POLAR SPOTS ON MARS 173 



ascribed to their being exposed to the sun. In the 

 year 1781 the south polar spot was extremely large, 

 which we might well expect, since that pole had but 

 lately been involved in a whole twelvemonth's dark- 

 ness and absence of the sun; but in 1783 I found it 

 considerably smaller than before, and it decreased 

 continually from the 20th of May till about the 

 middle of September, when it seemed to be at a 

 stand. During this last period the south pole had 

 already been above eight months enjoying the benefit 

 of summer, and still continued to receive the sunbeams ; 

 though, towards the latter end, in such an oblique 

 direction as to be but little benefited by them. On 

 the other hand, in the year 1781, the north polar spot, 

 which had then been its twelvemonth in the sunshine, 

 and was but lately returning to darkness, appeared 

 small, though undoubtedly increasing in size." The 

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

 and the distance from us varies from about 230 to 50 

 millions of miles. 



Astronomers, previous to Herschel's time, had found 

 that Mars was surrounded by an atmosphere like the 

 earth. One of them, Cassini, seems to have suspected 

 the existence of an atmosphere of great density, and 

 rising to a height of about 70,000 miles above the 

 planet's surface. 1 Herschel used the same means as 

 Cassini to determine the height of the atmosphere of 

 Mars by watching the fading or going out of starlight, 

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

 30,000 miles there was no indication of an atmosphere. 

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



1 Thirty-six semi-diameters of the planet. The atmosphere of the 

 earth is now supposed to be about 500 miles in height. 



