"As for the earth, out of it cometh bread; and under it is turned up as it were fire."—Job xxviii.
222. What becomes of the remaining two-thirds of the solar heat?
They are absorbed chiefly by the earth, the great medium of calorific absorption; but some portions are taken up by living things, both animal and vegetable. When the rays of heat strike upon the earth's surface, they are passed from particle to particle into the interior of the earth's crust. Other portions are distributed through the air and water by convection, and a third portion is thrown back into space by radiation. These latter phenomena will be duly explained as we proceed.
223. How do we know that heat is absorbed, and conducted into the internal earth?
It is found that there is a given depth beneath the surface of the globe at which an equal temperature prevails. The depth increases as we travel south or north from the equator, and corresponds with the shape of the earth's surface, sinking under the valleys, and rising under the hills.
224. Why may we not understand that this internal heat of the earth arises, as has been supposed by many philosophers, from internal combustion?
Because recent investigations have thrown considerable and satisfactory light upon the subject. It has been ascertained that the internal temperature of the earth increases to a certain depth, one degree in every fifty feet. But that below that depth the temperature begins to decline, and continues to do so with every increase of depth.