1824.] Jupiter's Third and Fourth Satellites. 221 



That observations of eclipses of the fourth satellite, are the 

 most incongruous it would be silly to doubt, and unless the 

 immersion and emersion can be procured, they cannot without 

 much difficulty be brought to bear, upon any useful point. The 

 difference of the telescopes employed will alway need a correc- 

 tion ; this may indeed be found by previous comparisons at the 

 same stations ; but as the dissimilarity of the weather at the 

 places of observation will materially influence the results, when 

 only the immersion or emersion be observed, an equation is 

 wanting, not so easily to be found. Fortunately, however, the 

 immersions and emersions of this, and the third satellite, are occa- 

 sionally observable within a space of time little more than two 

 hours ; let us, therefore, inquire a little into this matter. 



We have already noticed that the fourth satellite having 

 entered the shadow, is a very considerable time before it becomes 

 lost in it ; hence its disappearance will be extremely gradual ; 

 let us suppose that during this time it passes through three dis- 

 tinct gradations of lustre ; that at the first, it resembles the small 

 star of { Ursse Majoris ; at the second, the small star of Polaris ; 

 and at the third, the small star of a Lyra. Let us then have three 

 telescopes ; the one able only to show the small star of £; the 

 second only that of Polaris ; the third adequate to show the 

 small star of a. Lyra. On a fine night, all the telescopes show 

 their respective objects very well — say at 10 o'clock ; provided 

 the stars have considerable altitude, and the weather be equally 

 good, why should they not show them equally as well at 12 

 o'clock ? it would puzzle most persons, I think, to determine 5 if 

 so, provided the analogy hold good, why may not the evanes- 

 cence, and re-appearance of the fourth satellite be observed, 

 within reasonable limits? I confess I see no reason. I know it 

 is said, an observer will have an impression left on his mind, that 

 the satellite continues visible when it really is not so ; but this 

 is not distinctly proved ; and again on the emersion, knowing 

 the point at which he is to look for it, he thinks he sees it ear- 

 lier than he actually does ; this again is not proved : we will, 

 however, allow them both, and we shall be as if they were not 

 allowed at all. The only difference between the three telescopes 

 would be, that A would give the immersion earlier than B ; and 

 B sooner than C. And at the emersion, A would show the satel- 

 lite Inter than B, and B later than C ; circumstances, as we shall 

 hereafter see, not of the least importance. 



Entertaining then sentiments such as these, I determined on 

 the first favourable opportunity, to observe the immersion and 

 emersion of the fourth satellite, with every possible care ; and the 

 first day of the present month enabled me to do so. Jupiter's 

 meridian altitude was about 62° ; the immersion occurred when 

 the planet was l 1 ' 34' east of the meridian ; the emersion when 

 it was 34 minutes west of it. The memoranda relative to the 

 observations, as entered in the Journal at the time are as follow : 



