economy. Still, nothing could be plainer than this. In our day, when it is probable that force, motion, gravity, heat, light, electricity, magnetism, are simply modifications of one and the same agent, and the effect of the vibrations of that impalpable and invisible fluid known as ether, the assertion that the sun is the only fuel, the only force, must not call forth anywhere the smile of incredulity. All fuels, all forces, are to be regarded as only parts of the sun's heat. What is coal? Fossil carbon. And was not this carbon fixed in plants by the sun's heat, of which it is the equivalent? Under the action of solar radiations the carbonic acid in the atmosphere is decomposed on contact with plants; the carbon is fixed in the plant, and the oxygen goes back into the air to serve for the respiration of animals. Hence, no sun, no vegetation; no vegetation, no carbon; no carbon, no coal. Coal, in burning, gives up the solar heat which was stored up in it, and therefore it was that, on seeing a locomotive engine move, Stephenson said: "It is not the coal that drives this engine, it is the sun's heat stored up in the coal thousands of ages ago; locomotives are but the horses of the sun." We might make a like comparison with respect to wine and the alcohol it contains; and the Bordelais use no mere figure of speech when they speak of their admirable Sauterne wine as being "bottled sunshine."
When water rises in the shape of vapor, what is it that causes it to ascend? The heat of the sun. If it comes down as rain, forming torrents and brooks which feed our mill-races and drive our mills, what is it that turns the wheel? The sun, for it was the sun that in the first place raised the water. When the wind blows upon the sails of a windmill, or on the sails of a ship, what is it that drives the mill or propels the ship? The sun, for wind is simply an atmospheric current produced by the heating of a stratum of air which, being dilated by the sun, tends to an equilibrium with strata of the same density, and hence rises, while a volume of cooler air takes its place. And what are the tides, the propulsive power of which there is some thought of utilizing, whether directly by means of water-wheels, or indirectly by compressing air and so producing a constant supply of force? They are a portion of the heat of the sun, for the seas are formed by the coming together of all those torrents and rivers which descend into their common reservoir, the ocean. Then, too, the tides are the result of the combined attraction of sun and moon upon the earth. Thus we find that the sun is always and everywhere active.
It is, therefore, no paradox to regard the sun as the one source of fuel in the future, and as the reservoir of force to which generations to come will at no distant day have recourse. Hence it is that savants and great engineers, as Euclid, Archimedes, Hero, Salomon de Cans, Buffon, Saussure, Bélidor, Evans, Herschel, Pouillet, Ericsson, have in every age put to themselves the question how it might be possible to take from the sun a part of its heat for the benefit of this poor globe.
