VALVES AND COCKS.
The word valve comes from the Latin—valva—a leaf, fold or valve of a door (as of a folding door).
A valve may act automatically so as to be opened by the effort of a fluid to pass in one direction and closed by its effort to pass in the other direction, as a clack valve; or it may be opened or closed by hand or mechanism, as a screw valve or a slide valve. In the glossary at the beginning of this work, the word has been carefully defined and several illustrations have been given of various designs of the device which have come into general use.
Valves are of several classes.
1. Rotary; such as cocks, faucets, plug throttle-valves.
2. Lifting; raised clear from the seat by power beneath; such as ball, conical, cup, safety, poppet.
3. Hinged; such as clack, butterfly.
4. Sliding; such as the slide, D, B and box.
5. Spring; such as some forms of safety-valves, Snifting and Relief valves.
6. Inverted-cup; such as quicksilver valve, air trap, etc.