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ASTRONOMICAL PAPERS. 



77 



An Aroma of the Trnn/it «/M E R C U R Y over the SUN, 



on Noven4)er gth, 1769. N. S. 



N the judgment of moft aftronomers, the tranfits of 

 Mercury and Venus over the Sun afford the heft op- 

 portunities, for fctthng the longitudes of phices on the 

 earth, even preferahle to that derived from, the ecHpfes of 

 Jupiter's latellites, when the parallax of the Sun is pre- 

 vioufly knoxvn. Thofe of Mercury happen frequently, 

 and although they are of but little importance in deter- 

 mining the parallax of the Sun and the dimenfions of the 

 folar fyftem, by reafon of his great diftance from the earth, 

 and the diifercnce of their parallaxes being lefs than that 

 of the Sun; yet they have been carefully obfcrved, for the 

 purpofc of fettling his theory, and the longitudes of the 

 places of obfervation. The fociety therefore fenfible of the 

 importance of this phcenomenon, both to the perfedion of 

 aftronomy in general, and particularly for completing the 

 purpofes defigned to be anfwered by tlie obfervation of 

 the tranfit of Venus, have appointed the fame committee, 



with the addition of two other gentlemen, to obfcrve the 

 tranfit of Mercury on the 9th of November, 1769, in Phi- 

 ladelphia, that had been before appointed to obfcrve that 

 of Venus. 



Having flill the fame inftruments in our obfcrvatory, 

 which we iifed on the former occafion, together with a 



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an ingenious contrivance of his, in the conftrudion of the 

 pendulum, to remedy the irregularities arifmg from heat 

 and cold; we paid the utmoft attention to the going of. 

 the clock both before and after the tranfit. From com- 

 paring afufficient number of correfponding altitudes of the 

 Sun's limbs, we found that our clock was too flow for mean 



" and the equation of time being 15'. 49",(5 or 



time i'. 20 



to avoid fraaions 15'. 50'^- 17'. 10" were added to the 

 times of all our obfervations, as they were v/rittcn down in 

 the obfervatory, to reduce them to apparent time. In this 



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