But as the friction of this common piston is necessarily very con siderable, the better class of engines have usually what are called metallic pistons, of which there are different kinds, invented by Cart wright, 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, owing to its construction, can adapt itself to the irregularities of the cylinder. It is found in practice that these metallic pistons wear for a long time, and do not of course require the frequent repacking necessary to those with tow or hempen stuffing. Figs. 3 and 4 represent a plan and elevation of an improved form of Barton's piston, to explain the principle.
In Newcomen's engine the steam was admitted to the cylinder, and the communication again cut off by means of a cock of the common construction ; but a more efficient contrivance is requisite when the steam is to be admitted alternately both above and below the piston, and to the condenser, as it is in engines since the introduction of Watt's improvement^ This can be accomplished by a fosr.pasiage cot*, originally invented by Leupold In 1720, and 'since improved by Bramah and others, The principle of a four-way cock will be under stood by the annexed figure (fig. 5) of the plan of one employed by Meese,. Maudelay and Field In their small engines.
mnnication with the upper part of the cylinder by means of the tube T of the elide.
A is a portion of the cylinder; the steam-pipe ; c, D, s:, three passages, one communicating with the top, another with the bottom of the cylinder, and the third with the condenser ; r is the four-passage cock, which, by turning alternately to the right and left, establishes a communication between one of the former with the latter passage. Fig. 6 represents the conical valve with its aide apertures, and that nt the top, by which the steam enters.
Watt employed flat conical valves for the purpose under considera tion, which were raised or depressed by cranks acting on a guide-rod at right angles to the plane of the valve, which therefore did not turn on a hinge like the common clack-ralre of a pump. In some of his engines the valves were raised or depressed by toothed sectors acting on a rack in the guide-rod, so that the valve might rise from its seat without altering the parallelism of its plane. Two such valves were mounted in one box, one above the other, the guide-rod of the lowest passing through that of the upper.
Fig. 7 shows the valves of this construction, of the large engine erected by Messrs. Mandalay for the Chelsea Water-Works. c, part of the cylinder; P, the piston ; T, the " plug-tree ;" o, the gearing handles, which are struck by the tappets on the plug-tree, and thus open and close the valves v ; s, the steam-passage to the upper and lower parts of the cylinder ; n, the passage to the condenser.
In most engines of the present day, however, the elide-ralre, as it is termed, has superseded the use of the others, excepting in large pumping-engines : a perfectly flat surface slides on another, termi nating the orifices which are to be opened and shut; such is the general principle, but the forms and arrangements are too numerous to be mentioned. Fi.1. 8 shows a part of the cylinder of an engine with
box-slide rakes, now much used.
s, the orifice of the steam-pipe; the eteam passes to the upper part of the cylinder at D, the lower passage x being shut off in the position of the valve shown and shaded In the figure ; the elide is moved by the rod n, and it is shown in its second position in dotted lines, in which position it will be seen that the steam can then enter benrath the piston, while the passage r to the condenser is in turn in coin The characteristic and most valuable part of this principle is this, of making part of the elide act as a pipe to connect the two parts of tho cylinder alternately with the condenser. The steam, by pressing on the slide in the common form of slide, enormously increases the friction with the surface against which it acts, and also produces rapid wear of the parts; this defect is remedied in the box-slide and all others which possess this peculiarity.
Slide-valves were proposed by Murray, in 1799, but were abandoned, till improved workmanship allowed of their being more perfectly made; they have been successively improved in principle by Murdoch,13111MRII, Millington, Maudelsy, and Seward, the slides of the last-named being now much used in marine engines.
It has been mentioned that the alternate action of the valves in the atmospheric and Watt's engines was produced by pins, or tappets, ad justed on a rod called the plug-tree, suspended from tho beam ; as the plug-tree moved up and down with the beam, the tappets struck the ends of bent levers or cranks, which raised or depressed the valves in proper succession : some of these levers were so formed that the tappet by pressing against them might keep the valve closed during the greater part of the stroke* of the piston, and others required an inter mediate shorter lever, or claw, to act on the valve-rod; so that the whole arrangement was inevitably complicated and cumbrous. But when the slide-valve superseded Watt's double conical valves, and the steam passages could be opened and closed by the motion of one rod only, connected with the elide, this motion could be readily produced by what is termed an cxcentrie, which for this purpose usually consists of a circular plate of metal, keyed excentrically on the shaft of the fly wheel, and working within a ring attached to the end of a frame in tended to move a crank directly connected with the slide-rod at its other extremity. As the shaft revolves, the excentrie plate imparts an alternating motion to tho frame, which, transmitted by the crank, alternately raises and depresses the slide-rod. The principle of the excentrie is one of the most valuable of those mechanical contrivances by means of which a continuous circular can be converted into an alter. tinting rectilinear motion.