VALVE. 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 aide, and to be perfectly closed by a force when acting on the oppoaite side; and thus either admit the entrance of a fluid into a 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 i 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 Cocas.) 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.
nrottk-valves usually consist of a thin disc, or circular plate of metal which entirely crosses the area of the steam-pipe, when closed, being sup ported by an axis or spindle, which passes diame trically 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 intro duced by Mr. Joshua Field, of the firm of Mandalay and Co. ; the object of which is to regulate the sup ply of the 'team 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 admittesi 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 courseof steam, water, or other fluids, is the four-way cock. It was
adopted by Leupold, upwards of a hundred years ago, and has been subse quently applied in very numerous instances; particularly by Mr. Trevithick, in his locomotive high-pressure engines, and by most of the 18comotionists 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 I, 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 con denser.
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 uncles side of the piston ; and f, 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 apace between them; the block is generally faced with brass, and grooved upon the pipe, so as to slide over it tight; it is moved by a rod, which passes through a stuffing-box k. 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•passage f. On raising the slide, d becomes open to the eduction-pamage, and e to the steam.