We speak of plants on Mars, of the snows at its poles, of its seas, atmosphere, and clouds, as though we had seen them. Are we justified in tracing all these analogies? In fact, we see only blotches of red, green, and white, upon the little disk of the planet; but, is the red terra firma; the green, water; or the white, snow? Yes, we are now justified in saying that they are. For two centuries astronomers were in error with regard to spots on the moon, which were taken for seas, whereas they are motionless deserts, desolate regions where no breeze ever stirs. But it is otherwise as regards the spots on Mars.
The unvarying aspect of the moon never exhibits the slightest cloud upon its surface, nor do the occultations of stars by its disk reveal even the slightest traces of an atmosphere. Contrariwise, the aspect of Mars is ever varying. White spots move about over its disk, very often modifying its apparent configuration. These spots can be nothing else but clouds. The white spots at its poles increase or diminish with the seasons, exactly like the circumpolar ice of earth, which, for an observer on Venus, would have the same aspect and the same variations as the polar spots of Mars have for us. Hence we conclude that these Martial white polar spots are masses of frozen water. Each hemisphere of Mars is harder to observe during its winter than during its summer, being often covered with clouds over its greater part. This would be precisely the aspect of the earth if observed from Venus. But what causes these clouds over Mars? Plainly nothing else but the evaporation of water. As for the ice, that is the same water frozen. But is the water there the same as here? Down to a few years ago, this question remained unanswered, but now it admits of a reply, thanks to the spectroscope, and the observations especially of Mr. Huggins.
The planets reflect the light they receive from the sun; on examining their spectra, we find the solar spectrum as though it had been reflected from a mirror. If we direct the spectroscope on Mars, we get, first of all, an image perfectly identical with that produced by the central star of our system. But, by the employment of more exact methods, Mr. Huggins found, during the last opposition of the planet, that the spectrum of Mars is crossed, in its orange portion, by a group of black lines coincident with the lines which appear in the solar spectrum at sunset when the sun's light passes through the denser strata of our atmosphere. Now, are these tell-tale rays produced by our atmosphere? To decide this question, the spectroscope was turned on the moon, which was at the time nearer the horizon than Mars. If the lines in question were produced by our atmosphere, they must have appeared in the lunar as well as in the Martial spectrum, and with greater intensity in the former. Yet they were not to be seen at all in the lunar spectrum; and hence it is plain that they are owing to the atmosphere of Mars.
The atmosphere of that planet, therefore, adds these special char-