In all the attempts at pumping-engines hitherto made, including Savory's, the steam acted directly upon the water to be moved without any intervening part. To Dr. Papin, a celebrated Frenchman, is due the idea of the piston. It was first used by him in a model constructed in 1690, where the cylinder was still made to do duty also as a boiler; but in an improved steam-pump invented about 1700 he used it as a diaphragm floating on the top of the water in a separate vessel, or cylinder, and the steam, by pressing on the top of it, forced the water out of the cylinder at the other end.
The next great step in advance was made about 1705 in the " atmospheric" engine, conjointly invented by Newcomen, Cawley, and Savory. This machine held its own for nearly seventy years, aud was very largely applied to mines, so that it will be worth while to give a somewhat more detailed description of it than of the others.
In this engine, which is shown in fig. 1, the previous inventions of the separate boiler, and of the cylinder with its movable steam-tight piston, are utilized, although in a new form. The " beam," which has ever since been used in pumping-engines, was lased for the first time, and for the first time also the condensation of the steam was made an instantaneous process, instead of a slow and gradual one. Newcomen's ongine was chiefly used, like all former steam-engines, in raising water. To one end of a beam moving on an axis I, was attached the rod, N, of the pump t) be worked; to the other, the rod, N, of a piston P, moving in a cylinder C, below. The cylinder was placed over a boiler B, and was counected with it by a pipe provided with a stop-cock V, to cut off or admit the steam. Suppose the pump-rod de pressed, and the piston raised to the top of the cylinder—which was effected by weights suspended at the pump-end of the beam—the steam-cock was then turned to cut off the steam, and a dash of cold water was thrown into the cylin der by turning a cock R, on a pipe A, connected with a cistern C', This condensed the steam in the cylin der, and caused a vacuum below the pis ton, which was then forced down by the pressure of the atmosphere, bringing with it the end of the beam to which it was attached, aud raising the other along with the pump-rod. The cock was then turned to admit fresh steam below the piston, which was raised by the counterpoise; and thus the motion began anew. The opening shutting of the cocks was at first performed by an attendant, but subsequently a boy named Humphrey Potter (to save, it is said, the trouble of personal superintendence) devised a system of strings and levers by which the engine was made to work its own • valves. In 1717 Henry Beighton, an F.u.s., invented a simpler and more scientific system of which rendered the engine completely selfacting. During the latter part of the time that elapsed before Watt's discoveries changed everything, Stneaton brought Newcomer's engine to a very high degree of perfection. As the result of study and experiment fie made many improvements in it, in the form of the boiler, the proportions of the cylinder, etc. It
was he, too, who invented the cataract, a very ingenious self-acting valve arrangement, which is still universally used in Cornish engines: It is worth mentioning that, in 1725, Leupold invented an engine in which steam of a higher pressure thau that of the atmos phere was employed in the cylinder, but his engine possessed defects that prevented its practical use. .
The next essential improvements on the steam-engine were those of Watt, which began a new era in the history of steam-power. The first and most important improve ment made by Watt was the separate condenser, patented in 1769. He had observed that the jet of cold water thrown into the cylinder to condense the steam, necessarily reduced the temperature of the cylinder so much that a great deal of the steam flowing in at each upward stroke of the piston was condensed before the cylinder got back the heat abstracted from it by the spurt of cold water used for condensing the steam in the cylinder. The loss of stmn arising from this was so great that only about one-fourth of what was admitted into the cylinder was actually available as motive-power. Watt, therefore, provided a separate vessel in which.to condense the steam, and which could be kept constantly in a state of vacuum, without the loss which arose when the cylinder 'itself was used as a condenser. This device, which now looks simple enough, was the greatest of Watt's inventions, and forms the foundation of his fame. His genius was such that in a few years he changed the steam-engine from a clumsy, wasteful, almost impracticable machine into a machine practically the same as we now have. T he prin cipal improvements since his time have been either in matters relating to the boiler; in details of construction consequent on our increased facilities, improved machinery, and greater knowledge of the strength of materials; in the enlarged application of his princi ple of expansive working; or in the application of the steam-engine to the propulsion of carriages and vessels. His principal inventions were: 1. The condensation of steam in a vessel separate from the cylinder, so as to avoid the cooling of the latter; 2. The use of a pump, called an " air-pump," to withdraw the-condensed water, and mixed steam and air, from the condenser; 3. To surround the cylinder either with a steam-jacket, or with some non-conducting body, in order to prevent radiation of heat (these three, with others, were inclnded in the specification of 1'769); 4. To use the steam expansively in the way explained further on in this article (this was invented before 1769, but not published till 1782); and 5. The now universally used double-acting engine. and the con version of the reciprocating motion of the beam into a rotary motion by means of a crank (both these were invented before 1778, the engine being patented in 1782. hut the crank. having before that date been pirated and patented by another). In 1784 Watt also patented and published his parallel motion, throttle-valve, governor, and indicator; all four of which are in substance still used.