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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 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 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 inhabitant* in the moon, the earth must

• Sec Edinburgh llcvicw, No. 175. p. 192.

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 rugged with clouds.

(437.) The best charts of the lunar surface are those of Oassini, 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 Maedler'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 Selcnographia, 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 GRAVITATION. — PATHS OF PROJECTILES; APPARENT REAL

THE MOON RETAINED IN HER ORBIT BT 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 8UN COMPARED. — DENSITY OF THE SUN. — FORCE OF GRAVITY AT ITS SURFACE. — DISTURBING EFFECT OF THE SUN ON THE MOON*S MOTION.

(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 centers.

(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 f, 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

• Princip. Lex. i.

•f Sec 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 tile 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 rotition on the one hand, and from mere spasmodic contraction of muscles on the other. Urovtn, 3d edit. Edin. 1818, p. 47. (Note to edition of 1833.)

to say) been regarded ae 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 power and causation 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 center; 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 center; and having a highest point, vertex, or apogee, just as the moon has in its orbit, where the direction 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 center, 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 center 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 center 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 deflects her at every instant from the tangent of her orbit, and keeps her in the elliptic path which experience teaches us she actually pursues?

(442.) If a stone be whirled round at the end of a string it will stretch the string by a centrifugal force, which, if the speed of rotation be sufficiently increased, will at length break the string, and let the stone escape. However strong the string, it may, by a sufficient rotary velocity of the stone, be brought to the utmost tension it will bear without breaking; and if we know what weight it is capable of carrying, the velocity necessary for this purpose is easily calculated. Suppose, now, a string to connect the earth's center with a weight at its surface, whose strength should be just sufficient to sustain that weight suspended from it. Let us, however, for a moment imagine gravity to have no existence, and that the weight is made to revolve with the limiting velocity which that string can barely counteract: then will its tension be just equal to the weight of the revolving body; and any power which should continually urge the body towards the center with a force equal to its weight would perform the office, and might supply the place of the string, if divided.

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