20 



THE ANIMAL AS A MACHINE. 



of potential energy during its fall, and the equivalent 

 of the quantity so surrendered is found in the work 

 done upon the soil ; it finally passes away as the equiv- 

 alent energy of heat motion produced by friction and 

 impact. The potential chemical energy of the explo- 

 sive is the equivalent of the kinetic energy of the flying 

 projectile, and the latter has its equivalent in the work 

 done at the instant of striking and coming to rest, and 

 in the heat produced by the final change of mass- 

 motion into molecular or heat motion. 



Work may be defined as that operation which re- 

 sults in a change in the method of manifestation of 

 energy, and Energy as that which is transferred or 

 transformed, when work is done. The motion of a 

 projectile is the transfer of energy from one place to 

 another. It is generated at the point of departure, 

 stored as actual or kinetic energy, transferred to the 

 point of destination, and there restored and applied to 

 the production of work. 



Acceleration and retardation of masses in motion 

 can only be produced by doing work upon them, or by 

 causing them to do work, and thus, by the communi- 

 cation of energy to them or by its absorption from them, 

 in precisely the amount which measures the variation 

 of their actual energy as so produced. Every body 

 which is increasing in velocity of motion thus receives 

 and stores energy ; every mass undergoing retardation 

 must perform work, and thus must restore energy pre- 

 viously communicated to it. In every machine which 

 works continuously, and in which parts are alternately 

 accelerated and retarded, energy is stored at one period 

 and restored at another, in precisely equal amounts. 



Work done upon any machine may thus be expended 



