112 GENEEAL DESCRIPTION OF THE MOON. 



Perhaps the most puzzling feature of the radiating streaks, where 

 everything is perplexing, is found in the way the}" come into view and 

 disappear in each lunar period. When the surface is illuminated by 

 the very oblique rays of the sun they are quite invisible; as the lunar 

 dav advances they become faintly discernible, but are onl}" seen in 

 perfect clearness near the full moon. The reason for this peculiar 

 appearance of these light bands under a high sun has been a matter 

 of much conjecture; it is the subject of di.scussion in a later chapter 

 of this memoir, where it is shown that inasmuch as these bands appear 

 when the earth light falls upon the moon at a high angle, the effect 

 must be due to the angle of incidence of the rays on the shining sur- 

 faces. It should be noted that the light bands in most instances 

 diverge from more or less broad fields of light color about the crater- 

 like pits, fields which have the same habit of glowing under a high 

 illumination; in fact, a large part of the surface of the moon, perhaps 

 near one-tenth of its visible area, becomes thus relatively brilliant at 

 full moon, though it lacks that quality at the earlier and later stages 

 of the lunar day. 



In the above-considered statement concerning the visible phenomena 

 of the moon no account is taken of a great variety of obscure features 

 which, though easily seen with fairly good instruments, have received 

 slight attention from selenographers. As can readily be imagined, 

 observers find it difficult to discern dindy seen features which can not 

 be classed in any group of terrestrial objects. Whosoever will nar- 

 rowly inspect any part of the lunar surface, noting everything that 

 meets his eye, will find that he observes much that can not be explained 

 by what is seen on the earth. It is evident, indeed, that while in the 

 earlier stages of development this satellite in good part followed the 

 series of changes undergone by its planet there came a stage in which 

 it ceased to continue the process of evolution that the parent body has 

 undergone. The reason for this arrest in development appears to have 

 been the essential if not complete absence of an atmosphere and of 

 water. 



The difference in height between the lowest and highest points on 

 the lunar surface is not determined. To the most accented reliefs, 

 those of the higher crater walls, elevations of more than 25,000 feet 

 have been assigned; it is, however, to be noted that all these deter- 

 minations are made from the length of the shadows cast by the emi- 

 nences, with no effective means of correcting for certain errors 

 incidental to this method. It ma}" be assumed as tolerably certain 

 that a number of these elevations have their summits at least 20,000 

 feet above their bases, and that a few are yet higher. We do not 

 know how much lower than the ground about these elevations are the 

 lowest parts of the moon. My own observations incline me to the 



