The water being again supposed at its original temperature, and the piston in its first position, let a weight be placed upon the piston equal to the pressure of the atmosphere, so that the water beneath the piston will be pressed down by double the atmospheric pressure. If the lamp be once more applied, the water will, as before, be converted into vapour; but the piston will now be raised to the height of only six inches[48] from the bottom, the steam expanding into only half its former bulk. The temperature at which the water would commence to be converted into vapour, instead of being 212°, would be 250°; but the time elapsed between the moment of the first application of the lamp, and the complete conversion of the water into steam, will still be fifteen minutes.
Again, if the piston be loaded with a weight equal to double the atmospheric pressure, the water will be pressed down by the force of three atmospheres. If the lamp be applied as before, the water would be converted into steam in the same time; but the piston will now be raised only four inches above its first position, and the steam will consequently be three times as dense as when the piston was pressed down only by the atmosphere.
From these and similar experiments we infer:—
First, That the elastic pressure of steam is equal to the mechanical pressure under which the water producing the steam has been boiled.
Secondly, That the bulk which steam fills is diminished in the same proportion as the pressure of the steam is increased; or, in other words, that the density of steam is always in the same proportion as its pressure.
Thirdly, That the same quantity of heat is sufficient to convert the same weight of water into steam, whatever be the pressure under which the water is boiled, or whatever be the density and pressure of the steam produced.
Fourthly, That the same quantity of water being converted into steam, produces the same mechanical effect, whatever be the pressure or the density of the steam. Thus, in the first case, the weight of one atmosphere was raised a foot high; in the second case, the weight of two atmospheres was raised through half a foot; and, in the third case, the weight of three atmospheres was raised through the third of a foot; the weight raised being in every case increased in the same proportion as the height through which it is elevated is diminished. Every increase of the weight is, therefore, compensated by a proportionate diminution of the height through which it is raised, and the mechanical effect is consequently the same.
Fifthly, That the same quantity of heat or fuel is necessary and sufficient to produce the same mechanical effect, whatever be the pressure of the steam which it produces.
If steam be used to raise a piston against the atmospheric pressure only, although a definite physical force will be exerted by it, and a mechanical effect produced, yet under such circumstances it will exert no directly useful efficiency; but after the piston has been raised, and the tube beneath it filled with steam balancing the atmosphere above it, a useful effect to the same amount may be obtained by cooling the tube, and thereby reconverting the steam into water. The piston will thus be urged downwards by the unresisted force of the atmosphere, and any chain or rod attached to it will be drawn downwards with a corresponding force. If the area of the piston be, as already supposed, equal to the magnitude of one square foot, the atmospheric pressure upon it, being 15 pounds for each square inch, will amount to 144 times 15 pounds, or 2160 pounds. By drawing down a chain or rope acting over a pulley, the piston would in its descent (omitting the consideration of friction, &c.) raise a weight of 2160 pounds a foot high. Since 2160 pounds are nearly equal to one ton, it may, for the sake of round numbers be stated thus:—
"A cubic inch of water, being converted into steam, will, by the condensation of that steam, raise a ton weight a foot high." Such is the way in which the force of steam is rendered practically available in the atmospheric engine.