Newcomen's was reserved for the Englishmen New comen and his assistant Cawley to make a practical application of Papin's plan of using steam, which was effected in 17°5 by connecting with a steam-boiler a cylinder containing a piston. It was a steam-engine, but employed the pressure of the atmosphere to move the piston and to do the work, and hence was called an "atmospheric steam-engine," which proved to he well adapted for working the main rods of pumps in mines, and, later on, for driving revolving shafts. Figure 8 represents a Newcomen machine in its higher form of development. Immediately upon the dome shaped boiler, half sunk in a brick furnace, is seen the cylinder with its movable piston. A chain fastened to the piston is placed over a segment attached to a wooden double-armed lever or " beam." This beam at its centre swung upon a pivot, and was united with a connecting-rod, a crank, and a revolving shaft provided with a fly-wheel; and suspended to it were two rods properly secured by chains to two segments. The cyl inder, being entirely open on top, allowed the atmospheric pressure to act upon the piston and to set it in motion, together with the described mech anism, when a vacuum was formed in the lower space by the condensation of the steam admitted into it. It is, therefore, worthy of notice that the actual motive-power was not steam, but atmospheric pressure left free to act by the condensation of steam. The use of the chain referred to did not even allow of the direct action of the steam. This machine took twenty pounds of coal per horse-power per hour.
In the first machines the condensation of the steam was effected by simply throwing cold water in a shower over the outside of the cylinder. In an improved form of construction it was effected by injecting into the cylinder a jet of water taken from a reservoir, shown on top of the Fig ure, and kept constantly filled by the machine itself. This improvement was suggested by an accident: "As they were at first working, they were surprised to see the engine go several strokes, and very quick together, when, after a search, they found a hole in the piston, which let the cold water in, condensing the steam in the inside of the cylinder." The alter nate admission and exclusion of steam, originally effected by a work man opening and closing a cock with his hand, were here effected by a mechanism connected with a vertical rod, seen in the illustration in front. The valve-gear was first made to work thus automatically in 17r3 by a boy, Humphrey Potter, who caused the beam itself to open and close the valves by means of suitable catches and strings; but in T7IS, Henry Beighton sub stituted for the latter a plug-rod, which worked the valves by means of tappets. Smeaton improved this type, in 1774, by oakum cylinder-pack ing, and by raising the water used for condensation by a pump worked from the main beam; he also covered the lower side of the piston with wooden 'plank, to reduce unnecessary and untimely cylinder-condensation.
Walt s Newcomcn's engine had been in use more than fifty years, and had been much improved in its mechanical details, it was entirely superseded by the condensing steam-engine invented by James Watt, a mathematical-instrmnent maker at the University of Glasgow, who in 1763, having put a model Newconmen engine in order, and having been struck with its enormous consumption of steam, began a series of improvements which finally rendered the steam-engine univer sally applicable. Watt's improvements consisted in lagging the boiler, pipes, and cylinder with non-conductors, in condensing in a separate ves sel, and in making the engine double-acting by closing the cylinder at the top and passing the piston-rod through a steam-tight stuffing-box. In
1774 he produced a beam-engine in which the steam passed above the piston and depressed it, raising the weight of the pump-rods, the lower end of the cylinder being in communication with a separate condenser; Then a valve was opened, allowing the steam which was above the piston to flow beneath the piston, which was raised by the weight of the, pump rod. He introduced the " air-pump " to relieve the condenser of air and of an excess of water, and used oil and tallow for lubricating the piston instead of water, which caused excessive cylinder-condensation. In 1781 (in order to avoid the payment of royalty upon the crank, which was patented) he employ&I the " sun-and-planet " movement, to produce a rotary from a reciprocating motion, and added a fly-wheel and a shaft, so that it could drive machinery. In 1782 he patented the use of the expansion of steam—the application of steam on each side of the pis ton alternately, the opposite side being in communication with the con denser—the double or coupled engine, and the use of a rack upon the piston-rod, working upon a sector on the beam, to give perfect straight line motion to the rod. For guiding the piston-rod in a straight line Watt also provided the so-called " parallel motion." In 1784 he added the poppet-valve, the centrifugal ball-governor acting on a throttle-valve (pl. 82, fig. 5), and the steam-jacket. He also invented the indicator by which the occurrences in the cylinder might be made known and regu lated; and patented a locomotive steam-engine.
Watt's most important improvement on Newco men's machine was the addition of the condenser in 1765. Figure 9 (p1. 8o) illustrates such an apparatus. Instead of effecting the condensation in the cylinder itself, there was placed under it an hermetically-closed iron box into which the steam from the,cylinder was introduced and condensed by an injected spray of cold water. But as the injected water as well as the con densed steam would in a short time entirely fill the box, a pump (seen on the right in the cut) was connected with it, by means of which the water and also the air contained in it could be constantly sucked up and removed. This pump is therefore called the "air-pump." adding the three-port slide-valve, invented in 1799 by Murray, and intended to replace the distributing-cock originally used and the valves later on substituted for it, we have the modernized form of the principal part of the machine—namely, the cylinder with its immediate mechanisms—shown in Figures 11 and 12 ( pl. So), in which the fresh inflowing steam is indicated by the whitish color in the engrav ing. It will be seen that the steam is conducted from the boiler, by means of the steam-pipe D, first into the steam-chest C, E, and passes thence in Figure 12 above, and in Figure 11 below, the piston A' by means of the steam-passages a', e and f, g in the direction of the arrows. Hence in Fig ure 12 the piston is forced down, in Figure i f upward, being, however, operative only if the steam already used and standing above or below the piston can pass into the condenser. For the latter purpose again serve the passages g,f and e, a', further the exhaust-pipe 0, and finally, as also for the alternate admission of the steam, the three-port valve A, P. This valve is given the positions required for the distribution of the steam, which are indicated in Figures i r and r 2 (fi/. So), by the machine itself, by means of a mechanism connected with the valve-rod E. The other parts arc the piston-rod G and the stuffing-box .5', while the aperture seen in the centre of the bottom is closed by a cock through which the water collected in the cylinder is from time to time discharged.