208 THE WORLD-ENERGY 



energy has been transformed into that of the moving mass 

 of the bail. 



It is true the force acts through time. But it is equally 

 true that the effect produced by the progressively devel- 

 oped force is unfolded in precisely the same gradual way, 

 so that the force considered as cause is not precedent to, 

 but simultaneous with, the various phases of the complex 

 effect produced disturbance of the air, killing of the 

 bird, etc. 



It is worth noticing, too, that the example chosen pre- 

 sents the two phases of force commonly called "constant" 

 and " impulsive " forces. Gravity is a ' ' constant " force. 

 The explosion of gun-powder is an " impulsive " force. 

 The doctrine of the correlation of forces, however, shows 

 that this distinction is in reality the same as that between 

 continuous and discrete quantity, which we have already 

 considered. Indeed, physicists themselves treat this dis- 

 tinction as a fiction, when, for the sake of convenience, 

 they consider a continuous force as made up of minute 

 impulsive forces; which seems a little like reducing every- 

 thing to infinitesimals preliminary to omitting the incon- 

 venient parts. Though, of course, it must be admitted 

 that in any of the "exact" sciences, quantity is scarcely 

 manageable, otherwise than in its discrete aspect. 



With a moment's reflection, however, it is evident that 

 in all cases the exertion of force through time must be 

 constant, that is, continuous. At the same time it is 

 not less true that the known correlation of the various 

 phases of force shows with equal plainness that force is at 

 the same time impulsive. A constant force is not merely 

 made up of a series of independent impulses. It gives 

 impulse because it is constant. And the changes produced 



