MAES. 15 



planets, having a diameter of but 4085 miles, and being only 

 about one-seventh the magnitude of the earth ; it makes its 

 revolution in 687 days, at a distance of 144,780,000 miles 

 from the sun. 



Owing to the brilliancy and proximity to the sun of Venus 

 and Mercury, together with other causes, no rotation has 



PIG. 13. 



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 



