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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 the steam was produced separately in a boiler below the cylinder containing the piston. The condensation also was effected by cold water being injected into the cylinder through a cock. This was opened when the piston was to descend, and was closed after the descent; a second one 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 followed it was partially condensed, until the sides were again heated: there was thus a loss of steam, and therefore of combustibles, which was considerable.
249. Watt's improvements in the steam engine.-James Watt, a mathematical instrument maker in Glasgow, had to repair the model of a Newcomen's machine belonging to the physical cabinet of the University. He was struck by the enormous quantity of steam and of condensing water used by this machine. This notion became the starting point of 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 metamorphosed in all its parts, ultimately really became Watt's machine.
Condenser. Watt's first and principal invention was the condenser. 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 sides of the pump are not cooled, all the steam which enters there is utilised. Thus there was effected so great an economy of steam, and therefore of combustibles, 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 machine.
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 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
245 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 the piston rod produced an upward movement. This machine, into which air did not enter, and where the atmospheric pressure did not act, was called the single-acting 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 ; and he was not long in finding another plan.
Double-acting engine. In this engine, which we shall presently describe, and which, as represented in fig. 191, the cylinder is closed both at top and at the bottom, but the steam 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 (250).
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 (240), 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 machine a suction-pump, which continually withdrew from the condenser the air and water which tended to accumulate there.
Feed-pump and cold-water-pump. The two other pumps which Watt added are the feed-pump and the cold-water-pump. The 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, which imparts to the piston rod a rectilinear motion. We may add that Watt, who had commenced life as a philosophical instrument maker, carried into the execution of these great mechanisms the same geometric rigour and the same perfection, as for the best instruments for scientific use.
250. Description of the double-acting engine. — We have already seen that the double-action engine is that in which the steam acts alternately above and below the piston (249). Fig. 191 represents an engine of this kind, and fig. 194 gives a section of the cylinder, of the piston, and of the distribution of steam. The entire machine is of iron. To the piston T is fixed a rod A, which slides with gentle friction in a tubulure U placed at the centre of the plate which closes the cylinder (fig. 195). As it is very important that no steam shall escape between the piston rod and this tubulure, the latter is formed of two pieces, one attached to the plate, while the other, which fits in the first, can be pressed as tightly as is desired, so as to compress the material soaked with fat which is between the two tubulures. This arrangement is called a stuffing box; it prevents the escape of steam without interfering with the motion of the piston.
On the two sides of the cylinder are two columns h h, which guide the piston rod in its upward and downward motion. The end
247 of the piston rod is connected with a long piece B, called the connecting rod, which in turn is jointed with a shorter piece M, called
the crank, the length of which is just half that of the stroke of the piston. This is rigidly fixed to a horizontal shaft, D, so that it cannot move without transmitting its motion.
By means of this connecting rod and crank, the alternating rectilinear motion of the piston and of the rod is changed into a continuous circular motion. For if we look at the rod during the ascent of the piston, it acts upwards upon the crank, making it turn in the direction of the arrow. When the piston is at the top of its stroke, the motion rod and the crank are one in front of the other. As the piston descends the motion rod again acts, so as always to turn it in the same direction; and when the piston is at the bottom of the stroke, they are again vertical, but one in the prolongation of the other, Hence it follows that the axle which has made half a turn during the ascent, makes a second one during the descent, and thus a complete revolution during each double oscillation of the piston.
To transmit the motion to machinery, on the axle D is fixed a sheave on which works an endless band XY of leather, or gutta percha, which works on another sheave fixed to the machinery to be turned. Moved by the first sheave, this band communicates its motion to the second; in this manner the motion is transmitted to all the workshops of a large factory. On the right of the fixed sheave, G, there is a second, which is not fixed to the horizontal shaft ; this is the movable sheave. Its object is to suspend all the motion in the machine without stopping the steam engine. By means of an iron fork not seen in the figure, which encloses the band, the latter may be slid from the fixed to the movable sheave. As this latter is not connected with the horizontal shaft, it does not turn with it, and does not transmit its motion to the band.
On the horizontal shaft is a large iron wheel V, called the flywheel. This wheel, which is very large, is necessary for keeping up the motion. For each turn that the piston is at the top or bottom of its stroke, there is a momentary arrest, during which the motion of the whole machine tends to stop. It is then that the flywheel, in virtue of its inertia and of its acquired velocity, moves the horizontal shaft, and thus keeps up a regular motion.
251. Excentric. Valve-chest. The excentric is an arrangement by which a continuous circular motion is changed into an alternating rectilinear motion. It is very frequently used in machinery.
One of these is fitted to the horizontal shaft at E, and the other