H 



THE POPULAR SCIENCE MONTHLY. 



Ilelmholtz, is stored away by animals and vegetables, and constitutes 

 an abundant revenue of power for the whole human race.* 



If we inquire what becomes of that principal portion of the solar 

 heat w^hich misses the planet, and passes off into space, no certain 

 answer can be given. Remembering, however, that space is full of 



isolated particles of matter (which we encoun- 

 ter 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 rest- 

 ing-place. Some have attempted to maintain 

 that the sun sends heat only toward its plan- 

 ets ; 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 slight- 

 est reason to suppose the sun any way differ- 

 ent in this respect from every other incandes- 

 cent mass. 



Pouillet's experiments were made about 

 the same time as Herschel's, but with a differ- 

 ent apparatus, though based on the same prin- 

 ciples. He named his instrument the pyrhe- 

 liometer, or measurer of solar fire. Fig, 1 

 rejDresents it. The little snuffbox-like vessel, 

 a b, of silver-plated coppei-, blackened on the 

 upper surface, contains a weighed quantity of water, and a thermome- 

 ter is immersed in it, the mercury in its stem being visible at cl. 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 care- 

 ful manipulation. 



* Several experimenters have contrived machines for the purpose of utilizing the solar 

 heat as a source of mechanical energy, among whom Ericsson and Moucbot have been 

 most successful. M. Pifre describes in a recent number of the "Comptes Eendus " 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 applica- 

 tion in the rainless I'cgions like Egypt and Peru. 



Fig. 1. 



