*268 MOTIVE POWER EMPLOYED

ratio of the intensity of the two pressures, andwill continue to be diminished as they approachmore and more towards the same ; and, whenthey become equal, the piston will not bemoved at all. In all these cases, the readerwill perceive that the elasticity of the steam inthe boiler and that in the cylinder are widelydifferent; the one being that indicated by thepressure on the safety-valve of the boiler, andthe other by the intensity of the load upon thepiston.

We have all along supposed the productionof steam in the boiler to be a constant regularsupply, and adequate to form steam in sufficientquantity to keep up the same degree of densityin the boiler, whatever be the rate of efflux intothe cylinder by the different, velocities of thepiston. But, if this is not the case, if the forma-tion of steam in the boiler be not equal to thequantity escaping through the throttle-valve,then the density in the boiler will be diminish-ed, and the rate of efflux, and consequently thevelocity of the piston, will also be proportion-ably lessened.

Two causes thus operate in limiting the ve-locity of allhigh-pressure engines, viz.theinten-sity of the pressure upon the piston comparedwith that in the boiler; and the quantity of