VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. jg5 



hardly be necessary to take notice of a single deviation which occurs at 163°, fig. 

 2; as, from the proximity of the satellite to the conjunction, a mistake in the esti- 

 mation may easily take place. I generally made it a rule not to make allowance for 

 the influence of the superior light of the planet ; but it seems that we can hardly 

 abstract sufficiently on such occasions. Two similar cases occur, in fig. 3, at 179; 

 and fig. 4, at 5°. It is indeed not impossible but that occasional changes, on the 

 bodies of the satellites themselves, may occasion some temporary irregularity of 

 their apparent brightness: it will however not be necessary to make such an hypo- 

 thesis, till we have better authority for it. The brighter side of this satellite is 

 turned towards us when it is between the greatest eastern elongation and the con- 

 junction. It revolves consequently on its axis in 3 d 18 h I7 m .g. 



The 3d satellite suffers but little diminution of its brightness, and is in full lustre 

 at the time of both its elongations. It is however not impossible but that, after 

 having recovered its light, on the return from the opposition, it may suffer a 2d 

 defalcation of it in the nearest quadrant about half way towards the conjunction. 

 The 2 independent observations at 151 and 152°, fig. 3, seem to give some sup- 

 port to this surmise. It revolves on its axis in 7 d 3 h 5Q m .6. 



The 4th satellite presents us with a few bright views when it is going to its oppo- 

 sition, and on its return towards the greatest eastern elongation ; but otherwise it 

 is generally overcast. Its colour also is considerably different from that of the 

 other 3; and it revolves on its axis in 10 d 18 h 5 m .l. 



It will not be amiss to collect into one view, all the observations that relate to 

 the colour of the satellites. The 1st is white; but sometimes more intensely so 

 than at others. The 2d is white, bluish, and ash-coloured. The 3d is always 

 white; but the colour is of different intensity, in different situations. The 4th is 

 dusky, dingy, inclining to orange, reddish and ruddy at different times; and these 

 tints may induce us to surmise that this satellite has a considerable atmosphere. 



I shall conclude this paper with a result of the observation of the diameter of 

 the 2d satellite, taken by its entrance on the disc of the planet, July 28, 17 Q4 

 and marked in fig. 2, at 176 . The duration by the observation is fixed at 4 mi- 

 nutes; in which time it passes over an arch in its orbit of 16' 52 ff .Q. Now as its 

 distance from the planet is to its distance from the earth, so is 16' 52 /; . 9 to the 

 diameter of the satellite; or the mean distance of the 2d satellite may be rated, 

 with M. de Lalande, at 2' $7",- or 177". Then putting this equal to radius, we 

 shall have the following analogy : Radius is to 177 // » as the tangent of l6' 52 7/ .g 

 is to the angle, in seconds, which the diameter of the 2d satellite subtends when 

 seen from the earth. And by calculation, this comes out 0' / .87 ; that is less than 

 _%. of a second. 



I have not been scrupulously accurate in this calculation, as the real distance of 

 Jupiter at the time of observation should have been computed, whereas I have con- 

 tented myself with the mean distance. Nor am I very confident that the angle of 

 the greatest elongation, admitted to be 2' 57", is quite accurate; but I judged it 



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