56 Remarks on Inertia. 



would possess any self-moving power; but from what we 

 know of the mutual repulsion between the particles of mat- 

 ter, so that no two are actually in contact, if gravity were 

 destroyed, there can be no doubt that the particles, mu- 

 tually repelling each other, would be dissipated throughout 

 the universe. Nor is it true that any force is lost on the im- 

 pact of bodies : in all cases, the equation v = (when 



M-f N 

 V is the velocity of the body M before it impinged against N 

 at rest : v is the common velocity after the stroke) shows 

 that v will have a positive value, unless N be infinitely great ; 

 and universally uX(M-)-N)=VM : Therefore there is no 

 force lost ; and consequently there is no force in matter 

 which must be overcome in causing motion. We grant that 

 this meaning is not applicable to professor F's hypo- 

 thesis. 



A different statement of what Brisson deems his decisive 

 argument, may perhaps place it in a clearer light. Two bo- 

 dies, elevated above the surface of the earth, if they be free, 

 will be put in motion by the force of gravity, as they have 

 no self-directive power : they will move towards the earth 

 with a certain velocity ; and if we wish to increase the velo- 

 city of either of them, we must apply an additional force : 

 not because of any force in matter, but because there is 

 none. The same observation is applicable to a similar illus- 

 tration of Prof. Farrar's : "If, while a body is falling freely, 

 it be forced forward by the hand with a velocity greater than 

 that of its natural descent, the hand will experience on over- 

 taking the body, a blow, or resistance :"* — that is, to increase 

 the velocity of a falling body, we must, increase the cause of 

 this velocity ; — we must augment the force : and the resis- 

 tance, which the hand experiences is a natural consequence 

 of the reaction of the body ; and the blow is exactly equal to 

 what would have been received, if the hand had been at rest, 

 and the body had struck against it with a velocity equal to 

 the difference of the velocities of the hand and the body. 



* Cambridge Mechanics, p. 180. " Supon gamos un cuerpo que cal libremente 

 a impulsos de su pesantez ; si le damos con la mano paraque cayga mas aprisa, 

 esperimentarerJos tambien resistencia." Elementos de Mat. vol. 4, p. 9. The 

 verbal coincidence of this with the quotation given above from the " Cambridge 

 Mathematics," arises from Bails' having also used the " Cours de Mathematiques 

 par Bezout" in the composition of his work. The same may be said of the quo- 

 tations respecting the communication of motion. 



