STEAM-ENGINE. The earlier steam. engines, even Captain Savery's, which was long employed in this country, were only pumps for raising water ; a partial vacuum was formed in close vessels by the condensa tion of steam within them, and the atmospheric pressure raised the water to a certain height, whence it was forced higher by the elasticity of the steam admitted to act on its surface.
The first steam-engine which formed the connecting link between the steam-pumps and the modern steam-engines, was invented by Newcomen (1705). It contained a cylinder open at the upper end, fitted with a piston ; and the upward movement of the piston was occasioned by the pressure of steam beneath it ; whereas the downward movement was caused by the pressure of the air. Newcomen's engine was successively improved upon by Smeaton, Brindley, and other engineers, pre vious to Watt's time ; and from its intrinsic merits it remained in general use under the appropriate name of the Ainwspheric Engine during the greater part of the last century, but was only used for pumping water. Its guiding principle was, that the steam was solely employed to produce a partial vacuum by its condensation, its elastic force at high temperatures not being made use of.
The first and most important of Watt's improvements on the steam-engine consisted in effecting the condensation in a separate vessel, termed the Condenser, which commu nicated with the cylinder ; whereas Newcomen condensed his steam by a jet of cold water in the cylinder itself. This condenser being filled with steam from the boiler at the same time with the cylinder, the jet of cold water, admitted into the former only, effected the condensation of the whole volume of steam, both of that in the cylinder as well as of that in the condenser. As the cylinder was thus not c000led by a jet, a vast economy of heat resulted. The second of Watt's improvements consisted in closing the cylinder at top, the piston-rod being made to pass steam-tight through a cylindrical neck in that top, termed a Stging-Box. The object of this alteration was to admit of the elastic force of the steam being employed to impel the piston downwards, instead of simple atmospheric pressure. For this purpose the steam was admitted from the boiler above the piston at the same moment the condensation took place in the condenser; the steam-passage being made double for tne purpose, so that the communication with the condenser could be cut off when that with the cylinder was opened, alternately. Such is
the general principle of Watt's Single-Acting Engine, which hence became a Steam-Engine, and was no longer an Atmospheric Engine ; it became a Double-Acting Engine by removing the counterpoise, and producing the upward motion of the piston by admitting steath below as well as above it.
The changes in the engine introduced by Watt created the necessity for two pumps, and commonly three, which are worked by rods attached to the beam. The first of these is the Hot-Water or Air-Pump, intended to remove the air, condensed water, and steam from the condenser, in which they would otherwise accumulate, and finally stop the action ; the second is a Force-Pump, required to return the water, drawn from the condenser by the first, back to the boiler ; and the third, termed the Cold-Water Pump, supplies the cold-water cistern which contains the con denser.
In the mechanism of a steam-engine, the Piston is one of the most important parts, as it must be steam-tight, and yet work easily in the cylinder. The better class of engines have now usually what are called Metallic Pistons, of which there are different kinds, invented by Cartwright, Jessop, Barton, and others. The body of these pistons is metal, made in pieces or segments, acted on by springs radiating from a centre ; so that while the friction is diminished by both surfaces being metallic, the piston, ()Wing to its con struction, can adapt itself to the irregularities of the cylinder. The steam is admitted to the by various ingenious contrivances, among the four-passage cock, the conical valve, and the slide valve are the chief. For the opening and shutting of the valves, an Eccentric is now generally used; this is a peculiar contrivance, worked from the fly wheel. This fly-wheel is necessary when the engine is employed to drive machinery of any kind ; it is a heavy wheel, whose weight tends to equalize the motion of the beam and piston.