The Steam Engine Explained and Illustrated (Seventh Edition) / With an Account of Its Invention and Progressive Improvement, and Its Application to Navigation and Railways; Including Also a Memoir of Watt

by Dionysius Lardner

(65.)

1. A cubic inch of water evaporated under the ordinary atmospheric pressure, is converted into a cubic foot of steam.

2. A cubic inch of water evaporated under the atmospheric pressure, gives a mechanical force equal to what would raise about a ton weight a foot high.

(66.)

In like manner, if the piston were loaded with thirty pounds in addition to the atmosphere, the whole pressure on the water being then three times the pressure first supposed, the piston would be raised to somewhat more than one third of its first height by the evaporation of the water. This would give a mechanical force equivalent to three times the original weight raised a little more than one third of the original height.

In general, as the pressure on the piston is increased, the height to which the piston would be raised by the evaporation of the water will be diminished in a proportion somewhat less than the proportion in which the pressure on the piston is increased. If the temperature at which the water is converted into steam under these different pressures were the same, then the height to which the piston would be raised by the evaporation of the water would be diminished in precisely [Pg118] the same proportion as the pressure on the piston is increased; and, in that case, the whole mechanical force developed by the evaporation of the water would remain exactly the same under whatever pressure the water might be boiled. We shall explain hereafter the extent to which the variation of temperature in the water and steam corresponding to the variation of pressure modifies this law; but, as the effect of the difference of temperatures is not considerable, it will be convenient to register in the memory the following important practical conclusion:—

(67.)

GLASGOW.