18Q4.] Jupiter's Third and Fourth Satellites. 225 



Difference of Longitude of my Observatory and Col. Beaufoy's, 

 by Observations of\4 Eclipses of Jupiter's Satellites, made at 

 our respective Stations. 



Mean differ, of long, of the two Observatories . . = 59-34 

 Known difference of the two Observatories . . . . = 69' 17 

 Error of observation at the two Stations =0 0*1 7 



Thus it seems that 14 observations, nine of the first satellite, 

 two of the second, and three of the third, gave the difference of 

 longitude to seventeen-hundredths of a second, and that it was 

 the work of seven months ; while in the observation before nar- 

 rated, the accuracy is three times as great, and is obtained in a 

 few minutes more than two hours. Some perhaps will contend, 

 that this accuracy is the offspring of accident ; it is however at 

 least as probable, that accident has had nothing at all to do with 

 it ; on the same supposition, and with equal plausibility, might it 

 be urged that accident has prevented accuracy in each of the 

 14 observations, principally of the favourite satellite; the two 

 nearest of which to accuracy, are 50 or 100 times more remote 

 from it, than are the results of those observations, which are the 

 immediate objects of this communication. 



From this however let it not be supposed, that I imagine 

 equal accuracy will always be procured ; an observer may and 

 occasionally does obtain the right ascension of a star by one 

 wire of his transit instrument just as correctly as if he employ 

 the five or the seven; yet he must not suppose he will .always do 

 it; just so with the observations of immersion and emersion I 

 have alluded to; all I contend for is, provided the same care — 

 the same instrument — the same magnifying power— the same 



New Series, vol. vn. Q 



