ion by the aid of an embankment or aqueduct, while a piece
of rising ground or a hill may involve a cutting or a tunnel. Brindley
took the Bridgewater canal over the Irwell at Barton by means of an
aqueduct of three stone arches, the centre one having a span of 63 ft.,
and T. Telford arranged that the Ellesmere canal should cross the Dee
valley at Pont-y-Cysyllte partly by embankment and partly by aqueduct.
The embankment was continued till it was 75 ft. above the ground, when
it was succeeded by an aqueduct, 1000 ft. long and 127 ft. above the
river, consisting of a cast iron trough supported on iron arches with
stone piers. Occasionally when a navigable stream has to be crossed, a
swing viaduct is necessary to allow shipping to pass. The first was that
built by Sir E. Leader Williams to replace Brindley's aqueduct at
Barton, which was only high enough to give room for barges (see
MANCHESTER SHIP CANAL). One of the earliest canal tunnels was made in
1766-1777 by Brindley at Harecastle on the Trent and Mersey canal; it is
2880 yds. long, 12 ft. high and 9 ft. wide, and has no tow-path, the
boats being propelled by men lying on their backs and pushing with their
feet against the tunnel walls ("leggers"). A second tunnel, parallel to
this but 16 ft. high and 14 ft. wide, with a tow-path, was finished by
Telford in 1827. Standedge tunnel, on the Huddersfield canal, is over 3
m. long, and is also worked by leggers.

Dimensions.

The dimensions of a canal, apart from considerations of water-supply,
are regulated by the size of the vessels which are to be used on it.
According to J.M. Rankine, the depth of water and sectional area of
waterway should be such as not to cause any material increase of the
resistance to the motion of the boats beyond what would be encountered
in open water, and he gives the following rules as fulfilling these
conditions:--

Least breadth of bottom = 2 X greatest breadth of boat.
Least depth of water = 1-1/2 ft. + greatest draught of boat.
Least area of waterway = 6 X greatest midship section of boat.

The ordinary inland canal is commonly from 25 to 30 ft. wide at the
bottom, which is flat, and from 40 to 50 ft. at the water level, with a
depth of 4 or 5 ft., the angle of slope of the sides varying with the
nature of the soil. To retain the water in porous ground, and especially
on embankments, a strong watertight lining of puddle or tempered clay
must be provided on the bed and