The boiler a communicates, by means of a four-way cock ar, with the bottom of two open-topped cylinders, r s, having pistons, cd, moving in them. These pistons are fitted with lead, that they may act as counterpoises, se%erally, to the pump-buckets op ; and are attached by rods, e f, to the beams g h. To the opposite ends of the beams are fixed the pump-rods k 1, which work two force pumps, op, placed in the well. q is a perpendicular pipe, open at both ends ; the lower end being in the well, and the upper end bent over the cistern, t; the centres of the beams.
In the situation of the machine, as here represented, the steam in the boiler flows through the open passage into the cylinder r, and presses the piston, c, upwards ; this action, of course, depresses the puinp-rod lc, and forces the water under the plunger up the pipe q. When the steam has raised the piston c to nearly the top of the cylinder, the four-way cock z. is turned one-fourth a revolution, and thus opens a communication between the cylinder s and the boiler, and between the cylinder r and the open air. The weight of the rodf, (attached to the piston c,) and of the lead, in that piston, being greater than k and o, the piston descends by its gravity to the bottom of the cylinder, driving out the steam which raised it into the atmosphere. At the moment of closing the passage into the cylinder r, another passage was opened between the boiler and cylinder a; the elasticity of the steam forces the piston d upwards, and downwards, and produces the same effects as the action of the first cylinder.
As the remarks naturally arising from the subject of high-pressure steam will be more applicable at a future stage of the history than at this, we will now reserve them, and proceed, as far as it is possible, in chronological order.
The atmospheric engine of Newcomen, with the great improvements by Beighton, and notwithstanding the valuable key supplied by his calculations, continued for many years in the state just previously described. Modifications were occasionally suggested by various individuals, but of too unimportant a character to require particular mention. Its imperfections, however, had for a long period excited the attention of Mr. JOHN SMEATON, the most celebrated engi. neer of his day, and under his charge the engine ultimately attained, perhaps, as great a degree of perfection as its principle admitted. Having constant occasion to employ large steam-enginesin the great works he was called upon to execute, he directed his mind to the removal of its defects, and particularly to the im portant point of economising fuel. In calculating the proportions for an engine for the New River Company, in 1767, he considered that the stoppage of the water at every stroke, as well as putting the lever-beam, piston, heavy rode and chains, from a state of rest into motion, twice at every stroke, was a great loss of power; he therefore determined to work the engine slower, and with larger pumps, and put upon the piston all the load it would bear. To reduce the velocity of the column of water still more, he placed the fulcrum of the beam out of the centre, and made the stroke of the piston nine feet, whilst the pump, which lifted 36 feet, should work only with a six-feet stroke. This arrangement obliged him
to employ a long narrow cylinder, of only 18 inches diameter, and from this he also expected to obtain other advantages; viz., that every part of the steam, being nearer the surface of the cylinder, would be more readily condensed ; and, in consequence, that a less quantity of injection water would serve the cylinder, which would itself be more heated. Under all these appearances of advantage, he ventured to burden the piston with a pressure of 10.4 lbs. per inch. Thus, area of piston, [18 inches diameter,] 254 ; weight of the column of water, 36 feet in the pumps, [18 inches diameter,] 3060 lbs. ; of which take six-ninths for the difference in length of stroke, and it gives 2640 lbs. for the weight to be lifted by the piston ; and, dividing 2640 by 254, the area of the piston gives 10.4 lbs. pressure per inch. " Having once seen a common engine struggle under this burden," he writes, " I thought myself quite secure under those advantages; but how great was my surprise and mortification to find, that instead of requir ing less injection water than common, although the injection pump was calcu lated to afford as much injection water as usual, in proportion to the area of the cylinder, with a sufficient overplus to answer all Imaginable wants, it was unable to support the engine with injection, and that two men were obliged to assist to raise the injection water quicker by hand, to keep the engine in motion ; at the same time that the cylinder was so cold, I could keep my hand upon any part of it, and bear it for a length of time in the hot-well. By good fortune the engine performed the work it was appointed to do, as to the raising of the water; but the coals by no means answered my calculation. The injection pump being enlarged, the engine was in a state of doing business, and I tried many smaller experiments, but without any good effect, till I altered the fulcrum of the beam so much as reduced the load upon the piston from 10} to 8} lbs. per inch. Under this load, though it shortened the stroke at the pump-end, the engine went so much quicker as not only to raise more water, but consume less coal, took less injection water, the cylinder became hot, and the injection water cams out at 1800 of Fahrenheit ; and the engine, in every respect, not only did its work better, but went more pleasantly. This at once convinced me that a con siderable degree of condensation of the steam took place in entering the cylinder, and that I had lost more this way, by the coldness of the cylinder, than I had gained by the increase of load. In short, this single alteration seemed to have unfettered the engine ; but in what degree this condensation took place under different circumstances of heat, and where to strike the medium, so as, upon the whole, to do beat, was still unknown to me. But resolving, if possible, to make myself master of the subject, I immediately began to build a small fire engine at home, that I could easily convert into different shapes for experi ments, and which engine was very nearly ready to set to work in the winter of 1769.