i88 



THE POPULAR SCIENCE MONTHLY. 



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 be- 

 tween the sun and us, so that, when she is nearest, her illuminated 

 hemisphere is toward the sun, and we see only her dark hemisphere 

 ecVed 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. 



Aspect of Mars, with its Cap op Polab Snow. 



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 di- 

 mensions. These regions of ice vary in extent, according to the sea- 

 son 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 char- 

 acteristic dissimilarity between the configuration of Mars and that of 

 the earth. On our planet the seas have greater extent than the con- 



