After encountering many difficulties, which he had foreseen with great acuteness, and obviated with equal ingenuity, Buffon at length succeeded in repeating Archimedes's performance. In the spring of 1747 he laid before the French Academy a memoir which, in his collected works, extends over upwards of eighty pages. In this paper he described himself as in possession of an apparatus by means of which he could set fire to planks at the distance of 200 and even 210 feet, and melt metals and metallic minerals at distances varying from 25 to 40 feet. This apparatus he describes as composed of 168 plain glasses, silvered on the back, each six inches broad by eight inches long. These, he says, were ranged in a large wooden frame, at intervals not exceeding the third of an inch, so that, by means of an adjustment behind, each should be movable in all directions independent of the rest; the spaces between the glasses being further of use in allowing the operator to see from behind the point on which it behooved the various disks to be converged. In this last statement there is a parallel with that of Tzetzes, who speaks of the division of Archimedes's mirrors. At the present moment naturalists are paying great attention to plans for the using of the heat of the sun. It is said that on any county in the United States, twenty by thirty miles square, there is wasted as much heat of the sun as would drive, if we knew how to use it, all the steam-engines in the world. Fergus asked Uncle Fritz if he believed that Archimedes threw seven hundred pounds of stone from one of his machines. The largest modern guns throw shot of one thousand pounds, and it is only quite recently that any such shot have been used. Uncle Fritz told him that in the museum at St. Germain-en-Laye he would one day see a modern catapult, made by Colonel de Reffye from the design of a Roman catapult on Trajan's Column. This is supposed to be of the same pattern which is called an "Onager" in the Latin books. This catapult throws, when it is tested, a shot of twenty-four pounds, or it throws a sheaf of short arrows. In one catapult the power is gained by twisting ox-hide very tightly, and suddenly releasing it. Another is a very stout bow, worked with a small windlass. Of course this will give a great power. Seven hundred pounds, however, seems beyond the ability of any such machines as this; but from his higher walls Archimedes could, of course, have rolled such stones down on the decks of the ships below. And if he were throwing other stones or leaden balls to a greater distance with his _Onagers_, it may well be that Plutarch or Livy did not take very accurate account of the particular engine which threw one stone or another.