14 STABILITY OF SYSTEM. [SECT. rr. 



turbances it experiences, on the general mechanical prin- 

 ciple, that the sum of any number of small oscillations is 

 nearly equal to their simultaneous and joint effect. 



On account of the reciprocal action of matter, the stability 

 of the system depends upon the intensity of the primitive 

 momentum (N. 59) of the planets, and the ratio of their 

 masses to that of the sun ; for the nature of the conic sec- 

 tions in which the celestial bodies move depends upon the 

 velocity with which they were first propelled in space. Had 

 that velocity been such as to make the planets move in orbits 

 of unstable equilibrium (N. 60), their mutual attractions 

 might have changed them into parabolas, or even hyper- 

 bolas (N. 22) ; so that the earth and planets might, ages 

 ago, have been sweeping far from our sun through the abyss 

 of space." But as the orbits differ very little from circles, the 

 momentum of the planet, when projected, must have been 

 exactly sufficient to ensure the permanency and stability of 

 the system. Besides, the mass of the sun is vastly greater 

 than that of any planet ; and, as their inequalities bear the 

 same ratio to their elliptical motions that their masses do to 

 that of the sun, their mutual disturbances only increase or 

 diminish the excentricities of their orbits by very minute 

 quantities ; consequently the magnitude of the sun's mass 

 is the principal cause of the stability of the system. There 

 is not in the physical world a more splendid example of the 

 adaptation of means to the accomplishment of an end, than 

 is exhibited in the nice adjustment of these forces, at once 

 the cause of the variety and of the order of Nature. 



