114 FROM NEBULA TO NEBULA 



in this ease the "currents" are gravitational instead of 

 atmospheric in their nature. 



PROBLEM OF THE LUNAR SURFACE 



Generally speaking, the moon has a mottled appear- 

 ance, consisting of dark splotches on a silver shield. In 

 the telescope the brighter part is seen to be exceedingly 

 irregular and, as might be expected, contains the most 

 interesting objects. The dark spots, before the day of 

 the telescope, were called seas, and are still so named 

 notwithstanding that they have ceased to be looked 

 upon as such. These cover about two thirds of the en- 

 tire disk, and present in a sort of bas-relief the same 

 characteristic formations of the brighter surface. Were 

 it not for this latter peculiarity, they would probably 

 still be regarded as seas, in spite of other considerations 

 to the contrary. 



SPECIAL FEATURES 



1. CRATERS. These are ordinarily likened to a 

 saucer in shape, but misleadingly, doubtless from ignor- 

 ance of the interpretative significance of the detail. 

 They more nearly resemble a pie-tin, being practically 

 flat on the bottom and having steep walls extending all 

 around, in some cases, but usually exhibiting one or 

 more gaps of greater or less extent. The resemblance 

 is not complete, however, inasmuch as the walls are 

 terraced. The outer slopes of the containing walls are 

 not only much less steep, but invariably the vertical 

 descent on the inner side of the wall is greater than 

 that measured from the same summit to the average level 

 of the plane without. This peculiarity is in some cases 

 so accentuated as to inspire the conviction that the 

 mouth of the crater does not stop at the surface but 

 extends deep doirn into it. 



