VOL. XLVII.] PHILOSOPHICAL TRANSACTIONS. \f;S 



this was at 1 1** 13"' 15^ by the watch, which still kept exact pace with the clock; 

 and his eye had not been removed more than a minute. 



Venus passed the meridian in the transitory at l*" 37"" 55' afternoon by the 

 clock: the sun passed this day at ll'^ 57'" 27% and yesterday, the 15th, at ll'' 

 57"" 28^; whence it is easy to reduce all to apparent time, as follows: 



Total ingress of Venus 1751, April 15'^ '22^ 42'" 2' 



Her total emersion 15 23 15 47 



Her meridian transit l6 1 40 29 



Now, supposing the whole disk to have taken up one minute, 



as it seemed thereabout, both in the ingress and egress, 



the middle of the occultation must have been 15 22 58 24-1- 



And the duration, with respect to the centre of Venus .... 33 45 



P. S. Mr. John Canton sent notice that he observed the occultation of Venus 

 by the moon last Tuesday, at his house in Spital-square, and found the immer- 

 sion at 10'' 42*" 20" a. m. emersion at ll'' 15"" 40*. 



XXIII. Of a remarkable ^"fppearance in the Moon, Jjjril 22, 1751. By 



James Short, F.R.S. p. 164. 



In N° 396 of the Phil. Trans, there is an account of an observation made on 

 an uncommon appearance of the lunar spot called Plato in the nomenclature of 

 Riccioli's and Grimaldi's "Selenography, and Lacus niger major in that of Heve- 

 lius. Signor Bianchini, to whom we owe this communication, says, that it was 

 the 16th of August, 1725, N. s. about an hour after sun-set, when he took his 

 observation with a dioptric telescope, of 1 10 feet, made by the famous Campani, 

 the air being very serene, and the moon, as he says, speaking of the same phe- 

 nomenon in his book of Venus, a day past the first quarter: so that the said 

 spot then lay in the common section of light and darkness. The mountainous 

 oval margin, with which it is surrounded, was brightly illumined with the sun's 

 rays; but the plain bottom looked darkish, as having not yet received his light. 

 There was however extended along its area, from end to end, a track of reddish 

 light, as if a beam had been admitted through some perforation in that side of 

 the margin, which was then exposed to the sun. M. Bianchini proposes the so- 

 lution of this matter in two different ways: first, by supposing an aperture in the 

 margin, as just now mentioned: or secondly, by conceiving the moon to have 

 an atmosphere, and that thereby the rays passing near the summit of the margin 

 might be so refracted as to be thrown on the plain area or bottom. 



Mr. S. having lately had an opportunity of observing something of the same 

 nature himself, he here lays it before the Society, with a conjecture concerning 

 its cause. Monday, April 22, 1751, o. s. being at Marlborough house, and 

 having directed the great reflector to the moon, he perceived a single streak of 



