ECLIPSES ©F THE SATELLITES OF JUPITER. 15£ 



planet: An eclipse observed under such circumstances will, 

 1 apprehend, be as perfect as it well can be; and a corre- 

 spondent observation, taken under the like circumstances, 

 will give a correct difference of longitude of the two places, 

 provided the eclipse be observed' with telescopes of the same 

 powers. 



Taking- the eclipses in the following tables observed un- Much affected 

 der these favourable circumstances as the standard, and j^ unfavoura « 

 comparing their results, as given in the last column, with stances, 

 those of the others, it will be found, how much the latter 

 are affected, by the eclipses having been observed when the 

 atmosphere was hazy, or the planet very low, during twi- 

 light, or when the moon was near the planet, or the satellite 

 not far from the body of Jupiter; and that even if correspon- 

 dent observations had been taken, under favourable circum- 

 stances, at a known meridian, the difference of longitude 

 given by the comparison would have been far from correct: 

 the same eclipse observed at two places, under similar un- 

 favourable circumstances, would possibly give a result near 

 the truth ; as the observations at both places would be af- 

 fected in the same way, but probably not in an equal de- 

 gree, as it is not likely there would be exactly the same de- 

 gree of haze, the same strength of twilight, &c. at both 

 places ; and therefore those observations taken under the 

 same favourable circumstances can only be relied upon with 

 certainty. 



It may not be an easy matter to have telescopes at both ^- ffi » Itt 

 places of precisely the same powers for these observations: telescopes 

 at Madras we had two telescopes in use, constructed at the alike - 

 tame time, in appearance precisely alike, and intended by 

 Dollond to have been so in all respects; yet on repeated 

 trials, one was found to have a decided advantage of several 

 seconds over the other, showing the emersions sooner, and 

 the immersions later by that quantity. In order to do away % ^ ln • 

 the errour arising from a difference in the powers of tele- mersion and 



scones, immersions and emersions should be observed at both emer S I ° n 



. ,.„, ,. , • i -ii i , should be ob* 



places ; the diiierence ot longitude will be as much greater served. 



than it ought to be by one series as less by the other, but the 



medium will be the correct difference of longitude of the 



places: it is possible also there may be some difference in Difference in 



the eyes, 



