398 FROM NEBULA TO NEBULA 



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 (40 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 

 crater, let us say, is directly open to view, and then the 

 pool ought to appear at its largest and blackest, were it 

 not that just then the snow cloud is probably at its den- 

 sest. With the passing of the meridian, the eastern wall 

 is now being shone upon by the afternoon sun, and inas- 

 much as the rays start this time with the vertical instead 

 of with a slant, the eastern melting proceeds much more 

 rapidly at the outset, increasing, say, till 48 hours past 

 noon of the lunar day, and thereafter fading to the mini- 

 mum at sunset. During this half of the process we see 

 the original pool now in cold shadow lose its black as- 

 pect and freeze over and whiten with a skim of snow, 

 while, diametrically opposite on the crater floor, another 

 pool forms from the draining of thaw water off the east- 

 ern slope, giving off fresh clouds of vapor which, being 

 immediately transformed into flakes, settles wherever it 

 can. I use the word immediately advisedly, for Arctic ex- 

 plorers tell us that in those regions the transition of at- 

 mospheric vapor into snow is accomplished directly and 

 without the formality of clouds. Commenting on this 

 phenomenon, Prof. W. B. Wright (The Quaternary Ice 

 Age, p. 19) says: 



All the explorers who have persisted into the interior (of 

 the Arctic and Antarctic regions) are agreed as to the fine-grained 

 character of the snow which falls there. Among the most inter- 

 esting of the observations made by Nansen are those bearing on 

 the origin of this snow. He calls attention to the great clearness 

 of the atmosphere during nearly the whole of the time occupied 

 by his journey. On only a few days was the sky overcast, and 



