OBSERVATIONS OF LUNAR OBJECTS. 289 



remark : — " The effect of a low power on ^ and e (the east and middle arms 

 of trident) is |to show their southern extremities as bright hazy spots, and to 

 hide their character as lines of light ; a higher power shows the whole Une as 

 a nearly (or quite) uniformly bright streak." 



Fnder this interval and date I have the following memorandum : — " It is 

 a little remarkable that the streaks ft, r}, and, stiU later, y should exhibit 

 such variations as to accord with a decrease of brightness, becoming lost to 

 Mr. Elger, but stiU lingering in the Halifax refractor. With regard to the 

 tint of the floor, observation has established that it becomes darker under a 

 high sun. Three hypotheses maybe suggested in explanation: — First, Will 

 the heating of a bare surface produce a darkening of that surface ? Second, 

 Increase of angle of illumination, we know, occasions a darkening of the 

 vegetable covering of the earth : is it so with the moon ? Third, Can there 

 exist within the mountainous enclosure of Plato absorptive clouds the results 

 of vaporization by long-continued sunshine ?" 



1869, July 23. — Mr. Gledhill recorded the sector as fairly well defined. 



1870, August 11. — Mr. Elger described the markings on the east side of 

 the floor ri and ft as very faint ; the sector and y were not so faint ; the three 

 arms of the trident, ^, e, and e, were plain ; a, with Webb's elbow, were seen, 

 as in April and May last, very distinct. The drawing gives thp elbow and 

 the part of c as forming a sharp angle with a. 



Fig. 19. 

 Kg. 18. 



Plato, 1870, Aug. 11.— T. G. E. Elger. 



Plato, 1870, Oct. 9.— T.G.E. Elger. 



a 



1869, September 20. — Mr. GledhiU recorded the floor as dark,= 0-66, not 

 quite so dark as the south part of Grimaldi ; the sector easily seen. Same 

 evening Mr. Elger registered the N. boundary of the sector as extending 

 from spot No. 4, just past N"o. 7, to the east border. This appears to have 

 been an extension of the sector, including the streak y. For Mr. Elger's 

 drawing of this day, see history of streak a, p. 263, and for remarks, see 

 ante, p. 262. 



1870, October 9. — l^Ir. Elger recorded the sector as plain, »; faint, trident 

 faint. " I again suspect," says Mr. Elger, " the connexion between the 

 eastern and central arms of the trident observed by Mr. Pratt." Mr. Elger 

 gives a sketch of a and Webb's elbow, with c in a line with the elbow, join- 

 ing e, the west arm of the trident. The streak c, from Webb's elbow to the 

 west arm of trident, is curved, the concavity towards the west border (see In- 

 terval 168 to 156 hours, 2505*5 p. 291). Nothing appeared to occupy the area 

 between the border and streak except the plain floor ; the streak " o " en- 

 tirely absent. See intervals 132 to 144 hours, and 168 to meridian, ante, 

 pp. 285, 288). 



Summary.— Sxm's altitude 39° 50'-5 to 40° O'-O ; tint of floor 0-66, 

 estimated from curve. Streaks generally visible — the sector, the middle 

 arm of trident most frequent, the eastern arm next, and the western arm 



1872. X 



