1824.] Jupiter's Third and Fourth Satellites. 223 



Now if we add the difference of longitude to the Bushey obser- 

 vation of immersion, we shall have in Blackman-street time, the 

 instant at which the phenomenon was observed at Bushey ; and 

 the difference, if any, between this and the Blackman-street 

 observation, will show the time by which the satellite was seen 

 longer, at one station than at the other. 



4 h 32' 27-09" = Immersion at Bushey. 

 + 59*17 = Difference of longitude. 



4 33 26*26 = Blackman-street time when the immersion 



was observed at Bushey. 

 4 35 3*89 = Immersion at Blackmaa-street. 



-f-1 37*63 = the time that the satellite was seen longer at 

 Blackman-street than at Bushey. 



A mere inspection of the times shows, that the differences are 

 totisiderable : let us see if they are reconcileable. As the tele- 

 scopes at the two stations are nearly similar ; as the same pre- 

 caution of excluding adventitious light was employed at both ; 

 we must seek for the cause of the discrepancy, either in a dearer 

 atmosphere atone observatory than at the other — in a greater sen- 

 sibility to minute pai tides of light, which one observer has than 

 the other — or in the superior steadiness of the one instrument 

 over the other. As to difference of atmosphere, we have no 

 proof that there was any ; indeed the probability is in favour of 

 Bushey ; it being situated far from any frequented neighbour- 

 hood ; while the Blackman-street observatory is surrounded by 

 buildings in every direction. As to increased sensibility to small 

 particles of light, we have no good grounds to suspect that one 

 observer possesses this, more than the other. Hence we are left 

 to the only remaining source of discrepancy, namely, the greater 

 steadiness which one instrument has than the other; and this 

 there can be no doubt the Blackman-street instrument possesses : 

 itcanbe moved in right ascension by the finger and thumb ; astar 

 may be kept bisected by one of its micrometer wires any reason- 

 able time ; nor will any tremulous motion be communicated to 

 the star, although a power of 500 or 600 be employed. Hence 

 the experience of daily observation would authorize us to declare, 

 cateris paribus, that in Blackman-street the immersion ought to 

 be seen later than at Bushey : perhaps also the trifling difference 

 of focal length and magnifying power of the Blackman-street 

 instrument may contribute some little to the result ; which is, 

 that the immersion was seen later in Blackman-street than at 

 Bushey by 1' 37*63." 



But if our reasoning be correct, the emersion should be seen 

 earlier in Blackman-street ; and if the weather at the emersion 

 be the same at each station, as it was at the time of immersion, 

 by as much as the immersion was seen later in Blackman-street, 



