75) REPORT 1871. 



the shadows of the three peaks y, I, and e of 13. & M., which are represented 

 by Schroter in t. xxi. The. shadows of these and other peaks on the W. waU 

 have been very frequently observed of late years. 



I am not aware that liianchini's observation has been verified. The pecu- 

 liar ap2)earance which he has delineated depends not only on libration, but 

 also on the angle which the terminator makes with the meridian ; for it is 

 clear that the direction of the terminator must form a tangent to a line pass- 

 ing equally through the depression in the wall to produce the appearance 

 seen by Biauchini ; and it is highly probable that it is of very rare occur- 

 rence, as seen from the earth, the variation in the angle of terminator with 

 meridian being as much as 3°. 



While transcribing the above (April 22, 1871) I have considered the Bian- 

 chini phenomenon more closely. During the year 1870 the opportimities 

 for observing sunrise on Plato were comparatively numerous, and certainly 

 not the slightest appearance of Bianchini's streak was detected ; on the other 

 hand, the positions of the earliest rays of sunlight on the floor have been 

 determined, with some degree of precision, for the portion of the luni-solar 

 year during the period of the observations. If the configiu'ation of the W. 

 wall is different now from what it was in Bianchini's time, the phenomenon 

 may be explained by the supposition that the gap or pass iS". of the peak h 

 was lower than at present, and has been raised \>y " landslips " on one or 

 both sides, which are of extensive occuiTence on the moon as recognized by 

 Nasmyth ; the absence of fui-ther observations by Biauchini on the same 

 evening, however, leaves the matter in doubt. 



Short records, in the Phil. Trans, for 1751, p. 175, that on April 22, 

 1751, he saw a streak projected along the flat bottom of Plato. Soon after 

 he saw another streak parallel to the first, but somewhat lower [or northerly], 

 which in a very short time divided into two. He found a gap in the wall 

 opposite the first streak, and also one in the direction of the lower one. 



Not only is Bianchini's observation at variance with modern observations, 

 but Short's also. The order of appearance of the streaks of sunlight on the 

 floor on Jan. 10, 1870, is, first, the broad streak through the wide gap ; 

 second, the southern streak north of the peak y. The appearances of Short's 

 streaks were in the reverse order. 



The following record of observations by Schroter on July 30, 1789, at 

 O*" 48'", kindly translated by Mr. Gledhill, will illustrate Mr. Elger's obser- 

 vation on January 10, 1870 : — 



' Selenotopographische Fragmeute,' § 250, vol. i. p. 329. " A difl^erent, 

 more beautiful, and more magnificent view of Plato is obtained when, with 

 the rising sun, the first traces of an extremely faint twilight are seen on the 

 grey fioor of the crater, and when the first beams of light are thrown over 

 the mountains into the plain below. This view of Plato, which lasts only 

 for a few minutes during the slow monthly rotation, and for which one may 

 wait for a year and yet not see it, I saw on the 30th of July, 1789, 9'' 48'". 

 As in the 8th figure of t. xxi., the terminator had advanced from W. to E. as 

 far as a, /3. To the W. of this the greatest part of the border lay in the 

 light of day [or on the day side], and only the small portion to the E. of a, 

 /3 was illuminated on the night side. The whole inner grey sin-face, on the 

 contrary, was still hidden by the shadows of the lofty mountains on the 

 border, and on the S. border there was also a low spot filled with shadow. 

 "Wliile I was observing the shadows of the inner surface with power 161, I 

 became aware of something to the E. of the middle of the floor, as if the dark 

 surface were in a kind of fermentation. A few seconds later I saw here in 



