22 



THE ANIMAL AS A MACHINE. 



absorbed by the friction and other prejudicial resist- 

 ances of the mechanism, and which thus wastes energy 

 which might otherwise be usefully appHed. These two 

 quantities, together, constitute the Total Work or the 

 Gross Work of a machine, or of a train of mechanism. 

 In every case some energy is wasted, and the work done 

 by the machine is by that amount less than the work 

 performed in driving it. In badly proportioned ma- 

 chines the lost work is often partly expended in the 

 deformation and destruction of the members of the 

 construction ; in well-designed and properly worked 

 machinery loss occurs wholly through friction. In ma- 

 chines acting upon fluids this work is usually partly 

 wasted in the production of fluid friction — i.e., of cur- 

 rents and eddies ; thus producing new forms of actual 

 energy in ways which are not advantageous : even this 

 waste energy is finally converted, like the preceding 

 form, by molecular friction, into heat, and is dissipated 

 in that form of molecular energy. Thus all wasted 

 work is lost by conversion from the energy of mass- 

 motion into molecular energy, and ultimately disappears 

 as heat. 



The Efficiency of Mechanism is measured by the quan- 

 tity obtained by dividing the amount of useful work 

 performed, by the gross work of the piece or of the sys- 

 tem. It is always, therefore, a fraction, and is less than 

 unity ; which latter quantity constitutes a limit which 

 may be approached more and more nearly as the wastes 

 of energy and work are reduced, but can never be quite 

 reached. If the mean useful resistance be R, and the 

 space through which it is overcome be s , and if the 

 mean effort driving the machine be P, and the space 

 through which it acts be s, the total and the net or nse- 



