354 FROM NEBULA TO NEBULA 



plains, these snows are now quiescently restricted to the 

 immediate neighborhood of the ancient pools, accentuat- 

 ing thus the magnitudes and salient features of the lunar 

 sculpturings, more particularly of the craters, at the ex- 

 pense of the inland prairies. 



In elucidating a general principle it is usually much 

 simpler and more satisfactory to choose a concrete ex- 

 ample. Let us picture to ourselves, therefore, a circular 

 cistern twelve feet in depth and a hundred feet in diame- 

 ter. Instead of the walls being vertical, however, let 

 us imagine them sloping downward toward the center of 

 the bottom, but so as to leave a circular flat space, say 

 25 feet in diameter. Now, for the sake of easy reference, 

 conceive of this slope as cut into four steps, each a yard 

 in height. We shall then have a structure resembling 

 the pit of a Eoman amphitheater. Again, let us picture 

 standing on the bottom a number of granite blocks res- 

 pectively 2, 4, 8 and 10 feet in height, to represent natural 

 protuberances. Let us now fill the cistern to the brim 

 with water, thus submerging all the blocks, and start the 

 natural process of evaporation and snowing, limiting at 

 the same time the snowfall radius to, say, a thousand 

 feet ; for we must not overlook the present quiescence of 

 the moon's atmosphere, on account of which the snow 

 spreads but slightly. 



Now, until the water in the cistern has evaporated 

 down to the level of the first tread (beginning from the 

 top), the falling snow cannot gain a footing on the water 

 surface, but must settle only on the brim and a slight dis- 

 tance beyond, its depth gradually decreasing to our pre- 

 scribed snowfall limit. A yard's depth of water, area for 

 area, being equivalent to ten times that heighth of snow, 

 we shall have gained by the evaporation of the first yard 

 a snow rampart all around the brim, say five feet high, 

 sloping gradually backward to nothing. I have just 

 stated that the snow could settle only on the brim, but this 

 is not precisely true, for we must not forget the tallest 

 of the granite blocks, which for one-third of this time has 



