THE PAST HISTORY OF OUR MOON. 97 



atmosphere, so that on the moon's unprotected globe with 

 its surface one thirteenth of the earth's about 30 millions 

 fall each day, even at the present time. Of large meteoric 

 masses only a few hundreds fall each year on the earth, and 

 ] erhaps about a hundred on the moon ; but still, even at the 

 present rate of downfall, millions of large masses must have 

 fallen on the moon during the time when her surface was 

 plastic, while probably a much larger number including 

 many much larger masses must have fallen during that 

 period. Thus, not only without straining probabilities, but 

 by taking only the most probable assumptions as to the 

 past, we have arrived at a result which compels us to 

 believe that the moon's surface has been very much marked 

 by meteoric downfall, while it renders it by no means un- 

 likely that a large proportion of the markings so left would 

 be discernible under telescopic scrutiny. 



I would, in conclusion, invite those who have the requisite 

 leisure to a careful study of the distribution of various orders 

 of lunar marking. It would be well if the moon's surface 

 were isographically charted, and the distribution of the seas, 

 mountain-ranges, and craters of different dimensions and 

 character, of rills, radiating streaks, bright and dark regions, 

 and so on, carefully compared inter se, with the object of 

 determining whether the different parts of the moon's sur- 

 face were probably brought to their present condition during 

 earlier or later periods, and of interpreting also the signifi- 

 cance of the moon's characteristic peculiarities. In this 

 department of astronomy, as in some others, the effective- 

 ness of well-devised processes of charting has been hitherto 

 overlooked. 



