Cimmerium, Mare Sirenum, and Solis Lacus, which during the years 1877 to 1879 could be numbered among the most sombre on the planet, during the succeeding oppositions became less and less black, and in 1888 were of a light gray hardly sufficient to render them visible in the oblique position in which all three found themselves.... On the other hand, at the very same moment, the Mare Acidalium and the Lacus Hyperboreus showed very dark; the latter indeed appeared nearly black, although seen as tilted as the Syrtis and the equatorial bays. The condition of the regions called seas is therefore not constant: so much is unquestionable. Perhaps the change produced in them has to do with the season of the planet’s year.” Holding as he did the then prevailing view that the blue-green regions were bodies of water, he regarded those of intermediate tint as vast marshes or swamps, and he accounted for change of hue in them as due to inundations and occasions of drying up. In consequence of losing their water, the seas, he thought, had in places become so shallow that the bottom showed through. Plausible on the surface, this theory breaks down so soon as it is subjected to quantitative criticism. For the moment we try to track the water, we detect the inadequacy of the clew. The enormous areas over which the phenomenon occurs necessitates the establishing an alibi for all the lost water that has gone. Drying up on such a scale would mean the removal of many feet of liquid over hundreds of thousands of miles in extent. To produce any such change in appearance as we witness, even on the supposition that these seas were none too deep to start with, would involve lowering the level of the water by from five to twenty feet throughout two thirds of the whole surface of the southern hemisphere. This would leave a heap of waters to be accounted for, bewildering in its immensity. The myriad tons of it must be disposed of; either by drainage into other regions or by being caught up into the sky.