In its days and seasons, then, Mars is wonderfully like the Earth; except for the length of the year we should hardly know the difference in reckoning of time could we some morning wake up there instead of here. Only in one really unimportant respect should we feel strange; in months we should find ourselves turned topsy-turvy. But lunations have nothing to do with climate nor with the alternation between night and day; and in these two important respects we should certainly feel at home. Though the axis could be determined by the daily march of any marking and thus the planet’s tropic, temperate and polar regions marked out, the process is made easier by the presence of white patches covering the planet’s poles and known, in consequence, as the polar caps. It is from measures of the patches that the position of the Martian poles has actually been determined. These polar caps are exactly analogous in general position to those which bonnet our own Earth. They reproduce the appearance of the ice and snow of our arctic and antarctic regions seen from space, in a very remarkable manner. In truth they are things of note in more ways than one and would claim precedence on many counts. Priority of recognition, however, alone entitles them to premier consideration. Among the very first of the disk’s detail to be made out by man, they justly demand description first. With peculiar propriety the polar caps have thus the _pas_. Not only do they stand first in order of visibility, but they prove to occupy a like position logically when it comes to an explanation of the planet’s present physical state. It is not matter of hazard that the most evident of all the planet’s markings should also be the most fundamental, the fountainhead from which everything else flows. It is of the essence of the planet’s condition and furnishes the key to its comprehension. The steps leading to this conclusion are as interesting as they are cogent. They start at the polar caps’ visibility. For their size first riveted man’s attention and then attention to them disclosed that most vital of the characteristics of the planet’s surface: change.