INDUSTRIAL APPLICATIONS OF SOLAR HEAT. 553 



the shorter the time. But later the inventor has employed only coni- 

 cal mirrors, and in these the insolation surface is quadrupled when the 

 diameter of the mirror is doubled. 



Mouchot's researches did not end here. He proposed further to 

 obtain mechanical effects with solar heat, and in the beginning of 

 August, 1866, he put in operation at Paris the first machine of this 

 kind. 



In the mean time Ericsson was studying these same problems, 

 without knowing anything about Mouchot's experiments. Starting 

 from the facts collected by. Herschel and Pouillet, Ericsson, in the 

 first place, estimated the action of the sun upon a surface of nine 

 square metres to be sufficient to vaporize eight litres of water ; con- 

 sequently it would be equal to one horse-power. From these premises 

 he deduces striking consequences, as, for instance, that the solar heat 

 falling on the roofs of Philadelphia alone would suffice to drive 5,000 

 steam-engines of twenty horse-power each. Then, having demon- 

 strated that upon one square mile, using only one-half of the surface 

 and devoting the remainder to buildings, roads, etc., we can drive 

 64,800 steam-engines, each of a hundred horse-power, simply by the 

 heat radiating from the sun, he adds these remarkable words : " Archi- 

 medes, having completed his calculation of the force of a lever, said 

 that he could move the earth ; I affirm that the concentration of the 

 heat radiated by the sun would produce a force capable of stopping 

 the earth in its course." Again : " In England they are beginning to 

 calculate the time when the coal will give out, though coal-mines are, 

 so to speak, of recent exploitation. A few thousands of years drops 

 in the ocean of time will exhaust the coal-mines of Europe, unless, 

 meanwhile, recourse is had to the aid of the sun. True, the sun's 

 beams do not every day reach the surface of the earth; but, when the 

 great magazine is opened which shall supply heat gratuitously with- 

 out cost of transportation, the prudent engineer will know how to pro- 

 vide a reserve against cloudy days. At the same time we would observe 

 that a large proportion of the earth's surface is illumined by an ever- 

 radiant sun. The solar engine's sphere of activity is as great as its 

 dynamic power is considerable." Mr. Ericsson, who, besi'des genius, 

 possesses wealth and a long experience, will doubtless some day take 

 up again his studies upon the mechanical application of solar heat. 

 Meanwhile, we must state what has been done in this direction by a 

 Frenchman. 



The traveler who visits the library of Tours sees in the court-yard 

 in front a strange-looking apparatus. Imagine an immense truncated 

 cone, a mammoth lamp-shade, with its concavity directed skyward. 

 This apparatus is of copper, coated on the inside with a very thin sil- 

 ver-leaf. On the small base of the truncated cone rests a copper cyl- 

 inder, blackened on the outside, its vertical axis being identical with 

 that of the cone. This cylinder, surrounded as it were by a great 



