FIG. 3.
FIG. 4.
FIG. 5.
Valves are contrivances to admit the passage of fluids or gases by their own pressure in one direction, and in such a manner that the same pressure shall of itself prevent their return or passage in the opposite direction, as in [fig. 1], which is the piston of a common pump. There are almost innumerable varieties of valves, one consists of a ball of metal fitting into a cup which has a hole at the bottom ([fig. 2]). Another ([fig. 3]), is a plug of a conical shape fitting in the same way, and having a rod affixed to the top which passes through a hole in a piece of metal so as to guide it in its ascent and descent; this is the kind of valve used as a “safety-valve” in steam boilers, but having the pressure regulated by a spring or weights. [Figs. 4] and [5] are representations of a kind of valve which forms the piston itself, and is very useful as a piston for a square wooden tube for temporary purposes, as on board ship, where any number may be fitted up at little trouble, time, or expense. There are many other valves besides these, as the sliding valves of steam-engines, &c.
WHEELS.
FIG. 1.