Closely linked with the question of the existence of water on the planet, and indeed a fundamental point in the settlement of it, is the further question of whether there is any aqueous vapour in the Martian atmosphere. The evidence is somewhat conflicting. It is quite apparent that in the atmosphere of Mars there is nothing like the volume of water vapour which is present in that of the earth, for if there were, his features would be much more frequently obscured by cloud than is found to be the case. Still there are many observations on record which seem quite unaccountable unless the occasional presence of clouds is allowed. Thus on May 21, 1903, Mr. Denning records that the Syrtis Major (see Chart, Plate XXI.) being then very dark and sharply outlined, a very bright region crossed its southern extremity. By May 23, the Syrtis Major, 'usually the most conspicuous object in Mars, had become extremely feeble, as if covered with highly reflective vapours.' On May 24, Mr. Phillips observed the region of Zephyria and Aeolis to be also whitened, while the Syrtis Major was very faint; and on the 25th, Mr. Denning observed the striking whiteness of the same region observed by Mr. Phillips the day before. Illusion, so often invoked to explain away inconvenient observations, seems here impossible, in view of the prominence of the markings obscured, and the experience of the observers; and the evidence seems strongly in favour of real obscuration by cloud. It might have been expected that the evidence of the spectroscope would in such a case be decisive, but Campbell's negative conclusion is balanced by the affirmative result reached by Huggins and Vogel. It is safe to say, however, that whatever be the constitution of the Martian atmosphere, it is considerably less dense than our own air mantle.