moon than to that of the earth, and we may reasonably conclude that its surface is formed of volcanic matter in a light and porous condition, and therefore highly favourable for the rapid loss of surface heat by radiation. The surface-conditions of Mars are therefore, presumably, much more like those of the moon than like those of the earth.
The next condition favourable to the storing up of heat—a covering of vegetation—is almost certainly absent from Mars except, possibly, over limited areas and for short periods. In this feature also the surface of Mars approximates much nearer to lunar than to earth-conditions. The third condition—a dense, vapour-laden atmosphere—is also wanting in Mars. For although it possesses an atmosphere it is estimated by Mr. Lowell (in his latest article) to have a pressure equivalent to only 2 1/2 inches of mercury with us, giving it a density of only one-twelfth part that of ours; while aqueous vapour, the chief accumulator of heat, cannot permanently exist in it, and, notwithstanding repeated spectroscopic observations for the purpose of detecting it, has never been proved to exist.
I submit that I have now shown from the statements—and largely as the result of the long-continued observations—of Mr. Lowell himself, that, so far as the physical conditions of Mars are
