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diff --git a/old/56305-0.txt b/old/56305-0.txt deleted file mode 100644 index 53361f6..0000000 --- a/old/56305-0.txt +++ /dev/null @@ -1,7448 +0,0 @@ -The Project Gutenberg EBook of The Moon, by James Nasmyth and James Carpenter - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Moon - considered as a planet, a world, and a satellite. - -Author: James Nasmyth - James Carpenter - -Release Date: January 4, 2018 [EBook #56305] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE MOON *** - - - - -Produced by Eric Hutton, Stephen Hutcheson, and the Online -Distributed Proofreading Team at http://www.pgdp.net - - - - - - - [Illustration: GASSENDI. - Nov. 7. 1867 10 P.M.] - - - - - THE MOON: - CONSIDERED AS - A PLANET, A WORLD, and A SATELLITE. - - - BY - JAMES NASMYTH, C.E. - AND - JAMES CARPENTER, F.R.A.S. - LATE OF THE ROYAL OBSERVATORY, GREENWICH. - - - WITH TWENTY-FOUR ILLUSTRATIVE PLATES OF LUNAR OBJECTS, PHENOMENA, AND - SCENERY; NUMEROUS WOODCUTS, &c. - - LONDON: - JOHN MURRAY, ALBEMARLE STREET. - 1874. - - LONDON: - BRADBURY, AGNEW, & CO., PRINTERS, WHITEFRIARS. - - - TO - HIS GRACE THE DUKE OF ARGYLL, - IN RECOGNITION OF HIS LONG CONTINUED INTEREST IN THE SUBJECT OF WHICH - IT TREATS, - _This Volume_ - IS MOST RESPECTFULLY DEDICATED - BY - THE AUTHORS. - - - - - PREFACE. - - -The reason for this book’s appearance may be set forth in a few words. A -long course of reflective scrutiny of the lunar surface with the aid of -telescopes of considerable power, and a consequent familiarity with the -wonderful details there presented, convinced us that there was yet -something to be said about the moon, that existing works on Astronomy -did not contain. Much valuable labour has been bestowed upon the -topography of the moon, and this subject we do not pretend to advance. -Enough has also been written for the benefit of those who desire an -acquaintance with the intricate movements of the moon in space; and -accordingly we pass this subject without notice. But very little has -been written respecting the moon’s physiography, or the causative -phenomena of the features, broad and detailed, that the surface of our -satellite presents for study. Our observations had led us to some -conclusions, respecting the cause of volcanic energy and the mode of its -action as manifested in the characteristic craters and other eruptive -phenomena that abound upon the moon’s surface. We have endeavoured to -explain these phenomena by reference to a few natural laws, and to -connect them with the general hypothesis of planet formation which is -now widely accepted by cosmologists. The principal aim of our work is to -lay these proffered explanations before the students and admirers of -astronomy and science in general; and we trust that what we have deduced -concerning the moon may be taken as referring to a certain extent to -other planets. - -Some reflections upon the moon considered as a world, in reference to -questions of habitability, and to the peculiar conditions which would -attend a sojourn on the lunar surface, have appeared to us not -inappropriate. These, though instructive, are rather curious than -important. More worthy of respectful consideration are the few remarks -we have offered upon the moon as a satellite and a benefactor to the -inhabitants of this Earth. - -In reference to the Illustrations accompanying this work, more -especially those which represent certain portions of the lunar surface -as they are revealed by the aid of powerful telescopes, such as those -which we employed in our scrutiny, it is proper that we should say a few -words here on the means by which they have been produced.—— - -During upwards of thirty years of assiduous observation, every -favourable opportunity has been seized to educate the eye not only in -respect to comprehending the general character of the moon’s surface, -but also to examining minutely its marvellous details under every -variety of phase, in the hope of rightly understanding their true nature -as well as the causes which had produced them. This object was aided by -making careful drawings of each portion or object when it was most -favourably presented in the telescope. These drawings were again and -again repeated, revised, and compared with the actual objects, the eye -thus advancing in correctness and power of appreciating minute details, -while the hand was acquiring, by assiduous practice, the art of -rendering correct representations of the objects in view. In order to -present these Illustrations with as near an approach as possible to the -absolute integrity of the original objects, the idea occurred to us that -by translating the drawings into models which, when placed in the sun’s -rays, would faithfully reproduce the lunar effects of light and shadow, -and then photographing the models so treated, we should produce most -faithful representations of the original. The result was in every way -highly satisfactory, and has yielded pictures of the details of the -lunar surface such as we feel every confidence in submitting to those of -our readers who have made a special study of the subject. It is hoped -that those also who have not had opportunity to become intimately -acquainted with the details of the lunar surface, will be enabled to -become so by aid of these Illustrations. - -In conclusion, we think it desirable to add that the photographic -illustrations above referred to are printed by well-established pigment -processes which ensure their entire permanency. - - - - - CONTENTS. - - - PAGE - - - CHAPTER I. - ON THE COSMICAL ORIGIN OF THE PLANETS OF THE SOLAR SYSTEM. 1 - Origination of Material Things—Celestial Vapours—Nebulæ—Their - vast Numbers—Sir W. Herschel’s Observations and - Classification—Buffon’s Cosmogony—Laplace’s Nebular - Hypothesis—Doubts upon its Validity—Support from Spectrum - Analysis - - - CHAPTER II. - THE GENERATION OF COSMICAL HEAT. 11 - Conservation of Force—Indestructibility of Force—Its - Convertibility into Heat—Dawn of the Doctrine—Mayer’s - Deductions—Joule’s Experiments—Mechanical Equivalent of - Heat—Gravitation the Source of Cosmical Heat—Calculations - of Mayer and Helmholtz—The Moon as an Incandescent - Sphere—Not necessarily Burning—Loss of Heat by - Radiation—Cooling of External Crust—Commencement of - Selenological History - - - CHAPTER III. - THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 19 - Cooling commenced from Outer Surface—Contraction by - Cooling—Expansion of Molten Matter upon - Solidification—Water not exceptional—Similar Behaviour of - Molten Iron—Floating of Solid on Molten Metal—Currents in - a Pot of Molten Metal—Bursting of Iron Bottle by - Congelation of Bismuth within—Evidence from Furnace - Slag—From the Crater of Vesuvius—Effects of Contraction of - Moon’s Crust and Expansion of Interior—Production of - Ridges and Wrinkles—Theory of Wrinkles—Examples from - shrivelled Apple and Hand - - - CHAPTER IV. - THE FORM, MAGNITUDE, WEIGHT, AND DENSITY OF THE LUNAR GLOBE. 31 - Form of Moon—Not perfectly Spherical—Bulged towards - Earth—Diameter—Angular Measure—Linear Measure—Parallax of - Moon—Distance—Area of Lunar Sphere—Solid Contents—Mass of - Moon—Law of Gravitation—Mass determined by Tides and other - Means—Density—How obtained—Specific Gravity of Lunar - Matter—Force of Gravity at Surface—How determined—Weights - of similar Bodies on Earth and Moon—Effects of like Forces - acting against Gravity on Earth and Moon - - - CHAPTER V. - ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR ATMOSPHERE. 39 - Subject of Controversy—Phenomena of Terrestrial Atmosphere—No - Counterparts on Moon—Negative Evidence from Solar - Eclipses—No Twilight on Moon—Evidence from Spectrum - Analysis—From Occultations of Stars—Absence of Water or - Moisture—Cryophorus—No Reddening of Sun’s Rays by Vapours - on Moon—No Air or Water to complicate Discussions of Lunar - Volcanic Phenomena - - - CHAPTER VI. - THE GENERAL ASPECT OF THE LUNAR SURFACE. 51 - Pre-Telescopic Ideas—Human Countenance—Other supposed - Resemblances—Portrait of Full Moon—Permanence of - Features—Rotation of Moon—Solar Period and Solar Day on - Moon—Libration—Diurnal—In Latitude—In Longitude—Visible - and Invisible Hemispheres—Telescopic Scrutiny—Galileo’s - Views—Features Visible with Low Power—Low Powers on small - and large Telescopes—Salient - Features—Craters—Plains—Bright Streaks—Mountains—Higher - Telescopic Powers—Detail Scrutiny of Features - therewith—Discussion of High Powers—Education of - Eye—Highest practicable Power—Size of smallest Visible - Objects - - - CHAPTER VII. - TOPOGRAPHY OF THE MOON. 65 - Reasons for Mapping the Moon—Early Maps—Labours of - Langreen—Hevelius—Riccioli—Cassini—Schroeter—Modern - Maps—Lohrman’s—Beer and Maedler’s—Excellence of the - last—Measurement of Mountain Heights—Need of a Picture - Map—Formation of our own—Skeleton Map—Table of conspicuous - Objects—Descriptions of special - Objects—Copernicus—Gassendi—Eudoxus and - Aristotle—Triesnecker—Theophilus, Cyrillus, and - Catharina—Thebit—Plato—Valley of the - Alps—Pico—Tycho—Wargentin—Aristarchus and - Herodotus—Walter—Archimedes and the Apennines - - - CHAPTER VIII. - ON LUNAR CRATERS. 89 - Use of term Crater for Terrestrial and Lunar Formations—Truly - Volcanic Nature of Lunar Craters—Terrestrial and Lunar - Volcanic Areas compared—Similarity—Difference only in - Magnitude—Central Cone—Found in great and small Lunar - Craters—Formative Process of Terrestrial Volcanoes—Example - from Vesuvius—Vast Size of Lunar Craters—Reasons - assigned—Origin of Moon’s Volcanic Force—Aqueous Vapour - Theory untenable—Expansion upon Solidification - Theory—Formative Process of a Lunar Crater—Volcanic - Vent—Commencement of Eruption—Erection of - Rampart—Hollowing of Crater—Formation of Central Cone—Of - Plateau—Various Heights of Plateaux—Coneless - Craters—Filled-up Craters—Multiple Cones—Craters on - Plateau—Double Ramparts—Landslip Terraces—Rutted - Ramparts—Overlapping and Superposition of - Craters—Source-Connection of such—Froth-like Aggregations - of Craters—Majestic Dimensions of Larger Craters - - - CHAPTER IX. - ON THE GREAT RING-FORMATIONS NOT MANIFESTLY VOLCANIC. 117 - Absence of Central Cones—Vast Diameters—Difficult of - Explanation—Hooke’s Idea—Suggested Cause of True - Circularity—Scrope’s Hypothesis of Terrestrial - Tumescences—Rozet’s Tourbillonic Theory—Dana’s Ebullition - Theory - - - CHAPTER X. - PEAKS AND MOUNTAIN RANGES. 124 - Paucity of extensive Mountain Systems on Moon—Contrast with - Earth—Lunar Mountains found in less disturbed - Regions—Lunar Apennines, Caucasus, and Alps—Valley of - Alps—“Crag and Tail” Contour—Isolated Peaks—How - produced—Analogy from Freezing Fountain—Terrestrial - Counterparts and their Explanation by Scrope—Blowing Cone - on Teneriffe—Comparative Gentleness of Mountain-forming - Action—Relation between Mountain Systems and Crater - Systems—Wrinkle Ridges - - - CHAPTER XI. - CRACKS AND RADIATING STREAKS. 133 - Description—Divergence from Focal Craters—Experimental - Explanation of their Cause—Radial Cracking of - Crust—Outflow of Matter therefrom—Analogy from “Starred” - Ice—No Shadows cast by Streaks—Their probable Slight - Elevation—Open Cracks—Great Numbers—Length—Depth—In-fallen - Fragments—Shrinkage a Cause of Cracks—Lateness of their - Production - - - CHAPTER XII. - COLOUR AND BRIGHTNESS OF LUNAR DETAILS: CHRONOLOGY OF FORMATIONS, - AND FINALITY OF EXISTING FEATURES. 143 - Absence of Conspicuous Colour—Slight Tints of - “Seas”—Cause—Probable Variety of Tints in small - Patches—Diversity of Brightness of Details—Most - Conspicuous at Full Moon—Classification of - Shades—Exaggerated Contrasts in Photographs—Brightest - Portions probably the latest formed—Chronology of - Formations—Large Craters older than Small—Mountains older - than Craters—Bright Streaks comparatively recent—Cracks - most recent of all Features—Question of existing - Change—Evidence from Observation—Paucity of such - Evidence—Supposed Case of _Linné_—Theoretical - Discussion—Relative Cooling Tendencies of Earth and - Moon—Earth nearly assumed its final Condition—Moon - probably cooled Ages upon Ages ago—Possible slight Changes - from Solar Heating—Disintegrating Action - - - CHAPTER XIII. - THE MOON AS A WORLD: DAY AND NIGHT UPON ITS SURFACE. 155 - Existence of Habitants on other Planets—Interest of the - Question—Conditions of Life—Absence of these from Moon—No - Air or Water and intense Heat and Cold—Possible Existence - of Protogerms of Life—A Day on the Moon - imagined—Instructiveness of the Realization—Length of - Lunar Day—No Dawn or Twilight—Sudden Appearance of - Light—Slowness of Sun in Rising—No Atmospheric - Tints—Blackness of Sky and Visibility of Stars and fainter - Luminosities at Noon-day—Appearance of the Earth as a - Stationary Moon—Its Phases—Eclipse of Sun by - Earth—Attendant Phenomena—Lunar Landscape—Height essential - to secure a Point of View—Sunrise on a Crater—Desolation - of Scene—No Vestige of Life—Colour of Volcanic Products—No - Atmospheric Perspective—Blackness of Shadows—Impressions - on other Senses than Sight—Heat of Sun untempered—Intense - Cold in Shade—Dead Silence—No Medium to conduct - Sound—Lunar Afternoon and Sunset—Night—The Earth a - Moon—Its Size, Rotation, and Features—Shadow of Moon upon - it—Lunar Night-Sky—Constellations—Comets and Planets—No - Visible Meteors—Bombardment by Dark Meteoric Masses—Lunar - Landscape by Night—Intensity of Cold - - - CHAPTER XIV. - THE MOON AS A SATELLITE: ITS RELATION TO THE EARTH AND MAN. 171 - The Moon as a Luminary—Secondary Nature of Light-giving - Function—Primary Office as a Sanitary Agent—Cleansing - Effects of the Tides—Tidal Rivers and Transport - thereby—The Moon a “Tug”—Available Power of - Tides—Tide-Mills—Transfer of Tidal Power Inland—The Moon - as a Navigator’s Guide—Longitude found by the Moon—Moon’s - Motions—Discovered by Observations—Grouped into - Theories—Represented by Tables—The Nautical Almanac—The - Moon as a Long-Period Timekeeper—Reckoning by - “Moons”—Eclipses the Starting-Points of - Chronologies—Furnish indisputable Dates—Solar Surroundings - revealed by Eclipses when Moon screens the Sun—Solar - Corona—Moon as a Medal of Creation, a Half-formed - World—Abuses of the Moon—Superstitions—Erroneous Ideas - regarding Moonlight pourtrayed by Artists and Authors—The - Moon and the Weather—Errors and Facts—Atmospheric - Tides—Warmth from Moon—Paradoxical Effect in cooling the - Earth - - - CHAPTER XV. - CONCLUDING SUMMARY 184 - - - - - LIST OF PLATES. - - - PLATE PAGE - Gassendi _Frontispiece_ - I.—Summit of Vesuvius 26 - II.—Wrinkled Hand and Apple 30 - III.—Full Moon Photograph 52 - IV.—Picture-Map of the Moon {_To face each other._} - V.—Skeleton Map 68 - VI.—Terrestrial and Lunar Volcanic Areas Compared 88 - VII.—Progressive Series of Craters 92 - VIII.—Copernicus 96 - IX.—The Lunar Apennines, &c., &c. 100 - X.—Aristotle and Eudoxus 104 - XI.—Triesnecker 108 - XII.—Theophilus, Cyrillus, and Catharina 112 - XIII.—Arzachael, Ptolemy, and the Railway 116 - XIV.—Plato, the Valley of the Alps, Pico, &c. 120 - XV.—Mercator and Campanus 124 - XVI.—Tycho and its Surroundings 128 - XVII.—Wargentin 132 - XVIII.—Aristarchus and Herodotus 136 - XIX.—Glass Globe Cracked by Internal Pressure 140 - XX.—Overlapping Craters 148 - XXI.—Lunar Crater. Ideal Landscape 156 - XXII.—Solar Eclipse as it would be seen from the Moon 164 - XXIII.—Group of Mountains. Ideal Lunar Landscape 170 - - - - - THE MOON. - - - - - CHAPTER I. - ON THE COSMICAL ORIGIN OF THE PLANETS OF THE SOLAR SYSTEM. - - -In this Chapter we propose to treat briefly of the probable formation of -the various members of the solar system from matter which previously -existed in space in a condition different from that in which we at -present find it—_i.e._, in the form of planets and satellites. - -It is almost impossible to conceive that our world with its satellite, -and its fellow worlds with their satellites, and also the great centre -of them all, have always, from the commencement of time, possessed their -present form: all our experiences of the working of natural laws rebel -against such a supposition. In every phenomenon of nature upon this -earth—the great field from which we must glean our experiences and form -our analogies—we see a constant succession of changes going on, a -constant progression from one stage of development to another taking -place, a perpetual mutation of form and nature of the same material -substance occurring: we see the seed transformed into the plant, the -flower into the fruit, and the ovum into the animal. In the inorganic -world we witness the operation of the same principle; but, by reason of -their slower rate of progression, the changes there are manifested to us -rather by their resulting effects than by their visible course of -operation. And when we consider, as we are obliged to do, that the same -laws work in the greatest as well as the smallest processes of nature, -we are compelled to believe in an antecedent state of existence of the -matter that composes the host of heavenly bodies, and amongst them the -earth and its attendant moon. - -In the pursuit of this course of argument we are led to inquire whether -there exists in the universe any matter from which planetary bodies -could be formed, and how far their formation from such matter can be -explained by the operation of known material laws. - -Before the telescope revealed the hidden wonders of the skies, and -brought its rich fruits into our garner of knowledge concerning the -nature of the universe, the philosophic minds of some early astronomers, -Kepler and Tycho Brahe to wit, entertained the idea that the sun and the -stars—the suns of distant systems—were formed by the condensation of -celestial vapours into spherical bodies; Kepler basing his opinion on -the phenomena of the sudden shining forth of new stars on the margin of -the Milky Way. But it was when the telescope pierced into the depths of -celestial space, and brought to light the host of those marvellous -objects, the nebulæ, that the strongest evidence was afforded of the -probable validity of these suppositions. The mention of “nebulous stars” -made by the earlier astronomers refers only to clusters of telescopic -stars which the naked eye perceives as small patches of nebulous light; -and it does not appear that even the nebula in Andromeda, although so -plainly discernible as to be often now-a-days mistaken by the -uninitiated for a comet, was known, until it was discovered by means of -a telescope, in 1612, by Simon Marius, who described it as resembling a -candle shining through semi-transparent horn, as in a lantern, and -without any appearance of stars. Forty years after this date Huygens -discovered the splendid nebula in the sword handle of Orion, and in 1665 -another was detected by Hevelius. In 1667 Halley (afterwards Astronomer -Royal) discovered a fourth; a fifth was found by Kirsch in 1681, and a -sixth by Halley again in 1714. Half a century after this the labours of -Messier expanded the list of known nebulæ and clusters to 103, a -catalogue of which appeared in the “Connaissance du Temps” (the French -“Nautical Almanac”) for the years 1783-1784. But this branch of -celestial discovery achieved its most brilliant results when the rare -penetration, the indomitable perseverance, and the powerful instruments -of the elder Herschel were brought to bear upon it. In the year 1779 -this great astronomer began to search after nebulæ with a seven-inch -reflector, which he subsequently superseded by the great one of forty -feet focus and four feet aperture. In 1786 he published his first -catalogue of 1000 nebulæ; three years later he astonished the learned -world by a second catalogue containing 1000 more, and in 1802 a third -came forth comprising other 500, making 2500 in all! This number has -been so far increased by the labours of more recent astronomers that the -last complete catalogue, that of Sir John Herschel, published a few -years ago, contains the places of 5063 nebulæ and clusters. - -At the earlier periods of Herschel’s observations, that illustrious -observer appears to have inclined to the belief that all nebulæ were but -remote clusters of stars, so distant, so faint, and so thickly -agglomerated as to affect the eye only by their combined luminosity, and -at this period of the nebular history it was supposed that increased -telescopic power would resolve them into their component stars. But the -familiarity which Herschel gained with the phases of the multitudinous -nebulæ that passed in review before his eyes, led him ultimately to -adopt the opinion, advanced by previous philosophers, that they were -composed of some vapoury or elementary matter out of which, by the -process of condensation, the heavenly bodies were formed; and this led -him to attempt a classification of the known nebulæ into a cosmical -arrangement, in which, regarding a chaotic mass of vapoury matter as the -primordial state of existence, he arranged them into a series of stages -of progressive development, the individuals of one class being so nearly -allied to those in the next that, to use his own expression, not so much -difference existed between them “as there would be in an annual -description of the human figure were it given from the birth of a child -till he comes to be a man in his prime.” (_Philosophical Transactions, -Vol. CI., pp. 271_, _et seq._) - -His category comprises upwards of thirty classes or stages of -progression, the titles of a few of which we insert here to illustrate -the completeness of his scheme. - - Class 1. Of extensive diffused nebulosity. (A table of 52 - patches of such nebulosity actually observed is given, - some of which extend over an area of five or six square - degrees, and one of which occupies nine square degrees.) - ” 6. Of milky nebulosity with condensation. - ” 15. Of nebulæ that are of an irregular figure. - ” 17. Of round nebulæ. - ” 20. Of nebulæ that are gradually brighter in the middle. - ” 25. Of nebulæ that have a nucleus. - ” 29. Of nebulæ that draw progressively towards a period of - final condensation. - ” 30. Of planetary nebulæ. - ” 33. Of stellar nebulæ nearly approaching the appearance of - stars. - -In a walk through a forest we see trees in every stage of growth, from -the tiny sapling to the giant of the woods, and no doubt can exist in -our minds that the latter has sprung from the former. We cannot at a -passing glance discern the process of development actually going on; to -satisfy ourselves of this, we must record the appearance of some single -tree from time to time through a long series of years. And what a walk -through a forest is to an observer of the growth of a tree, a lifetime -is to the observer of changes in such objects as the nebulæ. The -transition from one state to another of the nebulous development is so -slow that a lifetime is hardly sufficient to detect it. Nor can any -precise evidence of change be obtained by the comparison of drawings or -descriptions of nebulæ at various epochs, with whatever care or skill -such drawings be made, for it will be admitted that no two draughtsmen -will produce each a drawing of the most simple object from the same -point of view, in which every detail in the one will coincide exactly -with every detail in the other. There is abundant evidence of this in -the existing representations of the great nebula in Orion; a comparison -of the drawings that have been lately made of this object, with the most -perfect instruments and by the most skilful of astronomical draughtsmen, -reveals varieties of detail and even of general appearance such as could -hardly be imagined to occur in similar delineations of one and the same -subject; and any one who himself makes a perfectly unbiassed drawing at -the telescope will find upon comparison of it with others that it will -offer many points of difference. The fact is that the drawing of a man, -like his penmanship, is a personal characteristic, peculiar to himself, -and the drawings of two persons cannot be expected to coincide any more -than their handwritings. The appearance of a nebula varies also to a -great extent with the power of the telescope used to observe it and the -conditions under which it is observed; the drawings of nebulæ made with -the inferior telescopes of a century or two centuries ago, the only ones -that, by comparison with those made in modern times, could give -satisfactory evidence of changes of form or detail, are so rude and -imperfect as to be useless for the purpose, and it is reasonable to -suppose that those made in the present day will be similarly useless a -century or two hence. Since then we can obtain no evidence of the -changes we must assume these mysterious objects to be undergoing, _ipso -facto_, by observation of _one nebula_ at _various periods_, we must for -the present accept the _primâ facie_ evidence offered (as in the case of -the trees in a forest) by the observation of _various nebulæ_ at _one -period_. - -“The total dissimilitude,” says Herschel at the close of the -observations we have alluded to, “between the appearance of a diffusion -of the nebulous matter and of a star, is so striking, that an idea of -the conversion of the one into the other can hardly occur to any one who -has not before him the result of the critical examination of the -nebulous system which has been displayed in this [his] paper. The end I -have had in view, by arranging my observations in the order in which -they have been placed, has been to show that the above mentioned -extremes may be connected by such nearly allied intermediate steps, as -will make it highly probable that every succeeding state of the nebulous -matter is the result of the action of gravitation upon it while in a -foregoing one, and by such steps the successive condensation of it has -been brought up to the planetary condition. From this the transit to the -stellar form, it has been shown, requires but a very small additional -compression of the nebulous matter.” - -Where the researches of Herschel terminated those of Laplace commenced. -Herschel showed how a mass of nebulous matter so diffused as to be -scarcely discernible might be and probably was, by the mere action of -gravitation, condensed into a mass of comparatively small dimensions -when viewed in relation to the immensity of its primordial condition. -Laplace demonstrated how the known laws of gravitation could and -probably did from such a partially condensed mass of matter produce an -entire planetary system with all its subordinate satellites. - -The first physicist who ventured to account for the formation of the -various bodies of our solar system was Buffon, the celebrated French -naturalist. His theory, which is fully detailed in his renowned work on -natural history, supposed that at some period of remote antiquity the -sun existed without any attendant planets, and that a comet having -dashed obliquely against it, ploughed up and drove off a portion of its -body sufficient in bulk to form the various planets of our system. He -suggests that the matter thus carried off “at first formed a torrent the -grosser and less dense parts of which were driven the farthest, and the -densest parts, having received only the like impulsion, were not so -remotely removed, the force of the sun’s attraction having retained -them:” that “the earth and planets therefore at the time of their -quitting the sun were burning and in a state of liquefaction;” that “by -degrees they cooled, and in this state of fluidity they took their -form.” He goes on to say that the obliquity of the stroke of the comet -might have been such as to separate from the bodies of the principal -planets small portions of matter, which would preserve the same -direction of motion as the principal planets, and thus would form their -attendant satellites. - -The hypothesis of Buffon, however, is not sufficient to explain all the -phenomena of the planetary system; and it is imperfect, inasmuch as it -begins by assuming the sun to be already existing, whereas any theory -accounting for the primary formation of the solar system ought -necessarily to include the origination of the most important body -thereof, the sun itself. Nevertheless, it is but due to Buffon to -mention his ideas, for the errors of one philosophy serve a most useful -end by opening out fields of inquiry for subsequent and more fortunate -speculators. - -Laplace, dissatisfied with Buffon’s theory, sought one more probable, -and thus was led to the proposition of the celebrated _nebular -hypothesis_ which bears his name, and which, in spite of its -disbelievers, has never been overthrown, but remains the only probable, -and, with our present knowledge, the only possible explanation of the -cosmical origin of the planets of our system. Although Laplace puts -forth his conjectures, to use his own words, “with the deference which -ought to inspire everything that is not a result of observation and -calculation,” yet the striking coincidence of all the planetary -phenomena with the conditions of his system gives to those conjectures, -again to use his modest language, “a probability strongly approaching -certitude.” - -Laplace conceived the sun to have been at one period the nucleus of a -vast nebula, the attenuated surrounding matter of which extended beyond -what is now the orbit of the remotest planet of the system. He supposed -that this mass of matter in process of condensation possessed a rotatory -motion round its centre of gravity, and that the parts of it that were -situated at the limits where centrifugal force exactly counterbalanced -the attractive force of the nucleus were abandoned by the contracting -mass, and thus were formed successively a number of rings of matter -concentric with and circulating around the central nucleus. As it would -be improbable that all the conditions necessary to preserve the -stability of such rings of matter in their annular form could in all -cases exist, they would break up into masses which would be endued with -a motion of rotation, and would in consequence assume a spheroidal form. -These masses, which hence constituted the various planets, in their turn -condensing, after the manner of the parent mass, and abandoning their -outlying matter, would become surrounded by similarly concentric rings, -which would break up and form the satellites surrounding the various -planetary masses; and, as a remarkable exception to the rule of the -instability of the rings and their consequent breakage, Laplace cited -the case of Saturn surrounded by his rings as the only instances of -unbroken rings that the whole system offers us; unless indeed we include -the zodiacal light, that cone of hazy luminosity that is frequently seen -streaming from our luminary shortly before and after sunset, and which -Laplace supposed to be formed of molecules of matter, too volatile to -unite either with themselves or with the planets, and which must hence -circulate about the sun in the form of a nebulous ring, and with such an -appearance as the zodiacal actually presents. - -This hypothesis, although it could not well be refuted, has been by many -hesitatingly received, and for a reason which was at one time cogent. In -the earlier stages of nebular research it was clearly seen, as we have -previously remarked, that many of the so-called nebulæ, which appeared -at first to consist of masses of vapoury matter, became, when -scrutinised with telescopes of higher power, resolved into clusters -containing countless numbers of stars, so small and so closely -agglomerated, that their united lustre only impressed the more feeble -eye as a faint nebulosity; and as it was found that each accession of -telescopic power increased the numbers of nebulæ that were thus -resolved, it was thought that every nebula would at some period succumb -to the greater penetration of more powerful instruments; and if this -were the case, and if no real nebulæ were hence found to exist, how, it -was argued, could the nebular hypothesis be maintained? One of the most -important nebulæ bearing upon this question was the great one in the -sword handle of Orion, one of the grandest and most conspicuous in the -whole heavens. On account of the brightness of some portions of this -object, it seemed as though it ought to be readily resolvable, supposing -all nebulæ to consist of stars, but all attempts to resolve it were in -vain, even with the powerful telescopes of Sir John Herschel and the -clear zenithal sky of the Cape of Good Hope. At length the question was -thought to be settled, for upon the completion of Lord Rosse’s giant -reflector, and upon examination of the nebula with it, his lordship -stated that there could be little, if any, doubt as to its -resolvability, and then it was maintained, by the disbelievers in the -nebular theory, that the last stronghold of that theory had been broken -down. - -But the truths of nature are for ever playing at hide and seek with -those who follow them:—the dogmas of one era are the exploded errors of -the next. Within the past few years a new science has arisen that -furnishes us with fresh powers of penetration into the vast and secret -laboratories of the universe; a new eye, so to speak, has been given us -by which we may discern, by the mere light that emanates from a -celestial body, something of the chemical elements of which it is -composed. When Newton two hundred years ago toyed with the prism he -bought at Stourbridge fair, and projected its pretty rainbow tints upon -the wall, his great mind little suspected that that phantom riband of -gorgeous colours would one day be called upon to give evidence upon the -probable cosmical origin of worlds. Yet such in truth has been the case. -Every substance when rendered luminous gives off light of some colour or -degree of refrangibility peculiar to itself, and although the eye cannot -detect any difference between one character of light and another, the -prism gives the means of ascertaining the quality and degree of -refrangibility of the light emanating from any source however distant, -and hence of gaining some knowledge of the nature of that source. If, -for instance, a ray of light from a solid body in combustion is passed -through a prism, a spectrum is produced which exhibits light of all -colours or all degrees of refrangibility; if the light from such a body, -before passing through the prism, be made to pass through gases or -certain metallic vapours, the resulting spectrum is found to be crossed -transversely by numbers of fine dark lines, apparently separating the -various colours, or cutting the spectrum into bands. The solar spectrum -is of this class; the once mysterious lines first observed by Wollaston, -and subsequently by Fraunhoffer, and known as “Fraunhoffer’s lines,” -have now been interpreted, chiefly by the sagacious German chemist -Kirchoff, and identified as the effects of absorption of certain of the -sun’s rays by chemical vapours contained in his atmosphere. The fixed -stars yield spectra of the same character, but varying considerably in -feature, the lines crossing the stellar spectra differing in position -and number from those of the sun, and one star from another, proving the -stars to possess varied chemical constitutions. But there is another -class of spectra, exhibited when light from other sources is passed -through the prism. These consist, not of a luminous riband of light like -the solar spectrum, but of bright isolated lines of coloured light with -comparatively wide dark spaces separating them. Such spectra are yielded -only by the light emitted from luminous gases and metals or chemical -elements in the condition of incandescent vapour. Every gas or element -in the state of luminous vapour yields a spectrum peculiar to itself, -and no two elements when vapourized before the prism show the same -combinations of luminous lines. - -Now in the course of some observations upon the spectra of the fixed -stars by Dr. Huggins, it occurred to that gentleman to turn his -telescope, armed with a spectroscope, upon some of the brighter of the -nebulæ, and great was his surprise to find that instead of yielding -continuous spectra, as they must have done had their light been made up -of that of a multitude of stars, they gave spectra containing only two -or three isolated bright lines; such a spectrum could only be produced -by some luminous gas or vapour, and of this form of matter we are now -justified in declaring, upon the strength of numerous modern -observations, these remarkable bodies are composed; and it is a curious -and interesting fact that some of the nebulæ styled resolvable, from the -fact of their exhibiting points of light like stars, yield these gaseous -spectra, whence Dr. Huggins concludes that the brighter points taken for -stars are in reality nuclei of greater condensation of the nebular -matter: and so the fact of the apparent resolvability of a nebula -affords no positive proof of its non-nebulous character. - -These observations—which have been fully confirmed by Father Secchi of -the Roman College—by destroying the evidence in favour of nebulæ being -remote clusters, add another attestation to the probability of the truth -of the nebular hypothesis, and we have now the confutation of the -luminologist to add to that of the astronomers who, in the person of the -illustrious Arago, asserted that the ideas of the great author of the -“Mécanique Céleste” “were those only which by their grandeur, their -coherence, and their mathematical character could be truly considered as -forming a physical cosmogony.” - -Confining, then, our attention to the single object of the universe it -is our task to treat of—the Moon—and without asserting as an -indisputable fact that which we can never hope to know otherwise than by -inference and analogy, we may assume that that body once existed in the -form of a vast mass of diffused or attenuated matter, and that, by the -action of gravitation upon the particles of that matter, it was -condensed into a comparatively small and compact planetary body. - -But while the process of condensation or compaction was going on, -another important law of nature—but recently unfolded to our -knowledge—was in powerful operation, the discussion of which law we -reserve for a separate Chapter. - - - - - CHAPTER II. - THE GENERATION OF COSMICAL HEAT. - - -In the preceding Chapter we endeavoured to show how the action of -gravitation upon the particles of diffused primordial matter would -result in the formation, by condensation and aggregation, of a spherical -planetary body. We have now to consider another result of the -gravitating action, and for this we must call to our aid a branch of -scientific enquiry and investigation unrecognized as such at the period -of Laplace’s speculations, and which has been developed almost entirely -within the past quarter of a century. - -The “great philosophical doctrine of the present era of science,” as the -subject about to engage our attention has been justly termed, bears the -title of the “Conservation of Force,” or—as some ambiguity is likely to -attend the definition of the term “Force”—the “Conservation of Energy.” -The basis of the doctrine is the broad and comprehensive natural law -which teaches us that the quantity of force comprised by the universe, -like the quantity of matter contained in it, is a fixed and invariable -amount, which can be neither added to nor taken from, but which is for -ever undergoing change and transformation from one form to another. That -we cannot create force ought to be as obvious a fact as that we cannot -create matter; and what we cannot create we cannot destroy. As in the -universe we see no new matter created, but the same matter constantly -disappearing from one form and reappearing in another, so we can find no -new force ever coming into action—no description of force that is not to -be referred to some previous manner of existence. - -Without entering upon a metaphysical discussion of the term “force,” it -will be sufficient for our purpose to consider it as something which -produces or resists motion, and hence we may argue that the ultimate -effect of force is motion. The force of gravity on the earth results in -the motion or tendency of all bodies towards its centre, and, similarly, -the action of gravitation upon the atoms or particles of a primeval -planet resulted in the motion of those particles towards each other. We -cannot conceive force otherwise than by its effects, or the motion it -produces. - -And force we are taught is indestructible; therefore motion must be -indestructible also. But when a falling body strikes the earth, or a -gunshot strikes its target, or a hammer delivers a blow upon an anvil, -or a brake is pressed against a rotating wheel, motion is arrested, and -it would seem natural to infer that it is destroyed. But if we say it is -indestructible, what becomes of it? The philosophical answer to the -question is this—that the motion of the mass becomes transferred to the -particles or molecules composing it, and transformed to molecular -motion, and this molecular motion manifests itself to us as heat. The -particles or atoms of matter are held together by cohesion, or, in other -words, by the action of molecular attraction. When heat is applied to -these particles, motion is set up among them, they are set in vibration, -and thus, requiring and making wider room, they urge each other apart, -and the well-known _expansion by heat_ is the result. If the heat be -further continued a more violent molecular motion ensues, every increase -of heat tending to urge the atoms further apart, till at length they -overcome their cohesive attraction and move about each other, and a -_liquid or molten condition_ results. If the heat be still further -increased, the atoms break away from their cohesive fetters altogether -and leap off the mass in the form of vapour, and the matter thus assumes -the _gaseous or vaporous form_. Thus we see that the phenomena of heat -are phenomena of motion, and of motion only. - -This mutual relation between heat and work presented itself as an embryo -idea to the minds of several of the earlier philosophers, by whom it was -maintained in opposition to the _material theory_ which held heat to be -a kind of matter or subtle fluid stored up in the inter-atomic spaces of -all bodies, capable of being separated and procured from them by rubbing -them together, but not generated thereby. Bacon, in his “Novum Organum,” -says that “heat itself, its essence and quiddity, is motion and nothing -else.” Locke defines heat as “a very brisk agitation of the insensible -parts of an object, which produces in us that sensation from whence we -denominate the object hot; so what in our sensation is _heat_, in the -object is nothing but _motion_.” Descartes and his followers upheld a -similar opinion. Richard Boyle, two hundred years ago, actually wrote a -treatise entitled “The Mechanical Theory of Heat and Cold,” and the -ingenious Count Rumford made some highly interesting and significant -experiments on the subject, which are described in a paper read before -the Royal Society in 1798, entitled “An Inquiry concerning the Source of -Heat excited by Friction.” But the conceptions of these authors remained -isolated and unfruitful for more than a century, and might have passed, -meantime, into the oblivion of barren speculation, but for the impulse -which this branch of inquiry has lately received. Now, however, they -stand forth as notable instances of truth trying to force itself into -recognition while yet men’s minds were unprepared or disinclined to -receive it. The key to the beautiful mechanical theory of heat was found -by these searching minds, but the unclasping of the lock that should -disclose its beauty and value was reserved for the philosophers of the -present age. - -Simultaneously and independently, and without even the knowledge of each -other, three men, far removed from probable intercourse, conceived the -same ideas and worked out nearly similar results concerning the -mechanical theory of heat. Seeing that motion was convertible into heat, -and heat into motion, it became of the utmost importance to determine -the exact relation that existed between the two elements. The first who -raised the idea to philosophic clearness was Dr. Julius Robert Mayer, a -physician of Heilbronn in Germany. In certain observations connected -with his medical practice it occurred to him that there must be a -necessary equivalent between work and heat, a necessary numerical -relation between them. “The variations of the difference of colour of -arterial and venous blood directed his attention to the theory of -respiration. He soon saw in the respiration of animals the origin of -their motive powers, and the comparison of animals to thermic machines -afterwards suggested to him the important principle with which his name -will remain for ever connected.” - -Next in order of publication of his results stands the name of Colding, -a Danish engineer, who about the year 1843 presented a series of memoirs -on the steam engine to the Royal Society of Copenhagen, in which he put -forth views almost identical with those of Mayer. - -Last in publication order, but foremost in the importance of his -experimental treatment of the subject, was our own countryman, Dr. Joule -of Manchester. “Entirely independent of Mayer, with his mind firmly -fixed upon a principle, and undismayed by the coolness with which his -first labours appear to have been received, he persisted for years in -his attempts to prove the invariability of the relation which subsists -between heat and ordinary mechanical power.” (We are quoting from -Professor Tyndall’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 would be generated if a pound weight, after having fallen -through a height of 772 feet, had its moving force destroyed by -collision with the earth. Conversely, the amount of heat necessary to -raise a pound of water one degree in temperature, would, if all applied -mechanically, be competent to raise a pound weight 772 feet high, or it -would raise 772 pounds one foot high. The term ‘foot pounds’ has been -introduced to express in a 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 converged under the -like influence. What would follow? Obviously the generation of an -intense heat by the arrest of converging motion, such a heat as would -result in the fusion of the whole into one mass. Mayer, in one of his -most remarkable papers (“Celestial Dynamics”) remarks that the -“Newtonian theory of gravitation, whilst it enables us to determine, -from its present form, the earth’s state of aggregation in ages past, at -the same time points out to us a source of heat powerful enough to -produce such a state of aggregation—powerful enough to melt worlds: it -teaches us to consider the molten state of a planet as the result of the -mechanical union of cosmical masses, and to derive the radiation of the -sun and the heat in the bowels of the earth from a common origin.” - -And the same laws that governed the formation of the earth, 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 -unmistakeable evidence of its original fluid or molten condition, and -the appearance 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 -conjecture and established upon the firm ground of direct calculation: -for the absolute amount of heat generated by the collision of a given -amount of matter is (of course, with some little uncertainty) deducible -from a mathematical formula. Mayer has computed the amount of heat that -the matter of the earth would have generated, if it had been formed -originally of only two parts drawn into collision by their mutual -attraction, and has found that it would be from 0 to 32,000 or 47,000[1] -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 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 -independently of such external matter as an atmosphere. The complex -mixture of gases and vapours which we term “air,” has nothing whatever -to do with the fusion of substances, whatever it may have to do with -their combustion. Combustion is a chemical phenomenon, due to the -combination of the oxygen of that air with the heated particles of the -combustible matter: oxygen is the sole supporter of 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 accumulating -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 solidification of that part of the mass which was most -free to radiate its heat into surrounding space: that part would -obviously be the outer surface. - -With the solidification of this external crust began the “year one” of -selenological history. - -The phenomena attendant upon the cooling of the mass we will consider in -the next Chapter. - - - - - CHAPTER III. - THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. - - -In the foregoing Chapters we have endeavoured to show, by the light of -modern science, first, how diffused cosmical matter was probably -condensed into a planetary mass by the mutual 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 generation of such sufficient heat as to reduce the whole mass to -a molten condition. Our present task is to consider the subsequent -cooling of the mass, and the phenomena attendant upon or resulting -therefrom. This brief Chapter is important to our subject, as we shall -have frequent occasion to refer to the leading principle we shall -endeavour to illustrate in it, in subsequently treating of the causes to -which the special selenological features are to be attributed. - -First, then, as regards the cooling of the igneous mass that 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 central part first and so began to solidify from its centre, or if -it had been possible for the mass to have cooled uniformly and -simultaneously throughout its whole depth, or that each substratum had -cooled before its superstratum, we should have had a moon whose surface -would have been smooth and without any such remarkable asperities and -excresences 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 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 -diminution of bulk or contraction of volume which accompanies the -cooling of solidified masses of previously molten substances; the second -would arise from a phenomenon which we may here observe is by no means -so generally known as from its importance it deserves to be: and as we -shall have 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 reference to it hereafter may be more clearly understood. - -The broad general principle of the phenomenon here referred to is -this:—that fusible substances are (with a few exceptions) 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 -contract 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 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 -commotion: the thin coat of scoria or molten oxide which forms on the -otherwise bright surface of the metal is seen, as fast as it forms at -the circumference of the pot, to be swept by active convergent currents -towards the centre, 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. - - [Illustration: Fig. 1.] - -Our illustration (Fig. 1) is intended, so far as such means can do so, -to convey some idea of this remarkable appearance at one instant of its -continued occurrence. To interpret our illustration rightly it is -necessary to imagine this vermicular freckling to be constantly and -rapidly streaming from all points of the periphery of the pot towards -the centre, where, as we have said, it accumulates in the form of a -floating island. We may observe that the motion is most rapid when the -hot metal is first put into the cool ladle: as the fluid metal parts -with some of its heat and the ladle gets hot by absorbing it, this -remarkable surface disturbance becomes less energetic. - -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 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 elevatory appearance as shown in the figure. It is difficult -to assign to this effect any other cause than that of an expansion and -consequent reduction of the specific gravity of the fluid metal in -contact with or in close proximity to the cooler sides of the pot, as, -according to the generally entertained idea that contraction universally -accompanies cooling, it would be impossible for the cooler to float on -the hotter metal, and the curious surface-currents above referred to -would be in contrary direction to that which they invariably take, -_i.e._, they would diverge from the centre instead of converging to it. -The external arrows in the figure represent the radiation of the heat -from the outer sides of the pot, which is the chief cause of cooling. - - [Illustration: Fig. 2.] - -Turning from cast-iron to other metals we find further manifestations of -this expansive solidification. Bismuth is a notable example. In his -lectures on Heat, Dr. Tyndall exhibited an experiment in which a stout -iron bottle was filled with molten bismuth, and the stopper tightly -closed. The whole was set aside to cool, and as the metal within -approached consolidation the bottle was rent open by its expansion, just -as would have been the case had the bottle been filled with water and -exposed to freezing temperature. Mercury affords another example. -Thermometers which have to be exposed to Arctic temperatures are -generally filled with spirit instead of quicksilver, because the latter -has been found to burst the bulbs when the cold reached the congealing -point of the metal, the 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[2] 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 5½ 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.” - - [Illustration: Fig. 3.] - -Here then we have a substance closely allied to volcanic material which -manifests the expansile principle in question; but we may go still -further and give evidence from the very fountain-head by instancing what -appears to be a most cogent example of its operation which we observed -on the occasion of a visit to the crater of Vesuvius in 1865 while a -modified eruption was in progress. On this occasion we observed -white-hot lava streaming down from apertures in the sides of a central -cone within the crater and 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. -Reference to Fig. 3, 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.) - - [Illustration: Fig. 4.] - - [Illustration: PLATE I. - CRATER OF VESUVIUS. - 1865.] - -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 circumstances, and many of the characteristic features of -volcanic action would be thus produced: the thickness of the outer -shell, the size of the vent made by the expanding matter for its escape, -and other conditions conspiring to modify the nature and extent of the -eruption. Thus there would result vast floodings of the exterior surface -of the shell by the so extruded molten matter, volcanoes, extruded -mountains, and other manifestations of eruptive phenomena. The sectional -diagram (Fig. 5) will help to convey a clear idea of this action. Basing -our reasoning on the principle we have thus enunciated, namely, that -molten telluric matter expands on nearing the point of solidification, -and which we have endeavoured to illustrate by reference to actual -examples of its operation, we consider we are justified in assuming that -such a course of volcanic phenomena has very probably occurred again and -again upon the moon; that this expansion of volume which accompanies the -solidification of molten matter furnishes a key to the solution of the -enigma of volcanic action; and that such theories as depend upon the -agency of gases, vapour, or water are at all events untenable with -regard to the moon, where no gases, vapour, or water, appear to exist. - - [Illustration: Fig. 5. A A. The solidified crust cooling, - contracting, and cracking; the cracking action enhanced by the - expansion of the substratum of molten matter, B B B, 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.] - -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 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 -terrestrial 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. - - [Illustration: Fig. 6.] - - [Illustration: Fig. 7.] - -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 underneath, is that the skin, so to term the outer stratum -of solid matter, becomes shrivelled up into alternate ridges and -depressions, or wrinkles. In its attempt to crush down and follow the -contracting substratum, 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 undulating ridges as in Fig. 6, or -broken elevated ridges as in Fig. 7, or overlappings of the outer crust -as in Fig. 8, or ridges capped by more or less fluid molten matter -extruded from beneath, as indicated in Fig. 9, a class of action which -might occur contemporaneously with the elevation of the ridge or -subsequently to its formation. - - [Illustration: Fig. 8.] - - [Illustration: 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 -mountainous part of our globe affords abundant evidence of such a cause -having been in action; such maps are pictures of wrinkles. Several parts -of the lunar surface, as we shall by-and-by see, present us with the -same appearances in a modified degree. - -To the few primary causes we have set forth in this chapter—to the -alternate expansion and contraction of successive strata of the lunar -sphere, when in a state of transition from an igneous and molten to a -cooled and solidified condition, we believe we shall be able to refer -well nigh all the remarkable and characteristic features of the lunar -surface which will come under our notice in the course of our survey. - - [Illustration: PLATE II. - BACK of HAND & SHRIVELLED APPLE. - TO ILLUSTRATE the ORIGIN of CERTAIN MOUNTAIN RANGES BY SHRINKAGE of - the GLOBE.] - - - - - CHAPTER IV. - THE FORM, MAGNITUDE, WEIGHT, AND DENSITY OF THE LUNAR GLOBE. - - -We have not hitherto had occasion to refer to what we may term the -physical elements of the moon: by which we mean the various data -concerning form, size, weight, density, &c. of that body, derived from -observation and calculation. To this purpose, therefore, we will now -devote a few pages, 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 therefore led to infer that the real form of the moon is that of a -perfect sphere. We know that the earth and the rest of the planets of -our system are spheroidal, or more or less flattened at the poles, and -we also know that this flattening is a consequence of axial rotation; -the extent of the flattening, or the oblateness of the spheroid, -depending upon the speed of that 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 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 -protuberant 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 measurement, -it is necessary to know either the distance of the moon from the earth, -or in astronomical language as leading to a knowledge of that distance, -what is the amount of the moon’s parallax. Parallax, generally, is an -apparent change of position of an object arising from change of the -point of view. The parallax of a heavenly body is the angle which the -earth would subtend if it were seen from that body. Supposing an -observer on the moon could measure the earth’s angular diameter, just as -we measure that of the moon, his measurement would represent what is -called the parallax of the moon. But we cannot go to the moon to make -such a measurement; nevertheless there is a simple method, explained in -most treatises on astronomy, which consists in 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 determination, 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:— - - 1°. 54′. 5″ : 31′. 9″ :: 7912 miles. : 2160 miles. - -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. - -Comparing these data with corresponding dimensions of the earth, we find -that the diameter of the moon is 1/3·665; the area 1/13·4245; and the -volume 1/49·1865, 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). To gain an -idea of their distance from each other it is necessary to suppose the -two discs in the diagram to be 30 inches apart; the real distance of the -moon from the earth being about 238,790 miles at its mean position. - - [Illustration: Fig. 10.] - -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 proportional to the mass_, and inversely proportional to -the 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 is -_proportional to its mass_. Knowing, then, the mass of the earth, which -we do with considerable 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 phenomena 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. - -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 5½ times that of water; that is to -say, the earth weighs 5½ times heavier than would a sphere of water of -equal size. Now a cubic foot of water weighs 62·3211 pounds, and from -this we can find by simple multiplication what is the weight of a cubic -mile of water, and, similarly, what would be the weight of 259,360 cubic -miles of water, and the last result multiplied by 5½ 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 21½ 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 5½ -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 aërolites 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. - -Reverting, now, to the mass of the moon: we must bear in mind that the -mass or weight of a planetary body determines the weight of all objects -on its surface. What we call a pound on the earth, would not be a pound -on the moon; for the following reason:—When we say that such and such an -object weighs so much, we 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 1/80 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 1/80 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 1/3·665 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 1/80 multiplied by -the square of 1/3·665; and this, worked out, is equal to 1/6. The force -of gravity upon the moon is, therefore, 1/6 of that on the earth; and -hence a pound upon the earth would be little more than 2½ 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 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 1/3·665 that of earth. - Area 14,657,000 square miles 1/13·424 ” ” - Area of the 7,328,500 square miles - visible - hemisphere - Solid contents 5276 millions of cubic miles 1/49·186 ” ” - Mass 73 trillions of tons 1/80 ” ” - Density 3·39 (water = 1) 0·62 ” ” - Force of 1/6 ” ” - gravity at - surface - Mean distance 238,790 miles. - from earth - - - - - CHAPTER V. - ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR ATMOSPHERE. - - -At the close of the preceding chapter we stated that any force acting in -opposition to that of gravity would be six times more effective on the -moon than on the earth. But, in fact, it would in many cases be still -more so; at all events, so far as projectile forces are concerned; for -the reason that “the powerful coercer of projectile range,” as the -earth’s atmosphere has been termed, has no counterpart, or at most a -very disproportionate one, upon the moon. - -The existence of an atmosphere surrounding the moon has been the subject -of considerable controversy, and a great deal of evidence on both sides -of the question has been offered from time to time, and is to be found -scattered through the records of various classes of observations. Some -of the more important items of this evidence it is our purpose to set -forth in the course of the present chapter. - -With the phenomena of the terrestrial atmosphere, with the effects that -are attributable to it, we are all well familiar, and our best course -therefore is to examine, as far as we are able, whether counterparts of -any of these effects are manifested upon the moon. For instance, the -clouds that are generated in and float through our air would, to an -observer on the moon, appear as ever changing bright or dusky spots, -obliterating certain of the permanent details of the earth’s surface, -and probably skirting the terrestrial disc, like the changing belts we -perceive on the planet Jupiter, or diversifying its features with less -regularity, after the manner exhibited by the planet Mars. If such -clouds existed on the moon it is evident that the details of its surface -must be, from time to time, similarly obscured; but no trace of such -obscuration has ever been detected. When the moon is observed with high -telescopic powers, all its details come out sharp and clear, without the -least appearance of change or the slightest symptoms of cloudiness other -than the occasional want of general definition, which may be proved to -be the result of unsteadiness or want of homogeneity in our own -atmosphere; for we must tell the uninitiated that nights of pure, good -definition, such as give the astronomer opportunity of examining with -high powers the minute details of planetary features, are very few and -far between. Out of the three hundred and sixty-five nights of a year -there are probably not a dozen that an astronomer can call really fine: -usually, even on nights that are to all common appearance superbly -brilliant, some strata of air of 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 confidently 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. - -This is but one proof against the existence of a lunar atmosphere, and, -it may be argued, not a very conclusive one; because there may still be -an atmosphere, though it be not sufficiently aqueous to condense into -clouds and not sufficiently dense to obscure the lunar details. The -probable existence of an atmosphere of such a character used to be -inferred from a phenomenon seen during total eclipses of the sun. On -these occasions the black body of the moon is invariably surrounded by a -luminous halo, or glory, to which the name “corona” has been applied; -and, further, besides this corona, apparently floating in it and -sometimes seemingly attached to the black edge of the moon, are seen -masses of cloud-like matter of a bright red colour, which, from the form -in which they were first seen and from their flame-like tinge, have -become universally known as the “red-flames.” It used to be said that -this corona could only be the consequence of a lunar atmosphere lit up -as it were by the sun’s rays shining through it, after the manner of a -sunbeam lighting up the atmosphere of a dusty chamber; and the red -flames were held by those who first observed them to be clouds of denser -matter floating in the said atmosphere, and refracting the red rays of -solar light as our own clouds are seen to do at sunrise and sunset. But -the evidence obtained, both by simple telescopic observation and by the -spectroscope, from recent extensively observed eclipses of the sun has -set this question quite at rest; for it has been settled finally and -indisputably that both the above appearances pertain to the sun, and -have nothing whatever to do with the moon. - - [Illustration: Fig. 11.] - -The occurrence of a solar eclipse offers other means in addition to the -foregoing whereby a lunar atmosphere would be detected. We know that all -gases and vapours absorb some portion of any light which may shine -through them. If then our satellite had an atmosphere, its black nucleus -when seen projected against the bright sun in an eclipse would be -surrounded by a sort of penumbra, or zone of shadow, in contact with its -edge, somewhat like that we have shown in an exaggerated degree in the -annexed cut (Fig. 11), and the passage of this penumbra over solar spots -and other features of the solar photosphere would to some extent obscure -the more minute details of such features. No such dusky band has however -been at any time observed. On the contrary, a band somewhat brighter -than the general surface of the sun has frequently been seen in contact -with the black edge of the moon: this in its turn was held to indicate -an atmosphere about the moon; but Sir George Airy has shown that a lunar -atmosphere, if it really did exist, could not produce such an -appearance, and that the cause of it must be sought in other directions. -If this effect were really due to the passage of the solar rays through -a lunar atmosphere a 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 -distinctness of the details of a solar spot, it would still manifest its -existence in another way. As the moon advances upon the sun’s disc the -latter assumes, of course, a crescent form. Now if air or vapour -enveloped the moon, the exceedingly delicate cusps of this crescent -would be distorted or turned out of shape. Instead of 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. - -Reverting 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 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 actually illuminated by direct sunlight. Twilight is -a consequence of this reflexion of light by the atmosphere when the sun -is below the horizon. If, then, an atmosphere enveloped the moon, we -should see by diffused light those parts of the lunar details that are -not receiving the direct solar beams; and before the sun rose and after -it had set upon any region of the moon, that region would still be -partially illuminated by a twilight. But, on the contrary, the shadowed -portions of a lunar landscape are pitchy black, without a trace of -diffused-light illumination, and the effects that a twilight would -produce are entirely absent from the moon. Once, indeed, one observer, -Schroeter, noticed something which he suspected was due to an effect of -this kind: when the moon exhibited itself as a very slender crescent, he -discovered a faint crepuscular light, extending from each of the cusps -along the circumference of the 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. - - [Illustration: Fig. 12.] - -Spectrum analysis would also betray the existence of a lunar atmosphere. -The solar rays falling on the moon are reflected from its surface to the -earth. If, then, an atmosphere existed, it is plain that the solar rays -must first pass through such atmosphere to reach the reflecting surface, -and returning from thence, again pass through it on their way to the -earth; so that they must in reality pass through virtually twice the -thickness of any atmosphere that may cover the moon. And if there be any -such atmosphere, the spectrum formed by the moon’s light, that is, by -the sun’s light reflected from the moon, would be modified in such a -manner as to exhibit absorption-lines different from those found in the -spectrum of the direct solar rays, just as the absorption-lines vary -according as the sun’s rays have to pass through a thinner or a denser -stratum of the terrestrial atmosphere. Guided by this reasoning, 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 -atmosphere.” - -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 violent and blue rays would lay behind the -red rays.” - -“I carefully observed the disappearance of the spectrum of η 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 _brought up_ by the refraction of its light -by the dense stratum of atmosphere through which the rays have to pass. -The sun is, therefore, exactly represented by the coin at the bottom of -the basin in the boy’s experiment, the atmosphere answers to the water, -and the horizon to the rim or edge of the basin. If there were no -atmosphere about the earth, the sun would not be so brought up above the -horizon, and, as a consequence, it would set earlier and rise later by -about a minute than it really does. This, of course, applies not merely -to the sun, but to all celestial bodies that rise and set. Every planet -and every star remains a shorter time below the horizon than it would if -there were no atmosphere surrounding the earth. - -To apply this to the point we are discussing. The moon in her orbital -course across the heavens is continually passing before, or occulting, -some of the stars that so thickly stud her apparent path. And when we -see a star thus pass behind the lunar disc on one side and come out -again on the other side, we are virtually observing the setting and -rising of that star upon the moon. If, then, the moon had an atmosphere, -it is clear, from analogy to the case of the earth, that the star must -disappear later and reappear sooner than if it has no atmosphere: just -as a star remains too short a time below the earth’s horizon, or behind -the earth, in consequence of the terrestrial atmosphere, so would a star -remain too short a time behind the moon if an atmosphere surrounded that -body. The point is settled in this way:—The moon’s apparent diameter has -been measured over and over again and is known with great accuracy; the -rate of her motion across the sky is also known with perfect accuracy: -hence it is easy to calculate how long the moon will take to travel -across a part of the sky exactly equal in length to her own diameter. -Supposing, then, that we observe a star pass behind the moon and out -again, it is clear that, if there be no atmosphere, the interval of time -during which it remains occulted ought to be exactly equal to the -computed time which the moon would take to pass over the star. If, -however, from the existence of a lunar atmosphere, the star disappears -too late and reappears too soon, as we have seen it would, these two -intervals will not agree; the computed time will be greater than the -observed time, and the 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-Royal several years ago, and it was based upon no less -than 296 occultation observations. In this determination the measured or -telescopic semidiameter of the moon was compared with the semidiameter -deduced from the occultations, upon the above principle, and it was -found that the telescopic semidiameter was greater than the occultation -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 it is -due to irradiation[3] 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 1/2000 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 Séjour, who made a rigorous examination of the -subject founded on observations of the solar eclipse of 1764. He -concluded that the horizontal refraction produced by a possible lunar -atmosphere amounted to 1″·5—a second and a half—or about 1/1400 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 1/1000 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 discussing the physical phenomena of the lunar surface, we -are, therefore, perfectly justified in omitting all considerations of an -atmosphere, and adapting our arguments to the non-existence of such an -appendage. - -And if there be no air upon the moon, we are almost forced to conclude -that there can be no water; for if water covered any part of the lunar -globe it must be vapourised under the influence of the long period of -uninterrupted sunshine (upwards of 300 hours) that constitutes the lunar -day, and would manifest itself in the form of clouds or mists obscuring -certain parts of the surface. But, as we have already said, no such -obliteration of details ever takes place; and, as we have further seen, -no evidence of aqueous vapour is manifested upon the occasion of -spectrum observations. Since, then, the effects of watery vapour are -absent, we are forced to conclude that the cause is absent also. - -Those parts of the moon which the ancient astronomers assumed, from -their comparatively smooth and dusky appearance, to be seas, have long -since been discovered to be merely extensive regions of less reflective -surface material; for the telescope reveals to us irregularities and -asperities covering well nigh the whole of them, which asperities could -not be seen if they were covered with water; unless, indeed, we admit -the possibility of seeing to the bottom of the water, not only -perpendicularly, but obliquely. Some observers have noticed features -that have led them to suppose that water was at one time present upon -the moon, and has left its traces in the form of appearances of erosive -action in some parts. But if water ever existed, where is it now? One -writer, it is true, has suggested as possible, that whatever air, and we -presume he would include whatever water also, the moon may possess, is -hidden away in sublunarean caves and hollows; but even if water existed -in these places it must sometimes assume the vapoury form, and thus make -its presence known. - - [Illustration: Fig. 13.] - - [Illustration: Fig. 14.] - -Sir John Herschel pointed out that if any moisture exists upon the moon, -it must be in a continual state of migration from the illuminated or -hot, to the unilluminated or cold side of the lunar globe. The -alternations of temperature, from the heat produced by the unmitigated -sunshine of 14 days’ duration, to the intensity of cold resulting from -the absence of any sunshine whatever for an equal period, must, he -argued, produce an action similar to that of the _cryophorus_ 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 takes place within it, and as a -consequence the water in the bulb A gives off more vapour. The -abstraction of heat from the water, which is a natural consequence of -this evaporation, causes it to freeze into a solid mass of ice. Now upon -the moon the same phenomenon would occur did the material exist there to -supply it. In the accompanying diagram let A represent the illuminated -or heated hemisphere of the moon, and B the dark or cold hemisphere; the -former being probably at a temperature of 300° above, and the latter -200° below Fahrenheit’s zero. Upon the above principle, if moisture -existed upon A it would become vapourised, and the vapour would migrate -over to B, and deposit itself there as hoarfrost; it would, therefore, -manifest itself to us while in the act of migrating by clouding or -dimming the details about the boundary of the illuminated hemisphere. -The sun, rising upon any point upon the margin of the dark hemisphere, -would have to shine through a bed of moisture, and we may justly -suppose, if this were the case, that the tops of mountains catching the -first beams of sunlight would be tinged with colour, or be lit up at -first with but a faint illumination, just as we see in the case of -terrestrial mountains whose summits catch the first, or receive the last -beams of the rising or setting sun. Nothing of this kind is, however, -perceptible: when the solar rays tip the lofty peaks of lunar mountains, -these shine at once with brilliant light, quite as vivid as any of those -parts that receive less horizontal illumination, or upon which the sun -is almost perpendicularly shining. - -All the evidence, then, that we have the means of obtaining, goes to -prove that neither air nor water exist upon the moon. Two complicating -elements affecting all questions relating to the geology of the -terraqueous globe we inhabit may thus be dismissed from our minds while -considering the physical features of the lunar surface. Fire on the one -hand and water or the other, are the agents to which the configurations -of the earth’s surface are referrable: 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 one which has the best pretensions to a rude accuracy is that first -mentioned; for the resemblance of the full moon to a human countenance, -wearing a painful or lugubrious expression, is very striking. Our -illustration of the full moon (Plate III.) is derived from an actual -photograph;[4] the relative intensities of light and shade are hence -somewhat exaggerated; otherwise it represents the full moon very nearly -as the naked eye sees it, and by gazing at the plate from a short -distance,[5] 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 philosophers, to the -effect that the spots on the moon were the shadows of opaque bodies -floating in space between it and the sun. - - [Illustration: PLATE III. - FULL MOON.] - -An observer watching the forms and positions of the lunar face-marks, -from night to night and from lunation to lunation, cannot fail to notice -the circumstance that they undergo no easily perceptible change of -position with respect to the circular outline of the disc; that in fact -the face of the moon presented to our view is always the same, or very -nearly so. If the moon had no orbital motion we should be led from the -above phenomenon to conclude that she had no axial motion, no movement -of rotation; but when we consider the orbital motion in connection with -the permanence of aspect, we are driven to the conclusion—one, 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. 11s.) 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 near the horizon he could see spots upon her uppermost -edge, which disappeared as she approached the zenith, or highest point -of her nightly path; and as she neared this point, other spots, before -invisible, came into view, near to what had been her lower edge. Galileo -was not long in referring this phenomenon to its true cause. The centre -of motion of the moon being the centre of the earth, it is clear that an -observer on the surface of the latter, looks down upon the rising moon -as from an eminence, and thus he is enabled to see more or less over or -around her. As the moon increases in altitude, the line of sight -gradually becomes parallel to the line joining the observer and the -centre of the earth, and at length he looks her full in the face: he -loses the full view and catches another side face view as she nears the -horizon in setting. This phenomenon, occurring as it does, with a daily -period, is known as the _diurnal 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 versâ_.[6] - -The extent of the moon’s librations, taking them all and in combination -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 inhabitants. 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 grâce_ from the revelations -of Galileo’s glasses. Our satellite was one of the earliest objects, if -not actually the first, upon which the Florentine turned his telescope; -and he found that the inequalities upon her surface were due to -differences in its configuration analogous to the continents and -islands, and (as might then have been thought) the seas of our globe. He -could trace, even with his moderate means, the semblance of -mountain-tops upon which the sun shone while their lower parts were in -shadow, of hills that were brightly illuminated upon their sides towards -the sun, of brightly shining elevations, and deeply shadowed -depressions, of smooth plains, and regions of mountainous ruggedness. He -saw that the boundary of sunlight upon the moon was not a clearly -defined line, as it would be if the lunar globe were a smooth sphere, as -the Aristotelians had asserted, but that the terminator was uneven and -broken into an irregular outline. From these observations the Florentine -astronomer concluded that the lunar world was covered not only with -mountains like our globe, but with mountains whose heights far surpassed -those existing upon the earth, and whose forms were strangely limited to -circularity. - -Galileo’s best telescopes magnified only some thirty times, and the -views which he thus obtained, must have been similar to those exhibited -by the smaller photographs of the moon produced in late years by Mr. De -la Rue and now familiar to the scientific public. Of course there is in -the natural moon as viewed with a small 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 photographers understand as “solarization.” - -In speaking of a view of the moon with a magnifying power of thirty, it -is necessary to bear in mind that the visible features will differ -considerably with the diameter of the object-glass of the telescope to -which this power is applied. The same details would not be seen alike -with the same power upon an object-glass of 10 inches diameter and one -of 2 inches. The superior illumination of the image in the former case -would bring into view minute details that could not be perceived with -the smaller aperture. He who would for curiosity wish to see the moon, -or any other object, as Galileo saw it, must use a telescope of the same -size and character in all respects as Galileo’s: it will not do to put -his magnifying power upon a larger telescope. With large telescopes, and -low 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 productive of no -inconvenience, it is clearly a waste of light, and it points to a rule -that the lowest power that a telescope should bear is that which gives a -pencil of light equal in diameter to the pupil of the eye under the -circumstances of brightness attendant upon the object viewed. In -observing faint objects this point assumes more importance, since it is -then necessary that all available light should enter the pupil. The -thought suggests itself that an artificial enlargement of the pupil, as -by a dose of belladonna, might be of assistance in searching for faint -objects, such as nebulæ 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 features. In the -first place we cannot help being struck with the immense preponderance -of circular or craterform asperities, and with the general tendency to -circular shape which is apparent in nearly all the lunar surface -markings; for even the larger regions known as the “seas” and the -smaller patches of the same character seem to repeat in their outlines -the round form of the craters. It is at the boundary of sunlight on the -lunar globe that we see these craterform spots to the best advantage, as -it is there that the rising or setting sun casts long shadows over the -lunar landscape, and brings elevations and asperities into bold relief. -They vary greatly in size, some are so large as to bear an estimable -proportion to the moon’s diameter, and the smallest are so minute as to -need the most powerful telescopes and the finest conditions of -atmosphere to perceive them. It is doubtful whether the smallest of them -have ever been seen, for there is no reason to doubt that there exist -countless numbers that are beyond the revealing powers of our finest -telescopes. - -From the great number and persistent character of these -circumvallations, 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_ and -_fens_. We see that many of these “seas” are partially surrounded by -ramparts or bulwarks which, under closer examination, and having regard -to their real magnitude, resolve themselves into immense mountain -chains. The general resemblance in form which the bulwarked plains thus -exhibit to the circular craters of large size, would lead us to suppose -that the two classes of objects had the same formative origin, but when -we take into account the immense size of the former, and the process by -which we infer the latter to have been developed, the supposition -becomes untenable. - -Another of the prominent features which we notice as highly curious, and -in some phases of the moon—at about the time of full—the most remarkable -of all, are certain bright lines that appear to radiate from some of the -more conspicuous craters, and extend 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 surrounding surface, -others are basins hollowed out of that surface and with low surrounding -ramparts; some are merely like walled plains or amphitheatres with flat -plateaux, while the majority have their lowest point of hollowness -considerably below the general level of the surrounding surface; some -are isolated upon the plains, others are aggregated into a thick crowd, -and overlapping and intruding upon each other; some have elevated peaks -or cones in their centres, and some are without these central cones, -while the plateaux of others again contain several minute craters -instead; some have their ramparts whole and perfect, others have them -breached or malformed, and many have them divided into terraces, -especially on their inner sides. - -In the plains, what with a low power appeared smooth as a water surface -becomes, under greater magnification, a rough and furrowed area, here -gently undulated and there broken into ridges and declivities, with now -and then deep rents or cracks extending for miles and spreading like -river-beds into numerous ramifications. Craters of all sizes and classes -are scattered over the plains; these appear generally of a different -tint to the surrounding surface, for the light reflected from the plains -has been observed to be slightly tinged with colour, The tint is not the -same in all cases: one large sea has a dingy greenish tinge, others are -merely grey and some others present a pale reddish hue. The cause of -this 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 of 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 subsequent Chapter. - -A high power gives us little more evidence than a low one upon the -nature of the long bright streaks that radiate from some of the more -conspicuous craters, but it enables us to see that those streaks do not -arise from any perceptible difference of level of the surface—that they -have no very definite outline, and that they do not present any sloping -sides to catch more sunlight, and thus shine brighter, than the general -surface. Indeed, one great peculiarity of them is that they come out -most forcibly where the sun is shining perpendicularly upon them; hence -they are best seen where the moon is at full, and they are not visible -at all at those regions upon which the sun is rising or setting. We also -see that they are not diverted by elevations in their path, as they -traverse in their course craters, mountains, and plains alike, giving a -slight additional brightness to all objects over which they pass, but -producing no other effect upon them. To employ a commonplace simile, -they look as though, after the whole surface of the moon had assumed its -final configuration, a vast brush charged with a whitish pigment had -been drawn over the globe in straight lines radiating from a central -point, leaving its trail upon everything it touched, but obscuring -nothing. - -Whatever may be the cause that produces this brightness of certain parts -of the moon without reference to configuration of surface, this cause -has not been confined to the formation of the radiating lines, for we -meet with many isolated spots, streaks and patches of the same bright -character. Upon some of the plains there are small areas and lines of -luminous matter possessing peculiarities similar to those of the -radiating streaks, as regards visibility with the high sun, and -invisibility when the solar rays fall upon them horizontally. Some of -the craters also are surrounded by a kind of aureole of this highly -reflective matter. A notable specimen is that called _Linné_, 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 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 reasonably demand -some information upon this point. It might have been instructive to have -cited the various details that may be said to come into view with -progressive increases of magnification. But this would be an all but -impossible task, on account of the varying conditions under which all -astronomical observations must necessarily be made. When we come to -delicate tests, there are no standards of telescopic power and -definition. Assuming the instrument to be of good size and high optical -character, there is yet a powerful 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 development 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 realize 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 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 magnification -shows it, the result would leave little to be wished for. - -But it may be asked by some, what is the absolute effect of such powers -as those we have spoken of, in bringing the moon apparently nearer to -our eyes? and what is the actual size of the smallest object visible -under the most favourable circumstances? A linear mile upon the moon -corresponds to an angular interval of 0·87 of a second; this refers to -regions about the centre of the disc; near the circumference the -foreshortening makes a difference, very great as the edge is approached. -Perhaps the smallest angle that the eye can without assistance -appreciate is half a minute; that is to say, an object that subtends to -the eye an arc of less than a half a minute can scarcely be seen.[7] -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. - - - - - CHAPTER VII. - TOPOGRAPHY OF THE MOON. - - -It is scarcely necessary to seek the reasons which prompted astronomers, -soon after the invention of the telescope, to map the surface features -of the moon. They may have considered it desirable to record the -positions of the spots upon her disc, for the purpose of facilitating -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. Riccioli, 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 engraved -about the year 1692. It appears to have been regarded as a standard -work, for a reduced copy of it was repeatedly issued with the yearly -volumes of the _Connaissance des Temps_, (the “Nautical Almanac” 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 -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 -(conspicuous craters) were accurately laid down by reference to the -edges of the disc: one hundred and seventy-six of these triangles, -plotted accurately upon an orthographic 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 -“terminator” in the following manner. In the annexed figure (Fig. 15) -let the circle represent the moon and M a mountain upon it: let S A be -the line of direction of the sun’s rays, passing the normal surface of -the moon at A and just tipping the mountain top. A will be the -terminator, and there will be darkness between it and the star-like -mountain summit M. The distance between A and M is measured: the -distance A B is known, for it is the moon’s radius. And since the line S -M is a 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 -6·39 English feet. The nomenclature of lunar features adopted by Beer -and Maedler is that introduced by Riccioli: mountains and features -hitherto undistinguished were named by them after ancient and modern -philosophers, in continuance of Riccioli’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. - - [Illustration: Fig. 15.] - - [Illustration: PLATE IV. - PICTURE MAP OF THE MOON.] - - [Illustration: PLATE V. - Skeleton Map of Moon - To Accompany Picture Map, Chap. VII] - -The excellent map resulting from the arduous labours of these -astronomers is simply a map: it does not pretend to be a picture. The -asperities and depressions are symbolized by a conventional system of -shading and no attempt is made to exhibit objects as they actually -appear in the telescope. A casual observer comparing details on the map -with the same details on the moon itself would fail to identify or -recognize them except where the features are very conspicuous. Such an -observer would be struck by the shadows by which the lunar objects -reveal themselves: he would get to know them mostly by their shadows, -since it is mainly by those that their forms are revealed to a -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 -identification 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 Ordinates. - - Abulfeda 107 30·0 120·7 - Agrippa 151 31·2 110·0 - Airy 93 34·7 123·0 - Albategnius 109 35·5 119·7 - Aliacensis 61 35·8 131·0 - Almanon 94 29·0 122·3 - Alpetragius 92 40·8 122·4 - Alphonsus 110 39·6 120·9 - Apianus 62 33·6 129·3 - Apollonius 154 6·5 109·5 - Arago 152 24·7 108·7 - Archimedes 191 40·3 95·8 - Aristarchus 176 62·3 99·2 - Aristillus 190 37·0 93·3 - Aristotle 209 30·0 84·6 - Arzachael 84 39·5 124·0 - Atlas 228 20·7 86·6 - Autolycus 189 36·8 95·5 - Azophi 76 30·7 126·8 - Bacon 17 32·5 142·0 - Baily 207 26·0 85·4 - Barocius 34 31·8 138·5 - Bessel 179 27·4 100·1 - Bettinus 11 48·8 144·9 - Bianchini 215 51·6 86·3 - Billy 121 64·3 121·4 - Blancanus 12 43·7 144·8 - Bonpland 110 48·5 117·6 - Borda 56 15·2 131·0 - Boscovich 160 31·1 106·8 - Bouvard 40 66·6 134·3 - Briggs 196 68·0 97·2 - Bullialdus 86 50·1 125·5 - Burg 206 25·5 87·5 - Calippus 199 32·4 90·3 - Campanus 71 52·3 129·0 - Capella 104 17·8 118·0 - Capuanus 43 50·5 132·8 - Casatus 7 43·7 147·0 - Cassini 200 35·5 89·7 - Catherina 95 24·7 124·0 - Cavalerius 144 71·2 109·5 - Cavendish 88 63·5 127·4 - Cichus 44 47·3 132·8 - Clavius 13 41·8 143·5 - Cleomides 183 10·7 97·0 - Colombo 98 12·8 122·7 - Condamine 214 48·7 84·2 - Condorcet 164 4·5 104·7 - Copernicus 147 49·8 107·0 - Cyrillus 96 23·5 121·3 - Damoiseau 124 69·2 117·2 - Davy 113 43·2 119·8 - Deambrel 129 26·8 113·5 - Delisle 195 55·7 95·2 - Descartes 106 28·5 119·3 - Diophantus 194 55·5 96·3 - Doppelmayer 70 58·6 129·6 - Encke 140 59·7 110·6 - Endymion 227 20·6 83·8 - Epigenes 223 39·0 79·5 - Erastothenes 168 44·6 104·0 - Eudoxus 208 29·7 88·0 - Fabricius 35 20·0 136·8 - Fernelius 37 35·1 134·8 - Firmicus 156 5·8 107·7 - Flamsteed 126 62·8 114·5 - Fontana 122 65·9 123·0 - Fontenelle 221 43·0 81·3 - Fourier 67 62·5 130·7 - Fracastorius 78 20·5 127·0 - Furnerius 52 11·7 133·0 - Gambart 138 47·2 112·2 - Gartner 224 26·5 82·3 - Gassendi 90 59·7 123·3 - Gauricus 46 43·5 132·5 - Gauss 201 10·3 90·3 - Gay Lussac 169 50·1 103·8 - Geber 83 29·6 124·8 - Geminus 187 13·0 93·0 - Gérard 218 63·7 88·8 - Goclenius 101 11·8 118·5 - Godin 135 31·3 111·7 - Grimaldi 125 70·8 116·3 - Gruemberger 6 41·4 145·8 - Gueriké 114 46·5 119·6 - Guttemberg 102 13·9 118·3 - Inghirami 27 61·3 138·9 - Isidorus 103 16·7 118·0 - Kant 105 25·8 118·5 - Kepler 146 60·0 108·0 - Kies 72 49·7 128·8 - Kircher 10 47·5 145·8 - Klaproth 8 43·5 146·7 - La Caille 74 37·5 126·8 - Lagrange 68 67·0 131·3 - La Hire 177 54·3 99·3 - Lalande 117 43·4 115·3 - Lambert 193 49·6 97·8 - Landsberg 127 54·0 113·0 - Langreen 100 6·3 117·7 - Letronne 120 62·0 119·0 - Licetus 21 34·1 139·6 - Lichtenberg 197 66·5 94·9 - Linnæus 188 31·7 95·7 - Littrow 185 20·5 99·4 - Lohrman 143 71·3 112·8 - Longomontanus 23 45·7 140·6 - Lubiniezky 91 51·3 123·5 - Macrobius 182 13·7 100·2 - Maginus 22 40·0 140·4 - Mairan 217 56·7 89·5 - Manilius 167 32·2 103·9 - Manzinus 4 31·3 146·0 - Maraldi 181 18·6 100·8 - Marius 171 65·0 105·5 - Maskelyne 132 19·5 111·0 - Mason 204 23·7 88·8 - Maupertius 213 48·7 85·8 - Maurolycus 33 31·8 137·0 - Menelaus 165 28·3 103·0 - Mercator 65 51·4 130·2 - Mersenius 89 61·7 125·7 - Messala 202 14·0 90·5 - Messier 131 10·8 114·0 - Metius 36 18·8 105·9 - Moretus 5 39·5 146·5 - Moesting 128 41·6 113·2 - Neander 57 18·7 131·0 - Nearchus 18 26·8 142·0 - Newton 1 41·0 147·7 - Nonius 49 36·5 133·2 - Olbers 172 73·0 107·7 - Pallas 149 38·6 109·5 - Parrot 108 35·8 121·6 - Petavius 80 9·5 127·5 - Phocylides 25 55·5 141·6 - Piazzi 41 65·0 133·5 - Picard 163 8·3 104·7 - Piccolomini 58 21·7 131·0 - Pico 211 41·9 87·3 - Pitatus 63 44·1 130·2 - Plana 205 24·8 88·8 - Plato 210 41·8 84·8 - Playfair 75 33·5 127·5 - Pliny 165 24·2 103·4 - Poisson 60 32·8 131·0 - Polybius 82 24·5 125·6 - Pontanus 59 29·0 130·2 - Posidonius 186 22·2 94·3 - Proclus 162 11·4 104·5 - Ptolemy 111 39·5 118·2 - Purbach 73 38·7 128·4 - Pythagoras 220 53·0 81·2 - Pytheas 178 49·7 100·4 - Ramsden 42 52·9 132·5 - Reamur 118 37·3 114·6 - Reiner 145 67·3 108·5 - Reinhold 139 51·5 111·2 - Repsold 219 60·2 85·7 - Rheita 51 16·1 134·2 - Riccioli 142 72·7 113·8 - Riccius 50 23·7 133·5 - Ritter 134 26·0 111·6 - Roemer 184 18·3 97·6 - Ross 161 25·0 105·3 - Sabine 133 25·0 112·0 - Sacrobosco 77 27·5 127·7 - Santbech 79 15·7 126·8 - Saussure 31 39·6 137·7 - Scheiner 14 45·5 143·5 - Schickard 28 59·0 137·5 - Schiller 24 51·3 141·0 - Schroeter 137 42·3 110·7 - Schubert 155 2·3 110·8 - Segner 16 51·3 143·5 - Seleucus 174 69·0 99·8 - Sharp 216 54·2 87·7 - Short 2 39·7 147·4 - Silberschlag 157 32·0 108·1 - Simpelius 3 35·8 147·7 - Snell 55 11·3 129·6 - Soemmering 136 42·8 112·2 - Stadius 148 45·6 107·0 - Stevinus 53 11·9 130·7 - Stoefler 32 35·6 136·8 - Strabo 226 23·2 81·6 - Struve 203 18·3 88·7 - Taruntius 153 11·7 109·0 - Taylor 130 27·6 116·2 - Thales 225 24·3 81·8 - Thebit 85 40·8 126·8 - Theophilus 97 22·3 120·0 - Timæus 222 38·3 80·8 - Timocharis 192 45·1 97·0 - Tobias Mayer 170 54·5 103·0 - Triesnecker 150 35·5 109·8 - Tycho 30 43·0 142·3 - Ukert 159 37·1 107·5 - Vasco de Gama 173 72·8 104·9 - Vendelinus 99 6·8 121·6 - Vieta 69 64·3 129·7 - Vitello 66 55·8 130·7 - Vitruvius 180 20·1 102·0 - Vlacq 19 25·0 140·1 - 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 -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 débris 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 magnitude of such objects should -be, so to speak, educated upon a special standard. It is for this reason -we are anxious our reader, when examining our illustrations, should -constantly refer 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; 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 -impossible to establish the fact of coincidence of the sites of the two -by actual simultaneous observation. - -At the east side of the rampart, multitudes of these 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 -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. - - - 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 mountains. This -great crack is worthy of attention, as giving evidence of the -deep-seated nature of the force which occasioned it, inasmuch as it -disregards all surface impediments, traversing plain and group of -mountains alike. - -There are several other features in and around these two magnificent -craters well worthy of careful observation and scrutiny, all of them -excellent 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 -illustrations, is the very remarkable display of chasms or cracks which -may be seen to the west side of it. Several of these great cracks -obviously diverge from a small crater near the west external bank of the -great one, and they subdivide or branch out, as they extend from the -apparent point of divergence, while they are crossed or intersected by -others. These cracks or chasms (for their width merits the latter -appellation) are nearly one mile broad at the widest part, and after -extending to fully 100 miles, taper away till they become invisible. -Although they are not test objects of the highest order of difficulty, -yet to see them with perfect distinctness requires an instrument of some -perfection and all the conditions of good vision. When such are present, -a keen and practised eye will find many details to rivet its attention, -among which are certain portions of the edges of these cracks or chasms -which have fallen in and caused interruptions to their continuity. - - - THEOPHILUS, 97 (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:— - -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 débris. - -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. - - - THEBIT, 85—(40·8—126·8). - -A crater about 32 miles in diameter and about 9700 feet deep, devoid of -a central cone. It appears on the upper part and near the middle of -Plate XIII. The plateau has five minute craters upon it. On the east -outside are two small craters, the lesser of which, about 2·75 miles -diameter, has a central cone. We specially note this fact, because it is -the smallest crater but one in which we have detected a central cone: no -doubt, however, many smaller craters possess this unmistakable stamp of -true volcanic origin, but so minute are the specks of light which the -central cones of such very small craters reflect, that they fail to be -visible to us. - -East of Thebit is a very remarkable straight cliff 60 miles long by -about 1000 feet high, called by some observers the “Railway,” and -apparently the result either of an upheaval or of a down-sinking of the -surface of the circular area across whose diameter it stretches. - -Under moderate magnifying power, this cliff appears straight, but with -higher power and under favourable conditions, its face is seen to be -serrated, and along the upper edge may be detected several very minute -craters. A more conspicuous small crater is seen at the north end of the -cliff. To the east of the cliff nearly opposite the centre are two -craters, from the east side of the larger of which proceeds a fine crack -parallel to the cliff and passing through a dome-shaped hill of low -eminence. - - - 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 interesting 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 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 magnifying power of from 200 to -300, it is seen to be a vast flat-bottomed valley bordered by gigantic -mountains, some of which attain heights upwards of 10,000 feet; towards -the south-east of this remarkable valley, and on both sides of it, are -groups of 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 -circumstances, 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 -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 -sunshine by which it is illuminated, contrasted with the intensely black -lunar heavens studded with stars shining with a steady brightness of -which, by reason of _our_ atmosphere intervening, we can have no -adequate conception save by the aid of a well-directed 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 XI. - -The interior of this fine crater presents striking examples of the -concentric terrace-like formations that we have elsewhere assigned to -vast landslip actions. Somewhat similar concentric terraces may be -observed in other lunar craters; some of these, however, appear to be -the results of some temporary modification of the ejective force, which -has caused the formation of more or less perfect inner 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 finding a comparatively free exit by the vent of Tycho, -so relieved the district immediately around him as to have thereby -reduced, in amount, the exit of the molten matter, and so left a zone -comparatively free from the extruded lava which, 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. - - - “WARGENTIN,” 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 -parallel to the surface of the plateau. - -To the north of this interesting object is the magnificent ring -formation Schickard, whose vast diameter of 123 miles contrasts -strikingly with that of the sixteen small craters within his rampart, -and equally so with a multitude of small craters scattered around. There -are many objects of interest on the portion of the lunar surface -included within our illustration, but as they are all of the usual type, -we shall not fatigue the attention of our readers by special -descriptions of them. - - -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 characteristic a feature in the structure of lunar craters. Several -very notable cracks or chasms may be seen to the north of these two -craters. They are contorted in a very unusual and remarkable manner, the -result probably of the force which formed them having to encounter very -varying resistance near the surface. - -Some parts of these chasms gape to the width of two to three miles, and -when closely scrutinized are seen to be here and there partly filled by -masses which have fallen inward from their sides. Several smaller -craters are scattered around, which, together with the great chasms and -neighbouring ridges, give evidence of varied volcanic activity in this -locality. We must not omit to draw attention to the parallelism or -general similarity of “strike” in the ridges of extruded matter; this -appearance has special interest in the eyes of geologists, and is well -represented in our illustration. - -Aristarchus is specially remarkable for the extraordinary 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 -unpractized 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 Intrusive 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 practized eye -to detect them. In the interior of the great crater Walter a remarkable -group of small craters may be observed surrounding his central cone, -which in this instance is not so perfectly in the centre of the rampart -as is usually the case. The number of small craters which we have -observed within the rampart is 20, exclusive of those on the rampart -itself. The entire group represented in the Plate suggests in a striking -manner the wild scenery which must characterize many portions of the -lunar surface; the more so if we keep in mind the vast proportions of -the objects which they comprise, upon which point we may remark that the -smallest crater represented in this Plate is considerably larger than -that of Vesuvius. - - - ARCHIMEDES, 191 (40·3—95·8), AUTOLYCUS, 189 (36·8—95·5), ARISTILLUS, - 190 (37·0—93·3), and the APENNINES. 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 -interesting portions of the lunar surface. If the reader is not -acquainted with what the telescope can reveal as to the grandeur of the -effect of sunrise on this very remarkable portion of the moon’s surface, -he should carefully inspect and study our illustration of it; and if he -will pay due regard to our previously repeated suggestion concerning the -attached scale of miles, he will, should he have the good fortune to -study the actual objects by the aid of a telescope, be well prepared to -realize and duly appreciate the magnificence of the scene which will be -presented to his sight. - -Were we to attempt an adequate detail description of all the interesting -features comprised within our illustration, it would, of itself, fill a -goodly volume; as there is included within the space represented every -variety of feature which so interestingly characterizes the lunar -surface. All the more prominent details are types of their class; and -are so favourably situated in respect to almost direct vision, as to -render their nature, forms, and altitudes above and depths below the -average surface of the moon most distinctly and impressively cognizable. - -Archimedes is the largest crater in the group; it has a diameter of -upwards of 52 miles, measuring from summit to summit of its vast -circular rampart or crater wall, the average height of which, above the -plateau, is about 4300 feet; but some parts of it rise considerably -higher, and, in consequence, cast steeple-like shadows across the -plateau when the sun’s rays are intercepted by them at a low angle. The -plateau of this grand crater is devoid of the usual central cone. Two -comparatively minute but beautifully-formed craters may be detected -close to the north-east interior side of the surrounding wall of the -great crater. Both outside and 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 -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 upheaving -action, which there is every reason to suppose originated at a profound -depth, since the general surface on each side of the crater does not -appear to be disturbed as to altitude, which would have been the case -had the upheaving action been at a moderate depth beneath. We would -venture to ascribe a depth of not less than ten miles as the most -moderate 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 détour, 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 special circumstances -which add greatly to the impressiveness of lunar scenery, such as the -unchanging pitchy-black aspect of the heavens and the death-like silence -which reigns unbroken there. - -These digressions are, in some respects, a 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 Apennines, extending to a -length of upwards of 450 miles. This mountain group rises gradually from -a comparatively level surface towards the south-west, in the form of -innumerable comparatively small 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 -interspersed 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 landscape -view of a small portion of this mountain range. - - [Illustration: PLATE VI. - TERRESTRIAL AND LUNAR VOLCANIC AREAS COMPARED. - PORTION OF THE MOON’S SURFACE.] - - [Illustration: VESUVIUS AND NEIGHBOURHOOD OF NAPLES.] - - - - - CHAPTER VIII. - ON LUNAR CRATERS. - - -As we stated in our brief general description of the visible hemisphere -of the moon, and as a cursory glance at our map and plates will have -shown, the predominant features of the lunar surface are the circular or -amphitheatrical formations that, by their number, and from their almost -unnatural uniformity of design, induced the belief among early observers -that they must have been of artificial origin. In proceeding now to -examine the details of our subject with more minuteness than before, -these annular formations claim the first share of our attention. - -By general acceptation the term “crater” has been used to represent -nearly all the circular hollows that we observe upon the moon; and -without doubt the word in its literal sense, as 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 height 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 conditions which have determined the -surface-formation have been identical with those that have obtained upon -the moon; and as a result we have some terrestrial volcanic districts -that, could we view them under the same circumstances, would be -identical in character with what we see by telescopic aid upon our -satellite. The most remarkable case of this similarity is offered by a -certain tract of the volcanic area about Naples, known from classic -times as the _Campi Phlegræi_, 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 -contains 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 neighbourhood of Naples, and the lunar a -portion of the surface about the crater Theophilus. - -In comparing these volcanic circles together, we are however brought -face to face with a striking difference that exists between the lunar -and terrestrial craters. This is the difference of magnitude. None of -those Plutonian amphitheatres included in the terrestrial area depicted -exceed a mile in diameter, and few larger volcanic vents than these are -known upon the earth. Yet when we turn to the moon, and measure some of -the larger craters there, we are astonished to find them ranging from an -almost invisible minuteness to 74 miles in diameter. The same -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[8] 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 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 Vesuvius (Fig. 3) in the middle of its -present crater—where can we say that the action in question ceased to -manifest itself on the moon, seeing that there is no break in the -continuity of the crater-and-cone system upon the moon anywhere between -craters of 1¾ 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. - - [Illustration: Fig. 16.] - -And in the first place let us take a passing glance at the probable -formative process of a terrestrial volcano. Rejecting the hypothesis of -Von Buch, which geologists have on the whole found to be untenable, and -which ascribes the formation of all mountains to the elevation of the -earth’s crust by some thrusting power beneath, we are led to regard a -volcano as a pyramid of ejected matter, thrown out of and around an -orifice in the external solid shell of the earth by commotions -engendered in its molten nucleus. What is the precise nature and source -of the ejective force geologists have not perfectly agreed upon, but we -may conceive that highly expanded vapour, in all probability steam, is -its primary cause. The escaping aperture may have been a weak place -since the foundations of the earth were laid, or it may have been formed -by a local expansion of the nucleus in the act of cooling, upon the -principle enunciated in our Third Chapter; or, again, the expansile -vapour may have forced its own way through that point of the confining -shell that 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 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 Strabo, in the century preceding the Christian -Era. - - [Illustration: PLATE VII - DIAGRAM OF LUNAR CRATERS FORMING A SERIES RANGING FROM 1¾ MILES TO - 78 MILES DIAMETER. ALL CONTAINING CENTRAL CONES.] - - [Illustration: Fig. 17.] - - [Illustration: Fig. 18.] - -Now this form may have been assumed under two conditions. If, as -Phillips has surmised, the mountain originally had a peaked summit with -but a small crater-orifice at the point, then we must ascribe its -decapitation to a subsequent eruption which in its violence carried away -the upper portion, either suddenly, or through a comparatively slow -process of grinding away or widening out of the sides of the orifice by -the chafing or fluxing action of the out-going materials. But it is -probable that the mountain never had the perfect summit indicated in our -first outline. The violent outburst that caused the great 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 eruption than that which -originally produced it is well established. We have only to glance at -the sketch, Fig. 18, of the outline of Vesuvius as it appeared between -the years A.D. 79 and 1631 to see how the old crater was enlarged by the -terrible Pompeian eruption of the first-mentioned year. Here we have a -crater ground and blown away till its original diameter of a mile and -three-quarters has been increased to nearly three miles. Scrope had no -hesitation in expressing his conviction that the external rings, such as -those of Santorin, St. Jago, St Helena, the Cirque of Teneriffe, the -Curral of Madeira, the cliff range that surrounds the island of Bourbon, -and others of similar form and structure, however wide the area they -enclose, are truly the “basal wrecks” of volcanic mountains that have -been blown into the air 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 débris. Still the vent strives to keep -itself open, and now and then gives out a small delivery of cindery -matter, which, being piled around the vent, after the manner of its -great prototype, forms the inner cone. 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. - - [Illustration: Fig. 19.] - -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. Then the appearance was that shown in Fig. 19, and which does -not differ greatly from that presented in the present day. The summit of -the Vesuvian cone, however, has been variously altered; it has been -blown away, leaving a large crateral hollow, and it has rebuilt itself -nearly upon its former model. - -When we transfer our attention to the volcanoes of the moon, we find -ourselves not quite so well favoured with means for studying the process -of their formation; for the sight of the building up of a volcanic -mountain such as man has been permitted to behold upon the earth has not -been allowed to an observer of the moon. The volcanic activity, -enfeebled though it now be, of which we are witnesses from time to time -on the earth, has altogether ceased upon our satellite, and left us only -its effects as a clue to the means by which they were produced. If we in -our time could have seen the actual throwing up of a lunar crater, our -task of description would have been simple; as it is we are compelled to -infer the constructive action from scrutiny of the finished structure. - -We can scarcely doubt that where a lunar crater bears general -resemblance to a terrestrial crater, the process of formation has been -nearly the same in the one case as in the other. Where variations -present themselves they may reasonably be ascribed to the difference of -conditions pertaining to the two spheres. The greatest dissimilarity is -in the point of dimensions; the projection of materials to 20 or more -miles distance from a volcanic vent appears almost incredible, until we -realize the full effect of the conditions which upon the moon are so -favourable to the dispersive action of an eruptive force. In the first -place, the force of gravity upon our satellite is only one-sixth of that -to which bodies are subject upon the earth. Secondly, by reason of the -small 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 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 -terrestrial volcanoes; the contact of water with the molten nucleus of -our globe is accepted as a probable means whereby volcanic commotions -are set up and ejective action is generated. But we are debarred from -referring to steam as an element of lunar 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. - - [Illustration: PLATE VIII. - COPERNICUS.] - -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 -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, to 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 -explanation, 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 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 -explanation, but it is compatible with the expansion on consolidation -theory, since it is reasonable to suppose that in the process of -consolidation the viscous nucleus would manifest its increase of bulk -over considerable areas, 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 scarcely 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 “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 probability, supposing every lunar crater to have once been a -mountain, that in every case the mountain’s summit should have been -blown away; and we are therefore compelled to consider that the moon’s -volcanic craters were formed by one continuous outburst, and that their -“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. - - [Illustration: Fig. 20.] - - [Illustration: Fig. 21.] - - [Illustration: PLATE IX. - THE LUNAR APENNINES, ARCHIMEDES &c., &c.] - -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 -subsurface matter of the moon, resulting in the creation of a mere -“star” or crack in and through the outermost and solid crust. As we -shall have to rely upon diagrams to explain the more complicated -features, we give one of this elementary stage also as a commencement of -the series; and Fig. 20 therefore represents a probable section of the -lunar surface at a point which was subsequently the location of a -crater. From the vent thus formed we conceive the pent-up matter to have -found its escape, not necessarily at a single outburst, but in all -probability in a paroxysmal manner, as volcanic action manifests itself -on our globe. The first outflow of molten material would probably -produce no more than a mere hill or tumescence as shewn sectionally in -Fig. 21; and if the ejective force were small this might increase to the -magnitude of a mountain by an exudative process to be alluded to -hereafter. But if the ejective force were violent, either at the moment -of the first outburst or at any subsequent paroxysm, an action -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 continuance of -this action would be accompanied by an enlargement of the conical cavity -or crater, not only by the outward rush of the violently discharged -material, but also by the “sweating” or grinding action of such of it as -in descending fell 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. - - [Illustration: Fig. 22.] - -So long as each succeeding paroxysm was greater than its predecessor, -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 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-given -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. - - [Illustration: Fig. 23.] - - [Illustration: Fig. 24.] - -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 hypothetically for the moon in Fig. 25; and the parallel -phenomena for the earth are shewn by the actual case (represented in -Fig. 26 and on Plate I.) of Vesuvius as it was seen by one of the -authors in 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. - - [Illustration: Fig. 25.] - -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 all such, however: there may have been many craters, -the formation of 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, 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. - - [Illustration: PLATE X. - ARISTOTLE & EUDOXUS.] - - [Illustration: Fig. 26.] - - [Illustration: Fig. 27.] - - [Illustration: Fig. 28.] - -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 basin’s 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 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 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). - - [Illustration: Fig. 29.] - -The question occurs whether other craters may not have been similarly -filled and have emptied themselves by the bursting of the wall under the -pressure of the accumulated lake of lava within. We know that this -breaching is a common phenomenon in the volcanoes of our globe; the -district of Auvergne furnishing us with many examples; and there are -some suspicious instances upon the 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. Erastothenes, Bulialdus, -Maurolicus, Petavius, Langreen, and Gassendi, are a few among many -instances of craters possessing more than a central single cone. This -multiplication of peaks upon the moon doubtless arose from similar -causes to those which produce the same feature in terrestrial volcanoes. -Our sketch of Vesuvius in 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 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. - - [Illustration: PLATE XI. - TRIESNECKER.] - - [Illustration: Fig. 30.] - -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 -secondary eruption has been of somewhat violent character, for it may -almost be regarded as an axiom that violent eruptions excavate craters -and weak ones pile up cones. In the cases referred to it seems -reasonable to suppose that the main vent has been the channel for an -up-cast of material, but that at some depth below the surface this -material met with some obstruction or cause of diversion, and that it -took a course which brought it out far away from the cone upon the floor -of the plateau. It might even be carried so far as to be upon the -rampart, and it is no uncommon thing to see small craters in such a -situation, though when they appear at such a distance from the primary -vent, it seems more reasonable to suppose that they do not belong to it -but have arisen from a subsequent and an independent action. - - [Illustration: Fig. 31.] - -We find scarcely an instance of a small crater occurring just in the -centre of a large one, or taking the place of the cone. This is a -curious circumstance. Whenever we have any central feature in a great -crater, that feature is a cone. The tendency of this fact is to prove -that cones were produced by very weak efforts of this expiring force, -for had there been any strength in the last paroxysm it is presumable -that it would have blown out and left a crater. No very violent -eruptions have therefore taken place from the vents that were connected -with the great craters of the moon, nothing more powerful than could -produce a cone of exudation or a cinder-heap. And with regard to cones, -it is noteworthy that whether they be single or multiple, they never -rise so high as the circular ramparts of their respective craters. This -supports the inferred connexion between the crater origin and the cone -origin, for supposing the two to have been independent, a supposition -untenable in view of the universality of the central position of the -cone, it is scarcely conceivable that the mountains should have 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 other cases very large segments have slid down and -lie in segmental masses on the plateaux or form terraces around the -rampart. Aristarchus, 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-bye; 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 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. - - [Illustration: PLATE XII. - THEOPHILUS CYRILLUS & CATHARINA. - SUNSET ASPECT.] - -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 -neighbourhood and blew a portion of it away; or it may have been that -the same deep-seated vent at different times gave forth discharges of -material the courses of which were more or less diverted on their way to -the surface. - -We have before alluded to the frequent occurrence of lines of craters -upon the moon. In these lines the overlapping is frequently visible; it -is seen in Plate XII. before referred to, 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 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 disturbing the earth by its writhings, is a mythological attempt to -explain the particular case in that region. - -The capricious manner in which these intrusions occur is very curious. -Very commonly a small crater appears upon the very rampart of a greater -one, and a more diminutive one still will appear upon the rampart of the -parasite. Stoeffler presents us with one example of this character, -Hipparchus with another, Maurolycus with a third, and these are but a -few cases of many. Here and there we observe several craters ranged in a -line with their rims in one direction all perfect, and the whole -appearing like a row of coins that have fallen from a heap. There is an -example near to Tycho which we reproduce in Plate XX. In this case one -is led to conjecture that the ejective agency, after exerting itself in -one spot, travelled onward and renewed itself for a time; that it ceased -after forming crater number two, and again journeyed forward in the same -line, recommencing action some miles further, and again subsiding; yet -again pushing forward and repeating its outburst, till it produced the -fourth crater, when its power became expended. In each of these -successive eruptions the centre of 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 itself in disturbing -greater or less surface-areas in proportion as it acts from greater or -less depths. Or, _mutatis mutandis_, if we assume an uniform depth for -the source of action, the greater or less surface disturbance will be a -measure of greater or less eruptive intensity. - -Perhaps the most remarkable case of a vast number of craters, which, -from their uniform dimensions, suggest the idea of 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 -_Hornitos_ 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.[9] Although upon our plate, and in -comparison with the great crater that is its chief feature, these -countless hollows appear so small as at first sight to appear -insignificant, we must remember that the minutest of them must be grand -objects, each probably equal in dimensions to Vesuvius. For since, as we -have shown in an early chapter, the smallest discernible telescopic -object must subtend an angle to our eye of about a second, and since -this angle extended to the moon represents a mile of its surface, it -follows that these tiny specks of shadow that besprinkle our picture, -are in the reality craters of a mile diameter. This comparison may help -the conception of the stupendous magnitude of the moon’s volcanic -features; for it is a conception most difficult to realize. It is hard -to bring the mind to grasp the fact that that hollow of Copernicus is -fifty miles in diameter. We read of an army having encamped in the once -peaceful crater of Vesuvius, and of one of the extinct volcanoes of the -_Campi Phlegræi_ 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. - - [Illustration: THE FORMATION OF THE CENTRAL CONE. FINAL ACTION OF A - LUNAR VOLCANO.] - - [Illustration: PLATE XIII - ARZACHAEL, PTOLEMY, and the RAILWAY.] - - - - - 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, 1¾ 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 central 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 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 _débris_, 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. - - [Illustration: Fig. 32.] - -The true circularity of these objects appears at first view a remarkable -feature. But it ceases to be so if we suppose them to have been produced -by some very concentrated sublunar force of an upheaving nature, and if -only we admit the homogeneity of the moon’s crust. For if the crust be -homogeneous, then _any_ 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_, F _f_, -will be of equal length, and they will form, so to speak, radii of a -sphere of force. This sphere is cut by the plane at _s s s s_, and as -the intersection necessarily takes place everywhere at the extremity of -these radii, the figure of intersection is demonstrably a circle (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 homogeneous 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. - - [Illustration: Fig. 33.] - - [Illustration: Fig. 34.] - -We confess to a difficulty in accounting for such a very local -generation of a deep-seated force; and, granting its occurrence, we are -unprepared with a satisfactory theory to explain the resultant effect of -such a force in producing a raised ring at the limit of the circular -disturbance. We may indeed, suppose that a vast circular cake or conical -frustra would be temporarily upraised as in Fig. 33, and that upon its -subsidence a certain extrusion of subsurface matter would occur around -the line or zone of rupture as in Fig. 34. This supposition, however, -implies such a peculiarly cohesive 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. - - [Illustration: 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 compression. 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.] - -Mr. Scrope in his work on volcanoes has given a hypothetical section of -a portion of the earth’s crust, which presents a bulging or tumescent -surface in some measure resembling the effect which such a cause as we -have been considering would produce. We give a slightly modified version -of his sketch in Fig. 35, showing what would be the probable phenomena -attending such an upheaval as regards the behaviour of the disturbed -portion of the crust, and also that of the lava or semifluid matter -beneath: and, as will be seen by the sketch, a possible phase of the -phenomena is the production of an elevated ridge or rampart at the -points of disruption _c c_; and where there is a ring of disruption, as -by our hypothesis there would be, the ridge or rampart _c c_ would be a -circle. In this 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. - - [Illustration: PLATE XIV. - PLATO.] - -Those who have offered other explanations of these vast ring-formed -mountain ranges, have been no more happy in their conjectures. M. Rozet, -who communicated a paper on selenology to the French Academy in 1846, -put forth the following theory. He argued that during the formation of -the solid scoriaceous pelicules of the moon, circular or tourbillonic -movements were set up; and these, by throwing the scoria from the centre -to the circumference, caused an accumulation thereof at the limit of the -circulation. He considered that this phenomenon continued during the -whole process of solidification, but that the amplitude of the whirlpool -diminished with the decreasing fluidity of the surface material. -Further, he suggested that when many vortices were formed, and the -distances of their centres, taken two and two, were less than the sums -of their radii, there resulted closed spaces terminated by arcs of -circles; and when for any two centres the distance was greater than the -sum of the radii of action, two separate and complete rings were formed. -We have only to remark on this, that we are at a loss to account for the -origination of such vorticose movements, and M. Rozet is silent on the -point. If the great circles are to be referred to an original sea of -molten matter, it appears to us more feasible to consider that wherever -we see one of them there has been, at the centre of the ring, a great -outflow of lava that has flooded the surrounding surface. Then, if from -any cause, and it is not difficult to assign one, the outflow became -intermittent, or spasmodic, or subject to sudden impulses, concentric -waves would be propagated over the pool and would throw up the scoria or -the solidifying lava in a circular bank at the limit of the fluid area. - -This hypothesis does not differ greatly from the _ebullition_ theory -proposed by Professor Dana, the American geologist, to explain these -formations. He considered that the lunar ring-mountains were formed by -an action analogous to that which is exemplified on the earth in the -crater of Kilauea, in the Hawaiian islands. This crater is a large open -pit exceeding three miles in its longer diameter, and nearly a thousand -feet deep. It has clear bluff walls round a greater part of its circuit, -with an inner ledge or plain at their base, raised 340 feet above the -bottom. This bottom is a plain of solid lavas, entirely open to day, -which may be traversed with safety (we are quoting Professor Dana’s own -statement written in 1846, and therefore not correctly applying to the -present time): over it there are pools of boiling lava in active -ebullition, and one is more than a thousand feet in diameter. There are -also cones at times, from a few yards to two or three thousand feet in -diameter, and varying greatly in angle of inclination. The largest of -these cones have a circular pit or crater at the summit. The great pit -itself is oblong, owing to its situation on a fissure, but the lakes -upon its bottom are round, and in them, says Professor Dana, “the -circular or slightly elliptical form of the moon’s craters is -exemplified to perfection.” - -Now Dana refers this great pit crater and its contained lava-lakes to -“the fact that the action at Kilauea is simply _boiling_, owing to the -extreme fluidity of the lavas. The gases or vapours which produce the -state of active ebullition escape freely in small bubbles, with little -commotion, like jets over boiling water; while at Vesuvius and other -like cones they collect in immense bubbles before they accumulate force -enough to make their way through; and consequently the lavas in the -latter case are ejected with so much violence that they rise to a height -often of many thousand feet and fall around in cinders. This action -builds up the pointed mountain, while the simple boiling of Kilauea -makes no cinders and no cinder cones.” - -Professor Dana continues, “If the fluidity of lavas, then, is sufficient -for this active ebullition, we may have boiling going on over an area of -an indefinite extent; for the size of a boiling lake can have no limits -except such as may arise from a deficiency of heat. The size of the -lunar craters is therefore no mystery. Neither is their circular form -difficult of explanation; for a boiling pool necessarily, by its own -action, extends itself circularly around its centre. The combination of -many circles, and the large sea-like areas are as readily -understood.”[10] - -In justice to Professor Dana it should be stated that he included in -this theory of formation all lunar craters, even those of small size and -possessing central cones; and he put forth his views in opposition to -the eruptive theory which we have set forth, and which was briefly given -to the world more than twenty-five years ago. As regards the smallest -craters with cones, we believe few geologists will refuse their -compliance with the supposition that they were formed as our -crater-bearing volcanoes were formed: and we have pointed out the -logical impossibility of assigning any limit of size beyond which the -eruptive action could not be said to hold good, so long as the central -cone is present. But when we come to ring-mountains having no cones, and -of such enormous size that we are compelled to hesitate in ascribing -them to ejective action, we are obliged to face the possibility of some -other causation. And, failing an explanation of our own that satisfied -us, we have alluded to the few hypotheses proffered by others, and of -these Professor Dana’s appears the most rational, since it is based upon -a parallel found on the earth. In citing it, however, we do necessarily -not endorse it. - - - - - CHAPTER X. - PEAKS AND MOUNTAIN RANGES. - - -The lunar features next in order of conspicuity are the mountain ranges, -peaks, and hill-chains, a class of eminences more in common with -terrestrial formations than the craters and circular structures that -have engaged our notice in the preceding chapters. - -In turning our attention to these features, we are at the outset struck -with the paucity on the lunar surface of extensive mountain systems as -compared with its richness in respect of crateral formations; and a -field of speculation is opened by the recognition of the remarkable -contrast which the moon thus presents to the earth, where mountain -ranges are the rule, and craters like the lunar ones are decidedly -exceptional. Another conspicuous but inexplicable fact is that the most -important ranges upon the moon occur in the northern half of the visible -hemisphere, where the craters are fewest and the comparatively -featureless districts termed “seas” are found. The finest range is that -named after our Apennines and which is included in our illustrative -Plate, No. IX. It extends for about 450 miles and has been estimated to -contain upwards of 3000 peaks, one of which—Mount Huyghens—attains the -altitude of 18,000 feet. The Caucasus is another lunar range which -appears like a diverted northward extension of the Apennines, and, -although a far less imposing group than the last named, contains many -lofty peaks, one of which approaches the altitude assigned to Mount -Huyghens while several others range between 11,000 and 14,000 feet high. -Another considerable range is the Alps, situated between the Caucasus -and the crater Plato, and reproduced on Plate XIV. It contains some 700 -peaked mountains and is remarkable for the immense valley, 80 miles long -and about five broad, that cuts it with seemingly artificial -straightness; and that, were it not for the flatness of its bottom, -might set one speculating upon the probability of some extraneous body -having rushed by the moon at an enormous velocity, gouging the surface -tangentially at this point and cutting a channel through the impeding -mass of mountains. There are other mountain ranges of less magnitude -than the foregoing; but those we have specified will suffice to -illustrate our suggestions concerning this class of features. - - [Illustration: PLATE XV. - MERCATOR & CAMPANUS.] - -We remark, too, that there is a prevailing tendency of the ranges just -mentioned to present their loftiest constituents in abrupt terminal -lines, facing nearly the same direction, the reverse of that towards -which they are carried by the moon’s rotation; and as they recede from -the high terminal line, the mountains gradually fall off in height, so -that in bulk the ranges present the “crag and tail” contour which -individual hills upon the earth so frequently exhibit. - -Isolated peaks are found in small numbers upon the moon; there are a few -striking examples of them nevertheless, and these are chiefly situated -in the mountainous region just alluded to. Several are seen to the east -(right hand) of the Alpine range depicted on Plate XIV. The best known -of these is Pico, which rises abruptly from a generally smooth plain to -a height of 7000 feet. It may be recognized as the lower of the two long -shadowing spots located almost centrally above the crater Plato in the -illustration just mentioned. Above it, at an actual distance of 40 -miles, there is another peak (unnamed) about 4000 feet high; and away to -the west, beyond the small crater joined by a hill-ridge to Plato, is a -third pyramidal mountain nearly as high as Pico. - -It seems natural to regard the great mountain chains as agglomerations -of those peaks of which we have isolated examples in Pico and its -compeers, and thus to consider that the formation of a mountain chain -has been a multiplication of the process that formed the single -pyramid-shaped eminences. At first thought it might appear that the -great mountain ranges were produced by bodily upthrustings of the crust -of the moon by some subsurface convulsions. But such an explanation -could hardly hold in relation to the isolated peaks, for it is -difficult, if not impossible, to conceive that these abrupt mountains, -almost resembling a sugarloaf in steepness, could have been protruded en -masse through a smooth region of the crust. On the contrary it is quite -consistent with probability to suppose that they were built up by a slow -process somewhat analogous to that to which we have ascribed the piling -of the central cones of the great craters. We believe they may be -regarded as true mountains of exudation, produced by the comparatively -gentle oozing of lava from a small orifice and its solidification around -it; the vent however remaining open and the summit or discharging -orifice continually rising with the growth of the mountain, as indicated -in the annexed cut, Fig. 36. This process is well exemplified in the -case of a water fountain playing during a severe frost; the water as it -falls around the lips of the orifice freezes into a hillock of ice, -through the centre of which, however, a vent for the fluid is preserved. -As the water trickles over the mound it is piled higher and higher by -accumulating layers of ice, till at length a massive cone is formed -whose height will be determined by the force or “head” of the water. -Substitute lava for water and we have at once a formative process which -may very fairly be considered as that which has given rise to the -isolated mountains of the moon. - - [Illustration: Fig. 36.] - - [Illustration: Fig. 37.] - - [Illustration: Fig. 38.] - - [Illustration: Fig. 39.] - -There are upon the earth mountainous forms resembling the isolated peaks -of the moon, and which have been explained by a similar theory to the -above. We reproduce a figure of one observed by Dana at Hawaii (Fig. -37), and a sketch of another observed on the summit of the Volcano of -Bourbon, (Fig. 38); we also reproduce (Fig. 39) an ideal section of the -latter, given by Mr. Scrope, and showing the successive layers of lava -which would be disposed by just such an action as that manifested in the -case of the freezing fountain; and we quote that author’s words in -reference to this explanation of the formation of Etna and other -volcanic mountains. “On examining,” says Mr. Scrope,[11] “the structure -of the mountain (Etna) we find its entire mass, so far as it is exposed -to view by denudation or other causes (and one enormous cavity, the Val -de Bove penetrates deeply into its very heart), to be composed of beds -of lava-rock alternating more or less irregularly with layers of scoriæ, -lapillo and ashes, almost precisely identical in mineral character, as -well as in general disposition, with those erupted by the volcano at -known dates within the historical period. Hence we are fully justified -in believing the whole mountain to have been built up in the course of -ages in a similar manner by repeated intermittent eruptions. And the -argument applies by the rules of analogy to all other volcanic -mountains, though the history of their recent eruptions may not be so -well recorded, provided that their structure corresponds with, and can -be fairly explained by this mode of production. It is also further -applicable, under the same reservation, to all mountains composed -entirely, or for the most part, of volcanic rocks, even though they may -not have been in eruption within our time.” - -To these illustrations furnished from Scrope’s work we add another, -copied from a photograph by Professor Piazzi Smyth, of a “blowing cone” -at the base of Teneriffe (Fig. 40), which is but one of many that are to -be found on that mountain and which has been formed by a process similar -to that we have been considering, but acting upon a comparatively small -scale. Professor Smyth describes this cone as about 70 feet high and of -parabolic figure, composed of hard lava and with an upper aperture still -yawning, “whence the burning breath of fires beneath once issued in fury -and with destruction.” - - [Illustration: PLATE XVI - TYCHO, - AND ITS SURROUNDINGS.] - -Reverting now to the moon, we remark that, if the foregoing explanation -of the isolated lunar peaks be tenable, it should hold equally for the -groups of them which we see in the lunar Apennines, Alps, Caucasus and -other ranges of like character. There occur in some places intermediate -groups which link the one to the other. Just above the crater -Archimedes, on Plate IX., for instance, we see several single peaks and -small clumps of them leading by successive multiple-peak examples to -what may be called chains of mountains like many that are included in -the contiguous Apennine system. And, in view of this connexion between -the single peaks and the mountain ranges formed of aggregations of such -peaks, it seems to us reasonable to conclude that the latter were formed -by the comparatively slow escape of lava through multitudinous openings -in a weak part of the moon’s crust, rather than to suppose that the -crust itself has been bodily upheaved and retained in its disturbed -position. The high peaks that many mountains in such a chain exhibit -accord better with the former than the latter explanation; for it is -difficult to imagine how such lofty eminences could be erected by an -upheaval, and we must remember that the moon has none of the denuding -elements which are at work upon the earth, weather-wearing its mountain -forms into sharpness and steepness.[12] - - [Illustration: Fig. 40. - SMALL VOLCANIC MOUNTAIN AT THE END OF A STREET AT TENERIFFE.] - - [Illustration: Fig. 41.] - -And we have ground for believing the mountain-forming process on the -moon to have been a comparatively gentle one, in the fact that the -mountain systems appear in regions otherwise little disturbed, and where -craters, which have all the appearances of violent origin, are few and -far between. Evidently the mountain and crater-forming processes, -although both due to extrusive action, were in some measure different, -and it is reasonable to suppose that the difference was in degree of -intensity; so that while a violent ejection of volcanic material would -give rise to a crater, a more gradual discharge would pile up a -mountain. In this view craters are evidences of _eruptive_, and -mountains of comparatively gentle _exudative_ action. - -We can hardly speculate with any degree of safety upon the cause of this -varying intensity of volcanic discharge. We may ascribe it to variation -of _depth_ of the initial disturbing force, or to suddenness of its -action; or it may be that different degrees of fluidity of the lava have -had modifying effects; or on the other hand different qualities of the -crust-material; or yet again differences of period—the quieter -extrusions having occurred at a time when the volcanic forces were dying -down. There is an alliance between lunar craters and mountains that goes -far to show that there has been no radical difference in their origins. -For instance, as we have previously pointed out, craters in some cases -run in linear groups, as if in those cases they had been formed along a -line of disruption or of least resistance of the crust; and the mountain -chains have a corresponding linear arrangement. Then we see craters and -mountain chains disposed in what seem obviously the same arcs of -disturbance. Thus Copernicus (No. 147), Erastothenes (No. 168), and the -Apennines appear to belong to one continuous line of eruption; and it -requires no great stretch of imagination to suppose that the Caucasus, -Eudoxus (No. 208) and Aristotle (No. 209) form a continuation of the -same line. Then around the Mare Serenetatis we see mountainous ridges -and craters alternating one with the other as though the exuding action -there, normally sufficient to produce the ridges, had at some points -become forcible enough to produce a crater. Again, upon the very -mountain ranges themselves, as for instance among the Apennines, we find -small craters occurring. We see, too, that the great craters are in many -cases surrounded by radiating systems of ridges which almost assume -mountainous proportions, and which are doubtless exuded matter from -“starred” cracks, the centres of which are occupied by the craters. The -same kind of ridges here and there occur apart from craters (see for -instance Plate XVIII., below Aristarchus and Herodotus) and sometimes -they occur in the neighbourhood of extensive cracks, to which they also -seem allied. We must indeed regard a linear crack as the origin either -of a ridge (if the exudation is slight) or of a mountain chain (if the -exudation is more copious) or a string of craters (if the extrusion -rises to eruptive violence). But the subject of cracks is important -enough to be treated in a separate chapter. - -We alluded in Chap. III. to the phenomena of wrinkling or puckering as -productive of certain mountainous formations; and we pointed out the -striking similarity in character of configuration between a shrivelled -skin and a terrestrial mountain region. We do not perceive upon the moon -such a decided coincidence of appearances extending over any -considerable portion of her surface; but there are numerous limited -areas where we behold mountainous ridges which partake strongly of the -wrinkle character; and in some cases it is difficult to decide whether -the puckering agency or the exudative agency just discussed has produced -the ridges. The district bordering upon Aristarchus and Herodotus, above -referred to, is of this doubtful character; and a similar district is -that contiguous to Triesnecker (Plate XI.) There are, however, abundant -examples of less prominent lines of elevation, which may, with more -probability, be ascribed to a veritable wrinkling or puckering action; -they are found over nearly the whole lunar surface, some of them -standing out in considerable relief, and some merely showing gentle -lines of elevation, or giving the surface an undulating appearance. A -close examination of our picture-map (Plate IV.) will reveal very -numerous examples, especially in the south-east (right-hand-upper) -quadrant. Some of these lines of tumescence are so slightly prominent -that we may suppose them to have been caused by the action indicated by -Fig. 6 (p. 28), while others, from their greater boldness, appear to -indicate a formative action analogous to that represented by Fig. 9 (p. -29). - - [Illustration: IDEAL SKETCH OF PICO AS IT WOULD PROBABLY APPEAR IF - SEEN BY A SPECTATOR LOCATED ON THE MOON.] - - [Illustration: PLATE XVII. - WARGENTIN.] - - - - - 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 phenomena -resulting in part from that reaction and in part from other effects of -cooling, which have been accompanied by comparatively little ejection or -upflow of molten matter, and in some cases by none at all. Of such the -most conspicuous examples are those bright streaks that are seen, under -certain conditions of illumination, to radiate in various directions -from single craters, and some of the individual radial branches of which -extend from four to seven hundred miles in a great arc on the moon’s -surface. - -There are several prominent examples of these bright streak systems upon -the visible hemisphere of the moon; the focal craters of the most -conspicuous are Tycho, Copernicus, Kepler, Aristarchus, Menelaus, and -Proclus. Generally these focal craters have ramparts and interiors -distinguished by the same peculiar bright or highly reflective material -which shows itself with such remarkable brilliance, especially at full -moon: under other conditions of 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 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 experimental -illustration; the disruptive force in the case of the moon being that to -which we have frequently referred as due to the expansion which precedes -the solidification of molten substances of volcanic character. - -Our illustration, Plate XIX., is a photograph from one of many glass -globes which we have cracked in the manner described: a careful -comparison between the arrangement of the divergent cracks represented -in the photograph and those seen spreading from Tycho and other lunar -craters 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 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 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. - - [Illustration: Fig. 42. - ILLUSTRATIVE OF THE RADIATING CRACKS WHICH PRECEDE THE FORMATION OF - THE BRIGHT STREAKS.] - -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 “star” into radiating or diverging cracks, up through which the -water immediately issues, making its appearance on the surface of the -ice simultaneously along the entire course of every crack, and on -reaching the surface, spreading on both sides to a width much exceeding -that of the crack itself. - - [Illustration: Fig. 43. - ILLUSTRATIVE OF THE RADIATING BRIGHT STREAKS.] - -If this familiar illustration be duly considered, we doubt not it will -be found to throw considerable light on the nature of those actions -which have resulted in the bright streaks on the moon’s surface. Some -have attempted to explain the cause of these bright streaks by assigning -them to streams of lava, issuing from the crater at the centre of their -divergence and flowing over the surface, but we consider such an -explanation totally untenable, as any idea of lava, be it ever so fluid -at its first issue from its source, flowing in streams of nearly equal -width, through courses several hundred miles long, up hills, over -mountains, and across plains, appears to us beyond all rational -probability. - - [Illustration: PLATE XVIII. - ARISTARCHUS & HERODOTUS.] - -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 sideway-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 favourable 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 from it, are not only of a common origin, but are so -far identical that the only difference in the structure of the one as -compared with the other is due to the more copious egress of the -extruded or erupted matter in the case of the crater, while the -restricted outflow or ejection of the matter up through the cracks would -cause its dispersion to be so comparatively gentle as to flood the sides -of the cracks and spread in a thin sheet more or less sideways -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 XI.), the principal -members of which converge to or cross at a small crater, and thus point -to a continuity of causation therewith analogous to the evident relation -between the bright streaks and their focal craters. Less remarkable, but -no less interesting, are those individual examples that appear in the -region north of (below) the Apennines (Plate IX.), and some of which by -their parallelism of direction with the mountain-chain appear to point -to a causative relation also. There is one long specimen, and several -shorter in the immediate neighbourhood of Mercator and Campanus (Plate -XV.); and another curious system of them, presenting suggestive -contortions, occurs in connection with the mountains Aristarchus and -Herodotus (Plate XVIII.). Others, again, appear to be identified with -the radial excrescences about Copernicus (Plate VIII.). Capuanus, -Agrippa, and Gassendi, among other craters, have more or less notable -cracks in their vicinities. - -Some of these chasms are conspicuous enough to be seen with moderate -telescopic means, and from this maximum degree of visibility there are -all grades downwards to those that require the highest optical powers -and the best circumstances for their detection. The earlier -selenographers detected but a few of them. Schroeter noted only 11; -Lohrman recorded 75 more; Beer and Maedler added 55 to the list, while -Schmidt of Athens raised the known number to 425, of which he has -published a descriptive catalogue. We take it that this increase of -successive discoveries has been due to the progressive perfection of -telescopes, or, perhaps, to increased education, so to speak, of the -eye, since Schmidt’s telescope is a much smaller instrument than that -used by Beer and Maedler, and is regarded by its owner as an inferior -one for its size. We doubt not that there are hundreds more of these -cracks which more perfect instruments and still sharper eyes will bring -to knowledge in the future. - -While these chasms have all lengths from 150 miles (which is about the -extent of those near 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 -profundity. In a few cases, and under very favourable circumstances, we -have observed their generally black interiors to be interrupted here and -there with bright spots suggestive of fragments from the sides of the -cracks having fallen into the opening. - -In seeking an explanation of these cracks, two possible causes suggest -themselves. One is the expansion of 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 -asperities which thus obviously existed before the cracking commenced; -and thus, so far, the hypothesis of the expansion-cracking is supported -by absolute fact. - -It may be objected that such an upheaving force as we are invoking, -being transitory, would allow the distended surface to collapse again -when it ceased to operate, and so close the cracks or chasms it -produced. But we consider it not improbable that in some cases, as a -consequence of the expansion of 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 produced some sort of elevation along the line of the -crack, like that represented sectionally in Fig. 45. It is probable that -some of the slightly tumescent lines on the moon’s surface have been -thus produced. - - [Illustration: PLATE XIX. - GLASS GLOBE CRACKED BY INTERNAL PRESSURE.] - - [Illustration: Fig. 44.] - - [Illustration: Fig. 45.] - -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 -speak weakening it, may have virtually increased the strength of an -upheaving force and thus have aided and localized its action. We see, -however, no reason why the inevitable ultimate contraction which must -have attended the cooling of the moon’s crust, even when all internal -reactions upon it had ceased, should not have created a class of cracks -without accompanying craters, while it would doubtless have a tendency -to increase the length and width of those already existing from any -other cause. Some of the more minute clefts, which presumably exist in -greater numbers than we yet know of, may doubtless be ascribed to this -effect of cooling contraction. In this view we should have to regard -such cracks as the latest of all lunar features. Whether the agency that -produced them is still at work—whether the cracks are on the increase—is -a question impossible of solution: for reasons to be presently adduced, -we incline to believe that all cosmical heat passed from the moon, and -therefore that it arrived at its present, and apparently final, -condition ages upon ages ago. - -Besides the ridges spoken of on p. 140, and regarded as cracks up -through which matter has been extruded, there are numerous ridges of -greater or less extent, which we conceive are of the nature of wrinkles, -and have been produced by tangential compression due to the collapse of -the moon’s crust upon the shrunken interior, as explained and -illustrated in Chap. III. The distinguishing feature of the two classes -of phenomena we consider to be the presence of a serrated summit in -those of the extruded class, while those produced by “wrinkling” action -have their summits comparatively free from serration or marked -irregularity. - - - - - CHAPTER XII. - COLOUR AND BRIGHTNESS OF LUNAR DETAILS: CHRONOLOGY OF FORMATIONS, AND - FINALITY OF EXISTING FEATURES. - - -Speaking generally, the details of the lunar surface seem to us to be -devoid of colour. To the naked eye of ordinary sensitiveness the moon -appears to possess a silvery whiteness: more critical judges of colour -would describe it as presenting a yellowish tinge. Sir John Herschel, -during his sojourn at the Cape of Good Hope, had frequent opportunities -of comparing the moon’s lustre with that of the weathered sandstone -surface of Table Mountain, when the moon was setting behind it, and both -were illuminated under the same direction of sunlight; and he remarked -that the moon was at such times “scarcely distinguishable from the rock -in apparent contact with it.” Although his observations had reference -chiefly to brightness, it can hardly be doubted that similarity of -colour is also implied; for any difference in the tint of the two -objects would have precluded the use of the words “scarcely -distinguishable;” a difference of colour interfering with a comparison -of lustre in such an observation, though it must be remembered that he -observed through a dense stratum of atmosphere. Viewed in the telescope, -the same general yellowish-white colour prevails over all the moon, with -a few exceptions offered by the so-called seas. The _Mare Crisium_, -_Mare Serenetatis_, and _Mare Humorum_ have somewhat of a greenish tint; -the _Palus Somnii_ and the circular area of Lichtenberg incline to -ruddiness. These tints are, however, extremely faint, and it has been -suggested by Arago that they may be mere effects of contrast rather than -actual colouration of the surface material. This, however, can hardly be -the case, since all the “seas” are not alike affected; those that are -slightly coloured are, as we have said, some green and some red, and -contrast could scarcely produce such variations. The supposition of -vegetation covering these great flats and giving them a local colour is -in our view still more untenable, in the face of the arguments that we -shall presently adduce against the possibility of vegetable life -existing upon the moon. - -It appears to us more rational to consider the tints due to actual -colour of the material (presumably lava or some once fluid mineral -substance) that has covered these areas; and it may well be conceived -that the variety of tint is due to different characters of material, or -even various conditions of the same material coming from different -depths below the lunar surface; and we may reasonably suppose that the -same variously-coloured substances occur in the rougher regions of the -lunar surface, but that they exist there in patches too small to be -recognized by us, or are “put out” by the brightness to which polyhedral -reflexion gives rise. - -Seeing that volcanic action has had so large a share in giving to the -moon’s surface its structural character, analogy of the most legitimate -order justifies us in concluding not only that the materials of that -surface are of kindred nature to those of the unquestionably volcanic -portions of the earth, but also that the tints and colours that -characterize terrestrial volcanic and Plutonian products have their -counterparts on the moon. Those who have seen the interior and -surroundings of a terrestrial volcano after a recent eruption, and -before atmospheric agents have exercised their dimming influences, must -have been struck with the colours of the erupted materials themselves -and the varied brilliant tints conferred on these materials by the -sublimated vapours of metals and mineral substances which have been -deposited upon them. If, then, analogy is any guide in enabling us to -infer the appearance of the invisible from that which we know to be of -kindred nature and which we have seen, we may justly conclude that were -the moon brought sufficiently near to us to exhibit the minute -characteristics of its surface, we should behold the same bright and -varied colours in and around its craters that we behold in and about -those of the earth; and in all probability the coloured materials of -lunar volcanoes would be more fresh and vivid than those of the earth by -reason of the absence of those atmospheric elements which tend so -rapidly to impair the brightness of coloured surfaces exposed to their -influence. - -Situated as we are, however, as regards distance from the moon, we have -no chance of perceiving these local colours in their smaller masses; but -it is by no means improbable, as we have suggested, that the faint tints -exhibited by the great plains are due to broad expanses of coloured -volcanic material. - -But if we fail to perceive diversity of colour upon the lunar surface, -we are in a very different position in regard to diversity of brightness -or variable light-reflective power of different districts and details. -This will be tolerably obvious to those casual observers who have -remarked nothing more of the moon’s physiography than the resemblance to -a somewhat lugubrious human countenance which the full moon exhibits, -and which is due to the accidental disposition of certain large and -small areas of surface material which have less of the light-reflecting -property than other portions; for since all parts seen by a terrestrial -observer may be said to be equally shone upon by the sun, it is clear -that apparently bright and shaded parts must be produced by differences -in the nature of the surface as regards power of reflecting the light -received. - -When we turn to the telescope and survey the full disc of the moon with -even a very moderate amount of optical aid, the meagre impression as to -variety of degree of brightness which the unassisted eye conveys is -vastly extended and enhanced, for the surface is seen to be diversified -by shades of brilliancy and dullness from almost glittering white to -sombre grey: and this variety of shading is rendered much more striking -by shielding the eye with a dusky glass from the excessive glare, which -drowns the details in a flood of light. Under these circumstances the -varieties of light and shade become almost bewildering, and defy the -power of brush or pencil to reproduce them. - -We may, however, realize an imperfect idea of this characteristic of the -lunar surface by reference to the self-drawn portrait of the full moon -upon Plate III. This is, in fact, a photograph taken from the full moon -itself, and enlarged sufficiently to render conspicuous the spots and -large and small regions that are strikingly bright in comparison with -what may in this place be described as the “ground” of the disc. As an -example of a wide and irregularly extensive district of highly -reflective material, the region of which Tycho is the central object, is -very remarkable. We may refer also to the bright “splashes” of which -Copernicus and Kepler are the centres. So brilliant are these spots that -they can easily be detected by the unassisted eye about the time of full -moon. Still brighter but less conspicuous by its size is the crater -Aristarchus, which shines with specular brightness, and almost induces -the belief that its interior is composed of some vitreous-surfaced -matter: the highly reflective nature of this object has often caused it -to become conspicuous when in the dark hemisphere of the moon, -unilluminated by the sun, and lighted only by the light reflected from -the earth. At these times it appears so bright that it has been taken -for a volcano in actual eruption, and no small amount of popular -misconception at one time arose therefrom concerning the conditions of -the moon as respects existing volcanic activity—a misconception that -still clings to the minds of many. - -The parts of the surface distinguished by deficiency of reflecting power -are conspicuous enough. We may cite, however, as an example of a detail -portion especially remarkable for its dingy aspect, the interior of the -crater Plato, which is one of the darkest spots (the darkest well -defined one) upon the hemisphere of the moon visible to us. For -facilitating reference to shades of luminosity, Schroeter and Lohrman -assorted the variously reflective parts into 10 grades, commencing with -the darkest. Grades 1 to 3 comprised the various deep greys; 4 and 5 the -light greys; 6 and 7 white; and 8 to 10 brilliant white. The spots -Grimaldi and Riccioli came under class 1 of this notation; Plato between -1 and 2. The “seas” generally ranged from 2 to 3; the brightest -mountainous portions mostly between degrees 4 and 6; the crater walls -and the bright streaks came between these and the bright peaks, which -fell under the 9th grade. The maximum brightness, the 10th grade, is -instanced only in the ease of Aristarchus and a point in Werner, though -Proclus nearly approaches it, as do many bright spots, chiefly the sites -of minute craters, which make their appearance at the time of full moon. - -In photographic pictures produced by the moon of itself, there is always -an apparent exaggeration in the relation of light to dark portions of -the disc. The dusky parts look, upon the photograph, much darker than to -the eye directed to the moon itself, whether assisted or not by optical -appliances. It may be that the real cause of this discrepancy is that -the eye fails to discover the actual difference upon the moon itself, -being insensible to the higher degrees of brightness or not estimating -them at their proper brilliance with respect to parts less bright. On -the other hand, it is probable that the enhanced contrast in the -photograph is due to some peculiar condition of the darker surface -matter affecting its power of reflecting the actinic constituent of the -rays that fall upon it. - -The study of the varying brightness or reflective power of different -regions and spots of the lunar disc leads us to the consideration of the -relative antiquity of the surface features; for it is hardly possible to -regard these variations attentively without being impressed with the -conviction that they have relation to some chronological order of -formation. We cannot, in the first place, resist the conviction that the -brightest features were the latest formed; this strikes us as evident on -_primâ facie_ grounds; but it becomes more clearly so when we remark -that the bright formations, as a rule, overlie the duller features. The -elevated parts of the crust are brighter than the “seas” and other -areas; and it is pretty clear that the former are newer than the latter, -upon which they appear to be super-imposed, or through which they seem -to have extruded.[13] The vast dusky plains are in every instance more -or less sprinkled with spots and minute craters, and these last were -obviously formed after the area that contains them. One is almost -disposed to place the order of formations in the order of relative -brightness, and so consider the dingiest parts the oldest and the -brightest spots and craters the newest features, though, in the absence -of an atmosphere competent to impair the reflective power of the surface -materials, we are unable to justify this classification by suggesting a -cause for such a deterioration by time as the hypothesis pre-supposes. - -As we have entered upon the question of relative age of the lunar -features, we may remark that there are evidences of various epochs of -formation of particular classes of details, irrespective of their -condition in respect of brightness, or, as we may say, freshness of -material. As a rule, the large craters are older than the small ones. -This is proved by the fact that a large object of this class is never -seen to interfere with or overlap a small one. Those of nearly equal -size are, however, seen to overlap one another as though several -eruptions of equal intensity had occurred from the same source at -different points. This is strikingly instanced in the group of craters -situated in the position 35-141 on our map, the order of formation of -each of which is clearly apparent. The region about Tycho offers an -inexhaustible field for study of these phenomena of overlapping or -interpolating craters, and it will be found, with very few exceptions, -that the smaller crater is the impinging or parasitical one, and must -therefore have been formed after the larger, upon which it intrudes or -impinges. There are frequent cases in which a large crater has had its -rampart interrupted by a lesser one, and this again has been broken into -by one still smaller; and instances may be found where a fourth crater -smaller than all has intruded itself upon the previous intruder. The -general tendency of these examples is to show that the craters -diminished in size as the moon’s volcanic energy subsided: that the -largest were produced in the throes of its early violence, and that the -smallest are the results of expiring efforts possibly impeded through -the deep-seatedness of the ejective source. - -Another general fact of this chronological order is that the mountain -chains are never seen to intrude upon formations of the crater order. We -do not anywhere find that a mountain chain runs absolutely into or -through a crater; but, on the other hand, we do find that craters have -formed on mountain chains. This leads unmistakably to the inference that -the craters were not formed _before_ their allied mountain chains; and -we might assume therefore that the mountains generally are the older -formations, but that there is nothing to prove that the two classes of -features, where they intermingle, as in the Apennines and Caucasus, were -not erupted cotemporaneously. - - [Illustration: PLATE XX. - OVERLAPPING CRATERS.] - -Upon the assumption that the latest ejected or extruded matter is that -which is brightest, we should place the bright streaks among the more -recent features. Be this as it may, it is tolerably certain that the -cracks, whose apparently close relation to the radiating streaks we have -endeavoured to point out, are relatively of a very late formative -period. We are indeed disposed to consider them as the most recent -features of all: the evidence in support of this consideration being the -fact that they are sometimes found intersecting small craters that, from -the way in which they are cut through by the cracks, must have been _in -situ_ before the cracking agency came into operation. It is in -accordance with our hypothesis of the moon’s transition from a fluid to -a solid body to consider that a cracking of the surface would be the -latest of all the phenomena produced by contraction in final cooling. - -The foregoing remarks naturally lead us to the question whether changes -are still going on upon the surface of our satellite: whether there is -still left in it a spark of its volcanic activity, or whether that -activity has become totally extinct. We shall consider this question -from the observational and theoretical point of view. First as regards -observations. This much may be affirmed indisputably—that no object or -detail visible to the earliest selenographers (whose period may be dated -200 years back) has altered from the date of their maps to the present. -When we pass from the bolder features to the more minute details we find -ourselves at a loss for materials for forming an inference; the only map -pretending to accuracy even of the larger among small objects being that -of Beer and Maedler, which, truly admirable as it is, is not very safely -to be relied upon for settling any question of alleged change, on -account of the conventional system adopted for exhibiting the forms of -objects, every object being mapped rather than drawn, and shown as it -never is or can be presented to view on the moon itself. This difficulty -would present itself if a question of change were ever raised upon the -evidence of Beer and Maedler’s map: it may indeed have prevented such a -question being raised, for certainly no one has hitherto been bold -enough to assert that any portion or detail of the map fails to -represent the actual state of the moon at the present time. - -In default of published maps, we are thrown for evidence on this -question upon observations and recollections of individual observers -whose familiarity with the lunar details extends over lengthy periods. -Speaking for ourselves, and upon the strength of close scrutinies -continued with assiduity through the past thirty years, we may say that -we have never had the suspicion suggested to our eye of any actual -change whatever having taken place in any feature or minute detail of -the lunar surface; and our scrutinies have throughout been made with -ample optical means, mostly with a 20-inch reflector. This experience -has made us not unnaturally in some slight degree sceptical concerning -the changes alleged to have been detected by others. Those asserted by -Schroeter and Gruithuisen were long ago rejected by Beer and Maedler, -who explained them, where the accuracy of the observer was not -questioned, by variations of illumination, a cause of illusory change -which is not always sufficiently taken into account. A notable instance -of this deception occurred a few years ago in the case of the minute -bright crater _Linné_, which was for a considerable period declared, -upon the strength of observations of very promiscuous character, to be -varying in form and dimensions almost daily, but the alleged constant -changes of which have since been tacitly regarded as due to varying -circumstances of illumination induced by combinations of libratory -effects with the ordinary changes depending upon the direction of the -sun’s rays as due to the age of the moon. This explanation does not, -however, dispose of the question whether the crater under notice -suffered any actual change before the hue and cry was raised concerning -it. Attention was first directed to it by Schmidt, of Athens, whose -powers of observation are known to be remarkable, and whose labours upon -the moon are of such extent and minuteness as to claim for his -assertions the most respectful consideration.[14] He affirmed in 1866 -that the crater at that date presented an appearance decidedly different -from that which it had had since 1841: that whereas it had been from the -earlier epoch always easily seen as a very deep crater, in October 1866 -and thenceforward it presented only a white spot, with at most but a -very shallow aperture, very difficult to be detected. Schmidt is one of -the very few observers whose long familiarity with the moon entitles him -to speak with confidence upon such a question as that before us upon the -sole strength of his own experience; and this case is but an isolated -one, at least it is the only one he has brought forward. He is, however, -still firmly convinced that it is an instance of actual change, and not -an illusion resulting from some peculiar condition of illumination of -the object. It should be added also on this side of the discussion that -an English observer, the Rev. T. W. Webb, while apparently indisposed to -concede the supposition of any notable changes in the lunar features, -has yet found from his own observations that, after all due allowance -for differences of light and shade upon objects at different times, -there is still a “residuum of minute variations not thus disposed of” -which seem to indicate that eruptive action in the moon has not yet -entirely died out, though its manifestation at present is very limited -in extent. It appears to us that, if evidence of continuing volcanic -action is to be sought on the moon, the place to look for it is around -the circumference of the disc, where eruption from any marginal orifice -would manifest itself in the form of a protruding haziness, somewhat as -illustrated to an exaggerated extent in the annexed cut. - - [Illustration: Fig. 46.] - -The theoretical view of the question, which we have now to consider, has -led us, however, to the strong belief that no vestige of its former -volcanic activity lingers in the moon—that it assumed its final -condition an inconceivable number of ages ago, and that the high -interest which would attach to the close scrutiny of our satellite if it -_were_ still the theatre of volcanic reactions cannot be hoped for. If -it be just and allowable to assume that the earth and the moon were -condensed into planetary form at nearly the same epoch (and the only -rational scheme of cosmogony justifies the assumption) then we may -institute a comparison between the condition of the two bodies as -respects their volcanic age, using the one as a basis for inference -concerning the state of the other. We have reason to believe that the -earth’s crust has nearly assumed its final state so far as volcanic -reactions of its interior upon its exterior are concerned: we may affirm -that within the historical period no igneous convulsions of any -considerable magnitude have occurred; and we may consider that the -volcanoes now active over the surface of the globe represent the last -expiring efforts of its eruptive force. Now in the earth we perceive -several conditions wherefrom we may infer that it parted with its -cosmical heat (and therefore with its prime source of volcanic agency) -at a rate which will appear relatively very slow when we come to compare -the like conditions in the moon. We may, we think, take for granted that -the surface of a planetary body generally determines its _heat -dispersing_ power, while its volume determines its _heat retaining_ -power. Given two spherical bodies of similar material but of unequal -magnitude and originally possessing the same degree of heat, the smaller -body will cool more rapidly than the larger, by reason of the greater -proportion which the surface of the smaller sphere bears to its volume -than that of the larger sphere to its volume—this proportion depending -upon the geometrical ratio which the surfaces of spheres bear to their -volumes, the contents of spheres being as the _cubes_ and the surfaces -as the _squares_ of their diameters. The volume of the earth is 49 times -as great as that of the moon, but its surface is only 13 times as great; -there is consequently in the earth a power of retaining its cosmical -heat nearly four times as great as in the case of the moon; in other -words, the moon and earth being supposed at one time to have had an -equally high temperature, the moon would cool down to a given low -temperature in about one fourth the time that the earth would require to -cool to the same temperature. But the earth’s cosmical heat has without -doubt been considerably conserved by its vaporous atmosphere, and still -more by the ocean in its antecedent vaporous form. Yet notwithstanding -all this, the earth’s surface has nearly assumed its final condition so -far as volcanic agencies are concerned: it has so far cooled as to be -subject to no considerable distortions or disruptions of its surface. -What then must be the state of the moon, which, from its small volume -and large proportionate area, parted with its heat at the above -comparatively rapid rate? The matter of the moon is, too, less dense -than the earth, and hence doubtless from this cause disposed to more -rapid cooling; and it has no atmosphere or vaporous envelope to retard -its radiating heat. We are driven thus to the conclusion that the moon’s -loss of cosmical heat must have been so rapid as to have allowed its -surface to assume its final conformation ages on ages ago, and hence -that it is unreasonable and hopeless to look for evidence of change of -any volcanic character still going on. - -We conceive it possible, however, that minute changes of a non-volcanic -character may be proceeding in the moon, arising from the violent -alternations of temperature to which the surface is exposed during a -lunar day and night. The sun, as we know, pours down its heat -unintermittingly for a period of fully 300 hours upon the lunar surface, -and the experimental investigations of Lord Rosse, essentially confirmed -by those of the French observer, Marie Davy, show that under this -powerful insolation the surface becomes heated to a degree which is -estimated at about 500° of Fahrenheit’s scale, the fusing point of tin -or bismuth. This heat, however, is entirely radiated away during the -equally long lunar night, and, as Sir John Herschel surmised, the -surface probably cools down again to a temperature as low as that of -interstellar space: this has been assumed as representing the absolute -zero of temperature, which has been calculated from experiments to be -250° below the zero of Fahrenheit’s scale. Now such a severe range of -heat and cold can hardly be without effect upon some of the component -materials of the lunar surface.[15] If there be any such materials as -the vitreous lavas that are found about our volcanoes, such as obsidian -for instance, they are doubtless cracked and shivered by these extreme -transitions of temperature; and this comparatively rapid succession of -changes continued through long ages would, we may suppose, result in a -disintegration of some parts of the surface and at length somewhat -modify the selenographic contour. It is, however, possible that the -surface matter is mainly composed of more crystalline and porous lavas, -and these might withstand the fierce extremes like the “fire-brick” of -mundane manufacture, to which in molecular structure they may be -considered comparable. Lavas as a rule are (upon the earth) of this -unvitreous nature, and if they are of like constitution on the moon, -there will be little reason to suspect changes from the cause we are -considering. Where, however, the material, whatever its nature, is piled -in more or less detached masses, there will doubtless be a grating and -fracturing at the points of contact of one mass with another, produced -by alternate expansions and contractions of the entire masses, which in -the long run of ages must bring about dislocations or dislodgments of -matter that might considerably affect the surface features from a close -point of view, but which can hardly be of sufficient magnitude to be -detected by a terrestrial observer whose best aids to vision give him no -perception of minute configurations. And it must always be borne in mind -that changes can only be _proved_ by reference to previous observations -and delineations of unquestionable accuracy. - -Speaking by our own lights, from our own experience and reasoning, we -are disposed to conclude that in all visible aspects the lunar surface -is unchangeable, that in fact it arrived at its terminal condition -_eons_ of ages ago, and that in the survey of its wonderful features, -even in the smallest details, we are presented with the sight of objects -of such transcendent antiquity as to render the oldest geological -features of the earth modern by comparison. - - - - - CHAPTER XIII. - THE MOON AS A WORLD: DAY AND NIGHT UPON ITS SURFACE. - - -A wide interest, if not a deep one, attaches to the general question as -to the existence of living beings, or at least the possibility of -organic existence, on planetary bodies other than our own. The question -has been examined in all ages, by the lights of the science peculiar to -each. With every important accession to our astronomical knowledge it -has been re-raised: every considerable discovery has given rise to some -new step or phase in the discussion, and in this way there has grown up -a somewhat extensive literature exclusively relating to mundane -plurality. It will readily be understood that the moon, from its -proximity to the earth, has from the first received a large, perhaps the -largest, share of attention from wanderers into this field of -speculation: and we might add greatly to the bulk of this volume by -merely reviewing some of the more curious and, in their way, instructive -conjectures specially relating to the moon as a world—to imaginary -journeys towards her, and to the beings conjectured to dwell upon and -within her. This, however, we feel there is no occasion to do, for it is -our purpose merely to point out the two or three almost conclusive -arguments against the possibility of any life, animal or vegetable, -having existence on our satellite. - -We well know what are the requisite conditions of life on the earth; and -we can go no further for grounds of inference; for if we were to start -by assuming forms of life capable of existence under conditions widely -and essentially different from those pertaining to our planet, there -would be no need for discussing our subject further: we could revel in -conjectures, without a thought as to their extravagance. The only -legitimate phase of the question we can entertain is this:—can there be -on the moon any kind of living things analogous to any kind of living -things upon the earth? And this question, we think, admits only of a -negative answer. The lowest forms of vitality cannot exist without air, -moisture, and a moderate range of temperature. It may be true, as recent -experiments seem to show, that organic germs will retain their vitality -without either of the first, and with exposure to intense cold and to a -considerable degree of heat; and it is conceivable that the mere germs -of life may be present on the moon.[16] But this is not the case with -living organisms themselves. We have, in Chapter V., specially devoted -to the subject, cited the evidence from which we know that there can be -at the most, no more air on the moon than is left in the receiver of an -air-pump after the ordinary process of exhaustion. And with regard to -moisture, it could not exist in any but the vaporous state, and we know -that no appreciable amount of vapour can be discovered by any -observation (and some of them are crucial enough) that we are capable of -making. We may suppose it just within the verge of possibility that some -low forms of vegetation might exist upon the moon with a paucity of air -and moisture such as would be beyond even our most severe powers of -detection: but granting even this, we are met by the temperature -difficulty; for it is inconceivable that any plant-life could survive -exposure first to a degree of cold vastly surpassing that of our arctic -regions, and then in a short time (14 days) to a degree of heat capable -of melting the more fusible metals—the total range being equal, as we -have elsewhere shown, to perhaps 600 or 700 degrees of our thermometric -scale. - -The higher forms of vegetation could not reasonably be expected to exist -under conditions which the lower forms could not survive. And as regards -the possibility of the existence of animal life in any form or condition -on the lunar surface, the reasons we have adduced in reference to the -non-existence of vegetable life bear still more strongly against the -possibility of the existence of the former. We know of no animal that -could live in what may be considered a vacuum and under such thermal -conditions as we have indicated. - - [Illustration: PLATE XXI. - NORMAL LUNAR CRATER.] - -As to man, aëronautic experience teaches us that human life is -endangered when the atmosphere is still sufficiently dense to support 12 -inches of mercury in the barometer tube; what then would be his -condition in a medium only sufficiently dense to sustain one-tenth of an -inch of the barometric column? We have evidence from the most delicate -tests that no atmosphere or vapour approaching even this degree of -attenuation exists around the moon’s surface. - -Taking all these adverse conditions into consideration we are in every -respect justified in concluding that there is no possibility of animal -or vegetable life existing on the moon, and that our satellite must -therefore be regarded as a barren world. - - * * * * * * * * - -After this disquisition upon lunar uninhabitability it may appear -somewhat inconsistent for us to attempt a description of the scenery of -the moon and some other effects that would be visible to a spectator, -and of which he would be otherwise sensible, during a day and a night -upon her surface. But we can offer the sufficient apology that an -imaginary sojourn of one complete lunar day and night upon the moon -affords an opportunity of marshalling before our readers some phenomena -that are proper to be noticed in a work of this character, and that have -necessarily been passed over in the series of chapters on consecutive -and special points that have gone before. It may be urged that, in -depicting the moon from such a standpoint as that now to be taken, we -are describing scenes that never have been such in the literal sense of -the word, since no eye has ever beheld them. Still we have this -justification—that we are invoking the conception of things that -actually exist; and that we are not, like some imaginary voyagers to the -moon, indulging in mere flights of fancy. Although it is impossible for -a habitant of this earth fully to realise existence upon the moon, it is -yet possible, indeed almost inevitable, for a thoughtful -telescopist—watching the moon night after night, observing the sun rise -upon a lunar scene, and noting the course of effects that follow till it -sets—it is almost inevitable, we say, for such an observer to identify -himself so far with the object of his scrutiny, as sometimes to become -in thought a lunar being. Seated in silence and in solitude at a -powerful telescope, abstracted from terrestrial influences, and gazing -upon the revealed details of some strikingly characteristic region of -the moon, it requires but a small effort of the imagination to suppose -one’s self actually upon the lunar globe, viewing some distant landscape -thereupon; and under these circumstances there is an irresistible -tendency in the mind to pass beyond the actually _visible_, and to fill -in with what it knows must exist those accessory features and phenomena -that are only hidden from us by distance and by our peculiar point of -view. Where the material eye is baffled, the clairvoyance of reason and -analogy comes to its aid. - -Let us then endeavour to realize the strange consequences which the -position and conditions of the moon produce upon the aspect of a lunar -landscape in the course of a lunar day and night. - -The moon’s day is a long one. From the time that the sun rises upon a -scene[17] till it sets, a period of 304 hours elapses, and of course -double this interval passes between one sunrise and the next. The -consequences of this slow march of the sun begin to show themselves from -the instant that he rises above the lunar horizon. Dawn, as we have it -on earth, can have no counterpart upon the moon. No atmosphere is there -to reflect the solar beams while the luminary is yet out of actual -sight, and only the glimmer of the zodiacal light heralds the approach -of day. From the black horizon the sun suddenly darts his bright -untempered beams upon the mountain tops, crowning them with dazzling -brilliance while their flanks and valleys are yet in utter darkness. -There is no blending of the night into day. And yet there is a growth of -illumination that in its early stages may be called a twilight, and -which is caused by the slow rise of the sun. Upon the earth, in central -latitudes, the average time occupied by the sun in rising, from the -first glint of his upper edge till the whole disc is in sight, is but -two minutes and a quarter. Upon the moon, however, this time is extended -to a few minutes short of an hour, and, therefore, during the first few -minutes a dim light will be shed by the small visible chord of the solar -disc, and this will give a proportionately modified degree of -illumination upon the prominent portion of the landscape, and impart to -it something of the weird aspect which so strikes an observer of a total -solar eclipse on earth when the scene is lit by the thin crescent of the -re-appearing sun. This impaired illumination constitutes the only dawn -that a lunar spectator could behold. And it must be of short duration; -for when, in the course of half an hour, the solar disc has risen half -into view the lighting would no doubt appear nearly as bright to the eye -as when the entire disc of the sun is above the horizon. In this lunar -sunrise, however, there is none of that gilding and glowing which makes -the phenomenon on earth so gorgeous. Those crimson sky-tints with which -we are familiar are due to the absorption of certain of the polychromous -rays of light by our atmosphere. The blue and violet components of the -solar beams are intercepted by our envelope of vapour, and only the red -portions are free to pass; while on the moon, as there is no atmosphere, -this selective absorption does not occur. If it did, an observer gazing -from the earth upon the regions of the moon upon which the sun is just -rising would see the surface tinted with rosy light. This, however, is -not the case; the faintest lunar features just catching the sun are seen -simply under white light diluted to a low degree of brightness. Only -upon rare occasions is the lunar scenery suffused with coloured -illumination, and these are when, as we shall presently have to -describe, the solar rays reach the moon after traversing the earth’s -atmosphere during an eclipse of the sun. - -This atmosphere of ours is the most influential element in beautifying -our terrestrial scenery, and the absence of such an appendage from the -moon is the great modifying cause that affects lunar scenery as compared -with that of the earth. We are accustomed to the sun with its dazzling -brightness—overpowering though it be—subdued and softened by our -vaporous screen. Upon the moon there is no such modification. The sun’s -intrinsic brilliancy is undiminished, its apparent distance is -shortened, and it gleams out in fierce splendour only to be realised, -and then imperfectly, by the conception of a gigantic electric light a -few feet from the eye. And the brightness is rendered the more striking -by the blackness of the surrounding sky. Since there is no atmosphere -there can be no sky-light, for there is nothing above the lunar world to -diffuse the solar beams; not a trace of that moisture which even in our -tropical skies scatters some of the sun’s light and gives a certain -degree of opacity or blueness, deep though it be, to the heavens by day. -Upon the moon, with no light-diffusing vapour, the sky must be as dark -or even darker than that with which we are familiar upon the finest of -moonless nights. And this blackness prevails in the full blaze of the -lunar noon-day sun. If the eye (upon the moon) could bear to gaze upon -the solar orb (which would be less possible than upon earth) or could it -be screened from the direct beams, as doubtless it could by intervening -objects, it would perceive the nebulous and other appendages which we -know as the corona, the zodiacal light, and the red solar protuberances: -or if these appendages could not be viewed with the sun above the -horizon they would certainly be seen in glorious perfection when the -luminary was about to rise or immediately after it had set. - -And, notwithstanding the sun’s presence, the planets and stars would be -seen to shine more brilliantly than we see them on the clearest of -nights; the constellations would have the same configurations, though -they would be differently situated with respect to the celestial pole -about which they would appear to turn, for the axis of rotation of the -moon is directed towards a point in the constellation Draco. The stars -would never twinkle or change colour as they appear to us to do, for -scintillation or twinkling is a phenomenon of atmospheric origin, and -they would retain their full brightness, down even to the horizon, since -there would be no haze to diminish their light. The planets, and the -brighter stars at least, would be seen even when they were situated very -near to the sun. The planet Mercury, so seldom detected by terrestrial -gazers, would be almost constantly in view during the lunar day, -manifesting his close attendance on the central luminary by making only -short excursions of about two (lunar) days’ length, first on one side -and then on the other. Venus would be nearly as continuously visible, -though her wanderings would be more extensive on either side. The -zodiacal light also, which in our English latitude and climate is but -rarely seen and in more favourable climes appears only when the sun -itself is hidden beneath the horizon, would upon the moon be seen as a -constant accompaniment to the luminary throughout his daily course -across the lunar sky. The other planets would appear generally as they -do to us on earth, but, never being lost in daylight, their courses -among the stars could be traced with scarcely any interruption. - -One planet, however, that adorns the sky of the lunar hemisphere which -is turned towards us deserves special mention from the conspicuous and -highly interesting appearance it must present. We allude to the earth. -To nearly one-half of the moon (that which we never see) this imposing -object can never be visible; but to the half that faces us the -terrestrial planet must appear almost fixed in the sky. A lunar -spectator in (what is to us) the centre of the disc, or about the region -north of the lunar mountains Ptolemy and Hipparchus, would have the -earth in his zenith. From regions upon the moon a little out of what is -to us the centre, a spectator would see the earth a little declining -from the zenith, and this declination would increase as the regions -corresponding to the (to us) apparent edge of the moon were approached, -till at the actual edge it would be seen only upon the horizon. From the -phenomena of libration (explained in Chap. VI.) the earth would appear -from nearly all parts of the lunar hemisphere to which it is visible at -all to describe a small circle in the sky. To an observer, however, upon -the (to us) marginal regions of the lunar globe, it would appear only -during a portion of the lunar day—being visible in fact only in that -part of its small circular path which happened to lie above the -observer’s horizon: in some regions only a portion of the terrestrial -disc would make its brief appearance. From the lunar hemisphere beyond -this marginal line the earth can never be seen at all. - -The lunar spectator whose situation enabled him to view the earth would -see it as a moon; and a glorious moon indeed it must be. Its diameter -would be four times as great as that of the moon itself as seen by us, -and the area of its full disc 13 times as great. It would be seen to -pass through its phases, just as does our satellite, once in a lunar day -or a terrestrial month, and during that cycle of phases, since 29 of our -days would be occupied by it, the axial rotation would bring all the -features of its surface configuration into view so many times in -succession. But the greatest beauty of this noble moon would be seen -during the lunar night, in considering which we shall again allude to -it; for when it is full-moon to the earth it is new-earth to the moon. -At lunar midnight this globe of ours is fully illuminated; as morning -nears, the earth-moon wanes, its disc slowly passing through the gibbous -phases until at sunrise it would be just half-illuminated. During the -long forenoon it assumes a crescent which narrows and narrows till at -midday the sun is in line with the earth and the latter is invisible, -save perhaps by a thin line of light marking its upper or lower edge, -accordingly as the sun is apparently above or below it. In the lunar -afternoon an illuminated crescent appears upon the opposite side of the -terrestrial globe, and this widens and widens till it becomes a half -disc by lunar sunset and a full disc by lunar midnight. - -The sun in his daily course passes at various distances, sometimes above -and sometimes below, the nearly stationary earth. Obviously it will at -times pass actually behind it, and then the lunar spectator would behold -the sublime spectacle of a total solar eclipse, and that under -circumstances which render the phenomenon far more imposing than its -counterpart can appear from the earth; for whereas, when we see the moon -eclipse the sun, the nearly similar (apparent) diameters of the two -bodies renders the duration of totality extremely short—at most 7 -minutes—a lunar spectator, the earth appearing to him four times the -diameter of the sun, and he and the earth being relatively stationary, -would enjoy a view of the totality extending over several hours. During -the passage of the solar disc behind that of the earth, a beautiful -succession of luminous phenomena would be observed to follow from the -refractions and dispersions which the sunbeams would suffer in passing -tangentially through those parts of our atmospheric envelope which lie -in their course; those, for instance, on the margin of the earth, as -seen from the moon. As the sun passed behind the earth, the latter would -be encircled upon the in-going side with a beautiful line of golden -light, deepening in places to glowing crimson, due to the absorption, -already spoken of, of all but the red and orange rays of the sun’s light -by the vapours of our atmosphere. As the eclipse proceeded and totality -came on, this ruddy glow would extend itself nearly, if not all, around -the black earth, and so bright would it be, that the whole lunar -landscape covered by the earth’s shadow would be illuminated with faint -crimson light,[18] save, perhaps, in some parts of the far distance, -upon which the earth had not yet cast its shadow, or off which the -shadow had passed. Although the crimson light would preponderate, it -would not appear bright and red alike all around the earth’s periphery. -The circle of light would be, in fact, _the ring of twilight_ round our -globe, and it would only appear red in those places where the atmosphere -chanced to be in that condition favourable for producing what on earth -we know as red sunset and sunrise. We know that the sun, even in clear -sky, does not always set and rise with the beautiful red glow, which may -be determined by merely local causes, and will therefore vary in -different parts of the earth. Now a lunar spectator watching the sun -eclipsed by the earth, would see, during totality and at a _coup d’œil_, -every point around our world upon which the sun is setting on one side -and rising upon the other. To every part of the earth around what is -then the margin, as seen from the moon, the sun is upon the horizon, -shining through a great thickness of atmosphere, reddening it, and being -reddened by it wherever the vaporous conditions conduce to that -colouration. And at all parts where these conditions obtain, the lunar -eclipse-observer would see the ring of light around the black -earth-globe brilliantly crimsoned; at other parts it would have other -shades of red and yellow, and the whole effect would be to make the -grand earth-ball, hanging in the lunar sky, like a dark sphere in a -circle of glittering gold and rubies. - -During the early stages of the eclipse, this chaplet of -brilliant-coloured lights would be brightest upon the side of the -_disappearing_ sun; at the time of central eclipse the radiance -(supposing the sun to pass centrally behind the earth) would be equally -distributed, and during the later stages it would preponderate upon the -side of the _reappearing_ sun. We have endeavoured to give a pictorial -realization of this phenomenon and of the effect of the eclipse upon the -lunar landscape, but such a picture cannot but fall very, very far short -of the reality. (See Plate XXII.) - - -And now for a time let us turn attention from the lunar sky to the -scenery of the lunar landscape. Let us, in imagination, take our stand -high upon the eastern side of the rampart of one of the great craters. -Height, it must be remarked, is more essential on the moon to command -extent of view than upon the earth, for on account of the comparative -smallness of the lunar sphere the dip of the horizon is very rapid. Such -height, however, would be attained without great exercise of muscular -power, since equal amounts of climbing energy would, from the smallness -of lunar gravity, take a man six times as high on the moon as on the -earth. Let us choose, for instance, the hill-side of Copernicus. The day -begins by a sudden transition. The faint looming of objects under the -united illumination of the half-full earth and the zodiacal light is the -lunar precursor of day-break. Suddenly the highest mountain peaks -receive the direct rays of a portion of the sun’s disc as it emerges -from below the horizon. The brilliant lighting of these summits serves -but to increase, by contrast, the prevailing darkness, for they seem to -float like islands of light in a sea of gloom. At a rate of motion -twenty-eight times slower than we are accustomed to, the light tardily -creeps down the mountain-sides, and in the course of about twelve hours -the whole of the circular rampart of the great crater below us, and -towards the east, shines out in brilliant light, unsoftened by a trace -of mountain-mist. But on the opposite side, looking into the crater, -nothing but blackness is to be seen. As hour succeeds hour, the sunbeams -reach peak after peak of the circular rampart in slow succession, till -at length the circle is complete and the vast crater-rim, 50 miles in -diameter, glistens like a silver-margined abyss of darkness. By-and-by, -in the centre, appears a group of bright peaks or bosses. These are the -now illuminated summits of the central cones, and the development of the -great mountain cluster they form henceforth becomes an imposing feature -of the scene. From our high standpoint, and looking backwards to the -sunny side of our cosmorama, we glance over a vast region of the wildest -volcanic desolation. Craters from five miles diameter downwards crowd -together in countless numbers, so that the surface, as far as the eye -can reach, looks veritably frothed over with them. Nearer the base of -the rampart on which we stand, extensive mountain chains run to north -and to south, casting long shadows towards us; and away to southward run -several great chasms a mile wide and of appalling blackness and depth. -Nearer still, almost beneath us, crag rises on crag and precipice upon -precipice, mingled with craters and yawning pits, towering pinnacles of -rock and piles of scoria and volcanic _débris_. But we behold no sign of -existing or vestige of past organic life. No heaths or mosses soften the -sharp edges and hard surfaces: no tints of cryptogamous or lichenous -vegetation give a complexion of life to the hard fire-worn countenance -of the scene. The whole landscape, as far as the eye can reach, is a -realization of a fearful dream of desolation and lifelessness—not a -dream of death, for that implies evidence of preexisting life, but a -vision of a world upon which the light of life has never dawned. - - [Illustration: PLATE XXII. - ASPECT OF AN ECLIPSE OF THE SUN BY THE EARTH, AS IT WOULD APPEAR AS - SEEN FROM THE MOON.] - -Looking again, after some hours’ interval, into the great crateral -amphitheatre, we see that the rays of the morning sun have crept down -the distant side of the rampart, opposite to that on which we stand, and -lighted up its vast landslipped terraces into a series of seeming -hill-circles with all the rude and rugged features of a terrestrial -mountain view, and none of the beauties save those of desolate grandeur. -The plateau of the crater is half in shadow 10,000 feet below, with its -grand group of cones, now fully in sight, rising from its centre. -Although these last are twenty miles away and the base of the opposite -rampart fully double that distance, we have no means of judging their -remoteness, for in the absence of an atmosphere there can be no aërial -perspective, and distant objects appear as brilliant and distinct as -those which are close to the observer. Not the brightness only, but the -various colours also of the distant objects are preserved in their full -intensity; for colour we may fairly assume there must be. Mineral -chlorates and sublimates will give vivid tints to certain parts of the -landscape surface, and there must be all the more sombre colours which -are common to mineral matters that have been subjected to fiery -influence. All these tints will shine and glow with their greater or -less intrinsic lustres, since they have not been deteriorated by -atmospheric agencies, and far and near they will appear clear alike, -since there is no aërial medium to veil them or tarnish their pristine -brightness. - -In the lunar landscape, in the line of sight, there are no means of -estimating distances; only from an eminence, where the intervening -ground can be seen, is it possible to realize _magnitude_ in a lunar -cosmorama and comprehend the dimensions of the objects it includes. - -And with no air there can be no diffusion of light. As a consequence, no -illumination reaches those parts of the scene which do not receive the -direct solar rays, save the feeble amount reflected from contiguous -illuminated objects, and a small quantity shed by the crescent earth. -The shadows have an awful blackness. As we stand upon our chosen point -of observation, we see on the lighted side of the rampart almost -dazzling brightness, while beneath us, on the side away from the sun, -there is a region many miles in area impenetrable to the sight, for -there is no object within it receiving sufficient light to render it -discernible; and all around us, far and near, there is the violent -contrast between intense brightness of insolated parts and deep gloom of -those in equally intense shadow. The black though starlit sky helps the -violence of this contrast, for the bright mountains in the distance -around us stand forth upon a background formed by the darkness of -interplanetary space. The visible effects of these conditions must be in -every sense unearthly and truly terrible. The hard, harsh glowing light -and pitchy shadows; the absence of all the conditions that give -tenderness to an earthly landscape; the black noonday sky, with the -glaring sun ghastly in its brightness; the entire absence of vestiges of -any life save that of the long since expired volcanoes—all these -conspire to make up a scene of dreary, desolate grandeur that is -scarcely conceivable by an earthly habitant, and that the description we -have attempted but insufficiently pourtrays. - -A legitimate extension of the imagination leads us to impressions of -lunar conditions upon other senses than that of sight, to which we have -hitherto confined our fancy. We are met at the outset with a difficulty -in this extension; for it is impossible to conceive the sensations which -the absence of an atmosphere would produce upon the most important of -our bodily functions. If we would attempt the task we must conjure up -feelings of suffocation, of which the thoughts are, however, too -horrible to be dwelt upon; we must therefore maintain the delusion that -we can exist without air, and attempt to realize some of the less -discomforting effects of the absence of this medium. Most notable among -these are the untempered heat of the direct solar rays, and the -influence thereof upon the surface material upon which we suppose -ourselves to stand. During a period of over three hundred hours the sun -pours down his beams with unmitigated ferocity upon a soil never -sheltered by a cloud or cooled by a shower, till that soil is heated, as -we have shown, to a temperature equal nearly to that of melting lead; -and this scorching influence is felt by everything upon which the sun -shines on the lunar globe. But while regions directly insolated are thus -heated, those parts turned from the sun would remain intensely cold, and -that scorching in sunshine and freezing in shade with which mountaineers -on the earth are familiar would be experienced in a terribly exaggerated -degree. Among the consequences, already alluded to, of the alternations -of temperature to which the moon’s crust is thus exposed, are doubtless -more or less considerable expansions and contractions of the surface -material, and we may conceive that a cracking and crumbling of the more -brittle constituents would ensue, together with a grating of contiguous -but disconnected masses, and an occasional dislocation of them. We refer -again to these phenomena to remark that if an atmospheric medium existed -they would be attended with noisy manifestations. There are abundant -causes for grating and crackling sounds, and such are the only sources -of noise upon the moon, where there is no life to raise a hum, no wind -to murmur, no ocean to boom and foam, and no brook to plash. Yet even -these crust-cracking commotions, though they might be felt by the -vibrations of the ground, would not manifest themselves audibly, for -without air there can be no communication between the grating or -cracking body and the nerves of hearing. Dead silence reigns on the -moon: a thousand cannons might be fired and a thousand drums beaten upon -that airless world, but no sound could come from them: lips might quiver -and tongues essay to speak, but no action of theirs could break the -utter silence of the lunar scene. - -At a rate twenty-eight times slower than upon earth, the shadows shorten -till the sun attains his meridian height, and then, from the tropical -region upon which we have in imagination stood, nothing is to be seen on -any side, save towards the black sky, but dazzling light. The relief of -afternoon shadow comes but tardily, and the darkness drags its slow -length along the valleys and creeps sluggishly up the mountain-sides -till, in a hundred hours or more, the time of sunset approaches. This -phenomenon is but daybreak reversed, and is unaccompanied by any of the -gorgeous sky tints that make the kindred event so enrapturing on earth. -The sun declines towards the dark horizon without losing one jot of its -brilliancy, and darts the full intensity of its heat upon all it shines -on to the last. Its disc touches the horizon, and in half an hour dips -half-way beneath it, its intrinsic brightness and colour remaining -unchanged. The brief interval of twilight occurs, as in the morning, -when only a small chord of the disc is visible, and the long shadows now -sharpen as the area of light that casts them decreases. For a while the -zodiacal light vies with the earth-moon high in the heavens in -illuminating the scene; but in a few hours this solar appendage passes -out of view, and our world becomes the queen of the lunar night. - -At this sunset time the earth, nearly in the zenith of us, will be at -its half-illuminated phase, and even then it will shed more light than -we receive upon the brightest of moonlight nights. As the night -proceeds, the earth-phase will increase through the gibbous stages until -at midnight it will be “full,” and our orb will be seen in its entire -beauty. It will perform at least one of its twenty-four-hourly rotations -during the time that it appears quite full, and the whole of its surface -features will in that time pass before the lunar spectator’s eye. At -times the northern pole will be turned towards our view, at times the -southern; and its polar ice-caps will appear as bright white spots, -marking its axis of rotation. If our lunar sojourn were prolonged we -should observe the northern ice-cap creep downwards to lower latitudes -(during our winter) and retreat again (during our summer); and this -variation would be perceptible in a less degree at the southern pole, on -account of the watery area surrounding it. The seas would appear (so far -as can be inferred) of pale blue-green tint; the continents -parti-coloured: and the tinted spots would vary with the changing -terrestrial seasons, as these are indicated by the positions and -magnitudes of the polar ice-caps. The permanent markings would be ever -undergoing apparent modification by the variations of the white -cloud-belts that encircle the terrestrial sphere. Of the nature of these -variations meteorological science is not as yet in a position to speak: -it would indeed be vastly to the benefit of that science if a view of -the distribution of clouds and vapours over the earth’s surface, as -comprehensive as that we are imagining, could really be obtained. - -It might happen at “full-earth,” that a black spot with a fainter -penumbral fringe would appear on one side of the illuminated disc and -pass somewhat rapidly across it. This would occur when the moon passed -exactly between the sun and the earth, and the shadow of the moon was -cast upon the terrestrial disc. We need hardly say that these -shadow-transits would occur upon those astronomically important -occasions when an eclipse of the sun is beheld from the earth. - -The other features of the sky during the long lunar night would not -differ greatly from those to which we alluded in speaking of its day -aspects. The stars would be the more brightly visible, from the greater -power of the eye-pupil to open in the absence of the glaring sun, and on -this account the milky-way would be very conspicuous and the brighter -nebulæ would come into view. The constellations would mark the night by -their positions, or the hours might be told off (in periods of -twenty-four each) by the successive reappearances of surface features on -certain parts of the terrestrial disc. The planets in opposition to the -sun would now be seen, and a comet might appear to vary the monotony of -the long lunar night. But a meteor would never flash across the sky, -though dark meteoric particles and masses would continually bombard the -lunar surface, sometimes singly, sometimes in showers. And these would -fall with a compound force due to their initial velocity added to that -of the moon’s attraction. As there is no atmosphere to consume the -meteors by frictional heat or break by its resistance the velocity of -their descent, they must strike the moon with a force to which that of a -cannon-ball striking a target is feeble indeed. A position on the moon -would be an unenviable stand-point from this cause alone. - -The lunar landscape by night needs little description: it would be lit -by the earth-moon sufficiently to allow salient features, even at a -distance, to be easily made out, for its moon (_i.e._ the earth) has -thirteen times the light-reflecting area that our’s has. But the night -illumination will change in intensity, since the earth-moon varies from -half-full to full, and again to half-full, between sunset and the next -sunrise. The direction of the light, and hence the positions of the -shadows, will scarcely alter on account of the apparent fixity of the -earth in the lunar sky. A slight degree of warmth might possibly be felt -with the reflected earth-light; but it would be insufficient to mollify -the intensity of the prevailing cold. The heat accumulated by the ground -during the three hundred hours’ sunshine radiates rapidly into space, -there being no atmospheric coat to retain it, and a cooling process -ensues that goes on till, all warmth having rapidly departed, the -previously parched soil assumes a temperature approaching that of -celestial space itself, and which has been, as we have stated, estimated -at about 200° below the Fahrenheit zero. If moisture existed upon the -moon, its night-side would be bound in a grip of frost to which our -Arctic regions would be comparatively tropical. But since there is no -water, the aspect of the lunar scenery remains unmodified by effects of -changing temperature. - -Such, then, are the most prominent effects that would manifest -themselves to the visual and other senses of a being transported to the -moon. The picture is not on the whole a pleasant one, but it is -instructive; and our rendering of it, imperfect though it be, may serve -to suggest other inferences that cannot but add to the interest which -always attaches to the contemplation of natural scenes and phenomena -from points of view different from those which we ordinarily occupy. - - [Illustration: PLATE XXIII. - GROUP of LUNAR MOUNTAINS. ideal lunar landscape.] - - - - - CHAPTER XIV. - THE MOON AS A SATELLITE: ITS RELATION TO THE EARTH AND MAN. - - -Apart from the recondite functions of the moon considered as one of the -interdependent members of the solar family, into which it would be -beyond our purpose to inquire, there are certain means by which it -subserves human interests and ministers to the wants of civilized man to -which we deem it desirable to call attention, especially as some of them -are not so self-apparent as to have attracted popular attention. - -The most generally appreciated because the most evident of the uses of -the moon is that of a luminary. Popular regard for it is usually -confined to its service in that character, and in that character poets -and painters have never tired in their efforts to glorify it. And -obviously this service as a “lesser light” is sufficiently prominent to -excite our warmest admiration. But moonlight is, from the very -conditions of its production, of such a changeable and fugitive nature, -and it affords after all so partial and imperfect an alleviation of -night’s darkness, that we are fain to regard the light-giving office of -the moon as one of secondary importance. Far more valuable to mankind in -general, so estimable as to lead us to place it foremost in our category -of lunar offices, is the duty which the moon performs in the character -of a sanitary agent. We can conceive no direful consequences that would -follow from a withdrawal of the moon’s mere light; but it is easy to -imagine what highly dangerous results would ensue if the moon ceased to -produce the tides of the ocean. Motion and activity in the elements of -the terraqueous globe appear to be among the prime conditions in -creation. Rest and stagnation are fraught with mischief. While the sun -keeps the atmosphere in constant and healthy circulation through the -agency of the winds, the moon performs an analogous service to the -waters of the sea and the rivers that flow into them. It is as the chief -producer of the tides—for we must not forget that the sun exercises -_its_ tidal influences, though in much lesser degree—that we ought to -place the highest value on the services of the moon: but for its aid as -a mighty scavenger, our shores, where rivers terminate, would become -stagnant deltas of fatal corruption. Twice (to speak generally) a day, -however, the organic matter which rivers deposit in a decomposing state -at their embouchures is swept away by the tidal wave; and thus, thanks -to the moon, a source of direful pestilence is prevented from arising. -Rivers themselves are providentially cleansed by the same means, where -they are polluted by bordering towns and cities which, from the nature -of things, are sure to arise on river banks; and it seems to be also in -the nature of things that the river traversing a city must become its -main sewer. The foul additions may be carried down by the stream in its -natural course towards the ocean, but where the river is large there -will be a decrease in velocity of the current near the mouth or where it -joins the sea, thus causing partial stagnation and consequent deposition -of the deleterient matters. All this, however, is removed, and its -inconceivable evils are averted by our mighty and ever active “sanitary -commissioner,” the moon. We can scarcely doubt that a healthy influence -of less obvious degree is exerted in the wide ocean itself; but, -considering merely human interests, we cannot suppress the conviction -that man is more widely and immediately benefited by this purifying -office of the moon than by any other. - -But the sanitary service is not the only one that the moon performs -through the agency of the tides. There is the work of tidal transport to -be considered. Upon tidal rivers and on certain coasts, notwithstanding -wind and the use of steam, a very large proportion of the heavy -merchandize is transported by that slow but powerful “tug” the -flood-tide; and a similar service, for which, however, the moon is not -to be entirely credited, is done by the down-flow of the ebb-tide. Large -ships and heavily-laden rafts and barges are quietly taken in tow by -this unobtrusive prime mover, and moved from the river’s mouth to the -far-up city, and from wharf to wharf along its banks; and a vast amount -of mechanical work is thus gratuitously performed which, if it had to be -provided by artificial means, would represent an amount of money value -which for such a city as London would have to be counted by thousands, -possibly millions, of pounds yearly. For this service we owe the moon -the gratitude that we ought to feel for a direct pecuniary benefactor. - -In the existing state of civilization and prosperity, we do not, -however, utilize the power of the tides nearly to the extent of their -capabilities. Our coal mines, rich with “the light of other days”—for -coal was long ago declared by Stevenson to be “bottled sunshine”—at -present furnish us with so abundant a supply of power-generating -material that in our eagerness to use it upon all possible occasions we -are losing sight, or putting out of mind, many other valuable prime -movers, and amongst them that of the rise and fall of the waters, which -can be immediately converted into any form of mechanical power by the -aid of tide-mills. Such mills may be found in existence here and there, -but for the present they are generally out-rivalled by the steam-engine -with all its conveniences and adaptabilities; and hence they have not -shared the benefits of that inventive ingenuity which has achieved such -wonders of mechanical appliance while steam has been in the ascendant. -But it must be remembered that in our extravagant use of coal we are -drawing from a bank into which nothing is being paid. We are consuming -an exhaustive store, and the time must come when it will be needful to -look around in quest of “powers that may be.” Then an impetus may be -given to the application of the tides to mechanical purposes as a prime -mover.[19] For the people of the British Islands the problem would have -an especial importance, viewing the extent of our seaboard and the -number of our tidal rivers. The source of motion that offers itself is -of almost incalculable extent. There is not merely the onward flowing -motion of streams to be utilized, but also the _lift_ of water, which, -if small in extent, is stupendous in amount; and within certain limits -it matters little to the mechanician whether the “foot-pounds” of work -placed at his disposal are in the form of a great mass lifted to a small -height or a small mass lifted to a great height. There is no reason -either why the utilization of the tides should be confined to rivers. -The sea-side might well become the circle of manufacturing industry, and -the millions of tons of water lifted several feet twice daily on our -shores might be converted, even by schemes already proposed, to furnish -the prime movement of thousands of factories. And we must not forget how -completely modern science has demonstrated the inter-convertibility of -all kinds of force, and thus opened the way for the introduction of -systems of transporting power that, in such a state of things as we are -for the moment considering, might be of immense benefit. Gravity, for -instance, can be converted into electricity; and electricity gives us -that wonderful power of transmitting _force_ without transmitting (or -even moving) _matter_, which power we use in the telegraph, where we -generate a force at one end of a wire and _use_ it to ring bells or -deflect needles at the other end, which may be thousands of miles away. -What we do with the slight amount of force needful for telegraphy is -capable of being done with any greater amount. A tide-mill might convert -its mechanical energy by an electro-magnetic engine, and in the form of -electricity its force could be conveyed inland by proper wires and there -reconverted back to mechanical or moving power. True, there would be a -considerable loss of power, but that power would cost nothing for its -first production. Another means ready to hand for transporting power is -by compressed air, which has already done good service; another is the -system so admirably worked out by Sir W. Armstrong, of transmitting -water-power through the agency of an “accumulator,” now so generally -used at our Docks and elsewhere, for working cranes and such other uses. -And as the whole duty of the engineer is to _convert_ the forces of -nature, there is a rich field open for his invention, and upon which he -may one day have to enter, in adapting the pulling force of the moon to -his fellow man’s mechanical wants through the intermediation of the -tides. - -Another of the high functions of the moon is that by which she subserves -the wants of the navigator, and enables him to track his course over the -pathless ocean. Of the two co-ordinates, Latitude and Longitude, that -are needful to determine the position of a ship at sea (or of any -standpoint upon the earth’s surface) the first is easily found, inasmuch -as it is always equal to the altitude of the celestial pole at the place -of observation. But the determination of the longitude has always been a -difficult problem, and one upon which a vast amount of ingenuity has -been expended. When it was first attacked it was soon discovered that -the moon was the object of all others by which it could be most -accurately and, all things considered, most readily determined. We must -premise that the longitude of one place from another is in effect the -difference between the local times at the two places, so that when we -say that a place or a ship is, for instance, seven hours, twenty-four -minutes, ten seconds, west of Greenwich, we mean that the time-o’-day at -the place or ship is seven hours twenty-four minutes ten seconds earlier -than that at Greenwich. Hence, finding the longitude at sea or at any -place and moment means finding what time it is at Greenwich at that -moment. Of course this could be most easily done if we could set a -timekeeper at Greenwich and rely upon its keeping time during a long sea -voyage; and this plan appeared so feasible that our Government long ago -offered a prize of £20,000 for a timekeeper which would perform to a -stated degree of accuracy after a certain sea voyage. One John Harrison -did make such a timekeeper, that actually satisfied the conditions, and -obtained the prize: and chronometers are now largely used for longitude, -their construction having been brought to great perfection, especially -in England, owing to a continuance (in a less liberal degree, however,) -of Government inducement. But chronometers are not entirely to be relied -on, even where several are carried, which in other than Government ships -is rarely the case: recourse must be had to the heavenly bodies for -check upon the timekeeper. And the moon is, as we have said, the body -that best serves the requirements of the problem. - -The lunar method for longitude amounts practically to this. The stars -are fixed; the sun, moon, and planets move amongst them; the sun and -planets with very slow rates of apparent motion, the moon with a very -rapid one. If, then, it be predicted that at a certain instant of -Greenwich time the moon will be a certain distance from a fixed star, -and if the mariner at sea observes _when_ the moon has that exact -distance, he will know the Greenwich time at the instant of his -observation.[20] The moon thus becomes to him as the hand of a -timepiece, whereof the stars are the hour and minute marks, the whole -being, as it were, set to Greenwich time. Then if he knows (which he -does by other observations easily obtained) the local time at his ship, -he can take the difference between the Greenwich time and his time, -which difference is in fact his longitude from Greenwich. The requisite -predictions of the distance of the moon from several fixed stars near -her are given to the utmost exactness for every three hours of every day -and night (when the moon can possibly be seen) in the navigator’s _vade -mecum_, the “Nautical Almanac,” and from these given distances the -navigator can, by a simple process of differencing, obtain the distance, -and hence the Greenwich time, for any intermediate instant at which he -may chance to make his observation. Whenever he can see the moon he can -obtain Greenwich time. Of course the whole value of this method depends -upon the exactitude of the predicted distances corresponding to the -given Greenwich times. These distances are obtained by tables of the -moon’s motions, which must be found from observations. The motions in -question are of an intricacy almost past comprehension, on account of -the disturbing forces to which the moon is subjected by the sun and -planets. The powers of the profoundest mathematicians, from Newton -downwards, have been severely exercised in efforts to group them into a -theory, and represent them by tables capable of furnishing the requisite -exact predictions of lunar positions for nautical purposes. Accurate -observations of the moon’s place night after night have, from the dawn -of this lunar method for longitude, been in urgent request by -mathematicians for the purposes specified, and it was solely to procure -these observations that the Observatory at Greenwich was established, -and mainly for their continued prosecution (and for the stellar -observations necessary for their utilization) that it is sustained. For -two centuries the moon has been unremittingly observed at Greenwich, and -the tables at present used for making the “Nautical Almanac” (those -formed by Prof. Hansen) depend upon the observations there obtained. The -work still goes on, for even now the degree of exactitude is not what is -desired, and astronomers are looking forward with some interest to new -lunar tables which were left complete by the late M. Delaunay, formerly -the head of astronomy in France, based upon a theory which he evolved. -This use of the moon is the grandest of all in respect of the results to -which it has led. - -Then, too, regarding the moon as a timekeeper, we must not forget the -service that it renders in furnishing a division of time intermediate -between the day—which is measured by the earth’s rotation—and the year, -which is defined by the earth’s orbital revolution. Notwithstanding the -survival of lunar reckoning in our religious services, we, in our time -and country, scarcely need a moon to mark our months; but we must not -forget that with many ancient people the moon was, and with some is -still, the chief timekeeper, the calendars of such people being lunar -ones, and all their events being reckoned and dated by “moons.” To us, -however, the moon is of great service in this department by enabling us -to fix dates to many historical events, the times of occurrence of which -are uncertain, by reason of defective records or by dependence upon such -uncertain data as “lives of emperors,” years of this or that king’s -reign, or generations of one or another family. The moon now and then -clears up a mystery, or decides a disputed point in chronology, by -furnishing the accurate date of an ancient eclipse, which was a -phenomenon that always inspired awe and secured for itself careful -record. The chronologer is continually applying to the astronomer for -the date and place of visibility of some total eclipse, of which he has -found an imperfect record, veritable as to the fact, but dated only by -reference to some year of a so-and-so’s reign, or by some battle or -other historical occurrence. The eclipses that occurred near the time -are then examined, and when one is found that tallies with recorded -conditions in other respects (such as the time of day and the place of -observation), its indisputable date becomes a starting-point from which -the chronologer works backwards and forwards in safety. There is one -famous eclipse—that predicted by Thales six centuries before Christ, -which put an end to the battle between the Medes and Lydians by the -terror its darkness created in both armies—which is most intimately -associated with ancient chronology, and has been used to rectify a -proximate date (the first year of Cyrus of Babylon) which forms the -foundation of all Scripture chronology. Sacred and profane history alike -are continually receiving assistance from the accurate dates which the -moon, by having caused eclipses of the sun, enables the astronomer to -fix beyond cavil or doubt. - -The mention of eclipses reminds us, too, of the use which the moon has -been in increasing, through them, our knowledge of the physical -condition of the sun. If the moon had never intervened to cut off the -blinding glare of the solar disc, we should have been to this day left -to assume that the sun is all-contained by the dazzling globe that we -ordinarily see. But, thanks to the moon’s intervention, we now know that -the sun is by no means the mere naked sphere we should have suspected. -Eclipses have taught us that it is surrounded by an envelope of glowing -gases, and that it has a vast vaporous surrounding, beyond its glowing -atmosphere, which appears to be composed of matter streaming away from -the sun into surrounding space. With these discoveries still in their -infancy, it is impossible to foresee the knowledge to which they will -eventually lead, but they can hardly be barren of fruit, and whatever -they ultimately teach will be so much insight gained into the sublimest -problem that human science has before it—the determination of the source -and maintaining power of the light and heat and vivifying agency of the -sun. In according our thankful reflections to the moon for these -revelations, we must not forget that, should there be inhabitants upon -our neighbouring worlds, Mercury, Venus, and Mars, which have no -satellites, they, the supposed inhabitants, can gain no such knowledge -upon the surroundings of the ruler of the solar system. On the other -hand, any rational being who may be supposed to dwell upon Saturn or -Jupiter, would, through the intervention of their numerous moons, have, -in the latter case especially, far more abundant opportunities of -acquiring the knowledge in question than we have. - -Finally, there is a use of the moon which touches us, author and reader, -very closely. It has taught us of a world in a condition totally -different from our own; of a planet without water, without air, without -the essentials to life development, but rather with the conditions for -life destruction; a planet left by the Creator—for wise purposes that we -cannot fully know—as it were but half-formed, with all the igneous -foundations fresh from the cosmical fire, and with its rough-cast -surface in its original state, its fire and mould-marks exposed to our -view. From these we have essayed to resolve some of the processes of -formation, and thus to learn something of the cosmical agencies that are -called forth in the purely igneous era of a planet’s history. We trust -that we, on our part, have shown that the study of the moon may be a -benefit not merely to the astronomer, but to the geologist; for we -behold in it a mighty “medal of creation” doubtless formed of the same -material and struck with the same die that moulded our earth; but while -the dust of countless ages and the action of powerful disintegrating and -denuding elements have eroded and obliterated the earthly impression, -the superscriptions on the lunar surface have remained with their -pristine clearness unsullied, every vestige sharp and bright as when it -left the Almighty Maker’s hands. The moon serves no second-rate or -insignificant service when it teaches us of the variety of creative -design in the worlds of our system, and exalts our estimation of this -peopled globe of ours by showing us that all the planetary worlds have -_not_ been deemed worthy to become the habitations of intelligent -beings. - - -Reflections upon the uses of the moon not unnaturally lead our thoughts -to some matters that may be regarded as abuses. These mainly take the -form of superstitions, erroneous beliefs in the moon’s influence over -terrestrial conditions, and occasionally of erroneous ideas upon the -moon’s functions as a luminary. The first-mentioned are almost beneath -notice, for they include such mythical suspicions as that the moon -influences human sanity and other affections of mind and body; that the -moon’s rays have a decomposing effect upon organic matter; that they -produce blindness by shining upon a sleeper’s eyes; that the moon -determines the hours of human death, which is supposed to occur with the -change of the tide, etc. All such, having no foundation on fact, are put -beyond our consideration. The third matter we have mentioned may also be -dismissed in a very few words. The erroneous ideas upon the moon’s -functions as a luminary, to which we allude, are those which are -manifested by poets and painters, and even historians, who do not -hesitate to bring the moon upon a scene in any form and at any time they -please without reference to actual lunar circumstances. It is no -uncommon thing to see, in a picture representing an evening scene, a -moon introduced which can only be seen in the morning—a waning moon -instead of a waxing one; and astronomical critics have, indeed, caught -artists so far tripping as to put a moon in a picture representing some -event that occurred upon a date when the moon was new, and therefore -invisible. Writers take the same liberties very frequently. A newspaper -correspondent, during the Franco-Prussian war, described the full moon -as shining upon a scene of desolation on a particular night, when really -there was no moon to be seen. One of the most flagrant cases of this -kind, however, occurs in Wolfe’s ballad on “The death of Sir John -Moore,” where it is written that the hero was buried “By the struggling -moonbeam’s misty light.” But the interment actually took place at a time -when the moon was out of sight. We mention these abuses of the moon in -the hope of promoting a better observance of the moon’s luminary office. -They who wish to bring the moon upon a scene, not knowing _ipso facto_ -that it was there, should first take the advice of Nick Bottom in the -“Midsummer Night’s Dream,” and make sure of their object by consulting -an almanac. - -The second of the specified abuses to which the moon is subject refers -to its supposed influence on the weather; and in the extent to which it -goes this is one of the most deeply rooted of popular errors. That there -is an infinitesimal influence exerted by the moon on our atmosphere will -be seen from the evidence we have to offer, but it is of a character and -extent vastly different from what is commonly believed. The popular -error is shown in its most absurd form when the mere _aspect_ of the -moon, the mere transition from one phase of illumination to another, is -asserted to be productive of a change of weather; as if the gradual -passage from first quarter to second quarter, or from that to third, -could of itself upset an existing condition of the atmosphere; or as if -the conjunction of the moon with the sun could invert the order of the -winds, generate clouds, and pour down rains. A moment’s reasoning ought -to show that the supposed cause and the observed effect have no -necessary connection. In our climate the weather may be said to change -at least every three days, and the moon changes—to retain the popular -term—every seven days; so that the probability of a coincidence of these -changes is very great indeed: when it occurs, the moon is sure to be -credited with causing it. But a theory of this kind is of no use unless -it can be shown to apply in every case; and, moreover, the change must -always be in the same direction: to suppose that the moon can turn a -fine day to a wet one, and a wet day to a fine morrow indiscriminately, -is to make our satellite blow hot and cold with the same mouth, and so -to reduce the supposition to an absurdity. If any marked connection -existed between the state of the air and the aspect of the moon, it must -inevitably have forced itself unsought upon the attention of -meteorologists. In the weekly return of Births, Deaths, and Marriages, -issued by the Registrar-General, a table is given, showing all the -meteorological elements at Greenwich for every day of the year, and a -column is set apart for noting the changes and positions of the moon. -These reports extend backwards nearly a quarter of a century. Here, -then, is a repertory of data that ought to reveal at a glance any such -connection, and would certainly have done so had it existed. But no -constant relation between the moon columns and those containing the -instrument readings has ever been traced. Our meteorological -observatories furnish continuous and unbroken records of atmospheric -variations, extending over long series of years: these afford still more -abundant means for testing the validity of the lunar hypothesis. The -collation has frequently been made for special points in the inquiry, -and certainly _some_ connection has been found to obtain between certain -positions of the moon in her orbit and certain instrumental averages; -but so small are the effects traceable to lunar influence, that they are -almost inappreciable among the grosser irregularities that arise from -other and as yet unexplained causes. - -The lunar influences upon our atmosphere most likely to be detected are -those of a tidal character, and those due to the radiation of the heat -which the moon receives from the sun. The first would be shown by the -barometer, which may be called an “atmospheric tide gauge.” Some years -ago Sir Edward Sabine instituted a series of observations at St. Helena, -to determine the variations of barometric indications from hour to hour -of the lunar day. The greatest differences were found to occur between -the times when the moon was on the meridian, and when it was six hours -away from the meridian; in other words, between atmospheric high tide -and low tide. But the average of these differences amounted only to the -four-hundredth part of an inch on the instrument’s scale; a quantity -that no weather observer would heed, that none but the best barometers -would show, and that can have no perceptible effect on weather changes. -The distance of the moon from the earth varies, as is well known, in -consequence of the elliptical form of her orbit: this variation ought -also to produce an effect upon the instrument’s indications; but Colonel -Sabine’s analysis showed that it was next to insensible; the mean -reading at apogee differing from that at perigee by only the -two-thousandth part of an inch. Schubler, a German meteorologist, had -arrived at similarly negative results some years previously. Hence it -appears that the great index of the weather is not sensibly affected by -the state of the moon: the conclusion to be drawn with regard to the -weather itself is obvious enough. As regards the heat received from the -moon, we know, from the recent experiments of Lord Rosse in England, and -Marie Davy in France, elsewhere alluded to, that a degree of warmth -appreciable to the highly sensitive thermopile is exerted by the moon -upon the earth near to the time of full moon, when the sun’s rays have -been pouring their unmitigated heat upon the lunar surface continuously -for fourteen days. And as it is improbable that the whole of the heat -sent earthwards from the moon reaches the earth’s surface, we must infer -that a considerable amount is absorbed in the higher atmosphere, and -does work in evaporating the lighter clouds and thinning the denser -ones. The effect of this upon the earth is to facilitate the radiation -of its heat into space, and so to cool the lower atmospheric strata. And -this effect has been shown to be a veritable one by an exhaustive -tabulation of temperature records from various observatories, which was -undertaken by Mr. Park Harrison. The general conclusion from these was, -that the temperature at the earth’s surface is lower by about 2½ degrees -at moon’s last quarter than at first quarter; the paradoxical result -being what would naturally follow from the foregoing consideration. The -tendency of the full moon to clear the sky has been remarked by several -distinguished authorities, to wit, Sir John Herschel, Humboldt, and -Arago; and in general the clearing may be accepted as a meteorological -fact, though in one case of close examination it has been negatived. It -cannot be doubted that a full moon sometimes shows a night to be clear -that would in the absence of the moon be called cloudy. - -When close comparisons are made between the moon’s positions and records -of rain-fall and wind-direction, dim indications of relation exhibit -themselves, which may be the feeble consequences of the change of -temperature just spoken of; but in every case where an effect has been -traced it has been of the most insignificant kind, and no apparent -connexion has been recognized between one effect and another. Certainly -there is nothing that can support the extensive popular belief in lunar -influence on weather, and nothing that can modify the conviction that -this belief as at present maintained is an absurd delusion. Yet its -acceptance is so general, and runs through such varied grades of -society, that we have felt it our duty to dwell upon it to the extent -that we have done. - - - - - CHAPTER XV. - CONCLUDING SUMMARY. - - -Having arrived at the conclusion of our subject, it appears to us -desirable that we should recall to the reader, by a rapid review, its -salient features. - -Our main object being to attempt what we conceived to be a rational -explanation of the surface details of the moon which should be in -accordance with the generally received theory of planetary formation, -and with the peculiar physical conditions of the lunar globe—the opening -of our work was a summary of the nebular hypothesis as it was started by -the first Herschel and systemised by Laplace. Following these -philosophers we endeavoured to show how a chaotic mass of primordial -matter existing in space would, under the action of gravitation, become -transformed into a system of planetary bodies circulating about a common -centre of gravity; and further, how, in some cases, the circulating -planetary masses would themselves become sub-centres of satellitic -systems; our earth being one of these sub-centres with only one -satellitic attendant—to wit, the moon, the subject of our study. - -The moon being thus considered as evolved from the parent nebulous mass, -and existing as an isolated and compact body, we had next to consider -what was the effect of the continued action of the gravitating force. By -the light of the beautiful “mechanical theory of heat” we argued that -this force, not being _destructible_, but being _convertible_, was -turned into heat; and that whatever may have been the original condition -of the parent nebulous mass, as regards temperature, its planetary -offspring became elevated to an intense degree of heat as they assumed -the form of spheres under the influence of gravitation. - -The incandescent sphere having attained its maximum degree of heat by -the total conversion thereinto of the gravitating force it embodied, we -explained how there must have ensued a dispersion of that heat by -radiation into surrounding space, resulting in the cooling and -consequent solidification of the outermost stratum of the lunar sphere, -and subsequently in the continuation of the cooling process downwards or -inwards to the centre. And here we essayed to prove that in this second -stage of the cooling process, when the crust was solid and the subjacent -portion of the molten sphere was about to solidify, there would come -into operation a principle which appears to govern the behaviour of -certain fusible substances, and which may be concisely termed the -principle of pre-solidifying expansion. We adduced several examples of -the manifestation of this principle, soliciting for it the careful -consideration of physicists and geologists, and looking to it as -furnishing the key to the mystery of volcanic action upon the moon, -since, without needing recourse to aqueous or gaseous sources of -eruptive power, it afforded a rationale of the ejection of the fluid and -semifluid matter of the moon through the solidified crust thereof, and -also of the dislocations of that crust, unattended by actual ejection of -subsurface matter, of which our satellite presents a variety of -examples, and which the earth also appears to have experienced at some -period of its formative history. - -Arrived at this stage of our subject we thought it needful to introduce -some pages of data and descriptive detail. Accordingly in one chapter we -discussed the form, magnitude, weight, and density of the moon, and the -force of gravity at its surface: and the more soundly to fix these data -in the mind, we devoted a few lines to explanation of the methods -whereby each has been ascertained. We then examined the question (so -important to our subject) of the existence or non-existence of a lunar -atmosphere, giving the evidence, which may be regarded as conclusive, in -proof of the absence of both air and water from the moon, and, -therefore, refuting the claim of these elements to be considered as -sources or influants of the moon’s volcanic manifestations. A general -_coup-d’œil_ of the lunar hemisphere facing the earth next engaged our -attention, and we considered the aspect of the disc as it is viewed by -the naked eye and with telescopes of various powers. From this general -survey we passed to the topography of the moon, tracing briefly the -admirable labours of those who have advanced this subject, and, by aid -of picture and skeleton maps and a table of position co-ordinates, -placing it within the reader’s power to become more than sufficiently -acquainted for the purposes of this work with the names and positions of -detail objects and features of interest. Special descriptions of -interesting and typical spots and regions were given in some few cases -where such appeared to be called for. - -These descriptive matters disposed of, we proceeded to discuss the -various classes of surface features with a view to explaining the -precise actions which appear to us to have led to their formation. -Naturally the craters first demanded our attention. We pointed out the -reasons for regarding the great majority of the circular formations of -the moon as craters, as truly volcanic as those of which we have -examples, modified by obvious causes, upon the earth; and, tracing the -causative phenomena of terrestrial volcanoes, we showed how the -explanations which have been offered to account for them scarcely apply -to those of the moon: and thus, driven to other hypotheses, we -endeavoured to demonstrate the probability of the lunar craters having -been produced by eruptive force, generated by that pre-solidifying -expansion of successive portions of the moon’s molten interior, which we -enunciated in our third chapter. The precise course of phenomena which -resulted in the production of a crater of the normal lunar type, with or -without the significant central cone, were then illustrated by a series -of step-by-step diagrams with accompanying descriptive paragraphs. And -after treating of craters of the normal type we pointed out and -explained some variations thereupon that are here and there to be met -with, and likewise those curious complications of arrangement which -exhibit craters superimposed one upon another and intermingled in -strange confusion. - -From craters manifestly volcanic we passed to the consideration of those -circular formations which, from their vastness of size, scarcely admit -of satisfactory explanation by a volcanic hypothesis. We summarized -several proffered theories of their origin, and pointed out what we -considered might be a possible key to the solution of the selenological -enigma which they constitute, without, however, expressing ourselves -entirely satisfied with the validity of our suggestion. The less -mysterious features presented by peaks and mountain ranges were then -discussed to the extent that we considered requisite, viewing their -comparatively simple character and the secondary position they occupy in -point of numerical importance upon the moon. At greater length we dealt -with the cracks and chasms and the allied phenomena of radiating -streaks, pointing out with regard to these latter the strikingly -beautiful correspondence in effect (and therefore presumably in cause) -between them and crack-systems of a glass globe “starred” by an -expanding internal medium. - -The more notable objects and features of the lunar surface being -disposed of, we had next to say a few words upon some residual -phenomena, chiefly upon the colour of lunar surface details, and upon -their various degrees of brightness or reflective power. And, inasmuch -as varying brightness seemed to us to be related to varying antiquity, -we were thence led to the question of the chronology of selenological -formations, and to the disputation upon the continuance of volcanic -action upon the moon in recent years. We regarded this question from the -observational and the inferential points of view, and were led to the -conclusion that the moon’s surface arrived at its terminal condition -ages ago, and that it is next to hopeless to look for evidence of -existing change. - -Thus far our work dealt with the moon as a planetary body merely. It -occurred to us, however, that we might add to the interest attaching to -our satellite were we to regard it for a time as a world, and consider -its conditions as respects fitness for habitation by beings like -ourselves. The arguments against the possibility of the moon being thus -fitted for human creatures, or, indeed, for any high organism, were -decisive enough to require little enforcing. It appeared to us, -nevertheless, that much might be learnt by imagining one’s self located -upon the moon during a period embracing one lunar day (a month of our -reckoning), with power to comprehend the peculiar circumstances and -conditions of such a situation. We therefore attempted a description of -an imaginary sojourn upon the moon, and pointed out some of the more -striking aspects and phenomena which we know by legitimate inference -would be there manifested. We trust, that while our modest efforts in -the chapter referring to this branch of our subject may prove in some -degree entertaining, they may be in a greater degree instructive, -inasmuch as certain facts are brought into prominence which would not -unnaturally be overlooked in contemplating the moon from the earth, the -only _real_ stand-point that is available to us. - -In our final chapter we considered the moon as a satellite, and sought -to enhance popular regard for it on account of certain high functions -which it performs for man’s benefit on this earth; but which are in -great risk of being overlooked. We showed that, notwithstanding the -moon’s occasionally useful service as a nocturnal luminary, it fills a -far higher office as a sanitary agent by cleansing the shores of our -seas and rivers through the agency of the tides. We pointed out the vast -amount of absolutely mechanical work and commercial labour which the -same tidal agency executes in transporting merchandize up and down our -rivers—an amount that, to take the port of London alone, represents a -money value _per annum_ that may be reckoned in millions sterling, -seeing that if our river was tideless all transport would have to be -done by manual or steam power. We then hinted at the stupendous -reservoir of power that the tidal waters constitute, a form of power -which has not as yet been sufficiently called into operation, but which -may be invoked by-and-by, when we have begun to feel more acutely the -consequences of our present prodigal use of the fuel that was stored up -for us by bountiful nature ages upon ages ago. The moon’s services to -the navigator, in affording him a ready means of finding his longitude -at sea; to the chronologist and historian, as a timekeeper, counting -periods too vast for accurate reckoning by other means; to the -astronomer and student of nature, in revealing certain wonderful -surroundings of the solar globe, which, but for the phenomena of -eclipses caused by the moon’s interposition, would never have been -suspected to exist—these were other functions that we dwelt upon, all -too briefly for their deserts; and, lastly, we spoke of the moon as a -medal of creation fraught with instructive suggestions, which it has -been our endeavour to bring to notice in the course of this work. And -from uses we passed to abuses, directing attention to a few popular -errors and widespread illusions relating to lunar influence upon and in -connection with things terrestrial. This part of our work might have -been considerably expanded, for, in truth, the moon has been a -misunderstood and misjudged body. Some justice we trust we have done to -her: we have brought her face to the fireside; we have analysed her -features, and told of virtues that few of her admiring beholders -conceived her to possess. We have traced out her history, fraught with -wonderful interest, and doubtless typical of the history of other -spheres that in countless numbers pervade the universe: and now, having -done our best to make all these points familiar, we commend the moon to -still further study and still more intimate acquaintance, confident that -she will repay all attentions, be they addressed to her as - - A PLANET, A WORLD, OR A SATELLITE. - - - THE END. - - - - - FOOTNOTES - - -[1]The melting temperature of iron is 1500° Centigrade. - -[2]Mr. T. Heunter, Manager of the Iron-works of James Murray, Esq., of - Dalmellington, Ayrshire. Another authority (Mr. Snelus, of the West - Cumberland Iron Company), writes as follows: “I had a hole dug on - the ‘cinder-fall,’ and allowed the running slag to flow through it - so as to form a tolerably large pool and yet keep fluid. Any crust - that formed was skimmed off. A portion of the same slag was cooled, - and the solid lump thrown into the pool. It floated just at the - surface.” Mr. Snelus adds, by the way, that he tried “Bessemer-Pig” - in the same way, and that the solid pig sunk in the molten for a - minute and then _rose and floated_ just at the surface, with about - one-twentieth of its bulk above the level of the fluid. - -[3]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. - -[4]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. - -[5]The proper distance for realising the conditions under which the moon - itself is seen will be that at which our disc is just covered by a - wafer about a quarter of an inch in diameter, held at arm’s length. - This will subtend an angle of about half a degree, which is nearly - the angular diameter of the moon. - -[6]The libratory movement has been taken advantage of, at the suggestion - of Sir Chas. Wheatstone, for producing stereoscopic photographs of - the moon. In the early days of stereoscopic photography the object - to be photographed was placed upon a kind of turn-table, and, after - a picture had been taken of it in one position, the table was turned - through a small angle for the taking of the second picture; the two - placed side by side then represented the object as it would have - been seen by two eyes widely separated, or whose visual rays - inclined at an angle equal to that through which the table was - turned; and when the pictures were viewed through a stereoscope, - they combined to produce the wonderful effect of solidity now - familiar to every one. The moon, by its librations, imitates the - turn-table movement; and, from a large number of photographs of her, - taken at different points of her orbit and at different seasons of - the year, it is possible to select two which, while they exhibit the - same phase of illumination, at the same time present the requisite - difference in the points of view from which they are taken to give - the effect of stereoscopicity when viewed binocularly. Mr. De la - Rue, the father of celestial photography, has been enabled to - produce several such pairs of pictures from the vast collection of - lunar photographs that he has accumulated. Any one of these pairs of - portraits, when stereoscopically combined, reproduces, to quote the - words of Sir John Herschel, “_the 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 when one view was taken, and that to which it would have to be - removed (our moon being fixed) to get the other. Nothing can surpass - the impression of _real corporeal form_ thus conveyed by some of - these pictures as taken by Mr. De la Rue with his powerful - reflector, the production of which (as a step in some sort taken by - man outside of the planet he inhabits) is one of the most remarkable - and unexpected triumphs of scientific art.” - -[7]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.” - -[8]Plate VIII. - -[9]“Cosmos,” Bohn’s Edition, Vol. V. p. 322. - -[10]_American Journal of Science, Second Series, Vol. II._ - -[11]“Volcanoes,” page 155. - -[12]In reference to such prominences on the lunar surface as cast - steeple-like shadows, it is well to remark that we must not in all - cases infer, from the acute spire-like form of the shadow, that the - object which casts the shadow is of a similar sharp or spire-like - form, which the first impression would naturally lead us to suppose. - A comparatively blunt or rounded eminence will project a long and - pointed shadow when the rays of light fall on the object at a low - angle, and especially so when the shadow is projected on a convex - surface. We illustrate this with a copy of an actual photograph of - the shadow cast by half a pea, Fig. 41. - -[13]We meet a difficulty in reconciling this idea with the partial - craters of which we have a conspicuous example in Fracastorius, No. - 78, of our Map, which seem to be partially sunk below the contiguous - surface. This looks as though the crater-rim belonged to an older - epoch than the plain from which it rises. - -[14]We are informed by a friend, who has lately visited Athens, that - Schmidt’s detail drawings of the Moon, comprising the work of forty - years, form a small library in themselves. The map embodying them is - so large (6 ft. 6 in. in diameter) and so full of detail that there - is small hope of its complete publication, unless there should be - such a wide extension of interest in the minute study of our - satellite as to justify the cost of reproducing it. - -[15]It is conceivable that the alleged changes in the crater Linné may - have been caused by a filling of the crater by some such crumbling - action as we are here contemplating. - -[16]Is it not conceivable that the protogerms of life pervade the whole - universe, and have been located upon every planetary body therein? - Sir William Thomson’s suggestion that life came to the earth upon a - seed-bearing meteor was weak, in so far that it shifted the locus of - life-generation from one planetary body to another. Is it not more - philosophical, more consistent with our conception of Creative - omnipotence and impartiality, to suppose that the protogerms of life - have been sown broadcast over all space, and that they have fallen - here upon a planet under conditions favourable to their development, - and have sprung into vitality when the fit circumstances have - arrived, and there upon a planet that is, and that may be for ever, - unfitted for their vivification? - -[17]Our remarks have general reference to a region of the moon near her - equator; near the poles some of the conditions we shall describe - would be somewhat modified. - -[18]We see this reddening during an eclipse of the moon (when the event - we are describing—an eclipse of the sun visible from the moon—really - takes place). The blood-red colour has often struck observers very - forcibly, and it has indeed been suggested that the appearance may - be the innocent and oft-repeated fulfilment of the prophetic - allusion to the moon being “turned into blood.” - -[19]About 100 years ago London was supplied with water chiefly by pumps - worked by tidal mills at London Bridge. - -[20]The sun and planets are comparatively useless for this object, - because of their slow movement among the stars; the change of their - positions from hour to hour is so small as to render uncertain the - Greenwich times deducible therefrom. Their use would be comparable - to taking the time from the hour-hand of a clock. - - - BRADBURY, AGNEW, & CO., PRINTERS, WHITEFRIARS. - - - Albemarle Street, - _December, 1873_. - - - - - WORKS ON SCIENCE, &c. - - -A NATURALIST’S VOYAGE ROUND THE WORLD. By Charles Darwin, F.R.S. With -Illustrations. 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