The intensely heated and liquid earth, revolving in the cold and empty spaces of the planetary system, gradually must have lost its excess of heat. Cooling most rapidly at the surface, it there solidified, and generated the first rocks. The loss of heat still going on, the solidification proceeded to a greater and greater depth, and should ultimately have reduced the earth to the same temperature as the empty space among the stars. The temperature of space has been calculated to be almost the same as that in the winter at Melville Island, in northernmost America, that is, 56 deg. below zero, or as far below the freezing point of water as the temperature of the hottest water that the hand can bear is above it. The earth is, however, not allowed to cool to that degree. It receives from the sun by radiation a quantity of heat which counteracts its tendency to cool, and hence the mean temperature of the surface of the earth has remained the same from the earliest historical epochs. In fact, at the surface we can find no trace of that original and internal great heat, the temperature of the surface of the earth being regulated altogether by the effect of the sun’s rays; but if we dig down to a moderate depth, about 45 feet, the influence of the sun becomes insensible. Within that space also we can detect a very curious and important arrangement of the heat. It is not that the whole surface becomes warmed in summer and cold in winter, but the heat which is received from the sun in one summer travels by conduction beneath the surface, and is succeeded by the heat of the next summer, an intervening and cooler layer corresponding to the winter time, so that at a depth of 20 feet we may detect the heat which had fallen upon the surface four or five years before, this space of 45 feet being formed of numerous layers like the coatings of an onion, one for each year, until becoming less and less distinct, according as the depth increases, they join together in forming the layer of invariable temperature in which all the effect of the sun’s heat is lost. If we dig down still farther, the earth, though having lost the heating power of the sun, becomes sensibly warmer. The greater the depth to which we descend, the higher is the temperature found to be. Thus, where deep sinkings have been made for mines or wells, the air or water at the bottom is found to be much higher in temperature than at the invariable layer which gives the mean temperature of the place. This observation was first made in the case of the deep mines in Cornwall, and, although for some time ascribed to the presence of the workmen and the burning lamps, has since been verified by observations in all parts of Europe, and such agreement found, that the law connecting the temperature with the depth has been at least approximately determined.