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contains little or no aqueous vapour. This conclusion is reached from observations both with the telescope and the spectroscope. The most careful eye observations of the planet show that the features are rarely, if ever, obscured by anything which can be considered as clouds in the Martian atmosphere. It is true that the features are not always seen with the same distinctness; but the variations in the appearance are no greater than would be due to the changes in the steadiness and purity of our own atmosphere, through which the astronomer necessarily makes his observations. Although, near the edge of the apparent disk of the planet, the features appear to be softened, as if seen through a greater thickness of the atmosphere, this appearance is, at least in part, due to the obliquity of the line of sight, which prevents our getting so good a view of the edge of the disk as of its centre. Something of the same sort may be noticed when the moon is viewed with the naked eye or an opera glass. Yet it is quite possible that a certain amount of the softening may be due to a rare atmosphere on Mars.

The most careful spectroscopic examination of the planet was made by Campbell, who compared its spectrum with that of the moon. He could not detect the slightest difference between the two spectra. Now, if Mars had an atmosphere capable of exerting a strong selective absorption on light, we should see lines in the spectrum due to this absorption or, at least, some of the lines would be strengthened. Our general conclusion therefore must be that, while it is quite probable that Mars has an atmosphere, it is one of considerable rarity,