A history of the growth of the steam-engine . he stroke is performed with-out further addition of steam. The variation of steam-pressure is approximately mversely proportional to the vari-ation of its volume. Thus, at half-stroke, the pressure be-comes one-half that at which the steam was supplied to thecylinder. At the end of the stroke it has fallen to one-fourth the initial pressure. The pressure is always nearlyequal to the product of the initial pressure and volumedivided by the volume at the given instant. In symbols, PV It is true that the condensation of steam doing workchanges this la
A history of the growth of the steam-engine . he stroke is performed with-out further addition of steam. The variation of steam-pressure is approximately mversely proportional to the vari-ation of its volume. Thus, at half-stroke, the pressure be-comes one-half that at which the steam was supplied to thecylinder. At the end of the stroke it has fallen to one-fourth the initial pressure. The pressure is always nearlyequal to the product of the initial pressure and volumedivided by the volume at the given instant. In symbols, PV It is true that the condensation of steam doing workchanges this law in a marked manner; but the condensationand reevaporation of steam, due to the transfer of heat to 108 THE DEVELOPMENT OF THE MODERN STEAM-ENGINE. and from the metal of the cylinder, tends to compensatethe first variation hy a reverse change of pressure withchange of volume. The sketch shows this progressive variation of pressureas expansion proceeds. It is seen that the work done perimit of volume of steam as taken from the boiler is much. Fig. 28.—Expansion of Steam. greater than when working without expansion. The prod-uct of the mean pressure by the volume of the cylinder isless, but the quotient obtained by dividing this quantity bythe volume or weight of steam taken from the boiler, ismuch greater with than without expansion. For the caseassumed and illustrated, the work done during expansion isone and two-fifths times that done previous to cutting offthe steam, and the work done per pound of steam is that done without expansion. Were there no losses to be met with and to be exagger-ated by the use of steam expansively, the gain would be- JAMES WATT AND HIS INVENTIONS. 109 come very great with moderate expansion, amounting totwice the work done when following full stroke, whenthe steam is cut off at one-seventh. The estimated gain is,however, never realized. Losses by friction, by conductionand radiation of heat, and by condensation and reevapora-tion in the
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