IRRIGATION AND NA lIGA TION 31 7
be expected under these conditions, the simpler forms of
irrigation works have been limited in number and capacity
only by the resources and engineering skill of the people, by
the insecurity of tenure, and by the interruptions caused by
invasion or internal dissension. Some of these works were
constructed by the former rulers of the country; but it was not
until after the establishment of British rule that the larger
and more important works, many of which irrigate annually
several hundred thousands of acres, were undertaken. As an
instance of the magnitude of some of these later works may
be mentioned the Chenab Canal, which irrigates nearly
2,000,000 acres, or about two-fifths of the whole cultivable area
of Egypt, with an ordinary discharge of II,ooo cubic feet per
second, or about six times that of the Thames at Teddington.
'No similar works in other countries approach in magnitude
the irrigation works of India, and no public works of nobler
utility have ever been undertaken in the world1.'
Irrigation works may be conveniently divided into three Different
great types: namely, lift, storage, and river works, which are tirypigs of
represented by wells, tanks or reservoirs, and canals. In lift works.
irrigation the water is raised from a lower level to that which
will command the area to be irrigated, the raising being
effected either by manual labour or by animal or mechanical
power. The source of supply is usually the subsoil water into
which wells have been sunk, but lift appliances are often erected
on the banks of rivers or pools from which water is raised to
the lands to be irrigated. Storage works are reservoirs, formed
by the construction of dams across drainage lines, for the
purpose of storing the supply which passes down after every
heavy fall of rain for subsequent use during long breaks in the
rains or in seasons of drought. River works consist essentially
of canals, drawing their supplies from rivers which are in
continuous flow during the whole or the greater portion of the
year. In most cases they include a weir, which is constructed
across the bed of the river immediately below the off-take of
the canal, for the purpose of holding up water to the full
supply level of the canal during low stages in the river. The
canals are usually graded with a much lower bed-fall than that
of the river from which they take off, and, by a suitable
alignment, can often be carried out of the river valley on to
high land, or even on to the watershed separating the catch-
ment of the river from that of the drainage line lying next to it.
The great canals in Northern India, and in the Orissa and
1 Sir J. Strachey's India (1903', p. 222.
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