OBSERVATIONS Or LUNAR OBJECTS. 91 



much hrigTiter relativehj to their best state thaa were the spots, of •which 

 generally at those times lew have been discernible." 



1870, May 13. Mr. Pratt has not only specified the order of brightness as 

 follows : — 



which we can compare with the degree of visibility for the 18 lunations as 

 given immediately under the number of each spot (trom this comparison we 

 see that the brightness ou May 13 was not strictly accordant with the 

 visibility), but he has described the character of visibility by the words easy, 

 conspicuous, &c., thus forming with the spots 7iot seen eight classes of objectSj 

 au analysis of which may be interesting. 



Class I. contains one spot only, Jfo. 1, deg. of -ds. = 1-000. 



Pratt. Exceedingly bright and dense. 



Elgcr. Unusually bright. 



Gledhill. Bright spot. 

 Class II. contains one spot only, No. 4, deg. of vis. = •892. 



Pratt. Bright but hazy. 



Elger. No remark. 



Gledhill. S^Dot. 

 Class III. contains one sjjot only, No. 3, deg. of vis. = •897. 



Pratt. Distinct ; he inserts 5 between 3 and 17. 



Elger. 3 and 17 nearly equal. 



Gledhill. Bright spot. 



Mr. Pratt observed the three components of the group 3, 30, 31 : he 

 described 30 and 31 as steadily seen ; they occur in Class VI. Mr. Pratt 

 accorded to spot No. 22 a high degree of brightness on this evening, and 

 described it as " conspicuous : " neither Mr. Elger nor Mr. Gledhill detected 

 it ; this doubtless depended upon the state of our own atmosphere. It may, 

 however, be remarked that the spot was less visible on May 13, 1870, as 

 compared with its visibility in August 1869, when it was seen by every 

 observer. 



The position of spot No. 5, as observed by Mr. Pratt on August 26, 1869, 

 was on the ivest border of the eastern arm of the " trident." The spot No 5, 

 discovered by Challis, and possessing a normal visibility of -510, has been so 

 frequently observed as almost to warrant its stability of position ; and should 

 its relative position, as regards the eastern arm of the trident, be found to 

 vary, it will afford evidence of a probable variation in the position of the 

 arm. Schroter's drawings of the Mare Crisium indicate similar movements 

 of the streaks from Proclus over the Mare, 



