CHAPTER X. STEAM ENGINES. 266. Invention of the steam engine.-Steam engines are undoubtedly the most important of the applications of the physical sciences to the arts. Based on the very great elastic force which aqueous vapour assumes at a high temperature (244) and on the condensation of this vapour by cooling (258), steam engines have created, in a small volume and at a small expense, very considerable motive powers. Their importance has caused much discussion and investigation as to their inventor, or rather inventors; for it is only by the successive efforts of several men of genius that these machines have attained their present simplicity and precision. The history of the steam engine commences with Hero, the inventor of the fountain which bears his name, who invented, nearly two thousand years ago, a steam tourniquet, known as the eolipyle, analogous to the hydraulic tourniquet (fig. 64). The names of Salomon of Caux, and then of the Marquis of Worcester, are mentioned in the history of the steam engine. Denis Papin, a French physicist, to whom is due the apparatus already described (252), was the first who caused a piston to ascend in a vertical cylinder, closed at the bottom and open at the top, by means of the elastic force of steam, and to descend by condensing this vapour by cooling; so that the piston which descended in virtue of atmospheric pressure had an up and down motion in the cylinder, which is still the principle of all steam engines. Papin, who was a Protestant, was obliged to fly from France in consequence of the revocation of the Edict of Nantes, and the description and plan of his machine was published in Germany in 1690. He even made a model large enough to move a boat by means of paddle-wheels. In this model there was water underneath the piston at the bottom of the cylinder. When a furnace was placed under this, the water was converted into steam, and its elastic force raised the piston; when the piston was at the top of -267] Steam Engines. 279 its course the furnace was withdrawn: the cylinder cooling, the steam was condensed and the piston sank. In 1705 Newcomen and Cawley constructed a steam engine, or 'fire-pump,' as it was then called, the object of which was to drain mines. In this engine (fig. 234) the steam was produced separately in a boiler m, below the cylinder c, containing the piston p. The condensation also was effected by cold water from a cistern, n, being injected into the cylinder through a cock, b. This was opened when the piston was to descend, and was closed after the descent; a second one, a, was opened through which steam entered, and so on. But the sides of the cylinder being cooled by this injection of cold water, the steam which filled it was partially condensed, until the sides were again heated; there was thus a considerable loss of steam, and therefore of fuel. The condensed water flowed out by a pipe, at the end of which was the valve v, which opened as the piston p descended. w wis the beam by which Fig. 234. the motion is transmitted to the pump rod, d. 267. Watt's improvements in the steam engine.-James Watt, a mathematical instrument maker in Glasgow, had to repair the model of a Newcomen's engine belonging to the physical cabinet of the University. Struck by the enormous quantity of steam and of condensing water used by this engine, he entered upon a long series of researches and improvements, which he pursued with admirable perseverance for fifty years, without ever being content with the success he obtained. Thus it was that Newcomen's machine, successively changed and improved in all its parts, at last really became Watt's machine. Condenser. Watt's first and principal invention was the con denser. This name is given to a closed vessel quite distinct from the cylinder in which the piston moves, and only connected with it by a tube provided with a stopcock. In this vessel cold water is injected, and the vapour is condensed by opening the connecting stopcock. Thus, as the side of the cylinder is not cooled, all the steam which enters there is utilised. Thus there was effected so great an economy of steam, and therefore of fuel, that Watt, and Boulton his partner, having taken a patent, realised great profits by only requiring, for a certain number of years, a third of the saving in the consumption of coal, as compared with Newcomen's engine. Single-acting engine. In Newcomen's engine, the cylinder of which was open at the top, the steam only lifted the piston; and then, when the steam was condensed, the pressure of the atmosphere brought it down again; whence the name atmospheric engine, by which it was designated. As the piston descended, air penetrated into the cylinder and cooled the sides, in consequence of which a portion of the vapour which penetrated into the cylinder was condensed until the sides were again heated. To remove this source of loss, Watt closed the cylinder altogether, and caused the vapour to act above the piston, so as to make it descend; then by an arrangement of stopcocks, alternately opened and closed by the action of the engine itself, the steam passed simultaneously above and below the piston. This, being pressed equally in opposite directions, remained in equilibrium; so that a simple counterpoise acting by means of a lever at the end of a piston rod raised the piston again, and so on. This machine, into which air did not enter, and where the atmospheric pressure did not act, was called the singleacting engine, to express that the steam had a useful action on only one side of the piston. The single-acting engine had the great disadvantage that it had no real force except when the piston was descending. It could transmit motion to pumps for emptying mines, because, for that, effort in only one direction was required; but it would not furnish a sufficiently regular motion for many industries—for cotton manufactures, for instance. Hence Watt's task was not completed, but he was not long in finding another plan. Double-acting engine. In this engine, one form of which we shall presently describe, and which is represented in fig. 235, the cylinder is closed both at the top and at the bottom, but the steam -268] Steam Engines. 281 acts alternately on the two faces of the piston; that is to say, that by a system of stopcocks, opened and closed by the engine itself, when the lower part of the cylinder communicates with the condenser, the upper part, on the contrary, is connected with the boiler, and the steam, acting in all its force on the piston, causes it to descend. Then when this is at the bottom of its stroke the parts change; the top of the cylinder is in connection with the condenser, and the bottom with the boiler; the piston rises again, and so forth, whence results an alternating rectilinear motion which is changed into a continuous circular motion, as will be presently described (268). Air-pump. Watt completed his engine by the addition of three pumps, which are worked by the engine, and play an important part. For the cold water of the condenser becomes rapidly heated by the heat which the steam gives up to it (258), and this water, soon reaching 100 degrees, would no longer condense the steam. Moreover, the air, which is always dissolved in cold water, is liberated in the boiler, owing to the increase in temperature. Now this air, passing both above and below the piston, would soon stop its motion. To prevent these two injurious effects, Watt applied to the engine a suction-pump, which continually withdrew from the condenser the air and water which tended to accumulate there. The Feed-pump and cold-water-pump. The two other pumps which Watt added are the feed-pump and the cold-water-pump. first is a force-pump which sends into the boiler the hot water withdrawn from the condenser by the air-pump, thus producing a considerable saving in fuel. The other is a suction-pump, which raises, either from a well or a river, or some other source, the cold water intended to replace that heated in the condenser and withdrawn by the air-pump. Besides the important parts which have thus been described, we owe to Watt the arrangement for distributing the steam alternately above and below the piston: the regulator, whose function, when the machine works too slowly, is to admit more steam into the cylinder, and, on the other hand, to diminish the quantity when the velocity is too great. Lastly, the parallelogram, devised by Watt, which imparts to the piston rod a rectilinear motion. It may be added that Watt, who had begun life as a philosophical instrument maker, carried into the execution of these great pieces of machinery the same perfection as is required for the best scientific instruments. 268. Description of the double-acting engine. We have already seen that the double-acting engine is that in which the |