VALVE, in hydraulics, pneumatics, &c, is a kind
of lid, or cover of a tube or vessel so contrived as to
open one way, but which, the more forcibly it is pres-
sed the other way, the closer it shuts the aperture; so
that it either admits the entrance of a fluid into the
tube or vessel, and prevents its return; or admits its
escape, and prevents its re-entrance.