If the Martian atmosphere be of the same proportionate mass as that of our earth, it can possess no more than one-seventh its superficial density. That is to say, it is more than twice as tenuous as the air at the summits of the Himalayas. The corresponding height of a terrestrial barometer would be four and a half inches. Owing, however, to the reduced strength of gravity on Mars, this slender envelope is exceedingly extensive. In the pure sky scarcely veiled by it, the sun, diminished to less than half his size at our horizons, probably exhibits his coronal streamers and prominences as a regular part of his noontide glory; atmospheric circulation proceeds so tranquilly as not to trouble the repose of a land “in which it seemeth always afternoon”; no cyclones traverse its surface, only mild trade-winds flow toward the equator, to supply for the volumes of air gently lifted by the power of the sun, to carry reinforcements of water-vapor north and south. Aerial movements are, in fact, by a very strong presumption, of the terrestrial type, but executed with greatly abated vigor. Brilliant projections above the terminator of Mars were first distinctly perceived at the Lick Observatory in 1890. They have been reobserved at Nice, Arequipa, and Flagstaff (Mr. Lowell’s observatory), coming into view, as a rule, when circumstances concur to favor their visibility. They strictly resemble lunar peaks and craters, catching the first rays of the sun, while the ground about them is still immersed in darkness; and Professor Campbell connects them with “mountain chains lying _across_ the terminator of the planet,” and in some cases possibly snow-covered. He calculates their height at about ten thousand feet. Their presence was unlooked for, since a flat expanse is a condition _sine quâ non_ for the minute intersection of land by water, which seems to prevail on Mars. Although the sun is less than half as powerful on Mars as it is here, the Martian climate, to outward appearance, compares favorably with our own. Polar glaciation is less extensive and more evanescent, and little snow falls outside the arctic and antarctic regions. Yet the theoretical mean temperature is minus 4° C., or 61° of Fahrenheit below freezing. This means a tremendous ice-grip. The coldest spot on the earth’s surface is considerably warmer than this cruel average. Fortunately, it exists only on paper. Some compensatory store of warmth must then be possessed by Mars, and it can scarcely be provided by its attenuated air. Possibly, internal heat may still be effective, and we see exemplified in Mars the geological period when vines and magnolias flourished in Greenland, and date-palms ripened their fruit on the coast of Hampshire.