If the turning of the E. on its axis is thus proved to be the cause of the apparent daily motion of the heavens, it is an easy step to consider the annual motion of the sun through the constellations of the zodiac as also apparent, and arising from a revolution of the E. about the sun in the same direction of w. to east. If we consider that the mass of the sun is about 359,000 times greater than that of the E., and that laws of mechanics, two bodies that revolve round each other, must revolve about 'their common center of gravity, the idea of the sun revolving about the E. is seen to be simply impossible. The common center of gravity of the two bodies being distant from the center of each inversely as their respective masses, is calculated to be only 267 m. from the center of the tun, and thefefore far within his Which lies a diameter of 882,000 miles. But by help of a figure, it is easy to show that the apparent motion of the sun on the ecliptic naturally arises from a motion of the E. about the sun. The motions of the planets also, that appear so complicated and irregular as seen by us, can only be satisfactorily explained by assuming that they too revolve round the sun in the same direction as the earth. See PRECESSION and NOTATION for an account of a Jrnall peri odic motion of the E.'s axis and its effects.
4, The Earth's Temperature. See METEOROLOGY and TEMPERATURE aS to the phe nomena of heat on the E.'s surface. As we go below the surface, we reach a depth beyond which the interior of the E. seems to have no sympathy with the external causes of heat or cold, and its heat appears to be its own, and to increase according to a fixed law the deeper we descend. The average rate of observed increase is 1° F. for a descent of between 40 and 50 feet. If this law were universal—which we do not know it to be—at a depth of less than 30 miles the heat would be such as to hold in fusion all known substances, and the E. would have to be regarded as a very thin crust or shell full of molten liquid. This theory of a molten interior obtained at one time extensive currency among philosophers, being indorsed with the names of Fourier and Humboldt; but it has since been shown to be inconsistent with the rigidity which astronomical observations prove the E. to possess. A. liquid nucleus would be subject to' tides like the ocean, and the crust would partake of the motion. Granting the increase of heat to be constant, we do not know what effect the increasing pressure may have in -preventing fusion.