THE MOON 333 



gerated and believes that the apparent length is due to 

 lines of small craters from which they emerge. The 

 most remarkable system is that starting from the crater 

 Tycho, itself of a brilliant whiteness, and giving the 

 whole region the appearance of a globe cracked by inter- 

 nal pressure a suggestion made by Nasmyth, who 

 actually cracked a glass globe in this way and obtained 

 a striking resemblance". 



SEAS. Of these the same writer says, "The so-called 

 seas of the moon are simply portions of the surface 

 darker in color than the average and very much less 

 broken up by craters or mountain ranges. These form 

 the main features of the face seen at full moon. They 

 are crossed by thin lines known as rills or clefts which 

 run in all directions, sometimes straight and unbroken 

 for hundreds of miles, even intersecting ranges of moun- 

 tains and craters and reappearing on the other side. 

 These rills are generally two and rarely exceed ten miles 

 in width, their depth varying from 100 yards up. A curi- 

 ous feature of a different kind is an absolutely straight 

 cut the great Alpine valley some 83 miles long, which 

 crosses a range of mountains and under low magnifica- 

 tion looks as if some wandering celestial body had grazed 

 the surface". 



MOUNTAINS. These are described in the classical 

 work of Nasmyth and Carpenter (The Moon, Chap. X) 

 in these words, which I would ask the reader to study 

 carefully in preparation for the explanation presently to 

 be offered: 



In turning our attention to these features [mountain ranges, 

 peaks, and hillchains] we are at the outset struck with the paucity 

 on the lunar surface of extensive mountain systems as compared 

 with its richness in respect of crateral formations, and a field of 

 speculation is opened by the recognition of the remarkable con- 

 trast which the moon thus presents to the earth, where mountain 

 ranges are the rule and craters like the lunar ones are decidedly 

 exceptional. Another conspicuous but inexplicable fact is that 

 the most important ranges upon the moon occur in the northern 

 half of the visible hemisphere where the craters are fewest and 

 the comparatively featureless districts termed seas are found. The 

 finest range is that named after our Appennines. It extends for 



