A WHEWELLITE ESSAY ON MARS. 1 5 1 



of the coating of hoarfrost (or of light snow, perhaps) 

 from the ruddy soil of the planet and from the frozen 

 surface of his oceans in the forenoon, and the precipi- 

 tation of fresh snow or hoarfrost when evening is 

 approaching. Throughout the day the air remains toler- 

 ably clear, so far as can be judged from the telescopic 

 aspect of the planet, though there is nothing to prevent 

 the occasional accumulation of light cirrus or snow- 

 clouds, especially in the forenoon. I believe, in fact, 

 that the phenomena which have commonly been re- 

 garded as due to the precipitation of rain from true 

 nimbus clouds over Martian oceans and continents 

 must be ascribed to the dissipation of cirrus clouds by 

 solar heat. 



But we must not fall into the mistake of supposing 

 that because the Martian atmosphere is at so low a 

 pressure that Martian barometers (mercurial) probably 

 stand at only four or five inches, the atmosphere is, 

 therefore, exceedingly shallow. Even on our earth an 

 atmosphere producing this amount of pressure would 

 extend many miles above the sea-level, for as a matter 

 of fact we know that at the height of eight or nine 

 miles, only, the atmospheric pressure is thus reduced, 

 and even the lowest estimates assign to the atmosphere 

 a height of fifty miles, or roughly some forty miles 

 above the height where the pressure corresponds to 

 five inches of the common barometer. But in the case 

 of Mars the atmospheric pressure diminishes much 

 more slowly with altitude than on our own earth. We 

 have only to climb to a height of three-and-a-half miles 



