POLAR SPOTS ON MARS 



73 



ascribed to their being exposed to the ran. In the 

 year 17si th. south polar spot was extremely Urge, 

 which we might well expect, since that pole had but 

 ly UMMI involved in a whole twelvemonth's dark- 

 and absence of the sun. foun-l it 



ihly HinalliT than before, and it decreased 

 lually from the 20th of May till about the 

 lie of September, when it seemed to be at a 

 Din mi: this last period the south pole had 

 ly been above ei >ths enj- '10 benefit 



i timer, and still continued to receive the sunbeams ; 

 though, towards i> 1 in such an o!.li.|u.- 



lut Intl, benefited by them 



the other hai> \ .u 1 lar spot, 



h had then been its twelvemonth in tin- sunvhin. , 

 and waa but lately i > 'l;irkn*8s, appeared 



though undoubtedly increasing in size." The 

 of the year in Mars is nearly two of our years, 

 the distance from us varies from about 230 to 50 

 of miles. 



previous to Herachel's time, had found 

 Man was surrounded by an atmosphere lik 



One of them, Cassini, seems to have suspected 

 existence of an atmosphere of great density, and 

 rising to a h< iu'ht of about 70,000 miles abov. 



sin EHhd UM.I tli.- .same means as 



>ini to determine the height < t the atmosphere of 

 by watching the fading or going out of starl 

 a star came up to its limb. At a distance of 

 1 miles there was no indication of an atmosphere. 

 It appears, however, that the planet is not without a 



.;- HM 



of th pUnet The 

 U now anppOMd to be about 500 mile* in height 



