The earth is a huge oblate or orange-shaped sphere, spinning on its shorter axis like a humming-top, yet at such a rate of speed as to seem standing still; it goes once round in twenty-four hours, its rotation being both the cause and the measure of day and night. The highest mountains range from four to five miles in height; the greatest depth of the ocean is probably little more than five miles, although Ross let down 27,000 feet of sounding-line in vain on one occasion. So that the earth's surface is very irregular; but its mountainous ridges and oceanic valleys are no greater things in proportion to its whole bulk, than the roughness of the rind of the orange it resembles in shape. The geological crust--that is to say, the total depth to which geologists suppose themselves to have reached in the way of observation--is no thicker in proportion than a sheet of thin writing paper pasted on a globe two feet in diameter. The surface of the earth is some 148,500,000 of miles in extent; and only one-fourth of that large space is dry land, the rest being ocean and ice. The atmosphere rises all round to a height between forty-five and fifty miles above the sea-level. The solar radiance sends such heat as it brings no deeper any where than 100 feet into the surface or scurfskin of the dry land--from forty to a hundred feet, one-third of the sun's heat being absorbed by the air. Yet the deeper man digs beyond the hundred feet, the warmer he finds the earth, and that at a somewhat determinable rate of increase. Supposing that rate of increment to go on toward the centre, it is computable that the solid underwork of the world, say granite by way of conjecture, must be in a state of fusion at no vast depth from the ground on which we tread. Let the scientific imagination descend a little lower, and it will find the melted granite in the form of a fiery vapor or gas--the dry steam of a red-hot liquid, in which the rock-built foundations of "the everlasting hills" melt like icebergs. But this is conjectural and probable, not observable and proved.