•. -I —^—-^ — 



60 



M A T H E M A T re A L and 



in the following tabic, taking a mean femldiameter of the 

 earth 3985 Englifli miles, 



36693417 Mercury's 

 68564850 Venus's 



1 



i 



M 

 J 



, M 



in Englifli miles 



904307200 Saturn's 



On account of the difficulty of afcertaining the precife 

 moment of the middle of the tranfit, from the menfurations 

 of the neareftdiflances of the limbs of the Sun and Ve- 



I 



nus, and thefmall difference of time between thecontaft: 

 happening, at the center of the earth, and at any particu- 

 lar place on its furface; aftronomers have generally pre- 

 ferred the comparifon of two obfervations at proper places, 

 where the effeds of parallax will be contrary to each other^ 

 retarding the contads at one place and accelerating them 

 at the other, for the purpofe of deducing the parallax and 

 diftanceof theSun from them- We have an opportunity of 



confirming theformerconclufions, by comparingour obfer- 

 vations with thofe that have been made at the royal obfcrva- 

 tory at Greenwich, as they have lately come to hand. They 

 differ indeed confiderably among themfelves, probably ow- 

 ing to the various methods, which the obfervers took to 

 judge of the contads, the account of which is not yet arrived 

 here; yet they give a mean parallax of the Sun nearly the 

 fame that we have deduced from our own obfervations at 

 Philadelphia. I have therefore inferted them in this ac- 

 count of the tranfit, as they ferve to fhew that we have 

 notioft our labour and expence on this occafion. The 

 method I have ufed is firft to reduce the Greenwich obfer- 

 vations of the contafts to the meridian of our obfervatory 

 in Philadelphia, by deduding from them the difference of 

 longitude converted into time; and then to calculate the 

 cfl'ea of parallax for both places at the apparent times of 

 the contads, upon the fuppofition of the Sun's horizontal 



parallax 



f 



/ 



'^ 



