From The Edinburgh Review. NASMYTH'S PHYSICAL HISTORY OF THE MOON.*

being himself the son and grandson of two eminent Scottish artists, and he fostered this taste under the facilities and THE earth's bright satellite has always training which were at his command in been an object of affectionate reverence the courses of the then recently-formed among the sons of men. In the early school of arts of the Scotch metropolis. days of human history magnificent tem- Not very long after the completion of his ples were reared in expression of this university career he began to give some feeling. In the present age no less attention to the investigations of astroncostly buildings are erected and main- omy, and was very soon deep in the contained at the public charge, where large struction of reflecting telescopes of large bands of carefully-trained and well-ap- size and considerable power. On June pointed ministrants keep watch and cele- 14, 1844, just thirty years ago, he combrate their solemn rites, night after night, municated, as one of the first fruits of his in the same service. But perhaps no labours in this direction, a paper to the more noteworthy illustration of the charm Royal Astronomical Society, describing which this particular devotion has, even" Certain Telescopic Appearances of the for unimaginative and unimpressionable Moon," and exhibited, in illustration of philosophers, could be found in the annals this memoir, a drawing and model repreof human history than is expressed in senting the aspect of a part of the lunar the beautifully illustrated volume which surface as it appeared in his telescopes has just been published under the con-under high magnifying power. The tract joint authorship of James Nasmyth and of the lunar surface which was dealt with James Carpenter, and which really rep-in these illustrations is a broken region resents more than thirty years' almost immediately surrounding the large crater unintermittent study and application on known as Maurolicus, and both drawing the part of a mechanical engineer, who and model were made by a telescope of is distinguished amongst his contempora- twelve inches' aperture magnifying in ries and compeers alike for the hard linear dimensions 360 times, and were practicality of his head, the adroit readi- upon the scale of one-eighth of an inch to ness of his hands, and the finished culti-the mile. About a couple of years after vation of his taste. The James Nasmyth this time, we ourselves had in our posalluded to in this remark, it will scarcely session a copy of a very large drawing of be necessary to say, is the civil engineer some lunar craters, that had been used so well known as the inventor of the by Captain Owen Stanley in one of his steam-hammer and of the steam-machin- lectures, and which was also made by ery for driving piles. In his schoolboy Mr. Nasmyth. This sketch was the prime days, while still attending the classes of original of the remarkable group of crathe high school at Edinburgh, James Nas-ters associated with Theophilus, Cyrillus, myth was led, by an accidental acquaint- and Catharina, which appears among the ance with the son of an ironfounder, to illustrations of the book now under study closely the various processes of notice. When the memoir on the "Telecasting and forging iron. He had also scopic Appearances of the Moon" was inherited a strong taste for drawing,

communicated to the Royal Astronomical Society, its author had been already in close observation and study of the lunar disc for some years, in the conviction that it furnished a very admirable and instructive means of illustrating certain grand features of volcanic operations. In one part of the memoir he drew pointed attention to the "brimful" crater, which is now presented in the illustrations of the book as Wargentin. The memoir of

1844, which occupied two pages of the afterwards photographed in appropriate sixth volume of the Monthly Notices of positions in strong sunshine, and from the Astronomical Society, was indeed these photographs the prints that appear essentially the protogerm of the noble in the book were for the most part finally quarto volume now before us. In the made by the heliotype process, in permabrief early memoir there are traces of the nent pigments which are as fixed and leading thoughts that have been devel- enduring as the ink of ordinary copperoped in the finished book. plate engravings. The finest of these pictures actually reproduce to the eye the appearances that are seen in the moon by the aid of high powers of the telescope, applied under the most favourable conditions of lighting and atmosphere. They nearly all of them deal with bold characteristics of lunar scenery, and in many of them the reproduction is so perfect that it seems to an experienced eye as if the old familiar reality were before it when it rests intently upon the pictorial rendering. Even the peculiar frosted-silver texture, and the indescribably delicate frettings and frecklings that start out from the lunar surface in passing instants of the nicest telescopic defi

coin. Ready proofs of this statement may be especially found in the delineations which represent the terraced landslips, and circumambient mottling of froth-craters, that surround Copernicus

Two distinguishing characters mark Mr. Nasmyth's monograph from the hundred and one treatises that have touched upon the same theme. These are, in the first place, the marvellous beauty and accuracy of the pictorial illustrations, which are altogether without parallel in this branch of art; and in the second place, the lucidity and completeness with which the author's views of the moon's physical condition, and probable formative history, have been put into words. The treatment of the subject in this monograph is that of a mind which has been trained in the methods and discipline of mechanical and engineering, rather than of astronomical and mathematical, sci-nition, are there, as mint-marks of the ence, and which has acquired a very firm grasp of the matter on what may be termed its practical side. It is not too much to say that, under the impress of these characteristics, the book is the most complete and intelligible description the lunar Etna of Hevelius - and the of the physical condition of the moon that walled hollows of Aristotle and Eudoxus; has yet been published. the long yawning void chasms that shatIn alluding to the exquisite delineations ter the ground near Triesnecker; the bulof the typical features of the lunar physi-warked floors of Shickard and Wargenognomy with which his volume is illus-tin; the serrated shadows of the sombre trated, Mr. Nasmyth explains that these abyss of Plato, with its sentinel peak are the results of more than thirty years Pico, and its clustered outwork of Alpine of continued study and work. Drawings summits ploughed through by a broad of the various objects here represented flat-bottomed valley; and, perhaps, before were, in the first instance, made at all, the clustered peaks and shadowfavourable opportunities when high fringed chain of the mighty Apennines, powers of the telescope could be satis- with the fissured crackings of the surfactorily and advantageously used, and rounding plain. These particular drawthese drawings were then subsequently ings are certainly as successful an re-examined in comparison with their attempt to present, in a pictorial form, originals, and retouched, corrected, and what the highest powers of the telescope amplified, time after time, until they at reveal in this weird field of investigation, last seemed to the practised eye of the as it is possible for the most sanguine artist as perfect as the equally practised enthusiasm to conceive. The result is in hand could render them. In this com- these instances beyond all praise. The pleted form they were next turned into long, patient, painstaking labour, and the models in bold relief; these models were consummate skill of the artist, alone can

explain how such marvels of pictorial occultations of fixed stars by the moon, verisimilitude have been produced by photographing artificial modellings.

and demonstrated that in every case the star was out of sight behind the moon Before entering definitely upon the within two seconds of the time that it consideration of Mr. Nasmyth's views of ought to have been under the circumthe physical condition and history of the stance of the moon being without any moon, it may be well to ask the reader to external investment of a gaseous or vaplace compactly before his mind, in a porous kind which could have bent the broad, general form, an idea of what the rays of star-light as they shot through body is that is concerned in the explana- the transparent space in the immediate tion. The moon, it will be remembered, neighbourhood of the solid limb of the is a solid sphere of material substance moon. If these two seconds of differhaving nearly the intrinsic density of flint-ence, the utmost allowance that can posglass, and of such size that it reaches to sibly be made in the face of this test, about the forty-ninth part of the volume were entirely due to a thin film of atmosof the earth, and has therefore a surface-phere enveloping the moon, that film area something less in extent than a must be, under the circumstance, two fourteenth part of the surface of the earth, thousand times less substantial and comprising in exact numbers 14.567,000 dense than the atmosphere of the earth, square miles. The size and density of and that would be as rare again as the this sphere, thus apportioned, are of such most perfect so-called vacuum that has amount that the force of gravity upon its ever been artificially produced by the airouter surface must be not more than a pump. Upon this showing, therefore, sixth part of the same force upon the the moon is held to be a virtually airless earth, so that a heavy body shot off from sphere. The absence of water is also the outer surface of the moon by any proved by similar unmistakable evigiven projectile effort would go six times dence. If it were present upon the suras far under the impulse as it would if face of the moon, even in the most limitstarted from the earth's surface in the ed amount, there would of necessity be same way. But the spherical mass thus an envelope of vapour about the sphere circumstanced in the matter of size and that would manifest its presence by the density, is a bare round ball of solid sub-influence it would exert upon such rays stance, destitute of all trace of atmos- of light as passed through its substance. pheric investment, whether of vapour or The naked moon, with these condiair. The absence of gaseous atmosphere, tions of volume and mass, is carried in of whatever kind, in the moon is definite- an even sweep around the earth at a disly proved by the simple fact that when- tsnce of about thirty earths' diameters. ever its opaque body passes along in the At this distance any tract of the moon's' sky in front of a fixed star, the shining surface twenty-six miles across, or about point is concealed by the passage of the as large again as the county of Middleintervening dark body within an immate- sex, would be visible to the eye as an rial trifle of the time that it ought to be immeasurably small speck or sizeless upon the assumption that the occulting point. With the aid of a telescope such body is bare of all gaseous or vaporous a visible speck would be more or less investment, which, if present, would have spread into perceptible dimensions. If kept the star for some time in sight when the magnifying power of the telescope actually behind the moon, as the sun is were enough to enlarge 6,000 diameters — brought into sight by atmospheric refrac- an extreme conception of the case which tion when below the earth's horizon. is now sometimes spoken of as being With a view to the final settlement of this within the possible achievements of optiquestion, among other examinations cal skill- the moon would be looked at which have been made, the astronomer as if it were not more than forty miles royal some little time ago put together away, and even small natural objects the results of 296 carefully observed upon its face, not more than twenty or

thirty yards wide, would be visible as ifestations of some form of eruptive force having dimensions and form. Such which has pitted and blistered the lunar powers of the telescope are, however, not surface. In many of them there still rereally available for practical purposes, mains the central peak which has been because they increase the disturbing piled up on the middle of the floor by effects of the air's imperfect and irregu- the expiring effort of the outburst. Vollar transparency to such an extent that canic action of some kind has, at some distinct vision ceases to be possible. The time, ruled rampant upon the moon. It highest telescopic power that can be is the one pervading power that has left effectually brought to bear upon the its stamp there in every feature and linmoon, even under the most favourable eament. condition of the atmosphere, is one that magnifies not more than 400 times across, and with that power an object on the moon 200 yards, or something less than an eighth of a mile, broad would become visible as a speck, and a square mile of surface would be of a measurable size. It is with such optical aid as this that the pictorial work of Mr. Nasmyth has been accomplished.

But volcanic action upon the moon has not been the same thing as volcanic action upon the earth. The leading energy or motive power in the volcanic eruptions of the earth is the expansive force of steam. On the moon there is no water, and therefore steam can have nothing to do with the matter. What then can have been the potential agency that has been operative in sculpturing the volcanic frettings of the moon? This is the primary question upon which Mr. Nasmyth brings his mechanical training and experience to bear in dealing with the moon's physical history. As a first step in his demonstration and argument, Mr. Nasmyth assumes that in its early infancy, in common with the other solid spheres of space, the moon was in the state of a huge drop-like ball of molten liquid, that this drop-like condition was due to great heat, and that at a later period of its history a solid surface, or shell, was formed round the spherical liquid mass, in consequence of the more rapid cooling and consequent solidification of this outer layer. In building his argument upon this ground he takes up what must at least be admitted to be a reasonable and justifiable position, The red-hot molten nucleus and cooled-crusttheory of the formation of the planetary spheres has, at least, more probability and is more satisfying to the reason than any other view that has yet been presented in its place. It perfectly and easily accounts for the globular, or quasi-globu lar, form of these mighty spheres, and in recent years it has acquired increased consistency and strength, as a theory, from the researches that have been happily made into the correlative physical conditions of the sun and stars.

With these telescopic appliances, bringing the moon in effect 400 times nearer to the earth, and so placing it for purposes of examination as if it were only 600, instead of 240,000, miles away, the moon is at once seen to be not a smooth ball, but a rough sphere, covered all over with rugosities and carved projections that are of sufficient dimensions to cast long black shadows beyond them when the sunshine falls obliquely upon their sides, and are thus thrown into very bold relief. These lunar projections are of different kinds, but there is one predominant type which is so abundantly and so continually reproduced that it may be taken to be the most important and characteristic of the series. This wears the form of a circular cup, raised, broken, and often terraced and serrated at the rim, but hollowed more or less down into the moon's substance, and is what is familiarly and technically known as the annular "crater" of the moon. Of these circular ridge-brimmed "craters" the number upon the moon is so vast that it is literally impossible for them to be counted. They exist in untold thousands, and in some parts are so crowded together that the moon's surface for large stretches of distance is entirely made up of them. But they vary materially in size, from something less than a mile, the smallest circular speck that is In this theory of the primitive formadiscernible, to something more than tion of the world-spheres of space, it is eighty, or even one hundred, miles across. understood that the solid crust first deThe most casual glance at these ring- posited as the outer shell of the molten shaped and bulwark-surrounded hollows mass must of necessity contract continupon the moon is sufficient to establish uously, more and more, upon the impris their claim to be properly called "cra- oned molten nucleus, as it scatters its ters." They are clearly the visible man-own elevated temperature more and more

into space, because all material substance, how strikingly the starred and fissured occupies a larger volume at high temper-condition of the globe imitates the apatures than at low ones. But, simulta-pearance of certain bright streaks in the neously with this contraction of the outer moon that are seen radiating round some cooling shell, another influence influence is of the principal craters when they are brought into play, which Mr. Nasmyth, illuminated by direct sunshine. with some claim to originality, boldly Mr. Nasmyth refers the cup-shaped seizes upon, and enlists into his service and wall-surrounded hollows that are so as the great prime mover in the subse-abundant upon the surface of the moon quent changes that have been effected immediately to the effect of this particuin the moon. The liquid mass which lies lar operation. He considers that as soon immediately within the consolidated as the brittle shell of the cooling sphere shell undergoes, in a less, but still very cracked, under the impulse of this inner important, degree, a similar process of expansion, and opened into fissures, porcooling, as it passes successive portions tions of the internal expanded molten of its inherent heat out through the ex-substance welled up through the cracks, ternal crust. But as it does this, and before it finally assumes the actually solid state, it expands, as water increases in volume as it cools even before it is frozen into ice. Mr. Nasmyth shows, by a reference to various processes with which ironfounders are familiar, that in this particular molten metals and molten slags behave exactly as water behaves, and he lays it down as a fundamental natural law not only that all molten bodies are specifically denser and heavier than solids of the same substance, and therefore expand in passing from the liquid to the solid state, but that the expansion invariably begins as the molten liquid approaches the condition of change. One very neat and instructive experiment is referred to in an illustration of the great energy that is brought into play when this expansion of cooling bodies occurs. If a strong iron bottle is filled with molten bismuth, and the neck of the bottle is firmly and tightly closed with a screwplug, the iron bottle is soon afterwards torn asunder by the solidification of the bismuth. In cases of spheres that have been circumstanced like the moon, the contraction of the outer shell, and the expansion of the imprisoned material, lying immediately within, must have gone on simultaneously, and on that account the force which has been finally brought into play to effect the rending of the outer case must have been a very violent one. Mr. Nasmyth alludes to another pretty experiment in speaking of the nature of this shattering-power. He fills a thin glass globe with cold water, hermetically seals, and then drops the globe into a warm bath, and in a very brief interval the brittle globe is starred by the internal expansion into radiating cracks, through which the water oozes. A glass globe fissured in this way has been photographed in this book to show

and in places jetted out in fountain-like streams, scattering a plentiful liquid shower around. In a communication to the Royal Astronomical Society, some little time since, Mr. Mattieu Williams described an experiment with the cooling of the tap-cinder from a puddlingfurnace, in which he has seen spirts of the molten cinder cjected through holes broken in the consolidating crust, to a height of four or five diameters of the mass, in this very way, and by the instrumentality of these very forces. In the case of the moon the spirted-out liquid was piled up where it fell, into heaps of pasty lava and consolidating scoriæ. In many instances the piled-up bulwarks were as far as fifty or sixty miles away from the orifice of eruption, on account of the distance to which the lava was shot under the relatively slight restraining power of gravitation in the moon, and they were universally of a regular circular form because the erupted jet was scattered to the same distance in all directions around the jet, as water is scattered from the vents of a dome-shaped fountain, and because there was no wind, under any circumstances, to divert or distort the regularity of the shower. Whenever there were successive eruptions of this character, of progressively diminishing violence, successive bulwarks were formed in concentric and narrowing rings, as in the instance of Tycho and Copernicus, so beautifully delineated in these illustrations. When the last failing throes of the volcanic outburst died gently away, the last failing streams of the ejected matter were piled into small cones of eruption choking and closing the vents. In some instances the piled-up accumulations of the erupted matter were so abundant, and rose so high, that lower terraces were formed by the slipping down of the top-heavy deposits,