Remarks on Inertia. 55 



la boule resiste done a une force reelle, et la detruit avant 

 de pouvoir agir comme pesante ; elle resiste done par une 

 force independante de sa pesanteur, et e'est cette force 

 qu'on appelle force d'inertie."* 



If any are not yet convinced, the following is supposed by- 

 its author sufficient to remove all doubts. Two homogene- 

 ous balls, similar in every respect, fall from the same height 

 to the earth, in exactly the same length of time: "Si Ton 

 veut que Tun des deux precede l'autre dans sa chute, il faut 

 a l'effort de. sa pesanteur ajouter une autre force ; il faut lui 

 donner une nouvelle impulsion, qu'il ne peut pas recevoir de 

 sa pesanteur puisque nos supposons qu'il lui, obeit complete- 

 ment. Or tout ce qui exige une force pour etre produit, est 

 une veritable resistance. Ce corps qui en tombant libre- 

 ment, obeit complement a la pesanteur, resiste done a un 

 mouvement plus prompt que celui qui vient de la pesanteur : 

 il y resiste done par une force independante de sa pesanteur, 

 e'est cette force qu'on appelle force d'inertie."t 



Now we do not contend that gravity and inertia are the 

 same : on the contrary, we believe the former to be an active 

 force, and the latter to be nothing more than the negation of 

 action : but we do say, that all the phenomena may be ex- 

 plained from ascertained facts, — that matter is absolutely 

 passive, and at the same time acted upon by the mutual at- 

 traction which influences all its particles. In the case of the 

 suspended ball, which has been given, it is not accurate to 

 say that any force would be too small to move it, nor that 

 any force is destroyed in causing its displacement. If P be 

 power applied to cause the motion of the suspended ball B, 

 and a be the angle made by the direction of the line of sus- 

 pension with the vertical, P will always be as B sin. «, while 

 the angle a has a real value : that is, P : B sin. a. Now 

 however small we take the angle a, this formula will be true, 

 so long as the angle exists. When a is nothing, though we 

 cannot say that P would also become nothing, — that is that 

 B would move without a force because matter is inert, we 

 are well assured that the force required to cause motion in 

 such circumstances, would be indefinitely small. If the 

 gravity of matter were annihilated, on the supposition that 

 it would still exist in masses, we cannot conaeive that it 



* Traite Ele. de Physique, tome 1, pp. 46, 47. t Ibid. p. 47. 



