VALVE. In mechanics, a close lid affixed to a tube, or hollow piston, or opening in a vessel, by means of a hinge or other sort of movable joint, in such a manner that it can be opened only in one direction. For the passage of water, the clack-valve, made of leather and wood, is generally used, working on a side-axle, or a central-axle, or pyramidi cal. A steam-valve is a flat metal plate, bevelled at the edges and guided by a pin, as an axis. In air-pumps they are of oiled silk, with a grating. But, for every purpose, caoutchouc is the best material. In the great London works, valves are as much as 30 inches in dia meter; and the great column of water makes them act with astonishing force. A spherical ball of metal laid on the end of a tube properly formed, and retained in its place by its weight alone, is some times used advantageously instead of a valve, as in the hydraulic ram. The caj'ety value of the steam engine is a valve at tached to the boiler, to obviate the dan ger of its bursting in case the steam should become too strong. The valve is so loaded that its weight, added to that of the atmosphere, exceeds the pressure of the steam, when of a sufficient force. When the expansive force increases so far that its pressure preponderates over the pressure of the atmosphere and the weight of the safety valve, the valve opens, and the steam escapes from the boiler till its elastic force is sufficiently diminished, and the valve shuts by its own weight. By opening the safety valve the engine may be stopped at pleasure, for which purpose a particular apparatus is attached to the valve. As any acciden tal derangement of the safety valve might be attended with the most disastrous con sequences, it has been proposed to make the valve of a metal which melts at a par ticular temperature, by which means the elastic force of the steam could never ex ceed that which corresponds to the tem perature at which the valve would melt.
This method has been adopted in France.
the proper limit, the small valve is drawn down, and the other valve up, thus blowing off at once a great deal of steam. This valve is somewhat interesting as being a very neat modification of the equilibrium valve.
Frequently a narrow bar of metal is made fast across a circular orifice in the direction of the diameter, and 2 semicir cular valves of leather, each of which is covered above and below with a brass plate of the same form, turn on the sides of the bar as on hinges. This is called the double cheek or butterfly valve.
When instead of bars of metal the re sistance of the valve is afforded by a spring, this is the spring valve, an il lustration of which is given here.
Duplex Safety Valve. This valve is the invention of Mr. S. A. Williams, of Corn wall,England. Its object is to prevent the ordinary valve from adhering to its seat, when, from any cause, the internal pres sure falls below the atmospheric pressure, and also to indicate the quantity of water in the boiler, to prevent it from getting too low. The common safety valve opens in the usual way, and is linked to a weighted lever d, g. The weighted lever is extended to the other side, beyond the fulcrum, to a distance equal to that of its distance from the valve. There is a small valve linked to this opposite end of the lever. It is placed inside of the man hole lid, opening downwards. Any up ward movement of the valve produces a corresponding downward movement of the smaller valve. The internal pressure tending to open the valves, will be ex erted only on the excess of the area of the larger valve over the area of the smaller one. A chain is connected to a float and the small valve. When the float is kept up by the water, the chain is loose, but when the water falls below