A WHEWELL1TE ESSAY ON MARS. 141 



remember that lie has polar snow-caps ; but we are no 

 longer bound to regard these snow-covered regions as 

 in any sense resembling our arctic regions. Again, 

 the seas and oceans of Mars may be permanently frozen 

 thoughout the greater part of their depth. The water- 

 vapour which is certainly present in his atmosphere 

 may be raised only by the midday sun, to be precipi- 

 tated in early evening. Winds and currents may 

 equally well prevail in a rare as in a dense atmosphere* 

 The white masses which have been compared to clouds, 

 and whose dissipation has been held to imply the 

 downfall of rain on Mars, may not be rain-clouds, but 

 snow-clouds ; or, where there is no downfall, they may 

 be not cumulus-clouds, but cirrus-clouds, that is, not 

 such clouds as are raised in our dense air near the 

 sea-level by the sun's warmth, but such light fleecy 

 clouds as are suspended high above the loftiest moun- 

 tain summits. 



It appears to us, indeed, that if we make any change 

 at all in our views about Mars, we must make a great 

 change. If we suppose the Martian air moderately 

 dense, comparable in density at any rate with our own 

 air, then since we know that considerable quantities of 

 aqueous vapour are raised into that air, we seem 

 compelled to conclude that there would be a precipita- 

 tion of snow (under the circumstancces already con- 

 sidered) which should keep the surface of Mars as 

 permanently snow-covered as our mountain-heights 

 above the snow-line. As this is not the case, for Mars 

 is not a white planet, we must assume so great a rarity 



