SUN S DISTANCE FROM THE EARTH. 1 3 



surprising to some ; but yet if an accurate observation be had 

 in both places above mentioned it has aheady been shewn that 

 the duration of these eclipses of Venus differ from each other 

 by 17 entire minutes on the supposition that the sun's paral- 

 lax \s 12% seconds and if this difference be found to be greater 

 or less by observation, the sun's parallax will be found to be 

 greater or less in nearly the same ratio. And since 17 minutes 

 of time answer to 12^ seconds of the sun's parallax, for each 

 second of the parallax there will arise a difference of upwards 

 of 80 seconds of time : — 



17 X 60 



~ = 81 



12.5 



Therefore if this distance be obtained true within two seconds, 

 the quantity of the sun's parallax will be got to within the 

 40th part of one second and consequently his distance will be 

 determined to within its 500th part. For 



40 ', 12] 2 is 500." 



End of Extract. 



The following is a note to Dr. Whalley's paper, dated 

 May 26th, 1803. 



"The Transit of Venus in 1761 proved much less favour- 

 able to the purpose than Dr. Halley expected. The motion 

 of Venus' node not being well known she passed much nearer 

 to the sun's centre than he supposed she would ; which made 

 the places he pointed out for observing the total duration not 

 proper for the purpose ; indeed the entrance of V^enus on the 

 sun could not be observed at Hudson's Bay. He made a mis- 

 take, too, in the calculation in taking the sum instead of the 

 difference of the angle of the ecliptic with the parallel to the 

 equator and the angle of Venus' path, which affected the 

 accuracy of his conclusions. He was mistaken, too, in think- 

 ing the external contact might be observed to a second of 

 time. Astronomers have disaarreed 20 seconds in observing the 



