392 FROM NEBULA TO NEBULA 



they are, are yet beyond the range of our telescopes to 

 reveal, and rest solely upon deduction. Taken in connec- 

 tion with the continuous repair work being performed by 

 the ever-rising snow, they suffice to keep the lunar struc- 

 tures looking remarkably clean-cut and permanent, a cir- 

 cumstance which explains the absence of signs of "weath- 

 ering" on the moon. 



When the process of glaciation originally began, the 

 lunar atmosphere was, of course, very much denser than 

 it is now, because the snow-that-was-to-be had not yet ab- 

 sorbed it. As a consequence, the meteorological condi- 

 tions at the outset were much different from now, for 

 whereas the winds doubtless then played a strong role 

 in scattering the snows far and wide over the continental 

 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. 



