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.

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.

CHAPTER II.
THE GENERATION OF COSMICAL HEAT.

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

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

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

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

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

Simultaneously and independently, and without even the knowledge of each other, three men, far removed from probable intercourse, conceived the same ideas and worked out nearly similar results concerning the mechanical theory of heat. Seeing that motion was convertible into heat, and heat into motion, it became of the utmost importance to determine the exact relation that existed between the two elements. The first who raised the idea to philosophic clearness was Dr. Julius Robert Mayer, a physician of Heilbronn in Germany. In certain observations connected with his medical practice it occurred to him that there must be a necessary equivalent between work and heat, a necessary numerical relation between them. “The variations of the difference of colour of arterial and venous blood directed his attention to the theory of respiration. He soon saw in the respiration of animals the origin of their motive powers, and the comparison of animals to thermic machines afterwards suggested to him the important principle with which his name will remain for ever connected.”