Again, as to the prominences, it seems to be demonstrated that they are due to some form of eruption and only assume the cloud form after the eruption which gave them birth has ceased. But what are the circumstances which give birth to these eruptions, what the nature of the layer (Zöllner's Trennungschicht") beneath which the eruptive action is prepared, and what the actual depth whence the erupted matter springs, we have very little to show.

And lastly, as to the corona and the general relations involved in the access of external matter from the interplanetary and intersidereal spaces to the neighbourhood of the Sun's globe, we have, I apprehend, small means of forming an opinion. The condition, indeed, of the space which lies immediately around the Sun is very little understood by us. It may be that in the study of the corona during total eclipses we may find a means of answering the many perplexing questions associated with this matter. It may even be that new appliances may enable us to study the corona when the Sun is not eclipsed, and so to learn whether systematic processes affecting the Sun's economy are at work in the region immediately surrounding him. At present our information on this subject is meagre in the extreme; and our means for acquiring information are far from promising. Here, as in so many matters related to the physical constitution of the Sun, we must perforce wait until our experimental knowledge and our instrumental means have been very largely increased.

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CHAPTER VIII.

THE SUN OUR FIRE, LIGHT, AND LIFE.

FEW of the results of modern scientific research are more remarkable than the recognition of the real extent of the influence which the Sun exerts upon the Earth. Of old the Sun's power as ruler over the seasons, his action upon vegetation, and other like influences, were recognised in a vague and general way. But men were far from regarding the Sun as the true source of many forms of force which seem almost equally important. Still less were they prepared to trace his influence in nearly every kind of action or mode of motion taking place upon our globe. It is the most striking feature of recent scientific research that it has taught us to see in nearly all terrestrial phenomena the action of a certain proportion of Sunforce.

We owe to the greatest astronomer of our time— Sir John Herschel--the first definite enunciation of this great principle. The Sun's rays,' he wrote in 1833, 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

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the electric equilibrium of the atmosphere which give rise to the phenomena of lightning, and probably also to terrestrial action 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 man, and the source 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 change consists, is almost entirely due-on the one hand to the abrasion of wind or rain and the alternation of heat and frost, on the other to the continual beating of 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 force of subterranean fires, thus

repressed on the one hand and released on the other, may break forth in points where the resistance is barely adequate to their retention, and thus bring the phenomena of even volcanic activity under the general law of solar influence.'

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Since this was written men of science have learned to enounce the complete law of the conservation of solar energy,' as applied to the organic and inorganic world. That which was put forward in a general way by Sir John Herschel has been made the subject of special scrutiny. We have learned how to weigh and measure the Sun's action and the force-supplies which we derive from it.

Let us take first the supply of heat the Earth derives from the Sun. We shall have much to excite our wonder, whether we regard the real vastness or the relative minuteness of this supply.

From the researches of Sir John Herschel it appears that the direct heat of the Sun, if' received on a surface capable of absorbing it and retaining it, would suffice to melt an inch of ice in thickness in 2h. 13m.;' and he thence calculates that no less than 26,000 tons of ice would be melted per hour by the heat actually thrown on a square mile exposed at noon under the equator. This amount must be multiplied fifty million times to correspond to the heat actually received by the Earth's globe during a single hour. Pouillet obtained results not differing very greatly from these. He calculated that an interval somewhat greater than 2h. 13m. would be required to melt a layer of

ice one inch thick.

Expressing his result according to a somewhat different method, he states, again, that if the Sun's heat were distributed uniformly over the Earth's surface it would in one year suffice to liquefy a layer of ice 100 feet thick, or to heat an ocean of fresh water sixty-six miles deep from the temperature of melting ice to the boiling point.'

Yet this enormous annual supply of heat is but the 1-2,138,000,000th † part of that which the Sun actually radiates into space in the course of a year. All the planets of the solar system are able to intercept but about the 227-millionth part of the heat actually emitted by the Sun. There is a fine passage in Herschel's 'Outlines of Astronomy' which shows how enormous is the amount of heat deduced by increasing in the ratio above indicated the supply of heat actually received from the Sun by the Earth: Supposing a cylinder of ice fortyfive miles in diameter to be continually darted into the Sun with the velocity of light, the heat now given off constantly by radiation would then be wholly expended

The actual relation between Pouillet's and Herschel's results may be thus expressed. Sir John Herschel deduced 43:39 feet as the thickness of ice which the Sun is capable of melting per minute, supposing the ice continually applied to the Sun's surface (and the water produced by its fusion continually carried off). Pouillet deduced 38·7 feet per second. Sir John Herschel says that 40 feet may be regarded as a probable mean. (It will be noticed that, with characteristic modesty and generosity, he places the mean much nearer to Pouillet's value than to his own.)

It is singular how persistently the number 2,300,000,000, calculated by Mayer, maintains its ground in scientific treatises. This number was correctly deduced from the old value of the Sun's distance. But the above is the true value, according to the best modern estimates of the solar parallax.