Page:Popular Science Monthly Volume 18.djvu/24

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Helmholtz, is stored away by animals and vegetables, and constitutes an abundant revenue of power for the whole human race.[1]

If we inquire what becomes of that principal portion of the solar heat which misses the planet, and passes off into space, no certain answer can be given.PSM V18 D024 The pouillet pyrheliometer.jpgFig. 1.Remembering, however, that space is full of isolated particles of matter (which we encounter from time to time as shooting-stars), we can see that nearer or more remotely in its course each solar ray is sure to reach a resting-place. Some have attempted to maintain that the sun sends heat only toward its planets; that the action of radiant heat, like that of gravitation, is only between masses. But all scientific investigation so far shows that this is not the case. The energy radiated from a heated globe is found to be alike in all directions, and wholly independent of the bodies which receive it, nor is there the slightest reason to suppose the sun any way different in this respect from every other incandescent mass.

Pouillet's experiments were made about the same time as Herschel's, but with a different apparatus, though based on the same principles. He named his instrument the pyrheliometer, or measurer of solar fire. Fig, 1 represents it. The little snuffbox-like vessel, a b, of silver-plated copper, blackened on the upper surface, contains a weighed quantity of water, and a thermometer is immersed in it, the mercury in its stem being visible at d. The disk e e makes it easy to point the instrument squarely to the sun by directing it so that the shadow of a falls concentrically upon this disk. The button at the lower end is for the purpose of agitating the water in the vessel a a, by simply turning the whole thing on its axis in the collar c c. The instrument is much more convenient than Herschel's apparatus, but hardly as accurate, except under very careful manipulation.

  1. Several experimenters have contrived machines for the purpose of utilizing the solar heat as a source of mechanical energy, among whom Ericsson and Mouchot have been most successful. M. Pifre describes in a recent number of the "Comptes Rendus" some results from a machine of Mouchot's construction, claiming to have utilized more than eighty per cent, of the heat which falls on the mirrors of the instrument: something over twelve calories to a square metre. We do not mean, of course, that this percentage of the total solar energy appeared as mechanical power in the engine, but only in its boiler. The machine had a mirror surface of nearly one hundred square feet, and gave not quite a horse-power. It is very possible that such machines will find useful application in the rainless regions like Egypt and Peru.