VOL. LIII.] PHILOSOPHICAL TRANSACTIONS. At^ 



might readily acquiesce, when he considers the accuracy of the observers, and 

 the nice agreement of ahuost all the observations. 



And such was the state of the sun's parallax as deduced from the latest and 

 best observations, when the approaching transit of Venus in 1761 engaged the 

 attention of the curious of all nations. Dr. Halley, in Philos. Trans., N" 348, 

 had proposed a method of determining the sun's parallax, by procuring observa- 

 tions to be made on this transit in such places where the difference of time 

 between the ingress and egress would be the greatest possible ; namely near the 

 mouth of the Ganges, where the sun would be vertical at the middle of the 

 transit, and at Port Nelson in Hudson's Bay, where the planet would enter on 

 the sun's disk about the time of sun-set, and leave it soon after sun-rising ; for 

 in the former place, says Dr. Halley, the planet would be equally distant from 

 noon both at ingress and egress, and the apparent motion of Venus on the 

 sun would be accelerated by almost double the quantity of the horizontal paral- 

 lax of Venus from the sun : because Venus is at that time retrograde, and 

 moves in a direction contrary to that of the eye of an observer on the earth's 

 surface. Whereas in Hudson's Bay, under an opposite meridian, the eye of an 

 observer will be carried, while the sun seems to move under the pole from 

 setting to rising, in a direction contrary to the motion of the observer's eye at 

 the Ganges ; that is, in the direction of the planet's retrograde motion from 

 east to west. — From these considerations, and supposing with Dr. Halley the 

 axis of the planet's path to be inclined to the axis of the equator in an angle of 

 a° 18' only, the interval lietween the two contacts would have been IS"" lO* 

 longer in Hudson's Bay than at the mouth of the Ganges. 



But on examination, the case is found to be somewhat different. The axis of 

 the equator on the 6th of June 1761 made an angle of 6° 10' with the axis of 

 the ecliptic on one side, and the axis of the planet's path an angle of 8° 30' 10* 

 on the other ; the axis of the planet's path therefore made an angle with the 

 equator of 14° 40' 10". — The planet's latitude was 34. minutes greater, both 

 from observation and the Doctor's own tables, than he had supposed, in his cal- 

 culation made from the Rodolphine tables corrected : and therefore the planet's 

 egress could not have been observed at Port Nelson. Having made a computa- 

 tion for a place in North America situated 5'' 30"" to the west of Greenwich, 

 and in the 60th degree of latitude ; and also for a place to the east of the Gran- 

 ges, and 6'' 30"' to the east of Greenwich, in the latitude of 22° 42' north 

 that the places might be nearly situated in the same circumstances with the 

 mouth of the Ganges and Port Nelson, it appears that the interval between the 

 two contacts would be but 4'" 50' longer in America than in the East Indies 

 supposing the sun's parallax ]2".5, and the inclination of Venus's path 14° 40' 

 to the equator. 



