486 | Artificial Irrigation. [ Oct., 
conclude this brief historic account of artificial irrigation by a short 
description of some of the methods adopted in their construction. 
All canals taken off from rivers require a dam to be constructed 
across the stream, just below the pomt of their debouchure, and 
there are two distinct systems of this canal irrigatiou in ordinary 
practice, according to the nature of the country to be irrigated, and 
its physical peculiarities. The high table-land lying at the foot of 
mountain ranges, as in Northern Italy and North-western India, 
requires the adoption of a system very different from that pursued 
in the delta lands of the coast, as in Eastern Spain and Madras. 
The latter system consists in throwing a dam across the bed of the 
river, to raise the surface level of the water, which is then con- 
ducted along canals, whose mouths are above the dams, to the lands 
requiring it. This system is necessarily confined to alluvial tracts, 
which have been formed by deposits from rivers in a state of flood. 
Tn the former case, it is necessary to go back to some point high up 
in the river’s course, whence the water can be brought on to the high 
land by excavation of a moderate depth, and by which suflicient 
command of level may be obtaimed to overflow the surface. 
The simplest kinds of canals are those which are generally 
known as Inundation Canals, many of which exist in the Punjab 
and in Sindh, and by which the low-lands adjoming the rivers in 
those provinces are irrigated. Cuts are made from the river inland, 
for a certain distance, and are then carried in a direction generally 
parallel to the fall of the country, or the course of the river. By 
these, when the latter is in flood, the autumn crop is watered, but 
in the cold season, when the river is low, the canals run dry, and 
the spring crop thus derives no benefit from them. There are no 
works at the head of such canals to control the supply of water, 
for the course of the river Indus is so uncertain that it may com- 
pletely desert the head, and the water may have to be brought in by 
a new mouth excavated for one season, which again may be useless 
in the next. 
The Lake system of Irrigation is common to the plains of 
Lombardy as well as to the Madras Presidency. Where its intro- 
duction 1s practicable, this is, no doubt, a far more economical 
method of applying artificial irrigation, than the construction of 
large canal works; the former requiring generally but small expen- 
diture, whilst the cost of the latter is run up by the numerous 
subsidiary works required for their completion. It does not appear 
necessary to say more with reference to these works; we shall 
therefore conclude the present article with a further notice of those 
subterranean channels to which we have already referred. Wellsted, 
in his ‘ Travels in Arabia,’ describes this method of irrigation, as it 
is practised in the Bediah and other oases of Oman. He says :— 
“The oases of Bediah, and nearly all those of the interior of Oman, 
