Illustration: DOUBLE LOCK.
Canals are artificial water-courses, either for the purpose of
connecting rivers, or for forming water communication for the
conveyance of goods. There are about 2200 miles of canal-way in
England, which is still in complete requisition, and but little
affected by the enormous goods traffic of the railways. Canals afford
a means of slow but cheap conveyance for heavy or bulky goods, not
requiring a rapid transit, for the more rapid the pace the greater
the resistance of the water. The usual rate of transit is somewhere
about two-and-a-half miles an hour, at which pace a horse can draw
about four times as much on water as he can on a railway, and about
thirty times as much as on a level turnpike road; but if a greater
speed were to be obtained, it is found that the resistance of the
water would impede it so much, that at the rate of five miles an
hour a horse could draw no more than he could on a railway, and at
ten miles only a quarter as much.
In constructing canals, it is important to have a good supply of
water, and this is generally secured by turning all the springs and
streams in its course into it, or deriving its source at its highest
level from a large river. The same works have often to be constructed
on the line of canals that are required on railways, such as bridges,
cuttings, embankments, tunnels, &c., and besides these, contrivances
peculiar to canals, called “locks,” which are now to be described.
Locks are barriers or doors constructed so that these artificial
rivers may be carried over rising ground and through valleys, without
the labour and expense of cutting through the hills and filling
up the hollows as would be required without them. Railways can be
constructed on ground which is not quite level without any embankment
or cutting, as it is not absolutely required that railways shall
be perfectly level. But water always will be level, unless it is
constantly flowing, as is the case with streams, and these only
sink a few inches in a mile, or else they become so rapid, that if
attempted to be imitated in canals, they would be useless; for it
would require too much power to draw any vessel up the canal against
such a stream, and would moreover require more water to supply
them than can commonly be obtained. The means therefore adopted to
overcome this difficulty are gates, or in other words, a pair of
“locks.” The canal is constructed in such a manner that it shall
be perfectly level for a certain distance, then sink down some ten
or twelve feet at once, and again flow on a level and sink down.
These sudden lowerings are effected as follows: two pairs of thick
solid doors of wood are fitted to shut in the water, and another
pair a short distance further on; behind these the bed of the canal
is lowered the required distance. When a barge or other vessel has
to pass down the canal the first pair of gates are opened, and the
barge floated in between them and the second pair, the first pair are
now closed, and the water beyond the second pair being lower, that
between the gates in which the barge floats is let out by means of
a valve worked by a rack and wheel; when this valve is raised the
water flows out and sinks down to the level of the water beyond,
carrying the barge with it; in a few minutes it is so low that, the
second gates being opened, the barge is drawn out and continues on
its way. But suppose the barge had to be brought up the canal, then
it is floated into the space between the gates (as before), and the
one behind the barge closed, the water beyond the gate in front being
higher, is let into the space where the barge is by a valve, and this
filling, lifts up the barge to the level of that in front, the front
gates are then opened and the barge proceeds onwards. These gates are
never made to shut level, but meet at an angle with the point towards
the highest water, which is done that they may resist the great
pressure which the water exerts, and for turning this to advantage,
for this very pressure shuts the gates and keeps them close together.
The canals which have much traffic on them have double locks, as
in the engraving, that barges may go up and down at the same time
without having to wait for each other. The barges and boats are
generally “towed” or drawn by a horse attached to a rope, and walking
on a “towing path” or road at the side of the canal.