it might fairly be held to be wrong for the present inhabitants of the Earth to exhaust, in contrivances intended to add to the luxuries or conveniences of life, those stores which are absolutely necessary to the wellbeing of future races.

In dealing, for example, with the question of terres trial coal supplies, it will not suffice to point out that for a thousand or several thousand years they may be drawn upon as at present, or even more largely, without exhaustion. The thousand or thousands of years will pass as surely as those which have already passed, and the wants entailed by our wastefulness will be felt none the less, that for so many years there had been no failure in the supplies contained within the great terrestrial storehouse. What must be done, then, is to show that by the progress of that very course of events which results in the rapid use of those stores, the means will spring into existence of obtaining fresh and inexhaustible supplies. This is no idly speculative view, but the plain and obvious duty of the scientific world. Precisely as the superiority of civilised races over barbarous tribes is shown in nothing more clearly than in the fact that the former are not content, as the latter are, merely to supply the wants of the moment, or of a few days, but seek to make provision, not only for future years, but for the wants of their immediate descendants, so it behoves the leaders of the great movement which during the last few years has so greatly changed the aspect of the human race, to show the superiority of the new order of things by a careful provision for, and

anticipation of, the wants of the races which will inhabit the Earth thousands of years hence.

Without discussing the various forms of work which are being done upon the Earth, or considering the various agents employed in producing the motive power by which those forms of work are set in action, it may be simply stated that at present nearly all our motive force is obtained from stored Sun-force. It would be difficult to point to a single work accomplished by the aid of modern scientific appliances which has not resulted in exhausting to a greater or less degree the force which the Earth has been garnering up in long-past ages for our use. It is in this all-important respect that the more modern forms of machine-work differ from other forms of work. I refer, of course, to machines driven by inanimate motive powers, and not to those worked by the direct action of animal force. The machine draws upon the Earth's garnered stores,* while the living worker draws upon the Earth's periodical supplies of force. In the former case, that is being used up which cannot be replaced; in the latter, what is consumed will be restored in the ordinary course of nature. In one case it is our force-principal,' in the other it is our 'force-income' we are consuming. The distinction is all-important.

Those appliances in which advantage is taken of the action of the wind, rainfall (rivers), tidal action, and a few other natural processes, are to be excepted. Modern invention, however, is but seldom directed to the utilisation of these old-fashioned force-supplies.

This is not the place to enter into a discussion of the methods by which the great problem-a problem not requiring immediate solution, but which in the long run will surpass all others in interest and importance is to be solved. But I may indicate what is, I take it, the direction in which a solution will be found. We are now utilising what Professor Tyndall calls the Sun of the Carboniferous Epoch: our descendants will have to employ the Sun of their own epoch. The heat of the solar rays-mayhap also their light and their actinic energy-must one day be applied to work our machinery. Already men have felt the advantage of thus employing solar energy. They have not, indeed, as yet applied the direct action of the Sun systematically to their purposes. But in an indirect manner they have utilised solar energy. The ships which sail upon our seas, the mills which are turned by water or by wind-these and many other devices of man have been contrived to utilise a portion of the Sun's heat. But the proportion thus utilised is almost indefinitely small by comparison with that which is actually available. It is only necessary to translate some of the ordinary phenomena of nature into the language of the familiar forces in order to see that this is so. For instance, the amount of energy involved in the production of rain is startlingly great

* Ericsson has constructed a machine in which the solar rays supply the primary motive force. It has not, however, yet been demonstrated (though I have not the least doubt it will be at some future epoch) that the solar heat can be employed in a profitable—that is, a mechanically advantageous,' manner.

when compared with our ordinary estimates of force. I have calculated that the force expended in the production of a day's steady rain over an area equal to that of the county of Middlesex would be equivalent to a mechanical power competent to raise 1,000,000,000 tons to a height of three miles!

Professor Tyndall has put in a striking form the relation which exists between the simpler processes of nature and those effects which seem to us the most apt exponents of power. I have seen,' he says, 'the wild stone-avalanches of the Alps, which smoke and thunder down the declivities with a vehemence almost sufficient to stun the observer. I have also seen snowflakes descending so softly as not to hurt the fragile spangles of which they were composed ;—yet to produce from aqueous vapour a quantity which a child could carry of that tender material, demands an exertion of energy competent to gather up the shattered blocks of the largest stone-avalanches I have ever seen, and pitch them to twice the height from which they fell.'

And when we have thus seen what a tremendous amount of energy is involved in such processes as the formation of snow or rain in comparatively small quantities, we begin to recognise, though we are far from being able to conceive, how enormous is the potential energy which supplies the rainfall of the 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 awhile, 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