244 THE OCEAN. [Book X. 



PEOPOSITION XVIII. 



Problem. — To determine the normal period in wMcli 

 the force which causes the sun's rotation endea- 

 vours to revolve the planets, as indicated by the 

 actual relative periods in which they perform their 

 orbital revolutions. 



If the revolving force be supposed to act from a 

 mathematical point in the centre of the sun, then, as 

 the actual periods decrease as the square root of the 

 cube of the distance decreases, both the actual and 

 the attempted periods would at that point be infinitely 

 decreased, making the effort of gravitation to be, to 

 give the planets an infinite number of revolutions in 

 an infinitely short period. 



A slight removal of the source of the revolving 

 force from the centre of the sun allows of the existence 

 of a definite period of attempted revolution, and the 

 greater that distance the greater the period. 



Allowing the maximum distance from which the 

 sun's gravitation can act, which is from the surface of 

 the sun, the period would then be '1169 of a day. 



The normal period sought for is therefore between 

 the two extremes above indicated. 



