On Machines in Genei-al. 30 1 



ment. We perceive that there is a medium, by means of 

 which we shall determine, if not in an absolute manner, at 

 least, having regard to the nature of the machine, that me- 

 thod which will be capable of the greatest effects. 



LX. Another general condition, which is not less impor- 

 tant when wc wish that machines should produce the greatest 

 possible effect, is, to contrive that the soliciting forces should 

 give rise to r.o movement inapplicable to the object in view. 

 If my object, for example, is to raise to a given height the 

 greatest quantity of water possible, whether with a pump or 

 otherwise, I should contrive that the water on flowing into 

 the upper reservoir should only have precisely as much ve- 

 locity as was necessary and no more, for all beyond this 

 quantity would uselessly consume the effort of the motive 

 power. It is clear in fact (XLV.), that in this case this 

 power would have to consume an useless momentum of ac- 

 tivity, and which would be equal to the half of the real 

 force with which the water would have arrived in the re- 

 servoir. 



It is not less evident, that in order to give the machines 

 the greatest effect possible, we should avoid or diminish, 

 at least as much as possible, the passive powers, such as 

 friction, rubbino- of cords, the resistance of the air, which 

 are always, in whatever direction the machine moves, among 

 the nuniber of the forces I have called resisting*. 



It would be easy to extend these particular remarks, but 

 my object is not to enter at present into any larger detail. 



LXI. It may be concluded, from what has been said on 

 the subject of friction and other passive bodies, that per- 

 petual motioTi is a thing absolutely impossible, by only em- 

 ploying in order to produce it bodies which would not be 

 solicited by any mplrix force, and even heavy bodies ; for 



♦ We often liear of pas-jve forces ; Imt where is the diiTercnce between aa 

 active and a passive force ? I think this question has never yet been an- 

 swered. Now it appears to me tliat the distinctive character of passive forces 

 consists ip this, that they never can become soliciting forces, w!:atevcr may 

 be the movement of the machine, while active forces can act sometimes in 

 the quality of soliciting and sometimes as resisting forces. In tiiis view, 

 ol)Stacies and fixed points are evidently passive forces, siacc they can neither 

 act as soliciting nor as rcisang forces (XXXI). 



these 



