34 On Machines in General. 



they really are, and as nature presents them to us, i. e. 

 which are of a finite density. 9d. Those to which we ascribe 

 a density infinitely great, and which, for this reason, must 

 be regarded as sensibly fixed and immoveable. 3d. Those 

 to which we ascribe a density infinitely small, or null, 

 and which, consequently, by their inertness, oppose no re- 

 sistance to their change of state. In practice we generally 

 regard as such, wires, rods, and generally all bodies which 

 do not inHuence sensibly, by their proper mass, the changes 

 which happen in the system ; but which are solely regarded 

 as means of communication between the different agents 

 which compose it. 



Tlwd Remark. 

 XXX. After having treated of equilibrium and of move- 

 ment in general, as much as my principal object permitted, 

 I shall pass to what regards more particularly what we com- 

 monly understand by machines ; for although the theory of 

 every kind of equilibrium and movement always enters into 

 the preceding principles, since there are only, according to 

 the first law, bodies which can destroy or modify the move- 

 ment of other bodies ; nevertheless there are cases where 

 we make abstraction of the mass of these bodies, merely 

 for the purpose of considering the eflort they make : for ex- 

 ample, when a man draws a body by a wire, or pushes it 

 by a rod, we do not introduce into the calculation the mass 

 of this man, nor even the eflfort of which he is capable, but 

 solely that which he exerts upon the point to which he ap- 

 plies it ; i. e. the tension of the wire, if it is by drawing that 

 it acts, or the pressure, if it be by pushing j and without 

 considering whether it be a man, an animal, a weight, a 

 sprinff, or a resistance occasioned by any obstacle, or by the 

 Vis inertice of a moveable body*, a friction,- au impulse caused 



by 



* Any body which we force to -change its state of repose, er of movement, 

 resists (XI.) the agent which produces the change; and it u this resistance 

 •which we call lis iiKrlice. Ir» order to find the value of this force, we must 

 decompose the acruaJ movement of the body into two, one of which is that 

 which it will have the instant afterwards ; for the other will be evidently 

 tliat wiiich must be destroyed, in order to force the body to change its state; 



i, e, the 



