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the apogee of the moon will occur where the perigee occurred before.

(361.) The best way to form a distinct conception of the moon's motion is to regard it as describing an ellipse about the earth in the focus, and, at the same time, to regard this ellipse itself to be in a twofold state of revolution ; 1st, in its own plane, by a continual advance of its axis in that plane; and 2dly, by a continual tilting motion of the plane itself, exactly similar to, but much more rapid than, that of the earth's equator produced by the conical motion of its axis described in art. 266.

(362.) The physical constitution of the moon is better known to us than that of any other heavenly body. By the aid of telescopes, we discern inequalities in its surface which can be no other than mountains and valleys,- for this plain reason, that we see the shadows cast by the former in the exact proportion as to length which they ought to have, when we take into account the inclination of the sun's rays to that part of the moon's surface on which they stand. The convex outline of the limb turned towards the sun is always circular, and very nearly smooth ; but the opposite border of the enlightened part, which (were the moon a perfect sphere) ought to be an exact and sharply defined ellipse, is always observed to be extremely ragged, and indented with deep recesses and prominent points. The mountains near this edge cast long black shadows, as they should evidently do, when we consider that the sun is in the act of rising or setting to the parts of the moon so circumstanced. But as the enlightened edge advances beyond them, i. e. as the sun to them gains altitude, their shadows shorten ; and at the full moon, when all the light falls in our line of sight, no shadows are seen on any part of her surface. From micrometrical measures of the lengths of the shadows of many of the more conspicuous mountains, taken, under the most favourable circumstances, the heights of many of them have been calculated,- the highest being about 12 English miles in perpendicular altitude. The existence of such mountains is corroborated by their appearance as small points or islands of light beyond the extreme edge of the enlightened part, which are their tops catching the sun-beams before the intermediate plain, and which, as the light advances, at length connect themselves with it, and appear as prominences from the general edge.

(363.) The generality of the lunar mountains present a striking uniformity and singularity of aspect. They are wonderfully numerous, occupying by far the larger portion of the surface, and almost universally of an exactly circular or cup-shaped form, foreshortened, however, into ellipses towards the limb; but the larger have for the most part flat bottoms within, from which rises centrally a small, steep, conical hill. They offer, in short, in its highest perfection, the true volcanic character, as it may be seen in the crater of Vesuvius, and in a map of the volcanic districts of the Campi Phlegræi * or the Puy de Dôme. And in some of the principal ones, decisive marks of volcanic stratification, arising from successive deposits of ejected matter, may be clearly traced with powerful telescopes. + What is, moreover, extremely singular in the geology of the moon is, that although nothing having the character of seas can be traced, (for the dusky spots which are commonly called seas, when closely examined, present appearances incompatible with the supposition of deep water,) yet there are large regions perfectly level, and apparently of a decided alluvial character.

(364.) The moon has no clouds, nor any other indi. cations of an atmosphere. Were there any, it could not fail to be perceived in the occultations of stars and the phænomena of solar eclipses. Hence its climate must be very extraordinary; the alternation being that of unmitigated and burning sunshine fiercer than an equatorial noon, continued for a whole fortnight, and the keenest severity of frost, far exceeding that of our polar winters, for an equal time. Such a disposition of things must produce a constant transfer of whatever moisture may exist on its surface, from the point beneath the sun to that opposite, by distillation in vacuo after the manner of the little instrument called a cryophorus. The consequence must be absolute aridity below the vertical sun, constant accretion of hoar frost in the opposite region, and, perhaps, a narrow zone of running water at the borders of the enlightened hemisphere. It is possible, then, that evaporation on the one hand, and condensation on the other, may to a certain extent preserve an equilibrium of temperature, and mitigate the extreme severity of both climates.

* See Breislak's map of the environs of Naples, and Desmarest's of Au. vergne. +From my own observations. -Author.

(365.) A circle of one second in diameter, as seen from the earth, on the surface of the moon, contains about a square mile. Telescopes, therefore, must yet be greatly improved, before we could expect to see signs of inhabitants, as manifested by edifices or by changes on the surface of the soil. It should, however, be observed, that, owing to the small density of the materials of the moon, and the comparatively feeble gravitation of bodies on her surface, muscular force would there go six times as far in overcoming the weight of materials, as on the earth. Owing to the want of air, however, it seems impossible that any form of life analogous to those on earth can subsist there. No appearance indicating vegetation, or the slightest variation of surface which can fairly be ascribed to change of season, can any where be discerned.

(366.) The lunar summer and winter arise, in fact, from the rotation of the moon on its own axis, the period of which rotation is exactly equal to its sidereal revolution about the earth, and is performed in a plane 1° 30' 11" inclined to the ecliptic, and therefore nearly coincident with her own orbit. This is the cause why we always see the same face of the moon, and have no knowledge of the other side. This remarkable coincidence of two periods, which at first sight would seem

perfectly distinct, is said to be a consequence of the general laws to be explained hereafter.

(367.) The moon's rotation on her axis is uniform ; but since her motion in her orbit (like that of the sun) is not so, we are enabled to look a few degrees round the equatorial parts of her visible border, on the eastern or western side, according to circumstances; or, in other words, the line joining the centers of the earth and moon fluctuates a little in its position, from its mean or average intersection with her surface, to the east or westward. And, moreover, since the axis about which she revolves is not exactly perpendicular to her orbit, her poles come alternately into view for a small space at the edges of her disc. These phenomena are known by the name of librations. In consequence of these two distinct kinds of libration, the same identical point of the moon's surface is not always the center of her disc, and we therefore get sight of a zone of a few degrees in breadth on all sides of the border, beyond an exact hemisphere.

(368.) If there be inhabitants in the moon, the earth must present to them the extraordinary appearance of a moon of nearly 2' in diameter, exhibiting the same phases as we see the moon to do, but immoveably fixed in their sky, (or, at least, changing its apparent place only by the small amount of the libration,) while the stars must seem to pass slowly beside and behind it. It will appear clouded with variable spots, and belted with equatorial and tropical zones corresponding to our tradewinds; and it may be doubted whether, in their perpetual change, the outlines of our continents and seas can ever be clearly discerned.

CHAP. VII.

OF TERRESTRIAL GRAVITY OF THE LAW OF UNIVERSAL GRA

VITATION, — PATHS OF PROJECTILES; APPARENT - REAL. THE MOON RETAINED IN HER ORBIT BY GRAVITY. -ITS LAW OF DIMINUTION. LAWS OF ELLIPTIC MOTION. - ORBIT OF THE EARTH ROUND THE SUN IN ACCORDANCE WITH THESE LAWS. - MASSES OF THE EARTH AND SUN COMPARED. — DENSITY OF THE SUN. --FORCE OF GRAVITY AT ITS SURFACE. DISTURBING EFFECT OF THE SUN ON THE MOON'S MOTION.

(369.) The reader has now been made acquainted with the chief phenomena of the motions of the earth in its orbit round the sun, and of the moon about the earth.

- We come next to speak of the physical cause which maintains and perpetuates these motions, and causes the massive bodies so revolving to deviate continually from the directions they would naturally seek to follow, in pursuance of the first law of motion *, and bend their courses into curves concave to their centers.

(370.) Whatever attempts may have been made by metaphysical writers to reason away the connection of cause and effect, and fritter it down into the unsatisfactory relation of habitual sequence t, it is certain that the conception of some more real and intimate connection is quite as strongly impressed upon the human mind as that of the existence of an external world, — the vindication of whose reality has (strange to say) been regarded as an achievement of no common merit in the annals of this branch of philosophy. It is our own immediate consciousness of effort, when we exert

* See Cab. Cyc. MECHANICS, chap. iii.

+ See Brown“ On Cause and Effect," - a work of great acuteness and subtlety of reasoning on some points, but in which the whole train of argument is vitiated by one enormous oversight; the omission, namely, of a distinct and immediate personal consciousness of causation in his enumeration of that sequence of events, by which the volition of the mind is made to terminate in the motion of material objects. I mean the consciousness of effort, as a thing entirely distinct from mere desire or volition on the one hand, and from mere spasmodic contraction of muscles on the other. Brown, 3d edit. Edin. 1818, p. 47. - Author.

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