The Steam Engine Familiarly Explained and Illustrated / With an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain axioms for railway speculators

by Dionysius Lardner

(132.) We have here, however, only considered the mechanical effect produced by the condensation of steam. Let us now examine its direct action.

Let the piston P be supposed to be connected by a rod with a load or resistance which it is intended to raise, and let the load placed upon it be supposed to amount to one ton, the total pressure on the piston will then be two tons; one due to the atmospheric pressure, and the other to the amount of the load. Upon applying heat to the water, steam will be produced; and when the water has been completely evaporated, the piston will rise to the height of six inches from the bottom of the tube. The total mechanical effect thus produced will be one ton weight raised through six perpendicular inches, which is equivalent to half a ton raised through one foot.

Again, let the load upon the piston be two tons; this will produce a total pressure upon the water below it amounting to three tons, including the atmospheric pressure. The water, when converted into vapour under this pressure, will raise the piston and its load through four perpendicular inches: the useful mechanical effect will then be two tons raised through the third of a foot, which is equivalent to two thirds of a ton raised one foot. In the same manner, if the piston were loaded with three tons, the mechanical effect would be equivalent to three fourths of a ton raised through one foot, and so on.

It appears therefore from this reasoning, that when the direct force of steam of greater pressure than the atmosphere is used without condensation, the total mechanical effect is always less than that produced by the condensation of atmospheric steam without expansion; but that the greater the pressure under which the steam is produced, the less will be the difference between these effects. In general, the proportion of the mechanical effect of high-pressure steam to the effect produced by the condensation of atmospheric steam, will be as the number of atmospheres expressing the pressure of the steam to the same number increased by one. Thus, if steam be produced under the pressure of six atmospheres, the proportion of its effect to that of the condensation of atmospheric steam will be as six to seven.

(133.) Another method of applying the power of steam mechanically is, to combine its direct action with condensation but without expansion.

The piston being, as before, loaded with one ton, the evaporation of the water will raise it through six perpendicular inches, and the result so far will be equivalent to a ton raised half a foot; but if the piston-rod be supposed also to act by a chain or cord over a wheel, so as to pull a weight up, the steam which has just raised the ton weight through six inches, may be condensed, and the piston will descend with a force of one ton into the vacuum thus produced, and another ton may be thus raised through half a foot. The total mechanical power thus yielded by the steam, adding to its direct action its effect by condensation, will then be one ton raised through one foot, being an effect exactly equal to that obtained by the condensation of atmospheric steam.

If the piston be loaded with two tons, its direct action will, as we have shown, raise these two tons through four inches, which is equivalent to two thirds of a ton raised a foot. By condensing this steam a ton weight may be raised in the same manner, by the descent of the piston through a third of a foot, which is equivalent to the third of a ton raised through one foot.

By pursuing like reasoning, it will appear that, if the direct force of high-pressure steam be combined with the indirect force produced by its condensation, the total mechanical effect will be precisely equal to the mechanical effect by the mere condensation of atmospheric steam.

(134.) In applying the principle of expansion to the direct action of high-pressure steam, advantages are gained analogous to those already explained with reference to the method of condensation.

Let the piston be supposed to be loaded with three tons: the evaporation of the water beneath it will raise this weight, including the atmospheric pressure, through three perpendicular inches. Let one ton be now removed, and the remaining two tons will be raised, by the expansion of the steam, through another perpendicular inch. Let the second ton be now removed, and the piston loaded with the remaining ton will rise, by the expansion of the steam, to the height of six inches from the bottom. These consequences follow immediately from the principle that steam will expand in proportion as the pressure upon it is diminished, observing that in this case the atmospheric pressure, amounting to one ton, must always be added to the load. In this process three separate effects are produced: one ton is raised through three inches, which is equivalent to a quarter of a ton raised through one foot; another ton is raised through four inches, which is equivalent to a third of a ton through a foot, and the third ton is raised through six inches, which is equivalent to half a ton raised through a foot. The total of these effects amounts to one and one-twelfth of a ton raised through one foot, while the same load, raised by the high-pressure steam without expansion, would be equivalent to only half a ton raised through one foot.