EQUILIBRATION. 505 



the Solar System is undergoing. Each planet, satellite, 

 and comet, exhibits to us at its aphelion a momentary equi- 

 librium between that force which urges it further away from 

 its primary, and that force which retards its retreat; since 

 the retreat goes on until the last of these forces exactly 

 counterpoises the first. In like manner at perihelion a con- 

 verse equilibrium is momentarily established. The varia- 

 tion of each orbit in size, in eccentricity, and in the position 

 of its plane, has similarly a limit at which the forces pro- 

 ducing change in the one direction, are equalled by those 

 antagonizing it; and an opposite limit at which an opposite 

 arrest takes place. Meanwhile, each of these simple per- 

 turbations, as well as each of the complex ones resulting from 

 their combination, exhibits, besides the temporary equilibra- 

 tion at each of its extremes, a certain general equilibra- 

 tion of compensating deviations on either side of a mean 

 state. That the moving equilibrium thus constituted, 



tends, in the course of indefinite time, to lapse into a com- 

 plete equilibrium, by the gradual decrease of planetary mo- 

 tions and eventual integration of all the separate masses com- 

 posing the Solar System, is a belief suggested by certain 

 observed cometary retardations, and entertained by some of 

 high authority. The received opinion that the appreciable 

 .diminution in the period of Encke's comet, implies a loss of 

 momentum caused by resistance of the ethereal medium, 

 commits astronomers who hold it, to the conclusion that this 

 same resistance must cause a loss of planetary motions a 

 loss which, infinitesimal though it may be in such periods as 

 we can measure, will, if indefinitely continued, bring these 

 motions to a close. Even should there be, as Sir John 

 Herschel suggests, a rotation of the ethereal medium in the 

 same direction with the planets, this arrest, though im- - 

 mensely postponed, would not be absolutely prevented. 

 Such an eventuality, however, must in any case be so incon- 

 ceivably remote as to have no other than a speculative inter- 

 est for us. It is referred to here, simply as illustrating the 

 34 



