ciable difference being perceived, we are entitled to conclude the nonexistence of any atmosphere 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 appreciable 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, 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 has been 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 As observed by myself in the eclipse of Oct. 13, 1837. So in ed. of 1833. 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 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 hastily2) to be a consequence of the general laws to be explained hereafter. Be that as 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 centres of the earth and moon fluctuates a little in its position, from its mean or average intersection with her surface, to the 'From my own observation, made quite independently of any knowledge of such a tendency having been observed by others. Humboldt, however, in his personal nar rative, speaks of it as well known to the pilots and seamen of Spanish America: see note at the end of the chapter (h.) See Edinburgh Review, No. 175, p. 192. east or westward. And, moreover, since the 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 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 centre 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 moon, the earth must present to them the extraordinary appearance of a moon of nearly 2° degrees in diameter, exhibiting phases complementary to those which 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 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 rugged with clouds. (437.) The best charts of the lunar surface are those of Cassini, of Russel (engraved from drawings, made by the aid of a seven feet reflecting telescope,) the seleno-topographical charts of Lohrmann, and the very elaborate projection of Beer and Maedler accompanying their work already cited.' Madame Witte, a Hanoverian lady, has recently succeeded in producing from her own observations, aided by Maedlar's charts, more than one complete model of the whole visible lunar hemisphere, of the most perfect kind, the result of incredible diligence and assiduity. Single craters have also been modelled on a large scale, both by her and Mr. Nasmyth. [Still more recently (1851) photography has been successfully applied to the exact delineation of the lunar surface by Mr. Whipple, using for the purpose the great Fraunhofer equatorial of the Observatory at Cambridge, U. S.] The representations of Hevelius in his Selenographia, though not without great merit at the time, and fine specimens of his own engraving, are now become antiquated. Additional Note on Art. 432. M. Arago has shown, from a comparison of rain, registered as having fallen during a long period, that a slight preponderance in respect of 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. CHAPTER VIII. OF TERRESTRIAL GRAVITY.—OF THE LAW OF UNIVERSAL GRAVITA- TION. (438.) 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 centres. 2 (439.) 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, 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 force to put matter in motion, or to oppose and neutralize force, which gives us this internal conviction of and causa Princip. Lex. i. power * 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, accompanied with intention thereby to accomplish an end, 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. (Note to edition of 1833.) tion so far as it refers to the material world, and compels us to believe that whenever we see material objects put in motion from a state of rest, or deflected from their rectilinear paths and changed in their velocities if already in motion, it is in consequence of such an EFFORT somehow exerted, though not accompanied with our consciousness. That such an effort should be exerted with success through an interposed space, is no more difficult to conceive, than that our hand should communicate motion to a stone, with which it is demonstrably not in contact. (440.) All bodies with which we are acquainted, when raised into the air and quietly abandoned, descend to the earth's surface in lines perpendicular to it. They are therefore urged thereto by a force or effort, which it is but reasonable to regard as the direct or indirect result of a consciousness and a will existing somewhere, though beyond our power to trace, which force we term gravity, and whose tendency or direction, as universal experience teaches, is towards the earth's centre; or rather, to speak strictly, with reference to its spheroidal figure, perpendicular to the surface of still water. But if we cast a body obliquely into the air, this tendency, though not extinguished or diminished, is materially modified in its ultimate effect. The upward impetus we give the stone is, it is true, after a time destroyed, and a downward one communicated to it, which ultimately brings it to the surface, where it is opposed in its further progress, and brought to rest. But all the while it has been continually deflected or bent aside from its rectilinear progress, and made to describe a curved line concave to the earth's centre; and having a highest point, vertex, or apogee, just as the moon has in its orbit, where the direc tion of its motion is perpendicular to the radius. (441.) When the stone which we fling obliquely upwards meets and is stopped in its descent by the earth's surface, its motion is not towards the centre, but inclined to the earth's radius at the same angle as when it quitted our hand. As we are sure that, if not stopped by the resistance of the earth, it would continue to descend, and that obliquely, what presumption, we may ask, is there that it would ever reach the centre towards which its motion, in no part of its visible course, was ever directed? What reason have we to believe that it might not rather circulate round it, as the moon does round the earth, returning again to the point it set out from, after completing an elliptic orbit of which the earth's centre occupies the lower focus? And if so, is it not reasonable to imagine that the same force of gravity may (since we know that it is exerted at all accessible heights above the surface, and even in the highest regions of the atmosphere) extend as far as 60 radii of the earth, or to the moon? and may not this be the power, for some power there must be, which |