Lowric] 224 [Xov. 19, 



nation, unless there be some cause that affects the fixedness of one of 

 them. 



We are to seek the cause of recession of nodes in a system so con- 

 structed and so operating, by the force of cosmical attraction, that this 

 very force will appear to be the cause, and that we may see its mode of 

 operation, if it be really there. To illustrate such a structure, we may 

 take any planet or satellite of the solar system ; for all alike have this 

 cosmical force and this inclination of orbital plane, and this recession of 

 nodes. 



We take the moon in its revolution. Because of this inclination, one 

 half of its orbit is above and the other half below the plane of the earth's 

 orbit. While the earth is sweeping around in its great orbit, it swings 

 the moon around it, as upon an epicycle of which the earth is the centre. 

 A proper conception of these two inotions in relation to each other gives 

 us the direction of the central force which produces the moon's motion. 



It is never directed from a point fixed as the centre of the orbit, nor 

 from a straight line, constituted by a motion of such a point, but from a 

 centre always moving in a line curving eastwardly, and in a direction dif- 

 fering from that of the moon's orbit according to the different inclinations 

 of the earth's and the moon's orbits. It is the very force which bears the 

 moon forward in space, and yet, by reason of the form of their connec- 

 tion, it is always moving laterally and eastwardly out of the centre of the 

 moon's plane, and tending also to push forward through and beyond the 

 plane, and thus it is aU the v/hile exerting its force in a sort of twisting of 

 the moon's orbit into perpetual accommodation to the curve of the orbit 

 of its primary. 



The result of this is, that no matter what may be the position of the 

 moon's plane, this force, always departing from a right line, constantly 

 draws the moon down or up through the plane of the earth's orbit sooner 

 in each successive revolution ; and this is equivalent to a westward warp- 

 ing or tilting motion of the moon's plane, so that it cuts that of the earth 

 more and more westward in each revolution ; and this would constitute a 

 constant recession of the nodes, even if there were no other causes of it ; 

 and it ought not to be overlooked. 



If this is a correct reading of this force and its dependent motions, 

 which I submit to those who may consider the subject Vt^orth thinking 

 about, then the central force of every planet operates in precisely the 

 same form on its sa,tellites, where their orbital planes are inclined, varied 

 only according to their degrees of inclination. And, of course, the. sun 

 (assuming its motion to be as heretofore stated) operates in the same way 

 upon all the planets, so as to produce a recession of their nodes ; and the 

 phenomenon of recession of nodes, even if not entirely normal, has a per- 

 fectly normal cause. 



It follows also, that wherever we find a constant recession of the nodes 

 of a secondary body, we may, naturally infer that its primary is itself re- 

 volving ai-ound some central body ; though it will be impossible to say that 



