By this hypothesis, so beautiful in its simplicity, an explanation was afforded embracing all the more important facts of our system; the revolution of all the planets in nearly circular orbits and in the same direction as that in which the sun rotates, and the revolution of their satellites, also in circular orbits and in the same direction as their primaries; the comparatively high temperature and consequent low density of the larger planets and the sun, as well as a variety of other phenomena, all seem to follow naturally from it. The fundamental assumption seems to be in harmony with a number of known facts. Thus in the case of our own planet the volcanoes distributed around the margins of the oceans, and the hot springs scattered irregularly over the whole terrestrial surface, suggest that great stores of heat exist beneath our feet, a presumption which finds confirmation in the fact that whenever we descend towards the interior of the earth, as in deep mines or wells, the temperature continues steadily to rise after we have passed a depth below which seasonal and diurnal changes of temperature cease to be felt, the rise being in some cases as much as 3 deg. for 100 ft., in others only 1 deg. for the same distance, but on the average 1 deg. for 60 ft. or 70 ft. If this increase of temperature continues down to great depths, and there seems to be no reason why it should not, then a point will be reached, say, at thirty or forty miles down, where the interior will attain a white heat. [Sidenote: The Earth as a Star]