VOL. XXXIV.] PHILOSOPHICAL TRANSACTIONS. 141 



The Longitude of Lisbon, and the Fort of Neiv York, from Wansted and Lon- 

 don, determined by Eclipses of the First Satellite of Jupiter. By the Rev. 

 Mr. James Bradley, M. A. Astron. Prof Savil. R.S.S. N"394, p. 85. 



The immersion of the first satellite was observed by Mr. Pound at Wansted, 

 with Mr. Hadley's reflecting telescope, August 4, N. S. 1725, about 45* after 

 the time of the immersion, as calculated from his tables. By another observa- 

 tion made August 1Q, N. S. the true immersion preceded the calculation from 

 the same tables I'" 10^ So that in 25 days the satellite's motion was accelerated 

 as much as answered to 1"" 55^ in time. Supposing therefore the acceleration 

 to have been in the same proportion between July 28, and August 4, N. S. 

 then the true immersion July 28, N. S. would have happened at Wansted about 

 1"" 15* after the time by the tables; which make the immersion at 12*^ 48™ 45* 

 app. time. The true immersion therefore was at Wansted, July 28, N. S. 11^ 

 50"" 0' app. time ; and at Lisbon it was observed at 12'' 12"" 26* app. time, the 

 difference being 37"* 34*. 



Sept. 28, N. S. the firsi satellite was seen emerging in the reflector at Wan- 

 sted 3*" 50* sooner than the tables make the emersion ; and by the mean of two 

 more observations made at the same place, and with the same telescope, on the 

 14th and l6th of October, N. S. the true emersion preceded the calculation 

 4"^ 30*. We may therefore from hence conclude, that on Sept. 21, N. S. the 

 true emersion at Wansted preceded the calculation by the tables about 3™ 35*, 

 and that the true emersion there was at 12'^ 1"' 15* April 1 ; but this emersion 

 was observed at Lisbon at 1 1** 24™ 55% the difference being 36' 20". 



The observations at Wansted being made with Mr. Hadley's reflecting teles 

 cope, by which one may see the first satellite near ^ of a minute sooner when 

 emerging, than in a refracting telescope of 15 feet, and the contrary when im- 

 merging, there ought to be some allowance made on account of different teles- 

 copes used at Lisbon and Wansted, by deducting 10 or 15* from the difference 

 of time collected from the immersions, and adding as much to the difference 

 deduced from the emersions. Such correction being made, the difference of 

 meridians by the immersion observed July 28, will be 37"" 20% and by the 

 emersion, Sept. 2J, 36"" 35*. 



The emersion observed at Lisbon, Dec. 8, N. S. at 8*^ 32"' 40' apparent time, 

 was likewise seen at Wansted in a 15-foot tube at g^ lO"' 5* apparent time, the 

 air being a little hazy, which may probably make the difference 37"" 25* a little 

 too great. 



The emersion seen at Lisbon, Jan. J6, 1726, N. S. at &' bi"" 10% which 



