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.