it  must  be  vaporised  under  the  influence  of  the 
long  period  of  uninterrupted  sunshine  (upwards  of  300  hours)  that 
constitutes  the  lunar  day,  and  would  manifest  itself  in  the  form  of 
clouds  or  mists  obscuring  certain  parts  of  the  surface.  But,  as  we 
have  already  said,  no  such  obliteration  of  details  ever  takes  place  ; 
and,  as  we  have  further  seen,  no  evidence  of  aqueous  vapour  is 
manifested  upon  the  occasion  of  spectrum  observations.  Since, 
then,  the  effects  of  watery  vapour  are  absent,  we  are  forced  to 
conclude  that  the  cause  is  absent  also. 

Those  parts  of  the  moon  which  the  ancient  astronomers  assumed, 
from  their  comparatively  smooth  and  dusky  appearance,  to  be  seas, 
have  long  since  been  discovered  to  be  merely  extensive  regions  of 
less  reflective  surface  material ;  for  the  telescope  reveals  to  us 
irregularities  and  asperities  covering  well-nigh  the  whole  of  them, 
which  asperities  could  not  be  seen  if  they  were  covered  with  water  ; 
unless,  indeed,  we  admit  the  possibility  of  seeing  to  the  bottom  of 
the  water,  not  only  perpendicularly,  but  obliquely.  Some  observers 
have  noticed  features  that  have  led  them  to  suppose  that  water 
was  at  one  time  present  upon  the  moon,  and  has  left  its  traces  in 
the  form  of  appearances  of  erosive  action  in  some  parts.  But  if 
water  ever  existed,  where  is  it  now  ?  One  writer,  it  is  true,  has 
suggested  as  possible,  that  whatever  air,  and  we  presume  he  would 
include  whatever  water  also,  the  moon  may  possess,  is  hidden 
away  in  sublunarean  caves  and  hollows  ;  but  even  if  water  existed 
in  these  places  it  must  sometimes  assume  the  vapoury  form,  and 
thus  make  its  presence  known. 

Sir  John  Herschel  pointed  out  that  if  any  moisture  exists  upon 
the  moon,  it  must  be  in  a  continual  state  of  migration  from  the 
illuminated  or  hot^  to  the  unilluminated  or  cold  side  of  the  lunar 
globe.  The  alternations  of  temperature,  from  the  heat  produced 
by  the  unmitigated  sunshine  of  14  days'  duration,  to  the  intensity 
of  cold  resulting  from  the  absence  of  any  sunshine  whatever  for  an 


CHAP,  v.]      NON-EXISTENQE    OF   A    LUNAR    ATMOSPHERE. 


55 


equal  period,  must,  he  argued,  produce  an  action  similar  to  that  of 
the  cryophorus  in  transporting  the  lunar  moisture  from  one  hemi- 
sphere to  the  other.  The  cryophorus  is  a  little  instrument 
invented  by  the  late  Dr.  WoUaston ;  it  consists  of  two  bulbs  of 
glass  connected  by  a  bent  tube,  in  the  manner  shown  in  the 
annexed  illustration.  Fig.  13.     One  of  the  bulbs.  A,  is  half-filled 


Fig.  13. 


with  water,  and,  all  air  being  exhausted,  the  instrument  is  her- 
metically sealed,  leaving  nothing  within  but  the  water  and  the 
aqueous  vapour  which  rises  therefrom  in  the  absence  of  atmos- 


Fio.  14. 


pheric  pressure.  When  the  empty  bulb,  B,  is  placed  in  a  freezing 
mixture,  a  rapid  condensation  of  this  vapour  takes  place  within  it, 
and  as  a  consequence  the  water  in  the  bulb  A  gives  off  more 
vapour.  The  abstraction  of  heat  from  the  water,  which  is  a 
natural  consequence  of  this  evaporation,  causes  it  to  freeze  into  a 
solid  mass  of  ice.     Now  upon  the  moon  the  same  phenomenon 


56  THE    MOON.  [chap.  v. 

would  occur  did  the  material  exist  there  to  supply  it.  In  the 
accompanying  diagram  let  A  represent  the  illuminated  or  heated 
hemisphere  of  the  moon,  and  B  the  dark  or  cold  hemisphere ;  the 
former  being  probably  at  a  temperature  of  300°  above,  and  the 
latter  200°  below  Fahrenheit's  zero.  Upon  the  above  principle,  if 
moisture  existed  upon  A  it  would  become  vaporised,  and  the 
vapour  would  migrate  over  to  B,  and  deposit  itself  there  as  hoar- 
frost ;  it  would,  therefore,  manifest  itself  to  us  while  in  the  act  of 
migrating  by  clouding  or  dimming  the  details  about  the  boundary 
of  the  illuminated  hemisphere.  The  sun,  rising  upon  any  point 
upon  the  margin  of  the  dark  hemisphere,  would  have  to  shine 
through  a  bed  of  moisture,  and  we  may  justly  suppose,  if  this 
were  the  case,  that  the  tops  of  mountains  catching  the  first  beams 
of  sunlight  would  be  tinged  with  colour,  or  be  lit  up  at  first  with 
but  a  faint  illumination,  just  as  we  see  in  the  case  of  terrestrial 
mountains  whose  summits  catch  the  first,  or  receive  the  last  beams 
of  the  rising  or  setting  sun.  Nothing  of  this  kind  is,  however, 
perceptible  :  when  the  solar  rays  tip  the  lofty  peaks  of  lunar 
mountains,  these  shine  at  once  with  brilliant  light,  quite  as  vivid 
as  any  of  those  parts  that  receive  less  horizontal  illumination,  or 
upon  which  the  sun  is  almost  perpendicularly  shining. 

All  the  evidence,  then,  that  we  have  the  means  of  obtaining, 
goes  to  prove  that  neither  air  nor  water  exists  upon  the  moon. 
Two  complicating  elements  affecting  all  questions  relating  to  the 
geology  of  the  terraqueous  globe  we  inhabit  may  thus  be  dismissed 
from  our  minds  while  considering  the  physical  features  of  the 
lunar  surface.  Fire  on  the  one  hand  and  water  on  the  other,  are  ? 
the  agents  to  which  the  configurations  of  the  earth's  surface  are 
referable  :  the  first  of  these  produced  the  igneous  rocks  that  form 
the  veritable  foundations  of  the  earth,  the  second  has  given  rise  to 
the  superstructure  of  deposits  that  constitute  the  secondary  and 
tertiary  formations :  were  these  last  removed  from  the  surface  of 
our  planet,  so  as  to  lay  bare  its  original  igneous  crust,  that  crust, 
so  far  as  reasoning  can  picture  it  to  us,  would  probably  not  differ 


CHAP,  v.]       NON-EXISTENCE    OF   A    LUNAR    ATMOSPHERE.  57 

essentially  from  the  visible  surface  of  the  moon.  In  considering 
the  causes  that  have  given  birth  to  the  diversified  features  of  that 
surface,  we  may,  therefore,  ignore  the  influence  of  air  and  water 
action  and  confine  our  reasoning  to  igneous  phenomena  alone :  our 
task  in  this  matter,  it  is  hardly  necessary  to  remark,  is  materially 
simplified  thereby. 


CHAPTER    VI. 

THE    GENERAL   ASPECT    OF    THE    LUNAR    SURFACE. 

We  have  now  reached  that  stage  of  our  subject  at  which  it 
behoves  us  to  repair  to  the  telescope  for  the  purpose  of  examining 
and  familiarising  ourselves  with  the  various  classes  of  detail  that 
the  lunar  surface  presents  to  our  view. 

That  the  moon  is  not  a  smooth  sphere  of  matter  is  a  fact  that 
manifested  itself  to  the  earliest  observers.  The  naked  eye 
perceives  on  her  face  spots  exhibiting  marked  differences  of 
illumination.  These  variations  of  light  and  shade,  long  before  the 
invention  of  the  telescope,  induced  the  belief  that  she  possessed 
surface  irregularities  like  those  that  diversify  the  face  of  the  earth, 
and  from  analogy  it  was  inferred  that  seas  and  continents  alter- 
nated upon  the  lunar  globe.  It  was  evident,  from  the  persistence 
and  invariability  of  the  dusky  markings,  that  they  were  not  due  to 
atmospheric  peculiarities,  but  were  veritable  variations  in  the 
character  or  disposition  of  the  surface  material.  Fancy  made 
pictures  of  these  unchangeable  spots  :  untutored  gazers  detected  in 
them  the  indications  of  a  human  countenance,  and  perhaps  the 
earliest  map  of  the  moon  was  a  rough  reproduction  of  a  man's  face, 
the  eyes,  nose  and  mouth  representing  the  more  salient  spots 
discernible  upon  the  lunar  disc.  Others  recognised  in  these  spots 
the  configuration  of  a  human  form,  head,  arms  and  legs  complete, 
which  a  French  superstition  that  lingers  to  the  present  day  held  to 
be  the  image  of  Judas  Iscariot  transported  to  the  moon  in  punish- 
ment for  his  treason.     Again,  an  Indian  notion  connects  the  lunar 


PLATE  IV. 


■Vroodburytype" 


FULL       MOON 


CHAP.  VI.]      GENERAL   ASPECT    OF   THE   LUNAR    SURFACE.  59 

spots  with  a  representation  of  a  roebuck  or  a  hare,  and  hence  the 
Sanskrit  names  for  the  moon,  mrigadhara,  a  roebuck-bearer,  and 
^sa'sabhrit,  a  hare-bearer.  Of  these  similitudes  the  one  which  has 
the  best  pretensions  to  a  rude  accuracy  is  that  first  mentioned ; 
for  the  resemblance  of  the  full  moon  to  a  human  countenance, 
wearing  a  painful  or  lugubrious  expression,  is  very  striking.  Our 
illustration  of  the  full  moon  (Plate  IV.)  is  derived  from  an  actual 
photograph  ;  *  the  relative  intensities  of  light  and  shade  are  hence 
somewhat  exaggerated  ;  otherwise  it  represents  the  full  moon  very 
nearly  as  the  naked  eye  sees  it,  and  by  gazing  at  the  plate  from  a 
short  distance,  t  the  well-known  features  will  manifest  themselves, 
while  they  who  choose  may  amuse  themselves  by  arranging  the 
markings  in  their  imagination  till  they  conform  to  the  other 
appearances  alluded  to. 

We  may  remark  in  passing  that  by  one  sect  of  ancient  writers 
the  moon  was  supposed  to  be  a  kind  of  mirror,  receiving  the  image 
of  the  earth  and  reflecting  it  back  to  terrestrial  spectators. 
Humboldt  affirmed  that  this  opinion  had  been  preserved  to  his 
day  as  a  popular  belief  among  Ihe  people  of  Asia  Minor.  He  says, 
*'I  was  once  very  much  astonished  to  hear  a  very  well  educated 
Persian  from  Ispahan,  who  certainly  had  never  read  a  Greek  book, 
mention  when  I  showed  him  the  moon's  spots  in  a  large  telescope 
in  Paris,  this  hypothesis  as  a  widely  diflused  belief  in  his  country  : 
V  *  What  we  see  in  the  moon,'  said  the  Persian,  *  is  ourselves ;  it  is 
the  map  of  our  earth.'  "  *'  Quite  as  extravagant  an  idea,  though 
perhaps  a  more  excusable  one,  was  that  held  by  some  ancient 
philosophers,  to  the  effect  that  the  spots  on  the  moon  were  the 
shadows  of  opaque  bodies  floating  in  space  between  it  and  the  sun. 

*  For  the  original  photograph  from  which  this  plate  was  produced,  and  for 
permission  to  reproduce  it,  we  owe  our  acknowledgments  to  Warren  De  la  Kue 
and  Joseph  Beck,  Esquires. 

f  The  proper  distance  for  realising  the  conditions  under  which  the  moon  itself 
is  seen  will  be  that  at  which  our  disc  is  just  covered  by  a  wafer  about  a  quarter  of 
an  inch  in  diameter,  held  at  arm's  length.  This  will  subtend  an  angle  of  about 
half  a  degree,  which  is  nearly  the  angular  diameter  of  the  moon. 


60  THE   MOON.  [chap.  vi. 

An  observer  watching  the  forms  and  positions  of  the  lunar  face- 
marks,  from  night  to  night  and  from  lunation  to  lunation,  cannot 
fail  to  notice  the  circumstance  that  they  undergo  no  easily 
perceptible  change  of  position  with  respect  to  the  circular  outline 
of  the  disc ;  that  in  fact  the  face  of  .the  moon  presented  to  our 
view  is  always  the  same,  or  very  nearly  so.  If  the  moon  had  no 
orbital  motion  we  should  be  led  from  the  above  phenomenon  to 
conclude  that  she  had  no  axial  motion,  no  movement  of  rotation ; 
but  when  we  consider  the  orbital  motion  in  connection  with  the 
permanence  of  aspect,  we  are  driven  to  the  conclusion — one,  how- 
ever, which  superficial  observers  have  some  difficulty  in  recognising 
— ^that  the  moon  has  an  axial  rotation  equal  in  period  to  her  orbital 
revolution.  Since  the  moon  makes  the  circuit  of  her  orbit  in 
twenty-seven  days  and  one -third  (more  exactly  27d.  7h.  43m.  lis.), 
it  follows  that  this  is  the  time  of  her  axial  rotation,  as  referred 
to  the  stars,  or  as  it  would  be  made  out  by  an  observer  located  at 
a  fixed  position  in  space  outside  the  lunar  orbit.  But  if  referred 
to  the  sun  this  period  appears  different ;  because  the  moon  while 
revolving  round  the  earth  is,  with  the  earth,  circulating  around  the 
sun.  Suppose  the  three  bodies,  moon,  earth,  and  sun,  to  be  in  a 
line  at  a  certain  period  of  a  lunation,  as  they  are  at  full  moon :  by 
the  time  the  moon  has  completed  her  twenty-seven  days'  journey 
around  the  earth,  the  latter  will  have  moved  along  twenty-seven 
days'  march  of  its  orbit,  which  is  about  twenty- seven  degrees  of 
celestial  longitude  :  the  sun  will  apparently  be  that  much  distant 
from  a  straight  line  passing  through  earth  and  moon,  and  the 
moon  must  therefore  move  forward  to  overtake  the  sun  before  she 
can  assume  the  full  phase  again.  She  will  take  something  over 
two  days  to  do  this ;  hence  the  solar  period  of  her  revolution 
becomes  more  than  twenty-nine  days  (to  be  exact,  29d.  12h.  44m. 
2s.  '87).  This  is  the  length  of  a  solar  day  upon  the  moon — the 
interval  from  one  sunrise  to  another  at  any  spot  upon  the  equator 
of  our  satellite,  and  the  interval  between  successive  reappearances 
of  the  same  phase  to  observers  on  the  earth.     The  physical  cause 


CHAP.  VI.]       GENERAL   ASPECT    OF   THE   LUNAR   SURFACE.  61 

of  the  coincidence  of  times  of  rotation  and  revolution  was  touched 
upon  in  a  previous  chapter. 

We  have  said  that  the  moon  continuously  presents  to  us  the  same 
hemisphere.  This  is  generally  true,  but  not  entirely  so.  Galileo, 
by  long  scrutiny,  familiarised  himself  with  every  detail  of  the  lunar 
disc  that  came  within  the  limited  grasp  of  his  telescopes,  and  he 
recognised  the  fact  that  according  as  the  position  of  the  moon  varied 
in  the  sky,  so  the  aspect  of  her  face  altered  to  a  slight  degree ;  that 
certain  regions  at  the  edge  of  her  disc  alternately  came  in  sight  and 
receded  from  his  view.  He  perceived,  in  fact,  an  appa7'e7it  rocking 
to  and  fro  of  the  globe  of  the  moon  ;  assort  of  balancing  or  libratory 
motion.  "When  the  moon  was  near  the  horizon  he  could  see  spots 
upon  her  uppermost  edge,  which  disappeared  as  she  approached  the 
zenith,  or  highest  point  of  her  nightly  path  ;  and  as  she  neared  this 
point,  other  spots,  before  invisible,  came  into  view,  near  to  what 
had  been  her  lower  edge.  Galileo  was  not  long  in  referring  this 
phenomenon  to  its  true  cause.  The  centre  of  motion  of  the  moon 
being  the  centre  of  the  earth,  it  is  clear  that  an  observer  on  the 
surface  of  the  latter,  looks  down  upon  the  rising  moon  as  from  an 
eminence,  and  thus  he  is  enabled  to  see  more  or  less  over  or  around 
her.  As  the  moon  increases  in  altitude,  the  line  of  sight  gradually 
becomes  parallel  to  the  line  joining  the  observer  and  the  centre  of 
the  earth,  and  at  length  he  looks  her  full  in  the  face  :  he  loses  the 
full  view  and  catches  another  side  face  view  as  she  nears  the  horizon 
in  setting.  This  phenomenon,  occurring  as  it  does,  with  a  daily 
period,  is  known  as  the  diurnal  lihration. 

But  a  kindred  phenomenon  presents  itself  in  another  period,  and 
from  another  cause.  The  moon  rotates  upon  her  axis  at  a  speed 
that  is  rigorously  uniform.  But  her  orbital  motion  is  not  uniform, 
sometimes  it  is  faster,  and  at  other  times  slower  than  its  average 
rate.  Hence,  the  angle  through  which  she  moves  along  her  orbit 
in  a  given  time,  now  exceeds,  and  now  falls  short  of  the  angle 
through  which  she  turns  upon  her  axis.  Her  visible  hemisphere 
thus  changes  to  an  extent  depending  upon  the  difference  between 


62  THE    MOON.  [chap.  vi. 

these  orbital  and  axial  angles,  and  the  apparent  balancing  thus 
produced  is  called  the  lihration  in  longitude.  Then  there  is  a  libra- 
tion  in  latitude  due  to  the  circumstance  that  the  axis  of  the  moon 
is  not  exactly  perpendicular  to  the  plane  of  her  orbit  ;  the  effect  of 
this  inclination  being,  that  we  sometimes  see  a  little  more  of  the 
north  than  of  the  south  polar  regions  of  our  satellite,  and  vice 
versa.* 

The  extent  of  the  moon's  librations,  taking  them  all  and  in  com- 
bination into  account,  amounts  to  about  seven  degrees  of  arc  of 
latitude  or  longitude  upon  the  moon,  both  in  the  north- south  and 
east- west  directions.  And  taking  into  account  the  whole  effect  of 
them,  we  may  conclude  that  our  view  of  the  moon's  surface,  instead 
of  being  confined  to  one  half,  is  extended  really  to  about  four- 
sevenths  of  the  whole  area  of  the  lunar  globe.  The  remaining 
three- sevenths   must   for   ever   remain  a  terra  incognita    to   the 

*  The  libratory  movement  has  been  taken  advantage  of,  at  the  suggestion  of  Sir 
Chas.  Wheatstone,  for  producing  stereoscopic  photographs  of  the  moon.  In  the 
early  days  of  stereoscopic  photography  the  object  to  be  photographed  was  placed 
upon  a  kind  of  turn-table,  and,  after  a  picture  had  been  taken  of  it  in  one  position, 
the  table  was  turned  through  a  small  angle  for  the  taking  of  the  second  picture  ; 
the  two  placed  side  by  side  then  represented  the  object  as  it  would  have  been  seen 
by  two  eyes  widely  separated,  or  whose  visual  rays  inclined  at  an  angle  equal  to 
that  through  which  the  table  was  turned  ;  and  when  the  pictures  were  viewed 
through  a  stereoscope,  they  combined  to  produce  the  wonderful  effect  of  solidity 
now  familiar  to  every  one.  The  moon,  by  its  librations,  imitates  the  turn-table 
movement;  and,  from  a  large  number  of  photographs  of  her,  taken  at  different 
points  of  her  orbit  and  at  different  seasons  of  the  year,  it  is  possible  to  select  two 
which,  while  they  exhibit  the  same  phase  of  illumination,  at  the  same  time  present 
the  requisite  difference  in  the  points  of  view  from  which  they  are  taken  to  give  the 
effect  of  stereoscopicity  when  viewed  binocularly.  Mr.  De  la  Rue,  the  father  of 
celestial  photography,  has  been  enabled  to  produce  several  such  pairs  of  pictures 
from  the  vast  collection  of  lunar  photographs  that  he  has  accumulated.  Any  one 
of  these  pairs  of  portraits,  when  stereoscopically  combined,  reproduces,  to  quote 
the  words  of  Sir  John  Herschel,  "  the  spTierical  form  just  as  a  giant  might  see  it 
whose  stature  were  such  that  the  interval  between  his  eyes  should  equal  the  dis- 
tance between  the  place  where  the  earth  stood  when  one  view  was  taken,  and  that 
to  which  it  would  have  to  be  removed  (our  moon  being  fixed)  to  get.  the  other. 
Nothing  can  surpass  the  impression  of  real  corporeal  form  thus  conveyed  by  some 
of  these  pictures  as  taken  by  Mr.  De  la  Rue  with  his  powerful  reflector,  the 
production  of  which  (as  a  step  in  some  sort  taken  by  man  outside  of  the  planet  he 
inhabits)  is  one  of  the  most  remarkable  and  unexpected  triumphs  of  scientific  art." 


CHAP.  VI.]      GENERAL    ASPECT   OF   THE   LUNAR   SURFACE.  68 

habitants  of  this  earth,  unless,  indeed,  from  some  catastrophe  which 
it  would  be  wild  fancy  to  anticipate,  a  period  of  rotation  should  be 
given  to  the  moon  different  from  that  which  it  at  present  possesses. 
Some  highly  fanciful  theorists  have  speculated  upon  the  possible 
condition  of  the  invisible  hemisphere,  and  have  propounded  the 
absurd  notion  that  the  opposite  side  of  the  moon  is  hollow,  or  that 
the  moon  is  a  mere  shell ;  others  again  have  urged  that  the  hidden 
half  is  more  or  less  covered  with  water,  and  others  again, 
that  it  is  peopled  with  inhabitants.  There  is,  however,  no  good 
reason  for  supposing  that  what  we  may  call  the  back  of  the  moon 
has  a  physical  structure  essentially  diiferent  from  the  face  presented 
towards  us.  So  far  as  can  be  judged  from  the  peeps  that  libration 
enables  us  to  obtain,  the  same  characteristic  features  (though  of 
course  with  different  details)  prevail  over  the  whole  lunar  surface. 

The  speculative  ideas  held  by  the  philosophers  of  the  pre-tele- 
scopic  age,  touching  the  causes  which  produced  the  inequalities  of 
light  and  shade  upon  the  moon,  received  their  coup  de  grace  from 
the  revelations  of  Galileo's  glasses.  Our  satellite  was  one  of  the 
earliest  objects,  if  not  actually  the  first,  upon  which  the  Florentine 
turned  his  telescope  ;  and  he  found  that  the  inequalities  upon  her 
surface  were  due  to  differences  in  its  configuration  analogous  to  the 
continents  and  islands,  and  (as  might  then  have  been  thought)  the 
seas  of  our  globe.  He  could  trace,  even  with  his  moderate  means, 
the  semblance  of  mountain-tops  upon  which  the  sun  shone  while 
their  lower  parts  were  in  shadow,  of  hills  that  were  brightly 
illuminated  upon  their  sides  towards  the  sun,  of  brightly  shining 
elevations,  and  deeply  shadowed  depressions,  of  smooth  plains,  and 
regions  of  mountainous  ruggedness.  He  saw  that  the  boundary  of 
sunlight  upon  the  moon  was  not  a  clearly  defined  line,  as  it  would 
be  if  the  lunar  globe  were  a  smooth  sphere,  as  the  Aristotelians  had 
asserted,  but  that  the  terminator  was  uneven  and  broken  into 
an  irregular  outline.  From  these  observations  the  Florentine 
astronomer  concluded  that  the  lunar  world  was  covered  not  only 
with  mountains  like  our  globe,  but  with  mountains  whose  heights 


64  THE    MOON.  [chap.  vi. 

far  surpassed  those  existing  upon  the  earth,  and  whose  forms  were 
strangely  limited  to  circularity. 

Galileo's  best  telescopes  magnified  only  some  thirty  times,  and 
the  views  which  he  thus  obtained,  must  have  been  similar  to  those 
exhibited  by  the  smaller  photographs  of  the  moon  produced  in  late 
years  by  Mr.  De  la  Rue  and  now  familiar  to  the  scientific  public. 
Of  course  there  is  in  the  natural  moon  as  viewed  with  a  small  tele- 
scope a  vivid  brilliancy  which  no  art  can  imitate,  and  in  photographs 
especially  there  is  a  tendency  to  exaggeration  of  the  depths  of 
shade  in  a  lunar  picture.  This  arises  from  the  circumstance  that 
various  regions  of  the  moon  do  not  impress  a  chemically  sensitized 
plate  as  they  impress  the  retina  of  the  eye.  Some  portions, 
notably  the  so-called  *'  seas  "  of  the  moon,  which  to  the  eye  appear 
but  slightly  duller  than  the  brighter  parts,  give  off  so  little  actinic 
light  that  they  appear  as  nearly  black  patches  upon  a  photograph, 
and  thus  give  an  undue  impression  of  the  relative  brightness  of 
various  parts  of  the  lunar  surface.  Doubtless  by  sufficient  exposure 
of  the  plate  in  the  camera-telescope  the  dark  patches  might  be  ren- 
dered lighter,  but  in  that  case  the  more  strongly  illuminated  por- 
tions, which  after  all  are  those  most  desirable  to  be  preserved, 
would  be  lost  by  the  effect  which  photographers  understand  as 
**  solarization." 

In  speaking  of  a  view  of  the  moon  with  a  magnifying  power  of 
thirty,  it  is  necessary  to  bear  in  mind  that  the  visible  features  will 
differ  considerably  with  the  diameter  of  the  object-glass  of  the 
telescope  to  which  this  power  is  applied.  The  same  details  would 
not  be  seen  alike  with  the  same  power  upon  an  object-glass  of  10 
inches  diameter  and  one  of  2  inches.  The  superior  illumination  of 
the  image  in  the  former  case  would  bring  into  view  minute  details' 
that  could  not  be  perceived  with  the  smaller  aperture.  He  who 
would  for  curiosity  wish  to  see  the  moon,  or  any  other  object,  as 
Galileo  saw  it,  must  use  a  telescope  of  the  same  size  and  character 
in  all  respects  as  Galileo's  :  it  will  not  do  to  put  his  magnifying 
power  upon  a  larger  telescope.     With  large  telescopes,  and  low 


CHAP.  VI.]      GENERAL  ASPECT  OF  THE   LUNAR   SURFACE.  65 

powers  used  upon  bright  objects  like  the  moon,  there  is  a  blinding 
flood  of  light  which  tends  to  contract  the  pupil  of  the  eye  and  pre- 
vent the  passage  of  the  whole  of  the  pencil  of  rays  coming  through 
the  eye-piece.  Although  this  last  result  may  be  productive  of  no 
inconvenience,  it  is  clearly  a  waste  of  light,  and  it  points  to  a  rule 
that  the  lowest  power  that  a  telescope  should  bear  is  that  which 
gives  a  pencil  of  light  equal  in  diameter  to  the  pupil  of  the  eye 
under  the  circumstances  of  brightness  attendant  upon  the  object 
viewed.  In  observing  faint  objects  this  point  assumes  more 
importance,  since  it  is  then  necessary  that  all  available  light  should 
enter  the  pupil.  The  thought  suggests  itself  that  an  artificial 
enlargement  of  the  pupil,  as  by  a  dose  of  belladonna,  might  be  of 
assistance  in  searching  for  faint  objects,  such  as  nebulae  and 
comets  :  but  we  prefer  to  leave  the  experiment  for  those  to  try 
who  pursue  that  branch  of  astronomical  observation. 

A  merely  cursory  examination  of  the  moon  with  the  low  power  to 
which  we  have  alluded  is  sufficient  to  show  us  the  more  salient  fea- 
tures. In  the  first  place  we  cannot  help  being  struck  with  the 
immense  preponderance  of  circular  or  craterform  asperities,  and 
with  the  general  tendency  to  circular  shape  which  is  apparent  in 
nearly  all  the  lunar  surface  markings ;  for  even  the  larger  regions 
known  as  the  "  seas  "  and  the  smaller  patches  of  the  same  character 
seem  to  repeat  in  their  outlines  the  round  form  of  the  craters.  It 
is  at  the  boundary  of  sunlight  on  the  lunar  globe  that  we  see  these 
craterform  spots  to  the  best  advantage,  as  it  is  there  that  the  rising 
or  setting  sun  casts  long  shadows  over  the  lunar  landscape,  and 
brings  elevations  and  asperities  into  bold  relief.  They  vary  greatly 
in  size,  some  are  so  large  as  to  bear  an  estimable  proportion  to  the 
moon's  diameter,  and  the  smallest  are  so  minute  as  to  need  the 
most  powerful  telescopes  and  the  finest  conditions  of  atmosphere  to 
perceive  them.  It  is  doubtful  whether  the  smallest  of  them  have 
ever  been"  seen,  for  there  is  no  reason  to  doubt  that  there  exist 
countless  numbers  that  are  beyond  the  revealing  powers  of  our 
finest  telescopes. 


66  THE    MOON.  [chap.  vi. 

From  tlie  great  number  and  persistent  character  of  tliese  circum- 
vallations,  Kepler  was  led  to  think  that  they  were  of  artificial  con- 
struction. He  regarded  them  as  pits  excavated  by  the  supposed 
habitants  of  the  moon  to  shelter  themselves  from  the  long  and 
intense  action  of  the  sun.  Had  he  known  their  real  dimensions,  of 
which  we  shall  have  to  speak  when  we  come  to  describe  them  more 
in  detail,  he  would  have  hesitated  in  propounding  such  a  hypothesis ; 
nevertheless  it  was,  to  a  certain  extent,  justified  by  the  regular  and 
seemingly  unnatural  recurrence  of  one  particular  form  of  structure, 
the  like  of  which  is,  too,  so  seldom  met  with  as  a  structural  feature 
of  the  surface  of  our  own  globe. 

The  next  most  striking  features,  revealed  by  a  low  telescopic 
power  upon  the  moon,  are  the  seemingly  smooth  plains  that  have 
the  appearance  of  dusky  spots,  and  that  collectively  cover  a  con- 
siderable portion — about  two-thirds — of  the  entire  disc.  The 
larger  of  these  spots  retain  the  name  of  seas,  the  term  having  been 
given  when  they  were  supposed  to  be  watery  expanses,  and  having 
been  retained,  possibly  to  avoid  the  confusion  inevitable  from  a 
change  of  name,  after  the  existence  of  water  upon  the  moon  was 
disproved.  Following  the  same  order  of  nomenclature,  the  smaller 
spots  have  received  the  appellations  of  lakes,  hays,  and  fens.  We 
see  that  many  of  these  "seas"  are  partially  surrounded  by  ramparts 
or  bulwarks  which,  under  closer  examination,  and  having  regard  to 
their  real  magnitude,  resolve  themselves  into  immense  mountain 
chains.  The  general  resemblance  in  form  which  the  bulwarked 
plains  thus  exhibit  to  the  circular  craters  of  large  size,  would  lead 
us  to  suppose  that  the  two  classes  of  objects  had  the  same  formative 
origin,  but  when  we  take  into  account  the  immense  size  of  the 
former,  and  the  process  by  which  we  infer  the  latter  to  have  been 
developed,  the  supposition  becomes  untenable. 

Another  of  the  prominent  features  which  we  notice  as  highly 
curious,  and  in  some  phases  of  the  moon — at  about  the  time  of  full 
— the  most  remarkable  of  all,  are  certain  bright  lines  that  appear 
to  radiate  from  some  of  the  more  conspicuous  craters,  and  extend 


CHAP.  VI.]      GENERAL   ASPECT  OF   THE   LUNAR   SURFACE.  67 

for  hundreds  of  miles  around.  No  selenological  formations  have 
so  sorely  puzzled  observers  as  these  peculiar  streaks,  and  a  great 
deal  of  fanciful  theorizing  has  been  bestowed  upon  them.  As  we 
are  now  only  glancing  at  the  moon,  we  do  not  enter  upon  explana- 
tions concerning  them  or  any  other  class  of  details  ;  all  such  will 
receive  due  consideration  in  their  proper  order  in  succeeding 
chapters. 

We  thus  see  that  the  classes  of  features  observable  upon  the 
moon  are  not  great  in  number  :  they  may  be  summed  up  as  craters 
and  their  central  cones,  mountain  chains^  with  occasional  isolated 
peaks,  smooth  plains^  with  more  or  less  of  irregularity  of  surface, 
and  bright  radiating  streaks.  But  when  we  come  to  study  with 
higher  powers  the  individual  examples  of  each  class  we  meet  with 
considerable  diversity.  This  is  especially  the  case  with  the  craters, 
which  appear  under  very  numerous  variations  of  the  one  order  of 
structure,  viz.,  the  ring-form.  A  higher  telescopic  power  shows  us 
that  not  only  do  these  craters  exist  of  all  magnitudes  within  a  limit 
of  largeness,  but  seemingly  with  no  limit  of  smallness,  but  that  in 
their  structure  and  arrangement  they  present  a  great  variety  of 
points  of  difference.  Some  are  seen  to  be  considerably  elevated 
above  the  surrounding  surface,  others  are  basins  hollowed  out  of 
that  surface  and  with  low  surrounding  ramparts ;  some  are  merely 
like  walled  plains  or  amphitheatres  with  flat  plateaux,  while  the 
majority  have  their  lowest  point  of  hollowness  considerably  below 
the  general  level  of  the  surrounding  surface ;  some  are  isolated 
upon  the  plains,  others  are  aggregated  into  a  thick  crowd,  and 
overlapping  and  intruding  upon  each  other;  some  have  elevated 
peaks  or  cones  in  their  centres,  and  some  are  without  these  central 
cones,  while  the  plateaux  of  others  again  contain  several  minute 
craters  instead ;  some  have  their  ramparts  whole  and  perfect,  others 
have  them  breached  or  malformed,  and  many  have  them  divided 
into  terraces,  especially  on  their  inner  sides. 

In  the  plains,  what  with  a  low  power  appeared  smooth  as  a  water 
surface  becomes,  under  greater  magnification,  a  rough  and  furrowed 

F  2 


68  THE   MOON.  [chap.  vi. 

area,  here  gently  undulated  and  there  broken  into  ridges  and 
declivities,  with  now  and  then  deep  rents  or  cracks  extending  for 
miles  and  spreading  like  river-beds  into  numerous  ramifications. 
Craters  of  all  sizes  and  classes  are  scattered  over  the  plains ;  these 
appear  generally  of  a  different  tint  to  the  surrounding  surface,  for 
the  light  reflected  from  the  plains  has  been  observed  to  be  slightly 
tinged  with  colour.  The  tint  is  not  the  same  in  all  cases  :  one 
large  sea  has  a  dingy  greenish  tinge,  others  are  merely  grey,  and 
some  others  present  a  pale  reddish  hue.  The  cause  of  this  diver- 
sity of  colour  is  mysterious  ;  it  has  been  supposed  to  indicate  the 
existence  of  vegetation  of  some  sort ;  but  this  involves  conditions 
that  we  know  do  not  exist. 

The  mountains,  under  higher  magnification,  do  not  present  such 
diversity  of  formation  as  the  craters,  or  at  least  the  points  of 
difference  are  not  so  apparent ;  but  they  exhibit  a  plentiful  variety 
of  combinations.  There  are  a  few  perfectly  isolated  examples  that 
cast  long  shadows  over  the  plains  on  which  they  stand  like  those  of 
a  towering  cathedral  in  the  rising  or  setting  sun.  Sometimes  they 
are  collected  into  groups,  but  mostly  they  are  connected  into 
stupendous  chains.  In  one  of  the  grandest  of  these  chains,  it  has 
been  estimated  that  a  good  telescope  will  show  3000  mountains 
clustered  together,  without  approach  to  symmetrical  order.  The 
scenery  which  they  would  present,  could  we  get  any  other  than  the 
"  bird's  eye  view  "  to  which  we  are  confined,  must  be  imposing  in 
the  extreme,  far  exceeding  in  sublime  grandeur  anything  that  the 
Alps  or  the  Himalayas  offer ;  for  while  on  the  one  hand  the  lunar 
mountains  equal  those  of  the  earth  in  altitude,  the  absence  of  an 
atmosphere,  and  consequently  of  the  effects  produced  thereby,  must 
give  rise  to  alternations  of  dazzling  light  and  black  depths  of  shade 
combining  to  form  panoramas  of  wild  scenery  that,  for  want  of  a 
parallel  on  earth,  we  may  well  call  unearthly.  But  we  are  debarred 
the  pleasure  of  actually  contemplating  such  pictures  by  the  circum- 
stance that  we  look  down  upon  the  mountain  tops  and  into  the 
valleys,  so  that  the  great  height  and  close  aggregation  of  the  peaks 


CHAP.  VI.]      GENERAL   ASPECT   OF   TEE   LUNAR   SURFACE.  69 

and  hills  are  not  so  apparent.  To  compare  the  lunar  and  terrestrial 
inountain  scenery  would  be  **  to  compare  the  different  views  of  a 
town  seen  from  the  car  of  a  balloon  with  the  more  interesting 
prospects  by  a  progress  through  the  streets."  Some  of  the  pecu- 
liarities of  the  lunar  scenery  we  have,  however,  endeavoured  to 
realize  in  a  subsequent  chapter. 

A  high  power  gives  us  little  more  evidence  than  a  low  one  upon 
the  nature  of  the  long  bright  streaks  that  radiate  from  some  of  the 
more  conspicuous  craters,  but  it  enables  us  to  see  that  those  streaks 
do  not  arise  from  any  perceptible  difference  of  level  of  the  surface 
— that  they  have  no  very  definite  outline,  and  that  they  do  not 
present  any  sloping  sides  to  catch  more  sunlight,  and  thus  shine 
brighter,  than  the  general  surface.  Indeed,  one  great  peculiarity 
of  them  is  that  they  come  out  most  forcibly  where  the  sun  is 
shining  perpendicularly  upon  them ;  hence  they  are  best  seen 
where  the  moon  is  at  full,  and  they  are  not  visible  at  all  at  those 
regions  upon  which  the  sun  is  rising  or  setting.  We  also  see  that 
they  are  not  diverted  by  elevations  in  their  path,  as  they  traverse  in 
their  course  craters,  mountains,  and  plains  alike,  giving  a  slight 
additional  brightness  to  all  objects  over  which  they  pass,  but 
producing  no  other  effect  upon  them.  To  employ  a  commonplace 
simile,  they  look  as  though,  after  the  whole  surface  of  the  moon 
had  assumed  its  final  configuration,  a  vast  brush  charged  with  a 
whitish  pigment  had  been  drawn  over  the  globe  in  straight  lines 
radiating  from  a  central  point,  leaving  its  trail  upon  everything  it 
touched,  but  obscuring  nothing. 

Whatever  may  be  the  cause  that  produces  this  brightness  of 
certain  parts  of  the  moon  without  reference  to  configuration  of 
surface,  this  cause  has  not  been  confined  to  the  formation  of  the 
radiating  lines,  for  we  meet  with  many  isolated  spots,  streaks,  and 
patches  of  the  same  bright  character.  Upon  some  of  the  plains 
there  are  small  areas  and  lines  of  luminous  matter  possessing 
peculiarities  similar  to  those  of  the  radiating  streaks,  as  regards 
visibility  with  the  high  sun,  and  invisibility  when  the  solar  rays 


70  THE    MOON.  [chap.  vi. 

fall  upon  tliem  liorizontally.  Some  of  the  craters  also  are  sur- 
rounded by  a  kind  of  aureole  of  this  highly  reflective  matter.  A 
notable  specimen  is  that  called  Linne,  concerning  which  a  great 
hue  and  cry  about  change  of  appearance  and  inferred  continuance 
of  volcanic  action  on  the  moon  was  raised  some  years  ago.  This 
object  is  an  insignificant  little  crater  of  about  a  mile  or  two  in 
diameter,  in  the  centre  of  an  ill- defined  spot  of  the  character 
referred  to,  and  about  eight  or  ten  miles  in  diameter.  With  a  low 
sun  the  crater  alone  is  visible  by  its  shadow  ;  but  as  the  luminary 
rises  the  shadow  shortens  and  becomes  all  but  invisible,  and  then 
the  white  spot  shines  forth.  These  alternations,  complicated  by 
variations  of  atmospheric  condition,  and  by  the  interpretations  of 
difi'erent  observers,  gave  rise  to  statements  of  somewhat  exagge- 
rated character  to  the  effect  that  considerable  changes,  of  the  nature 
of  volcanic  eruptions,  were  in  progress  in  that  particular  region  of 
the  moon. 

In  the  foregoing  remarks  we  have  alluded  somewhat  indefinitely 
to  high  powers ;  and  an  enquiring  but  unastronomical  reader  may 
reasonably  demand  some  information  upon  this  point.  It  might 
have  been  instructive  to  have  cited  the  various  details  that  may  be 
said  to  come  into  view  with  progressive  increases  of  magnification. 
But  this  would  be  an  all  but  impossible  task,  on  account  of  the 
varying  conditions  under  which  all  astronomical  observations  must 
necessarily  be  made.  When  we  come  to  delicate  tests,  there  are 
no  standards  of  telescopic  power  and  definition.  Assuming  the 
instrument  to  be  of  good  size  and  high  optical  character,  there  is  yet 
a  powerful  influencer  of  astronomical  definition  in  the  atmosphere 
and  its  variable  state.  Upon  two-thirds  of  the  clear  nights  of  a 
year  the  finest  telescopes  cannot  be  used  to  their  full  advantage, 
because  the  minute  flutterings  resulting  from  the  passage  of  the 
rays  of  light  through  moving  strata  of  air  of  different  densities  are 
magnified  just  as  the  image  in  the  telescope  is  magnified,  till  all 
minute  details  are  blurred  and  confused,  and  only  the  grosser 
features  are  left  visible.      And  supposing  the  telescope  and  atmo- 


CHAP.  VI.]      GENERAL   ASPECT    OF   THE   LUNAR   SURFACE.  71 

sphere  in  good  state,  there  is  still  an  important  point,  the  state  of 
the  observer's  eye,  to  be  considered.  After  all  it  is  the  eye  that 
sees,  and  the  best  telescopic  assistance  to  an  untrained  eye  is  of 
small  avail.  The  eye  is  as  susceptible  of  education  and  develop- 
ment as  any  other  organ ;  a  skilful  and  acute  observer  is  to  a  mere 
casual  gazer  what  a  watchmaker  would  be  to  a  ploughman,  a 
miniature  painter  to  a  whitewasher.  This  fact  is  not  generally 
recognized ;  no  man  would  think  of  taking  in  hand  an  engraver's 
burin,  and  expecting  on  the  instant  to  use  it  like  an  adept,  or  of 
going  to  a  smithy  and  without  previous  preparation  trying  to  forge 
a  horse-shoe.  Yet  do  folks  enter  observatories  with  uneducated 
eyes,  and  expect  at  once  to  realise  all  the  wonderful  things  that 
their  minds  have  pictured  to  themselves  from  the  perusal  of  astro- 
nomical books.  We  have  over  and  over  again  remarked  the 
dissatisfaction  which  attends  the  first  looks  of  novices  through  a 
powerful  telescope.  They  anticipate  immediately  beholding 
wonders,  and  they  are  disappointed  at  finding  how  little  they  can 
see,  and  how  far  short  the  sight  falls  of  what  they  had  expected. 
Courtesy  at  times  leads  them  to  express  wonder  and  surprise, 
which  it  is  easy  to  see  is  not  really  felt,  but  sometimes  honesty 
compels  them  to  give  expression  to  their  disappointment.  This 
arises  from  the  simple  fact  that  their  eyes  are  not  fit  for  the  work 
which  is  for  the  moment  imposed  upon  them  ;  they  know  not  what 
to  look  for,  or  how  to  look  for  it.  The  first  essay  at  telescopic 
gazing,  like  first  essays  generally,  serves  but  to  teach  us  our 
incapability. 

To  a  tutored  eye  a  great  deal  is  visible  with  a  comparatively  low 
power,  and  practised  observers  strive  to  use  magnifying  powers  as 
low  as  possible,  so  as  to  diminish,  as  far  as  may  be,  the  evils 
arising  from  an  untranquil  atmosphere.  With  a  power  so  small  as 
30  or  40,  many  exceedingly  delicate  details  on  the  moon  are  visible 
to  an  eye  that  is  familiar  with  them  under  higher  powers.  With 
200  we  may  say  that  every  ordinary  detail  will  come  out  under 
favourable  conditions ;  but  when  minute  points  of  structure,  mere 


72  THE   MOON.  [chap.  vi. 

nooks  and  corners  as  it  were,  are  to  be  scrutinised,  300  may  be 
used  with  advantage.  Another  hundred  diameters  almost  passes 
the  practical  limit.  Unless  the  air  be  not  merely  fine,  but  super- 
fine, the  details  become  "  clothy  "  and  tremulous  ;  the  extra  points 
brought  out  by  the  increased  power  are  then  only  caught  by 
momentary  glimpses,  of  which  but  a  very  few  are  obtained  during 
a  lengthy  period  of  persistent  scrutiny.  We  may  set  down  250  as 
the  most  useful,  and  350  the  utmost  effective  power  that  can  be 
employed  upon  the  particular  work  of  which  we  are  treating. 
Could  every  detail  on  the  moon  be  thoroughly  and  reliably  repre- 
sented as  this  amount  of  magnification  shows  it,  the  result  would 
leave  little  to  be  wished  for. 

.  But  it  may  be  asked  by  some,  what  is  the  absolute  effect  of  such 
powers  as  those  we  have  spoken  of,  in  bringing  the  moon  apparently 
nearer  to  our  eyes?  and  what  is  the  actual  size  of  the  smallest 
object  visible  under  the  most  favourable  circumstances  ?  A  linear 
mile  upon  the  moon  corresponds  to  an  angular  interval  of  0*87  of  a 
second ;  this  refers  to  regions  about  the  centre  of  the  disc ;  near 
the  circumference  the  foreshortening  makes  a  difference,  very  great 
as  the  edge  is  approached.  Perhaps  the  smallest  angle  that  the 
eye  can  without  assistance  appreciate  is  half  a  minute ;  that  is  to 
say,  an  object  that  subtends  to  the  eye  an  arc  of  less  than  half  a 
minute  can  scarcely  be  seen.*  Since  there  are  60  seconds  in  a 
minute,  it  follows  that  we  must  magnify  a  spot  a  second  in  diameter 
upon  the  moon  thirty  times  before  we  can  see  it ;  and  since  a 
second  represents  rather  more  than  a  mile,  really  about  2000 
yards,  on  the  moon,  as  seen  from  the  earth,  the  smallest  object 
visible  with  a  power  of  30  will  be  this  number  of  yards  in  diameter 
or  breadth.  To  see  an  object  200  yards  across,  we  should  require 
to  magnify  it  300  times,  and  this  would  only  bring  it  into  view  as  a 

*  This  is  a  point  of  some  uncertainty.  Dr.  Young  stated  (Lectures  Vol.  II. 
p.  575)  that  "  a  minute  is  perhaps  nearly  the  smallest  interval  at  which  two  objects 
can  be  distinguished,  although  a  line  subtending  only  a  tenth  of  a  minute  in  breadth 
may  sometimes  be  perceived  as  a  single  object." 


CHAP.  VI.]      GENERAL  ASPECT   OF   THE   LUNAR   SURFACE.  73 

point ;  20  yards  would  require  a  power  of  3000,  and  1  yard  60,000 
to  effect  the  same  thing.  Since,  as  we  have  said,  the  highest 
practicable  power  with  our  present  telescopes,  and  at  ordinary 
terrestrial  elevations,  is  350,  or  for  an  extreme  say  400,  it  is 
evident  that  the  minutest  lunar  object  or  detail  of  which  we  can 
perceive  as  a  point  must  measure  about  150  yards  :  to  see  the  form 
of  an  object,  so  as  to  discriminate  whether  it  be  round  or  square,  it 
would  require  to  be  probably  twice  this  size ;  for  it  may  be  safely 
assumed  that  we  cannot  perceive  the  outline  of  an  object  whose 
average  breadth  subtends  a  less  angle  than  a  minute. 

Arago  put  this  question  into  another  shape  : — The  moon  is 
distant  from  us  237,000  miles  (mean).  A  magnifying  power  of  a 
thousand  would  show  us  the  moon  as  if  she  were  distant  237  miles 
from  the  naked  eye. 

2000  would  bring  her  within  118  miles. 
4000      „  „  „         59      „ 

6000      „  „  „         39      „ 

Mont  Blanc  is  visible  to  the  naked  eye  from  Lyons,  at  the  distance 
of  about  100  miles ;  so  that  to  see  the  mountains  of  the  moon  as 
Mont  Blanc  is  seen  from  Lyons  would  require  the  impracticable 
power  of  2500. 


CHAPTER    VII. 

TOPOGRAPHY    OF    THE    MOON. 

It  is  scarcely  necessary  to  seek  the  reasons  which  prompted 
astronomers,  soon  after  the  invention  of  the  telescope,  to  map  the 
surface  features  of  the  moon.  They  may  have  considered  it  desir- 
able to  record  the  positions  of  the  spots  upon  her  disc,  for  the  pur- 
pose of  facilitating  observations  of  the  passage  of  the  earth's 
shadow  over  them  in  lunar  eclipses  ;  or  they  may  have  been  actu- 
ated by  a  desire  to  register  appearances  then  existing,  in  order  that 
if  changes  took  place  in  after  years  these  might  be  readily  detected. 
Scheiner  was  one  of  the  earliest  of  lunar  cartographers  ;  he  worked 
about  the  middle  of  the  seventeenth  century ;  but  his  delineations 
were  very  rough  and  exaggerated.  Better  maps — ^the  best  of  the 
time,  according  to  an  old  authority — were  engraved  by  one  Mellan, 
about  the  years  1634  or  1635.  At  about  the  same  epoch  Langreen 
and  Hevelius  were  working  upon  the  same  subject.  Langreen 
executed  some  thirty  maps  of  portions  of  the  moon,  and  introduced 
the  practice  of  naming  the  spots  after  philosophers  and  eminent 
men.  Hevelius  spent  several  years  upon  his  task,  the  results  of 
which  he  published  in  a  bulky  volume  containing  some  50  maps  of 
the  moon  in  various  phases,  and  accompanied  by  500  pages  of 
letter-press.  He  rejected  Langreen's  system  of  nomenclature,  and 
called  the  spots  after  the  seas  and  continents  of  the  earth  to  which 
he  conceived  they  bore  resemblance.  Riccioli,  another  seleno- 
grapher,  whose  map  was  compiled  from  observations  made  by 
Grimaldi,  restored  Langreen's  nomenclature,  but  he  confined  him- 


CHAP.  VII.]  TOPOGRAPHY    OF    THE    MOON.  75 

self  to  the  names  of  eminent  astronomers,  and  his  system  has 
gained  the  adhesion  of  the  map-makers  of  later  times.  Cassini 
prepared  a  large  map  from  his  own  observations,  and  it  was 
engraved  about  the  year  1692.  It  appears  to  have  been  regarded 
as  a  standard  work,  for  a  reduced  copy  of  it  was  repeatedly  issued 
with  the  yearly  volumes  of  the  Connaissance  des  Temps  (the 
*'  Nautical  Almanack  "  of  France)  some  time  after  its  publication. 
These  small  copies  have  no  great  merit :  the  large  copper  plate  of 
the  original  was,  we  are  told  by  Arago,  who  received  the  statement 
from  Bouvard,  sold  to  a  brazier  by  a  director  of  the  French  Govern- 
ment Printing- Office,  who  thought  proper  to  disembarrass  the 
stores  of  that  establishment,  by  ridding  them  of  what  he  considered 
lumber  !  La  Hire,  Mayer,  and  Lambert  followed,  during  the 
succeeding  century,  in  this  branch  of  astronomical  delineation.  At 
the  commencement  of  the  present  century,  the  subject  was  very 
earnestly  taken  up  by  the  indefatigable  Schroeter,  who,  although 
he  does  not  appear  to  have  produced  a  complete  map,  produced  a 
topograph  of  the  moon  in  a  large  series  of  partial  maps  and  draw- 
ings of  special  features.  Schroeter  was  a  fine  observer,  but  his 
delineations  show  him  to  have  been  an  indifferent  draughtsman. 
Some  of  his  drawings  are  but  the  rudest  representations  of  the 
objects  he  intended  to  depict;  many  of  the  bolder  features  of  con- 
spicuous objects  are  scarcely  recognizable  in  them.  A  bad  artist  is 
as  likely  to  mislead  posterity  as  a  bad  historian,  and  it  cannot  be 
surprising  if  observers  of  this  or  future  generations,  accepting 
Schroeter's  drawings  as  faithful  representations,  should  infer  from 
them  remarkable  changes  in  the  lunar  details.  It  is  much  to  be 
regretted  that  Schroeter's  work  should  be  thus  depreciated.  Lohr- 
man  of  Dresden,  was  the  next  cartographer  of  the  moon ;  in  1824 
he  put  forth  a  small  but  very  excellent  map  of  15  inches  diameter, 
and  published  a  book  of  descriptive  text,  accompanied  by  sectional 
charts  of  particular  areas.  His  work,  however,  was  eclipsed  by  the 
great  one  which  we  owe  to  the  joint  energy  of  MM.  Beer  and 
Maedler,  and  which  represents  a  stupendous  amount  of  observing 


76  THE    MOON.  [chap.  vii. 

work  carried  on  during  several  years  prior  to  1836,  the  date  of 
their  publication.  The  long  and  patient  labour  bestowed  upon 
their  map  and  upon  the  measures  on  which  it  depends,  deserve  the 
highest  praise  which  those  conversant  with  the  subject  can  bestow, 
and  it  must  be  very  long  before  their  efforts  can  be  superseded. 

Beer  and  Maedler's  map  has  a  diameter  of  37  inches :  it  repre- 
sents the  phase  of  the  moon  visible  in  the  condition  of  mean  libra- 
tion.  The  details  were  charted  by  a  careful  process  of  triangula- 
tion.  The  disc  was  first  divided  into  "  triangles  of  the  first  order,*' 
the  points  of  which  (conspicuous  craters)  were  accurately  laid  down 
by  reference  to  the  edges  of  the  disc  :  one  hundred  and  seventy-six 
of  these  triangles,  plotted  accurately  upon  an  orthographic  projec- 
tion of  the  hemisphere,  formed  the  reliable  basis  for  their  charting 
work.  From  these  a  great  number  of  "  points  of  the  second  order  " 
were  laid  down,  by  measuring  their  distance  and  angle  of  position 
with  regard  to  points  first  established.  The  skeleton  map  thus 
obtained  was  filled  up  by  drawings  made  at  the  telescope :  the 
diameters  of  the  measurable  craters  being  determined  by  the 
micrometer. 

Beer  and  Maedler  also  measured  the  heights  of  one  thousand 
and  ninety-five  lunar  mountains  and  crater- summits  :  the  resulting 
measures  are  given  in  a  table  contained  in  the  comprehensive  text- 
book which  accompanies  their  map.  These  heights  are  found  by 
one  of  two  methods,  either  by  measuring  the  length  of  the  shadow 
which  the  object  casts  under  a  known  elevation  of  the  sun  above  its 
horizon,  or  by  measuring  the  distance  between  the  illuminated 
point  of  the  mountain  and  the  "terminator"  in  the  following 
manner.  In  the  annexed  figure  (Fig.  15)  let  the  circle  represent 
the  moon  and  m  a  mountain  upon  it :  let  s  a  be  the  line  of  direction 
of  the  sun's  rays,  passing  the  normal  surface  of  the  moon  at  a  and 
just  tipping  the  mountain  top.  a  will  be  the  terminator,  and  there 
will  be  darkness  between  it  and  the  star-like  mountain  summit  m. 
The  distance  between  a  and  m  is  measured :  the  distance  a  b  is 
known,  for  it  is  the  moon's  radius.     And  since  the  line  s  m  is  a 


CHAP.  VII.] 


TOPOGRAPHY    OF    THE    MOON, 


77 


tangent  to  the  circle  the  angle  b  a  m  is  a  right  angle.  We  know 
the  length  of  its  two  sides  ab,  am,  and  we  can  therefore  hy  the 
known  properties  of  the  right-angled  triangle  find  the  length  of  the 


Fm.  15. 

hypothenuse  bm  :  and  since  bm  is  made  up  of  the  radius  ba  plus  the 
mountain  height,  we  have  only  to  subtract  the  moon's  radius  from 
the  ascertained  whole  length  of  the  hypothenuse  and  we  have  the 
height  of  the  mountain.  MM.  Beer  and  Maedler  exhibited  their 
measures  in  French  toises  :  in  the  heights  we  shall  have  occasion 
to  quote,  these  have  been  turned  into  English  feet,  upon  the  assump- 
tion that  the  toise  is  equal  to  6*39  English  feet.  The  nomencla- 
ture of  lunar  features  adopted  by  Beer  and  Maedler  is  that  intro- 
duced by  Riccioli :  mountains  and  teatures  hitherto  undistinguished 
were  named  by  them  after  ancient  and  modern  philosophers,  in 


78  THE   MOON.  [chap.  vii. 

Biccioli's  system,  and  occasionally  after  terrestial  features.  Some 
minute  objects  in  the  neighbourliood  of  large  and  named  ones  were 
included  under  the  name  of  the  large  one  and  distinguished  by 
Greek  or  Roman  letters. 

The  excellent  map  resulting  from  the  arduous  labours  of  these 
astronomers  is  simply  a  map  :  it  does  not  pretend  to  be  a  picture. 
The  asperities  and  depressions  are  symbolized  by  a  conventional 
system  of  shading  and  no  attempt  is  made  to  exhibit  objects  as 
they  actually  appear  in  the  telescope.  A  casual  observer  comparing 
details  on  the  map  with  the  same  details  on  the  moon  itself  would 
fail  to  identify  or  recognize  them  except  where  the  features  are  very 
conspicuous.  Such  an  observer  would  be  struck  by  the  shadows 
by  which  the  lunar  objects  reveal  themselves  :  he  would  get  to 
know  them  mostly  by  their  shadows,  since  it  is  mainly  by  those 
that  their  forms  are  revealed  to  a  terrestial  observer.  But  such  a 
map  as  that  under  notice  indicates  no  shadows,  and  objects  have  to 
be  identified  upon  it  rather  by  their  positions  with  regard  to  one 
another  or  to  the  borders  of  the  moon  than  by  any  notable  features 
they  actually  present  to  view.  This  inconvenience  occurred  to  us 
in  our  early  use  of  Beer  and  Maedler's  chart,  and  we  were  induced 
to  prepare  for  ourselves  a  map  in  which  every  object  is  shown  some- 
what, if  imperfectly,  as  it  actually  appears  at  some  period  of  a 
lunation.  This  was  done  by  copying  Beer  and  Maedler's  outlines 
and  filling  them  up  by  appropriate  shading.  To  do  justice  to  our 
task  we  enlarged  our  map  to  a  diameter  of  six  feet.  Upon  a  circle 
of  this  diameter  the  positions  and  dimensions  of  all  objects  were 
laid  down  from  the  German  original.  Then  from  our  own  observa- 
tions we  depicted  the  general  aspect  of  each  object :  and  we  so 
adjusted  the  shading  that  all  objects  should  be  shown  under  about 
the  same  angle  of  illumination — a  condition  which  is  never  fulfilled 
upon  the  moon  itself,  but  which  we  consider  ourselves  justified  in 
exhibiting  for  the  purpose  of  conveying  a  fair  impression  of  how  the 
various  lunar  objects  actually  appear  at  some  one  or  other  part  of  a 
lunation. 


SM.iFom'^b^ 


o 


CHAP.  VII.] 


TOPOGRAPRT    OF    TUil    MOON, 


79 


SKELETON  MAP  OF  THE  MOON. 
TO   ACCOMPANY    PICTURE   MAP    (Plate  V.). 


80 


THE    MOON. 


[chap.  VII. 


The  picture-map  thus  produced  has  been  photographed  to  a 
size  convenient  for  this  work :  and  in  order  to  make  it  available  for 
the  identification  of  such  objects^-chiefly  lunar  craters — as  we 
may  have  occasion  to  refer  to,  we  have  prepared  a  skeleton  map 
(p.  79)  which  includes  the  more  conspicuous  objects  of  that 
nature.  The  progressive  numbers  in  the  annexed  list  refer  to  the 
skeleton  map  on  page  79,  and  the  description  of  the  object  to  which 
they  are  annexed  will  be  found  on  pp.  82 — 100. 


No.         Name. 

1.  Newton. 

2.  Short. 

3.  Simpelius. 

4.  Manzinus. 

5.  Moretus. 

6.  Gruemberger. 

7.  Casatus. 

8.  Klaproth. 

9.  Wilson. 

10.  Kircher. 

11.  Bettinus. 

12.  Blancanus. 

13.  Clavius. 

14.  Scheiner. 

15.  Zuchius. 

16.  Segner. 

17.  Bacon. 

18.  Nearchus. 

19.  Vlacq. 

20.  Hommel, 

21.  Licetus. 

22.  Maginus. 

23.  Longomontanus. 

24.  Schiller. 

25.  Phocylides. 

26.  Wargentin. 

27.  Inghirami. 

28.  Schickard. 

29.  Wilhelm  I. 

30.  Tycho. 

31.  Saussure. 

32.  Stoefler. 

33.  Maurolycus. 

34.  Barocius. 

35.  Fabricius. 


No.  Name, 

36.  Metius. 

37.  Fernelius. 

38.  Heinsius, 

39.  Hainzel. 

40.  Bouvard. 

41.  Piazzi. 

42.  Ramsden. 

43.  Capuanus. 

44.  Cichus. 

45.  Wurzelbauer. 

46.  Gauricus. 

47.  Hell. 

48.  Walter. 

49.  Nonius. 

50.  Eiccius. 

51.  Rheita. 

52.  Furnerius. 

53.  Stevinus. 

54.  Hase. 

55.  Snell. 

56.  Borda. 

57.  Neander. 

58.  Piccolomini. 

59.  Pontanus. 

60.  Poisson. 

61.  Aliacensis. 

62.  Werner. 

63.  Pitatus. 

64.  Hesiodus. 

65.  Mercator. 

66.  Vitello. 

67.  Fourier. 

68.  Lagrange. 

69.  Vieta. 

70.  Doppelmajer, 


No.  Name. 

71.  Campanus. 

72.  Kies. 

73.  Purbach. 

74.  La  Caille. 

75.  Playfair. 

76.  Azophi. 

77.  Sacrobosco. 

78.  Fracastorius. 

79.  Santbech. 

80.  Petavius. 

8 1 .  Wilhelm  Humboldt. 

82.  Polybius. 

83.  Geber. 

84.  Arzachael. 

85.  Thebit. 

86.  Bullialdus. 

87.  Hippalus. 

88.  Cavendish. 

89.  Mersenius. 

90.  Gassendi. 

91.  Lubiniezky. 

92.  Alpetragius, 

93.  Airy. 

94.  Almanon. 

95.  Catharina. 

96.  Cyrillus. 

97.  Theophilus. 

98.  Colombo. 

99.  Vendelinus. 

100.  Langreen. 

101.  Goclenius. 

102.  Guttemberg. 

103.  Isidorus. 

104.  Capella. 

105.  Kant. 


CHAP.  VII.] 


TOPOGRAPHY    OF    THE    MOON. 


81 


No.  Name. 

106.  Descartes. 

107.  Abulfeda. 

108.  Parrot. 

109.  Albategnius. 

110.  Alphons. 

111.  Ptolemy. 

112.  Herschel. 

113.  Davy. 

114.  Guerik^. 

116.  Bonpland. 

117.  Lalande. 

118.  Reaumur. 

120.  Letronne. 

121.  Billy. 

122.  Fontana. 

123.  Hansteen. 

124.  Damoiseau. 

125.  Grimaldi. 

126.  Flamsteed. 

127.  Landsberg. 

128.  Moesting. 

129.  Deambrel. 

130.  Taylor. 

131.  Messier. 

132.  Maskelyne. 

133.  Sabine. 

134.  Ritter. 

135.  Godin. 

136.  Soemmering. 

137.  Schroeter. 

138.  Gambart.. 

139.  Reinhold. 

140.  Encke. 

141.  Hevelius. 

142.  Riccioli. 

143.  Lohrman. 

144.  Cavalerius. 

145.  Eeiner. 

146.  Kepler. 

147.  Copernicus. 


No. 

148.  Stadius. 

149.  Pallas. 

150.  Triesnecker. 

151.  Agrippa. 

152.  Arago. 

153.  Taruntius. 

154.  Apollonius. 

155.  Schubert. 

156.  Firmicus. 

157.  Silberschlag. 

158.  Hyginus. 

159.  Ukert. 

160.  Boscovicli. 

161.  Ross. 

162.  Proclus. 

163.  Picard. 

164.  Condorcet. 

165.  Pliny  or  Menelaus. 

167.  Manilius. 

168.  Erastothenes. 

169.  Gay  Lussac. 

170.  Tobias  Mayer. 

171.  Marius. 

172.  Gibers. 

173.  Vasco  de  Gama. 

174.  Seleucus. 

175.  Herodotus. 

176.  Aristarchus. 

177.  La  Hire. 

178.  Pytheas. 

179.  Bessel. 

180.  Vitruvius. 

181.  Maraldi. 

182.  Macrobius. 

183.  Cleomides. 

184.  Eoemer. 

185.  Littrow. 

186.  Posidonius. 

187.  Geminus. 

188.  Linnseus. 


No.  Name. 

189.  Autolycus. 

190.  Aristillus. 

191.  Archimedes. 

192.  Timocharis. 

193.  Lambert. 

194.  Diophantus. 

195.  Delisle. 

196.  Briggs. 

197.  Lichtenberg. 

199.  Calippus. 

200.  Cassini. 

201.  Gauss. 

202.  Messala. 

203.  Struve. 

204.  Mason. 

205.  Plana. 

206.  Burg. 

207.  Baily. 

208.  Eudoxus. 

209.  Aristotle. 

210.  Plato. 

211.  Pico. 

212.  Helicon. 

213.  Maupertuis. 

214.  Condamine. 

215.  Bianchini. 

216.  Sharp. 

217.  Mairan. 

218.  Gerard. 

219.  Repsold. 

220.  Pythagoras. 

221.  Fontenelle. 

222.  Timaeus. 

223.  Epigenes. 

224.  Gartner. 

225.  Thalee. 

226.  Strabo. 

227.  Endymion. 

228.  Atlas. 

229.  Hercules. 


The  strong  family  likeness  pervading  the  craters  of  the  moon 
renders  it  unnecessary  that  we  should  attempt  a  description  of  each 
one  of  them  or  even  of  one  in  twenty.  We  have,  however,  thought 
that  a  few  remarks  upon  the  salient  features  of  a  few  of  the  most 


82  THE    MOON.  [chap.  vii. 

important  may  be  acceptable  in  explanation  of  our  illustrative 
plates ;  and  what  we  have  to  say  of  the  few  may  be  taken  as  repre- 
sentative of  the  many. 


COPEKNICUS,  147.    Plate  VIII. 

This  may  deservedly  be  considered  as  one  of  the  grandest  and 
most  instructive  of  lunar  craters.  Although  its  vast  diameter  (46 
miles)  is  exceeded  by  others,  yet,  taken  as  a  whole,  it  forms  one  of 
the  most  impressive  and  interesting  objects  of  its  class.  Its  situa- 
tion, near  the  centre  of  the  lunar  disc,  renders  all  its  wonderful 
details,  as  well  as  those  of  its  immediately  surrounding  objects,  so 
conspicuous  as  to  establish  it  as.  a  very  favourite  object.  Its  vast 
rampart  rises  to  upwards  of  12,000  feet  above  the  level  of  the 
plateau,  nearly  in  the  centre  of  which  stands  a  magnificent  group 
of  cones,  three  of  them  attaining  the  height  of  upwards  of  2400 
feet. 

The  rampart  is  divided  by  concentric  segmental  terraced  ridges, 
which  present  every  appearance  of  being  enormous  landslips,  result- 
ing from  the  crushing  of  their  over-loaded  summits,  which  have 
slid  down  in  vast  segments  and  scattered  their  debris  on  to  the 
plateau.  Corresponding  vacancies  in  the  rampart  may  be  observed 
from  whence  these  prodigious  masses  have  broken  away.  The  same 
may  be  noticed,  although  in  a  somewhat  modified  degree,  around 
the  exterior  of  the  rampart.  In  order  to  approach  a  realization  of 
the  sublimity  and  grandeur  of  this  magnificent  example  of  a  lunar 
volcanic  crater,  our  reader  will  do  well  to  endeavour  to  fix  his  atten- 
tion on  its  enormous  magnitude  and  attempt  to  establish  in  his 
mind's  eye  a  correct  conception  of  the  scale  of  its  details  as  well 
as  its  general  dimensions,  which,  as  they  so  prodigiously  transcend 
those  of  the  largest  terrestrial  volcanic  craters,  require  that  our  ideas 
as  to  magnitude  of  such  objects  should  be,  so  to  speak,  educated 
upon  a  special  standard.  It  is  for  this  reason  we  are  anxious  our 
reader,  when  examining  our  illustrations,  should  constantly  refer 


CHAP.  VII.]  TOPOGRAPHY    OF    THE    MOON,  83 

the  objects  represented  in  them  to  the  scale  of  miles  appended  to 
each  plate,  otherwise  a  just  and  true  conception  of  the  grandeur  of 
the  objects  will  escape  him. 

Copernicus  is  specially  interesting,  as  being  evidently  the  result 
of  a  vast  discharge  of  molten  matter  which  has  been  ejected  at  the 
focus  or  centre  of  disruption  of  an  extensively  upheaved  portion  of 
the  lunar  crust.  A  careful  examination  of  the  crater  and  the  dis- 
trict around  it,  even  to  the  distance  of  more  than  100  miles  on  every 
side,  will  supply  unmistakable  evidence  of  the  vast  extent  and  force 
of  the  original  disruption,  manifested  by  a  wonderfully  complex 
reticulation  of  bright  streaks  which  diverge  in  every  direction  from 
the  crater  as  their  common  centre.  These  streaks  do  not  appear 
on  our  plate,  nor  are  they  seen  upon  the  moon  except  at  and  near 
the  full  phase.  They  show  conspicuously,  however,  by  their  united 
lustre  on  the  full  moon,  Plate  IV.  Every  one  of  those  bright 
streaks,  we  conceive,  is  a  record  of  what  was  originally  a  crack  or 
chasm  in  the  solid  crust  of  the  moon,  resulting  from  some  vastly 
powerful  upheaving  agency  over  the  site  of  whose  focus  of  energy 
Copernicus  stands.  The  cracking  of  the  crust  must  have  been 
followed  by  the  ejection  of  subjacent  molten  matter  up  through  the 
reticulated  cracks ;  this,  spreading  somewhat  on  either  side  of  them, 
has  left  these  bright  streaks  as  a  visible  record  of  the  force  and 
extent  of  the  upheaval;  while  at  the  focus  of  disruption  from 
whence  the  cracks  diverge,  the  grand  outburst  appears  to  have 
taken  place,  leaving  Copernicus  as  its  record  and  result. 

Many  somewhat  radial  ridges  or  spurs  may  be  observed  leading 
away  from  the  exterior  banks  of  the  great  rampart.  These  appear 
to  be  due  to  the  more  free  egress  which  the  extruded  matter  would 
find  near  the  focus  of  disruption.  The  spur-ridges  may  be  traced 
fining  away  for  fully  100  miles  on  all  sides,  until  they  become  such 
delicate  objects  as  to  approach  invisibility.  Several  vast  open 
chasms  or  cracks  may  be  observed  around  the  exterior  of  the  ram- 
part. They  appear  to  be  due  to  some  action  subsequent  to  the 
formation  of  the  great  crater — probably  the  result  of  contraction  on 

G  2 


84  THE    MOON.  [chap.  vii. 

the  cooling  of  the  crust,  or  of  a  deep-seated  upheaval  long  subse- 
quent to  that  which  resulted  in  the  formation  of  Copernicus  itself, 
as  they  intersect  objects  of  evidently  prior  formation. 

Under  circumstances  specially  favourable  for  "  fine  vision,"  for 
upwards  of  70  miles  on  all  sides  around  Copernicus,  myriads  of 
comparatively  minute  but  perfectly-formed  craters  may  be  observed. 
The  district  on  the  south-east  side  is  specially  rich  in  these 
wonderfully  thickly  scattered  craters,  which  we  have  reason  to 
suppose  stand  over  or  upon  the  reticulated  bright  streaks;  but, 
as  the  circumstances  of  illumination  which  are  requisite  to  enable 
us  to  detect  the  minute  craters  are  widely  adverse  to  those  which 
render  the  bright  streaks  visible,  namely,  nearly  full  moon  for  the 
one  and  gibbous  for  the  other,  it  is  next  to  impossible  to  establish 
the  fact  of  coincidence  of  the  sites  of  the  two  by  actual  simultaneous 
observation. 

At  the  east  side  of  the  rampart,  multitudes  of  these  compara- 
tively minute  craters  may  also  be  detected,  although  not  so  closely 
crowded  together  as  those  on  the  west  side  ;  but  among  those  on 
the  east  may  be  seen  myriads  of  minute  prominences  roughening 
the  surface  ;  on  close  scrutiny  these  are  seen  to  be  small  mounds  of 
extruded  matter  which,  not  having  been  ejected  with  sufficient 
energy  to  cause  the  erupted  material  to  assume  the  crater  form 
around  the  vent  of  ejection,  have  simply  assumed  the  mound  form 
so  well  known  to  be  the  result  of  volcanic  ejection  of  moderate  force. 

Were  we  to  select  a  comparatively  limited  portion  of  the  lunar 
surface  abounding  in  the  most  unmistakable  evidence  of  volcanic 
action  in  every  variety  that  can  characterize  its  several  phases,  we 
could  not  choose  one  yielding  in  all  respects  such  instructive 
examples  as  Copernicus  and  its  immediate  surroundings. 

GASSENDI,  90.    Frontispiece. 

An  interesting  crater  about  54  miles  in  diameter ;  the  height  of 
the  most  elevated  portion  of  the  surrounding  wall  from  the  plateau 


CHAP.  VII.]  TOPOGRAPHY    OF    THE    MOON.  85 

being  about  9600  feet.  The  centre  is  occupied  by  a  group  pf 
conical  mountains,  three  of  which  are  most  conspicuous  objects  and 
rise  to  nearly  7000  feet  above  the  level  of  the  plateau.  As  in  other 
similar  cases,  these  central  mountains  are  doubtless  the  result  of 
the  expiring  effort  of  the  eruption  which  had  formed  the  great 
circular  wall  of  the  crater.  The  plateau  is  traversed  by  several 
deep  cracks  or  chasms  nearly  one  mile  wide. 

Both  the  interior  and  exterior  of  the  wall  of  the  crater  are 
terraced  with  the  usual  segmental  ridges  or  landslips.  A  remark- 
able detached  portion  of  the  interior  bank  is  to  be  seen  on  the  east 
side,  while  on  the  west  exterior  of  the  wall  may  be  seen  an  equally 
remarkable  example  of  an  outburst  of  lava  subsequent  to  the 
formation  of  the  wall  or  bank  of  the  crater  ;  it  is  of  conical  form 
and  cannot  fail  to  secure  the  attention  of  a  careful  observer. 

Interpolated  on  the  north  wall  of  the  crater  may  be  seen  a  crater 
of  about  18  miles  diameter  which  has  burst  its  bank  in  towards  the 
great  crater,  upon  whose  plateau  the  lava  appears  to  have  discharged 
itself. 

The  neighbourhood  of  Gassendi  is  diversified  by  a  vast  number  of 
mounds  and  long  ridges  of  exudated  matter,  and  also  traversed  by 
enormous  chasms  and  cracks,  several  of  which  exceed  one  mile  wide 
and  are  fully  100  miles  in  length,  and,  as  is  usual  with  such  cracks, 
traverse  plain  and  mountain  alike,  disregarding  all  inequalities. 

Numbers  of  small  craters  are  scattered  around  ;  the  whole  form- 
ing an  interesting  and  instructive  portion  of  the  lunar  surface. 

EUDOXUS,  208,  and  ARISTOTLE,  209.     Plate  X. 

Two  gigantic  craters,  Eudoxus  being  nearly  35  miles  in  diameter 
and  upwards  of  11,000  feet  deep,  while  Aristotle  is  about  48  miles 
in  diameter,  and  about  10,000  feet  deep  (measuring  from  the 
summit  of  the  rampart  to  the  plateau).  These  two  magnificent 
craters  present  all  the  true  volcanic  characteristics  in  a  remarkable 
degree.     The  outsides,  as  well  as  the  insides  of  their  vast  surround' 


86  THE    MOON.  [chap.  vii. 

ing  walls  or  banks  display  on  the  grandest  scale  the  landslip  feature, 
the  result  of  the  over-piling  of  the  ejected  material,  and  the  conse- 
quent crushing  down  and  crumbling  of  the  substructure.  The 
true  eruptive  character  of  the  action  which  formed  the  craters  is 
well  evinced  by  the  existence  of  the  groups  of  conical  mountains 
which  occupy  the  centres  of  their  circular  plateaux,  since  these 
conical  mountains,  there  can  be  little  doubt,  stand  over  what 
were  once  the  vents  from  whence  the  ejected  matter  of  the  craters 
was  discharged. 

On  the  west  side  of  these  grand  craters  may  be  seen  myriads  of 
comparatively  minute  ones  (we  use  the  expression  *'  comparatively 
minute,"  although  most  of  them  are  fully  a  mile  in  diameter).  So 
thickly  are  these  small  craters  crowded  together,  that  counting 
them  is  totally  out  of  the  question ;  in  our  original  notes  we  have 
termed  them  "  Froth  craters  "  as  the  most  characteristic  description 
of  their  aspect. 

The  exterior  banks  of  Aristotle  are  characterized  by  radial  ridges 
or  spurs  ;  these  are  most  probably  the  result  of  the  flowing  down 
of  great  currents  of  very  fluid  lava.  To  the  east  of  the  craters  some 
very  lofty  mountains  of  exudation  may  be  seen,  and  immediately 
beyond  them  an  extensive  district  of  smaller  mountains  of  the  same 
class,  so  thickly  crowded  together  as  under  favourable  illumination 
to  present  a  multitude  of  brilliant  points  of  light  contrasted  by 
intervening  deep  shade.  On  the  west  bank  of  Aristotle  a  very 
perfect  crater  may  be  seen,  27  miles  in  diameter,  having  all  the 
usual  characteristic  features. 

About  40  miles  to  the  east  of  Eudoxus  there  is  a  fine  example  of 
a  crack  or  fissure  extending  fully  50  miles — 30  miles  through  a 
plain,  and  the  remaining  20  miles  cutting  through  a  group  of  very 
lofty  mountains.  This  great  crack  is  worthy  of  attention,  as  giving 
evidence  of  the  deep-seated  nature  of  the  force  which  occasioned  it 
inasmuch  as  it  disregards  all  surface  impediments,  traversing  plain 
and  group  of  mountains  alike. 

There  are  several  other  features  in  and  around  these  two  mag- 


CHAP.  VII.]  TOPOGRAPHY    OF    THE    MOON.  87 

nificent  craters  well  worthy  of  careful  observation  and  scrutiny,  all 
of  them  excellent  types  of  their  respective  classes. 


TKIESNECKER,  150.    Plate  XI. 

A  fine  example  of  a  normal  lunar  volcanic  crater,  having  all  the 
usual  characteristic  features  in  great  perfection.  Its  diameter  is 
about  20  miles,  and  it  possesses  a  good  example  of  the  central  cone 
and  also  of  interior  terracing. 

The  most  notable  feature,  however,  in  connection  with  this 
crater,  and  on  account  of  which  we  have  chosen  it  as  a  subject  for 
one  of  our  illustrations,  is  the  very  remarkable  display  of  chasms 
or  cracks  which  may  be  seen  to  the  west  side  of  it.  Several  of 
these  great  cracks  obviously  diverge  from  a  small  crater  near  the 
west  external  bank  of  the  great  one,  and  they  subdivide  or  branch 
out,  as  they  extend  from  the  apparent  point  of  divergence,  while 
they  are  crossed  or  intersected  by  others.  These  cracks  or  chasms 
(for  their  width  merits  the  latter  appellation)  are  nearly  one  mile 
broad  at  the  widest  part,  and  after  extending  to  fully  100  miles, 
taper  away  till  they  become  invisible.  Although  they  are  not  test 
objects  of  the  highest  order  of  difficulty,  yet  to  see  them  with 
perfect  distinctness  requires  an  instrument  of  some  perfection  and 
all  the  conditions  of  good  vision.  When  such  are  present,  a  keen 
and  practised  eye  will  find  many  details  to  rivet  its  attention,  among 
which  are  certain  portions  of  the  edges  of  these  cracks  or  chasms 
which  have  fallen  in  and  caused  interruptions  to  their  continuity. 

THEOPHILUS,  97  ;  (5yRILLUS,  96  ;  CATHARINA,  95.    Plate  XII. 

These  three  magnificent  craters  form  a  very  conspicuous  group 
near  the  middle  of  the  south-east  quarter  of  the  lunar  disc. 
Their  respective  diameters  and  depths  are  as  follows : — 
Theophilus,  64  miles  diameter;  depth  of  plateau  from  summit 
of  crater  wall,  16,000  feet ;  central  cone  5200  feet  high. 


88  THE    MOON.  [chap.  vii. 

Cyrillus,  60  miles  diameter ;  depth  of  plateau  from  summit  of 
crater  wall,  15,000  feet ;  central  cone,  5800  feet  high. 

Catharina,  65  miles  diameter ;  depth  of  plateau  from  summit  of 
crater  wall,  13,000  feet ;  centre  of  plateau  occupied  by  a  confused 
group  of  minor  craters  and  debris. 

Each  of  these  three  grand  craters  is  full  of  interesting  details, 
presenting  in  every  variety  the  characteristic  features  which  so 
fascinate  the  attention  of  the  careful  observer  of  the  moon's 
wonderful  surface,  and  affording  unmistakable  evidence  of  the 
tremendous  energy  of  the  volcanic  forces  which  at  some  incon- 
ceivably remote  period  piled  up  such  gigantic  formations. 

Theophilus  by  its  intrusion  within  the  area  of  Cyrillus  shows  in 
a  very  striking  manner  that  it  is  of  comparatively  more  recent 
formation  than  the  latter  crater.  There  are  many  such  examples 
in  other  parts  of  the  lunar  disc,  but  few  of  so  very  distinct  and 
marked  a  character. 

The  flanks  or  exterior  banks  of  Theophilus,  especially  those  on 
the  west  side,  are  studded  with  apparently  minute  craters,  all  of 
which  when  carefully  scrutinized  are  found  to  be  of  the  true  volcanic 
type  of  structure ;  and  minute  as  they  are,  by  comparison,  they 
would  to  a  beholder  close  to  them  appear  as  very  imposing  objects ; 
but  so  gigantic  are  the  more  notable  craters  in  the  neighbourhood, 
that  we  are  apt  to  overlook  what  are  in  themselves  really  large 
objects.  It  is  only  by  duly  training  the  mind,  as  we  have  pre- 
viously urged,  so  as  ever  to  keep  before  us  the  vast  scale  on  which 
the  volcanic  formations  of  the  lunar  surface  are  displayed,  that  we 
can  do  them  the  justice  which  their  intrinsic  grandeur  demands. 
We  trust  that  our  illustrations  may  in  some  measure  tend  to 
educate  the  mind's  eye,  so  as  to  derive  to  the  full  the  tranquil 
enjoyment  which  results  from  the  study  of  the  manifestation  of 
one  of  the  Creator's  most  potent  agencies  in  dealing  with  the 
materials  of  his  worlds,  namely,  volcanic  force.  So  rich  in 
wonderful  features  and  characteristic  details  is  this  magnificent 
group   and    its    neighbourhood,   that  a  volume    might   be   filled 


CHAP.  VII.]  TOPOGRAPHY    OF    THE    MOON.  89 

in  the  attempt  to  do  justice,  by  description,  to  objects  so   full 
of  suggestive  subject  for  study. 


PTO]LEMY,  111  ;  ALPHONS,  110;  ARZACHAEL,  84,  ETC.— Plate  XIII. 

The  portion  of  tbe  lunar  surface  comprised  within  the  limits  of 
this  illustration  being  situated  nearly  in  the  centre  of  the  moon's 
disc,  is  very  favourably  placed  for  revealing  the  multitude  of 
interesting  features  and  details  therein  represented.  They  consist 
of  every  variety  of  volcanic  crater  from  "  Ptolemy,"  whose  vast 
rampart  is  eighty- six  miles  diameter,  down  to  those  whose  dimen- 
sions are,  comparatively,  so  minute  as  to  render  them  at  the  extreme 
limits  of  visibility. 

Alphons  and  Arzachael,  two  of  the  next  largest  craters  in  our 
illustration,  situated  immediately  above  Ptolemy,  are  sixty  and 
fifty-five  miles  in  diameter  respectively,  and  are  possessed,  in  a 
remarkable  degree,  of  all  the  distinctive  characteristic  features 
of  lunar  craters,  having  magnificent  central  cones,  lofty  ragged 
ramparts,  together  with  very  striking  manifestations  of  landslip 
formations  as  appear  in  the  great  segmental  terraces  within  their 
ramparts,  together  with  several  minor  craters  interpolated  on  their 
plateau.  *'  Alphons,"  the  middle  crater  of  this  fine  group,  has  its 
plateau  specially  distinguished  by  several  cracks  or  chasms  fully 
one  mile  wide,  the  direction  or  **  strike  "  of  which  coincide  in  a  very 
remarkable  manner  with  several  other  similar  cracks  which  form 
conspicuous  features  among  the  multitude  of  interesting  details 
comprised  within  the  limits  of  our  illustration, — the  most  notable 
of  these  is  an  enormous  straight  cliff  traversing  the  diameter  of  a 
low-ridged  circular  formation,  seen  in  the  upper  right-hand  corner 
of  our  plate.  This  great  clifi"  is  sixty  miles  long  and  from  1000  to 
2000  feet  high ;  it  is  a  well-known  object  to  lunar  observers,  and 
has  been  termed  "  The  Eailway,"  on  account  of  its  straightness  as 
revealed  by  the  distinct  shadow  projected  by  it  on  the  plateau  when 
seen  under  its  sunrise  aspect.     The  face  of  this  vast  cliff",  although 


90  THE    MOON.  [chap.  vii. 

generally  straight,  is  seen,  when  minutely  scrutinized,  to  be  some- 
what serrated  in  its  outline,  while  on  its  upper  edge  may  be 
detected  some  very  minute  but  perfectly  formed  craters.  The 
existence  of  this  remarkable  cliflf  appears  to  be  due  either  to  an 
upheaval  or  a  down-sinking  of  portion  of  the  surface  of  the  circular 
area  across  whose  diameter  it  extends. 

To  the  right-hand  side  of  the  cliff  are  two  small  craters,  from 
the  side  of  which  a  fine  example  of  a  crack  may  be  detected  passing 
through  in  its  course  a  low  dome-formed  hill;  this  crack  is  parallel 
to  the  cliff,  having  in  that  respect  the  same  general  strike  or 
parallel  direction  which  so  remarkably  distinguishes  the  other 
cracks  observable  in  this  portion  of  the  moon's  surface. 

On  the  left  hand  of  this  great  cliff  is  situated  a  coneless  crater, 
named  *'  Thebit,"  on  the  right-hand  rampart  of  which  may  be 
observed  two  small  craters,  the  lesser  of  which  is  2*75  miles 
diameter  and  has  a  central  cone.  We  specially  remark  this  fact, 
as  it  is  the  smallest  lunar  crater  but  one,  in  which  we  have,  with 
perfect  distinctness,  detected  a  central  cone.  Not  but  that  many 
smaller  lunar  craters  exist  possessed  of  this  unmistakable  evidence 
of  their  volcanic  origin ;  but  so  minute  are  the  specks  of  light  which 
the  central  cones  of  such  small  craters  reflect,  that  they,  for  that 
reason,  most  probably  fail  to  reveal  themselves. 

PLATO,  210.    Plate  XIV. 

This  crater,  besides  being  a  conspicuous  object  on  account  of  its 
great  diameter,  has  many  interesting  details  in  and  around  it  requir- 
ing a  fine  instrument  and  favourable  circumstances  to  render  them 
distinctly  visible.  The  diameter  of  the  crater  is  70  miles ;  the 
surrounding  wall  or  rampart  varies  in  height  from  4000  to  upwards 
of  8000  feet,  and  is  serrated  with  noble  peaks  which  cast  their 
black  shadows  across  the  plateau  in  a  most  picturesque  manner, 
like  the  towers  and  spires  of  a  great  cathedral.  Reference  to  our 
illustration  will  convey  a  very  fair  idea  of  this  interesting  appearance. 


CHAP.  VII.]  TOPOGRAPHY    OF    THE    MOON.  91 

On  the  north-east  inside  of  the  circular  wall  or  rampart  may  be 
observed  a  fine  example  of  landslip,  or  sliding  down  of  a  considerable 
mass  of  the  interior  side  of  the  crater's  wall.  The  landslip  nature 
of  this  remarkable  detail  is  clearly  established  by  the  fact  of  the 
bottom  edge  of  the  downslipped  mass  projecting  in  towards  the 
centre  of  the  plateau  to  a  considerable  extent.  Other  smaller  land- 
slip features  may  be  seen,  but  none  on  so  grand  and  striking  a 
scale  as  the  one  referred  to.  A  number  of  exceedingly  minute 
craters  may  be  detected  on  the  surface  of  the  plateau.  The  plateau 
itself  is  remarkable  for  its  low  reflective  power,  which  causes  it  to 
look  like  a  dingy  spot  when  Plato  is  viewed  with  a  small  mag- 
nifying power.  The  exterior  of  the  crater  wall  is  remarkable  for 
the  rugged  character  of  its  formation,  and  forms  a  great  contrast  in 
that  respect  to  the  comparatively  smooth  unbroken  surface  of  the 
plateau,  which  by  the  way  is  devoid  of  a  central  cone.  The  sur- 
rounding features  and  objects  indicated  in  our  illustration  are  of 
the  highest  interest,  and  a  few  of  them  demand  special  description. 

THE  VALLEY  OF  THE  ALPS.  Plate  XIV. 

This  remarkable  object  lies  somewhat  diagonally  to  the  west  of 
Plato ;  when  seen  with  a  low  magnifying  power  (80  to  100),  it 
appears  as  a  rut  or  groove  tapering  towards  each  extremity.  It 
measures  upwards  of  75  miles  long  by  about  six  miles  wide  at  the 
broadest  part.  When  examined  under  favourable  circumstances, 
with  a  magnifying  power  of  from  200  to  300,  it  is  seen  to  be  a  vast 
flat-bottomed  valley  bordered  by  gigantic  mountains,  some  of  which 
attain  heights  upwards  of  10,000  feet ;  towards  the  south-east  of 
this  remarkable  valley,  and  on  both  sides  of  it,  are  groups  of  iso- 
lated mountains,  several  of  which  are  fully  8000  feet  high.  This 
flat-bottomed  valley,  which  has  retained  the  integrity  of  its  form 
amid  such  disturbing  forces  as  its  immediate  surroundings  indicate, 
is  one  of  the  many  structural  enigmas  with  which  the  lunar  surface 
abounds.     To  the  north-west  of  the  valley  a  vast  number  of  isolated 


92  THE    MOON.  [chap.  vii. 

mounds  or  small  mountains  of  exudation  may  be  seen ;  so 
numerous  are  they  as  to  defy  all  attempts  to  count  them  with  any- 
thing like  exactness ;  and  among  them,  a  power  of  200  to  300  will 
enable  an  observer,  under  favourable  circumstances,  to  detect  vast 
numbers  of  small  but  perfectly-formed  craters. 

PICO,  211.    Plate  XIV. 

This  is  one  of  the  most  interesting  examples  of  an  isolated  vol- 
canic "mountain  of  exudation,"  and  it  forms  a  very  striking  object 
when  seen  under  favourable  circumstances.  Its  height  is  upwards 
of  8000  feet,  and  it  is  about  three  times  as  long  at  the  base  as  it  is 
broad.  The  summit  is  cleft  into  three  peaks,  as  may  be  ascertained 
by  the  three-peaked  shadow  it  casts  on  the  plain.  Five  or  six 
minute  craters  of  very  perfect  form  may  be  detected  close  to  the 
base  of  this  magnificent  mountain.  There  are  several  other  iso- 
lated peaks  or  mountains  of  the  same  class  within  30  or  40  miles 
of  it  which  are  well  worthy  of  careful  scrutiny,  but  Pico  is  the 
master  of  the  situation,  and  offers  a  glorious  subject  for  realizing 
a  lunar  day-dream  in  the  mind's  eye,  if  we  can  only  by  an  effort  of 
imagination  conceive  its  aspect  under  the  fiercely  brilliant  sunshine 
by  which  it  is  illuminated,  contrasted  with  the  intensely  black  lunar 
heavens  studded  with  stars  shining  with  a  steady  brightness  of 
which,  by  reason  of  our  atmosphere  intervening,  we  can  have  no 
adequate  conception  save  by  the  aid  of  a  well-directed  imagin  ation. 

TYCHO,  30.     PLATE  XVI. 

This  magnificent  crater,  which  occupies  the  centre  of  the  crowded 
group  in  our  Plate,  is  54  miles  in  diameter,  and  upwards  of  16,000 
feet  deep,  from  the  highest  ridge  of  the  rampart  to  the  surface  of 
the  plateau,  whence  rises  a  grand  central  cone  5000  feet  high.  It 
is  one  of  the  most  conspicuous  of  all  the  lunar  craters,  not  so  much 
on  account  of  its  dimensions  as  from  its  occupying  the  great  focus 
of  disruption  from  whence  diverge  those  remarkable  bright  streaks, 


CHAP.  VII.]  TOPOGRAPHY    OF    THE   MOON.  93 

many  of  which  may  he  traced  over  1000  miles  of  the  moon's  surface, 
disregarding  in  their  course  all  interposing  ohstacles.  There  is 
every  reason  to  conclude  that  Tycho  is  an  instance  of  a  vast  disrup- 
tive action  which  rent  the  solid  crust  of  the  moon  into  radiating 
fissures,  which  were  subsequently  occupied  by  extruded  molten 
matter,  whose  superior  luminosity  marks  the  course  of  the  cracks  in 
all  directions  from  the  crater  as  their  common  centre  of  divergence. 
So  numerous  are  these  bright  streaks  when  examined  by  the  aid  of 
the  telescope,  and  they  give  to  this  region  of  the  moon's  surface 
such  an  extra  degree  of  luminosity,  that,  when  viewed  as  a  whole, 
their  locality  can  be  distinctly  seen  at  full  moon  by  the  unassisted 
eye  as  a  bright  patch  of  light  on  the  southern  portion  of  the  disc. 
(See  Plate  IV.)  The  causative  origin  of  the  streaks  is  discussed 
and  illustrated  in  Chapter  XI. 

The  interior  of  this  fine  crater  presents  striking  examples  of  the 
concentric  terrace-like  formations  that  we  have  elsewhere  assigned 
to  vast  landslip  actions.  Somewhat  similar  concentric  terraces  may 
be  observed  in  other  lunar  craters ;  some  of  these,  however,  appear  to 
be  the  results  of  some  temporary  modification  of  the  ejective  force, 
which  has  caused  the  formation  of  more  or  less  perfect  inner  ram- 
parts :  what  we  conceive  to  be  true  landslip  terraces  are  always  dis- 
tinguished from  these  by  their  more  or  less  fragmentary  character. 

On  reference  to  Plate  IV.,  showing  the  full  moon,  a  very  remark- 
able and  special  appearance  will  be  observed  in  a  dingy  district  or 
zone  immediately  surrounding  the  exterior  of  the  rampart  of  Tycho, 
and  of  which  we  venture  to  hazard  what  appears  to  us  a  rational 
explanation  :  namely,  that  as  Tycho  may  be  considered  to  have 
acted  as  a  sort  of  safety-valve  to  the  rending  and  ejective  force  which 
caused,  in  the  first  instance,  the  cracking  of  this  vast  portion  of  the 
moon's  crust — the  molten  matter  that  appears  to  have  been  forced 
up  through  these  cracks,  on  finding  a  comparatively  free  exit  by  the 
vent  of  Tycho,  so  relieved  the  district  immediately  around  him  as 
to  have  thereby  reduced,  in  amount,  the  exit  of  the  molten  matter, 
and  so  left  a  zone  comparatively  free  from  the  extruded  lava  which, 


94  THE   MOON.  [chap.  vii. 

according  to  our  view  of  the  subject,  came  up  simultaneously 
through  the  innumerable  fissures,  and,  spreading  sideways  along 
their  courses,  left  everlasting  records  of  the  original  positions  of  the 
radiating  cracks  in  the  form  of  the  bright  streaks  which  we  now 
behold. 

"  WAKGENTIN,"  26.    Plate  5¥i»s 

This  object  is  quite  unique  of  its  kind — a  crater  about  53  miles 
across,  that  to  all  appearance  has  been  filled  to  the  brim  with  lava 
that  has  been  left  to  consolidate.  There  are  evidences  of  the 
remains  of  a  rampart,  especially  on  the  south-west  portion  of  the 
rim.  The  general  aspect  of  this  extraordinary  object  has  been  not 
unaptly  compared  to  a  **  thin  cheese."  The  terraced  and  rutted 
exterior  of  the  rampart  has  all  the  usual  characteristic  details  of 
the  true  crater.  The  surface  of  the  high  plateau  is  marked  by  a 
few  ridges  branching  from  a  point  nearly  in  its  centre,  together  with 
some  other  slight  elevations  and  depressions ;  these,  however,  can 
only  be  detected  when  the  sun's  rays  fall  nearly  parallel  to  the  sur- 
face of  the  plateau. 

To  the  north  of  this  interesting  object  is  the  magnificent  ring 
formation  Schickard,  whose  vast  diameter  of  123  miles  contrasts 
strikingly  with  that  of  the  sixteen  small  craters  within  his  rampart, 
and  equally  so  with  a  multitude  of  small  craters  scattered  around. 
There  are  many  objects  of  interest  on  the  portion  of  the  lunar 
surface  included  within  our  illustration,  but  as  they  are  all  of  the 
usual  type,  we  shall  not  fatigue  the  attention  of  our  readers  by 
special  descriptions  of  them. 


AKISTARCHUS,  176,  and  HERODOTUS,  175.     Plate 


^ 


These  two  fine  examples  of  lunar  volcanic  craters  are  conspicu- 
ously situated  in  the  north-east  quarter  of  the  moon's  disc. 
Aristarchus  has  a  circular  rampart  nearly  28  miles  diameter,  the 
summit  of  which  is  about  7500  feet  above  the  surface  of  the  plateau, 


CHAP.  VII.]  TOPOGRAPHY    OF    THE    MOON.  95 

while  its  height  ahove  the  general  surface  of  the  moon  is  2600  feet. 
A  central  cone  having  several  subordinate  peaks  completes  the  true 
volcanic  character  of  this  crater  :  its  rampart  banks,  both  outside 
and  inside,  have  fine  examples  of  the  segmental  crescent- shaped 
ridges  or  landslips,  which  form  so  constant  and  characteristic  a 
feature  in  the  structure  of  lunar  craters.  Several  very  notable 
cracks  or  chasms  may  be  seen  to  the  north  of  these  two  craters. 
They  are  contorted  in  a  very  unusual  and  remarkable  manner,  the 
result  probably  of  the  force  which  formed  them  having  to  encounter 
very  varying  resistance  near  the  surface. 

Some  parts  of  these  chasms  gape  to  the  width  of  two  to  three 
miles,  and  when  closely  scrutinized  are  seen  to  be  here  and  there 
partly  filled  by  masses  which  have  fallen  inward  from  their  sides. 
Several  smaller  craters  are  scattered  around,  which,  together  with 
the  great  chasms  and  neighbouring  ridges,  give  evidence  of  varied 
volcanic  activity  in  this  locality.  We  must  not  omit  to  draw 
attention  to  the  parallelism  or  general  similarity  of  "strike" 
in  the  ridges  of  extruded  matter ;  this  appearance  has  special 
interest  in  the  eyes  of  geologists,  and  is  well  represented  in  our 
illustration. 

Aristarchus  is  specially  remarkable  for  the  extraordinary  capa- 
bility which  the  material  forming  its  interior  and  rampart  banks 
has  of  reflecting  light.  Although  there  are  many  portions  of  the 
lunar  surface  which  possess  the  same  property,  yet  few  so  remark- 
ably as  in  the  case  of  Aristarchus,  which  shines  with  such  bright- 
ness, as  compared  with  its  immediate  surroundings,  as  to  attract 
the  attention  of  the  most  unpractised  observer.  Some  have 
supposed  this  appearance  to  be  due  to  active  volcanic  discharge  still 
lingering  on  the  lunar  surface,  an  idea  in  which,  for  reasons  to  be 
duly  adduced,  we  have  no  faith.  Copernicus,  in  the  remarkable 
bright  streaks  which  radiate  from  it,  and  Tycho  also,  as  well  as 
several  other  spots,  are  apparently  composed  of  material  very 
nearly  as  highly  reflective  as  that  of  Aristarchus.  But  the 
comparative  isolation  of  Aristarchus,  as  well  as  the  extraordinary 


96  THE    MOON.  [chap.  vii. 

light-reflecting  property  of  its  material,  renders  it  especially 
noticeable,  so  much  so  as  to  make  it  quite  a  conspicuous  object 
when  illuminated  only  by  earth-light,  when  but  a  slender  crescent 
of  the  lunar  disc  is  illuminated,  or  when,  as  during  a  lunar  eclipse, 
the  disc  of  the  moon  is  within  the  shadow  of  the  earth  and  is 
lighted  only  by  the  rays  refracted  through  the  earth's  atmosphere. 

There  are  no  features  about  Herodotus  of  any  such  speciality  as 
to  call  for  remark,  except  it  be  the  breach  of  the  north  side  of  its 
rampart  by  the  southern  extremity  of  a  very  remarkable  contorted 
crack  or  chasm,  which  to  all  appearance  owes  its  existence  to  some 
great  disruptive  action  subsequent  to  the  formation  of  the  crater. 

WALTER,  48,  and  adjacent  Intrusfv^e  Craters.    Plate  XXII. 

This  plate  represents  a  southern  portion  of  the  moon's  surface, 
measuring  170  by  230  miles.  It  includes  upwards  of  200  craters 
of  all  dimensions,  from  Walter,  whose  rampart  measures  nearly  70 
miles  across,  down  to  those  of  such  small  apparent  diameter  as  to 
require  a  well-practised  eye  to  detect  them.  In  the  interior  of  the 
great  crater,  Walter,  a  remarkable  group  of  small  craters  may  be 
observed  surrounding  his  central  cone,  which  in  this  instance  is 
not  so  perfectly  in  the  centre  of  the  rampart  as  is  usually  the  case. 
The  number  of  small  craters  which  we  have  observed  within  the 
rampart  is  20,  exclusive  of  those  on  the  rampart  itself.  The  entire 
group  represented  in  the  Plate  suggests  in  a  striking  manner  the 
wild  scenery  which  must  characterize  many  portions  of  the  lunar 
surface ;  the  more  so  if  we  keep  in  mind  the  vast  proportions  of 
the  objects  which  they  comprise,  upon  which  point  we  may  remark 
that  the  smallest  crater  represented  in  this  Plate  is  considerably 
larger  than  that  of  Vesuvius. 


ARCHIMEDES,  191  ;  AUTOLYCUS,  189 ;  ARISTILLUS,  190,  AND  the 
APENNINES.    Plate  IX. 

This  group   of  three  magnificent  craters,  together  with  their 


CHAP.  VII.]  TOPOGRAPHY   OF   THE   MOON.  97 

remarkable  surroundings,  especially  including  the  noble  range  of 
mountains  termed  the  Apennines,  forms  on  the  whole  one  of  the 
most  striking  and  interesting  portions  of  the  lunar  surface.  If  the 
reader  is  not  acquainted  with  what  the  telescope  can  reveal  as  to 
the  grandeur  of  the  effect  of  sunrise  on  this  very  remarkable 
portion  of  the  moon's  surface,  he  should  carefully  inspect  and 
study  our  illustration  of  it ;  and  if  he  will  pay  due  regard  to  our 
previously  repeated  suggestion  concerning  the  attached  scale  of 
miles,  he  will,  should  he  have  the  good  fortune  to  study  the  actual 
objects  by  the  aid  of  a  telescope,  be  well  prepared  to  realize  and 
duly  appreciate  the  magnificence  of  the  scene  which  will  be 
presented  to  his  sight. 

Were  we  to  attempt  an  adequate  detailed  description  of  all  the 
interesting  features  comprised  within  our  illustration,  it  would,  of 
itself,  fill  a  goodly  volume ;  as  there  is  included  within  the  space 
represented  every  variety  of  feature  which  so  interestingly  charac- 
terizes the  lunar  surface.  All  the  more  prominent  details  are  types 
of  their  class ;  and  are  so  favourably  situated  in  respect  to  almost 
direct  vision,  as  to  render  their  nature,  forms,  and  altitudes  above 
and  depths  below  the  average  surface  of  the  moon  most  distinctly 
and  impressively  cognizable. 

Archimedes  is  the  largest  crater  in  the  group ;  it  has  a  diameter 
of  upwards  of  52  miles,  measuring  from  summit  to  summit  of  its 
vast  circular  rampart  or  crater  wall,  the  average  height  of  which, 
above  the  plateau,  is  about  4300  feet ;  but  some  parts  of  it  rise 
considerably  higher,  and,  in  consequence,  cast  steeple-like  shadows 
across  the  plateau  when  the  sun's  rays  are  intercepted  by  them  at 
a  low  angle.  The  plateau  of  this  grand  crater  is  devoid  of  the 
usual  central  cone.  Two  comparatively  minute  but  beautifully- 
formed  craters  may  be  detected  close  to  the  north-east  interior  side 
of  the  surrounding  wall  of  the  great  crater.  Both  outside  and  in- 
side of  the  crater  wall  may  be  seen  magnificent  examples  of  the 
landslip  subsidence  of  its  overloaded  banks ;  these  landslips  form 
vast  concentric  segments  of  the  outer  and  inner  circumference  of 


98  TEE   MOON.  [chap.  vii. 

the  great  circular  rampart,  and  doubtless  belong  to  its  era  of 
formation.  Two  very  fine  examples  of  cracks,  or  chasms,  may  be 
observed  proceeding  from  the  opposite  external  sides  of  the  crater, 
and  extending  upwards  of  100  miles  in  each  direction ;  these  cracks, 
or  chasms,  are  fully  a  mile  wide  at  their  commencement  next  the 
crater,  and  narrow  away  to  invisibility  at  their  further  extremity. 
Their  course  is  considerably  crooked,  and  in  some  parts  they  are 
partially  filled  by  masses  of  the  material  of  their  sides,  which  have 
fallen  inward  and  partially  choked  them.  The  depths  of  these 
enormous  chasms  must  be  very  great,  as  they  probably  owe  their 
existence  to  some  mighty  upheaving  action,  which  there  is  every 
reason  to  suppose  originated  at  a  profound  depth,  since  the  general 
surface  on  each  side  of  the  crater  does  not  appear  to  be  disturbed 
as  to  altitude,  which  would  have  been  the  case  had  the  upheaving 
action  been  at  a  moderate  depth  beneath.  We  would  venture  to 
ascribe  a  depth  of  not  less  than  ten  miles  as  the  most  moderate  esti- 
mate of  the  profundity  of  these  terrible  chasms.  If  the  reader  would 
realize  the  scale  of  them,  let  him  for  a  moment  imagine  himself  a 
traveller  on  the  surface  of  the  moon  coming  upon  one  of  them,  and 
finding  his  onward  progress  arrested  by  the  sudden  appearance  of 
its  vast  black  yawning  depths ;  for  by  reason  of  the  angle  of  his 
vision  being  almost  parallel  to  the  surface,  no  appearance  of  so 
profound  a  chasm  would  break  upon  his  sight  until  he  came  com- 
paratively close  to  its  fearful  edge.  Our  imaginary  lunar  traveller 
would  have  to  make  a  very  long  detour,  ere  he  circumvented  this 
terrible  interruption  to  his  progress.  If  the  reader  will  only 
endeavour  to  realize  in  his  mind's  eye  the  terrific  grandeur  of  a 
chasm  a  mile  wide  and  of  such  dark  profundity  as  to  be,  to  all 
appearance,  fathomless — portions  of  its  rugged  sides  fallen  in  wild 
confusion  into  the  jaws  of  the  tortuous  abyss,  and  catching  here 
and  there  a  ray  of  the  sun  sufficient  only  to  render  the  darkness  of 
the  chasm  more  impressive  as  to  its  profundity — he  will,  by  so 
doing,  learn  to  appreciate  the  romantic  grandeur  of  this,  one  of  the 
many  features  which  the  study  of  the  lunar  surface  presents  to  the 


CHAP.  VII.]  TOPOaBAPHY    OF   THE   MOON.  99 

careful  observer,  and  which  exceed  in  sublimity  the  wildest  efforts 
of  poetic  and  romantic  imagination.  The  contemplation  of  these 
views  of  the  lunar  world  are,  moreover,  vastly  enhanced  by  especial 
circumstances  which  add  greatly  to  the  impressiveness  of  lunar 
scenery,  such  as  the  unchanging  pitchy-black  aspect  of  the  heavens 
and  the  death-like  silence  which  reigns  unbroken  there. 

These  digressions  are,  in  some  respects,  a  forestalment  of  what 
we  have  to  say  by-and-by,  and  so  far  they  are  out  of  place ;  but 
with  the  illustration  to  which  the  above  remarks  refer  placed  before 
the  reader,  they  may,  in  some  respects,  enhance  the  interest  of  its 
examination. 

The  upper  portion  of  our  illustration  is  occupied  by  the  magnifi- 
cent range  of  volcanic  mountains  named  after  our  Apennines, 
extending  to  a  length  of  upwards  of  450  miles.  This  mountain 
group  rises  gradually  from  a  comparatively  level  surface  towards  the 
south-west,  in  the  form  of  innumerable  comparatively  small  moun- 
tains of  exudation,  which  increase  in  number  and  altitude  towards 
the  north-east,  where  they  culminate  and  suddenly  terminate  in  a 
sublime  range  of  peaks,  whose  altitude  and  rugged  aspect  must 
form  one  of  the  most  terribly  grand  and  romantic  scenes  which 
imagination  can  conceive.  The  north-east  face  of  the  range 
terminates  abruptly  in  an  almost  vertical  precipitous  face,  and  over 
the  plain  beneath  intense  black  steeple  or  spire-like  shadows  are 
cast,  some  of  which  at  sunrise  extend  fully  90  miles,  till  they  lose 
themselves  in  the  general  shading  due  to  the  curvature  of  the  lunar 
surface.  Nothing  can  exceed  the  sublimity  of  such  a  range  of 
mountains,  many  of  which  rise  to  heights  of  18,000  to  20,000  feet 
at  one  bound  from  the  plane  at  their  north-east  base.  The  most 
favourable  time  to  examine  the  details  of  this  magnificent  range  is 
from  about  a  day  before  first  quarter  to  a  day  after,  as  it  is  then 
that  the  general  structure  of  the  range  as  well  as  the  character  of 
the  contour  of  each  member  of  the  group  can,  from  the  circum- 
stances of  illumination  then  obtaining,  be  most  distinctly  inferred. 

Several  comparatively  small  perfectly-formed  craters   are   seen 

H  2 


100  THE    MOON.  [chap.  vii. 

interspersed  among  the  mountains,  giving  evidence  of  the  truly 
volcanic  character  of  the  surrounding  region,  which,  as  hefore  said, 
comprises  in  a  comparatively  limited  space  the  most  perfect  and 
striking  examples  of  nearly  every  class  of  lunar  volcanic  phe- 
nomena. 

We  have  endeavoured  on  Plate   XXV.  to  give  some  idea  of  a 
landscape  view  of  a  small  portion  of  this  mountain  range. 


Plate  v 


"'."  'c  o  db  urytyp  e 


VESUVIUS. 

AND     NEIGHBOURHOOD    OF    i\APL.ES. 


PORTION     OF   THE    MOCNS   SURFACE 

OF  THE  SAME    AREA    AS    THAT 

GIVEN    IN   THE     ILLUSTRATION 

OF    VESUVIUS    AND    NEIGHBOURHOOD 

OF     N  A  P  I,  E  S. 


^ 


CHAPTER   VIII. 

ON    LUNAR    CRATERS. 

As  we  stated  in  our  brief  general  description  of  the  visible 
hemisphere  of  the  moon,  and  as  a  cursory  glance  at  our  map  and 
plates  will  have  shown,  the  predominant  features  of  the  lunar 
surface  are  the  circular  or  amphitheatrical  formations  that,  by  their 
number,  and  from  their  almost  unnatural  uniformity  of  design, 
induced  the  belief  among  early  observers  that  they  must  have  been 
of  artificial  origin.  In  proceeding  now  to  examine  the  details  of 
our  subject  with  more  minuteness  than  before,  these  annular 
formations  claim  the  first  share  of  our  attention. 
■  By  general  acceptation  the  term  "  crater  "  has  been  used  to 
represent  nearly  all  the  circular  hollows  that  we  observe  upon  the 
moon;  and  without  doubt  the  word  in  its  literal  sense,  as  indicat- 
ing a  cup  or  circular  cavity,  is  so  far  aptly  applied.  But  among 
geologists  it  has  been  employed  in  a  more  special  sense  to  define 
the  hollowing  out  that  is  found  at  the  summit  of  some  extinct,  and 
the  majority  of  active,  volcanoes.  In  this  special  sense  it  may  be 
used  by  the  student  of  the  lunar  surface,  though  in  some,  and 
indeed  in  the  majority  of  cases,  the  lunar  crater  difi'ers  materially 
in  its  form  with  respect  to  its  surroundings  from  those  on  the 
earth ;  for  while,  as  we  have  said,  the  terrestrial  crater  is  generally 
a  hollow  on  a  mountain  top  with  its  flat  bottom  high  above  the 
level  of  the  surrounding  country,  those  upon  the  moon  have  their 
lowest  points  depressed  more  or  less  deeply  below  the  general 
surface  of  the  moon,  the  external  height  being  frequently  only  a 


102  THE   MOON.  [chap.  viii. 

half  or  one-third  of  the  internal  depth.  Yet  are  the  lunar  craters 
truly  volcanic  ;  as  Sir  John  Herschel  has  said,  they  offer  the  true 
volcanic  character  in  its  highest  perfection.  We  have  upon  the 
earth  some  few  instances  in  which  the  geological  conditions  which 
have  determined  the  surface-formation  have  been  identical  with 
those  that  have  obtained  upon  the  moon  ;  and  as  a  result  we  have 
some  terrestrial  volcanic  districts  that,  could  we  view  them  under 
the  same  circumstances,  would  be  identical  in  character  with  what 
we  see  by  telescopic  aid  upon  our  satellite.  The  most  remarkable 
case  of  this  similarity  is  offered  by  a  certain  tract  of  the  volcanic 
area  about  Naples,  known  from  classic  times  as  the  Campi 
Phlegrceij  or  burning  fields,  a  name  given  to  them  in  early  days, 
either  because  they  showed  traces  of  ancient  earth-fire,  or  because 
there  were  attached  to  the  localities  traditions  concerning  hot- 
springs  and  sulphurous  exhalations,  if  not  of  actual  fiery  eruptions. 
The  resemblance  of  which  we  are  speaking  is  here  so  close  that 
Professor  Phillips,  in  his  work  on  Vesuvius,  which  by  the  way  con- 
tains a  historical  description  of  the  district  in  question,  calls  the 
moon  a  grand  Phlegreian  field.  How  closely  the  ancient  craters  of 
this  famous  spot  resemble  the  generality  of  those  upon  the  moon  may 
be  judged  from  Plates  VI.  and  VII.,  in  which  representations  of  two 
areas,  terrestrial  and  lunar,  of  the  same  extent,  are  exhibited  side 
by  side,  the  terrestrial  region  being  the  volcanic  neighbourhood  of 
Naples,  and  the  lunar  a  portion  of  the  surface  about  the  crater 
Theophilus. 

In  comparing  these  volcanic  circles  together,  we  are  however 
brought  face  to  face  with  a  striking  difference  that  exists  between 
the  lunar  and  terrestrial  craters.  This  is  the  difference  of 
magnitude.  None  of  those  Plutonian  amphitheatres  included  in 
the  terrestrial  area  depicted  exceed  a  mile  in  diameter,  and  few 
larger  volcanic  vents  than  these  are  known  upon  the  earth.  Yet 
when  we  turn  to  the  moon,  and  measure  some  of  the  larger  craters 
there,  we  are  astonished  to  find  them  ranging  from  an  almost 
invisible  minuteness  to  74  miles  in  diameter.     The  same  dispro- 


CHAP.  VIII.]  LUNAR    CRATERS.  108 

portion  exists  between  the  depths  of  the  two  classes  of  craters. 
To  give  an  idea  of  relative  dimensions,  we  would  refer  to  our 
illustration  of  Copernicus*  and  its  hundreds  of  comparatively 
minute  surrounding  craters.  Our  terrestrial  Vesuvius  would  be 
represented  by  one  of  these  last,  which  upon  the  plate  measures 
about  the  twentieth  of  an  inch  in  diameter !  And  this  dispro- 
portion strikes  us  the  more  forcibly  when  we  consider  that  the 
lunar  globe  has  an  area  only  one- thirteenth  of  that  of  the  earth. 
In  view  of  this  great  apparent  discrepancy  it  is  not  surprising  that 
many  should  have  been  incredulous  as  to  the  true  volcanic 
character  of  the  lunar  mountains,  and  have  preferred  to  designate 
them  by  some  *' non-committal  "  term,  as  an  American  geologist 
(Professor  Dana)  has  expressed  it.  But  there  is  a  feature  in  the 
majority  of  the  ring-mountains  that,  as  we  conceive,  demonstrates 
completely  the  fact  of  volcanic  force  having  been  in  full  action,  and 
that  seems  to  stamp  the  volcanic  character  upon  the  crater-forms. 
This  special  feature  is  the  central  cone,  so  well  Imown  as  a 
characteristic  of  terrestrial  volcanoes,  accepted  as  the  result  of  the 
last  expiring  effort  of  the  eruptive  force,  and  formed  by  the 
deposit,  immediately  around  the  volcanic  orifice,  of  matter  which 
there  was  not  force  enough  to  project  to  a  greater  distance.  Upon 
the  moon  we  have  the  central  cone  in  small  craters  comparable  to 
those  on  the  earth,  and  we  have  it  in  progressively  larger  examples, 
upon  all  scales,  up  to  craters  of  74  miles  in  diameter,  as  we  have 
shown  on  p.  106.  Where,  then,  can  we  draw  the  line  ?  Where 
can  we  say  the  parallel  action  to  that  which  placed  Vesuvius  in  or 
near  the  centre  of  the  arc  of  Somma,  or  the  cone  figured  in  our 
sectional  drawing  of  Vesuvius  (Fig.  3)  in  the  middle  of  its  present 
crater — where  can  we  say  that  the  action  in  question  ceased  to 
manifest  itself  on  the  moon,  seeing  that  there  is  no  break  in  the 
continuity  of  the  crater-and-cone  system  upon  the  moon  anywhere 
between  craters  of  If  miles  and  74  miles  in  diameter  ?    We  have, 

*  Plate  VIII. 


104  THE   MOON.  [chap.  viii. 

it  is  true,  many  examples  of  coneless  craters,  but  these  are  of  all 
sizes,  down  to  tlie  smallest,  and  up  to  a  magnitude  that  would 
almost  seem  to  render  untenable  the  ejective  explanation  :  of  these 
we  shall  specially  speak  in  turn,  but  for  the  present  we  will  confine 
ourselves  to  the  normal  class  of  lunar  craters,  those  that  have 
central  cones,  and  that  are  in  all  reasonable  probability  truly 
volcanic. 

And  in  the  first  place  let  us  take  a  passing  glance  at  the 
probable  formative  process  of  a  terrestrial  volcano.  Rejecting  the 
hypothesis  of  Von  Buch,  which  geologists  have  on  the  whole  found 
to  be  untenable,  and  which  ascribes  the  formation  of  all  mountains 
to  the  elevation  of  the  earth's  crust  by  some  thrusting  power 
beneath,  we  are  led  to  regard  a  volcano  as  a  pyramid  of  ejected 
matter,  thrown  out  of  and  around  an  orifice  in  the  external  solid 
shell  of  the  earth  by  commotions  engendered  in  its  molten  nucleus. 
What  is  the  precise  nature  and  source  of  the  ejective  force 
geologists  have  not  perfectly  agreed  upon,  but  we  may  conceive 
that  highly  expanded  vapour,  in  all  probability  steam,  is  its 
primary  cause.  The  escaping  aperture  may  have  been  a  weak 
place  since  the  foundations  of  the  earth  were  laid,  or  it  may  have 
been  formed  by  a  local  expansion  of  the  nucleus  in  the  act  of 
cooling,  upon  the  principle  enunciated  in  our  third  chapter;  or, 
again,  the  expansile  vapour  may  have  forced  its  own  way  through 
that  point  of  the  confining  shell  that  ofi'ered  it  the  least  resistance. 
The  vent  once  formed,  the  building  of  the  volcanic  mountain 
commenced  by  the  out-belching  of  the  lava,  ashes,  and  scoriae,  and 
the  dispersion  of  these  around  the  vent  at  distances  depending 
upon  the  energy  with  which  they  were  projected.  As  the  action 
continued,  the  ejected  matter  would  accumulate  in  the  form  of  a 
mound,  through  the  centre  of  which  communication  would  be 
maintained  with  the  source  of  the  ejected  materials  and  the  seat  of 
the  explosive  agency.  The  height  to  which  the  pile  would  rise 
must  depend  upon  several  conditions :  upon  the  steady  sustenance 
of  the  matter,  and  upon  the  form  and  weight  of  the  component 


CHAP.  VIII.] 


LUNAR    CRATERS. 


105 


masses,  which  will  determine  the  slope  of  the  mountain's  sides. 
Supposing  the  action  to  subside  gradually,  the  tapering  form  will 
be  continued  upwards  by  the  comparatively  gentle  deposition  of 
material  around  the  orifice,  and  a  perfect  cone  will  result  of  some 
such  form  as  that  represented  below,  which  is  the  outline  ascribed 


Fig.  16. 

by  Professor  Phillips  to  Vesuvius  in  pre-historic,  or  even  pre- 
traditional  times,  and  which  may  be  seen  in  its  full  integrity  in 
the  cases  of  Etna,  Teneriffe,  Fusi  -  Yama,  the  great  volcanic 
mountain  of  Japan,  and  many  others.  The  earliest  recorded  form 
of  Vesuvius  is  that  of  a  truncated  cone  represented  in  Fig.  17, 


Fig.  17. 


which  shows  its  condition,  according  to  Strabo,  in  the  century 
preceding  the  Christian  Era.  Now  this  form  may  have  been 
assumed  under  two  conditions.  If,  as  Phillips  has  surmised,  the 
mountain  originally  had  a  peaked  summit  with  but  a  small  crater- 
orifice,  at  the  point,  then  we  must  ascribe  its  decapitation  to  a 
subsequent  eruption  which  in  its  violence  carried  away  the  upper 
portion,  either  suddenly,  or  through  a  comparatively  slow  process  of 
grinding  away  or  widening  out  of  the  sides  of  the  orifice  by  the 
chafing  or  fluxing  action  of  the  out-going  materials.  But  it  is 
probable  that  the  mountain  never  had  the  perfect  summit  indicated 
in  our  first  outline.     The  violent  outburst  that  caused  the  great 


106 


THE   MOON. 


[chap.  VIII. 


COMPAHIDHTO  HtU.. 

IViMUesWaaaT 


CoMe«MiaMToTll£BlT 


Small  CiuTefc 
1ii»ibi*Waijej»" 


#• 


t^HPANIOH  TO  PABpr 

llJ«Iile3PfamT 


Heriscuel 


,<<*•  'i<'-/,,. 


,v.^-> 


ERATOSTHNES. 

Sa  Mies  Item? 


A6RIPPA. 

SOMJles'lJlan? 


THE0P1<ILUS. 

ClilfilcallSain'' 


Petavius. 


DIAGRAM   OP 
78 


LUNAR   CRATERS,    FORMING   A   SERIES  RANGING   FROM   If   MILES   TO 
MILKS  IN   DIAMETER,    ALL   CONTAINING   CENTRAL   CONES. 


CHAP.  VIII.]  LUNAR    CRATERS.  107 

crater-opening  of  our  second  figure  may  have  been  but  one 
paroxysmal  phase  of  the  eruption  that  built  the  mountain  :  a 
sudden  cessation  of  the  eruptive  force  when  at  its  greatest 
intensity,  and  when  the  orifice  was  at  its  widest,  would  leave 
matters  in  an  opposite  condition  to  that  suggested  as  the  result 
of  a  slow  dying  out  of  the  action  :  instead  of  the  peak  we  should 
have  a  wide  crater-mouth.  It  is  of  small  consequence  for  our 
present  purpose  whether  the  crater  was  contemporaneous  with  the 
primitive  formation  of  the  mountain,  or  whether  it  was  formed 
centuries  afterwards  by  the  blowing  away  of  the  mountain's  head  ; 


Fia.  18. 


for  upon  the  vast  scale  of  geological  time,  intervals  such  as  those 
between  successive  paroxysms  of  the  same  eruption,  and  those 
between  successive  eruptions,  are  scarcely  to  be  discriminated,  even 
though  the  first  be  days  and  the  second  centuries.  We  may 
remark  that  the  widening  of  a  crater  by  a  subsequent  and  probably 
more  powerful  eruption  than  that  which  originally  produced  it  is 
well  established.  We  have  only  to  glance  at  the  sketch.  Fig.  18, 
of  the  outline  of  Vesuvius  as  it  appeared  between  the  years  a.d.  79 
and  1631  to  see  how  the  old  crater  was  enlarged  by  the  terrible 
Pompeian  eruption  of  the  first-mentioned  year.  Here  we  have  a 
crater  ground  and  blown  away  till  its  original  diameter  of  a  mile 
and  three-quarters  has  been  increased  to  nearly  three  miles. 
Scrope  had  no  hesitation  in  expressing  his  conviction  that  the 
external  rings,  such  as  those  of  Santorin,  St.  Jago,  St.  Helena,  the 
Cirque  of  Teneriffe,  the  Curral  of  Madeira,  the  cliff  range  that 
surrounds  the  island  of  Bourbon,  and  others  of  similar  form  and 
structure,  however  wide  the  area  they  enclose,  are  truly  the  "  basal 
wrecks  "  of  volcanic  mountains  that  have  been  blown  into  the  air 


108  THE   MOON.  [chap.  viii. 

each  by  some  eruption  of  peculiar  paroxysmal  violence  and 
persistence ;  and  that  the  circular  or  elliptical  basins  which  they 
wholly  or  in  part  surround  are  in  all  cases  true  craters  of  eruption. 
When  the  violent  outburst  that  produces  a  great  crater  in  a 
volcanic  mountain-top  more  or  less  completely  subsides,  the  funnel 
or  escaping  orifice  becomes  choked  with  debris.  Still  the  vent 
strives  to  keep  itself  open,  and  now  and  then  gives  out  a  small 
delivery  of  cindery  matter,  which,  being  piled  around  the  vent, 
after  the  manner  of  its  great  prototype,  forms  the  inner  cone. 


Fig.  19. 

This  last  may  in  its  turn  bear  an  open  crater  upon  its  summit, 
and  a  still  smaller  cone  may  form  within  it.  As  the  action  further 
dies  away,  the  molten  lava,  no  longer  seething  and  boiling,  and 
spirting  forth  with  the  rest  of  the  ejected  matter,  wells  upwards 
slowly,  and  cooling  rapidly  as  it  comes  in  contact  with  the  atmo- 
sphere, solidifies  and  forms  a  flat  bottom  or  floor  to  the  crater. 

It  may  happen  that  a  subsequent  eruption  from  the  original  vent 
will  be  comparable  in  violence  to  the  original  one,  and  then  the 
inner  cone  assumes  a  magnitude  that  renders  it  the  principal 
feature  of  the  mountain,  and  reduces  the  old  crater  to  a  secondary 
object.  This  has  been  the  case  with  Vesuvius.  During  the  erup- 
tion of  1631  the  great  cone  which  we  now  call  Vesuvius  was  thrown 
up,  and  the  ancient  crater  now  distinguished  as  Monte  Somma 
became  a  subsidiary  portion  of  the  whole  mountain.  Then  the 
appearance  was  that  shown  in  Fig.  19,  and  which  does  not  difi'er 
greatly  from  that  presented  in  the  present  day.  The  summit  of 
the  Vesuvian  cone,  however,  has  been  variously  altered ;  it  has 
been  blown  away,  leaving  a  large  crateral  hollow,  and  it  has  rebuilt 
itself  nearly  upon  its  former  model. 


CHAP.  VIII.]  LUNAR    CRATERS.  109 

When  we  transfer  our  attention  to  the  volcanoes  of  the  moon,  we 
find  ourselves  not  quite  so  well  favoured  with  means  for  studying 
the  process  of  their  formation ;  for  the  sight  of  the  building  up  of 
a  volcanic  mountain  such  as  man  has  been  permitted  to  behold 
upon  the  earth  has  not  been  allowed  to  an  observer  of  the  moon. 
The  volcanic  activity,  enfeebled  though  it  now  be,  of  which  we  are 
witnesses  from  time  to  time  on  the  earth,  has  altogether  ceased 
upon  our  satellite,  and  left  us  only  its  effects  as  a  clue  to  the  means 
by  which  they  were  produced.  If  we  in  our  time  could  have  seen 
the  actual  throwing  up  of  a  lunar  crater,  our  task  of  description 
would  have  been  simple ;  as  it  is  we  are  compelled  to  infer  the  con- 
structive action  from  scrutiny  of  the  finished  structure. 

We  can  scarcely  doubt  that  where  a  lunar  crater  bears  general 
resemblance  to  a  terrestrial  crater,  the  process  of  formation  has 
been  nearly  the  same  in  the  one  case  as  in  the  other.  Where 
variations  present  themselves  they  may  reasonably  be  ascribed  to 
the  difference  of  conditions  pertaining  to  the  two  spheres.  The 
greatest  dissimilarity  is  in  the  point  of  dimensions  ;  the  projection 
of  materials  to  20  or  more  miles  distance  from  a  volcanic  vent 
appears  almost  incredible,  until  we  realize  the  full  effect  of  the 
conditions  which  upon  the  moon  are  so  favourable  to  the  dispersive 
action  of  an  eruptive  force.  In  the  first  place,  the  force  of  gravity 
upon  our  satellite  is  only  one- sixth  of  that  to  which  bodies  are 
subject  upon  the  earth.  Secondly,  by  reason  of  the  small  mag- 
nitude of  the  moon  and  its  proportionally  much  larger  surface  in 
ratio  to  its  magnitude,  the  rate  at  which  it  parted  with  its  cosmical 
heat  must  have  been  much  more  rapid  than  in  the  case  of  the  earth, 
especially  when  enhanced  by  the  absence  of  the  heat-conserving 
power  of  an  atmosphere  of  air  or  water  vapour ;  and  the  disruptive 
and  eruptive  action  and  energy  may  be  assumed  to  be  greater  in 
proportion  to  the  more  rapid  rate  of  cooling ;  operating,  too,  as 
eruptive  action  would  on  matter  so  much  reduced  in  weight  as  it  is 
on  the  surface  of  the  moon,  we  thus  find  in  combination  conditions 
most  favourable  to  the  display  of  volcanic  action  in  the  highest 


110  THE   MOON.  [chap.  VIII. 

degree  of  violence.  Moreover,  as  the  ejected  material  in  its  passage 
from  the  centre  of  discharge  had  not  to  encounter  any  atmospheric 
resistance,  it  was  left  free  to  continue  the  primary  impulse  of  its 
ejection  without  other  than  gravitative  diminution,  and  thus  to 
deposit  itself  at  distances  from  its  source  vastly  greater  than  those 
of  which  we  have  examples  on  the  earth. 

We  can  of  course  only  conjecture  the  source  or  nature  of  the 
moon's  volcanic  force.  If  geologists  have  had  difficulty  in  assign- 
ing an  origin  to  the  power  that  threw  up  our  earthly  volcanoes, 
into  whose  craters  they  can  penetrate,  whose  processes  they  can 
watch,  and  w^hose  material  they  can  analyze,  how  vastly  more 
difficult  must  be  the  inquiry  into  the  primary  source  of  the  power 
that  has  been  at  work  upon  the  moon,  which  cannot  be  virtually 
approached  by  the  eye  within  a  distance  of  six  or  eight  hundred 
miles,  and  the  material  of  which  we  cannot  handle  to  see  if  it  be 
compacted  by  heat,  or  distended  by  vapours.  Steam  is  the  agent 
to  which  geologists  have  been  accustomed  to  look  for  explanation 
of  terrestrial  volcanoes  ;  the  contact  of  water  with  the  molten  '^ 
nucleus  of  our  globe  is  accepted  as  a  probable  means  whereby 
volcanic  commotions  are  set  up  and  ejective  action  is  generated. 
But  we  are  debarred  from  referring  to  steam  as  an  element  of 
lunary  geology,  by  reason  of  the  absence  of  water  from  the  lunar 
globe.  We  might  suppose  that  a  small  proportion  of  water  once 
existed ;  but  a  small  proportion  would  not  account  for  the  immense 
display  of  volcanic  action  which  the  whole  surface  exhibits.  If  we 
admitted  a  Neptunian  origin  to  the  disturbances  of  the  moon's 
crust,  we  should  be  compelled  to  suppose  that  water  had  existed 
nearly  in  as  great  quantity,  area  for  area,  there  as  upon  our  globe  ; 
but  this  we  cannot  reasonably  do. 

Aqueous  vapour  being  denied  us,  we  must  look  in  other  direc- 
tions for  an  ejective  force.  Of  the  nature  of  the  lunar  materials 
we  can  know  nothing,  and  we  might  therefore  assume  anything ; 
some  have  had  recourse  to  the  supposition  of  expansive  vapours 
given  off  by  some  volatile  component  of  the  said  material  while  in 


PLATIY    \^'! 


■/-•  ■    .    ■  .\  '  y/fer-?^:  fV,v    :v\^\v    ■:Ov^:^■.- 


J.Kai-.myt.K. 


C  0  P  E  Pv  N  !  C  U  S. 

/p5        0  rC  2p  30  f-0  50  Cp  7,0  <3p 

^^''■^^         '  Scale 


]  LVWAM   CEATEBS.  Ul 

EBtateof  fiMBOii,oggciifigmtedby  fthfimiflilcnfmhm  PrafieflBor 

Dioft  refes  to  so^lnir  as  proboifafy  an  important  dement  in  the 
moon's  geology,  soggesting  this  solnAanee  becaose  of  the  part 
idiidi  it  appeals  to  play  in  the  Tokanie  or  igneoos  opeations  of 
onr  ^he,  and  on  aoeoont  of  its  preaence  in  eosmical  meteors  that 
have  ecme  ivithin  range  of  onr  anatysia.  Ai^  matter  soUimated 
hy  heat  in  the  sabstnita  of  the  moon  would  he  condensed  i^on 
reaching  the  edd  smroonding  spaee,  and  woold  he  deposited  in  a 
state  of  fine  powder,  or  otherwise  in  a  soGd  fionn.  Maedler  has 
attiibnted  the  highly  leflectiTe  portions  of  some  parts  <tftiie  smfiMe^ 
snch  as  the  hright  streams  that  radiate  from  some  of  the  ciaten^ 
Gopemieas  and  Tydio  fiir  instance,  to  the  Titzificatum  of  tiie 
sorfiuie  matter  hy  gaseous  cnncnts.  But  in  suppoi^taoaDS  fiks 
these  we  must  remember  that  the  proibahility  of  truth  diminishes 
as  the  free  ground  for  peculation  widois.  It  does  not  appear 
dear  how  ei^anslTe  Ts^urs  could  hsTe  lain  dormant  till  the  moon 
assumed  a  solid  crust,  as  aQ  such  would  douhtiess  make  tiidr 
escape  before  asj  shell  was  fimned,  and  at  an  epodi  idien  there 
was  ample  fiidlity  far  tbdr  expansion. 

While  we  are  not  insensible  of  ibe  Talne  of  an  eipansJve  Tiponr 
eai^kiiation,  if  it  could  be  based  on  anything  beyond  mere  eonjee- 
ture,  we  are  disposed  to  attadi  greater  weig^  to  that  afforded  hy 
the  princ^le  sketched  in  our  third  chapter,  Tix.,  of  eipansion  i^on 
solidification.  We  ga^e,  as  we  think,  ample  proof  tiiat  moiten 
matter  of  Tdcanie  nature,  when  about  passing  to  the  solid  states 
increases  its  bulk  to  a  considerable  degree,  and  we  suggested  that 
the  lunar  globe  at  one  period  of  its  histoiy  must  hafe  been,  wbat 
our  earth  is  now,  a  sotid  shell  encon^assing  a  mdten  nucleus; 
and  further,  that  this  last,  in  i^proaching  its  solid  condition, 
expanded  and  burst  open  or  rent  its  confining  crust.  At  first 
si^t  it  may  seem  that  we  are  ascribing  too  great  a  degree  of 
energy  to  the  eipansiro  fiiroe  which  molten  substances  exhibit  in 
passing  to  the  solid  condition,  seeing  that  in  generd  such  forces 
are  slow  and  giadud  in  thdr  action ;  but  this  anomaly  disiqppears 


112  THE   MOON.  [chap.  viii. 

when  we  consider  the  vast  bulk  of  the  so  expanding  matter,  and 
the  comparatively  small  amount  that  in  its  expansion  it  had  to 
displace.  It  is  true  that  there  are  individual  mountains  on  the 
moon  covering  many  square  miles  of  surface,  that  as  much  as  a 
thousand  cubic  miles  of  material  may  have  been  thrown  up  at  a 
single  eruption ;  but  what  is  this  compared  to  the  entire  bulk  of 
the  moon  itself?  A  grain  of  mustard-seed  upon  a  globe  three 
feet  in  diameter  represents  the  scale  of  the  loftiest  of  terrestrial 
mountains ;  a  similar  grain  upon  a  globe  one  foot  in  diameter, 
would  indicate  the  proportion  of  the  largest  upon  the  moon.  A 
model  of  our  satellite  with  the  elevations  to  scale  would  show 
nothing  more  than  a  little  roughness,  or  superficial  blistering. 
Turn  for  a  moment  to  our  map  (Plate  V.),  upon  which  the 
shadows  give  information  as  to  the  heights  of  the  various 
irregularities,  and  suppose  it  to  represent  the  actual  size  of  some 
sphere  whose  surface  has  been  broken  up  by  reactions  of  some 
kind  of  the  interior  upon  the  exterior — suppose  it  to  have  been  a 
globe  of  fragile  material  filled  with  some  viscous  substance,  and 
that  this  has  expanded,  cracked  its  shell,  oozed  out  in  the  process 
of  solidification,  and  solidified :  the  irregularity  of  surface  which 
the  small  sphere,  roughened  by  the  out-leaking  matter,  would 
present,  would  not  be  less  than  that  exhibited  in  the  map  under 
notice.  "When  we  say  that  a  lunar  crater  has  a  diameter  of  30 
miles,  we  raise  astonishment  that  such  a  structure  could  result 
from  an  eruption  by  the  expansive  force  of  solidifying  matter  ;  but 
when  we  reflect  that  this  diameter  is  less  than  the  two-hundreth 
part  of  the  circumference  of  the  moon,  we  need  have  no  difficulty 
in  regarding  the  upheaval  as  the  result  of  a  force  slight  in 
comparison  to  the  bulk  of  the  material  giving  rise  to  it.  We  have 
upon  the  moon  evidence  of  volcanic  eruptions  being  the  final  result 
of  most  extensive  dislocations  of  surface,  such  as  could  only  be 
produced  by  some  widely  difi'used  uplifting  force.  We  allude  to 
the  frequent  occurrence  of  chains  of  craters  lying  in  a  nearly 
straight  line,  and  of  craters  situated  at  the  converging  point  of 


CHAP.  VIII.]  LUNAR    CRATERS.  113 

visible  lines  of  surface  disturbance.  Our  map  will  exhibit  many 
examples  of  both  cases.  An  examination  of  the  upper  portion 
(the  southern  hemisphere  of  the  moon)  will  reveal  abundant 
instances  of  the  linear  arrangement,  three,  four,  five  or  even  more 
crateral  circles  will  be  found  to  lie  with  their  centres  upon  the  same 
great-circle  track,  proving  almost  undoubtedly  a  connexion  between 
them  so  far  as  the  original  disturbing  force  which  produced  them 
is  concerned.  Again,  in  the  craters  Tycho  (30),  Copernicus  (147), 
Kepler  (146),  and  Proclus  (162),  we  see  instances  of  the  situation 
of  a  volcanic  outburst  at  an  obvious  focus  of  disturbance.  These 
manifest  an  up-thrusting  force  covering  a  large  sub- surface  area, 
and  escaping  at  the  point  of  least  resistance.  Such  an  extent  of 
action  almost  precludes  the  gaseous  explanation,  but  it  is  compatible 
with  the  expansion  on  consolidation  theory,  since  it  is  reasonable 
to  suppose  that  in  the  process  of  consolidation  the  viscous  nucleus 
would  manifest  its  increase  of  bulk  over  considerable  areas,  dis- 
turbing the  superimposed  crust  either  in  one  long  crack,  out  of  the 
wider  opening  parts  of  which  the  expanded  material  would  find  its 
escape,  or  "  starring "  it  with  numerous  cracks,  from  the  con- 
verging point  of  which  the  confined  matter  would  be  ejected  in 
greatest  abundance  and,  if  ejected  there  with  great  energy  and 
violence,  would  result  in  the  formation  of  a  volcanic  crater. 

The  actual  process  by  which  a  lunar  crater  would  be  formed 
would  differ  from  that  pertaining  to  a  terrestrial  crater  only  to  the 
extent  of  the  different  conditions  of  the  two  globes.  We  can 
scarcely  accept  Scrope's  term  ''basal  wrecks"  (of  volcanic  moun- 
tains that  have  had  the  summits  blown  away)  as  applicable  to  the 
craters  of  the  moon,  for  the  reason  that  the  lunar  globe  does  not 
offer  us  any  instance  of  a  mountain  comparable  in  extent  to  the 
great  craters  and  whose  summit  has  not  been  blown  away. 
Scrope's  definition  implies  a  double,  or  divided  process  of  forma- 
tion :  first  the  building  up  of  a  vast  conical  hill  and  then  the 
decapitation  and  "  evisceration  "  of  it  at  some  later  period.  There 
are  grounds  for  this  inferred  double  action  among  the  terrestrial 


114 


THE    MOON. 


[chap.  VIII. 


volcanoes,  since  both  tlie  perfect  cone  and  its  summitless  counter- 
part are  numerously  exemplified.  But  upon  the  moon  we  have 
no  perfect  cone  of  great  size,  we  have  no  exception  whereby  the 
rule  can  be  proved.  It  is  against  probability,  supposing  every 
lunar  crater  to  have  once  been  a  mountain,  that  in  every  case  the 
mountain's  summit  should  have  been  blown  away ;  and  we  are 
therefore  compelled  to  consider  that  the  moon's  volcanic  craters 
were  formed  by  one  continuous  outburst,  and  that  their  "  eviscera- 


Fia.  20. 


tion  "  was  a  part  of  the  original  formative  process.  We  do  not, 
however,  include  the  central  cone  in  this  consideration  :  that  may 
be  reasonably  ascribed  to  a  secondary  action  or  perhaps,  better,  to  a 
weaker  or  modified  phase  of  the  original  and  only  eruption. 

Under  these  circumstances  we  conceive  the  upcasting  and 
excavating  of  a  normal  lunar  crater  to  have  been  primarily  caused 
by  a  local  manifestation  of  the  force  of  expansion  upon  solidification 
of  the  sub-surface  matter  of  the  moon,  resulting  in  the  creation  of 
a  mere  "  star  "  or  crack  in  and  through  the  outermost  and  solid 
crust.  As  we  shall  have  to  rely  upon  diagrams  to  explain  the 
more  complicated  features,  we  give  one  of  this  elementary  stage 
also  as  a  commencement  of  the   series;    and  Fig.  20  therefore 


-  ^^>/i:^ 


THE    LUMA^    APFNN'N^S.     AP  CH  !  \^  E  D  E  S,  &c,  &c. 


CHAP.  VIII.] 


LUNAR    CRATERS. 


115 


represents  a  probable  section  of  the  lunar  surface  at  a  point  which 
was  subsequently  the  location  of  a  crater.  From  the  vent  thus 
formed  we  conceive  the  pent-up  matter  to  have  found  its  escape, 
not  necessarily  at  a  single  outburst,  but  in  all  probability  in  a 
paroxysmal  manner,  as  volcanic  action  manifests  itself  on  our  globe. 
The  first  outflow  of  molten  material  would  probably  produce  no  more 
than  a  mere  hill  or  tumescence  as  shewn  sectionally  in  Fig.  21;  and 
if  the  ejective  force  were  small  this  might  increase  to  the  magnitude 


Fig.  21. 

of  a  mountain  by  an  exudative  process  to  be  alluded  to  hereafter. 
But  if  the  ejective  force  were  violent,  either  at  the  moment  of  the 
first  outburst  or  at  any  subsequent  paroxysm,  an  action  repre- 
sented in  Fig.  22  would  result :  the  unsupported  edges  or  lips  of 
the  vent-hole  would  be  blown  and  ground  or  fluxed  away,  and  a 
funnel-formed  cavity  would  be  produced,  the  ejected  matter  (so 
much  of  it  as  in  falling  was  not  caught  by  the  funnel)  being 
deposited  around  the  hollow  and  forming  an  embryo  circular 
mountain.  The  continuance  of  this  action  would  be  accompanied 
by  an  enlargement  of  the  conical  cavity  or  crater,  not  only  by  the 
outward  rush  of  the  violently  discharged  material,  but  also  by  the 
"  sweating  "  or  grinding  action  of  such  of  it  as  in  descending  fell 

I  2 


116 


THE    MOON. 


[chap.  VIII. 


within  the  hollow.  And  at  the  same  time  that  the  crater  en- 
larged the  rampart  would  extend  its  circumference,  for  it  would 
he  formed  of  such  material  as  did  not  fall  hack  again  into  the 
crater.     Upon  this  view  of  the  crater- forming  process  we  hase  the 


Fio.  22. 


sketch,  Fig.  23,  of  the  probable  section  of  a  lunar  crater  at  one 
period  of  its  development. 

So  long  as  each  succeeding  paroxysm  was  greater  than  its  prede- 
cessor, this  excavating  of  the  hollow  and  widening  of  its  mouth  and 
mound  would  he  extended.  But  when  a  weaker  outburst  came,  or 
when  the  energy  of  the  last  eruption  died  away,  a  process  of  slow 
piling  up  of  matter  close  around  the  vent  would  ensue.  It  is 
obvious  that  when  the  ejective  force  could  no  longer  exert  itself  to  a 
great  distance  it  must  merely  have  lifted  its  burden  to  the  relieving 
vent  and  dropped  it  in  the  immediate  neighbourhood.  Even  if  the 
force  were  considerable,  the  eifect,  so  long  as  it  was  insufficient  to 


CHAP.  VIII.] 


LUNAR    CRATERS. 


117 


throw  the  ejecta  beyond  the  rim  of  the  crater,  would  be  to  pile 
material  in  the  lowermost  part  of  the  cavity  ;  for  what  was  not  cast 
over  the  edge  would  roll  or  flow  down  the  inner  slope  and  accumu- 
late at  the  bottom.  And  as  the  eruption  died  away,  it  would  add 
little  by  little  to  the  heap,  each  expiring  effort  leaving  the  out-giving 


Fio.  23. 


matter  nearer  the  orifice,  and  thus  building  up  the  central  cone  that 
is  so  conspicuous  a  feature  in  terrestrial  volcanoes,  and  which  is  also 
a  marked  one  in  a  very  large  proportion  of  the  craters  of  the  moon. 
This  formation  of  the  cone  is  pictorially  described  by  Fig.  24. 

In  the  volcanoes  of  the  earth  we  observe  another  action  either 
concurrent  with  or  immediately  subsequent  to  the  erection  or  forma- 
tion of  the  cone :  this  is  the  outflow  or  the  welling  forth  of  fluid 
lava,  which  in  cooling  forms  the  well-known  plateau.  We  have 
this  feature  copiously  represented  upon  the  moon,  and  it  is 
presumable  that  it  has  in  general    been  produced  in  a  manner 


118 


THE   MOON, 


[chap.  VIII. 


analogous  to  its  counterparts  upon  the  earth.  We  may  conceive 
that  the  fluid  matter  was  either  spirted  forth  with  the  solid  or  semi- 
solid constituents  of  the  cone,  in  which  case  it  would  drain  down 
and  fill  the  bottom  of  the  crater ;  or  we  may  suppose  that  it  issued 
from  the  summit  of  the  cone  and  ran  down  its  sides,  or  that,  as  we 
see  upon  the  earth,  it  found  its  escape  before  reaching  the  apex,  by 


FiQ.  24. 


forcing  its  way  through  the  basal  parts.  These  actions  are  indi- 
cated hypothetically  for  the  moon  in  Fig.  25 ;  and  the  parallel 
phenomena  for  the  earth  are  shewn  by  the  actual  case  (represented 
in  Fig.  26  and  on  Plate  I.)  of  Vesuvius  as  it  was  seen  by  one  of  the 
authors  in  1864,  when  the  principal  cone  was  vomiting  forth  ashes, 
stones,  and  red-hot  lava,  while  a  vent  at  the  side  emitted  very 
fluid  lava  which  was  settling  down  and  forming  the  plateau. 

Although  we  cannot,  obviously,  see  upon  the  moon  evidence  of 
a  cone  actually  overtopped  by  the  rising  lake  of  lava,  yet  it  is  not 


CHAP.  VIII.] 


LUNAB    CRATERS. 


119 


unreasonable  to  suppose  that  such  a  condition  of  things  actually 
occurred  in  many  of  those  instances  in  which  we  observe  craters 
without  central  cones,  but  with  plateaux  so  smooth  as  to  indicate 
previous  fluidity  or  viscosity.  From  the  state  of  things  exhibited 
in  Fig.  25  the  transition  to  that  shewn  in  Fig.  27  is  easily,  and  to 
our  view  reasonably,  conceivable.     We  are  in  a  manner  led  up  to 


Fig.  25. 

this  idea  by  a  review  of  the  various  heights  of  central  cones  above 
their  surrounding  plateaux.  For  instance,  in  such  examples  as 
Tycho  or  Theophilus,  we  have  cones  high  above  the  lava  floor  ;  in 
Copernicus,  Arzachael  and  Alphons  they  are  comparatively  lower ; 
the  lava  in  these  and  some  other  craters  does  not  appear  to  have 
risen  so  high ;  while  in  Aristotle  and  Eudoxus  among  others,  we 
have  only  traces  of  cones,  and  it  is  supposable  that  in  these  cases 
the  lava  rose  so  high  as  nearly  to  overtop  the  central  cones.  Why 
should  it  not  have  risen  so  far  as  to  overtop  and  therefore  conceal 
some  cones  entirely  ?     We  oifer  this  as  at  least  a  feasible  explana- 


120 


THE   MOON. 


[chap.  VIII. 


tion  of  some  coneless  craters  :  it  is  not  necessary  to  suppose  that  it 
applies  to  all  such,  however  :  there  may  have  been  many  craters, 
the  formation  of  which  ceased  so  abruptly  that  no  cone  was  pro- 


.-    ^    ^N'> 


1^    /)V^^ 


duced,  though  the  welling  forth  of  lava  occurred  from  the  vent, 
which  may  have  been  left  fully  open,  as  in  Fig.  28,  or  so  far  choked 
as  to  stay  the  egress  of  solid  ejecta  and  yet  allow  the  fluid  material 
to  ooze  upwards  through  it,  and  so  form  a  lake  of  molten  lava  which 
on  consolidation  became  the  plateau.  As  most  of  the  examples  of 
coneless  craters  exhibit  on  careful  examination  minute  craters  on 
the  surface  of  the  otherwise  smooth  plateaux,  we  may  suppose  that 
such  minute  craters  are  evidences  of  the  upflow  of  lava  which 
resulted  in  the  plateaux. 


P  L  A  T  E   X. 


Mmmm 


JKasirlyth. 


"Vvbodiiurjrtype ' 


ARISTOTLE      &     EUDOXUS 


ro      SO  7p  20  30  40  £0  W 

MILES  ^  ~ 

Scale. 


CHAP.  VIII.] 


LUNAR    CRATERS. 


121 


We  have  strong  evidence  in  support  of  this  upflow  of  lava 
offered  by  the  case  of  the  crater  Wargentin  (No.  29),  situated 
near  the  south-east  border  of  the  disc,  and  of  which  we  give 
a  special  plate.  (Plate  XVIII.)  It  appears  to  be  really  a  crater 
in  which  the  lava  has  risen  almost  to  the  point  of  overflowing, 
for  the  plateau  is  nearly  level  with  the  edge  of  the  rampart.  This 
edge  appears  to  have  been  higher  on  one  side  than  the  other,  for  on 
the  portion  nearest  the  centre  of  the  visible  disc  we  may,  under 
favourable  circumstances,  detect  a  segment  of  the  basin's  brim 
rising  above  the  smooth  plateau  as  indicated  in  our  illustration. 


Fig.  27. 


Upon  the  opposite  side  there  is  no  such  feature  visible,  the  plateau 
forms  a  sharp  table-like  edge.  It  is  just  possible  that  an  actual 
overflow  of  lava  took  place  at  this  part  of  the  crater,  but  from  the 
unfavourable  situation  of  this  remarkable  object  it  is  impossible  to 
decide  the  point  by  observation.  There  is  no  other  crater  upon  the 
visible  hemisphere  of  the  moon  that  exhibits  this  filled- up  con- 
dition ;  but,  unique  as  it  is,  it  is  sufficient  to  justify  our  conclusion 
that  the  plateau-forming  action  upon  the  moon  has  been  a  flowing- 
up  of  fluid  matter  from  below  subsequent  to  the  formation  of  the 
crater-rampart,  and  not,  as  a  casual  glance  at  the  great  smooth- 
bottom  craters  might  lead  us  to  suspect,  a  result  of  some  sort  of 
diluvial  deposit  which  has  filled  hollows  and  cavities  and  so  brought 


122 


THE    MOON. 


[chap.  VIII. 


up  an  even  surface.  The  elevated  basin  of  Wargentin  could  not 
have  been  filled  thus  while  the  surrounding  craters  with  ramparts 
equally  or  less  high  remained  empty  :  its  contained  matter  must 
have  been  supplied  from  within,  we  must  conjecture  by  the  upflow 
of  lava  from  the  orifice  which  gave  forth  the  material  to  form  the 


Fig.  28. 

crater al  rampart  in  the  first  instance.  We  are  free  to  conjecture 
that  at  some  period  of  this  table-mountain's  formation  it  was  a 
crater  with  a  central  cone,  and  that  the  rising  lava  over-topped  this 
last  feature  in  the  manner  shewn  by  Fig.  29. 

The  question  occurs  whether  other  craters  may  not  have  been 
similarly  filled  and  have  emptied  themselves  by  the  bursting  of  the 
wall  under  the  pressure  of  the  accumulated  lake  of  lava  within. 
We  know  that  this  breaching  is  a  common  phenomenon  in  the 
volcanoes  of  our  globe ;  the  district  of  Auvergne  furnishing  us  with 
many  examples ;  and  there  are  some  suspicious  instances  upon  the 


CHAP.  VIII.] 


LUNAR    CRATERS. 


123 


moon.  Copernicus  exhibits  signs  of  such  disruption,  as  also  does 
the  smaller  crater  intruding  upon  the  great  circle  of  Gassendi. 
(See  Frontispiece.)  But  the  existence  of  such  discharging  breaches 
implies  the  outpouring  of  a  body  of  fluid  or  semi-fluid  material, 
comparable  in  cubical  content  to  the  capacity  of  the  crater,  and  of 
this  we  ought  to  see  traces  or  evidence  in  the  form  of  a  bulky  or 


Fia.  29. 

extensive  lava  stream  issuing  from  the  breach.  But  although  there 
are  faint  indications  of  once  viscous  material  lying  in  the  direction 
that  escaping  fluid  would  take,  we  do  not  find  anything  of  the 
extent  that  we  should  expect  from  the  mass  of  matter  that  would 
constitute  a  craterfull.  It  is  true  that  if  the  escaping  fluid  had 
been  very  limpid  it  might  have  spread  over  a  large  area  and  have 
formed  a  stratum  too  thin  to  be  detected.  Such  a  degree  of 
limpidity  as  would  be  required  to  fulfil  this  condition  we  are  hardly, 
however,  justified  in  assuming. 


124 


THE    MOON. 


[chap.  Vtll. 


To  return  to  the  subject  of  central  cones.  Although  there  are 
cases  in  which  the  simple  condition  of  a  single  cone  exists,  yet  in 
the  majority  we  see  that  the  cone-forming  process  has  been  divided 
or  interrupted,  the  consequence  being  the  production  of  a  group  of 
conical  hills  instead  of  a  single  one.  Copernicus  offers  an  example 
of  this  character,  six,  some  observers  say  seven,  separate  points  of 


light,  indicating  as  many  peaks  tipped  with  sunshine,  having  been 
seen  when  the  greater  part  of  the  crater  has  been  buried  in  shadow. 
Eratosthenes,  Bulialdus,  Maurolicus,  Petavius,  Langreen,  and 
Gassendi,  are  a  few  among  many  instances  of  craters  possessing 
more  than  a  central  single  cone.  This  multiplication  of  peaks  upon 
the  moon  doubtless  arose  from  similar  causes  to  those  which  produce 
the  same  feature  in  terrestrial  volcanoes.  Our  sketch  of  Vesuvius  in 
1864  (Plate  I.  and  Fig.  26)  shews  the  double  cone  and  the  probable 
source  of  the  secondary  one  in  the  diverted  channel  of  the  out- 
coming  material.     A  very  slight  interruption  in  the  first  instance 


PLATE    X 


J.Nasmyth 


""Woodtiirytype' 


TRIESNECKER. 


fO         s  o 


CHAP.  VIII.] 


LUNAR    GRATMRS. 


125 


would  suffice  to  divert  the  stream  and  form  another  centre  of  action, 
or  a  choking  of  the  original  vent  would  compel  the  issuing  matter 
to  find  a  less  resisting  thoroughfare  into  open  space,  and  the  process 
of  cone-building  would  be  continued  from  the  new  orifice,  perhaps 
to  be  again  interrupted  after  a  time  and  again  driven  in  another 
direction.     In  this  manner,  by  repeated  arrests  and  diversions  of 


Fig.  31. 

the  ejecta,  cone  has  grown  upon  the  side  of  cone,  till,  ere  the  force 
has  entirely  spent  itself,  a  cluster  of  peaks  has  been  produced.  It 
may  have  been  that  this  action  has  taken  place  after  the  formation 
of  the  plateau,  in  the  manner  indicated  by  Fig.  30 ;  a  spasmodic 
outburst  of  comparatively  slight  violence  having  sought  relief  from 
the  original  vent,  and  the  flowing  matter,  finding  the  one  orifice 
not  sufficiently  open  to  let  it  pass,  having  forced  other  exit  through 
the  plateau. 

In  frequent  instances  we  observe  the  state  of  things  represented 
in  Fig.  31,  in  which  the  plateau  is  studded  with  few  or  many  small 


126  THE    MOON.  [chap.  viii. 

craters.  This  is  the  case  with  Plato,  with  Arzachael,  Hipparchus, 
Clavius  (which  contains  about  15  small  internal  craters),  and  many 
others.  It  is  probable  that  these  subsidiary  craters  were  produced 
by  an  after-action  like  that  which  has  produced  duplicated  cones, 
but  in  which  the  secondary  eruption  has  been  of  somewhat  violent 
character,  for  it  may  almost  be  regarded  as  an  axiom  that  violent 
eruptions  excavate  craters  and  weak  ones  pile  up  cones.  In  the 
cases  referred  to  it  seems  reasonable  to  suppose  that  the  main  vent 
has  been  the  channel  for  an  up-cast  of  material,  but  that  at  some 
depth  below  the  surface  this  material  met  with  some  obstruction  or 
cause  of  diversion,  and  that  it  took  a  course  which  brought  it  out 
far  away  from  the  cone  upon  the  floor  of  the  plateau.  It  might 
even  be  carried  so  far  as  to  be  upon  the  rampart,  and  it  is  no 
uncommon  thing  to  see  small  craters  in  such  a  situation,  though 
when  they  appear  at  such  a  distance  from  the  primary  vent,  it 
seems  more  reasonable  to  suppose  that  they  do  not  belong  to  it, 
but  have  arisen  from  a  subsequent  and  an  independent  action. 

We  find  scarcely  an  instance  of  a  small  crater  occurring  just  in 
the  centre  of  a  large  one,  or  taking  the  place  of  the  cone.  This  is 
a  curious  circumstance.  Whenever  we  have  any  central  feature  in 
a  great  crater  that  feature  is  a  cone.  The  tendency  of  this  fact  is 
to  prove  that  cones  were  produced  by  very  weak  efforts  of  this 
expiring  force,  for  had  there  been  any  strength  in  the  last  paroxysm 
it  is  presumable  that  it  would  have  blown  out  and  left  a  crater. 
No  very  violent  eruptions  have  therefore  taken  place  from  the  vents 
that  were  connected  with  the  great  craters  of  the  moon,  nothing 
more  powerful  than  could  produce  a  cone  of  exudation  or  a  cinder- 
heap.  And  with  regard  to  cones,  it  is  noteworthy  that  whether 
they  be  single  or  multiple,  they  never  rise  so  high  as  the  circular 
ramparts  of  their  respective  craters.  This  supports  the  inferred 
connexion  between  the  crater  origin  and  the  cone  origin,  for 
supposing  the  two  to  have  been  independent,  a  supposition 
untenable  in  view  of  the  universality  of  the  central  position  of  the 
cone,  it  is  scarcely  conceivable  that    the  mountains  should  have 


CHAP.  VIII.]  LUNAR    CBATEB8.  127 

always  been  located  within  ramparts  higher  than  themselves.  The 
less  height  argues  less  power  in  the  upcasting  agency,  and  the 
diminished  force  may  well  be  considered  as  that  which  would 
almost  of  necessity  precede  the  expiration  of  the  eruption. 

Occasionally  a  crater  is  met  with  that  has  a  double  rampart,  and 
the  concentricity  suggests  that  there  have  been  two  eruptions  from 
the  same  vent :  one  powerful,  which  formed  the  exterior  circle,  and 
a  second  rather  less  powerful  which  has  formed  the  interior  circle. 
It  is  not,  however,  evident  that  this  duplication  of  the  ring  has 
always  been  due  to  a  double  eruption.  In  many  cases  there  is 
duplication  of  only  a  portion :  a  terrace  exhibits  itself  around  a 
part  of  the  circular  range,  sometimes  upon  the  outside  and  some- 
times upon  the  inside.  These  terraces  are  not  likely  to  have  been 
formed  by  any  freak  of  the  eruption,  and  we  are  led  to  ascribe  them 
in  general  to  landslip  phenomena.  When,  in  the  course  of  a 
volcano's  formation,  the  piling-up  of  material  about  the  vent  has 
continued  till  the  lower  portions  have  been  unable  to  support  the 
upper,  or  when  from  any  cause  the  material  composing  the  pile 
has  lost  its  cohesiveness,  the  natural  consequence  has  been  a 
breaking  away  of  a  portion  of  the  structure  and  its  precipitation 
down  the  inclined  sides  of  the  crater.  Vast  segments  of  many  of 
the  lunar  mountain-rings  appear  to  have  been  thus  dislodged  from 
their  original  sites  and  cast  down  the  flanks  to  form  crescent  ranges 
of  volcanic  rocks  either  within  or  without  the  circle.  Nearly  every 
one  of  our  plates  contains  craters  exhibiting  this  feature  in  more 
or  less  extensive  degree.  Sometimes  the  separated  portion  has 
been  very  small  in  proportion  to  the  circumference  of  the  crater  : 
Plato  is  an  instance  in  which  a  comparatively  small  mass  has  been 
detached.  In  other  cases  very  large  segments  have  slid  down  and 
lie  in  segmental  masses  on  the  plateaux  or  form  terraces  around 
the  rampart.  Aristarchus,  Triesnecker,  and  Copernicus  exhibit 
this  larger  extent  of  dislocation. 

It  is  possible  that  these  landslips  occurred  long  after  the  forma- 
tion of  the  craters  that  have  been  subject  to  them.     They  are 


128  TH:E    moon.  [chap.  viii. 

probably  attributable  to  recent  disintegration  of  the  lunar  rocks, 
and  we  have  a  powerful  cause  for  this  in  the  alternations  of  tem- 
perature to  which  the  lunar  crust  is  exposed.  We  shall  have  occa- 
sion to  revert  to  this  subject  by-and-by  ;  at  present  it  must  suffice 
to  point  out  that  the  extremes  of  cold  and  heat,  between  which 
the  lunar  soil  varies,  are,  with  reasonable  probability,  assumed 
to  be  on  the  one  hand  the  temperature  of  space  (which  is  supposed 
to  be  between  200°  and  250''  below  zero),  and,  on  the  other  hand,  a 
degree  of  heat  equal  to  about  twice  that  of  boiling  water.  A  range 
of  at  least  500°  must  work  great  changes  in  such  heterogeneous 
materials  as  we  may  conjecture  those  of  the  lunar  crust  to  be,  by 
the  alternate  contractions  and  expansions  which  it  must  engender, 
and  which  must  tend  to  enlarge  existing  fissures  and  create  new 
ones,  to  grind  contiguous  surfaces  and  to  dislodge  unstable  masses. 
This  cause  of  change,  it  is  to  be  remarked,  is  one  which  is  still 
exerting  itself. 

In  a  few  cases  we  have  an  entirely  opposite  interruption  of  the 
uniformity  of  a  crater's  contour.  Instead  of  the  breaking  away  of 
the  ring  in  segments,  we  see  the  entire  circuit  marked  with  deep 
ruts  that  run  down  the  flanks  in  a  radial  direction,  giving  us 
evidence  of  a  downward  streaming  of  semi-fluid  matter,  instead  of  a 
disruption  of  solid  masses.  "We  cannot  doubt  that  these  ruts  have 
been  formed  by  lava  currents,  and  they  indicate  a  condition  of 
ejected  material  different  from  that  which  existed  in  the  cases 
where  the  landslip  character  is  found.  In  these  last  the  ejecta 
appears  to  have  been  in  the  form  of  masses  of  solidified  or  rapidly 
solidifying  matter,  which  remained  where  deposited  for  a  time  and 
then  gave  way  from  overloading  or  loss  of  cohesiveness,  whereas 
the  substances  thrown  out  in  the  case  of  the  rutted  banks  were 
probably  mixed  solid  and  fluid,  the  former  remaining  upon  the 
flanks  while  the  latter  trickled  away.  Nothing  so  well  represents, 
upon  a  small  scale,  this  radial  channelling  as  a  heap  of  wetted 
sand  left  for  a  while  for  the  water  to  drain  off  from  it.  The  solid 
grains  in  such  a  heap  sustain  its  general  mass-form,  but  the  liquid 


p  i  A  T  I"   y  1 1 


J.ITafiTT/tb 


"  WooAburytype" 


THEOPHiLUS,CYRiLLUS,&    CATHARINA 


6      0  W         20         30  'K}         SO         60         TO         80 


CHAP.  VIII.]  LUNAR    CRATERS.  129 

in  passing  away  cuts  the  surface  into  fissures  running  from  the  sum- 
mit to  the  base,  and  forms  it  into  a  model  of  a  volcanic  mountain 
with  every  feature  of  peak,  crag,  and  chasm  reproduced.  This  simi- 
larity of  efi'ect  leads  us  to  suspect  a  parallelism  of  cause,  and  thus  to 
the  inference  that  the  material  which  originally  formed  such  a  crater- 
mountain  as  Aristillus  (which  is  a  most  prominent  example  of  this 
rutted  character,  and  appears  in  Plate  IX.,  side  by  side  with  a  crater 
that  has  its  banks  segmentally  broken),  must  have  been  of  the  com- 
pound nature  indicated ;  and  that  an  action  analogous  to  that  which 
ruts  a  damp  sand-heap,  rutted  also  the  banks  of  the  lunar  crater. 

Before  passing  from  the  subject  of  craters  it  behoves  us  to  say  a 
few  words  upon  the  curious  manner  in  which  these  formations  are 
complicated  by  intermingling  and  superposition.  Yet,  upon  this 
point,  we  may  be  brief,  for  in  the  way  of  description  our  plates 
speak  more  forcibly  than  is  possible  by  words.  In  particular  we 
would  refer  to  Plate  XII.,  which  represents  the  conspicuous  group 
of  craters  of  which  the  three  largest  members  have  been  respectively 
named  Theophilus,  Cyrillus,  and  Catharina.  But  the  area  included 
in  this  plate  is  by  no  means  an  extraordinary  one ;  there  are  regions 
about  Tycho  wherein  the  craters  so  crowd  and  elbow  each  other  that, 
in  their  intricate  combinations,  they  almost  defy  accurate  depiction. 
Our  map  and  Plate  XVI.  will  serve  to  give  some  idea  of  them.  This 
intermingling  of  craters  obviously  show^s  that  all  the  lunar  volcanoes 
were  not  simultaneously  produced,  but  that  after  one  had  been  formed, 
an  eruption  occurred  in  its  immediate  neighbourhood  and  blew  a 
portion  of  it  away ;  or  it  may  have  been  that  the  same  deep-seated 
vent  at  different  times  gave  forth  discharges  of  material  the  courses 
of  which  were  more  or  less  diverted  on  their  way  to  the  surface. 

We  have  before  alluded  to  the  frequent  occurrence  of  lines  of 
craters  upon  the  moon.  In  these  lines  the  overlapping  is  frequently 
visible ;  it  is  seen  in  Plate  XII.  before  referred  to,  w^here  the  ring 
mountains  are  linked  into  a  chain  slightly  curved,  and  upon  the 
map,  Plate  V.,  the  nearly  central  craters  Ptolemy  and  Alphons, 
the  latter  of  which  overlaps  the  former,  are  seen  to  form  part  of  a 


130  THE    MOON.  [chap.  viii. 

line  of  craters  marking  a  connection  of  primary  disturbance.  An 
extensive  crack  suggests  itself  as  a  favourable  cause  for  the  produc- 
tion of  this  overlaying  of  craters,  for  it  would  serve  as  a  sort  of 
**  line  of  fire  "  from  various  points  at  which  eruptions  would  burst 
forth,  sometimes  weak  or  far  apart,  when  the  result  would  be  lines 
of  isolated  craters,  and  sometimes  near  together,  or  powerful,  when 
the  consequence  would  be  the  intrusion  of  one  upon  the  other,  and 
the  perfect  production  of  the  latest  formed  at  the  expense  or  to  the 
detriment  of  those  that  had  been  formed  previously.  The  linear 
grouping  of  volcanoes  upon  the  earth  long  ago  struck  observant 
minds.  The  fable  of  the  Typhon  lying  under  Sicily  and  the 
Phlegreian  fields  and  disturbing  the  earth  by  its  writhings,  is  a 
mythological  attempt  to  explain  the  particular  case  in  that  region. 

The  capricious  manner  in  which  these  intrusions  occur  is  very 
curious.  Very  commonly  a  small  crater  appears  upon  the  very 
rampart  of  a  greater  one,  and  a  more  diminutive  one  still  will 
appear  upon  the  rampart  of  the  parasite.  Stoefiler  presents  us 
with  one  example  of  this  character,  Hipparchus  with  another, 
Maurolycus  with  a  third,  and  these  are  but  a  few  cases  of  many. 
Here  and  there  we  observe  several  craters  ranged  in  a  line  with 
their  rims  in  one  direction  all  perfect,  and  the  whole  appearing  like 
a  row  of  coins  that  have  fallen  from  a  heap.  There  is  an  example 
near  to  Tycho  which  we  reproduce  in  Plate  XXEE.  In  this  case  one 
is  led  to  conjecture  that  the  ejective  agency,  after  exerting  itself 
in  one  spot,  travelled  onward  and  renewed  itself  for  a  time  ;  that  it 
ceased  after  forming  crater  number  two,  and  again  journeyed  for- 
ward in  the  same  line,  recommencing  action  some  miles  further, 
and  again  subsiding ;  yet  again  pushing  forward  and  repeating  its 
outburst,  till  it  produced  the  fourth  crater,  when  its  power  became 
expended.  In  each  of  these  successive  eruptions  the  centre  of  dis- 
charge has  been  just  outside  the  crater  last  formed  ;  and  the  close 
connexion  of  the  members  of  the  group,  together  with  the  fact  of 
their  nearly  similar  size,  appears  to  indicate  a  community  of  origin. 
For  it  seems  feasible  that  as  a  general  rule  the  size  of  a  crater  may 


CHAP.  VIII.]  LUNAR    CRATERS.  131 

be  taken  as  a  measure  of  the  depth  of  force  that  gave  rise  to  the 
eruption  producing  it.  This  may  not  be  true  for  particular  cases, 
but  it  will  hold  where  a  great  number  are  collectively  considered  ; 
for  if  we  assume  the  existence  of  an  average  disturbing  force,  it  is 
apparently  clear  that  it  will  manifest  itself  in  disturbing  greater  or 
less  surface-areas  in  proportion  as  it  acts  from  greater  or  less 
depths.  Or,  mutatis  mutandis,  if  we  assume  an  uniform  depth 
for  the  source  of  action,  the  greater  or  less  surface  disturbance  will 
be  a  measure  of  greater  or  less  eruptive  intensity. 

Perhaps  the  most  remarkable  case  of  a  vast  number  of  craters, 
which,  from  their  uniform  dimensions,  suggest  the  idea  of  com- 
munity of  source-power  or  source-depth,  is  that  offered  by  the 
region  surrounding  Copernicus,  which,  as  will  be  seen  by  our  plate 
of  that  object,  is  a  vast  Phlegreian  field  of  diminutive  craters.  So 
countless  are  the  minute  craters  that  a  high  magnifying  power 
brings  into  view  when  atmospheric  circumstances  are  favourable, 
and  so  closely  are  they  crowded  together,  that  the  resulting 
appearance  suggests  the  idea  of  froth,  and  we  should  be  disposed 
to  christen  this  the  "  frothy  region  "  of  the  moon,  did  not  a  danger 
exist  in  the  tendency  to  connect  a  name  with  a  cause.  The  craters 
that  are  here  so  abundant  are  doubtless  the  remains  of  true 
volcanoes  analogous  to  the  parasitical  cones  that  are  to  be  found  on 
several  terrestrial  mountains,  and  not  such  accidental  formations  as 
the  Hornitos  described  by  Humboldt  as  abounding  in  the  neigh- 
bourhood of  the  Mexican  volcano,  Jurillo,  but  which  the  traveller 
did  not  consider  to  be  true  cones  of  eruption.*  Although  upon  our 
plate,  and  in  comparison  with  the  great  crater  that  is  its  chief 
feature,  these  countless  hollows  appear  so  small  as  at  first  sight  to 
appear  insignificant,  we  must  remember  that  the  minutest  of  them 
must  be  grand  objects,  each  probably  equal  in  dimensions  to 
Vesuvius.  For  since,  as  we  have  shown  in  an  early  chapter,  the 
smallest  discernible  telescopic  object  must  subtend  an  angle  to  our 

*  «  Cosmos,"  Bohn's  Edition,  Vol.  V.  p.  322. 

K  2 


132 


THE    MOON. 


[chap.  VIII. 


eye  of  about  a  second,  and  since  this  angle  extended  to  the  moon 
represents  a  mile  of  its  surface,  it  follows  that  these  tiny  specks 
of  shadow  that  besprinkle  our  picture,  are  in  the  reality  craters  of 
a  mile  diameter.  This  comparison  may  help  the  conception  of  the 
stupendous  magnitude  of  the  moon's  volcanic  features  ;  for  it  is  a 
conception  most  difficult  to  realize.  It  is  hard  to  bring  the  mind 
to  grasp  the  fact  that  that  hollow  of  Copernicus  is  fifty  miles  in 
diameter.  We  read  of  an  army  having  encamped  in  the  once 
peaceful  crater  of  Vesuvius,  and  of  one  of  the  extinct  volcanoes  of 
the  Camj)i  Phlegrcei  being  used  as  a  hunting  preserve  by  an  Itahan 
king.  These  facts  give  an  idea  of  vastness  to  those  who  have  not 
the  good  fortune  to  see  the  actual  dimensions  of  a  volcanic  orifice 
themselves.  But  it  is  almost  impossible  to  conjure  up  a  vision  of 
what  that  fifty-mile  crater  would  look  like  upon  the  moon  itself  ; 
and  for  want  of  a  terrestrial  object  as  a  standard  of  comparison,  our 
picture,  and  even  the  telescopic  view  of  the  moon  itself,  fails  to 
render  the  imagination  any  help.  We  may  try  to  realize  the 
vastness  by  considering  that  one  of  our  average  English  counties 
could  be  contained  within  its  ramparts,  or  by  conceiving  a  moun- 
tainous amphitheatre  whose  opposite  sides  are  as  far  apart  as  the 
cathedrals  of  London  and  Canterbury,  but  even  these  comparisons 
leave  us  unimpressed  with  the  true  majesty  which  the  object  would 
present  to  a  spectator  upon  the  surface  of  our  satellite. 


TUB   FORMATION   OF   THB  CENTRAL  CONK,      FINAL  ACTION   OF   A  LUNAR  VOLCANO. 


PLATE    XIII. 


"  "V/bo  dburytype" 

PTOLEMY,    ALPHONS,ARZACHAEL&c 


10    so  'p  'iO  jp  w  so         6p  7p  sp  9f> 

SCALF. 


TJNIVEiiSITj:)) 


CHAPTER    IX. 

ON    THE    GREAT    RING-FORMATIONS    NOT    MANIFESTLY    VOLCANIC. 

In  our  previous  chapter  we  have  given  a  reason  for  regarding  as 
true  volcanic  craters  all  those  circular  formations,  of  whatever  size, 
that  exhibit  that  distinctive  feature  the  central  cone.  Between  the 
smallest  crater  with  a  cone  that  we  can  detect  under  the  best  tele- 
scopic conditions,  namely,  the  companion  to  Hell,  If  mile  diameter, 
and  the  great  one  called  Petavius,  78  miles  in  diameter,  we  find  no 
break  in  the  continuity  of  the  crater- cum- cone  system  that  would 
justify  us  in  saying  that  on  the  one  side  the  volcanic  or  eruptive 
cause  ceased,  and  on  the  other  side  some  other  causative  action 
began.  But  there  are  numerous  circular  formations  that  surpass 
the  magnitude  of  Petavius  and  its  peers,  but  that  have  no  circular 
cone,  and  are,  therefore,  not  so  manifestly  volcanic  as  those  which 
possess  this  feature.  Our  map  will  show  many  striking  examples 
of  this  class  at  a  glance.  We  may  in  particular  refer  tnier  alia  to 
Ptolemy  near  the  centre  of  the  moon,  to  Grimaldi  (No.  125), 
Schickard  (No.  28),  Schiller  (No.  24),  and  Clavius  (No.  13),  ail  of 
which  exceed  100  miles  in  diameter.  Even  the  great  Mare 
Crisium^  nearly  300  miles  in  diameter,  appears  to  be  a  formation 
not  distinct  from  those  which  we  have  just  named.  These  present 
little  of  the  generic  crater  character  in  their  appearance ;  and  they 
have  been  distinguished  therefrom  by  the  name  of  Walled  or  JRam- 
parted  Plains,  Their  actual  origin  is  beyond  our  explanation,  and 
in  attempting  to  account  for  them  we  must  perforce  allow  consider- 
able freedom  to  conjecture.     They  certainly,  as  Hooke  suggested, 


134 


THE    MOON, 


[chap.  IX. 


present  a  "  broken  bubble  "-like  aspect ;  but  one  cannot  reasonably 
imagine  the  existence  of  any  form  of  mineral  matter  that  would 
sustain  itself  in  bubble  form  over  areas  of  many  hundreds  of  square 
miles.  And  if  it  were  reasonable  to  suppose  the  great  rings  to  be 
the  foundations  of  such  vast  volcanic  domes,  we  must  conclude 
these  to  have  broken  when  they  could  no  longer  sustain  themselves, 
and  in  that  case  the  surface  beneath  should  be  strewed  with  debrisy 


^-5^ 
-/-- 


X 


Fig.  32. 

of  which,  however,  we  can  find  no  trace.  Moreover,  we  might  fairly 
expect  that  some  of  the  smaller  domes  would  have  remained  stand- 
ing :  we  need  hardly  say  that  nothing  of  the  kind  exists. 

The  true  circularity  of  these  objects  appears  at  first  view  a 
remarkable  feature.  But  it  ceases  to  be  so  if  we  suppose  them  to 
have  been  produced  by  some  very  concentrated  sublunar  force  of  an 
upheaving  nature,  and  if  only  we  admit  the  homogeneity  of  the 
moon's  crust.  For  if  the  crust  be  homogeneous,  then  any  up- 
heaving force,  deeply  seated  beneath  it,  will  exert  itself  \cith  equal 
effects  at  equal  distances  from  the  source :  the  lines  of  equal  effect 
will  obviously  be  radii  of  a  sphere  with  the  source  of  the  disturb- 


CHAP.  IX.]    RING-FORMATIONS  NOT  MANIFESTLY  VOLCANIQ. 


135 


ance  for  its  centre,  and  they  will  meet  a  surface  over  the  source  in 
a  circle.  This  will  be  evident  from  Fig.  32,  in  which  a  force  is 
supposed  to  act  at  F  below  the  surface  s  s  s  s.  The  matter  com- 
posing s  s  being  homogeneous,  the  action  of  F  will  be  equal  at 
equal  distances  in  all  directions.  The  lines  of  equal  force,  F/,  F/, 
will  be  of  equal  length,  and  they  will  form,  so  to  speak,  radii  of  a 
sphere  of  force.  This  sphere  is  cut  by  the  plane  at  s  s  s  s,  and  as 
the  intersection  necessarily  takes  place  everywhere  at  the  extremity 
of  these  radii,  the  figure  of  intersection  is  demonstrably  a  circle 


Fia.  33. 


Fig.  34. 


V^:\-;t-f-r/.=  /"^ 


(shown  in  perspective  as  an  ellipse  in  the  figure).  Thus  we  see 
that  an  intense  but  extremely  confined  explosion,  for  instance, 
beneath  the  moon's  crust  must  disturb  a  circular  area  of  its  surface, 
if  the  intervening  material  be  homogeneous.  If  this  be  not  homo- 
geneous there  would  be,  where  it  offered  less  than  the  average 
resistance  to  the  disturbance,  an  outward  distortion  of  the  circle  ; 
and  an  opposite  interruption  to  circularity  if  it  offers  more  than  the 
average  resistance.  This  assumed  homogeneity  may  possibly  be  the 
explanation  of  the  general  circularity  of  the  lunar  surface  features, 
small  and  great. 

We  confess  to  a  difficulty  in  accounting  for  such  a  very  local 
generation  of  a  deep-seated  force ;  and,  granting  its  occurrence,  we 
are  unprepared  with  a  satisfactory  theory  to  explain  the  resultant 
effect  of  such  a  force  in  producing  a  raised  ring  at  the  limit  of  the 
circular  disturbance.     We  may,  indeed,  suppose  that  a  vast  circular 


136  THE   MOON.  [chap.  ix. 

cake  or  conical  frustra  would  be  temporarily  upraised  as  in  Fig.  33, 
and  that  upon  its  subsidence  a  certain  extrusion  of  subsurface 
matter  would  occur  around  the  line  or  zone  of  rupture  as  in  Fig.  34. 
This  supposition,  however,  implies  such  a  peculiarly  cohesive  con- 
dition of  the  matter  of  the  uplifted  cake,  that  it  is  doubtful  whether 
it  can  be  considered  tenable.  We  should  expect  any  ordinary  form 
of  rocky  matter  subjected  to  such  an  upheaval  to  be  fractured  and 
distorted,  especially  when  the  original  disturbing  force  is  greater  in 
the  centre  than  at  the  edge,  as,  according  to  the  above  hypothesis, 
it  would  be ;  and  in  subsiding,  the  rocky  plateau  would  thus  retain 
some  traces  of  its  disturbance ;  but  in  the  circular  areas  upon  the 
moon  there  is  nothing  to  indicate  that  they  have  been  subjected  to 
such  dislocations. 

Mr.  Scrope  in  his  work  on  volcanoes  has  given  a  hypothetical 
section  of  a  portion  of  the  earth's  crust,  which  presents  a  bulging 
or  tumescent  surface  in  some  measure  resembling  the  effect  which 
such  a  cause  as  we  have  been  considering  would  produce.  We  give 
a  slightly  modified  version  of  his  sketch  in  Fig.  35,  showing  what 
would  be  the  probable  phenomena  attending  such  an  upheaval  as 
regards  the  behaviour  of  the  disturbed  portion  of  the  crust,  and  also 
that  of  the  lava  or  semifluid  matter  beneath:  and,  as  will  be  seen 
by  the  sketch,  a  possible  phase  of  the  phenomena  is  the  production 
of  an  elevated  ridge  or  rampart  at  the  points  of  disruption  c  c ;  and 
where  there  is  a  ring  of  disruption,  as  by  our  hypothesis  there 
would  be,  the  ridge  or  rampart  c  c  would  be  a  circle.  In  this  draw- 
ing we  see  the  cracking  and  distortion  to  which  the  elevated  area 
would  be  subjected,  but  of  which,  as  previously  remarked,  the  cir- 
cular areas  of  the  moon  present  no  trace  of  residual  appearance. 

Those  who  have  offered  other  explanations  of  these  vast  ring- 
formed  mountain  ranges,  have  been  no  more  happy  m  their  conjec- 
tures. M.  Bozet,  who  communicated  a  paper  on  selenology  to  the 
French  Academy  in  1846,  put  forth  the  following  theory.  He 
argued  that  during  the  formation  of  the  solid  scoriaceous  pelicules 
of  the  moon,  circular  or  tourbillonic  movements  were  set  up ;  and 


<|s^ 


CHAP.  IX,]    RING-FORMATIONS  NOT  MANIFESTLY  VOLCANIC. 


137 


these,  by  throwing  the  scoriae  from  the  centre  to  the  circumference, 
caused  an  accumulation  thereof  at  the  limit  of  the  circulation.  He 
considered  that  this  phenomenon  continued  during  the  whole  pro- 
cess of  solidification,  but  that  the  amplitude  of  the  whirlpool 
diminished  with  the  decreasing  fluidity  of  the  surface  material. 
Further,  he  suggested  that  when  many  vortices  were  formed,  and 


Fig.  35.  "  " 

A  A.  Fissures  gaping  downwards  and  injected  by  intumescent  lava 
beneath,  b  b  b.  Fissures  gaping  upwards  and  allowing  wedges  of  rock 
to  drop  below  the  level  of  the  intervening  masses,  g  c.  Wedges  forced 
upwards  by  horizontal  compression,  e  p.  Neutral  plane  or  pivot  axis, 
above  and  below  which  the  directions  of  the  tearing  strain  and  horizontal 
compression  are  severally  indicated  by  the  smaller  arrows  ;  the  larger 
arrows  beneath  represent  the  direction  of  the  primary  expansive  force. 


the  distances  of  their  centres,  taken  two  and  two,  were  less  than 
the  sums  of  their  radii,  there  resulted  close  spaces  terminated  by 
arcs  of  circles  ;  and  when  for  any  two  centres  the  distance  was 
greater  than  the  sum  of  their  radii  of  action,  two  separate  and  com- 
plete rings  were  formed.  We  have  only  to  remark  on  this,  that  we 
are  at  a  loss  to  account  for  the  origination  of  such  vorticose  move- 
ments,, and  M.  Kozet  is  silent  on  the  point.  If  the  great  circles 
are  to  be  referred  to  an  original  sea  of  molten  matter,  it  appears  to 


138  THE    MOON.  [chap.  ix. 

US  more  feasible  to  consider  that  wherever  we  see  one  of  them  there 
has  been,  at  the  centre  of  the  ring,  a  great  outflow  of  lava  that  has 
flooded  the  surrounding  surface.  Then,  if  from  any  cause,  and  it 
is  not  difficult  to  assign  one,  the  outflow  became  intermittent,  or 
spasmodic,  or  subject  to  sudden  impulses,  concentric  waves  would 
be  propagated  over  the  pool  and  would  throw  up  the  scoria  or  the 
solidifying  lava  in  a  circular  bank  at  the  limit  of  the  fluid  area. 

This  hypothesis  does  not  difi'er  greatly  from  the  ebullition  theory 
proposed  by  Professor  Dana,  the  American  geologist,  to  explain 
these  formations.  He  considered  that  the  lunar  ring-mountains 
were  formed  by  an  action  analogous  to  that  which  is  exemplified  on 
the  earth  in  the  crater  of  Kilauea,  in  the  Hawaiian  islands.  This 
crater  is  a  large  open  pit  exceeding  three  miles  in  its  longer 
diameter,  and  nearly  a  thousand  feet  deep.  It  has  clear  bluff  walls 
round  a  greater  part  of  its  circuit,  with  an  inner  ledge  or  plain  at 
their  base,  raised  340  feet  above  the  bottom.  This  bottom  is  a 
plain  of  solid  lavas,  entirely  open  to-day,  which  may  be  traversed 
with  safety  (we  are  quoting  Professor  Dana's  own  statement 
written  in  1846,  and  therefore  not  correctly  applying  to  the  present 
time) :  over  it  there  are  pools  of  boiling  lava  in  active  ebulHtion, 
and  one  is  more  than  a  thousand  feet  in  diameter.  There  are  also 
cones  at  times,  from  a  few  yards  to  two  or  three  thousand  feet  in 
diameter,  and  varying  greatly  in  angle  of  inclination.  The  largest 
of  these  cones  have  a  circular  pit  or  crater  at  the  summit.  The 
great  pit  itself  is  oblong,  owing  to  its  situation  on  a  fissure,  but 
the  lakes  upon  its  bottom  are  round,  and  in  them,  says  Professor 
Dana,  "  the  circular  or  slightly  elliptical  form  of  the  moon's 
craters  is  exemplified  to  perfection." 

Now  Dana  refers  this  great  pit  crater  and  its  contained  lava- 
lakes  to  "  the  fact  that  the  action  at  Kilauea  is  simply  boiling, 
owing  to  the  extreme  fluidity  of  the  lavas.  The  gases  or  vapours 
which  produce  the  state  of  active  ebullition  escape  freely  in  small 
bubbles,  with  little  commotion,  like  jets  over  boiling  water ;  while 
at  Vesuvius  and  other  like  cones  they  collect  in  immense  bubbles 


CHAP.  IX.]    RING-FORMATIONS  NOT  MANIFESTLY  VOLCANIC.  139 

before  they  accumulate  force  enough  to  make  their  way  through ; 
and  consequently  the  lavas  in  the  latter  case  are  ejected  with  so 
much  violence  that  they  rise  to  a  height  often  of  many  thousand 
feet  and  fall  around  in  cinders.  This  action  builds  up  the  pointed 
mountain,  while  the  simple  boiling  of  Kilauea  makes  no  cinders 
and  no  cinder  cones." 

Professor  Dana  continues,  *'If  the  fluidity  of  lavas,  then,  is 
sufficient  for  this  active  ebullition,  we  may  have  boiling  going  on 
over  an  area  of  an  indefinite  extent ;  for  the  size  of  a  boiling  lake 
can  have  no  limits  except  such  as  may  arise  from  a  deficiency  of 
heat.  The  size  of  the  lunar  craters  is  therefore  no  mystery. 
Neither  is  their  circular  form  difficult  of  explanation  ;  for  a  boiling 
pool  necessarily,  by  its  own  action,  extends  itself  circularly  around 
its  centre.  The  combination  of  many  circles,  and  the  large  sea- 
like areas,  are  as  readily  understood."  * 

In  justice  to  Professor  Dana  it  should  be  stated  that  he  included 
in  this  theory  of  formation  all  lunar  craters,  even  those  of  small 
size  and  possessing  central  cones ;  and  he  put  forth  his  views  in 
opposition  to  the  eruptive  theory  which  we  have  set  forth,  and  which 
was  briefly  given  to  the  world  more  than  twenty-five  years  ago.  As 
regards  the  smallest  craters  with  cones,  we  believe  few  geologists  will 
refuse  their  compliance  with  the  supposition  that  they  were  formed 
as  our  crater-bearing  volcanoes  were  formed :  and  we  have  pointed 
out  the  logical  impossibility  of  assigning  any  limit  of  size  beyond 
which  the  eruptive  action  could  not  be  said  to  hold  good,  so  long  as 
the  central  cone  is  present.  But  when  we  come  to  ring-mountains 
having  no  cones,  and  of  such  enormous  size  that  we  are  compelled 
to  hesitate  in  ascribing  them  to  ejective  action,  we  are  obliged  to 
face  the  possibility  of  some  other  causation.  And,  failing  an 
explanation  of  our  own  that  satisfied  us,  we  have  alluded  to  the 
few  hypotheses  proffered  by  others,  and  of  these  Professor  Dana's 
appears  the  most  rational,  since  it  is  based  upon  a  parallel  found  on 
the  earth.     In  citing  it,  however,  we  do  necessarily  not  indorse  it. 

*  American  Journal  of  Science,  Second  Series,  Vol.  IT. 


CHAPTER  X. 

PEAKS  AND  MOUNTAIN  EANGES. 

The  lunar  features  next  in  order  of  conspicuity  are  the  mountain 
ranges,  peaks,  and  hill- chains,  a  class  of  eminences  more  in 
common  with  terrestrial  formations  than  the  craters  and  circular 
structures  that  have  engaged  our  notice  in  the  preceding' chapters.- 

In  turning  our  attention  to  these  features,  we  are  at  the  outset 
struck  with  the  paucity  on  the  lunar  surface  of  extensive  mountain 
systems  as  compared  with  its  richness  in  respect  of  crateral 
formations  ;  and  a  field  of  speculation  is  opened  by  the  recognition 
of  the  remarkable  contrast  which  the  moon  thus  presents  to  the 
earth,  where  mountain  ranges  are  the  rule,  and  craters  like  the 
lunar  ones  are  decidedly  exceptional.  Another  conspicuous  but 
inexplicable  fact  is  that  the  most  important  ranges  upon  the  moon 
occur  in  the  northern  half  of  the  visible  hemisphere,  where  the 
craters  are  fewest  and  the  comparatively  featureless  districts 
termed  *'  seas  "  are  found.  The  finest  range  is  that  named  after 
our  Apennines  and  which  is  included  in  our  illustrative  Plate, 
No.  IX.  It  extends  for  about  450  miles,  and  has  been  estimated 
to  contain  upwards  of  3000  peaks,  one  of  which — Mount  Huyghens 
— attains  the  altitude  of  18,000  feet.  The  Caucasus  is  another 
lunar  range  which  appears  like  a  diverted  northward  extension  of 
the  Apennines,  and,  although  a  far  less  imposing  group  than  the 
last  named,  contains  many  lofty  peaks,  one  of  which  approaches 
the  altitude  assigned  to  Mount  Huyghens  while  several  others 
range  between  11,000  and  14,000  feet  high.     Another  consider- 


PLATE  XV 


J.Naarayth. 


'"WbocLbTirytype" 


MERCATOR      &     CAMPANUS 

'lO      S       0                m               20              30              fC              50             60               70 
^ ' 1 1 1 J '  1 > 

Scale 


CHAP.  X.]  PEAKS    AND    MOUNTAIN   RANGES.  141 

able  range  is  the  Alps,  situated  between  the  Caucasus  and  the 
crater  Plato,  and  reproduced  on  Plate  XIV.  It  contains  some 
700  peaked  mountains  and.  is  remarkable  for  the  immense  valley, 
80  miles  long  and  about  five  broad,  that  cuts  it  with  seemingly 
artificial  straightness  ;  and  that,  were  it  not  for  the  flatness  of  its 
bottom,  might  set  one  speculating  upon  the  probability  of  some 
extraneous  body  having  rushed  by  the  moon  at  an  enormous 
velocity,  gouging  the  surface  tangentially  at  this  point  and  cutting 
a  channel  through  the  impeding  mass  of  mountains.  There  are 
other  mountain  ranges  of  less  magnitude  than  the  foregoing ;  but 
those  we  have  specified  will  sufiice  to  illustrate  our  suggestions 
concerning  this  class  of  features. 

We  remark,  too,  that  there  is  a  prevailing  tendency  of  the  ranges 
just  mentioned  to  present  their  loftiest  constituents  in  abrupt 
terminal  lines,  facing  nearly  the  same  direction,  the  reverse  of  that 
towards  w^hich  they  are  carried  by  the  moon's  rotation ;  and  as 
they  recede  from  the  high  terminal  line,  the  mountains  gradually 
fall  off  in  height,  so  that  in  bulk  the  ranges  present  the  "  crag  and 
tail "  contour  which  individual  hills  upon  the  earth  so  frequently 
exhibit. 

Isolated  peaks  are  found  in  small  numbers  upon  the  moon ;  there 
are  a  few  striking  examples  of  them  nevertheless,  and  these  are 
chiefly  situated  in  the  mountainous  region  just  alluded  to.  Several 
are  seen  to  the  east  (right  hand)  of  the  Alpine  range  depicted  on 
Plate  XIV.  The  best  known  of  these  is  Pico,  which  rises  abruptly 
from  a  generally  smooth  plain  to  a  height  of  7000  feet.  It  may  be 
recognized  as  the  lower  of  the  two  long  shadowing  spots  located 
almost  centrally  above  the  crater  Plato  in  the  illustration  just 
mentioned.  Above  it,  at  an  actual  distance  of  40  miles,  there  is 
another  peak  (unnamed),  about  4000  feet  high  ;  and  away  to  the 
west,  beyond  the  small  crater  joined  by  a  hill-ridge  to  Plato,  is  a 
third  pyramidal  mountain  nearly  as  high  as  Pico. 

It  seems  natural  to  regard  the  great  mountain  chains  as  agglo- 
merations of  those  peaks  of  which  we  have  isolated  examples  in 


142 


THE    MOON. 


[chap.  X. 


Pico  and  its  compeers,  and  thus  to  consider  that  the  formation  of  a 
mountain  chain  has  been  a  multiplication  of  the  process  that  formed 
the  single  pyramid- shaped  eminences.  At  first  thought  it  might 
appear  that  the  great  mountain  ranges  were  produced  by  bodily 
upthrustings  of  the  crust  of  the  moon  by  some  subsurface  convul- 
sions. But  such  an  explanation  could  hardly  hold  in  relation  to 
the  isolated  peaks,  for  it  is  difficult,  if  not  impossible,  to  conceive 


Fio.  36. 


that  these  abrupt  mountains,  almost  resembling  a  sugarloaf  in 
steepness,  could  have  been  protruded  6?i  masse  through  a  smooth 
region  of  the  crust.  On  the  contrary,  it  is  quite  consistent  with 
probability  to  suppose  that  they  were  built  up  by  a  slow  process 
somewhat  analogous  to  that  to  which  we  have  ascribed  the  piling 
of  the  central  cones  of  the  greater  craters.  We  believe  they  may 
be  regarded  as  true  mountains  of  exudation,  produced  by  the  com- 
paratively gentle  oozing  of  lava  from  a  small  orifice  and  its  solidifi- 
cation around  it ;  the  vent  however  remaining  open  and  the  summit 
or  discharging  orifice  continually  rising  with  the  growth  of  the 


CHAP.  X.] 


PEAKS    AND    MOUNTAIN   RANGES. 


143 


mountain,  as  indicated  in  the  annexed  cut,  Fig.  36.  This  process 
is  well  exemplified  in  the  case  of  a  water  fountain  playing  during 
a  severe  frost ;  the  water  as  it  falls  around  the  lips  of  the  orifice 
freezes  into  a  hillock  of  ice,  through  the  centre  of  which,  however, 
a  vent  for  the  fluid  is  preserved.  As  the  water  trickles  over  the 
mound  it  is  piled  higher  and  higher  hy  accumulating  layers  of  ice, 


Fia.  37. 


till  at  length  a  massive  cone  is  formed  whose  height  will  be  deter- 
mined by  the  force  or  "  head  "  of  the  water.  Substitute  lava  for 
water,  and  we  have  at  once  a  formative  process  which  may  very 
fairly  be  considered  as  that  which  has  given  rise  to  the  isolated 
mountains  of  the  moon. 

There  are  upon  the  earth  mountainous  forms  resembling  the 
isolated  peaks  of  the  moon,  and  which  have  been  explained  by  a 
similar  theory  to  the  above.  We  reproduce  a  figure  of  one  observed 
by  Dana  at  Hawaii  (Fig.  37),  and  a  sketch  of  another  observed  on 
the  summit  of  the  volcano  of  Bourbon  (Fig.  38) ;  we  also  repro- 
duce (Fig.  39)  an  ideal  section  of  the  latter,  given  by  Mr.  Scrope, 


144  THE    MOON.  [chap.  x. 

and  showing  the  successive  layers  of  lava  which  would  be  disposed 


Fig.  38. 


by  just  such  an  action  as  that  manifested  in  the  case  of  the  freezing 


Fig.  39. 


fountain ;  and  we  quote  that  author's  words  in  reference  to  this 
explanation  of  the  formation  of  Etna  and  other  volcanic  mountains. 


PLATE   XV i 


J.¥asrayth 


"Wbodbiirytype.' 


TYCHO    AND    ITS    SURROUNDINGS 

TO   5    0         JO         -20        30        iW         SO        60       70        SO 


Scale 


CHAP.  X.]  PEAKS    AND    MOUNTAIN   RANGES.  145 

"  On  examining,"  says  Mr.  Scrope,*  "  the  structure  of  the  moun- 
tain (Etna)  we  find  its  entire  mass,  so  far  as  it  is  exposed  to  view 
by  denudation  or  other  causes  (and  one  enormous  cavity,  the  Yal  de 
Bove  penetrates  deeply  into  its  very  heart),  to  be  composed  of  beds 
of  lava-rock  alternating  more  or  less  irregularly  with  layers  of  scoriae, 
lapillo  and  ashes,  almost  precisely  identical  in  mineral  character,  as 
well  as  in  general  disposition,  with  those  erupted  by  the  volcano  at 
known  dates  within  the  historical  period.  Hence  we  are  fully 
justified  in  believing  the  whole  mountain  to  have  been  built  up  in 
the  course  of  ages  in  a  similar  manner  by  repeated  intermittent 
eruptions.  And  the  argument  applies  by  the  rules  of  analogy  to 
all  other  volcanic  mountains,  though  the  history  of  their  recent 
eruptions  may  not  be  so  well  recorded,  provided  that  their  structure 
corresponds  with,  and  can  be  fairly  explained  by,  this  mode  of  pro- 
duction. It  is  also  further  applicable,  under  the  same  reservation, 
to  all  mountains  composed  entirely,  or  for  the  most  part,  of  volcanic 
rocks,  even  though  they  may  not  have  been  in  eruption  within  our 
time." 

To  these  illustrations  furnished  from  Scrope's  work  we  add 
another,  copied  from  a  photograph  by  Professor  Piazzi  Smyth,  of 
a  "  blowing  cone  "  at  the  base  of  Teneriffe  (Fig.  40),  which  is  but 
one  of  many  that  are  to  be  found  on  that  mountain,  and  which  has 
been  formed  by  a  process  similar  to  that  we  have  been  considering, 
but  acting  upon  a  comparatively  small  scale.  Professor  Smyth 
describes  this  cone  as  about  70  feet  high  and  of  parabolic  figure, 
composed  of  hard  lava  and  with  an  upper  aperture  still  yawning, 
*'  whence  the  burning  breath  of  fires  beneath  once  issued  in  fury 
and  with  destruction." 

Reverting  now  to  the  moon,  we  remark  that,  if  the  foregoing 
explanation  of  the  isolated  lunar  peaks  be  tenable,  it  should  hold 
equally  for  the  groups  of  them  which  we  see  in  the  lunar  Apennines, 
Alps,  Caucasus,  and  other  ranges  of  like  character.  There  occur 
in  some  places  intermediate  groups  which  link  the  one  to  the  other. 

*  "  Volcanoes,"  page  155. 


146 


THE    MOON. 


[chap.  X. 


Just  above  the  crater  Archimedes,  on  Plate  IX.,  for  instance,  we 
see  several  single  peaks  and  small  clumps  of  them  leading  by  suc- 
cessive multiple-peak  examples  to  what  may  be  called  chains  of 
mountains  like  many  that  are  included  in  the  contiguous  Apennine 


Fig.   40. 

SMALL   VOLCANIC   MOUNTAIN   AT   THE   END   OF   A   STREET   AT   TENERIFFE. 


system.  And,  in  view  of  this  connexion  between  the  single  peaks 
and  the  mountain  ranges  formed  of  aggregations  of  such  peaks,  it 
seems  to  us  reasonable  to  conclude  that  the  latter  were  formed  by 
the  comparatively  slow  escape  of  lava  through  multitudinous  open- 
ings in  a  weak  part  of  the  moon's  crust,  rather  than  to  suppose 
that  the  crust  itself  has  been  bodily  upheaved  and  retained  in  its 
disturbed  position.     The  high  peaks  that  many  mountains  in  such 


CHAP.  X.] 


PEAKS    AND    MOUNTAIN   RANGES. 


147 


a  chain  exhibit  accord  better  with  the  former  than  the  latter 
explanation  ;  for  it  is  difficult  to  imagine  how  such  lofty  eminences 
could  be  erected  by  an  upheaval,  and  we  must  remember  that  the 
moon  has  none  of  the  denuding  elements  which  are  at  work  upon 
the  earth,  weather  wearing  its  mountain  forms  into  sharpness  and 
steepness.* 

And  we  have  ground  for  believing  the  mountain-forming  process 
on  the  moon  to  have  been  a  comparatively  gentle  one,  in  the  fact 
that  the  mountain  systems  appear  in  regions  otherwise  little 
disturbed,  and  where  craters,  which  have  all  the  appearances  of 
violent  origin,  are  few  and  far  between.  Evidently  the  mountain 
and  crater-forming  processes,  although  both  due  to  extrusive 
action,  were  in  some  measure  difierent,  and  it  is  reasonable  to 
suppose  that  the  difference  was  in  degree  of  intensity ;  so  that 
while  a  violent  ejection  of  volcanic  material  would  give  rise  to  a 
crater,  a  more  gradual  discharge  would  pile  up  a  mountain.     In 

*  In  reference  to  such  prominences  on  the  lunar  surface  as  cast  steeple-like 
shadows,  it  is  well  to  remark  that  we  must  not  in  all  cases  infer,  from  the  acute 
spire-like  form  of  the  shadow,  that  the  object  which  casts  the  shadow  is  of  a 
similar  sharp  or  spire-like  form,  which  the  first  impression  would  naturally  lead  us 
to  suppose.  A  comparatively  blunt  or  rounded  eminence  will  project  a  long  and 
pointed  shadow  when  the  rays  of  light  fall  on  the  object  at  a  low  angle,  and 
especially  so  when  the  shadow  is  projected  on  a  convex  surface.  We  illustrate  this 
with  a  copy  of  an  actual  photograph  of  the  shadow  cast  by  half  a  pea,  Fig.  41. 


Fia.  41. 


L  2 


148  THE    MOON.  [chap.  x. 

this  view  craters  are  evidences  of  erujptive,  and  mountains  of  com- 
paratively gentle  exudative  action. 

We  can  hardly  speculate  with  any  degree  of  safety  upon  the 
cause  of  this  varying  intensity  of  volcanic  discharge.  We  may 
ascribe  it  to  variation  of  depth  of  the  initial  disturbing  force,  or  to 
suddenness  of  its  action ;  or  it  may  be  that  different  degrees  of 
fluidity  of  the  lava  have  had  modifying  effects ;  or  on  the  other 
hand  different  qualities  of  the  crust-material ;  or  yet  again 
differences  of  period — the  quieter  extrusions  having  occurred  at  a 
time  when  the  volcanic  forces  were  dying  down.  There  is  an 
alliance  between  lunar  craters  and  mountains  that  goes  far  to  show 
that  there  has  been  no  radical  difference  in  their  origins. 
For  instance,  as  we  have  previously  pointed  out,  craters  in  some 
cases  run  in  linear  groups,  as  if  in  those  cases  they  had  been 
formed  along  a  line  of  disruption  or  of  least  resistance  of  the 
crust  ;  and  the  mountain  chains  have  a  corresponding  linear 
arrangement.  Then  we  see  craters  and  mountain  chains  disposed 
in  what  seem  obviously  the  same  arcs  of  disturbance.  Thus  Coper- 
nicus (No.  147),  Erastothenes  (No.  168),  and  the  Apennines  appear 
to  belong  to  one  continuous  line  of  eruption  ;  and  it  requires  no 
great  stretch  of  imagination  to  suppose  that  the  Caucasus,  Eudoxus 
(No.  208),  and  Aristotle  (No.  209)  form  a  continuation  of  the  same 
line.  Then  around  the  Mare  Serenetatis  we  see  mountainous  ridges 
and  craters  alternating  one  with  the  other  as  though  the  exuding 
action  there,  normally  sufficient  to  produce  the  ridges,  had  at  some 
points  become  forcible  enough  to  produce  a  crater.  Again,  upon 
the  very  mountain  ranges  themselves,  as  for  instance  among  the 
Apennines,  we  find  small  craters  occurring.  We  see,  too,  that  the 
great  craters  are  in  many  cases  surrounded  by  radiating  systems  of 
ridges  which  almost  assume  mountainous  proportions,  and  which 
are  doubtless  exuded  matter  from  "  starred  '*  cracks,  the  centres  of 
which  are  occupied  by  the  craters.  The  same  kind  of  ridges  here 
and  there  occur  apart  from  craters  (see  for  instance  Plate  XX., 
below  Aristarchus  and  Herodotus)  and  sometimes  they  occur  in  the 


CHAP.x.]  PEAKS    AND    MOUNTAIN   RANGES.  149 

neighbourhood  of  extensive  cracks,  to  which  they  also  seem  allied. 
We  must  indeed  regard  a  linear  crack  as  the  origin  either  of  a 
ridge  (if  the  exudation  is  slight)  or  of  a  mountain  chain  (if  the 
exudation  is  more  copious)  or  a  string  of  craters  (if  the  extrusion 
rises  to  eruptive  violence).  But  the  subject  of  cracks  is  important 
enough  to  be  treated  in  a  separate  chapter. 

We  alluded  in  Chap.  III.  to  the  phenomena  of  wrinkhng  or 
puckering  as  productive  of  certain  mountainous  formations;  and 
we  pointed  out  the  striking  similarity  in  character  of  configuration 
between  a  shrivelled  skin  and  a  terrestrial  mountain  region.  We 
do  not  perceive  upon  the  moon  such  a  decided  coincidence  of 
appearances  extending  over  any  considerable  portion  of  her  surface ; 
but  there  are  numerous  limited  areas  where  we  behold  mountainous 
ridges  which  partake  strongly  of  the  wrinkle  character ;  and  in 
some  cases  it  is  difficult  to  decide  whether  the  puckering  agency  or 
the  exudative  agency  just  discussed  has  produced  the  ridges.  The 
district  bordering  upon  Aristarchus  and  Herodotus,  above  referred 
to,  is  of  this  doubtful  character ;  and  a  similar  district  is  that  con- 
tiguous to  Triesnecker  (Plate  XI.).  There  are,  however,  abundant 
examples  of  less  prominent  lines  of  elevation,  which  may,  with 
more  probability,  be  ascribed  to  a  veritable  wrinkling  or  puckering 
action ;  they  are  found  over  nearly  the  whole  lunar  surface,  some 
of  them  standing  out  in  considerable  relief,  and  some  merely 
showing  gentle  lines  of  elevation,  or  giving  the  surface  an 
undulating  appearance.  A  close  examination  of  our  picture-map 
(Plate  V.)  will  reveal  very  numerous  examples,  especially  in  the 
south-east  (right-hand-upper)  quadrant.  Some  of  these  lines  of 
tumescence  are  so  slightly  prominent  that  we  may  suppose  them 
to  have  been  caused  by  the  action  indicated  by  Fig.  6  (p.  32), 
while  others,  from  their  greater  boldness,  appear  to  indicate  a 
formative  action  analogous  to  that  represented  by  Fig.  9  (p.  33). 


CHAPTER    XI. 

CRACKS  AND   RADIATING    STREAKS. 

We  have  hitherto  confined  our  attention  to  those  reactions  of  the 
moon's  molten  interior  upon  its  exterior  which  have  been  accom- 
panied by  considerable  extrusions  of  sub-surface  material  in  its 
molten  or  semi- solid  condition.  We  now  pass  to  the  consideration 
of  some  phenomena  resulting  in  part  from  that  reaction  and  in  part 
from  other  effects  of  cooling,  which  have  been  accompanied  by 
comparatively  little  ejection  or  upflow  of  molten  matter,  and  in 
some  cases  by  none  at  all.  Of  such  the  most  conspicuous  examples 
are  those  bright  streaks  that  are  seen,  under  certain  conditions  of 
illumination,  to  radiate  in  various  directions  from  single  craters, 
and  some  of  the  individual  radial  branches  of  which  extend  from 
four  to  seven  hundred  miles  in  a  great  arc  on  the  moon's  surface. 

There  are  several  prominent  examples  of  these  bright  streak 
systems  upon  the  visible  hemisphere  of  the  moon ;  the  focal  craters 
of  the  most  conspicuous  are  Tycho,  Copernicus,  Kepler,  Aristarchus, 
Menelaus,  and  Proclus.  Generally  these  focal  craters  have 
ramparts  and  interiors  distinguished  by  the  same  peculiar  bright  or 
highly  reflective  material  which  shows  itself  with  such  remarkable 
brilliance,  especially  at  full  moon  :  under  other  conditions  of  illu- 
mination they  are  not  so  strikingly  visible.  At  or  nearly  full  moon 
the  streaks  are  seen  to  traverse  over  plains,  mountains,  craters,  and 
all  asperities;  holding  their  way  totally  disregardful  of  every 
object  that  happens  to  lay  in  their  course. 

The   most  remarkable  bright  streak  system  is  that  diverging 


PLATE   XVH 


'  ^^)c  J.'b-orytypc 


GLASS       GLOBE 
CRACKED      BY    INTERNAL     PRESSURE' 
ILLUSTRATING    THE    CAUSE     OF    THE    BRIGHT    STREAKS 
RADIATING      FROM      TYCHO. 


PLATE   X!X. 


-i^ 


''VVbodburytype" 


FULL       MOON 

EXHIBIT!  r:G     THE     BRIGHT     STREAKS 
RADIATING     FROM     TYCHO. 


^ 


CHAP.  XI.]  CRACKS    AND    RADIATING    8TREAK8.  151 

from  the  great  crater  Tycho.  The  streaks  that  can  be  easily 
individualized  in  this  group  number  more  than  one  hundred,  while 
the  courses  of  some  of  them  may  be  traced  through  upwards  of  six 
hundred  miles  from  their  centre  of  divergence.  Those  around 
Copernicus,  although  less  remarkable  in  regard  to  their  extent  than 
those  diverging  from  Tycho,  are  nevertheless  in  many  respects  well 
deserving  of  careful  examination :  they  are  so  numerous  as  utterly 
to  defy  attempts  to  count  them,  while  their  intricate  reticulation 
renders  any  endeavour  to  delineate  their  arrangement  equally 
hopeless. 

The  fact  that  these  bright  streaks  are  invariably  found  diverg- 
ing from  a  crater,  impressively  indicates  a  close  relationship  or 
community  of  origin  between  the  two  phenomena :  they  are 
obviously  the  result  of  one  and  the  same  causative  action.  It  is 
no  less  clear  that  the  actuating  cause  or  prime  agency  must  have 
been  very  deep-seated  and  of  enormous  disruptive  power  to  have 
operated  over  such  vast  areas  as  those  through  which  many  of  the 
streaks  extend.  With  a  view  to  illustrate  experimentally  what  we 
conceive  to  have  been  the  nature  of  this  actuating  cause,  we  have 
taken  a  glass  globe,  and,  having  filled  it  with  water  and  hermetically 
sealed  it,  have  plunged  it  into  a  warm  bath :  the  enclosed  water, 
expanding  at  a  greater  rate  than  the  glass,  exerts  a  disruptive  force 
on  the  interior  surface  of  the  latter,  the  consequence  being  that  at 
the  point  of  least  resistance,  the  globe  is  rent  by  a  vast  number  of 
cracks  diverging  in  every  direction  from  the  focus  of  disruption. 
The  result  is  such  a  strikingly  similar  counterpart  of  the  diverging 
bright  streak  systems  which  we  see  proceeding  from  Tycho  and  the 
other  lunar  craters  before  referred  to,  that  it  is  impossible  to  resist 
the  conclusion  that  the  disruptive  action  which  originated  them 
operated  in  the  same  manner  as  in  the  case  of  our  experimental 
illustration ;  the  disruptive  force  in  the  case  of  the  moon  being  that 
to  which  we  have  frequently  referred  as  due  to  the  expansion 
which  precedes  the  solidification  of  molten  substances  of  volcanic 
character. 


152  THE    MOON,  [chap.  xi. 

On  Plate  XVITE.  we  present  a  photograph  from  one  of  many  glass 
glohes  which  we  have  cracked  in  the  manner  described  :  a  careful 
comparison  between  the  arrangement  of  divergent  cracks  displayed 


FiQ.  42. 

ILLUSTRATIVE   OF   THE   RADIATING  CRACKS  WHICH   PRECEDE   THE    FORMATION   OF    THE 

BRIGHT    STREAKS. 


by  this  photograph  with  the  streaks  seen  spreading  from  Tycho 
upon  the  contiguous  lunar  photograph  -will,  we  trust,  justify  us  in 
what  we  have  stated  as  to  the  similarity  of  the  causes  which  have 
produced  such  identical  results. 


CHAP.  XI.]  CRACKS   AND    RADIATING    STREAKS.  153 

The  accompanying  figures  will  further  illustrate  our  views  upon 
the  causative  origin  of  the  hright  streaks.  The  primary  action 
rent  the  solid  crust  of  the  moon  and  produced  a  system  of  radiating 


Fia.  43. 

ILLUSTRATIVE   OP   THE   RADIATING   BRIGHT  STREAKS. 

fissures  (Fig.  42) :  these  immediately  afforded  egress  for  the  molten 
matter  heneath  to  make  its  appearance  on  the  surface  simul- 
taneously along  the  entire  course  of  every  crack,  and  irrespective  of 
all  surface  inequalities  or  irregularities  whatever  (Fig.  43).  We 
conceive  that  the  upflowing  matter  spread  in  hoth  directions  side- 


154  THE    MOON.  [chap.  xi. 

ways,  and  in  this  manner  produced  streaks  of  very  much  greater 
width  than  the  cracks  or  fissures  up  through  which  it  made  its  way 
to  the  surface. 

In  further  elucidation  of  this  part  of  our  suhject  we  may  refer  to 
a  familiar  hut  as  we  conceive  cogent  illustration  of  an  analogous 
action  in  the  behaviour  of  water  beneath  the  ice  of  a  frozen  pond, 
which,  on  being  fractured  by  some  concentrated  pressure,  or  by  a 
blow,  is  well  known  to  "  star  "  into  radiating  or  diverging  cracks, 
up  through  which  the  water  immediately  issues,  making  its 
appearance  on  the  surface  of  the  ice  simultaneously  along  the  entire 
course  of  every  crack,  and  on  reaching  the  surface,  spreading  on 
both  sides  to  a  width  much  exceeding  that  of  the  crack  itself. 

If  this  familiar  illustration  be  duly  considered,  we  doubt  not  it 
will  be  found  to  throw  considerable  light  on  the  nature  of  those 
actions  which  have  resulted  in  the  bright  streaks  on  the  moon's 
surface.  Some  have  attempted  to  explain  the  cause  of  these  bright 
streaks  by  assigning  them  to  streams  of  lava,  issuing  from  the 
crater  at  the  centre  of  their  divergence  and  flowing  over  the  surface, 
but  we  consider  such  an  explanation  totally  untenable,  as  any  idea 
of  lava,  be  it  ever  so  fluid  at  its  first  issue  from  its  source,  flowing 
in  streams  of  nearly  equal  width,  through  courses  several  hundred 
miles  long,  up  hills,  over  mountains,  and  across  plains,  appears  to 
us  beyond  all  rational  probability. 

It  may  be  objected  to  our  explanation  of  the  formation  of  these 
bright  streaks,  that  so  far  as  our  means  of  observation  avail  us,  we 
fail  to  detect  any  shadows  from  them  or  from  such  marginal  edges 
as  might  be  expected  to  result  from  a  side-way  spreading  outflow  of 
lava  from  the  cracks  which  afforded  it  exit  in  the  manner  described. 
Were  the  edges  of  these  streaks  terminated  by  cliff-like  or  craggy 
margins  of  such  height  as  30  or  40  feet,  we  might  just  be  able  at 
low  angles  of  illumination  and  under  the  most  favourable  circum- 
stances of  vision,  to  detect  some  slight  appearance  of  shadows ;  but 
so  far  as  we  are  aware,  no  such  shadows  have  been  observed.  We 
are  led  to  suppose  that  the  impossibility  of  detecting  them  is  due  not 


PLATE  XX. 


^i; 


0     43 


'¥' 


0 


•  ) 


0,:^v*>M;r^v. 


'.IJasmyth. 


'  Woodburytype '.' 


Vv'ARGENTIN  , 

JO    5     O  10        20         30        40         SO        60         70        SO 

lA  1  LES 

Scale     • 


(l  XJKIVEBSITY  » 


CHAP.  XI.]  CRACKS    AND    RADIATING    STREAKS.  155 

to  their  absence  but  to  the  height  of  the  margins  being  so  moderate 
as  not  to  cast  any  cognizable  shadow,  inasmuch  as  an  abrupt  craggy 
margin  of  10  or  15  feet  high  would,  under  even  the  most  favour- 
able circumstances,  fail  to  render  such  visible  to  us.  Reference  to 
our  ideal  section  of  one  of  these  bright  streaks  (Fig.  45),  will  shew 
how  thin  their  edges  may  be  in  relation  to  their  spreading  width. 

The  absence  of  cognizable  shadows  from  the  bright  streaks  has 
led  some  observers  to  conclude  that  they  have  no  elevation  above 
the  surface  over  which  they  traverse,  and  it  has  therefore  been 
suggested  that  their  existence  is  due  to  possible  vapours  which  may 
have  issued  through  the  cracks,  and  condensed  in  some  sublimated 
or  pulverulent  form  along  their  courses,  the  condensed  vapours  in 
question  forming  a  surface  of  high  reflective  properties.  That 
metallic  or  mineral  substances  of  some  kinds  do  deposit  on  conden- 
sation very  white  powders,  or  sublimates,  we  are  quite  ready  to 
admit,  and  such  explanation  of  the  high  luminosity  of  the  bright 
streaks,  and  of  the  craters  situated  at  the  foci  or  centres  of  their 
divergence  is  by  no  means  improbable,  so  far  as  concerns  their  mere 
brightness.  But  as  we  invariably  find  a  crater  occupying  the 
centre  of  divergence,  and  such  craters  are  possessed  of  all  the 
characteristic  features  and  details  which  establish  their  true 
volcanic  nature  as  the  results  of  energetic  extrusions  of  lava  and 
scoria,  we  cannot  resist  the  conclusion  that  the  material  of  the 
crater,  and  that  of  the  bright  streaks  diverging  from  it,  are  not 
only  of  a  common  origin,  but  are  so  far  identical  that  the  only 
difference  in  the  structure  of  the  one  as  compared  with  the  other 
is  due  to  the  more  copious  egress  of  the  extruded  or  erupted 
matter  in  the  case  of  the  crater,  while  the  restricted  outflow  or 
ejection  of  the  matter  up  through  the  cracks  would  cause  its 
dispersion  to  be  so  comparatively  gentle  as  to  flood  the  sides  of  the 
cracks  and  spread  in  a  thin  sheet  more  or  less  sideways  simul- 
taneously along  their  courses.  There  are  indeed  evidences  in  the 
wider  of  the  bright  streaks  of  their  being  the  result  of  the  outflow 
of  lava  through  systems  of  cracks  running  parallel  to  each  other, 


166  THE    MOON.  [chap.  xi. 

the  confluence  of  the  lava  issuing  from  which  would  naturally  yield 
the  appearance  of  one  streak  of  great  width.  Some  of  those 
diverging  from  Tycho  are  of  this  class ;  many  other  examples 
might  be  cited,  among  which  we  may  name  the  wide  streaks 
proceeding  from  the  crater  Menelaus  and  also  those  from  Proclus. 
Some  of  these  occupy  widths  upwards  of  25  miles — amply  sufficient 
to  admit  of  many  concurrent  cracks  with  confluent  lava  outflows. 

We  are  disposed  to  consider  as  related  to  the  forementioned 
radiating  streaks,  the  numerous,  we  may  say  the  multitudinous, 
long  and  narrow  chasms  that  have  been  sometimes  called  "  canals" 
or  '*  rills,"  but  which  are  so  obviously  cracks  or  chasms,  that  it  is 
desirable  that  this  name  should  be  applied  to  them  rather  than  one 
which  may  mislead  by  implying  an  aqueous  theory  of  formation. 
These  cracks,  singly  and  in  groups,  are  found  in  great  numbers  in 
many  parts  of  the  moon's  surface.  As  a  few  of  the  more  con- 
spicuous examples  which  our  plates  exhibit  we  may  refer  to  the 
remarkable  group  west  of  Triesnecker  (Plate  XI.),  the  principal 
members  of  which  converge  to  or  cross  at  a  small  crater,  and  thus 
point  to  a  continuity  of  causation  therewith  analogous  to  the  evident 
relation  between  the  bright  streaks  and  their  focal  craters.  Less 
remarkable,  but  no  less  interesting,  are  those  individual  examples 
that  appear  in  the  region  north  of  (below)  the  Apennines  (Plate  IX.), 
and  some  of  which  by  their  parallelism  of  direction  with  the 
mountain-chain  appear  to  point  to  a  causative  relation  also.  There 
is  one  long  specimen,  and  several  shorter  in  the  immediate 
neighbourhood  of  Mercator  and  Campanus  (Plate  XV.) ;  and 
another  curious  system  of  them,  presenting  suggestive  contortions, 
occurs  in  connection  with  the  mountains  Aristarchus  and 
Herodotus  (Plate  XXI.).  Others,  again,  appear  to  be  identified 
with  the  radial  excrescences  about  Copernicus  (Plate  VIII.). 
Capuanus,  Agrippa,  and  Gassendi,  among  other  craters,  have  more 
or  less  notable  cracks  in  their  vicinities. 

Some  of  these  chasms  are  conspicuous  enough  to  be  seen  with 
moderate  telescopic  means,  and  from   this  maximum   degree  of 


CHAP.  XI.]  CRACKS    AND    RADIATING    STREAKS.  157 

visibility  there  are  all  grades  downwards  to  those  that  require  the 
highest  optical  powers  and  the  best  circumstances  for  their  detection. 
The  earlier  selenographers  detected  but  a  few  of  them.  Schroeter 
noted  only  11 ;  Lohrman  recorded  75  more  ;  Beer  and  Maedler  added 
55  to  the  list,  while  Schmidt  of  Athens  raised  the  known  number 
to  425,  of  which  he  has  published  a  descriptive  catalogue.  We  take 
it  that  this  increase  of  successive  discoveries  has  been  due  to  the 
progressive  perfection  of  telescopes,  or,  perhaps,  to  increased 
education,  so  to  speak,  of  the  eye,  since  Schmidt's  telescope  is  a 
much  smaller  instrument  than  that  used  by  Beer  and  Maedler,  and 
is  regarded  by  its  owner  as  an  inferior  one  for  its  size.  We  doubt 
not  that  there  are  hundreds  more  of  these  cracks  which  more 
perfect  instruments  and  still  sharper  eyes  will  bring  to  knowledge 
in  the  future. 

While  these  chasms  have  all  lengths  from  150  miles  (which  is 
about  the  extent  of  those  near  Triesnecker)  down  to  a  few  miles,  they 
appear  to  have  a  less  variable  breadth,  since  we  do  not  find  many 
that  at  their  maximum  openings  exceed  two  miles  across ;  about  a 
mile  or  less  is  their  usual  width  throughout  the  greater  part  of 
their  length,  and  generally  they  taper  off  to  invisibility  at  their 
extremities,  where  they  do  not  encounter  and  terminate  at  a  crater 
or  other  asperity,  which  is,  however,  sometimes  the  case.  Of  their 
depth  we  can  form  no  precise  estimate,  though  from  the  sharpness 
of  their  edges  w^  may  conclude  that  their  sides  approach  perpen- 
dicularity, and,  therefore,  that  their  depth  is  very  great ;  we  have 
elsewhere  suggested  ten  miles  as  a  possible  profundity.  In  a  few 
cases,  and  under  very  favourable  circumstances,  we  have  observed 
their  generally  black  interiors  to  be  interrupted  here  and  there  with 
bright  spots  suggestive  of  fragments  from  the  sides  of  the  cracks 
having  fallen  into  the  opening. 

In  seeking  an  explanation  of  these  cracks,  two  possible  causes 
suggest  themselves.  One  is  the  expansion  of  sub-surface  matter, 
already  suggested  as  explanatory  of  the  bright  streaks ;  the  other, 
a  contraction  of  the  crust  by  cooling.     We  doubt  not  that  both 


158 


THE    MOON. 


[chap.  XI. 


causes  have  been  at  work,  one  perhaps  enhancing  the  other.  Where, 
as  in  the  cases  we  have  pointed  out,  there  are  cracks  which  are  so 
connected  with  craters  as  to  imply  relationship,  we  may  conclude 
that  an  upheaving  or  expansive  force  in  the  sublunar  molten 
matter  has  given  rise  to  the  cracks,  and  that  the  central  craters 
have  been  formed  simultaneously,  by  the  release,  with  ejective 
violence,  of  the  matter  from  its  confining  crust.  The  nature  of  the 
expansive  force  being  assumed  that  of  solidifying  matter,  the  wide 


J.N. 

Fig.  44. 


extent  of  some  chasms  indicates  a  deep  location  of  that  force.  And 
depth  in  this  matter  implies  lateness  (in  the  scale  of  selenological 
time)  of  operation,  since  the  central  portions  of  the  globe  would  be 
the  last  to  cool.  Now,  we  have  evidence  of  comparative  lateness 
afforded  by  the  fact  that  in  many  cases  the  cracks  have  passed 
through  craters  and  other  asperities  which  thus  obviously  existed 
before  the  cracking  commenced ;  and  thus,  so  far,  the  hypothesis  of 
the  expansion-cracking  is  supported  by  absolute  fact. 

It  may  be  objected  that  such  an  upheaving  force  as  we  are 
invoking,  being  transitory,  would  allow  the  distended  surface  to 
collapse  again  when  it  ceased  to  operate,  and  so  close  the  cracks  or 
chasms  it  produced.  But  we  consider  it  not  improbable  that  in 
some   cases,  as  a  consequence   of  the  expansion   of  sub- surface 


CHAP.  XI.] 


CRACKS  AND    RADIATING    STREAKS. 


159 


matter,  an  upflow  thereof  may  have  partially  filled  the  crack,  and  by 
solidifying  have  held  it  open ;  and  it  is  rational  to  suppose  that 
there  have  been  various  degrees  of  filling  and  even  of  overflow — that 
in  some  cases  the  rising  matter  has  not  nearly  reached  the  edge  of 
the  crack,  as  in  Fig.  44,  while  in  others  it  has  risen  almost  to  the 


Fig.  45. 


surface,  and  in  some  instances  has  actually  overrun  it  and  produced 
some  sort  of  elevation  along  the  line  of  the  crack,  like  that  repre- 
sented sectionally  in  Fig.  45.  It  is  probable  that  some  of  the 
slightly  tumescent  lines  on  the  moon's  surface  have  been  thus 
produced. 

We  have  suggested  shrinkage  as  a  possible  explanation  of  some 
cracks.  It  could  hardly  have  been  the  direct  cause  of  those  com- 
pound ones  which  are  distinguished  by  focal  craters,  though  it  may 
have  been  a  co-operative  cause,  since  the  contracting  tendency  of 
any  area  of  the  crust,  by  so  to  speak  weakening  it,  may  have 


160  THE    MOON.  [chap.  xi. 

virtually  increased  the  strength  of  an  upheaving  force  and  thus 
have  aided  and  localized  its  action.  We  see,  however,  no  reason 
why  the  inevitable  ultimate  contraction  which  must  have  attended 
the  cooling  of  the  moon's  crust,  even  when  all  internal  reactions 
upon  it  had  ceased,  should  not  have  created  a  class  of  cracks  with- 
out accompanying  craters,  while  it  would  doubtless  have  a  tendency 
to  increase  the  length  and  width  of  those  already  existing  from  any 
other  cause.  Some  of  the  more  minute  clefts,  which  presumably 
exist  in  greater  numbers  than  w^e  yet  know  of,  may  doubtless  be 
ascribed  to  this  effect  of  cooling  contraction.  In  this  view  we  should 
have  to  regard  such  cracks  as  the  latest  of  all  lunar  features. 
Whether  the  agency  that  produced  them  is  still  at  work — whether 
the  cracks  are  on  the  increase — is  a  question  impossible  of  solution  : 
for  reasons  to  be  presently  adduced,  we  incline  to  believe  that  all 
cosmical  heat  passed  from  the  moon,  and  therefore  that  it  arrived  at 
its  present,  and  apparently  final,  condition  ages  upon  ages  ago. 

Besides  the  ridges  spoken  of  on  p.  157,  and  regarded  as  cracks 
up  through  which  matter  has  been  extruded,  there  are  numerous 
ridges  of  greater  or  less  extent,  which  we  conceive  are  of  the 
nature  of  wrinkles,  and  have  been  produced  by  tangential  com- 
pression due  to  the  collapse  of  the  moon's  crust  upon  the 
shrunken  interior,  as  explained  and  illustrated  in  Chap.  III. 
The  distinguishing  feature  of  the  two  classes  of  phenomena  we 
consider  to  be  the  presence  of  a  serrated  summit  in  those  of 
the  extruded  class,  while  those  produced  by '' wrinkling"  action 
have  their  summits  comparatively  free  from  serration  or  marked 
irregularity. 


PLATE    XXI. 


J.lSIasirLytlx 


'  WoocLbiiry-cype" 


ARISTARCHUS    &     HERODOTUS 

70         £  0  10  20  30  *0  SO 


Scale 


CHAPTER    XII. 

COLOUR  AND  BRIGHTNESS   OF  LUNAR  DETAILS  :  CHRONOLOGY  OF 
FORMATIONS,   AND    FINALITY   OF    EXISTING    FEATURES. 

Speaking  generally,  the  details  of  the  lunar  surface  seem  to  us 
to  be  devoid  of  colour.     To  the  naked  eye  of  ordinary  sensitiveness 
the  moon  appears  to  possess  a  silvery  whiteness  :  more   critical 
judges  of  colour  would  describe  it  as  presenting  a  yellowish  tinge. 
Sir  John  Herschel,  during  his  sojourn  at  the  Cape  of  Good  Hope, 
had  frequent  opportunities  of  comparing  the  moon's  lustre  with 
that  of  the  weathered  sandstone  surface  of  Table  Mountain,  when 
the  moon  was  setting  behind  it,  and  both  were  illuminated  under 
the  same  direction  of  sunlight ;  and  he  remarked  that  the  moon 
was  at  such  times  "  scarcely  distinguishable   from   the  rock   in 
apparent  contact  with  it."     Although  his  observations  had  reference 
chiefly  to  brightness,  it  can  hardly  be  doubted  that  similarity  of 
colour  is  also  implied ;  for  any  diiference  in  the  tint  of  the  two 
objects  would  have  precluded  the  use  of  the  words  "  scarcely  dis- 
tinguishable ;  "  a  difference  of  colour  interfering  with  a  comparison 
of  lustre  in  such  an  observation,  though  it  must  be  remembered 
that  he  observed  through  a  dense  stratum  of  atmosphere.     Viewed 
in  the  telescope,  the  same  general  yellowish-white  colour  prevails 
over  all  the  moon,  with  a  few  exceptions  offered  by  the  so-called 
seas.     The  Mare  Crisium,  Mare  Serenetatis,  and  Mare  Humorum 
have  somewhat  of  a  greenish  tint;  the  Palus  Somnii   and  the 
circular  area  of  Lichtenberg  incline  to  ruddiness.     These  tints  are, 
however,  extremely  faint,  and  it  has  been  suggested  by  Arago  that 


162  THE    MOON.  [chap.  xii. 

they  may  be  mere  effects  of  contrast  rather  than  actual  colouration 
of  the  surface  material.  This,  however,  can  hardly  be  the  case, 
since  all  the  "  seas  "  are  not  alike  affected;  those  that  are  slightly 
coloured  are,  as  we  have  said,  some  green  and  some  red,  and  con- 
trast could  scarcely  produce  such  variations.  The  supposition  of 
vegetation  covering  these  great  flats  and  giving  them  a  local  colour 
is  in  our  view  still  more  untenable,  in  the  face  of  the  arguments 
that  we  shall  presently  adduce  against  the  possibility  of  vegetable 
life  existing  upon  the  moon. 

^^It  appears  to  us  more  rational  to  consider  the  tints  due  to  actual 
colour  of  the  material  (presumably  lava  or  some  once  fluid  mineral 
substance)  that  has  covered  these  areas  ;  and  it  may  well  be  con- 
ceived that  the  variety  of  tint  is  due  to  different  characters  of 
material,  or  even  various  conditions  of  the  same  material  coming 
from  different  depths  below  the  lunar  surface ;  and  we  may 
reasonably  suppose  that  the  same  variously-coloured  substances 
occur  in  the  rougher  regions  of  the  lunar  surface,  but  that  they 
exist  there  in  patches  too  small  to  be  recognized  by  us,  or  are 
**  put  out "  by  the  brightness  to  which  polyhedral  reflexion  gives 
rise. 

Seeing  that  volanic  action  has  had  so  large  a  share  in  giving  to 
the  moon's  surface  its  structural  character,  analogy  of  the  most 
legitimate  order  justifies  us  in  concluding  not  only  that  the 
materials  of  that  surface  are  of  kindred  nature  to  those  of  the  un- 
questionably volcanic  portions  of  the  earth,  but  also  that  the  tints 
and  colours  that  characterize  terrestrial  volcanic  and  Plutonian 
products  have  their  counterparts  on  the  moon.  Those  who  have 
seen  the  interior  and  surroundings  of  a  terrestrial  volcano  after  a 
recent  eruption,  and  before  atmospheric  agents  have  exercised  their 
dimming  influences,  must  have  been  struck  with  the  colours  of  the 
erupted  materials  themselves  and  the  varied  brilliant  tints  conferred 
on  these  materials  by  the  sublimated  vapours  of  metals  and  mineral 
substances  which  have  been  deposited  upon  them.  If,  then, 
analogy  is  any  guide  in  enabling  us  to  infer  the  appearance  of  the 


CHAP.  XII.]  CHRONOLOGY    OF   FORMATIONS.  163 

invisible  from  that  which  we  know  to  be  of  kindred  nature  and 
which  we  have  seen,  we  may  justly  conclude  that  were  the  moon 
brought  sufficiently  near  to  us  to  exhibit  the  minute  characteristics 
of  its  surface,  we  should  behold  the  same  bright  and  varied  colours 
in  and  around  its  craters  that  we  behold  in  and  about  those  of  the 
earth ;  and  in  all  probability  the  coloured  materials  of  lunar 
volcanoes  w^ould  be  more  fresh  and  vivid  than  those  of  the  earth 
by  reason  of  the  absence  of  those  atmospheric  elements  which 
tend  so  rapidly  to  impair  the  brightness  of  coloured  surfaces 
exposed  to  their  influence. 

Situated  as  we  are,  however,  as  regards  distance  from  the  moon, 
we  have  no  chance  of  perceiving  these  local  colours  in  their  smaller 
masses ;  but  it  is  by  no  means  improbable,  as  we  have  suggested, 
that  the  faint  tints  exhibited  by  the  great  plains  are  due  to  broad 
expanses  of  coloured  volcanic  material. 

But  if  we  fail  to  perceive  diversity  of  colour  upon  the  lunar 
surface,  we  are  in  a  very  different  position  in  regard  to  diversity  of 
brightness  or  variable  light-reflective  power  of  difi'erent  districts 
and  details.  This  will  be  tolerably  obvious  to  those  casual  ob- 
servers who  have  remarked  nothing  more  of  the  moon's  physio- 
graphy than  the  resemblance  to  a  somewhat  lugubrious  human 
countenance  which  the  full  moon  exhibits,  and  which  is  due  to  the 
accidental  disposition  of  certain  large  and  small  areas  of  surface 
material  which  have  less  of  the  light-reflecting  property  than 
other  portions ;  for  since  all  parts  seen  by  a  terrestrial  observer 
may  be  said  to  be  equally  shone  upon  by  the  sun,  it  is  clear  that 
apparently  bright  and  shaded  parts  must  be  produced  by  differences 
in  the  nature  of  the  surface  as  regards  power  of  reflecting  the  light 
received. 

When  we  turn  to  the  telescope  and  survey  the  full  disc  of  the 
moon  with  even  a  very  moderate  amount  of  optical  aid,  the  meagre 
impression  as  to  variety  of  degree  of  brightness  which  the 
unassisted  eye  conveys  is  vastly  extended  and  enhanced,  for  the 
surface  is  seen  to  be  diversified  by  shades  of  brilliancy  and  dulness 

M    2 


164  THE    MOON.  [chap.  xii. 

from  almost  glittering  white  to  sombre  grey  :  and  this  variety  of 
shading  is  rendered  much  more  striking  by  shielding  the  eye  with 
a  dusky  glass  from  the  excessive  glare,  which  drowns  the  details  in 
a  flood  of  light.  Under  these  circumstances  the  varieties  of  light 
and  shade  become  almost  bewildering,  and  defy  the  power  of  brush 
or  pencil  to  reproduce  them. 

We  may,  however,  realize  an  imperfect  idea  of  this  characteristic 
of  the  lunar  surface  by  reference  to  the  self- drawn  portrait  of  the 
full  moon  upon  Plate  IV.  This  is,  in  fact,  a  photograph  taken 
from  the  full  moon  itself,  and  enlarged  sufficiently  to  render 
conspicuous  the  spots  and  large  and  small  regions  that  are 
strildngly  bright  in  comparison  with  what  may  in  this  place  be 
described  as  the  "  ground  "  of  the  disc.  As  an  example  of  a  wide 
and  irregularly  extensive  district  of  highly  reflective  material,  the 
region  of  which  Tycho  is  the  central  object,  is  very  remarkable. 
We  may  refer  also  to  the  bright  **  splashes  "  of  which  Copernicus 
and  Kepler  are  the  centres.  So  brilliant  are  these  spots  that  they 
can  easily  be  detected  by  the  unassisted  eye  about  the  time  of  full 
moon.  Still  brighter  but  less  conspicuous  by  its  size  is  the  crater 
Aristarchus,  which  shines  with  specular  brightness,  and  almost 
induces  the  belief  that  its  interior  is  composed  of  some  vitreous- 
surfaced  matter  :  the  highly-reflective  nature  of  this  object  has 
often  caused  it  to  become  conspicuous  when  in  the  dark  hemisphere 
of  the  moon,  unilluminated  by  the  sun,  and  lighted  only  by  the 
light  reflected  from  the  earth.  At  these  times  it  appears  so  bright 
that  it  has  been  taken  for  a  volcano  in  actual  eruption,  and  no 
small  amount  of  popular  misconception  at  one  time  arose  therefrom 
concerning  the  conditions  of  the  moon  as  respects  existing  volcanic 
activity — a  misconception  that  still  clings  to  the  mind  of  many. 

The  parts  of  the  surface  distinguished  by  deficiency  of  reflecting 
power  are  conspicuous  enough.  We  may  cite,  however,  as  an 
example  of  a  detailed  portion  especially  remarkable  for  its  dingy 
aspect,  the  interior  of  the  crater  Plato,  which  is  one  of  the  darkest 
spots  (the  darkest  well-defined  one)  upon  the  hemisphere  of  the 


OHAP.  XII.]  CHRONOLOGY    OF   FORMATIONS.  165 

moon  visible  to  us.  For  facilitating  reference  to  shades  of 
luminosity,  Schroeter  and  Lohrman  assorted  the  variously  reflective 
parts  into  10  grades,  commencing  with  the  darkest.  Grades  1  to  3 
comprised  the  various  deep  greys  ;  4  and  5  the  light  greys  ;  6  and 
7  white ;  and  8  to  10  brilliant  white.  The  spots  Grimaldi  and 
Eiccioli  came  under  class  1  of  this  notation  ;  Plato  between  1  and 
2.  The  "  seas  "  generally  ranged  from  2  to  3  ;  the  brightest 
mountainous  portions  mostly  between  degrees  4  and  6  ;  the  crater 
walls  and  the  bright  streaks  came  between  these  and  the  bright 
peaks,  which  fell  under  the  9th  grade.  The  maximum  brightness, 
the  10th  grade,  is  instanced  only  in  the  case  of  Aristarchus  and  a 
point  in  Werner,  though  Proclus  nearly  approaches  it,  as  do  many 
bright  spots,  chiefly  the  sites  of  minute  craters,  which  make  their 
appearance  at  the  time  of  full  moon. 

In  photographic  pictures  produced  by  the  moon  of  itself  there  is 
always  an  apparent  exaggeration  in  the  relation  of  light  to  dark 
portions  of  the  disc.  The  dusky  parts  look,  upon  the  photograph, 
much  darker  than  to  the  eye  directed  to  the  moon  itself,  whether 
assisted  or  not  by  optical  appliances.  It  may  be  that  the  real 
cause  of  this  discrepancy  is  that  the  eye  fails  to  discover  the  actual 
difference  upon  the  moon  itself,  being  insensible  to  the  higher 
degrees  of  brightness  or  not  estimating  them  at  their  proper 
brilliance  with  respect  to  parts  less  bright.  On  the  other  hand,  it 
is  probable  that  the  enhanced  contrast  in  the  photograph  is  due  to 
some  peculiar  condition  of  the  darker  surface  matter  afl'ecting  its 
power  of  reflecting  the  actinic  constituent  of  the  rays  that  fall 
upon  it. 

The  study  of  the  varying  brightness  or  reflective  power  of 
different  regions  and  spots  of  the  lunar  disc  leads  us  to  the  con- 
sideration of  the  relative  antiquity  of  the  surface  features  ;  for  it  is 
hardly  possible  to  regard  these  variations  attentively  without  being 
impressed  with  the  conviction  that  they  have  relation  to  some 
chronological  order  of  formation.  We  cannot,  in  the  first  place, 
resist  the  conviction  that  the  brightest  features  were  the  latest 


166  THE    MOON.  [chap.  xii. 

formed ;  this  strikes  us  as  evident  on  pvimd  facie  grounds  ;  but  it 
becomes  more  clearly  so  when  we  remark  that  the  bright  forma- 
tions, as  a  rule,  overlie  the  duller  features.  The  elevated  parts  of 
the  crust  are  brighter  than  the  **  seas  "  and  other  areas  ;  and  it  is 
pretty  clear  that  the  former  are  newer  than  the  latter,  upon  which 
they  appear  to  be  super-imposed,  or  through  which  they  seem  to 
have  extruded.*  The  vast  dusky  plains  are  in  every  instance  more 
or  less  sprinkled  with  spots  and  minute  craters,  and  these  last 
were  obviously  formed  after  the  area  that  contains  them.  One  is 
almost  disposed  to  place  the  order  of  formations  in  the  order  of 
relative  brightness,  and  so  consider  the  dingiest  parts  the  oldest 
and  the  brightest  spots  and  craters  the  newest  features,  though,  in 
the  absence  of  an  atmosphere  competent  to  impair  the  reflective 
power  of  the  surface  materials,  we  are  unable  to  justify  this 
classification  by  suggesting  a  cause  for  such  a  deterioration  by 
time  as  the  hypothesis  pre-supposes. 

As  we  have  entered  upon  the  question  of  relative  age  of  the 
lunar  features,  we  may  remark  that  there  are  evidences  of  various 
epochs  of  formation  of  particular  classes  of  details,  irrespective  of 
their  condition  in  respect  of  brightness,  or,  as  we  may  say,  fresh- 
ness of  material.  As  a  rule,  the  large  craters  are  older  than  the 
small  ones.  This  is  proved  by  the  fact  that  a  large  object  of  this 
class  is  never  seen  to  interfere  with  or  overlap  a  small  one.  Those 
of  nearly  equal  size  are,  however,  seen  to  overlap  one  another  as 
though  -several  eruptions  of  equal  intensity  had  occurred  from  the 
same  source  at  different  points.  This  is  strikingly  instanced  in 
the  group  of  craters  situated  in  the  position  35 — 141  on  our  map, 
the  order  of  formation  of  each  of  which  is  clearly  apparent.  The 
region  about  Tycho  offers  an  inexhaustible  field  for  study  of  these 
phenomena  of  over-lapping  or  interpolating  craters,  and  it  will  be 


*  We  meet  a  difficulty  in  reconciling  this  idea  with  the  partial  craters  of  which 
we  have  a  conspicuous  example  in  Fracastorius,  No.  78,  of  our  Map,  which  seem  to 
be  partially  sunk  below  the  contiguous  surface.  This  looks  as  though  the  crater- 
rim  belonged  to  an  older  epoch  than  the  plain  from  which  it  rises. 


OVERLAPPING    CRATERS 


so  10  20 


50  60  70 


CHAP.  XII.]  CHRONOLOGY    OF   FORMATIONS.  167 

found,  with  very  few  exceptions,  that  the  smaller  crater  is  the 
impinging  or  parasitical  one,  and  must  therefore  have  been  formed 
after  the  larger,  upon  which  it  intrudes  or  impinges.  There  are 
frequent  cases  in  which  a  large  crater  has  had  its  rampart  inter- 
rupted by  a  lesser  one,  and  this  again  has  been  broken  into  by  one 
still  smaller ;  and  instances  may  be  found  where  a  fourth  crater 
smaller  than  all  has  intruded  itself  upon  the  previous  intruder. 
The  general  tendency  of  these  examples  is  to  show  that  the  craters 
diminished  in  size  as  the  moon's  volcanic  energy  subsided :  that 
the  largest  were  produced  in  the  throes  of  its  early  violence,  and 
that  the  smallest  are  the  results  of  expiring  efforts  possibly 
impeded  through  the  deep-seatedness  of  the  ejective  source. 

Another  general  fact  of  this  chronological  order  is  that  the 
mountain  chains  are  never  seen  to  intrude  upon  formations  of  the 
crater  order.  We  do  not  anywhere  find  that  a  mountain  chain 
runs  absolutely  into  or  through  a  crater  ;  but,  on  the  other  hand, 
we  do  find  that  craters  have  formed  on  mountain  chains.  This 
leads  unmistakably  to  the  inference  that  the  craters  were  not 
formed  before  their  allied  mountain  chains  ;  and  we  might  assume 
therefore  that  the  mountains  generally  are  the  older  formations, 
but  that  there  is  nothing  to  prove  that  the  two  classes  of  features, 
where  they  intermingle,  as  in  the  Apennines  and  Caucasus,  were 
not  erupted  cotemporaneously. 

Upon  the  assumption  that  the  latest  ejected  or  extruded  matter 
is  that  which  is  brightest,  we  should  place  the  bright  streaks 
among  the  more  recent  features.  Be  this  as  it  may,  it  is  tolerably 
certain  that  the  cracks,  whose  apparently  close  relation  to  the 
radiating  streaks  we  have  endeavoured  to  point  out,  are  relatively 
of  a  very  late  formative  period.  We  are  indeed  disposed  to 
consider  them  as  the  most  recent  features  of  all ;  the  evidence  in 
support  of  this  consideration  being  the  fact  that  they  are  sometimes 
found  intersecting  small  craters  that,  from  the  way  in  which  they 
are  cut  through  by  the  cracks,  must  have  been  in  situ  before  the 
cracking  agency  came  into  operation.     It  is  in  accordance  with  our 


168  THE    MOON.  [chap.  xii. 

hypothesis  of  the  moon's  transition  from  a  fluid  to  a  solid  body  to 
consider  that  a  cracking  of  the  surface  would  he  the  latest  of  all 
the  phenomena  produced  by  contraction  in  final  cooling. 

The  foregoing  remarks  naturally  lead  us  to  the  question  whether 
changes  are  still  going  on  upon  the  surface  of  our  satellite  :  whether 
there  is  still  left  in  it  a  spark  of  its  volcanic  activity,  or  whether 
that  activity  has  become  totally  extinct.  We  shall  consider  this 
question  from  the  observational  and  theoretical  point  of  view. 
First  as  regards  observations.  This  much  may  be  affirmed  indis- 
putably— that  no  object  or  detail  visible  to  the  earliest  seleno- 
graphers  (whose  period  may  be  dated  200  years  back)  has  altered 
from  the  date  of  their  maps  to  the  present.  When  we  pass  from 
the  bolder  features  to  the  more  minute  details  we  find  ourselves  at 
a  loss  for  materials  for  forming  an  inference ;  the  only  map  pre- 
tending to  accuracy  even  of  the  larger  among  small  objects  being 
that  of  Beer  and  Maedler,  which,  truly  admirable  as  it  is,  is  not 
very  safely  to  be  relied  upon  for  settling  any  question  of  alleged 
change,  on  account  of  the  conventional  system  adopted  for  exhibit- 
ing the  forms  of  objects,  every  object  being  mapped  rather  than 
drawn,  and  shown  as  it  never  is  or  can  be  presented  to  view  on  the 
moon  itself.  This  difficulty  would  present  itself  if  a  question  of 
change  were  ever  raised  upon  the  evidence  of  Beer  and  Maedler's 
map  :  it  may  indeed  have  prevented  such  a  question  being  raised,, 
for  certainly  no  one  has  hitherto  been  bold  enough  to  assert  that 
any  portion  or  detail  of  the  map  fails  to  represent  the  actual  state 
of  the  moon  at  the  present  time. 

In  default  of  published  maps,  we  are  thrown  for  evidence  on  this 
question  upon  observations  and  recollections  of  individual  observers 
whose  familiarity  with  the  lunar  details  extends  over  lengthy  periods. 
Speaking  for  ourselves,  and  upon  the  strength  of  close  scrutinies 
continued  with  assiduity  through  the  past  thirty  years,  we  may  say 
that  we  have  never  had  the  suspicion  suggested  to  our  eye  of  any 
actual  change  whatever  having  taken  place  in  any  feature  or  minute 
detail  of  the  lunar  surface;  and  our  scrutinies  have  throughout 


CHAP.  XII.]  CHRONOLOGY    OF   FORMATIONS.  169 

been  made  with   ample   optical   means,   mostly  with    a   20-inch 
reflector.     This  experience  has  made  us  not  unnaturally  in  some 
slight  decree  sceptical  concerning  the  changes  alleged  to  have  been 
detected  by  others.     Those  asserted  by  Schroeter  and  Gruithuisen 
were  long  ago  rejected  by  Beer  and  Maedler,  who  explained  them, 
where  the  accuracy  of  the  observer  was  not  questioned,  by  varia- 
tions of  illumination,  a   cause  of  illusory  change  which   is   not 
always  sufficiently  taken  into  account.     A  notable  instance  of  this 
deception  occurred  a  few  years  ago  in  the  case  of  the  minute  bright 
crater  Linnet  which  was  for  a  considerable  period  declared,  upon 
the  strength  of  observations  of  very  promiscuous  character,  to  be 
varying  in  form  and   dimensions  almost   daily,  but  the   alleged 
constant  changes  of  which  have  since  been  tacitly  regarded  as  due 
to  varying  circumstances  of  illumination  induced  by  combinations 
of  libratory  effects  with  the  ordinary  changes  depending  upon  the 
direction  of  the  sun's  rays  as  due  to  the  age  of  the  moon.     This 
explanation  does  not,  however,  dispose  of  the  question  whether  the 
crater  under  notice  suffered  any  actual  change  before  the  hue  and 
cry  was  raised  concerning  it.     Attention  was  first  directed  to  it  by 
Schmidt,  of  Athens,  whose  powers  of  observation  are  known  to  be 
remarkable,  and  whose  labours  upon  the  moon  are  of  such  extent 
and  minuteness  as  to  claim  for  his  assertions  the  most  respectful 
consideration.*     He  affirmed  in  1866  that  the  crater  at  that  date 
presented  an  appearance  decidedly  different  from  that  which  it  had 
had  since  1841 :  that  whereas  it  had  been  from  the  earlier  epoch 
always  easily  seen  as  a  very  deep  crater,  in  October,  1866,  and 
thenceforward  it  presented  only  a  white  spot,  with  at  most  but  a 
very  shallow  aperture,  very  difficult  to  be  detected.    Schmidt  is  one 
of  the  very  few  observers  whose  long  familiarity  with  the  moon 

*  We  are  informed  by  a  friend,  who  has  lately  visited  Athens,  that  Schmidt's 
detail  drawings  of  the  Moon,  comprising  the  work  of  forty  years,  form  a  small 
library  in  themselves.  The  map  embodying  them  is  so  large  (6  ft.  6  in.  in  diameter) 
and  so  full  of  detail  that  there  is  small  hope  of  its  complete  publication,  unless 
there  should  be  such  a  wide  extension  of  interest  in  the  minute  study  of  our  satellite 
as  to  justify  the  cost  of  reproducing  it. 


170 


THE    MOON, 


[chap.  XII. 


entitles  him  to  speak  with  confidence  upon  such  a  question  as  that 
before  us  upon  the  sole  strength  of  his  own  experience ;  and  this 
case  is  but  an  isolated  one,  at  least  it  is  the  only  one  he  has 
brought  forward.  He  is,  however,  still  firmly  convinced  that  it  is 
an  instance  of  actual  change,  and  not  an  illusion  resulting  from 


Fig.  46. 


some  peculiar  condition  of  illumination  of  the  object.  It  should  be 
added  also  on  this  side  of  the  discussion  than  an  English  observer, 
the  Kev.  T.  W.  Webb,  while  apparently  indisposed  to  concede  the 
supposition  of  any  notable  changes  in  the  lunar  features,  has  yet 
found  from  his  own  observations  that,  after  all  due  allowance  for 
differences  of  light  and  shade  upon  objects  at  different  times,  there 
is  still  a  "residuum  of  minute  variations  not  thus  disposed  of" 
which  seem  to  indicate  that  eruptive  action  in  the  moon  has  not 
yet  entirely  died  out,  though  its  manifestation  at  present  is  very 
limited  in  extent.  It  appears  to  us  that,  if  evidence  of  continuing 
volcanic  action  is  to  be  sought  on  the  moon,  the  place  to  look  for  it 


CHAP.  XII.}  CHRONOLOGY    OF   FORMATIONS,  171 

is  around  the  circumference  of  the  disc,  where  eruptions  from  any 
marginal  orifice  would  manifest  itself  in  the  form  of  a  protruding 
haziness,  somewhat  as  illustrated  to  an  exaggerated  extent  in  the 
annexed  cut  (Fig.  46). 

The  theoretical  view  of  the  question,  which  we  have  now  to 
consider,  has  led  us,  however,  to  the  strong  belief  that  no  vestige 
of  its  former  volcanic  activity  lingers  in  the  moon — that  it  assumed 
its  final  condition  an  inconceivable  number  of  ages  ago,  and  that 
the  high  interest  which  would  attach  to  the  close  scrutiny  of  our 
satellite  if  it  ivere  still  the  theatre  of  volcanic  reaction  cannot  be 
hoped  for.  If  it  be  just  and  allowable  to  assume  that  the  earth  and 
the  moon  were  condensed  into  planetary  form  at  nearly  the  same 
epoch  (and  the  only  rational  scheme  of  cosmogony  justifies  the 
assumption)  then  we  may  institute  a  comparison  between  the  con- 
dition of  the  two  bodies  as  respects  their  volcanic  age,  using  the 
one  as  a  basis  for  inference  concerning  the  state  of  the  other.  We 
have  reason  to  believe  that  the  earth's  crust  has  nearly  assumed  its 
final  state  so  far  as  volcanic  reactions  of  its  interior  upon  its 
exterior  are  concerned :  we  may  affirm  that  within  the  historical 
period  no  igneous  convulsions  of  any  considerable  magnitude  have 
occurred ;  and  we  may  consider  that  the  volcanoes  now  active  over 
the  surface  of  the  globe  represent  the  last  expiring  efforts  of  its 
eruptive  force.  Now  in  the  earth  we  perceive  several  conditions 
wherefrom  we  may  infer  that  it  parted  with  its  cosmical  heat  (and 
therefore  with  its  prime  source  of  volcanic  agency)  at  a  rate  which 
will  appear  relatively  very  slow  when  we  come  to  compare  the  like 
conditions  in  the  moon.  We  may,  we  think,  take  for  granted  that 
the  surface  of  a  planetary  body  generally  determines  its  heat  dis- 
persing power,  while  its  volume  determines  its  heat  retaining 
power.  Given  two  spherical  bodies  of  similar  material  but  of 
unequal  magnitude  and  originally  possessing  the  same  degree  of 
heat,  the  smaller  body  will  cool  more  rapidly  than  the  larger,  by 
reason  of  the  greater  proportion  which  the  surface  of  the  smaller 
sphere  bears  to  its  volume  than  that  of  the  larger  sphere  to  its 


172  THE    MOON.  [chap.  xii. 

volume — this  proportion  depending  upon  the  geometrical  ratio 
which  the  surfaces  of  spheres  hear  to  their  volumes,  the  contents  of 
spheres  heing  as  the  cubes  and  the  surfaces  as  the  squares  oi  their 
diameters.  The  volume  of  the  earth  is  49  times  as  great  as  that 
of  the  moon,  hut  its  surface  is  only  13  times  as  great;  there  is 
consequently  in  the  earth  a  power  of  retaining  its  cosmical  heat 
nearly  four  times  as  great  as  in  the  case  of  the  moon ;  in  other 
words,  the  moon  and  earth  heing  supposed  at  one  time  to  have  had 
an  equally  high  temperature,  the  moon  would  cool  down  to  a  given 
low  temperature  in  ahout  one -fourth  the  time  that  the  earth  would 
require  to  cool  to  the  same  temperature.  But  the  earth's  cosmical 
heat  has  without  doubt  been  considerably  conserved  by  its  vaporous 
atmosphere,  and  still  more  by  the  ocean  in  its  antecedent  vaporous 
form.  Yet  notwithstanding  all  this,  the  earth's  surface  has  nearly 
assumed  its  final  condition  so  far  as  volcanic  agencies  are  con- 
cerned :  it  has  so  far  cooled  as  to  be  subject  to  no  considerable  dis- 
tortions or  disruptions  of  its  surface.  What  then  must  be  the 
state  of  the  moon,  which,  from  its  small  volume  and  large  propor- 
tionate area,  parted  with  its  heat  at  the  above  comparatively  rapid 
rate  ?  The  matter  of  the  moon  is,  too,  less  dense  than  the 
earth,  and  from  this  cause  doubtless  disposed  to  more  rapid 
cooling ;  and  it  has  no  atmosphere  or  vaporous  envelope  to  retard 
its  radiating  heat.  We  are  driven  thus  to  the  conclusion  that  the 
moon's  loss  of  cosmical  heat  must  have  been  so  rapid  as  to  have 
allowed  its  surface  to  assume  its  final  conformation  ages  on  ages 
ago,  and  hence  that  it  is  unreasonable  and  hopeless  to  look  for 
evidence  of  change  of  any  volcanic  character  still  going  on. 

We  conceive  it  possible,  however,  that  minute  changes  of  a  non- 
volcanic  character  may  be  proceeding  in  the  moon,  arising  from  the 
violent  alternations  of  temperature  to  which  the  surface  is  exposed 
during  a  lunar  day  and  night.  The  sun,  as  we  know,  pours  down 
its  heat  unintermittingly  for  a  period  of  fully  300  hours  upon  the 
lunar  surface,  and  the  experimental  investigations  of  Lord  Kosse, 
essentially  confirmed  by  those  of  the  French  observer,  Marie  Davy, 


CHAP.  XII.]  CHRONOLOGY    OF   FORMATIONS.  173 

show  that  under  this  powerful  insolation  the  surface  becomes  heated 
to  a  degree  which  is  estimated  at  about  500°  of  Fahrenheit's  scale, 
the  fusing  point  of  tin  or  bismuth.  This  heat,  however,  is  entirely 
radiated  away  during  the  equally  long  lunar  night,  and,  as  Sir  John 
Herschel  surmised,  the  surface  probably  cools  down  again  to  a 
temperature  as  low  as  that  of  interstellar  space :  this  has  been 
assumed  as  representing  the  absolute  zero  of  temperature  which  has 
been  calculated  from  experiments  to  be  250°  below  the  zero  of 
Fahrenheit's  scale.  Now  such  a  severe  range  of  heat  and  cold  can 
hardly  be  without  effect  upon  some  of  the  component  materials  of 
the  lunar  surface.*  If  there  be  any  such  materials  as  the  vitreous 
lavas  that  are  found  about  our  volcanoes,  such  as  obsidian  for  in- 
stance, they  are  doubtless  cracked  and  shivered  by  these  extreme 
transitions  of  temperature ;  and  this  comparatively  rapid  succession 
of  changes  continued  through  long  ages  would,  we  may  suppose, 
result  in  a  disintegration  of  some  parts  of  the  surface  and  at  length 
somewhat  modify  the  selenographic  contour.  It  is,  however, 
possible  that  the  surface  matter  is  mainly  composed  of  more 
crystalline  and  porous  lavas,  and  these  might  withstand  the  fierce 
extremes  like  the  "  fire-brick  "  of  mundane  manufacture,  to  which 
in  molecular  structure  they  may  be  considered  comparable.  Lavas 
as  a  rule  are  (upon  the  earth)  of  this  unvitreous  nature,  and  if  they 
are  of  like  constitution  on  the  moon,  there  will  be  little  reason  to 
suspect  changes  from  the  cause  we  are  considering.  Where,  how- 
ever, the  material,  whatever  its  nature,  is  piled  in  more  or  less 
detached  masses,  there  will  doubtless  be  a  grating  and  fracturing 
at  the  points  of  contact  of  one  mass  with  another,  produced  by 
alternate  expansions  and  contractions  of  the  entire  masses,  which  in 
the  long  run  of  ages  must  bring  about  dislocations  or  dislodgments 
of  matter  that  might  considerably  affect  the  surface  features  from 
a  close  point  of  view,  but  which  can  hardly  be  of  sufficient  magni- 


*  It  is  conceivable  that  the  alleged  changes  in  the  crater  Linne  may  have 
been  caused  by  a  filling  of  the  crater  by  some  such  crumbling  action  as  we  are 
here  contemplating. 


174  THE    MOON.  [chap.  xii. 

tude  to  be  detected  by  a  terrestrial  observer  whose  best  aids  to 
vision  give  him  no  perception  of  minute  configurations.  And  it 
must  always  be  borne  in  mind  that  changes  can  only  be  proved  by 
reference  to  previous  observations  and  delineations  of  unquestion- 
able accuracy. 

Speaking  by  our  own  lights,  from  our  own  experience  and 
reasoning,  we  are  disposed  to  conclude  that  in  all  visible  aspects 
the  lunar  surface  is  unchangeable,  that  in  fact  it  arrived  at  its 
terminal  condition  ceons  of  ages  ago,  and  that  in  the  survey  of  its 
wonderful  features,  even  in  the  smallest  details,  we  are  presented 
with  the  sight  of  objects  of  such  transcendent  antiquity  as  to  render 
the  oldest  geological  features  of  the  earth  modern  by  comparison. 


CHAPTER    XII L 

THE   MOOK   AS  A   WORLD:    DAY  AND  NIGHT   UPON  ITS  SURFACE. 

A  WIDE  interest,  if  not  a  deep  one,  attaches  to  the  general  ques- 
tion as  to  the  existence  of  living  beings,  or  at  least  the  possibility 
of  organic  existence,  on  planetary  bodies  other  than  our  own.  The 
question  has  been  examined  in  all  ages,  by  the  lights  of  the  science 
peculiar  to  each.  With  every  important  accession  to  our  astrono- 
mical knowledge  it  has  been  re-raised  :  every  considerable  discovery 
has  given  rise  to  some  new  step  or  phase  in  the  discussion,  and  in 
this  way  there  has  grown  up  a  somewhat  extensive  literature  ex- 
clusively relating  to  mundane  plurality.  It  will  readily  be  under- 
stood that  the  moon,  from  its  proximity  to  the  earth,  has  from  the 
first  received  a  large,  perhaps  the  largest,  share  of  attention  from 
wanderers  in  this  field  of  speculation  :  and  we  might  add  greatly  to 
the  bulk  of  this  volume  by  merely  reviewing  some  of  the  more 
curious  and,  in  their  way,  instructive  conjectures  specially  relating 
to  the  moon  as  a  world — to  imaginary  journeys  towards  her,  and  to 
the  beings  conjectured  to  dwell  upon  and  within  her.  This,  how- 
ever, we  feel  there  is  no  occasion  to  do,  for  it  is  our  purpose  merely 
to  point  out  the  two  or  three  almost  conclusive  arguments  against 
the  possibility  of  any  life,  animal  or  vegetable,  having  existence  on 
our  satellite. 

We  well  know  what  are  the  requisite  conditions  of  life  on  the 
earth ;  and  we  can  go  no  further  for  grounds  of  inference ;  for  if  w^e 
were  to  start  by  assuming  forms  of  life  capable  of  existence  under 
conditions  widely  and  essentially  different  from  those  pertaining  to 


1^6  THE    MOON.  [chap.  XIII. 

our  planet,  there  would  be  no  need  for  discussing  our  subject 
further :  we  could  revel  in  conjectures,  without  a  thought  as  to 
their  extravagance.  The  only  legitimate  phase  of  the  question  we 
can  entertain  is  this : —  can  there  be  on  the  moon  any  kind 
of  living  things  analogous  to  any  kind  of  living  things  upon 
the  earth?  And  this  question,  we  think,  admits  only  of  a 
negative  answer.  The  lowest  forms  of  vitality  cannot  exist 
without  air,  moisture,  and  a  moderate  range  of  temperature.  It 
may  be  true,  as  recent  experiments  seem  to  show,  that  organic 
germs  will  retain  their  vitality  without  either  of  the  first,  and  with 
exposure  to  intense  cold  and  to  a  considerable  degree  of  heat ;  and 
it  is  conceivable  that  the  mere  germs  of  life  may  be  present  on  the 
moon.*  But  this  is  not  the  case  with  living  organisms  themselves. 
We  have,  in  Chapter  Y.,  specially  devoted  to  the  subject,  cited  the 
evidence  from  which  we  know  that  there  can  be  at  the  most,  no 
more  air  on  the  moon  than  is  left  in  the  receiver  of  an  air-pump 
after  the  ordinary  process  of  exhaustion.  And  with  regard  to 
moisture,  it  could  not  exist  in  any  but  the  vaporous  state,  and  we 
knew  that  no  appreciable  amount  of  vapour  can  be  discovered  by 
any  observation  (and  some  of  them  are  crucial  enough)  that  we  are 
capable  of  making.  We  may  suppose  it  just  within  the  verge  of 
possibility  that  some  low  forms  of  vegetation  might  exist  upon  the 
moon  with  a  paucity  of  air  and  moisture  such  as  would  be  beyond 
even  our  most  severe  powers  of  detection :  but  granting  even  this, 
we  are  met  by  the  temperature  diifficulty ;  for  it  is  inconceivable 
that  any  plant-life  could  survive  exposure  first  to  a  degree  of  cold 
vastly  surpassing  that  of  our  arctic  regions,  and  then  in  a  short  time 

*  Is  it  not  conceivable  that  the  protogerms  of  life  pervade  the  whole  universe, 
and  have  been  located  upon  every  planetary  body  therein  ?  Sir  William  Thomson's 
suggestion  that  life  came  to  the  earth  upon  a  seed-bearing  meteor  was  weak,  in  so 
far  that  it  shifted  the  locus  of  life-generation  from  one  planetary  body  to  another. 
Is  it  not  more  philosophical,  more  consistent  with  our  conception  of  Creative  omni- 
potence and  impartiality,  to  suppose  that  the  protogerms  of  life  have  been  sown 
broadcast  over  all  space,  and  that  they  have  fallen  here  upon  a  planet  under  con- 
ditions favourable  to  their  development,  and  have  sprung  into  vitality  when  the 
fit  circumstances  have  arrived,  and  there  upon  a  planet  that  is,  and  that  may  be  for 
ever,  unfitted  for  their  vivification. 


< 
or 
o 

cc: 

13 


O 

22: 


CHAP.  XIII.]  THE    MOON    AS    A    WORLD.  Ill 

(14  days)  to  a  degree  of  heat  capable  of  melting  the  more  fusible 
metals — the  total  range  being  equal,  as  we  have  elsewhere  shown, 
to  perhaps  600  or  700  degrees  of  our  thermometric  scale. 

The  higher  forms  of  vegetation  could  not  reasonably  be  expected 
to  exist  under  conditions  which  the  lower  forms  could  not  survive. 
And  as  regards  the  possibility  of  the  existence  of  animal  life  in  any 
form  or  condition  on  the  lunar  surface,  the  reasons  we  have  adduced 
in  reference  to  the  non-existence  of  vegetable  life  bear  still  more 
strongly  against  the  possibility  of  the  existence  of  the  former.  We 
know  of  no  animal  that  could  live  in  what  may  be  considered  a 
vacuum  and  under  such  thermal  conditions  as  we  have  indicated. 

As  to  man,  aeronautic  experiences  teaches  us  that  human  life  is 
endangered  when  the  atmosphere  is  still  sufficiently  dense  to  support 
12  inches  of  mercury  in  the  barometer  tube ;  what  then  would  be 
his  condition  in  a  medium  only  sufficiently  dense  to  sustain  one- 
tenth  of  an  inch  of  the  barometric  column?  We  have  evidence 
from  the  most  delicate  tests  that  no  atmosphere  or  vapour 
approaching  even  this  degree  of  attenuation  exists  around  the 
moon's  surface. 

Taking  all  these  adverse  conditions  into  consideration,  we  are  in 
every  respect  justified  in  concluding  that  there  is  no  possibility  of 
animal  or  vegetable  life  existing  on  the  moon,  and  that  our  satellite 
must  therefore  be  regarded  as  a  barren  world. 

******* 

After  this  disquisition  upon  lunar  uninhabitability  it  may  appear 
somewhat  inconsistent  for  us  to  attempt  a  description  of  the  scenery 
of  the  moon  and  some  other  effects  that  would  be  visible  to  a  spec- 
tator, and  of  which  he  would  be  otherwise  sensible,  during  a  day 
and  a  night  upon  her  surface.  But  we  can  offer  the  sufficient 
apology  that  an  imaginary  sojourn  of  one  complete  lunar  day  and 
night  upon  the  moon  affords  an  opportunity  of  marshalling  before 
our  readers  some  phenomena  that  are  proper  to  be  noticed  in  a 
work  of  this  character,  and  that  have  necessarily  been  passed  over 
in  the  series  of  chapters  on  consecutive  and  special  points  that  have 


178  THE    MOON.  [chap.  xiii. 

gone  before.  It  may  be  urged  that,  in  depicting  tbe  moon  from 
such  a  standpoint  as  that  now  to  be  taken,  we  are  describing  scenes 
that  never  have  been  such  in  the  literal  sense  of  the  word,  since  no 
eye  has  ever  beheld  them.  Still  we  have  this  justification — that  we 
are  invoking  the  conception  of  things  that  actually  exist ;  and  that 
we  are  not,  like  some  imaginary  voyagers  to  the  moon,  indulging 
in  mere  flights  of  fancy.  Although  it  is  impossible  for  a  habitant 
of  this  earth  fully  to  realize  existence  upon  the  moon,  it  is  yet 
possible,  indeed  almost  inevitable,  for  a  thoughtful  telescopist — 
watching  the  moon  night  after  night,  observing  the  sun  rise  upon  a 
lunar  scene,  and  noting  the  course  of  eiBfects  that  follow  till  it  sets 
— it  is  almost  inevitable,  we  say,  for  such  an  observer  to  identify 
himself  so  far  with  the  object  of  his  scrutiny,  as  sometimes  to  be- 
come in  thought  a  lunar  being.  Seated  in  silence  and  in  solitude 
at  a  powerful  telescope,  abstracted  from  terrestrial  influences,  and 
gazing  upon  the  revealed  details  of  some  strikingly  characteristic 
region  of  the  moon,  it  requires  but  a  small  effort  of  the  imagination 
to  suppose  one's  self  actually  upon  the  lunar  globe,  viewing  some 
distant  landscape  thereupon ;  and  under  these  circumstances  there 
is  an  irresistible  tendency  in  the  mind  to  pass  beyond  the  actually 
visible,  and  to  fill  in  with  what  it  knows  must  exist  those  accessory 
features  and  phenomena  that  are  only  hidden  from  us  by  distance 
and  by  our  peculiar  point  of  view.  Where  the  material  eye  is 
baffled,  the  clairvoyance  of  reason  and  analogy  comes  to  its  aid. 

Let  us  then  endeavour  to  realize  the  strange  consequences  which 
the  position  and  conditiqns  of  the  moon  produce  upon  the  aspect 
of  a  lunar  landscape  in  the  course  of  a  lunar  day  and  night. 

The  moon's  day  is  a  long  one.  From  the  time  that  the  sun  rises 
upon  a  scene*  till  it  sets,  a  period  of  304  hours  elapses,  and  of 
course  double  this  interval  passes  between  one  sunrise  and  the 
next.     The  consequences  of  this  slow  march  of  the  sun  begin  to 

*  Our  remarks  have  general  reference  to  a  region  of  the  moon  near  her  equator  j 
near  the  poles  some  of  the  conditions  we  shall  describe  would  be  somewhat 
modified. 


CHAP.  XIII.]  THE    MOON   AB    A    WORLD.  179 

show  themselves  from  the  instant  that  he  rises  above  the  lunar 
horizon.  Dawn,  as  we  have  it  on  earth,  can  have  no  counterpart 
upon  the  moon.  No  atmosphere  is  there  to  reflect  the  solar  beams 
while  the  luminary  is  yet  out  of  actual  sight,  and  only  the  glimmer 
of  the  zodiacal  light  heralds  the  approach  of  day.  From  the  black 
horizon  the  sun  suddenly  darts  his  bright  untempered  beams  upon 
the  mountain  tops,  crowning  them  with  dazzling  brilliance  Vv^hile 
their  flanks  and  valleys  are  yet  in  utter  darkness.  There  is  no 
blending  of  the  night  into  day.  And  yet  there  is  a  growth  of 
illumination  that  in  its  early  stages  may  be  called  a  twilight,  and 
which  is  caused  by  the  slow  rise  of  the  sun.  Upon  the  earth,  in 
central  latitudes,  the  average  time  occupied  by  the  sun  in  rising, 
from  the  first  glint  of  his  upper  edge  till  the  whole  disc  is  in  sight, 
is  but  two  minutes  and  a  quarter.  Upon  the  moon,  however,  this 
time  is  extended  to  a  few  minutes  short  of  an  hour,  and  therefore, 
during  the  first  few  minutes  a  dim  light  will  be  shed  by  the  small 
visible  chord  of  the  solar  disc,  and  this  will  give  a  proportionately 
modified  degree  of  illumination  upon  the  prominent  portion  of  the 
landscape,  and  impart  to  it  something  of  the  weird  aspect  which  so 
strikes  an  observer  of  a  total  solar  eclipse  on  earth  when  the  scene 
is  lit  by  the  thin  crescent  of  the  re -appearing  sun.  This  impaired 
illumination  constitutes  the  only  dawn  that  a  lunar  spectator  could 
behold.  And  it  must  be  of  short  duration ;  for  when,  in  the  course 
of  half  an  hour,  the  solar  disc  has  risen  half  into  view  the  lighting 
would  no  doubt  appear  nearly  as  bright  to  the  eye  as  when  the 
entire  disc  of  the  sun  is  above  the  horizon.  In  this  lunar  sunrise, 
however,  there  is  none  of  that  gilding  and  glowing  which  makes  the 
phenomenon  on  earth  so  gorgeous.  Those  crimson  sky-tints  with 
which  we  are  familiar  are  due  to  the  absorption  of  certain  of  the 
polychromous  rays  of  light  by  our  atmosphere.  The  blue  and 
violet  components  of  the  solar  beams  are  intercepted  by  our  enve- 
lope of  vapour,  and  only  the  red  portions  are  free  to  pass ;  while  on 
the  moon,  as  there  is  no  atmosphere,  this  selective  absorption  does 
not  occur.     If  it  did,  an  observer  gazing  from  the  earth  upon  the 

N  2 


180  THE    MOON.  [chap.  xiii. 

regions  of  the  moon  upon  which  the  sun  is  just  rising  would  see 
the  surface  tinted  with  rosy  light.  This,  however,  is  not  the  case  : 
the  faintest  lunar  features  just  catching  the  sun  are  seen  simply 
under  white  light  diluted  to  a  low  degree  of  brightness.  Only  upon 
rare  occasions  is  the  lunar  scenery  suffused  with  coloured  illumina- 
tion, and  these  are  when,  as  we  shall  presently  have  to  describe, 
the  solar  rays  reach  the  moon  after  traversing  the  earth's  atmos- 
phere during  an  eclipse  of  the  sun. 

This  atmosphere  of  ours  is  the  most  influential  element  in 
beautifying  our  terrestrial  scenery,  and  the  absence  of  such  an 
appendage  from  the  moon  is  the  great  modifying  cause  that  affects 
lunar  scenery  as  compared  with  that  of  the  earth.  We  are 
accustomed  to  the  sun  with  its  dazzling  brightness — overpowering 
though  it  be— subdued  and  softened  by  our  vaporous  screen.  Upon 
the  moon  there  is  no  such  modification.  The  sun's  intrinsic 
brilliancy  is  undiminished,  its  apparent  distance  is  shortened,  and 
it  gleams  out  in  fierce  splendour  only  to  be  realized,  and  then 
imperfectly,  by  the  conception  of  a  gigantic  electric  light  a  few  feet 
from  the  eye.  And  the  brightness  is  rendered  the  more  striking 
by  the  blackness  of  the  surrounding  sky.  Since  there  is  no  atmo- 
sphere there  can  be  no  sky-light,  for  there  is  nothing  above  the 
lunar  world  to  diffuse  the  solar  beams  ;  not  a  trace  of  that  moisture 
which  even  in  our  tropical  skies  scatters  some  of  the  sun's  light 
and  gives  a  certain  degree  of  opacity  or  blueness,  deep  though  it  be, 
to  the  heavens  by  day.  Upon  the  moon,  with  no  light-difi'using 
vapour,  the  sky  must  be  as  dark  or  even  darker  than  that  with 
which  we  are  familiar  upon  the  finest  of  moonless  nights.  And 
this  blackness  prevails  in  the  full  blaze  of  the  lunar  noon-day  sun. 
If  the  eye  (upon  the  moon)  could  bear  to  gaze  upon  the  solar  orb 
(which  would  be  less  possible  than  upon  earth)  or  could  it  be 
screened  from  the  direct  beams,  as  doubtless  it  could  by  intervening 
objects,  it  would  perceive  the  nebulous  and  other  appendages  which 
we  know  as  the  corona,  the  zodiacal  light,  and  the  red  solar  pro- 
tuberances :  or  if  these  appendages  could  not  be  viewed  with  the 


CHAP.  XIII.]  THE    MOON   AS    A    WORLD  181 

sun  above  the  horizon  they  would  certainly  be  seen  in  glorious  per- 
fection when  the  luminary  was  about  to  rise  or  immediately  after 
it  had  set. 

And,  notwithstanding  the  sun's  presence,  the  planets  and  stars 
would  be  seen  to  shine  more  brilliantly  than  we  see  them  on  the 
clearest  of  nights ;  the  constellations  would  have  the  same  configu- 
rations, though  they  would  be  differently  situated  with  respect  to 
the  celestial  pole  about  which  they  would  appear  to  turn,  for  the 
axis  of  rotation  of  the  moon  is  directed  towards  a  point  in  the  con- 
stellation Draco.  The  stars  would  never  twinkle  or  change  colour 
as  they  appear  to  us  to  do,  for  scintillation  or  twinkling  is  a 
phenomenon  of  atmospheric  origin,  and  they  would  retain  their  full 
brightness,  down  even  to  the  horizon,  since  there  would  be  no  haze 
to  diminish  their  light.  The  planets,  and  the  brighter  stars  at 
least,  would  be  seen  even  when  they  were  situated  very  near  to  the 
sun.  The  planet  Mercury,  so  seldom  detected  by  terrestrial  gazers, 
would  be  almost  constantly  in  view  during  the  lunar  day,  manifest- 
ing his  close  attendance  on  the  central  luminary  by  making  only 
short  excursions  of  about  two  (lunar)  days'  length,  first  on  one  side 
and  then  on  the  other.  Venus  would  be  nearly  as  continuously 
visible,  though  her  wanderings  would  be  more  extensive  on  either 
side.  The  zodiacal  light  also,  which  in  our  English  latitude  and 
climate  is  but  rarely  seen  and  in  more  favourable  climes  appears 
only  when  the  sun  itself  is  hidden  beneath  the  horizon,  would  upon 
the  moon  be  seen  as  a  constant  accompaniment  to  the  luminary 
throughout  his  daily  course  across  the  lunar  sky.  The  other 
planets  would  appear  generally  as  they  do  to  us  on  earth,  but,  never 
being  lost  in  daylight,  their  courses  among  the  stars  could  be  traced 
with  scarcely  any  interruption. 

One  planet,  however,  that  adorns  the  sky  of  the  lunar  hemisphere 
which  is  turned  towards  us  deserves  special  mention  from  the  con- 
spicuous and  highly  interesting  appearance  it  must  present.  We 
allude  to  the  earth.  To  nearly  one-half  of  the  moon  (that  which 
we  never  see)  this  imposing  object  can  never  be  visible  ;  but  to  the 


182  THE    MOON.  [chap.  xiii. 

half  that  faces  us  the  terrestrial  planet  must  appear  almost  fixed 
in  the  sky.  A  lunar  spectator  in  (what  is  to  us)  the  centre  of  the 
disc,  or  ahout  the  region  north  of  the  lunar  mountains  Ptolemy  and 
Hipparchus,  would  have  the  earth  in  his  zenith.  From  regions 
upon  the  moon  a  little  out  of  what  is  to  us  the  centre,  a  spectator 
would  see  the  earth  a  little  declining  from  the  zenith,  and  this 
declination  would  increase  as  the  regions  corresponding  to  the  (to 
us)  apparent  edge  of  the  moon  were  approached,  till  at  the  actual 
edge  it  w^ould  be  seen  only  upon  the  horizon.  From  the  phenomena 
of  libration  (explained  in  Chap.  VI.)  the  earth  would  appear  from 
nearly  all  parts  of  the  lunar  hemisphere  to  which  it  is  visible  at  all 
to  describe  a  small  circle  in  the  sky.  To  an  observer,  however, 
upon  the  (to  us)  marginal  regions  of  the  lunar  globe,  it  would 
appear  only  during  a  portion  of  the  lunar  day — ^being  visible  in 
fact  only  in  that  part  of  its  small  circular  path  which  happened  to 
lie  above  the  observer's  horizon  :  in  some  regions  only  a  portion  of 
the  terrestrial  disc  would  make  its  brief  appearance.  From  the 
lunar  hemisphere  beyond  this  marginal  line  the  earth  can  never  be 
seen  at  all. 

The  lunar  spectator  whose  situation  enabled  him  to  view  the 
earth  would  see  it  as  a  moon  ;  and  a  glorious  moon  indeed  it  must 
be.  Its  diameter  would  be  four  times  as  great  as  that  of  the  moon 
itself  as  seen  by  us,  and  the  area  of  its  full  disc  13  times  as  great. 
It  would  be  seen  to  pass  through  its  phases,  just  as  does  our 
satellite,  once  in  a  lunar  day  or  a  terrestrial  month,  and  during 
that  cycle  of  phases,  since  29  of  our  days  would  be  occupied  by  it, 
the  axial  rotation  would  bring  all  the  features  of  its  surface 
configuration  into  view  so  many  times  in  succession.  But  the 
greatest  beauty  of  this  noble  moon  would  be  seen  during  the  lunar 
night,  in  considering  which  we  shall  again  allude  to  it ;  for  when  it 
is  full-moon  to  the  earth  it  is  new-earth  to  the  moon.  At  lunar 
midnight  this  globe  of  ours  is  fully  illuminated ;  as  morning 
nears,  the  earth-moon  wanes,  its  disc  slowly  passing  through  the 
gibbous  phases  until  at  sunrise  it  would  be  just  half-illuminated. 


CHAP.  XIII.]  THE    MOON   AS    A    WORLD.  183 

During  the  long  forenoon  it  assumes  a  crescent  which  narrows  and 
narrows  till  at  midday  the  sun  is  in  line  with  the  earth  and  the 
latter  is  invisible,  save  perhaps  by  a  thin  line  of  light  marking  its 
upper  or  lower  edge,  accordingly  as  the  sun  is  apparently  above  or 
below  it.  In  the  lunar  afternoon  an  illuminated  crescent  appears 
upon  the  opposite  side  of  the  terrestrial  globe,  and  this  widens  and 
widens  till  it  becomes  a  half  disc  by  lunar  sunset  and  a  full  disc  by 
lunar  midnight. 

The  sun  in  his  daily  course  passes  at  various  distances,  some- 
times above  and  sometimes  below,  the  nearly  stationary  earth. 
Obviously  it  will  at  times  pass  actually  behind  it,  and  then  the 
lunar  spectator  would  behold  the  sublime  spectacle  of  a  total  solar 
eclipse,  and  that  under  circumstances  which  render  the  phenomenon 
far  more  imposing  than  its  counterpart  can  appear  from  the  earth  ; 
for  whereas,  when  we  see  the  moon  eclipse  the  sun,  the  nearly 
similar  (apparent)  diameters  of  the  two  bodies  render  the  duration 
of  totality  extremely  short — at  most  7  minutes — a  lunar  spectator, 
the  earth  appearing  to  him  four  times  the  diameter  of  the  sun,  and 
he  and  the  earth  being  relatively  stationary,  would  enjoy  a  view  of 
the  totality  extending  over  several  hours.  During  the  passage  of 
the  solar  disc  behind  that  of  the  earth,  a  beautiful  succession  of 
luminous  phenomena  would  be  observed  to  follow  from  the  refrac- 
tions and  dispersions  which  the  sunbeams  would  suffer  in  passing 
tangentially  through  those  parts  of  our  atmospheric  envelope 
which  lie  in  their  course  ;  those,  for  instance,  on  the  margin  of  the 
earth,  as  seen  from  the  moon.  As  the  sun  passed  behind  the 
earth,  the  latter  would  be  encircled  upon  the  in-gomg  side  wdth  a 
beautiful  line  of  golden  light,  deepening  in  places  to  glowing 
crimson,  due  to  the  absorption,  already  spoken  of,  of  all  but  the 
red  and  orange  rays  of  the  sun's  light  by  the  vapours  of  our 
atmosphere.  As  the  eclipse  proceeded  and  totality  came  on,  this 
ruddy  glow  would  extend  itself  nearly,  if  not  all,  around  the  black 
earth,  and  so  bright  would  it  be,  that  the  whole  lunar  landscape 
covered  by  the  earth's  shadow  would  be  illuminated  with   faint 


184  THE    MOON.  [chap.  xiii. 

crimson  light,*  save,  perhaps,  in  some  parts  of  the  far  distance, 
upon  which  the  earth  had  not  yet  cast  its  shadow,  or  off  which  the 
shadow  had  passed.  Although  the  crimson  light  would  prepon- 
derate, it  would  not  appear  bright  and  red  alike  all  around  the 
earth's  periphery.  The  circle  of  light  would  be,  in  fact,  the  ring 
of  twilight  round  our  globe,  and  it  would  only  appear  red  in  those 
places  where  the  atmosphere  chanced  to  be  in  that  condition  favour- 
able for  producing  what  on  earth  we  know  as  red  sunset  and  sunrise. 
We  know  that  the  sun,  even  in  clear  sky,  does  not  always  set  and 
rise  with  the  beautiful  red  glow,  which  may  be  determined  by 
merely  local  causes,  and  will  therefore  vary  in  different  parts  of  the 
earth.  Now  a  lunar  spectator  watching  the  sun  eclipsed  by  the 
earth,  would  see,  during  totality  and  at  a  coup  d'oeil,  every  point 
around  our  world  upon  which  the  sun  is  setting  on  one  side  and 
rising  upon  the  other.  To  every  part  of  the  earth  around  what  is 
then  the  margin,  as  seen  from  the  moon,  the  sun  is  upon  the 
horizon,  shining  through  a  great  thickness  of  atmosphere,  reddening 
it,  and  being  reddened  by  it  wherever  the  vaporous  conditions 
conduce  to  that  colouration.  And  at  all  parts  where  these  con- 
ditions obtain,  the  lunar  eclipse-observer  would  see  the  ring  of  light 
around  the  black  earth -globe  brilliantly  crimsoned ;  at  other  parts 
it  would  have  other  shades  of  red  and  yellow,  and  the  whole  effect 
would  be  to  make  the  grand  earth -ball,  hanging  in  the  lunar  sky, 
like  a  dark  sphere  in  a  circle  of  glittering  gold  and  rubies. 

During  the  early  stages  of  the  eclipse,  this  chaplet  of  brilliant- 
coloured  lights  would  be  brightest  upon  the  side  of  the  clisap' 
pearing  sun ;  at  the  time  of  central  eclipse  the  radiance  (supposing 
the  sun  to  pass  centrally  behind  the  earth)  would  be  equally 
distributed,  and  during  the  later  stages  it  would  preponderate  upon 


*  We  see  this  reddening  during  an  eclipse  of  the  moon  (when  the  event  we  are 
describing — an  eclipse  of  the  sun  visible  from  the  moon — really  takes  place).  The 
blood-red  colour  has  often  struck  observei's  very  forcibly,  and  it  has  indeed  been 
suggested  that  the  appearance  may  be  the  innocent  and  oft-repeated  fulfilment  of 
the  prophetic  allusion  to  the  moon  being  "  turned  into  blood." 


a:  z 

<  S 

ui  O 

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T.  Ui 

r-  X 

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ll; 


CHAP.  XIII.]  THE    MOON   A8    A    WORLD,  185 

the  side  of  the  reappearing  sun.  We  have  endeavoured  to  give  a 
pictorial  realization  of  this  phenomenon  and  of  the  effect  of  the 
eclipse  upon  the  lunar  landscape,  hut  such  a  picture  cannot  hut 
fall  very,  very  far  short  of  the  reality.     (See  Plate  XXIV.) 

And  now  for  a  time  let  us  turn  attention  from  the  lunar  sky  to 
the  scenery  of  the  lunar  landscape.  Let  us,  in  imagination,  take 
our  stand  high  upon  the  eastern  side  of  the  rampart  of  one  of  the 
great  craters.  Height,  it  must  he  remarked,  is  more  essential  on 
the  moon  to  command  extent  of  view  than  upon  the  earth,  for  on 
account  of  the  comparative  smallness  of  the  lunar  sphere  the  dip  of 
the  horizon  is  very  rapid.  Such  height,  however,  would  he  attained 
without  great  exercise  of  muscular  power,  since  equal  amounts  of 
climbing  energy  would,  from  the  smallness  of  lunar  gravity,  take  a 
man  six  times  as  high  on  the  moon  as  on  the  earth.  Let  us  choose, 
for  instance,  the  hill-side  of  Copernicus.  The  day  begins  by  a 
sudden  transition.  The  faint  looming  of  objects  under  the  united 
illumination  of  the  half-full  earth,  and  the  zodiacal  light  is  the 
lunar  precursor  of  daybreak.  Suddenly  the  highest  mountain  peaks 
receive  the  direct  rays  of  a  portion  of  the  sun's  disc  as  it  emerges 
from  below  the  horizon.  The  brilliant  lighting  of  these  summits 
serves  but  to  increase,  by  contrast,  the  prevailing  darkness,  for  they 
seem  to  float  like  islands  of  light  in  a  sea  of  gloom.  At  a  rate  of 
motion  twenty-eight  times  slower  than  we  are  accustomed  to,  the 
light  tardily  creeps  down  the  mountain-sides,  and  in  the  course  of 
about  twelve  hours  the  whole  of  the  circular  rampart  of  the  great 
crater  below  us,  and  towards  the  east,  shines  out  in  brilliant  light, 
unsoftened  by  a  trace  of  mountain- mist.  But  on  the  opposite  side, 
looking  into  the  crater,  nothing  but  blackness  is  to  be  seen.  As 
hour  succeeds  hour,  the  sunbeams  reach  peak  after  peak  of  the 
circular  rampart  in  slow  succession,  till  at  length  the  circle  is  com- 
plete and  the  vast  crater-rim,  50  miles  in  diameter,  glistens  like  a 
silver-margined  abyss  of  darkness.  By-and-by,  in  the  centre, 
appears  a  group  of  bright  peaks  or  bosses.     These  are  the  now 


186  THE    MOON.  [chap.  xiii. 

illuminated  summits  of  the  central  cones,  and  the  development  of 
the  great  mountain  cluster  they  form  henceforth  becomes  an 
imposing  feature  of  the  scene.  From  our  high  standpoint,  and 
looking  backwards  to  the  sunny  side  of  our  cosmorama,  we  glance 
over  a  vast  region  of  the  wildest  volcanic  desolation.  Craters  from 
five  miles  diameter  downwards  crowd  together  in  countless  numbers, 
so  that  the  surface,  as  far  as  the  eye  can  reach,  looks  veritably 
frothed  over  with  them.  Nearer  the  base  of  the  rampart  on  which 
we  stand,  extensive  mountain  chains  run  to  north  and  to  south, 
casting  long  shadows  towards  us;  and  away  to  southward  run 
several  gi'eat  chasms  a  mile  wide  and  of  appalling  blackness  and 
depth.  Nearer  still,  almost  beneath  us,  crag  rises  on  crag  and 
precipice  upon  precipice,  mingled  with  craters  and  yawning  pits, 
towering  pinnacles  of  rock  and  piles  of  scoriae  and  volcanic  debris. 
But  we  behold  no  sign  of  existing  or  vestige  of  past  organic  life. 
No  heaths  or  mosses  soften  the  sharp  edges  and  hard  surfaces  :  no 
tints  of  cryptogamous  or  lichenous  vegetation  give  a  complexion  of 
life  to  the  hard  fire-worn  countenance  of  the  scene.  The  whole 
landscape,  as  far  as  the  eye  can  reach,  is  a  realization  of  a  fearful 
dream  of  desolation  and  lifelessness — not  a  dream  of  death,  for 
that  implies  evidence  of  pre-existing  life,  but  a  vision  of  a  world 
upon  which  the  light  of  life  has  never  dawned. 

Looking  again,  after  some  hours'  interval,  into  the  great  crateral 
amphitheatre,  we  see  that  the  rays  of  the  morning  sun  have  crept 
down  the  distant  side  of  the  rampart,  opposite  to  that  on  which  we 
stand,  and  lighted  up  its  vast  landslipped  terraces  into  a  series  of 
seeming  hill-circles  with  all  the  rude  and  rugged  features  of  a 
terrestrial  mountain  view,  and  none  of  the  beauties  save  those  of 
desolate  grandeur.  The  plateau  of  the  crater  is  half  in  shadow 
10,000  feet  below,  with  its  grand  group  of  cones,  now  fully  in 
sight,  rising  from  its  centre.  Although  these  last  are  twenty 
miles  away  and  the  base  of  the  opposite  rampart  fully  double  that 
distance,  we  have  no  means  of  judging  their  remoteness,  for  in  the 
absence  of  an  atmosphere  there  can  be  no  aerial  perspective,  and 


CHAP.  XIII.]  THE    MOON   AS    A    WORLD.  187 

distant  objects  appear  as  brilliant  and  distinct  as  those  which  are 
close  to  the  observer.  Not  the  brightness  only,  but  the  various 
colours  also  of  the  distant  objects  are  preserved  in  their  full 
intensity  ;  for  colour  we  may  fairly  assume  there  must  be. 
Mineral  chlorates  and  sublimates  will  give  vivid  tints  to  certain 
parts  of  the  landscape  surface,  and  there  must  be  all  the  more 
sombre  colours  which  are  common  to  mineral  matters  that  have 
been  subjected  to  fiery  influence.  All  these  tints  will  shine  and 
glow  with  their  greater  or  less  intrinsic  lustres,  since  they  have  not 
been  deteriorated  by  atmospheric  agencies,  and  far  and  near  they 
will  appear  clear  alike,  since  there  is  no  aerial  medium  to  veil  them 
or  tarnish  their  pristine  brightness. 

Cin  the  lunar  landscape,  in  the  line  of  sight,  there  are  no  means 
of  estimating  distances  ;  only  from  an  eminence,  where  the  inter- 
vening ground  can  be  seen,  is  it  possible  to  realize  magnitude  in  a 
lunar  cosmorama  and  comprehend  the  dimensions  of  the  objects  it 
includes. 

And  with  no  air  there  can  be  no  diffusion  of  light.  As  a  con- 
sequence, no  illumination  reaches  those  parts  of  the  scene  which 
do  not  receive  the  direct  solar  rays,  save  the  feeble  amount  reflected 
from  contiguous  illuminated  objects,  and  a  small  quantity  shed  by 
the  crescent  earth.  The  shadows  have  an  awful  blackness.  As  we 
stand  upon  our  chosen  point  of  observation,  we  see  on  the  lighted 
side  of  the  rampart  almost  dazzling  brightness,  while  beneath  us, 
on  the  side  away  from  the  sun,  there  is  a  region  many  miles  in 
area  impenetrable  to  the  sight,  for  there  is  no  object  within  it 
receiving  sufficient  light  to  render  it  discernible ;  and  all  around 
us,  far  and  near,  there  is  the  violent  contrast  between  intense 
brightness  of  insulated  parts  and  deep  gloom  of  those  in  equally 
intense  shadow.  The  black  though  starlit  sky  helps  the  violence 
of  this  contrast,  for  the  bright  mountains  in  the  distance  around 
us  stand  forth  upon  a  background  formed  by  the  darkness  of  inter- 
planetary space.  The  visible  effects  of  these  conditions  must  be  in 
every  sense  unearthly  and  truly  terrible. )  The  hard,  harsh,  glowing 


188  THE    MOON.  [chap.  xiii. 

light  and  pitchy  shadows ;  the  absence  of  all  the  conditions  that 
give  tenderness  to  an  earthly  landscape ;  the  black  noonday  sky, 
with  the  glaring  sun  ghastly  in  its  brightness ;  the  entire  absence 
of  vestiges  of  any  life  save  that  of  the  long  since  expired  volcanoes 
— all  these  conspire  to  make  up  a  scene  of  dreary,  desolate 
grandeur  that  is  scarcely  conceivable  by  an  earthly  habitant,  and 
that  the  description  we  have  attempted  but  insufficiently  pourtrays. 
A  legitimate  extension  of  the  imagination  leads  us  to  impressions 
of  lunar  conditions  upon  other  senses  than  that  of  sight,  to  which 
we  have  hitherto  confined  our  fancy.  We  are  met  at  the  outset 
with  a  difficulty  in  this  extension ;  for  it  is  impossible  to  conceive 
the  sensations  which  the  absence  of  an  atmosphere  would  produce 
upon  the  most  important  of  our  bodily  functions.  If  we  would 
attempt  the  task  we  must  conjure  up  feelings  of  suffocation,  of 
which  the  thoughts  are,  however,  too  horrible  to  be  dwelt  upon ; 
we  must  therefore  maintain  the  delusion  that  we  can  exist  without 
air,  and  attempt  to  realize  some  of  the  less  discomforting  effects  of 
the  absence  of  this  medium.  Most  notable  among  these  are  the 
untempered  heat  of  the  direct  solar  rays,  and  the  influence  thereof 
upon  the  surface  material  upon  which  we  suppose  ourselves  to 
stand.  During  a  period  of  over  three  hundred  hours  the  sun  pours 
down  his  beams  with  unmitigated  ferocity  upon  a  soil  never 
sheltered  by  a  cloud  or  cooled  by  a  shower,  till  that  soil  is  heated, 
as  we  have  shown,  to  a  temperature  equal  nearly  to  that  of  melting 
lead ;  and  this  scorching  influence  is  felt  by  everything  upon  which 
the  sun  shines  on  the  lunar  globe.y  But  while  regions  directly 
isolated  are  thus  heated,  those  parts  turned  from  the  sun  w^ould 
remain  intensely  cold,  and  that  scorching  in  sunshine  and  freezing 
in  shade  with  which  mountaineers  on  the  earth  are  familiar  would 
be  experienced  in  a  terribly  exaggerated  degree.  Among  the 
consequences,  already  alluded  to,  of  the  alternations  of  temperature 
to  which  the  moon's  crust  is  thus  exposed,  are  doubtless  more  or 
less  considerable  expansions  and  contractions  of  the  surface 
material,  and  we  may  conceive  that  a  cracking  and  crumbling  of 


CHAP.  XIII.]  THE   MOON   A8    A    WORLD.  189 

the  more  brittle  constituents  would  ensue,  together  with  a  grating 
of  contiguous  but  disconnected  masses,  and  an  occasional  dislocation 
of  them.  We  refer  again  to  these  phenomena  to  remark  that  if  an 
atmospheric  medium  existed  they  would  be  attended  with  noisy 
manifestations.  fThere  are  abundant  causes  for  grating  and 
crackling  sounds,  and  such  are  the  only  sources  of  noise  upon  the 
moon,  where  there  is  no  life  to  raise  a  hum,  no  wind  to  murmur, 
no  ocean  to  boom  and  foam,  and  no  brook  to  plash.  Yet  even 
these  crust- cracking  commotions,  though  they  might  be  felt  by  the 
vibrations  of  the  ground,  would  not  manifest  themselves  audibly, 
for  without  air  there  can  be  no  communication  between  the  grating 
or  cracking  body  and  the  nerves  of  hearing.  Dead  silence  reigns 
on  the  moon  :  a  thousand  cannons  might  be  fired  and  a  thousand 
drums  beaten  upon  that  airless  world,  but  no  sound  could  come 
from  them  :  lips  might  quiver  and  tongues  essay  to  speak,  but  no 
action  of  theirs  could  break  the  utter  silence  of  the  lunar  scene. 

At  a  rate  twenty-eight  times  slower  than  upon  earth,  the  shadows 
shorten  till  the  sun  attains  his  meridian  height,  and  then,  from  the 
tropical  region  upon  which  we  have  in  imagination  stood,  nothing 
is  to  be  seen  on  any  side,  save  towards  the  black  sky,  but  dazzling 
light.  The  relief  of  afternoon  shadow  comes  but  tardily,  and  the 
darkness  drags  its  slow  length  along  the  valleys  and  creeps 
sluggishly  up  the  mountain-sides  till,  in  a  hundred  hours  or  more, 
the  time  of  sunset  approaches.  This  phenomenon  is  but  daybreak 
reversed,  and  is  unaccompanied  by  any  of  the  gorgeous  sky  tints 
that  make  the  kindred  event  so  enrapturing  on  earth.  The  sun 
declines  towards  the  dark  horizon  without  losing  one  jot  of  its 
brilliancy,  and  darts  the  full  intensity  of  its  heat  upon  all  it  shines 
on  to  the  last.  Its  disc  touches  the  horizon,  and  in  half  an  hour 
dips  half-way  beneath  it,  its  intrinsic  brightness  and  colour 
remaining  unchanged.  *  The  brief  interval  of  twilight  occurs,  as  in 
the  morning,  when  only  a  small  chord  of  the  disc  is  visible,  and 
the  long  shadows  now  sharpen  as  the  area  of  light  that  casts  them 
decreases.     For  a  while  the  zodiacal  light  vies  with  the  earth-moon 


190  THE    MOON.  [chap.  xiii. 

high  in  the  heavens  in  illuminating  the  scene ;  but  in  a  few  hours 
this  solar  appendage  passes  out  of  view,  and  our  world  becomes  the 
queen  of  the  lunar  night. 

At  this  sunset  time  the  earth,  nearly  in  the  zenith  of  us,  will  be 
at  its  half-illuminated  phase,  and  even  then  it  will  shed  more  light 
than  we  receive  upon  the  brightest  of  moonlight  nights.  As  the 
night  proceeds,  the  earth-phase  will  increase  through  the  gibbous 
stages  until  at  midnight  it  will  be  "  full,"  and  our  orb  will  be  seen 
in  its  entire  beauty.  It  will  perform  at  least  one  of  its  twenty-four- 
hourly  rotations  during  the  time  that  it  appears  quite  full,  and  the 
whole  of  its  surface  features  will  in  that  time  pass  before  the  lunar 
spectator's  eye.  At  times  the  northern  pole  will  be  turned  towards 
our  view,  at  times  the  southern ;  and  its  polar  ice-caps  will  appear 
as  bright  white  spots,  marking  its  axis  of  rotation.  If  our  lunar 
sojourn  were  prolonged  we  should  observe  the  northern  ice-caps 
creep  downwards  to  lower  latitudes  (during  our  winter)  and  retreat 
again  (during  our  summer) ;  and  this  variation  would  be  perceptible 
in  a  less  degree  at  the  southern  pole,  on  account  of  the  watery  area 
surrounding  it.  The  seas  would  appear  (so  far  as  can  be  inferred) 
of  pale  blue-green  tint ;  the  continents  parti- coloured  :  and  the 
tinted  spots  would  vary  with  the  changing  terrestrial  seasons,  as 
these  are  indicated  by  the  positions  and  magnitudes  of  the  polar 
ice-caps.  The  permanent  markings  would  be  ever  undergoing 
apparent  modification  by  the  variations  of  the  white  cloud- belts 
that  encircle  the  terrestrial  sphere.  Of  the  nature  of  these 
variations  meteorological  science  is  not  as  yet  in  a  position  to 
speak :  it  would  indeed  be  vastly  to  the  benefit  of  that  science  if 
a  view  of  the  distribution  of  clouds  and  vapours  over  the  earth's 
surface,  as  comprehensive  as  that  we  are  imagining,  could  really  be 
obtained. 

It  might  happen  at  "  full-earth,"  that  a  black  spot  with  a  fainter 
penumbral  fringe  would  appear  on  one  side  of  the  illuminated  disc 
and  pass  somewhat  rapidly  across  it.  This  would  occur  when  the 
moon  passed  exactly  between   the   sun   and  the  earth,  and  the 


CHAP.  XIII.]  THE    MOON   AS    A    WORLD.  191 

shadow  of  the  moon  was  cast  upon  the  terrestrial  disc.  We  need 
hardly  say  that  these  shadow-transits  would  occur  upon  those 
astronomically  important  occasions  when  an  eclipse  of  the  sun  is 
beheld  from  the  earth. 

The  other  features  of  the  sky  during  the  long  lunar  night  would 
not  differ  greatly  from  those  to  which  we  alluded  in  speaking  of  its 
day  aspects.  The  stars  would  be  the  more  brightly  visible,  from 
the  greater  power  of  the  eye-pupil  to  open  in  the  absence  of  the 
glaring  sun,  and  on  this  account  the  milky-way  would  be  very 
conspicuous  and  the  brighter  nebulse  would  come  into  view.  The 
constellations  would  mark  the  night  by  their  positions,  or  the  hours 
might  be  told  off  (in  periods  of  twenty-four  each)  by  the  successive 
reappearances  of  surface  features  on  certain  parts  of  the  terrestrial 
disc.  The  planets  in  opposition  to  the  sun  would  now  be  seen, 
and  a  comet  might  appear  to  vary  the  monotony  of  the  long  lunar 
night.  But  a  meteor  would  never  flash  across  the  sky,  though 
dark  meteoric  particles  and  masses  would  continually  bombard  the 
lunar  surface,  sometimes  singly,  sometimes  in  showers.  And  these 
would  fall  with  a  compound  force  due  to  their  initial  velocity  added 
to  that  of  the  moon's  attraction.  As  there  is  no  atmosphere  to 
consume  the  meteors  by  frictional  heat  or  break  by  its  resistance 
the  velocity  of  their  descent,  they  must  strike  the  moon  with  a  force 
to  which  that  of  a  cannon-ball  striking  a  target  is  feeble  indeed. 
A  position  on  the  moon  would  be  an  unenviable  stand-point  from 
this  cause  alone. 

The  lunar  landscape  by  night  needs  little  description  :  it  would 
be  lit  by  the  earth-moon  sufiiciently  to  allow  salient  features,  even 
at  a  distance,  to  be  easily  made  out,  for  its  moon  {i.e.  the  earth) 
has  thirteen  times  the  light-reflecting  area  that  ours  has.  But  the 
night  illumination  will  change  in  intensity,  since  the  earth-moon 
varies  from  half-full  to  full,  and  again  to  half-full,  between  sunset 
and  the  next  sunrise.  The  direction  of  the  light,  and  hence  the 
positions  of  the  shadows,  will  scarcely  alter  on  account  of  the 
apparent  fixity  of  the  earth  in  the  lunar  sky.     A  slight  degree  of 


192  THE    MOON.  [chap.  xiii. 

warmth  might  possibly  be  felt  with  the  reflected  earth-light ;  but 
it  would  be  insufficient  to  mollify  the  intensity  of  the  prevailing 
cold.  The  heat  accumulated  by  the  ground  during  the  three 
hundred  hours'  sunshine  radiates  rapidly  into  space,  there  being 
no  atmospheric  coat  to  retain  it,  and  a  cooling  process  ensues  that 
goes  on  till,  all  warmth  having  rapidly  departed,  the  previously 
parched  soil  assumes  a  temperature  approaching  that  of  celestial 
space  itself,  and  which  has  been,  as  we  have  stated,  estimated  at 
between  200°  and  250°  below  the  Fahrenheit  zero.  If  moisture 
existed  upon  the  moon,  its  night-side  would  be  bound  in  a  grip  of 
frost  to  which  our  Arctic  regions  would  be  comparatively  tropical. 
But  since  there  is  no  water,  the  aspect  of  the  lunar  scenery 
remains  unmodified  by  effects  of  changing  temperature. 

Such,  then,  are  the  most  prominent  effects  that  would  manifest 
themselves  to  the  visual  and  other  senses  of  a  being  transported 
to  the  moon.  The  picture  is  not  on  the  whole  a  pleasant  one, 
but  it  is  instructive  ;  and  our  rendering  of  it,  imperfect  though  it 
be,  may  serve  to  suggest  other  inferences  that  cannot  but  add  to 
the  interest  which  always  attaches  to  the  contemplation  of  natural 
scenes  and  phenomena  from  points  of  view  different  from  those 
which  we  ordinarily  occupy. 


CHAPTER    XIV. 

THE    MOON    AS    A    SATELLITE:    ITS    RELATION    TO    THE    EARTH 

AND    MAN. 

Apart  from  the  recondite  functions  of  the  moon  considered  as 
one  of  the  interdependent  members  of  the  solar  family,  into  which 
it  would  be  beyond  our  purpose  to  inquire,  there  are  certain  means 
by  which  it  subserves  human  interests  and  ministers  to  the  wants 
of  civilized  man  to  which  we  deem  it  desirable  to  call  attention, 
especially  as  some  of  them  are  not  so  self-apparent  as  to  have 
attracted  popular  attention. 

The  most  generally  appreciated  because  the  most  evident  of  the 
uses  of  the  moon  is  that  of  a  luminary.  Popular  regard  for  it  is 
usually  confined  to  its  service  in  that  character,  and  in  that 
character  poets  and  painters  have  never  tired  in  their  efforts  to 
glorify  it.  And  obviously  this  service  as  a  *'  lesser  light "  is 
sufficiently  prominent  to  excite  our  warmest  admiration.  But 
moonlight  is,  from  the  very  conditions  of  its  production,  of  such  a 
changeable  and  fugitive  nature,  and  it  affords  after  all  so  partial 
and  imperfect  an  alleviation  of  night's  darkness,  that  we  are  fain 
to  regard  the  light-giving  office  of  the  moon  as  one  of  secondary 
importance.  Far  more  valuable  to  mankind  in  general,  so  estim- 
able as  to  lead  us  to  place  it  foremost  in  our  category  of  lunar 
offices,  is  the  duty  which  the  moon  performs  in  the  character  of  a 
sanitary  agent.  We  can  conceive  no  direful  consequences  that 
would  follow  from  a  withdrawal  of  the  moon's  mere  light ;  but  it 
is  easy  to  imagine  what  highly  dangerous  results  would  ensue  if 


194  THE    MOON.  [chap.  xiv. 

the  moon  ceased  to  produce  the  tides  of  the  ocean.  Motion  and 
activity  in  the  elements  of  the  terraqueous  globe  appear  to  be 
among  the  prime  conditions  in  creation.  Rest  and  stagnation  are 
fraught  with  mischief.  While  the  sun  keeps  the  atmosphere  in 
constant  and  healthy  circulation  through  the  agency  of  the  winds, 
the  moon  performs  an  analogous  service  to  the  waters  of  the  sea 
and  the  rivers  that  flow  into  them.  It  is  as  the  chief  producer  of 
the  tides — for  we  must  not  forget  that  the  sun  exercises  its  tidal 
influences,  though  in  much  lesser  degree — that  we  ought  to  place 
the  highest  value  on  the  services  of  the  moon  :  but  for  its  aid  as 
a  mighty  scavenger,  our  shores,  where  rivers  terminate,  would 
become  stagnant  deltas  of  fatal  corruption.  Twice  (to  speak 
generally)  a  day,  however,  the  organic  matter  which  rivers  deposit 
in  a  decomposing  state  at  their  embouchures  is  swept  away  by  the 
tidal  wave ;  and  thus,  thanks  to  the  moon,  a  source  of  direful 
pestilence  is  prevented  from  arising.  Rivers  themselves  are  pro- 
videntially cleansed  by  the  same  means,  where  they  are  polluted 
by  bordering  towns  and  cities  which,  from  the  nature  of  things, 
are  sure  to  arise  on  river  banks ;  and  it  seems  to  be  also  in  the 
nature  of  things  that  the  river  traversing  a  city  must  become  its 
main  sewer.  The  foul  additions  may  be  carried  do\\Ti  by  the 
stream  in  its  natural  course  towards  the  ocean,  but  where  the 
river  is  large  there  will  be  a  decrease  in  velocity  of  the  current 
near  the  mouth  or  where  it  joins  the  sea,  thus  causing  partial 
stagnation  and  consequent  deposition  of  the' deleterient  matters. 
All  this,  however,  is  removed,  and  its  inconceivable  evils  are 
averted  by  our  mighty  and  ever  active  "  sanitary  commissioner," 
the  moon.  We  can  scarcely  doubt  that  a  healthy  influence  of  less 
obvious  degree  is  exerted  in  the  wide  ocean  itself ;  but,  considering 
merely  human  interests,  we  cannot  suppress  the  conviction  that 
man  is  more  widely  and  immediately  benefited  by  this  purifying 
office  of  the  moon  than  by  any  other. 

But  the  sanitary  service  is  not  the  only  one  that  the  moon 
performs  through  the  agency  of  the  tides.     There  is  the  work  of 


CHAP.  XIV.]  THE    MOON    AS    A    SATELLITE.  195 

tidal  transport  to  be  considered.  Upon  tidal  rivers  and  on  certain 
coasts,  notwithstanding  wind  and  the  use  of  steam,  a  very  large 
proportion  of  the  heavy  merchandize  is  transported  by  that  slow 
but  powerful  "  tug "  the  flood-tide ;  and  a  similar  service,  for 
which,  however,  the  moon  is  not  to  be  entirely  credited,  is  done  by 
the  down-flow  of  the  ebb-tide.  Large  ships  and  heavily-laden  rafts 
and  barges  are  quietly  taken  in  tow  by  this  unobtrusive  prime 
mover,  and  moved  from  the  river's  mouth  to  the  far -up  city,  and 
from  wharf  to  wharf  along  its  banks;  and  a  vast  amount  of 
mechanical  work  is  thus  gratuitously  performed  which,  if  it  had  to 
be  provided  by  artificial  means,  would  represent  an  amount  of 
money  value  which  for  such  a  city  as  London  would  have  to  be 
counted  by  thousands,  possibly  millions,  of  pounds  yearly.  For 
this  service  we  owe  the  moon  the  gratitude  that  we  ought  to  feel 
for  a  direct  pecuniary  benefactor. 

In  the  existing  state  of  civilization  and  prosperity,  we  do  not, 
however,  utilize  the  power  of  the  tides  nearly  to  the  extent  of  their 
capabilities.  Our  coal  mines,  rich  with  the  ** light  of  other  days" 
— for  coal  was  long  ago  declared  by  Stevenson  to  be  "bottled  sun- 
shine " — at  present  furnish  us  with  so  abundant  a  supply  of  power- 
generating  material  that  in  our  eagerness  to  use  it  upon  all  possible 
occasions  we  are  losing  sight,  or  putting  out  of  mind,  many  other 
valuable  prime  movers,  and  amongst  them  that  of  the  rise  and  fall 
of  the  waters,  which  can  be  immediately  converted  into  any  form  of 
mechanical  power  by  the  aid  of  tide-mills.  Such  mills  may  be  found 
in  existence  here  and  there,  but  for  the  present  they  are  generally 
outrivalled  by  the  steam  engine  with  all  its  conveniences  and  adapta- 
bilities ;  and  hence  they  have  not  shared  the  benefits  of  that  in- 
ventive ingenuity  which  has  achieved  such  wonders  of  mechanical 
appliance  while  steam  has  been  in  the  ascendant.  But  it  must  be 
remembered  that  in  our  extravagant  use  of  coal  we  are  drawing  from 
a  bank  into  which  nothing  is  being  paid.  We  are  consuming  an 
exhaustible  store,  and  the  time  must  come  when  it  will  be  needful 
to  look  around  in  quest  of  **  powers  that  may  be."     Then  an  im- 

o  2 


196  THE    MOON.  [chap.  xiv. 

petus  may  be  given  to  the  application  of  the  tides  to  mechanical 
purposes  as  a  prime  mover.*  For  the  people  of  the  British  Islands 
the  problem  would  have  an  especial  importance,  viewing  the  extent 
of  our  seaboard  and  the  number  of  our  tidal  rivers.  The  source  of 
motion  that  offers  itself  is  of  almost  incalculable  extent.  There  is 
not  merely  the  onward  flowing  motion  of  streams  to  be  utilized,  but 
also  the  lift  of  water,  which,  if  small  in  extent,  is  stupendous  in 
amount ;  and  within  certain  limits  it  matters  little  to  the  mecha- 
nician whether  the  ** foot-pounds"  of  work  placed  at  his  disposal 
are  in  the  form  of  a  great  mass  lifted  to  a  small  height  or  a  small 
mass  lifted  to  a  great  height.  There  is  no  reason  either  why  the 
utilization  of  the  tides  should  be  confined  to  rivers.  The  sea-side 
might  well  become  the  circle  of  manufacturing  industry,  and  the 
millions  of  tons  of  water  lifted  several  feet  twice  daily  on  our  shores 
might  be  converted,  even  by  schemes  already  proposed,  to  furnish 
the  prime  movement  of  thousands  of  factories.  And  we  must  not 
forget  how  completely  modern  science  has  demonstrated  the  inter- 
convertibility  of  all  kinds  of  force,  and  thus  opened  the  way  for  the 
introduction  of  systems  of  transporting  power  that,  in  such  a  state 
of  things  as  we  are  for  the  moment  considering,  might  be  of  im- 
mense benefit.  Gravity,  for  instance,  can  be  converted  into  elec- 
tricity; and  electricity  gives  us  that  wonderful  power  of  trans- 
mitting force  without  transmitting  (or  even  moving)  matter,  which 
power  we  use  in  the  telegraph,  where  we  generate  a  force  at  one  end, 
of  a  wire  and  use  it  to  ring  bells  or  deflect  needles  at  the  other  end, 
which  may  be  thousands  of  miles  away.  What  we  do  with  the 
slight  amount  of  force  needful  for  telegraphy  is  capable  of  being 
done  with  any  greater  amount.  A  tide-mill  might  convert  its 
mechanical  energy  by  an  electro-magnetic  engine,  and  in  the  form 
of  electricity  its  force  could  be  conveyed  inland  by  proper  wires  and 
there  reconverted  back  to  mechanical  or  moving  power.  True, 
there  would  be  a  considerable  loss  of  power,  but  that  power  would 

♦  About  100  years  ago  London  was  supplied  with  water  chiefly  by  pumps  worked 
by  tidal  mills  at  London  Bridge. 


CHAP.  XIV.]  THE    MOON    AS    A    SATELLITE.  197 

cost  nothing  for  its  first  production.  Another  means  ready  to  hand 
for  transporting  power  is  by  compressed  air,  which  has  already  done 
good  service ;  another  is  the  system  so  admirably  worked  out  by 
Sir  W.  Armstrong,  of  transmitting  water-power  through  the  agency 
of  an  "  accumulator,"  now  so  generally  used  at  our  Docks  and  else- 
where for  working  cranes  and  such  other  uses.  And  as  the  whole 
duty  of  the  engineer  is  to  convert  the  forces  of  nature,  there  is  a 
rich  field  open  for  his  invention,  and  upon  which  he  may  one  day 
have  to  enter,  in  adapting  the  pulling  force  of  the  moon  to  his 
fellow  man's  mechanical  wants  through  the  intermediation  of  the 
tides. 

Another  of  the  high  functions  of  the  moon  is  that  by  which  she 
subserves  the  wants  of  the  navigator,  and  enables  him  to  track  his 
course  over  the  pathless  ocean.  Of  the  two  co-ordinates.  Latitude 
and  Longitude,  that  are  needful  to  determine  the  position  of  a  ship 
at  sea  (or  of  any  standpoint  upon  the  earth's  surface)  the  first  is 
easily  found,  inasmuch  as  it  is  always  equal  to  the  altitude  of  the 
celestial  pole  at  the  place  of  observation.  But  the  determination  of 
the  longitude  has  always  been  a  difficult  problem,  and  one  upon 
which  a  vast  amount  of  ingenuity  has  been  expended.  When  it  was 
first  attacked  it  was  soon  discovered  that  the  moon  was  the  object 
of  all  others  by  which  it  could  be  most  accurately  and,  all  things 
considered,  most  readily  determined.  We  must  premise  that  the 
longitude  of  one  place  from  another  is  in  eff'ect  the  difference 
between  the  local  times  at  the  two  places,  so  that  when  we  say  that 
a  place  or  a  ship  is,  for  instance,  seven  hours,  twenty-four  minutes, 
ten  seconds,  west  of  Greenwich,  we  mean  that  the  time-o'-day  at 
the  place  or  ship  is  seven  hours  twenty-four  minutes  ten  seconds 
earlier  than  that  at  Greenwich.  Hence,  finding  the  longitude  at  sea 
or  at  any  place  and  moment  means  finding  what  time  it  is  at  Green- 
wich at  that  moment.  Of  course  this  could  be  most  easily  done  if 
we  could  set  a  timekeeper  at  Greenwich  and  rely  upon  its  keeping 
time  during  a  long  sea  voyage ;  and  this  plan  appeared  so  feasible 
that  our  Government  long  ago  offered  a  prize  of  ^620, 000  for  a  time- 


198  THE    MOON.  [chap.  xiv. 

keeper  which  would  perform  to  a  stated  degree  of  accuracy  after  a 
certain  sea  voyage.  One  John  Harrison  did  make  such  a  timekeeper, 
that  actually  satisfied  the  conditions,  and  obtained  the  prize :  and 
chronometers  are  now  largely  used  for  longitude,  their  construction 
having  been  brought  to  great  perfection,  especially  in  England, 
owing  to  a  continuance  (in  a  less  liberal  degree,  however)  of 
Government  inducement.  But  chronometers  are  not  entirely  to  be 
relied  on,  even  where  several  are  carried,  which  in  other  than 
Government  ships  is  rarely  the  case  :  recourse  must  be  had  to  the 
heavenly  bodies  for  check  upon  the  timekeeper.  And  the  moon  is, 
as  we  have  said,  the  body  that  best  serves  the  requirements  of  the 
problem. 

The  lunar  method  for  longitude  amounts  practically  to  this.  The 
stars  are  fixed ;  the  sun,  moon,  and  planets  move  amongst  them  ; 
the  sun  and  planets  with  very  slow  rates  of  apparent  motion,  the 
moon  with  a  very  rapid  one.  If,  then,  it  be  predicted  that  at  a 
certain  instant  of  Greenwich  time  the  moon  will  be  a  certain  dis- 
tance from  a  fixed  star,  and  if  the  mariner  at  sea  observes  ivhen  the 
moon  has  that  exact  distance,  he  will  know  the  Greenwich  time  at 
the  instant  of  his  observation.*  The  moon  thus  becomes  to  him  as 
the  hand  of  a  timepiece,  whereof  the  stars  are  the  hour  and  minute 
marks,  the  whole  being,  as  it  were,  set  to  Greenwich  time.  The 
requisite  predictions  of  the  distance  (as  seen  from  the  earth's 
centre)  of  the  moon  from  convenient  fixed  stars,  or  from  the  sun,  or 
any  of  the  principal  planets — whose  calculated  places  are  so 
accurate  that  they  may  for  this  purpose  be  used  as  fixed  stars— are 
given  to  the  utmost  exactness  in  the  navigators'  vade  mecum,  the 
"  Nautical  Almanac,"  for  every  third  hour,  day  and  night,  of 
Greenwich  time  (except  for  a  few  days  near  new-moon,  when  the 
moon  cannot  be  seen) ;  and  from  these  given  distances  the  navigator 


*  The  sun  and  planets  are  comparatively  useless  for  this  object,  because  of  their 
slow  movement  among  the  stars ;  the  change  of  their  positions  from  hour  to  hour 
is  so  small  as  to  render  uncertain  the  Greenwich  times  deducible  therefrom. 
Their  use  would  be  comparable  to  taking  the  time  from  the  hour-hand  of  a  clock. 


CHAP.  XIV.]  THE    MOON    AS    A    SATELLITE.  199 

can,  by  a  simple  process  of  differencing,  obtain  the  Greenwich  time 
corresponding  to  the  distance  which  he  may  have  observed.*  Then 
knowing,  as  he  does  by  other  observations  easily  obtained,  the  local 
or  ship's  time  of  his  observation,  he  takes  the  difference  between 
this  and  the  corresponding  Greenwich  time,  and  this  difference  is 
his  longitude  from  Greenwich.  Of  course  the  whole  value  of  this 
method  depends  upon  the  exactitude  of  the  predicted  distances 
corresponding  to  the  given  Greenwich  times.  These  distances  are 
obtained  by  tables  of  the  moon's  motions,  which  must  be  found 
from  observations.  The  motions  in  question  are  of  an  intricacy 
almost  past  comprehension,  on  account  of  the  disturbing  forces  to 
which  the  moon  is  subjected  by  the  sun  and  planets.  The  powers 
of  the  profoundest  mathematicians,  from  Newton  downwards,  have 
been  severely  exercised  in  efforts  to  group  them  into  a  theory,  and 
represent  them  by  tables  capable  of  furnishing  the  requisite  exact 
predictions  of  lunar  positions  for  nautical  purposes.  Accurate  ob- 
servations of  the  moon's  place  night  after  night  have,  from  the 
dawn  of  this  lunar  method  for  longitude,  been  in  urgent  request  by 
mathematicians  for  the  purposes  specified,  and  it  was  solely  to  pro- 
cure these  observations  that  the  Observatory  at  Greenwich  was 
established,  and  mainly  for  their  continued  prosecution  (and  for  the 
stellar  observations  necessary  for  their  utilization)  that  it  is  sus- 
tained. For  two  centuries  the  moon  has  been  unremittingly 
observed  at  Greenwich,  and  the  tables  at  present  used  for  making 
the  ** Nautical  Almanac"  (those  formed  by  Prof.  Hansen)  depend 
upon  the  observations  there  obtained.  The  work  still  goes  on,  for 
even  now  the  degree  of  exactitude  is  not  what  is  desired,  and 
astronomers  are  looking  forward  with  some  interest  to  new  lunar 
tables  which  were  left  complete  by  the  late  M.  Delaunay,  formerly 
the  head  of  astronomy  in  France,  based  upon  a  theory  which  he 
evolved.  This  use  of  the  moon  is  the  grandest  of  all  in  respect  of 
the  results  to  which  it  has  led. 

*  Certain  corrections  are  necessary  to  clear  his  observed  distance  of  the  effects  pf 
parallax  and  refraction  ;  upon  these,  however,  we  cannot  enter  here, 


200  THE    MOON.  [chap.  xiv. 

Then,  too,  regarding  the  moon  as  a  timekeeper,  we  must  not 
forget  the  service  that  it  renders  in  furnishing  a  division  of  time 
intermediate  between  the  day — ^which  is  measured  by  the  earth's 
rotation — and  the  year,  which  is  defined  by  the  earth's  orbital 
revolution.  Notwithstanding  the  survival  of  lunar  reckoning  in 
our  religious  services,  we,  in  our  time  and  country,  scarcely  need 
a  moon  to  mark  our  months  ;  but  we  must  not  forget  that  with 
many  ancient  people  the  moon  was,  and  with  some  is  still,  the 
chief  timekeeper,  the  calendars  of  such  people  being  lunar  ones, 
and  all  their  events  being  reckoned  and  dated  by  "  moons."  To 
us,  however,  the  moon  is  of  great  service  in  this  department  by 
enabling  us  to  fix  dates  to  many  historical  events,  the  times  of 
occurrence  of  which  are  uncertain,  by  reason  of  defective  records  or 
by  dependence  upon  such  uncertain  data  as  *' lives  of  emperors," 
years  of  this  or  that  king's  reign,  or  generations  of  one  or  another 
family.  The  moon  now  and  then  clears  up  a  mystery,  or  decides  a 
disputed  point  in  chronology,  by  furnishing  the  accurate  date  of  an 
ancient  eclipse,  which  was  a  phenomenon  that  always  inspired  awe 
and  secured  for  itself  careful  record.  The  chronologer  is  continually 
applying  to  the  astronomer  for  the  date  and  place  of  visibility  of 
some  total  eclipse,  of  which  he  has  found  an  imperfect  record, 
veritable  as  to  the  fact,  but  dated  only  by  reference  to  some  year 
of  a  so-and-so's  reign,  or  by  some  battle  or  other  historical 
occurrence.  The  eclipses  that  occurred  near  the  time  are  then 
examined,  and  when  one  is  found  that  tallies  with  recorded  condi- 
tions in  other  respects  (such  as  the  time  of  day  and  the  place  of 
observation),  its  indisputable  date  becomes  a  starting-point  from 
which  the  chronologer  works  backwards  and  forwards  in  safety. 
There  is  one  famous  eclipse — that  predicted  by  Thales  six  centuries 
before  Christ,  which  put  an  end  to  the  battle  between  the  Medes 
and  Lydians  by  the  terror  its  darkness  created  in  both  armies — 
which  is  most  intimately  associated  with  ancient  chronology,  and 
has  been  used  to  rectify  a  proximate  date  (the  first  year  of  Cyrus 
of  Babylon)  which  forms  the  foundation  of  all  Scripture  chronology. 


CHAP.  XIV.]  THE    MOON   AS    A    SATELLITE.  201 

Sacred  and  profane  history  alike  are  continually  receiving  assist- 
ance from  the  accurate  dates  which  the  moon,  by  having  caused 
eclipses  of  the  sun,  enables  the  astronomer  to  fix  beyond  cavil  or 
doubt. 

The .  mention  of  eclipses  reminds  us,  too,  of  the  use  which  the 
moon  has  been  in  increasing,  through  them,  our  knowledge  of  the 
physical  condition  of  the  sun.  If  the  moon  had  never  intervened 
to  cut  off  the  blinding  glare  of  the  solar  disc,  we  should  have  been 
to  this  day  left  to  assume  that  the  sun  is  all-contained  by  the 
dazzling  globe  that  we  ordinarily  see.  But,  thanks  to  the  moon's 
intervention,  we  now  know  that  the  sun  is  by  no  means  the  mere 
naked  sphere  we  should  have  suspected.  Eclipses  have  taught  us 
that  it  is  surrounded  by  an  envelope  of  glowing  gases,  and  that  it 
has  a  vast  vaporous  surrounding,  beyond  its  glowing  atmosphere, 
which  appears  to  be  composed  of  matter  streaming  away  from  the 
sun  into  surrounding  space.  With  these  discoveries  still  in  their 
infancy,  it  is  impossible  to  foresee  the  knowledge  to  which  they 
will  eventually  lead,  but  they  can  hardly  be  barren  of  fruit,  and 
whatever  they  ultimately  teach  will  be  so  much  insight  gained  into 
the  sublimest  problem  that  human  science  has  before  it — the  deter- 
mination of  the  source  and  maintaining  power  of  the  light  and  heat 
and  vivifying  agency  of  the  sun.  In  according  our  thankful  reflec- 
tions to  the  moon  for  these  revelations,  we  must  not  forget  that, 
should  there  be  inhabitants  upon  our  neighbouring  worlds. 
Mercury,  Venus,  and  Mars,  which  have  no  satellites,  they,  the 
supposed  inhabitants,  can  gain  no  such  knowledge  upon  the 
surroundings  of  the  ruler  of  the  solar  system.  On  the  other 
hand,  any  rational  being  who  may  be  supposed  to  dwell  upon 
Saturn  or  Jupiter,  would,  through  the  intervention  of  their 
numerous  moons,  have,  in  the  latter  case  especially,  far  more 
abundant  opportunities  of  acquiring  the  knowledge  in  question 
than  we  have. 

Finally,  there  is  a  use  of  the  moon  which  touches  us,  author  and 
reader,  very  closely.     It  has  taught  us  of  a  world  in  a  condition 


202  THE    MOON.  [chap.  xiv. 

totally  different  from  our  own  ;  of  a  planet  without  water,  without 
air,  without  the  essentials  to  life  development,  hut  rather  with  the 
conditions  for  life  destruction ;  a  planet  left  hy  the  Creator — for 
wise  purposes  that  we  cannot  fully  know — as  it  were  hut  half-formed, 
with  all  the  igneous  foundations  fresh  from  the  cosmical  fire,  and 
with  its  rough-cast  surface  in  its  original  state,  its  fire  and  mould- 
marks  exposed  to  our  view.  From  these  we  have  essayed  to  resolve 
some  of  the  processes  of  formation,  and  thus  to  learn  something  of 
the  cosmical  agencies  that  are  called  forth  in  the  purely  igneous 
era  of  a  planet's  history.  We  trust  that  we,  on  our  part,  have 
shown  that  the  study  of  the  moon  may  he  a  benefit  not  merely  to 
the  astronomer,  but  to  the  geologist ;  for  we  behold  in  it  a  mighty 
"  medal  of  creation  "  doubtless  formed  of  the  same  material  and 
struck  with  the  same  die  that  moulded  our  earth ;  but  while  the 
dust  of  countless  ages  and  the  action  of  powerful  disintegrating  and 
denuding  elements  have  eroded  and  obliterated  the  earthly  impres- 
sion, the  superscriptions  on  the  lunar  surface  have  remained  with 
their  pristine  clearness  unsullied,  every  vestige  sharp  and  bright  as 
when  it  left  the  Almighty  Maker's  hands.  The  moon  serves  no 
second-rate  or  insignificant  service  when  it  teaches  us  of  the  variety 
of  creative  design  in  the  worlds  of  our  system,  and  exalts  our  esti- 
mation of  this  peopled  globe  of  ours  by  showing  us  that  all  the 
planetary  worlds  have  not  been  deemed  worthy  to  become  the 
habitations  of  intelligent  beings. 


Keflections  upon  the  uses  of  the  moon  not  unnaturally  lead  our 
thoughts  to  some  matters  that  may  be  regarded  as  abuses.  These 
mainly  take  the  form  of  superstitions,  erroneous  beliefs  in  the 
moon's  influence  over  terrestrial  conditions,  and  occasionally  of 
erroneous  ideas  upon  the  moon's  functions  as  a  luminary.  The 
first-mentioned  are  almost  beneath  notice,  for  they  include  such 
mythical  suspicions  as  that  the  moon  influences  human  sanity  and 
other  affections  of  mind  and  body;  that  the  moon's  rays  have  ft 


CHAP.  XIV.]  THE    MOON    AS    A    SATELLITE.  203 

decomposing  effect  upon  organic  matter  ;  that  they  produce  blind- 
ness by  shining  upon  a  sleeper's  eyes  ;  that  the  moon  determines 
the  hours  of  human  death,  which  is  supposed  to  occur  with  the 
change  of  the  tide,  etc.  All  such,  having  no  foundation  on  fact, 
are  put  beyond  our  consideration.  The  third  matter  we  have  men- 
tioned may  also  be  dismissed  in  a  very  few  words.  The  erroneous 
ideas  upon  the  moon's  functions  as  a  luminary,  to  which  we  allude, 
are  those  which  are  manifested  by  poets  and  painters,  and  even 
historians,  who  do  not  hesitate  to  bring  the  moon  upon  a  scene  in 
any  form  and  at  any  time  they  please  without  reference  to  actual 
lunar  circumstances.  It  is  no  uncommon  thing  to  see,  in  a  picture 
representing  an  evening  scene,  a  moon  introduced  which  can  only 
be  seen  in  the  morning — a  waning  moon  instead  of  a  waxing  one  ; 
and  astronomical  critics  have,  indeed,  caught  artists  so  far  tripping 
as  to  put  a  moon  in  a  picture  representing  some  event  that  occurred 
upon  a  date  when  the  moon  was  new,  and  therefore  invisible. 
Writers  take  the  same  liberties  very  frequently.  A  newspaper 
correspondent,  during  the  Franco-Prussian  war,  described  the  full 
moon  as  shining  upon  a  scene  of  desolation  on  a  particular  night, 
when  really  there  was  no  moon  to  be  seen.  One  of  the  most  flagrant 
cases  of  this  kind,  however,  occurs  in  Wolfe's  ballad  on  "  The  death 
of  Sir  John  Moore,"  where  it  is  written  that  the  hero  was  buried 
*'  By  the  struggling  moonbeam's  misty  light."  But  the  interment 
actually  took  place  at  a  time  when  the  moon  was  out  of  sight.  We 
mention  these  abuses  of  the  moon  in  the  hope  of  promoting  a  better 
observance  of  the  moon's  luminary  office.  They  who  wish  to  bring 
the  moon  upon  a  scene,  not  knowing  ipso  facto  that  it  was  there, 
should  first  take  the  advice  of  Nick  Bottom  in  the  "  Midsummer 
Night's  Dream,"  and  make  sure  of  their  object  by  consulting  an 
almanac. 

The  second  of  the  specified  abuses  to  which  the  moon  is  subject 
refers  to  its  supposed  influence  on  the  weather ;  and  in  the  extent 
to  which  it  goes  this  is  one  of  the  most  deeply  rooted  of  popular 
errors.     That  there  is  an  infinitesimal  influence  exerted  by  the 


204  THE    MOON.  [chap.  xiv. 

moon  on  our  atmosphere  will  be  seen  from  the  evidence  we  have 
to  offer,  but  it  is  of  a  character  and  extent  vastly  different  from 
what  is  commonly  believed.  The  popular  error  is  shown  in  its 
most  absurd  form  when  the  mere  aspect  of  the  moon,  the  mere 
transition  from  one  phase  of  illumination  to  another,  is  asserted 
to  be  productive  of  a  change  of  weather  ;  as  if  the  gradual  passage 
from  first  quarter  to  second  quarter,  or  from  that  to  third,  could  of 
itself  upset  an  existing  condition  of  the  atmosphere  ;  or  as  if  the 
conjunction  of  the  moon  with  the  sun  could  invert  the  order  of  the 
winds,  generate  clouds,  and  pour  down  rains.  A  moment's  reason- 
ing ought  to  show  that  the  supposed  cause  and  the  observed  effect 
have  no  necessary  connection.  In  our  climate  the  weather  may  be 
said  to  change  at  least  every  three  days,  and  the  moon  changes — 
to  retain  the  popular  term — every  seven  days  ;  so  that  the  proba- 
bility of  a  coincidence  of  these  changes  is  very  great  indeed  :  when 
it  occurs,  the  moon  is  sure  to  be  credited  with  causing  it.  But  a 
theory  of  this  kind  is  of  no  use  unless  it  can  be  shown  to  apply  in 
every  case ;  and,  moreover,  the  change  must  always  be  in  the  same 
direction ;  to  suppose  that  the  moon  can  turn  a  fine  day  to  a  wet 
one,  and  a  wet  day  to  a  fine  morrow  indiscriminately,  is  to  make 
our  satellite  blow  hot  and  cold  with  the  same  mouth,  and  so  to 
reduce  the  supposition  to  an  absurdity.  If  any  marked  connection 
existed  between  the  state  of  the  air  and  the  aspect  of  the  moon,  it 
must  inevitably  have  forced  itself  unsought  upon  the  attention  of 
meteorologists.  In  the  weekly  return  of  Births,  Deaths,  and 
Marriages,  issued  by  the  Registrar -General,  a  table  is  given,  show- 
ing all  the  meteorological  elements  at  Greenwich  for  every  day  of 
the  year,  and  a  column  is  set  apart  for  noting  the  changes  and 
positions  of  the  moon.  These  reports  extend  backwards  nearly  a 
quarter  of  a  century.  Here,  then,  is  a  repertory  of  data  that  ought 
to  reveal  at  a  glance  any  such  connection,  and  would  certainly  have 
done  so  had  it  existed.  But  no  constant  relation  between  the  moon 
columns  and  those  containing  the  instrument  readings  has  ever 
been  traced.     Our  meteorological  observatories  furnish  continuous 


CHAP.  XIV.]  THE   MOON   AS   A    SATELLITE.  205 

and  unbroken  records  of  atmospheric  variations,  extending  over 
long  series  of  years  :  these  afford  still  more  abundant  means  for 
testing  the  validity  of  the  lunar  hypothesis.  The  collation  has 
frequently  been  made  for  special  points  in  the  inquiry,  and  certainly 
some  connection  has  been  found  to  obtain  between  certain  positions 
of  the  moon  in  her  orbit  and  certain  instrumental  averages  ;  but  so 
small  are  the  effects  traceable  to  lunar  influence,  that  they  are 
almost  inappreciable  among  the  grosser  irregularities  that  arise 
from  other  and  as  yet  unexplained  causes. 

^  The  lunar  influences  upon  our  atmosphere  most  likely  to  be 
detected  are  those  of  a  tidal  character,  and  those  due  to  the  radiation 
of  the  heat  which  the  moon  receives  from  the  sun.  The  first  would 
be  shown  by  the  barometer,  which  may  be  called  an  "  atmospheric 
tide  gauge."  Some  years  ago  Colonel  Sir  Edward  Sabine  instituted 
a  series  of  observations  at  St.  Helena,  to  determine  the  variations 
of  barometric  indications  from  hour  to  hour  of  the  lunar  day. 
The  greatest  differences  were  found  to  occur  between  the  times 
when  the  moon  was  on  the  meridian,  and  when  it  was  six  hours 
away  from  the  meridian ;  in  other  words,  between  atmospheric 
high  tide  and  low  tide.  But  the  average  of  these  differences 
amounted  only  to  the  four-hundredth  part  of  an  inch  on  the  instru- 
ment's scale;  a  quantity  that  no  weather  observer  would  heed, 
that  none  but  the  best  barometers  would  show,  and  that  can  have 
no  perceptible  effect  on  weather  changes.  The  distance  of  the 
moon  from  the  earth  varies,  as  is  well  known,  in  consequence  of 
the  elliptical  form  of  her  orbit :  this  variation  ought  also  to 
produce  an  effect  upon  the  instrument's  indications ;  but  Colonel 
Sabine's  analysis  showed  that  it  was  next  to  insensible  ;  the  mean 
reading  at  apogee  differing  from  that  at  perigee  by  only  the  two- 
thousandth  part  of  an  inch.  Schubler,  a  German  meteorologist, 
had  arrived  at  similarly  negative  results  some  years  previously. 
Hence  it  appears  that  the  great  index  of  the  weather  is  not 
sensibly  affected  by  the  state  of  the  moon ;  the  conclusion  to  be 
drawn  with  regard  to  the  weather  itself  is  obvious  enough.     As 


206  THE    MOON.  [chap.  xiv. 

regards  the  lieat  received  from  the  moon,  we  know,  from  the  recent 
experiments  of  Lord  Rosse  in  England,  and  Marie  Davy  in  France, 
elsewhere  alluded  to,  that  a  degree  of  warmth  appreciable  to  the 
highly  sensitive  thermopile  is  exerted  by  the  moon  upon  the  earth 
near  to  the  time  of  full  moon,  when  the  sun's  rays  have  been 
pouring  their  unmitigated  heat  upon  the  lunar  surface  continuously 
for  fourteen  days.  And  as  it  is  improbable  that  the  whole  of  the 
heat  sent  earthwards  from  the  moon  reaches  the  earth's  surface, 
we  must  infer  that  a  considerable  amount  is  absorbed  in  the  higher 
atmosphere,  and  does  work  in  evaporating  the  lighter  clouds  and 
thinning  the  denser  ones.  The  effect  of  this  upon  the  earth  is  to 
facilitate  the  radiation  of  its  heat  into  space,  and  so  to  cool  the 
lower  atmospheric  strata.  And  this  effect  has  been  shown  to  be  a 
veritable  one  by  an  exhaustive  tabulation  of  temperature  records 
from  various  observatories,  which  was  undertaken  by  Mr.  Park 
Harrison.  The  general  conclusion  from  these  was,  that  the 
temperature  at  the  earth's  surface  is  lower  by  about  2J  degrees  at 
moon's  last  quarter  than  at  first  quarter ;  the  paradoxical  result 
being  what  would  naturally  follow  from  the  foregoing  consideration. 
The  tendency  of  the  full  moon  to  clear  the  sky  has  been  remarked 
by  several  distinguished  authorities,  to  wit,  Sir  John  Herschel, 
Humboldt,  and  Arago ;  and  in  general  the  clearing  may  be  accepted 
as  a  meteorological  fact,  though  in  one  case  of  close  examination  it 
has  been  negatived.  It  cannot  be  doubted  that  a  full  moon  some- 
times shows  a  night  to  be  clear  that  would  in  the  absence  of  the 
moon  be  called  cloudy. 

When  close  comparisons  are  made  between  the  moon's  positions 
and  records  of  rain-fall  and  wind-direction,  dim  indications  of 
relation  exhibit  themselves,  which  may  be  the  feeble  consequences 
of  the  change  of  temperature  just  spoken  of;  but  in  every  case 
where  an  effect  has  been  traced  it  has  been  of  the  most  insignifi- 
cant kind,  and  no  apparent  connexion  has  been  recognized  between 
one  effect  and  another.  Certainly  there  is  nothing  that  can  support 
the  extensive  popular  belief  in  lunar  influence  on  weather,  and 


CHAP.  XIV.]  THE   MOON  AS    A    SATELLITE.  207 

nothing  that  can  modify  the  conviction  that  this  belief  as  at 
present  maintained  is  an  absurd  delusion.  Yet  its  acceptance  is  so 
general,  and  runs  through  such  varied  grades  of  society,  that  we 
have  felt  it  our  duty  to  dwell  upon  it  to  the  extent  that  we  have 
done. 


CHAPTER  XV. 

CONCLUDING    SUMMARY. 

Having  arrived  at  the  conclusion  of  our  subject,  it  appears  to  us 
desirable  that  we  should  recall  to  the  reader,  by  a  rapid  review, 
its  salient  features. 

Our  main  object  being  to  attempt  what  we  conceive  to  be  a 
rational  explanation  of  the  surface  details  of  the  moon  which 
should  be  in  accordance  with  the  generally  received  theory  of 
planetary  formation,  and  with  the  peculiar  physical  conditions  of 
the  lunar  globe — the  opening  of  our  work  was  a  summary  of  the 
nebular  hypothesis  as  it  was  started  by  the  first  Herschel  and 
systemised  by  Laplace.  Following  these  philosophers  we  en- 
deavoured to  show  how  a  chaotic  mass  of  primordial  matter 
existing  in  space  would,  under  the  action  of  gravitation,  become 
transformed  into  a  system  of  planetary  bodies  circulating  about  a 
common  centre  of  gravity ;  and  further,  how,  in  some  cases,  the 
circulating  planetary  masses  would  themselves  become  sub-centres 
of  satellitic  systems;  our  earth  being  one  of  these  sub-centres 
with  only  one  satellitic  attendant — to  wit,  the  moon,  the  subject  of 
our  study. 

The  moon  being  thus  considered  as  evolved  from  the  parent 
nebulous  mass,  and  existing  as  an  isolated  and  compact  body,  we 
had  next  to  consider  what  was  the  effect  of  the  continued  action  of 
the  gravitating  force.  By  the  light  of  the  beautiful  **  mechanical 
theory  of  heat "  we  argued  that  this  force,  not  being  destructible, 
but  being  convertible,  was  turned  into  heat ;  and  that  whatever 


CHAP.  XV.]  CONCLUDING    SUM3IABY.  209 

may  have  been  the  original  condition  of  the  parent  nebulous  mass, 
as  regards  temperature,  its  planetary  offspring  became  elevated  to 
an  intense  degree  of  heat  as  they  assumed  the  form  of  spheres 
under  the  influence  of  gravitation. 

The  incandescent  sphere  having  attained  its  maximum  degree  of 
heat  by  the  total  conversion  thereinto  of  the  gravitating  force  it 
embodied,  we  explained  how  there  must  have  ensued  a  dispersion 
of  that  heat,  by  radiation  into  surrounding  space,  resulting  in  the 
cooling  and  consequent  solidification  of  the  outermost  stratum  of 
the  lunar  sphere,  and  subsequently  in  the  continuation  of  the  cool- 
ing process  downwards  or  inwards  to  the  centre.  And  here  we 
essayed  to  prove  that  in  this  second  stage  of  the  cooling  process, 
when  the  crust  was  solid  and  the  subjacent  portion  of  the  molten 
sphere  was  about  to  solidify,  there  would  come  into  operation  a 
principle  which  appears  to  govern  the  behaviour  of  certain  fusible 
substances,  and  which  may  be  concisely  termed  the  principle  of 
pre-solidifying  expansion.  We  adduced  several  examples  of  the 
manifestation  of  this  principle,  soliciting  for  it  the  careful  con- 
sideration of  physicists  and  geologists,  and  looking  to  it  as  furnish- 
ing the  key  to  the  mystery  of  volcanic  action  upon  the  moon,  since, 
without  needing  recourse  to  aqueous  or  gaseous  sources  of  eruptive 
power,  it  afforded  a  rationale  of  the  ejection  of  the  fluid  and  semi- 
fluid matter  of  the  moon  through  the  soHdified  crust  thereof,  and 
also  of  the  dislocations  of  that  crust,  unattended  by  actual 
ejection  of  subsurface  matter,  of  which  our  satellite  presents  a 
variety  of  examples,  and  which  the  earth  also  appears  to  have 
experienced  at  some  period  of  its  formative  history. 

Arrived  at  this  stage  of  our  subject  we  thought  it  needful  to 
introduce  some  pages  of  data  and  descriptive  detail.  Accordingly 
in  one  chapter  we  discussed  the  form,  magnitude,  weight,  and 
density  of  the  moon,  and  the  force  of  gravity  at  its  surface  :  and 
the  more  soundly  to  fix  these  data  in  the  mind,  we  devoted  a  few 
lines  to  explanation  of  the  methods  whereby  each  has  been  ascer- 
tained.    We. then  examined  the  question  (so  important  to  our  sub- 


210  THE    MOON.  [chap.  xv. 

ject)  of  the  existence  or  non-existence  of  a  lunar  atmosphere,  giving 
the  evidence,  which  may  he  regarded  as  conclusive,  in  proof  of  the 
absence  of  both  air  and  water  from  the  moon,  and,  therefore,  refut- 
ing the  claim  of  these  elements  to  be  considered  as  sources  or 
influencers  of  the  moon's  volcanic  manifestations.  A  general  coup 
d'oeil  of  the  lunar  hemisphere  facing  the  earth  next  engaged  our 
attention,  and  we  considered  the  aspect  of  the  disc  as  it  is  viewed 
by  the  naked  eye  and  with  telescopes  of  various  powers.  From  this 
general  survey  we  passed  to  the  topography  of  the  moon,  tracing 
briefly  the  admirable  labours  of  those  who  have  advanced  this  sub- 
ject, and,  by  aid  of  picture  and  skeleton  maps,  placing  it  within 
the  reader's  power  to  become  more  than  sufficiently  acquainted  for 
the  purposes  of  this  work  with  the  names  and  positions  of  detailed 
objects  and  features  of  interest.  Special  descriptions  of  interesting 
and  typical  spots  and  regions  were  given  in  some  few  cases  where 
such  appeared  to  be  called  for. 

These  descriptive  matters  disposed  of,  we  proceeded  to  discuss 
the  various  classes  of  surface  features  with  a  view  to  explaining  the 
precise  actions  which  appear  to  us  to  have  led  to  their  formation. 
Naturally  the  craters  first  demanded  our  attention.  We  pointed 
out  the  reasons  for  regarding  the  great  majority  of  the  circular 
formations  of  the  moon  as  craters,  as  truly  volcanic  as  those  of 
which  we  have  examples,  modified  by  obvious  causes,  upon  the 
earth ;  and,  tracing  the  causative  phenomena  of  terrestial  volcanoes, 
we  showed  how  the  explanations  which  have  been  offered  to  account 
for  them  scarcely  apply  to  those  of  the  moon :  and  thus,  driven  to 
other  hypotheses,  we  endeavoured  to  demonstrate  the  probability  of 
the  lunar  craters  having  been  produced  by  eruptive  force,  generated 
by  that  pre-solidifying  expansion  of  successive  portions  of  the 
moon's  molten  interior,  which  we  enunciated  in  our  third  chapter. 
The  precise  cause  of  phenomena  which  resulted  in  the  production 
of  a  crater  of  the  normal  lunar  type,  with  or  without  the  significant 
central  cone,  were  then  illustrated  by  a  series  of  step-by-step 
diagrams  with  accompanying  descriptive   paragraphs.     And  after 


CHAP.  XV.]  CONCLUDING    SUMMARY.  211 

treating  of  craters  of  the  normal  type  we  pointed  out  and  explained 
some  variations  thereupon  that  are  here  and  there  to  be  met  with, 
and  likewise  those  curious  complications  of  arrangement  which 
exhibit  craters  superimposed  one  upon  another  and  intermingled 
in  strange  confusion. 

From  craters  manifestly  volcanic  we  passed  to  the  consideration 
of  those  circular  formations  which,  from  their  vastness  of  size, 
scarcely  admit  of  satisfactory  explanation  by  a  volcanic  hypothesis. 
We  summarized  several  proffered  theories  of  their  origin,  and 
pointed  out  what  we  considered  might  be  a  possible  key  to  the 
solution  of  the  selenological  enigma  which  they  constitute,  without 
however,  expressing  ourselves  entirely  satisfied  with  the  validity  of 
our  suggestion.  The  less  mysterious  features  presented  by  peaks 
and  mountain  ranges  were  then  discussed  to  the  extent  that  we  con- 
sidered requisite,  viewing  their  comparatively  simple  character  and 
the  secondary  position  they  occupy  in  point  of  numerical  import- 
ance upon  the  moon.  At  greater  length  we  dealt  with  the  cracks 
and  chasms  and  the  allied  phenomena  of  radiating  streaks,  pointing 
out  with  regard  to  these  latter  the  strikingly  beautiful  correspond- 
ence in  effect  (and  therefore  presumably  in  cause)  between  them 
and  crack- systems  of  a  glass  globe  *'  starred  "  by  an  expanding 
internal  medium. 

The  more  notable  objects  and  features  of  the  lunar  surface  being 
disposed  of,  we  had  next  to  say  a  few  words  upon  some  residual 
phenomena,  chiefly  upon  the  colour  of  lunar  surface  details,  and 
upon  their  various  degrees  of  brightness  or  reflective  power.  And, 
inasmuch  as  varying  brightness  seemed  to  us  to  be  related  to 
varying  antiquity,  we  were  thence  led  to  the  question  of  the 
chronology  of  selenological  formations,  and  to  the  disputation  upon 
the  continuance  of  volcanic  action  upon  the  moon  in  recent  years. 
We  regarded  this  question  from  the  observational  and  the  infer- 
ential points  of  view,  and  were  led  to  the  conclusion  that  the 
moon's  surface  arrived  at  its  terminal  condition  ages  ago,  and  that 
it  is  next  to  hopeless  to  look  for  evidence  of  existing  change. 


212  THE    MOON.  [chap.  xv. 

Thus  far  our  work  dealt  with  the  moon  as  a  planetary  body 
merely.  It  occurred  to  us,  however,  that  we  might  add  to  the 
interest  attaching  to  our  satellite  were  we  to  regard  it  for  a  time  as 
a  world,  and  consider  its  conditions  as  respects  fitness  for  habita- 
tion by  beings  like  ourselves.  The  arguments  against  the  possi- 
bility of  the  moon  being  thus  fitted  for  human  creatures,  or, 
indeed,  for  any  high  organism,  were  decisive  enough  to  require 
little  enforcing.  It  appeared  to  us,  nevertheless,  that  much  might 
be  learnt  by  imagining  one's  self  located  upon  the  moon  during  a 
period  embracing  one  lunar  day  (a  month  of  our  reckoning),  with 
power  to  comprehend  the  peculiar  circumstances  and  conditions  of 
such  a  situation.  We  therefore  attempted  a  description  of  an 
imaginary  sojourn  upon  the  moon,  and  pointed  out  some  of  the 
more  striking  aspects  and  phenomena  which  we  know  by  legitimate 
inference  would  be  there  manifested.  We  trust,  that  while  our 
modest  efforts  in  the  chapter  referring  to  this  branch  of  our  subject 
may  prove  in  some  degree  entertaining,  they  may  be  in  a  greater 
degree  instructive,  inasmuch  as  certain  facts  are  brought  into 
prominence  which  would  not  unnaturally  be  overlooked  in  contem- 
plating the  moon  from  the  earth,  the  only  real  stand-point  that  is 
available  to  us. 

In  our  final  chapter  we  considered  the  moon  as  a  satellite,  and 
sought  to  enhance  popular  regard  for  it  on  account  of  certain  high 
functions  which  it  performs  for  man's  benefit  on  this  earth ;  but 
which  are  in  great  risk  of  being  overlooked.  We  showed  that,  not- 
withstanding the  moon's  occasionally  useful  service  as  a  nocturnal 
luminary,  it  fills  a  far  higher  office  as  a  sanitary  agent  by  cleansing 
the  shores  of  our  seas  and  rivers  through  the  agency  of  the  tides. 
We  pointed  out  the  vast  amount  of  absolutely  mechanical  work  and 
commercial  labour  which  the  same  tidal  agency  executes  in  trans- 
porting merchandize  up  and  down  our  rivers — an  amount  that,  to 
take  the  port  of  London  alone,  represents  a  money  value  pei'  annum 
that  may  be  reckoned  in  millions  sterling,  seeing  that  if  our  river 
was  tideless  all  transport  would  have  to  be  done  by  manual  or 


CHAP.  XV.]  CONCLUDING    SUMMARY.  213 

steam  power.  We  then  hinted  at  the  stupendous  reservoir  of 
power  that  the  tidal  waters  constitute,  a  form  of  power  which  has 
not  as  yet  been  sufficiently  called  into  operation,  but  which  may  be 
invoked  by-and-by,  when  we  have  begun  to  feel  more  acutely  the 
consequences  of  our  present  prodigal  use  of  the  fuel  that  was 
stored  up  for  us  by  bountiful  nature  ages  upon  ages  ago.  The 
moon's  services  to  the  navigator,  in  affording  him  a  ready  means  of 
finding  his  longitude  at  sea ;  to  the  chronologist  and  historian,  as 
a  timekeeper,  counting  periods  too  vast  for  accurate  reckoning  by 
other  means ;  to  the  astronomer  and  student  of  nature,  in  revealing 
certain  wonderful  surroundings  of  the  solar  globe,  which,  but  for 
the  phenomena  of  eclipses  caused  by  the  moon's  interposition, 
would  never  have  been  suspected  to  exist — these  were  other 
functions  that  we  dwelt  upon,  all  too  briefly  for  their  deserts  ;  and, 
lastly,  we  spoke  of  the  moon  as  a  medal  of  creation  fraught  with 
instructive  suggestions,  which  it  has  been  our  endeavour  to  bring 
to  notice  in  the  course  of  this  work.  And  from  uses  we  passed  to 
abuses,  directing  attention  to  a  few  popular  errors  and  wide- spread 
illusions  relating  to  lunar  influence  upon,  and  in  connection  with 
things  terrestrial.  This  part  of  our  work  might  have  been  con- 
siderably expanded,  for,  in  truth,  the  moon  has  been  a  misunder- 
stood and  misjudged  body.  Some  justice  we  trust  we  have  done  to 
her :  we  have  brought  her  face  to  the  fireside ;  we  have  analysed 
her  features,  and  told  of  virtues  that  few  of  her  admiring  beholders 
conceived  her  to  possess.  We  have  traced  out  her  history,  fraught 
with  wonderful  interest,  and  doubtless  typical  of  the  history  of 
other  spheres  that  in  countless  numbers  pervade  the  universe :  and 
now,  having  done  our  best  to  make  all  these  points  familiar,  we 
commend  the  moon  to  still  further  study  and  still  more  intimate 
acquaintance,  confident  that  she  will  repay  all  attentions,  be  they 
addressed  to  her  as 

A  PLANET,  A  WORLD,  OE  A  SATELLITE. 


BRADBURV,    AUNEW,    <!c   OO.,    I'KINTKRS,    WHITKFRIARS. 


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