When the attendant sees the piston as low as he thinks proper, he shuts the injection cock by depressing the lever q, and at the same time he opens the regulator, by forcing down the handle o, which oversees the tumbling bob, and its kg, catching the cross pin of the spanner i, opens the regulator.
The steam has been accumulating above the water in the boiler during the whole time of the piston's descent. The moment, therefore, that the steam-cock is opened, the steam having an elasticity of rather more than one pound pet square inch greater than that of the air, rushes into the cylinder, when it immediately blows open the sniffing valve, and assists the water which had coins in by the fonner injection, and what arose from the condensed steam, to descend by its own weight through the eduction pipe it, and open the valve to run out into the hot well s.
This water is nearly boiling hot, or at least its surface; for, while lying in the bottom of the cylinder, it will condense steam till it acquires this temperature, and therefore cannot run down till it will condense no more. There is a cause of some waste of steam at its first admission, in order to heat the inside of the cylinder and the injected water to the boiling temperature ; but the space being entail, and the whole being already very warm, it is very soon done ; and when things are properly constructed, little more is wanted than what will warm the cylinder, for the eduction pipe is made of large dimensions, and receives some of the injection water even during the descent of the piston, and this portion will be removed out of the way of the steam.
The first effect of the entering steam is of great service; it drives out of the cylinder the vapour which it finds there. This Is seldom pure steam or watery vapour, because all water contains a quantity of air in a state of chemi cal union ; but the union is only feeble, and a boiling heat is sufficient for dis engaging the greatest part of it by increasing its elasticity. It may also be disengaged by simply removing the external pressure of the atmosphere. There fore the small space below the piston contains watery vapour, mixed with all the air which had been disengaged from the water in the boiler by ebullition) and all that :ran separated from the injection water by tine diminution of exter nal preuure, in addition to any which may enter by leakage.
Let us now consider the state of the piston when setting out on its return : as it is evident that it will start, or begin to rise by the counter-weight, the mo ment the steam-cock is opened ; for at that instant the excess of the asmo spherical pressure, by which it was kept down in opposition to the preponderancy of the outer end of the beam, is diminished. At the fist instant of the return
of the pump-rods, they draw up the piston with great violence, all the weight of the water in the pumps acting in addition to the counter-weight ; but the falling of the lower valves in the pumps, after an inch or two of motion, arrests the further descent of the water, and bears the weight of the column of water; and after this the piston will rise gradually by the action of the counter-weight.
The action of the counter-weight is very different in the two motions of the engine; for while the engine is making a working stroke it is lifting not only the column of water in the pump, but the absolute weight of the bucket-rods also ; and while the pump-rods are descending, there is a diminution of the counter weight, by the whole weight lost by the immersion of the rod in water. The wooden rods which are generally used being soaked in water and joined by iron straps, are heavier, and but a little heavier than water, and they are generally about one-third of the bulk of the water in the pumps.
By this counter-weight the piston is drawn upwards ; and it would even rise although the steam which is admitted was not quite so elastic as common air.
Suppose the mercury in the barometer to stand at .30 inches, and that the preponderancy at the outer end of the beam was equal to 1-9th of the pressure of the air on the piston, the piston would not rise until the elasticity of the steam was equal to 30 1-9th,_ that is, to 261 inches nearly; but if the steam was just equal to this quantity, the pistou would rise as fast as the steam of that density could be supplied to the cylinder through the eteam.pipe ; and on this sup position, the velocity of the ascent would depend on the velocity of that supply. But this is not the case in practice, because the steam must be stronger than the air, in order to blow out and discharge the air ; it will therefore enter the cylinder without any effort on the piston to draw or suck it in. At the same time the counter-weight must not be so great as to draw up the piston with that force which will cause a suction within the cylinder greater than the steam-pipe can supply, or it would diminish the pressure of the steam within the cylinder lower than the atmosphere, and prevent it from snifting or blowing out the air.