25. We may now return towards the Sun, and direct our attention to the planet Mars. Here we have some approximation to the condition of the Earth, in circumstances, as in position. It is true, his light and heat, so far as distance from the Sun affects them, are less than half those at the Earth. His density appears to be nearly equal to that of the Earth, but his mass is so much smaller, that gravity at his surface is only one-half of what it is here. Then, as to his physical condition, so far as we can determine it, astronomers discern in his face[11] the outlines of continents and seas. The ruddy color by which he is distinguished, the red and fiery aspect which he presents, arise, they think, from the color of the land, while the seas appear greenish. Clouds often seem to intercept the astronomer's view of the globe, which with its continents and oceans thus revolves under his eye; and that there is an atmosphere on which such clouds may float, appears to be further proved, by brilliant white spots at the poles of the planet, which are conjectured to be snow; for they disappear when they have been long exposed to the sun, and are greatest when just emerging from the long night of their polar winter; the snow-line then extending to about six degrees (reckoned upon the meridian of the planet) from the pole. Moreover, Mars agrees with the earth, in the period of his rotation; which is about 24 hours; and in having his axis inclined to his orbit, so as to produce a cycle of long and short days and nights, a return of summer and winter, in every revolution of the planet.