To work this engine, the boiler is first filled two or three feet deep with wdter, from which steam is generated by the furnace fire, of sufficient strength to exert a pressure of about one pound beneath each square inch of the safety valve; the steam will then lift up the valve, and escape. In this condition we will suppose the machine to be in a state of rest, having both the steam-cock and injection cock shut, and just as represented in the figure.
The man who attends the engine depresses the handle p, so as to throw the tumbling bob into the position there shown ; and the leg of the fork thrusting back the spanner i i, opens the regulators or steam-cock, when the steam from the boiler immediately rushes in, and, dispersing itself throughout the cylinder, mixes with the air contained therein : much will be condensed by the cold surface of the cylinder and piston, and the condensed water will trickle down the sides, and run off at the eduction pipe R. This condensation will be re peated until the whole cylinder and piston are made as hot as boiling water.
When this happens, the steam will begin to open the enifting-valve a, and issue through the pipe ; at first slowly, and very cloudy, being mixed with much air. The blast at a will grow stronger by degrees, and more transparent, in pro portion as the common air becomes exhausted. When the attendant perceives the engine to be ready for starting, he lifts up the handle o or p, till the tumbling bob Y falls over the perpendicular, towards the cylinder, and its leg, striking the cross pin of the spanner i, draws it forwards, and shuts the steam regulator ; at the same instant, he lifts up the handle q, of the F, which opens the injection cock. The pressure of the column of water in the injection pipe P immediately forces some water through the spout d, by the jets. The cold water, coming in contact with some of the pure vapour which now fills the cylinder, condenses it, and thus makes a partial void, into which the more distant steam immediately expands; and by this very expansion its capacity for heat is increased, or, in other words, as it grows cold, it abstracts the heat more powerfully from the steam situated immediately beyond it.
In this expansion and refrigeration the steam is itself partly condensed, or converted into water,•and leaves a void, into which the circumjacent steam im mediately expands, and produces the same effect on the steam beyond it ; and thus it happens, that the abstraction of a small quantity of heat, from an incon aiderable twos of steam, produces a condensation throughout a cylinder which is extensive.
• What remains in the cylinder no longer balances the atmospheric pressure on the surface of the water in the injection cistern, and therefore, the water spouts rapidly through the holds d, by the joint action of the column P, and the un balanced pressure of the atmosphere ; at the same time, the sniffing valve e, and the eduction valve R, are shut by the externalpressure of the atmosphere, and prevent the entrance of air or water into the cylinder. The velocity of the injection water must therefore rapidly increase, and the jets dash against the bottom of the piston, and be scattered through the whole capacity of the cylinder. In a very short space of time, therefore, the condensation of the steam becomes universal, and the elasticity of what remains is very small. The whole pressure
of the atmosphere, therefore, being exerted on the upper surface of the piston, while there is hardly any on its under side, if the load on the outer end of the working beam is inferior to this pressure, it must yield to it. The piston G must descend, and the pump piston in the well, or mine, must ascend, bringing along with it the water therein ; but the motion does not begin at the instant the injection is made.
The piston Was kept at the top by the preponderancy of the outer end of the working beam, and the load of water in the pumps, and it must remain there till the difference between the elasticity of the steam below it, and the pressure of the atmosphere, exceed this preponderancy. There must, therefore, be a small space of time between the beginning of the condensation and the begin ning of the motion ; this is very small, not exceeding the third or fourth part of a second; but it may be very distinctly observed by an attentive spectator, who may perceive that the instant the injection cock is opened, if the cylinder has the slightest yielding in its suspension, it will heave upwards a little by the pressure of the air on the bottom. Its own weight is not at all equal to this pressure ; and instead of its being necessary to support it by a strong floor, it must be kept down by large beams, loaded at the end with heavy walls. This heaving of the cylinder shows the instantaneous commencement of the conden sation ; and it is not till after this has passed, that the piston is seen to start, and begins to descend. The motion must continue till the great piston reaches the bottom of the cylinder, because it is not like the motion which would take place in a cylinder of air rarefied to the same degree. In this latter case, the impelling force would be continually diminished, because the capacity of the -cylinder diminishing by the descent of the piston, the air in it would continu ally become more dense and elastic, until the piston would stop at a certain height, where the elasticity of the included air, together with the load upon the pump-rod at the well end of the beam, would balance the atmospherical pres sure on the piston ; but when the contents of the cylinder are pure vapour, and the continued stream of injected cold water keeps down its temperature to the same pitch as at the beginning, the elasticity of the remaining steam can never increase by the descent of the piston. The impelling, or accelerating force, remains therefore the same • and the descent of the piston will be accelerated almost uniformly, unless ;here be an increase of resistance arising from the nature of the work performed at the other end of the beam. And it may fre quently be observed in a good steam-engine, where every part is air-tight, that if the cylinder has been completely purged of common air before the steam cock is shut, and none has entered since, the piston will descend to the very bottom of the cylinder. It sometimes happens, by the great pump drawing air, or some part of the communication chains giving way, that the piston descends with such violence as to beat out the bottom of the cylinder at a blow : to prevent which accidents, the catch-pins x are applied at the extremity of the beam.