120 FROM NEBULA TO NEBULA 



plete. Perhaps if the day and night were now each a 

 month instead of only a fortnight long, the sun could 

 entirely thaw the snow every recurring day; but the 

 potent fact is that it does not so suffice, and that the 

 thaw never succeeds in baring any part of the natural 

 surface. Hence now, despite the daily tliaws y no notice- 

 able clouds of vapor can arise, because the universal 

 snow absorbs its own liquefactions as soon as formed, 

 like a sponge, and keeps them chilled below the point of 

 evaporation. However, we have here to do with no com- 

 mon terrestrial thaw that is quickly checked by an 

 early nightfall, but with one whereby perhaps a depth 

 of 10 feet of snow is made to disappear between dawn 

 and sundown. Most of this melting is doubtless drunk 

 up and retained, but a good deal of it manages, to seep 

 down through the snow blanket into the original chan- 

 nels and basins , thus periodically flushing them and 

 keeping them clear of snow accumulations. In this 

 way the snow is prevented from reducing the whole 

 prospect to a dead level, but, on the direct contrary, 

 it greatly accentuates the asperities by adding the depth 

 of the empty basins to that of the snow mountains 

 risen genii-like out of them. Hence we have the ex- 

 planation of the cavernous effect so conspicuous in the 

 deeper craters. 



The general flatness of the moon's true surface pre 

 dudes such great glacial movements as our earth once 

 experienced ; but snow-slides of considerable size are 

 nevertheless liable to occur and thereby obliterate 

 prominent features. This is doubtless the explanation 

 of the curious transformations of the great crater Lin- 

 naeus, whose vicissitudes have been the subject of so 

 many controversies between selenologists. 



Lands nearly level with the sea are always exten- 

 sively diversified by shallow, rough-bottomed ponds and 

 straits, and deep snows resting upon such a base should 



