diameter of Mars; besides, she is nearer to us, her least distance being about 30,000,000 miles. But, one objection is, that Venus revolves between the sun and us, so that, when she is nearest, her illuminated hemisphere is toward the sun, and we see only her dark hemisphere edged by a slight luminous crescent, or, rather, we do not see it at all. Hence it is that the surface of Venus is harder to observe than that of Mars, and hence, too, it is that Mars has the preeminence, and that in the sun's whole family he is the one with which we shall first gain acquaintance.
The geography of Mars has been studied and mapped out. What principally strikes one on studying this planet is that its poles, like those of the earth, are marked by two white zones, two caps of snow, one of which is shown in the cut. Sometimes both of these poles are so bright that they seem to extend beyond the true bounds of the planet. This is owing to that effect of irradiation which makes a white circle appear to us larger than a black circle of the same dimensions. These regions of ice vary in extent, according to the season of the year; they grow in thickness and superficial extent around both poles in the winter, melting again and retreating in the summer. They have a larger extension than our glacial regions, for sometimes they descend as far as Martial latitude 45°, which corresponds with the terrestrial latitude of France.
This first view of Mars shows an analogy with our own planet, in the distribution of climates into frigid, temperate, and torrid zones. The study of its topography will, on the other hand, show a very characteristic dissimilarity between the configuration of Mars and that of the earth. On our planet the seas have greater extent than the con-