old books that relate to this matter are unfortunately filled with exaggerations, and the lack of recent evidence prevents our extracting from them the portion of truth they perhaps contain. Pontoppidan, in his "Natural History of Norway," describes a cavity in the vicinity of Frederickshall, in which the duration of the fall of a stone appeared to be two minutes. Assuming, says Arago, that the stone fell clear, without hitting and being retarded by projections in the walls of the cavity, the total depth indicated by its two minutes' fall would be over 4,000 metres, exceeding by 800 metres the height of the highest mountain in the Pyrenees. But it would appear that the noise of the stone's falling was heard for two minutes—that it consequently rolled and bounded from point to point; and modern travelers have nothing further to say of the famous Frederickshall hole. Another account, of the legendary cavern of Dolsteen, in the Island of Herroe, Norway, is likewise doubtful. According to a belief among the inhabitants, this cavern extended to and under Scotland. It is told that, in 1750, two priests ventured far into it and heard the rumbling of the sea above them. Coming to the brink of a precipice, they threw over a large stone, which was heard to fall a minute after. Without, however, attaching importance to accounts from such unreliable sources, it may still be admitted that natural cavities exist which might be made use of in exploring the deeper strata of the earth's crust. M. Babinet, who cherished the idea of forming a society for digging a deep hole for such purposes, thought the question had its industrial side which ought not to be lost sight of. "This is no longer," he somewhere says, "the time of Voltaire, who so bitterly berated Maupertuis, whom he described as having wished to pierce the earth that we might see our antipodes by leaning over the edge of the well of this antagonist of the irascible king of literature. Nobody would today deny the possibility of sinking the shafts of mines to a depth of several thousand metres, when we have such choice of ground, dimensions, and, above all, time. Let us suppose that we have reached a depth of four kilometres only, and cleared a suitable space. If men can not support the heat, machines, which are not so delicate, can. We see ourselves in possession of a vast space, the walls of which are of the temperatures of our ovens and stoves. Conducting thereto a stream of water, it issues hotter than boiling water, and is a veritable mine of heat, as truly so as are the precious coal-mines of England and Belgium." It is a fact that the heat of the springs of Chaudes-Aigues, which reaches 80°, is made use of by the inhabitants for purposes of cooking, heating their houses, washing, etc. By conduits of wood, in all the streets of the village, reservoirs on the ground-floor of each house are supplied, and these serve the purpose of heating-stoves in cold weather, fires and chimneys being dispensed with. In summer, the inflow is stopped by little sluice-gates at the inlet of each supply-pipe, the water then flowing to the brook at the border of the
