The bellows was probably the first instrument of which they formed a part. No other machine equally ancient can be pointed out in which they were required.
By far the most important improvement on the primitive bellows or bag was the admission of air by a separate opening—a contrivance that led to the invention of the valve, one of the most essential elements of steam, of water, as well as pneumatic machinery.
Valves and Cocks.—Generally described, a valve is a lid or cover to an opening, so formed as to open a communication in one direction and close it in another by lifting, turning, or sliding—among the varieties may be classed as, the cock, the slide-valve, the poppet valve and the clack-valve. A common form of this valve is shown in [Fig. 139], page 261.
An every day example of a valve, and almost the simplest known, is that of an ordinary pump where the valve opens upward to admit the water and closes downward to prevent its return.
A valve has a seat, whether it be a gate or circular valve, and is generally turned by a circular handle fitted to the spindle.
Difference between a cock and valve.—The cock is a valve, but a valve is not a cock; the cock is a conical plug slotted and fitted with a handle for turning the cone-shaped valve, with its opening in line, or otherwise, with the opening of the pipe.
Globe Valve is a valve enclosed in a globular chamber, [Fig. 135]. This, like many other valves, takes its name from its shape.
Globe valves, whenever possible, should be placed so that the pressure comes under the valve, or at the side, for if the valve should become loose from the stem (which they often do) if the pressure is on top, there would be a total stoppage of the steam.
Relief Valve is a valve so arranged that it opens outward when a dangerous pressure or shock occurs; a valve belonging to the feeding apparatus of a marine engine, through which the water escapes into the hot well when it is shut off from the boiler.