(130.) The single clack valve is the most simple example of the first class. It is usually constructed by attaching to a plate of metal larger than the aperture which the valve is intended to stop, a piece of leather, and to the under side of this leather another piece of metal smaller than the aperture. The leather [Pg228] extending on one side beyond the larger metallic plate, and being flexible, forms the hinge on which the valve plays. Such a valve is usually closed by its own weight, and opened by the pressure of the fluid which passes through it. It is also held closed more firmly by the pressure of the fluid whose return it is intended to obstruct. An example of this valve occurs in the steam engine, in the passage between the condenser and the air-pump. The aperture which it stops is there a seat inclined at an angle whose inclination is such as to render the weight of the valve sufficient to close it. In cases where the valve is exposed to heat, as in the example just mentioned, where it is continually in contact with the hot water flowing from the condenser to the air-pump, the use of leather is inadmissible, and in that case the metallic surface of the valve is ground smooth to fit its seat. The extent to which such a valve should be capable of opening, ought to be such that the aperture produced by it shall be equal to the aperture which it stops. This will be effected if the angle through which it rises be about 30°. [Illustration: _Fig._ 46.] The valve by which the air and water collected in the bottom of the air-pump are admitted to pass through the air-pump piston is a double clack, consisting of two semicircular plates, having the hinges on the diameters of these semicircles, as represented in _fig._ 46. (131.) Of the valves which are opened by a motion perpendicular to their seat, the most simple is a flat metallic plate, made larger than the orifice which it is intended to stop, and ground so as to rest in steam-tight contact with the surface surrounding the aperture. Such a valve is usually guided in its perpendicular motion by a spindle passing through its centre, and sliding in holes made in cross bars extending above and below the seat of the valve. The conical steam-valves, which have been already described (116.), usually called spindle-valves, are the most common of this class. The best angle to be given to the conical seat is found in practice to be 45°. With a less inclination the valve has a tendency to be fastened in its seat, and a greater inclination would cause the top of the valve to occupy [Pg229] unnecessary space in the valve-box. The area, or transverse section of the valve-box, should be rather more than double the magnitude of the upper surface of the valve, in order to allow a sufficiently free passage for the steam, and the play of the valve should be such as to allow it to rise from its seat to a height not less than one fourth of the diameter of its upper surface.