78 



MATHEMATICAL and 



manner we obtained tlie time of the fubfequent obfervati* 

 ons. Dr. WUliamfon, Mr. Shippen and myfelf ufed the 

 fame telcfcopcs, wc had ufed before in oblerving the tr^nfit 

 of Venus; excepting that on this occafion 1 chofe that 

 power of the telefcope which magnifies the diameters of 

 objeds an hundred times. Mr. Evans ufed the refleding 

 telefcope formerly ufed by Mr. Biddle at the Capes. 



On the day of the tranfit, we aflembled together at the 

 obfervatory, adjufted our telefcopes to diftind vifion, ap- 

 pointed an afhftanc to count the clock with an audible voice, 

 and agreed that no other perfon Ihould fpeak, nor move 

 from his telefcope, until both contads were over; but 

 write down his own obfervation feparately by himfelf, that 



it might be compared with the others. The fky being 

 very ferene, and the limb of the Sun well defined in our 

 telefcopes, we obferved the contads, as they are exhibited 

 In the following table, 



Obfcrvcrs. 



Dr. JViUmyj'fon^ 

 JMr, Shipperiy 

 Mr. Evans_j 



Myfdf, ■ 



^ExtcriiLil Cont. 



1 



1 

 1 



h, m. fee 



Int. Cont. 

 h. m, fee. 



Par. in 



Vcrt. 

 II 



1 



j Par. p. 



tohlsP. 

 II 



a, iC-y. ^Ap.l". 

 a. 36. I a 



a. 3^>- 9 

 %, 36. 9 



a. 37- 30 

 a. 11' 40 



^- Zh 38 

 ■ a. zi. 30 



3.74 

 3.745 



3»44 

 3*44 



Par, !n his 



.// 



1,48-at the External 



Contad^L 

 1,49 at the Internal 



Contatft. 



T i 



r 



I happened to have that part of the limb of the Sun, on 

 which Mercury entered, In the middle of the field of my 

 telefcope, with my eye intent upon it; fo that I am cer- 

 tain, that there was not the leaft impreffion on the Sun's 

 limb, perceptible by my telefcope, a fingle fecond of time 

 before I difcovered it. So that I am not furprized that 

 Dr. Halley, who had obferved a tranfit of Mercury in the 

 Ifland of St. Helena, concluding that, that of Venus would 

 be equally inftantaneous, expedcd, that the contad of 

 her limb with the Sun might be determined to a fingle ^ 

 fecond of time. The atmofphere of Venus renders it 

 quite otherwife, and produces an uncertainty of 5 or 6 

 feconds of time, in judging of the contads; whereas no 

 fuch thing was perceptible in Mercury. The firft appear- 

 ance of Mercury, on the Sun's limb, was a Heady fmall 



fpeck, 



\ 



N 



/ 



