398 Prof. De Volson Wood on 



again conclude that the density of the aether may be con- 

 sidered as sensibly uniform throughout space, providing its 

 temperature be essentially uniform. 



If we assume that the law of the resistance by which the 

 aether opposes the motion of a body varies as the square of the 

 velocity of the body, we are still unable to assign the coeffi- 

 cient which will give the numerical value ; but it is safe to 

 assume that the entire mass of the aether occupying the path 

 of a body moving through it, will not have a velocity imparted 

 to it exceeding that of the body ; but to be on the safe side, 

 we will assume that it imparts a velocity equal to itself. The 

 energy thus imparted will be lost to the body. To simplify 

 the case, consider a planet moving in a circular orbit : r the 

 radius of the planet, d its distance from the sun, D its specific 

 gravity compared with water as unity, v x the velocity in its 

 orbit; then the mass of aether occupying the place of the 

 planet during one revolution about the sun will be, using 

 equation (9), 



w£w^ x27rd > 



which, multiplied by \v 2 v will give the energy imparted to it. 

 The kinetic energy of a planet, neglecting its rotation, will 

 be 



v 2 



Dividing the former, after multiplying it by ^v 2 , by the latter 

 gives 



1 d 



7 x 10 24 ' rD 



(18) 



for the fraction of the energy lost during one revolution about 

 the sun. Applying this to the earth, we have 



d-hrD = 93,000,000-r-3912x 5^ = 43000, 



and (18) becomes 



nearly, . (19) 



10 22 



for the fraction of the energy lost in one year; and hence 

 at this rate would require more than 1,666,#6# trillion 

 (1,666,000,000,000,000,000,000) years to bring it to rest. 



Equation (18) is not applicable to the resistance offered to 

 a comet, on account of the elongated orbit of the latter ; but 

 some idea of the effect of the resistance of the aether to the 

 movement of a comet may be found by considering what 



