It would seem, then, that either some important difference exists, by which the Martian air is enabled to retain the sun's heat even more effectively than our air does (for the climate as indicated by the limits of the polar snows seems the same, though the distance from the sun is greater); or else there is some mistake in the supposition that the same general state of things prevails on Mars as on our own earth. I confess that though Professor Tyndall has shown clearly how the atmosphere of a more distant planet _might_ make up for the deficient supply of solar heat, by more effectively retaining the heat, I know of nothing in either the telescopic or the spectroscopic evidence respecting any of the planets which tends to show, or even renders it likely, that any such arrangement exists,--excepting always the peculiarity in Mars's case which we are now endeavouring to explain. Insomuch that should any other explanation of the difficulty be suggested, and appear to have weight in its favour, I apprehend that the mere possibility of an atmospheric arrangement, such as has been suggested, should not prevent our admitting this other explanation. I am inclined to think that there is such an explanation. It seems to me that there are good reasons for regarding Mars as a planet which has passed to a much later stage of planetary life than that through which our earth is now passing, and that in this circumstance some of the peculiarities of his appearance find their explanation. As a planet outside the earth, Mars must probably be regarded as one formed somewhat before the earth. As a much smaller planet, he would be not only less heated when first found (whatever theory of planetary formation we adopt), but would also have parted much more rapidly (relatively) with his heat, according to the same law which makes a small mass of metal cool more quickly than a large one. If he has a rarer atmosphere he would be a colder planet on that account also. Being also remoter from the sun, he receives less heat from that orb, and we thus have a fourth reason for regarding Mars as a much colder planet than our earth, both as to inherent heat and as to heat received from without. It seems to me that we may in this consideration find the real meaning of the comparatively limited extension of the Martian snows. It has been well pointed out by Professor Tyndall that for the formation of great glacial masses, not great cold only, but great heat also is required. The snows which fall on mountain slopes, to be compacted into ice and afterwards to form great glaciers, were raised into the air by the sun's heat. Every ice particle represents the action of that heat upon the particles of water at the surface of ocean, sea, or lake, or of wet soil. If the sun's heat suddenly died out, there would prevail an intense cold, and the snows and ice now existing would assuredly remain. The waters also of the earth would congeal. But no new snows would fall. The congealed