266 MOTIVE POWER EMPLOYED

all elastic fluids whatever, when those fluidsare subjected to great degrees of pressure, andare thus made highly elastic, there is a ten-dency in them to resume their original elasti-city, or the density of the surrounding atmos-phere.

Steam, when confined, may be highly elastic,from the continual addition of successive par-ticles produced in the evaporation of water ;and air, from the expansion or compression ofthe same particles ; but the law of their elasti-city, when subjected to pressure, remains thesame, and each has the same tendency toexpand, when the pressure w hich keeps themin that state is removed.

When, therefore, an aperture is made in avessel or boiler, containing steam highly elastic,the tendency which fluids of different densitieshave to assume a state of equilibrium, willcause the steam in the boiler to rush out intothe atmosphere; and the velocity of efflux willbe proportionate to the difference betweenthe density of the steam in the boiler, and thatof the space into which it rushes.

It is a well-known law of pneumatics, thatair rushes into a void with the velocity whicha heavy body would acquire by falling fromthe top of a homogeneous atmosphere ; and thatthe velocity with which a fluid of greater den-