PHYSICAL ASTRONOMY* 289 



gative, extends to = A, and passes from thence 

 to again ; the period of all those changes de- 

 pending on n the multiplier of t. 



If into the value of any of the inequalities, a term of 



the form, A tan nt, -. r , or of the form Ant, 



smnt 



were to enter, the inequality so expressed, would 

 continually increase, and the order of the system 

 might finally be displayed. 



LA GRANGE and LA PLACE, in demonstrating that 

 no such terms as these last can enter into the ex- 

 pression of the disturbances of the planets, made 

 known one of the most important truths in physi- 

 cal astronomy. They proved, that the planetary 

 system is stable ; that it does not involve any 

 principle of destruction in itself, but is calcula- 

 ted to endure for ever, unless the action of an 

 external power be introduced. 



290. This accurate compensation of the ine- 

 qualities of the planetary motions, depends on 

 three conditions, belonging to the primitive and 

 original constitution of the system : 



I. That the eccentricities of the orbits are all 

 inconsiderable, or contained within very 

 narrow limits : 



VOL. II. T II. That 



