whole Earth. We must remember, too, that a large amount of rain falls where it is not wanted, and that the energy of the Sun expended in the production of wind is in large part wasted. Clouds are raised by evaporation from the sea surface to fall on another part of the self-same waters. Storms are roused which blow with vehemence for a while, and then sink into rest without having accomplished any purpose necessary to the wants of terrestrial races. Here at once we see a large amount of energy not fully utilised. I do not indeed say that this apparently useless expenditure of force has no purpose in the economy of nature. Doubtless every natural event has its end and object. What I would dwell upon is, that if the energy which thus seems wasted could be made available to subserve human wants, it might be used without any fear that the economy of nature would suffer from such an application of her energies. And if this is true of the application of the indirect effects of solar energy, it is à fortiori true of the utilisation of the Sun's direct action--that is, of those solar rays to which the winds and the rains are due.

Now, if we assume that with the progress of science the power of thus employing to the full-or much more fully than at present-the forces which the Sun really expends upon the Earth will be acquired by man, we recognise the probability that science viewed generally is one of the means by which the efficiency of the solar energies is enlarged and extended. What is true of our Earth may be regarded as in all probability true

of other worlds than ours. As on our Earth so probably in other worlds there are or have been eras during which the beneficent power given to our great luminary is used without any consciousness of its value. There are or have been more advanced eras when the return of day and night, the progress of the seasons, the nourishment of the lately sown seed by spring rains, and the whitening of the fields into harvest under the summer Sun, are watched with anxious. interest. Then later follow the eras when the annual supply of truly vital energies seems to become insufficient, and when garnered stores of force are utilised by the thoughtful or ransacked by the too eager. And lastly, it may well be that in other worlds, as one day doubtless on our Earth, there will be eras when a more advanced degree of science will enable intelligent beings to derive from direct solar action the means of obtaining even larger supplies of force than they had been able to gather from the Sun-work of past epochs garnered by nature for their benefit.

The ideas of the Sun's true position in the solar system thus suggested lend an enhanced interest to the question whence the Sun himself derives and recruits his energies. As Tyndall has finely written, How is the perennial loss made good? We are apt to overlook the wonderful in the common. Possibly to many of usand even to some of the most enlightened among us— the Sun appears as a fire, differing from our terrestrial fires only in the magnitude and intensity of its combustion. But what is the burning matter which can thus

maintain itself? All that we know of cosmical phenomena declares our brotherhood with the Sun-affirms that the same constituents enter into the composition of his mass as those already known to chemistry. But no earthly substance with which we are acquaintedno substance which the fall of meteors has landed on the Earth-would be at all competent to maintain the Sun's combustion. The chemical energy of such substances would be too weak, and their dissipation too speedy. Were the Sun a block of burning coal, and were it supplied with oxygen sufficient for the observed emission, it would be utterly consumed in 5,000 years. On the other hand, to imagine it a body originally endowed with a store of heat-a hot globe now cooling -necessitates the ascription to it of qualities wholly different from those possessed by terrestrial matter. If we knew the specific heat of the Sun* we could calculate its rate of cooling. Assuming the specific heat to be the same as that of water—the terrestrial substance which possesses the highest specific heat— then, at its present rate of emission, the entire mass of the Sun would cool down 15,000 degrees in 5,000 years. In short, if the Sun be formed of matter like

The absolute quantity of heat necessary to raise the average temperature of the Sun by any given amount-say one degree, and therefore the absolute quantity of heat which corresponds to the loss of say one degree of temperature from the average temperature of the Sunwill depend on the physical constitution of the Sun. To say merely that the Sun's substance subsists at such and such a temperature, is by no means sufficient to indicate the amount of heat which the Sun is capable of imparting. A mass of iron, for example, may be at a temperature precisely equalling that of an equal mass of boiling water; yet the boiling water will give out far more heat while passing to any given lower temperature than the heated iron will.

our own, some means must exist of restoring to it its wasted power.'

We have not as yet the means of satisfactorily answering the question thus suggested. Answers have been suggested, but no answer has yet seemed so satisfactory that men could regard the problem as disposed of. Indeed the facts are so extraordinary,' as Tyndall has said, 'that the soberest hypothesis must appear wild.' Whether we conceive, with Mayer and Thomson, that the Sun's heat is maintained by the incessant downfall of cosmical bodies gathered out of space by the Sun's mighty attractive energies; or whether we follow Helmholtz in supposing that the gradual contraction of the solar orb is the mainspring of the solar energies; or whether we believe, with Secchi, that the dissociation of compound bodies in the Sun's substance is a fund of force to be gradually exhausted only as the dissociated elements unite in chemical combinations; or, lastly, whether we prefer the idea thrown out by Sir John Herschel that mayhap the vital energies of monstrous creatures—the willow-leaves of Nasmyth-are the true source of the great luminary's might, we have not overpassed by a step the amazing field of conjecture appertaining to our subject. Startling as these theories appear, they are not a whit more startling than the known facts which they are intended to interpret.

I have no wish to enter here into a detailed consideration of any of the theories above referred to; but I wish to make a few remarks respecting those of Helmholtz and Thomson.

When we consider the evidence forced upon us by the present condition of the solar system, and by the nature of the observed motions taking place within it, we find it difficult not to believe that two great processes have for uncounted ages been at work within its limits. On the one hand, the evidence is very strong in favour of the view that a process of contraction from a nebulous condition has taken place not only in the case of the Sun, but in that of the planets and other members of the solar system. On the other hand, the evidence is absolutely demonstrative that at the present time uncounted millions of minute cosmical bodies are streaming in upon the Sun. We have, then, clear evidence that at least some portion of the Sun's energy is derived from each of the two sources now dealt with. We can hardly conceive that the process of solar contraction has come to an end; and certainly we have no proof in the apparent constancy of the Sun's volume that the process has ceased, since Helmholtz has shown 'that the shrinking of the Sun's diameter by one10,000th part of its present length would generate an amount of heat competent to cover the solar emission for 2,000 years.' Nor can we question that whatever energy may correspond to the velocity, mass, and distance of a meteoric body at any epoch must have been transferred to the Sun if at some later epoch the mass of the meteor has come (after whatever processes) to form part of the solar globe. Now, without committing myself to the opinion that the whole solar emission can be accounted for by combining these two