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TERRESTRIAL EFFECTS OF THE SUN'S RADIATION. 237

agitation in which the fluids composing the visible surface are maintained, and the continual generation and filling in of the pores, without having recourse to internal causes. The mode of action here alluded to is perfectly represented to the eye in the disturbed subsidence of a precipitate, as described in art. 387., when the fluid from which it subsides is warm, and losing heat from its surface.

(399.) The sun's rays are the ultimate source of almost every motion which takes place on the surface of the earth. By its heat are produced all winds, and those disturbances in the electric equilibrium of the atmosphere which give rise to the phenomena of lightning, and probably also to those of terrestrial magnetism and the aurora. By their vivifying action vegetables are enabled to draw support from inorganic matter, and become, in their turn, the support of animals and of man, and the sources of those great deposits of dynamical efficiency which are laid up for human use in our coal strata.* By them the waters of the sea are made to circulate in vapour through the air, and irrigate the land, producing springs and rivers. By them are produced all disturbances of the chemical equilibrium of the elements of nature, which, by a series of compositions and decompositions, give rise to new products, and originate a transfer of materials. Even the slow degradation of the solid constituents of the surface, in which its chief geological changes consist, is almost entirely due on the one hand to the abrasion of wind and rain, and the alternation of heat and frost; on the other to the continual beating of the sea waves, agitated by winds, the results of solar radiation. Tidal action (itself partly due to the sun's agency) exercises here a comparatively slight influence. The effect of oceanic currents (mainly originating in that influence), though slight in abrasion, is powerful in diffusing and transporting the matter abraded; and when we consider the immense transfer of matter so produced, the increase of pressure over large spaces in the bed of the ocean, and diminution over corresponding portions of the land, we are not at a loss to perceive how the elastic power of subterranean fires, thus repressed on the one hand and relieved on the other, may break forth in points when the resistance is barely adequate to their retention, and thus bring the phenomena of even volcanic activity under the general law of solar influence. *

* So in the edition of 1833.

(400.) The great mystery, however, is to conceive how so enormous a conflagration (if such it be) can be kept up. Every discovery in chemical science here leaves us completely at a loss, or rather, seems to remove farther the prospect of probable explanation. If conjecture might be hazarded, we should look rather to the known possibility of an indefinite generation of heat by friction, or to its excitement by the electric discharge, than to any actual combustion of ponderable fuel, whether solid or gaseous, for the origin of the solar radiation.t

* So in the edition of 1833.

f Electricity traversing excessively rarefied air or vapours, gives out light, and, doubtless, also beat. May not a continual current of electric matter be constantly circulating in the sun's immediate neighbourhood, or traversing the planetary spaces, and exciting, in the upper regions of its atmosphere, those phenomena of which, on however diminutive a scale, we have yet an unequivocal manifestation in our aurora borcalis. The possible analogy of the solar light to that of the aurora has been distinctly insisted on by the late Sir W. Herschel, in his paper already cited. It would be a highly curious subject of experimental inquiry, how far a mere reduplication of sheets of flame, at a distance one behind the other (by which their light might be brought to any required intensity), would communicate to the heat of the resulting compound ray the penetrating character which distinguishes the solar calorific rays. We may also observe, that the tranquillity of the sun's polar, as compared with its equatorial regions (if its spots be really atmospheric), cannot be accounted for by its rotation on its axis only, but mutt arise from some cause external to the luminous surface of the sun, as we see the belts of Jupiter and Saturn, and our tradewinds, arise from a cause, external to these planets, combining itself with their rotation, which alone can produce no motions when once the form of equilibrium is attained.

The prismatic analysis of the solar beam exhibits in the spectrum a series of *' fixed lines," totally unlike those which belong to the light of any known terrestrial flame. This may hereafter lead us to a clearer Insight into its origin. But, before we can draw any conclusions from such an indication, we must recollect, that previous to reaching us it has undergone the whole absorptive action of our atmosphere, as well as of the sun's. Of the latter we know nothing, and may conjecture every thing; but of the blue colour of the former we arc sure; and if this be an inherent (i. e. an absorptive) colour, the air must be expected to act on the spectrum after the analogy of other coloured media, which often (and especially light blue media) leave unabsorbed portions separated by dark intervals. It deserves enquiry, therefore, whether some or all the fixed lines observed by Wollaston and Fraunhofer may not have their origin in our own atmosphere. Experiments made on lofty mountains, or the cars of balloons, on ttie one hand, and on the other with reflected beams which have been made to traverse several miles of additional air near the surface, would decide this point. The absorptive efll-ct of the sun's atmosphere, and possibly also of the medium surrounding it (whatever it be) which resists the motions of comets, cannot be thus eliminated. — Note to the edition of 1833.

239

CHAPTER VII.

OF THE MOON.—ITS SIDEREAL PERIOD. —ITS APPARENT DIAMETER.

— ITS PARALLAX, DISTANCE, AND REAL DIAMETER — FIRST

APPROXIMATION TO ITS ORBIT AN ELLIPSE ABOUT THE EARTH

IN THE FOCUS. — ITS EXCENTRICITY AND INCLINATION. MOTION

OF ITS NODES AND APSIDES OF OCCULTATIONS AND SOLAR

ECLIPSES GENERALLY. — LIMITS WITHIN WHICH THEY ARE POSSIBLE.— THEY PROVE THE MOON TO BE AN OPAKE SOLID — ITS LIGHT DERIVED FROM THE SUN. — ITS PHASES. — SYNODIC REVOLUTION OR LUNAR MONTH.—OF ECLIPSES MORE PARTICULARLY.

— THEIR PHENOMENA. THEIR PERIODICAL RECURRENCE PHYSICAL CONSTITUTION OF THE MOON.— ITS MOUNTAINS AND OTHER SUPERFICIAL FEATURES. — INDICATIONS OF FORMER VOLCANIC

ACTIVITY. — ITS ATMOSPHERE CLIMATE.— RADIATION OF HEAT

FROM ITS SURFACE.— ROTATION ON ITS OWN AXIS. — LIBRATION.

— APPEARANCE OF THE EARTH FROM IT.

(401.) The moon, like the sun, appears to advance among the stars with a movement contrary to the general diurnal motion of the heavens, but much more rapid, so as to be very readily perceived (as we have before observed) by a few hours' cursory attention on any moonlight night. By this continual advance, which, though sometimes quicker, sometimes slower, is never intermitted or reversed, it makes the tour of the heavens in a mean or average period of 27d 7h 43m 11"^, returning, in that time, to a position among the stars nearly coincident with that it had before, and which would be exactly so, but for reasons presently to be stated.

(402.) The moon, then, like the sun, apparently describes an orbit round the earth, and this orbit cannot be very different from a circle, because the apparent angular diameter of the full moon is not liable to any great extent of variation.

(403.) The distance of the moon from the earth is concluded from its horizontal parallax, which may be found either directly, by observations at remote geographical stations, exactly similar to those described in art. 355., in the case of the sun, or by means of the phenomena called occultations, from which also its apparent diameter is most readily and correctly found. From such observations it results that the mean or average distance of the center of the moon from that of the earth is 59*9643 of the earth's equatorial radii, or about 237,000 miles. This distance, great as it is, is little more than one-fourth of the diameter of the sun's body, so that the globe of the sun would nearly twice include the whole orbit of the moon; a consideration wonderfully calculated to raise our ideas of that stupendous luminary!

(404.) The distance of the moon's center from an observer at any station on the earth's surface, compared with its apparent angular diameter as measured from that station, will give its real or linear diameter. Now, the former distance is easily calculated when the distance from the earth's center is known, and the apparent zenith distance of the moon also determined by observation; for if we turn to the figure of art. 339., and suppose S the moon, A the station, and C the earth's center, the distance S C, and the earth's radius C A, two sides of the triangle ACS are given, and the angle CAS, which is the supplement of Z A S, the observed zenith distance, whence it is easy to find A S, the moon's distance from A. From such observations and calculations it results, that the real diameter of the moon is 2160 miles, or about 0*2729 of that of the earth, whence it follows that, the bulk of the latter being considered as 1, that of the former will be 0*0204, or about ?"5. The difference of the apparent diameter of the moon, as seen from the earth's center and from any point of its surface, is technically called the augmentation of the apparent diameter, and its maximum occurs when the moon is in the zenith of the spectator. Her mean angular diameter, as seen from the center, is 31' 7", and is always 0*545 x her horizontal parallax.

(405.) By a series of observations, such as described in art. 403., if continued during one or more revolutions of the moon, its real distance may be ascertained at every point of its orbit; and if at the same time its apparent places in the 241

heavens be observed, and reduced by means of its parallax to the earth's center, their angular intervals will become known, so that the path of the moon may then be laid down on a chart supposed to represent the plane in which its orbit lies, just as was explained in the case of the solar ellipse (art. 349.) Now, when this is done, it is found that, neglecting certain small, though very perceptible, deviations of which a satisfactory account will hereafter be rendered, the form of the apparent orbit, like that of the sun, is elliptic, but considerably more eccentric, the eccentricity amounting to O05484 of the mean distance, or the major semi-axis of the ellipse, and the earth's centre being situated in its focus.

(406.) The plane in which this orbit lies is not the ecliptic, however, but is inclined to it at an angle of 5° 8' 48", which is called the inclination of the lunar orbit, and intersects it in two opposite points, which are called its nodes—the ascending node being that in which the moon passes from the southern side of the ecliptic to the northern, and the descending the reverse. The points of the orbit at which the moon is nearest to, and farthest from, the earth, are called respectively its perigee and apogee, and the line joining them and the earth the line of apsides.

(407.) There are, however, several remarkable circumstances which interrupt the closeness of the analogy, which cannot fail to strike the reader, between the motion of the moon around the earth, and of the earth around the sun. In the latter case, the ellipse described remains, during a great many revolutions, unaltered in its position and dimensions; or, at least, the changes which it undergoes are not perceptible but in a course of very nice observations, which have disclosed, it is true, the existence of "perturbations," but of so minute an order, that, in ordinary parlance, and for common purposes, we may leave them unconsidered. But this cannot be done in the case of the moon. Even in a single revolution, its deviation from a perfect ellipse is very sensible. It does not return to the same exact position among the stars from which it set out, thereby indicating a continual change in the plane of its orbit. And, in effect,

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