OBSERVATIONS OF LUNAK OBJECTS. 273 



nearest the western border, was darker than the eastern ; and there, where 

 the sun's rays were more obstructed than further east, the peculiar appearance 

 of something reflecting the stronger light from tlie brighter border above the 

 surface was not seen. The most expressive description that I can give, after 

 twenty-four hours' consideration, is, in'the words of Schroter, " a kind of fer- 

 mentation." It is certainly very unusual for the clearness of objects near the 

 terminator to be interfered with ; but should there have been *' vapours " in 

 motion, catching momentai'ily the reflected rays and, as the sun rose higher, 

 the direct rays over the mountain-border, such an appearance as I witnessed 

 must have been produced ; and the presence of such vapours may occasion 

 the darker tint of the floor, and especially the indistinctness of the boundary 

 of the streaks of sunlight and the edges of the shadows. I never before 

 observed the floor of Plato to be so dark ; but I have seen it once only 

 under similar circumstances, except that of aperture. 



Interval 24 to 36 hours. 

 1870, May 9. — Mr. Elger's record is as follows: — "Markings not well 

 seen" (but he does not specify them) ; " the sector was the brightest." He 

 also says, " the northern portion of the floor [that which on August 26, 1869, 

 was dark and extended between the streak c and the border] was noted as 

 equally light ; the streak a could not be traced." On the same evening Mr. 

 GledhiU recorded the floor as light, =0-33, and that streak a was not to be 

 distinguished from the bright floor all along the north border. He described 

 the streaks as faint and rather diffuse, the sector faint, not sharp at edges, 

 and seemed broader than usual. Libration in latitude S. in August and N. 

 in May would tend to throw streak a apparently nearer the N. border in 

 May ; but Mr. Gledhill could not distinguish it from the general brightness, 



Clironological progression of increase of hrightness on the N. W. part of the 



floor of Flato. 



On referring to Mr. Elger's drawings of January 12, 1870, interval 60, to 

 72 hours, and January 14, 1870, interval 108 to 120 hours, I find the N.W. 

 part of the floor extending from Webb's elbow to very nearly the position 

 of the west arm of the trident equally light ; indeed presenting on the 12th 

 a similar contour to Mr. Elger's sketch of May 10, 1870, interval 60 to 72 

 hours, the diff'erence being that on the 12th of January, 1870, the streak 

 a was distinctly separated from the border. The streak a was first recorded 

 by Mr. Elger on September 20, 1869, interval 168 hours to meridian passage, 

 and his diagram of that date is strikingly in contrast with those of Jan. 12 

 and 14, and May 9 and 10 (see fig. 8, p. 263, and fig. 11, p. 274). 



On September 25, 1869, interval 72 to 60 hours, we have a diagram (see fig, 

 9, ante, p. 263) of Mr. Gledhill's in which the N.W. part of the floor is figured 

 as nearly similar to Mr. Elger's of the 20th, the streak including the three spots 

 Nos. 13, 19, and 16. On October 17, 1869, interval 108 to 120 hours, Mr. 

 Gledhill again saw the streak, and described it as a " well seen streak which 

 covers 13, 19, and 16;" he aligned it thus: "the streak produced E.N.E, 

 would cut the north border of B. & M.'s crater A outside Plato." On October 

 25, 1869, interval 60 to 48 hours, Mr. GledhiD gives a diagram in which a and 

 " " occur with a dark space between the streak and the border. On November 

 19, 1869, interval 168 to 156 hours, Mr. Gledhill saw the streak a with its 

 continuation " o," Webb's elbow, and the streak c, " o " and c diverging from 

 the western side of Webb's elbow. The earliest instance of an increase of light 

 on the N.W. part of the floor, and of the observation of Webb's elbow during 



1872. TJ 



