the first canal was made
in 1792-1796 at South Hadley, Massachusetts, and the canal-system,
though its expansion was checked by the growth of railways, has attained
a length of 4200 m., most of the mileage being in New York, Ohio, and
Pennsylvania. The splendid inland navigation system of Canada mainly
consists of natural lakes and rivers, and the artificial waterways are
largely "lateral" canals, cut in order to enable vessels to avoid rapids
in the rivers. (See the articles on the various countries for accounts
of the canal-systems they possess.)

The canals that were made in the early days of canal-construction were
mostly of the class known as _barge_ or _boat canals_, and owing to
their limited depth and breadth were only available for vessels of small
size. But with the growth of commerce the advantage was seen of cutting
canals of such dimensions as to enable them to accommodate sea-going
ships. Such _ship-canals_, which from an engineering point of view
chiefly differ from barge-canals in the magnitude of the works they
involve, have mostly been constructed either to shorten the voyage
between two seas by cutting through an intervening isthmus, or to
convert important inland places into seaports. An early example of the
first class is afforded by the Caledonian Canal (q.v.), while among
later ones may be mentioned the Suez Canal (q.v.), the Kaiser Wilhelm,
Nord-Ostsee or Kiel Canal, connecting Brunsbuttel at the mouth of the
Elbe with Kiel (q.v.) on the Baltic, and the various canals that have
been proposed across the isthmus that joins North and South America (see
PANAMA CANAL). Examples of the second class are the Manchester Ship
Canal and the canal that runs from Zeebrugge on the North Sea to Bruges
(q.v.).

_Construction._--In laying out a line of canal the engineer is more
restricted than in forming the route of a road or a railway. Since water
runs downhill, gradients are inadmissible, and the canal must either be
made on one uniform level or must be adapted to the general rise or fall
of the country through which it passes by being constructed in a series
of level reaches at varying heights above a chosen datum line, each
closed by a lock or some equivalent device to enable vessels to be
transferred from one to another. To avoid unduly heavy earthwork, the
reaches must closely follow the bases of hills and the windings of
valleys, but from time to time it will become necessary to cross a
sudden depress