The direct heat of the sun, shining through so thin an atmosphere, must be considerable wherever the sun is at a sufficient elevation; 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. Accordingly, during the greater part of the Martian 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 terrestral observer.