Owing to the brilliancy and proximity to the sun of Venus and Mercury, together with other causes, no rotation has been observed in them, but as this exists in all the other planets there is no doubt they also rotate, but the nearness of Mars to the earth when in opposition (that is, when the earth is between it and the sun), has caused its rotation to be distinctly visible; the observation of this has been from time to time greatly favoured by certain dark spots which have remained stationary a sufficient time to determine the question very accurately. Mars takes 24 hours and 37 minutes to turn on its axis, or pretty nearly the time the earth does. Its resemblance is still more increased by the axis of rotation being oblique, from all which it is inferred that there is a day and night, winter and summer, and variation of climate very nearly resembling that of our own world, and there are round bright parts situated at the poles of Mars, which enlarge when it is winter there and diminish when summer, just as would the snows of arctic regions, and these are therefore supposed to be portions of the surface of Mars which are actually covered with snow (fig. 13). As to the question of the planets being inhabited, of course it can never be answered with certainty, but it is a great deal more likely that they are than that the moon is, which, having neither water nor atmosphere, can hardly be supposed to give habitation to any beings similar or analogous to those on earth, while Mars possesses a climate not greatly differing from that of the earth, and has both air and water. Mars has, at certain positions of the earth with respect to it, a partially "gibbous" form, that is, a small portion of the non-illuminated part comes within the lines of our vision it is then of the form seen in (fig. 14), but this can only occur when the earth and planet occupy positions somewhat near to that represented in fig. 15, in which S is the sun, E E opposite positions of the earth, and M Mars. [Illustration: FIG. 14.] [Illustration: FIG. 15.] [Illustration: FIG. 16.] [Illustration: FIG. 17.]