372 PHILOSOPHICAL TKANSACTIONS. [anNO 1765. 



egress will Imppen towards the time of sunset, when the apparent altitude of the 

 8UI1 will not much exceed 4 degrees. Observers therefore should be stationed 

 farther to the west, about Cape Corientes; at which place and at Mexico 

 though in about '20° of n. latitude, the effect of parallax will still be considerable 

 particularly at the egress, as appears from the following table. 



Places. ^'""^'^ internal Second internal Total effect of Difference in total du- 



contact, contact. par. = S".T. ration from Tornea°. 



Mexico I'" 7» 5"" 3' 6"* 1 0' 1 7™ 50' 



Cape Corientes O 26 5 O 5 26 17 6 



By comparing the observations to be made at either of the above places, or in 

 any of the neighbouring parts, with those of Tornea°, a difference of more than 

 17 minutes in the total duration may commodiously be obtained; by which the 

 quantity of the sun's parallax may be determined agreeably to the method pro- 

 posed by Dr. Halley in the case of the last transit, and in which no error but 

 that of the observation can take place, supposing the situation of the two places 

 to be nearly known. 



When Dr. Halley's computation was examined, and it was found that so great 

 a difference in the total duration of the transit at any two places as had been ex- 

 pected, could not conveniently be obtained;' another method was proposed, and 

 was accordingly carried into execution, viz. to station two observers in such a 

 manner that one of the internal contacts might be observed with the greatest 

 difference possible arising from a contrary effect of parallax at the two places. 

 This method, though necessarily inadequate, because the longitude of the two 

 stations must be rigorously known, may be practised at both contacts in 1769. 



It appears by computation that the time of the first internal contact in the 

 evening, is accelerated as much as possible by parallax in 48° 42' of n. latitude, 

 and 6° to the east of Greenwich, at or near Nancy in France. But this compu- 

 tation is framed on a supposition that the sun's centre is in the very horizon: in 

 which circumstances no observation can be taken. If the sun's altitude at the 

 time of the contact should be required equal to 5 or 10 degrees, then it will ap- 

 pear that Greenwich and Dublin are stations very advantageous; and we have 

 already seen that the time of ingress at the former place will happen 7'" 12' sooner 

 than if seen from the earth's centre, on account of par^jHax. Indeed the effect 

 of parallax will be nearly the same to every part of GreAt Britain. The part of 

 the earth's surface, where tVie effect of parallax on the planet at the same con- 

 tact will be as great as possible in a contrary direction, when the sun's altitude is 

 about 5°, is in 46° of s. latitude nearly, and in l68° or 169° of w. longitude 

 from London, in the great Pacific Ocean, where it does not at present appear 

 that there is any land. If however an observer should be stationed in any of the 

 islands in the South sea, for which a computation has already been made, even 



