4S ANIMAL MECHANISM. 



be estimated by enlarging the definition, of mechanical work. 

 Thus, a horse which tows a boat, a man who planes a board, 

 a bird which strikes the air with its wing, does mechanical 

 work, and yet they do not lift weights. In order to reduce 

 cases of this kind to a general definition, we must admit as 

 the expression of work, the effort wuhijilied by the spnce traversed. 

 This effort, besides, may always be compared with the weight, 

 the lifting of which would necessitate an equal effort, so that 

 we say of a traction or an impulse, that it corresponds with 

 10 or 20 kilogrammes. When a workman planes or turns a 

 piece of metal, if the tool which he drives into it penetr.ctes 

 only on condition of receiving an impulse of one kilogramme, 

 the workman, in order to have effected a kilogrammetre of 

 work, ought to have detached from the mass a shaving of a 

 metre in length. A horse which tows a boat with 20 kilo- 

 gramme force, will have employed a force of 20,000 kilo- 

 grammetres when he has gone 1,000 metres. 



But still that is not yet sufficient to be applied to all the 

 forms of mechanical labour. If, for example, force be em- 

 ployed to displace a mass, the effort necessary for the move- 

 ment will vary with the speed which is given to that mass. 

 Let us imagine a block of stone suspended freely at the end 

 of a very long rope; the lightest pressure applied to this 

 block for a few instants will produce movement in it, while 

 the strongest blow of the fist will scarcely cause any sensible 

 displacement, because the force requisite to displace masses 

 increases according to the square of the speed which is com- 

 municated to them.* 



A force of very short duration applied to a mass, produces 

 only a shock incapable of displacing it. But this same shock, 

 if it be exerted by means of an elastic medium, is transformed 

 into an act of longer duration, and without having added 

 anything to the quantity of motion, becomes capable of pro- 

 ducing work. 



This elasticity intervenes in the animal economy to permit 

 the utilization of the very brief act which constitutes the 

 formation of the muscular wave. The formation of the wave, 



* Tins action is expressed liy 



m 



