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fires in the direct proportion of their intensity.* 3dly, From the fact, that the most vivid flames disappear, and the most intensely ignited solids appear only as black spots on the disc of the sun when held between it and the eye. From the last remark it follows, that the body of the sun, however dark it may appear when seen through its spots, may, nevertheless, be in a state of most intense ignition. It does not, however, follow of necessity that it must be so. The contrary is at least physically possible. A perfectly reflective canopy would effectually defend it from the radiation of the luminous regions above its atmosphere, and no heat would be conducted downwards through a gaseous medium increasing rapidly in density. That the penumbral clouds are highly reflective, the fact of their visibility in such a situation can leave no doubt.

(397.) As the magnitude of the sun has been measured, and (as we shall hereafter see) its weight, or quantity of ponderable matter, ascertained, so also attempts have been made, and not wholly without success, from the heat actually communicated by its rays to given surfaces of material bodies exposed to their vertical action on the earth's surface, to estimate the total expenditure of heat by that luminary in a given time. The result of such experiments has been thus announced. Supposing a cylinder of ice 45 miles in diameter, to be continually darted into the sun with the velocity of light, and that the water produced by its fusion were continually carried off, the heat now given off constantly by radiation would then be wholly expended in its liquefaction, on the one hand, so as to leave no radiant surplus; while on the other, the actual temperature at its surface would undergo no diminution. I

* By direct measurement with the actinometer, I find that out of 1000 calorific solar rays, 816 penetrate a sheet of plate glass 0·12 inch thick; and that of 1000 rays which have passed through one such plate, 859 are capable of passing through another.

† The ball of ignited quicklime, in Lieutenant Drummond's oxy-hydrogen lamp, gives the nearest imitation of the solar splendour which has yet been produced. The appearance of this against the sun was, however, as described in an imperfect trial at which I was present. The experiment ought to be repeated under favourable circumstances. Note to the ed. of 1833. According to the more recent experiments of Messrs. Fizeau and Foucault, the intensity of the light at the surface of Drummond's lime-ball is only one-146th part of that at the surface of the sun !- (Note added 1858.)

" Results of Astronomical Observations at the Cape of Good Hope," p. 444.

H. 1827.

(397 a.) Another mode of expressing the heat generated and radiated off from the sun's surface, well calculated to impress us with an overwhelming idea of the tremendous energies there constantly in action, is that employed by Professor Thomson, who estimates the dynamical effect which would be produced in our manufactories by a consumption of fuel competent to evolve the heat given out by each individual square yard of that surface, at 63000 horse-power, to maintain which would require the combustion of 13500 pounds of coal per hour.*

(398.) This immense escape of heat by radiation, we may remark, will fully explain the constant state of tumultuous 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 phænomena of lightning, and probably also to those of terrestrialmagnetism and the aurora. By their vivifying action vegetables are enabled to draw support from inorganic matter,

* See Trans. R. S. Edin, xxi. p. 69. “ On the Mechanical Energies of the Solar System,” by W. Thomson, Esq., Prof. Nat. Phil., Glasgow. The Pro. fessor grounds this estimate on M. Pouillet's determination of the amount of solar radiation and Mr. Joule's estimate of the mechanical equivalent of a centigrade thermal unit. The author of this work found at the Cape of Good Hope, by experiments made on six summer days, from December 23rd, 1836, to January 9th, 1837, the sun being nearly vertical in each experiment, that in that latitude at midsummer, at noon, and at 140 feet above the sea level, the solar radiation is competent to melt an inch in thickness from a sheet of ice exposed perpendicularly to its rays (if wholly so employed) in 2h. 12m. 42°. Estimating the heat absorbed in traversing our atmosphere at one-third of the total quantity incident on it, this gives, all reductions made, 43:39 feet in thickness of ice per minute melted at the sun's surface. M. Pouillet's experi. ments (made in June, 1837), give 11.80 metres or 38-7 feet per minute. Forty feet may therefore be taken as a probable mean, and from this the result in (art. 397.) is calculated.

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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 where the resistance is barely adequate to their retention, and thus bring the phænomena of even volcanic activity under the general law of solar influence.t

(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.*

* So in the edition of 1833.

† So in the edition of 1833.

• Electricity traversing excessively rarefied air or vapours gives out light, and, doubtless, also heat. May not a continual current of electric matter be constantly çirculating in the sun's immediate neighbourhood, or traversing the planetary spaces, and exciting, in the upper regions of its atmosphere, those phænomena of which, on however diminutive a scale, we have yet an unequivocal manifestation in our aurora borealis. 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 enquiry, 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 must 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 are sure; and if this be an inherent (i. e. an absorptive) colour, the air must be expected to act on the spectruin after the analogy of other coloured media, wbich 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 the 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 effect 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. The idea of referring the origin of the solar heat to friction has been worked out into an elaborate theory by Prof. Thomson, in his paper already cited, of which some account will be given in a more advanced portion of this work. (1858.)

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NOTE ON ART. (394 a.)- The year 1856 was remarkable for the absence of spots in the sun (in exact accordance with Wolf's period); during 1857 the phase of increased activity came on; and the present year (1858) is ushered in with a magnificent display of spots in the sun's southern hemisphere. - (Note udded Jan. 4. 1858.)

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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 POS

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SIBLE. THEY PROVE THE MOON TO BE AN OPAQUE SOLID. -ITS
LIGHT DERIVED FROM TIME SUN. ITS PHASES. SYNODIC RE-
VOLUTION OR LUNAR MONTH. ILARVEST MOON. OF ECLIPSES
MORE PARTICULARLY..

-THEIR PHÆNOMENA.—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.- PROBABLE ELONGATION OF THE MOON'S FIGURE IN THE

DIRECTION OF THE EARTH.

ITS HABITABILITY NOT IMPOSSIBLE.

(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 118.5, 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 con

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