NOTES. 



439 



NOTE 59, p. 11. Momentum. Force measured by the weight of a body 

 and its speed, or simple velocity, conjointly. The primitive momentum 

 of the planets is, therefore, the quantity of motion which was impressed 

 upon them when they were first thrown into space. 



NOTE 60, p. 11. Unstable equilibrium. A body is said to be in equi- 

 librium when it is so balanced as to remain at rest. But there are two kinds 

 of equilibrium, stable and unstable. If a body balanced in stable equili- 

 brium be slightly disturbed, it will endeavour to return to rest by a number 

 of movements to and fro, which will continually decrease till they cease 

 altogether, and then the body will be restored to its original state of repose. 

 But, if the equilibrium be unstable, these movements to and fro, or oscil- 

 lations, will become greater and greater till the equilibrium is destroyed. 



NOTE 61, p. 14. Retrograde. Going backwards, as from east to west, 

 contrary to the motion of the planets. 



NOTE 62, p. 14. Parallel directions. Such as never meet, though 

 prolonged ever so far. 



NOTE 63, pp. 14, 16. The whole force, $c. Let S, fig. 13, be the sun, 

 N m n the plane of the ecliptic, p the disturbed planet moving in its orbit 



Fig. 13. 



, and d the disturbing planet. Now, d attracts the sun and the 

 planet p with different intensities in the directions d S, dp : the difference 

 only of these forces disturbs the motion of p ; it is therefore called the dis- 

 turbing force. But this whole disturbing force may be regarded as equi- 

 valent to three forces, acting in the directions p S,p T, and p m. The force 

 acting in the radius vector p S, joining the centres of the sun and planet, 

 is called the radial force. It sometimes draws the disturbed planet p from 

 the sun, and sometimes brings it nearer to him. The force which acts in 



