i 



40 



M 



A K D 



I 



9 fcconds of a minute; fo that the fubtenfe of it, were it 

 much larger than it is, muft be invifible to the naked eye 

 at the diftance of 6 inches, and it is hardly poffible to dif- 

 tinguifti 10 feconds by inftruments, let them be ever fo 



fkilfuUy made, M 



have been devifed by 



aftronomers, which fliew the ingenuity of the inventors; 

 but the difadvantage of them all was, that they depended 

 upon obfervations to be made with a precifion, which no 

 inftruments hitherto conftru£ted could poffibly accomplifh. 

 The tranfits of Venus alone afford an opportunity of de- 

 termining this problem with fufficient certainty, and thefe, 

 from the ftrl£t laws of her motion, happen fo feldom, that 

 there cannot be more of them than two in one century, and 

 in fome centuries none at all. Three only have been ob- 

 ferved fince the creation, and the firft of them by two 

 perfons only. The peculiar advantage of this phcEno- 



menon for determining the parallax of the Sun with a 

 precifion which is not to be expeded from any other me- 

 thod, confifts in its being deduced from the abfolute time 

 that elapfes between the inftants of the contads with the 

 Sun's limb, as feen from different parts of the earth; or 

 irom the difference of total durations as noted by diftant 

 obfervers, properly ftationed for that purpofe. A fecond 

 of time being eafily diflinguifhedbyawell regulated clock, 

 if the aforefaid abfolute difference of time be carefully no- 

 ted, in places where it will amount to 24 minutes, it will 

 give the parallax, fmall as it is, within the hundreth part 

 of a fecond of a degree, and confequcntly the diftance of 

 the Sun and planets within the feven hundredth part of 



hole. In fome tranfits this difference of time will 



the v 



be greater, and in others lefs, in certain places on the earth, 



which renders thofe that happen on the northern part of 

 the Sun's difc, in general, more favourable to our purpofe, 

 than thofe that happen on the fouthern hemifphere. Hence 

 it is, that although much was done in this matter by the fe- 

 dulity and care of aftronomers at the tranfit in the year 

 1761, when Venus paffcd fouth of the Sun's center, yet 



our 



