A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 59 



the point where they are distinctly gaping by slender streaks of shining material, 

 which appear, from his description, to be like the rays. It may be that vapors 

 ascending through open clefts would not be sufficiently concentrated to produce 

 a distinct band of color on their margins, while such would be the. case when 

 they mounted through an incipient fissure, as I have supposed to be the fact with 

 the radiating streaks. 



In considering the succession of the ray systems, it should be noted that, 

 beside those which are definitely to be observed, there are evidently others in 

 part destroyed by later developed groups. In the best conditions of seeing, these 

 faintly indicated and evidently ancient sets of rays may be seen in all stages 

 of obsolescence, down to the state where they are conjectured rather than 

 observed. This, together with the phenomena of interference of one set of rays 

 with another, suggests that the process of their formation may have been continued 

 for a very considerable time, though the development of the larger bands appears 

 to have been brought about only in the later state of the surface, yet, as remarked 

 above, not to the very latest time of activity. 



It is evident that the distribution of the several ray systems is not equal on 

 all parts of the moon. Thus the first quadrant has thirteen recognized groups, 

 while the fourth, just south of it, has but six. The second quadrant has eleven 

 and the third eight. Thus the eastern and western halves of the surface together 

 have the same number, but the northern hemisphere has twenty-four and the 

 southern fourteen systems of rays. Moreover, the greater number of the groups 

 are situated on that half of the visible surface wherein lie by far the greater part 

 of the maria, and on the surfaces of those lava fields none of the distinct centers of 

 radiation are found. This predominance of the rays in the regions of high country 

 near the maria may possibly be due to the extensive heating of the northern half 

 of the moon by the lavas which formed them, and to the consequent refrigeration 

 which would tend to develop crevices and thus lead to the production of rays.' 



THE PRESERVATION OF THE RAY SYSTEM.S. 



The facts already set forth clearly show that the ray systems are fairly to be 

 regarded as features which have been somewhat gradually developed, and are, as 

 a whole, of ancient origin. It is, indeed, difficult to escape the conclusion that 

 they are, when measured in terms of geological ages, all exceedingly old. They 



' In the earlier years of my work on the moon, the results of which are here set forth, I noted 

 certain very faint rays which aijpeared to point to centers of radiation on the unseen side of the 

 moon. I have been unable to find the note-book in which these observations were recorded, and 

 my eyes, damaged by studies on that brilliant surface, no longer enable me to trace them. Accord- 

 ing to my memory, these streaks, as are all others near the limb, were faintly though distinctly 

 traceable, in the course of some years' observation, to the number of about a score, indicating 

 about half a dozen such invisible centers. The impression left upon my mind is that the very best 

 vision and opportunity might prove the existence of at least a dozen of these groups where the rays 

 converged to a point in the invisible field. The studies needed to determine this matter will 

 be difficult to make, for the reason that all these rays are faint in the regions near the limb. 



