THE MOON 355 



been exposed. It, too, is capped with snow, say to the 

 depth of two feet. " 



Now suppose another equal period to elapse, and the 

 cistern's depth reduced to the level of the second tread. 

 This time the brim's cap has increased by another five 

 feet in heighth, and the slope behind it in slightly less pro- 

 portion, because the rising snow brim has in a measure 

 further blocked the spread of the snow. Again, the first 

 tread having all this time been clear of the water, it, too, 

 is covered, and to a depth of five feet ; the tallest granite 

 island has also gained six more feet in height; and the 

 second one of eight feet, having been above water two- 

 thirds of the period, disports a snow cap of four feet. 

 Thus the process goes on till we reach the circular area at 

 the bottom. In this area we have finally four islands 

 standing in a shallow pool of water less than a yard deep. 

 But note that the first block uncovered is not the tallest 

 now, for its pinnacle being, let us say, only two feet 

 square, lacks the area necessary to sustain a taller col- 

 umn. As a matter of fact the third block has much the 

 broadest top of any, and though still shorter than either 

 the first or second, must in time, as the result of future 

 processes of evaporation and snowfall, grow to a great 

 height; yet, being itself limited in area also, it can per- 

 haps never reach the altitude of the encircling rampart, 

 which by now, let us say, has risen to the imposing alti- 

 tude of possibly thirty feet, and will continue rising, 

 given the necessary supply of snow, until it telescopes 

 upon itself by its own weight. But to return to the moon : 



Naturally, the melted water that manages to seep 

 back into the basins must, while the sun is still high, im- 

 mediately undergo again the process of evaporation. Ow- 

 ing, however, to the frigidity of the planet's quiescent at- 

 mosphere, as well as to its own self -built prison walls, the 

 rising mist changes into snow before it escapes from the 

 caverns, in which condition it is, of course, undetectable 

 as vapor. Some of this snow doubtless spreads promis- 

 cuously over the planet's surface, but a major share of it 

 remains to restore and heighten the old walls. This pil- 



