440 



PHYSICAL SCIENCES. 



the direction 01 tne tangent p T is called the tangential force. It disturbs 

 the motion of p in longitude, that is, it accelerates its motion in some 

 parts of its orbit and retards it in others, so that the radius vector S p 

 does not move over equal areas in equal times. (See note 26.) For ex- 

 ample, in the position of the bodies in fig. 14, it is evident that, in con- 

 sequence of the attraction of d, the planet p will have its motion accelerated 

 from Q to C, retarded from C to D, again accelerated from D to 0, and 

 lastly retarded from to Q. The disturbing body is here supposed to be 

 at rest, and the orbit circular ; but, as both bodies are perpetually moving 

 with different velocities in ellipses, the perturbations or changes in the 

 motions of p are very numerous. Lastly, that part of the disturbing force 

 which acts in the direction of a linepm, fig. 13, at right angles to the 

 plane of the orbit Np n, may be called the perpendicular force. It some- 

 times causes the body to approach nearer, and sometimes to recede farther 

 from, the plane of the ecliptic Nmw, than it would otherwise do. The 

 action of the disturbing forces is admirably explained in a work on gravi- 

 tation, by Mr. Airy, the Astronomer Royal. 



NOTE 64, pp. 16, 74. Perihelion. Fig. 10, P, the point of an orbit 

 nearest the sun. 



NOTE 65, p. 16. Aphelion. Fig. 10, A, the point of an orbit farthest 

 from the sun. 



NOTE 66, pp. 16, 17. In fig. 15 the central force is 'greater than 

 the exact law of gravity ; therefore the curvature Pp a is greater than 

 Pp A the real ellipse ; hence the planet p comes to the point a, called the 

 aphelion, sooner than if it moved in the orbit P p A, which makes the line 



Fig. 15. 



Fig. 16. 



PSA advance to a. In fig. 16, on the contrary, the curvature Pp a is 

 less than in the true ellipse, so that the planet p must move through more 



than the arc P p A, or 180, before it comes 

 J'ig. 17. ^0 ^g aphelion a, which causes the greater 



axis P S A to recede to a. 



NOTE 67, pp. 16, 17. Motion of ap- 

 sides. Let P S A, fig. 17, be the position 

 of the elliptical orbit of a planet, at any 

 time ; then, by the action of the disturbing 

 forces, it successively takes the position 

 F S A', P" S A", &c., till by this direct 

 motion it has accomplished a revolution, 

 and then it begins again ; so that the mo- 

 tion is perpetual. 



NOTE 68, p. 17. Sidereal revolution. 

 The consecutive return of an object to the same star. 



