ENERGY AND ITS TRANSFORMATIONS. 35 



in a gaseous or vaporous fluid, the variations of tem- 

 perature, pressure, and volume of which result in the 

 production, more or less efficiently, of mechanical power 

 in readily applicable form. 



The heat-engines do by far the greater part of the 

 work of the world, and the steam-engine the main por- 

 tion of that performed by thermodynamic operations. 



Solar engines^ so-called, are heat-engines in which the 

 direct heat energy of the sun, instead of the stored heat 

 energy of a combustible, is utilized through the action of 

 a working fluid, as with other forms of machine of this 

 class. 



(3) The ivater-whcelsy including the various classes of 

 so-called vertical wheels, and the turbines ; in which 

 latter, the water, instead of entering buckets," to be 

 again poured out of them, passes continuously through 

 channels, without reversal of motion. 



These machines effect no transformations of energy ; 

 but simply turn it out of its natural course into an arti- 

 ficial channel of application. It is kinetic, as found, 

 and remains kinetic until transformed in the course of 

 its application to its intended purpose. 



(4) Tidal machines are simply floats, rising and fall- 

 ing with the tides ; or they are vertical water-wheels, 

 working in tidal currents in precisely the same manner 

 as those operated by ordinary running streams. They 

 transfer, but do not transform, energy. 



(5) Windmills are pneumatic turbines, especially 

 fitted to take up the energy of moving air, and to transfer 

 it, without transformation, to machinery of transmission, 

 through which it is conveyed to its point of application. 



(6) Electrical engines, electro-dynamic machines, 

 dynamos and motors, as they are variously called, are 



