472 



NOTES. 



Fig. 20. 



spheres balance one another on each side of this plane, and would still do so if 

 all the particles of which they consist were moveable among themselves, pro- 

 vided the earth were not disturbed by the action of the sun and moon, which 

 alters the parallelism of the equator by the small variation calledjiutation, to 

 be explained hereafter. 



NOTE 81, p. 27. If each particle, #c. Let P, P', P", &c., fig. 21, be planets 

 moving in their orbits about the centre of gravity of the system. Let P S M, 



P' S M', &c. be portions of these orbits moved over by the radii vectores S P, 

 S P', &c., in a given time, and let pS m, p'Sm', &c. be their shadows or pro- 

 jections on the invariable plane. Then, if the numbers which represent the 

 masses of the planets P, P', &c., be respectively multiplied by the numbers 

 representing the areas or spaces pSm, p'Sm', &c., the sum of the whole will 

 be greater .for the invariable plane than it would be for any plane that could 

 pass through S, the centre of gravity of the system. 



NOTB 82, p. 28. The centre of gravity of the solar system lies within the 



