the shorter the time. But later the inventor has employed only conical 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; consequently 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 demonstrated 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: "Archimedes, 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 without cost of transportation, the prudent engineer will know how to provide 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, besides 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 silver-leaf. On the small base of the truncated cone rests a copper cylinder, blackened on the outside, its vertical axis being identical with that of the cone. This cylinder, surrounded as it were by a great
