Lovvrie.] -^-^^ [Xov. 19, 



principal orbit, and, so far as we know, this form i^ervades the whole 

 solar system. 



True, we know not yet the direction of the real orbit of the sun, and, 

 therefore, cannot tell how the other orbits ai-e inclined to it. But we 

 know that all the others have different inclinations, and that, therefore, 

 not more than one, and probably none, of them coincides with the sun's. 

 And if the observations of Sir William Herschel and his successors, on 

 the course of the sun, are near the truth, then it is proved that all are so 

 inclined; and we do not mark recessions on the sun's orbit because we 

 have not yet found where it is. Finding the law that recession of nodes 

 always accompanies their existence, we naturally expect a like cause for 

 all cases, a cause growing out of like relations to the main force of the 

 system or sub-system ; and therefore we ought to study how the central 

 force operates on a dependent body moving in that form. 



Let us be sure, even at the risk at an unnecessary presentation of rudi- 

 ments, that we have a right possession of this phenomenon of the reces- 

 sion of the nodes, and that it is a phenomenon of the earth's motion. It 

 is, of course, difficult for a person unused to the study of the motions of 

 the solar system to form or retain veiy clear conceptions of all their 

 changing complications. He will often be mistaken in his geometry of 

 the heavens, and may seldom have the pleasure of more than a transient 

 confidence in his conceptions about it. Occupying a revolving and rotat- 

 ing position, and obliged to find from it the courses and velocities of the 

 shifting currents of the cosmical ocean, and fix them by the floating land- 

 marks of the skies, he will often get confused and suspect himself in- 

 competent. 



We shall not need to go beyond the instances of the earth and moon to 

 get illustrations of this motion sufficient to show its unity of form and 

 unity of relation to the central body. It is involved in the geometrical 

 conception of a cosmical system, that, where its orbital planes differ in 

 inclination, each must internode with all the others by a line passing 

 through their common centre, and this is the line of its nodes. But if 

 the planes always maintained the same direction in space, there could be 

 no motion of nodes, and these cross-roads of the skies would be less im- 

 portant and interesting than they now are . 



It is admitted that the axes of rotation of all the planets and satellites, 

 except the earth, are fixed and stable, so that they change direction only 

 with their orbital planes and not in them, and it is supposed that the 

 earth alone tilts in its plane. It is admitted also that all these planes ex- 

 cept the earth's have a constant warping or tilting motion westward, and 

 that their bodies tilt iciilh them, and this causes these planes to cut through 

 any fixed plane further westward in each revolution, and the lines of their 

 nodes to recede on any such plane, and the ecliptic is taken as such a 

 one ; but it is supposed that a similar appearance is produced, relative to 

 the earth, by a tilting of the earth itself in its plane, marked by its equa^ 

 tor on the ecliptic, and not by a tilting of its plane. If this be so, then 



