LIGHT SCIENCE* FOR
LEISURE HOURS.
FAMILIAR ESSAYS ON
SCIENTIFIC SUBJECTS, NATURAL PHENOMENA. &c.
WITH A
SKETCH of tke LIFE of XAKY SOMERVILLE.
RICHARD A. PROCTOR, B.A. CAMB.,
n
HONORARY SECRETARY OP THE ROYAL ASTRONOMICAL SOCIETY;
AUTHOR OP
'OTHER WORLDS' 'SATURN*' 'ESSAYS ON ASTROXO.MY' 'THE ORBS AROUND US'
ETC.
Truths of Science waiting to be canght." TEXNYSOS.
LONDON :
LOXGMAXS, GREEX, AXD CO.
1873.
A II rights reserved.
0/71
T43
1273
PBEFACE.
THE First Series of Light Science Essays met with a
success so far beyond my expectations, that I should
have found in that circumstance alone a reason for
adding the present volume to the series. But I have
also felt a wish to publish these essays because they con-
tain facts collected at the cost of much labour and care-
fully discussed, useful, therefore, I trust, to others as
well as to myself, when thus gathered into a volume.
Those who have read my former series of essays, viz.,
' Light Science, Series I.,' 'The Orbs around Us,' and
' Essays on Astronomy,' will perceive that even when
I treat here of subjects already dealt with by me else-
where, I have been careful to avoid the repetition of
any statements, except those few without which a sub-
ject would be incomplete. For instance, it will not
be easy to find in my two papers on comets in ' The
Orbs around Us,' statements or reasoning repeated in
the two papers on comets in the present volume.
However, for the most part, the papers in this series
23/0
vi PREFACE.
are distinct in subject as well as in treatment from any
of my essays which have formerly appeared.
I invite special attention to the second essay on the
Transit of Venus. The time is drawing near when it
will be too late to take action to extend and render
complete and satisfactory our preparations for this im-
portant phenomenon the most important, I venture
to assert, of all the astronomical phenomena of the
present century. Without imputing blame to any per-
son, I must dwell strongly on the fact that the share
proposed to be taken by Great Britain in the observa-
tions of the transit, is unworthy of her position in the
scientific world, and as a nation. There is great risk
that, for want of an adequate number of southern sta-
tions, the whole series of observations by all countries
engaged in the work tvill result in failure; and it
appears to me nay, more positively it certainly is
a deplorable circumstance, that while Russia and
America are providing for more than thirty northern
stations, whereof sixteen are Halleyan, Great Britain
will supply but three southern stations, of which only
one chances to be Halleyan as well as Delislean ; while
even as respects this one station, Mr. Groschen has told
the country, speaking in his place in Parliament, that
either Halley's method c will not be applied at all,
or at least very little reliance will be placed upon it.'
Yet at sixteen northern stations, some of them most
PEE FACE. VI 1
difficult of occupation, Russia and America will apply
this very method ; while even the criterion devised to
minimise the value of the method, leaves it superior,
when applied at the stations I have indicated, to De-
lisle's, as applied at selected stations.
I appeal to all who have influence in these matters to
examine the evidence for themselves (whether as pre-
sented here, or with charts in my Essays in Astronomy,
or in recent numbers of the Monthly Notices of the
Astronomical Society), and to form their own judgment
as to the position of affairs. That is all I ask, since
I am satisfied that that will be altogether sufficient to
suggest the promptest and most energetic action.*
RICHD. A. PROCTOR.
LOXDOX: May 1873.
* Since this was written I have received letters from the greatest master
of mathematical astronomy this country has produced since Newton's day,
strongly confirming my views as to the extreme importance of providing
many southern stations for applying Hall ey's method in 1874, and urging
me, moreover, to appeal to America to take part in this special work, for
which she is peculiarly fitted, because of the bravery and enterprise of her
seamen, the skill and ingenuity of her astronomers and physicists, and
her singular liberality as a nation in all scientific matters.
CONTENTS.
PAGE
LIFE OF MRS. SOMERVILLE 1
THE COMING TRANSIT OF \ r ENUS, AND BRITISH PREPARATIONS
FOR OBSERVING IT 15
THE EVER-WIDENING WORLD OF STABS 40
MOVEMENTS IN THE STAR-DEPTHS 55
THE GREAT NEBULA IN ORION 78
THE SUN'S TRUE ATMOSPHERE .95
SOMETHING WRONG WITH THE SUN 118
NEWS FROM HERSCHEL'S PF.ANKT 122
THE Two COMETS OF THE YEAR 1 868 :
PART I. BRORSEN'S COMET 147
PAST II. WINNECKE'S COMET 163
COMETS OF SHORT PERIOD 181
THE GULF STREAM 195
OCEANIC CIRCULATION . . . .. . . . .212
ADDENDUM IN REPLY TO DR. CARPENTER 245
THE CLIMATE OF GREAT BRITAIN 260
THE Low BAROMETER OF THE ANTARCTIC TEMPERATE ZONE . 285
LIST OF ILLUSTRATIONS.
PAGE
'CHAET OF THE NORTH ATLANTIC ON AN EQUAL- SURFACE PRO-
JECTION, SHOWING THE GULF STREAM, &C 217
CHART OF THE NORTHERN HEMISPHERE, SHOWING THE CURVES
OF EQUAL MEAN ANNUAL TEMPERATURE AND EQUAL MID-
WINTER TEMPERATURE FOR LONDON 264
THE SAME, SHOWING THK CUP.VES OF EQUAL MEAN ANNUAL
TEMPERATURE AND EQUAL MID-SUMMER TEMPERATURE FOR
LONDON ........... 265
DIAGRAM SHOWING THE ANNUAL VARIATION OF MEAN DIURNAL
TEMPERATURE AT GREENWICH 277
DIAGRAM SHOWING THE BAROMETRIC PRESSURE OVER SOUTHERN
HEMISPHERE 287
DIAGRAM SHOWING THE BAROMETRIC PRESSURE OVER NORTHERN
HEMISPHERE ib.
FIGURE ILLUSTRATING A THEORY IN EXPLANATION OF THE Low
BAROMETER OF THE ANTARCTIC ZONE 305
LIGHT SCIENCE
FOR LEISURE HOURS.
SECOND SERIES.
MRS. SOMERVILLE.
MARY SOMERVILLE (nee FAIRFAX) was born at Jedburgh
on December 26, 1780, and died on November 30, 1872,
at Naples, aged nearly ninety-two years. In consider-
ing her education, we have not to mention important
seminaries, where skilled teachers make it their chief
business to impart to others the knowledge for which
they are themselves eminent, but to speak only of
studies pursued in the calm of a quiet home. This,
rightly understood, is perhaps the most remarkable
feature of her career. There are few mathematicians
so eminent as she deservedly was, in whose fame great
public schools and universities do not in some degree
partake. But we owe almost to accident the discovery
of the powers of Mary Fairfax's mind, while the gradual
development of those powers proceeded under the
guidance of tutors unknown to fame, and with access
only to such assistance as could be given by the friends
of her own family.
2 LIGHT SCIENCE FOR LEISURE HOURS.
Mrs. Somerville has herself described how it chanced
that the peculiar powers of her mind came first to be
recognised. She was in the habit of working at her
needle in the window-seat, while her brother took his
lessons in geometry and arithmetic. Fortunately (in
her case) the work which is regarded as most suitable
to the capacity of women leaves the mind unoccupied ;
and consequently there was nothing to prevent Mary
Fairfax from attending to the lessons intended for her
brother. She gradually became interested in the subject
of these lessons, and took care not only to be present
regularly, but to study her brother's books in her own
room. It happened that, on one occasion, young Fair-
fax failed to answer a question addressed to him, and
his sister involuntarily prompted him. The tutor was
naturally surprised that the quiet Mary Fairfax should
have any ideas beyond the needlework which had ap-
parently engaged her attention ; but, being a sensible
man, he was at the pains to ascertain the degree arid
soundness of her knowledge, and, finding that she had
really grasped the first principles of mathematics, he
6 took care that she should have liberty to go on in
her own way.' If a boy had shown similar fitness for
mathematical research, anxious attention would have
been devoted to the choice of books and teachers,
school and university ; but the case of a girl showing
such tastes seemed to be adequately met by according
to her the privilege of following her own devices. We
shall never know certainly, though it may be that
hereafter we shall be able to guess, what science lost
through the all but utter neglect of the unusual powers
MRS. SOMERVILLE. 3
of Mary Fairfax's mind. We may rejoice that, through
an accident, she was permitted to reach the position
she actually attained ; but there is scarcely a line of
her writings which does not, while showing what she
was, suggest thoughts of what she might have been.
While studying mathematics ' in her own way,' she
found a difficulty which for a time threatened to inter-
fere with her progress. She was unable to read the
P-rincipia, because she could not understand Latin.
In this strait, she applied, c after much hesitation/ to
Prof. Playfair. She asked if a woman might, without
impropriety, learn Latin. After ascertaining the pur-
pose which the young lady had in view possibly in
doubt lest she might follow in the steps of Anne Dacier
Prof. Playfair told her that it would not, in his
opinion, do her any harm to learn Latin in order to
read the Principia. It is noteworthy, as having pro-
bably a bearing on the course which Mrs. Somerville's
reading subsequently took, that Playfair was one of
the few in this country who at that time appreciated
the methods of 'the higher mathematical analysis, and
had formed a just opinion of their power ' a power,
however,' as Sir John Herschel well remarks, ' which
he was content to admire and applaud rather than
ready to wield.' His excellent review of the Mecanique
Celeste probably gave (as Herschel suggests) a stronger
impulse to the public mind in the direction of the
higher analysis than he could have communicated by
. any researches of his own.
It was not. however, as a mathematician that Mrs.
B 2
4 LIGHT SCIENCE FOR LEISURE HOURS.
Somerville first became known to the world. A subject
of research, exceedingly difficult and only to be pursued
successfully under very favourable conditions, was
undertaken by her during the life of her first husband,
Captain Grreig, son of High-Admiral Greig of the Eus-
sian Navy. She sought to determine by experiment
the magnetising influence of the violet rays of the
solar spectrum. ' It is not surprising,' says Sir John
Herschel on this subject, ' that the feeble though
unequivocal indications of magnetism which she
undoubtedly obtained should have been regarded by
many as insufficient to decide the question at issue.'
Nevertheless it was justly regarded as a noteworthy
achievement that, in a climate so unsuitable as ours,
any success should have been attained in a research of
such extreme difficulty. That she achieved, and, what
is more, deserved success, will be inferred from the
words in which Sir John Herschel indicates his own
opinion of the value of her results : ' To us,' he says,
6 their evidence appears entitled to considerable weight ;
but it is more to our immediate purpose to notice the
simple and rational manner in which her experiments
were conducted, the absence of needless complication
and refinement in their plan, and of unnecessary or
costly apparatus in their execution, and the perfect
freedom from all pretension or affected embarrassment
in their statement.'
In 1832 Mrs. Somerville published the work on
which, in our opinion, her fame in future years will be
held mainly to depend. The Mechanism of the Heavens
MXS. SOMERVILLE. 5
was originally intended to form one of the works
published by the Society for the Diffusion of Useful
Knowledge, though it soon outgrew the dimensions
suited for such a purpose. Indeed, it is remarkable
that either Mrs. Somerville herself or Lord Brougham,
at whose suggestion the work was undertaken, should
suppose it possible to epitomise Laplace's magnum
opus, or so to popularise ft as to bring it within the
scope of the Society's publications.
It will be well, in weighing the value of the book,
to consider it first with reference to the purpose of its
author, though a judgment based on that consideration
alone would not be a fair one. These, then, are the
words in which Mrs. Somerville presents the scope and
purpose of her work :
' A complete acquaintance with physical astronomy
can only be attained by those who are well versed in
the highest branches of mathematical and mechanical
science : such alone can appreciate the extreme beauty
of the results, and the means by which these results
are obtained. Nevertheless, a sufficient skill in ana-
lysis to follow the general outline, to see the mutual
dependence of the several parts of the system, and to
comprehend by what means some of the most extraor-
dinary conclusions have been arrived at, is within the
reach of many who shrink from the task, appalled by
difficulties which perhaps are not more formidable than
those incident to the study of the elements of every
"branch of knowledge, and possibly overrating them by
not making a sufficient distinction between the degree
6 LIGHT SCIENCE FOR LEISURE HOURS.
of mathematical acquirement necessary for making
discoveries and that which is requisite for understand-
ing what others have done. That the study of mathe-
matics, and their application to astronomy, are full of
interest, will be allowed by all who have devoted their
time and attention to these pursuits ; and they only
can estimate the delight of arriving at truth, whether
it be the discovery of a world or of a new property of
numbers.'
It cannot be doubted that Mrs. Somerville here indi-
cates her belief in the possibility of presenting her
subject in a form suited to the capacities of a large
number of readers, and to some extent advocates this
as her object. Whether she succeeded or failed in this
purpose must therefore be the first question to engage
our attention. Sir John Herschel considers that she
succeeded, 4 for all those parts of her subject, at least,
which the work ' professes to embrace, that is to say,
the general exposition of the mechanical principles
employed, the planetary and lunar theories, and those
of Jupiter's satellites, with the incidental points natu-
rally arising out of them.' With the utmost respect for
the authority of one who was so thorough a master of
the subject which Mary Somerville endeavoured to
popularise, I venture to express a different opinion.
I find it impossible to come to any other conclusion
than that, as respects the main purpose of her work,
Mrs. Somerville failed entirely; though I hasten to
qualify this statement by the remark that, in my
opinion, success was altogether impossible. I believe,
MRS. SOMERVILLE. 7
in fact, that neither Mrs. Somerville nor Sir John
Herschel thoroughly apprehended the difficulty of con-
veying to the general reader clear ideas respecting even
the elements of the subjects they severally endeavoured
to expound. But I feel bound to add that Mis.
Somerville's failure, inevitable from the very nature of
her task, would in any case have been brought about
by the manner in which the task was accomplished.
It will presently be seen that, in saying this, I am, in
fact, touching on the most remarkable and distin-
guishing quality of Mrs. Somerville's mind.
There are two essential requisites in a treatise in-
tended to introduce a difficult subject to general readers.
First, there must be a clear apprehension of the position
of such readers, of what they can and of what they
cannot understand, and of the form in which what is
written for them may most usefully be presented. It
is not too much to say that if just ideas had been en-
tertained by Mrs. Somerville on this point, the attempt
to present the Mechanism of the Heavens in a popular
form would never have been made. But, secondly, it is
essential that in any work of the kind each statement
each sentence, in fact should be presented in terms
so precise as to be absolutely unmistakable. This is
not so necessary in advanced treatises indeed, it is
too well known how large a proportion of our works on
advanced science are wanting in strict precision of
expression. But it is absolutely necessary in works
intended to popularise science. It is a somewhat
remarkable circumstance that in the Mechanism of the
8 LIGHT SCIENCE FOR LEISURE HOURS.
Heavens the boldest attempt ever made, perhaps, in
this direction not only is precision of expression not
a notable feature, but, on the contrary, the most strik-
ing fault in the work is the inexactness of the language.
Even Sir John Herschel, whose perfect familiarity with
the subject of the work would tend to render the fault
less obvious to him, was nevertheless truck by it :
'The most considerable fault we have to find,' he
wrote, ' with the work before us consists in an habitual
laxity of language, evidently originating in so complete
a familiarity with the quantities concerned as to induce
a disregard of the words by which they are designated,
but which, to any one less intimately conversant with
the actual analytical operations than its author, must
infallibly become a source of serious errors, and which,
at all events, renders it necessary for the reader to be
constantly on his guard.'
These words form the penultimate sentence of Sir
John Herschel's critique. I have preferred to speak
first of the subject touched on, so as to pass without
reservation to a more pleasing topic the real and
unquestionable value of Mrs. Somerville's chief work.
And, after all, the good qualities of the work are intrin-
sic, while its main fault relates to a purpose which the
work never could have fulfilled, no matter how care-
fully the fault had been avoided.
It is in this sense regarding the work apart from
its special purpose, and judging of it only as a contri-
bution to advanced scientific literature that we may
fairly say, with Sir John Herschel, that the work is
MRS. SOMERVILLE. 9
one of which any geometer might be proud. There is,
indeed, ample evidence of the disadvantage under
which Mrs. Somerville laboured, in the want of tho-
rough mathematical training ; but so much the more
wonderful is it that she should have completely mas-
tered her subject. Every page indicates her apprecia-
tion of the methods employed by Laplace and Lagrange.
Where she does not strictly follow the Mecanique
Celeste, she evidences a clear recognition of the pur-
poses to be subserved by adopting a different course.
I would not be understood as commending all the
departures thus made ;-on the contrary, there are cases
where it appears to me that on the whole it would have
been preferable to have followed the processes of the
Mecanique Celeste more closely, while there are others
where certain more modern processes might perhaps
with advantage have been introduced. But even in
such instances we recognise in the course pursued by
Mrs. Somerville the decision of one perfectly familiar
with the subject in hand. And many of the changes
must undoubtedly be regarded either as improvements,
or else as altogether desirable when the scale of Mrs.
Somerville's treatise is taken into account. Amongst
instances of the former kind must be classed the method
employed in the investigation of the equations of con-
tinuity of a fluid ; amongst instances of the latter, I
would specially cite the treatment of the theory of
elliptic motion, in the opening chapters of the second
book.
If however I were asked to point out the feature of
10 LIGHT SCIENCE FOR LEISURE HOURS.
this work which, in my opinion, most strikingly indi-
cated the powers of Mrs Somerville's mind, I should
unhesitatingly select the preliminary dissertation. In
this we have an abstract of the Newtonian philosophy
such as none but a master-mind could have produced.
Apart from its scientific value and it has great scien-
tific value it is a work of great literary merit. If it
is not in plan and purpose altogether original, inas-
much as it must be regarded as to some degree an
abstract of Laplace's Systeme du Monde, it is never-
theless, as Herschel has well remarked, ' an abstract so
vivid and judicious as to have all the merit of origin-
ality, and such as could have been produced only by
one accustomed to large and general views, as well as
perfectly familiar with the particulars of the subject.'
Three years after the appearance of the Mechanism
of the Heavens, Mrs. Sornerville published the work by
which she is probably best known to general readers.
The Connexion of the Physical Sciences was, I believe,
written at the suggestion of Lord Brougham, as an
expansion of the admirable introduction to the Celestial
Mechanism. It is a work full of interest, not only to
the student of advanced science, but to the general
reader. In saying this we indicate its chief merit and
its most marked defect. It is impossible to conceive
that any reader, no matter how advanced or how limited
his knowledge, could fail to find many most instructive
pages in this work; but it is equally impossible to
conceive that any one reader could find the whole work,
or even any considerable portion, instructive or useful.
MRS. SOMERVILLE. II
The fact was that Mrs. Somerville recognised, or which
is practically the same thing, wrote as if she recognised,
no distinction between the recondite and the simple.
She makes no more attempt at explanation when
speaking of the perturbations of the planets or discuss-
ing the most profound problems of motecular physics,
than when she is merely running over a series of state-
ments respecting geographical or climatic relations.
It would almost seem as though her mind was so con-
stituted that the difficulties which ordinary minds ex-
perience in considering complex mathematical problems
had no existence for her. A writer, to whom we owe
one of the best obituary notices of Mrs. Somerville
which hitherto have appeared, tells us that the sort of
pressure Mrs. Somerville underwent from her publisher
as the earlier editions of the Connexion of the Physical
Sciences passed through the press ' convinced her of her
own unfitness for popularising science. When there was
already no time to lose in regard to her proof sheets, she
had hint upon hint from Mr. Murray that this and that
and the other paragraph required to be made plainer to
popular comprehension. She declared that she tried very
hard to please Mr. Murray and others who made the same
complaint, but that every departure from scientific
terms and formulas appeared to her a departure from
clearness and simplicity ; so that, by the time she had
explained and described to the extent required, her
statements seemed to her cumbrous and confused. In
other words, this was not her proper work.'
Eespecting her two other works, I shall merely
12 LIGHT SCIENCE FOR LEISURE HOURS.
remark that the Physical Geography appeared in 1848,
and the Molecular and Microscopic Science in 1869,
when she had reached the advanced age of eighty-eight
years.
I may be excused for regarding Mary Soinerville's life
with reference rather to her astronomical and mathemati-
cal researches than to her proficiency in other branches
of science. In this aspect of her career it is difficult,
great as was the reputation she deservedly obtained, not
to contemplate with regret those circumstances, the
effects of unfortunate prejudices, whereby she was pre-
vented from applying the full powers of her mind to
the advancement of science. It is certain that no
department of mathematical research was beyond her
powers, and that in any she could have done original
work. In mere mental grasp few men have probably
surpassed her ; but the thorough training, the scholarly
discipline, which can alone give to the mind the power
of advancing beyond the point up to which it has
followed the guidance of others, had unfortunately been
denied to her. Accordingly, while her writings show
her power and her thorough mastery of the instruments
of mathematical research, they are remarkable less for
their actual value, though their value is great, than as
indicating what, under happier auspices, she might
have accomplished.
I have mentioned that Mrs. Somerville was twice
married. By her first marriage she had one son, Mr.
Woronzow Greig, since .deceased. A few years after
Captain Greig's death she married her cousin, Dr.
MRS. SOMERVILLE. I -
\j
Somerville, by which marriage she had three daughters,
two of whom survive her. The latter years of her life
(twenty-three years, we believe) were passed in Italy.
It has been said by one who was well acquainted with
the circumstances that ' the long exile which occupied
the latter portion of her life was a weary trial to her.
She carried a thoroughly Scotch heart in her breast ; and
the true mountaineer's longing for her native country
sickened many an hour of many a tedious year. She
liked London life, too, and the equal intercourses which
students like herself can there enjoy ; whereas, in Italy,
she was out of place. She seldom met any one with
whom she could converse on the subjects which in-
terested her most ; and if she studied, it could be for no
further end than her own gratification. It was felt by
her friends to be a truly pathetic incident that, of all
people in the world, Mrs. Somerville should be debarred
the sight of the singular comet of 1 843 ; and the cir-
cumstance was symbolical of the whole case of her
exile. The only Italian observatory which afforded the
necessary implements was in a Jesuit establishment,
where no woman was allowed to pass the threshold. At
the same hour her heart yearned towards her native
Scotland, and her intellect hungered for the congenial
intercourse of London ; and she looked up at the sky
with the mortifying knowledge of what was to be seen
there but for the impediment which barred her access
to the great telescope at hand. With all her gentleness
of temper and her lifelong habit of acquiescence, she
14 LIGHT SCIENCE FOR LEISURE HOURS.
suffered deeply, while many of her friends were indig-
nant at the sacrifice.'
I shall venture to quote, in conclusion, some remarks
by Sir Henry Holland on features of Mrs. Somerville's
character and life which have been hidden from general
knowledge : ' She was a woman not of science only,'
he tells us, ' but of refined and cultivated tastes. Her
paintings and musical talents might well have won
admiration, even had there been nothing else beyond
them. Her classical attainments were considerable,
derived probably from that early part of life when the
gentle Mary Fairfax gentle she must ever have been
was enriching her mind by quiet study in her Scotch
home. ... A few words more on the moral part
of Mrs. Somerville's character ; and here, too, I speak
from intimate knowledge. She was the gentlest and
kindest of human beings qualities well attested even
by her features and conversation, but expressed still
more in all the habits of her domestic and social life.
Her modesty and humility were as remarkable as those
talents which they concealed from common observation.
Scotland,' he justly adds, ' is proud of
having produced a Crichton. She may be proud, also,
in having given birthplace to Mary Somerville.'
(From Monthly Notices of the Royal Astronomical Society
for February, 1873.)
THE COMING TRANSIT OF VENUS. 15
THE COMING TRANSIT OF VENUS, AND BRITISH
PREPARATIONS FOR OBSERVING IT.
BY far the most important of all the phenomena which
astronomers are now expecting is the transit of Venus,
which will take place on December 8, 1874. Even
the eclipses of the last few years, though they have
attracted so much attention, and have been observed so
carefully ,have in reality been regarded as altogether less
important than the next transit of Venus. Total eclipses
are almost every-year phenomena, but transits of Venus
occur only at average intervals of more than half a
century. The last took place in 1769, and after the
transit of 1882 none will occur till 2004. Apart from
this circumstance, a transit of Venus is of extreme
importance in the science of astronomy. It admittedly
affords the most satisfactory means of determining the
distance of the sun in other words, the dimensions of
the solar system itself. And such determination of the
scale on which our system is constructed affords the
only means we possess of measuring the vast spaces
which separate us from the fixed stars. So that the
observations which are to be made in December, 1874,
and renewed (but under somewhat different conditions)
in December, 1882, bear directly on the fundamental
problem of astronomy, so far as astronomy relates to
1 6 LIGHT SCIENCE FOR LEISURE HOURS.
the determination of the distances and the magnitude of
the celestial bodies.*
I propose here, after inquiring briefly into the
general question of the determination of the sun's
distance, to describe the nature of the opportunities
which will be afforded during the transit of 1874, and
to discuss the preparations which are being made by
this country to take her part in the work of observation.
It will be seen, as I proceed, that this discussion of the
* I venture to quote here the appeal made by Halley (when Astro-
nomer "Roy*!") forty-five years before the transit of 1761, the earlier of
the pair of transits then looked forward to. It will show that, in dealing
with a transit 21 months before the date of its occurrence, I am not
looking forward so inordinately as might be supposed by those unfami-
liar with the nature of these inquiries. I should remark, however, that
since Halley's day other methods for determining the sun's distance have
been devised and employed. Six methods are described in my treatise
on the ' Sun,' and a seventh has, within the last few months, been
suggested by the great French astronomer Leverrier. Thus, then, wrote
Halley in 1716 : ' I could wish, indeed, that observations of the transit
should be undertaken by many persons in different places : first, because
of the greater confidence which could be placed in well-according obser-
vations ; and, secondly, lest a single observer should, by the interven-
tion of clouds, be deprived of that spectacle which, so far as I know,
will not be visible again to the men of this and the next century, and
on which depends the certain and sufficient solution of a most noble and
otherwise intractable problem. I therefore again and again urge upon
those inquiring observers of the celestial bodies, who, when I have de-
parted this life, will be reserved to observe these things, that, mindful
of my counsel, they should devote themselves strenuously and with all
their energies to conduct the observation ; I desire and pray that they
may be favoured in every way, and especially that they may not be
deprived of that most desirable spectacle by the inopportune darkness
of a clouded sky; and that, finally, the magnitudes of the celestial
bodies, forced into narrower limits (of exactness), may, as it were, make
submission to the glory and eternal fame of those observers.'
These hopes were not fulfilled, so far as the transit of 1761 was con-
cerned; but the transit of 1769 was observed with great care at no less
than seventy-four stations, fifty of which, however, were in Europe.
THE COMING TRANSIT OF VENUS. 17
subject does not labour under the fault of being prema-
ture. On the contrary, the time is now at hand when
a final decision must be made as to the course which
this country is to pursue ; and inasmuch as my purpose
is not solely to describe what is being done, but to
point out what (in my opinion) should be done, the
present is the proper time to speak.
A surveyor, who wishes to determine the distance of
an inaccessible object, measures a convenient base-line
and observes the direction of the object as seen from
either end of the line. He thus has the base and
the two base-angles of a triangle ; and the simplest
geometrical considerations teach that the other two
sides of the triangle can thence be determined. These
sides are, of course, the distances of the inaccessible
object from the two ends of the base-line. Now this is
the fundamental method employed by astronomers to
determine the distances of the celestial bodies. It is
applied directly to the moon. An observer at Green-
wich (let us say), notes the direction of the moon when
at her highest, or due south ; another at Cape Town
(let us say), does the like ; then a line joining Green-
wich and Cape Town is a base-line of known length,
and the two directions give the base-angles. The
triangle is a very long one, its vertical angle (that is,
the angle opposite the base) being one of about a degree
and a half, or about the angle swept out by the hand
of a clock or watch during a quarter of a minute ; but
such a triangle is quite within the methods of treat-
ment available to astronomers.
c
1 8 LIGHT SCIENCE FOR LEISURE HOURS
In applying this method to the sun, a serious diffi-
culty comes in. He is so far off that, instead of a
triangle with a respectable vertical angle, there is a
triangle having a vertical angle of about the 240th
part of a degree (under the most favourable conditions
which can be conveniently obtained). To know how
small such an angle is, let the reader note the minute
hand of a clock or watch, and observe how little it
shifts around its centre in a single second of time ; yet
this angular shift is twenty-four times as great as that
we have mentioned.
It must not be forgotten that, in all such cases, the
question is not whether the astronomer can recognise
such and such an effect, but whether he can measure
it. It is not the whole quantity about which astrono-
mers are troubled. Unquestionably the observer at
Greenwich can recognise the depression of the mid-day
sun, 1 due to the fact that Greenwich lies above (or
north of) the earth's centre. For this depression is an
element which he has to take into account in his obser-
vations. The corresponding depression, even in the
case of bodies far more distant than the sun, as the
planets Jupiter and Saturn, is announced systematically
in our national astronomical almanac. But the direct
measurement of the depression is altogether out of the
question.
If the stars which really bestrew the heavens beyond
1 Only observations of the mid-day sun would avail, because the only
instruments having the requisite delicacy of adjustment are meridional.
There is an instrument suitable for observing the moon when she is not
on the meridian ; but it is quite unfit for the purpose we are considering.
THE COMING TRANSIT OF VENUS. 19
the sun could be seen, the case would be different, for
they would serve as index points, by means of which to
estimate the sun's displacement. But although stars
not lying near the sun's place on the heavens can be
seen by day with powerful telescopes, those close around
him are quite invisible. This method failing, the
astronomer has to look for other means of solving the
problem. The planet Venus, which comes at times
much nearer to the earth than the sun is, and in fact
nearer than any celestial body except the moon, na-
turally claims attention as a suitable object for the
astronomer's purpose. For it is to be remembered that
the proportions of the solar system have long been
accurately determined ; so that as soon as the distance
of any one planet is ascertained, the scale of the whole
solar system becomes also known.
Venus, however, when at her nearest, is lost in the
sun's light, and, though discernible in powerful tele-
scopes, is quite unsuitably placed for the delicate obser-
vations which would alone avail to determine her
distance.
This brings us at once to the recognition of the
importance of a transit of Venus. When Venus passes
between the sun and earth, in such a way as not to
cross the sun's face, that is, when she passes above
or below the long and almost linear portion of space
lying actually between the earth and sun, she cannot
be well observed ; but when, in making the passage,
she comes so close to the line joining the earth and sion
as actually to be seen on the sun's face, she can be
c 2
20 LIGHT SCIENCE FOR LEISURE HOURS.
observed to great advantage. For she is then seen as
a round black spot on the sun's face ; this face is thus
as a sort of dial-plate on which the black disc of Venus is
as an index. The sharply-defined edge of this black
disc presents the same advantage which a neatly-cut
index possesses, enabling the observer to measure satis-
factorily the place of the planet. All the circumstances
are favourable, except two : first, the index, that
is, the black disc, is not even for an instant at rest ;
and, secondly, the index-plate, that is, the sun's disc,
is itself displaced by any difference in the position of
the terrestrial observers.
Nothing can be done to remedy the latter circum-
stance. Its effects are easily seen. Suppose an observer
at some northern station sees Venus in reality depressed
by a third of a minute of arc, which is about the hun-
dredth part of the sun's apparent diameter. Then the
sun, being farther away in the proportion of about
ten to three, is depressed by about the tenth of a
minute. Accordingly, Venus only seems to be depressed
by the difference of these amounts, or by little more
than a quarter of a minute. Nevertheless it is far
easier to measure this reduced displacement on the
sun's face, than to measure the larger displacement
without his face as an index-plate.
The other circumstance has been dealt with in two
ways.
First, in accordance with a suggestion of Halley's,
instead of attempting to measure the position of Venus
on the sun's face, the astronomer may simply time her
THE COMING TRANSIT OF VENUS. 21
as she crosses that face, and so judge how long the
chord is which she has traversed. This shows how
nearly the chord approaches the sun's centre, and thus
gives a determination as satisfactory as an actual mea-
surement. Of course, there are many details to be
taken into account : for instance, the apparent path of
Venus is not a straight line in reality, because the
observer's station is not at rest, but carried round the
axis of the rotating earth. But the mathematician
finds no difficulty in taking such considerations fully
into account.
Secondly, Delisle proposed that astronomers should
note the actual moment (of absolute, not local time)
when Venus seems to enter or leave the sun's face, as
seen from different stations on the earth. It will be
manifest, on a moment's consideration of the actual
circumstances of the case, that the transit will not
seem to begin or end at the same instant, as seen from
different parts of the earth. There is the great globe
of the sun at one side, and the smaller globe of the
earth on the other ; and Venus passes between. Now,
in order to show more clearly what must happen, let
us take an illustrative case drawn from an event which
in a few weeks from the present time will interest a
large proportion of our population. Suppose that on
one side of the river Thames there is a long building
whose extremeties- we call A and B. Suppose that just
opposite there is a barge whose corresponding extreme-
ties we call a and b. Now suppose the winning boat to
be coming along so as to pass between the house and
22 LIGHT SCIENCE FOR LEISURE HOURS.
the barge (coming first between the ends A, a). And
for simplicity of description let us confine our remarks
to the little flag carried at the bow of the boat. It is
manifest that an observer at a will see the little flag
cross his line of vision towards A before an observer at
b sees the like. And the observer at a will in like
manner see the light blue flag (I beg pardon, I should
say the blue flag simply) crossing his line of vision
towards B before an observer at b sees the like. The
flag will traverse the range A B as seen both from a
and from 6, but both its ingress on this range and its
egress from it will be earlier as seen from a than as
seen from b. Now our earth may be compared to the
barge; the sun to the building A B.; and Venus to the
boat. There is one spot on the earth at which Venus
will seem to enter earliest on the sun's face, and an-
other spot (on the opposite side, just as b is farthest
away from a) where Venus will seem to enter latest ;
and in like manner there is one spot at which Venus
will seem to leave the sun's face earliest, and another
(on the opposite side) at which Venus will seem to leave
the sun's face latest.
And as our illustrative case explains the nature of
Delisle's method, so also it illustrates the rationale of the
method. Of course, the two cases are not exactly similar ;
but they are sufficiently so to make the illustration in-
structive. Suppose that the length of the barge a b is
known (as the dimensions of the earth are known) ;
thus, say that it is 24 yards in length. Now suppose
that the course of the boat is known to be in mid-
THE COMING TRANSIT OF VENUS. 23
stream, or exactly midway between the house and the
barge. Then a moment's consideration will show that
the boat traverses 12 yards between the moments when
the spectators at a and b severally see it towards A.
Now suppose that the observer at a indicates by a call
or other signal the moment when the flag is thus seen
by him, and that the observer at 6, provided with a
stop-watch, notes that two seconds elapse before he sees
the flag towards A. This, then, is the time occupied
by the boat in traversing 1 2 yards ; so that she is mov-
ing at the rate of six yards per second. Similar remarks
apply to the apparent transit of the flag past B as seen
from a and 6. * In like manner, the astronomer can
gather from observations by Delisle's method the rate
at which Venus is moving in her orbit, that is, the
exact number of miles over which she moves per minute.
So that, since he knows exactly how long she is in com-
pleting the circuit of her orbit, he learns, in fact, the
exact circumference of her orbit in miles, whence its
radius (or her distance from the sun) follows at once.
It is manifest that Delisle's method can be applied
with equal advantage either to the ingress or to the
egress of Venus. The comparison of two observations
in one of which her ingress happens as early as possible,
while in the other it happens as late as possible is
quite sufficient to determine the sun's distance. So also
the comparison of two observations of egress (most
accelerated and most retarded) is separately sufficient
to determine the sun's distance. This is an important
advantage of the method. Because while, as in Halley's
24 LIGHT SCIENCE FOR LEISURE HOURS.
method, two stations are absolutely necessary, there is
but a single observation to be made at each, whereas in
Halley's the beginning and end of the transit must be
observed at both stations. This introduces a double
difficulty. For first, there is the necessity for a longer
continuance of clear sky, since the transit may last
several hours ; and, secondly, there is the difficulty of
securing a station where the sun is well placed on the
sky, both at the beginning and end of the transit. It
will not suffice, in applying Halley's method, to have
the sun well above the horizon at the moment of ingress
if he is low down at the moment of egress, or to have
the sun high at egress if he is low at ingress. Accord-
ingly, the condition has to be secured that at stations
where the day is short (that is, in December, at north-
erly stations) the middle of the transit shall occur
nearly at mid-day. This limits the choice for northern
stations considerably.
On the other hand, Delisle's method has this disad-
vantage, that the exact moment at which ingress or
egress occurs must be known. A mistake, even of a
second or two, would be of serious moment. So that
the clocks made use of at each station where this
method is applied, must not only have good rates, but
must show absolutely true time at the moment of the
observed phenomenon. Moreover, the latitude and
longitude of the place of observation must be known,
the latter (the only difficult point) with especial
accuracy, since on its determination depends the change
of local time into (say) Greenwich time; and this
THE COMING TRANSIT OF VENUS. 25
change must be accurately effected before two observa-
tions made in different longitudes can be compared as
respects the absolute time of their occurrence. On the
contraiy, Halley's method, while only requiring a rela-
tively rough determination of the longitude, can be
satisfactorily applied when the clocks employed are
simply well rated ; for it depends only on the duration
of the transit as seen at different stations. A clock
must be badly rated indeed utterly unfit, in fact, for
any astronomical use whatever which should lose a
single second in four or five hours.
But the most important point to be noticed is, that
both methods ought to be employed, if possible, apart
from all nice considerations of their relative value. It is
certain that astronomers will place much more confidence
in closely concordant results obtained by the application
of these two methods, differing wholly as they do in
principle, than in as many and equally concordant
results all obtained by one method. A third method is
indeed to be applied, viz., a method based on the
ingenious use of photography. But as yet too little is
known respecting the chances of success by this method
to warrant too implicit reliance upon it.
Let us inquire what preparations are being made by
astronomers, and especially by the astronomers of Eng-
land, to make adequate use of the opportunities pre-
sented by the coming transit.
It has first, unfortunately, to be noted, that, so far
as this country is concerned, no provision whatever has
been hitherto made for the employment of Halley's
26 LIGHT SCIENCE FOR LEISURE HOURS.
method. If this resulted from the simple preference of
Delisle's method, there would be little to say. Most
assuredly, speaking for myself, I should be very loth to
urge the advantages of Halley's method, if I found
against such a view the practical experience of those
astronomers who are continually testing the value of
various methods of observation. But the rejection of
Halley's method for the transit of 1874 was not origin-
ally, and is not now, based on any objection to the
principle of the method, but on certain mathematical
considerations, which appeared to prove that the method
could not be advantageously applied in 1874, while it
could be applied successfully in 1882. It was accord-
ingly reserved for the latter transit, and all the stations
for observing the transit of 1874 were selected with
special reference to the method of Delisle.
Now it happened that early in 1869 I was attracted
to the examination of the subject of the coming tran-
sits, by the circumstance that the investigation applied
to the matter by the Astronomer Royal had struck me
as imperfect in method. I was interested, viewing the
matter merely as a mathematical problem, to inquire
what corrections might occur if all the niceties of re-
search of which the question admitted were applied
throughout the investigation. Working with this sole
object in view, I analysed the whole matter in two
independent ways, viz., first as a problem of calculation,
and secondly as a geometrical problem. The results,
perfectly concordant, differed so remarkably from those
published by the Astronomer Royal, that I was con-
THE COMING TRANSIT OF VENUS. 2>]
strained (in mere fealty to the cause of science) to sub-
mit them to the examination of the scientific world.
To begin with : Halley's method, of which, in 1857,
and again in 1864, and yet again in 1868, the Astro-
nomer Eoyal had said that it is totally inapplicable in
1874, was found to be applicable under circumstances
altogether more favourable than those which will exist
in 1882. 1 It was found not only to be applicable with
1 The origin of this mistake on the Astronomer Royal's part is
thus explained in an article in the 'Spectator' for March 1, 1873:
' Everyone is asking whether it is possible that an astronomer so emi-
nent and so skilful as Sir George Airy for the time is past when
names need be concealed can have made any serious mistake in a matter
of this importance. And again, everyonfc is anxious to know precisely
what mistake is imputed, and how it arose (granting that a mistake has
been made).
' To this last question the reply is easy. It chanced unfortunately
that in 1857 the Astronomer Royal delivered a lecture on the subject of
the now approaching Transits. In that lecture his great mistake had
its origin. Intent on presenting the more striking and popular features
of his subject, and in a way which would be clear and convincing to
everyone, he was led to adopt a method of reasoning which on the face
of it seems convincing enough (and which, indeed, is sound in itself) ;
but the conclusions derived from which may be, and in the actual case
are, dependent on certain details into which the Astronomer Royal
neither then entered nor has ever entered since. It is the palpably con-
vincing nature of the evidence at a first view which led to all the
mischief. We will endeavour to give a brief but sufficient sketch of
the line of argument.
' Let it be premised that, for applying Halley's method or the
English method, as it is often called with advantage, what is wanted
is that at some station the transit shall last as long as possible, while
at another it shall last as short a time as possible. It matters nothing
whether the increase or reduction of the time be obtained by a seeming
change in the length of the line traversed by Venus, or by a change in
the rate at which she seems to move during transit. So much premised,
let it be noted that in 1874 Venus will cross the sun's face on a line
placed somewhat as a line from the figure X to the figure I on a
clock-face. As seen from northern stations, the line of transit will be
28 LIGHT SCIENCE FOE LEISURE HOURS.
advantage, but even more advantageously than De-
lisle's.
Lowered, and therefore manifestly will be lengthened. From southern
stations, the line will be raised, and therefore shortened. We therefore
set an observer at as northerly a station as we can, to get as great a
lengthening as we can, and that is one point gained. We set an observer
at as southerly a station as we can, and so get as great a shortening as
possible, and that is a second point gained. But it is easily shown (we
do not trouble our readers with the proof) that our northerly observer
is so shifted by the earth's rotation while the transit is in progress that
Venus is seemingly hastened on her course in transit. This shortens
the time of transit at the northern station, and is discordant with the
lengthening obtained by setting an observer as far north as possible.
Here, then, is one point against us. Lastly, the southern station can
be taken so as to give either a hastening or a retarding of Venus's
motion, simply because the transit occurs in the southern summer, when
places far south have no night, so that we can set the observer either
where he will have the sun moving from east to west during the transit,
or where he will have the sun moving from west to east. We set our
observer so that Venus is hastened (which is secured by taking a station
where, during the transit, the sun moves from east to west). This
hastening is manifestly accordant with the shortening of her path at
southern stations, and thus we get a third point in our favour. We
have, then, three points in our favour and one against us, or , balance
of only two favourable points.
' Now, in 1882, Venus crosses the lower part of the sun's face, or some-
what as from figure VII to figure IV on a clock-face. In this case, the
northern station gives the lowest or shortest course, while the southern
gives the highest or longest course. As before, we get two points in our
favour by setting an observer far to the north and another far to the
south. As before, the northern observer sees Venus hastened on her
course ; but now this is a favourable point, since it manifestly accords
with the shortening of the northern line of transit. This makes point
three in our favour. And again, as before, we can set our southern
observer where the motion of Venus can be hastened or retarded as we
please. We assign him a station where she will be retarded (which is
secured when the sun moves from west to east during the transit) : this
manifestly accords with the lengthening of her path. Thus we have
four favourable points in all in 1882; whereas in 1874 we can secure
only three or (one being unfavourable), a majority of only two favour-
able points.
'It seems manifest, then, that the transit of 1882 is twice as favour-
THE COMING TRANSIT OF VENUS. 29
On this point all doubts should have been very
quickly removed. For, almost simultaneously with
able for applying Halley's method as the transit of 1874. So the
Astronomer Eoyal concluded. He did not enter into details, but after
summing up the evidence much as we have done above, he said " the
observable difference of duration in 187 4 will probably not be half of that
in 1882." It was in 1857 that he thus spoke; and he has never said a
word or written a line since that time implying that he had gone into
the details of the matter. When he next touched on the subject (in 1864)
he referred to the lecture of 1857 as showing the suitability of Halley's
method in 1882, and he left the transit of 1874 wholly unnoticed.
Again, in December 1868, he touched on the matter, simply saying that
Halley's method fails totally in 1874. That fatal lecture, or rather the
error suggested in the process of popularising the subject for that occasion,
led to so established a conviction as to the uselessness of Halley's
method in 1874, that it had never seemed worth while to re-examine the
matter. But now let us consider details a little, and see how the matter
will then appear.
' In the first place, the transit of 1882 at once loses its apparent
superiority. The southern observer must have the sun moving from west
to east during the transit, or in other words, he must have the sun on
the night side (so to speak) of the sky. There is, of course, no night
near the Antarctic Pole on December 6, but at nominal midnight the
sun is at its lowest ; and the sun must be towards this part of his diurnal
course, if the observer is to get the advantage we are considering. There
is no known Antarctic station where this can be, the sun being also
fairly high at the beginning and end of the transit. This at once dis-
poses of the superiority of the transit of 1882. If an Antarctic station
is sought at all, there will be a hastening instead of a retarding of the
planet's transit, or an unfavourable point, as in the case of the earlier
transit. In reality, the loss thus accruing is found to be much more
serious in 1882 than the corresponding loss in 1874, when we inquire
into actual details.
' But in 1874, as we have seen, there must be an unfavourable hastening
of Venus's motion as seen from a northern station ; and this hastening
seems to cancel the effect due to the lengthened transit-path. When we
inquire, however, to what extent this cancelling takes place, we at once
see that the Astronomer Kcyal was frightened away from Halley's
method without sufficient reason. He manifestly (see the italicised re-
mark quoted above) supposed that the duration would scarcely be in-
creased at all at the northern station. Let us see, however, whether
Mr. Proctor has been right or not in saying that the duration is con-
30 LIGHT SCIENCE FOR LEISURE HOURS.
the announcement of my result, the news arrived that
the French astronomer, Puiseux, had obtained almost
siderably increased at a suitable northern station, notwithstanding the
undoubted partial cancelling which takes place from the cause indicated.
Not to favour one side or the other, we go direct to the Nautical Alma-
nack for 1874. We take Nertchinsk, the place pointed to by Mr. Proc-
tor so far back as March 1869 ; and we note that he then assigned to
this station a lengthening of the duration of transit by 15^ minutes (a
very considerable amount, much more in fact than at the most favourable
station in 1882). Now, what says the Nautical Almanack for 1874 ? At
page 434, it states that the mean duration of transit is 3 hours 42 min. 2
sec. At page 20 of the appendix, it states that at Nertchinsk the duration
is 3 hours 57 min. 6 sec., exceeding the former duration by 15 min.
4 sec. This is very close indeed to Mr. Proctor's result, and shows
how nearly the values obtained by his graphic constructions accord
with those deduced by rigid calculation. (Moreover, a part even of the
slight difference is due to a difference in the adopted value of Venus's
diameter.) Here, then, instead of that complete cancelling of the value
of the northern station which Sir G. Airy too hastily assumed, we have
a lengthening of the transit period by more than 15 minutes, which in
this problem is an unusually large amount. To show that this is so,
and how slightly the northern station is affected by the peculiarity which
Sir G. Airy had hastily regarded as introducing a fatal objection, we
have only to remark that at Possession Island, the most favourable
southern station (where the two conditions conspire, instead of opposing
each other^, the shortening of the transit amounts only to 17^ minutes.
Combining this shortening with the lengthening at Nertchinsk, we have
a difference of duration of fully 32^ minutes. And now observe how
greatly this result differs from Sir G. Airy's anticipation ! He thought
the difference of duration would probably " not be half of that in 1882 " ;
but his own estimate of the greatest difference of duration in 1882 (to
be obtained only by seeking an inaccessible station, where the sun will
be but four degrees high at egress) amounts only to 28 minutes. In-
stead of being less than half, the difference of duration in 1874 is
greater in the proportion of about 7 to 6. Add to this that in 1874 the
solar elevation, both at ingress and egress will exceed twenty degrees,
and the importance of having a station at Possession Island becomes
manifest. Eussia has occupied Nertchinsk, and it is Great Britain's
duty (and that of no other country) to occupy Possession Island. If
she shrinks from this duty, it will be no answer to the reproach which
she will hereafter incur, that she occupied stations in other respects ad-
vantageous. Other countries are occupying these stations, the Papelotte
THE COMING TRANSIT OF VENUS. 31
exactly the same conclusion. The sole difference be-
tween his result and mine was, that he simply an-
nounced that Halley's method was advantageously
applicable, whereas I showed that it was more advan-
tageously applicable than Delisle's. Even this differ-
ance, however, is readily accounted for, since, in
Puiseux's investigation, several of the niceties to which
I had attended were neglected as unimportant. 1
To show how completely the application of Halley's
method has been neglected in the choice of stations
for English observing parties, let the following con-
siderations be noticed :
At northern stations Venus will be seen lower down
that at southern stations, so that as she transits the
upper part of the sun's disc, her chord of transit is
necessarily longer at northern than at southern stations.
Now Russia occupies the best northern stations, as is
her due, since they fall in Russian territory. At
Nertchinsk, near Lake Baikal, Russia will have an
observing party ; and here the transit will last longer
than as supposed to be seen from the earth's centre, by
fully 15^ minutes. For at this place the transit will
begin nearly 6 minutes early, and end nearly 10
minutes late. Now, if we had only a southern station
and La Haye Sainte of the scientific Waterloo ; this country's duty calls
her to a post so important and so difficult of tenure, that it may fairly
be described as the Hougoumount of the position.
1 For example, Puiseux left out of consideration the dimensions of
Venus's disc, regarding her transit as that of her centre. He omitted
also, as unimportant, the fact that mean time and apparent time are not
coincident on December 8. The correction due to this cause is consi-
derable.
*2 LIGHT SCIENCE FOR LEISURE HOURS.
O
where the transit began several minutes late, and ended
several minutes early, we should have a transit lasting
for a shorter time than as seen from the earth's centre :
and then, comparing what was observed at such a
station with what was observed at Nertchinsk, we should
have Halley's method applied under effective and
favourable conditions. But the southern stations to
which England sends observing parties areKodriguez and
Christ Church (New Zealand) ; * and at the former station
the transit begins late and ends late, while at the latter it
begins early and ends early ; so that at neither is there
the combination of a late beginning and an early end-
ing, required for the effective application of Halley's
method.
Now there is a station a station which this country
ought unquestionably to occupy where the transit
would be even more shortened than it is lengthened at
Nertchinsk. This station is an Antarctic island on
which Sir James Eoss landed a party in 1846, and to
which he gave the name of Possession Island. It lies
due south of the southernmost extremity of New Zea-
land, close by the rugged shore-line of Victoria Land,
and within 18 degrees of the south pole. At this
station the transit will begin 6 minutes late and end
11-J- minutes early, or be shortened altogether no less
than 17^ minutes. Adding to this the lengthening of
the transit by 15^ minutes at Nertchinsk, we obtain a
1 There has been a change as to the station selected in New Zealand,
from Auckland to Christ Church. The change is in accordance with
my own suggestions, so far as the application of Delisle's method is
concerned.
THE COMING TRANSIT OF VENUS. 33
difference of duration of fully 33 minutes. Nothing
like this difference was available in the transit of
1769; nothing like it will be available in 1882. I
do not know the circumstances of the transits of
2004 and 2012, but it is altogether unlikely that the
opportunity of applying Halley's method will be so
favourable during either of these transits as in 1874. Be
that as it may, however, it is absolutely certain that
no opportunity equal to that which will be afforded
during the transit of 1874 will recur for one hundred
and thirty-two years, nor has such an opportunity been
ever before offered to astronomers. Absolutely the
best opportunity of applying Halley's ingenious method
which has ever been afforded, or will be afforded for
more than a century and a quarter, is available to astro-
nomers during the approaching transit. The duty of
seizing this opportunity belongs assuredly to our
country, which alone has colonial possessions close to
the station in question, and which alone also has sea-
men stilt living who have actually set their foot on
Possession Island.
I must confess that when, four years ago, I indicated
this opportunity, I thought that it would have been
seized at once. I thought that reconnoitring expedi-
tions would quickly have been prepared, and that by
the present time complete arrangements would have
been made for landing an observing party on Posses-
sion Island in due season for the required observations.
It would have been a matter of complete indifference
to me whether this had been done with or without
D
34 LIGHT SCIENCE FOR LEISURE HOURS.
acknowledgment of the source whence the suggestion
had come. But assuredly I hoped that some steps
would have been taken without delay to seize an
opportunity so important, the loss of which could not
but reflect some degree of discredit upon the science
of this country.
For up to that very time the spring of 1869 the
importance of an Antarctic expedition for observing
the transit of 1882 by Halley's method had been in-
sisted upon over and over again by leading astrono-
mical and geographical authorities. Nay, this very
station, Possession Island, had been selected as the
most suitable. The feasibility of reaching it and land-
ing on it had been insisted upon. The superior
meteorological chances presented by the station, as
compared with other southern stations, had been dwelt
on strongly. Everything promised that before long an
Antarctic reconnoitring expedition would set forth to
prepare the way. It was in the full height of these
anticipatory inquiries that I pointed out the inex-
pediency of any attempts to apply Halley's method at
an Antarctic station in 1882, dwelling earnestly on the
fact that when the transit began at Possession Island,
in 1882, the sun would be barely five degrees above
the horizon, an elevation utterly unfit for exact obser-
vations. Upon this all the plans for an Antarctic
expedition in 1882 were abandoned. But although
this was as it should be (for the lives of our seamen
are not to be endangered without the prospect of
valuable results), there was no necessity for abandon-
THE COMING TRANSIT OF VENUS. 35
ing all idea of an Antarctic expedition. The schemes
set afoot for observing the transit of 1882 should
have been transferred to the transit of 1874. Not a
single argument which had been argued in their favour
was wanting in the case of the latter transit. The
main argument was greatly strengthened ; for the
difference of duration in 1882 would only be twenty-
four minutes, if Possession Island were the selected
station ; whereas we have seen that in 1874, the corre-
sponding difference will be fully thirty-three minutes.
And the fatal objection to Possession Island as a sta-
tion in 1882, has no existence in the case of the transit
of 1874. Instead of the utterly insufficient solar ele-
vation of five degrees just mentioned, there will be, in
1874, a solar elevation of thirty-eight and a half
degrees when the transit begins, and of twenty-five
degrees when the transit ends. And necessarily all
the considerations which had been urged as to the
importance of Antarctic expeditions, per se, and
especially of the interest which would attach to the
experiences of a wintering party near the south pole of
the earth, remain unchanged.
While there is still a possibility of retrieving matters,
I would earnestly appeal to all who can assist in bring-
ing about such a result to spare no pains in the endea-
vour. I believe the scientific credit of this couutry to
be seriously imperilled. Hereafter, the very arguments
used in favour of the now abandoned scheme for ob-
serving the transit of 1882 from Possession Island, will
be urged, even as now (for a better purpose) I am
D 2
36 LIGHT SCIENCE FOR LEISURE HOURS.
urging them, to show that the importance of such
observations (if feasible) had not been overlooked. It
has been shown, and is now admitted, that they are
feasible in 1874. What, then, I ask, will be thought
of this country if the task which is her duty shall be
neglected ? It was sufficiently unfortunate that the
opportunity had been so long overlooked. But it will
be nothing less than a national calamity, if, having
been recognised in ample time to be employed, that
opportunity be altogether neglected.
Now, after four years' delay, time runs short indeed.
It is essential that any party intended to observe the
transit, should be landed before the Antarctic summer
of 1873-74 draws near its end certainly before the
middle of February 1874. There may not be time for
sending a suitably provided expedition from England.
On this point it is for others to speak. I should say,
however, that unquestionably there is time for sending
an expedition from Tasmania or New Zealand. It was
in fact proposed in 1868 by Captain Richards (Hydro-
grapher to the Admiralty) that New Zealand should be
made the head-quarters of the expedition then being
planned for observing the transit of 1882 from Posses-
sion Island. One can see no reason why this plan
should not now be resumed for securing the more
valuable observations which can be made during the
transit of 1874.
If we inquire what has been done towards preparing
for observations by Delisle's method, we shall see that
by a very slight modification of the Government
THE COMING TRANSIT OF VENUS. 37
arrangements, Possession Island might be taken as a
station without any great additional expense.
The transit begins earliest at a place in north lati-
tude 39 45', and west longitude 143 23'. Woahoo
has been selected as a suitable station near this spot ;
and in fact the transit begins more than 1 1 minutes
early at Woahoo, while the sun has an elevation at the
time of about 20 degrees. Nothing could be more
suitable than the station selected by England in this
neighbourhood. France takes the Marquesas, while
Russia has a station near the mouth of the Amoor
River.
The transit begins latest at a place in 44 27' south
latitude, and 26 27' east longitude. The best station
hereabout is Crozet Island, so far as astronomical con-
ditions are concerned ; but bad weather very commonly
prevails here. England will send an observing party
to Kerguelen's Land, and will also occupy the Mau-
ritius and Rodriguez Island, which are not so well
placed ; since the transit begins 12 J minutes late
at Crozet, 11 \ minutes late at Kerguelen, only 10 \
minutes late at Mauritius, and only 10 minutes late at
Rodriguez. The party at Mauritius will be that which
Lord Lindsay is preparing at his own expense ; and it
will be amply provided with all that is required for the
purposes of exact observation. Why should not the
Government expedition to Rodriguez be given up ?
Its cost will certainly not be well repaid, since the cir-
'cumstances of the transit at Mauritius and Rodriguez
are almost identical ; and if the money thus saved were
38 LIGHT SCIENCE FOR LEISURE HOURS.
devoted to an expedition to Possession Island, a good
step would have been made towards providing for the
cost of such an expedition.
The transit will end earliest at a place in south
latitude 64 47', and west longitude 114 37'. The
best station in this neighbourhood is that very place,
Possession Island, which affords the most favourable
opportunity for applying Halley's method. For at
Possession Island the transit will end 11J minutes
early. Next in value come several islands between
New Zealand and Victoria Land. It was originally
proposed to have an English observing party at Auck-
land or Wellington, New Zealand ; but the station
at present selected is Christ Church, where the
transit will end 9J minutes early. It is, in my
opinion, most unfortunate, that when Possession Island
affords the best station for the application of De-
lisle's method as well as Halley's, a station inferior
in both respects should be selected. Here again
expense might be saved which would go far towards
the preparation of an expedition (from New Zealand,
if need be) to winter in Possession Island.
Lastly, the transit will end latest at a place in north
latitude 62 5', and east longitude 48 22'. Here the
Eussians are in great force, as Orsk, Omsk, Tobolsk,
and other Russian towns are very suitably placed.
The selected station for an English observing party is
Alexandria, where the transit begins late by about 10
minutes. The sun will only be about 14 degrees high
at the time, and a greater elevation would be preferable.
THE COMING TRANSIT OF VENUS. 39
Amongst the mistakes pointed out by me in 1869 was
the complete omission of all notice of stations admir-
ably placed in Northern India for observing the
retarded end of the transit. Thus at Peshawur the
transit will begin 10^ minutes late, the sun having an
elevation of 31^ degrees. If Peshawur be not conve-
niently accessible, then Delhi and the country around
would serve nearly as well astronomically. I supposed,
until quite recently, that this suggestion, like the more
important one relating to Possession Island, would
receive no attention. But I was gratified a few weeks
ago, by hearing from the Astronomer Royal that my
discussion of the bubject had induced him to urge that
a station should be selected ' somewhere in the North
of India.' I may be permitted to add (since I do so
from no personal gratification, but to give a weight to
my present arguments, which otherwise they might
not possess) that in the same letter the Astronomer
Royal described my researches on the transit of Venus
as ; probably the best ' of all ' contributions from
Englishmen and foreigners.' Apart therefore from the
circumstance that though many have discussed my
researches not one astronomer has questioned the
accuracy of my chief conclusions, I have now the recog-
nition tardy indeed, but not the less sufficient
of the astronomer whose work I criticised. If I use
this as a lever to advance my present argument, it is
because I feel that the scientific credit of this country
is likely to be affected if England does not discharge
her duty in this matter. I am satisfied, moreover, that
4O LIGHT SCIENCE FOR LEISURE HOURS.
whereas the reputation of the eminent man of science
who stands at the head of the astronomy of this country
will in no degree be affected if the proposed expedition
be undertaken somewhat later than was desirable, it
will suffer seriously hereafter if that expedition should
not be undertaken at all.
Eraser's Magazine for March 1873.
THE EVER-WIDENING WORLD OF STARS.
As the science of astronomy has advanced, the ideas
men have formed respecting the extent of the universe
have gradually become more and more enlarged. In
far-off times, when astronomers were content to judge
of the conformation of the universe by the appearances
directly presented to their contemplation, the ideas
formed respecting the celestial bodies were singularly
homely. We read that Theophrastus looked upon the
Milky Way as the fastening of the stellar hemispheres,
which are ' so carelessly knitted together, that the fiery
heavens beyond them can be seen through the spaces.'
Anaximenes believed the heavens to be made of a kind
of fine earthenware, and that the stars are the heads of
nails driven through the domed vault formed of this
material. And even Lucretius, whose views of nature
were so noble, has referred without disapproval to the
bizarre theory of Xenophanes that the stars are fiery
clouds collected in the upper regions of air.
THE EVER-WIDENING WORLD OF STARS. 41
While the Ptolemaic system of astronomy was ac-
cepted there were no means of forming any trustworthy
views respecting the extent of the stellar universe. If
the earth were ever at rest we could never know how
far the stars are from us ; and therefore the old astrono-
mers were free to invent whatever theories they pleased
as to the scale on which the sidereal scheme is con-
structed. It was only when the earth was set free by
Copernicus from the imaginary chains which had been
conceived as holding it in the centre of the universe
that it became possible to form any conception of the
distances at which the stars lie from us. Indeed Tycho
Brahe immediately pointed this out as an overwhelm-
ing objection against the new theory. ' Are we to
suppose,' he argued, ' that the stars are placed at such
enormous distances from us as to seem wholly un-
changed in position while the earth sweeps round the
sun in an orbit millions of miles in diameter ? If this
amazing theory were true, the stars would be hundreds
of millions of miles from us, a view which is utterly
monstrous and incredible.'
But strange as this new view appeared, it gradually
gained ground. It became presently so well established
that when Cassini discovered that the earth travels in
a much wider orbit than Tycho Brahe had supposed
so that the stars were at once thrown many hundreds
of millions of miles farther from us astronomers still
held to the new order of things. 'With Briarean
arms,' as Humboldt has described their labours, the
fellow-workers of Cassini thrust farther and farther
42 LIGHT SCIENCE FOR LEISURE HOURS.
away the ' heaven of the fixed stars,' until the immensity
of the universe grew so great beneath their labours,
that new modes of expressing its dimensions had to be
adopted. They were not satisfied with the obvious
circumstance that the stars seem to remain unchanged
in position as the earth sweeps round the sun. They
tested this apparent fixity of position with instruments
of greater and greater power, yet always with the
same result. They made observations ten, twenty,
even fifty times more exact than Tycho Brahe's, and
the fact that they still detected no change of position
signified nothing less than the universe of the fixed
stars is ten, twenty, even fifty times farther from us
than Tycho Bralie had imagined.
Thus when Sir W. Herschel began the noble series
of researches amid the stellar depths which has rendered
his name illustrious, the world of stars was already
of inconceivably enormous extent. Yet so widely did
he increase our appreciation of the vastness of the
universe, that it has been thought no exaggeration to
say of him, that ' he broke through the barriers of the
heavens : ' ' Caelorum perrupit claustra,' says his monu-
ment at Upton, and every student of astronomy who
has carefully examined Herschel's labours understands
the justice of the expression. For consider what
Herschel did. When he began his survey of the
heavens, astronomers had proved indeed that the nearest
of the fixed stars lie at enormous distances from us, and
some of the more advanced thinkers had begun to form
noble speculations respecting the relations of the stars
THE EVER- WIDENING WORLD OF STARS. 43
which lie beyond the sphere of those visible to us.
But it was reserved for Sir W. Herschel to apply exact
observations to the unseen star- systems. He literally
gauged the celestial depths. With a telescope whose
light-gathering power extended the range of vision to
about eight hundred times its natural limit, he swept
the whole of the northern heavens. He estimated the
depth of the system of stars in every direction by a
simple and natural process. For, like all great thinkers,
he struck out modes of inquiry which, the moment they
were presented to the world, seemed so obvious, that
the wonder was how they could have remained so long
undetected. He said that precisely as the quantity of
water passed through by the sailor's lead-line marks
the depth of the sea, so the number of stars which can
be seen when a telescope of given power is turned
towards any part of the heavens is a measure of the
depth of the sidereal system in that direction. In in-
dividual cases, indeed, the law may not be true, just as
the sailor's lead-line may light on the peak of some
sunken rock, and so give no true measure of the general
depth of the sea in the neighbourhood. But when the
average of a great number of such ' star-gaugings ' is
taken, then we may feel tolerably certain that on
applying the simple rule devised by Herschel we shall
form no inaccurate estimates of our system's extent in
any direction.
Thence arose his great theoiy of the stellar system.
He showed that our sun is but one of an immense
number of suns, distributed in a region of space resem-
44 LIGHT SCIENCE FOR LEISURE HOURS.
bling a cloven disc in figure. When we look along
the thickness of the disc we see the enormous beds of
stars, which lie round us in that direction as a cloud
of milky light, which so comes to form a cloven ring
round the heavens. But when we look out towards
the sides of the disc, where the stars are less profusely
scattered, we see between them the black background
of the sky.
Then Herschel extended his researches to those
strange objects called the nebulae. He showed that
where astronomers had reckoned tens of these objects
there were in reality thousands. And he found that a
large proportion of the nebulae can be resolved into
stars. He held that these, therefore, may be looked
upon as external universes, resembling that great system
of stars of which our sun is a member. We need not,
at this point, dwell upon the distinction which Herschel
drew between nebulas of this sort, and those objects
which he held (and as we now know, justly) to be true
clouds, formed of some vaporous substance, of the
actual nature of which he forbore to express an opinion.
Let it suffice to remark that in whatever mode those
vaporous nebulae might be supposed to be formed, it
was clear to Herschel that they cannot be held to lie
necessarily beyond the system of the fixed stars, as he
held to be certainly the case with the stellar nebulae.
Since Herschel's day a multitude of important dis-
coveries have been made. His son, the present Sir
John Herschel, carried the system of star-gaugings over
the southern heavens, having first trained himself for
THE EVER-WIDENING WORLD OF STARS. 45
the work by verifying Sir William's northern star-gaug-
ings. The eminent astronomer Struve and others have
applied a series of tests to the basis of Herschel's theory
of the universe. Increased telescopic power has been
applied to the examination of the nebulae. And lastly,
a mode of research more wonderful than the boldest
pioneers of science had ventured to hope for has been
applied to determine what the stars and nebulae really
are, nay even the very elements of which they are con-
stituted.
Therefore we stand in a position so far in advance of
that to which it was in Herschel's power to attain, that
the attempt to modify his theories need no longer be
thought to savour of undue boldness. Half a century
does not pass without bringing a vast extension of
knowledge, and certainly the last half-century has been
no exception to this rule ; insomuch that could the
great astronomer take his place again among us, and
become cognisant of the vast strides which his favourite
science has made since he left us, he would be the first
to point out that many of his views require to be
modified or even to be wholly abandoned.
For instance, let us consider the meaning of the
following observation made by the younger Herschel.
While 'sweeping' the southern heavens, this eminent
astronomer noticed occasionally the existence of faint
outlying streamers belonging to the Milky Way, not
only irresolvable into stars, but so exceedingly distant
that he could scarcely speak of them as really visible.
He was sensible of their existence, but when the eye
46 LIGHT SCIENCE FOR LEISURE HOURS.
was turned directly upon them they vanished, insomuch
that, he says, ' the idea of illusion has repeatedly arisen
subsequently,' yet when he came to map down the
places where these phantom star-streams had been
detected, he found that they formed regular branches
of the galactic system.
Now these outlying star-streams prove first of all
that the star-system is not disc-shaped, but spiral in
figure. Between the stars which form the ordinary
streams of the Milky Way, and those which form the
phantom streams, there must lie regions in which stars
are either altogether wanting or strewn with much
less profusion than in either the nearer or the farther
stream.
But this is not the only nor the chief conclusion
which may be drawn from the existence of the almost
evanescent star-streams. According to Herschel's views
the stars which compose those streams are only faint
through enormity of distance. They may be as large
as our sun, many of them perhaps far larger. And
between them there may yawn distances as large as
those which separate us from Arcturus or Aldebaran.
Now, this being so the outlying parts of our own
sidereal system being removed so far from us as to be
all but evanescent in Herschel's splendid reflector
how much greater ought to be the faintness of the
sidereal systems which lie outside ours ! If the nebulae
are really such systems, and made up of suns like our
own, then not only ought Herschel's great reflector to
fail in rendering them visible, but even Lord Eosse's
THE EVER-WIDENING WORLD OF STARS. 47
noble mirror would require to be increasad a hundred-
fold in power before we could see them. For clearly
the nebulae, which appear as mere tiny specks upon the
vault of heaven, must be very much farther away than
the confines of our system, if they are comparable with
it in size.
Therefore we must have ' of two things one.' Either
the confines of our sidereal system are constituted very
differently from the parts in our neighbourhood; or
the nebulae are constituted very differently from the
sidereal system. We say, of two things one, meaning
that one of the two views must be true ; but it is plain
that there is nothing to prevent both being true.
We may next come to the inquiry whether these
views are severally supported by any special evidence.
Now as to the first, it happens that the southern
heavens surveyed by the younger Herschel afford
evidence such as Sir William Herschel was not pos-
sessed of. The former has seen places in the southern
skies where the outline of the Milky Way is so sharply
defined that even in the telescope the sudden change
from a background of black sky to the sprinkled light
of the galaxy is not lost. One half of the field of view
will exhibit the former aspect, the other the latter.
Now if we consider a cloud, or a dense flight of birds,
or any cluster of objects exhibiting a well-defined out-
line, we see at once what that well-defined outline
means. It signifies that the eye is directed along the
'edge or surface of a distinct cluster of objects in one
case globules of water, in another birds, and so on
48 LIGHT SCIENCE FOR LEISURE HOURS.
and the idea is at once precluded that the eye is within
the cluster, of whatever nature that cluster may be.
Therefore the theory that the sun forms one of a system
of stars spread pretty uniformly over a disc-shaped
space must be given up; for were it true, the ap-
proach to the Milky Way would always be gradual.
When we add that in the southern skies the Milky
Way presents the most fantastic configuration, here
expanding into fan-shaped masses, there winding about
in a multitude of strange convolutions, here suddenly
narrowing into a bright neck or isthmus, there exhi-
biting a nearly circular vacancy, it becomes clear that
the galaxy cannot have the figure assigned to it by Sir
W. Herschel. It must consist of streams and sprays
of stars at different distances. Such streams by their
fantastic convolutions serve to explain all the pecu-
liarities of the galaxy's structure.
And next, have we any evidence that the nebulae
are not really beyond the galaxy, but are mixed up
with the sidereal system ? It appears to me that we
have.
Sir William Herschel noticed that there are places
where the nebulae are much more densely crowded than
elsewhere, and he was disposed to suspect that pre-
cisely as the stars by their aggregation form the zone
of the Milky Way, so there is a zone of nebulas. But
when Sir John Herschel had completed the survey of
the heavens it was found that a very different law of
distribution made its appearance. Instead of being
collected in a zone or band around the heavens, the
THE EVER-WIDENING WORLD OF STARS. 49
nebulae are arranged in two distinct hut irregular
clusters, separated by a well-marked zone almost en-
tirely free from nebulae. And this zone coincides
almost exactly with the Milky Way.
What are we to understand by so special an arrange-
ment as this ? A modern astronomer says it clearly
proves that the nebulae do not belong to the star-
world ; but I can see no escape from an exactly oppo-
site view. A simple illustration will serve to exhibit
the nature of the case. Suppose a person found a
space of ground on which gravel was arranged in the
form of a ring, and that rough stones were thickly
spread over the whole space except the gravel ring,
would he conclude that there was no association be-
tween the arrangement of the gravel and the arrange-
ment of the stones, because few stones were to be
found on or near the gravel ? Would he not rather
find in this peculiarity distinct evidence that there
was some association ? He would, we think, argue
that the gravel had been collected into one place and
the stones into another, in pursuance of some one par-
ticular scheme. The corresponding conclusion in the
case of the stars and nebulae would clearly be that the
stars had been drawn together in one direction and the
nebulae in another, out of a common world of cosmical
matter.. In other words we should look on the nebulae
as members of the same system or scheme that the
stars belong to.
And here it may be asked how the conclusion thus
deduced from the arrangement of stars and nebulae can
50 LIGHT SCIENCE FOR LEISURE HOURS.
be said to tend to enlarge our views of the world of
stars. On the contrary, it might "he urged, the views
which had prevailed before, presented us with nobler
conceptions of the universe. For we were able to
recognise in the thousands of nebulae which fleck the
dark background of the sky, sidereal systems as noble
as that of which our sun is a member ; and in the
existence of countless star-systems we had a spectacle
to contemplate before which the human intellect was
compelled to bow in its utter powerlessness and insig-
nificance : whereas it seems as though the new views
would reduce the scope of our vision to a single
galaxy of stars, unless some few members of the nebular
system may still be looked on as outer star-schemes.
But on a closer inspection of the views I have been
maintaining, it will appear that they largely enhance
our conceptions of the scale on which the world of
stars is constructed. Until now it has been held that
the telescopes which man has been able to construct
enabled us to scan the limits of our sidereal system,
and to pass so readily beyond those limits as to become
sensible of the existence of thousands of other schemes
as noble as our own or nobler. But if the new views
should be established, we should be compelled to recog-
nise in the world of stars a system which our most
powerful instruments are not fully able to gauge. The
clusters of stars, whose splendour has so worthily ex-
cited the admiration of the Herschels, the Rosses, the
Struves, and the Bonds, must be looked upon as among
the glories of our own system, and indicative of the
THE EVER-WIDENING WORLD OF STARS. 51
multiplied forms of structure or of aggregation to be
found within its boundaries. As of late our concep-
tions of the wealth of the solar system have been
enhanced by the discovery of numberless new objects
and new forms of matter existing within its range,
and co-ordinating themselves in regular relations with
the earlier known members of the system, so we seem
now called on to recognise in the stellar world an un-
suspected wealth of material, a hitherto unrecognised
variety of cosmical forms, and an extension into regions
of space to which our most powerful telescopes have
not yet been able to penetrate.
But now I would call attention to a peculiarity of
the southern skies which, while apparently affording
conclusive testimony in favour of the new views, has
unaccountably (in my opinion) been urged as an argu-
ment tending in quite another direction. There are
to be seen in those skies two mysterious clouds of light,
which were called by the first Europeans who sailed
the southern seas the Magellanic clouds, and are now
commonly spoken of by astronomers as the Nubeculge.
Examined by the powerful telescope of Sir John Her-
schel, these objects have been found to consist of small
fixed stars and nebulae, grouped together without any
evidence of special arrangement, but still obviously
intermixed, not merely seen projected on the same
field of view.
These strange objects have given rise to many specu-
lations ; and among the definite views put forward
respecting them is one recently expressed in a most
E 2
52 LIGHT SCIENCE FOR LEISURE HOURS.
valuable communication to the Royal Astronomical
Society from the pen of Mr. Cleveland Abbe, an
astronomer who has laboured in the sound school of the
Poulkowa Observatory. Having recognised in the
peculiar arrangement of stars and nebulae above re-
ferred to, an argument that the nebulae lie beyond our
system, Mr. Abbe suggests that the Magellanic clouds
are two of the nearest of the nebular systems, which
thus exhibit larger dimensions than their fellow-
schemes.
The converse of this, which may be termed the positive
theory of the Nubeculae, is the hypothesis which may be
termed the negative theory. Whatever these objects
may be, astronomers have said, they are quite distinct
from the sidereal system, nor are the nebulae seen
within them to be looked upon as fellows of the other
nebulae. For in the Nubeculae we see what we recog-
nise nowhere else, the combination namely of clustering
groups of stars and freely scattered nebulae. It is the
characteristic (still I am quoting the theory) of the
sidereal system that where its splendours are greatest
nebulae are wanting ; it is the characteristic of nebular
aggregation that it withdraws itself in appearance from
the neighbourhood of clustering star groups. But in
the Magellanic clouds neither of these characteristics is
to be recognised ; therefore these objects are distinct
from either system.
Nor has another argument been wanting to indicate
the distinction that exists between the Magellanic clouds
and the other splendours of the celestial vault. Sir
THE EVER-WIDENING WORLD OF STARS. 53
John Herschel, sweeping over their neighbourhood with
his 18-inch reflector, was struck with the singular bar-
renness of the skies around them. With that expres-
sive verbiage which gives so great a charm to his astro-
nomical descriptions, he forces on our attention, again
and again, the poverty of the regions which lie around
the Nubeculse. ' Oppressively barren ' he describes
them in one place ; ' the access to the Nubeculse on all
sides is through a desert,' he says in another. And this
peculiarity, thus established by the certain evidence of
an observer so able and trustworthy, has been held by
many to imply in the clearest and most distinct manner
that there is no connection between the Nubeculae and
the stellar system.
To me the evidence afforded by the barrenness of the
regions round the Magellanic clouds points irresistibly
in the opposite direction. Why should some outer
system, free as is assumed of all association with our
own, occupy that peculiarly barren space which so at-
tracted the attention of Sir. John Herschel ? But if
we look on the coincidence as striking in the case of
one, how much more remarkable will it appear when
the only two outer systems of the sort, thus brought
within our ken, are associated in this way with the most
singularly barren region in the whole heavens ! Surely
the more natural conclusion to be drawn from the
phenomenon is that the richness of the Magellanic
clouds and the poverty of the surrounding districts
'stand to each other in the most intimate correlation.
Is there not reason for concluding that those districts
54 LIGHT SCIENCE FOR LEISURE HOURS.
are poor because of the action of the same process of
aggregation which has attracted within the Nubeculse
a larger share than usual of stellar and nebular glories ? l
It need hardly be mentioned that the former argu-
ment, on which the distinction between the Nubeculse
and other celestial objects has been founded, is disposed
of at once if we recognise the stellar and nebular
systems as in reality forming but a single scheme. Not
only so, but the Nubeculse afford a striking argument
in favour of the latter view. To return to the somewhat
homely illustration made use of above. Our conceptions
of the original association between the stones and the
gravel arranged in the manner indicated would certainly
be strengthened, or would even be changed into abso-
lute certainty, if we perceived in a part of the ground
two heaps in which stones and gravel were intermixed.
When I add that there are two distinctly marked
nebular streams leading towards the Nubeculse, as well
as several well-marked star-streams tending in the
same direction, the evidence of association seems greatly
strengthened.
If these views be accepted, we shall have to look upon
the world of stars as made up of all classes of clustering
aggregations, besides strange wisps and sprays extend-
ing throughout space in the most fantastic convolutions.
Then also, while dismissing the idea that the nebulae as
1 Sir William Herschel has recorded a peculiarity respecting nebulae
which is worthy of mention in connection with the facts above consi-
dered. ' I have found,' he says, ' that the spaces preceding nebulse were
generally quite deprived of stars, so as often to afford many fields
without a single star.'
THE EVER-WIDENING WORLD OF STARS. 55
a class are external systems, we may accept as highly
probable the conclusion that some of the spiral or whirl-
pool nebulae really lie far beyond the confines of our
system. For we see in these objects the very picture
of what the new views show our sidereal system to be.
There are the spiral whorls corresponding to the double
ring of the Milky Way ; there, are faint outlying
streamers corresponding to the phantom star-streams
traced by Sir John Herschel; there also, are bright
single stars and miniature clusters, nay, there also, may
even be recognised large knots or lobes of clustering stars,
forming no inapt analogue of the Magellanic clouds.
Eraser's Magazine for July 1869.
MOVEMENTS IN THE STAR-DEPTHS.
AMONG the many striking contrasts between the seeming
and the real suggested by the study of astronomy, there
is none more startling than the contrast which exists
between the apparent repose of the heavens and what is
really taking place among the star-depths. On a calm
clear night
When all the winds are laid,
And every height comes out, and jutting peak
And valley, and the immeasurable heavens
Break open to their highest
the stars seem set as emblems of eternal fixity and rest.
As such they have been regarded in all ages by the poet ;
nor has science, so far as it has been directed to the
56 LIGHT SCIENCE FOR LEISURE HOURS.
apparent movements of the stars, taught any other
lesson. It has, indeed, shown that the stars are even
more steadfast than they seem, in so far as it teaches
that their diurnal and annual motions are but apparent,
while the great precessional swaying of the star-sphere
is but the reflexion of the earth's gyration. More and
more just, so far as these motions are concerned, has
appeared the title of ' the fixed stars,' assigned by
astronomers to the suns which people space.
Yet the depths displayed to our view in the stillness of
the calmest and clearest night are, in reality, astir with
the most stupendous activity. The least of the orbs we
see some star so faint that it is only discerned by
momentary gleams is the abode of forces whose action
during a single instant surpasses in effect all the forces
at work upon the earth during a decade of years. All
the wonderful processes taking place within and around
the globe of our own sun have their analogues in that
distant orb. Let it be remembered also that our sun
himself presents an aspect which in no sense suggests
his real condition. If we would picture him as he
actually is, we must consider the uproar and tumult
which prevail where, to our ordinary perceptions, all
seems at perfect rest. The least movement on that
glowing photosphere represents the action of forces so
tremendous that they would be competent to destroy
in an instant this eaxth on which we live. The most
hideous turmoil, outvying a million-fold the roar of the
hurricane or the crash of the thunderbolt, must prevail
for ever in every part of the solar atmosphere. And in
MOVEMENTS IN THE STAR-DEPTHS. 57
whatever respects other suns may differ from our own,
in this at least we know that they resemble him. It is
the very charter of their existence as suns as real
living centres of energy to schemes of circling worlds
that they should thus continually pulsate with their
own vitality. Each is the central engine on whose
internal motions the life of a system of worlds depends
and each must, with persistent activity, work out its
purpose, until the fuel which supplies its forces shall be
exhausted.
All the evidence as yet obtained points to the conclu-
sion that our own sun, wonderful as is his structure and
stupendous his energy, is yet very far inferior in splen-
dour and power to most of his fellow suns. PJaced where
Sirius is, the sun would appear but as a third-rate star,
less bright than hundreds of the stars visible to the
unaided eye. But removed to the distance of Alde-
baran, or Castor, or Betelgeux, our sun would certainly
not shine more brightly than the fourth-magnitude
stars, while it is probable that his lustre would be so
reduced that he would be barely discernible. There
can be little doubt that of all the stars seen on the
clearest and darkest night, there are scarce fifty which
are not far larger suns than ours, and consequently the
scene of more tremendous processes of change.
But when we turn from the consideration of the
energy and vitality of individual stars to inquire into
the movements taking place within the star system, we
are yet more startlingly impressed by the contrast be-
tween the apparent rest prevailing in the star-depths
58 LIGHT SCIENCE FOR LEISURE HOURS.
and the inconceivable activity really present there. It
seems incredible that all those orbs which look so still
are speeding through space with a velocity compared
with which every form of motion familiar to us on
earth must be regarded as almost absolute rest. This
appears even more surprising when we consider that
during all those centuries with which history deals,
during the rise and fall of the nations of antiquity,
during the darkness of the Middle Ages, during the
more familiar scenes of recent centuries, the stars
have presented an aspect so constant that if the Chal-
dsean astronomers could be restored to life, they would
recognise scarce any change in the positions of the stars
forming the ancient constellations. Yet there are no
astronomical facts more thoroughly established than
those which relate to the motions of the stars. The
giant orb of Sirius, exceeding our sun a thousand times
in volume, Capella and Procyon, the glories of Orion,
the clustered Pleiads, Arcturus, Vega, and Aldebaran,
all the stars known to the astronomer, are urging their
way with inconceivable velocity, each on its own course,
though doubtless all these motions are subordinated to
some as yet unexplained system of movements whereby
all the stars of the galaxy are made to form parts of
one harmonious whole.
Until lately it had only been by one method of
observation that the astronomer could assure himself
that these motions were taking place. That method is
the simplest conceivable. If a star's place were accu-
rately determined, either with respect to ' neighbouring
MOVEMENTS IN THE STAR-DEPTHS. 59
stars or to the imaginary circles and points on the
sphere which are determined by the earth's movements
of rotation and revolution, then, if the star be really
in motion, a change of place must in the long run
manifest itself, not indeed to ordinary vision, but to
the piercing scrutiny and to the yet more remarkable
measuring powers of the astronomical telescope. A
hundred years may elapse before the motion is measure-
able, yet the astronomer can none the less certainly
assure himself that the motion is taking place, since he
has the records of those who have gone before him, and
the means of satisfying himself that those records are
trustworthy.
It had long been felt, however, that there was an
unfortunate gap in the evidence respecting stellar
motions. The astronomer could tell how much or how
little the stars were shifting on the heavens, but he
could obtain no measure whatever of other motions
which nevertheless must exist among the stars. If a
star were receding or approaching, no trace whatever
of such motion could be recognised. No instrumental
means could enable the astronomer to measure the
change of brightness due to the star's change of dis-
tance, since such changes must needs be infinitely small
compared with the actual lustre of the star.
So that it seemed as though the astronomer must for
ever remain ignorant of one most important portion of
the stellar motions. All he could do, as it appeared,
was to watch the aspect of the heavens, and, as it slowly
changed, to infer in what way the stars were moving
60 LIGHT SCIENCE FOR LEISURE HOURS.
athwart the line of vision ; and even this he could only
do most imperfectly, since his knowledge of the dis-
tances of the stars is so limited that he can form but
inexact notions of the rate at which the stars are so
moving. They may be very far away and moving very
swiftly, or they may be at a less (though still enormous)
distance and moving with a correspondingly reduced
velocity.
This source of difficulty was very strikingly illustrated
when the subject of the stellar motions was treated in
connection with the ideas respecting the sidereal uni-
verse promulgated by Sir W. Herschel. In the hy-
pothesis which regarded the stars as spread with a
certain general uniformity through a stratum or slice
of space, there was no feature which afforded any pro-
mise that by the study of the stellar motions the
mysteries of the sidereal universe might be interpreted.
The very basis of Sir W. Herschel's own researches into
the subject is the vague supposition that it is as likely
a, priori that any given star will move in one direction
as in another. Later we find Struve presenting his
results in the following form : 6 One may wager four
hundred thousand to one that a portion of the seeming
motions of the stars is due to the sun's motion, and it
is an even wager (on pent parier un contre uri) that the
latter motion takes place at the rate of between 1 35 and
175 millions of miles per annum.' The whole question
had become one of probabilities, based on more or less
trustworthy assumptions. We cannot wonder greatly
that, when Sir Gr. Airy undertook the complete re-
MOVEMENTS IN THE STAR-DEPTHS. 6 1
examination of the matter twenty years ago, the result
he obtained, while indicating the general probability
of the inferences before obtained, nevertheless exhibited
the whole problem as one needing further investigation. 1
It will be seen presently that we cannot too atten-
tively regard those earlier researches, if we would fully
estimate the importance of the results which have
recently been obtained. Let it be carefully noticed
that the earlier results flowed directly from the hypo-
thesis respecting the stars which have so long main-
tained their ground in our text-books of astronomy
If these hypotheses are sound, the results flowing from
them, even though only based on the general principles
of probability as applied to those hypotheses, might be
expected to be somewhat near the truth. If, on the
contrary, an independent and trustworthy series of
results should show that those earlier results are not
correct are indeed very far from correctness then
pro tanto the hypotheses which led to those earlier
results would be invalidated.
Let it then be clearly understood that, according to
the results in question, the stars were held to be in
motion at rates comparable in general with the velocity
of our sun, this velocity being estimated at about four
and three-quarter miles per second. We do not include
here the result that the sun is moving towards Hercules,
because that may be regarded as established, whatever
1 This part of my subject is fully discussed in a paper called ' The
Sun's Journey through Space,' which appeared in Fraser's Magazine for
September 1869, and will be found among my ' Essays on Astronomy.'
62 LIGHT SCIENCE FOR LEISURE HOURS.
opinion we may form as to the distribution of the stars
in space.
Before proceeding to indicate the bearing of recent
observations on these theoretical conclusions, I would
invite some t degree of attention to the circumstance
that the view I am here advancing as to the bearing of
new facts on the old hypotheses, is not a new one
framed to account for the new facts in a way agreeing
with my own theories respecting the stars. More than
three years ago in Fraser's Magazine, and earlier still
in the proceedings of scientific societies, I indicated my
belief that the real facts are precisely such as have now
been demonstrated.
Already when I so wrote, promise had been afforded
that the astronomer might come in time to know, 1 not
merely whether certain stars are approaching or reced-
ing, but at what rate (in miles per second) these
motions are taking place. I need not here enter into
an explanation of the method by which this was to be
accomplished, inasmuch as a full account of the prin-
ciple on which the method is based is given in the
paper called c News from Sirius,' in my Essays on Astro-
nomy. Suffice it to say, that it depends on the observed
displacement of some known dark line in the rainbow-
tinted streak forming the spectrum of a star, and that
when such a line is displaced towards the red end of the
spectrum it is known that the star is receding, while
1 See the closing words of the last paragraph but three in the essay
mentioned.
MOVEMENTS IN THE STAR-DEPTHS. 63
when the displacement is towards the violet end it is
known that the star is approaching.
Dr. Huggins, our great spectroscopist, had success-
fully applied this method to the star Sirius, and he had
found that that star is receding from the earth at the
rate of upwards of twenty-five miles per second. But
Sirius was the only star which could then be examined
by this method. The light of Sirius exceeds more
than five times that of the next star in order of bright-
ness, at least of those visible in our hemisphere ; and
with the instrument then at Dr. Huggins' disposal (his
own eight-inch refractor) it was found impossible to see
the dark lines of any other star-spectrum with a spec-
troscope dispersive enough to give any measurable
displacement of the lines.
But the importance of the inquiry (as well as of
those other spectroscopic researches in which Dr. Hug-
gins had been so successful) was manifest to our
scientific societies ; and accordingly a large sum was
granted by the Eoyal Society for the construction of a
refracting telescope, fifteen inches in aperture, to enable
Dr. Huggins to extend his researches to the leading
stars of our northern heavens. This fine instrument
was ready for use in the spring of this year, and before
many weeks had passed Dr. Huggins had obtained
results of surpassing interest and importance. He had
recognised motions of recession and approach in no less
than thirty stars, and had traced laws before unknown
in the phenomena of these stellar motions.
One of the most striking features in the series of
64 LIGHT SCIENCE FOR LEISURE HOURS.
star-motions observed and measured by Dr. Hugging,
is the amazing velocity with which some of the stars
are moving. Astronomers had ascertained that Sirius
is moving athwart the line of vision much more rapidly
than the sun is travelling through space. But Sirius
is so exceptional both in his brightness and in his esti-
mated bulk, that his enormous velocity did not appeal-
altogether surprising. It did not lead the generality
of astronomers to consider that the sun's velocity and
the average velocity of the stars had been greatly under-
estimated. But now we learn from a method of research
which is far more trustworthy than any applied to the
measurement of thwart motions, that some of the stars
are moving from or towards the earth with a velocity
far exceeding that of Sirius. If we take the thwart
motion of Sirius at twenty-five miles per second, and
his motion of recession at twenty miles (this being the
value assigned by the latest and best measurements),
we find for this absolute motion the amazing velocity
of about thirty-two miles per second. But Dr. Huggins
finds that Arcturus is receding from the sun at the rate
of 55 miles per second, Vega at the rate of about 50
miles, Arided (the chief brilliant of the Swan) at the
rate of 39 miles, Pollux 49 miles, and Dubhe of the
Great Bear at the rate of fr6m 46 to 60 miles per
second. Beside such motions as these, our sun's esti-
mated velocity of about 4| miles per second, which had
seemed so imposing when it was considered that he
bore with him at this enormous rate his whole family
of planets, sinks into relative insignificance. We here
MOVEMENTS IN THE STAR-DEPTHS. 65
recognise stellar rates of motion nearly equalling that
at which our earth circuits around the sun. But a
velocity which, considered with reference to a minute
orb like the earth, is intelligible, becomes altogether
startling in the case of orbs like Arcturus and Vega,
which undoubtedly exceed our own sun many times in
volume. I use the word ' intelligible ' with a purpose ;
for I am not considering here what is conceivable or
the reverse. We can in reality understand why the
earth should be possessed of the velocity she actually
displays. We know that the sun's attraction is com-
petent to generate such a velocity, or a much greater
velocity. But in the case of a star these swift motions
cannot be thus explained. The stars are too far apart
to be so influenced by their mutual attractions that
great velocities would be generated. And thus the
thoughtful mind cannot but recognise in the stellar
motions a subject of contemplation far more impressive
than the subordinate, though even swifter motions of
the Earth, Venus, or Mercury. Whence sprang that
amazing energy which is represented by the proper
motions of the suns ? If we admit the possibility that
forces of eruption or expulsion could account for the
observed motions, we shall have to answer the startling
question, Of what order are the orbs whence the giant
suns are expelled ? and the yet more difficult questions,
Where are these orbs? and, How is it that, inordinately
large though they must be, we are yet unable to distin-
guish them from ordinary suns ? If, on the other
hand, we prefer to regard the stellar velocities as gene-
F
66 LIGHT SCIENCE FOR LEISURE HOURS.
rated by the attractions of larger orders of bodies than
the stars (as planetary velocities may be regarded as
generated by their parent suns), we still have the last
two questions to answer; and, so far as can be judged,
these questions are at present unanswerable. 1
AE other striking feature in the results announced by
Dr. Huggins is the absence of any systematic agree-
ment between the stellar motions he has recognised,
and the motion of our sun towards Hercules. It is
manifest that if our sun were alone in motion, the
actual rates of approach and recession of all the stars
in the heavens would be at once determined when the
rate of the sun's motion was determined. If, for ex-
ample, he were moving at the rate of twenty miles per
second towards the star Lambda of Hercules, he would
be approaching every star lying in that direction at
1 In passing, however, I would venture to touch on this question of
central suns, or of central but opaque orbs round which the stars may
revolve, in order to remove a very prevalent misconception. It seems
to be commonly supposed that we cannot imagine such orbs to lie far
enough away to account for their not being discernible either as orbs of
light or by hiding more distant stars, without depriving them of the
attractive influence necessary to sway the motions of the stars. This,
however, is not the case. An orb looking as bright as Sirius, but ten
times as far away, if of equal density and inherent brightness, would be
a thousand times more massive, while the effect of distance would only
be to reduce its attraction one hundred times. It would, therefore,
attract our sun ten times as strongly as Sirius actually does. In like
manner, an orb one hundred times as far away as Sirius, but so large
as to appear as bright, would attract our sun one hundred times as
strongly, and so on. So that it cannot be positively asserted that
among the stars visible to us there may not be the central sun of the
sidereal scheme inordinately large and massive compared with the
rest, but reduced by di. stance to the same order of brightness.
MOVEMENTS IN THE STAR-DEPTHS. 67
the same rate ; he would be receding from all stars
lying in the opposite direction at the same rate ; and
he would be approaching or receding from stars lying
in opposite directions at a less rate (readily calculable).
A certain half of the heavens would contain all the
stars which the sun was approaching ; the other half
would contain all the stars from which he was receding ;
and the circle separating these halves would mark the
place of stars which the sun was neither receding from
nor approaching. But nothing of this sort can be
recognised in the observed stellar rates of approach
and recession. Sirius (which lies nearly opposite to
Hercules) is receding at the rate of about 20 miles per
second ; but Vega (which lies close to Hercules), instead
of approaching at about the same rate, is actually
approaching at the rate of about 50 miles per second.
Castor, which is very near the border line between the
two hemispheres just mentioned, and should therefore
neither be approaching nor receding, is in fact reced-
ing at the rate of about 25 miles per second ; while
Pollux, though similarly placed, is approaching the sun
at the rate of about 49 miles per second. Again, of the
seven bright stars forming Charles's Wain, six are
approaching (five of them at the rate of about 20 miles
per second), while the seventh is receding at a rate
probably exceeding 50 miles per second.
Thus we see that the sun cannot be regarded as an
orb moving within the scheme of stars, and by his own
movement causing the chief apparent motions of the
surrounding orbs. His motion is but part of a grand
F 2
68 LIGHT SCIENCE FOR LEISURE HOURS.
scheme of motions, whose laws are as yet unknown to
us. We may recognise in the method of research which
has now been so successfully applied, the sole means of
determining what those laws may be. We can now tell
the very rate, in miles per annum, at which the suns
are approaching or receding from us ; and though we
have no reason for believing that our sun occupies in
any sense a central position so that we have yet to
learn at what rate and in what way the stars move
around the true centre of their system, yet it is far
from unlikely that if we can but ascertain the motions
of a sufficient number of stars, we shall have the
means of judging where the centre lies round which
these motions are taking place.
The astronomer may well look with doubt, however,
on the efforts which are being made to solve this stu-
pendous problem. If we may judge from the analogy
of our own solar system, we can see that in the far
more complicated scheme of the stars there must exist
innumerable features to perplex the observer. If we
imagine a being placed in the midst of the solar system,
and enabled to study the various apparent motions
visible from his stand-point, and if we further suppose
him gifted with the power of measuring the rate at
which the various orbs are approaching him or receding
from him, then we know that if his scrutiny were but
continued long enough, he could not fail to recognise
the laws which exist within that system and regulate
all those motions. Where at first all had seemed con-
fusion, our imaginary observer would recognise in the
MOVEMENTS IN THE STAR-DEPTHS. 69
course of time a beautiful harmony; motions which had
appeared discordant would be found to be in reality
subordinated into one grand scheme. But if we suppose
our observer to occupy his imaginary stand-point for a
few hours, or even for a few days only, how imperfect
would be his ideas of the harmony of the celestial mo-
tions! He would see the primary planets moving
apparently in diverse directions and at inconsistent
rates ; the secondary planets apparently travelling with
non-accordant motions and on different paths; the
asteroids would perplex him by their wide range of
apparent distribution ; meteoric systems would appear
to conform to no recognisable law ; and the movements
of comets would seem altogether inexplicable.
Yet the terrestrial observer of the infinitely more
complicated sidereal system is in reality even less
favourably circumstanced than our imaginary observer
of the planetary scheme. The motions which come
within his ken are more minute, compared with the
real dimensions of the stellar paths, than the motion of
Saturn or Jupiter in a single second compared with the
wide orbits traversed by these planets. We cannot tell
whether the observed motion of a star is that by which
it is carried on some vast independent orbit ; or is its
motion within some subordinate scheme ; or, lastly, is
for the most part due to the sun's own motion within
the sidereal system. When we see the stars of the
same constellation carried in different directions, we
cannot tell whether the real motions are diverse in
character, or whether the diversity is but apparent,
70 LIGHT SCIENCE FOR LEISURE HOURS.
like the apparent advance and retrogression of planets
which, nevertheless, are travelling in a common direc-
tion around a common centre.
But precisely because the difficulties which surround
the problem of the stellar motions are so stupendous,
we must so much the more carefully examine every
feature which observation may reveal to us. To do
otherwise were to abandon the problem as altogether
hopeless.
Now it cannot but be recognised that in this respect
the new method of research is peculiarly promising.
For whereas all former methods have dealt only with
apparent motion, this method tells us of the real rate
of stellar displacements. We have seen how it has
disposed of the inferences which had been formed as to j
the sun's velocity, and the average velocities of stellar
motion ; let us inquire what has been its bearing on
the views of astronomers respecting the stellar universe
regarded as a scheme or system.
Other methods of dealing with the motions of the
stars had related chiefly to the question of the sun's
journey through space, until Madler was led to inquire
whether the motions of the stars might not afford the |
means of determining where the centre of the stellar
system may lie. Limiting his range of inquiry, in the
first instance, by certain preliminary considerations, he
proceeded to examine the direction of the apparent
stellar motions in a particular region of the heavens.
It seemed likely to him that the centre of the universe
would be near the Milky Way, and probably on that
MOVEMENTS IN THE STAR-DEPTHS. Jl
band of conspicuous stars which extends over the
Greater Dog, Orion, the Bull, Perseus, and Cassiopeia.
Still further, he reasoned that if the sun is circling
around the central orb, this body must lie on a line
square to the sun's path ; so that if we imagine a line
extending from the point in the heavens from which
the sun is travelling to the point towards which he is
travelling, then the central orb must lie somewhere on
or near to a plane through the sun and square to that
line. Now such a plane would cut the Milky Way in
two places, one in the northern heavens in Perseus, the
other in the southern heavens between the Altar and
the Centaur. Madler further indicates reasons for
believing that the centre of the sidereal universe lies
towards the northern region of the Milky Way. Lastly,
seeing that not far from the northern region there is a
remarkable star cluster, the Pleiades, he was led to
examine the region around the Pleiades for those signs
which he thought likely to exist towards that part of the
heavens where lies the centre of the sidereal universe.
We do not enter here into a consideration of the reason-
ing which led Madler to conclude that in that part of
the heavens the stars would all appear to be moving in
the same general direction, for they are rather recondite.
That, however, was his anticipation ; and as he found
that the stars in the constellation Taurus are nearly all
moving southwards, he was satisfied that he had not
been mistaken in setting the Pleiades as the central
region of the universe, and the star Alcyone, the
72 LIGHT SCIENCE FOR LEISURE HOURS.
brightest of the Pleiades, as the central orb around
which all the stars revolve.
Now to such a problem as this a problem whose
grandeur cannot but be recognised even by those who
reject the conclusions adopted by Madler the new
method of research is applicable with peculiar force.
For instance, if the stars of Taurus are circling round
a particular orb also in Taurus, it will be manifest, on
a moment's consideration, that they can have only a
slight motion either of recession or approach with re-
spect to the sun. When from our station on the earth
we see Venus or Mercury nearly in the same direction
as the sun, we know that at the moment either planet
has only a thwart motion, being then either at its
greatest or least distance from us. So that if the new
method were applied to stars in Taurus, and showed
that swift motions of recession or approach are there in
progress, it would at once dispose of the attractive but
too speculative theory of the German astronomer.
This has not yet been accomplished; in fact, since
Dr. Huggins' instrument was mounted and in order,
the constellation Taurus has not been well placed for
observation by the new method. But in the meantime,
evidence of the most convincing nature has been ob-
tained to show that Madler's theory is unsound.
We have seen that the theory was based, in the main,
on a certain general community of apparent motion
among the stars in Taurus. Madler took it for granted
that this community of motion is exceptional. It did
not occur to him to examine the motions of stars in
MOVEMENTS IN THE STAR-DEPTHS. 73
other parts of the heavens, to see whether perchance a
like feature might not present itself elsewhere.
Having been myself led by other inquiries than
Madler's to the conclusion that the stellar motions
might afford useful information as to the structure of
the heavens, I thought it desirable to make a chart
showing all the known stellar motions in such a way
that wherever a community of direction exists it would
be at once apparent in the chart. Little arrows affixed
to the star-discs on the map, showed by their direction
and length the nature and amount of the stellar thv^art
motions. When the map was completed, it was easy
to see that the community of motion in Taurus was
only one instance, and by no means the most striking
which could be recognised, of a phenomenon which I
have since called star-drift. Certain sets of stars are
seen to be moving athwart the heavens, nearly in the
same direction, and nearly at the same rate, in such
sort as to show that they form distinct families of suns,
travelling onwards each family as a single group
through the celestial spaces.
If this view is just, Madler's theory is at once shown
to be unsound ; since the stars in Taurus thus appear
as simply a drifting family of stars, one among several
such families.
All that was required to make the proof convincing
was, that one of these sets of drifting stars should be
shown to be either approaching the earth or receding
from it as a single group.
Now, among the instances of star-drift, there was
74 LIGHT SCIENCE FOR LEISURE HOURS.
one in the Great Bear which presented some very
striking features. Five stars in this constellation,
known as Beta, Gramma, Delta, Eta, and Zeta, were
seen to be travelling, not merely at the same rate and
in the same direction, but on a course precisely oppo-
site to that which they would have had if their apparent
motion had been due to the sun's motion in space.
Moreover, all these stars are large and conspicuous ;
while one of them, Zeta, is distinguished by having
two companions, one very close to it, and the other so
far away that its motion around Zeta is only completed
(according to Madler's computation) in a period of
about 2,000 years ; so that, if all the five large stars
form a single system, the cyclic revolutions of the
system must require millions of millions of years for
their completion.
I selected this family of stars as affording a con-
venient means of testing (crucially) the accuracy of
my theory of star-drift. If that theory is just, all
these stars must be either approaching or receding at
a common rate. If the theory is unsound, the chances
are enormous against their possessing a common motion
of approach or recession. I expressed a strong feeling
of confidence that whenever Dr. Huggins applied the
new method of research to these stars, he would find
that they are either all approaching or all receding,
and at one and the same rate. When I expressed this
opinion, I knew that before many months had passed,
the matter would be decided one way or the other.
Nothing could be more complete than the confirma-
MOVEMENTS IN THE STAR-DEPTHS. 75
tion of my views by Dr. Huggins' observations. In
his table of stellar motions, Dr. Huggins brackets
together the five stars in question as possessing a com-
mon motion of recession at the rate of about twenty
miles per second. Moreover he finds, from the nature
of their spectra, that they are all alike in physical
constitution.
It is hardly necessary to insist upon the importance
of this result. It proves, first, that in this instance
and therefore presumably in the other instances of
apparent star-drift, there is a distinct family or group
of stars, travelling bodily onwards amidst the star-
depths. It is shown that the motions taking place
within this star-family are small compared with the
common motion of the group. It can be inferred that
the group is relatively isolated, since otherwise we
should find other stars in the Great Bear sharing in
the motion of these five ; and also, if there had been a
disturbing orb at a moderate distance from the group,
the members of the family would ere this have lost
their uniformity of motion. Whatever may be the
centre around which these five stars are moving as a
single group, the distance of that centre must exceed
enormously the dimensions of the group, precisely as
the distance of the sun from Jupiter's satellite family
enormously exceeds the dimensions of that system.
Yet the distances separating the stars of the Great
Bear are themselves amazingly vast. The distance
between Beta and Zeta of the Great Bear cannot be
less than 100,000 times the distance separating our
76 LIGHT SCIENCE FOR LEISURE HOURS.
earth from the sun, and is probably far vaster. What
then must be the distance of the centre of motion, as
seen from which this enormous space is reduced to an
almost evanescent arc !
It seems not unlikely that we ought to regard the
family of stars here recognised as bearing the same
general relation to the stellar universe (or to that por-
tion of it to which our sun belongs) that a group of
meteors bears to the solar system. All the drifting
star-families may not indeed travel around one and the
same centre ; or there may be no true centre, but only
a central region, round which these movements take
place : but it is impossible to consider thoughtfully
any instance of community of stellar motions without
feeling that it implies a common influence affecting in
the same or nearly the same way each member of the
drifting star-family. If there is but one such centre,
whether it be a single orb, or a central region of thickly
clustering stars, there now seems to be at least a pos-
sibility that we may find where this centre lies. When
only a few more star-families have been recognised,
and their motions of approach or recession determined,
it will be a problem of no inordinate difficulty to
deduce the position in space of the regions round
which these motions are taking place, or else to prove
(which would equally be a solution of the problem now
before us) that no such region exists, and that the stars
drift around more centres than one.
Whatever success may attend the efforts made to
explain the stellar motions, there can be no doubt that
MOVEMENTS IN THE STAR-DEPTHS. 77
the problem is well worthy of the most thorough in-
vestigation. There is, indeed, something startling in
the thought that man, placed as he is on a tiny orb
an orb rotating swiftly on its axis, carried swiftly round
the sun, and borne along with him in his swift motion
through space man, shortlived and weak, and unable
by his unaided vision to perceive a thousandth part of
the star-system, should yet attempt (and not unhope-
fully) to master the secret of its structure and motions.
It may be that what has hitherto been done is but the
beginning of the series of labours by which, if ever,
that end will be accomplished ; or it may be that we
are nearer to the mastery of the problem than we at
present imagine: but, in any case, there is but one
course by which success can be achieved. Piece by
piece the facts on which our reasoning is to depend
must be gathered together ; while at every stage of the
inquiry, the full meaning of observed facts must be as
far as possible evolved. Success will not be obtained
by observation alone, nor by theorising alone ; but by
that combination only of observation and theory to
which we owe all the most important discoveries
hitherto effected by astronomers.
Eraser's Magazine for November 1872.
78 LIGHT SCIENCE FOR LEISURE HOURS.
THE GREAT NEBULA IN ORION.
DURING the first four months of the year, the constella-
tion Orion is very favourably situated for observation
in the evening. This magnificent asterism is more
easily recognised than the Great Bear, Cassiopeia's
Chair, or the fine festoon of stars which adorns the
constellation Perseus. There is, indeed, a peculiarity
about Orion which tends considerably to facilitate
recognition. The other constellations named above,
gyrate round the pole in a manner which presents
them to us in continually varying positions. It is not
so with Orion. Divided centrally by the equator, the
mighty hunter continues twelve hours above and twelve
hours below the horizon. His shoulders are visible
somewhat more, his feet somewhat less, than twelve
hours. When he is in the south, he is seen as a giant
with upraised arms, erect, and having one knee bent,
as if he were ascending a height. Before him, as if
raised on his left arm, is a curve of small stars, forming
the shield, or target of lion's skin, which he is repre-
sented as uprearing in the face of Taurus. When Orion
is in the east, his figure is inclined backwards ; when
he is setting, he seems to be bent forwards, as if rush-
ing down a height ; but he is never seen in an inverted
position, like the northern constellations.
And we may note in passing, that the figure of Orion,
THE GREAT NEBULA IN ORION. 79
as he sets, does not exactly correspond with the image
presented in that fine passage in Maud :
I arose, and all by myself, in my own dark garden ground,
Listening now to the tide, in its broadflung shipwrecking roar,
Now to the scream of a maddened beach dragged down by the wave,
"Walked in a wintry wind, by a ghastly glimmer, and found
The shining Daffodil dead, and Orion low in his grave ;
and again, towards the end of the poem :
It fell on a time of year
"When the face of night is fair on the dewy downs,
And the shining Daffodil dies, and the charioteer
And starry Gemini hang like glorious crowns
Over Orion's grave low down in the West.
I would not, however, for one moment be understood
as finding fault with these passages of Tennyson's finest
poem. Detached from the context, the image is un-
doubtedly faulty ; but there is a correctness in the very
incorrectness of the image, placed as it is in the mouth
of one
Raging alone as his father raged in his mood ;
brooding evermore on his father's self-murder :
On a horror of shattered limbs ....
Mangled and flattened and crushed.
Let us pass on, however, to the subject of our paper.
Beneath the three bright stars which form the belt
of Orion, are several small stars, ranged, when Orion is
in the south, in a vertical direction. These form the
sword of the giant. On a clear night it is easy to see
that the middle star of the sword presents a peculiarity
of appearance : it shines as through a diffused haze.
8o LIGHT SCIENCE FOR LEISURE HOURS.
In an opera-glass this phenomenon is yet more easily
recognisable. A very small telescope exhibits the
cause of the peculiarity, for it is at once seen, that
what seemed a star is in reality a mass of small stars
intermixed with a diffused nebulosity.
It is a very remarkable circumstance that Galileo,
whose small telescope, directed to the clear skies of
Italy, revealed so many interesting phenomena, failed
to detect
That marvellous round of milky light
Below Orion.
It would not, indeed, have been very remarkable if he
had simply failed to notice this object. But he would
seem to have directed his attention for some time
especially to the region in the midst of which Orion's
nebula is found. He says : ' At first I meant to de-
lineate the whole of this constellation ; but on account
of the immense multitude of stars being also hampered
through want of leisure- -I left the completion of this
design till I should have another opportunity.' He
therefore directed his attention wholly to a space of
about ten square degrees, between the belt and sword,
in which space he counted no less than four hundred
stars. What is yet more remarkable, he mentions the
fact that there are many small spots on the heavens
shining with a light resembling that of the Milky Way
(complures similis coloris areolce sparsim per cethera
subfulgeani) ; and he even speaks of nebulae of this
sort in the head and belt and sword of Orion. He
asserts, however, that by means of his telescope, these
THE GREAT NEBULA IN ORION. 8 1
nebulae were distinctly resolved into starsa circum-
stance which, as we shall see presently, renders his
description wholly inapplicable to the great nebula.
Yet the very star around which (in the naked-eye view)
this nebula appears to cling, is figured in Galileo's
drawing of the belt and sword of Orion !
It seems almost inconceivable that Galileo should
have overlooked the nebula, assuming its appearance
in his day to have resembled that which it has at pre-
sent. And as it appears to have been established, that
if the nebula has changed at all during the past century
it has changed very slowly indeed, one can scarcely
believe that in Galileo's time it should have presented
a very different aspect. Is it possible that the view
suggested by Humboldt is correct that Galileo did
not see the nebula because he did not wish to see it ?
6 Galileo,' says Humboldt, ' was disinclined to admit or
assume the existence of starless nebulae.' Long after
the discovery of the great nebula in Andromeda
known as 'the transcendently beautiful queen of the
nebulas ' Galileo omitted all mention in his works of
any but starry nebula?. The last-named nebula was
discovered in 1614, by Simon Marius, whose claims to
the discovery of Jupiter's satellites had greatly angered
Galileo, and had called forth a torrent of invective, in
which the Protestant German was abused as a heretic
by Galileo, little aware that he would himself before
long incur the displeasure of the Church. If we could
suppose that an unwillingness, either to confirm his
rival's discovery of a starless nebula, or to acknowledge
G
82 LIGHT SCIENCE FOE LEISURE HOURS.
that he had himself fallen into an error on the subject
of nebulae, prevented Galileo from speaking about the
great nebula in Orion, we should be compelled to form
but a low opinion of his honesty. It happens too
frequently that
The man of science himself is fonder of glory, and vain
An eye well practised in nature, a spirit bounded and poor.
That Hevelius, c the star-cataloguer,' should have
failed to detect the Orion nebula is far less remarkable;
for Hevelius objected to the use of telescopes in the
work of cataloguing stars. He determined the position
of each star by looking at the star through minute
holes or pinnules, at the ends of a long rod attached to
an instrument resembling the quadrant.
The actual discoverer of the great nebula was Huy-
ghens, in 1656. The description he gives of the dis-
covery is so animated and interesting, that we shal
translate it at length. He says :
' While I was observing the variable belts of Jupiter
a dark band across the centre of Mars, and some indis
tinct phenomena on his disc, I detected among the
fixed stars an appearance resembling nothing whic]
had ever been seen before, so far as I am aware
This phenomenon can only be seen with large tele
scopes such as I myself make use of. Astronomer
reckon that there are three stars in the sword of Orion
which lie very close to each other. But as I was look
ing, in the year 1656, through my 23-feet telescope, a
the middle of the sword, I saw, in place of one star
no less than twelve stars which indeed is no unusua
THE GREAT NEBULA IN ORION. 83
occurrence with powerful telescopes. Three of these
stars seemed to be almost in contact, and with these
were four others which shone as through a haze, so
that the space around shone much more brightly than
the rest of the sky. And as the heavens were serene
and appeared very dark, there seemed to be a gap in
this part, through which a view was disclosed of
brighter heavens beyond. All this I have continued
to see up to the present time [the work in which these
remarks appear the Sy 'sterna Satumium was publish-
ed in 1659], so that this singular object, whatever it is,
may be inferred to remain constantly in that part of
the sky. I certainly have never seen anything resem-
bling it in any other of the fixed stars. For other
objects once thought to be nebulous, and the Milky
Way itself, show no mistiness when looked at through
telescopes, nor are they anything but congeries of stars
thickly clustered together.'
Huyghens does not seem to have noticed that the
space bet\veen the three stars he described as close
together is perfectly free from nebulous light insomuch
that if the nebula itself is rightly compared to a gap in
the darker heavens, this spot resembles a gap within
the nebula. And indeed, it is not uninteresting to
notice how comparatively inefficient was Huyghens'
telescope, though it was nearly eight yards in focal
length. A good achromatic telescope two feet long
would reveal more than Huyghens was able to detect
with his unwieldy instrument.
Dominic Cassini soon after discovered a fourth star
G 2
84 LIGHT SCIENCE FOR LEISURE HOURS.
near the three seen by Huyghens. The four form the
celebrated trapezium, an object interesting to the pos-
sessors of moderately good telescopes, and which has
also been a subject of close investigation by professed
astronomers. Besides the four stars seen by Cassini,
there have been found five minute stars within and
around the trapezium. These tiny objects seem to
shine with variable brilliancy ; for sometimes one will
surpass the rest, while at others it will be almost
invisible.
After Cassini's discovery, pictures were made of the
great nebula by Picard, Le Grentil, and Messier, These
present no features of special interest. It is as we
approach the present time, and find the great nebula
the centre of quite a little warfare among astronomers
now claimed as an ally by one party, now by their
opponents that we begin to attach an almost romantic
interest to the investigation of this remarkable object.
In the year 1811, Sir W. Herschel announced that
he had (as he supposed) detected changes in the Orion
nebula. The announcement appeared in connection
with a very remarkable theory respecting nebulae gene-
rally Herschel's celebrated hypothesis of the conver-
sion of some nebulae into stars. The astronomical
world now heard for the first time of that self-luminous
nebulous matter, distributed in a highly attenuated
form throughout the celestial regions, which Herschel
looked upon as the material from which the stars have
been originally formed. There is an allusion to this
theory in those words of the Princess Ida :
THE GREAT NEBULA IN ORION. 85
There sinks the nebulous star we call the Sun,
If that hypothesis of theirs be sound.
And in the teaching of ' comely Psyche ' :
This world was once a fluid haze of light,
Till toward the centre set the starry tides,
And eddied into suns, that wheeling cast
The .planets.
Few theories have met with a stranger fate. Eeceived
respectfully at first on the authority of the great astro-
nomer who propounded it then in the zenith of his
fame the theory gradually found a place in nearly all
astronomical works. But, in the words of a distinguished
living astronomer, ' The bold hypothesis did not receive
that confirmation from the labours of subsequent in-
quirers which is so remarkable in the case of many of
Herschel's other speculations.' It came to pass at length
that the theory was looked upon by nearly all English
astronomers as wholly untenable. In Gfermany it was
never abandoned, however, and a great modern discovery
has suddenly brought it into general favour, and has in
this, as in so many other instances, vindicated Herschel's
claim to be looked upon as the most clear-sighted, as
well as the boldest and most original of astronomical
theorisers.
Herschel had pointed out various circumstances
which, in his opinion, justified a belief in the existence
of a nebulous substance fire-mist or star-mist, as it
has been termed throughout interstellar space. He
had discovered and observed several thousand nebulae,
and he considered that amongst these he could detect
traces of progressive development. Some nebulas were,
86 LIGHT SCIENCE FOR LEISURE HOURS.
he supposed, comparatively young ; they showed no
signs of systematic aggregation or of central condensa-
tion. In some nebulae he traced the approach towards
the formation of subordinate centres of attraction ;
while in others, again, a single centre began to be
noticeable. He showed the various steps by which
aggregation of the former kind might be supposed to
result in the formation of star-clusters, and condensa-
tion of the latter kind into the formation of conspicuous
single stars.
But it was felt that the strongest part of Herschel's
case lay in his reference to the great nebula of Orion.
He pointed out that amongst all the nebulae which
might be reasonably assumed to be star-systems, a cer-
tain proportionality had always been found to exist
between the telescope which first detected a nebula and
that which effected its resolution into stars. And this
was what might be expected to happen with star-
systems lying beyond our galactic system. But how
different is this from what was seen in the case of the
Orion nebula. Here is an object so brilliant as to be
visible to the naked eye, and which is found on exami-
nation to cover a large region of the heavens. And
yet the most powerful telescopes had failed to show the
slightest symptom of resolution. Were we to believe
that we saw here a system of suns so far off that no
telescope could exhibit the separate existence of the
component luminaries, and therefore (considering merely
the observed extent of the nebula, which is undoubtedly
but a faint indication of its real dimensions) so incon-
THE GREAT NEBULA IN ORION. 8 7
ceivably enormous in extent that the star-system of
which our sun is a member shrinks into nothingness in
comparison ? Surely it seemed far more reasonable to
recognise in the Orion nebula but a portion of our
galaxy, a portion very vast in extent, but far inferior
to that c limitless ocean of universes ' presented to us
by the other view.
And when Sir W. Herschel was able, as he thought >
to point to changes taking place within the Orion
nebula, it seemed yet more improbable that the object
was a star-system.
But now telescopes more powerful than those with
which the elder Herschel had scanned the great nebula
were directed to its examination. Sir John Herschel,
following in his father's footsteps, applied himself to
the diligent survey of the marvellous nebula with a
reflecting telescope eighteen inches in aperture. He
presented the nebula to us as an object of which ' the
revelation of the ten-feet telescope was but the mere
rudiment.' Strange outlying wisps and streamers of
light were seen, extending far out into space. Yet
more strange seemed the internal constitution of the
object. So strange, indeed, did the nebula appear, ' so
unlike what had hitherto been known of collections of
stars,' that Sir John Herschel considered the evidence
afforded by its appearance as sufficient to warrant the
conclusion of a non-stellar substance.
But this eminent astronomer obtained a yet better
view of the great nebula when he transported to the
Cape of Good Hope an instrument equal in power to
88 LIGHT SCIENCE FOR LEISURE HOURS.
that which he had applied to the northern heavens.
In the clear skies of the southern hemisphere the nebula
shines with a splendour far surpassing that which it has
in northern climes. It is also seen far higher above the
horizon. Thus the drawing which Sir J. Herschel was
able to execute during his three years' residence at the
Cape is among the best views of the great nebula that
have ever been taken. But even under these favourable
circumstances. Sir John records ' that the nebula,
through his great reflector, showed not a symptom of
resolution.'
Then Lassell turned his powerful mirror, two feet in
diameter, upon the unintelligible nebula. But though
he was able to execute a remarkable drawing of the
object, he could discern no trace of stellar constitution.
In 1845 Lord Eosse interrogated the great nebula
with his three-feet mirror. Marvellous was the com-
plexity and splendour of the object revealed to him, but
not the trace of a star could be seen.
The end was not yet, however. Encouraged by the
success of the three-feet telescope, Lord Rosse com-
menced the construction of one four times as powerful.
After long and persistent labours, and at a cost, it is
said, of thirty thousand pounds, the great Parsonstown
reflector, with its wonderful six-feet speculum, was
directed to the survey of the heavens. At Christmas,
1845, while the instrument was yet incomplete, and in
unfavourable weather, the giant tube was turned
towards the Orion nebula. Professor Nichol was the
first who saw the mysterious object as pictured by the
THE GREAT NEBULA IN ORION. 89
great mirror. Although the observation was not suc-
cessful so far as the resolution of the nebula was con-
cerned, yet Nichol's graphic account of the telescope's
performance is well worth reading :
4 Strongly attracted in youth by the lofty conceptions
of Herschel [he writes], I may be apt to surround the
incident I have to narrate with feelings in so far of a
personal origin and interest : but, unless I greatly
deceive myself, there are few who would view it other-
wise than I. With an anxiety natural and profound,
the scientific world watched the examination of Orion
by the six-feet mirror; for the result had either to
confirm Herschel's hypothesis, in so far as human
insight ever could confirm it ; or unfold among the
stellar groups a variety of constitution not indicated
by those in the neighbourhood of our galaxy. Although
Lord Eosse warned me that the circumstances of the
moment would not permit me to regard the decision
then given as absolutely final, I went in breathless
interest to the inspection. Not yet the veriest trace of
a star ! Unintelligible as ever, there the nebula lay ;
but how gorgeous its brighter parts ! How countless
those streamers branching from it on every side ! How
strange, especially that large horn on the north, rising
in relief from the black skies like a vast cumulous cloud !
It was thus still possible that the nebula was irresolv-
able by human art ; and so doubt remained. Why the
concurrence of every favourable condition is requisite for
success in such inquiries may be readily comprehended.
The object in view is to discern, singly, sparkling
90 LIGHT SCIENCE FOR LEISURE HOURS.
points, small as the point of a needle, and close as the
particles of a handful of sand ; so that it needs but the
smallest unsteadiness in the air, or imperfection in the
shape of the reflecting surface, to scatter the light of
each point, to merge them into each other, and present
them as one mass.'
Before long Lord Eosse, after regrinding the great
mirror, obtained better views of the mysterious nebula.
Even now, however, he could use but half the power of
the telescope, yet at length the doubts of astronomers
as to the resolvability of the nebula were removed.
' We could plainly see,' he wrote to Professor Mchol,
' that all about the trapezium was a mass of stars, the
rest of the nebula also abounding with stars, and ex-
hibiting the characteristics of resolvability strongly
marked.' These views were abundantly confirmed by
subsequent observations with the great mirror.
It will surprise many to learn that even Lord Kosse's
great reflector is surpassed in certain respects by some of
the exquisite refractors now constructed by opticians.
As a light-gatherer the mirror is, of course, unapproach-
able by refractors. Even if it were possible to construct
an achromatic lens six feet in diameter, yet, to prevent
flexure, a thickness would have to be given to the glass
which would render it almost impervious to light and
therefore utterly useless. But refractors have a power
of definition not possessed by large reflectors. With a
refractor eight inches only in aperture, for instance,
Mr. Dawes has obtained better views of the planets (and
specially of Mars), than Lord Rosse's six-feet speculum
THE GREAT NEBULA IN ORION. 91
would give under the most favourable circumstances.
And in like manner, the performance of Lord Eosse's
telescope on the Orion nebula has been surpassed so
far as resolution into discrete stars is concerned by
the exquisite denning power of the noble refractor of
Harvard College (U.S.). By means of this instrument
hundreds of stars have been counted within the confines
of the once intractable nebula.
It seemed, therefore, that all doubt had vanished
from the subject which had so long perplexed astrono-
mers. 'That was proved to be real,' Nichol wrote,
'which, with conceptions of space enlarged even as
Herschel's, we deemed incomprehensible. 9
Yet even at this stage of the inquiry there were
found minds bold enough to question whether a per-
fectly satisfactory solution of the great problem had
really been attained. Nor is it difficult, I think, to
point out strong reasons for such doubts. I shall con-
tent myself by naming one which has always appeared
to me irresistible.
The Orion nebula as seen in powerful telescopes
covers a large extent of the celestial sphere. According
to the Padre Secchi, who observed it with the great
Merz refractor of the observatory at Eome, the nebulous
region covers a triangular space, the width of whose
base is some eight times, while its height is more than
ten times as great as the moon's apparent diameter, a
space more than fifty times greater than that covered
by the moon. Now, I do not say that it is inconceivable
that an outlying star-system, so far off as to be irre-
92 LIGHT SCIENCE FOR LEISURE HOURS.
solvable by any but the most powerful telescopes, should
cover so large a space on the heavens. On the contrary,
I do not believe that a galaxy resembling our own
would be resolvable at all, unless it were so near as to
appear much larger than the Orion nebula. I believe
astronomers have been wholly mistaken in considering
any of the nebulae to be such systems as our own.
There may be millions of such systems in space, but I
am very certain no telescope we could make would
suffice to resolve any of them. But what I do consider
inconceivable, is, that a nebula extending so widely, and
placed (as supposed) beyond our system, should yet
appear to cling (as the Orion nebula undoubtedly does)
around the fixed stars seen in the same field with it.
So strongly marked is this characteristic, that Sir John
Herschel (who failed, apparently, to see its meaning)
mentions amongst others no less than four stars, one of
which is the bright middle star of the belt as ' involved
in strong nebulosity,' while the intermediate nebulosity
is only just traceable. The probability that this
arrangement is accidental is so small as to be almost
evanescent.
However, as I have said, English astronomers, almost
without a dissentient voice, accepted the resolution of
the nebula as a proof that it represents a distant star-
system resembling our own galactic system, but far
surpassing it in magnitude.
The time came, however, when a new instrument,
more telling even than the telescope, was to be directed
upon the Orion nebula, and with very startling results.
THE GEE AT NEBULA IN ORION. 93
The spectroscope had revealed much respecting the
constitution of the fixed stars. We had learned that they
are suns resembling our own. It remained only to
show that the Orion nebula consists of similar suns, in
order to establish beyond all possibility of doubt the
theories which had been so complacently accepted. A
very different result rewarded the attempt, however.
When Dr. Huggins turned his spectroscope towards the
great nebula, he saw, in place of a spectrum resembling
the sun's, three bright lines only ! A spectrum of this
sort indicates that the source of light is a luminous
gas, so that whatever the Orion nebula may be, it is
most certainly not a congeries of suns resembling our
own.
It would be unwise to theorise at present on a result
so remarkable. Nor can we assert that Herschel's
speculations have been confirmed, though his general
reasoning has been abundantly justified. Astronomers
have yet to do much before they can interpret the
mysterious entity which adorns Orion's sword. On every
side, however, observations are being made which give
promise of the solution of this and kindred difficulties.
\Ve have seen the light of comets analysed by the same
powerful instrument ; and we learn that the light from
the tail and coma is similar in quality (so far as obser-
vation has yet extended) to that emitted from the
Orion nebula. We see, therefore, that in our own solar
system we have analogues of what has been revealed in
external space. I would not, indeed, go so far as to
assert that the Orion nebula is a nest of cometic
94 LIGHT SCIENCE FOR LEISURE HOURS.
systems ; but I may safely allege that there is now
not a particle of evidence that the nebula lies beyond
our galaxy.
Nor need we doubt the accuracy of Lord Eosse's
observations. More than a year before his death,
indeed, he mentioned to Dr. Huggins ' that the matter
of the great nebula in Orion had not been resolved by
his telescope. In some parts of the nebula he observed
a large number of exceedingly minute red stars. These
red stars, however, though apparently connected with
the irresolvable blue material of the nebula, yet seemed
to be distinct from it.'
The whole subject seems to be as perplexing as any
that has ever been submitted to astronomers. Time,
however, will doubtless unravel the thread of the
mystery. We may safely leave the inquiry in the hands
of the able observers and physicists whose attention has
been for a long time directed towards it. And we need
only note, in conclusion, that in the southern hemi-
sphere there exists an object equally mysterious the
great nebula round 77 Argus which has yielded similar
results when tested with the spectroscope. The examina-
tion of this mysterious nebula, associated with the most
remarkable variable in the heavens a star which at one
time shines but as a fifth magnitude star, and at another
exceeds even the brilliant Canopus in splendour may,
for aught that is known, throw a new light on the con-
stitution of the great Orion nebula.
From Eraser's Magazine for February 1869.
THE SUN'S TRUE ATMOSPHERE. 95
THE SUN'S TRUE ATMOSPHERE.
So much attention was directed to the solar corona
during the discussions which preceded and followed
the late eclipse, that a discovery of extreme import-
ance but not at all associated with the corona has
received far less attention than it deserves. The dis-
covery I refer to is, in fact, more important in its bear-
ing on problems of solar physics than any which has
been made since Kirchhoff first told us how to inter-
pret the solar spectrum. It is also intimately con-
nected with the labours of that eminent physicist. I
propose briefly to describe the nature of the discovery,
and then to discuss some of the results to which it
seems to point.
Astronomers have long seen reason to believe that
the sun has an atmosphere. And by the word atmo-
sphere I mean something more than mere vaporous or
gaseous masses, such as the prominences have been
shown to be. A solar envelope, complete and con-
tinuous as our own atmosphere, seems undoubtedly
suggested by the appearance which the sun's image
presents when thrown on a suitably prepared screen in
a darkened room; for then the disc is seen to be
shaded off continuously towards the edge, where its
brilliancy is scarcely half as great as at the centre.
The phenomenon is so readily seen, and so unmistake-
96 LIGHT SCIENCE FOR LEISURE HOURS.
able, that it is with a sense of wonder one hears that
Arago called it in question. To use the words of Sir
John Herschel, 'the fact is so palpable that it is a
matter of some astonishment that it could ever fail to
strike the most superficial observer.' And, again, not
only the light but the heat of the outer portions of the
sun's image has been estimated. In this case we do
not depend upon the perhaps fallible evidence of the
eye, but on that of heat-measuring instruments. Fr.
Secchi, measuring the heat of different parts of the solar
image, has found that of the part near the centre nearly
double that from the borders. Lastly, photography
gives unmistakable evidence on the subject.
Now, when Kirchhoff discovered the meaning of the
solar spectrum, it seemed clear to him that he had
determined the nature and constitution of the solar
atmosphere. Let us consider the nature of Kirchhoff's
discovery.
He found that the dark lines across the rainbow-
tinted streak forming the background (as it were) of
the solar spectrum, are due to the action of absorbing
vapours. The vapours necessarily lie outside the source
of that part of the sun's light which produces the rain-
bow-tinted streak. If those vapours could be removed
for a while, we should see a simple rainbow-riband of
light. Or if the vapours could be so heated as to be
no less hot than the matter beneath them which pro-
duces the rainbow spectrum, they would no longer
cause any dark lines to appear ; but being cooler, and
so giving out less light than they intercept, they cut
THE SVN'S TRUE ATMOSPHERE. 97
out the dark spaces corresponding to their special
absorptive powers. To use Mr. Lockyer's striking,
though perhaps not strictly poetical, description of
their action, these vapours c gobble up the light on its
way to the observer, so that it comes out with a
balance on the wrong side of the account.' Each
vapour produces its own special set of lines, occupying
precisely those parts of the spectrum which the vapour's
light would illuminate if the vapour shone alone. For
these vapours, notwithstanding their action in inter-
cepting or absorbing portions of the sunlight, are
themselves in reality glowing with a light so intense
that the human eye could not bear to rest upon it. If
we could examine the vapours we supposed just now
removed from the sun, we should obtain the very lines
of light which are wanting in the spectrum of the
sun.
When Kirchhoff had recognised in this way the
presence of absorptive vapours around the real light-
globe of the sun, he judged that they form the solar
atmosphere. Because, although his mode of observa-
tion was not such as to assure him that these vapours
completely envelope the sun, yet the telescopic aspect
of the sun, and especially that darkening near the edge
to which I have just referred, seemed to leave room
; for no other conclusion. But at this stage of the
inquiry Kirchhoff fell into a mistake. He judged that
the solar corona was the atmosphere which produced
, the solar dark lines, as well as the darkening of the
sun's disc near the edge. The mistake is one which,
H
98 LIGHT SCIENCE FOR LEISURE HOURS.
as it seems to me, he would have avoided had he taken
into account the enormous pressure at which an atmo-
sphere so extensive as the corona would necessarily
exist under the influence of the sun's mighty attractive
energies. It may easily he shown that if the outer
parts of the corona were as rare as the contents of our
so-called vacuum-tubes, or even a thousand times rarer,
yet according to the la,ws which regulate atmospheric
pressure, the density even at vast heights above the
sun's surface would attain to many hundred times that of
our heaviest gases. The pressure would, indeed, be so
great that we can, see no way of escaping the conclu-
sion that, despite the enormous heat, the gases com-
posing the imagined atmosphere would be liquefied or
even solidified.
When the observers of the Indian eclipse of 1868
found that the coloured prominences are masses of
glowing hydrogen, with other gases intermixed, and
when the prominence-spectrum was found to show the
hydrogen lines as these appear when hydrogen exists
at very moderate pressures, Kirchhoff s view had to be
abandoned as altogether untenable. Wherever the
vapours exist which produce the solar dark lines, they
are undoubtedly not to be looked for in the corona.
But there the lines are. The absorptive action is
exerted somewhere. The question is Where are the
absorptive vapours ?
At this stage of the inquiry, a very strange view was
expressed 'by Mr. Lockyer a view which appears to
have been founded on a slight misapprehension of the
THE SUN'S TRUE ATMOSPHERE. 99
principles of spectrum analysis. He put forward the
theory that the absorptive action takes place below the
level of the sun's surface as we see it.
But observations made by Fr. Secchi at Rome pointed
to a view so different from Mr. Lockyer's, as to lead to
a controversy which filled many pages of the Comptes
Rendus, of the Philosopical Magazine, and of other
publications a controversy Conducted, as too many
philosophical discussions have been, with a somewhat
unphilosophical acrimony.
Fr. Secchi had noticed that when the very edge of
the sun's disc is examined with the spectroscope, the
dark lines disappear from the spectrum, which thus
becomes a simple rainbow-tinted streak. He judged,
accordingly, that the absorbing atmosphere exists above
the sun's real surface ; for he believed that just at the
edge the bright lines corresponding to the light from
the vapours themselves so nearly equal in intensity the
light of the solar spectrum, that no signs of difference
can be detected ; or, in other words, that the dark lines
are obliterated. On the other hand, the glowing atmo-
sphere cannot, he argued, reach much above the sun's
surface, since otherwise the spectroscope would show
the bright lines belonging to that atmosphere's light.
Now, no such lines are visible. So far as the spectro-
scopic evidence is concerned, it would appear as though
immediately above the sun's surface, as we see it, there
came the sierra that low range of prominence-matter,
which, strangely enough, some have regarded as an
atmospheric envelope. The spectrum of the sierra
H 2
IOO LIGHT SCIENCE FOR LEISURE HOURS.
shows beyond all question that, like the prominences,
this region consists of glowing hydrogen, mixed up
with a few, and at times with several other gases, but
certainly not capable of accounting for the thousands
of dark lines in the solar spectrum. It seems quite
clear, also, that the sierra is not of the nature of an
envelope at all.
Over the narrow layer which Secchi supposed to
exist between the sun's surface and the coloured sierra,
began, and presently waxed warm, the controversy
above referred to. Fr. Secchi was positive that he
could see the narrow continuous spectrum on which he
founded his view; Mr. Lockyer was equally positive
that the worthy father could see nothing of the kind.
Fr. Secchi urged that his telescope was better than
Mr. Lockyer's, and that he worked in a better atmo-
sphere ; Mr. Lockyer retorted that his spectroscope
was better than Fr. Secchi's, and that the imagined
superiority of the Roman atmosphere was a myth.
Something was said, too, by the London observer about
a large speculum, which was to decide the question,
though this mirror does not seem to have been actually
brought into action. Both the disputants expressed
full confidence that time would prove the justice of
their several views.
Soon after, an observation was made by Mr. Lockyer,
which seemed to prove the justice of Fr. Secchi's
opinion ; for, on a very favourable day for observations,
Mr. Lockyer was able to detect, not the narrow rain-
bow-tinted spectrum seen by Secchi, but a narrow strip
THE SUN'S TRUE ATMOSPHERE. IOI
of spectrum belonging to the region just outside the
sun's edge, which showed hundreds of bright lines.
Here seemed to be conclusive evidence of that shallow
atmosphere of glowing vapours in which Fr. Secchi
had faith. But Mr. Lockyer interpreted his observa-
tion differently. The presence of these vapours on
this particular occasion he regarded as wholly excep-
tional, and the cause of the exception he held to be
the energetic injection of vapours from beneath the
surface of the sun.
At about this stage of the controversy I had occasion
to consider the problems associated with the physical
condition of the sun and his surroundings ; and although
I took no part in the discussion between Fr. Secchi and
Mr. Lockyer, I expressed (in papers which I wrote upon
the subject) opinions which agreed with the views of
the Italian astronomer. It is necessary for me to pre-
sent in this place my own reasoning on the question at
issue, because it not only serves to introduce the special
observation made last December, by which the problem
has been finally solved, but also presents certain con-
siderations which must be attended to in interpreting
that observation.
In the first place, I noted that the darkening of the
sun's disc near the edge, or rather the marked nature
of that darkening, instead of showing (as had been so
often stated) that the sun has a very deep atmosphere,
proves, on the contrary, that his atmosphere must be
exceedingly shallow by comparison with the dimen-
sions of his globe. It is easy to show why this is ; and
102 LIGHT SCIENCE FOR LEISURE HOURS.
although the considerations on which the matter de-
pends are exceedingly simple, yet the case is by no
means the first in which exceedingly simple considera-
tions have been lost sight of by students of science.
Suppose we have a brightly-white globe encased sym-
metrically within a globe of some imperfectly trans-
parent substance as green glass. Now, if the white
globe is an inch in diameter and the green glass globe
a yard in diameter, the brightness of the white globe
will be more or less impaired according to the trans-
parency of the glass ; but it will not be much more
impaired at the edge of the inner globe's disc than
near the middle. For clearly, when we look at the
middle, we look through a foot and a half of glass
(wanting only half an inch), and when we look at the
edge of the inner globe's disc, we also look through a
foot and a half of glass (wanting only a small fraction
of an inch). Neither the half inch in the one case,
nor the small fraction of an inch in the other, can
make any appreciable difference, so that the enclosing
globe of glass cuts off as much light when we look at
the centre of the inner globe's disc as when we look at
the edge. But now suppose that the enclosing globe
forms a mere shell around the inner one. Suppose, for
instance, that the inner globe is a yard in diameter,
and the shell of glass only half an inch thick. Then
in this case, as in the former, the brightness of the
inner globe will be more or less impaired according to
the transparency of the glass ; but it will no longer be
affected equally whether we look at the middle or at
THE SUN'S TRUE ATMOSPHERE. 103
the edge of the inner globe's disc. In the former case
we only look though half an inch of glass, in the latter
we look through a much greater range of glass ; as the
reader will see at once if he draw two concentric circles
nearly equal in size to represent the inner globe and
its enclosing shell. It is easy to calculate how long
the range of glass actually is in the latter case. I have
just gone through the calculation, and find that when
the eye is directed to the edge of the enclosed globe,
its line of sight passes through rather more than four
inches and a quarter, so that more than eight times as
much light is absorbed as in the case where the eye
looks at the middle of the inner globe's disc, or directly
through half an inch of glass.
Xow we cannot tell what proportion holds in the
case of the sun's disc, because we do not know how
much light has been absorbed where we look at the
middle of the disc. All we know is that whatever re-
mains after such absorption is about twice as much as
we receive from near the edge of the disc. It is easily
seen that this knowledge is insufficient for our require-
ments. But there can be no question whatever that
the total absorption near the edge exceeds many times
that near the middle of the disc ; and on very reason-
able assumptions as to this excess, it may readily be
shown that the absorbing atmosphere cannot exceed
some five or six hundred miles in depth. Probably it
is even shallower.
' Now, there is a circumstance which perfectly ac-
counts for the non-recognition by spectroscopists of
104 LIGHT SCIENCE FOR LEISURE HOURS.
an atmosphere relatively so shallow as this. Let it be
remembered, in passing, that the average height of the
sierra may be set at about five thousand miles ; so that
the atmosphere we are dealing with would be at the
outside but one-fifth as high as that fine rim of red
light with saw-like edge which astronomers detected
around the eclipsed sun in the total eclipses of 1842,
1851, and 1860. Still it might be thought that
patience only would be needed to detect the signs of
such an atmosphere, shallow though it be. But there
is a peculiarity of telescopic observation which renders
the recognition of such an atmosphere, if of less than a
certain depth, not difficult merely, but impossible. It
may be well to exhibit the nature of the peculiarity at
length, because it is of considerable interest to all who
possess or use telescopes. I take an illustrative case,
which seems, at first, to have little connection with my
subject.
Every reader of this work has heard of the double
stars, and I dare say most of those who read this parti-
cular article have seen many of these beautiful objects.
It is known that some double stars are much closer
than others, and we commonly hear it mentioned as a
proof of the excellence of a telescope that it will divide
such and such a double star. But it might seem that
if a telescope of a certain size were constructed with
extreme care, it should be capable of dividing any
double star ; because we might use an eye-piece of any
magnifying power we pleased, and so, as it were, force
apart the two star-images formed by the object-glass.
THE SUN'S TRVE ATMOSPHERE. 105
Instead of this being the case, however, there is a limit
for every object-glass, beyond which no separation is
possible ; for this reason, simply, that the star-images
formed by the object-glass are not points of light, as
they would be if they correctly represented the stars of
which they are the optical images. The larger the
object-glass (assumed to be perfect in construction) the
smaller is the star-image ; 1 but it has always a definite
size, and if this size is such that the two images of the
stars forming a pair actually touch or overlap, we can-
not separate them by using highly-magnifying eye-
pieces.
Now what is true of a star is true of" every point of
any object we examine with a telescope. The image
of the point is always a circle of light, which, though
minute, has yet appreciable dimensions. The image
of the object is made up of all these circles, which
necessarily overlap. Nor let the reader suppose that
on this account telescopic observation is untrustworthy.
Precisely the same peculiarity affects ordinary vision.
There is no such thing as a perfect optical image of an
object; though neither eyesight nor telescopic vision
need be regarded as deceptive, on this account. Our
power of seeing minute details is limited by this
peculiarity, but we are not actually deceived. If
1 A curious illustration of this is given by the fact that a certain
astronomer of old, having reduced the aperture of his telescope to a
mere pin-hole, announced that he was thus enabled to measure the real
globes of the stars, for, instead of seeing the stars through his telescope
as minute points of light, he now saw them with discs like the planets.
He thought he was improving the defining qualities of his telescope,
instead of altogether destroying them.
106 LIGHT SCIENCE FOR LEISURE HOURS.
microscopic writing be shown us, for instance, we may
find ourselves, after poring over it for some time,
unable to make out its meaning, the letters seeming
all blended together ; but we know what our failure
really means, and do not fall into the mistake of
concluding that there are no details because the actual
details are inscrutable.
Let us apply this consideration to the sun, and more
particularly to the appearance presented by the edge
of the sun's disc. The image of every point of this
edge is a small circle ; the combination of all these
small circles must produce a ring of light all round
the true outline of the disc. If the sun's atmosphere
did not reach beyond this ring, then no contrivance
whatever could render the atmosphere discernible, let
the telescope be ever so perfect and the observer
ever so clear-sighted or skilful. Now, the actual ex-
tension of this ring will be greater or less according as
the object-glass of the telescope is less or greater. It
may readily be shown that neither Mr. Lockyer's tele-
scope nor Fr. Secchi's could possibly show any signs of
a solar atmosphere under two hundred miles in depth,
while in all probability an atmosphere four or five
times as deep would escape their scrutiny.
Are we then to remain altogether in ignorance of
such an atmosphere, supposing that it actually exists,
and that the dark lines in the solar spectrum are due
to its absorptive power ? Is there no way of obviating
the difficulty which has just been dealt with ?
So far as the method of observing the sun when
THE SUN'S TRUE ATMOSPHERE. 107
uneclipsed is concerned, the answer to these questions
must be negative ; or, rather, it must be answered that
our only hope of meeting the difficulty consists in
increasing the size of the telescopes with which the
sun is spectroscopically studied. And inasmuch as
Dr. Huggins is preparing to apply the powers of a much
larger telescope than either Mr. Lockyer's or Fr.
Secchi's, we may possibly still hope to hear that the
relatively shallow atmosphere can be studied when
the sun is not eclipsed. For we may now speak
of the existence of this atmosphere as a demonstrated
fact. The difficulty which seemed to present insuper-
able obstacles to the observers who study the uneclipsed
sun, has been overcome by the ingenuity of one of the
most skilful of those very observers Professor Young,
of America when studying the solar eclipse of last
December.
If during any total eclipse of the sun, the moon just
concealed the whole of the sun's disc (as may well
happen), and if our satellite were only complaisant
enough to stay still for a few minutes in such a position,
so that one of these exact total eclipses could be
studied as readily as one of greater extent (which never
can happen), then the shallow atmosphere I have been
speaking of could be recognised. The difficulty above
considered would no longer exist. For the ring of
light which actually hides the shallow atmosphere when
the sun is not eclipsed, is an extension of the bright
rim of the disc outwards : if the disc is completely
hidden, there is no bright rim to be extended, and any-
108 LIGHT SCIENCE FOR LEISURE HOURS.
thing existing close by the sun's globe can be recog-
nised.
But then, unfortunately, no total eclipse can present
these desirable features. If a total eclipse is to be
worth seeing at all, the moon's disc as seen at the time
must be appreciably larger than the sun's. When
totality begins the outlines of the two discs just touch
at a single point, and when totality ends the two discs
just touch at another point ; but during all the rest of
the totality the two outlines do not touch at all, that
of the moon surrounding without touching that of the
sun. The outlines of the two discs do twice touch,
however, in each case for one moment and at one point.
What Professor Young determined to do, therefore, was
to Bring under special examination that one point
where the outlines touch at the exact moment when
totality begins. In other words, he directed his special
attention to the point where the last trace of the sun's
disc was about to disappear. It is perhaps scarcely
necessary to say that he did not trust to the powers of
his telescope, but that he employed a powerful spec-
troscope. And further, he did not depend on his own
observations alone, but had adjusted a spectroscope for
the use of Mr. Pye, an English gentleman residing in
the part of Spain where the eclipse-observing parties
were stationed, so that that gentleman also might make
the required observations.
In his account, Professor Young does not mention
what he expected to see. It is probable that he had
in his thoughts the observations of Fr. Secchi, and
THE SUN'S TRUE ATMOSPHERE. 1 09
hoped to obtain evidence respecting that shallow at-
mospheric envelope which Secchi believed in and
Lockyer rejected ; though it is quite possible he merely
desired to ascertain whether the constitution of the
lower part of the sierra differed in any marked respect
from that of the upper portion. As the moment
approached when the last fine sickle of sunlight was to
be obscured, the solar spectrum which was visible in
the spectroscopic field of view grew rapidly fainter.
The region actually examined by Professor Young was
in reality a narrow, almost linear space, touching the
edge of the sun's disc; so that before totality had com-
menced he had the light from our own illuminated
atmosphere, and not direct sunlight, to deal with.
Thus he had just such a solar spectrum as is seen when
a spectroscope is directed to the sky in the daytime.
But as the moment of totality drew near, the illumina-
tion of the atmosphere, and with it the brightness of
the rainbow-tinted streak, rapidly diminished. At last
the solar spectrum vanished ; and then What was it
replaced by ? What was found to be the spectrum of
the solar atmosphere close by the sun's surface? In
place of the rainbow-tinted riband crossed by thousands
and thousands of dark lines, there appeared a new and
most beautiful spectrum a riband of rainbow-tinted
lines, thousands in number and of all degrees of thick-
ness, hundreds of red lines, and then, in order,
hundreds of orange lines, hundreds of yellow, green,
indigo, and violet lines, like coloured cross-threads on a
black riband, only infinitely more beautiful. A charm-
110 LIGHT SCIENCE FOR LEISURE HOURS.
ing spectacle, truly, but so short-lived that no man can
ever hope, though he lived to four-score years and ten,
to let his eyes rest in all his life for more than ten or
twelve seconds on the beautiful array of coloured lines
which two men only have as yet beheld. We may in-
crease the dimensions and power of our telescopes until
the existence of these lines can be recognised without
the aid of eclipse-darkness, but the lines can never be
seen, save during eclipse, as Young and his colleague
saw them last December. And these observers tell us
that in a second or two the lines vanished, the ad-
vancing moon hiding the shallow solar atmosphere. If
it should ever be given to any man to see six total
eclipses (which has never yet happened to any), and to
successfully apply in each instance the method em-
ployed by Professor Young, then in all, during his life,
that man would have seen the beautiful line-spectrum
to perfection for some ten or twelve seconds ; but not
otherwise can even so long a total period of observation
be secured. No single observer, then, can hope to
lear^i much about the thousands of lines which have
still to be mapped during eclipse opportunities.
But now let us consider the import of the observa-
tion. What are these myriads of coloured lines?
Every dark line of the solar spectrum, says Professor
Young, seemed to have its representative in this
bright-line spectrum. Many of the groups of lines
which had flashed so quickly into view and endured
but so brief a period, were familiar to him ; in other
words, his study of the solar spectrum had made him
THE SUN'S TRUE ATMOSPHERE. Ill
conversant with the corresponding groups of dark lines.
It follows, then, beyond all possibility of question, that
the source of light was a highly complex atmosphere,
formed of those very vapours which, by their absorptive
power, produce the dark lines formed, that is, of the
vapours of iron and of copper, of zinc, sodium, magne-
sium, and of all those elements whose presence in the
sun's substance had been inferred from the study of
the solar spectrum.
Here, then, at length we have the true solar atmo-
sphere, an atmosphere of a highly complex nature, and
doubtless exceedingly dense near the visible surface of
the sun, because subject to a pressure so enormous.
The upper limit of this atmosphere cannot lie very far
above the sun's surface, at least not very far compared
with the sun's dimensions. Supposing the actual time
during which the line-spectrum was visible to have been
two seconds, then it is easy to tell how deep the atmo-
sphere is. For in two seconds the moon must have
traversed a space corresponding to about three hun-
dred miles at the sun's distance. An atmosphere three
hundred miles deep is, therefore, indicated by Pro-
fessor Young's observations. It need hardly be said,
however, that in the excitement of eclipse observation,
the estimate of minute intervals of time can scarcely
be relied upon, unless checked by instrumental arrange-
ments, which was not the case in the present instance.
We may fairly conclude that the depth of the solar
atmosphere lies between some such limits as a hundred
miles and five hundred miles.
112 LIGHT SCIENCE FOR LEISURE HOURS.
In the above estimate, I have supposed the measure-
ment to be made from the sun's visible surface. But
it is very unlikely that that surface is the true lower
limit of the atmosphere. It seems far more probable
that the surface we see is merely a layer of clouds (as
Sir William Herschel suggested so long ago) in the
solar atmosphere, and that the actual depth of the
atmosphere is more truly indicated by the appearances
seen when large sun-spots are examined. That these
spots are cavities has been abundantly established.
That they are openings through layers of solar clouds
has not been indeed demonstrated, yet it is difficult to
conceive how they can otherwise be interpreted. As
to the way in which the spots are formed, theorists are
at issue, some urging that there is an uprush from
depths beneath the solar surface ; others, that there is
a downrush of matter from without. But neither of
these views is in any way incompatible with Herschel's
theory, that the spots are openings in solar cloud-
layers.
We might thus be led to compare the solar atmo-
sphere with our own, though it will of course be
obvious that there are many marked points of differ-
ence. But in our own atmosphere we have at least
two distinct cloud-levels, the region, namely, where the
cumulus or wool-pack clouds are formed, and that
where the cirrus or feathery clouds, make their ap-
pearance. There is air above the cirrus clouds, air be-
tween the cirrus and cumulus layers, and air between
the cumulus clouds and the earth. And precisely in
THE SUN'S TRUE ATMOSPHERE. 113
the same way we may conceive that there exists at all
times a solar atmospheric region beneath as well as
above the cloud-layer which forms the sun's visible
surface, and beneath and between the other cloud-
layers revealed by telescopic observations.
But passing from the very difficult question sug-
gested by the consideration of regions below the sun's
visible surface, let us discuss briefly the bearing of
Professor Young's discovery upon our views respecting
those outer regions the coloured prominences and
sierra, the corona itself, and, in fine, all the portions
of space which lie above the true atmosphere.
In the first place, it seems to me that the observa-
tions made during the late eclipse dispose finally of
the theory that the coloured sierra is an atmospheric
envelope, properly so-called. I had long since been
led to question whether the sierra could be so regarded.
Let me remind the reader that the sierra is nothing
more nor less than the region which Lockyer redis-
covered in 1868. It had, in fact, been recognised by
1 telescopists since 1806, the name sierra having been
given to it by the observers of the eclipse of 1842. It
is a red region, having (as its name implies) a serrated
upper surface, as seen in the telescope, and seemingly
extending all round the sun's disc. The red pro-
minences appear to spring from its upper surface.
Strangely enough, when Lockyer made his ingenious
observations of the coloured prominences, he had not
, heard of this discovery, or had forgotten it. Accord-
ingly, finding traces of prominence-matter all round the
I
114 LIGHT SCIENCE FOR LEISURE HOURS.
sun, he concluded that there was a continuous envelope
of hydrogen (mixed with some other gases) surround-
ing the whole of the sun's globe. It was probably
through being misled by this supposition that he gave
to the sierra a new name entitling it the chromo-
sphere announcing at the same time that its upper
surface was smooth in outline. Respighi, the eminent
Italian spectroscopist also working, it would seem,
in ignorance or forgetfulness of the prior recognition
of the layer announced presently that the upper
surface of the so-called chromosphere l was altogether
irregular more irregular, in fact, than the surface of
a tempest-tossed sea. On re-examining the sierra,
Mr. Lockyer found this to be the case. But perhaps
the most striking evidence as to the real aspect of the
sierra was afforded during the eclipse of last December,
when Fr. Secchi, towards the close of totality, saw
around the western half of the moon's disc a complete
semicircle of sierra, and noted that this beautiful
coloured crescent was formed of multitudes of minute
1 It affords strange evidence of the caution with which new names
should be suggested, that this name, embodying, as we see, an erroneous
theory, and also perpetuating the remembrance of a mistaken claim, is
scarcely yet beginning to fall into disuse. Perhaps its Greek origin and
its length may have something to do with this ; for although astronomy
at least descriptive astronomy has hitherto not been disfigured by
the hideous nomenclature which botanists and geologists seem to rejoice
in, yet there is always a large class of science students who delight in
sesquipedal names, as giving an air of profundity to their discourse.
It may even be dangerous to hint that the true form of the compound
for a colour-sphere is not chromo -sphere, but chromato-sphere, since the
extra syllable will multiply tenfold the favour with which the compound
is accepted. When will the tyro learn that the true lover of science
'Projicit ampullas et sesquipedalia verba'?
THE SUNS TRUE ATMOSPHERE. 115
prominences. This agrees very satisfactorily with my
own anticipatory description of the probable nature of
the sierra, when I suggested that the sun's surface is
probably ' covered at all times with small prominences,
bearing somewhat the same relation to the gigantic
" horns " and " boomerangs " seen during eclipses
that the bushes covering certain forest regions bear to
the trees.'
But the larger prominences have been shown by
Zollner and Respighi to be phenomena of eruption.
They are masses of glowing gas, which have been flung
from great depths beneath the visible surface of the
sun. May we not conclude that the smaller prominences
which constitute the sierra are of like nature ? that they
also have been flung from beneath the sun's visible sur-
face ? As respects the larger prominences we can have
no manner of doubt, because they have been seen to be
flung out in eruptive sort. And this refers to all orders
of prominences, except only those very numerous and
relatively very small prominences which crowd together
so as to form the seemingly continuous coloured sierra.
These cannot be watched as the others have been. But
it seems highly probable that those among them which
are not the remains of loftier prominences, are, like
their larger fellows, phenomena of eruption.
Again, as respects the corona, all the evidence we
have is opposed to the conception that the phenomenon
is atmospheric. It shows two regions, which, though
not separated by well-defined limits from each other,
may yet be regarded as, in a sense, distinct. There is
i 2
Il6 LIGHT SCIENCE FOR LEISURE HOURS.
an inner and brighter portion, which the sesquipedalians
have proposed to call the leucosphere, apparently on
the lucus a non lucendo principle, for it is neither
white nor spherical. And there is the outer portion,
much less brilliant, and much more strikingly radiated.
Neither one part nor the other presents a single feature
suggestive of an atmospheric nature; 1 and the cer-
tainty that the two portions belong to a single object
affords yet more conclusive evidence against this in-
terpretation of the corona. But the rays of the corona
are of a somewhat remarkable nature. When well seen,
as during the eclipse of 1868, they are pointed ; and
even during so unfavourable an eclipse as that of
December last, the dark spaces between the rays are
seen to widen rapidly with increased distance from the
sun. These pointed radiations serve to show that
coronal rays must be, in reality, shaped somewhat as
cones, having their bases towards the sun. The idea
is startling enough, but, admitting the accuracy of the
pictures made during well-seen eclipses, and of the
Astronomer-Eoyars account of the corona during
the eclipses of 1851 and 1860, there is no escape
from the conclusion here stated. It is not more certain
that the sun is a globe, and not a flat disc as he
seems to be, than that the coronal radiations are not
flat pointed rays, but cone-shaped. Yet no one will
suppose that there are a number of monstrous cone-
1 I am here referring to the possibility that the corona may be due to
some species of solar atmosphere. The theory that the corona is due
to light in our own atmosphere has now at length been definitely aban-
doned by all astronomers.
THE SUN'S TRUE ATMOSPHERE. 117
shaped masses atmospheric or otherwise standing,
as it were, upon the sun's surface. I can see no other
way of accounting for these conical extensions than by
regarding them as phenomena indicating some form of
repulsive action exerted by the sun.
But whatever opinion we may form on this and
kindred problems, it seems clear that we must regard
the envelope discovered by Professor Young as the only
true solar atmosphere : and a very strange and com-
plex atmosphere it is. Nothing yet learned respecting
the sun's surroundings surpasses in interest this fiery
envelope, in which some of the most familiar of our
metals appear as glowing vapours. If anything could
add to the interest attaching to the coloured pro-
minences and sierra, it is the fact now revealed that
they are propelled through this wonderful envelope,
' over which they float for a while with strangely chang-
ing figure. Truly the study of solar physics, which
twenty years ago seemed at a stand-still, is advancing
with rapid strides ; and it seems scarcely possible to
exaggerate the interest either of what has been already
revealed, or of the discoveries which are likely to be
effected during the approaching eclipse.
From the St. Pauls Magazine for May 1871.
ADDENDUM. Doubts were urged, for some time after
this paper appeared, as to the reality of Young's dis-
covery. But during the total eclipse of December 1871,
and yet again during the annular eclipse of 1872,
decisive evidence was obtained in its favour, and it is
now received by all.
Il8 LIGHT SCIENCE FOR LEISURE HOURS,
SOMETHING WRONG WITH THE SUN.
WHEN we consider the intense heat which has prevailed
in Europe during July, and the circumstance that in
America also the heat has been excessive, insomuch
that in New York the number of deaths during the
week ending July 6 was three times greater than the
average, we are naturally led to the conclusion that
the sun himself is giving out more heat than usual.
Though not endorsing such an opinion, which, indeed,
is not warranted by the facts, since terrestrial causes
are quite sufficient to explain the recent unusual heats,
we cannot refrain from noting, as at least a curious
coincidence, that at the very time when the heat has
been so great, the great central luminary of the solar
system has been the scene of a very remarkable dis-
turbance an event, in fact, altogether unlike any
which astronomers have hitherto observed.
Now certain Italian spectroscopists Eespighi, Sec-
chi, Tacchini, and others have set themselves the
task of keeping a continual watch upon the chromato-
sphere. They draw pictures of it, and of the mighty
coloured prominences which are from time to time
upreared out of, or through, the chromatospheric en-
velope. They note the vapours which are present, as
well as what can be learned of the heat at which these
vapours exist, their pressure, their rate of motion, and
SOMETHING WRONG WITH THE SUN. 119
other like circumstances. It was while engaged in
some of the more difficult and delicate of these tasks
that Tacchini noticed the strange occurrence now to
be described.
' I have observed a phenomenon,' he says, ' which is
altogether new in the whole series of my observations.
Since May 6, I had found certain regions in the sun
remarkable for the presence of magnesium.' Some of
these extended half-way round the sun. This state of
things continued, the extension of these magnesium
regions gradually growing greater, until at length, on
June 18,' says Tacchini, 'I was able to recognise the
presence of magnesium quite round the sun that is
to say, the chromatosphere was completely invaded
by the vapour of this metal. This ebullition was ac-
companied by an absence of the coloured prominences,
while, on the contrary, the flames of the chromato-
sphere were very marked and brilliant. It seemed to
me as though I could see the surface of our great
source of light renewing itself.' While this was going
on Tacchini noticed (as had frequently happened before
in his experience) that the bright streaks on the sun
which are called faculge were particularly brilliant
close to those parts of the edge of the disc where the
flames of the chromatosphere were most splendid and
characteristic. The granulations also, which the as-
tronomer can recognise all over the sun, when a large
telescope is employed, were unusually distinct.
Tacchini concludes (and the inference seems just)
that there had not been a number of local eruptions of
120 LIGHT SCIENCE FOR LEISURE HOURS.
magnesium vapour, but complete expulsions. Only
we would venture to substitute for the word * expul-
sion ' the expression c outflow ' or ' uprising,' since it
may well be that these vapours rise by a quiet process
resembling evaporation, and not by any action so violent
that it could properly be regarded as expulsive.
In whatever way, however, the glowing vapour of
magnesium thus streamed into the envelope of the sun,
it would seem that the aspect of our luminary was
modified by the process not indeed in a very striking
manner, or our observers in England would have noticed
the change, yet appreciably. ' More than one person,'
says Tacchini, ' has told me that the light of the sun
has not at present its ordinary aspect ; and at the
Observatory we have judged that we might make the
same remark. The change must be attributed to mag-
nesium.'
It is impossible to consider attentively the remark-
able occurrence recorded by Tacchini without being
struck by the evidence which it affords of solar muta-
bility. We know that during thousands of years our
sun has poured forth his light and heat upon the
worlds which circle around him, and that there has
been no marked intermittence of the supply. We hear,
indeed, of occasions when the sun has been darkened
for a while ; and we have abundant reasons for be-
lieving that he has at times been so spot-covered that
there has been a notable diminution of the supply of
light and heat for several days together. Yet we
have had no reasons for anticipating that our sun
SOMETHING WRONG WITH THE SUN. 121
might permanently lose so much of his heat and lustre
that the inhabitants of earth would suffer. Tacchini's
observation reminds us, however, that processes are at
work upon the sun which admit of being checked or
increased, interrupted altogether or exaggerated so
violently, that the whole aspect of the sun, his condition
as the fire and lamp of the planetary system, may
be seriously affected.
If we only remember that our sun is one of the stars,
not in any way distinguished, unless perhaps by relative
insignificance, from the greater number of the stars
which illuminate our skies at night 01 are revealed
by the telescope, we shall learn to recognise the possi-
bility that he may undergo marked changes. There
are stars which after shining with apparent steadiness
for thousands of years (possibly for millions of years
before astronomy was thought of), have become sud-
denly much reduced in brightness, or after a few
flickerings (as it were) have gone out altogether.
There are others which have shone with equal steadi-
ness, and have then suddenly blazed out for a while
with a lustre exceeding a hundredfold that which they
formerly possessed. It would be equally unpleasant
for ourselves whether the sun suddenly lost the best
part of his light, and presently went out altogether, or
whether he suddenly grew fiftyfold brighter and hotter
than he now is. Yet in the present position of sidereal
astronomy, it is quite impossible to assert confidently
that one event or the other might not take place at
any time.
122 LIGHT SCIENCE FOR LEISURE HOURS.
Fortunately, we may view this matter (just as
astronomers have learned to view the prospect of mis-
chievous collisions with comets) as a question of proba-
bilities. Among so many thousands of stars there have
been so many sudden outbursts of light and fire, so
many sudden defalcations of splendour. Our sun is
one of those thousands, and so far as we know takes
his chance with the rest.
From the Spectator for August 1872.
L-u " ' '
........ ,,r-..^
NEWS FROM HERSCHEVS PLANET.
SATURN the altissimus planeta of the ancients re-
mains still the most distant planet respecting whose
physical condition astronomers can obtain satisfactory
information. The most powerful telescopes yet con-
structed have been turned in vain towards those two
mighty orbs which circle outside the path of distant
Saturn : from bevond the vast depths which .separate us
from Uranus and Neptune, telescopists can obtain little
intelligence respecting the physical habitudes of either
planet. Nor need we be surprised at the failure of
astronomers, when we consider the difficulties under
which the inquiry has been conducted. In comparing
the telescopic aspect of Uranus with that of Saturn (for
example) we must remember that Uranus is not only
twice as far from the earth but also twice as far from
NEWS FROM HERSCHEL'S PLANET. 123
the sun as Saturn is. So that the features of Uranus
are not merely reduced in seeming dimensions, in the
proportion of about one to four, but they are less
brilliantly illuminated in the same proportion. And
therefore (roughly) any given portion of the surface of
Uranus say a hundred miles square near the middle of
his visible disc sends to us but about one-sixteenth
part of the light which an equal and similarly-placed
portion of the surface of Saturn would send to us.
Now every astronomer knows how difficult it is, even
with very powerful telescopes, to study the physical
features of Saturn. A telescope of moderate power will
show us his ring-system and some of his satellites ; but
to study the belts which mark his surface, the aspect of
his polar regions, and in particular those delicate tints
which characterise various portions of his disc, requires
a telescope of great power. It will be understood,
therefore, that in the case of Uranus, which receives so
much less light from the sun, and is so much farther
from us, even the best telescopes yet made by man must
fail to reveal any features of interest. We may add
also that Uranus is a much smaller planet than Saturn,
though far larger than the combined volume of all the
four planets, Mars, Venus, the Earth, and Mercury. If
Saturn (without his rings) and Uranus were both visible
together in the same telescopic field (a circumstance
which may from time to time happen) the Herschelian
planet would appear so small and faint that it might
readily be taken for one of Saturn's moons, the ringed
124 LI GHT SCIENCE FOR LEISURE HOURS.
planet sending us altogether some sixty times as much
light as Uranus.
But what the telescope had hitherto failed to accom-
plish, has just been achieved by means of that wonderful
ally of the telescope, the spectroscope, in the able hands
of the eminent astronomer and physicist, Dr. Huggins.
News has been received about the constitution of the
atmosphere of Uranus, and news so strange (apart from
the strangeness of the mere fact that any information
could be gained at all respecting a vaporous envelope
so far away) as to lead us to speculate somewhat curi-
ously respecting the conditions under which the Ura-
nians, if there are any, have their being.
Before describing the results of Dr. Huggins's late
study of the planet, it may be well to give a brief
account of what is known respecting Uranus.
The question has been raised whether Uranus was
known to the astronomers of old times. There
is nothing altogether improbable in the supposi-
tion that in countries where the skies are unusually
clear, the planet might have been detected by its
motions. Even in our latitude Uranus can be quite
readily seen on clear and moonless nights, when favour-
ably situated. He shines at such times as a star of
about the fifth magnitude that is, somewhat more
brightly than the faintest stars visible to the naked
eye. In the clear skies of more southerly latitudes he
would appear a sufficiently conspicuous object, though,
of course, it would be wholly impossible for even the
most keen-sighted observer to recognise any difference
NEWS FROM HERSCHEL'S PLANET. 125
between the aspect of the planet and that of a star of
equal brightness. The steadiness of the light of Saturn
causes this planet to present a very marked contrast
with the first magnitude stars whose lustre nearly
equals his own. But although the stars of the lower
orders of magnitude scintillate like the leading orbs,
their scintillations are not equally distinguishable by
the unaided eye. Nor is it unlikely that if Uranus
were carefully watched (without telescopic aid) he
would appear to scintillate slightly. Uranus would
only be recognisable as a planet by his movements.
There seems little reason for doubting, however, that
even the motions of so faint a star might have been
recognised by some of the ancient astronomers, whose
chief occupation consisted in the actual study of the
star groups. We might thus understand the Burmese
tradition that there are eight planets, the sun, the moon,
Mercury, Venus, Jupiter, and Saturn, and another
named Rahu which is invisible. If Uranus was actually
discovered by ancient astronomers, it seems far from un-
likely that the planet was onJy discovered to be lost
again, and perhaps within a very short time. For if any-
thing positive had been learned respecting the revolu-
tion of this distant orb, the same tradition which
recorded discovery of the planet would probably have
recorded the nature of its apparent motions.
Be this as it may, we need by no means accept the
opinion of Buchanan, that if the Burmese tradition
relates to Uranus, Sir William Herschel must be
' stripped of his honours.' The rediscovery of a lost
126 LIGHT SCIENCE FOR LEISURE HOURS.
planet, especially of one which had remained concealed
for so many centuries, must be regarded as at least as
interesting as the discovery of a planet altogether nn-
known. Nor was there any circumstance in the actual
discovery of Uranus, which would lose its interest, even
though we accepted quite certainly the conclusion that
the Herschelian planet was no other than old Kahu. 1
Let us turn to HerscheFs own narrative of his detec-
tion of Uranus. It is in many respects very instructive.
In the first place, we must note the nature of the
work he was engaged upon. He had conceived the idea
of measuring the distances of the stars, or at least of
the nearer stars, by noting whether as the earth circles
around the sun the relative positions of stars lying very
close to each other seemed to vary in any degree. To
this end he was searching the heavens for those objects
which we now call double stars, most of which were in
his day supposed to be not in reality pairs of stars
that is, not physically associated together but seen
near together only because lying nearly in the same
direction. The brighter star of a pair was in fact sup-
1 It is, after all, at least as likely that Kahu assuming there really
was a planet known under this name might have been Vesta, the
brightest of the small planets which circle between Mars and Jupiter,
as the distant and slow-moving Uranus. For although Vesta is not
nearly so bright as Uranus, shining, indeed, only as a star of the
seventh magnitude, yet she can at times be seen without telescopic aid
by persons of extremely good sight ; and her movements are far more
rapid than those of Uranus. In the high table-lands of those eastern
countries, where some place the birth of astronomy, keen-sighted ob-
servers might quite readily have discovered her planetary nature,
whereas the slow movements of Uranus would probably have escaped
their notice.
NEWS FROM HERSCHEL'S PLANET. 12 7
posed to lie very much nearer than the fainter ; and it
was because, being so much nearer, the brighter star
should be much more affected (seemingly) by the
earth's motion around the sun, that Herschel hoped to
learn much by studying the aspect of these unequal
double stars at different seasons of the year. He hoped
yet more from the study of such bright orbs as are
surrounded by several very faint stars. It was a case
of this kind that he was dealing with, when accident
led him to the discovery of Uranus. ' On Tuesday, the
13th of March (1781),' he writes, 'between ten and
eleven in the evening, while I was examining the small
stars in the neighbourhood of Eta in Gremini, I per-
ceived one that appeared visibly larger than the rest.
Being struck with its uncommon magnitude, I com-
pared it to Eta and the small stars in the quartile
between Auriga and Gemini, and finding it so much
larger than either of them, suspected it to be a comet.
I was then engaged in a series of observations (which
I hope soon to have the opportunity of laying before
the Royal Society) requiring very high powers, and I
had ready at hand the several magnifiers of 227, 660,
932, 1,536, 2,010, &c., all of which I have successfully
used on that occasion. The power I had on when I
first saw the (supposed) comet was 227. From experi-
ence I knew that the diameters of the fixed stars are
not proportionally magnified with higher powers, as
those of the planets are ; therefore I now put on the
powers of 660 and 932, and found the diameter of the
comet increased in proportion to the power, as it ought
128 LIGHT SCIENCE FOR LEISURE HOURS.
to be on a supposition of its not being a fixed star,
while the diameters of the stars to which I compared
it were not increased in the same ratio. Moreover, the
comet being magnified much beyond what its light
would admit of, appeared hazy and ill-defined with
these great powers, while the stars presented that lustre
and distinctness which from many thousand observa-
tions I knew they would retain. The sequel has shown
that my surmises were well-founded.'
There are three points to be specially noted in this
account. First, the astronomer was engaged in a
process of systematic survey of the celestial depths so
that the discovery of the new orb cannot be properly
regarded as accidental, although Herschel was not at
the time on the look-out for as yet unknown planets.
Secondly, the instruments he was employing were of
his own construction and device, and probably no
other in existence in his day would have led him to
the discovery that the strange orb was not a fixed star.
And thirdly, without the experience he had acquired
in the study of the heavens he would not have been
able to apply the test which, as we have seen, he found
so decisive. The fact that the stars are not magnified
by increased telescopic power to the same extent as
planets or comets, is, as Professor Pritchard has justly
remarked, c an important result of the undulatory theory
of light, and was unsuspected in Sir William Herschel's
day.' So that whether we consider the work Herschel
was engaged upon, the instruments he used, or the ex-
perience he had acquired, we recognise the fact that he
NEWS FROM HERSCHEL'S PLANET. 129
alone of the astronomers of his time was capable of dis-
covering Uranus otherwise than by a fortunate accident.
Others might have lighted on the discovery indeed,
we shall presently see that the real wonder is that
Uranus had not been for many years a recognised mem-
ber of the solar system but no one except Herschel
could within a few minutes of his first view of the planet
have pronounced confidently that the strange orb (what-
ever it might be) was not a fixed star.
I do not propose to enter here, at length, into the
series of researches by which it was finally demonstrated
that the newly-discovered body was not a comet but a
planet, travelling on a nearly circular path around the
sun, at about twice Saturn's distance from that orb.
With this part of the work Herschel had very little to
do. To use Professor Pritchard's words, having ascer-
tained the apparent size, position, and motion of the
stranger, ' Herschel very properly consigned it to the
care of those professional astronomers who possessed
fixed instruments of precision in properly constituted
observatories to Dr. Maskelyne, for instance, who was
then the Astronomer-Eoyal at Greenwich, and to
Lalande, who presided over the observatory in Paris.'
As the newly-discovered body travelled onwards upon
its apparent path, astronomers gradually acquired the
means of determining what its real path might be.
At first they were misled by erroneous measures of the
stranger's apparent size, which suggested that the sup-
posed comet had in the course of the first month after
its discovery approached to within half its original
K
130 LIGHT SCIENCE FOR LEISURE HOURS.
distance. At length, setting aside all these measures,
and considering only the movements of the stranger,
Professor Saron was led to the belief that it was no
comet, but a member of the solar system. It was
eventually proved, chiefly by the labours of Lexell,
Lalande, and the great mathematician Laplace, that
this theory fully explained all the observed motions of
the newly-discovered body, and before long (so complete
is the mastery which the Newtonian system gives astro-
nomers over the motions of the heavenly bodies) all the
circumstances of the new planet's real motions became
very accurately known. It was now possible, not only
to predict the future movements of the stranger, but
to calculate his motions during former years. This last
process was quickly applied to the planet, with the
object of determining whether among the records of
observations made on stars, any might be detected
which related in reality to the newly-discovered body.
The result will appear at first sight somewhat surpris-
ing. The new planet had actually been observed no
less than nineteen times before that night when Herschel
first showed that it was not a fixed star, and those
observations were made by astronomers no less eminent
than Flamstead, Bradley, Mayer, and Lemonnier.
Flamstead had seen the planet five several times, each
time cataloguing it as a star of the sixth magnitude, so
that five such stars had to be dismissed from Flam-
stead's lists. But the case of Lemonnier was even more
singular ; for he had actually observed the planet no
less than twelve times, several of his observations having
NEWS FROM HERSCHEL'S PLANET. ill
\J
been made within the space of a few weeks. ' M. Arago
naturally comments,' says Professor Pritchard, ' on the
want of system displayed by Lemonnier in 1769 ; had
he but reduced and arranged his observations in a
properly-constructed register, his name instead of
Herschel's would have been attached for all time to one
of the starry host. But Lemonnier was not a man of
order ; his astronomical papers are said to have been a
very picture of chaos ; and M. Bouvard, to whom we
have long been indebted for the best tables of the new
planet, narrates that he had seen one of Lemonnier's
observations of this very star written on a paper bag
which had contained hair powder I '
In our days, when fresh planets are being discovered
and named in the course of each year that passes, it
may appear strange that much difficulty was found in
assigning a suitable name to the stranger. But we
must remember that for ages the planetary system had
been supposed to comprise no other primary members
than those known to the ancients. The discovery of
Uranus was an altogether novel and unlooked-for cir-
cumstance. It was not supposed that fresh discoveries
of like nature would be made, still less that a planet
would hereafter be discovered under circumstances far
more interesting even than those which attended the
discovery of Uranus. Accordingly a mighty work was
made before Uranus was fitted with a name. Lalande
proposed the name of the discoverer, and the new planet
was indeed long known on the Continent by the name
of Herschel. The symbol of the planet ( $ ), the initial
K 2
132 LIGHT SCIENCE FOR LEISURE HOURS.
letter of Herschel's name with a small globe attached
to the cross-stroke, still reminds us of the honour which
Continental astronomers generously proposed to render
to their fellow-worker in England. 1 Lichtenberg pro-
posed the name of Astrsea, the goddess of justice for
this ' exquisite reason,' that since justice had failed to
establish her reign upon earth, she might be supposed
to have removed herself as far as possible from our
unworthy planet. Poinsinet suggested that Cybele
would be a suitable name ; for since Saturn and Jupiter,
to whom the gods owed their origin, had long held their
seat in the heavens, it was time to find a place for Cybele,
'the great mother of the gods.' Had the supposed
Greek representative of Cybele Ehsea been selected
for the honour, the name of the planet would have
approached somewhat nearly in sound, and perhaps in
signification, to the old name Kahu. But neither
Astraea nor Cybele were regarded as of sufficient dignity
and importance among the ancient deities to supply a
name for the new planet. 2 Prosperin proposed Neptune
as a suitable name, because Saturn would thus have the
1 There is a certain incongruity, accordingly, among the symbols of
the primary planets. Mercury is symbolised by his caduceus, Venus by
her looking-glass (I suppose), Mars by his spear and shield, Jupiter by
his throne, Saturn by his sickle ; and again, when we pass to the sym-
bols assigned to the planets discovered in the present century, we find
Neptune symbolized by his trident, Vesta by her altar, Ceres by her
sickle, Minerva by a sword, and Juno by a star-tipped sceptre. Uranus
alone is represented by a symbol which has no relation to his position
among the deities of mythology.
2 Both these names are found among the asteroids, the fifth of these
bodies (in order of discovery) being called Astraea, the eighty-ninth
being named after the great mother of gods and goddesses.
NEWS FROM HERSCHEL'S PLANET. 133
eldest of his sons on one side of him, and his second
son on the other. Bode at length suggested the name
of Uranus, the most ancient of the deities ; and as
Saturn, the father of Jupiter, travels on a wider orbit
than Jupiter, so it was judged fitting that an even wider
orbit than Saturn's should be adjudged to Jupiter's
grandfather. In accepting the name of Uranus for the
new planet, astronomers seemed to assert a belief that
no planet would be found to travel on a yet wider path ;
and accordingly when a more distant planet was dis-
covered, the suggestion of Prosperin had to be recon-
sidered; but it was too late to change the accepted
nomenclature, and accordingly the younger brother of
Jupiter has had assigned to him a planet circling out-
side the paths of the planets assigned to their father and
grandfather. It may be noted, also, that a more appro-
priate name for the new planet would have been Crelus,
since all the other planets have received the Latin
names of the deities.
Herschel himself proposed another name. As Gralileo
had called the satellites of Jupiter the Medicean planets,
while French astronomers proposed to call the spots on
the sun the Bourbonian stars, so Hersohel, grateful for
the kindness which he had received at the hands of
Greorge III., proposed that the new planet should be
called Greorgium Sidus. On account of the interest
attaching to all Herschel's remarks respecting his dis-
covery, I quote in full the letter in which he submitted
this proposition to Sir Joseph Banks, then the President
of the Royal Society. ' By the observations of the most
134 LIGHT SCIENCE FOR LEISURE HOURS.
eminent astronomers in Europe,' he remarks, ' it appears
that the new star, which I had the honour of pointing
out to them in March 1781, is a primary planet of our
solar system. A body so nearly related to us by its
similar condition and situation in the unbounded ex-
panse of the starry heavens, must often be the subject
of the conversation, not only of astronomers, but of
every lover of science in general. This consideration,
then, makes it necessary to give it a name, whereby it
may be distinguished from the rest of the planets and
fixed stars. In the fabulous ages of ancient times, the
appellations of Mercury, Venus, Mars, Jupiter, and
Saturn, were given to the planets, as being their princi-
pal heroes and divinities. In the present more philo-
sophical era, it would be hardly allowable to have
recourse to the same method, and call on Juno, Pallas,
Apollo, or Minerva, for a name to our new planet.
The first consideration in any particular event or re-
markable incident seems to be its chronology ; if, in
any future age it should be asked when this last-found
planet was discovered, it would be very satisfactory to
say, " In the reign of George III." As a philosopher,
then, the name of Greorgium Sidus presents itself to me
as an appellation which will conveniently convey the
information of the time and country where and when
it was brought to view. But as a subject of the best
of kings, who is the liberal protector of every art and
science ; as a native of the country from whence this
illustrious family was called to the British throne ; as
a member of that society which flourishes by the dis-
NEWS FROM HERSCHEL'S PLANET. 135
tinguished liberality of its royal patron ; and last of all
as a person now more immediately under the protection
of this excellent monarch, and owing everything to his
unlimited bounty, I cannot but wish to take this oppor-
tunity of expressing my gratitude by giving the name
of Greorgium Sidus
' Georgium sidus
-jam nunc assuesce vocari,'
to a star which, with respect to us, first began to shine
under his auspicious reign.' Herschel concludes by
remarking that, by addressing this letter to the Presi-
dent of the Royal Society, he takes the most effectual
method of communicating the proposed name to the
literati of Europe, which he hopes ' they will receive
with pleasure. 5
Herschel's proposition found little favour, however,
among Continental astronomers. Indeed it is some-
what singular that for some time two names came into
general use one in Great Britain and the other on
the Continent, neither being the name eventually
adopted for the planet. In books published in Eng-
land for more than a quarter of a century after the
discovery of Uranus we find the planet called either
the Greorgium Sidus, or the Georgian. For a shorter
season the planet was called on the Continent either
the Herschelian planet, or simply Herschel. Many
years elapsed before the present usage was definitely
established.
In considering Herschel's telescopic study of the
planet, we must remember that, owing to the enormous
136 LIGHT SCIENCE FOR LEISURE HOURS.
length of time occupied by Uranus in circling round
his orbit, the astronomer labours under a difficulty
distinct in character from the difficulties which have
already been considered. As Jupiter and Saturn circle
on their wide orbits they exhibit to us the former
in the course of eleven years, the latter in the course
of twenty-nine and a half years all those varying pre-
sentations which correspond to the seasons of these
planets. Jupiter, indeed, owing to the uprightness of
his axis (with reference to his path) presents but slight
changes. But Saturn's globe is at one time bowed
towards us, so that a large portion of his north polar
regions can be seen, and anon (fifteen years later) is so
bowed, that a large portion of his southern polar
regions can be seen ; while between these epochs we
see the globe of Saturn so posed that both poles are
.on the edge of his disc, and then only does the shape
of his disc indicate truly the compression or polar
flattening of the planet.
But although similar changes occur in the case of
Uranus, they occupy no less than eighty-four years in
running through their cycle, or forty-two years in com-
pleting a half cycle during which, necessarily, all
possible presentations of the planet are exhibited.
Now it is commonly recognised among telescopists that
the observing time of an astronomer's life that is, the
period during which he retains not merely his full skill,
but the energy necessary for difficult researches
continues but about twenty-five years at the outside.
So that few astronomers can hope to study Uranus in
NEWS FROM HERSCHEL' S PLANET. 137
all his presentations, as they can study Mars, or Jupiter,
or Saturn.
When we add to this circumstance the extreme
faintness of Uranus, we cannot wonder that Herschel
should have been unable to speak very confidently on
many points of interest. His measures of the planet's
globe were sufficiently satisfactory, and, combined with
modern researches, show that Uranus has a diameter
exceeding the earth's rather less than four and a half
times. Thus the surface of Uranus exceeds that of our
globe about twenty times, and his bulk is more than
eighty times as great as the earth's. His volume, in
fact, exceeds the combined volume of Mercury, Venus,
the Earth, and Mars, almost exactly forty times. But
Sir W. Herschel was unable to measure the disc of
Uranus in such a way as to determine whether the
planet is compressed in the same marked degree as
Jupiter and Saturn. All that he felt competent to
say was that the disc of the planet seemed to him to
be oval, whether he used his seven-feet, or his ten-feet,
or his twenty-feet reflector. Arago has expressed some
surprise that Herschel should have been content with
such a statement. But in reality the circumstance is
in no way surprising. For as a matter of fact
Herschel had been almost foiled by the difficulty of
measuring even the planet's mean diameter. The dis-
cordance between his earliest measures is somewhat
startling. His first estimate of the diameter made it
ten thousand miles too small (its actual value being
about thirty-four thousand miles); his next made it
138 LIGHT SCIENCE FOR LEISURE HOURS.
nearly three thousand miles too great ; while his third
made it ten thousand miles too great. His contem-
poraries were even less successful. Maskelyne, after a
long and careful series of observations, assigned to the
planet a diameter eight thousand miles too small ; the
astronomers of Milan gave the planet a diameter more
than twenty thousand miles too great ; and Mayer, of
Mannheim, was even more unfortunate, for he assigned
to the planet a diameter exceeding its actual diameter
of thirty-four thousand miles, by rather more than
fifty thousand miles. It will be understood, therefore,
that Herschel might well leave unattempted the task
of comparing the different diameters of the planet.
This task required that he should estimate a quantity
(the difference between the greatest and the least
diameters) which was small even by comparison with
the errors of his former measurements.
But besides this, a peculiarity in the axial pose of
Uranus has to be taken into account. I have spoken
of the uprightness of Jupiter's axis with reference to
his path ; and by this I have intended to indicate the
fact that if we regard Jupiter's path as a great level
surface, and compare Jupiter to a gigantic top spinning
upon that surface, this mighty top spins with a nearly
upright axis. In the case of Uranus the state of
things is altogether different. The axis of Uranus is
so bowed down from uprightness as to be nearly in the
level of the planet's path. The result of this is that
when Uranus is in one part of his path his northern
pole is turned almost directly towards us. At such a
NEWS FROM HBRSCHEVS PLANET. 139
time we should be able to detect no sign of polar
flattening even though Uranus were shaped like a
watch-case. At the opposite part the other pole is as
directly turned towards the earth. Only at the parts
of his path between these two can any signs of com-
pression be expected to manifest themselves; and Uranus
occupies these portions of his path only at intervals of
forty-two years.
Herschel would have failed altogether in determining
the pose of Uranus but for his discovery that the
planet has moons. For the moons of the larger planets
travel for the most part near the level of their planet's
equator. We can, indeed, only infer this in the case
of Uranus (for even the best modern measurements
cannot be regarded as satisfactorily determining the
figure of his globe), but the inference is tolerably safe.
For six years Herschel looked in vain for Uranian
satellites. His largest telescopes, supplemented by his
wonderful eyesight and his long practice in detecting
minute points of light, failed to reveal any trace of
such bodies. At length he devised a plan by which
the light-gathering power of his telescopes was largely
increased. On the llth of January, 1787, he detected
two satellites, though several days elapsed before he
felt j ustified in announcing the discovery. At intervals,
during the years 1790-1798, he repeated his observa-
tions ; and he supposed that he had discovered four
other satellites. He expresses so much confidence as to
the real existence of these four bodies, that it is very
difficult for those who appreciate his skill to understand
140 LIGHT SCIENCE FOR LEISURE HOURS.
how he could have been deceived. But he admits that
he was unable to watch any of these satellites through
a considerable part of its path, or to identify any of
them on different nights. All he felt sure about was
that certain points of light were seen which did not
remain stationary, as would have happened had they
been fixed stars. No astronomer, however, has since
seen any of these four additional satellites, though Mr.
Lassell has discovered two which Herschel could not
see (probably owing to their nearness to the body of the
planet). As Mr. Lassell has employed a telescope more
powerful than Herschel's largest reflector, and has
given much attention to the subject, no one has a
better right to speak authoritatively on the subject of
the four additional satellites. Since, therefore, he is very
confident that they have no existence, I feel bound to
represent that view as the most probable ; yet I am
unable to pass from the subject without expressing a
hope that one of these days new "[Iranian satellites will
be revealed.
The four known moons travel backwards; that is,
they circle in a direction opposed to that in which all
the planets of the solar system, and all the moons of
Jupiter and Saturn, as well as our own moon, are
observed to travel. Much importance has been attached
to this peculiarity; but in reality the paths of the
Uranian moons are so strangely situated with respect
to the path of Uranus, that the direction in which they
travel can hardly be compared with the common direc-
tion of the planetary motions. Imagine the path of
NJSJTS FROM HERSCHEL'S PLANET. 141
Uranus to be represented by a very large wooden hoop
floating on a sheet of water ; then, if a small wooden
hoop were so weighted as to float almost upright, with
one half out of the water, the position of that hoop
would represent the position of the path of one of the
planet's satellites. It will be seen at once that if we
suppose a body to travel round (and upon) the former hoop
in a certain direction, then a body travelling round the
latter hoop could scarcely be said to travel in the same
direction, whether it circled one way or the other.
Or to employ another illustration if a watch be laid
face upward on a table, we should correctly say that its
hands move from east through south to west ; but, if
it be held nearly upright and the face rather upwards,
we should scarcely say that the hands moved from
east through south to west ; nor if the face were tilted
a little further forward, so as to be inclined rather
downwards, should we say that the hands move from
east through north to west.
The great slope or tilt of the paths is undoubtedly a
more singular feature than the direction of motion.
Implying as it does that the planet's globe is similarly
tilted, it suggests the strangest conceptions as to the
seasonal changes of the planet. It seems impossible
to suppose that the inhabitants of Uranus, if there are
any, can depend on the sun for their supply of heat.
The vast distance of Uranus from the sun, although
reducing the heat-supply to much less than the three-
hundredth part of that which we receive, is yet an
insignificant circumstance by comparison with the axial
142 LIGHT SCIENCE FOR LEISURE HOURS.
tilt. One can understand at least the possibility that
some peculiarity in the atmosphere of the planet
might serve to remedy the effects of the former cir-
cumstance ; precisely as our English climate is tempered
by the abundant moisture with which the air is ordi-
narily laden. But while we can conceive that the
minute and almost starlike sun of the Uranian skies
may supply much more heat than its mere dimensions
would lead us to expect, it is difficult indeed to under-
stand how the absence of that sun for years from the
Uranian sky can be adequately compensated. Yet in
Uranian latitudes corresponding to the latitude of
London the sun remains below the horizon for about
twenty-three of our years in succession. Such is the
Arctic * night of regions in Uranus occupying a posi-
tion corresponding to that of places in our temperate
zone.
But the most important result of the discovery of
the satellites has been the determination of the mass
or weight of the planet, whence also the mean density
of its substance has been ascertained. It has been thus
discovered that, like Jupiter and Saturn, Uranus is con-
structed of much lighter materials than the earth. Our
earth would outweigh almost exactly six times a globe as
large as the earth but no denser than Uranus. It is to be
1 It has been remarked that there is some incongruity in the name
Arctic planets which I have assigned in my ' Other Worlds ' to Uranus
and Neptune, when considered with reference to the theory I have enun-
ciated that these planets still retain an enormous amount of inherent
heat. Many seem to imagine that the term arctic implies cold. I have,
of course, only used the name as indicating the distance of Uranus and
Neptune from the sun.
NEWS FROM HERSCHEL'S PLANET. 143
noticed that in this respect the outer planets resemble
the sun, whose density is but about one-fourth that of
the earth. It seems impossible that the apparent size
of any one of the outer planets can truly indicate the
dimensions of its real globe. An atmosphere of
enormous extent must needs surround, it would seem,
the liquid or solid nucleus which probably exists within
the orb we see.
In the case of Jupiter or Saturn, the telescope has
told us much which bears on this point ; and as I have
indicated in my ' Other Worlds,' and elsewhere, there is
an overwhelming mass of evidence in favour of the theory
that those orbs are still instinct with their primeval
fires. But in the case of Uranus, it might well be
deemed hopeless to pursue such inquiries, otherwise
than by considering the analogy of the two larger
planets. Direct evidence tending to show that the
atmosphere of Uranus is in a condition wholly differing
from that of our own atmosphere, cannot possibly be
obtained by means of any telescopes yet constructed by
men. Some astronomers assert that they have seen
faint traces of belts across the disc of Uranus ; but the
traces must be very faint indeed, since the best tele-
scopes of our day fail to show any marks whatever
upon the planet's face. Even if such belts can be
seen, their changes of appearance cannot be studied
systematically.
It is, however, on this very subject the condition
of the planet's atmosphere that the discovery I have
now to describe throws light.
144 LIGHT SCIENCE FOR LEISURE HOURS.
Faint as is the light of Uranus, yet, when a telescope
of sufficient size is employed, the spectrum of the
planet is seen as a faint rainbow-tinted streak. The
peculiarities of this streak, if discernible, are the means
whereby the spectroscopist is to ascertain what is the
condition of the planet's atmosphere. Now, Father
Secchi, studying Uranus with the fine eight-inch tele-
scope of the Roman Observatory, was able to detect
certain peculiarities in its spectrum, though it would
now appear that (owing probably to the faintness of
the light) he was deceived as to their exact nature.
He says : ' The yellow part of the spectrum is wanting
altogether. In the green and the blue there are two
bands, very wide and very dark.' But he was unable
to say what is the nature of the atmosphere of the
planet, or to show how these peculiarities might be
accounted for.
Recently, however, the Royal Society placed in the
hands of Dr. Huggins a telescope much more powerful
than either the Roman telescope or the instrument with
which Dr. Huggins had made his celebrated observa-
tions on sun and planets, stars and star- cloudlets. It
is fifteen inches in aperture, and has a light-gathering
power fully three times as great as that possessed by
either of the instruments just mentioned.
As seen by the aid of this fine telescope the spectrum
of Uranus is found to be complete, 'no part being
wanting, so far as the feebleness of its light permits it
to be traced.' But there are six dark bands, or strong
lines, indicating the absorptive action of the planet's
NEWS FROM HERSCHEZS PLANET. 145
atmosphere. One of these strong lines corresponds in
position with one of the lines of hydrogen. Now it
may seem at a first view that since the light of Uranus
is reflected solar light, we might expect to find in the
spectrum of Uranus the solar lines of hydrogen. But
the line in question is too strong to be regarded as
merely representing the corresponding line in the solar
spectrum ; indeed, Dr. Huggins distinctly mentions
that ' the bands produced by planetary absorption are
broad and strong in comparison with the solar lines.'
We must conclude, therefore, that there exists in the
atmosphere of Uranus the gas hydrogen, sufficiently
familiar to us as an element which appears in combi-
nation with others, but which we by no means recognise
as a suitable constituent (at least to any great extent)
of an atmosphere which living creatures are to breathe. 1
And not only must hydrogen be present in the atmo-
sphere of Uranus, but in such enormous quantities as
to be one of the chief atmospheric constituents. The
strength of the hydrogen line cannot otherwise be
accounted for. If by the action of tremendous heat
all the oceans of our globe could be changed into their
constituent elements, hydrogen and oxygen, it is pro-
bable that the signs by which an inhabitant of Venus
or Mercury could recognise that such a change had
taken place would be very much less marked than the
signs by which Dr. Huggins has discovered that hydro-
1 Traces of hydrogen can nearly always be detected in the air, but
the quantity of hydrogen thus shown to be present is almost infinitesi-
mally small compared with the amount of oxygen and nitrogen.
L
146 LIGHT SCIENCE FOR LEISURE HOURS.
gen exists in the atmosphere of Uranus. It will indeed
be readily inferred that this must be the case, when
the fact is noted that no signs whatever of the exist-
ence of nitrogen can be recognised in the spectrum of
Uranus, though it is difficult to suppose that nitrogen
is really wanting in the planet's atmosphere. Dr.
Huggins also notes that none of the lines in the spec-
trum of Uranus appear to indicate the presence of
carbonic acid. Nor are there any lines in the spectrum
of Uranus corresponding to those which make their
appearance in the solar spectrum when the sun is low
down, and is therefore shining through the denser
atmospheric strata. Most of these lines are due to
the presence of aqueous vapour in our atmosphere,
and it would seem to follow that if the vapour of
water exists at all in the atmosphere of Uranus its
quantity must be small compared with that of the free
hydrogen.
Admitting that the line seen by Dr. Huggins is
really due to hydrogen a fact of which he himself has
very little doubt we certainly have a strange discovery
to deal with. If it be remembered that oxygen, the
main supporter of such life as we are familiar with,
cannot be mixed with -hydrogen without the certainty
that the first spark will cause an explosion (in which
the whole of one or other of the gases will combine
with a due portion of the other to produce water), it
is difficult to resist the conclusion that oxygen must be
absent from the atmosphere of Uranus. If hydrogen
could be added in such quantities to our atmosphere
NEWS FROM HERSCHEL'S PLANET. 147
as to be recognisable from a distant planet by spectro-
scopic analysis, then no terrestrial fires could be lighted,
for a spark would produce a catastrophe in which all
living things upon the earth, if not the solid earth
itself, would be destroyed. A single flash of lightning
would be competent to leave the earth but a huge
cinder, even if its whole frame were not rent into a
million fragments by the explosion which would
ensue.
Under what strange conditions then must life exist
in Uranus, if there be indeed life upon that distant
orb. Either our life-sustaining element, oxygen, is
wanting ; or, if it exists in sufficient quantities (ac-
cording to our notions) for the support of life, then
there can be no fire, natural or artificial, on that giant
planet. It seems more reasonable to conclude that,
as had been suspected for other reasons, the planet is
not at present in a condition which renders it a suitable
abode for living creatures.
The St. PattTs Magazine for October 1371.
THE TWO COMETS OF THE YEAR 1868.
PART I. BRORSEN'S COMET.
TEN years ago, all that astronomers could hope to do
with comets was to note their appearance and changes
of appearance when viewed with high telescopic powers.
There was one instrument, indeed, the polariscope,
L 2
148 LIGHT SCIENCE FOR LEISURE HOURS.
which afforded doubtful evidence respecting the quality
of the light we receive from comets, and thus allowed
astronomers to form vague guesses respecting the struc-
ture of these mysterious wanderers. But beyond the
unsatisfactory indications of this instrument, astrono-
mers had no means whatever of ascertaining the phy-
sical nature of comets.
At present, however, an instrument of incomparably
higher powers is applicable to the inquiry. The spec-
troscope has the power of revealing, not only the
general character of any substance which is a source
of light, but even of exhibiting, in many instances,
the elementary constitution of such a substance. The
indications of this wonderful instrument of analysis
are not affected by the distance or dimensions of the
object under examination. So long as the object is
luminous the spectroscope will tell us with the utmost
certainty whether the light is inherent or reflected ;
and if the light is inherent that is, if the object is
self-luminous the spectroscope will tell us with the
utmost certainty what terrestrial elements (if any)
exist in the constitution of the object. It is with the
revelations of the spectroscope respecting Brorsen's
comet that I now propose to deal. I must make a
few preliminary remarks, however, respecting the vari-
ous peculiarities of structure which have been presented
by comets.
I assume that my readers are familiar with the
general appearance presented by comets at least by
those which are visible to the naked eye. It may be
THE TWO COMETS OF THE YEAR 1868. 149
necessary to note, however, of the three features com-
monly recognised in comets viz. the nucleus, coma,
and tail the coma alone is invariably exhibited. A
comet which has neither nucleus nor tail presents
simply a round mass of vapour slightly condensed
towards the centre. The nucleus, when seen, appears
as a bright point within the condensed part of a comet.
The tail, as every one knows, is a long train of light
issuing from the head.
It was noted in very early times that comets are
almost perfectly translucent. . This peculiarity has
been confirmed by modern and more exact observations.
Sir W. Herschel watched the central passage of a comet
over the fainter component of a double star ; and he
could detect no diminution of the star's brilliancy.
Similar observations were made by MM. Olbers and
Struve. Sir John Herschel watched the passage of
Biela's comet over a small cluster of very faint tele-
scopic stars. The slightest haze would have oblite-
rated the cluster, yet no appreciable effect was pro-
duced by the interposition of cometic matter having a
thickness (according to Herschel's estimate) of 50,000
miles. And there is another remarkable evidence of
tenuity. From recognised optical principles, a star
seen through the globular head of a comet, should
appear displaced from its true position just as any
object seen (non-centrally) through a globular decanter
full of water seems thrown out of its true place. The
astronomer Bessel made an observation on a star which
approached within about eight seconds of the nucleus
150 LIGHT SCIENCE FOR LEISURE HOURS.
of Halley's comet, and he found that the place of the
star was not affected to an appreciable extent. '
Whether the nucleus of a comet is solid or not had
long been a disputed point among astronomers. With
telescopes of moderate power the bright point within
the coma presents an appearance of solidity which
might readily deceive the observer. But with an
increase of power the nucleus assumes a different
appearance. Instead of having a well-defined outline
it appears to merge into the coma by a somewhat rapid
gradation but not by an abrupt variation of light.
Grood observers have reported the extinction of tele-
scopic stars behind the nuclei of comets, but there are
peculiar difficulties about an observation of this sort ;
and it is very difficult to determine whether a star is
really concealed from view by the interposition of the
nucleus or simply obliterated by the glare of light.
The tail of a comet appears nearly always as an
extension from the coma, and a dark interval is usually
seen between the head and the tail. But there is an
immense variety in the configuration of comets' tails.
The comet of 1744 had six tails spread out like a fan.
The comet of 1807 had two tails both turned from
the sun. The comet of 1823 had also two tails, but
one was turned almost directly towards the sun. Other
comets have had lateral tails.
The processes which seem to be passed through by
comets during their approach towards and recession
from the sun have proved very perplexing to astrono-
mers and physicists. When first seen a comet usually
THE TWO COMETS OF T&E YEAR 1868. 1 5 1
appears as a light roundish cloud with a point of
brighter light near the centre. As it approaches the
sun the comet appears to grow considerably brighter
on the side turned towards him. An emanation of
light seems to proceed towards the sun for a short
distance and then to curl backwards and stream out
in a contrary direction. Gradually the backward
streaming rays extend to a greater distance the
nucleus continuing to throw out matter towards the
sun. Thus the tail is formed ; and it is often thrown
out to a distance of many millions of miles in a few
hours.
One of the most singular facts connected with the
approach and recession of a comet, is the peculiarity
that the comet grows gradually smaller and smaller as
it approaches perihelion, and swells out in a corre-
sponding manner as it passes away from the sun. The
comet of 1652 was observed by Hevelius to increase so
rapidly in dimensions as it passed away from the sun,
that between December 20 and January 12 its volume
had increased in the proportion of about 13,800 to 1.
When it was last visible this comet exceeded the sun
in volume. This observation, on which much doubt
had been thrown, has been confirmed by the researches
of the best modern observers. M. Struve measured
Encke's comet as it approached the sun towards the
end of the year 1828. He found that between October
28 and December 24 the comet collapsed to about the
.sixteen-thousandth part of its original volume. Sir
John Herschel found in like manner that Halley's
152 LIGHT SCIENCE FOR LEISURE HOURS.
comet when passing away from the sun increased in
volume upwards of fortyfold in a single week.
The tremendous heat to which many comets are sub-
jected during perihelion passage is an important point
for consideration, in attempting to form an opinion of
the physical structure of comets. Newton calculated
that the comet of 1680 was subjected to a heat 2,000
times greater than that of red-hot iron. But comets
have been known to approach the sun even more closely.
Sir John Herschel estimates that the comet of 1843
was subjected to a heat exceeding in the proportion of
24^ to 1 the heat concentrated in the focus of Perkins'
great lens. Yet the heat thus concentrated had suf-
ficed to melt agate, rock-crystal, and cornelian.
We cannot wonder that so great an intensity of heat
should have produced remarkable effects upon many
comets. The great wonder is that any comet should
resist the effects of such heat without being dissipated
into space.
We learn from Seneca that Ephorus, an ancient
Greek author, mentions a comet which divided into
two distinct comets. Kepler considered that two
comets which were seen together in 1618 had been
produced by the division of a single comet. Cysatus
noticed that the great comet of 1618 showed obvious
signs of a tendency to break up into fragments. This
comet when first seen appeared as a circular nebulous
cloud. A few weeks later it seemed to be divided into
several distinct cloudlike masses. On December 20
6 it resembled a multitude of small stars.'
THE TWO COMETS OF THE YEAR 1868. 153
We might doubt whether these observations were
entitled to credit were it not that, quite recently,
Biela's comet has been seen to separate into two dis-
tinct comets, each having a nucleus, coma, and tail,
and each of which pursued its course independently
until distance concealed both from view.
It is clear that nothing but a long series of careful
observations can put us in a position to theorise with
confidence, respecting the nature of comets, the pro-
cesses of change which they undergo, and the functions
which they subserve in the economy of the solar system.
We may therefore dwell with particular satisfaction on
the fact that every comet which has appeared during the
last two years has been subjected to careful observa-
tion, and that at length, by means of spectroscopic
analysis we are beginning to get hold of positive facts
respecting comets, and have promise of shortly being
able to form consistent theories with regard to these
singular members of the solar system.
I have had occasion in other works to speak of
the principles on which spectroscopic analysis depends ;
but I think it best briefly to restate the most im-
portant points. When the light from a luminous
object is received upon a prism, there is formed what
is called the prismatic spectrum. According to the
nature of the source of light this spectrum varies in
appearance. If the source of light is an incandescent
body the spectrum is a continuous, rainbow-tinted
streak. Where the 1 ight comes from an incandescent mass
surrounded with cooler vapours, the streak of rainbow-
154 LIGHT SCIENCE FOE LEISURE HOURS.
coloured light is crossed by dark lines whose position
indicates the nature of the vapours which the light has
traversed. When the light comes from luminous
vapours the spectrum consists wholly of bright lines ;
and these have exactly the same position as the corre-
sponding dark lines which are seen when the same
vapours intercept light from an incandescent solid
mass. Lastly, when light is reflected from an opaque
substance, the spectrum is the same as that which
would be presented by the light before reflection, unless
the opaque substance is surrounded by vapours, in
which case the spectrum will be crossed by new dark
lines corresponding to the absorptive qualities exerted
by those particular vapours.
We see then the wonderful qualities of the new
analysis. Applied to the sun and stars it has enabled
our physicists and astronomers to pronounce as con-
fidently that certain elements exist in these far distant
orbs, as the chemist can pronounce on the constitution
of substances submitted to his direct analysis. The
questions, or some of them, which have been at issue
respecting comets, will undoubtedly yield to the powers
of the spectroscope. The great want, at present, is a
brilliant comet to work upon. Donati's comet (1859),
or the great comet of 1861 would have served this
purpose admirably, but the first came in the very year
in which the principles of spectroscopic analysis were
first discovered; and the powers of the spectroscope
were only just beginning to be recognised when the
comet of 1861 made its brief visit to our northern
skies.
THE TWO COMETS OF THE YEAR 1868. 155
Two small comets have been analysed with the spec-
troscope, and each presented similar results. The
spectrum in each case consisted of thin bright lines on
a faint continuous streak of light. And from the fact
that the bright lines did not extend across the whole
breadth of the faint streak of light, it became evident
that they formed the spectrum of the nucleus, the
faint continuous spectrum belonging to the coma.
Hence it resulted that the nucleus of each of these
small comets consisted of self-luminous gas, while the
coma either consisted of incandescent solid matter or
shone by reflecting the light of the sun. The latter is
far the more probable hypothesis. In fact, when we
consider the extreme tenuity of the substance of a
comet, and therefore the certainty that if composed of
solid matter such matter must be dispersed in very
minute fragments, we shall recognise the extreme
improbability that these fragments should be self-
luminous through intensity of heat. If the comets
had been brighter, I may remark, there would have
been no dubiety respecting this point, since it would
have been possible to compare the continuous streak of
light with the solar spectrum, and by the resemblance
or dissimilarity of the two spectra, to determine whether
the coma really shines by reflecting the sun's light or
not.
Brorsen's comet has now been examined with the
spectroscope, and with results quite different from
.those which attended the analysis of the other two.
Dr. Huggins, the physicist, who examined the latter,
says of Brorsen's comet :
156 LIGHT SCIENCE FOR LEISURE HOURS.
' It appears in the telescope as a nearly round nebu-
losity, in which the light increases rapidly towards the
centre, where, on some occasions, I detected, I believe,
a small stellar nucleus. Generally, this minute nucleus
was not to be distinguished in the bright central part
of the comet. The spectrum consists for the most
part of three bright bands. The length of the bands
in the instrument shows that they are not due alone
to the stellar nucleus, but are produced by the light
of the brighter portions of the coma. I took some
pains to learn the precise character of these luminous
bands. When the slit was wide they resembled the
expanded lines seen in some gases. As the slit was
made narrow the two fainter bands appeared to fade
out without becoming more denned. I was unable to
resolve them into lines. The middle band, which is so
much brighter than the others that it may be con-
sidered to represent probably three-fourths, or nearly
so,* of the whole of the light which we receive from the
comet, appears to possess similar characters. In this
band, however, I detected occasionally two bright lines
which appear to be shorter than the band, and may be
due to the nucleus itself. .... Besides these bright
bands there was a very faint continuous spectrum.'
Interpreting these observations according to the
principles which have been already stated, we deduce
the following interesting results.
The nucleus of Brorsen's comet consists of luminous
gas. The coma is also gaseous in the neighbourhood
of the nucleus, but its outer portions are of a different
THE TWO COMETS OF THE YEAR 1868. 157
character and shine by reflecting the solar light. This
part of the coma may be either liquid or solid. There
is nothing opposed to the supposition that it is of
the nature of cloud that is, that it is produced by
the condensation of true vapour into minute liquid
globules.
Eeturning to the consideration of the gaseous part
of the comet the question will at once suggest itself
what the gases may be which constitute the substance
of the nucleus and coma. Here our information is not
quite so satisfactory as could be desired.
The brightest band is in the green part of the spec-
trum, and agrees very nearly with the brightest line in
the spectrum of nitrogen. The want of exact agree-
ment prevents us from assuming that nitrogen really
exists (in any form) in the substance of the comet.
The other lines of the spectrum of nitrogen are not
present in the spectrum of the comet : but this pecu-
liarity is not so perplexing as the other, for it is well
known that certain lines will disappear from the spectra
of hydrogen, nitrogen, and other gases, under particular
circumstances of illumination, temperature, and so
on.
Nor is the circumstance that there are bands of light
instead of well marked lines a peculiarity which need
cause perplexity. For under certain circumstances of
temperature and pressure, the lines of the spectra of
various gases become expanded or diffused until they
appear as bands of light.
The two fainter bands are yellow and blue, respec-
158 LIGHT SCIENCE FOR LEISURE HOURS.
lively. They cannot be identified with the lines seen
in the spectra of any known terrestrial gases.
Of whatever gases the nucleus is composed it appears
that conditions wholly different from any with which
we are familiar on earth prevail in this, and doubtless
in all other comets. The gases which form the nucleus,
though self-luminous, are probably not incandescent.
Remembering that comets are luminous when situated
far out in space beyond the orbit of our own earth, we
are prevented from assuming the existence of an inten-
sity of heat (due to solar action) sufficient to account
for their inherent light. And if the light of a comet
were due to a state of incandescence in the component
gases, there would be a rapid consumption of the
substance of the comet, and we should be quite
unable to account for the fact that Halley's comet
has continued to shine, with no appreciable loss of
brilliancy, for upwards of three hundred years. We
seem forced therefore to surmise that the gases which
form the substance of comets owe their light to a species
of phosphorescence which is independent of the comet's
temperature, or else to some electrical properties the
nature of which it would not be easy to divine.
Our perplexity is increased when we see the gases
which form the nuclei assuming either the liquid or
the solid form in the outer part of the coma. The
change from gaseity to liquidity or solidity is an evi-
dence of loss of heat, whereas one would expect the
outer part of the coma, which is exposed to the full
THE TWO COMETS OF THE YEAR 1868. 159
intensity of the sun's action, to be the most heated por-
tion of a comet's volume.
None of the comets which have been examined have
had a tail, so that we are unable as yet to form any
certain opinion respecting the nature of this portion of
a comet's volume. It seems almost certain, however,
that the tail shines by reflected light, because in every
known instance the tail has appeared as an extension
from the outer part of the coma, and may therefore be
expected to resemble that portion of the comet in its
general characteristics.
One of the comets which has been examined with
the spectroscope, though it has not a visible tail, has
been shown to have an appendage of a very remarkable
character, respecting which, also, we have been able to
learn several interesting particulars.
In the year 1866 a telescopic comet was discovered
by M. Tempel. This was the first comet examined by
Dr. Huggins. Its orbit was carefully calculated by the
German astronomer Oppolzer, and found to pass very
near the orbit of our own earth. Soon after this, Pro-
fessor Adams calculated the orbit of the November
shooting stars ; and to the surprise of the astronomical
world it was found that these minute bodies travel
along the very path in space which had been already
assigned to Tempel's comet. We need not here discuss
the circumstances of this discovery. Let it suffice to
state that all astronomers who are competent to form
an opinion on the subject are agreed that the Novem-
ber shooting-stars are certainly due to the existence of
l6o LIGHT SCIENCE FOR LEISURE HOURS.
a long- extended flight of cosmical bodies travelling in
the track of Tempel's comet.
Now it appears clear that this flight of cosmical
bodies may be looked upon as constituting an ex-
tension of the comet an invisible train as it were.
But for the accident that the comet's track intersects
the earth's path in space, we should have remained
for ever ignorant of the fact that the comet has
any other extent than that which is indicated by
its telescopic figure. Now, however, that we know
otherwise, we recognise the probability that other
comets which have been looked upon as tailless may
have invisible extensions reaching far behind them into
space, or even completely around their orbit.
But the members of the November shooting-star
system have been subjected to spectroscopic analysis.
We know that they contain several terrestrial elements ;
and we recognise the probability that if we could
examine one of them before its destruction (in tra-
versing our own atmosphere) we should find a close
resemblance in its constitution to that of those aero-
lites or meteorites which have reached the surface of
the earth.
But here we encounter a new difficulty. One theory
respecting the tails of comets has accounted for them
by the supposition of a propulsive effect exerted by the
solar rays ; and another theory has ascribed them to the
action of vapours ascending in the solar atmosphere.
But if the tails of comets really consist of solid matter
very widely dispersed, it must be quite evident that
THE TWO COMETS OF THE YEAR 1868. 161
neither of these causes could suffice to account for the
great extension of these appendages. Then the rapid
manner in which the tails seem to be formed remains
wholly mysterious. And we are also left without any
explanation of the rapid change of position exhibited by
the tail while the comet is sweeping around the sun
at the time of nearest approach to that luminary.
Sir John Herschel compared this motion to that of a
stick whirled round by the handle the whole extent
of the tail partaking in the movement as if comet and
tail formed a rigid mass.
The difficulties here discussed seem in the present
state of our knowledge wholly insoluble. In fact, it
seems impossible even to conceive of a solution to the
last mentioned phenomenon, so long as we look upon
the comet's tail as a distinct unvarying entity. For
instance, if the tail, a hundred millions of miles long,
which extended backwards from Halley's comet before
perihelion passage, consisted of the same matter as the
tail which projected forwards to the same extent a few
days later, then certainly there is nothing in our pre-
sent experience of matter and its relations which can
enable us to deal with so astounding a phenomenon.
It will be understood, of course, Sir John Herschel does
not say in so many words, that the tail of Halley's
comet was brandished round in the manner described
above, but that, although it appeared to move in
this manner, it is impossible so to conceive of its
motion.
M
1 62 LIGHT SCIENCE FOR LEISURE HOURS.
We refrain, however, from speaking further on a
point respecting which we have no means of reasoning
satisfactorily. Mere guess-work is an altogether
unprofitable resource in the discussion of scientific
matters.
Now that we have so powerful an instrument of re-
search as the spectroscope, there really seems hope
that even the hitherto inscrutable mysteries presented
by comets' tails may one day be interpreted. Each
comet which has been subjected to spectroscopic
analysis has revealed something new. Observations,
such as those which have been made on Brorsen's comet,
and on the two telescopic comets previously examined
by Dr. Huggins, are not merely valuable in themselves,
but as affording promise of what may be achieved when
some brilliant comet shall be subjected to spectroscopic
analysis. When we consider that all the comets yet
examined have been absolutely invisible to the naked
eye on the darkest night, whereas several of the great
comets have blazed forth as the most conspicuous
objects in the heavens, and have even been visible in
the full splendour of the midday sun, we see good
reason for the hope that far fuller information will be
gained respecting the structure of comets so soon as
one of the more important members of the family shall
have paid us a visit.
Whenever such an event may happen it is not likely to
find our spectroscopists unprepared. It is probable that,
before long, every important observatory will be sup-
plied with spectroscopes. Already some of the most
THE TWO COMETS OF THE YEAR 1868. 163
powerful telescopes in use have been fitted with them.
We hear also, that the giant reflector of the Parsons-
town Observatory commonly known as the Sosse tele-
scope has been armed with a spectroscope especially
constructed for the purpose by Mr. Browning, F.R.A.S.,
the optician. Not only in England, but at the princi-
pal Continental observatories, spectroscopic work is in
progress, and observers are daily becoming more and
more familiar with the powers of the new analysis.
Stars which are far too small to be viewed by the
naked eye have already been examined with the spec-
troscope. The Padre Secchi at Rome has just pub-
lished a list of minute red stars thus examined. It is
such delicate work as this which will fit observers to
deal with the difficulties involved in the spectroscopic
analysis of comets.
We shall see when we come to deal with the second
cometof the year 1868, that wehave yet better reason than
the analysis of Brorsen's comet has afforded, for hoping
that before long we may have interesting and exact
information respecting the structure of these mysterious
wanderers. We may even hope to gain some know-
ledge respecting the purposes which comets subserve in
the economy of the solar and sidereal systems.
PAET II. WINNECKE'S COMET.
IN the preceding pages I have described the principal
features presented by comets as they approach and pass
away from the neighbourhood of the sun. The various
K 2
1 64 LIGHT SCIENCE FOR LEISURE HOURS.
hypotheses which have been put forward to account for
these peculiarities must now for a brief space claim our
attention. Although we are far from being in a posi-
tion to theorise with any confidence respecting the
nature of comets, and still less as to the purposes which
they subserve in the economy of nature, yet the observa-
tions made upon the second comet of the year 1868 have
resulted in a positive discovery which may serve as a
stand-point, so to speak, whence we can examine some-
what more confidently than of old, the various theories
which have suggested themselves to those who have
studied cometic phenomena.
In considering these hypotheses we have to distinguish
between the views which have been entertained respect-
ing the nucleus and coma, and those which regard the
less intelligible phenomena presented by the tail. This
remark may seem trite and obvious, but in reality
the two classes of hypotheses are found singularly con-
founded together in many works on popular astronomy.
Let it be understood then, that when, in speaking of
an hypothesis respecting comets no special mention is
made of the tail, it is to be assumed that the hypothe-
sis applies solely to the head of the comet. The same
holds, by the way, with reference to the phenomena
presented by comets. For instance, when we said in
the paper on Comet I. that comets grow smaller as
they approach the sun, the remark was to be under-
stood to apply to the volume of the head, not to the
whole space occupied by the head and tail. In fact, it
would have been impossible to assert anything with
THE TWO COMETS OF THE YEAR 1868. 165
respect to the volumes of comets" tails, inasmuch as the
apparent extent of these appendages varies according
to the atmospheric conditions (humidity, clearness, and
so on) under which the comet is observed, and also
according to the light-gathering power of the observer's
telescope.
To return, however, to the theories which have been
formed respecting comets.
It has been commonly admitted that the substance
of which comets are composed is either wholly or prin-
cipally gaseous. In no other way, it should seem, can
the remarkable variations of appearance which comets
present as they approach the sun or recede from him
be reasonably accounted for.
Kepler held that comets are wholly gaseous, and
that they are liable to be dissipated in space by the
sun's action. He supposed that the process of evapora-
tion which thus led to the destruction of a comet was
carried on through the medium of the tail. It need
hardly be said that modern observations are completely
opposed to this view. Comets have been seen to return
again and again to the neighbourhood of the sun with-
out any apparent diminution of volume, although at
each return a tail of considerable length has been
thrown out. For a long time, indeed, it was thought
that Halley's comet was gradually diminishing in
volume ; but at the last return this magnificent object
had recovered all its pristine splendour.
Newton held, on the contrary, that comets are partly
composed of solid matter. He supposed that only the
1 66 LIGHT SCIENCE FOR LEISURE HOURS.
gaseous matter was affected to any noteworthy extent
by the action of the sun's heat. Eaised from the solid
nucleus the vaporised particles passed first into the
coma, he imagined, and were thence carried off into
space to form the comet's tail. Others so far modified
Newton's views as to suggest that the vaporised matter
is not wholly carried off but partially re-precipitated
upon the head of the comet, just as the vapours raised
from the ocean are precipitated upon the earth in the
form of rain.
We have seen that a comet diminishes in volume as
it approaches the sun. It will be noticed that both
the theories which have been described would account
satisfactorily for the observed decrease of volume.
But neither of them gives any satisfactory explanation
of the fact that a comet recovers its original volume as
it departs from the sun's neighbourhood. Newton, in-
deed, put forward certain views respecting the emission
of smoke from the nucleus during perihelion passage,
and he surmised that the true dimensions of the comet
might in this manner be veiled to a certain extent :
but this part of his theory has the disadvantage of
being almost unintelligible, besides being wholly in-
sufficient to account for the regular diminution and
increase which attend the approach and recession of a
comet.
A theory has lately been put forward by M. Valz
which accounts for the variation of a comet's volume
by the supposition that the solar atmosphere exerts a
power of compression, which, varying with that atmo-
THE TWO COMETS OF THE YEAR 1868. 167
sphere's density, is most effective in the sun's neighbour-
hood. We know, for instance, that a balloon must not
be fully inflated at first rising, because when- it reaches
the upper regions of air, where there is less compres-
sion, the enclosed gas expands and would burst the
silk if the balloon had been fully filled at first. And
certainly, on the somewhat bold assumption that the
solar atmosphere extends outwards to those regions in
which the observed change of volume takes place, and
on the additional and equally bold supposition that
comets are surrounded with a film of some sort per-
forming the same function as the silk of the balloon
(or that in some other way the substance of the comet
is prevented from intermingling with the substance of
the solar atmosphere) the theory of M. Valz would
have a certain air of probability. Even then, however,
it would be insufficient to account for the enormous
extent to which the variation has been observed to
proceed.
The only probable explanation of the variation in
question is that which is put forward by Sir John
Herschel in his admirable work on the southern
heavens. During his stay at the Cape of (rood Hope
he had an opportunity of observing the recession of
Halley's comet, and he discusses the phenomena with
admirable acumen and judgment. The result at which
he arrives appears to afford a simple and rational ex-
planation of the observed phenomena. He supposes
that as a comet approaches the sun the action of the
solar heat transforms the nebulous substance of the
1 68 LIGHT SCIENCE FOR LEISURE HOURS.
comet into invisible vapour. This action progressing
from without inwards, of course produces an apparent
diminution of volume. The diminution continues as
long as the comet is approaching the sun, and for yet
a few days after perihelion passage ; but soon after the
comet has begun to leave the sun's neighbourhood the
transparent vapour begins to return to its original
condition, the solar action being insufficient to keep
the whole of the vaporised matter in the gaseous state.
Thus the comet gradually resumes its original apparent
dimensions.
There are few phenomena which have given rise to
more speculation than those presented by the tails of
comets. Astronomers who, in dealing with other
matters, have exhibited the soundest judgment, and
the most logical accuracy of argument, seem to feel free
to indulge in the most fancifuX speculations when deal-
ing with this subject.
A favourite theory with the earlier astronomers was
founded on the observed peculiarity that the tails of
comets are usually turned directly from the sun. It was
supposed that the tail is not a really existent entity,
but merely indicates the passage of the solar rays
through space, after their condensation by the spherical
head of the comet. Just as a light received into a dark
room through a small aperture appears as a long ray
extending in a straight line through the room, so,
according to this theory, the sun's light, concentrated
by the comet's head, throws a long luminous beam into
space. Unfortunately for this view there is a want of
THE TWO COMETS OF THE YEAR 1868. 169
analogy between the two cases thus brought into com-
parison. The light shining into a room produces the
appearance of a ray, because it illuminates the air and
the small particles of floating dust which it encounters
in its passage. There is nothing corresponding to this
in the interplanetary spaces. If there were, the sky
would never appear black, since the sun would always
be shining on matter capable of reflecting his rays*
Kepler was the first to form a reasonable hypothesis
respecting comets' tails. He supposed that the action of
the solar heat dissipates and breaks up a comet's substance.
The rarer portions are continually swept away, he
imagined, by the propulsive energy of the solar rays,
and are swept in this way to enormous distances from
the comet's tail. The denser portions remain around
the nucleus and form the coma.
The modern theory respecting light (according to
which there is no propulsion of matter from the sun,
but a simple propagation of wave-like motion), does not
affect Kepler's hypothesis so much as might be ima-
gined. Whatever theory of light we adopt we are
forced to assume an extreme tenuity in the matter
which forms the tails of comets. And when once we
have made this assumption, we are enabled to admit
that even the propagation of a wave-like motion through
the ether which is supposed to occupy the interplanetary
spaces, might suffice to carry off the attenuated nebu-
lous matter with tremendous rapidity.
The defect of Kepler's theory is that it appears
insufficient to account for those anomalous tail-for-
170 LIGHT SCIENCE FOR LEISURE HOURS.
mations which were referred to in our paper on
Comet I.
Newton's hypothesis respecting comets' tails was
somewhat different. He supposed that the intensely
heated comet communicated its heat to the surrounding
ether, which thus grew rarer and ascended in the solar
atmosphere that is, flowed away from the sun pre-
cisely as heated air ascends from the earth. The ether
thus displaced would carry away with it the rarer por-
tions of the comet's substance, just as smoke is carried
upwards by a current of heated air.
It will be seen at once that Newton's theory^ like
Kepler's, affords no explanation of lateral tails, or of tails
turned towards the sun.
In modern times a theory has been founded on the
supposition that cometic phenomena may be due to
electrical agency. The German astronomer Olbers was
one of the first to propound this view, and many emi-
nent astronomers amongst others the younger Herschel
have looked with favour upon the theory. As yet,
however, we do not know enough respecting electricity
to accept with confidence any theory of comets founded
upon its agency.
The comet respecting which I now have to treat
was discovered in the middle of June 1868, by Win-
necke. At first it was a telescopic object, but it
gradually increased in brilliancy until it became visible
to the unaided eye. In the telescope, at the end of
June, the comet appeared as a circular cloud rather
brighter in the middle, where there was a roundish spot
THE TWO COMETS OF THE YEAR 1868. 171
of light. A tail could be traced to a distance of about
one degree from the nucleus.
Dr. Huggins quickly subjected the new arrival to
spectroscopic analysis. The result, at first sight, seemed
to differ little from that which had been noticed in the
case of Brorsen's comet. Indeed the astronomers at
the Paris observatory and the Padre Secchi at Rome
were led to pronounce the spectra of the two comets to
be absolutely identical. The more powerful spectro-
scopic appliances employed by Dr. Huggins, however,
exhibited important differences.
The spectrum consisted of three bands of light sepa-
rated by dark intervals. Of these bands two were
greenish blue, the other greenish yellow. The two
former were tongue-shaped, the last was narrowed off at
both extremities.
From what I have said above respecting the nature
of spectroscopic analysis, it will be understood that the
distribution of the comet's light along the length of
the spectrum is the most important circumstance to be
attended to in endeavouring to form an estimate of the
substance of the comet. But as we see that there are,
in this instance, peculiarities affecting the breadth of the
spectrum, it will be well briefly to consider their mean-
ing. The matter is, in reality, simple enough, but
requires a little attention.
The breadth of the spectrum corresponds to the
breadth of the object which is the source of light. If
that object is uniformly bright the spectrum is also
uniformly bright across its breadth, whatever variations
172 LIGHT SCIENCE FOR LEISURE HOURS.
may exist in the direction of its length. But if the
object is brighter in some parts of its breadth than in
others, the spectrum will show corresponding variations
of brilliancy across its breadth. Hitherto we have been
assuming that all the light from the object is of the
same kind, however it may vary in brilliancy. Suppose,
however, that the light from the middle of the object
gives one kind of spectrum, the light from the outer
parts another ; then the spectrum will vary in character
as well as in brilliancy across its breadth. Suppose for
example, that the middle of the object is gaseous while
the outer parts are solid or liquid, then the appearance
presented would be two thin streaks of rainbow-tinted
light, separated by a dark space l across which would
be seen the bright lines belonging to the gaseous central
part of the luminous object.
Now the breadth of the spectrum seen by Dr. Huggins,
corresponded with the breadth of the coma so far as
the widest parts of the tongue-shaped bands were con-
cerned. But the narrower parts were about the width
of the nucleus. Therefore the first question to be de-
cided was 'this, Is the narrowing of these bands of
light towards one extremity, and of the other towards
both extremities, to be considered as indicative of any
1 Our readers will, of course, understand that a slice only of the object
is brought under spectroscopic analysis at once. If the whole of a cir-
cular object, whose centre was gaseous, were examined at once, the
middle streak of the spectrum would exhibit the compound spectrum of
the edge and centre of the object. Such an arrangement would clearly
be unfavourable to the formation of clear views respecting the character
of the object's light.
THE TWO COMETS OF THE YEAR 1868. 173
difference, in character, between the light emitted by
the nucleus and that emitted by the coma ? At first
sight it seems that no other conclusion could be come
to. But a little consideration enabled Dr. Huggins to
arrive at a different result. The tongue-shaped bands
were not only narrower but very much fainter towards
one end. They were also fainter along their outer
edges, on account, of course, of the faintness of the
coma as compared with the nucleus. Now it was
possible that the narrowing down of the bands might
be only apparent, and due to the fact that their outer
parts, though really existent, became invisible at the
fainter end. And there were two modes of attacking
the question. First the observer could determine by a
careful inspection whether the light at the narrower
end of the tongues was so faint that it ought to disap-
pear at the edges merely by undergoing the same sort
of reduction as the brighter light at the broader end of
the tongue : this would show that the coma does not
differ in constitution from the nucleus. Secondly, if
the strip brought under examination were narrowed by
any contrivance, it is clear that any difference which
might exist in the constitution of the coma and of the
nucleus ought to be exhibited in a more marked
manner.
Dr. Huggins applied both methods, and each resulted
in showing that the nucleus has the same constitution
as the coma, excepting only that the exterior part of
the coma seems to give a continuous spectrum. In
other words, the nucleus and all the coma except its
174 LIGHT SCIENCE FOR LEISURE HOURS.
outer shell consists of the same incandescent vapour ;
but the outer shell of the coma either consists of in-
candescent solid or liquid matter or shines by reflecting
the solar rays.
So far, however, there is little in the spectroscopic
analysis which differs in character from what had been
observed respecting Brorsen's comet. But we have now
to record one of the most startling discoveries ever made
respecting comets.
Dr. Huggins was reminded by the appearance of the
cometic spectrum of a form of the spectrum of carbon
which he had observed in the year 1864. It must be
premised that the spectrum of an element often assumes
a different form according to the circumstances under
which it is obtained. Amongst the objects which have
spectra thus variable is the element carbon. The par-
ticular form of carbon-spectrum which resembled that
of the comet, is that obtained when an electric spark is
taken through olefiant gas a substance which, as many
of my readers are doubtless aware, consists of carbon
and hydrogen, and is one of the constituents of ordinary
coal-gas. 1 Of course the spectrum of olefiant gas
exhibits the bright lines belonging to hydrogen ; but as
these are well known, the part of the spectrum belong-
ing to carbon also becomes determinable.
Having noticed, as we said, the resemblance between
the spectrum of the comet and a form of the carbon
1 The other constituent is ' fire-damp ;' also a compound of carbon
and hydrogen. Olefiant gas is commonly called heavy carburetted
hydrogen, while fire-damp is termed light carburetted hydrogen.
THE TWO COMETS OF THE YEAR 1868. 175
spectrum, Dr. Huggins determined to compare the two
spectra directly. We have not space to explain the
contrivances by which this was effected. Suffice it to
say, that when the two spectra were brought side by side
it appeared that in place of mere resemblance there was
absolute identity. The bands of light which formed
the comet's spectrum were found not only to coincide
in position with those which appeared in the spectrum
of olefiant gas, but to present the same relative bright-
ness. Two days later the observations were repeated
by Dr. Huggins in company with Professor Miller (who
had been associated with him in his earlier spectroscopic
labours), and both observers agreed in the opinion that
the coincidence between' the spectra could not be more
exact.
The reader will, of course, understand that the hydro-
gen lines belonging to the spectrum of olefiant gas are
not seen in the spectrum of the comet.
Now only one interpretation can be put on this re-
markable result, and that is that Winnecke's comet
consists of the incandescent vapour of carbon, not
of burning carbon, be it understood, but of volatilised
carbon.
But carbon, as we are acquainted with it on earth,
is a substance whose chief peculiarity, perhaps, is its
fixity at ordinary temperatures ; and no phenomenon
hitherto presented by comets is more perplexing than
the existence of volatilised carbon as the main or only
.constituent of a comet of enormous real bulk, when
that comet was not so near to the sun as to be raised
176 LIGHT SCIENCE FOR LEISURE HOURS.
(one could suppose) to an extraordinarily high tempera-
ture. There have been cases where comets have been
so near to the sun as to account for almost any con-
ceivable change in the constitution of their elements.
An intensity of heat of which we can form no concep-
tion must have been experienced, for example, by
Newton's comet ; and a still fiercer heat dissipated the
substance of the comet of 1843. But Winnecke's
comet at the time of observation was at far too great a
distance from the sun for us to assign to its mass a
temperature which under ordinary circumstances would
account for the volatilisation of carbon.
Nor does the rarity of the atmosphere in which the
comet was moving serve to help us in our difficulty.
Doubtless we are little familiar with the effects which
terrestrial elements would experience if they were dis-
tributed freely in the ether occupying the interplanetary
spaces. But so far as our experience enables us to
judge, we should rather look for intensity of cold than
of heat under such circumstances. We see the heights
of the Andes and of the Himalayas clothed in perpetual
snow, though day after day the fierce heat of the tropi-
cal sun pours down upon them, and though there is no
winter there (in our sense of the word) during which
the snows are accumulated. We know that the explana-
tion of this peculiarity lies in the extreme rarity of the
air at a great height. It seems, therefore, reasonable
to conclude that the cold of the interplanetary spaces
must be far greater. Yet here we have an object whose
light comes from the incandescent vapour of so fixed
THE TWO COMETS OF THE YEAR 1868- 177
and unchangeable a substance as carbon, and thus, in
place of an almost inconceivable intensity of cold we
find the evidence of intense heat.
It seems impossible, at present, to suggest any ex-
planation of the observed phenomena. That carbon
exists out yonder in space in the state of luminous gas
or vapour, is the one fact of which alone we can
be certain. Dr. Huggins in his treatment of this fact
suggests the possibility that the carbon may be divided
into particles so minute, that as the comet approaches
the sun, more of the sun's heat is gathered up, so to
speak, and that thus the carbon is volatilised. He also
points to phenomena of phosphorescence and fluores-
cence in illustration of the appearance presented by
the comet's spectrum ; but without suggesting any
association between these phenomena and those pre-
sented by comets.
One cannot help associating the new views thus
opened out to us respecting comets, with the discovery
recently made that the meteoric bodies which flash
singly or in showers across our skies belong in reality
to the trains of comets. We have now every reason to
believe that there is not a single member of the me-
teoric systems, not a single aerolite, bolide, or fire-ball,
that has not belonged once upon a time to a comet.
The evidence on which this view is founded, though it
may seem insufficient at a first glance, is almost irre-
sistible to those who can appreciate its significance.
Let us briefly recapitulate the facts. ,
It has been proved that shooting-stars come from
8
178 LIGHT SCIENCE FOR LEISURE HOURS.
the interplanetary spaces, that they form systems, and
that these systems travel in regular elliptical orbits
about the sun. Two of the systems produce striking
meteoric displays, viz. the system encountered by the
earth on or about August 10, and the system encoun-
tered on or about November 13. Now it had been
suggested that the members of the former system
follow the track of the conspicuous comet which made
its appearance in the year 1862 ; and it was proved
that, assuming the orbit of the meteors to be very
eccentric, and assigning to them a period of 147 years
(that of the comet), their motions corresponded in the
most remarkable manner with the orbital track of the
comet. In fact the agreement was so close that very
few who had examined the question could believe it to
be accidental. But there were two objections on which
some stress was laid. First, it had been necessary to
make assumptions respecting the motion of the meteors ;
secondly, those assumptions were not rendered probable
by anything which had been proved respecting any
meteoric system. The examination of the November
star-shower by a host of eminent mathematicians in
1866-7 led to results which at once removed these
objections, and brought new evidence and that of
the most striking character in favour of the theory
that comets and meteors are associated. It had been
supposed that the November meteors travelled in a
nearly circular orbit within a period of somewhat less
than a year. Adams proved that they travel in an orbit
extending far out into space beyond the orbit of distant
THE TWO COMETS OF THE YEAR 1868. 179
Uranus. And the period of this orbit was calculated
to be 33^ years. Here then was strong confirmatory
evidence in favour of the elliptic orbit and the long
period assigned, by way of assumption, to the August
meteors. But this was far from being all. Astrono-
mers looked for a comet to be associated with the
November meteors ; and they found one a small one,
it is true, but with a well-defined character an orbit
calculated beyond suspicion of important error, and
agreeing so closely in its motions with those of the
November meteors that the chances were millions on
millions to one against the coincidence being acci-
dental. It hardly required, after this, that an associa-
tion should be pointed out between other meteor-
systems and other comets. Yet this has been done,
and thus that which had already been demonstrated
was illustrated by new proofs. We may say that
nothing which men of science have dealt with has ever
been more satisfactorily proved than the fact that
meteors are the attendants on comets.
Now, how meteors are thrown off from cometic
nuclei we are not yet able to say. They differ wholly
in character from their source, and thus we learn that
the gaseous nature of cometic nuclei is due to the
action of causes connected with those to which the
nuclear (structure of the comet's head is due. But
whether the first formation of meteoric systems is asso-
ciated in any way with the processes which result in
the formation of a comet's tail, is not quite so clear.
As yet no comet which has had a brilliant tail has been
N 2
/8o LIGHT SCIENCE FOR LEISURE HOURS.
subjected to spectroscopic analysis, so that we cannot
pronounce with any certainty respecting the structure
of these singular appendages. Some astronomers are
disposed to look on the formation of a track of meteors
all round the orbit of a comet as due to the action of
influences by which parts of the comet's mass are
thrown into orbits of slightly longer period than that
of the head, though closely resembling that orbit in
figure. Be this as it may, it is certain that the great
contrast in character between the meteoric bodies
which form the train of a comet, and the gaseous
nucleus and coma, remains yet among the mysteries
which astronomers have been unable to clear up.
But so soon as it had been shown that a comet's head
is formed of a certain well-known terrestrial substance,
it was natural that the question should be asked
whether this substance is to be found in meteors.
Hitherto no great progress has been made in deter-
mining the elementary constitution of meteors which
have not actually fallen upon the earth. It is so diffi-
cult to catch them during their brief transit across our
skies that only a few substances, as sodium, phosphorus,
magnesium, and so on, have been shown with any
appearance of probability to exist in shooting-stars.
Certainly carbon is not among the number of those
elements which have been detected in this way. But
at a recent meeting of the Astronomical Society, it was
stated that several aerolites contain carbon in their
structure, and Dr. De la Rue offered a fragment of
one of these to Dr. Huggins for analysis. Certainly a
THE TWO COMETS OF THE YEAR 1868. i8l
strange circumstance that an astronomer who had
analysed the structure of a body millions of miles away
from the earth, should take into his hands and subject
to chemical analysis a fragment which had once in all
probability belonged to a similar comet!
In conclusion, I must notice that there has been a
remarkable absence during the past few years of those
brilliant and long-tailed comets which alone seem
calculated to afford the spectroscopist the means of
answering some of the difficult questions suggested
above. The tail of Winnecke's comet was too faint to
give a visible spectrum. In fact the comet itself was
only just visible to the naked eye. When a blazing
object like Donati's comet or the comet of 1861 comes
to be subjected to spectroscopic analysis, we may hope
for an amount of information compared with which
that hitherto obtained is probably altogether insig-
nificant.
From Frazer's Magazine for February and June 1869.
COMETS OF SHORT PERIOD.
IT is related by Apollonius the Myndian, that the
Chaldean astronomers held comets to be bodies which
travel in extended orbits around the solar system.
' The Chaldeans spoke of comets,' he says, ' as of tra-
vellers, penetrating far into the upper celestial spaces.'
He adds, that those ancient astronomers were even able
1 82 LIGHT SCIENCE FOR LEISURE HOURS.
to predict the return of comets. How far it may be
safe to accept the statements of Apollonius is uncer-
tain. He ascribed other powers to the Chaldeans, of
which we may fairly doubt their possession for in-
stance, the power of predicting earthquakes and floods.
In fact, there is so marked a disposition among ancient
writers to exaggerate the acquisitions of Chaldean
astronomers, that it becomes extremely difficult to dis-
tinguish truth from falsehood. Still, there is sufficient
evidence of their skill and patience as observers, to
render it fully possible that they may have discovered
the periodicity of one or two comets.
But until the rise of modern astronomy, the opinion
which was almost universally held respecting comets
was that of Aristotle, that they are of the same nature
as meteors or shooting-stars, existing either in the air
not far above the clouds, or certainly far below the
moon.
The discovery of the periodicity of Halley's comet
following quickly on Newton's announcement of the
law of gravitation, led astronomers to examine the
orbits of all the comets which became visible, with the
hope of finding that some of these bodies may be tra-
velling in re-entering paths. But inasmuch as none
of the brilliant comets of whose appearance records had
been preserved seemed to have ever revisited the earth
save Halley's alone, while even Halley's travelled in an
orbit of enormous extent, an orbit which reached out
in space more than three times as far as the orbit of
the most distant known planet, astronomers held that
COMETS OF SHORT PERIOD. 183
the only kind of path which they might expect a comet
to pursue was a long oval. They accordingly con-
fined their calculations, and limited the invention of
new mathematical processes, to the case of very eccen-
tric orbits.
But in 1770 a comet appeared which led astrono-
mers to form wholly new views. No orbit which could
be devised (subject to the above-mentioned condition)
could be reconciled with the motions of the new arrival.
At length the astronomer Lexell discovered that the
path of the comet was not an oval of extreme eccen-
tricity, but an ellipse of such a figure that the comet's
period of revolution was less than six years. But here
a difficulty arose. The comet was sufficiently conspi-
cuous ; and it was asked, how could such an object
have gone on circulating so rapidly around the sun,
and yet have remained undiscovered ? A very singular
result rewarded the inquiry into this question. It was
found that the aphelion of the comet's path lay just
outside the orbit of Jupiter ; and, further, that when
the comet was last in aphelion, Jupiter was quite close
to it. Thus it became clear that the comet had been
travelling in another, and doubtless much wider orbit,
when its motions had brought it into the neighbour-
hood of the planet Jupiter the giant of the solar
system. The comet had actually approached the planet
nearer than his fourth satellite. ' It had intruded,'
says Sir J. Herschel, ' an uninvited member into his
family circle.'
The result of this close appulse may be readily con-
184 LIGHT SCIENCE FOR LEISURE HOURS.
ceived. Just as Halley's comet, when close to the sun,
sweeps rapidly round him that is, in a sharply curved
path so the new comet's path was sharply bent around
the temporary focus formed by the great planet. But
just as Halley's comet, after perihelion passage, moves
away from the sun, so Lexell's comet, after what may
be termed perijovian passage, moved away from Jupi-
ter, and passed again within the sun's attraction. From
this time the comet began to follow a new orbit around
the sun. This new orbit was an oval of moderate
eccentricity, round which the comet travelled in about
five and a half years.
At the next return of the comet to perihelion, it was
not likely that astronomers would obtain a view of it ;
for, on account of the odd half-year in its period, it
came to perihelion when the earth held a point in her
orbit exactly opposite to that which she had occupied
at the comet's former perihelion passage ; therefore,
the comet, which before was favourably, was now un-
favourably situated for observation.
As the period for the comet's second return ap-
proached, astronomers looked out eagerly for its advent.
Again and again the heavens were 'swept' for the
faint speck of nebulous light which should have an-
nounced the return of the wanderer. But days, and
weeks, and months passed, until it became certain
that either the comet had been shorn of nearly all its
former brilliancy, and had thus escaped unnoticed, or
that something had happened to deflect it from its
course.
COM JETS OF SHORT PERIOD. 185
The last alternative appeared so much the more
probable one, that mathematicians began to examine
the path of the comet, to see whether it had approached
so near to any disturbing body as to have been driven
from its recently adopted orbit. The examination was
soon rewarded with success. If we consider the nature
of orbital motion, we shall at once see that, so long as
Lexell's comet was subjected to no new disturbing
attractions, it was compelled, once in every revolution,
to return to the scene of its former encounter with the
planet Jupiter. This return was fraught with danger
to the stability of the comet's motions. So long as
Jupiter was not near that particular part of his orbit
at which the encounter had taken place, the cornet was
free to pass the point of danger, and return towards
the sun ; but if ever it should happen that Jupiter was
close at hand when the comet approached his orbit,
then the comet would be as liable to have its motions
disarranged as at the original encounter. It happened
that the period of the comet's motion in its new orbit
was almost exactly one-half of Jupiter's period.
This was unfortunate ; since it clearly follows that,
when the comet had revolved twice, Jupiter had re-
volved once round the sun. Thus the comet again
encountered the planet, with what exact result has
never become known ; but certainly with this general
result, that the comet's movements were completely
disarranged. It has never returned to the neighbour-
hood of the earth.
We may look upon Lexell's as the first discovered
1 86 LIGHT SCIENCE FOR LEISURE HOURS.
comet of short period ; for although it was never seen
after its first visit, yet nothing can be more certain
than that it did actually return once, and that it
went twice round its new orbit. Indeed, if it has
not been absorbed by Jupiter a very unlikely con-
tingency it must still be revolving in space with an
orbit which brings it, once in each revolution, to the
scene of its former encounters. The figure of its orbit
may be altered again and again by encounters with
Jupiter ; but each new orbit must traverse this dan-
gerous point. This follows directly from the laws of
orbital motion around an attracting centre. A body
will continue to revolve in any orbit along which it
has once begun to move, unless it is acted upon by
some extraneous force. Accordingly, if at any point
of its path an extraneous force suddenly disturb its
motion, the disturbed orbit cannot fail to pass through
the point of disturbance. Thus the body may again
fall under the influence of the disturbing agent, and be
caused to move in yet another orbit through the same
point. And in the course of millions of years, a body
might thus travel in a hundred different orbits, all
passing through a common point. There is, indeed,
one way in which Lexell's comet might have escaped
from Jupiter's control. If after one of its encounters
with Jupiter, it happened to pursue a path which
brought it very nearly into contact with Saturn or some
other large planet, it might be compelled thenceforth
to abandon its allegiance to Jupiter. But the proba-
bility of this happening to a comet which had once got
COMETS OF SHORT PERIOD. 187
into the toils, may be reckoned ' almost at naked
nothing.'
We have been careful to dwell on this point for a
reason which will appear presently.
The search for LexelFs comet led to the discovery of
a considerable number of nebulae ; and the discovery
of nebulae led in turn to the discovery of another comet
of small period. In 1786 Mechain announced to Mes-
sier (who had constructed a list of 103 nebulae) that he
had discovered a nebulous object. This turned out to
be a telescopic comet. It was again seen by Miss
Caroline Herschel in 1795, by Thulis in 1805, and by
Pons in 1818. All this time no suspicion had arisen
that these observers had seen the same object. But in
1818 the comet remained in view so long that it became
possible to calculate its orbit. This was done by the
Grerman mathematician Encke, who found that the
orbit is an ellipse, and the period of revolution about
three years and four months. He found, after a labori-
ous process of calculation, that it could be no other
than the object that attracted attention in 1786, 1795,
and 1805. Encke then applied himself to calculate
the next return of the comet, which he did so success-
fully that astronomers have continued to call by his
name the object whose motions he had been the first to
interpret.
Encke's comet was seen by one observer only in
1822, as it was not favourably situated for observation
'in the northern hemisphere that observer was M.
Riimker, who followed the comet for three weeks at
1 88 LIGHT SCIENCE FOR LEISURE HOURS.
the private observatory of Sir T. M. Brisbane at Para-
matta. In 1825, the comet was detected by several
independent observers. It was seen again in 1828,
being detected by two observers Harding at Gottin-
gen, and Gambart at Marseilles. In 1832 and 1835,
it was seen from the observatory at the Cape of (rood
Hope.
At the next return of the comet, which took place
on December 9, it was visible to the naked eye for the
first time since its discovery. At this passage, also, a
very noteworthy peculiarity was remarked or rather
a peculiarity which had been remarked by Encke in
1818, was now, for the first time, placed beyond a
doubt. Encke had suspected that the comet's period
was slowly diminishing. Each return to perihelion oc-
curred about two and a half hours before the calculated
time. Such a discrepancy may appear very trifling,
and in fact it might seem that no certainty could be
felt respecting it ; and this is the case so far as one or
two revolutions are concerned. But when each succes-
sive revolution shows the same discrepancy, the defi-
ciency soon mounts up to a period respecting which
no doubt can be entertained. For example, between
the perihelion passage in 1789 and that of 1865, the
comet has made twenty-three revolutions, and each
has been less than the preceding by two and a half
hours (on the average). Hence, the last revolution of
the series occupied two days and a half less than the
first. But even this does not express the full effect of
the change ; for the comet having gained two and a
COMETS OF SHORT PERIOD. 189
half hours in the first revolution, five in the next, seven
and a half in the next, and so on it is the sum of all
these gains (and not the gain made in the last revolution)
which expresses the total gain of the comet in point of
time. Hence the last perihelion passage occurred
twenty-nine days before the time at which it would
have taken place, but for some unknown cause which
has interfered with the comet's motion. What that
cause may be, has not yet been certainly determined ;
but it is at least highly probable that Encke has as-
signed the true cause in suggesting that so light a
substance as the comet may be retarded in its passage
through the interplanetary spaces by the existence of
' a thin ethereal medium,' incapable of perceptibly
retarding the motion of the planets.
At first sight, it may seem strange that we should
speak of the acceleration of the comet as being caused
by the retarding influence of such a medium as has
been conceived to occupy the interplanetary spaces.
Yet it is strictly the case that, if a planet or comet be
continually checked in its onward course, its velocity
will continually grow greater and greater. For instance,
if our earth were so checked, it would move in a spiral
which would gradually bring its orbit to that of Venus,
by which time its motion would be as rapid as that of
Venus (which moves one-third faster than the earth) ;
then it would continue revolving in a spiral fill it
reached the orbit of Mercury, when it would be moving
as fast as this the swiftest of all the planets. And so
IQO LIGHT SCIENCE FOR LEISURE HOURS.
the earth would continue to approach the sun with
continually increasing velocity.
Returning to Encke's comet, we have to notice yet
another important discovery which was effected by its
means. The comet passed so near to Mercury in 1835
as to enable astronomers to form a much more satis-
factory estimate of this planet's mass than had hitherto
been obtained. It was found that the mass of Mercury
had been largely over-estimated.
No very long interval passed after the discovery of
Encke's comet before another comet of short period
was detected. M. Pons, who had discovered Encke's
comet, it will be remembered, in 1818, observed a faint
nebulous object on June 12, 1819. This object turned
out to be a comet ; and in this case, as in the former,
Encke calculated the stranger's orbit and period. He
found that it moves in an ellipse which extends slightly
beyond the orbit of Jupiter, and that it has a period of
about five and a half years. This object was not seen
again, however, until the year 1858, when M. Winnecke
discovered it, and at first supposed it to be a new comet.
Calculation soon showed the identity of the two objects,
and confirmed the results which had been obtained by
Encke in 1819.
The next comet of short period was discovered by
M. Biela in 1826. Perhaps nothing in the whole his-
tory of cometic observation is more surprising than
what has been recorded of this singular object. We
must premise that the comet had been seen in March
1772, and again in November 1805. But it was not
COMETS OF SHORT PERIOD. 191
until its re-discovery in 1826 that its orbit and period
were computed. An ellipse of moderate eccentricity,
extending beyond the orbit of Jupiter, was assigned
as the comet's orbit the period of revolution being
about six and a half years. The orbit was found to pass
within about twenty thousand miles of the earth's
orbit; and at the first return of the comet (in 1832),
some alarm was experienced lest the near approach of
the two bodies should lead to mischief of some sort.
The comet returned again in 1839 and 1845. It was
at the last-mentioned return that a singular pheno-
menon occurred, which is nearly unique in the
history of comets. On the 19th of December 1845,
Hind noticed a certain protuberance on the comet's
northern edge. Ten days later, observers in North
America noticed that the comet had separated into
two distinct comets, similar in form, and each having
a nucleus, a coma, and a tail. European observers did
not recognise the bi-partition of the comet until the
middle of January 1846. The new and smaller comet
appears to have sprung into existence from the pro-
tuberance observed by Hind, since this object moved
towards the north of the other. After a while, the new
comet became the brighter, but, shortly after, it re-
sumed its original relative brilliancy. Lieutenant
Maury noticed, on one occasion, a faint 'bridge-like
connection' between the two comets. The distance
between them gradually increased, until first the new
comet, and then the old one, had passed out of view.
In 1852, Biela's comet was again seen, and the
192 LIGHT SCIENCE FOR LEISURE HOURS.
Padre Secchi, at Rome, detected a faint comet pre-
ceding it. If, as is probable, this faint comet is the
companion, we may assume that the two bodies are
permanently separated.
At the two next returns the comet was not seen,
and much interest was felt by astronomers respecting
the anticipated return in January 1866. It was
searched for systematically at the principal European
observatories. In fact, so closely did Father Secchi
examine the calculated track of the comet, that he
detected several new nebulae in that region. But the
comet itself was not found. Astronomers are unable
to assign any satisfactory reasons for its disappearance.
It is known to have traversed the zone of the Novem-
ber meteors where that zone is richest our readers
will remember the display of shooting-stars in 1866
and Sir J. Herschel surmises that it may have been
destroyed in the encounter. Possibly this may be the
true solution of the difficulty ; or, it may be that the
comet was merely dispersed for a while during the
passage of the meteor-zone, and may yet gather itself
together and become visible to astronomers. 1
We pass over three or four comets belonging to this
class which present no special features of interest, to
1 The return of this comet in 1872 was eagerly looked for by astro-
nomers. But the comet was not seen. On November 27, 1872, there
was a fine display of meteors, as the earth passed through the comet's
track, and afterwards a cometic object was seen in the direction towards
which the meteors had been travelling. But this was not Biela's comet,
which, indeed, must have passed that place nearly twelve weeks earlier.
Indeed, some doubt exists whether the object was travelling in the
track either of the comet or of the meteors.
COMETS OF SHORT PERIOD. 193
come to an object which has recently been rediscovered,
and will continue visible (in good telescopes) for
several weeks. On February 6, 1846, M. Brorsen
discovered a telescopic comet, whose motions soon
showed it to belong to the class of objects we are now
dealing with. It was found to have an orbit of mode-
rate eccentricity, extending just beyond Jupiter's orbit,
and a period of about five and a half years. It was
not seen at its next return to perihelion ; but was re-
discovered by M. Bruhns on March 18, 1857. In
1862, it again escaped undetected; but at its present
return, it has been rediscovered (by three observers
simultaneously), and it is now being carefully tracked
across the northern skies.
In all, there have been recognised thirteen comets of
short period that is, having periods of less than seven
years. Amongst these are included several which have
only been seen once, and some which are known to have
been subjected to such disturbance as no longer to
travel in orbits of short period. Of these thirteen
comets, no less than ten have the aphelia of their
orbits just beyond the orbit of the planet Jupiter ; two
have their aphelia just within Jupiter's orbit; and
Encke's comet alone has its aphelion at a safe distance
from that orbit. It appears to us that the peculiarity
thus exhibited is not without meaning. Eemembering
the history of Lexell's comet, we seem to find a satis-
factory explanation of the peculiarity. We have seen
how Lexell's comet was first introduced into the system
of short-period comets by the giant planet Jupiter, and
194 LIGHT SCIENCE FOR LEISURE HOURS.
then summarily dismissed. So long as the comet re-
mained within that system, the aphelion of its orbit
lay just beyond the orbit of Jupiter, and this would
be the case with any comet introduced in a similar
manner. But for the coincidence which led to its
expulsion, Lexell's comet would have continued to
revolve as a short-period comet. It seems also clear,
that in the course of many ages, its period and orbit
would have grown gradually smaller, through the
operation of the same cause (whatever that may be)
which is now reducing the period and orbit of Encke's
comet. At length it must have attained a path safe
within the orbit of the great disturbing planet. In
the list of short-period comets, then, we seem to see
illustrations of the successive stages through which
Lexell's comet would have passed in attaining the sort
of orbit in which Encke's comet is now moving. And
it seems permissible to assume that all the short-period
comets have been introduced to their present position
within the solar system by the same cause which led
to the temporary appearance of Lexell's comet as a
comet of short period that is, by the attractive
energy of the planet Jupiter.
Chambers' 's Journal, July 1868.
195
THE GULF STREAM.
MAJOR EENNELL was the first, I believe, to whom we
owe the comparison of ocean-currents to rivers. He
spoke of them as ocean-rivers, and pointed out how
enormously their dimensions exceed those of such
streams even as the Amazon and the Mississippi. Some
of the ocean-currents are from 50 to 250 miles in
breadth, and flow more swiftly than the largest navi-
gable rivers. The banks and bottom of these currents
are not land, but water ; and so deep are the currents
that they are turned aside by shoals and banks whose
tops are ' 40, 50, or even 100 fathoms beneath the
surface of the ocean.' The outlines of ocean-currents
are sharply defined, insomuch that < often,' says Captain
Maury, ' one half of a vessel may be seen floating in the
current, while the other half is in common water of the
sea.' The border-line of the Gulf Stream can be traced
by the eye. Yet more remarkable is the distinction
between the moving water and that which is at rest,
when large masses of sea-weed carried along by the
former enable one to recognise the rapidity with which
it moves.
Of all the ocean-currents the most important, perhaps,
in its bearing on the destinies of men and nations, is
the great Gulf Stream. I propose to examine the
o 2
196 LIGHT SCIENCE FOR LEISURE HOURS.
course and habitudes of this current, and then to inquire
a little into the vexed question of its cause.
Major Eennell traced the Gulf Stream from a sup-
posed source in the Indian and Southern Oceans.
Modern geographers and physicists prefer to look for
the rise of the current somewhere near the Cape of
Good Hope. ' The commencement and first impulse
of the mighty Gulf Stream is to be sought,' writes
Humboldt, ' southward of the Cape of Good Hope.'
It appears to me, however, that the true source of the
great stream is to be looked for in the equatorial zone
of the Atlantic. When we come to inquire into the
cause or causes which give birth to the Gulf Stream,
we are led, as I imagine, to this region rather than to
any other (though, perhaps, in a stream which forms
part of a continuous system of circulation, we can
hardly speak of any one portion as the source) ; I shall
therefore trace the stream, and the system to which it
belongs, from the great equatorial waters which move,
as Columbus was the first to discover, 4 with the heavens
(las aguas van con los cielos)., that is, from east to
west, following in this the apparent motions of the sun,
moon, and stars.'
The map of the Atlantic Ocean on p. 217, is con-
structed upon one of those forms of isographic pro-
jection described in my Essays on Astronomy. It is
important, in dealing with the subject of currents, that
the question of area should be considered, and, there-
fore, that our illustrative charts should represent such
areas correctly. This Mercator's charts are far from
THE GULF STREAM. 197
doing. The portion of the Atlantic Ocean between
England and the United States of America is unduly
magnified, and still more is this the case with the
portion between Sweden and Greenland. On the other
hand the portion between Africa and the Gulf of Mexico
is unduly diminished. Thus it is scarcely possible to
form from such charts just notions of the actual cha-
racter of the oceanic circulation whereof the Gulf Stream
forms a part. (Compare the charts illustrating the
Essay on the Climate of Great Britain, pp. 264, 265.)
We see, in our map (p. 217), 1 that there is a great
equatorial stream extending in its eastern portion far to
the south of the equator, but passing to the north also
even here, and still further to the north between the
coasts of Africa and South America. Near here the great
equatorial current divides into two portions. One passes
southward and then returns towards the east, according
to some authorities, but, according to others, continues
its course southward until it is lost in the Antarctic
Ocean. We shall follow the northern bifurcation,
however. The course of this portion of the Atlantic
current system has been far more exactly traced out.
Taking a north-westerly course, the great current pours
itself against the barrier formed by the Leeward and
Windward Islands. Passing between these islands, it
sweeps around the shores of the G-ulf of Mexico, a por-
1 For the sake of completeness, and also that the present essay may
fairly represent my views when it was written, I leave the account of the
map and of the course of the Gulf Stream unchanged here. By com-
paring this essay with the following, it will be noticed that only very
few passages are repeated in substance.
198 LIGHT SCIENCE FOR LEISURE HOURS.
tion, however, of its volume passing probably outside
the West Indian Islands, to rejoin the other outside the
promontory of Florida. At this point the stream has
become, probably, somewhat diminished in volume,
but being still more diminished in breadth, it flows as
a deep, strong, and swift stream, known among sailors
as ' The Narrows of Bernini.' From hence the stream,
now become the true Gulf Stream, grows gradually
wider, less deep, and less swift. Off Hatteras it is
already twice as broad as in the Florida Straits, and
as it stretches with a wide easterly sweep across the
Atlantic towards the shores of Ireland and the Hebrides,
the current not only reassumes something of its ori-
ginal extent of surface, but again bifurcates ; a wide
but somewhat sluggish stream is sent southward
towards the shores of north-western Africa, to rejoin
the equatorial stream. The main portion of the
current, however, passes with a north-easterly course
up the Atlantic valley, between Iceland and Sweden
to the Polar seas. It seems uncertain whether Rennell's
current, which passes around the Bay of Biscay, and
the current which streams southward past the shores of
Spain, are forks of the Gulf Stream. They are usually
represented in maps as independent currents, and in
Captain Maury's large map of the Gulf Stream the
great southern bifurcation already mentioned is repre-
sented as a current impinging upon the flank of the
stream which flows past Sppin and north-western
Africa. Yet, if these streams have not their source in
the Gulf Stream, it will be found no easy problem to
THE GULF STREAM. 199
assign their origin ; and I cannot but think that the
Biscay and Guinea currents, as well as the current
which flows into the Mediterranean through the Straits
of Gibraltar, are as truly bifurcations of the Gulf
Stream as the current which laves the shores of Ireland
and Sweden.
There will be noticed also in the map three return
streams, one flowing southward outside Iceland, another
sweeping round the eastern shores of Greenland, and
the third flowing through Baffin's Bay and Davis's
Straits. The two last unite south of Davis's Straits, and
flow on together to meet the first stream outside New-
foundland, whence the three flow as a single current
past the shores of the United States. It is generally
assumed, and in all probability justly, that these three
streams are derived from the Gulf Stream, and are
different branches of its returning waters.
Between the single return-stream which laves the
shores of the United States and the Gulf Stream there
is an unshaded space in the map. It is not to be in-
ferred, however, that this space represents still (or
rather unflowing) water. On the contrary, it is the
c debatable ground ' between the opposite currents. In
spring the whole of this space is occupied by the south-
ward flowing waters of the cold return-current. In
autumn the whole of the space is occupied with the
waters of the Gulf Stream. Backwards and forwards
over this space the rival currents are continually sway-
ing, the period of an oscillation being one year.
In the widest part of the Atlantic Ocean that,
200 LIGHT SCIENCE FOR LEISURE HOURS.
namely, which extends between the most westerly part
of Africa and the West Indies there is a wide expanse
of waters unmoved by the flux or reflux of currents.
Surrounded on every side by the circulating waters of
the Central Atlantic current-system, this region re-
mains undisturbed save by winds and the tidal wave.
Accordingly its surface is covered with different forms
of marine vegetation. My readers will doubtless
remember the interest which the Great Sargasso Sea
excited in the mind of Christopher Columbus. Oviedo
termed this region the ' seaweed meadow.' ' A host of
small marine animals,' says Hurnboldt, ' inhabit this
ever-verdant mass of Fucus natans, one of the most
widely-diffused of the social plants of the ocean, con-
stantly drifted hither and thither by the tepid winds
that blow across its surface.'
In the South Atlantic there is a smaller and some-
what more sharply-defined Sargasso, covered chiefly
with rockweed and drift. A weedy space occurs also
about the Falkland Islands, but is probably not a true
Sargasso. Maury considers that the seaweed reported
there probably comes from the Straits of Magellan,
where it grows so thickly that steamers find great
difficulty in making their way through it; for it so
cumbers their paddles as to make frequent stoppages
necessary.
Such is the distribution of the surface of the Atlantic
Ocean. But now the question will at once suggest
itself: Is the complete system of oceanic circulation
exhibited on the surface ? It seems now quite certain
THE GULF STREAM. 2OI
that this question must be answered in the negative.
We might, indeed, at once point to the existence of the
important current which laves the shores of the United
States as an answer to the question ; for where can all
this water find an outlet ? It does not pass the Penin-
sula of Florida as a current ; it does not cross the Grulf
Stream ; where, then, can it go but underneath the
ocean's surface ? But we have positive evidence of the
existence of under- currents.
In the first place it is found that in deep-sea sound-
ings in many parts of the ocean, far more line may be
paid out without any sign of the bottom being reached
than the depth of the ocean in those parts would
account for. In places where it has been proved by
other methods than ordinary sounding that the depth
is not more than three miles, no less than ten miles of
line have been paid out, being carried out so strongly
that the slightest check in the paying-out apparatus
has sufficed to break the sounding-line.
In the second place, it has been found possible to
determine the depth at which a submarine current is
flowing, and the direction in which it flows. Thus
Lieuts. Walsh and Lee, in the American service, having
loaded a block of wood to sinking, and let it down to
different depths, had repeatedly the satisfaction of
seeing the work of under-currents. c It was wonderful,
indeed,' they write, ' to see the bawega ' (a float attached
to the upper end of the line) ' moving off, against wind,
sea, and surface current, at the rate of over one knot
an hour, as was generally the case, and on one occasion,
202 LIGHT SCIENCE FOE LEISURE HOURS.
as much as one and three-quarter knots. The men in
the boat could not repress exclamations of surprise, for
it really appeared as if some monster of the deep had
hold of the weight below, and was walking off with it.'
Lastly, we may mention that Captain Wilkes, of the
United States Exploring Expedition, established the
existence of a cold under-current no less than two hun-
dred miles broad at the equator.
We may assume, then, that a complete system of cir-
culation, vertical as well as horizontal, exists throughout
the whole of the waters contained within the great
Atlantic valley.
Where are we to look for the origin of this vast series
of movements ? The actual ' work done ' in the Atlantic
Ocean is so enormous in other words, the transfer of
such large volumes of water represents so enormous a
force, that we might well expect to be able at once to
assign the motive-power of this great machinery. For
it would seem that the giant which works such wonders
could not readily hide himself from our recognition.
It has not been found, however, that the solution of
the problem has been so simple as was to have been
anticipated.
Passing over the earlier guesses which marked the
Gulf Stream as the offspring of the Mississippi Kiver,
of the sun's motion in the ecliptic (a mysterious inter-
pretation of the phenomena), and of the tidal wave, we
may remark that but two explanations of the Atlantic
currents seem to merit discussion.
Sir John Herschel is the principal exponent of the
THE GULF STREAM. 203
first theory, which assigns to the trade-winds the princi-
pal almost the sole agency in the generation of the
Atlantic current-system. He refuses indeed, to look
on the subject as one of any doubt or difficulty. ' The
dynamics of the Grulf Stream have of late,' he writes,
' been made a subject of much (we cannot but think
misplaced) wonder, as if there could be any possible
ground for doubting that it owes its origin entirely to
the trade-winds.' ' If there were no atmosphere, there
would be no Gulf Stream, or any other considerable
oceanic current (as distinguished from a mere surface-
drift) whatever.' He presents his solution somewhat
as follows : The trade-winds are an actually existent
cause for an easterly motion in the tropical seas ; we
cannot ignore their action ; we know, also, that when
the trade-winds arrive at the equator, they have lost
their easterly momentum; and we know, therefore,
that that momentum must have been imparted to the
surface of the water (for where else can it have gone ?) ;
hence there arises the great easterly movement which
generates the whole system of circulation.
The second view, which attributes oceanic circulation
to differences of temperature and saltness in different
parts of the ocean, is supported by Humboldt and others,
but is taken up most unflinchingly by Captain Maury,
who assigns it as practically the sole cause of all oceanic
circulation. 6 The Grulf Stream,' he writes, ' as well as
all the constant currents of the sea, is due mainly to this
cause. Such differences are inconsistent with aqueous
equilibrium, and to maintain this equilibrium the
204 LIGHT SCIENCE FOR LEISURE HOURS.
great currents are set in motion. The agents which
derange equilibrium in the waters of the sea, by altering
specific gravity, reach from the equator to the poles,
and in their operations they are as ceaseless as heat and
cold ; consequently, they call for a system of perpetual
currents to undo their perpetual work.' c Other causes
help to cause currents,' he says, c but the currents created
by them are ephemeral?
Here we have what is ' a very pretty quarrel as it
stands.' Each of the disputants points to causes of
acknowledged importance, and also (whether efficient
or not in the particular matter under question) of
acknowledged general efficiency. Each has much to
say in favour of his own view, and each considers his
antagonist's agent as utterly insufficient for the work
ascribed to it. Each has heard his opponent's argu-
ments, and reiterates his own statement. Nor can it be
said that the opponents are unequally matched ; for, if
we must place Sir John Herschel far before Maury as a
mathematician and physicist, and if we must undoubt-
edly look upon the former as the more practised
reasoner, yet we must remember, in turn, the special
attention which Captain Maury has given to the subject
under discussion, and the practical acquaintance with it
which his experience as a seaman has given to him.
Let us briefly state the arguments adduced by
Herschel against Maury's view, and by Maury against
Herschel's.
Sir John Herschel asserts that, inasmuch as the sun's
heat warms the surface of the ocean most intensely, so
THE GULF STREAM.
205
that the water of least specific gravity is already upper-
most, there can be no tendency to motion. For the
heated water cannot descend, being buoyant; nor
ascend, being uppermost ; nor move laterally, having
no impulse to motion of that sort, and being only able
to move laterally ' by reason of a general declivity of
surface, the dilated portion occupying a higher level.'
He then applies to this declivity the test of quanti-
tative analysis. Taking a column of water at the
equator having at the base a temperature of 39 (at
which temperature fresh water attains its greatest
density, and which is also the temperature of water
7,200 feet beneath the surface at the equator), while its
top has a temperature of 84 (the warmth of equatorial
surface-water), he finds that such a column is 10 feet
higher than a similar column in latitude 56, where
39 is the surface temperature. And since from
the equator to latitude 56 the distance is 3,360
geographical miles, we have a declivity of barely one-
twenty-eighth of an inch per geographical, or one-
thirty-second of an inch per statute mile. Such a
declivity is utterly insufficient to account for the
existence of a strong current from the equator towards
the tropics ; while, so far from giving any account of
the motion of the equatorial current from east to west,
it would tend to form a north-easterly current.
This seems strongly opposed to Maury's view, and I
do not find that he does much to get over the force of
Herschel's reasoning. He points out, indeed, that sea-
water does not attain its greatest density at a tempera-
206 LIGHT SCIENCE FOR LEISURE HOURS.
ture of 39, but some 12 or 14 lower. This, however,
does not affect Herschel's argument. If he had taken
a column whose base had a temperature of 25 instead
of 39, he would have had to extend, also, the range of
the water-slope in latitude ; and, in fact, he would have
obtained a yet smaller declivity in this way than that
actually deduced by him. Maury does not seem to
have noticed the really weak point in Herschel's argu-
ment. I shall presently show where this seems to me
to lie.
But if Maury fails in efficiently defending his own
views, he certainly is sufficiently effective in his attack
upon Sir John Herschel's.
He asks, in the first place, the pertinent question
6 How can the north-easterly trade-winds, which blow
only 240 days out of 365, cause the equatorial current
to flow all through the year towards the north-west
without varying its velocity either to the force or to the
prevalence of the trade-winds ? ' ' That the winds do
make currents in the sea, no one,' he says, 6 will have
the hardihood to deny ; but currents that are born of
the winds are as unstable as the winds; uncertain
as to time, place, and direction, they are sporadic and
ephemeral.'
He then points to a fact which ' militates strongly
against the vast current-begetting power that has been
given by theory to the gentle trade-winds. In both
oceans, the Sargasso seas lie partly within the trade-
wind region ; but in neither do these winds give rise
to any current. The weeds are partly out of water, and
THE GULF STREAM. 2OJ
the wind has therefore more power upon them than it
has upon the water itself ; they tail to the wind. And
if the supreme power over the currents of the sea reside
in the winds, as Herschel would have it, then of all
places in the trade-wind region, we should here have
the strongest currents. Had there been currents here,
these weeds would have been borne away long ago ; but
so far from it, we know that they have been in the
Sargasso Sea of the Atlantic since the voyage of
Columbus.'
In another argument, Maury certainly falls into an
error. He says, How can the north-easterly winds
cause the Gulf Stream to flow towards the north-east ?
But, as he himself points out, the trade-winds do not
blow over the Gulf Stream proper, and there can be no
doubt that, if the trade-winds sufficed to keep up a
continual equatorial current, finding a passage towards
the north after encountering the barrier opposed by the
American continent, this resulting northerly current
would assume a north-easterly course, for the very same
reason that the air-currents flowing from the equator
towards the north pole become south-westerly or counter
trade-winds. But he seems justified in asking how it
is possible that the impulse imparted by the gentle
trade-winds to the equatorial current could suffice to
generate a stream which eventually travels far towards
the north pole, if it do not even circle completely around
Greenland. 'When we inject water into a pool,' he
says, ' be the force never so great, the jet is soon over-
come, broken up, and made to disappear. In this
208 LIGHT SCIENCE FOR LEISURE HOURS.
illustration, the Gulf Stream may be likened to the jet,
and the Atlantic to the pool. We remember to have
observed, as children, how soon the mill-tail loses its
current in the pool below ; or we may now see at any
time, and on a larger scale, how soon the Niagara, cur-
rent and all, is swallowed up in the lake below.'
Franklin, who was the originator of the theory sup-
ported by Herschel, had unnecessarily introduced the
supposition that the trade-winds maintain a 6 head of
water ' in the Gulf of Mexico, and that the Gulf Stream
flows downwards like a river from this ' head,' as a
fountain or source. Maury rightly attacks this view,
which is undoubtedly a mistaken one ; but in doing so,
he falls into an error which exhibits his weakness in
the treatment of hydrodynamical problems. He points
out that, inasmuch as the Gulf Stream grows wider as
it crosses the Atlantic, it necessarily grows shallower,
so that the water-bed in which the stream flows has a
higher level under the shallow than under the deep
part of the current, and therefore, says Maury, c the
current runs up hill. 9 Herschel terms this a strange
perversion of language, but perhaps it would be more
correct to speak of it as a strange blunder. The stream
could, of course, only be said to run up hill if its surface
were seeking a higher level, which does not and cannot
happen. That the spreading out of the water of the
current, so as to form a wider and shallower stream,
does not correspond to an upward flow, is evident from
this, that it happens often with rivers, which no one
will suspect of running up hill.
THE GULF STREAM. 2OQ
Herschel does not find an answer to the main objec-
tions urged by Maury against the trade- wind theory.
Content with urging an apparently unanswerable objec-
tion against his opponent's view, he leaves his own to
take care of itself.
In forming an opinion respecting the two theories,
one is struck with the immense superiority in the power
of Maury's agent. For, if we consider, we shall see that
almost the whole of the sun's action upon the ocean
goes to produce those variations in temperature and
saltness in which Maury sees the origin of the current-
system ; but a very moderate portion of the sun's action
is called into play in the production of the trade-wind^.
Now it is very doubtful whether any large proportion
even of the force expended in producing the trade-winds,
ever acts on the water. For we know that the north-
easterly and south-easterly air-currents of the northern
and southern hemispheres, do not wholly merge into
northern and southern currents meeting point-blank
near the equator, as Herschel's theory seems to imply.
On the contrary, there is a wide zone of calms at the
equator, and the two systems of trade-winds appear to
pass upwards above the calm air, without parting with
the whole of their easterly motion. When once they
begin to travel polewards, they lose their easterly motion
in the same way that they acquired it that is, through
the effects of the earth's rotation. And whatever portion
is lost in this way which, for aught we know, may be
a very considerable portion cannot be taken into
210 LIGHT SCIENCE FOR LEISURE HOURS.
account as available to generate the easterly equatorial
current.
And now let us consider for a moment the relation
which holds between cause and effect in the case sup-
posed by Herschel. We have more than a fourth part
of the Atlantic Ocean in a state of perpetual motion,
and it is assumed that the air immediately above the
ocean is responsible for this circulation. Now even if
we suppose that the whole of the vis viva in the aerial
circulation is imparted to the waters, and neglect all
consideration of the fact that for a large portion of the
year the winds do not act in the manner available for
the production of the currents we are considering, yet
even then, I apprehend that we shall find the vis viva
of the aerial very far below that of the aqueous circula-
tion. The volume of moving water is, of course, far
less than that of the moving air, and the mean velocity
of the water-currents is less than that of the air-cur-
rents ; but, on the other hand, the specific gravity of
water is some 830 or 840 times greater than that of
air, and this difference far more than counterbalances
the others.
But now, when we come to consider the forces called
into action in producing changes of temperature, etc.,
we no longer find such a disproportion between cause
and effect. The sun's action on the equatorial and
tropical regions of the Atlantic not only produces a
great change in the density of the water, but also
raises immense masses by evaporation. Now the buoy-
ancy caused by increase of temperature is partly
THE GULF STREAM. 211
diminished through increase of saltness ; still it is an
important motive force. A large portion of the evapo-
rated water is also precipitated over the equatorial
regions in the form of rain ; yet that a very large por-
tion is carried away from equatorial and tropical to
temperate zones is beyond dispute.
But now, how are we to get over the arguments by
which Herschel seeks to show that the buoyant water
will not rapidly move off, and that the effect of evapo-
ration is merely to produce opposing inrushes of water
which destroy each other's effect ? Easily, I take it, if
we remember that the buoyancy of the water does pro-
duce a surface-flow from the equator, however slight,
and that this is sufficient to destroy the balance of
forces which might otherwise make it doubtful whether
the place of the evaporated water would be supplied
from below or from above. I apprehend that there is
a continual under-flow of cooler water, rushing in
towards the equator on both sides, to supply the place
of the water evaporated by the sun's heat. Now there
can be no question that under-currents arriving in this
manner, whether from the north or from the south,
would acquire a strong westerly motion (just as the
trade-winds do). Thus they would generate from below
the great equatorial westerly current. In this up-flow
of cool currents having a strong westerly motion, I find
the mainspring of the series of motions. The water
thus pouring in towards the equator is withdrawn from
beneath the temperate and arctic zones, so that room is
continually being made for that north-easterly surface-
p 2
212 LIGHT SCIENCE FOR LEISURE HOURS.
stream which is the necessary consequence of the con-
tinual flow of the great western equatorial current
against the barrier formed by the American continent.
It would require much more space than I have at my
disposal to deal at length with the subject of my paper.
I therefore conclude by referring my readers to Maury's
interesting work on the 'Physical Geography of the
Sea,' with the remark that his views seem to me only
to require the mainspring or starting force towards the
west which I have ventured to suggest, to supply a
complete, efficient, and natural explanation of the whole
series of phenomena presented by the great ocean-
currents.
The Student for July 1868.
OCEANIC CIRCULATION.
THEKE are some questions, seemingly innocent enough,
which yet appear fated to rouse to unusual warmth all
who take part in their discussion. One cannot, for
instance, find anything obviously tending to warmth of
temper in the telescopic study of a planet ; yet the
elder Cassini was moved to passionate invective by cer-
tain observations of Mars not perfectly according with
his own ; and Sir W. Herschel, usually so philosophic,
was roused by Schroter's recognition of mountains in
Venus to deliver himself of a criticism justly described
OCEANIC CIRCULATION. 213
by Arago as 'fort vive, et, en apparence du moins,
quelque peu passionnee.' The question, again, whether
the ' Eozoon Canadense ' is a true * Rhizopod,' though
not altogether removed from the region of hard words,
might appear to be unlikely to excite warlike emotions ;
yet there has been some very pretty fighting over it.
The solar corona has in like manner given occasion for
rather strong writing; and if, on the one hand, the
supporters of a lately-abandoned theory said of their
opponents that 'they made themselves ridiculous,'
these, in their turn, at times used a tone reminding
one of the scholar who said of a rival, * May God con-
found him for his theory of the Irregular Verbs : ' yet
the corona seems at a first view rather calculated to
produce a sedative effect than to excite unphilosophic
wrath. The subject of oceanic circulation would appear
to belong to the class of questions here considered.
The very name of the Grulf Stream is to some phy-
sical geographers as a red cloth is to a bull. Even Sir
John Herschel, usually placidity itself, was moved when
he spoke on this point. But though he and Maury
grew warm enough in its discussion, their warmth wa$
ice-cold compared with the fire of more recent dis-
putants. We have before us the latest contribution to the
subject, a rather ponderous essay in one of our leading
quarterlies ; and herein we find pleasing references to
the ' stupidities ' of one set of opponents, the ' shallow
nonsense ' of a second, ' the wrong-headedness ' of a
third, with other similar amenities. More than once
214 LIGHT SCIENCE FOR LEISURE HOURS.
during the progress of this controversy the gentle public
has been reminded of Bret Harte's remarks
about the row
That broke up the Society upon the Stanislow ;
and has been inclined to urge, with 4 Truthful James,'
that they
Hold it is not decent for a scientific gent
To say another is an ass, at least to all intent ;
Nor should the individual who happens to be meant,
Reply by heaving rocks at him to any great extent.
The controversy has not, indeed, reached this last
stage of development, and we trust it never will ; but
it has gone so near to it as to suggest that the dis-
putants have wished to demonstrate, by example, the
justice of Darwin's theory about the human ' snarling
muscles.' !
I propose to inquire into the subject which has been
thus warmly discussed, trusting not to be myself in-
veigled by it into any warmth of expression. Indeed,
1 ' He who rejects with scorn the belief that the shape of his own
canine teeth, and their occasional great development in other men, are
due to our early progenitors having been provided with these formidable
weapons, will probably reveal, by sneering, the line of his own descent.
For though he no longer intends, nor has the power, to use these teeth
as weapons, he will unconsciously retract his " snarling muscles " (thus
named by Sir Charles Bell), so as to expose them ready for action, like
a dog prepared to fight/ Darwin's 'Descent of Man,' vol. i. p. 176.
"We may mention, by the way, that an instance has recently occurred,
in which the human teeth were used to some purpose against one of the
recognised masters in the art of biting. A man, proceeding in company
with several others through a wood, was attacked by a hyena (usually
one of the most cowardly of beasts). His companions fled, and having
no weapon he was reduced to the necessity of showing tooth for tooth,
and taking a good grip of the hyena's nose, he compelled that gentle-
man to howl with anguish. On this, the man's companions returned
and presently beat the hyena to death.
OCEANIC CIRCULATION. 215
but for the fate of others, I should feel no anxiety on
this point, though I have myself a favourite theory to
uphold respecting one branch of the subject. As it is,
I share something of the feeling of the Eed Cross
Knight when he was approaching ' Foul Error's den/
and his monitress said to him, ' The perils of this place
I better wot than thou ; therefore I rede, Beware.' I
am not without hope, however, that I may be able to
keep my snarling muscles quiescent.
I shall direct attention chiefly to the Atlantic cur-
rents, as being those whose real direction and extent
are best known, and those, moreover, whose character-
istics are most important to European nations.
Let us begin with the surface currents, and though
the system of surface circulation can scarcely be said to
have a real beginning, let us start with the great equa-
torial currents which flow westwards from the Gulf of
Guinea, 1 or more correctly from the Bight of Biafra.
We speak of the westwardly equatorial currents-, because
not unfrequently there is an equatorial eastward current
running between two much more important tropical
westward currents. Yet ordinarily there is one great
westward current running in an unbroken stream from
equatorial Africa to the shores of Brazil, and even when
this great current is divided into two by an eastward
current this last is only to be regarded as a sort of
' backwater.' The water moving westwards is relatively
cold, more especially on the African side of the Atlantic.
1 Along the shores of the Gulf of Guinea there flows an easterly
current, several degrees warmer than the equatorial current.
2l6 LIGHT SCIENCE FOR LEISURE HOURS.
The accompanying map exhibits the nature of the
surface circulation of the North Atlantic. It is con-
structed on one of the forms of equal-surface projection
described in my ' Essays on Astronomy,' and has the
advantage over the ordinary Mercator's charts of ex-
hibiting the true dimensions of the various currents.
I would, however, invite the student who wishes to fa-
miliarise himself with the true nature of the Atlantic
currents to construct other maps ; for instance, a polar
map on the first method of equal-surface projection
described in that essay (see pp. 264, 265), and a map of
the whole Atlantic on the second plan, taking the
meridian 40 west of Greenwich as the central one.
Of the water carried westwards by the great equa-
torial movement, the most important portion after
reaching Brazil is carried northwards towards the West
Indies. The reason of this is obviously to be found
in the fact that Cape San Roque, forming the jutting
angle of Brazil, lies several degrees south of the equa-
tor. The portion carried southward forms the Brazil
Current, and after travelling along the shores of South
America almost as far as the mouth of the La Plata,
acquires gradually an eastwardly motion which eventu-
ally carries it back across the Atlantic towards the Cape
of Good Hope, there to pass northwards, and so again
to traverse the Bight of Biafra. The surface-circula-
tion in the South Atlantic is thus seen to be com-
paratively simple.
The larger portion of the equatorial current is carried
less quickly northward, because the northern shore-line
2l8 LIGHT SCIENCE FOR LEISURE HOURS.
of Brazil and Guiana is inclined at a much smaller
angle than the south-eastern to the westwardly course
of the great equatorial currents. Thus the water which
is carried towards the West Indies has time to acquire
under the tropical sun a much higher temperature than
it had possessed when traversing the GKilf of Guinea.
It is divided into two parts by the guasi-barrier which
the West Indian Islands (or rather the semi-submerged
mountains of which they form the crests) oppose to its
progress. A comparatively small portion finds its way
into the Caribbean Sea, and making the circuit of the
Gulf of Mexico, passes out eastwards round the penin-
sula of Florida. We may fairly assume that this por-
tion is comparatively small ; simply because this true
gulf stream, passing between Cuba and Florida on an
eastern course, would continue so to move for at least
some considerable distance, were it not in some way
deflected. But it actually turns almost due northwards
after passing through the Bahama Sea, traversing the
Bernini Narrows on this course, and so onwards towards
Hatteras. This would seem to imply that the true Gulf
Stream is pressed northwards by the arrival of a much
larger body of water which has travelled outside the
West Indies. It is true that the diversion of the Gulf
Stream northwards may be really caused by the great
Bahama Bank. But this would equally establish our
position ; for if the Bahama Bank is thus effective in
diverting the whole of this now swiftly moving current,
the Windward Isles may be assumed to be correspond-
ingly effective in diverting the greater portion of the
OCEANIC CIRCULATION. 219
sluggish equatorial current. Moreover, if we remember
how shoals commonly take their origin, we may con-
sider that the very existence of the Bahama Bank is
probably due to the former encounter of the two im-
portant branches of the equatorial current the part
which had circled the Gulf of Mexico and the part
which had travelled outside the West Indies. Thus,
the northerly course finally taken by the Gulf Stream
implies that the latter portion had prevailed over the
former, and therefore that it is the most considerable
portion. I must mention, however, that the Edinburgh
Reviewer holds the part which enters the Caribbean Sea
to be the larger.
Be this as it may, the Gulf Stream proper has ac-
quired, during its circuit, characteristics perfectly
distinct from those which it had had when entering the
Caribbean Sea, or from those possessed by the remain-
ing portion when approaching the Bahamas. In the
first place, having traversed a much longer course under
the same intense tropical heat, the Gulf Stream has
become much warmer than the outer stream. In the
second place (probably from having traversed the outlets
of the Mississippi, and so carrying with it the finely-
divided matter brought down by that river), the Gulf
Stream has acquired a peculiar blue colour, somewhat
resembling that recognised in most of the Swiss lakes. 1
1 This explanation of the colour of the Gulf Stream seems the best
that has hitherto been offered. The Edinburgh Reviewer thus states the
matter : ' The remarkable blueness which distinguishes the water of the
Gulf Stream from the oceanic water through which it flows may be due
to its retaining in suspension the finest of the sedimentary particles
220 LIGHT SCIENCE FOR LEISURE HOURS.
Thirdly, its course having carried it into narrow chan-
nels, it has required a relatively rapid rate of outflow,
insomuch that the surface flow of the current on its
outward passage through the Narrows of Bernini, takes
place at the rate of from 2J to 4 miles per hour. Its
width here is at the surface not more than about 25
miles, its maximum depth rather more than a quarter
of a mile (about two-fifths of the channel's maximum
depth), and its mean rate of flow probably about 50
miles per day.
I shall not follow the Edinburgh Reviewer in con-
sidering the details of the progress of the Grulf Stream
from the Narrows of Bernini to Cape Hatteras, because,
though in themselves of the utmost interest and impor-
tance, these details throw no special light on the general
subject of oceanic circulation. Suffice it that as far as
Hatteras the Grulf Stream remains distinctly recognis-
able, and that even off Sandy Hook (New York) its
surface temperature is little reduced, and its velocity
still amounts to about one mile per hour. Off Nan-
brought down by that river, the coarser having been deposited near its
(the river's) mouth ; just as the intense blueness of the waters of Lake
Geneva depends on its retention of the finest sedimentary particles
brought down by the Ehone in the upper part of its course, while that
of the waters of the Mediterranean is due to its pervasion by the like
particles brought down by the river Rhone and other rivers, which dis-
charge themselves into its western basin, and by the Nile into its eastern.'
It will be remembered that Prof. Tyndall, by researches carried on
during the return of the Urgent from the eclipse expedition of 1870,
was enabled to throw considerable light on the cause of the colour and
shades of colour in water of greater or less depth. See also Dr. Car-
penter's ' Eeport of Researches in the Mediterranean,' in the ' Proceed-
ings of the Royal Society,' vol. xix. p. 200.
OCEANIC CIRCULATION. 221
tucket the breadth of the current is about 410 miles,
its winter surface temperature only 10 below that
which it had in the Florida Channel, and its rate of
flow still nearly one mile per hour. It has at this part
of its course acquired a good deal of easting, a circum-
stance which must (unquestionably, we conceive) be
ascribed to the fact that it brings from low latitudes
the more rapid easterly rotation movement of the earth.
The same would, of course, apply to the less character-
istic but larger current which has arrived at the same
latitudes without circuiting the Grulf of Mexico.
Now here we approach a critical part of our subject.
It is admitted by all that off Newfoundland the Gulf
Stream loses its special characteristics. As Dr. Hayes
remarks, ' its strength diminishes ; the air of a higher
latitude brings its temperature down to that of the
North Atlantic generally* (not, however, without
raising the temperature of the North Atlantic to some
extent) ; ' the water loses all its Grulf Stream character
as to course, warmth, and flow ' (and as to colour also) ;
' and it dies away into the sluggish Atlantic drift which
sets from a westerly to an easterly direction.' It is not
so generally noticed, but will scarcely, I suppose, be
disputed, that the Grulf Stream water strengthens, and
that appreciably, this sluggish Atlantic drift. Then it
is reinforced by the portion which has travelled outside
the West Indian Islands ; and we may assume (without
giving rise to objections) that the general prevalence of
south-westerly winds will further strengthen the east-
ward motion of the combined mass. At any rate, let
222 LIGHT SCIENCE FOR LEISURE HOURS.
the causes be what they may (and presently we shall
have a further cause to take into account), it is ad-
mitted by all physical geographers that a great, though
slow current, or drift, does pass eastwards from the
neighbourhood of Newfoundland. Moreover, it is ad-
mitted by all that the southern part of this current
(which the Edinburgh Reviewer actually regards as
identifiable with the Grulf Stream 1 ) traverses the At-
lantic until, nearing the Azores, it joins the southwardly
Guinea current ; while the northern part passes on a
north-easterly course, which carries it between Britain
and Iceland, between Sweden and Spitzbergen, onwards,
even as far as the very neighbourhood of Nova Zembla.
Lastly, it is admitted by all that, directly or indi-
rectly, this great north-easterly current causes the
climate of Great Britain, and of the north-western
parts of Europe generally, to be milder than that of
North American regions in corresponding latitudes.
It might appear, then, that all these things being
admitted, no question of any importance remains, so
far as the actual facts of the oceanic surface-circulation
are in question. We shall presently see that a question
has arisen as to the cause of the observed facts ; but as
to their nature everything that seems worth discussing
at ail appears to be satisfactorily disposed of.
Let those readers who in their simplicity have
adopted this notion hasten to dispossess themselves of
it by reading some remarks by Dr. Hayes, the American
1 He says that the great equatorial current is partly supplied ' by the
return of a portion of the Gulf Stream.'
OCEANIC CIRCULATION. 22$
explorer, quoted with approval by the Edinburgh Ke-
viewer. The latter having repeated from ' Lothair ' " a
sneer at the shallow nonsense which has been talked
about the Gulf Stream, and at the exaggerated esti-
mates of its potency which have been put forward by
men (as well as women) who ought to have known
better " (these are the reviewer's words, not Mr. Dis-
raeli's), proceeds as follows : ' As Dr. Hayes truly re-
marks, " Weather predictors without end have launched
upon it their stupidities ; meteorologists have deluged
the world (sic) with their assumptions -respecting it ;
theorists of all kinds have floated their notions upon it.
One whirls it away into the arctic regions, and opens a
passage to the pole with it ; another compels it to give
a climate to countries where otherwise there would be
no climate worth mentioning ; while still another spins
it round the Atlantic Ocean, and its wide-spread arms
close upon a stagnant sea. . . . Through means
such as these mankind has come to look upon the Gulf
Stream with a certain degree of awe. It is a * breeder
of storms ' ; the giver of heat ; it might become the
father of pestilence. Will it always continue to do its
duty as hitherto ? or will it start off suddenly with some
new fancy, and pursuing some new course, upset the
physical and moral status of the world ? "
Now we have seen that the writer who thus endorses
Dr. Hayes' diatribe, is among those who hold that a
southern offset from the Gulf Stream circles round
the Sargasso Sea to join the Guinea current. He says
farther on that he ' entirely accords ' with the opinion
224 LIGHT SCIENCE FOR LEISURE HOURS.
of Buchan, the meteorologist, that the north-easterly
current above (referred to produces an afflux of warmth
brought to the British Isles by the water that laves our
western coasts.' He proceeds : ' There is ample evidence
that the cold of some parts of the north polar area is
greatly mitigated by an afflux of water bringing with
it the comparative warmth of temperate seas. It has
long been known that cocoa-nuts, tropical seeds, trunks
of tropical trees, timbers and spars of ships wrecked far
to the south, and sometimes portions of their cargo,
are found on the shores of the Western Hebrides, the
Orkney, Shetland, and Faroe Islands, the north of
Norway, and even Spitzbergen ; and since their trans-
sport has taken place just in the course of the Gulf
Stream if prolonged to the north-east, their arrival has
been accepted almost without question as evidence of
its agency. The evidence furnished by the surface tem-
perature of that north-eastern portion of the Atlantic
Ocean which intervenes between Iceland and the North
Cape, and then stretches away to the eastward between
Spitzbergen and Nova Zembla, seems at first sight con-
clusive to the like effect. A large amount of additional
thermometric evidence has been collected of late years;
and this has been most ably digested by the eminent
German geographer, Dr. Petermann, who has recently
put forward a series of maps for different periods of the
year, in which these observations are embodied, and
their results made obvious to the eye by the course of
the * lines of equal temperature,' which in the summer
pass between Iceland and the Shetland Islands, a little
OCEANIC CIRCULATION. 22$
to the east of north towards Spitzbergen, and thence
with more of an easterly bend even beyond the seventy-
fifth degree of north latitude. The existence of a warm
stream in this direction has been confirmed still more
recently by two adventurous officers Lieutenant Julius
Payer, of the Austrian army, and Lieutenant Wey-
precht, of the German army who followed its path
last summer in a small sailing vessel hired by them-
selves, and state that they found open water from
east longitude 42 to east longitude 60, even beyond
the seventy-eighth parallel of north latitude, the
highest point they reached being north latitude 79, in
east longitude 43. A Eussian expedition under Prince
Alexis Alexandrovitch, of which the distinguished
savant. Von Mildendorf, had the scientific charge, was
about the same time exploring the Polar Sea between
Nova Zembla and Iceland ; and Von Mildendorf has
stated to the Imperial Academy of St. Petersburg
that ' the corvette Wajag has proved the extension of
the Gulf Stream to the west coast of Nova Zembla, and
that we find it on the meridian of Banin Xoss (in east
longitude 43^) still of a width equal to two degrees of
latitude, and of a temperature of fifty- four degrees
Fahrenheit, cooling down only four or six degrees at
depths of thirty and fifty fathoms.'
As if to remove all question as to his real opinion
the reviewer immediately adds that he fully accepts,
not only the great body of facts so ' industriously cor-
related by Dr. Petermann, but the inference Dr. Peter-
mann draws from them that an attempt to penetrate
Q
226 LIGHT SCIENCE FOR LEISURE HOURS.
the polar ice-wall to the north-east of Spitzbergen is;
more likely to be successful than the search for a pas-
sage in any other direction.'
So that ( 1 ) Dr. Petermann, regarded by our reviewer
as an eminent geographer ; (2) Von Mildendorf, whom
he regards as a distinguished savant ; and (3) the re-
viewer himself, who no doubt does not regard himself
as either shallow or stupid, seem all agreed as to the
very points which the reviewer has spoken of as in-
volving stupidities and shallow nonsense. Certainly
they all agree as to the only points which seem in the
least worthy of discussion.
What, then, the reader will ask, is the matter in
dispute? Over what momentous question have the
angry words quoted above been bandied ?
After diligent search for the apple of discord, the
student of the review will be led to the conclusion that
it is neither more nor less than the name ' Grulf Stream.'
We have seen that Von Mildendorf calls the warm
current which passes by Nova Zembla the Grulf Stream.
In this, it appears, he has shown shallowness and stu-
pidity. Dr. Petermann has equally committed himself,
or rather has committed a more serious offence. For
Von Mildendorf might have used the offensive epithet
only through inadvertence ; but Dr. Petermann not
only uses it, but has the hardihood (we might almost
say the cruelty) to maintain that ' it is a matter of no
consequence.' Moreover, as our reviewer sadly admits,
' other physical geographers ' agree with Dr. Petermann.
The reviewer is so grieved by the defection of the
OCEANIC CIRCULATION. 227
c distinguished savant,' the ' eminent geographer,' and
' the other physical geographers,' that for a moment his
confidence deserts him, and instead of applying afresh
to them, directly, the lash which has indirectly reached
them, he proceeds thus mildly : 'In oux belief, of which
we shall presently explain the grounds, the real Gulf
Stream has no more to do with the inflow into the
polar area than with the ripening of oranges at Naples,
or the maintenance of Catholicism at Eome, so that,
even if its current were to be entirely diverted by the
cutting of a wide channel through the Isthmus of Pan-
ama, not only would the climate of the British Islands
suffer very little, but a north-easterly stream of warm
water . . . would still mollify the severity of polar
cold, and help to render Spitzbergen and Nova Zembla
accessible to arctic voyagers.' This belief, in which I
cordially concur, would seem to afford excellent reason
for rejecting the name Gulf Stream whenever the course
of the stream shall thus have been diverted, but scarcely
seems to justify the disuse of the name under the actual
circumstances ; still less would it appear to afford good
grounds for using such hard words as ' shallow non-
sense ' and ' stupidity.' If the course of the Danube
were intercepted in Baden, it is tolerably certain that
a mighty river would continue to flow past Vienna,
Belgrade, and Ismail to the Black Sea ; nor would the
noble river which flows northward through Germany be
much reduced though the Ehine were diverted in the
Grisons : yet geographers are satisfied to call these
rivers the Danube' and the Ehine, not adopting new
Q 2
228 LIGHT SCIENCE FOR LEISURE HOURS.
names at every stage where some new influx changes
the size and character of either. And the title ' Gulf
Stream' has, in like manner, advantages in point of
convenience, which are likely to prevent geographers
from rejecting it yet awhile. It may mislead some few
into supposing that the whole of the great north-
easterly current has passed through the Grulf of Mexico,
just as we can conceive that some few students of geo-
graphy might imagine all the water which flows past
Cologne or Coblentz to have come from the Orisons, or
all that flows past Nikopolis to have come from Baden.
Almost every convenient name, however, is open to
some such disadvantage ; and the student of oceanic
circulation who finds he has been to some degree misled
by a name must not mistake the detection of his error
for a great geographical discovery.
Majora canamus.
We have hitherto considered surface-currents only.
We have not, indeed, considered all the surface currents
which traverse the North Atlantic ; but the principal
streams have been indicated. We must now direct our
attention to submarine currents.
It is impossible to consider carefully the nature and
distribution of the surface circulation without recog-
nising the fact that there must be currents beneath
the surface. It is true that one can conceive the
existence of a complete system of oceanic circulation
without any movement in the depths of the sea ; but
when we examine the actual surface currents we find
that either the commencement or the prolongation of
OCEANIC CIRCULATION. 229
some currents must necessarily be submarine. For
instance, the quantity of water carried by the great
north-easterly drift into the Arctic Ocean is very much
greater than that which flows out of the Arctic Ocean,
by the so-called Arctic current, past Greenland. Ex-
amining, indeed, the ordinary current charts, always
drawn on Mercator's projection (seemingly because this
projection is the very worst that could be devised for
the purpose), we might suppose that this arctic stream
was much more extensive than it really is. But what
can be expected of a projection which makes Green-
land (whose real area is not much greater than that of
the Scandinavian peninsula) actually as large as South
America. The Arctic current, however, affords yet
better evidence of the occurrence of submarine streams,
for the extension which passes between the Gulf Stream
and the United States, is in places completely lost
sight of (the Gulf Stream touching the American
shores), and reappears farther on. It is clear that
it must have passed under the Gulf Stream in such
cases.
Now, the study of the submarine currents has of late
years thrown considerable light on the whole question
of oceanic circulation, and has supplied the solution of
some problems which had formerly appeared altogether
perplexing.
We owe to Drs. Carpenter and Wyville Thomson
some of the most important facts recently ascertained.
Others, however, have shared in the work. I would,
indeed, particularly invite attention to the fact that I
230 LIGHT SCIENCE FOR LEISURE HOURS.
do not here pretend to give anything like a complete
history of recent investigations into the subject. I
select only those facts which bear most significantly on
the wider relations the more marked features of
oceanic circulation.
In the first place, a result which had long perplexed
physical geographers has been shown to be erroneous.
It had been supposed that the temperature of sea-water
below a certain depth is in all latitudes constant, and
about seven degrees above the temperature at which
fresh water freezes. Sir John Herschel, in his ' Phy-
sical Geography,' adopted this supposed discovery as
well established.. Now, let one consequence of such a
relation be carefully noted. The surface water in the
tropics is warmer than this supposed constant bottom-
temperature ; the surface water in arctic regions is
cooler ; at some intermediate latitude the surface water
has the same temperature as the water at the bottom.
Hence in this intermediate latitude the water is uni-
formly warm (according to the supposed relation) from
the surface to the bottom. We may therefore regard
the water in this latitude as constituting, in effect, a
constant barrier between the tropical waters and the
arctic waters. "Without regarding it as absolutely im-
movable we should yet be compelled to regard it as so
far steadfast as to negative the theory of the existence
of submarine currents of an importance corresponding
to that of the surface currents. Accordingly, the theory
put forward by Humboldt and Pouillet to the effect
that there is an interchange of waters between polar
OCEANIC CIRCULATION. 231
and equatorial regions was discredited by this supposed
discovery.
Drs. Carpenter and Wyville Thomson, however, have
been able to show that no such relation exists. There
are vast submarine regions of the Atlantic where the
temperature of the water is far lower than the constant
and uniform temperature believed in by Sir John Her-
schel. The temperature is, indeed, in places, as low,
or veiy nearly so, as the freezing-point of fresh water,
under a surface-temperature 20 degrees or so higher.
But in other regions having the same surface-tempera-
ture the depths are 10, 12, or 14 degrees higher than
that of freezing fresh water. Moreover these relations
are constant, so far as the deep water is concerned.
Now, there can be only one interpretation of the cir-
cumstances here mentioned. If the depths of the ocean
were unmoved by any process of submarine circulation
there can be no question that a general uniformity of
deep sea temperature would prevail in given latitudes.
We should not find the bottom water in one region 1 3
or 14 degrees warmer than the water in a closely adja-
cent region. We have only to inquire what is the case
in inland seas where no great influx of water of alien
temperature can take place, to see that this must be so.
Take, for instance, the Mediterranean. Here we learn
from Dr. Carpenter's recent researches that ' the tem-
perature at every depth beneath 100 fathoms is found
to be uniform, even down to a bottom of 1,900 fathoms ;
as had, indeed, been previously ascertained by Captain
Spratt, although his observations, made with thermo-
232 LIGHT SCIENCE FOR LEISURE HOURS.
meters not protected against pressure, set this uniform
temperature too high. In the western basin of the
Mediterranean, as shown by the Porcupine observations
of 1870, the uniform temperature is 54 or 55 degrees ;
being, in fact, the winter temperature of the entire
contents of the basin, from the surface downwards ; and
being also, it would appear, the mean temperature of
the crust of the earth in that region.' We learn, then,
two things viz., first, that where extensive submarine
motions are impossible, a constant submarine tempera-
ture may be expected to prevail in the same latitudes ;
and, secondly, that in the latitude of the Mediterranean
the submarine temperature is about 54 or 55 degrees
Fahr. Thus, it is clear, in the first place, that the
varieties of temperature observed in the depths of the
Atlantic must be due to the continual arrival of water
of the observed temperatures, at a rate great enough to
prevent the deep water from acquiring a constant tem-
perature ; and in the second place it becomes possible
to tell whence the submarine currents are flowing. If
they are cooler than they should be supposing latitude
alone in question, then they are travelling from arctic
towards tropical regions, and vice versa. On this last
point no doubt remains. In a latitude corresponding
to that of the Mediterranean basin, the depths of the
Atlantic are far colder, even in their warmest por-
tions, than they would be if latitude alone were in
question. ' In regard to surface-temperature,' says
Dr. Carpenter, 'there is no indication of any essen-
tial difference between the Mediterranean and the
OCEANIC CIRCULATION. 233
Eastern Atlantic ' in the same latitudes ; c and the
thickness of the stratum that undergoes superheating
during the summer is about the same. ... At
the depth of a hundred fathoms, in the Atlantic as in
the Mediterranean, the effect of the superheating seems
extinct, the thermometer standing at about 53 degrees ;
and beneath this ' (in the Atlantic only), 'there is a slow
and tolerably uniform reduction at the rate of about
two-thirds of a degree for every fathom, down to
700, at which depth the thermometer registers 49
degrees. But the rate of reduction then suddenly
changes in the most marked manner ; the thermometer
showing a fall of no less than nine degrees in the next
200 fathoms, so that at 900 fathoms it stands at 40
degrees. Beneath this depth the reduction again be-
comes very gradual ; the temperatures shown at 1,500,
2,000, and 2,435 fathoms (the last being the deepest
reliable temperature sounding yet obtained) being,
respectively, 38, 37, and 36 degrees.'
Thus, there can be no possible question that the
depths of the Atlantic are occupied by a vast current
much colder than the deep sea temperature due to the
latitude, and, therefore necessarily flowing from the
arctic towards the tropical seas.
Such are the broad facts of the Atlantic circulation.
We have a surface circulation whose general features
are such as have been described, and are generally ad-
.mitted, though a dispute has arisen as to a question of
nomenclature ; and then we have a general submarine
' set ' of water from the arctic regions towards the
234 LIGHT SCIENCE FOR LEISURE HOURS.
tropics, the existence of which is also generally ad-
mitted.
But now we again approach a subject of controversy,
and one which is certainly better worthy of discussion
than that which we considered above. It relates, in
fact, to the question how this wonderful system of
oceanic circulation is brought about.
Passing over several crude theories which have long
since been disposed of, we come first to the theory that
the system of oceanic circulation is due to the action of
the trade-winds. This theory has been maintained by
Franklin and others in former times, by Sir John
Herschel in our own, and is warmly advocated in the
present day, by many whose opinions are not to be
rashly contradicted.
Against this theory it has been urged by Captain
Maury c with singular wrongheadedness ' accordirg
to the Edinburgh Eeviewer, but as it seems to me with
no small degree of reason that the trade-winds are
neither powerful enough nor persistent enough to ac-
count for the great equatorial currents, or therefore for
the Grulf Stream. Maury says, ' with the view of ascer-
taining the average number of days during the year
that the north-east trade-winds of the Atlantic operate
upon the water between the equator and 25 degrees
north latitude, log-books containing no less than
380,284 observations on the force and direction of the
wind in that ocean were examined. The data thus
afforded were carefully compared and discussed. The
results show that within these latitudes and on the
OCEANIC CIRCULATION. 235
average the wind from the north-east is in excess of
the wind from the south-west only 111 days -out of the
365. Now, can the north-east trades, by blowing for
less than one-third of the time cause the Gulf Stream
to run all the time, and without varying its velocity
either to their force or prevalence.' Our reviewer
not only dwells on the wrongheadedness of this argu-
ment wholly irresistible as it appears but asserts
that ' the trade-wind origin of the Gulf Stream is about
as certain as the rotundity of the earth.' It could have
been wished that in place of abusing Captain Maury for
wrongheadedness, the reviewer would have devoted a
few lines to the demolition of Maury's argument.
Maury himself advanced the relative lightness of the
equatorial water as the true reason of the oceanic circu-
lation. But granting that the expansion of the equa-
torial water under the sun's heat, as well as the resulting
buoyancy, would cause an overflow of equatorial water
polewards, this overflow would be an exceedingly slow
movement, and it would result in an eastwardly instead
of a westwardly flow, for the very same reason that the
counter trade- winds travelling polewards assume an
eastwardly direction.
In the Student for July 1868, I advanced another
explanation. I urged that the sun's action on the
equatorial and tropical regions of the Atlantic, raising
immense quantities of water by evaporation, causes an
influx of water from below. ' There can be no question,'
I then wrote, ' that under-currents arriving in this
manner, whether from the north or from the south'
236 LIGHT SCIENCE FOR LEISURE HOURS.
(that is from arctic or from antarctic regions) 4 would
acquire a strong westerly motion (just as the trade-
winds do). Thus they would generate from below the
great equatorial westerly current. In this upflow of cool
currents having a strong westerly motion, we find the
mainspring of the series of motions. The water thus
pouring in towards the equator is withdrawn from
beneath the temperate and arctic zones, so that room is
continually being made for that north-easterly surface-
stream which is the necessary consequence of the con-
tinual flow of the great westerly equatorial current
against the barrier formed by the American continent.
. . . . Captain Maury's views seem only to require
the mainspring or starting- force towards the west
which has been here suggested, to supply a complete,
efficient, and natural explanation of the whole series of
phenomena presented by the great ocean currents.'
Four or five months later, and while the results on
which Dr. Carpenter subsequently based his theory of
the oceanic circulation were as yet unpublished, I drew
attention in the columns of the Daily News to the
comparatively limited extent of the influences due to
polar cold. This cause, I pointed out, ' scarcely has
any influence in latitudes lower than the parallel of 50
degrees.'
In the year 1869 Dr. Carpenter was first led to advo-
cate the theory that the continual descent of cold water
in the Arctic Seas is the mainspring of the system of
oceanic circulation. He reasoned that the Arctic Seas
being exposed to great cold, the surface water ; descends
OCEANIC CIRCULATION. 237
in virtue of its reduction in temperature and increase
of density, its place being taken, not by the rising up
of water from beneath, but by an inflow of water from
the neighbouring area; and since sea-water becomes
continually heavier in proportion to its reduction of
temperature, this cooling action will go on without the
check which is interposed in the case of fresh water.' 1
Thus the water becoming denser and heavier will
descend, and 'there will be a continual tendency to
the flowing off of its deepest portion into the warmer
area by which the polar basin is surrounded ; producing
a reduction in the level of the polar area, which must
create a fresh indraught of surface-water from the warmer
area around to supply its place. This, in its turn,
being subjected to the same cooling action, will descend
and flow off at the bottom, producing a fresh reduction
of level and a renewed indraught at the surface.'
Dr. Carpenter illustrated this theory, or rather the
combined action of polar cold and equatorial heat, by
an experiment, the plan of which had occurred also to
myself, and been described by me in conversation
somewhat earlier. ' A long narrow trough having glass
sides was filled with water, and a piece of ice was
wedged in at one end between its side plates just
beneath the top, whilst the surface of the water at the
other end was warmed by a piece of metal, of which a
part projected beyond the trough, and was heated by a
' Fresh water expands with reduction of temperature, near the
freezing point, and hence, becoming lighter, the descending motion above
described is interfered with in the case of fresh water.
238 LIGHT SCIENCE FOR LEISURE HOURS.
spirit lamp placed beneath it ; thus representing the
relative thermal conditions of the polar and equatorial
basins. A colouring liquid viscid enough to hold
together in the water, while mixing with it sufficiently
to move as its moves, being then introduced, the liquid
as it impinged on the ice was seen to sink rapidly to
the bottom, then, to flow slowly along the floor of the
trough towards the opposite extremity, then gradually
to rise beneath the heated plate, and then to flow slowly
along the surface towards the glacial end, repeating
the same movement until the ice had melted.'
It will be observed that in this experiment the effect
of cold is not exerted alone, so that it by no means
proves that the arctic cold is the chief agent in pro-
ducing the system of oceanic circulation. Moreover,
the conditions of the polar and equatorial basins are in
one respect not accurately (or even nearly) reproduced,
for the real arctic area is very much smaller, compared
with the real equatorial area, than in the case of the
experiment. Indeed it appears to me that Dr. Carpenter
paid far too little attention to the relative smallness of
the arctic area. This may have been partly due to the
erroneous ideas suggested by the ordinary maps on
Mercator's Projection, in which, as I have already
mentioned, the arctic regions are enormously exagger-
ated. It is almost impossible to study such a map as
that which illustrates this paper (see page 217) without
feeling that the theory presented by Dr. Carpenter will
scarcely hold water, or rather if this way of presenting
the argument be permitted that the arctic area does
OCEANIC CIRCULATION. 239
not hold water enough to produce the effects de-
scribed by Dr. Carpenter. For in that map the whole
area of the Arctic Ocean is presented ; l and from out
of that area, be it noted, must come the northern supply
of descending water, not only for the Atlantic equatorial
current, but for the much larger equatorial current of
the Pacific, if Dr. Carpenter's theory be sound.
The following letter, written by Sir John Herschel
only a few weeks before his lamented decease, has been
very widely quoted in favour of Dr. Carpenter's theory ;
yet if carefully studied it will be found rather to set
forth the strength of the theory advocated a year earlier
by the present writer. In this letter, at least, Sir John
Herschel appears to be disposed, in so far as he con-
cedes the efficiency of heat, cold, and evaporation, to
incline to the equatorial action as the most important.
Answering Dr. Carpenter, who had addressed a letter
to him on the subject, he says : ' After well considering
all you say, as well as the common-sense of the matter,
and the experience of our hot-water circulation pipes
in our green-houses, &c., there is no refusing to admit
that an oceanic circulation of some sort must arise from
mere heat, cold, and evaporation, as verce causce ; and
you have brought forward with singular emphasis 2 the
more powerful action of the polar cold, or rather, the
1 The bounding lines drawn from the pole on the right and left of
the white space represent one and the same meridian.
2 In Sir John Herschel's letters one can often recognise slight touches
w.e will not say of sarcasm (for he was incapable of saying aught that
could be considered bitter or unpleasant), but of what may be described
as a humorous suggestiVeness.
240 LIGHT SCIENCE FOR LEISURE HOURS.
more intense action, as its maximum effect is limited
to a much smaller area than that of the maximum of
equatorial heat. The action of the trade and counter-
trade winds, in like manner, cannot be ignored ; and
henceforward the question of ocean-currents will have
to be considered under a twofold point of view.'
It appears to me that not only is the equatorial or
rather tropical action much wider in range, but it is
also more intense than the polar action. For, let us
consider what happens during the heat of the day over
the tropical Atlantic. Here, over an area enormously
exceeding the whole arctic basin (we are considering,
be it understood, only the northern part of the system
of circulation) a process of evaporation is taking place
at so rapid a rate as to furnish almost the whole of
that rain-supply whence the rivers of Europe and North
America (east of the Eocky Mountains) take their
origin. There is thus produced a continual defect of
pressure, not merely along an equatorial strip, but so
far as 20 or even 30 degrees of north latitude, while
the downfall of rain which, taking one part with an-
other of the temperate and sub-arctic Atlantic, may be
regarded as incessant, continually adds to the pressure
in these last-mentioned regions. That on the whole
there must result a most effective excess of pressure
over the temperate zone of the Atlantic, as compared
with the tropical and equatorial portion, seems to me
indisputable. Now, if we compare this with the effects
of refrigeration over the relatively insignificant arctic
area, which as I have said has to supply the North
OCEANIC CIRCULATION. 241
Pacific submarine circulation (if Dr. Carpenter's theory
be true), as well as that of the North Atlantic, we can
scarcely doubt, as it seems to me, which cause is the
more effective. I would venture to predict that if
Dr. Carpenter's experiment were tried first with the
ice alone to produce circulation, and secondly with the
heat alone, the superior efficiency of the latter cause
would be at once recognised ; but I much more confi-
dently predict that if, as in the experiment I myself
proposed, the relative areas of the equatorial and arctic
basins were represented, there would be found to be
scarcely any comparison between the effects of arctic
cold and equatorial heat, so largely would the latter
predominate.
It is necessary to mention, however, that the prin-
ciple itself of the experiment has been objected to, on
the ground that the gradation of temperature must
always be much more rapid in such an experiment
than in the actual case of the Atlantic Ocean. This
objection, however, is, in rea)ity, based on a misappre-
hension. It is sufficient that the difference of tem-
perature at the two ends of the trough should corre-
spond to the difference between the temperature of the
arctic and equatorial seas ; and it is a matter of no im-
portance whatever that the real rate of gradation should
be represented. The case may be compared to the
illustration of the descent of water to form springs or
the like. Here an experiment would be valid in which
the outflow of the illustrative spring was obtained by
causing the vent to be so much below the level of the
242 LIGHT SCIENCE FOR LEISURE HOURS.
reservoir, though the slope from the reservoir to the
vent were very much greater than in the case of any
natural spring. Just as in the case of a spring it is
the difference of level, and not the rate of slope, which
is effective in causing the rate of outflow, so in the
case of the oceanic vertical circulation, it is the actual
difference of temperature, and not the rate of grada-
tion, which produces the activity of the circulation.
Another objection has been urged against the 'heat
and cold theory' by a very skilful mathematician, Mr.
Croll. He reasons on this wise : Since the water which
is carried from the equator to the latitude of England 1
(say) must have partaken, when at the equator, of the
earth's rotation there, which has a velocity of more
than 1,000 miles per hour eastwards, whereas, when it
reaches our latitudes, it partakes of a rotation-move-
ment reduced to about 620 miles per hour, it follows
that, neglecting the drift motions as relatively quite
insignificant, friction has deprived the water which has
thus travelled from the equator to our latitudes of a
velocity amounting to no less than 380 miles per hour.
If friction is thus effective, how utterly inconceivable
is it, says Mr. Croll, that the descending currents of
Dr. Carpenter's theory (or the ascending currents of
the evaporation theory) should maintain their motion.
Hence, Mr. Croll is an earnest advocate of the trade-
wind theory.
The worst of this reasoning is that it proves too
1 I present the general nature of Mr. Croll's reasoning, without fol-
lowing him in details.
OCEANIC CIRCULATION. 243
much. If friction is so effective, then when the trade-
winds flag, as we have seen that they do, the ocean
currents ought to be brought to a standstill. More-
over, the submarine currents exist, and the wind theory
leaves them unexplained. The fact really is that Mr.
Croll's reasoning has no application to a system of fluid
circulation, where the advance of one part of the fluid
is always made room for, so to speak, by the removal
of that which it replaces. We might equally well
apply Mr. Croll's reasoning to prove that a river cannot
flow because of the friction along its banks, as to show
that ocean currents cannot flow within their liquid
banks. Indeed, many of the points in dispute in this
matter of oceanic circulation may be excellently illus-
trated by considering the case of a river. I propose to
draw this paper to a conclusion by setting forth such
an illustration. My readers will not fail to recognise
the opinions here severally parodied, so to speak, and
so to infer the theory which I regard as affording,
on the whole, the best explanation of the observed
relations.
6 Shallow persons,' might one say, ' have launched
all sorts of stupidities upon the Mississippi Eiver.
Physical geographers have deluged the world with their
assumptions respecting it; theorists of all kinds have
floated their notions upon it. One says that it brings
down, past Baton Eouge and New Orleans, the drainage
of half the United States; others ascribe to it the
detritus around the delta of that great river which
flows into the Grulf of Mexico ; yet others consider that
R 2
244 LIGHT SCIENCE FOR LEISURE HOURS.
it breeds the fogs infesting the path of the great stream
which flows from Vicksburg to Placquemines.' All
this is utter nonsense. The Mississippi has no more
to do with the great stream flowing through Louisiana
than with the Thames at London. The real Mississippi
is a stream of singular purity, and presents other charac-
teristics clearly recognisable as far as its junction with
the Missouri ; but in the stream which runs past St.
Louis none of the characteristics of the Mississippi can
be traced. Here, to all intents and purposes, the Mis-
sissippi comes to an end. As for the cause of the
motion of the great stream itself there can be little
question. Some have urged that it is due to the gra-
dual slope of the land; but in all the experimental
illustrations of the effects of such slope which we have
yet seen, the inclination has been monstrously exag-
gerated. If slope were the cause of the river's flow,
then unquestionably the effective part of the action
must reside in the Eocky Mountains, and not in the
great reaches of the river. We admit that the chief
bulk of the river lies in the great reaches ; but this
fact has no bearing, we assert, on the question at issue.
However, it is demonstrable that no cause of this sort
can be in question. For let the following reasoning
be carefully marked. In Wisconsin, in 40 north
latitude, the river partakes of the earth's rotation
motion, there equal in rate to about 800 miles per
hour; in Louisiana, in 30 north latitude, the river
still partakes of the earth's rotation movement, here
equal to about 900 miles per hour. Hence, were it
OCEANIC CIRCULATION. 245
not for the friction exerted by the banks, the water of
the river in Louisiana would be flowing at the rate of
100 miles per hour westwards. If, then, friction de-
prives the river of this enormous velocity as it obvi-
ously does how much more must it deprive the river
of the minute velocity of four or five miles per hour
due to slope or inclination. It is certain, therefore,
that the flow of the stream is due to the prevalent
northerly winds of the so-called Mississippi valley.
There are not wanting those, indeed, who assert that
this cannot be the case, because northerly winds are
not prevalent in this region. But the singular wrong-
headedness of this reasoning renders reply unnecessary.
That the flow of the great stream is caused by these
winds is as certain as the rotundity of the earth.
From English Mechanic for July and August 1872.
ADDENDUM. 1
IT is impossible but that on a subject so difficult and
complicated as that of oceanic circulation, different
views should be entertained by students of science.
And it is clear that in the present stage of the inquiry
no useful purpose could be fulfilled by making the
problem a matter for controversy. Dr. Carpenter him-
self has shown that much more is to be gained by
1 This paper was written in reply to comments by Dr. Carpenter on
the former paper. The nature of these comments will be inferred from
my reply ; in fact I quote the most important passages.
546 LIGHT SCIENCE FOR LEISURE HOURS.
observation than by reasoning on imperfect knowledge.
If I venture to remark that his deep-sea researches
have led to the most important contribution which has
been added for many years to our information respect-
ing oceanic circulation, he will not, I trust, consider
that I am passing beyond the bounds of controversial
courtesy. But I am, indeed, not anxious to treat the
matter as one for controversy in any sense. It will be
perceived by those who have read my remarks on the
subject, that I have rather put them forward as sug-
gestions than as indicating theories which can be
maintained with any degree of assurance, far less with
conviction. Nor does it seem to me likely that one
explanation can suffice to account for all the pheno-
mena recognised in oceanic circulation. This is a case,
if ever such case were, in which more causes are in
operation than one ; so that it may very well happen
that excellent arguments can be adduced in main-
tenance of different views. If, therefore, I enter on
the defence of what I have already written on this
subject, it is not with the wish to show that one parti-
cular explanation of oceanic circulation is correct, and
all others erroneous. If I am desirous of dealing with
the considerations urged by Dr. Carpenter, it is not
because they seem to him to militate against the views
I have to some extent advocated. What I wish to
show is that I have not addressed your readers on
the subject of oceanic circulation without making
myself familiar with the facts which bear upon that
subject, and at the very least, with those compara-
OCEANIC CIRCULATION. 247
tively fundamental facts to which attention has been
invited.
And here I would remark that one who writes so
much and so often as I have had occasion to do on this
and kindred subjects, is placed to some degree at a
disadvantage. He cannot, on the one hand, assume
that the readers of any particular essay have also read
all that he has written on the subject ; yet, on the
other, he cannot assume that none have done so, and
that he is therefore free to repeat (in a more or less
modified form) much that he has formerly urged. I
was, perhaps, somewhat too careful in writing for your
pages to avoid touching at any length on any parts of
the subject which I had more particularly dealt with
elsewhere ; and accordingly I have laid myself open to
a method of attack, which in reality involves the sug-
gestion that I have written without due consideration
even of the elements of my subject. I have no doubt
that Dr. Carpenter has no wish to imply this directly,
yet indirectly it is implied in every paragraph of his
reply. I shall be able to show, however, that every one
of the points touched on by Dr. Carpenter had been
fully considered by me and, for the most part, several
months before he had turned his attention to this
subject.
First, there is the remark that I have left out of
view the circumstance that if there is excess of evapo-
ration in the intertropical area, the excess ought to
show itself, as in the Mediterranean, in an increase of
specific gravity, whereas the specific gravity of the
248 LIGHT SCIENCE FOR LEISURE HOURS.
equatorial water is lower than that of tropical water.
Now, it is unquestionably true that the effect of evapo-
ration is to increase the specific gravity of sea water ;
but it is equally true that the effect of the heat which
causes the evaporation is to diminish the specific
gravity. The point is considered in my essay entitled
'Is the Gulf Stream a Myth?' in the first series of
4 Light Science for Leisure Hours.' ' We recognise,' I
there say, ' two contrary effects as the immediate results
of the 'sun's action. In the first place, by warming the
equatorial waters it tends to make them lighter ; in
the second place, by causing evaporation it renders them
salter, and so tends to make them heavier.' And I
proceed to inquire which cause is likely to be the more
effective, arriving at the conclusion that the water is
made lighter. The case, indeed, appears to me to be
altogether different from that of the Mediterranean
Sea cited by Dr. Carpenter. In the Mediterranean we
have the same heating action as on the Atlantic in the
same latitudes, but not the same relatively enormous
quantity of water freely communicating with the region
so heated. We have, then, in the Mediterranean
evaporation as everywhere else, and evaporation to the
same degree, appreciably, as elsewhere in similar lati-
tudes ; but evaporation not compensated as in the open
Atlantic by the effects of free communication with
surrounding water. Hence we have in the Mediter-
ranean an increase of saltness ; in other words, an in-
crease of specific gravity. And precisely because this
increase takes place in the Mediterranean, whereas the
OCEANIC CIRCULATION. 249
water of the Atlantic in the same latitudes, exposed to
the same average degree of heat, is not rendered heavier,
it may be maintained not unreasonably that the
water of the equatorial Atlantic being unconfined,
will in like manner not be rendered heavier by
evaporation. It seems to me that we have here a
positive argument of great weight in favour of my
views. But independently of this I would ask whether
it can be questioned that enormous evaporation does
take place over the equatorial area. This is what I
contend for, and I should have imagined that few would
undertake to deny the proposition.
In passing, I must remark that I do not adopt the
distinction between equatorial and tropical water which
Dr. Carpenter appears to recognise. I have in view
the evaporation over an enormously larger area than he
considers no less an area, in fact, than the whole
ocean between latitudes 40 north and south of the
equator (at the equinoxes, and varying according to
the season). It by no means follows that because the
equatorial current does not cover this enormous area,
therefore the relation which I have suggested as
the mainspring of oceanic circulation has not that
extent. On the contrary, while it is on the one hand
certain that there is an excess of heat over this
enormous area, it is on the other almost a necessity
of my theory that the resulting current should be found
running along the middle only of the great region of
evaporation.
This brings me to Dr. Carpenter's second objection,
250 LIGHT SCIENCE FOR LEISURE HOURS.
that if the removal of equatorial water draws in polar
water from the bottom, the whole intermediate stratum
should first rise towards the surface. I do not hold
the view thus demolished, but simply that the inflow
is from below. The question whether the inflow would
be from above or below was dealt with by me in a
paper on c Oceanic Circulation' in the Student for
July 1868. I do not urge this as a proof that Dr.
Carpenter's objection is invalid. My reasoning may
admit of being refuted. But I wish to show that the
objection is not a new one to me. The inflow may be
from below without being from the bottom. If it were
from the bottom it would not have the effects I have
ascribed to it, that is, it would not result in a west-
wardly-flowing current. What I conceive is that since
the whole tropical and equatorial area is a region of
excessive evaporation (as surely no physicist will deny),
there is over the whole region a depression of the ocean
level. This depression may be, or rather must be,
exceedingly minute ; but the total quantity of water
thus, as it were, wanting, must be enormous. The
difference must by the laws of fluid equilibrium be
supplied, and though the immediate supply in equa-
torial regions may come from tropical regions, the
actual source of the total supply must be sought for in
higher latitudes. That the water drawn in under these
circumstances would traverse the surface of the Atlan-
tic, is by no means proved by the fact that the eminent
mathematicians cited by Dr. Carpenter consider that
an in-draught to replace water ; swept off from the
OCEANIC CIRCULATION. 251
surface,' by trade-wind action would be a surface cur-
rent. The two cases are wholly dissimilar, I must,
however, admit that my case is one of extreme diffi-
culty regarded as a problem in hydrodynamics. It is
so difficult that I do not believe it can be solved even
after the very imperfect fashion in which hydrodyna-
mical problems have hitherto perforce been dealt with.
When the physics of hydrodynamics have been treated
by mathematicians like the physics of astronomy, or
rather when they can be so treated, it may be possible
to deal with this problem. Unless I greatly mistake,
however, in such a then, we shall find a never.
I do not see how the action of the cause I have
considered is affected by the circumstance that the
equatorial heat does not show any effects below 200
fathoms ; for the cause is in its very nature a surface
one. But I would remark that so far as continuity of
action is concerned, the equatorial heat seems at least
on a par with the polar cold. For as the aqueous vapour
rises it finds its way to regions where the atmospheric
circulation is at work to carry it away (it is only the
surplus quantity which is condensed into clouds, and
even these are in great part carried away) ; and thus
the process of evaporation can hardly be exhausted.
Even at night, though in a modified manner, the eva-
poration must continue. But the action of the polar
cold, though it is continuous in the sense that the
increase of cold extends to great depths, yet has this
great difficulty to contend with, that the descending
water must perforce wait until room is made for it by
252 LIGHT SCIENCE FOR LEISURE HOURS.
the slow removal, the creeping away, as it were, of that
which it replaces. That this cause, per se, can ever
become one of sufficient activity 1 to generate a complete
system of vertical oceanic circulation seems at the least
open to grave question. It appears to me also that
when applied to the North Pacific this theory fails.
Very little water can pass through Behring's Straits, and
beyond Behring's Straits there is an island-locked and
shallow sea of enormous area, altogether unlike the deep
North Atlantic.
I would further point out that the interesting fact
above mentioned, namely that the equatorial heat exerts
no perceptible effect at a depth exceeding 200 fathoms,
is in reality almost a necessity for my theory. For if
the whole of the equatorial ocean were heated, and,
therefore, of reduced specific gravity, the water arriving
from higher latitudes would flow to the bottom, and so
have to force up the intervening strata, in order to pro-
duce the observed effects ; and this may be regarded as
impossible. As it is, such colder and heavier water
would be in dynamical equilibrium within a very short
distance of the surface.
Next, as to the question of rainfall. Dr. Carpenter
considers that I have overlooked the considerations (1)
that the rainfall of Europe and North America may be
accounted for by the evaporation in the Mid- Atlantic,
1 In passing I may notice that I did not suppose Sir J. Herschel to
be humorous in reference to the intensity of the polar action, but in his
use of the word ' emphasis.' I should not have touched on the point,
did I not thoroughly sympathise with the emphatic utterance of specula-
tive or theoretical opinions.
OCEANIC CIRCULATION. 253
beyond the region of the trade-winds, say between 20
and 40 north latitude ; and (2) that there is an enor-
mous rainfall in the region of equatorial calms, which
Sir John Herschel attributes to the deposit of waters
taken up by the N.E. and S.E. trades. To this I must
reply that in my essay on Eain in the ' Intellectual
Observer' for December 1867, I have weighed the
whole question of rainfall at least with great care, and
with constant reference to the best sources of informa-
tion. One circumstance I there note which seems at a
first view (or rather viewed as Dr. Carpenter appears to
consider the matter) much more fatal as an objection
to my theory than either of those noted by Dr. Car-
penter ; viz., that according to the observations of
Humboldt and others, the annual rainfall is at a maxi-
mum at the equator, and diminishes with increase of
latitude. But the whole question is, where does all
this rain come from ? If it comes from tropical and
equatorial evaporation it will surely not be argued that
what falls in or near the place of evaporation itself,
represents the total amount of such evaporation. It is
unquestionable, I conceive, that the rainfall is only the
excess of the aqueous vapour poured so copiously into
the air from the whole of this region. It is the quan-
tity which the air, as it were, rejects. It is a matter of
little importance where the rainfall of higher latitudes
comes from, though it should be noticed that the views
of Dove, Kaemtz, and other leading meteorologists re-
'specting the winds and rains of high and low latitudes,
support my remark about the great rivers.
254 LIGHT SCIENCE FOR LEISURE HOURS.
Now we have in the phenomena of the zone of calms
a crucial test of Sir J. Herschel's theory as to the origin
of the equatorial rains. It appears to me that this test
altogether negatives Herschel's theory. If the moisture
to which these equatorial rains are due came from the
trade-wind regions, we should certainly not expect the
fall of these rains to be associated in any marked degree
with the progress of the equatorial day ; or, if at all,
then the cooler parts of the day, when the point of
saturation is lower, would be the time of precipitation.
With the mid-day heat would come a cessation of pre-
cipitation. As a matter of fact the contrary is the
case. The sun (we are told by Dove, Kaemtz, Hum-
boldt, Maury, Buchan, and many more) rises commonly
in a clear sky in equatorial regions. As the day proceeds
clouds form, and towards mid-day they grow dense. It
is at noon that heavy showers fall, and towards evening
the skies again become clear. Now, any one who has
noticed what happens on calm summer days in any well-
, watered region can see that the equatorial phenomena
represent the same processes on a greatly enlarged scale.
On a summer's day in such regions we see how scattered
cumulus clouds begin to form in early morning, become
larger and more numerous as the day proceeds,, and in
the afternoon begin to be transformed into cumulo-
stratus. The explanation is simple. The sun's heat
has caused aqueous vapour to rise into the air, until
there is so much that not very far above the earth's
level the saturation point is reached. The further rise
of the vapour is followed by the process of condensation
OCEANIC CIRCULATION. 255
into clouds, much heat being given out in the process,
causing the air to expand in the neighbourhood of the
clouds so formed, and thus giving to these clouds their
peculiar rounded tops. (At least this feature seems
better explained thus than by De Saussure's theory.)
Now suppose the conditions changed to those existing
at the equator. The supply of vapour is very much
greater, the saturation point is very much higher near
the sea-surface, and the contrast between the conditions
prevailing there and in the region where condensation
begins is very much more marked. The air above the
equatorial and tropical seas contains, in the form of
invisible aqueous vapour, an enormous quantity of water ;
this vapour rises and extends itself, its place being con-
tinually supplied by fresh evaporation. What must
happen when the process has continued for several
hours, but precisely what is observed to happen ? There
is an overflow, so to speak, resembling, only much more
marked, that which causes the formation of our summer
clouds. Enormous cloud-masses are formed, which
cannot be carried away by the atmospheric circulation
(very high above the calm zone), so fast as they are
formed. Hence follows excessive accumulation, pre-
sently resulting in precipitation, accompanied by re-
markable electrical phenomena.
But to suppose that the whole quantity of water
evaporated at the equator and in tropical regions, is
precipitated there in the form of rain, corresponds to
such a supposition as that the water overflowing a dam
includes all that has risen to the level of the dam.
256 LIGHT SCIENCE FOR LEISURE HOURS.
I should not be greatly concerned if the result of the
experiments I spoke of should not accord with my predic-
tion. But merely to put ice in water capable of melting
it, is not in any sense to represent the conditions of the
actual case. The addition of water from the ice as it
melts is not in accordance with these conditions. It
cannot surely be maintained that the oceanic circulation
depends on the addition of water from the melting of
ice ; and yet I apprehend that the melting of ice is no
unimportant feature of Dr. Carpenter's experiment. At
any rate, the ice does melt, and the movement comes to
an end when all the ice has melted away. Let the ice
be packed outside the arctic end of the canal, so as
merely to produce a refrigeration corresponding to what
actually takes place with water carried into arctic lati-
tudes, and I conceive that a very feeble circulation
would result. Under the actual circumstances, the
melting of the ice produces effects much more nearly
corresponding to those due to rainfall than to the mere
effects of arctic cold. The very activity of the circu-
lation shows that the water which moves towards the
ice does not undergo refrigeration. Water does not
cool quite so quickly. It is the melted ice-water which
descends ; and nothing takes place in the arctic regions
which corresponds to this continued addition of water
to that already circulating. Otherwise, the arctic ice
would be continually diminishing, which, of course, is
not the case.
It will be gathered that I agree entirely with the
opinion which Sir W. Thomson expressed, as to the
OCEANIC CIRCULATION.
257
reason why heat is necessary for Dr. Carpenter's experi-
ment. Heat is necessary, because the ice must be
melted to make the experiment succeed. But comparing
the effects of heat and refrigeration (not of heat and
the continual inflow of ice-cold water), I conceive that
heat would be found altogether the more effective.
Lastly, as to the wind theory of the Gulf Stream, Dr.
Carpenter remarks that, so far as he knows, I am ' the
only man of science in this country agreeing with Capt.
Maury in attributing the Grulf Stream to some other
cause than the impelling force of the trade winds.' He
must be aware that there are not half a dozen students
of science in this country who have expressed definite
opinions on the subject after a thorough and independent
inquiry into the evidence. Amongst those who main-
tain the wind theory there is not one, so far as I know,
with whom Dr. Carpenter is in agreement. Mr. Laughton
disputes the very principle of Dr. Carpenter's reasoning,
holdiog that the change of temperature from equator to
poles proceeds too slowly mile for mile to produce the
effects which Dr. Carpenter indicates. Mr. Croll, in
like manner, has expressed his complete dissent from
Dr. Carpenter's reasoning. So also has Mr. Findlay. I
believe these gentlemen to be mistaken, and I conceive
that I have been able to put my finger on the precise
point where their respective lines of reasoning fail.
But, if Dr. Carpenter is to take general consent as an
argument, and to maintain that I am wrong because he
knows of no one who agrees with me, I may as well
point out that he is entering into a very questionable
258 LIGHT SCIENCE FOR LEISURE HOURS.
alliance, so far as his special views are concerned. So
far as I know, all the continental students of science
who share our common views as to vertical circulation,
reject the wind theory as solely sufficing to account for
the Gulf Stream. Again, he sets Sir J. Herschel's
opinion (thirty years ago) that ' the Gulf Stream is
entirely due to the trade winds ' as almost conclusive
against me. It is, at least, not new to me, since it
is cited in every paper I have written on the subject.
But is there no evidence to show that Sir J. Herschel
abandoned the view he formerly entertained ? I would
ask what Sir John Herschel implies when, in his letter
to Dr. Carpenter, he writes, * The action of the trade
and counter-trade winds, in like manner, cannot be
ignored ; and henceforward the question of ocean cur-
rents will have to be considered under a twofold point
of view.' The word 6 henceforward ' implies very dis-
tinctly that Sir J. Herschel was entertaining a new
opinion that is, an opinion new to him ; and I think
Dr. Carpenter would find it difficult to demonstrate that
this new opinion would not have enforced the omission
of the word entirely from the sentence quoted by Dr.
Carpenter.
I need hardly say that I do not agree with Captain
Maury, whose theory of oceanic circulation appears to
me to be wholly untenable. Nor do I for a moment
assert that the winds play no part in producing oceanic
circulation. I may have been mistaken in attaching
so much weight as I have to Maury's evidence as to the
trade wind zones, though it is known that science owes
more to him than to any man for our present knowledge
OCEANIC CIRCULATION. 259
of the winds prevalent in certain regions ; and when I
first wrote on the Grulf Stream there was no evidence on
the subject even approaching Maury's (or that collected
by Maury) in accuracy and completeness. But there is
one argument which those who have adopted the trade
winds as the primary cause of the Grulf Stream appear
to me to have overlooked, and it is on this argument
that my own view has been chiefly based. The trade
wind zone of the northern hemisphere is not constant
in position; but travels northwards and southwards
with the northerly and southerly motion of the sun in
declination. The change in the position of the zone of
calms is not, indeed, so great as is stated in Buchan's
meteorology, where it is said to travel from 25 north to
25 south of the equator ; but it is considerably greater
than was supposed by Dove, Kaemtz, and others. If we
set the extreme shift of the northern trade-zone at ten
degrees we are certainly not over-rating it. Taking
this zone as extending in spring or autumn from 10 to
25 north latitude, we should have it in winter extending
from 5 to 20, and in summer from 15 to 30, the
only part common to these two ranges being that from
15 to 20 that is to say, the northern five degrees
of the winter zone, and the southern five degrees of the
summer zone, each zone being 15 wide. Now, if any
one will mark these zones on the North Atlantic, he
will find that while the zone of winter trades would
produce a current flowing into the southern half of the
Gulf of Mexico, the zone of summer trades would pro-
duce a current flowing into the northern half. The
8 2
260 LIGHT SCIENCE FOR LEISURE HOURS.
former would produce a current flowing as the Grulf
Stream actually flows ; the latter would produce a cur-
rent flowing precisely in the opposite direction. This
being the case, I do not find the evidence for the trade
winds as the sole or even the main cause of the Gulf
Stream altogether convincing. The case does not, for
instance, seem quite 'as clear as the rotation of the
earth.' It seems, also, not undesirable to mention that
the equatorial current and the Gulf Stream are not
mere drift-currents, and that on a careful estimation of
the frictional action of such winds as the trades on the
surface of the ocean, the action will be found quite
unequal to the propulsion of so vast a body of water as is
actually carried westwards (not, by the way, before these
winds). Until difficulties such as these have been
removed from the trade wind theory as solely sufficient
to account for the Grulf Stream, I think I would rather
be the only student of science opposing that theory,
than one of a phalanx, however large, maintaining it.
There is, however, no such phalanx ; the subject being
regarded by nearly all students of science as a very
open one.
English Mechanic, Aug. 30, 1872.
THE CLIMATE OF GREAT BRITAIN.
IF there is one feature in the material relations of a
country which may be considered as characteristic as
of itself sufficient to define the qualities of the inhabit-
ants, and the position they are fitted to occupy in the,
THE CLIMATE OF GREAT BRITAIN. 261
world's history it is climate. ' It includes,' says Hum-
boldt, 'all those modifications of the atmosphere by
which our organs are affected such as temperature,
humidity, variations of barometric pressure, its tran-
quillity or subjection to foreign winds, its purity or
admixture with gaseous exhalations, and its ordinary
transparency that clearness of sky so important through
its influence, not only on the radiation of heat from the
soil, the development of organic tissue and the ripening
of fruits, but also on the outflow of moral sentiments in
the different races.' I do not propose, however, to deal
with the constitution of the climate of Great Britain
under this general view. To do so, indeed, would require
somewhat more space than can in this volume be con-
veniently allotted to a single subject. I wish chiefly
to consider the subject of temperature (mean annual
and extreme winter or summer temperature) ; though
I shall have a few words to say respecting that feature
of our climate which most foreigners consider to be its
chief defect the want of transparency or clearness in
our skies as compared with those of some other European
countries.
The mean annual temperature of a country is less
important to the welfare of the inhabitants than the
extreme range of temperature exhibited in the course
of the year. Of two countries which have the same
mean annual temperature, one may have a climate
most admirably adapted to the welfare of its inhabit-
ants, while the other may have a climate offering such
fierce and violent extremes of heat and cold that its
262 LIGHT SCIENCE FOR LEISURE HOURS.
inhabitants resemble the unfortunates described by
Dante, doomed
' a soffrir tormenti e caldi e geli.'
However, I shall deal first with this feature mean
annual temperature as affording a starting-point from
which to proceed to other considerations.
If the surface of the earth were perfectly uniform, or
symmetrically distributed into districts of land and
water arranged in zones along latitude-parallels, and if
the strata of the soil were throughout of like density,
radiating power, and elevation, the lines of equal mean
temperature would be parallels of latitude. This hypo-
thetical condition of things is, we know, very far from
representing the true condition of the earth's surface.
Land and water are distributed in a manner which
hardly presents the semblance of law ; elevations and
depressions, not merely of areas of limited extent, but
of whole countries, are exhibited in each hemisphere ;
and endless diversities of soil, contour, and distribution,
disturb that mathematical uniformity and exactness,
which could alone produce the co-ordination of climates
under latitude-parallels.
It is to Humboldt that we owe the valuable propo-
sition that maps of the world should exhibit parallels
of heat, as well as latitude-parallels ; and no atlas is
now considered complete without maps in which iso-
therms, or lines of equal mean annual temperature,
isochimenals or lines of equal winter heat, and iso-
therals or lines of equal heat in summer, are exhibited.
THE CLIMATE OF GREAT BRITAIN. 263
These lines are usually presented in maps on Mercator's
projection, an arrangement which has some advantages,
but is not, on the whole, very well suited to exhibit the
true conformation of the isothermal lines the study of
which, it has been well remarked, constitutes the basis
of all climatology.
In Figs. 1 and 2, the northern hemisphere of the
earth is presented on a projection (the equal surface)
which has been discussed in my ' Essays on Astronomy.'
The smallness of the scale would not readily permit of the
introduction of the system of isothermal lines usually
presented, therefore I have only introduced the isotherm
which passes through London. In both figures this
isotherm is represented by a dotted closed curve passing
across the south of England, thence across the Atlantic
in a south-westerly direction, and across the continent
of America nearly on the latitude of New York. After
it has entered the Pacific Ocean, the isotherm passes
somewhat northwards, but trends southwards again as
it nears the Asiatic continent, reaching its greatest
southerly range in the sea of Japan, traversing Asia
nearly on the latitude of the Aral Sea, and thence passing
somewhat northwards through the Crimea, Vienna, and
Brussels to London. Along its whole extent the iso-
therm nowhere has a higher latitude than where it
crosses the British Isles ; in other words, the mean
annual temperature of Great Britain is higher than
that of any country lying between the same latitude-
parallels. The advantage of this arrangement is second
only in importance to that which England will be seen
264 LIGHT SCIENCE FOE LEISURE HOURS.
to possess when we come to consider the extreme
range of temperature during the year. In fact, Eng-
FIG. 1.
Northern hemisphere on an equal-surface projection, showing curves of
mean annual and midwinter temperature through London.
land is thus brought to the centre of the true temperate
zone of the northern hemisphere ; and the considera-
tion of Figs. 1 and 2 will show that the isotherm of
London approaches as near to the tropic of Cancer
in one part of its course, as to the Arctic circle in
another.
Before leaving this part of the subject, let me note
a circumstance, not immediately connected with the
THE CLIMATE OF GREAT BRITAIN. 265
climate of Great Britain, but geographically interesting.
In examining the polar presentation of the London
FIG. 2.
Northern hemisphere on an equal-surface projection, showing curves of
mean annual and midsummer temperature through London.
isotherm, we see that in two parts of its course it
exhibits a tendency to travel northwards, and becomes,
in fact, convex towards the pole. If we laid down iso-
therms of greater mean temperature that is, nearer
the equator we should find this peculiarity gradually
diminishing. But if we laid down isotherms of lower
mean temperature, we should find the convexities
gradually becoming sharper and more defined, approach-
266 LIGHT SCIENCE FOR LEISURE HOURS.
ing each other more and more nearly, until finally they
would meet, and the isothermal curve be divided into
two irregular ovals. Proceeding to trace out curves of
still lower temperature, we should find the two ovals
closing in towards two poles of cold. These are indi-
cated in Figs. 1 and 2 by two black spots, one north
of the American, the other north of the Asiatic con-
tinent. It is to be noted, however, that at the American
pole the mean annual temperature is not quite so low
as at the Asiatic pole, the former temperature being
3i, the latter 1 Fahrenheit.
"Returning to our subject, let us consider the all-
important question of range of climate. The effects
of climate, unimportant to the stronger inhabitants of
a country, but largely influencing the health and com-
fort of the majority, are chiefly felt through the changes
that occur during the year. Now, we have seen that
the line of mean annual temperature of England departs
in a very marked manner from coincidence with a
latitude-parallel ; but we shall find the lines indicating
the extreme temperatures of the year much more
irregular ; and the peculiarity of climate, which their
conformation illustrates, much more important.
In Fig. 1 the isochimenal, or the line of equal winter
heat, through London, is indicated by a strongly marked
closed curve. Its form is remarkable. It passes nearly
in a north and south direction, along the length of
England arid Scotland, approaches singularly near
to Iceland, but turns sharply southwards and travels
across the Atlantic in a direction which brings it to
THE CLIMATE OF GEE AT BRITAIN. 267
the American continent near Washington. Still ap-
proaching the tropics, it travels through the northern
parts of Texas, where it reaches its greatest southerly
range. Passing gradually northwards to the neigh-
bourhood of the Aleutian Islands, it thence trends
southwards again, passes through the Corea, traverses
the Asiatic continent nearly on the latitude-parallel
of Nankin ; thence travelling slightly northwards, it
crosses the southern part of the Caspian Sea, the Black
Sea, and the north of Turkey, passing through Venice
and Paris to London. On the continents the isochi-
menal falls outside (that is, south of) the annual iso-
therm, while on the oceans the reverse holds. The
projection of the isochimenal thus appears as an
irregular oval, whose greatest length lies on the con-
tinents.
We see here, again, the indication of a tendency to
form two curves, and thus of the presence of two poles
of extreme winter cold in the northern hemisphere.
The isochimenals of greatest cold hitherto traced in
the two continents are shown by two broken curves
in Fig. 1. The cold of the Asiatic curve is very much
greater than that of the Amerfcan, the former curve
marking a winter cold of 40 Fahrenheit (72 below
freezing), the latter a winter cold of 26 5', only if
one may apply such an adverb to a cold of 58 5' below
freezing. Professor Nichol remarks that, 'if a polar
projection were made of these regions for January, it
would be found that the two coldest spaces of these
continents form a continuous band passing across the
268 LIGHT SCIENCE FOR LEISURE HOURS.
pole of the earth.' I cannot but think that this is a
mistake. I believe that if the isotherms traced, in
part, in Fig. 1 could be completed, they would be
found to form two ovals. The American oval would
enclose the American pole of mean temperature, but
very eccentrically, showing that the pole of extreme
winter temperature lay westwards and southwards, pro-
bably near Victoria Land. The Asiatic oval would
not probably enclose the Asiatic pole of mean tempera-
ture ; and the position indicated for the Asiatic pole
of extreme winter cold lies on or near the Arctic circle,
where it is crossed by the river Lena. At the true
pole of the earth the extreme winter cold is probably
not nearly so intense as the cold at either of the points
here indicated.
From the direction of the isochimenal through Lon-
don, it is evident that the Eastern Counties and Kent
experience the coldest winters of all places in the
British Isles, while Cornwall and the south-westerly
parts of Ireland enjoy the mildest winter climates.
In fact, winter in Cornwall is not more severe than in
Constantinople ; and in south-west Ireland the winter
is still milder, approaching, in this respect, to the
winter climate of Teheran.
The contrast, when we turn to the isotheral of Lon-
don, is remarkable. Instead of travelling nearly north-
wards, this curve passes in a south-westerly direction,
reaching its greatest southerly range in the central
part of the Atlantic Ocean; thence it travels with a
northerly sweep through Nova Scotia and Canada, till
THE CLIMATE OF GREAT BRITAIN. 269
it reaches its greatest northerly range near the Eocky
Mountains. 1 Thence it turns sharply southwards,
crosses Vancouver's Island, sweeps nearly to latitude
45 in the central part of the Pacific, whence passing
slightly northwards it crosses the southern part of
Saghalien Island. Here it turns sharply northwards,
crosses that very district of Siberia which, in Fig. 1,
is occupied by the isochimenal of intensest winter cold,
traverses Siberia, and passes near St. Petersburg, through
Berlin and Amsterdam to London.
The relations thus presented by the isotheral of
London are precisely the reverse of those exhibited
by the isochimenal. The isotheral forms a closed
irregular oval, whose greatest length lies on the two
oceans : here it falls outside the line of mean annual
heat, while on the continent it falls far within this
line.
In another respect the isotheral presents a note-
worthy contrast to the isochimenal. While the latter
encloses an area largely exceeding the area enclosed by
the mean annual line, the isotheral encloses an area
noticeably smaller. 8
A tendency to break up into two curves is exhibited
in the isotheral, even more markedly than in the two
other curves. But singularly enough, here, where one
1 It is noteworthy that the minimum distance of the isotheral from
the North Pole here attained is exactly equal to the minimum distance
of the isochimenal from the equator.
2 Here an important advantage of the isographic projection is ex-
hibited. The relation pointed out is altogether obliterated in Mercator's
projection, and could only be roughly inferred from any but an isographic
projection.
270 LIGHT SCIENCE FOR LEISURE HOURS.
would expect more certain information of the existence
of poles of cold, since so much more of the northern
hemisphere can be traversed in summer than in winter,
we have no satisfactory evidence. In fact, the irregular
curve marked in near the pole in Fig. 2 is the most
northerly isotheral yet determined. The temperature
corresponding to this isotheral is 36 Fahrenheit, or
four degrees above freezing. From a consideration of
the form-variations of the isotherals as they travel
northwards, I have been led to the opinion that there
exist three poles of summer cold, and that these fall
not very far from the positions indicated by the small
dark circles in Fig. 2.
From the direction of the isotheral line through
London, it is evident that along the south-eastern coast
of England the heat of summer is greater than in any
other part of the British Isles. On the other hand,
the northern parts of Scotland, which we have seen
enjoy a winter climate fully as warm as that of London,
have a much cooler summer climate. The south-
western parts of Ireland exhibit a change even more
remarkable For whereas the winter climate in these
parts is the same as that of Persia, the summer
climate is the same as that of the very portion of
Siberia in which (most probably) the greatest cold
ever observable in our northern hemisphere is ex-
perienced in winter. The summer of the Orkney
Islands, again, is no warmer than that of the southern
parts of Iceland.
THE CLIMATE OF GREAT BRITAIN. 27 1
It appears, then, that the inhabitants of England
enjoy three notable advantages as respects climate.
First, a higher mean annual temperature than that of
any other country so far from the equator ; secondly,
a moderate degree of cold in winter; and lastly, a
moderate degree of heat in summer. The last two
advantages resolve themselves into one, viz., small
range 6f temperature throughout the year. Our range
of climate is from about 36 in winter to 62^- in
summer, or in all, 26^ Fahrenheit. Compare with
this the climate of the country near Lake Winnipeg,
with a winter cold of 4 below zero, and a summer heat
scarcely inferior to that of London ; so that the range
of climate is no less than 65. Yet more remarkable
is the variation of climate in parts of Siberia, near
Yakutsk ; here the range is from 40 in winter to
62 in summer a variation of 102, or four times the
variation of our London climate. Other parts of the
British Isles have, however, a yet smaller range even
than that of London. Thus in the south-western parts
of Ireland, and in the Orkney Isles, the variation is less
than 19.
Nor is it difficult to assign sufficient reasons for the
mildness of the British climate for our warm winters
and cold summers. It will appear, on examination,
that nearly all the constant causes affecting the tem-
perature of a climate operate to raise the mean tem-
perature of our year, while, of variable causes those
which tend to generate increased heat operate in
272 LIGHT SCIENCE FOR LEISURE HOURS.
winter, while those which have a contrary effect operate
in summer.
Humboldt enumerates among the causes tending to
exalt temperature the following non- variables : The
vicinity of a west coast in the northern temperate zone ;
the configuration of a country cut up by numerous deep
bays and far-penetrating arms of the sea; the right
position of a portion of the dry land, i.e. its relation to
an ocean free of ice, extending beyond the polar circle
or to a continent of considerable extent which lies
beyond the same meridional lines under the equator, or
at least in part within the tropics ; the rarity of swamps
which continue covered with ice through the spring, or
even into summer; the absence of forests on a dry,
sandy soil ; and the neighbourhood of an ocean-current
of a higher temperature than that of the surrounding
sea.
All these causes, it will be observed except the
neighbourhood of a tropical continent on the same
meridian tend to increase the mean heat of the climate
in England. The great Grulf Stream probably exer-
cises a more important influence than any of the others.
Its position is indicated in Figs. 1 and 2. Humboldt
attaches a high importance to the presence of a tropical
continent on the same meridian ; and he considers that
the climate of Europe is warmer than that of Asia,
because Africa, with its extensive heat-radiating deserts,
lies to the south of Europe, while the Indian Ocean
lies to the south of Asia. There are objections, how-
ever, to the reasoning he adopts. In the first place, if
THE CLIMATE OF GREAT BRITAIN. 273
the heat-radiating power of a continent really influenced
the country lying to the north, it should tend to lower
rather than raise the temperature, for the ascending
currents of air would strengthen the currents of colder
air pouring in from the north, and these currents
on Humboldt's assumption that the country directly to
the north is that affected would lower the mean
annual temperature. It would only be exceptionally
that the warmer returning currents would descend, and
thus exalt the temperature. It seems clear, however,
that Asia is the continent chiefly affected by the heat-
radiating power of Africa ; since the cold currents from
the north travel eastwards, while the warm return-
current has a westerly motion. We should thus attri-
bute the milder climate of Europe rather to the
influence of the tropical parts of the Atlantic Ocean,
than to the cause assigned by Humboldt, and we
should invert the effects he attributes to oceans and
continents respectively. With this change somewhat
a bold one, I confess 1 we may say that all the non-
variable causes tending to exalt temperature operate in
England's favour.
The constant causes tending to lower temperature
are simply the converse of those above considered.
1 Not unsupported, however, by good authority. Thus Professor
Nichol, speaking of the climate of Europe, writes : ' The air that rises
in Africa blows rather over Asia than Europe. The cradle of our
winds is not in Sahara but in America.' Again, Kaemte notices, that
if the effects of oceans and continents were those assigned by Humboldt,
we should find in the western parts of America a colder climate than
in the eastern parts ; the reverse, however, is the case.
T
274 LIGHT SCIENCE FOR LEISURE HOURS.
Of variable causes increasing temperature, the
principal are a serene sky in summer, and a cloudy
sky in winter. It may appear, at first sight, para-
doxical to assign opposite effects to a cloudy sky. It
must be remembered, however, that clouds considered
with reference to temperature, have two functions :
they partially prevent the access of heat to the earth,
and they partially prevent the escape of heat from the
earth. Now, in summer the first-named influence is
more important than the second: the days are longer
than the nights ; that is, the earth is receiving heat
during the greater part of the time in summer. A
cause, therefore, which affects the receipt of heat is
more important than a cause affecting the escape of
heat. On the other hand, in winter the nights are
shorter than the days, and the influence of clouds in
preventing the escape of heat becomes more important
than their effect on the receipt of heat. 1 In fact, we
may compare the influence of clouds to the effects of
certain kinds of clothing ; flannel, for instance, is as
suitable an article of dress for the cricketer as for the
skater.
Now the climate of England is remarkably humid
both in winter and summer. And this humidity is
shown, not so much by the quantity of rain which falls,
1 Gilbert White noticed long ago apparently without understanding
the influence of a clouded sky on the temperature. ' We have often
observed,' he says, ' that cold seems to descend from above ; for, when a
thermometer hangs abroad on a frosty night, the intervention of a cloud
shall immediately raise the mercury ten degrees ; and a clear sky shall
again compel it to descend to its former gauge.'
THE CLIMATE OF GREAT BRITAIN. 275
as by the frequent presence of large quantities of
aqueous vapour in the atmosphere. Skies, even, which
we in England consider clear, are overcast compared
with the deep-blue skies of France or Italy. What the
influence of these humid palls may be ' on the out-flow
of moral sentiments ' which Humboldt considered to be
so favourably influenced by transparent skies, I shall
not here pause to inquire. It is clear, however, that
the influence of our cloudy skies tends to modify the
severity both of our winter and our summer seasons ;
and these benefits are so great that we may cheerfully
accept them as more than a counterpoise for hypo-
thetical injurious effects on ' the outflow of our moral
sentiments ' (whatever that may mean).
I proceed to consider the actual variations presented
in the course of a year in England. As some selection
must be made, I shall select a series of observations
which have been made at Greenwich during the present
century. It will be gathered from the preceding
pages that the range of temperature at Greenwich is at
least not less than the average range of the British
Isles. Greenwich, also, from its neighbourhood to
London, and from the number and accuracy of the
observations made there, is obviously the best selection
that could be made. It must not be forgotten, how-
ever, that the climate of Greenwich is not the climate
of the British Isles, and that careful observations made
in other places have sufficiently indicated the existence
of local peculiarities, which, therefore, it may fairly be
assumed, characterise also the Greenwich indications.
T 2
276 LIGHT SCIENCE FOR LEISURE HOURS.
In Fig. 3 the annual variations of mean diurnal
temperature are represented graphically. The figure
was formed in the following manner : A rectangle
having been drawn, each of the longer sides was
divided into 365 parts, and a series of parallel lines
joining every tenth of these divisions was pencilled in.
The spaces separating these lines represented successive
intervals of ten days throughout the year. The shorter
sides were divided into thirty-three parts and parallel
lines drawn, joining the points of division. Of these
longer parallels the lowest was taken to represent a
temperature of 32 Fahrenheit (i.e., the freezing point)
and the others, in order, successive degrees of heat up
to 65. Then, from the Greenwich tables, which have
been formed from the observations of forty-three years,
the temperature of each day was marked in, at its
proper level and at its proper distance from either end
of the rectangle. Thus 365 points were marked in, and
these being joined by a connected line, presented the
curve exhibited in Fig. 3. The lines bounding the
months, and the lines indicating 35, 40, &c., Fahren-
heit, were then inked in and the figure completed.
The resulting curve is remarkable in many respects.
In the first place, it was to have been expected that a
curve representing the average of so many years of
observation would be uniform ; that is, would only
exhibit variations in its rate of rise and fall, not such
a multiplicity of alternations as are observed in Fig. 3,
And this irregularity will appear the more remarkable
when it is remembered that the temperatures used as
"VI
i
278 LIGHT SCIENCE FOR LEISURE HOURS.
the Greenwich means are not the true average tem-
peratures. They were obtained by constructing a curve
from the true averages, and taking a curved line (the
curve of Fig. 3, in fact) in such a way as to take off
the most marked irregularities of the true curve of
averages ; or to use the words of the meteorologist who
constructed the Greenwich table of means, Mr. Glaisher,
a curved line was drawn which passed through or near
all the points determining the true curve of averages,
4 and in such a way that the area of the space above
the adopted line of mean temperature was equal to that
below the line.' Despite this process, the curve exhibits
no less than fourteen distinctly marked maxima of
elevation, and a much larger number of variations of
flexure. The sudden variations of temperature at the
beginning of February, early in April, and early in
May are very remarkable ; they have their counterparts
in the three variations which take place between the
latter part of November and the end of the year, only
these occur in much more rapid succession. The
nature of the curve between June and August is also
remarkable, as are the three convexities which are ex-
hibited in the September, October, and November
portions of the curve.
If we follow our leading meteorologists in taking the
curve of Fig. 3 as representing the true annual climate
of London, how are we to assign physical causes for the
remarkable variations above indicated ? Not easily, I
take it. It were, indeed, as easy as inviting to specu-
late on cosmical causes ; to follow Ertel, for instance,
THE CLIMATE OF GREAT BRITAIN. 279
in assigning effects to those zones of meteorities
which are known to. intersect the earth's orbit, and
others which may fairly be assumed to fall within or
without that orbit. It may be, perhaps, that the
recognised shooting-star periods have, some of them,
their counterparts in heat-changes ; but certainly the
time has not yet come to pronounce a consistent theory
of such effects. The evidence afforded by the Greenwich
curve on this point is unsatisfactory, to say the lease.
The elevation at the beginning of January, and the
marked irregularity in February, correspond to Ertel's
views ; so also the fact that large aerolites have fre-
quently fallen in the first week in April, about the
20th of April, about the 18th of May, early in August, 1
about the 19th of October, and early in December,
seems to correspond to elevations in the curve ; while
depression opposite the 1 2th of May, might be referred
to the intervention of the zone of meteors, which causes
the now celebrated November shower. But the nega-
tive evidence is almost equally strong. Where, for
instance, is the elevation which one would expect, on
Ertel's theory, in November ? Also, if the cause of the
observed irregularities were that suggested by Ertel,
the curves for other countries in the northern hemi-
sphere should exhibit similar irregularities on corre-
sponding dates, which does not appear to be the case.
In fact, if there really exist effects due to cosmical
causes, these are not likely to be educed from observa-
1 Keference is not made here to the August shooting-star shower,
which takes place a week later than the epoch alluaed to.
280 LIGHT SCIENCE FOR LEISURE HOURS.
tions of the variation of mean diurnal temperature,
since it is clear that a cause of variation due to objects
external to the earth could affect only the temperature
of certain hours of one day or of several days. A cluster
of meteors between the earth and the sun might
diminish the mid-day heat ; one external to the earth's
orbit might increase the nocturnal temperature ; and
though in either case the mean diurnal temperature
would be affected, yet it is obvious that the effect
would be masked in taking the mean, or even that two
or more opposing influences might cancel each other.
If it could be shown that the curve for mid-day, or for
midnight heat corresponded to the curve of mean heat,
Ertel's theory would be overthrown at once ; since, for
its support it is necessary to show that depressions in
the mean curve are due to mid-day loss of heat, and
elevations to midnight gain of heat.
There are, however, terrestrial causes to which the
irregularities of our curve (which irregularities, be it
remembered, represent regularly recurring irregulari-
ties of heat) may be ascribed. For instance, there can
be no doubt that our climate is considerably affected by
the changes which take place in the Polar seas ; and it
may not unfairly be assumed that the processes by
which different regions of Polar ice are successively
set adrift (to be carried southward by the strong under-
current known to exist in the northern Atlantic Ocean),
take place at epochs which recur with tolerable regu-
larity. And it may be that the irregularity of the
rising as compared with the falling of the heat-curve
THE CLIMATE OF GREAT BRITAIN. 281
is due to this cause ; since the breaking-up of ice-fields
and their rapid transport southwards would clearly
produce sudden changes, having no counterpart in the
effects due to the gradual process of freezing. 1
It may well be, however, that the observations of
forty-three years are not sufficient to afford the true
mean diurnal temperature for a climate so variable as
ours. Indeed, if the curves given by Kaemtz for
continental climates be as accurately indicative of
observed changes as that of Fig. 3, we must either
accept such an hypothesis, or else assume that the
English climate is marked by regularly recurring
variations altogether wanting in continental climates ;
and it is to be noted that the regular recurrence of
changes is a peculiarity wholly distinct from variability
of climate, properly so termed, and seems even incon-
sistent with such a characteristic. It may happen,
therefore, that the observations of the next thirty or
forty years will afford a curve of different figure ; and
that by comparing the observations of the eighty or
ninety years, which would then be available, many, or
all, of the irregularities exhibited in Fig. 3 might be
removed. In this case we might expect our climate-
curve to assume the form indicated by the light line
taken through the irregularities of Fig. 3. It will be
observed that this modified curve exhibits but one
maximum and one minimum. It is not wholly free,
1 Icebergs have been seen travelling southwards against a strong
northward surface-current, and even forcing their way through field-ice
in so travelling.
282 LIGHT SCIENCE FOR LEISURE HOURS.
however, from variations of flexure. It presents, indeed,
six well-marked convexities, and as many concavities ;
in other words, no less than twelve points of inflexion.
The most remarkable irregularity of this sort is that
exhibited near the end of November ; and it is note-
worthy that this irregularity is presented by continental
climate-curves also. It has been ascribed by Ertel to
the effect of the meteor-zone which causes the
November shower. But as it is exhibited by the curves
of horary as well as of diurnal means, while the
meteor-zone cannot by any possibility affect the tem-
perature of the earth's following hemisphere, and as,
further, it does not correspond to the true date of the
shower, this view may be looked upon as doubtful.
The August curve occurring near the maximum
elevation where slow change was to be expected, is
also well worthy of notice ; as are the January and
May flexures.
It will be noticed that nothing has been said of
extreme heat or cold occasionally experienced in
England. As such visits generally last but for a short
time, their effects are not very injurious, save on the
very weak, the aged, or the invalid. Corresponding to
the passage of an immense heat-wave or cold-wave,
there invariably occurs a sudden rise in the mortality-
returns ; but almost as invariably the rise is followed
by a nearly equivalent, but less sudden fall ; showing
conclusively that many of the deaths which marked the
epoch of severest weather occurred a few weeks only
before their natural time.
THE CLIMATE OF GREAT BRITAIN. 283
The weather during a part of the late winter was
somewhat severer than our average English winter-
weather. The thermometer, however, at no time
descended below zero, as it did on January 3, 1854;
and the diurnal mean did not descend at any time so
low as 10 7', as it did on January 20, 1838. There is
no foundation for the opinion, sometimes expressed,
that our winter weather is changing. An examination
of the columns in the Greenwich meteorological tables,
show that the successive recurrence of several mild
winters is not peculiar to the last decade or two. The
observations of Gilbert White, imperfect as they are
compared with modern observations, point the same
way.
Among severe, but short, intervals of cold weather may
be noted that which occurred in January 1768. The
frost was so intense, says Gilbert White, ' that horses
fell sick with an epidemic distemper which injured the
winds of many and killed some; meat was so hard
frozen that it could not be spitted, nor secured but in
cellars ; and bays, laurustines, and laurels were killed.'
White's account of the summer of 1783 will fitly
close our sketch of British weather-changes. ' This
summer,' he says, 'was an amazing and portentous one,
and full of horrible phenomena ; for besides the alarm-
ing meteors and tremendous thunder-storms that
affrighted and distressed the different counties of this
kingdom, the peculiar haze or smoky fog that prevailed
for many weeks in this island, and in every part of
Europe, and even -beyond its limits, was a most ex-
284 LIGHT SCIENCE FOR LEISURE HOURS.
traordinary appearance, unlike anything known within
the memory of man. By my journal, I find that I had
noticed this strange occurrence from June 23 to July
20, inclusive, during which period the wind varied to
every quarter, without making any alteration in the air.
The sun at noon looked as blank, and ferruginous as a
clouded moon, and shed a rust-coloured ferruginous
light on the ground and floors of rooms, but was par-
ticularly lurid and blood-coloured at rising and setting.
All the time the heat was so intense that butchers' meat
could hardly be eaten the day after it was killed ; and
the flies swarmed so in the lanes and hedges, that they
rendered the horses half frantic, and riding irksome.
The country people began to look with a superstitious
awe at the red, louring aspect of the sun. Milton's
noble simile, in his first book of " Paradise Lost," fre-
quently occurred to my mind ; and it is, indeed, par-
ticularly applicable, because, towards the end, it alludes
to a superstitious kind of dread, with which the minds
of men are always impressed by such strange and un-
usual phenomena :
As when the sun new risen,
Looks through the horizontal misty air,
Shorn of his beams ; or, from behind the moon,
In dim eclipse, disastrous twilight sheds
On half the nations, and with fear of change
Perplexes monarchs.'
Intellectual Observer, March 1867.
LOW BAROMETER OF ANTARCTIC ZONE. 285
THE LOW BAROMETER OF THE ANTARCTIC
TEMPERATE ZONE.
THE great difficulty presented by the science of
meteorology lies in the intricate combination of causes
producing atmospheric variations, and the impossi-
bility of determining by experiment the relative
efficiency even of the most important agents of change.
As Sir \V. Herschel well observed, we are in the
position of a man who hears at intervals a few frag-
ments of a long history narrated in a prosy, un-
methodical manner. ' A host of circumstances omitted
or forgotten, and the want of connection between the
parts, prevent the hearer from obtaining possession of
the entire history. Were he allowed to interrupt the
narrator, and ask him to explain the apparent contra-
dictions, or to clear up doubts at obscure points, he
might hope to arrive at a general view. The questions
that we would address to Nature, are the very experi-
ments of which we are deprived in the science of
meteorology.' l
It is, therefore, but seldom in the study of this
science that we meet with phenomena to which we can
assign a definite cause, or which we can explain on
simple principles. Even those marked phenomena,
which appear most easily referable to simple agencies,
1 Kaemtz's Meteord'.ogy.
286 LIGHT SCIENCE FOR LEISURE HOURS.
present difficulties on a close investigation which com-
pel us at once to recognise the efficiency of more causes
than one. For instance, the phenomenon of the trade-
winds, as explained by Halley, appears at first sight
easily intelligible ; but when we look on this phenome-
non as a part merely as indeed it is of the marvel-
lously complex circulation of the earth's atmosphere
when we come to inquire why these winds blow so
many days in one latitude, and so many in another, or
why they do not blow continually in any latitude
when we consider the character of these winds as
respects moisture and temperature, the variation of the
velocity with which they blow, and of the quantity of
air they transfer from latitude to latitude we
encounter difficulties which require for their elucida-
tion the comparison of thousands of observations, or
which baffle all attempts at elucidation.
There is, however, one atmospheric phenomenon
that which I have selected for the subject of this paper
which presents a grand simplicity, rendering the
attempt at a simple solution somewhat more hopeful
than is usually the case with meteorological phenomena.
The discovery of this phenomenon formed one of the
most interesting results of Captain Sir J. C. Eoss's
celebrated expedition to the Antarctic Ocean. He
found, as the result of observations conducted during
three years, that the mean barometric pressure varied
in the following manner at the latitudes and places
specified :
LOW BAROMETER OF ANTARCTIC ZONE. 287
South latitude Height of the barometer
Place
0'
29-974 in.
13
30-016
22
17
30-085
34
48
30-023
42
53
29-950
45
29-664
49
8
29-469
51
33
29-497
54
26
29-347
55
52
29-360
60
29-114
66
29-078
74
28-928
At sea
Cape of Good Hope and Sydney
Tasmania
At sea
Kerguelen and Auckland Isles
Falkland Isles
At sea
Cape Horn
At sea
"We see here a gradual increase of barometric pressure,
from the equator to about 30 south latitude, and from
this point at first a gradual diminution so that in
288 LIGHT SCIENCE FOR LEISURE HOURS.
about 40 south latitude we find the same pressure as
at the equator, and thence a more rapid diminution.
The rate of change is illustrated graphically in Fig. 1,
which represents the height of the barometer above
28 J inches for different southern latitudes. In the
northern hemisphere there is a similar increase of
pressure as we leave the equator, a maximum is there
also attained in about latitude 30 ; but from this
point towards the poles there is a marked difference in
the rate of diminution of pressure in the two hemi-
spheres. The following table by Schow is sufficient to
indicate this :
North latitude Barometric pressure
29-853 in.
10 30-002
20 30-004
30 30-069
40 30-006
45 30-011
60 29943
55 - 29-960
60 29-835
65 29-623
70 29-722
75 29-863
There are minor irregularities in this table, due,
doubtless, to local peculiarities, the arrangement of
land and water being so much more complicated in the
northern than in the southern hemisphere. Neglecting
these (as in Fig. 2, which represents for the northern
hemisphere the relations corresponding to those exhibited
for the southern hemisphere in Fig. 1), we see that
there is a much greater resemblance between the rise
LOW BAROMETER OF ANTARCTIC ZONE. 289
and fall of barometric pressure as we proceed north-
wards than as we proceed southwards. In fact, the
curve is almost exactly symmetrical on either side of
30 north latitude to the equator on one side, and to
latitude 60 on the other. From 60 the pressure con-
tinues to diminish for awhile, but appears to attain a
minimum in about latitude 73, and thence to increase.
In the southern hemisphere, if there is any correspond-
ing minimum, it must lie in a latitude nearer the south
pole than any yet attained.
The most marked feature in the comparison of the
two hemispheres is the difference of pressure over the
southern and northern zones, between latitudes 45 and
75. This is a peculiarity so remarkable, that for a
long time many meteorologists considered that the
observations of Captain Eoss were insufficient to warrant
our concluding that so important a difference really
exists between the two hemispheres. But not only has
Captain Maury from a comparison of 7,000 observations
confirmed the results obtained by Ross, but, in meteor-
ological tables published by the Board of Trade, the
same conclusions are drawn from 115,000 observations,
taken during a period of no less than 13,000 days. In
fact, it is now shown that the difference is yet greater
than it had been supposed to be from the observations
of Captain Ross. From a comparison of observations
made in the Antarctic Seas with those of Captain Sir
Leopold McClintock, it appears that the average dif-
ference of barometric height in the northern and southern
zones, between latitudes 40 and 60, is about one inch.
u
2QO LIGHT SCIENCE FOR LEISURE HOURS.
Figs. 1 and 2 exhibit a relation midway between these
later results and those tabulated above.
Assuming an average difference of only three-quarters
of an inch in the northern and southern zones, between
latitudes 40 and 60, let us consider what is the dif-
ference of pressure on these two zones of the earth's
surface. The area of either zone is 21,974,260*5 square
miles, and the pressure on a square mile due to a
barometric height of three-quarters of an inch is about
670,000 tons, therefore the pressure on the northern
zone, between the latitudes named, exceeds the pressure
on the southern zone by no less than 14,500,000,000,000
tons. Including all latitudes within which there has
been ascertained to be a difference of barometric pressure
in the two hemispheres, we shall probably be within the
mark if we say, that the atmospherical pressure on the
northern hemisphere is 20,000,000,000,000 tons greater
than the atmospherical pressure on the southern hemi-
sphere.
Such a peculiarity as this may almost deserve to be
spoken of in the terms applied by Sir J. Herschel to
the distribution of land and water upon our earth, it is
' massive enough to call for mention as an astronomical
feature.'' I propose to examine two theories which have
been suggested in explanation of this feature of the
earth's envelope. These theories are founded on local
peculiarities, and the feature considered appears as a
dynamical one that is, as a peculiarity resulting from
states of motion in the aerial envelope. I shall endea-
vour to establish a theory founded on a consideration
LOW BAROMETER OF ANTARCTIC ZONE. 291
of the earth's mass as a whole, and presenting the
atmospheric feature in question as a statical one.
The first theory I have to notice is one founded on
the configuration of land and water upon the northern
and southern hemispheres of the earth's globe. In the
northern hemisphere, and more especially in that part
of the northern hemisphere in which barometrical
observations have been most persistently and systemati-
cally conducted, there is much more land than in the
southern hemisphere. Now barometrical observations
are referred to the sea-level, and observations made in
Europe and America may be considered as referred to
the level of the northern parts of the Atlantic Ocean.
It is argued that the North Atlantic, compared with
southern oceans, is little more than ( a large lake, having
elevated banks east and west.' ' Practically, the air
there is a portion of the solid globe, so that the uncon-
fined air will rest upon and rise above the former, as if
it were solid and a portion of the earth ; so that the
altitude of the air over the North Atlantic will be in-
creased some hundreds of feet, and the barometer at
the sea-level will be pressed upon, not only by the free
air clear of the earth's banks, but also by the air con-
fined in the basin, much as if the air were at the bottom
of a mine.' !
Presented in the above form, the theory that the
higher northern barometer is due to the contour of the
northern hemisphere scarcely deserves serious comment.
1 From a letter addressed to the editor of the Athenaum by Dr. H.
Muirhead.
u 2
LIGHT SCIENCE FOR LEISURE HOURS.
To speak of the confined air of the North Atlantic
Ocean is surely unreasonable. An ocean 2,000 miles
across, swept by more frequent storms than are experi-
enced in any other part of the globe, cannot be very
aptly compared to ' the bottom of a mine.' An inelastic
fluid flowing steadily over a rugged surface shows no
trace, or but the slightest trace, of the nature of that
surface, by any variations of its own level. But it is
still less conceivable that an elastic fluid should be in-
fluenced in the manner suggested. In fact, if this
happened, we should no longer be enabled to determine
the heights of mountains by barometric observations ;
for according to the theory the air should extend to a
greater height above mountains than above plains ; and
as regards comparison between a barometer at the foot
of a mountain and one at the summit, we might argue
that the barometer in the valley, compared with a
barometer at the same level in a plane district, ' is
pressed upon, not only by the air clear of the mountain
tops, but also by the air confined within the valley,' so
that the altitude over the valley is greater by some
hundreds of feet than the altitude over a plain at the
same level as the valley ; and thus, before we could
determine the height of the mountain above the level
of the plain, we should have to determine the exact
effects due to the confinement of the air in the valley.
We know that, on the contrary, the average barometric
pressure in the most confined valley does not differ
appreciably from the average pressure over the most
widely extended plain at the same level.
LOW BAROMETER OF ANTARCTIC ZONE. 293
We may, however, reasonably inquire whether the
presence of continents in the northern hemisphere
might not operate in another manner. If we place
any mass within a vessel containing fluid, it is clear
that we increase the fluid pressure over every point
of the vessel's bottom, because this pressure depends
wholly on the depth of the bottom below the level of
the fluid, and the level rises when any solid substance
is placed within the vessel. Now if we suppose a globe
covered all over by water to be surrounded by a
perfectly uniform atmospheric envelope, the mean
pressure of this envelope at the water-level would
certainly be increased if continents were supposed to be
raised in any manner above the surface of the water ;
and if the atmosphere over one half of such a globe
were supposed to be prevented in any way from mixing
freely with the atmosphere over the other half, then it
is clear that the mean pressure at the water-level would
be greatest on that half-globe over which the most
extensive and highest continents had been raised. On
the assumption, then, of some such arrangement over
our own earth an arrangement, that is, which should
prevent the northern air from mixing with the southern
one might see in the northern continents a true
cause of increased barometric pressure at the sea-level
of the northern hemisphere.
We have, however, not only no evidence that such
an arrangement exists, but very strong evidence of an
atmospheric circulation which carries air from hemi-
sphere to hemisphere, and mixes in the most intimate
294 LIGHT SCIENCE FOR LEISURE HOURS.
manner the whole mass of gases which form the earth's
atmospheric envelope. The whole question of the
circulation of the air is investigated in Maury's in-
teresting work on the Physical Geography of the Sea,
and he appears to establish in the most convincing
manner the interchange of air between the northern
and southern hemispheres.
And even if we could assume that the atmospheric
covering of any portion of the earth's surface was in
any way prevented from passing freely to other
regions, yet the cause assigned would be inadequate
to account for the difference of barometric pressure
actually existing between the two hemispheres. All
the land above the sea-level in the northern hemisphere,
if uniformly distributed over the surface of that
hemisphere, would be raised to a height of less than
200 feet above the present sea-level, and the actual
difference of level corresponding to the observed dif-
ference of barometric pressure is more than four times
as great.
Passing over this theory as neither consistent with
the known laws regulating the motions of elastic fluids,
nor sufficient even if the consideration of those laws
were neglected, we come to the theory suggested by
Captain Maury a theory deserving of much more
attentive consideration. I shall quote his own words,
as the fairest method of presenting his theory; after
stating the observed difference of barometric pressure
in the two hemispheres, and mentioning the expulsion of
air from the northern hemisphere as the cause of this
LOW BAROMETER OF ANTARCTIC ZONE. 295
difference, lie writes: 'To explain the great and
grand phenomena of nature, by illustrations drawn from
the puny contrivances of human device, is often a feeble
resort, but nevertheless we may, in order to explain
this expulsion of air from the watery south, where all
is sea, be pardoned for the homely reference. We all
know, that, as the steam or vapour begins to form in
the tea-kettle, it expels air thence, and itself occupies
the space which the air occupied. If still more heat be
applied, as to the boiler of a steam-engine, the air will be
entirely expelled, and we have nothing but steam above
the water in the boiler. Now at the south over this
great waste of circumfluent waters, we do not have as
much heat for evaporation as in the boiler or the tea-
kettle ; but, as far as it goes, it forms vapour, which
has proportionately precisely the same tendency that
the vapour in the tea-kettle has to drive off the air
above, and occupy the space it held. Nor is this all.
This austral vapour, rising up, is cooled and condensed.
Thus a vast amount of heat is liberated in the upper
regions, which goes to heat the air there, expand it, and
thus, by altering the level, causes it to flow off.'
The theory thus divides itself into two parts : we
have first the expulsive effects due to the vapour
raised from southern oceans ; and, secondly, the expan-
sive effects due to the liberation of heat as the vapour
is condensed. Now I would, in the first place, submit
that we cannot assign to the second cause the effects
here considered. The amount of heat liberated as the
vapours rising from southern ocean are condensed is
296 LIGHT SCIENCE FOR LEISURE HOURS.
undoubtedly great, but it cannot be more than the
equivalent of the amount of heat rendered latent as the
vapours are formed, and therefore the expansive effects
due to the liberation of heat cannot be greater than the
contrary effects due to the prior imprisonment of heat.
It is quite true, and has been accepted as the undoubted
explanation of many climatic effects, that if vapour be
raised in one place and condensed over another, then
the temperature of the air over the latter place is raised.
But when we have to consider a phenomenon extending
over a zone twenty or thirty degrees in width, we can-
not argue in this manner. Nay, it is necessary to the
force of Maury's second cause that the condensation of
vapour should take place over the very zone in which
the vaporisation is proceeding. To assign similar effects
to both processes, is to require that the winding up and
the loosening of the spring should take place in the
same direction.
Whatever effects, then, are due to the constant
evaporation going on in the southern hemisphere, must
not be derived from changes of temperature. So far
as these are effective at all, they must depend on the
excess of evaporation over condensation (since the
excess cannot possibly lie the other way), and therefore
represent diminution of heat or increase of pressure, the
contrary effect to that we have to account for. We
have, therefore, only to consider the first cause men-
tioned by Maury ; that is the expulsive effects due to
the formation of aqueous vapour. ,
At first sight, this process of expulsion appears simple
LOW BAROMETER OF ANTARCTIC ZONE. 297
'enough, and seems further to coincide with many well-
known phenomena. The theory supposes that over a
wide zone of the southern hemisphere aqueous vapour
is continually rising ; that as it rises it displaces in
part the heavier air over these regions; and that
equilibrium being thus disturbed, the excess of air flows
off continually towards the- equator. Now we know
that the prevailing surface-winds over that zone of the
southern hemisphere in which the barometer exhibits
the peculiarity we are considering, blow from the
equator ; that is, they tend to sweep the lower strata of
the atmosphere towards the south pole. They therefore
tend to increase the quantity of humid air in high
southern latitudes. We know also that the prevailing
upper currents over the southern zone we are considering
blow towards the equator. They tend, therefore, to
carry the drier portion of the air towards the equator.
All this seems in accordance with Maury's theory, and
indeed if the prevailing upper and lower currents flowed
in directions contrary to those indicated, the theory
would fall at once.
Again, although we find no evidence in barometric
pressure over the south tropical zone of that increase
which Maury's theory would lead us to expect (since
the surplus air is carried first to this zone), yet it might
be argued that the surplus is so distributed as to appear
in another way. It is evident that if the atmospheric
envelope normally appertaining to the southern hemi-
sphere were, through the effects of the causes assigned
by Maury, increased in extent, this increase might show
298 LIGHT SCIENCE FOR LEISURE HOURS.
itself, not in an increase of pressure over the south
tropical zone that is, not in an increase of height there
but in the extension of the surplus atmosphere into
the northern hemisphere. This would be shown by the
extension of the southern trade-winds to or beyond the
equator, so that the (so-called) equatorial zone of calms
should lie north of the equator. As this is really the
position occupied by the belt of calms, Maury's theory
appears to gain additional force by the coincidence.
Another argument may be drawn from the analogy
of the low barometer in moist weather. In fact, it is
well known that Deluc explained this phenomenon in a
manner precisely accordant with the views expressed
by Maury.
Despite the apparent force of these arguments, and
others that might be adduced, it will not be difficult,
I think, to show that neither is Maury's theory con-
sistent with known physical laws, nor (passing over this
objection) is the theory sufficient to account for the
grand phenomenon under consideration.
It is quite true that a volume of aqueous vapour
weighs less than an equal volume of air ; it is equally true
that a volume of moist air weighs less than an equal
volume of dry air at the same tension.- But water,
quietly evaporating in the open air, does not displace
the air, but penetrates into its interstices, according
to the well-established law regulating the mixture
of vapours. The aqueous vapour which thus intimately
mixes itself with the air produces no effect whatever,
either by its weight or by its elasticity, on the move-
LOW BAROMETER OF ANTARCTIC ZONE. 299
ments of the atmosphere. The experiments of Gay-
Lussac, Dalton, and others, have long since proved that
the actual effects of the quiet evaporation of water are
those here described. It is on this account that Deluc's
hypothesis in explanation of the fall of the barometer
when the air is moist is now no longer accepted. It
has been shown that the observed fall is not due to the
moistness of the air, but to increase of temperature.
Hot winds bring (in Europe) moist air, and thus moist
air and a low barometer are found to be coexistent
phenomena. But they are not in the relation of cause
and effect. In fact, in New Holland, where hot winds
bring dry air, we find the barometer low when the air
is dry.
It follows from what has just been said of the manner
in which aqueous vapour associates itself with air, that
atmospheric pressure is increased instead of diminished
by the process of quiet evaporation, since the weight
of the vapour is added to that of the air. Therefore,
all things being equal, we should expect to find the
barometer higher in the southern or watery hemisphere
than in the northern.
It might seem unnecessary to consider Maury's theory
further, but as some doubts may still remain whether
some process of the kind conceived by him may not
take place, 1 I proceed to consider the efficiency of such
1 In fact, Sir J. Herschel, in his work on Meteorology, assigns as a
cause of the low barometric pressure near the equator, compared with
that near the tropics, a process similar to that conceived by Maury, only
depending on the excess of heat near the equator. I cannot but agree
300 LIGHT SCIENCE FOR LEISURE HOURS.
a process to account for the great phenomenon we are
dealing with.
It must be remembered, in the first place, that the
theory requires that there should be a greater volume
of mixed air and vapour over the southern temperate
zone than there is in the corresponding northern zone,
otherwise there would not be that continual overflow
towards the equator which is required by the theory.
So far as it goes, this increment of volume implies an
increment of weight. The increase of volume is more
than compensated (in theory) by diminution of specific
gravity, but it must be held in mind that the increase
of volume has to be accounted for by the theory as well
as the difference in barometric pressure.
Again, the theory requires that the upper regions of
air should be dry, for it is the upper air that is carried
towards the equator ; and if this air were moist, we
should no longer have the different proportions of moist
and dry air which are required by the theory. We must
have an aggregation of moist air in high southern lati-
tudes, and of dry air towards the equator.
Again, we must call to mind that one-half of the
northern hemisphere is covered by water, and a part of
the southern hemisphere is not so covered, so that the
effects suggested by Maury are (1) not peculiar to the
with those metereologists who consider that the notion of any appreci-
able uplifting of the air by the rising vapour of water is a mistaken one.
But whether it be so or not, it is evident that Herschel's view would re-
quire a regular increase of pressure from the equator to the antarctic
pole, and therefore is opposed to Maury's explanation.
LOW BAROMETER OF ANTARCTIC ZONE. 301
southern hemisphere, nor (2) do they prevail over the
whole of that hemisphere.
Lastly, we must remember that the process conceived
by Maury must be wholly or principally a diurnal pro-
cess, and so can only take place (on an average) over
one half of the southern zone at any one time.
All these considerations tend to diminish very im-
portantly the efficiency of the cause assigned by Maury,
Let us, however, consider what is the maximum value
that efficiency could have if all these circumstances were
neglected. We shall see that even in this case, which
assigns an efficiency at least three or four times as great
as would be consistent with actual facts, we shall still
find the cause assigned by Maury inadequate to the
production of the phenomenon under consideration.
The greatest weight of aqueous vapour which is ever
present in a given volume of air is equivalent to about
one-sixtieth part of the weight of the air. Now, if we
suppose the barometer at thirty inches, and the whole
column of air above the barometer to be impregnated
with air in the above-named proportion a view very
favourable to the theory, since the cold of the upper
regions of air largely diminishes the proportionate
weight of aqueous vapour it is clear that one-sixtieth
part or half an inch of the barometer's height is due
to the presence of aqueous vapour. Now, at mean
tensions the specific gravity of aqueous vapour is about
three-fifths of the specific gravity of air, so that the
proportion of one-sixtieth part of weight corresponds
to a proportion of one-thirty-sixth part of volume ; in
302 LIGHT SCIENCE FOR LEISURE HOURS.
other words, our column of air owes one-thirty-sixtli
part of its height to the presence of aqueous vapour.
If we suppose this thirty-sixth part to flow off not
from the upper regions only, but in such a manner that
one complete thirty-sixth part of the volume of the
column should pass off then, instead of standing at a
height of thirty inches, the barometer would stand at a
height of 29^ inches, less by only one-third of an inch
than the height of 29-J inches due to the dry air alone.
Now we cannot, in accordance with Maury's theory,
legitimately add the five-sixths of an inch of barometric
pressure to the height of the barometer under a neigh-
bouring column. For we have no evidence to show
that the air assumed to be expelled from the southern
temperate zone is heaped over the southern tropical
zone ; on the contrary, we have a barometer in the
latter zone not quite so high even as the barometer in
the corresponding northern zone. Therefore if air is
expelled in the manner supposed by Maury, it must be
distributed over a very much greater portion of the
globe's surface than it had been expelled from. Hence,
returning to our imaginary column of air, but a small
fraction of the five-sixths of an inch due to overflow
must be added to the barometer under a neighbouring
air-column. The latter barometer originally at 29J-
may be fairly assumed to rise at most to about 29f
inches. We have, then, a difference of 29f 29^-
inches, or two-thirds of an inch ; so that despite all the
opposing considerations we have neglected, we still
have a difference less by one-third than that for which
LOW BAROMETER OF ANTARCTIC ZONE. 303
we have to account ; and. indeed, so far as the com-
parison between the northern and southern temperate
zones is concerned (and this is the true question at
issue), we are only entitled to consider the third part
of an inch lost by overflow, as the true measure of the
efficiency of this cause.
So far as I am aware, the theory I am about to pre-
sent in explanation of the phenomenon of a low antarctic
barometer is original. It is sufficiently simple ;
perhaps, if we remember how very seldom physical
phenomena admit of a simple explanation, one may
say that the theory labours under the disadvantage of
simplicity.
It is obvious that the centre of gravity of the solid
portion of the earth's globe lies somewhat to the south
of the centre of figure. This arrangement has long
been accepted as the explanation of two remarkable
geographical features the prevalence of water over
the southern hemisphere, and the configuration of
nearly all the peninsulas over the whole globe. Whether
or not those parts within the antarctic regions which
have not yet been explored, are occupied by land
(chiefly) is a question which has little more bearing on
our views respecting this point than has the counter
question whether the unexplored north-polar regions
are or are not occupied by a north-polar ocean. 1 Sup-
1 Captain Maury holds the affirmative on both poiuts. I have already
had occasion to discuss in these pages his theory of a tidal north-polar
ocean, and I think the theory cannot be maintained. But the theory of
a polar ocean communicating with the Atlantic and Pacific is a sufficiently
304 LIGHT SCIENCE FOR LEISURE HOURS.
posing these arrangements to exist, it is evident that
they form mere local peculiarities. The general tend-
ency of water towards the southern hemisphere is very
obvious, and, so far as I am aware, no other explanation
of the peculiarity has ever been offered than that founded
on a slight displacement southwards of the earth's
centre of gravity. If, then, C is the centre of the black
circle in Fig. 3, representing the solid part of the
earth, the centre of gravity of this part lies (in the Fig.)
slightly below C between C and C', let us suppose.
Now we see that, owing to this slight displacement,
the watery envelope of the earth tends southward. If
the earth were a perfectly uniform spheroid, it is clear
that there would be a tendency to some such arrange-
ment as is represented (on a greatly exaggerated scale)
in Fig. 3, in which the shaded part represents the sea
that is, a shell of water, thicker towards the south,
probable one. The theory of an antarctic continent is hardly in the
same position, since antarctic explorations have given us but faint indi-
cations, here and there, of the habitudes of the south-polar regions.
But I may note, in passing, a very singular argument used by Captain
Maury in favour of the existence of such a continent. He states it as
a physical law that land is scarcely ever antipodal to land ; ' therefore,'
he says, ' since the north-polar regions are probably occupied by a vast
ocean, the south-polar regions are probably occupied by a vast con-
tinent.' He seems to forget that it by no means follows that because
land is seldom antipodal to land, water should seldom be antipodal to
water. Since the extent of water is nearly three times that of land, it
is absolutely necessary that nearly two-thirds of the water should be
antipodal to water. The supposed peculiarity that nearly all the land
is antipodal to water (one twenty-seventh only being antipodal to land),
is in reality no peculiarity at all. It would have been far more singular
if any large proportion of the land (which occupies little more than one-
fourth of the globe) had been antipodal to land.
LOW BAROMETER OF ANTARCTIC ZONE. 305
would surround the solid earth. For our present pur-
poses it is sufficient to consider this supposed arrange-
ment, as minor inequalities of the earth's surface-
contour have clearly nothing whatever to do with the
phenomenon" we are considering. .
Let C' be the centre of the spheroid which bounds
the earth's fluid envelope. Then it is very clear that
if this envelope were of the same specific gravity as the
solid portion of the earth, the centre of gravity of the
entire mass would lie very near C', but slightly soutli
of that point, on account of the slight southerly dis-
306 LIGHT SCIENCE FOR LEISURE HOURS.
placement of the centre of gravity of the solid portion.
But when we consider that the specific gravity of the
fluid envelope is less than one-fifth of that of the solid
globe, it is perfectly clear that the centre of gravity of
the entire mass will not be so far south as C'. For, of
the entire mass, the northern half is the heavier, and
therefore the centre of gravity must lie north of the
centre of the entire mass that is, north of C'. In
fact, it must lie much nearer to C than to C'.
Thus, the centre of gravity of the solid globe, and
that of the entire mass, solid and fluid, both lie be-
tween C and C'. Now it is evident that the central
point, about which the earth's atmospheric envelope
tends to form itself as a spherical or spheroidal shell, is
the centre of gravity of the entire solid and fluid
terrestrial globe that is, is a point north of C'. There-
fore, precisely as the effect of the fluid envelope collect-
ing itself centrally about a point south of C is to cause
the mean depth of water to be greatest in the southern
hemisphere, so the fact that the atmospheric envelope
collects itself centrally about a point north of C' should
result in giving a greater mean depth of air (referred
to the sea-level) over the northern hemisphere. This
arrangement is represented in Fig. 3, in which the un-
shaded part is supposed to represent the atmosphere.
I have endeavoured to make the above explanation
of my theory in explanation of the low antarctic baro-
meter as complete and exact as possible ; but there is
another way of presenting the theory, which, though
less complete, may appear clearer to some minds :
LOW BAROMETER OF ANTARCTIC ZONE, 307
Variation of mean barometric pressure, as we pro-
ceed from one place to another, may be due either to a
variation of circumstances of heat, moisture, and other
like relations, or to difference of level. Maury's ex-
planation assigns to the low antarctic barometer a cause
or causes falling under the former category. My theory
amounts to the supposition that the low barometer is
due to an absolute difference of level. I say that the
sea-level, to which we refer barometric pressure, is not
a just level of reference when atmospheric pressure over
the whole globe is the subject of inquiry, because the
southern seas stand out to a greater distance than the
northern seas from the true centre of gravity of the
earth's solid and fluid mass.
Assuming my theory to be correct, we have a means
rough, it may be, but not uninstructive of deter-
mining the displacement of the centre of gravity of the
earth's solid mass from the centre of figure. For, accept-
ing one inch as the difference of barometric height at
the two poles, it is easily calculated that this difference
amounts to a difference of level of about 850 feet. In other
words, the surface of the water at the south pole lies
farther than the surface of the water at the north pole
from the centre of gravity of the entire fluid and solid
globe, by about 850 feet. Hence this centre of gravity
must lie about 425 feet north of C' (which is the centre
of the bounding surface of the water). Now, it is
evident that both the centre of gravity of the entire
fluid and solid mass, and that of the solid mass, must
lie much nearer to C than to C'. Hence both these
308 LIGHT SCIENCE FOR LEISURE HOURS.
centres of gravity lie considerably within 400 feet of C,
and C' lies considerably within 825 feet of C. Thus
the centres of figure and the centres of gravity of the
earth's solid mass, and of the entire fluid and solid mass
are collected within a space less than one-eighth of
mile in length a distance almost evanescent in com-
parison with the dimensions of the earth's globe,
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INDEX.
Acton's Modem Cookery 39
A irds Blackstone Economised 39
Alpine Club Map of Switzerland 33
Alpine Guide (The) 33
A moss Jurisprudence :
Primer of the Constitution 10
Andersons Strength of Materials 20
A rmstrong's Organic Chemistry 20
Arnolds (Dr.) Christian Life 29
Lectures on Modern History 2
, Mi scellaneous Works 12
School Sermons 29
- Sermons 29
(T. ) Manual of English Literature 12
A moulds Life of Lord Denman 7
Atherstone Priory 39
Autumn Holidays of a Country Parson ... 13
Ay res Treasury of Bible Knowledge 38
Bacon's Essays, by Whately
Life and Letters, by Spedding ...
Works
Bain's Mental and Moral Science
on the Senses and Intellect
Bakers Two Works on Ceylon
Ball's Guide to the Central Alps
Guide to the Western Alps
Guide to the Eastern Alps
Beckers Charicles and Gallus
Blacks Treatise on Brewing
Blackhys German-English Dictionary
Elaine's Rural Sports
Bloxam's Metals
Boultbee on 39 Articles
Bourne's Catechism of the Steam Engine .
Handbook of Steam Engine
Treatise on the Steam Engine ...
Improvements in the same
Bawdier s Family Shakspeare
Bramley-Moore s Six Sisters of the Valley .
Brande's Dictionary of Science, Literature,
and Art
Bray's Manual of Anthropology
Philosophy of Necessity
Brinkley's Astronomy
Browne's Exposition of the 39 Articles
Brunei s Life of Brunei
Buckles History of Civilisation
Posthumous Remains
Bull's Hints to Mothers
Maternal Management of Children .
9
$
34
39
15
20
23
27
27
27
27
35
39
22
22
II
17
28
7
Burkes Rise of Great Families .
Vicissitudes of Families .
Busk's Folk-lore of Rome
Valleys of Tirol
Cabinet Lawyer
Campbells Norway
r a tes s Biographical Dictionary
and Woodward's Encyclopaedia ...
Changed Aspects of Unchanged Truths ...
Chesney's Indian Polity
. Modern Military Biography
Waterloo Campaign
dough's Lives from Plutarch
Colenso on M oabite Stone &c
"s Pentateuch and Book of Joshua.
. Speaker's Bible Commentary ...
Collins s Mineralogy of Cornwall
Perspective
Commonplace Philosopher in Town and
Country, by A. K. H. B
Comtes Positive Polity
Comyn's Elena
Congreves Essays
Politics of Aristotle
Conington's Translation of Virgil's ^Eneid
Miscellaneous Writings.
Contanseaus Two French Dictionaries ...
Conybeare and Howson's Life and Epistle;
of St. Paul
Cotton's Memoir and Correspondence
Counsel and Comfort from a City Pulpit..
Cox's (G. W.) Aryan Mythology
. Crusades
History of Greece
Tale of the Great Persiai
War
. Tales of Ancient Greece ..
and Jones's Teutonic Tale
Crawleys Thucydides
Creasy on British Constitution ...
Cresy's Encyclopaedia of Civil Engineerini
Critical Essays of a Country Parson
Crookes's Chemical Analysis
Dyeing and Calico-printing ,
Culley's Handbook of Telegraphy
Cusacks Student's History of Ireland ....
D'Aubigne's Reformation in the Time (
Cnhrin....
;
NEW WORKS PUBLISHED BY LONGMANS & CO.
idsotis Introduction to New Testament
\d Shot (The), by Marksman
aisne and Le Maoufs Botany
{Morgan's Paradoxes
^Tocquevilles Democracy in America...
b-aeli's Lord George Bentinck
Novels and Tales
k son on the Ox
'v's Law of Storms
fa's Fairyland
Reasons of Faith....
's Gothic Revival
Hints on Household Taste
yards's Rambles among the Dolomites
ihents of Botany
cott ' s Commentary on Ephesians
' Galatians
Pastoral Epist.
. Philippians, &c.
) Thessalonians .
> Lectures on Life of Christ
'chs of History
'ns's Ancient Stone Implements
Vfs History of Israel
'Minis Application of Cast and
Wrought Iron to Building... 28
Information for Engineers 28
: "1 Treatise on Mills and Millwork 27
ar's Chapters on Language 13
Families of Speech 13
wygram on Horses and Stables 37
Vth's Essays o
ter's Collieries and Colliers 38
\cis's Fishing Book 36
man's Historical Geography of Europe 5
i January to December 14
de's English in Ireland 2
1 History of England 2
Short Studies 12
'dners Houses of Lancaster and York 6
gee on Horse-Shoeing 36
fs Elementary Physics 19
Natural Philosophy 19
liter's Buckingham and Charles 3
> Life of Christ o 2
Thirty Years' War 6
rt and Churchill's Dolomites 32
\estones Bible Synonyms 29
rue's Mechanics 20
' Mechanism 20
t's Ethics of Aristotle 10
:r Thoughts of a Country Parson 14
'He's Journal !
it's Algebra and Trigonometry 20
th's Sermons for the Times 29
1 on Correlation of Physical Forces ... 18
"s Encyclopaedia of Architecture 26
on Election of R
Harrison's Political Problems .............
Hart-wig's Aerial World ............
-- Polar World . .................. !.'.'.'.
- -- Sea and its Living Wonders ...
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