The principal parts of Mr. Hornblower's engine are two cylinders, A, B, Plate DIX, Fig. 1, the largest of which is A. A piston moves in each, having their rods C and D moving through collars at E and F. These cylinders may be supplied with steam from the boiler by means of the square pipe G, which has a flinch to connect it with the rest of the steam-pipe. This square part is represented as branching off to both cylinders. c and d are two cocks, which have handles and tumblers as usual, worked by' the plug beam W. On the fore-side (that is the side next the eye) of the cylinders is represented another com municating pipe, whose section is also square or rectangular, having also two cocks a, b. The pipe Y, immediately under the cock b, establishes a commu nication between the upper and lower parts of the small cylinder B, by opening the cock b. There is a similar pipe on the other side of the cylinder A, im mediately under the cock d. When the cocks e and a are open, and the cocks b and d arc shut, the steam from the boiler has free admission into the upper part of the cylinder II, and the steam from the lower part of II has free admission into the upper part of A; but the upper part of each cylinder has no communi cation with its lower part.
From the bottom of the great cylinder proceeds the eduction-pipe K, having a valve at its opening into the cylinder, which bends downwards, and is connect ed with the conical condenser L. The condenser is fixed on a hollow box AI, on which stand the pumps N and 0 for extracting the air and water; which last runs along the trough T into a cistern U, from which it is raised by the pump V for recruiting the boiler, being already nearly boiling hot. Immediately under the condenser there is a spigot valve at S, over which is a small jet pipe, reaching to the bend of the educ tion-pipe. The whole of the condensing apparatus is contained in a cistern R of cold water. A small pipe P comes from the side of the condenser, and ter minates on the bottom of the trough 'I', and is there covered with a valve Q, which is kept tight by the water that is always running over it. Lastly, the pump-rods X cause the outer end of the beam to pre ponderate, so that the quiescent position of the beam is that represented in the figure, the pistons being at the top of the cylinders.
Suppose all the cocks open, and steam coming in copiously from the boiler, and no condensation going on in L; the steam must drive out all the air, and at last follow it through the valve Q. Now shut the valves b and d, and open the valve S of the condenser. The condensation will immediately commence. There is now no pressure on the under side of the piston of A, and it immediately descends. The communication
between the lower part of B and the upper part of A being open, the steam will go from B into the space left by the piston of A. It must therefore expand, and its elasticity must diminish, and will no longer balance the pressure of the steam above the piston of B. This piston, therefore, if not withheld by the beam, would descend till it is in eqnilibrio, having steam of equal density above and below it. But it cannot descend so far; for the cylinder A is wider than B, and the arm of the beam at which its piston hangs is longer than the arm which snpports the pis ton of B: therefore when the piston of B has descend ed as far as the beam will permit it, the steam between the two pistons occupies a larger space than it did when both pistons were at the tops of their cy linders. Its density, therefore, and its elasticity, di minish as its bulk increases. It is therefore not a ba lance; for the steam on the upper side of B, and the piston B, pulls at the beam with all the difference of these pressures. The slightest view of the subject must show the reader, that as the pistons descend, the steam that is between them will grow continually rarer and less elastic, and that both pistons will pull the beam downwards.
Suppose now that each has reached the bottom of its cylinder. Shut the cock a and the eduction cock at the bottom of A, and open the cocks b and d. The communication being now established between the upper and lower part of each cylinder, nothing hinders the counter weight from raising the pistons to the top. Let them arrive there. The cylinder B is at this time filled with steam of the ordinary density, and the cy linder A with an equal absolute quantity of steam, but expanded into a larger space.
Shut the cocks b and d, and open the cock a, and the eduction cock at the bottom of A; the condensa tion will again operate, and the pistons descend. And thus the operation may be repeated as long as steam is supplied; and one full of the cylinder B of ordinary steam is expended during each working stroke.
Let us now examine the power of this engine. It is evident, that when both pistons are at the top of their respective cylinders, the active pressure (that is, the difference of the pressure on its two sides) on the pis ton of B is nothing, while that on the piston of A is equal to the full pressure of the atmosphere on its area. This, multiplied by the length of the arm by which it is supported, gives its mechanical energy. As the pistons descend, the pressure on the piston of B increases, while that on the piston of A diminishes. When both are at the bottom, the pressure on the pis ton of B is at its maximum, and that on the piston of A at its minimum.