After a minute and exhaustive study of the polar snow caps by the combined observations of Lowell, Douglass and W. H. Pickering, Mr. Lowell says: "It is interesting that the cap should so simply tell us of these three important things: the presence of air, the presence of water, and the presence of a temperature, not incomparable with that of the Earth." Seasonal changes on Mars have long been recognized and admitted by astronomers, and these changes are on so vast a scale as to be distinctly visible from the earth. Without an atmosphere the surface of Mars would be inert. Schiaparelli was the first to notice that at successive oppositions the same regions showed different degrees of darkness and accounted for these variations by seasonal change. Mr. Denning believes that certain changes in the appearance of the markings to be due to vaporous condensations. Sir Norman Lockyer believed he saw the obscuration of a large region by clouds, this obscuration continuing for some hours. A bright spot on the terminator of Mars, discovered by Douglass at the Lowell Observatory, and which led to the newspaper excitement that signals were being made, was seen to move and finally disappear and its appearance, drift and disappearance is interpreted by Lowell as a cloud illuminated by the Sun and carried along by the wind. The presence of clouds, judging from my own brief experience, was certainly suggested at times by the peculiar way in which a large region known as Syrtis Major disappeared and flashed out again. This behavior might be expected of the tenuous lines as a result of refraction and other disturbances in our own atmosphere; when, however, a large, dark region at one time stands out firm, clear and sharp-cut as the stroke of a Japanese brush, then gradually fades out and remains obscure for some time we are inclined to believe that Sir Norman Lockyer's interpretation is true and that in such a case drifting clouds or sudden vaporous condensation produced the obscuration.