MARS IN HIS ICY CUIRASS 105 



late in the season, and then only at the equator where 

 the crust is thinnest. So cold must be the planet that 

 the snn would be powerless to cause evaporation were 

 it not for the accidental circumstance that, on account 

 of the diminished surface gravity and lower atmospheric 

 pressure, the boiling point of water there is reduced 

 from our 212 F. to about 111, and the thawing point 

 correspondingly. 



The process of evaporation takes place from the 

 surface of this glacial crust, or rather from the surface 

 of the thaw-sheet submerging that crust,* during the 

 daylight hours of Mars' protracted northern spring 

 of 199 days and his scarcely shorter summer of 183 

 days. The nights are relatively cold, and during 

 them much snow falls upon the thaw-softened surface, 

 thus giving birth in the course of time to a vast litter 

 of disintegrated ice, extending far into the high lati- 

 tudes. 



When this condition reaches a maximum, this super- 

 ficial sheet of thaw-water is of itself sufficiently deep (or 

 perhaps is reinforced by the subjacent waters breaking 

 through the softened crust at or near the equator) to 

 reflect in its movements the tidal effect of the sun. 

 War ex of wide scope arc formed, which progress with 

 a high degree of regularity over the smooth ice-cnist- 

 bottom sweeping the glacial detritus before them until, 

 having reached the limit of their range, they deposit 

 their cargoes in long, regular, geometrical lines, which 

 then form embryonic ridges, or dams, for future waves 

 and new cargoes to respect and add to, until in the 

 end the accumulations grow to a WIDTH capable of 

 arresting our attention even over the abyss of space. 



*Of course no evaporation can take place from the crystalline 

 surface of ice or snow; but when a pool is formed over a substratum 

 of ice, there is no bar, other thing's being favorable, to evaporation 

 taking place from the surface of the pool itself. 



