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.