The first vessel P being thus emptied of its air, open the cock again, and the force of steam from the boiler presses upon the surface of the water with an elastic quality like air, still increasing in elasticity or spring, till it counterpoises or rather exceeds the weight of water ascending in the pipe S, out of which the contained water will be immediately discharged when once gotten to the top, which takes up some time to recover that power; but having once got it, and being in work, it is easy for one that never saw the engine, after half an hour's experience, to keep a constant stream running out the full bore of the pipe. On the outside of the vessel you may see how the water goes out, as well as if the vessel were transparent; for as far as the steam continues within the vessel, so far is the vessel dry without, and so very hot as scarcely to endure the least touch of the band. But as far as the water is, the said vessel will be cold and wet where any water has fallen on it, which cold and moisture vanish as fast as the steam, in its descent, takes place of the water; but if you force all the water out, the steam, or a small part thereof, going through P, will rattle the clack, so as to give sufficient notice to change the cocks, and the steam will then begin to force upon the other vessel, without the least alteration in the stream; only sometimes the stream of water will be somewhat stronger than before, if you change the cocks before any considerable quantity of steam be gone up the clack R: but it is better to let none of the steam go off, for that is losing so much strength, and is easily prevented by altering the cocks some little time before the vessel is emptied.
De Caus's Engine, 1615.
Savery's Engine, 1699.
The wood-cut represents two reservoirs, P P, designed for alternate action; the tube E conveys water from the discharging pipe, to replenish the boiler L, when the water in it is almost consumed; and this is done by keeping D supplied with water, and (lighting the fire at B) generating a sufficiency of steam to press the water into L, through the pipe K. This will convey a tolerably correct idea of Savery's engine, and the mode of its operation. He gives no proportions of its parts, nor perhaps had he established any rule of action. He appears to have considered the strength of his machine to be the only limit to be observed; ".for," says he, "I will raise you water 500 or 1,000 feet high, could you find us a way to procure strength enough for such an immense weight as a pillar of water of that height. But my engine at 60, 70, or 80 feet, raises a full bore of water with much ease."
Such was the machine which first solved the problem, which had so long occupied the attention of many ingenious and talented men, of employing the power of steam as an auxiliary to, or substitute for, the other sources of mechanical power then known. In simplicity of construction and of action, and in its perfect freedom from friction, the machine has never perhaps been exceeded, although it was subsequently superseded by Newcomen's engine, when it was required as a machine to raise water from great depths; yet from the fewness of its parts, and from its little liability to derangement, it was occasionally employed, until a very recent period, in cases where great power was not required.
Various contrivances were supplied by different persons, by which it was rendered self-acting. In 1819 Mr. Pontifex of Shoe Lane, London, obtained a Patent for improvements in this description, and erected an engine upon this principle at the City Gas Works, which we shall subsequently notice.
Although Savery's invention proved of considerable utility, yet it possessed certain inherent defects which greatly" limited its sphere of action. The principal of these defects were, first, the great strain to which the boilers and steam vessels were subjected; for the force of the steam being exerted directly upon the surface of the water to be raised, the pressure of the steam was required in all cases to exceed that of the ascending column of water. The second, and perhaps even greater, defect was the enormous consumption of fuel occasioned by the steam vessel at each successive discharge, being alternately raised to the temperature of the steam, and cooled down to the temperature of the water.
Scarcely had Savery's engine been brought sufficiently into operation to call attention to the defects we have noticed, when by a different application of one of the principles, a machine of a totally different character was produced by which the first of these defects was completely obviated. This was the atmospheric engine invented by Thomas Newcomen and John Cawley, the former a blacksmith, and the latter a glazier, in the town of Dartmouth, in Devonshire.
Instead of applying the pressure of the steam directly upon the surface of the water, they employed the steam to produce by its condensation a vacuum beneath a piston, moving in a cylinder and exposed to the atmosphere, and the power thus acquired they applied to work pumps through the intervention of a beam. By this arrangement, the pressure of the steam was Dot in any case required to exceed that of the atmosphere, as by assigning suitable proportions to the steam cylinder and the pumps, or to the arms of the beam, they could raise the water to any required height.
The figure on the following page (from an old engraving), will perhaps assist the reader's comprehension.
a represents the boiler; b the safety-valve; c the cylinder, open at the top, but closed at the bottom, in which there are three holes, d e f; e the passage for steam from the boiler; d admits a jet of cold water from the reservoir g, for the condensation of the steam; f the exit passage for condensed steam and water; h the piston, working air-tight in the cylinder, by packing; i the beam or loggerhead, for the purpose of transmitting the motion, of the piston to the pumps in the mine.