A cover or stop to an aperture, to control or direct the course of fluids. They are usually contrived so as to be readily opened by a small force acting on one side, and to be perfectly closed by a force when acting on the opposite side; and thus either admit the entrance of a fluid intoa tube or vessel, and prevent its return; or else permit the fluid to escape, and prevent its re-entrance.

Valves are members of the utmost importance to steam-engines, pumps, and a variety of pneumatic, hydraulic, and hydrostatic machinery; and they are constructed in a great variety of forms, to adapt them to their several uses. Cocks employed for drawing off liquids are strictly valves; but this class of valves we have described under their usual distinctive name. (See Cocks.) Numerous valves have been described in different parts of this work, under the above-mentioned subjects, we shall therefore notice in this place several varieties which have not been elsewhere specified.

Throttle-valves usually consist of a thin disc, or circular plate of metal which entirely crosses the area of the steam-pipe, when closed, being supported by an axis or spindle, which passes diametrically through, or across it, and into the sides of the pipe. This spindle is either operated upon by the governor of the engine, or by hand, setting it open to such an extent as to intercept more or less of the steam in its passage to the engine.

Field's Regulating Valve, is a contrivance introduced by Mr. Joshua Field, of the firm of Maudslay and Co.; the object of which is to regulate the supply of the steam in a superior manner to the throttle-valve last described. "It consists," says Mr. Tredgold, "of a valve, placed in the situation usually assigned to the throttle-valve, that is, near to the place' where the steam is admitted to the cylinder. This valve is to be opened at once, at the commencement of the stroke, so as to afford full passage to the steam, and shut at once, after a certain part of the stroke is made, that the rest of it may be completed by the power of the steam." Thus, by causing the valve to be shut sooner or later during the stroke, the power of the engine may be regulated.

One of the earliest and simplest contrivances for completely reversing the direction or course of steam, water, or other fluids, is the four-way cock. It was adopted by Leupold, upwards of a hundred years ago, and has been subsequently applied in very numerous instances; particularly by Mr. Trevithick, in his locomotive high-pressure engines, and by most of the locomotionists of the present day. The annexed cut exhibits a vertical section of a four-way cock, considered as applied to a steam-engine: at a is represented the communication with the steam-pipe from the boiler; b, the passage to the upper side of the piston; c, the passage to the lower side of the piston; and d, the passage to the condenser. In the position represented, the steam is entering the upper part of the cylinder, and the lower part is open to the condenser; but if the plug, or central movable portion of the cock be moved one quarter of a revolution in either direction, then the steam is opened to the lower part of the cylinder, and the upper part is open to the condenser.

The D slide-valve is another invention of great simplicity, and has been much used for opening and changing the communications with the steam cylinder.

In the annexed vertical section, a is the steam-box, into which steam is admitted by the passage b. This box is bolted to a pipe, divided into three compartments; viz. d, a passage leading to the upper side of the piston; e, a similar passage to the under side of the piston; andf, a passage to the condenser. The apertures of this passage are faced with brass, and the space between each opening it is essentially necessary should not be less than each opening; g is a block of metal with a cross cast into it, equal in length to two of the apertures and the space between them; the block is generally faced with brass, and grooved upon the pipe, so as to slide over it steam-tight; it is moved by a rod, which passes through a stuffing-box k. In this position of the slide, the steam would pass through d to the top of the piston, whilst the steam beneath the piston would pass through e, to the eduction-passagef. On raising the slide, d becomes open to the eduction-passage, and e to the steam.

Valve 676Valve 677Valve 678

The D slide, and the four-way cock, however, equally possess a great defect, that of wasting the steam that fills the passages of the movable portion of the valves. Watt, Hornblower, Murdoch, and other steam mechanicians, devised modifications of the D and ether valves, by which the waste of steam was nearly obviated. The invention of Mr. Murray in 1789. for the same purpose, ranks very high in our estimation, being attended with less friction than the others; we accordingly give it insertion in this place.

Valve 679

o in the foregoing figure, is the pipe conveying steam from the boiler, and delivering it into the descending pipe p, which terminates in the valve q, opening to the lower part of the cylinder, by the side opening, marked as a shaded parallelogram, while the valve r opens a similar communication with the upper part of the cylinder; so that, by the successive opening and shutting of q and r, steam is admitted above and below the piston. s is the lower end of the eduction-pipe, joining on to the condenser, and this pipe opens first to the lower part of the cylinder by the valve t, and leads also by a perpendicular continuation of the same pipe v, to a valve a, by which a connexion is formed with the upper part of the cylinder. The two apertures into the cylinder, called nozzles, are therefore common both to the admission of steam and the formation of a vacuum, which is regulated simply by the working of the valves. For as the figure now stands, r is the only open valve in the steam-pipe; consequently steam would enter above the piston to depress it, while a vacuum would exist below it, on account of the valve t being open to the condenser.