al, and as the boat passes through
the lock is replaced by water flowing from the upper reach. A descending
boat in the same way displaces a volume of water equal to its submerged
capacity, but in this case the water flows back into the higher reach
where it is retained when the gates are closed.

Waste-weirs and stop-gates.

An essential adjunct to a canal is a sufficient number of waste-weirs to
discharge surplus water accumulating during floods, which, if not
provided with an exit, may overflow the tow-path, and cause a breach in
the banks, stoppage of the traffic, and damage to adjoining lands. The
number and positions of these waste-weirs must depend on the nature of
the country through which the canal passes. Wherever the canal crosses a
stream a waste-weir should be formed in the aqueduct; but independently
of this the engineer must consider at what points large influxes of
water may be apprehended, and must at such places form not only
waste-weirs of sufficient size to carry off the surplus, but also
artificial courses for its discharge into the nearest streams. These
waste-weirs are placed at the top water-level of the canal, so that when
a flood occurs the water flows over them and thus relieves the banks.

Stop-gates are necessary at short intervals of a few miles for the
purpose of dividing the canal into isolated reaches, so that in the
event of a breach the gates may be shut, and the discharge of water
confined to the small reach intercepted between two of them, instead of
extending throughout the whole line of canal. In broad canals these
stop-gates may be formed like the gates of locks, two pairs of gates
being made to shut in opposite directions. In small works they may be
made of thick planks slipped into grooves formed at the narrow points of
the canal under road bridges, or at contractions made at intermediate
points to receive them. Self-acting stop-gates have been tried, but have
not proved trustworthy. When repairs have to be made stop-gates allow of
the water being run off by "off-lets" from a short reach, and afterwards
restored with but little interruption of the traffic. These off-lets are
pipes placed at the level of the bottom of the canal and provided with
valves which can be opened when required. They are generally formed at
aqueducts or bridges crossing rivers, where the contents of the canal
between the stop-gates can be run off into the stream.

Locks.

Locks are chambers,