Just as almost all of the features we note are permanent in place, showing that they belong to the surface, so are they all impermanent in character. Change is the only absolutely unchanging thing except position about the features the planet presents to view. It was in the aspect of the polar caps that this important fact first came to light. Not only did they thus initially instance a general law, they have turned out to make it; for by themselves changing they largely cause change in all the rest. But for a long time they alone exemplified its workings. To Sir William Herschel we owe the first study of their change in aspect. This eminent observer noted that their varying size was subject to a regular rhythmic wax and wane timed to the course of the seasons of the planet’s year. The caps increased in the winter of their hemisphere and decreased in its summer and being situate in opposite hemispheres they did this alternately with pendulum-like precision. His observations were soon abundantly confirmed, for the phenomena take place upon a vast scale and are thus easy of recognition. At their maximum spread the caps cover more than one hundred times as much ground as when they have shrunk to their minimum. In the depth of winter they stretch over much more than the polar zone, coming down to 60° and even 50° of latitude north or south as the case may be, thence melting till by midsummer they span only five or six degrees across. In this they bear close analogue to the behavior of our own. Ours would show not otherwise were they viewed from the impersonal standpoint of space. Very little telescopic aid suffices to disclose the Martian polar phenomena in this their more salient characteristics and convince an observer of their likeness to those of the earth. Any one may note what is there going on by successive observations of the planet with a three-inch glass. Nor is the change by any means slow. A few days at the proper Martian season, or at most a couple of weeks, produces conspicuous and conclusive alterations in the size of these nightcaps of the planet’s winter sleep. Resembling our own so well they were early surmised to be of like constitution and composed, therefore, of ice and snow. Plausible on its face, this view of them was generally adopted and common sense has held to it ever since. It has encountered, of course, opposition, partly from very proper conservatism, but chiefly from that earth-centred philosophy which has doubted most advances since Galileo’s time, and carbonic acid has been put forward by this school of sceptics to take its place. We shall critically examine both objections; the latter first, because a certain physical fact enables us to dispose of it at once. In casual appearance there is not much to choose between the rival candidates of common sense and uncommon subtlety, water and frozen carbonic acid gas, both being suitably white and both going and coming with the temperature. But,