been made not only to quicken the rate of transit, but also
to move heavier loads, thus increasing the carrying capacity of the
waterways. But at speeds exceeding about 3-1/2 m. an hour the "wash" of
the boat begins to cause erosion of the banks, and thus necessitates the
employment of special protective measures, such as building side walls
of masonry or concrete. For a canal of given depth there is a particular
speed at which a boat can be hauled with a smaller expenditure of energy
than at a higher or a lower speed, this maximum being the speed of free
propagation of the primary wave raised by the motion of the boat (see
WAVE). About 1830 when, in the absence of railways, canals could still
aspire to act as carriers of passengers, advantage was taken of this
fact on the Glasgow and Ardrossan canal, and subsequently on some
others, to run fast passenger boats, made lightly of wrought iron and
measuring 60 ft. in length by about 6 ft. in breadth. Provided with two
horses they started at a low speed behind the wave, and then on a given
signal were jerked on the top of the wave, when their speed was
maintained at 7 or 8 m. an hour, the depth of the canal being 3 or 4 ft.
This method, however, is obviously inapplicable to heavy barges, and in
their case improved conditions of transport had to be sought in other
directions.

Mechanical power.

Steam towage was first employed on the Forth and Clyde canal in 1802,
when a tug-boat fitted with steam engines by W. Symington drew two
barges for a distance of 19-1/2 m. in 6 hours in the teeth of a strong
headwind. As a result of this successful experiment it was proposed to
employ steam tugs on the Bridgewater canal; but the project fell through
owing to the death of the duke of Bridgewater, and the directors of the
Forth and Clyde canal also decided against this method because they
feared damage to the banks. Steam tugs are only practicable on
navigations on which there are either no locks or they are large enough
to admit the tug and its train of barges simultaneously; otherwise the
advantages are more than counterbalanced by the delays at locks. On the
Bridgewater canal, which has an average width of 50 ft. with a depth of
5-1/2 ft., is provided with vertical stone walls in place of sloping
banks, and has no locks for its entire length of 40 m. except at
Runcorn, where it joins the Mersey, tugs of 50 i.h.p., with a draught of
4 ft., tow four barges, each weighing 60 tons