158 THE POPULAR SCIENCE MONTHLY. 



NEW LIGHT ON A LUNAR MYSTERY. 



By GAEEETT P. SEEVISS. 



EVERY possessor of a telescope knows that among the mount- 

 ains of the moon there are some to which the name of 

 " shining mountains " seems peculiarly applicable. The most cele- 

 brated of these is the huge extinct volcano Aristarchus, the 

 slopes of whose crater possess such extraordinary reflective power 

 that it is visible on the night-side of the moon by virtue of the 

 comparatively faint light received from the earth. Another fa- 

 mous bright mountain on the moon is Proclus, which rears its 

 crest high above the eastern shore of the so-called Crisian Sea. 

 With a telescope I have seen Proclus glittering above the brown- 

 ish plains surrounding it^ in the middle of a summer afternoon, 

 when, to the naked eye, the moon appeared as a faint silvery disk, 

 half blended with the blue of the sky. There are other natural 

 features of the moon's surface which shine with extraordinary 

 brightness, the most conspicuous being the systems of long " rays," 

 radiating from such crater-rings as Tycho and Copernicus. 



But in addition to these long-known and easily recognized ob- 

 jects, there have occasionally been seen upon the moon certain 

 bright points, which are even more curious and mysterious than 

 the shining mountains. The earliest observation of this kind ap- 

 pears to have been made by Herschel in 1783. It was repeated by 

 him in 1787, when he did not hesitate to report, in a communica- 

 tion to the Royal Society, that he had discovered three lunar vol- 

 canoes in a state of eruption. Astronomers have been consider- 

 ably puzzled ever since to account for Herschel's statement. 

 Nobody could question the accuracy of his observation, so far as 

 the power of his telescope enabled him to carry it. At the same 

 time, few, if any, especially in recent times, were willing to ad- 

 mit that that prince of telescopists had really seen volcanoes in 

 action upon the moon. The complete absence of any evidence that 

 volcanic activity did not cease upon our satellite ages upon ages 

 ago militated too strongly against Herschel's assertion. The gen- 

 eral conclusion finally was, that Herschel had been misled by the 

 extraordinary brightness of some of the shining mountains which 

 I have just described. It remained almost the only serious blot 

 upon Herschel's record as an observer. He had described the ap- 

 pearance of the supposed eruption too carefully to admit any 

 question as to his meaning. And yet, it seemed, a mere tyro in 

 astronomical observation could hardly be deceived in such a man- 

 ner, much less the most famous astronomer of his time. 



But just now new light has been thrown upon the mystery. 



