32 



TliE ANIMAL AS, A MACHINE. 



to the higher portions of the land, again to return in 

 the streams to the sea-level. 



(3) The kinetic energy of the air-currents, as utihzed 

 by windmills, which convert it to useful purposes pre- 

 cisely as the water-wheel intercepts the energy of fall- 

 ing water with a similar end. 



The primary source of this energy is again the heat 

 of the sun, which produces a convection of air-cur- 

 rents similar dynamically, in method and result, to 

 those of water in the ocean or over or on the land. 



(4) The energy of the tides, the rise and fall of which 

 constitute a source of power less easy of utilization 

 than that of streams, and for this reason, rarely applied 

 to the production of power. 



The origin of this energy is in the force of gravita- 

 tion as it acts upon the ocean, changing its level through 

 the attractions of the sun and the moon. This is seen 

 to be essentially different from the other cases. 



(5) The energy of electricity, originally exhibited 

 either in the form of molecular energy resulting from 

 chemical action, or produced by transformation directly 

 from the dynamic form. In this latter case, the ma- 

 chine transforming it is an intermediate or a secondary, 

 instead of a prime, motor. In the former case the 

 origin is potential, in the latter kinetic. 



(6) The energy of muscular action, the power of ani- 

 mals, derived from the chemical forces acting in the 

 production of vegetation and transformed for use in the 

 animal system, through either thermo-electric or thermo- 

 dynamic processes, or perhaps through the action of 

 both, each having its appropriate task. 



In the animal system, the vegetable matter employed 

 as food is converted by the natural forces of digestion 



