THE MOON 359 



GENERAL REFLECTIONS 



LUNAB SURFACE CHANGES. The author of all these is, 

 of course, the sun. At midnight on the moon what little 

 air there is should be absolutely dry, all of its moisture 

 having been frozen out of it and deposited upon the sur- 

 face in the form of frost and snow. On the deepest beds 

 of the sea this coverlet would be but a mere sprinkle, 

 having fallen after the descent of night as the last act in 

 the drama of the day. 



Now, on the moon, owing to the exceedingly light 

 atmospheric pressure, water boils at a much lower tem- 

 perature than here, let us say, at 32 F., though this can 

 scarcely be more than a close guess. For this reason the 

 thawing process begins early immediately with, or prob- 

 ably an hour before, dawn so that (as there is no light 

 refraction there) on the very lowest places such as the 

 maria floors, where it lies at its lightest, the snow may 

 be effectually flooded over before observers can even 

 glimpse its presence. Granting the reasonableness of this 

 inference, it is easy to understand why areas seen to be 

 just whitening as they pass into the night may at their 

 succeeding dawn appear to have lost their whiteness from 

 some nocturnal cause, instead of from the freshets caused 

 by the returning day. 



As the sun rises on the lunar landscape, he first 

 shines upon the western sides of the hills ; the eastern re- 

 maining in shadow, illuminated only by the reflection 

 from the slopes opposite. Wherever his rays impinge, 

 there the process of thawing goes on and the water of 

 thaw naturally flows down the icy slope or seeps down 

 through the porous snow, until it reaches the pool at the 

 bottom. Once there, the water is warmed to a point of 

 boiling (32 F., remember, on the moon), and, rising into 

 the thin atmosphere, is quickly reconverted into snow 

 flurries, which to us look to be clouds of vapor and which 

 selenographers have heretofore been construing as 

 "noxious gases from the moon's volcanic vents." 



All this, let it be understood, occurs in the morning 

 of the lunar day. At high noon, the whole bottom of the 



