PHYSICAL ASTRONOMY. 285 



It follows, that Long. 1st Sat , 3 Long. 2d Sat. + 

 2 Long. 3d Sat. = a constant quantity ; and it 

 has been found, since ever the satellites were ob- 

 served, that this constant quantity has been near- 

 ly equal to 180. This last must be the result of 

 original constitution. 



283. The first satellite moves nearly in the 

 plane of Jupiter's equator, and has no eccentri- 

 city except what is communicated to it from 

 the third and fourth ; the irregularities of one 

 of these small planets producing similar irregu- 

 larities in the rest. It has beside an inequality 

 chiefly produced by the action of the second, 

 and circumscribed by the period of 437.659 

 days. 



The orbit of the second satellite moves 

 on a fixed plane, to which it is inclined at an 

 angle of 27'. 13", and on which its nodes have a 

 retrograde motion, so that they complete a re- 

 volution in 29.914 years. 



The motion of the nodes of this satellite is one of the 

 principal data that have been used for determining 

 the masses of the satellites, which are so necessary 

 to be known in computing their disturbances. 

 This satellite has no eccentricity but what it de- 

 rives from the action of the third and fourth. 



285. The 



