28 EFFECTS OF JUPITER'S COMPRESSION. SECT. IV. 



One effect of this disturbing force in the spheroid of Jupiter is to 

 occasion a direct motion in the greater axes of the orbits of all 

 his satellites, which is more rapid the nearer the satellite is to 

 the planet, and very much greater than that part of their motion 

 which arises from the disturbing action of the sun. The same 

 cause occasions the orbits of the satellites to remain nearly in the 

 plane of Jupiter's equator (N. 87), on account of which the 

 satellites are always seen nearly in the same line (N. 88) ; and 

 the powerful action of that quantity of prominent matter is the 

 reason why the motions of the nodes of these small bodies are so 

 much more rapid than those of the planet. The nodes of the 

 fourth satellite accomplish a tropical revolution in 531 years, 

 while those of Jupiter's orbit require no less than 36,261 years ; 

 a proof of the reciprocal attraction between each particle of 

 Jupiter's equator and of the satellites. In fact, if the satellites 

 moved exactly in the plane of Jupiter's equator, they would not 

 be pulled out of that plane, because his attraction would be equal 

 on both sides of it. But, as their orbits have a small inclination 

 to the plane of the planet's equator, there is a want of symmetry, 

 and the action of the protuberant matter tends to make the nodes 

 regress by pulling the satellites above or below the planes of 

 their orbits ; an action which is so great on the interior satellites, 

 that the motions of their nodes are nearly the same as if no 

 other disturbing force existed. 



The orbits of the satellites do not retain a permanent inclina- 

 tion, either to the plane of Jupiter's equator, or to that of his 

 orbit, but to certain planes passing between the two, and through 

 their intersection. These have a greater inclination to his 

 equator the farther the satellite is removed, owing to the in- 

 fluence of Jupiter's compression ; and they have a slow motion 

 corresponding to secular variations in the planes of Jupiter's 

 orbit and equator. 



The satellites are not only subject to periodic and secular 

 inequalities from their mutual attraction, similar to those which 

 affect the motions and orbits of the planets, but also to others 

 peculiar to themselves. Of the periodic inequalities arising from 

 their mutual attraction the most remarkable take place in the 

 angular motions (N. 89) of the three nearest to Jupiter, the 

 second of which receives from the first a perturbation similar to 

 that which it produces in the third ; and it experiences from the 



