32 EFFECTS OF JUPITER'S COMPRESSION. [SECT. rv. 



exterior body is the same as if its mass were united in one 

 particle in its centre of gravity, and therefore inversely as 

 the square of the distance. In a spheroid, however, there 

 is an additional force arising from the bulging mass at its 

 equator, which, not following the exact law of gravity, acts 

 as a disturbing force. 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 actiCn of the sun. The same cause occasions the 

 orbits of the satellites to remain nearly in the plane of Jupi- 

 ter's equator (N. 87), on account of which the satellites are 

 always seen nearly in the same line (N. 88) ; and the pow- 

 erful 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 at- 

 traction 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 bypulling 

 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 incli- 

 nation, 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 



