The temperature of space being so low as -239° Fahrenheit, it becomes a matter of no small interest to ascertain whether the earth may not be ultimately reduced by radiation to the temperature of the surrounding medium; what the sources of heat are; and whether they be sufficient to compensate the loss, and to maintain the earth in a state fit for the support of animal and vegetable life in time to come. All observations that have been made under the surface of the ground concur in proving that there is a stratum at the depth of from 40 to 100 feet throughout the whole earth where the temperature is invariable at all times and seasons, and which differs but little from the mean annual temperature of the country above. According to M. Boussingault, that stratum at the equator is at the depth of little more than a foot in places sheltered from the direct rays of the sun; but in our climates it is at a much greater depth. In the course of more than half a century the temperature of the earth at the depth of 90 feet, in the cellars of the Observatory at Paris, has never been above or below 53° of Fahrenheit’s thermometer, which is only 2° above the mean annual temperature at Paris. This zone, unaffected by the sun’s rays from above, or by the internal heat from below, serves as an origin whence the effects of the external heat are estimated on one side, and the internal temperature of the globe on the other. As early as the year 1740 M. Gensanne discovered in the lead-mines of Giromagny, in the Vosges mountains, three leagues from Béfort, that the heat of the ground increases with the depth below the zone of constant temperature. A vast number of observations have been made since that time, in the mines of Europe and America, by MM. Saussure, Daubuisson, Humboldt, Cordier, Fox, Reich, and others, which agree, without an exception, in proving that the temperature of the earth becomes higher in descending towards its centre. The greatest depth that has been attained is in the silver-mine of Guanaxato, in Mexico, where M. de Humboldt found a temperature of 98° at the depth of 285 fathoms, the mean annual temperature of the country being only 61°. Next to that is the Dalcoath copper-mine, in Cornwall, where Mr. Fox’s thermometer stood at 68° in a hole in the rock at the depth of 230 fathoms, and at 82° in water at the depth of 240 fathoms, the mean annual temperature at the surface being about 50°. But it is needless to multiply examples, all of which concur in showing that there is a very great difference between the temperature in the interior of the earth and at its surface. Mr. Fox’s observations on the temperature of springs which rise at profound depths in mines afford the strongest testimony. He found considerable streams flowing into some of the Cornish mines at the temperature of 80° or 90°, which is about 30° or 40° above that of the surface, and also ascertained that nearly 2,000,000 gallons of water are daily pumped from