IN FORT WILLIAM, &C. 67 



taken by every perfon, who fhall obferve the fame phe- 

 nomenon at one and the fame rrtftaht of time; and 

 thence the diftance of Jupiter from the meridian of 

 each will be known to fecon'ds, if we fuppofe the lati- 

 tudes known before-hand. 



And if the telefcopes of quadrants could be made 

 fufficiently powerful to obferve the fatellites, then a 

 fingle obferver, at any place, could perform the whole 

 without trouble or difficulty, and would only need a 

 common watch, and a little more patience than would 

 be requifite if the watch were perfect, and calculation 

 true. 



But fuppofing the telefcopes and quadrants as they 

 are, and two oblervers at each place, one employed 

 with the fatellite, and the other with the quadrant, then 

 the latter muft carefully keep the body of Jupiter on the 

 line of altitude till the other tells him to Hop, which is 

 to be done at the initantof obferving the expected phe- 

 nomenon. 



By this mode a degree of longitude rriav be meafured 

 with as much accuracy as a degree of latitude; and it 

 is what I have in contemplation to perform, as foon as 

 I can get the requifite inllruments. 



Remarks on fome erroneous Obfervations of Jupiter's 

 Jirjl Satellite. 



In 1 778, 1 took notice, that when Jupiter is very near 

 the oppofition, the obfervations are not to be depended 

 on, and that the Satellite vanifhed without changing 

 colour. The fame happened in 1779, 1780, and again 

 in 1784, at Beemulwilfa; therefore I have put down the 

 times of fuch obfervations, as they are reduced to ap- 

 parent time, from the known deviation of the clock 

 from mean time. The tranfit inftrument was examined 



Vol. I. F by 



