A WHEWELLITE ESSAY ON MAES. 149 



the sun of Mars, though powerless to raise great quan- 

 tities of vapour into the planet's tenuous atmosphere, 

 is perfectly competent to melt and vaporize this thin 

 coating of snow or hoarfrost. The direct heat of the 

 sun, shining through so thin an atmosphere, must be 

 considerable wherever the sun is at a sufficient eleva- 

 tion ; and of course the very tenuity of the air renders 

 vaporization so much the easier, for the boiling-point 

 (and consequently all temperatures of evaporation at 

 given rates) would be correspondingly lowered.* Accord- 

 ingly, during the greater part of the Martial day the 

 hoar frost and whatever light snow might have fallen 

 on the preceding evening would be completely dissolved 

 away, and thus the ruddy earth or the greenish ice- 

 masses of the so-called oceans would be revealed to 

 the terrestrial observer. We may picture the result by 

 conceiving one of those Martial globes which Captain 

 Busk has recently caused Messrs. Malby to make from 

 my charts, to be first coated with thin hoarfrost, and 

 then held before a fire just long enough to melt the 

 hoarfrost on the part of the globe nearest to the fire, 



* Amongst other disadvantages presented by Mars, regarded as an 

 abode for beings like ourselves, is the circumstance that if his atmo- 

 sphere be in proportion to his mass, as we have assumed, it must be 

 impossible to boil food properly on the ruddy planet. For water would 

 boil at a temperature about seventy degrees below our boiling point, 

 so that it would barely be heated enough to parboil. A cup of good 

 tea is an impossibility in Mars, and equally out of the question is a well- 

 boiled potato. It does not make matters more pleasant that the tea- 

 plant and the potato are impossible, of themselves, on Mars, and that 

 therefore the possibility of boiling them may be regarded as a second- 

 ary consideration. 



