PRIME-MOVERS. 353 



men working at one time, we find that the prime-mover has another and 

 most important claim upon our interest, namely, that it enables us to 

 attain results it would be absolutely impossible to attain by any 

 aggregation of human or other muscular effort, however brutally indif- 

 ferent we might be to the misery of those who were engaged in that 

 effort. 



Excluding from our consideration light and even electricity as 

 not being up to the present sources of power on which we rely in 

 practice, there remain three principal groups into which our Prime- 

 Movers may be arranged : viz., those which work by the agency of 

 the wind, those which work by the agency of water, and those which 

 work by the agency of heat. 



Of these three great groups two, heat and water, are capable of 

 division, and indeed demand division into various branches. 



A water Prime-Mover may be actuated by the impact of water, as 

 in some kinds of water-wheels, turbines, and hydraulic rams, or by 

 water acting as a weight or pressure, as in other kinds of water- 

 \vheels, and in water pressure engines, or by streams of water in- 

 ducing currents, as in the case of the jet pump and of the " trombe 

 d'eau," or by its undulating movements, as in ocean waves. The 

 ability of water to give out motive power may arise from falls, from 

 the currents of rivers, from the tides, or, as has been said, from the 

 oscillation of the waves. 



Prime-movers which utilize the force of the wind, are few in number, 

 and in all cases act by impact. 



As regards those prime-movers which work by the aid of heat. We 

 may have that heat developed by the combustion of fuel, and being 

 so developed, applied to heating water, raising steam, and working 

 some of the numerous forms of steam-engines, or, as in the case of 

 the Giffard injector, performing work by currents induced by the 

 flow of steam ; or we may have the heat of fuel applied to vary 

 the density of the air, and thus to obtain motion as by the smoke-jack ; 

 or the fuel may be employed to augment the bulk and the pressure of 

 gases, as in the numerous caloric engines ; or we may have heat and 

 power developed in the combustion of gases, as in the forms of gas 

 engines, or in the combustion of explosives, as in gunpowder, dynamite, 

 and other like materials, used not only for the purposes of artillery and 



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