University of California • Berkeley THE MOON: CONSIDERED AS A PLANET, A WORLD, AND A SATELLITE. r JAMES NASMYTH DELT THO»0. BARLOW. SCULT CASSEN D I . N V ? 7. 18 6 7 10 P. M. THE MOON: &Ca?J 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. SECOND EDITION. 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, IS MOST RESPECTFULLY DEDICATED BY THE AUTHORS. PBEFACE. 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 forma- tion which is now widely accepted by cosmologists. The principal aim of our work is to lay these proffered explanations before the students viii PREFACE, 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 inappro- priate. 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 PREFACE. ix 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. CHAPTER I. ON THE COSMICAL ORIGIN OF THE PLANETS OF THE SOLAR SYSTEM. PAGE Origination of Material Things — Celestial Vapours — Nebulae— Their vast Numbers — Sir W. Herschel's Observations and Classification — Buffon's Cosmogony — La- place's Nebular Hypothesis — Doubts upon its Validity — Support from Spectrum Analysis 1 CHAPTER II. THE GENERATION OF COSMICAL HEAT. Conservation of Force — Indestructibility of Force — Its Convertibility into Heat — Dawn of the Doctrine — Mayer's Deductions — Joule's Experiments— Mechanical Equiva- lent 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 11 CHAPTER III. THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 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 . . . . . . 19 CHAPTER IV. THE FORM, MAGNITUDE, WEIGHT, AND DENSITY OF THE LUNAR GLOBE. 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 b 2 CONTENTS. PAGE 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 Eorces acting against Gravity on Earth and Moon 31 CHAPTER V. ON THE EXISTENCE OE NON-EXISTENCE OF A LUNAE ATMOSPHEEE. 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 Eeddening of Sun's Eays by Vapours on Moon — No Air or Water to complicate Discussions of Lunar Volcanic Phenomena . . 39 CHAPTER VI. THE GENEEAL ASPECT OF THE LUNAE SUEFACE. Pre-Telescopic Ideas — Human Countenance — Other supposed Eesemblances — Portrait of Full Moon — Permanence of Features — Eotation 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 . 51 CHAPTER VII. TOPOGEAPHY OF THE MOON. Eeasons for Mapping the Moon — Early Maps — Labours of Langreen — Hevelius — Eiccioli — 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 .......... 65 CHAPTER VIII. ON LUNAE CEATEES. Use of term Crater for Terrestrial and Lunar Formations — Truly Volcanic Nature of Lunar Craters — Terrestrial and Lunar Volcanic Areas compared — Similarity — Difference only in Magnitude — Central Cone— Found in great and small Lunar Craters — Formative Process of Terrestrial Volcanoes— Example from Vesuvius — Vast Size of Lunar Craters — Eeasons assigned — Origin of Moon's Volcanic CONTENTS. xiii PAGE Force — Aqueous Vapour Theory untenable — Expansion upon Solidification Theory — Formative Process of a Lunar Crater — Volcanic Vent — Commencement of Eruption — Erection of Eampart — Hollowing of Crater — Formation of Central Cone — Of Plateau — Various Heights of Plateaux — Coneless Craters — Filled-up Craters — Multiple Cones — Craters on Plateau — Double Eamparts — Landslip Terraces — "Rutted Eamparts — Overlapping and Superposition of Craters — Source- Connection of such — Froth-like Aggregations of Craters — Majestic Dimensions of Larger Craters 89 CHAPTEK IX. ON THE GEEAT BLNG-FOBMATIONS NOT MANIFESTLY VOLCANIC. Absence of Central Cones — Vast Diameters — Difficult of Explanation — Hooke's Idea — Suggested Cause of True Circularity — Scrope's Hypothesis of Terrestrial Tumescences — Bozet's Tourbillonic Theory— Dana's Ebullition Theory . ► 117 CHAPTER X. PEAKS AND MOUNTAIN BANGES. Paucity of extensive Mountain Systems on Moon — Contrast "with Earth — Lunar Mountains found in less disturbed Eegions — Lunar Apennines, Caucasus, and Alps — Valley of Alps — "Crag and Tail" Contour — Isolated Peaks — How produced — Analogy from Freezing Fountain — Terrestrial Counterparts and their Explanation by Scrope — Blowing Cone on Teneriffe — Comparative Gentleness of Mountain-forming Action — Eelation between Mountain Systems and Crater Systems — Wrinkle Eidges 124 CHAPTER XL CEACKS AND EADIATLNG STEEAKS. Description — Divergence from Focal Craters — Experimental Explanation of their Cause — Eadial 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 Frag- ments — Shrinkage a Cause of Cracks — Lateness of their Production .. . .133; CHAPTER XIL COLOUE AND BEIGHTNESS OF LUNAE DETAILS: CHEONOLOGT OF FOEMATIONS, AND FINALITY OF EXISTING FEATUEES. 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 — xiv CONTENTS. PAGE Supposed Case of LinnS — Theoretical Discussion — Eelative 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 — Dis- integrating Action 143 CHAPTER XIII. THE MOON AS A WOELD : DAY AND NIGHT UPON ITS SURFACE. 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 — Distinctiveness of the Realization — Length of Lunar Day — No Dawn or Twilight — Sudden Appearance of Light— Slowness of Sun in Rising — No Atmos- pheric 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 — Con- stellations — Comets and Planets — No Visible Meteors — Bombardment by Dark Meteoric Masses — Lunar Landscape by Night — Intensity of Cold . . . .155 CHAPTER XIV. THE MOON AS A SATELLITE: ITS RELATION TO THE EARTH AND MAN. The Moon as a Luminary — Secondary Nature of Light-giving Punction — 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 Surround- ings revealed by Eclipses when Moon screens the Sun — Solar Corona — Moon as a Medal of Creation, a Half-formed World — Abuses of the Moon — Super- stitions — Erroneous Ideas regarding Moonlight betrayed by Artists and Authors — The Moon and the Weather — Errors and Facts — Atmospheric Tides — Warmth from Moon — Paradoxical Effect in cooling the Earth . . . .171 CHAPTER XV. CONCLUDING SUMMARY 184 LIST OF PLATES. PLATE ' PAGK 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. 68 V. — Skeleton Map 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 XL — 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 XVHI. — Aristarchus and Herodotus 136 xvi LIST OF PLATES. PLATE PAGE XIX.— Full Moon and Cracked Glass Globe, illustrating the Cause of tiie Bright Eadiating Streaks 140 XX. — Overlapping Craters 148 XXI.— Lunar Crater. Ideal Landscape 156 XXII. — Solar Eclipse as it would be seen from the Moon . . . .164 XXIIL — 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 pro- cesses 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. B 2 THE MOON. [chap. i. In the pursuit of this course of argument we are led to inquire whether there exists in the universe any matter from which planetary bodies could be formed, and how far their formation from such matter can be explained by the operation of known material laws. Before the telescope revealed the hidden wonders of the skies, and brought its rich fruits into our garner of knowledge concerning the nature of the universe, the philosophic minds of some early astronomers, Kepler and Tycho Brahe to wit, entertained the idea that the sun and the stars — the suns of distant systems — were formed by the condensation of celestial vapours into spherical bodies ; Kepler basing his opinion on the phenomena of the sudden shining forth of new stars on the margin of the Milky Way. But it was when the telescope pierced into the depths of celestial space, and brought to light the host of those marvellous objects, the nebulae, that the strongest evidence was afforded of the probable validity of these suppositions. The mention of " nebulous stars " made by the earlier astronomers refers only to clusters of telescopic stars which the naked eye perceives as small patches of nebulous light ; and it does not appear that even the nebula in Andromeda, although so plainly discernible as to be often now-a-days mistaken by the uninitiated for a comet, was known, until it was discovered by means of a telescope, in 1612, by Simon Marius, who described it as resembling a candle shining through semi-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 Eoyal) discovered a fourth ; a fifth was found by Kirsch in 1681, and a sixth by Halley again in 1714. Half a century after this the labours of Messier expanded the list of known nebulae and clusters to 103, a catalogue of which appeared in the " Connaissance du Temps " (the French " Nautical Almanac") for the years 1783-1784. But this branch of celestial dis- covery 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 nebulae with a seven-inch reflector, which he subsequently superseded by the great one of forty feet focus chap, l] COSMICAL ORIGIN OF PLANETARY SYSTEM. 3 and four feet aperture. In 1786 he published his first catalogue of 1000 nebulae ; three years later he astonished the learned world by a second catalogue containing 1000 more, and in 1802 a third came forth com- prising other 500, making 2500 in all! This number has been so far increased by the labours of more recent astronomers that the last complete catalogue, that of Sir John Herschel, published a few years ago, contains the places of 5063 nebulae and clusters. At the earlier periods of Herschel's observations, that illustrious observer appears to have inclined to the belief that all nebulae were but remote clusters of stars, so distant, so faint, and so thickly agglomerated as to 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 nebulae that passed in review before his eyes, led him ultimately to adopt the opinion, advanced by previous philosophers, that they were composed of some vapoury or 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 nebulae 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. CL, pp. 271 et seq.) His category comprises upwards of thirty classes or stages of progres- sion, 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. b 2 4 THE MOON. [chap. i. Class 1 5. Of nebulae that are of an irregular figure. „ 1 7. Of round nebulae. „ 20. Of nebulae that are gradually brighter in the middle. „ 25. Of nebulae that have a nucleus. „ 29. Of nebulae that draw progressively towards a period of final condensation. „ 30. Of planetary nebulae. „ 33. Of stellar nebulae nearly approaching the appearance of stars. In a walk through a forest we see trees in every stage of growth, from the tiny sapling to the giant of the woods, and no doubt can exist in our minds that the latter has sprung from the former. We cannot at a passing glance discern the process of development actually going on ; to satisfy ourselves of this, we must record the appearance of some single tree from time to time through a long series of years. And what a walk through a forest is to an observer of the growth of a tree, a lifetime is to the observer of changes in such objects as the nebulae. The tran- sition 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 descrip- tions of nebulae at various epochs, with whatever care or skill such drawings be made, for it will be admitted that no two draughtsmen will produce each a drawing of the most simple object from the same point of view, in which every detail in the one will coincide exactly with every detail in the other. There is abundant evidence of this in the existing representations of the great nebula in Orion ; a comparison of the drawings that have been lately made of this object, with the most perfect instruments and by the most skilful of astronomical draughtsmen, reveals varieties of detail and even of general appearance such as could hardly be imagined to occur in similar delineations of one and the same subject ; and any one who himself makes a perfectly unbiassed drawing at the telescope will find upon comparison of it with others that it will offer many points of difference. The fact is that the drawing of a man, like his penmanship, is a personal characteristic, peculiar to himself, and the drawings of two persons cannot be expected to coincide any more chap, l] COSMICAL ORIGIN OF PLANETARY SYSTEM. 5 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 w T hich it is observed; the drawings of nebulae made with the inferior telescopes of a century or two centuries ago, the only ones that, by comparison with those made in modern times, could give satisfactory evidence of changes of form or detail, are so rude and imperfect as to be useless for the purpose, and it is reasonable to suppose that those made in the present day will be similarly useless a century or two hence. Since then we can obtain no evidence of the changes we must assume these mysterious objects to be undergoing, ipso facto, by observation of one nebula at various periods, we must for the present accept the primd facie evidence offered (as in the case of the trees in a forest) by the observation of various nebulw at one period. " The total dissimilitude," says Herschel at the close of the observa- tions 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 w T hich 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 suc- cessive 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 com- menced. 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. 6 THE MOON. [chap. i. 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 specu- lators. 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 every- thing that is not a result of observation and calculation/' yet the chap, i.] COSMICAL ORIGIN OF PLANETARY SYSTEM. 7 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 n fibula, 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 counter- balanced 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 nebulae, which appeared at first to consist of masses of vapoury matter, 8 THE MOON". [chap. i. 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 nebulas that were thus resolved, it was thought that every nebula would at some period succumb to the greater penetration of more powerful instruments ; and if this were the case, and if no real nebulae were hence found to exist, how, it was argued, could the nebular hypothesis be maintained ? One of the most important nebulae bearing upon this question was the great one in the sword handle of Orion, one of the grandest and most conspicuous in the whole heavens. On account of the brightness of some portions of this object, it seemed as though it ought to be readily resolvable, supposing all nebulae to consist of stars, but all attempts to resolve it were in vain, even with the powerful telescopes of Sir John Herschel and the clear zenethal sky of the Cape of Good Hope. At length the question was thought to be settled, for upon the completion of Lord 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 chap, i.] COSMICAL ORIGIN OF PLANETARY SYSTEM. 9 detect any difference between one character of light and another, the prism gives the means of ascertaining the quality and degree of refrangi- bility 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 Fraunhofer, and known as " Fraunhofer's lines," have now been interpreted, chiefly by the sagacious German chemist Kirchhoff, and identified as the effects of absorption of certain of the sun's rays by chemical vapours contained in his atmosphere. The fixed stars yield spectra of the same character, but varying considerably in feature, the lines crossing the stella spectra differing in position and number from those of the sun, and one star from another, proving the stars to possess varied chemical constitutions. But there is another class of spectra, exhibited when light from other sources is passed through the prism. These consist, not of a luminous riband of light like the solar spec- trum, 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 vaporized before the prism show the same combinations of lumi- nous 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 nebulae, and great was his surprise to find that instead of yielding continuous spectra, as they must have done had their light been made up of that of a multitude of stars, they gave spectra containing only two or three isolated c 10 THE MOOX. [chap. i. bright lines ; such a spectrum could only be produced by some luminous gas or vapour, and of this form of matter we are now justified in declaring, upon the strength of numerous modern observations, these remarkable bodies are composed ; and it is a curious and interesting fact that some of the nebulae styled resolvable, from the fact of their exhibit- ing 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 Eoman College — by destroying the evidence in favour of nebulae being remote clusters, add another attestation to the probability of the truth of the nebular hypothesis, and we have now the confutation of the luminologist to add to that of the astronomers who, in the person of the illustrious Arago, asserted that the ideas of the great author of the " M^canique Celeste " " 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 C 2 12 THE MOON. [ohap. ii. produces or resists motion, and hence we may argue that the ultimate effect of force is motion. The force of gravity on the earth results in the motion or tendency of all bodies towards its centre, and similarly, the action of gravitation upon the atoms or particles of a primeval planet resulted in the motion of those particles towards each other. We cannot conceive force otherwise than by its effects, or the motion it produces. And force we are taught is indestructible ; therefore motion must be indestructible also. But when a falling body strikes the earth, or a gun- shot strikes its target, or a hammer delivers a blow upon an anvil, or a brake is pressed against a rotating wheel, motion is arrested, and it would seem natural to infer that it is destroyed. But if we say it is indestructible, what becomes of it 1 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 chap, ii.] THE GENERATION OF COSMICAL HEAT. 13 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 recog- nition 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 dis- close 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 deter- mine 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 com- parison of animals to thermic machines afterwards suggested to him the important principle with which his name will remain for ever connected," 14 THE MOON. - [chap, be, Next in order of publication of his results stands the name of Colding, a Danish engineer, who about the year 1843 presented a series of memoirs on the steam-engine to the Eoyal Society of Copenhagen, in which he put forth views almost identical with those of Mayer. Last in publication order, but foremost in the importance of his experi- mental treatment of the subject, was our own countryman, Dr. Joule of Manchester. " Entirely independent of Mayer, with his mind firmly fixed upon a principle, and undismayed by the coolness with which his first labours appear to have been received, he persisted for years in his attempts to prove the invariability of the relation which subsists between heat and ordinary mechanical power." (We are quoting from Professor TyndaU's valuable work on " Heat considered as a Mode of Motion.") " He placed water in a suitable vessel, agitated the water by paddles, and determined both the amount of heat developed by the stirring of the liquid, and the amount of labour expended in its production. He did the same with mercury and sperm oil. He also caused discs of cast iron to rub against each other, and measured the heat produced by their friction, and the force expended in overcoming it. He urged water through capillary tubes, and determined the amount of heat generated by the friction, of the liquid against the sides of the tubes. And the results of his experiments leave no shadow of doubt upon the mind that, under all circumstances, the quantity of heat generated by the same amount of force is fixed and invariable. A given amount of force, in causing the iron discs to rotate against each other, produced precisely the same amount of heat as when it was applied to agitate water, mercury, or sperm oil. * * * * The absolute amount of heat generated by the same expenditure of power, was in all cases the same." "In this way it was found that the quantity of heat which would raise one pound of water one degree Fahrenheit in temperature, is exactly equal to what w r ould be generated if a pound weight, after having fallen through a height of 772 feet, had its moving force destroyed by collision with the earth. Conversely, the amount of heat necessary to raise a pound of w T ater one degree in temperature, would, if all applied mechanically, be competent to raise a pound weight 772 feet high, or it would raise 772 pounds one foot high. The term ' foot-pounds ' has been introduced to chap, ii.] THE GENERATION OF COSMICAL HEAT. 15 express in a convenient way the lifting of one pound to the height of a foot. Thus the quantity of heat necessary to raise the temperature of a pound of water one degree Fahrenheit being taken as a standard, 772 foot-pounds constitute what is called the mechanical equivalent of heat." By a process entirely different, and by an independent course of reasoning, Mayer had, a few months previous to Joule, determined this equivalent to be 771*4 foot-pounds. Such a remarkable coincidence arrived at by pursuing different routes gives this value a strong claim to accuracy, and raises the Mechanical Theory of Heat to the dignity of an exact science, and its enunciators to the foremost place in the ranks of physical philosophers. In linking together the labours of the two remarkable men above alluded to, Prof. Tyndall remarks, that " Mayer's labours have in some measure the stamp of profound intuition, which rose however to the energy of undoubting conviction in the author's mind. Joule's labours, on the contrary, are an experimental demonstration. Mayer thought his theory out, and rose to its grandest applications. Joule worked his theory out, and gave it the solidity of natural truth. True to the speculative instinct of his country, Mayer drew large and mighty conclusions from slender premises ; while the Englishman aimed above all things at the firm establishment of facts To each belongs a reputation which will not quickly fade, for the share he has had, not only in establishing the dynamical theory of heat, but also in leading the way towards a right appreciation of the general energies of the universe." But from these generalities we must pass to the application of the mechanical theory of heat to our special subject. We have learnt that every form of motion is convertible into heat. We know that the falling meteor or shooting star, whose motion is impeded by friction against the earth's atmosphere, is heated thereby to a temperature of incandescence. Let us then suppose that myriads of such cosmical particles came into collision from the effect of their mutual attraction, or that the component atoms of a vast nebulous mass violently con- verged under the like influence. What would follow ? Obviously the generation of an intense heat by the arrest of converging motion, such a 16 THE MOON. [chaf. 11. heat as would result in the fusion of the whole into one mass. Mayer, in one of his most remarkable papers (" Celestial Dynamics ") remarks that the " Newtonian theory of gravitation, whilst it enables us to deter- mine, from its present form, the earth's state of aggregation in ages past, at the same time points out to us a source of heat powerful enough to produce such a state of aggregation — powerful enough to melt worlds : it teaches us to consider the molten state of a planet as the result of the mechanical union of cosmical masses, and to derive the radiation of the sun and the heat in the bowels of the earth from a common origin." And the same laws that governed the formation of the earth, governed also the formation of the moon : the variations of Nature's operations are quantitative only and not qualitative. The Divine Will that made the earth made the moon also, and the means and mode of working were the same for both. The geological phenomena of the earth afford unmistake- able evidence of its original fluid or molten condition, and the appear- ance of the moon is as unmistakeably that of a body once in an igneous or molten state. The enigma of the earth's primary formation is solved by the application of the dynamical theory of heat. By this theory the generation of cosmical heat is removed from the quicksands of conjec- ture and established upon the firm ground of direct calculation : for the absolute amount of heat generated by the collision of a given amount of matter is (of course, with some little uncertainty) deducible from a mathematical formula. Mayer has computed the amount of heat that the matter of the earth would have generated, if it had been formed originally of only two parts drawn into collision by their mutual attraction, and has found that it would be from to 32,000 or 47,000* Centigrade degrees, according as one part was infinitely small as compared with the other, or as the two parts were of equal size. Professor Helmholtz, another labourer in the same field of science, has computed the amount of heat generated by the condensation of the whole of the matter composing the solar system; this he finds would be equivalent to the heat that would be required to raise the temperature of a mass of water equal to the sum of * The melting temperature of iron is 1500° Centigrade. chap, ir.] THE GENERATION OF COSMICAL HEAT. 17 the masses of all the bodies of the system to 28,000,000 (twenty-eight million) degrees of the Centigrade scale. These examples afford abundant evidence of sufficient heat having been generated by the aggregation of the matter of the moon to reduce it to a state of fusion, and so to produce, from a nebulous chaos of diffused cosmical matter, a molten body of definite outline and size. It is requisite here to remark that fusion does not necessarily imply combustion. It has been frequently asked, How can a volcanic theory of the lunar phenomena be upheld consistently with the condition that it possesses no atmosphere to support Fire ? To this we would reply that to produce a state of incandescence or a molten condition it is not necessary that the body be surrounded by an atmosphere. The intensely rapid motion of the particles of matter of bodies, which the dynamical theory shows to be the origin of the molten state, exists quite indepen- dently of such external matter as an atmosphere. The complex mixture of gases and vapours which we term " air/' has nothing whatever to do with the fusion of substances, whatever it may have to do with their combustion. Combustion is a chemical phenomenon, due to the com- bination of the oxygen of that air with the heated particles of the combustible matter : oxygen is the sole supporter of combustion, and hence combustion is to be regarded rather as a phenomenon of oxygen than as a phenomenon of the matter with which that oxygen combines. The greatest intensity of heat may exist without oxygen, and consequently without combustion. In support of this argument it will be sufficient to adduce, upon the authority of Dr. Tyndall, the fact that a platinum wire can be raised to a luminous temperature and actually fused in a perfect vacuum. But while the mass of condensing cosmical matter was thus accumu- lating and forming the globe of the moon, the heat consequent upon the aggregation of its particles was suffering some diminution from the effect of radiation. So long as the radiated heat lost fell short of the dynamical heat generated, no effect of cooling would be manifest ; but when the vis viva of the condensing matter was all converted into its equivalent of heat, or when the accession of heat fell short of that radiated, a necessary cooling must ensue, and this cooling would be accompanied by a solidifi- d 18 THE MOON. [chap. ii. cation of that part of the mass which was most free to radiate its heat into surrounding space : that part would obviously be the outer surface. With the solidification of this external crust began the " year one " of selenological history. The phenomena attendant upon the cooling of the mass we will con- sider in the next Chapter. CHAPTER III. THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. In the foregoing Chapters we have endeavoured to show, by the light of modern science, first, how diffused cosmical matter was probably condensed into a planetary mass by the mutual gravitation of its particles, and secondly, how, the after destruction of the gravitative force, by the collision of the converging particles of matter, resulted in the gene- ration of such sufficient heat as to reduce the whole mass to a molten condition. Our present task is to consider the subsequent cooling of the mass, and the phenomena attendant upon or resulting therefrom. This brief Chapter is important to our subject, as we shall have fre- quent occasion to refer to the leading principle we shall endeavour to illustrate in it, in subsequently treating of the causes to which the special selenological features are to be attributed. First, then, as regards the cooling of the igneous mass that constituted the moon at the inconceivably remote period when possibly that body was really " a lesser light " shining with a luminosity of its own, due to its then incandescent state, and not simply a reflector, as it is now, of light which it receives from the sun. If we could conceive it possible that the igneous mass in the act of cooling parted with its heat from the cen- tral part first and so began to solidify from its centre, or if it had been possible for the mass to have cooled uniformly and simultaneously through- out its whole depth, or that each substratum had cooled before its super- stratum, we should have had a moon whose surface would have been smooth and without any such remarkable asperities and excrescences as are now presented to our view. But these suppositions are inadmissible : on the contrary we are compelled to consider that the portion of the D 2 20 THE MOON. [chap. hi. igneous or molten body that first cooled was its exterior surface, which, radiating its heat into surrounding space, became solid and comparatively cool while the interior retained its hot and molten condition. So that at this early stage of the moon's history it existed in the form of a solid shell inclosing a molten interior. Now at this period of its formation, the moon's mass, partly cooled and solidified and partly molten, would be subject to the influence of two powerful molecular forces : the first of these would consist in the diminu- tion of bulk or contraction of volume which accompanies the cooling of solidified masses of previously molten substances ; the second would arise from a phenomenon which we may here observe is by no means so gene- rally known as from its importance it deserves to be : and as we shall have frequent occasion to refer to it as one of the chief agencies in producing the peculiar structural characteristics of the moon's surface, it may be well here to give a few examples of its action, that our refer- ence to it hereafter may be more clearly understood. The broad general principle of the phenomenon here referred to is this : — that fusible substances are (with a few exceptions) specifically heavier while in their molten condition than in the solidified state, or in other words, that molten matter occupies less space, weight for weight, than the same matter after it has passed from the melted to the solid condition. It follows as an obvious corollary that such substances con- tract in bulk in fusing or melting, and expand in becoming solid. It is this expansion upon solidification that now concerns us. Water, as is well known, increases in density as it cools, till it reaches the temperature of 39° Fahrenheit, after which, upon a further decrease of temperature, its density begins to decrease, or in other words its bulk expands, and hence the well-known fact of ice floating in water, and the inconvenient fact of water-pipes bursting in a frost. This action in water is of the utmost importance in the grand economy of nature, and it has been accepted as a marvellous exception to the general law of substances increasing in density (or shrinking) as they decrease in temperature. Water is, however, by no means the exceptional substance that it has been so generally considered. It is a fact perfectly familiar to iron- founders, that when a mass of solid cast-iron is dropped into a pot of chap, in.] SUBSEQUENT COOLING OF IGNEOUS BODY. 21 molten iron of identical quality, the solid is found to float persistently upon the molten metal — so persistently that when it is intentionally thrust to the bottom of the pot, it rises again the moment the submerging agency is withdrawn. As regards the amount of buoyancy we believe it may be stated in round numbers to be at least two or three per cent. It has been suggested by some who are familiar with this phenomenon that the solid mass may be kept up by a spurious buoyancy imparted to it by a film of adhering air, or that surface impurities upon the solid metal may tend to reduce the specific gravity of the mass and thereby prevent it sinking, and that the fact of floatation is not absolutely a proof of greater specific lightness. But in controversion of these suggestions, we can state as the result of experiment that pieces of cast-iron which have had their surface roughness entirely removed, leaving the bright metal exposed, still float on the molten metal, and further that when, under the influence of the great heat of the molten mass, the solid is gradually melted away, and consequently any possible surface impurities or adhering air must necessarily have been removed, the remaining portion continues to float to the last. The inevitable inference from this is that in the case of cast-iron the solid is specifically lighter than the molten, and, therefore, that in passing from the molten to the solid condition this substance undergoes expansion in bulk. We are able to offer a confirmation of this inference in the case of cast-iron by a remarkable phenomenon well known to iron-founders, but of which we have never met with special notice. When a ladle or pot of molten iron is drawn from the melting furnace and allowed to stand at rest, the surface presents a most remarkable and suggestive appearance. Instead of remaining calm and smooth it is the scene of a lively com- motion : the thin coat of scoria or molten oxide which forms on the otherwise bright surface of the metal is seen, as fast as it forms at the circumference of the pot, to be swept by active convergent currents towards the centre, where it accumulates in a patch. While this action is proceeding, the entire upper surface of the metal appears as if it were covered with animated vermicules of scoria, springing into existence at the circumference of the pot, and from thence rapidly streaming and wriggling themselves towards the centre. 22 THE MOON. [CHAP. III. Our illustration (Fig. 1) is intended, so far as such means can do so, to convey some idea of this remarkable appearance at one instant of its continued occurrence. To interpret our illustration rightly it is necessary Fig. 1. to imagine this vermicular freckling to he constantly and rapidly stream- ing from all points of the periphery of the pot towards the centre, where, as we have said, it accumulates in the form of a floating island. We may observe that the motion is most rapid when the hot metal is first put into the cool ladle : as the fluid metal parts with some of its heat and the ladle gets hot by absorbing it, this remarkable surface disturbance becomes less energetic. Now if we carefully consider this peculiar action and seek a cause for the phenomenon, we shall be led to the conclusion that it arises from the expansion of that portion of the molten mass which is in contact with or close proximity to the comparatively cool sides of the ladle, which sides act as the chief agent in dispersing the heat of the melted metal. The motion of the scoria betrays that of the fluid metal beneath, and careful observation will show that the motion in question is the result of an upward current of the metal around the circumference of the ladle, as indicated by the arrows a, b, c in the accompanying sectional drawing chap, in.] SUBSEQUENT COOLING OF IGNEOUS BODY. 23 of the ladle (Fig. 2). The upward current of the metal can actually be seen when specially looked for, at the rim of the pot, where it is deflected into the convergent horizontal direction and where it presents an eleva- Fig. 2. tory appearance as shown in the figure. It is difficult to assign to this effect any other cause than that of an expansion and consequent reduc- tion of the specific gravity of the fluid metal in contact with or in close proximity to the cooler sides of the pot, as, according to the generally entertained idea that contraction universally accompanies cooling, it would be impossible for the cooler to float on the hotter metal, and the curious surface-currents above referred to would be in contrary direction to that which they invariably take, i.e., they would diverge from the centre instead of converging to it. The external arrows in the figure represent the radiation of the heat from the outer sides of the pot, which is the chief cause of cooling. Turning from .cast-iron to other metals we find further manifesta-^ tions of this expansive solidification. Bismuth is a notable example. In his lectures on Heat, Dr. Tyndall exhibited an experiment in which a stout iron bottle was filled with molten bismuth, and the stopper tightly closed. The whole was set aside to cool, and as the metal within 24 THE MOON. [chap. m. approached consolidation the bottle was rent open by its expansion, just as would have been the case had the bottle been filled with water and exposed to freezing temperature. Mercury affords another example. Thermometers which have to be exposed to Arctic temperatures are generally filled with spirit instead of quicksilver, because the latter has been found to burst the bulbs when the cold reached the congealing point of the metal, the bursting being a consequence of the expansion which accompanies the act of congelation. Silver also expands in passing from the fluid to the solid state, for we are informed by a practical refiner that solid floats on molten silver as ice floats on water ; it also, as likewise do gold and copper, exhibits surface converging currents in the melting-pot like those depicted above for molten iron. It may, however, be objected that metals are too distantly related to volcanic substances to justify inferences being drawn from their behaviour in explanation of volcanic phenomena. With a view therefore of testing the question at issue with a substance admitted as closely allied to volcanic material, we appealed to the furnace slag of iron-works. The following are extracts from the letters of an iron manufacturer of great experience* to whom we referred the question : — " I beg to inform you that cold slag floats in molten slag in the same way cold iron floats in molten iron. " I filled a box with hot molten slag run quickly from a blast furnace; the box was about 5^ feet square by 2 feet deep, and I dropped into the slag a piece of cold slag weighing 16 lbs., when it came to the top in a second. I pushed it down to the bottom several times and it always made its appearance at the top : indeed a small portion of it remained above the molten slag." Here then we have a substance closely allied to volcanic material which manifests the expansile principle in question ; but we may go still * Mr. T. Heunter, Manager of the Iron- works of James Murray, Esq., of Dalmellington, Ayr- shire. Another authority (Mr. Snelus, of the West Cumberland Iron Company), writes as follows : " I had a hole dug on the ' cinder-fall,' and allowed the running slag to flow through it so as to form a tolerably large pool and yet keep fluid. Any crust that formed was skimmed off. A portion of the same slag was cooled, and the solid lump thrown into the pool. It floated just at the surface." Mr. Snelus adds, by the way, that he tried " Bessemer- Pig " in the same way, and that the solid pig sunk in the molten for a minute and then rose and floated just a,t the surface, with about one twentieth of its bulk above the level of the fluid. chap, m.] SUBSEQUENT COOLING OF IGNEOUS BODY. 25 further and give evidence from the very fountain-head by instancing what appears to be a most cogent example of its operation which we observed on the occasion of a visit to the crater of Vesuvius in 1865 while a modi- fied eruption was in progress. On this occasion we observed white-hot lava streaming down from apertures in the sides of a central cone within the crater and forming a lake of molten lava on the plateau or bottom of the crater ; on the surface of this molten lake vast cakes of the same lava which had become solidified were floating, exactly in the same manner as ice floats in water. The solidified lava had cracked, and divided into cakes, in consequence of its contraction and also of the uprising of the accumulating fluid lava on which it floated, more and more space being thus afforded for it to separate, on account of the crater widening upwards, while through the joints or fissures the fluid lava could be seen beneath. But for the decrease in density and consequent expansion in volume which accompanied solidification, this floating of the solidified lava on the molten could not have occurred. Eeference to Fig. 3, j Fig. 3. THE MOON. [chap. III. which represents a section of the crater of Vesuvius on the occasion above referred to, will perhaps assist the reader to a more clear idea of what we have endeavoured to describe, a a are the streams of white- hot lava issuing from openings in the sides of the central cone, 'and accumulating beneath the solidified crust b b in a lake of molten lava at c c ; the solidified crust b b as it was floated upwards dividing into separate cakes as represented in Fig. 4. (See also Plate I.) Fig. 4. Let us now consider what would be the effect produced upon a spherical mass of molten matter in progress of cooling, first under the action of the above described expansion which precedes solidification, and then by the contraction which accompanies the cooling of a solidified body. The first portion of such a mass to part with its heat being its external surface, this portion would expand, but there being no obstacle to resist the expansion there would be no other result than a temporary slight enlargement of the sphere. This external portion would on cooling form a solid shell encompassing a more or less fluid molten nucleus, but as this interior has in its turn, on approaching the point of solidification, to expand also, and there being, so to speak, no room for its expansion, by reason of its confinement within its solid casing, what would be the consequence ? — the shell would be rent or burst open, and a portion of the molten interior ejected with more or less violence according to circum- stances, and many of the characteristic features of volcanic action would < LU > cc chap, m] SUBSEQUENT COOLING OF IGNEOUS BODY. 27 be thus produced : the thickness of the outer shell, the size of the vent made by the expanding matter for its escape, and other conditions con- spiring to modify the nature and extent of the eruption. Thus there would result vast floodings of the exterior surface of the shell by the so extruded molten matter, volcanoes, extruded mountains, and other mani- festations of eruptive phenomena. The sectional diagram (Fig. 5) will Fig. 5. a a. The solidified crust cooling, contracting, and cracking ; the cracking action enhanced by the expansion of the substratum of molten matter, bbb, which, expanding as it approaches the point of solidification, injects portions of the molten matter up through the contractile cracks, and results in producing craters, mountains of exudation, and districts flooded with extruded lava, c c c. The nucleus of intensely hot molten matter. help to convey a clear idea of this action. Basing our reasoning on the principle we have thus enunciated, namely, that molten telluric matter expands on nearing the point of solidification, and which we have endeavoured to illustrate by reference to actual examples of its opera- tion, we consider we are justified in assuming that such a course of volcanic phenomena has very probably occurred again and again upon the moon ; that this expansion of volume which accompanies the solidifi- cation of molten matter furnishes a key to the solution of the enigma of volcanic action ; and that such theories as depend upon the agency of gases, vapour, or water are at all events untenable with regard to the moon, where no gases, vapour, or water, appear to exist. That an upheaving and ejective force has been in action with varying intensity beneath the whole of the lunar surface is manifest from the aspect of its structural details, and we are impressed with the conviction e 2 28 THE MOON. [chap. III. that the principle we have set forth, namely the paroxysms of expansion which successively occurred as portions of its molten interior approached solidification, supply us with a rational cause to which such vast ejective and upheaving phenomena may be assigned. Many features of terres- trial geology likewise require such an expansive force whereby to explain them; we therefore venture to recommend this source and cause of ejective action to the careful consideration of geologists. When the molten substratum had burst its confines, ejected its superfluous matter, and produced the resulting volcanic features, it would, after final solidification, resume the normal process of contraction upon cooling, and so retreat or shrink away from the external shell. Let us now consider what would be the result of this. Evidently the external shell or crust would become relatively too large to remain at all points in close contact with the subjacent matter. The consequence of too large a solid shell having to accommodate itself to a shrunken body under- neath, is that the skin, so to term the outer stratum of solid matter, becomes shrivelled up into alternate ridges and depressions, or wrinkles. In its attempt to crush down and follow the contracting substratum, it would have to displace the superabundant or superfluous material of its former larger surface by thrusting it (by the action of tangential force) into c Fig. 6. J pTLt -■» t t c Fig. 7. undulating ridges as in Fig. 6, or broken elevated ridges as in Fig. 7, or chap, in.] SUBSEQUENT COOLING OF IGNEOUS BODY. 29 overlappings of the outer crust as in Fig. 8, or ridges capped by more or less fluid molten matter extruded from beneath, as indicated in Fig. 9, a class of action which might occur contemporaneously with the elevation of the ridge or subsequently to its formation. Fig. 8. c Fig. 9. A long-kept shrivelled apple affords an apt illustration of this wrinkle theory ; another example may be observed in the human face and hand, when age has caused the flesh to shrink and so leave the comparatively unshrinking skin relatively too large as a covering for it. We illustrate both of these examples by actual photographs of the respective objects, which are reproduced on Plate II. Whenever an outer covering has to accommodate and apply itself to an interior body that has become too small for it, wrinkles are inevitably produced. The same action that shrivels the human skin into creases and wrinkles, has also shrivelled certain regions of the igneous crust of the earth. A map of a moun- tainous part of our globe affords abundant evidence of such a cause having been in action; such maps are pictures of wrinkles. Several parts of the lunar surface, as we shall by-and-by see, present us with the same appearances in a modified degree. 30 THE MOON. [chap. hi. To the few primary causes we have set forth in this chapter — to the alternate expansion and contraction of successive strata of the lunar sphere, when in a state of transition from an igneous and molten to a cooled and solidified condition, we believe we shall be able to refer well nigh all the remarkable and characteristic features of the lunar surface which will come under our notice in the course of our survey. LxJ f— < I Q_ Q_ CHAPTER IV. THE FORM, MAGNITUDE, WEIGHT, AND DENSIT7 OF THE LUNAR GLOBE. We have not hitherto had occasion to refer to what we may term the physical elements of the moon : by which we mean the various data concerning form, size, weight, density, &c. of that body, derived from observation and calculation. To this purpose, therefore, we will now devote a few pages, confining ourselves to such matters as specially bear upon the requirements of our subject, omitting such as are irrelevant to our purpose, and touching but lightly upon such as are commonly known, or are explained in ordinary elementary treatises on astronomy. First, then, as regards the form of the moon. The form of the lunar disc, when fully illuminated, we perceive to be a perfect circle ; that is to say, the measured diameters in all directions are equal ; and we are there- fore led to infer that the real form of the moon is that of a perfect sphere. We know that the earth and the rest of the planets of our system are spheroidal, or more or less flattened at the poles, and we also know that this flattening is a consequence of axial rotation ; the extent of the flattening, or the oblateness of the spheroid, depending upon the speed of that rotation. But in the case of the moon the axial rotation is so slow that the flattening produced thereby, although it must exist, is so slight as to be imperceptible to our observation. We might therefore conclude that the moon is a perfectly spherical body, did not theory step in to show us that there is another cause by which its form is disturbed. Assuming the moon to have been once in a fluid state, it is demonstrable that the attraction of the earth would accumulate a mass of matter, like a tidal elevation, in the direction of a line joining the centres of the two 32 THE MOON". [ohap. it. bodies : and as a consequence, the real shape of the moon must be an ellipsoid, or somewhat egg-shaped body, the major axis of which is directed towards the earth. That some such phenomenon has obtained is evident from the coincidence of the times of orbital revolution and axial rotation of the lunar sphere. " It would be against all probability," says Laplace, " to suppose that these two motions had been at their origin perfectly equal ; " but it is sufficient that their primitive difference was but small, in which case the constant attraction by the earth of the protruberant part of the moon would, establish the equality which at present exists. It is, however, sufficient for all purposes with which we are concerned to regard the moon as a sphere, and the next point to be considered is its size. To determine this, two data are necessary — its apparent or angular diameter, and its distance from the earth. The first of these is obtained by measuring the angle comprised between two lines directed from the eye to two opposite " limbs " or edges of the moon. If, for instance, we were to take a pair of compasses and, placing the joint at the eye, open out the legs till the two points appear to touch two opposite edges of the moon, the two legs would be inclined at an angle which would represent the diameter of the moon, and this angle we could measure by applying a divided arc or protractor to the compasses. In practice this measurement is made by means of telescopes attached to accurately divided circles ; the difference between the readings of the circle when the telescope is directed to opposite limbs of the moon giving its angular diameter at the time of the observation. But from the fact that the orbit of the moon is an ellipse, it is evident that she is at some times much nearer to us than at others, and, as a consequence, her apparent magnitude is variable : there is also a slight variation depending upon the altitude of the moon at the time of the measure ; the mean diameter, however, or the diameter at mean distance from the centre of the earth has, from long course of observation, been found to be 31' 9". To convert this apparent angular diameter into real linear measure- ment, it is necessary to know either the distance of the moon from the earth, or in astronomical language as leading to a knowledge of that distance, what is the amount of the moon's parallax. Parallax, generally, chap, iv.] FORM, MAGNITUDE, WEIGHT, AND DENSITY. 33 is an apparent change of position of an object arising from change of the point of view. The parallax of a heavenly body is the angle which the earth would subtend if it were seen from that body. Supposing an observer on the moon could measure the earth's angular diameter, just as we measure that of the moon, his measurement would represent what is called the parallax of the moon. But we cannot go to the moon to make such a measurement ; nevertheless there is a simple method, explained in most treatises on astronomy, which consists in observing the moon from stations on the earth widely separated, and by which we can obtain a precisely similar result. Without detailing the process, it is sufficient for us to know that the angle which would be subtended by the earth if seen from the moon, or the moon's parallax, is according to the latest deter- mination, equal to 1° 54' 5". This value, however, varies considerably with the variations of distance due to the elliptic orbit of the moon : the number we have given represents the mean parallax, or the parallax at mean distance. But we have to turn these angular measurements into miles. To effect this we have only to work a simple rule of three sum. It will easily be understood that, as the angular diameter of the earth seen from the moon is to the angular diameter of the moon seen from the earth, so is the diameter of the earth in miles to the diameter of the moon in miles. The diameter of the earth we know to be 7912 miles : putting this therefore in its proper place in the proportion sum, and duly working it out by the schoolboy's rule, we get : — miles. miles. 1° . 54' . 5" : 31' .9" : : 7912 : 2160. And 2160 miles is therefore the diameter of the lunar globe. Knowing the diameter, we can easily obtain the other elements of magnitude. According to the well-known relation of the diameter of a sphere to its area, we find the area of the moon to be 14,657,000 square miles : or half that number, 7,328,500 miles, as the area of the hemisphere at any one time presented to our view. And similarly, from the relation of the solidity of a sphere to its diameter, we find the solid contents of the moon to be 5276 millions of cubic miles of matter. 34 THE MOON. [OHAP. IV. Comparing these data with corresponding dimensions of the earth, we find that the diameter of the moon is ~ ; the area j^J^ ; and the volume ^^ t of the respective elements of the earth. Those who prefer a graphical to a numerical comparison, may judge of the sizes of the two bodies by the accompanying illustration (Fig. 10). Fig. 10. To gain an idea of their distance from each other it is necessary to suppose the two discs in the diagram to be five feet apart; the real distance of the moon from the earth being about 238,790 miles at its mean position. Next, we come to what is technically termed the mass, but what in common language we may call the weight of the moon. It is important to know this, because the weight of a body taken in connection with its size furnishes us with a knowledge of its density, or the specific gravity of the material of which it is composed. But it is not quite so easy to determine the mass as the dimensions of the moon : to measure it, we have seen is easy enough ; to weigh it is a comparatively difficult matter. To solve the problem we have to appeal to Newton's law of universal gravitation. This law teaches us that every particle of matter in the universe attracts every other particle with a force which is directly pro- portional to the mass, and inversely proportional to the square of the chap, iv.] FORM, MAGNITUDE, WEIGHT, AND DENSITY. 35 distance of the attracting particles. There are several methods by which this law is applied to the measurement of the mass of the moon. One of the simplest is by the agency of the Tides. We know that the moon, attracting the waters, produces a certain amount of elevation of the aqueous covering of the earth ; and we know that the sun produces also a like elevation, but to a much smaller extent, by reason of its much greater distance. Now measuring accurately the heights of the solar and lunar tides, and making allowance for the difference of distance of the sun and moon from the earth, we can compare directly the effect that is due to the sun with the effect that is due to the moon : and since the masses of the two bodies are just in proportion to the effects they produce, it is evident that we have a comparison between the mass of the sun and that of the moon ; and knowing what is the sun's mass we can, by simple proportion, find that of the moon. Another method is as follows : — The moon is retained in her orbital path by the attraction of the earth ; if it were not for this attraction she would fly off from her course in a tangential line. She has thus a constant tendency to quit her orbit, which the earth's attraction as constantly overcomes. It is evident from this that the earth pulls the moon towards itself by a definite amount in every second of time. But while the earth is pulling the moon, the moon is also pulling the earth : they are pulling each other together; and moreover each is exerting a pull which ia proportional to its mass. Knowing, then, the mass of the earth, which we do with con- siderable accuracy, we can find what share of the whole pulling force is due to it, the residue being the moon's share : the proportion which this residue bears to the earth's share gives us the proportion of the moon's mass to that of the earth, and hence the mass of the moon. There are yet two other methods : one depending upon the phe- nomena of nutation, or the attraction of the sun and moon upon the protruberant matter of the terrestrial spheroid; and the other upon a displacement of the centre of gravity of the earth and moon, which shows itself in observations of the sun. By each and all of these methods has the lunar mass been at various times determined, and it has been found, as the latest and best accepted value, that the mass of the moon is one-eightieth that of the earth. f 2 36 THE MOON. [chap. it. From the known diameter of the earth we ascertain that its volume is 259,360 millions of cubic miles : and from the various experiments that have been made to determine the mean density of the earth, it has been found that that mean density is about 5j times that of water ; that is to say, the earth weighs 5j times heavier than would a sphere of water of equal size. Now a cubic foot of water weighs 62'3211 pounds, and from this w T e can find by simple multiplication what is the weight of a cubic mile of water, and, similarly, what would be the weight of 259,360 cubic miles of water, and the last result multiplied by 5-J will give the weight of the earth in tons : The calculation, although extremely simple, involves a confusing heap of figures ; but the result, which is all that concerns us, is, that the weight of the earth is 5842 trillions of tons : and since, as we have above stated, the mass of the earth is 80 times that of the moon, it follows that the weight of the moon is 73 trillions of tons. The cubical contents of a body compared with its weight gives us its density. In the moon we have 5276 millions of cubic miles of matter, the total weight of which is 73 trillions of tons. Now, 5276 millions of cubic miles of water would weigh about 21 J trillions of tons ; and as this number is to 73 as 1 is to 3*4, it is clear that the density of the lunar matter is 3*4 greater than water : and inasmuch as the earth is 5^ times denser than water, we see that the moon is about 0*62 as dense as the earth, or that the material of the moon is lighter, bulk for bulk, than the mean material of the terraqueous globe in the proportion of 62 to 100, or, nearly, 6 to 10. This specific gravity of the lunar material (3*4) we may remark is about the same as that of flint glass or the diamond : and curiously enough it nearly coincides with that of some of the aerolites that have from time to time fallen to the earth ; hence support has been claimed for the theory that these bodies were originally fragments of lunar matter, probably ejected at some time from the lunar volcanoes with such force as to propel them so far within the sphere of the earth's attraction that they have ultimately been drawn to its surface. Keverting, now, to the mass of the moon : we must bear in mind that the mass or weight of a planetary body determines the weight of all objects on its surface. What we call a pound on the earth, would not be a pound on the moon ; for the following reason : — When we say that chap, iv.] FORM, MAGNITUDE, WEIGHT, AND DENSITY. 37 such and such an object weighs so much, we really mean that it is attracted towards the earth with a certain force depending upon its own weight. This attraction we call gravity ; and the falling of a weight to the earth is an example of the action of the law of universal gravitation. The earth and the weight fall together — or are held together if the weight is in contact with the earth — with a force which depends directly upon the mass of the two, and upon the distance between them. Newton proved that the attraction of a sphere upon external objects is precisely as if the whole of its matter were contained at its centre. So that the attractive force of the earth upon a ton weight at its surface, is the attraction which 5842 trillions of tons exert upon one ton situated 3956 miles (the radius of the earth) distant. If the weight of the earth were only half the above quantity, it is clear that the attraction would be only half what it is ; and hence the ton weight, being pulled by only half the force, would only be equal to half a ton ; that is to say, only half as much muscular force (or any other force but gravity) would be required to lift it. It is plain, therefore, that what weighs a pound on the earth could not weigh a pound on the moon, which is only ~ of the weight of the earth. What, then, is the relation between a pound on the earth and the same mass of matter on the moon ? It would seem, since the moon's mass is ^ of the earth, that the pound transported to the moon ought to weigh the eightieth part of a pound there ; and so it would if the distance from the centre of the moon to its surface were the same as the distance of the centre of the earth from its surface. But the radius of the moon is only -^ that of the earth ; and the force of gravity varies inversely as the square of the distance between the centres of the gravitating masses. So that the attraction by the moon of a body at its surface, as compared with that of the earth, is i divided by the square of ■— ; and this, worked out, is equal to J. The force of gravity upon the moon is, therefore, ^ of that on the earth; and hence a pound upon the earth would be little more than 2\ ounces on the moon ; and it follows as a consequence that any force, such as muscular exertion, or the energy of chemical, plutonic or explosive forces, would be six times more effective upon the moon than upon the earth. A man who could jump six feet from the earth, could with the same 38 THE MOON. [chap iv. *i muscular effort jump thirty-six feet from the moon ; the explosive energy that would project a body a mile above the earth would project a like body six miles above the surface of the moon. It is the practice, in elementary and popular treatises on astronomy, to state merely the numerical results in giving data such as those embodied in the foregoing pages; and uninitiated readers, not knowing the means by which the figures are arrived at, are sometimes disposed to regard them with a certain amount of doubt or uncertainty. On this account we have thought it advisable to give, in as brief and concise a form as possible, the various steps by which these seemingly unattainable results are obtained. The data explained in the foregoing text are here collected to facilitate reference. Diameter of Moon . . . 2160 miles .... -±- that of earth. 3663 13424 Area 14,657,000 square miles Area of the visible hemisphere 7,328,500 square miles Solid contents . . . . 5276 millions of cubic miles . -}— 49186 Mass 73 trillions of tons i Density 3'39 (water =1) . . . 0'62 Force of gravity at surface . . . . . . . | Mean distance from earth . . . 238j790 miles. CHAPTER V. ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR ATMOSPHERE. At the close of the preceding chapter we stated that any force acting in opposition to that of gravity would be six times more effective on the moon than on the earth. But, in fact, it would in many cases be still more so ; at all events, so far as projectile forces are concerned ; for the reason that "the powerful coercer of projectile range," as the earth's atmosphere has been termed, has no counterpart, or at most a very dis- proportionate one, upon the moon. The existence of an atmosphere surrounding the moon has been the subject of considerable controversy, and a great deal of evidence on both sides of the question has been offered from time to time, and is to be found scattered through the records of various classes of observations. Some of the more important items of this evidence it is our purpose to set forth in the course of the present chapter. With the phenomena of the terrestrial atmosphere, with the effects that are attributable to it, we are all well familiar, and our best course therefore is to examine, as far as we are able, whether counterparts of any of these effects are manifested upon the moon. For instance, the clouds that are generated in and float through our air would, to an observer on the moon, appear as ever changing bright or dusky spots, obliterating certain of the permanent details of the earth's surface, and probably skirting the terrestrial disc, like the changing belts we perceive on the planet Jupiter, or diversifying its features with less regularity, after the manner exhibited by the planet Mars. If such clouds existed on the moon it is evident that the details of its surface must be, from time to time, similarly obscured ; but no trace of such obscuration has ever been 40 THE MOON. [chap. v. detected. When the moon is observed with high telescopic powers, all its details come out sharp and clear, without the least appearance of change or the slightest symptoms of cloudiness other than the occasional want of general definition, which may be proved to be the result of un- steadiness or want of homogeneity in our own atmosphere; for we must tell the uninitiated that nights of pure, good definition, such as give the astronomer opportunity of examining with high powers the minute details of planetary features, are very few and far between. Out of the three hundred and sixty-five nights of a year there are probably not a dozen that an astronomer can call really fine : usually, even on nights that are to all common appearance superbly brilliant, some strata of air of different densities or temperatures, or in rapid motion, intervene between the observer and the object of his observation, and through these, owing to the ever-changing refractions which the rays of light coming from the object suffer in their course, observation of the delicate markings of a planet is impossible : all is blurred and confused, and nothing but bolder features can be recognized. It has in consequence sometimes happened that a slight indistinctness of some minute detail of the moon has been attributed to clouds or mists at the lunar surface, whereas the real cause has been only a bad condition of our own atmosphere. It may be con- fidently asserted that when all indistinctness due to terrestrial causes is taken account of or eliminated, there remain no traces whatever of any clouds or mists upon the surface of the moon. Thi3 is but one proof against the existence of a lunar atmosphere, and, it may be argued, not a very conclusive one ; because there may still be an atmosphere, though it be not sufficiently aqueous to con- dense into clouds and not sufficiently dense to obscure the lunar details. The probable existence of an atmosphere of such a character used to be in- ferred from a phenomenon seen during total eclipses of the sun. On these occasions the black body of the moon is invariably surrounded by a luminous halo, or glory, to which the name " corona " has been applied ; and, further, besides this corona, apparently floating in it and sometimes seemingly attached to the black edge of the moon, are seen masses of cloud- like matter of a bright red colour, which, from the form in which they were first seen and from their flame-like tinge, have become universally CHAP. V.] NON-EXISTEXCE OF A LUNAR ATMOSPHERE. 41 known as the " red-flames." It used to be said that this corona could only be the consequence of a lunar atmosphere lit up as it were by the sun's rays shining through it, after the manner of a sunbeam lighting up the atmosphere of a dusty chamber ; and the red flames were held by those who first observed them to be clouds of denser matter floating in the said atmosphere, and refracting the red rays of solar light as our own clouds are seen to do at sunrise and sunset. But the evidence obtained, both by simple telescopic observation and by the spectroscope, from recent extensively observed eclipses of the sun has set this question quite at rest; for it has been settled finally and indisputably that both the above appearances pertain to the sun, and have nothing whatever to do with the moon. The occurrence of a solar eclipse offers other means in addition to the foregoing whereby a lunar atmosphere would be detected. We know that all gases and vapours absorb some portion of any light which may shine through them. If then our satellite had an atmosphere, its black nucleus when seen projected against the bright sun in an eclipse would be sur- rounded by a sort of penumbra, or zone of shadow, in contact with its edge, somewhat like that we have shown in an exaggerated degree in the annexed cut (Fig. 11), and the passage of this penumbra over solar spots Fig. 11. and other features of the solar photosphere would to some extent obscure the more minute details of such features. No such dusky band has how- ever been at any time observed. On the contrary, a band somewhat brighter than the general surface of the sun has frequently been seen in G 42 THE MOON. [chap. v. contact with the black edge of the moon : this in its turn was held to indicate an atmosphere about the moon ; but Sir George Airy has shown that a lunar atmosphere, if it really did exist, could not produce such an appearance, and that the cause of it must be sought in other directions. If this effect were really due to the passage of the solar rays through a lunar atmosphere a similar effect ought to be produced by the passage of the sun's rays through the terrestrial atmosphere : and we might hence expect to see the shadow of the earth projected on the moon during a lunar eclipse surrounded by a sort of bright zone or halo : we need hardly say such an appearance has never manifested itself. Similarly as we stated that the delicate details of solar spots would be obscured by a lunar atmosphere, small stars passing behind the moon would suffer some diminution in brightness as they approached apparent contact with the moon's edge : this fading has been watched for on many occasions, and in a few cases such an appearance has been suspected, but in by far the majority of instances nothing like a diminution of brightness or change of colour of the stars has been seen ; stars of the smallest magnitude visible under such circumstances retain their feeble lustre unimpaired up to the moment of their disappearance behind the moon's limb. Again, in a solar eclipse, even if there were an atmosphere about the moon not sufficiently dense to form a hazy outline or impair the distinct- ness of the details of a solar spot, it would still manifest its existence in another way. As the moon advances upon the sun's disc the latter assumes, of course, a crescent form. Now if air or vapour enveloped the moon, the exceedingly delicate cusps of this crescent would be distorted or turned out of shape. Instead of remaining symmetrical, like the lower one in the annexed drawing (Fig. 12), they would be bent or deformed after the manner we have shown in the upper one. The slightest symptom of a distortion like this could not fail to obtrude itself upon an observer's eye ; but in no instance has anything of the kind been seen. Keverting to the consequences of the terrestrial atmosphere : one of the most striking of these is the phenomenon of diffused daylight, which we need hardly remind the reader is produced by the scattering or diffusion of the sun's rays among the minute particles of vapour composing or contained in that atmosphere. Were it not for this reflexion and diffusion chap, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 43 of the sun's light, those parts of our earth not exposed to direct sunshine would be hidden in darkness, receiving no illumination beyond the feeble amount that might be reflected from proximate terrestrial objects Fig. 12. actually illuminated by direct sunlight. Twilight is a consequence of this •reflexion of light by the atmosphere when the sun is below the horizon. If, then, an atmosphere enveloped the moon, we should see by diffused light those parts of the lunar details that are not receiving the direct solar beams ; and before the sun rose and after it had set upon any region of the moon, that region would still be partially illuminated by a twilight. But, on the contrary, the shadowed portions of a lunar landscape are pitchy black, without a trace of diffused-light illumina- tion, and the effects that a twilight would produce are entirely absent from the moon. Once, indeed, one observer, Schroeter, noticed some- thing which he suspected was due to an effect of this kind : when the moon exhibited itself as a very slender crescent, he discovered a faint crepuscular light, extending from each of the cusps along the circumfer- ence of the unenlightened part of the disc, and he inferred from estimates of the length and breadth of the line of light that there was an atmosphere about the moon of 5376 feet in height. This is the only instance on record, we believe, of such an appearance being seen. Spectrum analysis would also betray the existence of a lunar atmo- sphere. The solar rays falling on the moon are reflected from its surface to the earth. If, then, an atmosphere existed, it is plain that the solar rays must first pass through such atmosphere to reach the reflecting G 2 44 THE MOON. [chap. v. surface, and returning from thence, again pass through it on their way to the earth ; so that they must in reality pass through virtually twice the thickness of any atmosphere that may cover the moon. And if there be any such atmosphere, the spectrum formed by the moon's light, that is, by the sun's light reflected from the moon, would be modified in such a manner as to exhibit absorption-lines different from those found in the spectrum of the direct solar rays, just as the absorption-lines vary according as the sun's rays have to pass through a thinner or a denser stratum of the terrestrial atmosphere. Guided by this reasoning, Drs. Huggins and Miller made numerous observations upon the spectrum of the moon's light, which are detailed in the " Philosophical Transactions " for the year 1864 ; and their result, quoting the words of the report, was " that the spectrum analysis of the light reflected from the moon is wholly negative as to the existence of any considerable lunar atmo- sphere." Upon another occasion, Dr. Huggins made an analogous observation of the spectrum of a star at the moment of its occultation, which observation he records in the following words : — " When an observation is made of the spectrum of a star a little before, or at the moment of its occultation by the dark limb of the moon, several phenomena characteristic of the passage of the star's light through an atmosphere might possibly present themselves to the observer. If a lunar atmosphere exist, which either by the substances of which it is composed, or by the vapours diffused through it, can exert a selective absorption upon the star's light, this absorption would be indicated to us by the appearance in the spectrum of new dark lines immediately before the star is occulted by the moon." "If finely divided matter, aqueous or otherwise, were present about the moon, the red rays of the star's light would be enfeebled in a smaller degree than the rays of higher refrangibilities." " If there be about the moon an atmosphere free from vapour, and possessing no absorptive power, but of some density, then the spectrum would not be extinguished by the moon's limb at the same instant throughout its length. The violet and blue rays would lie behind the red rays." chap, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 45 " I carefully observed the disappearance of the spectrum of e Piscium at its occultation of January 4, 1865, for these phenomena ; but no signs of a lunar atmosphere were detected." But perhaps the strongest evidence of the non-existence of any appreciable lunar atmosphere is afforded by the non-refraction of the light of a star passing behind the edge of the lunar disc. Refraction, we know, is a bending of the rays of light coming from any object, caused by their passage through strata of transparent matter of different densities ; we have a familiar example in the apparent bending of a stick when half plunged into water. There is a simple schoolboy's experiment which illustrates refraction in a very cogent manner, but which we should, from its very simplicity, hesitate to recall to the reader's mind did it not very aptly represent the actual case we wish to exemplify. A coin is placed on the bottom of an empty basin, and the eye is brought into such a position that the coin is just hidden behind the basin's rim. Water is then poured into the basin and, without the eye being moved from its former place, as the depth of water increases, the coin is brought by degrees fully into view ; the water refracting or turning out of their course the rays of light coming from the coin, and lifting them, as it were, over the edge of the basin. Now a perfectly similar phenomenon takes place at every sunrise and sunset on the earth. When the sun is really below the horizon, it is nevertheless still visible to us because it is hvught up by the refraction of its light by the dense stratum of atmosphere through which the rays have to pass. The sun is, therefore, exactly represented by the coin at the bottom of the basin in the boy's experiment, the atmosphere answers to the water, and the horizon to the rim or edge of the basin. If there were no atmosphere about the earth, the sun would not be so brought up above the horizon, and, as a consequence, it would set earlier and rise later by about a minute than it really does. This, of course, applies not merely to the sun, but to all celestial bodies that rise and set. Every planet and every star remains a shorter time below the horizon than it would if there were no atmosphere surrounding the earth. To apply this to the point we are discussing. The moon in her orbital course across the heavens is continually passing before, or 46 THE MOON". [chap. v. occulting, some of the stars that so thickly stud her apparent path. And when we see a star thus pass behind the lunar disc on one side and come out again on the other side, we are virtually observing the setting and rising of that star upon the moon. If, then, the moon had an atmosphere, it is clear, from analogy to the case of the earth, that the star must disappear later and reappear sooner than if it has no atmosphere : just as a star remains too short a time below the earth's horizon, or behind the earth, in consequence of the terrestrial atmosphere, so would a star remain too short a time behind the moon if an atmosphere surrounded that body. The point is settled in this way : — The moon's apparent diameter has been measured over and over again and is known with great accuracy ; the rate of her motion across the sky is also known with perfect accuracy : hence it is easy to calculate how long the moon will take to travel across a part of the sky exactly equal in length to her own diameter. Supposing, then, that we observe a star pass behind the moon and out again, it is clear that, if there be no atmosphere, the interval of time during which it remains occulted ought to be exactly equal to the com- puted time which the moon would take to pass over the star. If, however, from the existence of a lunar atmosphere, the star disappears too late and reappears too soon, as we have seen it would, these two intervals will not agree; the computed time will be greater than the observed time, and the difference, if any there be, will represent the amount of refraction the star's light has sustained or suffered, and hence the extent of atmosphere it has had to pass through. Comparisons of these two intervals of time have been repeatedly made, the most recent and most extensive was executed under the direction of the Astronomer-Eoyal several years ago, and it was based upon no less than 296 occupation observations. In this determination the measured or telescopic semidiameter of the moon was compared with the semidiameter deduced from the occultations, upon the above principle, and it was found that the telescopic semidiameter was greater than the occupation semidiameter by two seconds of angular measurement or by about a thousandth part of the whole diameter of the moon. Sir George Airy, commenting on this result, says that it appears to him that the origin of this difference is to be sought in one of two causes. " Either chap, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 47 it is due to irradiation* of the telescopic semidiameter, and I do not doubt that a part at least of the two seconds is to be ascribed to that cause ; or it may be due to refraction by the moon's atmosphere. If the whole two seconds were caused by atmospheric refraction this would imply a horizontal refraction of one second, which is only jjjg part of the earth's horizontal refraction. It is possible that an atmosphere competent to produce this refraction would not make itself visible in any other way." This result accords well, considering the relative accuracy of the means employed, with that obtained a century ago by the French astronomer Du Sejour, who made a rigorous examination of the subject founded on observations of the solar eclipse of 1764. He concluded that the hori- zontal refraction produced by a possible lunar atmosphere amounted to 1" # 5 — a second and a half — or about -~^ of that produced by the earth's atmosphere. The greater weight is of course to be allowed to the more recent determination in consideration of the large number of accurate observations upon which it was based. But an atmosphere 2,000 times rarer than our air can scarcely be regarded as an atmosphere at all. The contents of an air-pump receiver can seldom be rarefied to a greater extent than to about -^ of the density of air at the earth's surface, with the best of pneumatic machines ; and the lunar atmosphere, if it exist at all, is thus proved to be twice as attenuated as what we are accustomed to recognise as a vacuum. In dis- cussing the physical phenomena of the lunar surface, we are, therefore, perfectly justified in omitting all considerations of an atmosphere, and adapting our arguments to the non-existence of such an appendage. And if there be no air upon the moon, we are almost forced to con- clude that there can be no water ; for if water covered any part of the lunar globe it must be vaporised under the influence of the long period of uninterrupted sunshine (upwards of 300 hours) that constitutes the lunar day, and would manifest itself in the form of clouds or mists obscur- ing certain parts of the surface. But, as we have already said, no such * Irradiation is an ocular phenomenon in virtue of which all strongly illuminated objects appear to the eye to be larger than they really are. The impression produced by light upon the retina appears to extend itself around the focal image formed by the lenses of the eye. It is from the effect of irradiation that a white disc on a black ground looks larger than a black disc of the same size on a white ground. 48 THE MOON. [chap. v. obliteration of details ever takes place ; and, as we have further seen, no evidence of aqueous vapour is manifested upon the occasion of spectrum observations. Since, then, the effects of watery vapour are absent, we are forced to conclude that the cause is absent also. Those parts of the moon which the ancient astronomers assumed, from their comparatively smooth and dusky appearance, to be seas, have long since been discovered to be merely extensive regions of less reflective surface material ; for the telescope reveals to us irregularities and asperities covering well nigh the whole of them, which asperities could not be seen if they were covered with water; unless, indeed, we admit the possibility of seeing to the bottom of the water, not only perpendicularly, but obliquely. Some observers have noticed features that have led them to suppose that water was at one time present upon the moon, and has left its traces in the form of appearances of erosive action in some parts. But if water ever existed, where is it now ? One writer, it is true, has suggested as possible, that whatever air, and we presume he would include whatever water also, the moon may possess, is hidden away in sublunarean caves and hollows ; but even if water existed in these places it must sometimes assume the vapour}' form, and thus make its presence known. Sir John Herschel pointed out that if any moisture exists upon the moon, it must be in a continual state of migration from the illumi- nated or hot, to the unilluminated or cold side of the lunar globe. The alternations of temperature, from the heat produced by the unmitigated sunshine of 14 days' duration, to the intensity of cold resulting from the absence of any sunshine whatever for an equal period, must, he argued, produce an action similar to that of the cryojjhorus in transporting the lunar moisture from one hemisphere to the other. The cryophorus is a little instrument invented by the late Dr. Wollaston ; it consists of two bulbs of glass connected by a bent tube, in the manner shown in the annexed illustration, fig. 13. One of the bulbs, A, is half-filled with water, and, all air being exhausted, the instrument is hermetically sealed, leaving nothing within but the water and the aqueous vapour which rises therefrom in the absence of atmospheric pressure. When the empty bulb, B, is placed in a freezing mixture, a rapid condensation of this vapour chap, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 49 takes place within it, and as a consequence the water in the bulb A gives off more vapour. The abstraction of heat from the water, which is a Fig. 13. natural consequence of this evaporation, causes it to freeze into a solid mass of ice. Now upon the moon the same phenomenon would occur did the material exist there to supply it. In the accompanying diagram let A represent the illuminated or heated hemisphere of the moon, and B Fig. 14. the dark or cold hemisphere ; the former being probably at a temperature of 300° above, and the latter 200° below Fahrenheit's zero. Upon the above principle, if moisture existed upon A it would become vaporised, and the vapour would migrate over to B, and deposit itself there as hoar- frost ; it would, therefore, manifest itself to us while in the act of migrating by clouding or dimming the details about the boundary of the illuminated hemisphere. The sun, rising upon any point upon the margin of the dark hemisphere, would have to shine through a bed of moisture, and we may justly suppose, if this were the case, that the tops of moun- tains catching the first beams of sunlight would be tinged with colour, or be lit up at first with but a faint illumination, just as we see in the case of terrestrial mountains whose summits catch the first, or receive the H 50 THE MOOS. [chap v. last beams of the rising or setting sun. Nothing of this kind is, however, perceptible : when the solar rays tip the lofty peaks of lunar mountains, these shine at once with brilliant light, quite as vivid as any of those parts that receive less horizontal illumination, or upon which the sun is almost perpendicularly shining. All the evidence, then, that we have the means of obtaining, goes to prove that neither air nor water exists upon the moon. Two compli- cating elements affecting all questions relating to the geology of the terraqueous globe we inhabit may thus be dismissed from our minds while considering the physical features of the lunar surface. Fire on the one hand and water on the other, are the agents to which the configura- tions of the earth's surface are referable : the first of these produced the igneous rocks that form the veritable foundations of the earth, the second has given rise to the superstructure of deposits that constitute the secondary and tertiary formations : were these last removed from the surface of our planet, so as to lay bare its original igneous crust, that crust, so far as reasoning can picture it to us, would probably not differ essentially from the visible surface of the moon. In considering the causes that have given birth to the diversified features of that surface, we may, therefore, ignore the influence of air and water action and confine our reasoning to igneous phenomena alone : our task in this matter, it is hardly necessary to remark, is materially simplified thereby. CHAPTER VI. THE GENERAL ASPECT OF THE LUNAR SURFACE. We have now reached that stage of our subject at which it behoves us to repair to the telescope for the purpose of examining and familiarising ourselves with the various classes of detail that the lunar surface presents to our view. That the moon is not a smooth sphere of matter is a fact that manifested itself to the earliest observers. The naked eye perceives on her face spots exhibiting marked differences of illumination. These variations of light and shade, long before the invention of the telescope, induced the belief that she possessed surface irregularities like those that diversify the face of the earth, and from analogy it was inferred that seas and continents alternated upon the lunar globe. It was evident, from the persistence and invariability of the dusky markings, that they were not due to atmospheric peculiarities, but were veritable variations in the character or disposition of the surface material. Fancy made pictures of these unchangeable spots : untutored gazers detected in them the indications of a human countenance, and perhaps the earliest map of the moon was a rough reproduction of a man's face, the eyes, nose and mouth representing the more salient spots discernible upon the lunar disc. Others recognised in these spots the configuration of a human form, head, arms and legs complete, which a French superstition that lingers to the present day held to be the image of Judas Iscariot transported to the moon in punishment for his treason. Again, an Indian notion connects the lunar spots with a representation of a roebuck or a hare, and hence the Sanskrit names for the moon, mrigadhara, a roebuck-bearer, and 'sa'sabhrit, a hare-bearer. Of these similitudes the h 2 52 THE MOON. [chap. vi. one which has the best pretensions to a rude accuracy is that first mentioned ; for the resemblance of the full moon to a human countenance, wearing a painful or lugubrious expression, is very striking. Our illustra- tion of the full moon (Plate III.) is derived from an actual photograph ; • the relative intensities of light and shade are hence somewhat exagge- rated ; otherwise it represents the full moon very nearly as the naked eye sees it, and by gazing at the plate from a short distance,")" the well-known features will manifest themselves, while they who choose may amuse themselves by arranging the markings in their imagination till they conform to the other appearances alluded to. We may remark in passing that by one sect of ancient writers the moon was supposed to be a kind of mirror, receiving the image of the earth and reflecting it back to terrestrial spectators. Humboldt affirmed that this opinion had been preserved to his day as a popular belief among the people of Asia Minor. He says, " I was once very much astonished to hear a very well educated Persian from Ispahan, who certainly had never read a Greek book, mention when I showed him the moon's spots in a large telescope in Paris, this hypothesis as a widely diffused belief in his country : ' What we see in the moon/ said the Persian, ' is ourselves ; it is the map of our earth/ " Quite as extravagant an idea, though perhaps a more excusable one, was that held by some ancient philoso- phers, to the effect that the spots on the moon were the shadows of opaque bodies floating in space between it and the sun. An observer watching the forms and positions of the lunar face-marks, from night to night and from lunation to lunation, cannot fail to notice the circumstance that they undergo no easily perceptible change of position with respect to the circular outline of the disc ; that in fact the face of the moon presented to our view is always the same, or very nearly so. If the moon had no orbital motion we should be led from the above phenomenon to conclude that she had no axial motion, no movement of * For the original photograph from which this plate was produced, and for permission to reproduce it, we owe our acknowledgments to Warren De la Rue and Joseph Beck, Esquires. t The proper distance for realising the conditions under which the moon itself is seen will he that at which our disc is just covered by a wafer about a quarter of an inch in diameter, held at arm's length. This will subtend an angle of about half a degree, which is nearly the angular diameter of the moon. PLATE ! iWbodkory! FULL MOON chap, n.] GENERAL ASPECT OF THE LUNAR SURFACE. 53 rotation ; but when we consider the orbital motion in connection with the permanence of aspect, we are driven to the conclusion — one, however, which superficial observers have some difficulty in recognising — that the moon has an axial rotation equal in period to her orbital revolution. Since the moon makes the circuit of her orbit in twenty-seven days and one-third (more exactly 27d. 7h. 43m. lis.) it follows that this is the time of her axial rotation, as referred to the stars, or as it would be made out by an observer located at a fixed position in space outside the lunar orbit. But if referred to the sun this period appears different ; because the moon while revolving round the earth is, with the earth, circulating around the sun. Suppose the three bodies, moon, earth, and sun, to be in a line at a certain period of a lunation, as they are at full moon : by the time the moon has completed her twenty-seven days' journey around the earth, the latter will have moved along twenty-seven days' march of its orbit, which is about twenty-seven degrees of celestial longitude : the sun will apparently be that much distant from a straight line passing through earth and moon, and the moon must therefore move forward to overtake the sun before she can assume the full phase again. She will take something over two days to do this ; hence the solar period of her revolution becomes more than twenty-nine days (to be exact, 29d. 12h. 44m. 2s. '87). This is the length of a solar day upon the moon — the interval from one sunrise to another at any spot upon the equator of our satellite, and the interval between successive reappearances of the same phase to observers on the earth. The physical cause of the coincidence of times of rotation and revolution was touched upon in a previous chapter. We have said that the moon continuously presents to us the same hemisphere. This is generally true, but not entirely so. Galileo, by long scrutiny, familiarised himself with every detail of the lunar disc that came within the limited grasp of his telescopes, and he recognised the fact that according as the position of the moon varied in the sky, so the aspect of her face altered to a slight degree ; that certain regions at the edge of her disc alternately came in sight and receded from his view. He perceived, in fact, an apparent rocking to and fro of the globe of the moon; a sort of balancing or libratory motion. When the moon was 54 THE MOON. [chap. vi. near the horizon he could see spots upon her uppermost edge, which disappeared as she approached the zenith, or highest point of her nightly path ; and as she neared this point, other spots, before invisible, came into view, near to what had been her lower edge. Galileo was not long in referring this phenomenon to its true cause. The centre of motion of the moon being the centre of the earth, it is clear that an observer on the sur- face of the latter, looks down upon the rising moon as from an eminence, and thus he is enabled to see more or less over or around her. As the moon increases in altitude, the line of sight gradually becomes parallel to the line joining the observer and the centre of the earth, and at length he looks her full in the face : he loses the full view and catches another side face view as she nears the horizon in setting. This phenomenon, occurring as it does, with a daily period, is known as the diurnal libration. But a kindred phenomenon presents itself in another period, and from another cause. The moon rotates upon her axis at a speed that is rigorously uniform. But her orbital motion is not uniform, sometimes it is faster, and at other times slower than its average rate. Hence, the angle through which she moves along her orbit in a given time, now exceeds, and now falls short of the angle through which she turns upon her axis. Her visible hemisphere thus changes to an extent depending upon the difference between these orbital and axial angles, and the apparent balancing thus produced is called the libration in longitude. Then there is a libration in latitude due to the circumstance that the axis of the moon is not exactly perpendicular to the plane of her orbit ; the effect of this inclination being, that we sometimes see a little more of the north than of the south polar regions of our satellite, and vice versd* * The libratory movement has been taken advantage of, at the suggestion of Sir Chas. Wheatstone, for producing stereoscopic photographs of the moon. In the early days of stereo- scopic photography the object to te photographed was placed upon a kind of turn-table, and, after a picture had been taken of it in one position, the table was turned through a small angle for the taking of the second picture ; the two placed side by side then represented the object as it would have been seen by two eyes widely separated, or whose visual rays inclined at an angle equal to that through which the table was turned ; and when the pictures were viewed through a stereoscope, they combined to produce the wonderful effect of solidity now familiar to every one. The moon, by its librations, imitates the turn-table movement ; and, from a large number of photographs of her, taken at different points of her orbit and at different seasons of the year, it is possible to select two which, while they exhibit the same phase of illumination, at the same time present the requisite difference in the points of view from which they are taken chap, ft ] GENERAL ASPECT OF THE LUNAR SURFACE. 55 The extent of the "moon's librations, taking them all and in com- bination into account, amounts to about seven degrees of arc of latitude or longitude upon the moon, both in the north-south and east-west directions. And taking into account the whole effect of them, we may conclude that our view of the moon's surface, instead of being confined to one half, is extended really to about four-sevenths of the whole area of the lunar globe. The remaining three-sevenths must for ever remain a terra incognita to the habitants of this earth, unless, indeed, from some catastrophe which it would be wild fancy to anticipate, a period of rotation should be given to the moon different from that which it at present possesses. Some highly fanciful theorists have speculated upon the possible condition of the invisible hemisphere, and have propounded the absurd notion that the opposite side of the moon is hollow, or that the moon is a mere shell ; others again have urged that the hidden half is more or less covered with water, and others again that it is peopled with inhabi- tants. There is, however, no good reason for supposing that what we may call the back of the moon has a physical structure essentially different from the face presented towards us. So far as can be judged from the peeps that libration enables us to obtain, the same characteristic features (though of course with different details) prevail over the whole lunar surface. The speculative ideas held by the philosophers of the pre- telescopic age, touching the causes which produced the inequalities of light and shade upon the moon, received their coup de grdce from the revelations of Galileo's glasses. Our satellite was one of the earliest objects, if not actually the first, upon which the Florentine turned his telescope ; and he found that the inequalities upon her surface were due to differences to give the effect of stereoscopicity when viewed binocularly. Mr. De la Rue, the father of celestial photography, has been enabled to produce several such pairs of pictures from the vast collection of lunar photographs that he has accumulated. Any one of these pairs of portraits, when stereoscopically combined, reproduces, to quote the words of Sir John Herschel, " the spherical form just as a giant might see it whose stature were such that the interval between his eyes should equal the distance between the place where the earth stood wben one view was taken, and that to which it would have to be removed' (our moon being fixed) to get the other. Nothing can surpass the impression of real corporeal form thus conveyed by some of these pictures as taken by Mr. De la Rue with his powerful reflector, the production of which (as a step in some sort taken by man outside of the planet he inhabits) is one of the most remarkable and unexpected triumphs of scientifie art." 56 THE MOON. [chap. vi. in its configuration analogous to the continents and islands, and (as might then have been thought) the seas of our globe. He could trace, even with his moderate means, the semblance of mountain-tops upon which the sun shone while their lower parts were in shadow, of hills that were brightly illuminated upon their sides towards the sun, of brightly shining elevations, and deeply shadowed depressions, of smooth plains, and regions of mountainous ruggedness. He saw that the boundary of sunlight upon the moon was not a clearly defined line, as it would be if the lunar globe were a smooth sphere, as the Aristotelians had asserted, but that the terminator was uneven and broken into an irregular outline. From these observations the Florentine astronomer concluded that the lunar world was covered not only with mountains like our globe, but with mountains whose heights far surpassed those existing upon the earth, and whose forms were strangely limited to circularity. Galileo's best telescopes magnified only some thirty times, and the views which he thus obtained, must have been similar to those exhibited by the smaller photographs of the moon produced in late years by Mr. De la Eue and now familiar to the scientific public. Of course there is in the natural moon as viewed with a small telescope a vivid brilliancy which no art can imitate, and in photographs especially there is a tendency to exaggeration of the depths of shade in a lunar picture. This arises from the circumstance that various regions of the moon do not impress a chemically sensitized plate as they impress the retina of the eye. Some portions, notably the so-called " seas " of the moon, which to the eye appear but slightly duller than the brighter parts, give off so little actinic light that they appear as nearly black patches upon a photograph, and thus give an undue impression of the relative brightness of various parts of the lunar surface. Doubtless by sufficient exposure of the plate in the camera-telescope the dark patches might be rendered lighter, but in that case the more strongly illuminated portions, which after all are those most desirable to be preserved, would be lost by the effect which photo- graphers understand as " solarization." In speaking of a view of the moon with a magnifying power of thirty, it is necessary to bear in mind that the visible features will differ considerably with the diameter of the object-glass of the chap, vi.] GENERAL ASPECT OF THE LUNAR SURFACE. 57 telescope to which this power is applied. The same details would not be seen alike with the same power upon an object-glass of 10 inches diameter and one of 2 inches. The superior illumination of the image in the former case would bring into view minute details that could not be perceived with the smaller aperture. He who would for curiosity wish to see the moon, or any other object, as Galileo saw it, must use a telescope of the same size and character in all respects as Galileo's : it will not do to put his magnifying power upon a larger telescope. With large telescopes, and low powers used upon bright objects like the moon, there is a blinding flood of light which tends to contract the pupil of the eye and prevent the passage of the whole of the pencil of rays coming through the eye-piece. Although this last result may be pro- ductive of no inconvenience, it is clearly a waste of light, and it points to a rule that the lowest power that a telescope should bear is that which gives a pencil of light equal in diameter to the pupil of the eye under the circumstances of brightness attendant upon the object viewed. In observing faint objects this point assumes more importance, since it is then necessary that all available light should enter the pupil. The thought suggests itself that an artificial enlargement of the pupil, as by a dose of belladonna, might be of assistance in searching for faint objects, such as nebulae and comets : but we prefer to leave the experiment for those to try who pursue that branch of astronomical observation. A merely cursory examination of the moon with the low power to which we have alluded is sufficient to show us the more salient fea- tures. In the first place we cannot help being struck with the immense preponderance of circular or craterform asperities, and with the general tendency to circular shape which is apparent in nearly all the lunar surface markings ; for even the larger regions known as the " seas " and the smaller patches of the same character seem to repeat in their outlines the round form of the craters. It is at the boundary of sunlight on the lunar globe that we see these craterform spots to the best advantage, as it is there that the rising or setting sun casts long shadows over the lunar landscape, and brings elevations and asperities into bold relief. They vary greatly in size, some are so large as to bear an estimable proportion to the moon's diameter, and the smallest are so i 58 THE MOON. [chap. vi. minute as to need the most powerful telescopes and the finest conditions of atmosphere to perceive them. It is doubtful whether the smallest of them have ever been seen, for there is no reason to doubt that there exist countless numbers that are beyond the revealing powers of our finest telescopes. From the great number and persistent character of these circumval- lations, Kepler was led to think that they- were of artificial construction. He regarded them as pits excavated by the supposed habitants of the moon to shelter themselves from the long and intense action of the sun. Had he known their real dimensions, of which we shall have to speak when we come to describe them more in detail, he would have hesitated in propounding such a hypothesis; nevertheless it was, to a certain extent, justified by the regular and seemingly unnatural recurrence of one particular form of structure, the like of which is, too, so seldom met with as a structural feature of the surface of our own globe. The next most striking features, revealed by a low telescopic power upon the moon, are the seemingly smooth plains that have the appearance of dusky spots, and that collectively cover a considerable portion — about two-thirds — of the entire disc. The larger of these spots retain the name of seas, the term having been given when they were supposed to be watery expanses, and having been retained, possibly to avoid the confusion inevitable from a change of name, after the existence of water upon the moon was disproved. Following the same order of nomenclature, the smaller spots have received the appellations of lakes, bays, &n.d fens. We see that many of these " seas " are partially surrounded by ramparts or bulwarks which, under closer examination, and having regard to their real magnitude, resolve themselves into immense mountain chains. The general resemblance in form which the bulwarked plains thus exhibit to the circular craters of large size, would lead us to suppose that the two classes of objects had the same formative origin, but when we take into account the immense size of the former, and the process by which we infer the latter to have been developed, the supposition becomes untenable. Another of the prominent features which we notice as highly curious, and in some phases of the moon — at about the time of full — the most remarkable of all, are certain bright lines that appear to radiate from some chap, vi.] GENERAL ASPECT OF THE LUNAR SURFACE. 59 of the more conspicuous craters, and extend for hundreds of miles around. No selenological formations have so sorely puzzled observers as these peculiar streaks, and a great deal of fanciful theorizing has been bestowed upon them. As we are now only glancing at the moon, we do not enter upon explanations concerning them or any other class of details ; all such will receive due consideration in their proper order in succeeding chapters. We thus see that the classes of features observable upon the moon are not great in number : they may be summed up as craters and their central cones, mountain chains, with occasional isolated peaks, smooth plains, with more or less of irregularity of surface, and bright radiating streaks. But when we come to study with higher powers the individual examples of each class we meet with considerable diversity. This is especially the case with the craters, which appear under very numerous variations of the one order of structure, viz., the ring-form. A higher telescopic power shows us that not only do these craters exist of all magnitudes within a limit of largeness, but seemingly with no limit of smallness, but that in their structure and arrangement they present a great variety of points of difference. Some are seen to be considerably elevated above the sur- rounding surface, others are basins hollowed out of that surface and with low surrounding ramparts ; some are merely like walled plains or amphitheatres with flat plateaux, while the majority have their lowest point of hollowness considerably below the general level of the surrounding surface ; some are isolated upon the plains, others are aggregated into a thick crowd, and overlapping and intruding upon each other ; some have elevated peaks or cones in their centres, and some are without these central cones, while the plateaux of others again contain several minute craters instead ; some have their ramparts whole and perfect, others have them breached or malformed, and many have them divided into terraces, especially on their inner sides. In the plains, what with a low power appeared smooth as a water surface becomes, under greater magnification, a rough and furrowed area, here gently undulated and there broken into ridges and declivities, with now and then deep rents or cracks extending for miles and spreading like river-beds into numerous ramifications. Craters of all sizes and classes i 2 60 THE MOON. [chap. vj. are scattered over the plains ; these appear generally of a different tint to the surrounding surface, for the light reflected from the plains has been observed to be slightly tinged with colour. The tint is not the same in all cases : one large sea has a dingy greenish tinge, others are merely grey, and some others present a pale reddish hue. The cause of this diversity of colour is mysterious ; it has been supposed to indicate the existence of vegetation of some sort ; but this involves conditions that we know do not exist. The mountains, under higher magnification, do not present such diversity of formation as the craters, or at least the points of difference are not so apparent ; but they exhibit a plentiful variety of combinations. There are a few perfectly isolated examples that cast long shadows over the plains on which they stand like those of a towering cathedral in the rising or setting sun. Sometimes they are collected into groups, but mostly they are connected into stupendous chains. In one of the grandest of these chains, it has been estimated that a good telescope will show 3000 mountains clustered together, without approach to symmetrical order. The scenery which they would present, could we get any other than the " bird's eye view " to which we are confined, must be imposing in the extreme, far exceeding in sublime grandeur anything that the Alps or the Himalayas offer ; for while on the one hand the lunar mountains equal those of the earth in altitude, the absence of an atmosphere, and consequently of the effects produced thereby, must give rise to alternations of dazzling light and black depths of shade combining to form panoramas of wild scenery that, for want of a parallel on earth, we may well call unearthly. But we are debarred the pleasure of actually contemplating such pictures by the circumstance that we look down upon the mountain tops and into the valleys, so that the great height and close aggregation of the peaks and hills are not so apparent. To compare the lunar and terrestrial mountain scenery would be " to compare the different views of a town seen from the car o£ a balloon with the more interesting prospects by a progress through the streets." Some of the peculiarities of the lunar scenery we have, however, endeavoured to realize in a subse- quent Chapter. A high power gives us little more evidence than a low one upon the chap, vi.] GENERAL ASPECT OF THE LUNAR SURFACE. 61 nature of the long bright streaks that radiate from some of the more conspicuous craters, but it enables us to see that those streaks do not arise from any perceptible difference of level of the surface — that they have no very definite outline, and that they do not present any sloping sides to catch more sunlight, and thus shine brighter, than the general surface. Indeed, one great peculiarity of them is that they come out most forcibly where the sun is shining perpendicularly upon them ; hence they are best seen where the moon is at full, and they are not visible at all at those regions upon which the sun is rising or setting. We also see that they are not diverted by elevations in their path, as they traverse in their course craters, mountains, and plains alike, giving a slight additional brightness to all objects over which they pass, but producing no other effect upon them. To employ a commonplace simile, they look as though, after the whole surface of the moon had assumed its final con- figuration, a vast brush charged with a whitish pigment had been drawn over the globe in straight lines radiating from a central point, leaving its trail upon everything it touched, but obscuring nothing. Whatever may be the cause that produces this brightness of certain parts of the moon without reference to configuration of surface, this cause has not been confined to the formation of the radiating lines, for we meet with many isolated spots, streaks and patches of the same bright character. Upon some of the plains there are small areas and lines of luminous matter possessing peculiarities similar to those of the radiating streaks, as regards visibility with the high sun, and invisibility when the solar rays fall upon them horizontally. Some of the craters also are sur- rounded by a kind of aureole of this highly reflective matter. A notable specimen is that called Linne, concerning which a great hue and cry about change of appearance and inferred continuance of volcanic action on the moon was raised some years ago. This object is an insignificant little crater of about a mile or two in diameter, in the centre of an ill-defined spot of the character referred to, and about eight or ten miles in diameter. With a low sun the crater alone is visible by its shadow ; but as the luminary rises the shadow shortens and becomes all but invisible, and then the white spot shines forth. These alternations, complicated by variations of atmospheric condition, and by the interpretations of different 62 THE MOON. [chap, vi. observers, gave rise to statements of somewhat exaggerated character to the effect that considerable changes, of the nature of volcanic eruptions, were in progress in that particular region of the moon. In the foregoing remarks we have alluded somewhat indefinitely to high powers ; and an enquiring but unastronomical reader may reason- ably demand some information upon this point. It might have been instructive to have cited the various details that may be said to come into view with progressive increases of magnification. But this would be an all but impossible task, on account of the varying conditions under which all astronomical observations must necessarily be made. When we come to delicate tests, there are no standards of telescopic power and definition. Assuming the instrument to be of good size and high optical character, there is yet a powerful influant of astronomical definition in the atmosphere and its variable state. Upon two-thirds of the clear nights of a year the finest telescopes cannot be used to their full advantage, because the minute flutterings resulting from the passage of the rays of light through moving strata of air of different densities are magnified just as the image in the telescope is magnified, till all minute details are blurred and confused, and only the grosser features are left visible. And supposing the telescope and atmosphere in good state, there is still an important point, the state of the observer's eye, to be considered. After all it is the eye that sees, and the best telescopic assistance to an untrained eye is of small avail. The eye is as susceptible of education and develop- ment as any other organ ; a skilful and acute observer is to a mere casual gazer what a watchmaker would be to a ploughman, a miniature painter to a whitewasher. This fact is not generally recognized ; no man would think of taking in hand an engraver's burin, and expecting on the instant to use it like an adept, or of going to a smithy and without previous preparation trying to forge a horse-shoe. Yet do folks enter observatories with uneducated eyes, and expect at once to realise all the wonderful things that their minds have pictured to themselves from the perusal of astronomical books. We have over and over again remarked the dissatisfaction which attends the first looks of novices through a powerful telescope. They anticipate immediately beholding wonders, and they are disappointed at finding how little they can see, and how far short chap, vi.] GENERAL ASPECT OF THE LUNAR SURFACE. 63 the sight falls of what they had expected. Courtesy at times leads them to express wonder and surprise, which it is easy to see is not really felt, but sometimes honesty compels them to give expression to their disappointment. This arises from the simple fact that their eyes are not fit for the work which is for the moment imposed upon them ; they know not what to look for, or how to look for it. The first essay at telescopic gazing, like first essays generally, serves but to teach us our incapability. To a tutored eye a great deal is visible with a comparatively low power, and practised observers strive to use magnifying powers as low as possible, so as to diminish, as far as may be, the evils arising from an untranquil atmosphere. With a power so small as 30 or 40, many exceedingly delicate details on the moon are visible to an eye that is familiar with them under higher powers. With 200 we may say that every ordinary detail will come out under favourable conditions ; but when minute points of structure, mere nooks and corners as it were, are to be scrutinised, 300 may be used with advantage. Another hundred diameters almost passes the practical limit. Unless the air be not merely fine, but superfine, the details become " clothy " and tremulous ; the extra points brought out by the increased power are then only caught by momentary glimpses, of which but a very few are obtained during a lengthy period of persistent scrutiny. We may set down 250 as the most useful, and 350 the utmost effective power that can be employed upon the particular work of which we are treating. Could every detail on the moon be thoroughly and reliably represented as this amount of magnifi- cation shows it, the result would leave little to be wished for. But it may be asked by some, what is the absolute effect of such powers as those we have spoken of, in bringing the moon apparently nearer to our eyes? and what is the actual size of the smallest object visible under the most favourable circumstances ? A linear mile upon the moon corresponds to an angular interval of 0'87 of a second ; this refers to regions about the centre of the disc ; near the circumference the foreshortening makes a difference, very great as the edge is approached. Perhaps the smallest angle that the eye can without assistance appreciate is half a minute ; that is to say, an object that subtends to the eye an 64 THE MOON. [chap. vi. arc of less than half a minute can scarcely be seen.* Since there are 60 seconds in a minute, it follows that we must magnify a spot a second in diameter upon the moon thirty times before we can see it ; and since a second represents rather more than a mile, really about 2000 yards, on the moon, as seen from the earth, the smallest object visible with a power of 30 will be this number of yards in diameter or breadth. To see an object 200 yards across, we should require to magnify it 300 times, and this would only bring it into view as a point ; 20 yards would require a power of 3000, and 1 yard 60,000 to effect the same thing. Since, as -we have said, the highest practicable power with our present telescopes, and at ordinary terrestrial elevations, is 350, or for an extreme say 400, it is evident that the minutest lunar object or detail of which we can perceive as a point must measure about 150 yards : to see the form of an object, so as to discriminate whether it be round or square, it would require to be probably twice this size ; for it may be safely assumed that we cannot perceive the outline of an object whose average breadth subtends a less angle than a minute. Arago put this question into another shape : — The moon is distant from us 237,000 miles (mean). A magnifying power of a thousand would show us the moon as if she were distant 237 miles from the naked eye. 2000 would bring her within 118 miles. 4000 „ „ „ 59 „ 6000 „ „ „ 39 „ Mont Blanc is visible to the naked eye from Lyons, at the distance of about 100 miles; so that to see the mountains of the moon as Mont Blanc is seen from Lyons would require the impracticable power of 2500. * This is a point of some uncertainty. Dr. Young stated (Lectures Vol. II. p. 575) that " a minute is perhaps nearly the smallest interval at which two objects can be distinguished, although a line subtending only a tenth of a minute in breadth may sometimes be perceived as a single object." CHAPTER VII. TOPOGRAPHY OF THE MOON. It is scarcely necessary to seek the reasons which prompted astro- nomers, soon after the invention of the telescope, to map the surface features of the moon. They may have considered it desirable to record the positions of the spots upon her disc, for the purpose of facilitat- ing observations of the passage of the earth's shadow over them in lunar eclipses ; or they may have been actuated by a desire to register appearances then existing, in order that if changes took place in after years these might be readily detected. Scheiner was one of the earliest of lunar cartographers ; he worked about the middle of the seventeenth century ; but his delineations were very rough and exaggerated. Better maps — the best of the time, according to an old authority — were engraved by one Mellan, about the years 1634 or 1635. At about the same epoch, Langreen and Hevelius were working upon the same subject. Langreen executed some thirty maps of portions of the moon, and introduced the practice of naming the spots after philosophers and eminent men. Hevelius spent several years upon his task, the results of which he published in a bulky volume containing some 50 maps of the moon in various phases, and accompanied by 500 pages of letter-press. He rejected Langreen's system of nomenclature, and called the spots after the seas and continents of the earth to which he conceived they bore resemblance. Eiccioli, another selenographer, whose map was compiled from observations made by Grimaldi, restored Langreen's nomenclature, but he confined himself to the names of eminent astronomers, and his system has gained the adhesion of the map-makers of later times. Cassini prepared a large map from his own observations, and it was 66 THE MOON. [chap. vn. engraved about the year 1692. It appears to have been regarded as a standard work, for a reduced copy of it was repeatedly issued with the yearly volumes of the Connaissance desTemps, (the "Nautical Almanack" of France) some time after its publication. These small copies have no great merit : the large copper plate of the original was, we are told by Arago, who received the statement from Bouvard, sold to a brazier by a director of the French Government Printing-Office, who thought proper to disembarrass the stores of that establishment, by ridding them of what he considered lumber ! La Hire, Mayer, and Lambert, followed during the succeeding century, in this branch of astronomical delineation. At the commencement of the present century, the subject was very earnestly taken up by the indefatigable Schroeter, who, although he does not appear to have produced a complete map, produced a topograph of the moon in *a large series of partial maps and drawings of special features. Schroeter was a fine observer, but his delineations show him to have been an indifferent draughtsman. Some of his drawings are but the rudest representations of the objects he intended to depict; many of the bolder features of conspicuous objects are scarcely recognizable in them. A bad artist is as likely to mislead posterity as a bad historian, and it cannot be surprising if observers of this or future generations, accepting Schroeter's drawings as faithful representations, should infer from them remarkable changes in the lunar details. It is much to be regretted that Schroeter's work should be thus depreciated. Lohrman of Dresden, was the next cartographer of the moon ; in 1824 he put forth a small but very excellent map of 15 inches diameter, and published a book of descriptive text, accompanied by sectional charts of particular areas. His work, however, was eclipsed by the great one which we owe to the joint energy of MM. Beer and Maedler, and which represents a stupendous amount of observing work carried on during several years prior to 1836, the date of their publication. The long and patient labour bestowed upon their map and upon the measures on which it depends, deserve the highest praise which those conversant with the subject can bestow, and it must be very long before their efforts can be superseded. Beer and Maedler's map has a diameter of 37 inches : it represents the chap, vii.] TOPOGRAPHY OF THE MOOtf. 67 phase of the moon visible in the condition of mean libration. The details were charted by a careful process of triangulation. The disc was first divided into "triangles of the first order," the points of which (con- spicuous craters) were accurately laid down by reference to the edges of the disc : one hundred and seventy-six of these triangles, plotted accu- rately upon an orthographic projection of the hemisphere, formed the reliable basis for their charting work. From these a great number of ".points of the second order" were laid down, by measuring their distance and angle of position with regard to points first established. The skeleton map thus obtained was filled up by drawings made at the telescope : the diameters of the measureable craters being determined by the micrometer. Beer and Maedler also measured the heights of one thousand and ninety-five lunar mountains and crater-summits : the resulting measures are given in a table contained in the comprehensive text-book which accompanies their map. These heights are found by one of two methods, either by measuring the length of the shadow which the object casts under a known elevation of the sun above its horizon, or by measuring the distance between the illuminated point of the mountain and the " termi- nator" in the following manner. In the annexed figure (Fig. 15) let the circle represent the moon and m a mountain upon it : let s A be the line of direction of the sun's rays, passing the normal surface of the moon at A and just tipping the mountain top. A will be the terminator, and there will be darkness between it and the star-like mountain summit M. The distance between A and M is measured : the distance a b is known, for it is the moon's radius. And since the line s Mis a tangent to the circle the angle b a m is a right angle. We know the length of its two sides ab, am, and we can therefore by the known properties of the right-angled triangle find the length of the hypothenuse bm : and since bm is made up of the radius ba plus the mountain height, we have only to subtract the moon's radius from the ascertained whole length of the hypothenuse and we have the height of the mountain. MM. Beer and Maedler exhibited their measures in French toises : in the heights we shall have occasion to quote, these have been turned into English feet, upon the assumption that the toise is equal to K 2 68 THE MOON. [CHAP. VII. G'39 English feet. The nomenclature of lunar features adopted by Beer and Maedler is that introduced by Kiccioli : mountains and features hitherto undistinguished were named by them after ancient and modern Fig. 15. philosophers, in continuance of Kiccioli's system, and occasionally after terrestrial features. Some minute objects in the neighbourhood of large and named ones were included under the name of the large one and distinguished by Greek or Roman letters. The excellent map resulting from the arduous labours of these astro- nomers is simply a map : it does not pretend to be a picture. The asperities and depressions are symbolized by a conventional system of shading and no attempt is made to exhibit objects as they actually appear in the telescope. A casual observer comparing details on the map with the same details on the moon itself would fail to identify or recognize them except where the features are very conspicuous. Such an observer would be struck by the shadows by which the lunar objects reveal themselves : he would get to know them mostly by their shadows, PLATE IV JNasmyth Woodbury; PICTURE MAP OF' THE MOON Skeleton Map of Moon To Accompany Picture Map, Chap : VI PLATE Published by John Murray. AlbemarU Street J*iccaAUlv Ymcent Brooks Day * Soi CHAP. VII.] TOPOGRAPHY OF THE MOON. 69 since it is mainly by those that their forms are revealed to a terrestrial observer. But such a map as that under notice indicates no shadows, and objects have to be identified upon it rather by their positions with regard to one another or to the borders of the moon than by any notable features they actually present to view. This inconvenience occurred to us in our early use of Beer and Maedler's chart, and we were induced to prepare for ourselves a map in which every object is shown somewhat, if imperfectly, as it actually appears at some period of a lunation. This was done by copying Beer and Maedler's outlines and filling them up by appropriate shading. To do justice to our task we enlarged our map to a diameter of six feet. Upon a circle of this diameter the positions and dimensions of all objects were laid down from the German original. Then from our own observations we depicted the general aspect of each object : and we so adjusted the shading that all objects should be shown under about the same angle of illumination — a condition which is never fulfilled upon the moon itself, but which we consider ourselves justified in exhibiting for the purpose of conveying a fair impression of how the various lunar objects actually appear at some one or other part of a lunation. The picture-map thus produced has been photographed to the size convenient for this work : and in order to make it available for the iden- tification of such objects as we may have occasion to refer to, we have placed around it a co-ordinate scale of arbitrary divisions by which any object can be found as by the latitude and longitude divisions upon a common geographical map. We have also prepared a skeleton map which includes the more conspicuous objects, and which faces the picture map. (Plates IV. and V.) The numbers on the skeleton map are those given in the second column of the accompanying table. The table also gives the co-ordinate positions of the various craters, the names of which are, for convenience of reference, printed in alphabetical order. Name. Number. Map Ordinate?. Name. Number. Map Ordinates. Abulfeda . . . 107 . . 30-0 120-7 Almanon . . . 94 . . 29-0 122-3 Agrippa . . . 151 . . 31-2 110-0 Alpetragius . . 92 . . 40*8 122-4 Airy . . . . 93 . . 34"7 1230 Alphonsus . 110 . . 39-6 120-9 Albategnius . . 109 . . 355 119-7 Apianus . . . 62 . . 33-6 129-3 Aliacensis . . . 61 . . 35-8 131-0 Apollonius . 154 . . 6-5 109-5 70 THE MOOK [CHAP. VII. Name. Number. Map Ordinates. Name. Number. Map Ordinates. Arago . . . . . 152 . . 24-7 108-7 Diophantus . . . 194 . . 55-5 96-3 Archimedes . . . 191 . . 40-3 95-8 Doppelmayer . . . 70 . . 58-6 129-6 Aristarchus . . . 176 . . 62-3 99-2 Aristillus . . . . 190 . . 37-0 933 Encke . . . . . 140 . . 59-7 110-6 Aristotle . . . . 209 . . 30'0 84-6 Endymion . . . . 227 . . 20-6 83-8 Arzachael . . . . 84 . . 39'5 124-0 Epigenes . . . . 223 . . 39-0 79-5 Atla3 . . . . . 228 . . 20*7 86-6 Erastothenes . . . 168 , . 44-6 104-0 Autolycus . . 189 . . 36-8 95-5 Eudoxus . . . . 208 . . 29-7 88-0 Azophi . . . . 76 . . 30-7 126-8 Fabricius . . . . 35 . . 20-0 136-8 Bacon . . . . . 17 . . 32 '5 142-0 Fernelius . . . . 37 . . 351 134-8 Baily . . . . . 207 . . 26-0 85-4 Firmicus . . . . 156 . . 5-8 107-7 Barocius . . .' . 34 . . 31 '8 138-5 Flamsteed . , . . 126 . . 62-8 114-5 Bessel . . . . . 179 . , 27-4 100-1 Fontana . . . . 122 . . 65-9 123-0 Bettinus . . . . 11 . . 48-8 144-9 Fontenelle , . 221 . . 43-0 81-3 Bianchini . . . . 215 . . 51-6 86'3 Fourier . . . . 67 . . 62-5 130-7 Billy . . . . . 121 . . 64-3 121-4 Fracastorius . . . 78 . . 20-5 127-0 Blancanus . . . . 12 . . 43'7 144*8 Furnerius . . , 52 . . 11-7 133-0 Bonpland . . . . 116 . , 48*5 117-6 Borda . . . . . 56 . . 15-2 131-0 Gambart . . . . 138 . . 47-2 112*2 Boscovich . . . . 160 . . 31-1 106-8 Gartner . . . 224 . . 26-5 82-3 Bouvard . . . . 40 . . 66-6 134-3 Gassendi . . . . 90 , . 59-7 123-3 Briggs . . . . . 196 , . 68-0 97-2 Gauricus . . . 46 . . 43-5 132-5 Bullialdus. . . . 86 . , 50-1 125-5 Gauss . . . . 201 . . 10-3 90-3 Burg . . . . . 206 . , 25'5 87-5 Gay Lussac . . 169 . . 50-1 103-8 Geber . . . . 83 . . 29-6 124-8 Calippus . . . . 199 . . 32-4 90-3 Geminus . . . 187 . . 13-0 93-0 Campanus . . 71 . . 52-3 129-0 Gerard . . . . 218 . . 63-7 88-8 Capella . . . . . 104 . . 17*8 118-0 Goclenius . . . 101 . . 11-8 118-5 Capuanus . . . . 43 . . 50-5 132'8 Godin . ... . 135 . . 313 111-7 Casatus . . . 7 . . 43'7 147-0 Grimaldi . . . . 125 . . 70-8 116-3 Cassini . . . . . 200 . . 35-5 89-7 Gruemberger . 6 . , 41-4 145-8 Catherina . . . . 95 . . 24-7 124-0 Guerike . . . 114 . . 46-5 119-6 Cavalerius . . 144 , . 712 109-5 Guttemberg . . . 102 . . 13-9 118-3 Cavendish . . . . 88 . . 63-5 127-4 . Cichus . . . . , 44 . , 47*3 132-8 Hainzel . . , . 39 . . 52-3 136-7 Clavius . . . . , 13 . . 41-8 143*5 Hansteen . , . 123 . . 65-5 119-9 Cleomides . . . . 183 . . 10-7 97-0 Hase . . . . . 54 . . 9-8 129-5 Colombo . . . . 98 . . 12-8 122-7 Heinsius . . . . 38 . . 45-5 136-0 Condamine . . , 214 . . 48-7 84-2 Helicon . . . . 212 . . 48-0 89-6 Condorcet . . . . 164 . . 4-5 104-7 Hell , 47 , , 41-7 131-6 Copernicus . . 147 . . 49-8 107-0 Hercules . . . . 229 . . 22-3 8&-7 Cyrillus . . , . 96 . . 23-5 121-3 Herodotus . . . . 175 . . 63 2 99-6 Herschel . . . . 112 . . 39-3 116-2 Damoiseau . . 124 . . 69-2 117-2 Hesiodus . . . . 64 . . 45-8 1300 Davy . . . . . 113 . . 43-2 119-8 Hevelius . . . . 141 . . 71-5 111-3 Deanibrel . . • . 129 . . 26-8 1135 Hippalus . . . . 87 . . 54-0 127-0 Delisle . . . . . 195 . . 55-7 95-2 Hommel . . . . 20 . . 26-8 140-0 Descartes . . . . 106 . . 28-5 119-3 Hyginus . . . . 158 . . 33-6 108-0 CHAP. VII.] TOPOGRAPHY OF THE MOON. 71 Name. Inghirami . Isidorus . Kant . . Kepler . . Kies . . . Kircher Klaproth . La Caille . Lagrange . La Hire Lalande Lambert . Landsberg . Langreen . Letronne . Licetus . . Lichtenberg Linnaeus . Littrow Lohrman . Longomontanus Lubiniezky Macrobius . Maginus . Mairan . . Manilius . Manzinus . Maraldi . . Marius . . Maskelyne . Mason . . Maupertius Maurolycus Menelaus . Mereator . Mersenius . Messala . . Messier . . Metius . . Moretus Moesting . Neander . Nearcbus . Newton Nonius . . Number. 27 , 103 . 105 , 146 72 10 8 74 68 177 117 193 127 100 120 21 197 188 185 143 23 91 , 182 22 , 217 , 167 4 . 181 . 171 . 132 . 204 . 213 . 33 . 165 . 65 . 89 . 202 . 131 . 36 5 . 128 . 57 . 18 1 . 49 Map Ordinates. 61-3 1389 167 118-0 25-8 60-0 49-7 475 43-5 375 670 543 434 49-6 54-0 6-3 620 341 66-5 317 20-5 71-3 45-7 51-3 137 40-0 56*7 32-2 313 18-6 65 19-5 23-7 487 31-8 28-3 51-4 61'7 140 10-8 18-8 39-5 41-6 18-7 26-8 41-0 365 118-5 108-0 128-8 145-8 146-7 126-8 1313 99-3 115-3 97-8 113-0 117-7 119 139-6 94-9 95-7 99-4 112-8 140-6 1235 100-2 140-4 89-5 1039 146-0 100-8 105-5 111-0 88-8 85-8 137-0 103 130 2 125-7 90-5 114-0 135-9 146-5 1132 1310 142-0 147-7 1332 Name. Olbers . Pallas . Parrot . Petavius Phocylides Piazzi . Picard . Piccolomini Pico . , Pitatus , Plana . Plato . Playfair Pliny . Poisson . Polybius Pontanus Posidonius Proclus . Ptolemy Purbacb Pytbagoraa Pytbeas Ramsden Eeamur Reiner . Reinhold Repsold Rbeita . Riccioli . Riccius . Ritter . Roemer . Sabine ". Sacrobosco Santbecb Saussure Scbeiner Schickard Schiller Scbroeter Schubert Segner . Seleucus Sharp . Number. Map Ordinates. 172 . . 73-0 107 7 149 . . 38-6 109-5 108 . . 35-8 121-6 80 . . 9-5 127-5 25 . . 55-5 141-6 41 . . 65-0 133-5 163 . . 8-3 104-7 58 . . 21-7 131-0 211 . . 41-9 87-3 63 . . 44-1 130-2 205 . . 24-8 88-8 210 . . 41-8 84-8 75 . . 33 5 127-5 165 . . 24-2 103-4 60 . . 32-8 1310 82 . . 24-5 125-6 59 . . 29 130-2 186 . . 22-2 94-3 162 . . 11-4 104-5 Ill . . 39-5 1182 73 . . 38-7 128-4 220 . . 53-0 812 178 . . 49-7 1004 42 . . 52-9 132-5 118 . . 373 114-6 145 . . 673 108-5 139 . . 51-5 111-2 219 . . 60-2 85-7 51 . . 16-1 134-2 142 . . 72-7 1138 50 . . 23-7 133 5 134 . . 26 111-6 184 . . 18-3 97 6 161 . . 25-0 105-3 133 . . 25 1120 77 . . 27-5 1277 79 . . 15-7 126-8 31 . . 396 137-7 14 . . 45-5 1435 28 . . 59-0 1375 24 . . 51-3 1410 137 . . 42-3 110-7 155 . . 2-3 110-8 16 . . 51-3 143-5 174 . . 69-0 998 216 . . 54-2 87-7 72 THE MOON. [CHAP. VII. Name. Number. Map Ordinates. Name. Number. Map Ordinates. Short .... 2 . . 39 7 147-4 Tycho 30 . . 430 142-3 Silberschlag . . 157 . . 32-0 1081 Simpelius . . . 3 . . 35 8 147-7 Ukert 159 . . 37-1 107-5 Snell .... . 55 . . 113 129 6 Soemmering . . . 136 . . 42-8 112-2 Vasco de Gama . . 173 . . 72-8 104-9 Stadius . . . 148 . . 45 6 107-0 Vendelinus . . . 99 . . 6-8 121-6 Stevinus . . . . 53 . . 119 130-7 69 . . 64-3 129-7 Stoefler . . . 32 . . 35 6 136 8 Vitello 66 . . 55-8 130-7 Strabo .... 226 . . 23 2 81-6 Vitruvius .... 180 . . 20-1 102-0 Struve .... 203 . . 183 88-7 Vlacq 19 . . 25-0 140-1 Taruntius . . . 153 . . 11-7 109-0 Walter 48 . . 37 8 1319 Taylor 130 . . 276 116-2 "Wargentin . . . 26 . . 57-5 140-2 Thales .... 225 . . 24-3 81-8 62 . . 36-4 129-3 Thebit 85 . . 40-8 126-8 Wilhelm Humboldt 81 . . 5-7 127-8 Theophilus . . 97 . . 22-3 120-0 Wilhelml. . . . 29 . . 45-9 1386 Timseus . . . . 222 . . 383 80-8 9 . . 45-7 146 4 Timocharis . . . 192 . . 45-1 97-0 Wurzelbauer . . . 45 . . 45-0 132-6 Tobias Mayer . 170 . . 545 103-0 Triesnecker . . . 150 . . 35 5 109-8 Zuchius .... 15 . . 50-7 144-2 The strong family likeness pervading the craters of the moon renders it unnecessary that we should attempt a description of each one of them or even of one in twenty. We have, however, thought that a few remarks upon the salient features of a few of the most important may be acceptable in explanation of our illustrative plates ; and what we have to say of the few may be taken as representative of the many. COPERNICUS, 147— (49-8— 107-0). Plate VIII. This may deservedly be considered as one of the grandest and most instructive of lunar craters. Although its vast diameter (46 miles) is exceeded by others, yet, taken as a whole, it forms one of the most impressive and interesting objects of its class. Its situation, near the centre of the lunar disc, renders all its wonderful details, as well as those of its immediately surrounding objects, so conspicuous as to establish it as a very favourite object. Its vast rampart rises to upwards of 12,000 feet above the level of the plateau, nearly in the centre of which stands a magnificent group of cones, three of them attaining the height of upwards of 2400 feet. The rampart is divided by concentric segmental terraced ridges, which chap, vii.] TOPOGRAPHY OF THE MOON. 73 present every appearance of being enormous landslips, resulting from the crushing of their over-loaded summits, which have slid down in vast segments and scattered their debris on to the plateau. Corresponding vacancies in the rampart may be observed from whence these prodigious masses have broken away. The same may be noticed, although in a somewhat modified degree, around the exterior of the rampart. In order to approach a realization of the sublimity and grandeur of this magnificent example of a lunar volcanic crater, our reader would do well to endeavour to fix his attention on its enormous magnitude and attempt to establish in his mind's eye a correct conception of the scale of its details as well as its general dimensions, which, as they so prodigiously transcend those of the largest terrestrial volcanic craters, require that our ideas as to mag- nitude of such objects should be, so to speak, educated upon a special standard. It is for this reason we are anxious our reader, when examining our illustrations, should constantly refer the objects represented in them to the scale of miles appended to each plate, otherwise a just and true conception of the grandeur of the objects will escape him. Copernicus is specially interesting, as being evidently the result of a vast discharge of molten matter which has been ejected at the focus or centre of disruption of an extensively upheaved portion of the lunar crust. A careful examination of the crater and the district around it, even to the distance of more than 100 miles on every side, will supply unmistakable evidence of the vast extent and force of the original disruption, manifested by a wonderfully complex reticulation of bright streaks which diverge in every direction from the crater as their common centre. These streaks do not appear on our plate, nor are they seen upon the moon except at and near the full phase. They show conspicuously, however, by their united lustre on the full moon, Plate III. Every one of those bright streaks, we conceive, is a record of what was originally a crack or chasm in the solid crust of the moon, resulting from some vastly powerful upheaving agency over the site of whose focus of energy Copernicus stands. The cracking of the crust must have been followed by the ejection of subjacent molten matter up through the reticulated cracks ; this, spreading somewhat on either side of them, has left these bright streaks as a visible record of the force and extent of the upheaval ; 74 . THE MOON. [chap. vii. while at the focus of disruption from whence the cracks diverge, the grand outburst appears to have taken place, leaving Copernicus as its record and result. Many somewhat radial ridges or spurs may be observed leading away from the exterior banks of the great rampart. These appear to be due to the more free egress which the extruded matter would find near the focus of disruption. The spur-ridges may be traced fining away for fully 100 miles on all sides, until they become such delicate objects as to approach invisibility. Several vast open chasms or cracks may be observed around the exterior of the rampart. They appear to be due to some action subsequent to the formation of the great crater — probably the result of contraction on the cooling of the crust, or of a deep-seated upheaval long subsequent to that which resulted in the formation of Copernicus itself, as they intersect objects of evidently prior formation. Under circumstances specially favourable for " fine vision," for upwards of 70 miles on all sides around Copernicus, myriads of comparatively minute but perfectly-formed craters may be observed. The district on the south-east side is specially rich in these wonderfully thickly scattered craters, which we have reason to suppose stand over or upon the reticulated bright streaks ; but, as the circumstances of illumination which are requisite to enable us to detect the minute craters are widely adverse to those which render the bright streaks visible, namely, nearly full moon for the one and gibbous for the other, it is next to im- possible to establish the fact of coincidence of the sites of the two by actual simultaneous observation. At the east side of the rampart, multitudes of these comparatively minute craters may also be detected, although not so closely crowded together as those on the west side ; but among those on the east may be seen myriads of minute prominences roughening the surface ; on close scrutiny these are seen to be small mounds of extruded matter which, not having been ejected with sufficient energy to cause the erupted material to assume the crater form around the vent of ejection, have simply assumed the mound form so well known to be the result of volcanic ejection of moderate force. Were we to select a comparatively limited portion of the lunar surface chap, vii.] TOPOGRAPHY OF THE MOON. 75 abounding in the most unmistakable evidence of volcanic action in every variety that can characterize its several phases, we could not choose one yielding in all respects such instructive examples as Copernicus and its immediate surroundings. GASSENDI, 90— (59'7— 123-3). Frontispiece. An interesting crater about 54 miles diameter ; the height of the most elevated portion of the surrounding wall from the plateau being about 9600 feet. The centre is occupied by a group of conical mountains, three of which are most conspicuous objects and rise to nearly 7000 feet above the level of the plateau. As in other similar cases, these central mountains are doubtless the result of the expiring effort of the eruption which had formed the great circular wall of the crater. The plateau is traversed by several deep cracks or chasms nearly one mile wide. Both the interior and exterior of the wall of the crater are terraced with the usual segmental ridges or landslips. A remarkable detached portion of the interior bank is to be seen on the east side, while on the west exterior of the wall may be seen an equally remarkable example of an outburst of lava subsequent to the formation of the wall or bank of the crater ; it is of conical form and cannot fail to secure the attention of a careful observer. Interpolated on the north wall of the crater may be seen a crater of about 18 miles diameter which has burst its bank in towards the great crater, upon whose plateau the lava appears to have discharged itself. The neighbourhood of Gassendi is diversified by a vast number of mounds and long ridges of exudated matter, and also traversed by enormous chasms and cracks, several of which exceed one mile wide and are fully 100 miles in length, and, as is usual with such cracks, traverse plain and mountain alike, disregarding all surface inequalities. Numbers of small craters are scattered around ; the whole forming an interesting and instructive portion of the lunar surface. l 2 76 THE MOON. [chap. vii. EUDOXUS, 208 (29-7—88-0), and ARISTOTLE, 209 (30-0—84-6). Plate X. Two gigantic craters, Eudoxus being nearly 35 miles in diameter and upwards of 11,000 feet deep, while Aristotle is about 48 miles in diameter,, and about 10,000 feet deep (measuring from the summit of the rampart to the plateau). These two magnificent craters present all the true volcanic characteristics in a remarkable degree. The outsides, as well as the insides of their vast surrounding walls or banks display- on the grandest scale the landslip feature, the result of the over- piling of the ejected material, and the consequent crushing down and crumbling of the substructure. The true eruptive character of the action which formed the craters is well evinced by the existence of the groups of conical mountains which occupy the centres of their circular plateaux, since these conical mountains, there can be little doubt, stand over what were once the vents from whence the ejected matter of the craters was discharged. On the west side of these grand craters may be seen myriads of comparatively minute ones (we use the expression " comparatively minute," although most of them are fully a mile in diameter). So thickly are these small craters crowded together, that counting them is totally out of *the question ; in our original notes we have termed them " Froth craters " as the most characteristic description of their aspect. The exterior banks of Aristotle are characterized by radial ridges or spurs : these are most probably the result of the flowing down of great currents of very fluid lava. To the east of the craters some very lofty mountains of exudation may be seen, and immediately beyond them an extensive district of smaller mountains of the same class, so thickly crowded together as under favourable illumination to present a multitude of brilliant points of light contrasted by intervening deep shade. On the west bank of Aristotle a very perfect crater may be seen, 27 miles in diameter, having all the usual characteristic features. About 40 miles to the east of Eudoxus there is a fine example of a crack or fissure extending fully 50 miles — 30 miles through a plain, and the remaining 20 miles cutting through a group of very lofty moun- tains. This great crack is worthy of attention, as giving evidence chap, vii.] TOPOGRAPHY OF THE MOOK 77 of the deep-seated nature of the force which occasioned it inasmuch as it disregards all surface impediments, traversing plain and group of moun- tains alike. There are several other features in and around these two magnificent craters well worthy of careful observation and scrutiny, all of them ex- cellent types of their respective classes. TRIESNEKER, 150 (35-5-109-8). Plate XI. A fine example of a normal lunar volcanic crater, having all the usual characteristic features in great perfection. Its diameter is about 20 miles, and it possesses a good example of the central cone and also of interior terracing. The most notable feature, however, in connection with this crater, and on account of which we have chosen it as a subject for one of our illustra- tions, is the very remarkable display of chasms or cracks which may be seen to the west side of it. Several of these great cracks obviously diverge from a small crater near the west external bank of the great one, and they subdivide or branch out, as they extend from the apparent point of divergence, while they are crossed or intersected by others. These cracks or chasms (for their width merits the latter appellation) are nearly one mile broad at the widest part, and after extending to fully 100 miles, taper away till they become invisible. Although they are not test objects of the highest order of difficulty, yet to see them with perfect distinctness requires an instrument of some perfection and all the con- ditions of good vision. When such are present, a keen and practised eye will find many details to rivet its attention, among which are certain portions of the edges of these cracks or chasms which have fallen in and caused interruptions to their continuity. THEOPHILUS, 97 (22-3—120-0). CYRILLUS, 96 (23-5—121-3). CATHARINA, 95 (24-7—124-0). Plate XII. These three magnificent craters form a very conspicuous group near the middle of the south-east quarter of the lunar disc. Their respective diameters and depths are as follows : — 78 THE MOON. [chap. vii. Theophilus, 64 miles diameter ; depth of plateau from summit of crater wall, 16,000 feet ; central cone, 5200 feet high. Cyrillus, 60 miles diameter; depth of plateau from summit of crater wall, 15,000 feet ; central cone, 5800 feet high. Catharina, 65 miles diameter ; depth of plateau from summit of crater wall, 13,000 feet ; centre of plateau occupied by a confused group of minor craters and debris. Each of these three grand craters is full of interesting details, presenting in every variety the characteristic features which so fascinate the attention of the careful observer of the moon's wonderful surface, and affording unmistakable evidence of the tremendous energy of the volcanic forces which at some inconceivably remote period piled up such gigantic formations. Theophilus by its intrusion within the area of Cyrillus shows in a very striking manner that it is of comparatively more recent formation than the latter crater. There are many such examples in other parts of the lunar disc, but few of so very distinct and marked a character. The flanks or exterior banks of Theophilus, especially those on the west side, are studded with apparently minute craters, all of which when carefully scrutinized are found to be of the true volcanic type of structure ; and minute as they are, by comparison, they would to a beholder close to them appear as very imposing objects ; but so gigantic are the more notable craters in the neighbourhood, that we are apt to overlook what are in themselves really large objects. It is only by duly training the mind, as we have previously urged, so as ever to keep before us the vast scale on which the volcanic formations of the lunar surface are displayed, that we can do them the justice which their intrinsic grandeur demands. We trust that our illustrations may in some measure tend to educate the mind's eye, so as to derive to the full the tranquil enjoyment which results from the study of the manifestation of one of the Creator's most potent agencies in dealing with the materials of his worlds, namely, volcanic force. So rich in wonderful features and characteristic details is this magnificent group and its neighbourhood, that a volume might be filled in the attempt to do justice, by description, to objects so full of suggestive subject for study. chap, vir.] TOPOGRAPHY OF THE MOON. 79 PTOLEMY, ALPHONS, ARZACHAEL, ETC. —Plate XLTI. Ill 110 84 The portion of the lunar surface comprised within the limits of this illustration being situated nearly in the centre of the moon's disc, is very favourably placed for revealing the multitude of interesting features and details therein represented. They consist of every variety of volcanic crater from " Ptolemy," whose vast rampart is eighty-six miles diameter, down to those whose dimensions are, comparatively, so minute as to render them at the extreme limits of visibility. Alphons and Arzachael, two of the next largest craters in our illustra- tion, situated immediately above Ptolemy, are sixty and fifty-five miles diameter respectively, and are possessed, in a remarkable degree, of all the distinctive characteristic features of lunar craters, having magnificent central cones, lofty ragged ramparts, together with very striking mani- festations of landslip formations as appear in the great segmental terraces within their ramparts, together with several minor craters interpolated on their plateau. " Alphons," the middle crater of this fine group, has its plateau specially distinguished by several cracks or chasms fully one mile wide, the direction or " strike " of which coincide in a very- remarkable manner with several other similar cracks which form con- spicuous features among the multitude of interesting details comprised within the limits of our illustration, — the most notable of these is an enormous straight cliff traversing the diameter of a low-ridged circular formation, seen in the upper right-hand corner of our plate. This great cliff is sixty miles long and from 1000 to 2000 feet high ; it is a well- known object to lunar observers, and has been termed " The Railway," on account of its straightness as revealed by the distinct shadow pro- jected by it on the plateau when seen under its sunrise aspect. The face of this vast cliff, although generally straight, is seen, when minutely scrutinized, to be somewhat serrated in its outline, while on its upper edge may be detected some very minute but perfectly formed craters. The existence of this remarkable cliff appears to be due either to an upheaval or a down-sinking of portion of the surface of the circular area across whose diameter it extends. To the right-hand side of the cliff are two small craters, from the side 80 THE MOON. [chap. vii. of which a fine example of a crack may be detected passing through in its course a low dome-formed hill; this crack is parallel to the cliff, having in that respect the same general strike or parallel direction which so remarkably distinguishes the other cracks observable in this portion of the moon's surface. On the left hand of this great cliff is situated a coneless crater, named "Thebit," on the right-hand rampart of which may be observed two small craters, the lesser of which is 2*75 miles diameter and has a central cone. We specially remark this fact, as it is the smallest lunar crater but one, in which we have, with perfect distinctness, detected a central cone. Not but that many smaller lunar craters exist possessed of this un- mistakable evidence of their volcanic origin ; but so minute are the specks of light which the central cones of such small craters reflect, that they, for that reason, most probably fail to reveal themselves. PLATO, 210 (41-8-81-8). Plate XIV. This crater, besides being a conspicuous object on account of its great diameter, has many interesting details in and around it requiring a fine instrument and favourable circumstances to render them distinctly visible. The diameter of the crater is 70 miles ; the surrounding wall or rampart varies in height from 4000 to upwards of 8000 feet, and is serrated with noble peaks which cast their black shadows across the plateau in a most picturesque manner, like the towers and spires of a great cathedral. Reference to our illustration will convey a very fair idea of this interest- ing appearance. On the north-east inside of the circular wall or rampart may be observed a fine example of landslip, or sliding down of a considerable mass of the interior side of the crater's wall. The landslip nature of this remarkable detail is clearly established by the fact of the bottom edge of the downslipped mass projecting in towards the centre of the plateau to a considerable extent. Other smaller landslip features may be seen, but none on so grand and striking a scale as the one referred to. A number of exceedingly minute craters may be detected on the surface of the plateau. The plateau itself is remarkable for its low reflective power, which causes it' to look like a dingy spot when Plato chap, vii.] TOPOGRAPHY OF THE MOON. 81 is viewed with a small magnifying power. The exterior of the crater wall is remarkable for the rugged character of its formation, and forms a great contrast in that respect to the comparatively smooth unbroken surface of the plateau, which by the way is devoid of a central cone. The surrounding features and objects indicated in our illustration are of the highest interest, and a few of them demand special description. THE VALLEY OF THE ALPS (37-0—86-0). Plate XIV. This remarkable object lays somewhat diagonally to the west of Plato; when seen with a low magnifying power, (80 or 100), it appears as a rut or groove tapering towards each extremity. It measures upwards of 75 miles long by about six miles wide at the broadest part. When examined under favourable circumstances, with a mag- nifying power of from 200 to 300, it is seen to be a vast flat-bottomed valley bordered by gigantic mountains, some of which attain heights upwards of 10,000 feet ; towards the south-east of this remarkable valley, and on both sides of it, are groups of isolated mountains, several of which are fully 8000 feet high. This flat-bottomed valley, which has retained the integrity of its form amid such disturbing forces as its immediate surroundings indicate, is one of the many structural enigmas with which the lunar surface abounds. To the north-west of the valley a vast number of isolated mounds or small mountains of exudation may be seen ; so numerous are they as to defy all attempts to count them with anything like exactness ; and among them, a power of 200 to 300 will enable an observer, under favourable cir- cumstances, to detect vast numbers of small but perfectly-formed craters. PICO, 211 (41-9— 87-3). Plate XIV, This is one of the most interesting examples of an isolated volcanic " mountain of exudation," and it forms a very striking object when seen under favourable circumstances. Its height is upwards of 8000 feet, and it is about three times as long at the base as it is broad. The summit is cleft into three peaks, as may be ascertained by the three-peaked shadow it casts on the plain. Five or six minute craters of very perfect M 82 THE MOON. [chap. vii. form may be detected close to the base of this magnificent mountain. There are several other isolated peaks or mountains of the same class within 30 or 40 miles of it which are well worthy of careful scrutiny, but Pico is the master of the situation, and offers a glorious subject for realizing a lunar day-dream in the mind's eye, if we can only by an effort of imagination conceive its aspect under the fiercely brilliant sun- shine by which it is illuminated, contrasted with the intensely black lunar heavens studded with stars shining with a steady brightness of which, by reason of our atmosphere intervening, we can have no ade- quate conception save by the aid of a well-directed imagination. TYCHO, 30 (43-0— 142-3). Plate XVI. This magnificent crater, which occupies the centre of the crowded group in our Plate, is 54 miles in diameter, and upwards of 16,000 feet deep, from the highest ridge of the rampart to the surface of the plateau, whence rises a grand central cone 5000 feet high. It is one of the most conspicuous of all the lunar craters, not so much on account of its dimensions as from its occupying the great focus of disruption from whence diverge those remarkable bright streaks, many of which may be traced over 1000 miles of the moon's surface, disregarding in their course all interposing obstacles. There is every reason to conclude that Tycho is an instance of a vast disruptive action which rent the solid crust of the moon into radiating fissures, which were subsequently occupied by extruded molten matter, whose superior luminosity marks the course of the cracks in all directions from the crater as their common centre of divergence. So numerous are these bright streaks when examined by the aid of the telescope, and they give to this region of the moon's surface such an extra degree of luminosity, that, when viewed as a whole, their locality can be distinctly seen at full moon by the unassisted eye as a bright patch of light on the southern portion of the disc. (See Plate III.) The causative origin of the streaks is discussed and illustrated in Chapter XL The interior of this fine crater presents striking examples of the concentric terrace-like formations that we have elsewhere assigned to vast landslip actions. Somewhat similar concentric terraces may be observed in other lunar craters ; some of these, however, appear to be the results chap, vii.] TOPOGRAPHY OF THE MOON. 83 of some temporary modification of the ejective force, which has caused the formation of more or less perfect inner ramparts : what we conceive to be true landslip terraces are always distinguished from these by their more or less fragmentary character. On reference to Plate III., showing the full moon, a very remarkable and special appearance will be observed in a dingy district or zone immediately surrounding the exterior of the rampart of Tycho, and of which we venture to hazard what appears to us a rational explanation : namely, that as Tycho may be considered to have acted as a sort of safety- valve to the rending and ejective force which caused, in the first instance, the cracking of this vast portion of the moon's crust — the molten matter that appears to have been forced up through these cracks, on rinding a comparatively free exit by the vent of Tycho, so relieved the district immediately around him as to have thereby reduced, in amount, the exit of the molten matter, and so left a zone comparatively free from the extruded lava which, according to our view of the subject, came up simultaneously through the innumerable fissures, and, spreading sideways along their courses, left everlasting records of the original positions of the radiating cracks in the form of the bright streaks which we now behold. " WAKGENTIN," 26 (57-5—140-2). Plate XVII. This object is quite unique of its kind — a crater about 53 miles across that to all appearance has been filled to the brim with lava that has been left to consolidate. There are evidences of the remains of a rampart, especially on the south-west portion of the rim. The general aspect of this extraordinary object has been not unaptly compared to a " thin cheese." The terraced and rutted exterior of the rampart has all the usual characteristic details of the true crater. The surface of the high plateau is marked by a few ridges branching from a point nearly in its centre, together with some other slight elevations and depressions ; these, however, can only be detected when the sun's rays fall nearly parallelto the surface of the plateau. To the north of this interesting object is the magnificent ring forma- tion Schickard, whose vast diameter of 123 miles contrasts strikingly with that of the sixteen small craters within his rampart, and equally m 2 84 THE MOON. [chap. m. so with a multitude of small craters scattered around. There are many objects of interest on the portion of the lunar surface included within our illustration, but as they are all of the usual type, we shall not fatigue the attention of our readers by special descriptions of them. ARISTARCHUS, 176 (6-3-99-2), and HERODOTUS, 175 (63 2— 99-6). Plate XVIII. These two fine examples of lunar volcanic craters are conspicuously situated in the north-east quarter of the moon's disc. Aristarchus has a circular rampart nearly 28 miles diameter, the summit of which is about 7500 feet above the surface of the plateau, while its height above the general surface of the moon is 2600 feet. A central cone having several subordinate peaks completes the true volcanic character of this crater : its rampart banks, both outside and inside, have fine examples of the segmental crescent-shaped ridges or landslips, which form so constant and charac- teristic a feature in the structure of lunar craters. Several very notable cracks or chasms may be seen to the north of these two craters. They are contorted in a very unusual and remarkable manner, the result probably of the force which formed them having to encounter very varying resistance near the surface. Some parts of these chasms gape to the width of two to three miles, and when closely scrutinized are seen to be here and there partly filled by masses which have fallen inward from their sides. Several smaller craters are scattered around, which, together with the great chasms and neighbouring ridges, give evidence of varied volcanic activity in this locality. We must not omit to draw attention to the parallelism or general similarity of " strike " in the ridges of extruded matter ; this appearance has special interest in the eyes of geologists, and is well represented in our illustration. Aristarchus is specially remarkable for the extraordinary capability which the material forming its interior and rampart banks has of reflecting light. Although there are many portions of the lunar surface which possess the same property, yet few so remarkably as in the case of Aristarchus, which shines with such brightness, as compared with its immediate surroundings, as to attract the attention of the most unpractised chap, vii.] TOPOGRAPHY OF THE MOON. 85 observer. Some have supposed this appearance to be due to active volcanic discharge still lingering on the lunar surface, an idea in which, for reasons to be duly adduced, we have no faith. Copernicus, in the remarkable bright streaks which radiate from it, and Tycho also, as well as several other spots, are apparently composed of material very nearly as highly reflective as that of Aristarchus. But the comparative isolation of Aristarchus, as well as the extraordinary light-reflecting property of its material, renders it especially noticeable, so much so as to make it quite a conspicuous object when illuminated only by earth-light, when but a slender crescent of the lunar disc is illuminated, or when, as during a lunar eclipse, the disc of the moon is within the shadow of the earth and is lighted only by the rays refracted through the earth's atmosphere. There are no features about Herodotus of any such speciality as to call for remark, except it be the breach of the north side of its rampart by the southern extremity of a very remarkable contorted crack or chasm, which to all appearance owes its existence to some great disruptive action subsequent to the formation of the crater. WALTER, 48 (37-8— 131-9), and adjacent Intkusive Craters. Plate XX. This Plate represents a southern portion of the moon's surface, measuring 170 by 230 miles. It includes upwards of 200 craters of all dimensions, from Walter, whose rampart measures nearly 70 miles across, down to those of such small apparent diameter as to require a well- practised eye to detect them. In the interior of the great crater Walter a remarkable group of small craters may be observed surrounding his central cone, which in this instance is not so perfectly in the centre of the rampart as is usually the case. The number of small craters which we have observed within the rampart is 20, exclusive of those on the rampart itself. The entire group represented in the Plate suggests in a striking manner the wild scenery which must characterize many portions of the lunar surface ; the more so if we keep in mind the vast proportions of the objects which they comprise, upon which point we may remark that the smallest crater represented in this Plate is considerably larger than that of Vesuvius. 86 THE MOON. [chap. vii. AKCHIMEDES, 191 (40-3— 95-8), AUTOLYCUS, 189 (36-8-95-5), APJSTILLUS, 190 (37-0— 93-3), and the APPENNINES. Plate IX. This group of three magnificent craters, together with their remarkable surroundings, especially including the noble range of mountains termed the Apennines, forms on the whole one of the most striking and interest- ing portions of the lunar surface. If the reader is not acquainted with what the telescope can reveal as to the grandeur of the effect of sun- rise on this very remarkable portion of the moon's surface, he should carefully inspect and study our illustration of it ; and if he will pay due regard to our previously repeated suggestion concerning the attached scale of miles, he will, should he have the good fortune to study the actual objects by the aid of a telescope, be well prepared to realize and duly appreciate the magnificence of the scene which will be presented to his sight. Were we to attempt an adequate detail description of all the interest- ing features comprised within our illustration, it would, of itself, fill a goodly volume ; as there is included within the space represented every variety of feature which so interestingly characterizes the lunar surface. All the more prominent details of types of their class ; and are so favour- ably situated in respect to almost direct vision, as to render their nature, forms, and altitudes above and depths below the average surface of the moon most distinctly and impressively cognizable. Archimedes is the largest crater in the group ; it has a diameter of upwards of 52 miles, measuring from summit to summit of its vast circular rampart or crater wall, the average height of which, above the plateau, is about 4300 feet ; but some parts of it rise considerably higher, and, in consequence, cast steeple-like shadows across the plateau when the sun's rays are intercepted by them at a low angle. The plateau of this grand crater is devoid of the usual central cone. Two comparatively minute but beautifully-formed craters may be detected close to the north-east interior side of the surrounding wall of the great crater. Both outside and inside of the crater wall may be seen magnificent examples of the landslip subsidence of its overloaded banks ; these landslips form vast concentric segments of the outer and inner chap, vii.] TOPOGRAPHY OF THE MOOK 87 circumference of the great circular rampart, and doubtless belong to its era of formation. Two very fine examples of cracks, or chasms, may be observed proceeding from the opposite external sides of the crater, and extending upwards of 100 miles in each direction ; these cracks, or chasms, are fully a mile wide at their commencement next the crater, and narrow away to invisibility at their further extremity. Their course is considerably crooked, and in some parts they are partially filled by masses of the material of their sides, which have fallen inward and partially choked them. The depths of these enormous chasms must be very great, as they probably owe their existence to some mighty up- heaving action, which there is every reason to suppose originated at a profound depth, since the general surface on each side of the crater does not appear to be disturbed as to altitude, which would have been the case had the upheaving action been at a moderate depth beneath. We would venture to ascribe a depth of not less than ten miles as the most moderate estimate of the profundity of these terrible chasms. If the reader would realize the scale of them, let him for a moment imagine himself a traveller on the surface of the moon coming upon one of them, and finding his onward progress arrested by the sudden appearance of its vast black yawning depths ; for by reason of the angle of his vision being almost parallel to the surface, no appearance of so profound a chasm would break upon his sight until he came comparatively close to its fearful edge. Our imaginary lunar traveller would have to make a very long detour, ere he circumvented this terrible interruption to his progress. If the reader will only endeavour to realize in his mind's eye the terrific grandeur of a chasm a mile wide and of such dark profundity as to be, to all appearance, fathomless — portions of its rugged sides fallen in wild confusion into the jaws of the tortuous abyss, and catching here and there a ray of the sun sufficient only to render the darkness of the chasm more impressive as to its profundity — he will, by so doing, learn to appreciate the romantic grandeur of this, one of the many features which the study of the lunar surface presents to the careful observer, and which exceed in sublimity the wildest efforts of poetic and romantic imagination. The contemplation of these views of the lunar world are, moreover, vastly enhanced by especial circumstances which 88 THE MOOK [chap. vii. add greatly to the impressiveness of lunar scenery, such as the unchanging pitchy-black aspect of the heavens and the death-like silence which reigns unbroken there. These digressions are, in some respects, a forestallment of what we have to say by-and-by, and so far they are out of place ; but with the illustration to which the above remarks refer placed before the reader, they may, in some respects, enhance the interest of its examination. The upper portion of our illustration is occupied by the magnificent range of volcanic mountains named after our Appennines, extending to a length of upwards of 450 miles. This mountain group rises gradually from a comparatively level surface towards the south-west, in the form of innumerable comparatively small mountains of exudation, which increase in number and altitude towards the north-east, where they culminate and suddenly terminate in a sublime range of peaks, whose altitude and rugged aspect must form one of the most terribly grand and romantic scenes which imagination can conceive. The north-east face of the range terminates abruptly in an almost vertical precipitous face, and over the plain beneath intense black steeple or spire-like shadows are cast, some of which at sunrise extend fully 90 miles, till they lose themselves in the general shading due to the curvature of the lunar surface. Nothing can exceed the sublimity of such a range of mountains, many of which rise to heights of 18,000 to 20,000 feet at one bound from the plane at their north-east base. The most favourable time to examine the details of this magnificent range is from about a day before first quarter to a day after, as it is then that the general structure of the range as well as the character of the contour of each member of the group can, from the circumstances of illumination then obtaining, be most distinctly inferred. Several comparatively small perfectly-formed craters are seen inter- spersed among the mountains, giving evidence of the truly volcanic character of the surrounding region, which, as before said, comprises in a comparatively limited space the most perfect and striking examples of nearly every class of lunar volcanic phenomena. We have endeavoured on Plate XXIII. to give some idea of a land- scape view of a small portion of this mountain range. _ O t/3 < or < CO < a: a: CHAPTER VIII. ON LUNAR CRATERS. As we stated in our brief general description of the visible hemisphere of the moon, and as a cursory glance at our map and plates will have shown, the predominant features of the lunar surface are the circular or amphitheatrical formations that, by their number, and from their almost unnatural uniformity of design, induced the belief among early observers that they must have been of artificial origin. In proceeding now to examine the details of our subject with more minuteness than before, these angular formations claim the first share of our attention. By general acceptation the term " crater " has been used to represent nearly all the circular hollows that we observe upon the moon ; and without doubt the word in its literal sense, as indicating a cup or circular cavity, is so far aptly applied. But among geologists it has been employed in a more special sense to define the hollowing out that is found at the summit of some extinct, and the majority of active, volcanoes. In this special sense it may be used by the student of the lunar surface, though in some, and indeed in the majority of cases, the lunar crater differs materially in its form with respect to its surroundings from those on the earth; for while, as we have said, the terrestrial crater is generally a hollow on a mountain top with its flat bottom high above the level of the surrounding country, those upon the moon have their lowest points depressed more or less deeply below the general surface of the moon, the external B eight being frequently only a half or one-third of the internal depth. Yet are the lunar craters truly volcanic ; as Sir John Herschel has said, they offer the true volcanic character in its highest perfection. We have upon the earth some few instances in which the geological 90 THE MOON. [chap, rat conditions which have determined the surface-formation have been iden- tical with those that have obtained upon the moon ; and as a result we have some terrestrial volcanic districts that, could we view them under the same circumstances, would be identical in character with what we see by telescopic aid upon our satellite. The most remarkable case of this similarity is offered by a certain tract of the volcanic area about Naples, known from classic times as the Campi Phlegrcei, or burning fields, a name given to them in early days, either because they showed traces of ancient earth-fire, or because there were attached to the localities traditions concerning hot-springs and sulphurous exhalations, if not of actual fiery eruptions. The resemblance of which we are speaking is here so close that Professor Phillips, in his work on Vesuvius, which by the way con- tains a historical description of the district in question, calls the moon a grand Phlegreian field. How closely the ancient craters of this famous spot resemble the generality of those upon the moon may be judged from Plate VI., in which representations of two areas, terrestrial and lunar, of the same extent, are exhibited side by side, the terrestrial region being the volcanic neighbourhoood of Naples, and the lunar a portion of the surface about the crater Theophilus. In comparing these volcanic circles together, we are however brought face to face with a striking difference that exists between the lunar and terrestrial craters. This is the difference of magnitude. None of those Plutonian amphitheatres included in the terrestrial area depicted exceed a mile in diameter, and few larger volcanic vents than these are known upon the earth. Yet when we turn to the moon, and measure some of the larger craters there, we are astonished to find them ranging from an almost invisible minuteness to 74 miles in diameter. The same disproportion exists between the depths of the two classes of craters. To give an idea of relative dimensions, we would refer to our illustration of Copernicus* and its hundreds of comparatively minute surrounding craters. Our terrestrial Vesuvius would be represented by one of these last, which upon the plate measures about the twentieth of an inch in diameter ! And this disproportion strikes us the more forcibly when * Plate VIII. chap, tiil] LUNAR CRATERS. 91 we consider that the lunar globe has an area only one-thirteenth of that of the earth. In view of this great apparent discrepancy it is not surprising that many should have been incredulous as to the true volcanic character of the lunar mountains, and have preferred to designate them by some " non-committal " term, as an American geologist (Professor Dana) has expressed it. But there is a feature in the majority of the ring-mountains that, as we conceive, demonstrates completely the fact of volcanic force having been in full action, and that seems to stamp the volcanic character upon the crater-forms. This special feature is the central cone, so well known as a characteristic of terrestrial volcanoes, accepted as the result of the last expiring effort of the eruptive force* and formed by the deposit, immediately around the volcanic orifice, of matter which there was not force enough to project to a greater distance. Upon the moon we have the central cone in small craters comparable to those on the earth, and we have it in progressively larger examples, upon all scales, up to craters of 74 miles in diameter, as we have shown in Plate VII. Where, then, can we draw the line ? Where can we say the parallel action to that which placed Vesuvius in or near the centre of the arc of Somma, or the cone figured in our sectional drawing of Vesu- vius (Fig. 3) in the middle of its present crater — where can we say that the action in question ceased to manifest itself on the moon, seeing that there is no break in the continuity of the crater-and-cone system upon the moon anywhere between craters of If miles and 74 miles in diameter? We have, it is true, many examples of coneless craters, but these are of all sizes, down to the smallest, and up to a largeness that would almost seem to render untenable the ejective explanation : of these we shall specially speak in turn, but for the present we will confine ourselves to the normal class of lunar craters, those that have central cones, and that are in all reasonable probability truly volcanic. And in the first place let us take a passing glance at the probable formative process of a terrestrial volcano. Eejecting the hypothesis of Von Buch, which geologists have on the whole found to be untenable, and which ascribes the formation of all mountains to the elevation of the earth's crust by some thrusting power beneath, we are led to regard a volcano as a pyramid of ejected matter, thrown out of and around an N 2 92 . THE MOON. [chap. viii. orifice in the external solid shell of the earth by commotions engendered in its molten nucleus. What is the precise nature and source of the ejective force geologists have not perfectly agreed upon, but we may conceive that highly expanded vapour, in all probability steam, is its primary cause. The escaping aperture may have been a weak place since the foundations of the earth were laid, or it may have been formed by a local expansion of the nucleus in the act of cooling, upon the principle enunciated in our Third Chapter ; or, again, the expansile vapour may have forced its own way through that point of the confining shell that offered it the least resistance. The vent once formed, the building of the volcanic mountain commenced by the out-belching of the lava, ashes, and scoria, and the dispersion of these around the vent at distances depending upon the energy with which they were projected. As the action continued, the ejected matter would accumulate in the form of a mound, through the centre of which communication would be maintained with the source of the ejected materials and the seat of the explosive agency. The height to which the pile would rise must depend upon several conditions : upon the steady sustenance of the matter, and upon the form and weight of the component masses, which will determine the slope of the mountain's sides. Supposing the action to subside gradually, the tapering form will be continued upwards by the comparatively gentle deposition of material around the orifice, and a perfect cone will result of some such form as that represented below, which is the outline ascribed by Fig. 16. Professor Phillips to Vesuvius in pre-historic, or even pre-traditional times, and which may be seen in its full integrity in the cases of Etna, Teneriffe, Fussi-Yamma, the great volcanic mountain of Japan, and many others. The earliest recorded form of Vesuvius is that of a truncated cone represented in Fig. 17, which shows its condition, according to PLATE VII Companion to Hell. \Vt MUesDiaanT Companion to Thebit. 2'/* Miles Dianf # Small Crater Inside "Walter" t Miles Dtfl-rri 1 Companion to Pa rot II Miles Diarn r Herschel 17 Miles Dim? Goo I N. 2Z3£Les Dia-nf Vit el lo. 243ElBElTa3if Delambre 26 Affiles IhsnE- Copernicus 4* Miles Diam' Werner . SBMlesDia-TJo? 17? E RATOSTHNES. 33 Mfle-i Jhajif •&-IS. Cam pan us. 27 Miles Diam T ^T^W Tycho. 50 Miles DiacraT Mu^^ Theophilus GtMilealhaiiif Petav I u s 78 "Miles Diam r /.Sfafmg'tii JB73 "^oentBroekB Day* Soft^np DIAGRAM OF LUNAR CRATERS FORMING A SERIES RANGING FROM 1 3 A MILES TO 78 MILES DIAMETER. ALL CONTAINING CENTRAL CONES. Published' by John Murray. Atbemarle Street Piccadilly CHAP. VIII.] LUNAR CRATERS. 93 Strabo, in the century preceding the Christian Era. Now this form may have been assumed under two conditions. If, as Phillips has surmised, the mountain originally had a peaked summit with but a small Fig. 17. crater-orifice, at the point, then we must ascribe its decapitation to a subsequent eruption which in its violence carried away the upper portion, either suddenly, or through a comparatively slow process of grinding away or widening out of the sides of the orifice by the chafing or fluxing action of the out-going materials. But it is probable that the mountain never had the perfect summit indicated in our first outline. The violent outburst that caused the great crater-opening of our second figure may have been but one paroxysmal phase of the eruption that built the mountain : a sudden cessation of the eruptive force when at its greatest intensity, and when the orifice was at its widest, would leave matters in an opposite condition to that suggested as the result of a slow dying out of the action : instead of the peak we should have a wide crater-mouth. It is of small consequence for our present purpose whether the crater was contemporaneous with the primitive formation of the mountain, or whether it was formed centuries afterwards by the blowing away of the mountain's head ; for upon the vast scale of geological time, intervals such as those between successive paroxysms of the same eruption, and those between successive eruptions, are scarcely to be discriminated, even though the first be days and the second centuries. We may remark that the widening of a crater by a subsequent and probably more powerful Fig. 18. eruption than that which originally produced it is well established. We have only to glance at the sketch, Fig. 18, of the outline of Vesuvius as it appeared between the years a.d. 79 and 1631 to see how the old crater was 94 THE MOON. [chap. viii. enlarged by the terrible Pompeian eruption of the first-mentioned year. Here we have a crater ground and blown away till its original diameter of a mile and three-quarters has been increased to nearly three miles. Scrope had no hesitation in expressing his conviction that the external rings, such as those of Santorin, St. Jago, St. Helena, the Cirque of Teneriffe, the.Curral of Madeira, the cliff range that surrounds the island of Bourbon, and others of similar form and structure, however wide the area they enclose, are truly the a basal wrecks " of volcanic mountains that have been blown into the air each by some eruption of peculiar paroxysmal violence and persistence ; and that the circular or elliptical basins which they wholly or in part surround are in all cases true craters of eruption. When the violent outburst that produces a great crater in a volcanic mountain-top more or less completely subsides, the funnel or escaping orifice becomes choked with de'bris. Still the vent strives to keep itself open, and now and then gives out a small delivery of cindery matter, which, being piled around the vent, after the manner of its great proto- type, forms the inner cone. This last may in its turn bear an open crater upon its summit, and a still smaller cone may form within it. As the action further dies away, the molten lava, no longer seething and boiling, and spirting forth with the rest of the ejected matter, wells upwards slowly, and cooling rapidly as it comes in contact with the atmosphere, solidifies and forms a flat bottom or floor to the crater. It may happen that a subsequent eruption from the original vent will be comparable in violence to the original one, and then the inner cone assumes a magnitude that renders it the principal feature of the mountain, and reduces the old crater to a secondary object. This has been the case with Vesuvius. During the eruption of 1631 the great cone which we now call Vesuvius was thrown up, and the ancient crater now distinguished as Monte Somma became a subsidiary portion of the whole mountain. Fig. 19. Then the appearance was that shown in Fig. 19, and which does not chap, viii.] LUNAR CRATERS. 95 differ greatly from that presented in the present day. The summit of the Vesuviah cone, however, has been variously altered ; it has been blown away, leaving a large crateral hollow, and it has rebuilt itself nearly upon its former model. When we transfer our attention to the volcanoes of the moon, we find ourselves not quite so well favoured with means for studying the process of their formation ; for the sight of the building up of a volcanic moun- tain such as man has been permitted to behold upon the earth has not been allowed to an observer of the moon. The volcanic activity, enfeebled though it now be, of which we are witnesses from time to time on the earth, has altogether ceased upon our satellite, and left us only its effects as a clue to the means by which they were produced. If we in our time could have seen the actual throwing up of a lunar crater, our task of description would have been simple ; as it is we are compelled to infer the constructive action from scrutiny of the finished structure. We can scarcely doubt that where a lunar crater bears general resem- blance to a terrestrial crater, the process of formation has been nearly the same in the one case as in the other. Where variations present them- selves they may reasonably be ascribed to the difference of conditions pertaining to the two spheres. The greatest dissimilarity is in the point of dimensions ; the projection of materials to 20 or more miles distance from a volcanic vent appears almost incredible, until we realize the full effect of the conditions which upon the moon are so favourable to the dispersive action of an eruptive force. In the first place, the force of gravity upon our satellite is only one-sixth of that to which bodies are subject upon the earth. Secondly, by reason of the small magnitude of the moon and its proportionally much larger surface in ratio to its magnitude, the rate at which it parted with its cosmical heat must have been much more rapid than in the case of the earth, especially when enhanced by the absence of the heat-conserving power of an atmosphere of air or water vapour; and the disruptive and eruptive action and energy may be assumed to be greater in proportion to the more rapid rate of cooling; operating, too, as eruptive action would on matter so much reduced in weight as it is on the surface of the moon, we thus find in combination conditions most favourable to the display of volcanic action in the highest 96 THE MOON. [chap. viii. degree of violence. Moreover, as the ejected material in its passage from the centre of discharge had not to encounter any atmospheric resistance, it was left free to continue the primary impulse of its ejection without other than gravitative diminution, and thus to deposit itself at distances from its source vastly greater than those of which we have examples on the earth. We can of course only conjecture the source or nature of the moon's volcanic force. If geologists have had difficulty in assigning an origin to the power that threw up our earthly volcanoes, into whose craters they can penetrate, whose processes they can watch, and whose material they can analyze, how vastly more difficult must be the inquiry into the primary source of the power that has been at work upon the moon, which cannot be virtually approached by the eye within a distance of six or eight hundred miles, and the material of which we cannot handle to see if it be compacted by heat, or distended by vapours. Steam is the agent to which geologists have been accustomed to look for explanation of terres- trial volcanoes ; the contact of water with the molten nucleus of our globe is accepted as a probable means whereby volcanic commotions are set up and ejective action is generated. But we are debarred from referring to steam as an element of lunary geology, by reason of the absence of water from the lunar globe. We might suppose that a small proportion of water once existed; but a small proportion would not account for the immense display of volcanic action which the whole surface exhibits. If we admitted a Neptunian origin to the disturbances of the moon's crust, we should be compelled to suppose that water, had existed nearly in as great quantity, area for area, there as upon our globe ; but this we cannot reasonably do. Aqueous vapour being denied us, we must look in other directions for an ejective force. Of the nature of the lunar materials we can know nothing, and we might therefore assume anything ; some have had recourse to the supposition of expansive vapours given off by some volatile component of the said material while in a state of fusion, or generated by chemical combinations. Professor Dana refers to sulphur as probably an important element in the moon's geology, suggesting this substance because of the part which it appears to play in the volcanic or igneous PLATE VHi. Wk *^ i • \ * «V *>; ,/ :m*^v -. 1 / '• '• COPERNICUS • 5 I » it SI St 7t it S CA LE Published by John Murray Albemarle Street. Piccadilly chap, viii.] LUNAR CRATERS. 97 operations of our globe, and on account of its presence in cosmical meteors that have come within range of our analysis. Any matter sublimated by heat in the substrata of the moon would be condensed upon reaching the cold surrounding space, and would be deposited in a state of fine powder, or otherwise in a solid form. Maedler has attributed the highly reflective portions of some parts of the surface, such as the bright streams that radiate from some of the craters, Copernicus and Tycho for instance, # the vitrification of the surface matter by gaseous currents. But in suppositions like these we must remember that the probability of truth diminishes as the free ground for speculation widens. It does not appear clear how expansive vapours could have lain dormant till the moon assumed a solid crust, as all such would doubtless make their escape before any shell was formed, and at an epoch when there was ample facility for their expansion. While we are not insensible of the value of an expansive vapour expla- nation, if it could be based on anything beyond mere conjecture, we are disposed to attach greater weight to that afforded by the principle sketched in our third chapter, viz., of expansion upon solidification. We gave, as we think, ample proof that molten matter of volcanic nature, when about passing to the solid state, increases its bulk to a considerable degree, and we suggested that the lunar globe at one period of its history must have been, what our earth is now, a solid shell encompassing a molten nucleus ; and further, that this last, in approaching its solid condition, expanded and burst open or rent its confining crust. At first sight it may seem that we are ascribing too great a degree of energy to the expansive force which molten substances exhibit in passing to the solid condition, seeing that in general such forces are slow and gradual in their action ; but this anomaly disappears when we consider the vast bulk of the so expanding matter, and the comparatively small amount that in its expansion it had to displace. It is true that there are individual mountains on the moon covering many square miles of surface, that as much as a thousand cubic miles of material may have been thrown up at a single eruption ; but what is this compared to the entire bulk of the moon itself ? A grain of mustard-seed upon a globe three feet in diameter represents the scale of the loftiest of terrestrial mountains ; a similar grain upon a globe one o 98 THE MOON. [chap. viii. foot in diameter, would indicate the proportion of the largest upon the moon. A model of our satellite with the elevations to scale would show nothing more than a little roughness, or superficial blistering. Turn for a moment to our map (Plate IV.), upon which the shadows give information as to the heights of the various irregularities, and suppose it to represent the actual size of some sphere whose surface has been broken up by reactions of some kind of the interior upon the exterior — suppose it to have been a globe of fragile material filled with some viscous substance, and that this has expanded, cracked its shell, oozed out in the process of solidification, and solidified : the irregularity of surface which the small sphere, roughened by the out-leaking matter, would present, would not be less than that exhibited in the map under notice. When we say that a lunar crater has a diameter of 30 miles, we raise astonishment that such a structure could result from an eruption by the expansive force of solidifying matter ; but when we reflect that this diameter is less than the two-hundredth part of the circumference of the moon, we need have no difficulty in regarding the upheaval as the result of a force slight in comparison to the bulk of the material giving rise to it. We have upon the moon evidence of volcanic eruptions being the final result of most extensive dislocations of surface, such as could only be produced by some widely diffused uplifting force. We allude to the frequent occurrence of chains of craters lying in a nearly straight line, and of craters situated at the converging point of visible lines of surface disturbance. Our map will exhibit many examples of both cases. An examination of the upper portion (the southern hemisphere of the moon) will reveal abundant instances of the linear arrangement, three, four, five or even more crateral circles will be found to lie with their centres upon the same great-circle track, proving almost undoubtedly a connexion between them so far as the original disturbing force which produced them is concerned. Again, in the craters Tycho (30), Copernicus (147), Kepler (146), and Proclus (162), we see instances of the situation of a volcanic outburst at an obvious focus of disturbance. These manifest an up-thrusting force covering a large sub-surface area, and escaping at the point of least resistance. Such an extent of action almost precludes the gaseous expla- nation, but it is compatible with the expansion on consolidation theory, chap. vin. ] LUNAR CEATERS. 99 since it is reasonable to suppose that in the process of consolidation the viscous nucleus would manifest its increase of bulk over considerable areas, disturbing the superimposed crust either in one long crack, out of the wider opening parts of which the expanded material would find its escape, or "starring" it with numerous cracks, from the converging point of which the confined matter would be ejected in greatest abundance and, if ejected there with great energy and violence, would result in the formation of a volcanic crater. The actual process by which a lunar crater would be formed would differ from that pertaining to a terrestrial crater only to the extent of the different conditions of the two globes. We can scarely accept Scrope's term " basal wrecks " (of volcanic mountains that have had the summits blown away) as applicable to the craters of the moon, for the reason that the lunar globe does not offer us any instance of a mountain comparable in extent to the great craters and whose summit has not been blown away. Scrope's definition implies a double, or divided process of formation: first the building up of a vast conical hill and then the decapitation and u evisceration " of it at some later period. There are grounds for this inferred double action among the terrestrial volcanoes, since both the perfect cone and its summitless counterpart are numerously exemplified. But upon the moon we have no perfect cone of great size, we have no exception whereby the rule can be proved It is against pro- bability, supposing every lunar crater to have once been a mountain, that in every case the mountain's summit should have been blown away ; and we are therefore compelled to consider that the moon's volcanic craters were formed by one continuous outburst, and that their " evisceration " was a part of the original formative process. We do not, however, include the central cone in this consideration : that may be reasonably ascribed to a secondary action or perhaps, better, to a weaker or modified phase of the original and only eruption. Under these circumstances we conceive the upcasting and excavating of a normal lunar crater to have been primarily caused by a local manifestation of the force of expansion upon solidification of the sub- surface matter of the moon, resulting in the creation of a mere " star " or crack in and through the outermost and solid crust. As we shall have to o 2 100 THE MOON. [chap. VIII. rely upon diagrams to explain the more complicated features, we give one of this elementary stage also as a commencement of the series ; and Fig. 20 therefore represents a probable section of the lunar surface at a point which was subsequently the location of a crater. From the vent thus formed we conceive the pent-up matter to have found its escape, not necessarily at a single outburst, but in all probability in a paroxysmal manner, as volcanic action manifests itself on our globe. The first outflow Fig. 21. of molten material would probably produce no more than a mere hill or tumescence as shewn sectionally in Fig. 21 ; and if the ejective force PLATt IX. hvfto&i Disk £ij^ THE LUNAR APENNINES, ARCHEMEDES fe, 4* ■» « K W ?• . (0 90 100 IC m ISO *40 IW S C A L F. PuLUtheJ. bi .7<>/i/i Murrtn Albemarle Street .Ficendillv CHAP. VIII.] LUNAR CRATERS. 101 were small this might increase to the magnitude of a mountain by an exudative process to be alluded to hereafter. But if tjie ejective force- were violent, either at the moment of the first outburst or at any subsequent Fig. 22. paroxysm, an action represented in Fig. 22 would result : the unsupported edges or lips of the vent-hole would be blown and ground or fluxed away, and a funnel-formed cavity would be produced, the ejected matter (so much of it as in falling was not caught by the funnel) being deposited around the hollow and forming an embryo circular mountain. The con- tinuance of this action would be accompanied by an enlargement of the conical cavity or crater, not only by the outward rush of the violently dis- charged material, but also by the " sweating " or grinding action of such of it as in descending fell within the hollow. And at the same time that the crater enlarged the rampart would extend its circumference, for it would be formed of such material as did not fall back again into the crater. Upon this view of the crater-forming process we base the sketch, Fig. 23, of the probable section of a lunar crater at one period of its development. So long as each succeeding paroxysm was greater than its predecessor, 102 THE MOON. [chap. VIII. this excavating of the hollow and widening of its mouth and mound would be extended. But when a weaker outburst came, or when the energy of the last eruption died away, a process of slow piling up of Fig. 23. matter close around the vent would ensue. It is obvious that when the ejective force could no longer exert itself to a great distance it must merely have lifted its burden to the relieving vent and dropped it in the immediate neighbourhood. Even if the force were considerable, the effect, so long as it was insufficient to throw the ejecta beyond the rim of the crater, would be to pile material in the lowermost part of the cavity ; for what was not cast over the edge would roll or flow down the inner slope and accumulate at the bottom. And as the eruption died away, it would add little by little to the heap, each expiring effort leaving the out-giving matter nearer the orifice, and thus building up the central cone that is so conspicuous a feature in terrestrial volcanoes, and which is also a marked one in a very large proportion of the craters of the moon. This formation of the cone is pictorially described by Fig. 24. CHAP. VIII.] LUNAR CRATERS. 103 In the volcanoes of the earth we observe another action either concurrent with or immediately subsequent to the erection or formation of the cone : this is the outflow or the welling forth of fluid lava, which in cooling forms the well-known plateau. We have this feature copiously- represented upon the moon and it is presumable that it has in general been produced in a manner analogous to its counterparts upon the earth. We may conceive that the fluid matter was either spirted forth with the solid or semisolid constituents of the cone, in which case it would drain down and fill the bottom of the crater ; or we may suppose that it issued from the summit of the cone and ran down its sides, or that, as we see upon the earth, it found its escape before reaching the apex; by forcing its way through the basal parts. These actions are indicated hypotheti- cally for the moon in Fig. 25 ; and the parallel phenomena for the earth are shewn by the actual case (represented in Fig. 26 and on Plate I.) of Vesuvius as it was seen by one of the authors in 1865, when the principal cone was vomiting forth ashes, stones, and red-hot lava, while a vent at the side emitted very fluid lava which was settling down and forming the plateau. 104 THE MOON. [chap. viti. Although we cannot, obviously, see upon the moon evidence of a cone actually overtopped by the -rising lake of lava, yet it is not unreasonable to suppose that such a condition of things actually occurred in many of those instances in which we observe craters without central cones, but with plateaux so smooth as to indicate previous fluidity or viscosity. From the state of things exhibited in Fig. 25 the transition to that shewn in Fig. 27 is easily, and to our view reasonably, conceivable. We are in a manner led up to this idea by a review of the various heights of central cones above their surrounding plateaux. For instance, in such examples as Tycho or Theophilus, we have cones high above the lava floor; in Copernicus, Arzachael and Alphonsus they are comparatively lower ; the lava in these and some other craters does not appear to have risen so high ; while in Aristotle and Eudoxus among others, we have only traces of cones, and it is supposable that in these cases the lava rose so high as nearly to overtop the central cones. Why should it not have risen so far as to overtop and therefore conceal some cones entirely ? We offer this as at least a feasible explanation of some coneless craters : it is not necessary to suppose that it applies to PLATE X J.Nasmyth Brooks TjtcfitSoa. ARISTOTLE & EUDOXUS X us SCALE. J^blifhed bv John Murray Albemarle Street. .Viccadillv CHAP. VIII.] LUNAR CRATERS. 105 w.; > 1 Fig. 26. all such, however : there may have been many craters, the formation of Fig. 27. which ceased so abruptly that no cone was produced, though the welling forth of lava occurred from the vent, which may have been left fully open, 106 THE MOON. [chap. vm. as in Fig. 28, or so far choked as to stay the egress of solid ejecta and yet allow the fluid material to ooze upwards through it, and so form a lake of molten lava which on consolidation became the plateau. As most of the examples of coneless craters exhibit on careful examination minute craters on the surface of the otherwise smooth plateaux, we may suppose that such minute craters are evidences of the upflow of lava which resulted in the plateaux. We have strong evidence in support of this up-flow of lava offered by the case of the crater Wargentin, (No. 26, 57'5 — 140'2) situated near the south-east border of the disc, and of which we give a special plate. (Plate XVII.) It appears to be really a crater in which the lava has risen almost to the point of overflowing, for the plateau is nearly level with the edge of the rampart. This edge appears to have been higher on one side than the other, for on the portion nearest the centre of the visible disc we may, under favourable circumstances, detect a segment of the basins brim rising above the smooth plateau as indicated in our illustration. Upon the opposite side there is no such feature visible, the plateau forms a sharp table-like edge. It is just possible that an actual overflow of lava took CHAP. VIII.] LUNAR CRATERS. 107 place at this part of the crater, but from the unfavourable situation of this remarkable object it is impossible to decide the point by observation. There is no other crater upon the visible hemisphere of the moon that exhibits this filled-up condition ; but, unique as it is, it is sufficient to justify our conclusion that the plateau-forming action upon the moon has been a flowing-up of fluid matter from below subsequent to the formation of the crater-rampart, and not, as a casual glance at the great Fig. 29. smooth-bottom craters might lead us to suspect, a result of some sort of diluvial deposit which has filled hollows and cavities and so brought up an even surface. The elevated basin of Wargentin could not have been filled thus while the surrounding craters with ramparts equally or less high remained empty : its contained matter must have been supplied from within, we must conjecture by the upflow of lava from the orifice which gave forth the material to form the crateral rampart in the first instance. We are free to conjecture that at some period of this table- mountain's formation it was a crater with a central cone, and that the rising lava over-topped this last feature in the manner shewn by the above figure (Fig. 29). P 2 108 THE MOON. ' [chap. vni. » The question occurs whether other craters may not have been similarly filled and have emptied themselves by the bursting of the wall under the pressure of the accumulated lake of lava within. We know that this breaching is a common phenomenon in the volcanoes of our globe ; the dis- trict of Auvergne furnishing us with many examples ; and there are some suspicious instances upon the moon. Copernicus exhibits signs of such disruption, as also does the smaller crater intruding upon the great circle of Gassendi. (See Frontispiece.) But the existence of such discharging breaches implies the outpouring of a body of fluid or semi-fluid material, comparable in cubical content to the capacity of the crater, and of this we ought to see traces or evidence in the form of a bulky or extensive lava stream issuing from the breach. But although there are faint indications of once viscous material lying in the direction that escaping fluid would take, we do not find anything of the extent that we should expect from the mass of matter that would constitute a craterfull. It is true that if the escaping fluid had been very limpid it might have spread over a large area and have formed a stratum too thin to be detected. Such a degree of limpidity as would be required to fulfil this condition we are hardly, however, justified in assuming. To return to the subject of central cones. Although there are cases in which the simple condition of a single cone exists, yet in the majority we see that the cone-forming process has been divided or interrupted, the consequence being the production of a group of conical hills instead of a single one. Copernicus offers an example of this character, six, some observers say seven, separate points of light, indicating as many peaks tipped with sunshine, having been seen when the greater part of the crater has been buried in shadow. Eratosthenes, Bulialdus, Maurolicus, Petavius, Langreen, and Gassendi, are a few among many instances of craters possessing more than a central single cone. This multiplication of peaks upon the moon doubtless arose from similar causes to those which produce the same feature in terrestrial volcanoes. Our sketch x)f Vesuvius in 1865 (Fig. 26) shows the double cone and the probable source of the secondary one in the diverted channel of the out- coming material. A very slight interruption in the first instance would suffice to divert the stream and form another centre of action, or a choking of the PLATE XI. J Vungrft RIESNECKER SCALE Pnhh'.ihfJ bvJoh/i Murrni Albemarle Street Ticcatiilh CHAP. VIII.] LUNAR CRATERS. 109 original vent would compel the issuing matter to find a less resisting thoroughfare into open space, and the process of cone-building would be continued from the new orifice, perhaps to be again interrupted after a time and again driven in another direction. In this manner, by- repeated arrests and diversions of the ejecta, cone has grown upon the side of cone, till, ere the force has entirely spent itself, a cluster of peaks has been produced. It may have been that this action has taken place after the formation of the plateau, in the manner indicated by Fig. 30 ; a spasmodic outburst of comparatively slight violence having sought relief from the original vent, and the flowing matter, finding the one orifice not sufficiently open to let it pass, having forced other exit through the plateau. In frequent instances we observe the state of things represented in Fig. 31, in which the plateau is studded with few or many small craters. This is the case with Plato, with Arzachael, Hipparchus, Clavius (which contains about 15 small internal craters), and many others. It is probable that these subsidiary craters were produced by an after-action like that which has produced duplicated cones, but in which the 110 THE MOON. [CHAP. VIII. secondary eruption has been of somewhat violent character, for it may almost be regarded as an axiom that violent eruptions excavate craters and weak ones pile up cones. In the cases referred to it seems reasonable to suppose that the main vent has been the channel for an up-cast of material, but that at some depth below the surface this material met with some obstruction or cause of diversion, and that it took a course which brought it out far away from the cone upon the floor of the plateau. It might even be carried so far as to be upon the rampart, and Fig. 31. it is no uncommon thing to see small craters in such a situation, though when they appear at such a distance from the primary vent, it seems more reasonable to suppose that they do not belong to it, but have arisen from a subsequent and an independent action. We find scarcely an instance of a small crater occurring just in the centre of a large one, or taking the place of the cone. This is a curious circumstance. Whenever we have any central feature in a great crater that feature is a cone. The tendency of this fact is to prove that cones were produced by very weak efforts of this expiring force, for had there been any strength in the last paroxysm it is presumable that it would chap, viii.] LUNAR CRATERS. Ill have blown out and left a crater. No very violent eruptions have there- fore taken place from the vents that were connected with the great craters of the moon, nothing more powerful than could produce a cone of exuda- tion or a cinder-heap. And with regard to cones, it is noteworthy that whether they be single or multiple, they never rise so high as the circular ramparts of their respective craters. This supports the inferred connexion between the crater origin and the cone origin, for supposing the two to have been independent, a supposition untenable in view of the universality of the central position of the cone, it is scarcely conceivable that the mountains should have always been located within ramparts higher than themselves. The less height argues less power in the upcasting agency, and the diminished force may well be considered as that which would almost of necessity precede the expiration of the eruption. Occasionally a crater is met with that has a double rampart, and the concentricity suggests that there have been two eruptions from the same vent : one powerful, which formed the exterior circle, and a second rather less powerful which has formed the interior circle. It is not, however, evident that this duplication of the ring has always been due to a double eruption. In many cases there is duplication of only a portion : a terrace exhibits itself around a part of the circular range, sometimes upon the outside and sometimes upon the inside. These terraces are not likely to have been formed by any freak of the eruption, and we are led to ascribe them in general to landslip phenomena. When, in the course of a volcano's formation, the piling-up of material about the vent has continued till the lower portions have been unable to support the upper, or when from any cause, the material composing the pile has lost its cohesiveness, the natural consequence has been a breaking away of a portion of the structure and its precipitation down the inclined sides of the crater. Vast segments of many of the lunar mountain-rings appear to have been thus dislodged from their original sites and cast down the flanks to form crescent ranges of volcanic rocks either within or without the circle. Nearly every one of our plates contains craters exhibiting this feature in more or less extensive degree. Sometimes the separated portion has been very small in proportion to the circumference of the crater : Plato is an instance in which a comparatively small mass has been detached. In 112 THE MOON. [chap vnx. other cases very large segments have slid down and lie in segmental masses on the plateaux or form terraces around the rampart. Aris- tarchus, Treisnecker, and Copernicus exhibit this larger extent of dislocation. It is possible that these landslips occurred long after the formation of the craters that have been subject to them. They are probably attributable to recent disintegration of the lunar rocks, and we have a powerful cause for this in the alternations of temperature to which the lunar crust is exposed. We shall have occasion to revert to this subject by-and-by ; at present it must suffice to point out that the extremes of cold and heat, between which the lunar soil varies, are, with reasonable probability, assumed to be on the one hand the temperature of space (which is supposed to be about 200° below zero), and, on the other hand, a degree of heat equal to about twice that of boiling water. A range of at least 500° must work great changes in such heterogeneous materials as we may conjecture those of the lunar crust to be, by the alternate contractions and expansions which it must engender, and which must tend to enlarge existing fissures and create new ones, to grind contiguous surfaces and to dislodge unstable masses. This cause of change, it is to be remarked, is one which is still exerting itself. In a few cases we have an entirely opposite interruption of the uniformity of a crater's contour. Instead of the breaking away of the ring in segments, we see the entire circuit marked with deep ruts that run down the flanks in a radial direction, giving us evidence of a downward streaming of semi-fluid matter, instead of a disruption of solid masses. We cannot doubt that these ruts have been formed by lava currents, and they indicate a condition of ejected material different from that which existed in the cases where the landslip character is found. In these last the ejecta appears to have been in the form of masses of solidified or rapidly solidifying matter, which remained where deposited for a time and then gave way from overloading or loss of cohesiveness, whereas the substances thrown out in the case of the rutted banks were probably mixed solid and fluid, the former remaining upon the flanks while the latter trickled away. Nothing so well represents, upon a small scale, this radial channelling as a heap of wetted sand left for a while for the water to LATE X THECPHILUS CYRILLUS & CATHARINA 10 5 10 Z0 30 40 50 6C SO w SCALE chap, vin.] LUNAR CRATERS. 113 drain off from it. The solid grains in such a heap sustain its general mass-form, but the liquid in passing away cuts the surface into fissures running from the summit to the base, and forms it into a model of a volcanic mountain with every feature of peak, crag, and chasm reproduced. This similarity of effect leads us to suspect a parallelism of cause, and thus to the inference that the material which originally formed such a crater-mountain as Aristillus (which is a most prominent example of this rutted character, and appears in Plate IX., side by side with a crater that has its banks segmentally broken), must have been of the compound nature indicated ; and that an action analogous to that which ruts a damp sand-heap, rutted also the banks of the lunar crater. Before passing from the subject of craters it behoves us to say a few words upon the curious manner in which these formations are complicated by intermingling and superposition. Yet, upon this point, we may be brief, for in the way of description our plates speak more forcibly than is possible by words. In particular we would refer to Plate XII., which represents the conspicuous group of craters of which the three largest members have been respectively named Theophilus, Cyrillus, and Catherina. But the area included in this plate is by no means an extraordinary one ; there are regions about Tycho wherein the craters so crowd and elbow each other that, in their intricate combinations, they almost defy accurate depiction. Our map and Plate XVI. will serve to give some idea of them. This intermingling of craters obviously shows that all the lunar volcanoes were not simultaneously produced, but that after one had been formed, an eruption occurred in its immediate neigh- bourhood and blew a portion of it away ; or it may have been that the same deep-seated vent at different times gave forth discharges of material the courses of which were more or less diverted on their way to the surface. We have before alluded to the frequent occurrence of lines of craters upon the moon. In these lines the overlapping is frequently visible ; it is seen in Plate XII. before referred to, where the ring mountains are linked into a chain slightly curved, and upon the map, Plate IV., the nearly central craters Ptolemy and Alphonsus, the latter of which overlaps the former, are seen to form part of a line of craters marking a connection Q 114 THE MOON. [chap. viii. of primary disturbance. An extensive crack suggests itself as a favourable cause for the production of this overlaying of craters, for it would serve as a sort of " line of fire " from various points at which eruptions would burst forth, sometimes weak or far apart, when the result would be lines of isolated craters, and sometimes near together, or powerful, when the consequence would be the intrusion of one upon the other, and the perfect production of the latest formed at the expense or to the detriment of those that had been formed previously. The linear grouping of volcanoes upon the earth long ago struck observant minds. The fable of the Typhon lying under Sicily and the Phlegreian fields and disturb- ing the earth by its writhings, is a mythological attempt to explain the particular case in that region. The capricious manner in which these intrusions occur is very curious. Very commonly a small crater appears upon the very rampart of a greater one, and a more diminutive one still will appear upon the rampart of the parasite. Stoeffler presents us with one example of this character, Hippar- chus with another, Maurolycus with a third, and these are but a few cases of many. Here and there we observe several craters ranged in a line with their rims in one direction all perfect, and the whole appearing like a row of coins that have fallen from a heap. There is an example near to Tycho which we reproduce in Plate XX. In this case one is led to conjec- ture that the ejective agency, after exerting itself in one spot, travelled onward and renewed itself for a time ; that it ceased after forming crater number two, and again journeyed forward in the same line, recommencing action some miles further, and again subsiding ; yet again pushing for- ward and repeating its outburst, till it produced the fourth crater, when its power became expended. In each of these successive eruptions the centre of discharge has been just outside the crater last formed ; and the close connexion of the members of the group, together with the fact of their nearly similar size, appears to indicate a community of origin. For it seems feasible that as a general rule the size of a crater may be taken as a measure of the depth of force that gave rise to the eruption producing it. This may not be true for particular cases, but it will hold where a great number are collectively considered ; for if we assume the existence of an average disturbing force, it is apparently clear that it will manifest chap, viil] LUNAR CRATERS. 115 itself in disturbing greater or less surface-areas in proportion as it acts from greater or less depths. Or, mutatis mutandis, if we assume an uniform depth for the source of action, the greater or less surface disturb- ance will be a measure of greater or less eruptive intensity. Perhaps the most remarkable case of a vast number of craters, which, from their uniform dimensions, suggest the idea of community of source- power or source-depth, is that offered by the region surrounding Copernicus, which, as will be seen by our plate of that object, is a vast Phlegreian field of diminutive craters. So countless are the minute craters that a high magnifying power brings into view when atmospheric circumstances are favourable, and so closely are they crowded together } that the resulting appearance suggests the idea of froth, and we should be disposed to christen this the " frothy region " of the moon, did not a danger exist in the tendency to connect a name with a cause. The craters that are here so abundant are doubtless the remains of true volcanoes analogous to the parasitical cones that are to be found on several terrestrial mountains, and not such accidental formations as the Uomitos described by Humboldt as abounding in the neighbourhood of the Mexican volcano, Jurillo, but which the traveller did not consider to be true cones of eruption .* Although upon our plate, and in comparison with the great crater that is its chief feature, these countless hollows appear so small as at first sight to appear insignificant, we must re- member that the minutest of them must be grand objects, each probably equal in dimensions to Vesuvius. For since, as we have shown in an early chapter, the smallest discernible telescopic object must subtend an angle to our eye of about a second, and since this angle extended to the moon represents a mile of its surface, it follows that these tiny specks of shadow that besprinkle our picture, are in the reality craters of a mile diameter. This comparison may help the conception of the stupendous magnitude of the moon's volcanic features ; for it is a conception most difficult to realize. It is hard to bring the mind to grasp the fact that that hollow of Copernicus is fifty miles in diameter. We read of an army having encamped in the once peaceful crater of Vesuvius, and of * "Cosmos," Bonn's Edition, VoL V. p. 322. Q 2 116 THE MOOS. [chap. VIII. one of the extinct volcanoes of the Campi Phlegrcei being used as a hunting preserve by an Italian king. These facts give an idea of vastness to those who have not the good fortune to see the actual dimensions of a volcanic orifice themselves. But it is almost impossible to conjure up a vision of what that fifty-mile crater would look like upon the moon itself ; and for want of a terrestrial object as a standard of comparison, our picture, and even the telescopic view of the moon itself, fails to render the imagination any help. We may try to realize the vastness by considering that one of our average English counties could be contained within its ramparts, or by conceiving a mountainous amphitheatre whose opposite sides are as far apart as the cathedrals of London and Canterbury, but even these comparisons leave us unimpressed with the true majesty which the object would present to a spectator upon the surface of our satellite. THE FORMATION OF THE CENTRAL CONE. FINAL ACTION OF A LUNAR VOLCANO. *m *^ TMajmjtil. PTO LEMY ALPHONS ARZACHAEL & c SO « S C A L E CHAPTER IX. ON THE GREAT RING-FORMATIONS NOT MANIFESTLY VOLCANIC. In our previous chapter we have given a reason for regarding as true volcanic craters all those circular formations, of whatever size, that exhibit that distinctive feature the central cone. Between the smallest crater with a cone that we can detect under the best telescopic conditions, namely, the companion to Hell, If mile diameter, and the great one called Petavius, 78 miles in diameter, we find no break in the continuity of the crater-cum-cone system that would justify us in saying that on the one side the volcanic or eruptive cause ceased, and on the other side some other causative action began. But there are numerous circular formations that surpass the magnitude of Petavius and its peers, but that have no circular cone, and are, therefore, not so manifestly volcanic as those which possess this feature. Our map will show many striking examples of this class at a glance. We may in particular refer inter alia to Ptolemy near the centre of the moon, to Grimaldi (No. 125), Shickard (No. 28), Schiller (No. 24), and Clavius (No. 13), all of which exceed 100 miles in diameter. Even the great Mare Crisium, nearly 300 miles in diameter, appears to be a formation not distinct from those which, we have just named. These present little of the generic crater character in their appearance ; and they have been distinguished therefrom by the name of Walled or Ramparted Plains. Their actual origin is beyond our explanation, and in attempting to account for them we must perforce allow considerable freedom to conjecture. They certainly, as Hooke suggested, present a "broken bubble "-like aspect; but one cannot reasonably imagine the existence of any form of mineral matter that would sustain itself in bubble form over areas of 118 THE MOON. [CHAP. IX. many hundreds of square miles. And if it were reasonable to suppose the great rings to be the foundations of such vast volcanic domes, we must conclude these to have broken when they could no longer sustain themselves, and in that case the surface beneath should be strewed with debris, of which, however, we can find no trace. Moreover, we might fairly expect that some of the smaller domes would have remained standing : we need hardly say that nothing of the kind exists. The true circularity of these objects appears at first view a remarkable feature. But it ceases to be so if we suppose them to have been produced by some very concentrated sublunar force of an upheaving nature, and if f~— / / / I \ \ \ ^ / / / r Fig. 32. only we admit the homogeneity of the moon's crust. For if the crust be homogeneous, then any upheaving force, deeply seated beneath it, will exert itself with equal effects at equal distances from the source : the lines of equal effect will obviously be radii of a sphere with the source of the disturbance for its centre, and they will meet a surface over the source in a circle. This will be evident from Fig. 32, in which a force is supposed to act at F below the surface s s s s. The matter composing s s being homogeneous, the action of F will be equal at equal distances in all directions. The lines of equal force, F/ F/ will be of equal length, and they will form, so to speak, radii of a sphere of force. This sphere is cut by the plane at s s s s, and as the intersection necessarily takes place every- chap, ix.] RING-FORMATIONS NOT MANIFESTLY VOLCANIC. 119 where at the extremity of these radii, the figure of intersection is demonstrably a circle (shown in perspective as an ellipse in the figure). Thus we see that an intense but extremely confined explosion, for instance, beneath the moon's crust must disturb a circular area of its surface, if the intervening material be homogeneous. If this be not homo- geneous there would be, where it offered less than the average resistance to the disturbance, an outward distortion of the circle ; and an opposite interruption to circularity if it offers more than the average resistance. This assumed homogeneity may possibly be the explanation of the general circularity of the lunar surface features, small and great. We confess to a difficulty in accounting for such a very local genera- tion of a deep-seated force ; and, granting its occurrence, we are unprepared with a satisfactory theory to explain the resultant effect of such a force in producing a raised ring at the limit of the circular disturbance. We may, indeed, suppose that a vast circular cake or conical frustra would be Fig. 33. Fig. 34. ^VEfs*5^? temporarily upraised as in Fig. 33, and that upon its subsidence a certain extrusion of subsurface matter would occur around the line or zone of rup- ture as in Fig. 34. This supposition, however, implies such a peculiarly cohesive condition of the matter of the uplifted cake, that it is doubtful whether it can be considered tenable. We should expect any ordinary form of rocky matter subjected to such an upheaval to be fractured and distorted, especially when the original disturbing force is greater in the centre than at the edge, as, according to the above hypothesis, it would be ; and in subsiding, the rocky plateau would thus retain some traces of its disturbance ; but in the circular areas upon the moon there is nothing to indicate that they have been subjected to such dislocations. 120 THE MOON. [chap. IX. Mr. Scrope in his work on volcanoes has given a hypothetical section of a portion of the earth's crust, which presents a bulging or tumescent surface in some measure resembling the effect which such a cause as we have been considering would produce. We give a slightly modified version of his sketch in Fig. 35, showing what would be the probable Fig. 35. A A. Fissures gaping downwards and injected by intumescent lava beneath. B B B. Fissures gaping upwards and allowing wedges of rock to drop below the level of the intervening masses, c c. Wedges forced upwards by horizontal com- pression. E f. Neutral plane or pivot axis, above and below which the directions of the tearing strain and horizontal compression are severally indicated by the smaller arrows ; the larger arrows beneath represent the direction of the primary expansive force. phenomena attending such an upheaval as regards the behaviour of the disturbed portion of the crust, and also that of the lava or semifluid matter beneath : and, as will be seen by the sketch, a possible phase of the phenomena is the production of an elevated ridge or rampart at the points of disruption c c ; and where there is a ring of disruption, as by our hypothesis there would be, the ridge or rampart c c would be a circle. In this drawing we see the cracking and distortion to which the elevated area would be subjected, but of which, as previously remarked, the circular areas of the moon present no trace of residual appearance. Those who have offered other explanations of these vast ring- formed mountain ranges, have been no more happy in their conjectures. M. Kozet, who communicated a paper on selenology to the French ':& " !i iir.;.T-*: . ///;. &v raft rt Vf /aK< ■■;- :, )V'"'- ■■':".■&?■: m ■■■■^■■■LJ j&*X»ksJDa.y&Son. WARGENTI N 1050 io ao so to so ee 7o so 90 S CA LE Piihlifhtii hv John Xurrnv Albemarle Street Ticcadillv CHAPTER XI. CRACKS AND RADIATING STREAKS. We have hitherto confined our attention to those reactions of the moon's molten interior upon its exterior which have been accompanied by considerable extrusions of sub-surface material in its molten or semi-solid condition. We now pass to the consideration of some phe- nomena resulting in part from that reaction and in part from other effects of cooling, which have been accompanied by comparatively little ejection or upflow of molten matter, and in some cases by none at all. Of such the most conspicuous examples are those bright streaks that are seen, under certain conditions of illumination, to radiate in various direc- tions from single craters, and some of the individual radial branches of which extend from four to seven hundred miles in a great arc on the moon's surface. There are several prominent examples of these bright streak systems upon the visible hemisphere of the moon ; the focal craters of the most conspicuous are Tycho, Copernicus, Kepler, Aristarchus, Menelaus, and Proclus. Generally these focal craters have ramparts and interiors dis- tinguished by the same peculiar bright or highly reflective material which shows itself with such remarkable brilliance, especially at full moon : under other conditions of illumination they are not so strikingly visible. At or nearly full moon the streaks are seen to traverse over plains, mountains, craters, and all asperities ; holding their way totally disregardful of every object that happens to lay in their course. The most remarkable bright streak system is that diverging from the great crater Tycho. The streaks that can be easily individualized in this group number more than one hundred, while the courses of some of them 134 THE MOON. [chap. xi. may be traced through upwards of six hundred miles from their centre of divergence. Those around Copernicus, although less remarkable in regard to their extent than those diverging from Tycho, are nevertheless in many respects well deserving of careful examination : they are so numerous as utterly to defy attempts to count them, while their intricate reticulation renders any endeavour to delineate their arrangement equally hopeless. The fact that these bright streaks are invariably found diverging from a crater, impressively indicates a close relationship or community of origin between the two phenomena : they are obviously the result of one and the same causative action. It is no less clear that the actuating cause or prime agency must have been very deep-seated and of enormous disruptive power to have operated over such vast areas as those through which many of the streaks extend. With a view to illustrate experimentally what we conceive to have been the nature of this actuating cause, we have taken a glass globe and, having filled it with water and hermetically sealed it, have plunged it into a warm bath : the enclosed water, expanding at. a greater rate than the glass, exerts a disruptive force on the interior surface of the latter, the consequence being that at the point of least resistance, the globe is rent by a vast number of cracks diverging in every direction from the focus of disruption. The result is such a strikingly- similar counterpart of the diverging bright streak systems which we see proceeding from Tycho and the other lunar craters before referred to, that it is impossible to resist the conclusion that the disruptive action which originated them operated in the same manner as in the case of our experi- mental illustration ; the disruptive force in the case of the moon being that to which we have frequently referred as due to the expansion which precedes the solidification of molten substances of volcanic character. On Plate XIX. we present a photograph from one of many glass globes which we have cracked in the manner described : a careful com- parison between the arrangement of divergent cracks displayed by this photograph with the streaks seen spreading from Tycho upon the conti- guous lunar photograph will, we trust, justify us in what we have stated as to the similarity of the causes which have produced such identical results. The accompanying figures will further illustrate our views upon the causative origin of the bright streaks. The primary action rent chap, xi.] CRACKS AND RADIATING STREAKS. 135 the solid crust of the moon and produced a system of radiating fissures (Fig. 42) : these immediately afforded egress for the molten matter beneath to make its appearance on the surface simultaneously along the entire course of every crack, and irrespective of all surface inequalities Fig. 42. illustrative op the radiating cracks which precede the formation op the bright streaks. or irregularities whatever (Fig. 43). We conceive that the upflowing matter spread in both directions sideways, and in this manner produced streaks of very much greater width than the cracks or fissures up through which it made its way to the surface. In further elucidation of this part of our subject we may refer to a familiar but as we conceive cogent illustration of an analogous action in the behaviour of water beneath the ice of a frozen pond, which, on being fractured by some concentrated pressure, or by a blow, is well known to 136 THE MOON. [chap. xi. " star " into radiating or diverging cracks, up through which the water immediately issues, making its appearance on the surface of the ice simultaneously along the entire course of every crack, and on reaching Fig. 43. illustrative of the radiating bright streaks. the surface, spreading on both sides to a width much exceeding that of the crack itself. If this familiar illustration be duly considered, we doubt not it will be found to throw considerable light on the nature of those actions which have resulted in the bright streaks on the moon's surface. Some have attempted to explain the cause of these bright streaks by assigning them to streams of lava, issuing from the crater at the centre of their divergence and flowing over the surface, but we consider such an expla- nation totally untenable, as any idea of lava, be it ever so fluid at its first PLATE XVIII. j.NasmyQi- ARISTARCHUS & HERODOTUS p. ...?.... y SCALE Fublithed bv Joltn Murray. Albemarle Street . Piccadilly chap, xl] CRACKS AND RADIATING STREAKS. 137 issue from its source, flowing in streams of nearly equal width, through courses several hundred miles long, up hills, over mountains, and across plains, appears to us beyond all rational probability. It may be objected to our explanation of the formation of these bright streaks, that so far as our means of observation avail us, we fail to detect any shadows from them or from such marginal edges as might be expected to result from a side- way spreading outflow of lava from the cracks which afforded it exit in the manner described. Were the edges of these streaks terminated by cliff-like or craggy margins of such height as 30 or 40 feet, we might just be able at low angles of illumination and under the most favourable circumstances of vision, to detect some slight appearance of shadows; but so far as we are aware, no such shadows have been observed. We are led to suppose that the impossibility of detecting them is due not to their absence but to the height of the margins being so moderate as not to cast any cognizable shadow, inasmuch as an abrupt craggy margin of 10 or 15 feet high would, under even the most favour- able circumstances, fail to render such visible to us. Reference to our ideal section of one of these bright streaks (Fig. 45), will show how thin their edges may be in relation to their spreading width. The absence of cognizable shadows from the bright streaks has led some observers to conclude that they have no elevation above the surface over which they traverse, and it has therefore been suggested that their existence is due to possible vapours which may have issued through the cracks, and condensed in some sublimated or pulverulent form along their courses, the condensed vapours in question forming a surface of high reflective properties. That metallic or mineral substances of some kinds do deposit on condensation very white powders, or sublimates, we are quite ready to admit, and such explanation of the high luminosity of the bright streaks, and of the craters situated at the foci or centres of their divergence is by no means improbable, so far as concerns their mere brightness. But as we invariably find a crater occupying the centre of divergence, and such craters are possessed of all the characteristic features and details which establish their true volcanic nature as the results of energetic extrusions of lava and scoria, we cannot resist the conclusion that the material of the crater, and that of the bright streaks diverging 138 THE MOON. [chap, xi- from it, are not only of a common origin, but are so far identical that the only difference in the structure of the one as compared with the other is due to the more copious egress of the extruded or erupted matter in the case of the crater, while the restricted outflow or ejection of the matter up through the cracks would cause its dispersion to be so com- paratively gentle as to flood the sides of the cracks and spread in a thin sheet more or less sideways simultaneously along their courses. There are indeed evidences in the wider of the bright streaks of their being the result of the outflow of lava through systems of cracks running parallel to each other, the confluence of the lava issuing from which would naturally yield the appearance of one streak of great width. Some of those diverging from Tycho are of this class ; many other examples might be cited, among which we may name the wide streaks proceeding from the crater Menelaus and also those from Proclus. Some of these occupy widths upwards of 25 miles — amply sufficient to admit of many concurrent cracks with confluent lava outflows. We are disposed to consider as related to the fore-mentioned radiating streaks, the numerous, we may say the multitudinous, long and narrow chasms that have been sometimes called " canals " or " rills," but which are so obviously cracks or chasms, that it is desirable that this name should be applied to them rather than one which may mislead by implying an aqueous theory of formation. These cracks, singly and in groups, are found in great numbers in many parts of the moon's surface. As a few of the more conspicuous examples which our plates exhibit we may refer to the remarkable group west of Treisnecker (Plate XL), the principal members of which converge to or cross at a small crater, and thus point to a continuity of causation therewith analogous to the evident relation between the bright streaks and their focal craters. Less remark- able, but no less interesting, are those individual examples that appear in the region north of (below) the Apennines (Plate IX.), and some of which by their parallelism of direction with the mountain-chain appear to point to a causative relation also. There is one long specimen, and several shorter in the immediate neighbourhood of Mercator and Campanus (Plate XV.) ; and another curious system of them, presenting suggestive contortions, occurs in connection with the mountains Aristarchus and chap, m.] CEACKS AND RADIATING STREAKS. 139 Herodotus (Plate XVIII.). Others, again, appear to be identified with the radial excrescences about Copernicus (Plate VIII.). Capuanus, Agrippa, and Grassendi, among other craters, have more or less notable cracks in their vicinities. Some of these chasms are conspicuous enough to be seen with moderate telescopic means, and from this maximum degree of visibility- there are all grades downwards to those that require the highest optical powers and the best circumstances for their detection. The earlier steno- graphers detected but a few of them. Schroeter noted only 1 1 ; Lohrman recorded 75 more ; Beer and Maedler added 55 to the list, while Schmidt of Athens raised the known number to 425, of which he has published a descriptive catalogue. We take it that this increase of successive dis- coveries has been due to the progressive perfection of telescopes, or, perhaps, to increased education, so to speak, of the eye, since Schmidt's telescope is a much smaller instrument than that used by Beer and Maedler, and is regarded by its owner as an inferior one for its size. We doubt not that there are hundreds more of these cracks which more perfect instruments and still sharper eyes will bring to knowledge in the future. While these chasms have all lengths from 150 miles (which is about the extent of those near Treisnecker) down to a few miles, they appear to have a less variable breadth, since we do not find many that at their maximum openings exceed two miles across ; about a mile or less is their usual width throughout the greater part of their length, and generally they taper off to invisibility at their extremities, where they do not encounter and terminate at a crater or other asperity, which is, however, sometimes the case. Of their depth we can form no precise estimate, though from the sharpness of their edges we may conclude that their sides approach perpendicularity, and, therefore, that their depth is very great ; we have elsewhere suggested ten miles as a possible pro- fundity. In a few cases, and under very favourable circumstances, we have observed their generally black interiors to be interrupted here and there with bright spots suggestive of fragments from the sides of the cracks having fallen into the opening. In seeking an explanation of these cracks, two possible causes suggest t 2 140 THE MOOX. [chap. att themselves. One is the expansion of subsurface matter, already suggested as explanatory of the bright streaks ; the other, a contraction of the crust by cooling. We doubt not that both causes have been at work, one perhaps enhancing the other. Where, as in the cases we have pointed out, there are cracks which are so connected with craters as to imply relationship, we may conclude that an upheaving or expansive force in the sublunar molten matter has given rise to the cracks, and that the central craters have been formed simultaneously, by the release, with ejective violence, of the matter from its confining crust. The nature of the expansive force being assumed that of solidifying matter, the wide extent of some chasms indicates a deep location of that force. And depth in this matter implies lateness (in the scale of selenological time) of operation, since the central portions of the globe would be the last to cool. Now, we have evidence of comparative lateness afforded by the fact that in many cases the cracks have passed through craters and other asperi- ties which thus obviously existed before the cracking commenced ; and thus, so far, the hypothesis of the expansion-cracking is supported by absolute fact. It may be objected that such an upheaving force as we are invoking, being transitory, would allow the distended surface to collapse again when it ceased to operate, and so close the cracks or chasms it produced. But we consider it not improbable that in some cases, as a consequence of the expansion of subsurface matter, an upflow thereof may have partially filled the crack, and by solidifying have held it open ; and it is rational to suppose that there have been various degrees of filling and even of overflow — that in some cases the rising matter has not nearly reached the edge of the crack, as in Fig. 44, while in others it has risen almost to the surface, and in some instances has actually overrun it and pro- duced some sort of elevation along the line of the crack, like that repre- sented sectionally in Fig. 45. It is probable that some of the slightly tumescent lines on the moon's surface have been thus produced. We have suggested shrinkage as a possible explanation of some cracks. It could hardly have been the direct cause of those compound ones which are distinguished by focal craters, though it may have been a co-operative cause, since the contracting tendency of any area of the crust, by so to X X ca