sides of the channel. Puddle is made of
clay which has been finely chopped up with narrow spades, water being
supplied until it is in a semi-plastic state. It is used in thin layers,
each of which is worked so as to be firmly united with the lower
stratum. The full thickness varies from 2 to 3 ft. To prevent the
erosion of the sides at the water-line by the wash from the boats, it
may be necessary to pitch them with stones or face them with brushwood.
In some of the old canals the slopes have been cut away and vertical
walls built to retain the towing-paths, with the result of adding
materially to the sectional area of the waterway.

Water supply.

A canal cannot be properly worked without a supply of water calculated
to last over the driest season of the year. If there be no natural lake
available in the district for storage and supply, or if the engineer
cannot draw upon some stream of sufficient size, he must form artificial
reservoirs in suitable situations, and the conditions which must be
attended to in selecting the positions of these and in constructing them
are the same as those for drinking-water supply, except that the purity
of the water is not a matter of moment. They must be situated at such an
elevation that the water from them may flow to the summit-level of the
canal, and if the expense of pumping is to be avoided, they must command
a sufficient catchment area to supply the loss of water from the canal
by evaporation from the surface, percolation through the bed, and
lockage. If the supply be inadequate, the draught of the boats plying on
the canal may have to be reduced in a dry season, and the consequent
decrease in the size of their cargoes will both lessen the carrying
capacity of the canal and increase the working expenses in relation to
the tonnage handled. Again, since the consumption of water in lockage
increases both with the size of the locks and the frequency with which
they are used, the difficulty of finding a sufficient water supply may
put a limit to the density of traffic possible on a canal or may
prohibit its locks from being enlarged so as to accommodate boats of the
size necessary for the economical handling of the traffic under modern
conditions. It may be pointed out that the up consumes more water than
the down traffic. An ascending boat on entering a lock displaces a
volume of water equal to its submerged capacity. The water so displaced
flows into the lower reach of the can