constructed of wood, brickwork, masonry or concrete,
and provided with gates at each end, by the aid of which vessels are
transferred from one reach of the canal to another. To enable a boat to
ascend, the upper gates and the sluices which command the flow of water
from the upper reach are closed. The sluices at the lower end of the
lock are then opened, and when the level of the water in the lock has
fallen to that of the lower reach, the boat passes in to the lock. The
lower gates and sluices being then closed, the upper sluices are opened,
and when the water rising in the lock has floated the boat up the level
of the upper reach the upper gates are opened and it passes out. For a
descending boat the procedure is reversed. The sluices by which the lock
is filled or emptied are carried through the walls in large locks, or
consist of openings in the gates in small ones. The gates are generally
of oak, fitting into recesses of the walls when open, and closing
against sills in the lock bottom when shut. In small narrow locks
single gates only are necessary; in large locks pairs of gates are
required, fitting together at the head or "mitre-post" when closed. The
vertical timber at the end of the gate is known as the "heel-post," and
at its foot is a casting that admits an iron pivot which is fixed in the
lock bottom, and on which the gate turns. Iron straps round the head of
the heel-post are let into the lock-coping to support the gate. The
gates are opened and closed by balance beams projecting over the lock
side, by gearing or in cases where they are very large and heavy by the
direct action of a hydraulic ram. In order to economize water canal
locks are made only a few inches wider than the vessels they have to
accommodate. The English canal boat is about 70 or 75 ft. long and 7 or
8 ft. in beam; canal barges are the same length but 14 or 15 ft. in
width, so that locks which will hold one of them will admit two of the
narrower canal boats side by side. In general canal locks are just long
enough to accommodate the longest vessels using the navigation. In some
cases, however, provision is made for admitting a train of barges; such
long locks have sometimes intermediate gates by which the effective
length is reduced when a single vessel is passing. The lift of canal
locks, that is, the difference between the level of adjoining reaches,
is in general about 8 or 10 ft., but sometimes is as little as 1-1/2 ft.
On the Canal du