"In gazing at our fellow-planets on a clear night, as we see them stand out with pre-eminent brightness among the twinkling stars, who has not longed to penetrate the mystery of their being, and to know whether they, like our own Earth, are worlds full of life and movement? The vast distance that intervenes between us forbids us to expect a direct solution of the question, for no instrument yet made, or that we can hope to make, will bring their possible inhabitants within the range of our vision. We are reduced, therefore, to survey them with the sifting force of intellect, and to rest contented with such circumstantial proof as is derived from a knowledge of their general structure, and the analogies subsisting between them and our Earth. "Among our nearest neighbors, _Venus_ is nearly the size of our Earth; and _Mercury_ and _Mars_, though considerably smaller, would still form worlds which, to our ideas, would not in their magnitude be so very different from our own. As before remarked, all the planets revolve in elliptical orbits round the Sun, and the time consumed in their journey constitutes their year. Their polar axis is not 'straight up and down,' but leans over or is inclined to the plane of their orbit, so that each pole is turned toward the Sun at one period of the year, and away from it at another. This arrangement insures the regular alternation of seasons and a variety of climates on their surface. The orbital inclination of _Mars_, for example, is much the same as that of the Earth, and, therefore, the relative proportion of his seasons must have a close resemblance to our own. It might be expected under these circumstances that ice would accumulate toward the poles in winter time, as on our Earth, and accordingly glacial accumulations have not only been observed by Astronomers, but it has been remarked that they occasionally diminish by melting during the heats of summer, while they increase in winter. Now as the planets, like the Earth, turn round on their axis with perfect regularity--and those just mentioned do so in very similar periods of time, hence, all have their days and nights. "We have already stated that the Earth and its fellow-planets are kept steadily in their orbits by the exact adjustment of _centrifugal_ and _centripetal_ forces. Hence each moves in its regular order.