SECT, in.] PERTURBATIONS. 15 



SECTION III. 



Perturbations, Periodic and Secular Disturbing Action equivalent to three 

 Partial Forces Tangential Force the cause of the Periodic Inequalities 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 Dis- 

 tance 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 Perturbations in Lati- 

 tude, 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 Inequality of Jupiter and Saturn. 



THE planets are subject to disturbances of two kinds, both 

 resulting from the constant operation of their reciprocal 

 attraction : one kind, depending upon their positions with 

 regard to each other, begins from zero, increases to a 

 maximum, decreases, and becomes zero again, when the 

 planets return to the same relative positions. In conse- 

 quence of these, the disturbed planet is sometimes drawn 

 away from the sun, sometimes brought nearer to him : 

 sometimes it is accelerated in its motion, and sometimes 

 retarded. At one time it is drawn above the plane of its 

 orbit, at another time below it, according to the position 

 of the disturbing body. All such changes, being accom- 

 plished in short periods, some in a few months, others in 

 years, or in hundreds of years, are denominated periodic 

 inequalities. The inequalities of the other kind, though 

 occasioned likewise by the disturbing energy of the planets, 

 are entirely independent of their relative positions. They 

 depend upon the relative positions of the orbits alone, whose 

 forms and places in space are altered by very minute quan- 



