ASTRONOMICAL PAPERS. 



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fpeck, black, well-defined, and not larger in my telefcope 

 than the dot of a pen. But that of Venus was tremulous, ob- 

 fcure, and ill-defined, growing gradually darker as fhe ad- 

 vanced on the Sun. If Mercury has an atmofphere, it 

 muft be lb rare and low, that his diftance from us renders 

 It abfolutely imperceptible with the telefcopes that we 

 ufed. At the internal contad, the crefcent of light round 

 the body of Mercury clofed inftantancoufly, fo that it 

 might be judged of with more precifion than that of Ve-^ 



nus; 



his atmofpl 



lere giving us no dillurbance in this cafe. 



We could not have a fairer opportunity, for afcertainlng 

 the truth of thcfc conclufions; as our telefcopes were in 

 good order, and well adjufted, and the fky was remarka- 

 bly clear and ferene, on both of thcfe days. On the firfl 

 of them, not a cloud appeared from morning till evening, 

 and on the latter, none till about four o'clock, when the 



Sun was very low; and both the tranfits began between 

 two and three o'clock, in the afternoon. 



About three o'clock, I applied myfelf tothe micrometer, 

 to meafure the diameters of the Sun and Mercury, and the 



ncareft diftance of their limbs; while Dr. Williamjon read 

 off the divifions of the micrometer, and a third perfon 

 wrote them down, with the times of making them- Thefc 

 meafures make the diameter of the Sun on the 9th of No- 



vember 1-769, 32 



' . 20', 



2 or his femidiameter 970^,1 fe- 



-^ 



conds, and the femidiameter of Mercury 4^^238. The 

 meafiircs of the leaft diftances of their limbs reduced to 

 minutes and feconds of a degree, with the parallaxes of 

 Mercury adapted to the apparent times of the obfervations, 

 as they are determined from a very large projection of two 

 inches to a fccond of his hor- parallax, are let down in 



the following 



table. 



Apparent 



t# 



\ 



