VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. igi 



satellite is very faint; it is not brighter than the belt, but is of a bluish, ruddy co- 

 lour. The apparent magnitude of the 2d satellite, after long looking, is very 

 nearly equal to that of the 1st; but at first sight it seems to be larger, owing to its 

 superior brightness. The apparent diameter of the 2d satellite is certainly larger 

 than that of the 4th. — At 23 h 55 m 42 s , the light of the 1st satellite, compared to 

 that of the 2d, is considerably increased since the last observation. It is now 

 nearly as bright as the 2d. 



October 16, 17 96, 23 m 49 s ; 10-feet reflector; power 600; the 1st, 2d, and 3d 

 satellites of Jupiter seem all considerably bright. The 3d is much larger than the 

 1st, and the 1st a little larger than the 2d. The intensity of the light seems to be 

 pretty equal in all the 3; that of the 2d however is perhaps a little stronger than 

 that of the 1st; for, notwithstanding its apparent less diameter, it seems to make 

 as strong an impression as the 1st. 



October 25, 1796, 21 h 44 m 48 s ; 10-feet reflector ; power 600; the 1st satellite 

 of Jupiter, compared to the 3d, is small. The 3d satellite is bright and large. 

 The 2d is brighter than the 1st. Compared to its usual brightness and magnitude, 

 it is very bright and small. The 1st satellite, compared to its usual brightness and 

 magnitude, is faint and small. The air is so tremulous that the power of 000 is 

 too high, and the necessary uniformity required in these observations will not per- 

 mit a lower to be used. Perhaps one of 400 might be more generally employed; 

 and it may be proper to use it constantly. 



November 3, 1796, 23 h 55 m 47 s ; 10-feet reflector; power 600; the 4th satellite 

 of Jupiter is large and bright. The 3d satellite is large and bright. The 1st 

 satellite is pretty small, and not very bright. The 2d satellite is small, and consi- 

 derably bright. The brightness and magnitude of each satellite refer to its own 

 usual brightness and magnitude. 



Before we can proceed to draw any conclusions from these observations, we ought 

 to take notice of many causes of deception, and of various difficulties that attend 

 the investigation of the brightness of the satellites. The difference in the state of 

 the atmosphere between 2 nights of observation, cannot influence much our esti- 

 mation of the brightness of a satellite, provided we adopt the method of compa- 

 rative estimations. If we endeavour by much practice to fix in our mind a general 

 ideal standard of the brightness of each satellite, we shall find the state of the at- 

 mosphere in different nights very much disposed to deceive us; but if we learn to 

 acquire a readiness of judging of the comparative brightness of each satellite with 

 respect to the other 3, we may arrive at much more precision, since the different 

 disposition of the air will nearly affect all the satellites alike. But here, as we get 

 rid of one cause of deception, we fall under the penalty of another. The situation 

 of those very satellites to which we are to refer the light of the satellite under esti- 

 mation, being changeable, permits us no longer to trust to their standard, without 

 a full scrutiny of the causes that may have produced an alteration in them. In the 

 foregoing observations it will also be seen, that I attempted to compare the intense- 



