12 STABILITY OF SYSTEM. SECT. III. 



primitive momentum (N. 59) of the planets, and the 

 ratio of their masses to that of the sun ; for the nature 

 of the conic sections 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 hyperbolas (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 mo- 

 mentum of the planet, when projected, must have been 

 exactly sufficient to insure the permanency and stability 

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

 greater than that of any planet ; and as their inequali- 

 ties bear the same ratio to their elliptical motions, that 

 their masses do to that of the sun, their mutual- disturb- 

 ances only increase or diminish the eccentricities of their 

 orbits, by very minute quantities ; consequently the mag- 

 nitude 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 exhib- 

 ited in the nice adjustment of these forces, at once the 

 cause of the variety and of the order of Nature. 



SECTION III. 



Perturbations, Periodic and Circular Disturbing Action equivalent to 

 three Partial Forces Tangential Force the Cause of the Periodic Ine 

 qualities in Longitude, and Secular Inequalities in the Form and Position 

 of the Orbit in its own Plane Radial Force the Cause of Variations in 

 the Planet's Distance from the Sun It combines with the Tangential 

 Force to produce the Secular Variations in the Form and Position of the 

 Orbit in its own Plane Perpendicular Force the Cause of Periodic Per- 

 turbations in Latitude, and Secular Variations in the Position of the 

 Orbit with regard to the Plane of the Ecliptic Mean Motion and Major 

 Axis Invariable Stability of System Effects of a Resisting Medium 

 Invariable Plane of the Solar System and of the Universe Great Ine- 

 quality of Jupiter and Saturn. 



THE planets are subject to disturbances of two kinds, 

 both resulting from the constant operation of their recip- 

 rocal attraction : one kind, depending upon their posi- 



