263 KEPORT— 1872. 



sunrise. Upon the supposition of the existence of an atmosjAere, and of the 

 exhalations arising from the intensely heating of the surface as the sun 

 approached the meridian, at a distance beyond 10° from the terminator, in 

 the absence of dense clouds, -we may have an undefined mistiness not fully 

 and entirely obscuring objects nor interfering with their distinct outlines, 

 yet rendering their aspects different as compared with those which they 

 present when they emerge from night. And this difference of aspect may 

 not only be increasinrj up to the epoch of maximum temperature, but diver- 

 gent, inasmuch as various substances will be variously affected by the degrees 

 of heat to which they may be subjected, some darlcening under the intensity 

 of the solar rays, others behaving in quite the opposite manner and exhibiting 

 something like luminositij , arising, perhaps, from heat-rays reverberated from 

 the sides of craters as the sun shines almost perpendicularly upon them. 

 That phenomena of the kind just alluded to are clearly recognizable upon the 

 moon there can be no question. "VVe have only to read "Webb's masterly 

 exposition of the progression of appearances during the luni-solar day to be 

 convinced that some such agencies as we have here supposed must be at 

 work.] 



On the probable success of continued observation of minute objects Mr. 

 Pratt writes : — " It has long appeared to me a reasonable expectation that 

 the effects of such a low-lying atmosphere (as only could be supposed) would 

 be much more visible in the search for minute detail on the surface than in 

 the observation of occulations ; while the former method might be expected 

 to yield results much more rapidly than the latter, as the duration of the 

 maximum effect to be expected in the latter case could not be much greater 

 than a second of time, while in the former case it might be observed con- 

 stantly for hours at a time." 



III. 



History of Streak a, with its adjuxcts, on the I^okth ajtd North-west 



Floor of Plato. 



Lunation September 1869, from 9 hours before meridian to 12 hours before 



sunset. 



This streak was not obsei-ved until the 20th of September, 1869, when it 

 was first seen by Mr. Elger (see fig. 8). It is figured by him as a short, 

 straight, narrow streak, stretching across the north-west part of the floor, 

 and forming, if produced, an acute angle with the longest diameter of Plato 

 drawn through spots Nos. 1 and 4. On the 25th of September, five days later, 

 Mr. Gledhill observed the streak, its form and direction being nearly iden- 

 tical with those of the streak seen by Mr. Elger. For the general distribu- 

 tion of streaks on the floor see 'fig. 9 and " Observers' Notes," interval 72 to 

 60 hours, post, p. 295. On the 27th of September, near sunset, interval 24 

 to 12 hours, the streak was seemingly lost, as Mr. Gledhill records only the 

 streak c as a broad baud of brightness, in width about one third the distance 

 from the north rim to spot No. 1 ; nor does he specify it separately on 

 October 27, 1869, under the same interval 24 to 12 hours.' 



Lunation October 1869, from 69 Jwurs before meridian to sunset. 



In this lunation the streak was picked up by Mr. GledhiU in the interval 

 108 to 120 hours, covering spots 13, 19, and 10 ; he aligned it, and found that 

 if produced it would cut the north border of B. and M.'s A (438, Webb's third 

 edition of ' Celestial Objects '). On this occasion it was deemed advisable to 



