PROGRESSIVE EFFECTS. 37 



gressive. The orbit of a planet is determined (omitting 

 perturbations) by two causes : first, the action of the 

 central body, a permanent cause, which alternately 

 increases and diminishes as the planet draws nearer to 

 or goes further from its perihelion, and which acts 

 moreover at every point in a different direction; and, 

 secondly, the tendency of the planet to continue 

 moving in the direction and with the velocity which it 

 has already acquired. This force also grows greater 

 as the planet draws nearer to its perihelion, because as 

 it does so its velocity increases ; and less, as it recedes 

 from its perihelion : and this force as well as the other 

 acts at each point in a different direction, because at 

 every point the action of the central force, by de- 

 flecting the planet from its previous direction, alters 

 the line in which it tends to continue moving. The 

 motion at each instant is determined by the amount 

 and direction of the motion and the amount and 

 direction of the sun's action at the previous instant : 

 and if we speak of the entire revolution of the planet 

 as one phenomenon (which, as it is periodical and 

 similar to itself, we often find it convenient to do) , 

 that phenomenon is the progressive effect of two 

 permanent and progressive causes, the central force 

 and the acquired motion. Those causes happening to 

 be progressive in the particular way which is called 

 periodical, the effect necessarily is so too; because, the 

 quantities to be added together returning in a regular 

 order, the same sums must also regularly return. 



This example is well worthy of consideration also 

 in another respect. Although the causes themselves 

 are permanent, and independent of all conditions 

 known to us, the changes which take place in the 

 quantities and relations of the causes are actually 

 caused by the periodical changes in the effects. The 



