When I began the course of lectures that resulted in this volume, I labored under the apprehension that an account of cosmic physics might prove dull. It soon threatened to prove too startling. I therefore hasten to reassure the timid by saying that we are outgrowing ice ages and probably deluges. Elevations of the Earth’s crust are likely to be less and less pronounced in the future, and meanwhile such as exist are being slowly worn down. Secondly, the Sun is sure to continue of much the same efficiency for many æons to come. And lastly, the essential ingredient of both prodigies, water, is daily becoming more scarce. To this latter point we now turn, and perhaps when it is explained to him the reader may think that he has been rescued from one fate only to fall into the hands of another. Geology is necessarily limited in its scope to what has happened; planetology is not so circumscribed in its domain. It may indulge in prognostication of the future, and find countenance for its conclusions in the physiognomy of other worlds. Thus one of the things which it foresees is the relative drying up of our abode. To those whose studies have never led them off this earth, the fact that the oceans are slowly evaporating into space may seem as incredible as would, to one marooned on a desert island, the march of mankind in the meantime. We live on an island in space, but can see something of the islands about us, and our conception of what is coming to our limited habitat can be judged most surely by what we note has happened to others more advanced than ourselves. Just as we look at Jupiter to perceive some likeness of what we once were, the real image of which has travelled by this time far into the depths of space beyond possibility of recall, so must we look to the Moon or Mars if we desire to see some faint adumbration of the pass to which we are likely to come. For from their lack of size they should have preceded us on the road we are bound to travel. Now, both these worlds to-day are water-lacking, in whole or part; the Moon practically absolutely so, Mars so far as any oceans or seas are concerned. We should do wisely then to take note. But we have more definite information than simply their present presentments. For both bear upon their faces marks of having held seas once upon a time. They were once, then, more as we are now. We cannot of course be sure, as we are unable to get near enough to scan their surfaces for signs of erosive action. But so far as we can make out, past seas best explain their appearance. [Illustration: THE MOON—PHOTOGRAPHED AT THE LOWELL OBSERVATORY.]