Centre (Belgium) there are locks with a lift of 17 ft.,
and on the St Denis canal near La Villette basins in Paris there is one
with a lift of 32-1/2 ft. In cases where a considerable difference of
level has to be surmounted the locks are placed close together in a
series or "flight," so that the lower gates of one serve also as the
upper gates of the next below. To save water, expecially where the lift
is considerable, side ponds are sometimes employed; they are reservoirs
into which a portion of the water in a lock-chamber is run, instead of
being discharged into the lower reach, and is afterwards used for
partially filling the chamber again. Double locks, that is, two locks
placed side by side and communicating by a passage which can be opened
or closed at will, also tend to save water, since each serves as a side
pond to the other. The same advantage is gained with double flights of
locks, and time also is saved since vessels can pass up and down
simultaneously.

Inclines.

A still greater economy of water can be effected by the use of inclined
planes or vertical lifts in place of locks. In China rude inclines
appear to have been used at an early date, vessels being carried down a
sloping plane of stonework by the aid of a flush of water or hauled up
it by capstans. On the Bude canal (England) this plan was adopted in an
improved form, the small flat-bottomed boats employed being fitted with
wheels to facilitate their course over the inclines. Another variant,
often adopted as an adjunct to locks where many small pleasure boats
have to be dealt with, is to fit the incline itself with rollers, upon
which the boats travel. In some cases the boats are conveyed on a
wheeled trolley or cradle running on rails; this plan was adopted on the
Morris canal, built in 1825-1831, in the case of 23 inclines having
gradients of about 1 in 10, the rise of each varying from 44 to 100 ft.
Between the Ourcq canal and the Marne, near Meaux, the difference of
level is about 40 ft., and barges weighing about 70 tons are taken from
the one to the other on a wheeled cradle weighing 35 tons by a wire rope
over an incline nearly 500 yards long. But heavy barges are apt to be
strained by being supported on cradles in this way, and to avoid this
objection they are sometimes drawn up the inclines floating in a tank or
caisson filled with water and running on wheels. This arrangement was
utilized about 1840 on the Chard canal (England), an