[Illustration: GASSENDI.
Nov. 7. 1867 10 P.M.]
THE MOON:
CONSIDERED AS
A PLANET, A WORLD, and A SATELLITE.
BY
JAMES NASMYTH, C.E.
AND
JAMES CARPENTER, F.R.A.S.
LATE OF THE ROYAL OBSERVATORY, GREENWICH.
WITH TWENTY-FOUR ILLUSTRATIVE PLATES OF LUNAR OBJECTS, PHENOMENA, AND
SCENERY; NUMEROUS WOODCUTS, &c.
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1874.
LONDON:
BRADBURY, AGNEW, & CO., PRINTERS, WHITEFRIARS.
TO
HIS GRACE THE DUKE OF ARGYLL,
IN RECOGNITION OF HIS LONG CONTINUED INTEREST IN THE SUBJECT OF WHICH
IT TREATS,
_This Volume_
IS MOST RESPECTFULLY DEDICATED
BY
THE AUTHORS.
PREFACE.
The reason for this book’s appearance may be set forth in a few words. A
long course of reflective scrutiny of the lunar surface with the aid of
telescopes of considerable power, and a consequent familiarity with the
wonderful details there presented, convinced us that there was yet
something to be said about the moon, that existing works on Astronomy
did not contain. Much valuable labour has been bestowed upon the
topography of the moon, and this subject we do not pretend to advance.
Enough has also been written for the benefit of those who desire an
acquaintance with the intricate movements of the moon in space; and
accordingly we pass this subject without notice. But very little has
been written respecting the moon’s physiography, or the causative
phenomena of the features, broad and detailed, that the surface of our
satellite presents for study. Our observations had led us to some
conclusions, respecting the cause of volcanic energy and the mode of its
action as manifested in the characteristic craters and other eruptive
phenomena that abound upon the moon’s surface. We have endeavoured to
explain these phenomena by reference to a few natural laws, and to
connect them with the general hypothesis of planet formation which is
now widely accepted by cosmologists. The principal aim of our work is to
lay these proffered explanations before the students and admirers of
astronomy and science in general; and we trust that what we have deduced
concerning the moon may be taken as referring to a certain extent to
other planets.
Some reflections upon the moon considered as a world, in reference to
questions of habitability, and to the peculiar conditions which would
attend a sojourn on the lunar surface, have appeared to us not
inappropriate. These, though instructive, are rather curious than
important. More worthy of respectful consideration are the few remarks
we have offered upon the moon as a satellite and a benefactor to the
inhabitants of this Earth.
In reference to the Illustrations accompanying this work, more
especially those which represent certain portions of the lunar surface
as they are revealed by the aid of powerful telescopes, such as those
which we employed in our scrutiny, it is proper that we should say a few
words here on the means by which they have been produced.——
During upwards of thirty years of assiduous observation, every
favourable opportunity has been seized to educate the eye not only in
respect to comprehending the general character of the moon’s surface,
but also to examining minutely its marvellous details under every
variety of phase, in the hope of rightly understanding their true nature
as well as the causes which had produced them. This object was aided by
making careful drawings of each portion or object when it was most
favourably presented in the telescope. These drawings were again and
again repeated, revised, and compared with the actual objects, the eye
thus advancing in correctness and power of appreciating minute details,
while the hand was acquiring, by assiduous practice, the art of
rendering correct representations of the objects in view. In order to
present these Illustrations with as near an approach as possible to the
absolute integrity of the original objects, the idea occurred to us that
by translating the drawings into models which, when placed in the sun’s
rays, would faithfully reproduce the lunar effects of light and shadow,
and then photographing the models so treated, we should produce most
faithful representations of the original. The result was in every way
highly satisfactory, and has yielded pictures of the details of the
lunar surface such as we feel every confidence in submitting to those of
our readers who have made a special study of the subject. It is hoped
that those also who have not had opportunity to become intimately
acquainted with the details of the lunar surface, will be enabled to
become so by aid of these Illustrations.
In conclusion, we think it desirable to add that the photographic
illustrations above referred to are printed by well-established pigment
processes which ensure their entire permanency.
CONTENTS.
PAGE
CHAPTER I.
ON THE COSMICAL ORIGIN OF THE PLANETS OF THE SOLAR SYSTEM. 1
Origination of Material Things—Celestial Vapours—Nebulæ—Their
vast Numbers—Sir W. Herschel’s Observations and
Classification—Buffon’s Cosmogony—Laplace’s Nebular
Hypothesis—Doubts upon its Validity—Support from Spectrum
Analysis
CHAPTER II.
THE GENERATION OF COSMICAL HEAT. 11
Conservation of Force—Indestructibility of Force—Its
Convertibility into Heat—Dawn of the Doctrine—Mayer’s
Deductions—Joule’s Experiments—Mechanical Equivalent of
Heat—Gravitation the Source of Cosmical Heat—Calculations
of Mayer and Helmholtz—The Moon as an Incandescent
Sphere—Not necessarily Burning—Loss of Heat by
Radiation—Cooling of External Crust—Commencement of
Selenological History
CHAPTER III.
THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 19
Cooling commenced from Outer Surface—Contraction by
Cooling—Expansion of Molten Matter upon
Solidification—Water not exceptional—Similar Behaviour of
Molten Iron—Floating of Solid on Molten Metal—Currents in
a Pot of Molten Metal—Bursting of Iron Bottle by
Congelation of Bismuth within—Evidence from Furnace
Slag—From the Crater of Vesuvius—Effects of Contraction of
Moon’s Crust and Expansion of Interior—Production of
Ridges and Wrinkles—Theory of Wrinkles—Examples from
shrivelled Apple and Hand
CHAPTER IV.
THE FORM, MAGNITUDE, WEIGHT, AND DENSITY OF THE LUNAR GLOBE. 31
Form of Moon—Not perfectly Spherical—Bulged towards
Earth—Diameter—Angular Measure—Linear Measure—Parallax of
Moon—Distance—Area of Lunar Sphere—Solid Contents—Mass of
Moon—Law of Gravitation—Mass determined by Tides and other
Means—Density—How obtained—Specific Gravity of Lunar
Matter—Force of Gravity at Surface—How determined—Weights
of similar Bodies on Earth and Moon—Effects of like Forces
acting against Gravity on Earth and Moon
CHAPTER V.
ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR ATMOSPHERE. 39
Subject of Controversy—Phenomena of Terrestrial Atmosphere—No
Counterparts on Moon—Negative Evidence from Solar
Eclipses—No Twilight on Moon—Evidence from Spectrum
Analysis—From Occultations of Stars—Absence of Water or
Moisture—Cryophorus—No Reddening of Sun’s Rays by Vapours
on Moon—No Air or Water to complicate Discussions of Lunar
Volcanic Phenomena
CHAPTER VI.
THE GENERAL ASPECT OF THE LUNAR SURFACE. 51
Pre-Telescopic Ideas—Human Countenance—Other supposed
Resemblances—Portrait of Full Moon—Permanence of
Features—Rotation of Moon—Solar Period and Solar Day on
Moon—Libration—Diurnal—In Latitude—In Longitude—Visible
and Invisible Hemispheres—Telescopic Scrutiny—Galileo’s
Views—Features Visible with Low Power—Low Powers on small
and large Telescopes—Salient
Features—Craters—Plains—Bright Streaks—Mountains—Higher
Telescopic Powers—Detail Scrutiny of Features
therewith—Discussion of High Powers—Education of
Eye—Highest practicable Power—Size of smallest Visible
Objects
CHAPTER VII.
TOPOGRAPHY OF THE MOON. 65
Reasons for Mapping the Moon—Early Maps—Labours of
Langreen—Hevelius—Riccioli—Cassini—Schroeter—Modern
Maps—Lohrman’s—Beer and Maedler’s—Excellence of the
last—Measurement of Mountain Heights—Need of a Picture
Map—Formation of our own—Skeleton Map—Table of conspicuous
Objects—Descriptions of special
Objects—Copernicus—Gassendi—Eudoxus and
Aristotle—Triesnecker—Theophilus, Cyrillus, and
Catharina—Thebit—Plato—Valley of the
Alps—Pico—Tycho—Wargentin—Aristarchus and
Herodotus—Walter—Archimedes and the Apennines
CHAPTER VIII.
ON LUNAR CRATERS. 89
Use of term Crater for Terrestrial and Lunar Formations—Truly
Volcanic Nature of Lunar Craters—Terrestrial and Lunar
Volcanic Areas compared—Similarity—Difference only in
Magnitude—Central Cone—Found in great and small Lunar
Craters—Formative Process of Terrestrial Volcanoes—Example
from Vesuvius—Vast Size of Lunar Craters—Reasons
assigned—Origin of Moon’s Volcanic Force—Aqueous Vapour
Theory untenable—Expansion upon Solidification
Theory—Formative Process of a Lunar Crater—Volcanic
Vent—Commencement of Eruption—Erection of
Rampart—Hollowing of Crater—Formation of Central Cone—Of
Plateau—Various Heights of Plateaux—Coneless
Craters—Filled-up Craters—Multiple Cones—Craters on
Plateau—Double Ramparts—Landslip Terraces—Rutted
Ramparts—Overlapping and Superposition of
Craters—Source-Connection of such—Froth-like Aggregations
of Craters—Majestic Dimensions of Larger Craters
CHAPTER IX.
ON THE GREAT RING-FORMATIONS NOT MANIFESTLY VOLCANIC. 117
Absence of Central Cones—Vast Diameters—Difficult of
Explanation—Hooke’s Idea—Suggested Cause of True
Circularity—Scrope’s Hypothesis of Terrestrial
Tumescences—Rozet’s Tourbillonic Theory—Dana’s Ebullition
Theory
CHAPTER X.
PEAKS AND MOUNTAIN RANGES. 124
Paucity of extensive Mountain Systems on Moon—Contrast with
Earth—Lunar Mountains found in less disturbed
Regions—Lunar Apennines, Caucasus, and Alps—Valley of
Alps—“Crag and Tail” Contour—Isolated Peaks—How
produced—Analogy from Freezing Fountain—Terrestrial
Counterparts and their Explanation by Scrope—Blowing Cone
on Teneriffe—Comparative Gentleness of Mountain-forming
Action—Relation between Mountain Systems and Crater
Systems—Wrinkle Ridges
CHAPTER XI.
CRACKS AND RADIATING STREAKS. 133
Description—Divergence from Focal Craters—Experimental
Explanation of their Cause—Radial Cracking of
Crust—Outflow of Matter therefrom—Analogy from “Starred”
Ice—No Shadows cast by Streaks—Their probable Slight
Elevation—Open Cracks—Great Numbers—Length—Depth—In-fallen
Fragments—Shrinkage a Cause of Cracks—Lateness of their
Production
CHAPTER XII.
COLOUR AND BRIGHTNESS OF LUNAR DETAILS: CHRONOLOGY OF FORMATIONS,
AND FINALITY OF EXISTING FEATURES. 143
Absence of Conspicuous Colour—Slight Tints of
“Seas”—Cause—Probable Variety of Tints in small
Patches—Diversity of Brightness of Details—Most
Conspicuous at Full Moon—Classification of
Shades—Exaggerated Contrasts in Photographs—Brightest
Portions probably the latest formed—Chronology of
Formations—Large Craters older than Small—Mountains older
than Craters—Bright Streaks comparatively recent—Cracks
most recent of all Features—Question of existing
Change—Evidence from Observation—Paucity of such
Evidence—Supposed Case of _Linné_—Theoretical
Discussion—Relative Cooling Tendencies of Earth and
Moon—Earth nearly assumed its final Condition—Moon
probably cooled Ages upon Ages ago—Possible slight Changes
from Solar Heating—Disintegrating Action
CHAPTER XIII.
THE MOON AS A WORLD: DAY AND NIGHT UPON ITS SURFACE. 155
Existence of Habitants on other Planets—Interest of the
Question—Conditions of Life—Absence of these from Moon—No
Air or Water and intense Heat and Cold—Possible Existence
of Protogerms of Life—A Day on the Moon
imagined—Instructiveness of the Realization—Length of
Lunar Day—No Dawn or Twilight—Sudden Appearance of
Light—Slowness of Sun in Rising—No Atmospheric
Tints—Blackness of Sky and Visibility of Stars and fainter
Luminosities at Noon-day—Appearance of the Earth as a
Stationary Moon—Its Phases—Eclipse of Sun by
Earth—Attendant Phenomena—Lunar Landscape—Height essential
to secure a Point of View—Sunrise on a Crater—Desolation
of Scene—No Vestige of Life—Colour of Volcanic Products—No
Atmospheric Perspective—Blackness of Shadows—Impressions
on other Senses than Sight—Heat of Sun untempered—Intense
Cold in Shade—Dead Silence—No Medium to conduct
Sound—Lunar Afternoon and Sunset—Night—The Earth a
Moon—Its Size, Rotation, and Features—Shadow of Moon upon
it—Lunar Night-Sky—Constellations—Comets and Planets—No
Visible Meteors—Bombardment by Dark Meteoric Masses—Lunar
Landscape by Night—Intensity of Cold
CHAPTER XIV.
THE MOON AS A SATELLITE: ITS RELATION TO THE EARTH AND MAN. 171
The Moon as a Luminary—Secondary Nature of Light-giving
Function—Primary Office as a Sanitary Agent—Cleansing
Effects of the Tides—Tidal Rivers and Transport
thereby—The Moon a “Tug”—Available Power of
Tides—Tide-Mills—Transfer of Tidal Power Inland—The Moon
as a Navigator’s Guide—Longitude found by the Moon—Moon’s
Motions—Discovered by Observations—Grouped into
Theories—Represented by Tables—The Nautical Almanac—The
Moon as a Long-Period Timekeeper—Reckoning by
“Moons”—Eclipses the Starting-Points of
Chronologies—Furnish indisputable Dates—Solar Surroundings
revealed by Eclipses when Moon screens the Sun—Solar
Corona—Moon as a Medal of Creation, a Half-formed
World—Abuses of the Moon—Superstitions—Erroneous Ideas
regarding Moonlight pourtrayed by Artists and Authors—The
Moon and the Weather—Errors and Facts—Atmospheric
Tides—Warmth from Moon—Paradoxical Effect in cooling the
Earth
CHAPTER XV.
CONCLUDING SUMMARY 184
LIST OF PLATES.
PLATE PAGE
Gassendi _Frontispiece_
I.—Summit of Vesuvius 26
II.—Wrinkled Hand and Apple 30
III.—Full Moon Photograph 52
IV.—Picture-Map of the Moon {_To face each other._}
V.—Skeleton Map 68
VI.—Terrestrial and Lunar Volcanic Areas Compared 88
VII.—Progressive Series of Craters 92
VIII.—Copernicus 96
IX.—The Lunar Apennines, &c., &c. 100
X.—Aristotle and Eudoxus 104
XI.—Triesnecker 108
XII.—Theophilus, Cyrillus, and Catharina 112
XIII.—Arzachael, Ptolemy, and the Railway 116
XIV.—Plato, the Valley of the Alps, Pico, &c. 120
XV.—Mercator and Campanus 124
XVI.—Tycho and its Surroundings 128
XVII.—Wargentin 132
XVIII.—Aristarchus and Herodotus 136
XIX.—Glass Globe Cracked by Internal Pressure 140
XX.—Overlapping Craters 148
XXI.—Lunar Crater. Ideal Landscape 156
XXII.—Solar Eclipse as it would be seen from the Moon 164
XXIII.—Group of Mountains. Ideal Lunar Landscape 170
THE MOON.
CHAPTER I.
ON THE COSMICAL ORIGIN OF THE PLANETS OF THE SOLAR SYSTEM.
In this Chapter we propose to treat briefly of the probable formation of
the various members of the solar system from matter which previously
existed in space in a condition different from that in which we at
present find it—_i.e._, in the form of planets and satellites.
It is almost impossible to conceive that our world with its satellite,
and its fellow worlds with their satellites, and also the great centre
of them all, have always, from the commencement of time, possessed their
present form: all our experiences of the working of natural laws rebel
against such a supposition. In every phenomenon of nature upon this
earth—the great field from which we must glean our experiences and form
our analogies—we see a constant succession of changes going on, a
constant progression from one stage of development to another taking
place, a perpetual mutation of form and nature of the same material
substance occurring: we see the seed transformed into the plant, the
flower into the fruit, and the ovum into the animal. In the inorganic
world we witness the operation of the same principle; but, by reason of
their slower rate of progression, the changes there are manifested to us
rather by their resulting effects than by their visible course of
operation. And when we consider, as we are obliged to do, that the same
laws work in the greatest as well as the smallest processes of nature,
we are compelled to believe in an antecedent state of existence of the
matter that composes the host of heavenly bodies, and amongst them the
earth and its attendant moon.
In the pursuit of this course of argument we are led to inquire whether
there exists in the universe any matter from which planetary bodies
could be formed, and how far their formation from such matter can be
explained by the operation of known material laws.
Before the telescope revealed the hidden wonders of the skies, and
brought its rich fruits into our garner of knowledge concerning the
nature of the universe, the philosophic minds of some early astronomers,
Kepler and Tycho Brahe to wit, entertained the idea that the sun and the
stars—the suns of distant systems—were formed by the condensation of
celestial vapours into spherical bodies; Kepler basing his opinion on
the phenomena of the sudden shining forth of new stars on the margin of
the Milky Way. But it was when the telescope pierced into the depths of
celestial space, and brought to light the host of those marvellous
objects, the nebulæ, that the strongest evidence was afforded of the
probable validity of these suppositions. The mention of “nebulous stars”
made by the earlier astronomers refers only to clusters of telescopic
stars which the naked eye perceives as small patches of nebulous light;
and it does not appear that even the nebula in Andromeda, although so
plainly discernible as to be often now-a-days mistaken by the
uninitiated for a comet, was known, until it was discovered by means of
a telescope, in 1612, by Simon Marius, who described it as resembling a
candle shining through semi-transparent horn, as in a lantern, and
without any appearance of stars. Forty years after this date Huygens
discovered the splendid nebula in the sword handle of Orion, and in 1665
another was detected by Hevelius. In 1667 Halley (afterwards Astronomer
Royal) discovered a fourth; a fifth was found by Kirsch in 1681, and a
sixth by Halley again in 1714. Half a century after this the labours of
Messier expanded the list of known nebulæ and clusters to 103, a
catalogue of which appeared in the “Connaissance du Temps” (the French
“Nautical Almanac”) for the years 1783-1784. But this branch of
celestial discovery achieved its most brilliant results when the rare
penetration, the indomitable perseverance, and the powerful instruments
of the elder Herschel were brought to bear upon it. In the year 1779
this great astronomer began to search after nebulæ with a seven-inch
reflector, which he subsequently superseded by the great one of forty
feet focus and four feet aperture. In 1786 he published his first
catalogue of 1000 nebulæ; three years later he astonished the learned
world by a second catalogue containing 1000 more, and in 1802 a third
came forth comprising other 500, making 2500 in all! This number has
been so far increased by the labours of more recent astronomers that the
last complete catalogue, that of Sir John Herschel, published a few
years ago, contains the places of 5063 nebulæ and clusters.
At the earlier periods of Herschel’s observations, that illustrious
observer appears to have inclined to the belief that all nebulæ were but
remote clusters of stars, so distant, so faint, and so thickly
agglomerated as to affect the eye only by their combined luminosity, and
at this period of the nebular history it was supposed that increased
telescopic power would resolve them into their component stars. But the
familiarity which Herschel gained with the phases of the multitudinous
nebulæ that passed in review before his eyes, led him ultimately to
adopt the opinion, advanced by previous philosophers, that they were
composed of some vapoury or elementary matter out of which, by the
process of condensation, the heavenly bodies were formed; and this led
him to attempt a classification of the known nebulæ into a cosmical
arrangement, in which, regarding a chaotic mass of vapoury matter as the
primordial state of existence, he arranged them into a series of stages
of progressive development, the individuals of one class being so nearly
allied to those in the next that, to use his own expression, not so much
difference existed between them “as there would be in an annual
description of the human figure were it given from the birth of a child
till he comes to be a man in his prime.” (_Philosophical Transactions,
Vol. CI., pp. 271_, _et seq._)
His category comprises upwards of thirty classes or stages of
progression, the titles of a few of which we insert here to illustrate
the completeness of his scheme.
Class 1. Of extensive diffused nebulosity. (A table of 52
patches of such nebulosity actually observed is given,
some of which extend over an area of five or six square
degrees, and one of which occupies nine square degrees.)
” 6. Of milky nebulosity with condensation.
” 15. Of nebulæ that are of an irregular figure.
” 17. Of round nebulæ.
” 20. Of nebulæ that are gradually brighter in the middle.
” 25. Of nebulæ that have a nucleus.
” 29. Of nebulæ that draw progressively towards a period of
final condensation.
” 30. Of planetary nebulæ.
” 33. Of stellar nebulæ nearly approaching the appearance of
stars.
In a walk through a forest we see trees in every stage of growth, from
the tiny sapling to the giant of the woods, and no doubt can exist in
our minds that the latter has sprung from the former. We cannot at a
passing glance discern the process of development actually going on; to
satisfy ourselves of this, we must record the appearance of some single
tree from time to time through a long series of years. And what a walk
through a forest is to an observer of the growth of a tree, a lifetime
is to the observer of changes in such objects as the nebulæ. The
transition from one state to another of the nebulous development is so
slow that a lifetime is hardly sufficient to detect it. Nor can any
precise evidence of change be obtained by the comparison of drawings or
descriptions of nebulæ at various epochs, with whatever care or skill
such drawings be made, for it will be admitted that no two draughtsmen
will produce each a drawing of the most simple object from the same
point of view, in which every detail in the one will coincide exactly
with every detail in the other. There is abundant evidence of this in
the existing representations of the great nebula in Orion; a comparison
of the drawings that have been lately made of this object, with the most
perfect instruments and by the most skilful of astronomical draughtsmen,
reveals varieties of detail and even of general appearance such as could
hardly be imagined to occur in similar delineations of one and the same
subject; and any one who himself makes a perfectly unbiassed drawing at
the telescope will find upon comparison of it with others that it will
offer many points of difference. The fact is that the drawing of a man,
like his penmanship, is a personal characteristic, peculiar to himself,
and the drawings of two persons cannot be expected to coincide any more
than their handwritings. The appearance of a nebula varies also to a
great extent with the power of the telescope used to observe it and the
conditions under which it is observed; the drawings of nebulæ made with
the inferior telescopes of a century or two centuries ago, the only ones
that, by comparison with those made in modern times, could give
satisfactory evidence of changes of form or detail, are so rude and
imperfect as to be useless for the purpose, and it is reasonable to
suppose that those made in the present day will be similarly useless a
century or two hence. Since then we can obtain no evidence of the
changes we must assume these mysterious objects to be undergoing, _ipso
facto_, by observation of _one nebula_ at _various periods_, we must for
the present accept the _primâ facie_ evidence offered (as in the case of
the trees in a forest) by the observation of _various nebulæ_ at _one
period_.
“The total dissimilitude,” says Herschel at the close of the
observations we have alluded to, “between the appearance of a diffusion
of the nebulous matter and of a star, is so striking, that an idea of
the conversion of the one into the other can hardly occur to any one who
has not before him the result of the critical examination of the
nebulous system which has been displayed in this [his] paper. The end I
have had in view, by arranging my observations in the order in which
they have been placed, has been to show that the above mentioned
extremes may be connected by such nearly allied intermediate steps, as
will make it highly probable that every succeeding state of the nebulous
matter is the result of the action of gravitation upon it while in a
foregoing one, and by such steps the successive condensation of it has
been brought up to the planetary condition. From this the transit to the
stellar form, it has been shown, requires but a very small additional
compression of the nebulous matter.”
Where the researches of Herschel terminated those of Laplace commenced.
Herschel showed how a mass of nebulous matter so diffused as to be
scarcely discernible might be and probably was, by the mere action of
gravitation, condensed into a mass of comparatively small dimensions
when viewed in relation to the immensity of its primordial condition.
Laplace demonstrated how the known laws of gravitation could and
probably did from such a partially condensed mass of matter produce an
entire planetary system with all its subordinate satellites.
The first physicist who ventured to account for the formation of the
various bodies of our solar system was Buffon, the celebrated French
naturalist. His theory, which is fully detailed in his renowned work on
natural history, supposed that at some period of remote antiquity the
sun existed without any attendant planets, and that a comet having
dashed obliquely against it, ploughed up and drove off a portion of its
body sufficient in bulk to form the various planets of our system. He
suggests that the matter thus carried off “at first formed a torrent the
grosser and less dense parts of which were driven the farthest, and the
densest parts, having received only the like impulsion, were not so
remotely removed, the force of the sun’s attraction having retained
them:” that “the earth and planets therefore at the time of their
quitting the sun were burning and in a state of liquefaction;” that “by
degrees they cooled, and in this state of fluidity they took their
form.” He goes on to say that the obliquity of the stroke of the comet
might have been such as to separate from the bodies of the principal
planets small portions of matter, which would preserve the same
direction of motion as the principal planets, and thus would form their
attendant satellites.
The hypothesis of Buffon, however, is not sufficient to explain all the
phenomena of the planetary system; and it is imperfect, inasmuch as it
begins by assuming the sun to be already existing, whereas any theory
accounting for the primary formation of the solar system ought
necessarily to include the origination of the most important body
thereof, the sun itself. Nevertheless, it is but due to Buffon to
mention his ideas, for the errors of one philosophy serve a most useful
end by opening out fields of inquiry for subsequent and more fortunate
speculators.
Laplace, dissatisfied with Buffon’s theory, sought one more probable,
and thus was led to the proposition of the celebrated _nebular
hypothesis_ which bears his name, and which, in spite of its
disbelievers, has never been overthrown, but remains the only probable,
and, with our present knowledge, the only possible explanation of the
cosmical origin of the planets of our system. Although Laplace puts
forth his conjectures, to use his own words, “with the deference which
ought to inspire everything that is not a result of observation and
calculation,” yet the striking coincidence of all the planetary
phenomena with the conditions of his system gives to those conjectures,
again to use his modest language, “a probability strongly approaching
certitude.”
Laplace conceived the sun to have been at one period the nucleus of a
vast nebula, the attenuated surrounding matter of which extended beyond
what is now the orbit of the remotest planet of the system. He supposed
that this mass of matter in process of condensation possessed a rotatory
motion round its centre of gravity, and that the parts of it that were
situated at the limits where centrifugal force exactly counterbalanced
the attractive force of the nucleus were abandoned by the contracting
mass, and thus were formed successively a number of rings of matter
concentric with and circulating around the central nucleus. As it would
be improbable that all the conditions necessary to preserve the
stability of such rings of matter in their annular form could in all
cases exist, they would break up into masses which would be endued with
a motion of rotation, and would in consequence assume a spheroidal form.
These masses, which hence constituted the various planets, in their turn
condensing, after the manner of the parent mass, and abandoning their
outlying matter, would become surrounded by similarly concentric rings,
which would break up and form the satellites surrounding the various
planetary masses; and, as a remarkable exception to the rule of the
instability of the rings and their consequent breakage, Laplace cited
the case of Saturn surrounded by his rings as the only instances of
unbroken rings that the whole system offers us; unless indeed we include
the zodiacal light, that cone of hazy luminosity that is frequently seen
streaming from our luminary shortly before and after sunset, and which
Laplace supposed to be formed of molecules of matter, too volatile to
unite either with themselves or with the planets, and which must hence
circulate about the sun in the form of a nebulous ring, and with such an
appearance as the zodiacal actually presents.
This hypothesis, although it could not well be refuted, has been by many
hesitatingly received, and for a reason which was at one time cogent. In
the earlier stages of nebular research it was clearly seen, as we have
previously remarked, that many of the so-called nebulæ, which appeared
at first to consist of masses of vapoury matter, became, when
scrutinised with telescopes of higher power, resolved into clusters
containing countless numbers of stars, so small and so closely
agglomerated, that their united lustre only impressed the more feeble
eye as a faint nebulosity; and as it was found that each accession of
telescopic power increased the numbers of nebulæ that were thus
resolved, it was thought that every nebula would at some period succumb
to the greater penetration of more powerful instruments; and if this
were the case, and if no real nebulæ were hence found to exist, how, it
was argued, could the nebular hypothesis be maintained? One of the most
important nebulæ bearing upon this question was the great one in the
sword handle of Orion, one of the grandest and most conspicuous in the
whole heavens. On account of the brightness of some portions of this
object, it seemed as though it ought to be readily resolvable, supposing
all nebulæ to consist of stars, but all attempts to resolve it were in
vain, even with the powerful telescopes of Sir John Herschel and the
clear zenithal sky of the Cape of Good Hope. At length the question was
thought to be settled, for upon the completion of Lord Rosse’s giant
reflector, and upon examination of the nebula with it, his lordship
stated that there could be little, if any, doubt as to its
resolvability, and then it was maintained, by the disbelievers in the
nebular theory, that the last stronghold of that theory had been broken
down.
But the truths of nature are for ever playing at hide and seek with
those who follow them:—the dogmas of one era are the exploded errors of
the next. Within the past few years a new science has arisen that
furnishes us with fresh powers of penetration into the vast and secret
laboratories of the universe; a new eye, so to speak, has been given us
by which we may discern, by the mere light that emanates from a
celestial body, something of the chemical elements of which it is
composed. When Newton two hundred years ago toyed with the prism he
bought at Stourbridge fair, and projected its pretty rainbow tints upon
the wall, his great mind little suspected that that phantom riband of
gorgeous colours would one day be called upon to give evidence upon the
probable cosmical origin of worlds. Yet such in truth has been the case.
Every substance when rendered luminous gives off light of some colour or
degree of refrangibility peculiar to itself, and although the eye cannot
detect any difference between one character of light and another, the
prism gives the means of ascertaining the quality and degree of
refrangibility of the light emanating from any source however distant,
and hence of gaining some knowledge of the nature of that source. If,
for instance, a ray of light from a solid body in combustion is passed
through a prism, a spectrum is produced which exhibits light of all
colours or all degrees of refrangibility; if the light from such a body,
before passing through the prism, be made to pass through gases or
certain metallic vapours, the resulting spectrum is found to be crossed
transversely by numbers of fine dark lines, apparently separating the
various colours, or cutting the spectrum into bands. The solar spectrum
is of this class; the once mysterious lines first observed by Wollaston,
and subsequently by Fraunhoffer, and known as “Fraunhoffer’s lines,”
have now been interpreted, chiefly by the sagacious German chemist
Kirchoff, and identified as the effects of absorption of certain of the
sun’s rays by chemical vapours contained in his atmosphere. The fixed
stars yield spectra of the same character, but varying considerably in
feature, the lines crossing the stellar spectra differing in position
and number from those of the sun, and one star from another, proving the
stars to possess varied chemical constitutions. But there is another
class of spectra, exhibited when light from other sources is passed
through the prism. These consist, not of a luminous riband of light like
the solar spectrum, but of bright isolated lines of coloured light with
comparatively wide dark spaces separating them. Such spectra are yielded
only by the light emitted from luminous gases and metals or chemical
elements in the condition of incandescent vapour. Every gas or element
in the state of luminous vapour yields a spectrum peculiar to itself,
and no two elements when vapourized before the prism show the same
combinations of luminous lines.
Now in the course of some observations upon the spectra of the fixed
stars by Dr. Huggins, it occurred to that gentleman to turn his
telescope, armed with a spectroscope, upon some of the brighter of the
nebulæ, and great was his surprise to find that instead of yielding
continuous spectra, as they must have done had their light been made up
of that of a multitude of stars, they gave spectra containing only two
or three isolated bright lines; such a spectrum could only be produced
by some luminous gas or vapour, and of this form of matter we are now
justified in declaring, upon the strength of numerous modern
observations, these remarkable bodies are composed; and it is a curious
and interesting fact that some of the nebulæ styled resolvable, from the
fact of their exhibiting points of light like stars, yield these gaseous
spectra, whence Dr. Huggins concludes that the brighter points taken for
stars are in reality nuclei of greater condensation of the nebular
matter: and so the fact of the apparent resolvability of a nebula
affords no positive proof of its non-nebulous character.
These observations—which have been fully confirmed by Father Secchi of
the Roman College—by destroying the evidence in favour of nebulæ being
remote clusters, add another attestation to the probability of the truth
of the nebular hypothesis, and we have now the confutation of the
luminologist to add to that of the astronomers who, in the person of the
illustrious Arago, asserted that the ideas of the great author of the
“Mécanique Céleste” “were those only which by their grandeur, their
coherence, and their mathematical character could be truly considered as
forming a physical cosmogony.”
Confining, then, our attention to the single object of the universe it
is our task to treat of—the Moon—and without asserting as an
indisputable fact that which we can never hope to know otherwise than by
inference and analogy, we may assume that that body once existed in the
form of a vast mass of diffused or attenuated matter, and that, by the
action of gravitation upon the particles of that matter, it was
condensed into a comparatively small and compact planetary body.
But while the process of condensation or compaction was going on,
another important law of nature—but recently unfolded to our
knowledge—was in powerful operation, the discussion of which law we
reserve for a separate Chapter.