ON RAIL-ROADS. 267

sity rushes into another fluid of less density, isas the respective densities of the fluids.

It is in this manner that the steam of thehigh-pressure engines acts in its passage fromthe boiler to the cylinders ; a passage is madebetween the two, the area of which is madegreater or less by opening or shutting thethrottle-valve by means of a regulator; whenthis opening is made, the steam issues throughit into the cylinder with a force proportionateto its density, and, expanding itself, wouldmake its escape into the air, if not preventedby the intervention of the piston.

If now the piston be made to move up anddown with a velocity equal to the rate of efflux,then the steam will exert no pressure upon itat all; and, the nearer the two velocities ap-proach towards each other, the less effective pres-sure will be exerted upon the piston. Again,if the piston be subjected to a certain pressureby the intensity of the load, then the rate ofefflux or velocity with which the steam willissue through the regulator, will be propor-tionate to the difference between the elasticity orpressure of the steam in the boiler, and the in-tensity of pressure upon the piston, and the ve-locity of the piston will then be proportionalityreduced; and that velocity will be in the precise