WEEKLY EVENING MEETING,

Friday, May 20, 1864.

The EARL OF ROSSE, F.R.S. M.R.I., in the Chair.

JAMES NASMYTH, ESQ. M.R.I.

On the Physical Aspects of the Moon's Surface.

As the Moon's hemisphere, which is ever turned towards us, has its features illuminated in opposite directions during her monthly passage in her orbit around the earth, every part of it is exposed in turn to the rays of the sun, which fall on the details of its features in constantly varying inclinations; and it is from this circumstance that we have such favourable opportunities afforded to us of obtaining a very correct knowledge of the configuration of the details in question, as well as of their height or depression above or below the mean level of the Moon's general surface. Thus it is that we are enabled most carefully to scrutinize her remarkable surface; and should we have drawn any hasty inferences from one set of observations, the opportunity is usually presented to us in the course of a fortnight, or at farthest a month, to correct them if erroneous, or to verify them if accurate, and to pursue further investigations that may be suggested by reflection on what we had last observed.

In these respects telescopic visits to the surface of the Moon yield more correct and reliable results than would many a visit to portions of our world where the scenery to be surveyed is not, perhaps, conveniently accessible and even when it is reached, the traveller may be surrounded by circumstances which very seriously interfere with his personal comfort, or disturb that tranquillity which is so requisite a condition for close and accurate observation, and thus lead him to hasty conclusions, which he has no future opportunity to rectify. In strong contrast with such circumstances is the position of the astronomer, comfortably placed beside his telescope, in the silence and tranquillity of a fine clear night, with all distracting objects excluded from his view. The whole of his attention is thus brought to focus, as it were, on the point under investigation there and then presented to his scrutiny, and ready to yield perfectly truthful replies to his questions; nothing being requisite for a correct interpretation of facts, other than a quick eye backed by a sound and unbiassed judgment.

It is from circumstances such as these that we have acquired, by a

long course of assiduous observation and reflection, an amount of intimate acquaintance with the physical structure of the Moon's exterior, in many important respects far more accurate than is our knowledge of that portion of the earth.

In order rightly to interpret the details of the Moon's surface, as revealed to us by the aid of the telescope, we ought, in the first place, to bear in mind the true nature of volcanic action, namely, that while it has reference to the existence of intense temperature and molten matter, it does not derive its origin from combustion, considered as such in a strictly chemical sense, but proceeds from an incandescent condition, induced in matter by the action of that great cosmical law which caused an intense heat to result from the gravitation of particles of matter towards a common centre. These particles, originally existing in a diffused condition, were, by the action of gravitation, made to coalesce, and so to form a planet. Volcanic action, then, has in all probability for its source the heat consequent upon the collapse of such diffused matter, resulting in that molten condition through which there is strong reason to believe all planetary bodies to have passed in their primitive state, and of which condition the geological history of our earth furnishes abundant evidence. Thus the molten lava which we see issuing from an active volcano on the earth, is really and truly a residual portion of that molten matter of which the entire globe once consisted.

In reference to the nature and origin of that eruptive force which had again and again, in the early periods of the Moon's history, caused the remaining molten matter of her interior to be ejected from beneath her solidified crust, and so to assume nearly every variety of volcanic formation in its most characteristic aspect, the key to these may be found in the action of that law which pervades almost all matter in a molten condition, namely, that "molten matter occupies less bulk, weight for weight, than the same material when it has ceased from the molten state; or, in other words, "that matter in a molten state is specifically more dense than the same material in a solidified condition." Thus it is that in passing from the molten to the solid state the normal law is resumed, and expansion of bulk either just immediately precedes or accompanies solidification. It is, therefore, in this expansion in the bulk of the solidifying matter, beneath the Moon's crust, that we are to look for the true cause of that eruptive or ejective action which has resulted in the displacement, surfaceward, of the fluid portion of the Moon's internal substance; a displacement which has manifested itself in nearly every variety of volcanic formation, such as circular craters with their central cones or mountains of exudation, cracked districts, &c.; all these variations of well-recognized volcanic phenomena being intermingled and overlaid one upon the other in the most striking and wonderful manner.

It may, however, be very reasonably and naturally asked, "What evidence have I that the features I refer to have any relation to volcanic action at all?" In reply to such a question I would direct

the inquirer's attention to one single feature which, I conceive, demonstrates more completely than any other the fact of volcanic action having (at however remote a period) existed in full activity in the Moon. The special feature to which I would refer is the central cone that may be observed within those "Ring-formed mountains," as they have been termed. "The central cone" is a well-known and distinctive feature in terrestrial volcanoes. It is the residue of the last expiring efforts of a once energetic eruptive volcanic action, which had thrown the ejected matter to such a considerable distance round about the volcanic vent, that in its descent it had accumulated around in the form of a ring-shaped mountain or crater; whilst on the subsidence of this volcanic energy, the ejected matter was deposited in the immediate vicinity of the vent or volcanic orifice, and thus arose the "central cone."

Anyone who is familiar with terrestrial volcanic craters must, at the first glance at those which are scattered in such infinite numbers over the Moon's surface, detect this well-known analogous feature, the central cone, and at once reasonably infer that these similar forms arose from a common cause, that cause being no other than volcanic action, accompanied by all its most marked characteristics.

Fig. 1 represents a fair average type of the structure of a Lunar Volcanic Crater with its central cone A.

FIG. 2.

The woodcuts have been kindly lent by Mr. Churchill, publisher of the 'Quarterly Journal of Science.' They illustrated Mr. J. Nasmyth's article in the July number of that work.

Fig. 3 is the section of a Lunar Crater, showing how by the eruption, and subsequent deposition of the ejected matter, the circular outer wall or crater had been formed.

Fig. 4. The section of the same, exhibiting the manner in which the central cone had resulted from the expiring efforts of the eruptive action.

In examining the Moon's surface, we cannot but be impressed with the vast dimensions of many of the volcanic craters with which her surface is studded. Craters of thirty miles and upwards in diameter are by no means uncommon, and the first impression on the mind in reference to such magnitudes is one of astonishment, that so small a planet as the Moon (whose magnitude is only about th that of the earth) should exhibit evidence of volcanic violence so far greater than any that we have on the earth. This apparent paradox will, however, disappear when we come to consider that in consequence of the Moon being so much less than the earth, the force of gravity on its exterior is not above 4th of that on the earth, and that the weight of the lunar materials on its surface is reduced in the latter proportion, while, on the other hand, by reason of the small magnitude of the Moon and its proportionately much larger surface in ratio to its magnitude, the rate at which it parted with its original cosmical heat must have been vastly more rapid than in the case of the earth. Now, as the disruptive and eruptive action and energy are in proportion to the

greater rate of cooling, those forces must have been much greater in the first instance; and, operating as they did on matter so much reduced in weight as it must be on the surface of the Moon, we thus find in combination two conditions most favourable to the display of volcanic force 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 to continue the primary impulse of the ejection in the most free and uninterrupted manner, and thus to deposit itself at distances from the volcanic vent so much greater than those of which we have any example in the earth, as to result in the formation of the craters of vast magnitude so frequently encountered in a survey of the Moon's surface. In like manner we find the ejected matter piled up to heights such as create the utmost astonishment. Lunar Mountains of 10,000 feet high are of frequent occurrence, while there are several of much greater altitude, some reaching the vast height of 28,000 feet, and that almost at one bound, as they start up directly from the plane over which they are seen to cast their long black, steeple-like shadows for many a mile; whilst at other times they intercept the rays of the sun upon their highest peaks many hours before their bases emerge from the profound darkness of the long lunar night.

Among the many terribly sublime scenes with which the Moon's surface must abound, none can be grander than that which would present itself to the spectator, were he placed inside of one of these vast volcanic craters (Tycho, for instance), surrounded on every side by the most terrific evidences of volcanic force in its wildest features.

In such a position he would have before him, starting up from the vast plane below, a mighty obelisk-shaped mountain of some 9,000 feet in height, casting its intense black shadow over the plateau; and partly up its slope he would see an amphitheatrical range of mountains beyond, which, in spite of their being about forty miles distant, would appear almost in his immediate proximity (owing to the absence of that "aerial perspective," which in terrestrial scenery imparts a softened aspect to the distant object), so near, indeed, as to reveal every cleft and chasm to the naked eye! This strange commingling of near and distant objects, the inevitable visual consequence of the absence of atmosphere or water, must impart to lunar scenery a terrible aspect; a stern wildness, which may aptly be termed unearthly. And when we seek to picture to ourselves, in addition to the lineaments and conditions of the lunar landscape, the awful effect of an absolutely black firmament, in which every star, visible above the horizon, would shine with a steady brilliancy (all causes of scintilTation or twinkling being absent, as these effects are due to the presence of variously heated strata, or currents in our atmosphere), or of the vivid and glaring sunlight, with which we have nothing to compare in our subdued solar illumination, made more striking by the contrast of an intensely black sky; if, we say, we would picture to ourselves the