THE STRUCTURE OF THE MOON. By J. P. NICHOL, Esq., LL.D. [Delivered at the Royal Manchester Institution, April, 1858.-From the "Manchester Courier."] ON contemplating the starry universe, one is struck by its immeasurable extent,- -an extent apparently unbounded as unbounded space, seeing that however remotely we penetrate by the aid of the telescope, it is only to feel ourselves on the mere threshold of further vastnesses; neither can we fail in the presence of those majestic movements, those slow and solemn evolutions, to be impressed with the immense duration of the works of Almighty Wisdom. But there is yet one other feature of the scheme, which is not so generally appreciated in its fulness-I mean its surpassing variety, and the complicacy of those evolutions of which its harmony is composed. In fact it is not the mere attributes of extent or duration which can bestow on any object the character of ma jesty or sublimity. Those two huge ideas, Time and Space are, in reality, little other than the dimensions of the canvas on which the picture of the external world has been drawn; but the inquiry remains as to the contents of the picture,—the question whether its parts are distinct in character, as well as infinite in number; whether those exquisite contrasts which the slightest glance over our own world reveals, are not also typical of a diversity of form and adjustment of parts pervading 'every region of that exhaustless domain. The Moon is the body belonging to our system by far the most within reach of the earth, its distance being only 240,000 miles. With a telescopic power of 1,000, you reduce that distance to 240 miles, and it is possible to reduce that to 80 miles under favourable circumstances. We cannot see an object distinctly on the earth 80 miles off, but that is partly owing to the unfavourable nature of the atmosphere; the moon, when viewed overhead through a clear air, appears with remarkable distinctness. The moon turns round upon its axis in the same time that it revolves round the earth; the consequence is that we never see but one face; the same face of the moon is always turned towards us. This circumstance doubles our opportunities of becoming acquainted with the face that is turned towards us, and the consequence is we know the side of the moon better than we know the earth's surface. We know its geographical outline, and the general nature of the objects it presents, much better than we know either hemisphere of our globe. There are no deserts of Sahara with the moon that cannot be traversed by the telescope, no inaccessible polar regions: the telescope defies both sandy deserts and polar snows. When we see the whole disc of the moon, a vast number of these marks are blotted out; the sun's rays fall directly upon the moon's elevations they cast no shadows. One cannot but be struck by the singular circumstance that the moon should turn round upon its axis in the same time that it turns round in its orbit upon the earth. We knew that until lately only as a simple fact; recently, however, the physical cause of that fact has been investigated, and it is a remarkable one. The fact could when not exist unless the moon were in construction quite unlike our globe. The earth is so constituted that the centre of the globe is the centre of gravity. In the earth, and all other spheres we know, the centre of gravity is exactly the same as the centre of form; in other words the two solid sides of the terrestrial sphere are of equal weight. If this were so with the moon there could not be this equality in the time of rotation. The two centres of the moon-of gravity and form-were ascertained by calculations based on the difference between the observed places of the moon and the calculated places, and their distance is about 32 miles. It is curious that the centre of gravity is further from us than the centre of form. From this curious calculation very important inferences will be found to follow. When we look narrowly at the moon we find that it is characterised by other two features which separate it from resemblance to the physical structure of the earth. In the first place, if there be an atmosphere it is an extremely small one. Nothing akin to any atmospheric influence has been seen between us and the moon. When stars come very near the moon's edge they are not sensibly refracted, as they would be if the moon had an atmosphere. The next characteristic is that on the side of the moon towards us there is no liquid observable. In former times those comparatively dark spots were thought to be oceans, and no doubt our oceans would so appear, viewed from the moon, from the fact that water absorbs more of the sun's rays than land. However, on narrowly examining these dark places, we find that they all show clear marks of inequality of surface. They remind one much of what the Americans call their "rolling prairies," and are small ranges of mountains. mode in which we ascertain these inequalities is by observing their shadows when the moon is in cresThe edge of the moon is then always covered with ruggednesses, from the land being uneven. It is manifest then that it is not water, and we further see that from the absence of clouds. In the planets cent. The Venus and Mars there is no doubt of the existence of liquid, for we see their atmospheres surcharged with clouds. If there were water in the moon, it would range itself round the centre of gravity. All we can say is, there is no water on the visible side of the moon, and but a very small atmosphere; there may be an ocean on the other side submerging the whole continent. The side of the moon towards us is high land or a plateau, raised 32 miles above the general level of the land. If the high table lands of Asia were raised 120 miles above the average level of the earth, we should not expect to find any water on the top, and only a very small atmosphere. This is precisely the condition of the side of the moon that is presented to us. This explains to us another set of phenomena that long puzzled astronomers. In examining minutely a number of those dark places on the moon a singular appearance was noticed--a series of lines of different colour, such as would probably be produced by the gradual retiring of an occan. Was this always a plateau? Is not its elevation akin to those we have on the surface of the earth? Let us now examine more minutely into other details of the moon's surface. A vast portion of its surface is covered with inequalities, or lunar mountains. These lunar mountains consist of three distinct classes. The first class may be termed isolated peaks. There is a mountain called Tycho ascending to a great elevation from the plain, and excessively steep on its sides, so that it looks under the telescope very much like a huge sugar loaf. The shadow that it casts enables us to find its height, and it is about 9,000 feet. There are many such isolated peaks upon the moon, but few upon our earth. For the most part, the earth's mountains consist of groups, or chains; isolated mountains have generally been severed from such groups. Such, I have no doubt, is the case with Ailsa Craig, and other peaks. Then again, we have mountain ranges on the moon. corresponding to our Alps and Andes, but very few. Here is one that is very easily seen by a good telescope it is called the Appenines, and reaches an ele vation of about 18,000 feet. There is a smaller range, termed the Alps, not quite so precipitous. On the other edge of the moon there is a range 28,000 feet high-an immense height for the moon, which is a comparatively small body-in fact, as lofty as any upon the earth. There are several features of these mountain ranges that correspond with what we see on the earth. Usually, our terrestrial mountains such as the Himalayas, are excessively steep on one side and sloping on the other Wherever we have a We range in the moon the same feature exists, showing similar action in the two orbs; but over these lunar mountains no refreshing cloud ever hangs, the wind never moans through their valleys; silently they rear their giant tops in the cloudless sky, bright for a period, then dark and cold. The third class of the mountain masses of the moon is the most peculiar and characteristic of that luminary. At least twofifths of its surface are studded with profound caverns, generally surrounded with a high rock. These singular manifestations of disruption are decidedly characteristic of the moon in its present state. know the whole nature and character of these craters very well; their breadth and depth may be easily measured by the shadows cast from the sun. These craters are not like those of our earth, but actual holes in the solid body of the moon, going down for a great depth, having usually a circular wall of rock at the top. They vary in magnitude from 60 to 70 miles in diameter, to the smallest possible space. The diameter of Tycho is about 55 miles; another, called Corpernicus, is about the same diameter. Their numbers increase as their size diminish, and they soon pass all enumeration. It is very difficult to give a general description of these curious formations: |