On the other hand, the force which urges the greater piston is continually decreasing, since there is a vacuum below it, and the steam which presses it is continually expanding into an increased bulk.

Impelled in this way, let us suppose the pistons to have arrived at the bottoms of the cylinders, and let the valves G, L, and o, be closed, and the valves I and N opened. No steam is allowed to flow from the boiler, G being closed, nor any allowed to pass into the condenser, since o is closed, and all communication between the cylinders is stopped by closing L. By opening the valve I,

a free communication is made between the top and bottom of the lesser piston through the tube H, so that the steam which presses above the lesser piston will exert the same pressure below it, and the piston is in a state of indifference. In the same manner the valve N being open, a free communication is made between the top and bottom of the greater piston, and the steam circulates above and below the piston, and leaves it free to rise. A counterpoise attached to the pump-rods, in this case, draws up the piston, as in Watt's single engine; and when they arrive at the top, the valves I and N are closed, and G, L, and o, opened, and the next descent of the pistons is produced in the manner already described, and so the process is continued.

The valves are worked by the engine itself, by means similar to some of those already described. By computation, we find the power of this engine to be nearly the same as a similar engine on Watt's expansive principle. It does not, however, appear, that any adequate advantage was gained by this modification of the principle, since no engines of this construction are now made.

(103.) The use of two cylinders was revived by Arthur Woolf in 1804, who, in this and the succeeding year, obtained patents for the application of steam raised under a high pressure to double-cylinder engines. The specification of his patent states, that he has proved by experiment that steam raised

under a safety-valve loaded with any given number of pounds upon the square inch will, if allowed to expand into as many times its bulk as there are pounds of pressure on the square inch, have a pressure equal to that of the atmosphere. Thus, if the safety-valve be loaded with four pounds on the square inch, the steam, after expanding into four times its bulk, will have the atmospheric pressure; if it be loaded with 5, 6, or 10 lbs. on the square inch, it will have the atmospheric pressure when it has expanded into 5, 6, or 10 times its bulk, and so on. It was, however, understood in this case, that the vessel into which it was allowed to expand should have the same temperature as the steam before it expands.

It is very unaccountable how a person of Mr. Woolf's experience in the practical application of steam could be led into errors so gross as those involved in the averments of this patent; and it is still more unaccountable how the experiments could have been conducted which led him to conclusions not only incompatible with all the established properties of elastic fluids, but even involving in themselves palpable contradiction and absurdity. If it were admitted that every additional pound avoirdupois which should be placed upon the safety-valve would enable steam, by its expansion into a proportionally enlarged space, to attain a pressure equal to the atmosphere, the obvious consequence would be, that a physical relation would subsist between the atmospheric pressure and the pound avoirdupois! It is wonderful that it did not occur to Mr. Woolf, that, granting his principle to be true at any given place, it would necessarily be false at another place, where the barometer would stand at a different height! Thus, if the principle were true at the foot of a mountain, it would be false at the top of it; and if it were true in fair weather, it would be false in foul weather, since these circumstances would be attended by a change in the atmospheric pressure, without making any change in the pound avoirdupois.*

*It is strange that this absurdity has been repeatedly given as unquestionable fact in various encyclopædias, as well as in by far the greater number of treatises expressly on the subject.

N

(104.) For several years after the extension of Watt's first patent had been obtained from parliament, he was altogether engrossed by the labour of bringing to perfection the application of the steam-engine to the drainage of mines, and in surmounting the numerous difficulties which presented themselves to its general adoption, even after its manifold advantages were established and admitted. When, however, these obstacles had been overcome, and the works for the manufacture of engines for pumping water, at Soho, had been organised and brought into active operation, he was relieved from the pressure of these anxieties, and was enabled to turn his attention to the far more extensive and important uses of which he had long been impressed with the conviction that the engine was capable. His sagacious mind enabled him to perceive that the machine he had created was an infant force, which by the fostering influence of his own genius would one day extend its vast power over the arts and manufactures, the commerce and the civilisation of the world. Filled with such aspirations, he addressed his attention about the year 1779, to the adaptation of the steam-engine to move machinery, and thereby to supersede animal power, and the natural agents, wind and water.

The idea that steam was capable of being applied extensively as a prime mover, had prevailed from a very early period; and now that we have seen its powers so extensively brought to bear, it will not be uninteresting to revert to the faint traces by which its agency was sketched in the crude speculations of the early mechanical inventors.

(105.) Papin, to whom the credit of discovering the method of producing a vacuum by the condensation of steam is due, was the earliest and most remarkable of those projectors. With very limited powers of practical application, he was, nevertheless, peculiarly happy in his mechanical conceptions; and had his experience and opportunities been proportionate to the clearsighted character of his mind, he would doubtless have anticipated some of the most memorable of his successors in the progressive improvement of the steam engine.

In his work already cited, after describing his method of imparting an alternate motion to a piston by the atmospheric

pressure acting against a vacuum produced by the condensation of steam, he stated that his invention, besides being applicable to pumping water, could be available for rowing vessels against wind and tide, which he proposed to accomplish in the following manner.

Paddle-wheels, such as have since been brought into general use, were to be placed at the sides, and attached to a shaft extending across the vessel. Within the vessel, and under this shaft, he proposed to place several cylinders supplied with pistons, to be worked by the atmospheric pressure. On the piston-rods were to be constructed racks furnished with teeth: these teeth were to work in the teeth of wheels or pinions, placed on the shaft of the paddlewheels. These pinions were not to be fixed on the shaft, but to be connected with it by a ratchet; so that when they turned in one direction, they would revolve without causing the shaft to revolve; but when driven in the other direction, the catch of the ratchet-wheel would act upon the shaft so as to compel the shaft and paddle-wheels to revolve with the motion of the pinion or wheel upon it. By this arrangement, whenever the piston of any cylinder was forced down by the atmospheric pressure, the rack descending would cause the corresponding pinion of the paddle-shaft to revolve; and the catch of the ratchet wheel, being thus in operation, would cause the paddle-shaft and paddle-wheels also to revolve; but whenever the piston would rise, the rack driving the pinion in the opposite direction, the catch of the ratchet wheel would merely fall from tooth to tooth, without driving the paddle-shaft.

It is evident that by such an arrangement a single cylinder and piston would give an intermitting motion to the paddleshaft, the motion of the wheel being continued only during the descent of the piston; but if several cylinders were provided, then their motion might be so managed, that when one would be performing its ascending stroke, and therefore giving no motion to the paddle-shaft, another should be performing its descending stroke, and therefore driving the paddle-shaft. As the interval between the arrival of the piston at the bottom of the cylinder and the commencement

of its next descent would have been, in the imperfect machine conceived by Papin, much longer than the time of the descent, it was evident that more than two cylinders would be necessary to insure a constantly acting force on the paddle-shaft, and, accordingly, Papin proposed to use several cylinders.

In addition to this, Papin proposed to construct a boiler having a fireplace surrounded on every side by water, so that the heat might be imparted to the water with such increased rapidity as to enable the piston to make four strokes per minute. These projects were promulged in 1690, but it does not appear that they were ever reduced to experiment.

(106.) Savery proposed, in his original patent, in 1698, to apply his steam engine as a general prime mover for all sorts of machinery, by causing it to raise water to make an artificial fall, by which overshot water-wheels might be driven. This proposal was not acted on during the lifetime of Savery, but it was at a subsequent period partially carried into effect. Mr. Joshua Rigley erected several steam engines on this principle at Manchester, and other parts of Lancashire, to impel the machinery of some of the earliest manufactories and cotton mills in that district. The engines usually raised the water from sixteen to twenty feet high, from whence it was conveyed to an overshot wheel, to which it gave motion. The same water was repeatedly elevated by the engine, so that no other supply was necessary, save what was sufficient to make good the waste. These engines continued in use for some years, until superseded by improved machines.*

(107.) In 1736, Jonathan Hulls obtained a patent for a method of towing ships into or out of harbour against wind and tide. This method was little more than a revival of that proposed by Papin in 1690. The motion, however, was to be communicated to the paddle-shaft by a rope passing over a pulley fixed on an axis, and was to be maintained during the returning stroke of the piston by the descent of a weight which was elevated during the descending stroke. There is no record, however, of this plan, any more than that of Papin, ever having been reduced to experiment.

(108.) During the early part of the last century the manu

* Farey, Treatise on the Steam Engine, p. 122.