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.

(430.) The generality of the lunar mountains present a striking uniformity and singularity of aspect. They are wonderfully numerous, especially towards the southern portion of the disc, 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, but with this remarkable peculiarity, viz. that the bottoms of many of the craters are very deeply depressed below the general surface of the moon, the internal depth being often twice or three times the external height. In some of the principal ones, decisive marks of volcanic stratification, arising from successive deposits of ejected matter, and evident indications of lava currents streaming outwards in all directions, may be clearly traced with powerful telescopes. (See Pl. V. fig. 2.)† In Lord Rosse's magnificent reflector, the flat bottom of the crater called Albategnius is seen to be strewed with blocks not visible in inferior telescopes, while the exterior of another (Aristillus) is all hatched over with deep gullies radiating towards its center. 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; as there are also here and there, chains of mountains whose appearance suggests no suspicion of volcanic origin.

* See Breislak's map of the environs of Naples, and Desmarest's of Auvergne. From a drawing taken with a reflector of twenty feet focal length (h).

(431.) We perceive on the moon no clouds, nor any other decisive indications of an atmosphere. Were there any, it could not fail to be perceived in the occultations of stars and the phænomena of solar eclipses, as well as in a great variety of other phænomena. The moon's diameter, for example, as measured micrometrically, and as estimated by the interval between the disappearance and reappearance of a star in an occultation, ought to differ by twice the horizontal refraction at the moon's surface. No appretiable difference being perceived, we are entitled to conclude the non-existence of any atmosphere at its edge dense enough to cause a refraction of 1" i. e. having one 1980th part of the density of the earth's atmosphere. In a solar eclipse, the existence of any sensible refracting atmosphere in the moon, would enable us to trace the limb of the latter beyond the cusps, externally to the sun's disc, by a narrow, but brilliant line of light, extending to some distance along its edge. No such phænomenon is seen. Very faint stars ought to be extinguished before occultation, were any appretiable amount of vapour suspended near the surface of the moon. But such is not the case; when occulted at the bright edge, indeed, the light of the moon extinguishes small stars, and even at the dark limb, the glare in the sky caused by the near presence of the moon, renders the occultation of very minute stars unobservable. But during the continuance of a total lunar eclipse, stars of the tenth and eleventh magnitude are seen to come up to the limb, and undergo sudden extinction as well as those of greater brightness. Hence, the climate of the moon 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,

*

* As observed by myself in the eclipse of Oct. 13. 1837.

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; but this process, which would imply the continual generation and destruction of an atmosphere of aqueous vapour, must, in conformity with what has been said above of a lunar atmosphere, be confined within very narrow limits.

(432.) Though the surface of the full moon exposed to us must necessarily be very much heated,— possibly to a degree much exceeding that of boiling water,-yet we feel no heat from it, and even in the focus of large reflectors, it fails to affect the thermometer. No doubt, therefore, its heat, (conformably to what is observed of that of bodies heated below the point of luminosity,) is much more readily absorbed in traversing transparent media than direct solar heat, and is extinguished in the upper regions of our atmosphere, never reaching the surface of the earth at all. Some probability is given to this by the tendency to disappearance of clouds under the full moon, a meteorological fact, (for as such we think it fully entitled to rank †,) for which it is necessary to seek a cause, and for which no other rational explanation seems to offer. As for any other influence of the moon on the weather, we have no decisive evidence in its favour. (433.) 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

*So in ed. of 1833.

From my own observations, made quite independently of any knowledge of such a tendency having been observed by others. Humboldt, however, in his Personal Narrative, speaks of it as well known to the pilots and seamen of Spanish America.

M. Arago has shown, from a comparison of rain, registered as having fallen during a long period, that a slight preponderance in respect ef quantity falls near the new moon over that which falls near the full. This would be a natural and necessary consequence of a preponderance of a cloudless sky about the full, and forms, therefore, part and parcel of the same meteorological fact.

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, in our opinion, fairly be ascribed to change of season, can any where be discerned.

(434.) 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, whose ascending node is always precisely coincident with the descending node of the lunar orbit. So that the axis of rotation describes a conical surface about the pole of the ecliptic in one revolution of the node. The remarkable coincidence of the two rotations, that about the axis and that about the earth, which at first sight would seem perfectly distinct, has been asserted (but we think somewhat too hastily *) to be a consequence of the general laws to be explained hereafter. Be that how it may, it is the cause why we always see the same face of the moon, and have no knowledge of the other side.†

(435.) 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

*See Edinburgh Review, No. 175. p. 192. Strange to say, there are persons who find it difficult to regard as a rotation on its own axis, that peculiarity of the moon's motion which consists in its keeping the same face always towards the earth. Should any of our readers be in this predicament, we recommend him to plant a staff upright in the ground, and, grasping it with both hands, walk round it, keeping as close to it as possible, with his face always turned towards it; when the unmistakeable sensation of giddiness will effectually satisfy him of the fact of his rotation on his own axis, or he may walk round a tree, always facing it, and carrying a compass in his hand, and while watching the needle during a few circuits endeavour to persuade himself that he does not turn upon his own center.

axis about which she revolves is neither exactly perpendicular to her orbit, nor holds an invariable direction in space, her poles come alternately into view for a small space at the edges of her disc. These phænomena 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.

(436.) If there be inhabitants in the side of the moon turned towards us, the earth must present to them the extraordinary appearance of a moon of nearly 2° in diameter, exhibiting phases complementary to those which we see the moon to do, but immovably 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 trade-winds; and it may be doubted whether, in their perpetual change, the outlines of our continents and seas can ever be clearly discerned. During a solar eclipse, the earth's atmosphere will become visible as a narrow, but bright, luminous ring of a ruddy colour, where it rests on the earth, gradually passing into faint blue, encircling the whole or part of the dark disc of the earth, the remainder being dark and ragged with clouds.

(436 a.) On the subject of the moon's habitability, the complete absence of air noticed in art. (431.), if general over her whole surface, would of course be decisive. Some considerations of a contrary nature, however, suggest themselves in consequence of a remark lately made by Prof. Hansen, viz., that the fact of the moon turning always the same face towards the earth is in all probability the result of an elongation of its figure in the direction of a line joining the centers of both the bodies acting conjointly with a noncoincidence of its center of gravity with its center of symmetry. To the middle of the length of a stick, loaded with a heavy weight at one end and a light one at the other, attach a string, and swing it round. The heavy weight will assume and