RESISTING MEDIUM. 199 



which come under our consideration in problems 

 of this kind, are enormous to common apprehen- 

 sion. Thus Encke's comet, according to the 

 results of the observations already made, will 

 lose, in ten revolutions, or thirty -three years, 

 less than 1- 1000th of its velocity: and if this 

 law were to continue, the velocity would not be 

 reduced to one-half its present value in less than 

 seven thousand revolutions or twenty-three thou- 

 sand years. If Jupiter were to lose one-millionth 

 of his velocity in a million years, (which, as has 

 been seen, is far more than can be considered in 

 any way probable,) he would require seventy 

 millions of years to lose l-1000tli of the velocity ; 

 and a period seven hundred times as long to re- 

 duce the velocity to one-half. These are periods 

 of time which quite overwhelm the imagination ; 

 and it is not pretended that the calculations are 

 made with any pretensions to accuracy. But at 

 the same time it is beyond doubt that though the 

 intervals of time thus assigned to these changes 

 are highly vague and uncertain, the changes them- 

 selves must, sooner or later, take place, in conse- 

 quence of the existence of the resisting medium. 

 Since there is such a retarding force perpetually 

 acting, however slight it be, it must in the end 

 destroy all the celestial motions. It may be 

 millions of millions of years before the earth's 

 retardation may perceptibly affect the apparent 

 motion of the sun ; but still the day will come (if 



