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46 



MATHEMATICAL and 



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I therefore conclude that the apparent firft internal contafl; 

 of Venus happened at 2''. 31^ 34^' ap. time. Yet it is 

 not improbable that her r^^a/ contad may have happened 

 a few feconds fooner, if It be certain that fhe has an atmos- 

 phere \ becaufe that might have obfcured the Sun's limb a 

 few feconds after Venus was entirely immerfed within the 

 difk; in the fame manner as I judged with refpedt to the 

 external conta£l, that the beginning of the obfcuration of 

 the Sun's limb was occafioned by the intervention of the 

 atmofphereof Venusa few feconds before her body adually 

 xame in conta£t with the Sun/* 



Account of the Contacts, by Dr. WILLIAMSON. 



" I made ufe of a refradling telefcope 24 feet long, 

 which magnifies ninety times. The glaflcs were in very 

 good order, and the air uncommonly ferene, fo that the 



Sun's limb appeared very diftindl and well defined, whence 

 I promifed myfelf the pleafure of fixing the external con- 

 ta£t to a fecond, but the event convinced me that I had 

 promifed too much. A dufky appearence once and again 

 drew my attention to a particular part of the Sun's limb, 



but I could fee no fuch dark fpot there as I thought Venus 

 muft produce, and it was not till 2\ II^ 31'' mean time, 



or 2\ 13 \ 46'^ apparent time, that I determined to flop a 

 watch which I had in my hand, to afcertain the time of my 



obfervation, leaft fome accident fhould prevent my hearing; 

 the affiftant, who flood at 5 or 6 yards diftance by the clock 

 counting feconds. At that very time I was doubtful, 

 whether the appearance on the limb of the Sun was cer- 

 tainly occafioned by the interpofition of the body of Venus ; 

 forthough the darknefs was of fome extent along the Sun's 

 limb, yet the impreflion was not proportionably deep, 

 fuppofing that it was made by a circle fo fmall as Venus 

 compared with the diameter of the Sun, nor was the dark- 

 nefs equally perfed; yet the fubfequent progrcfs of the 

 darknefs foon convinced me that I had not been much 

 too hafty in noting the time of the external contad. 



"When 







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