IRRIGATION.—NO. 1. 
217 
be effected, which, although not perfect, will 
have its advantages. A small rill, which is 
often quite dry in summer, by judicious man¬ 
agement, may still be made to improve a con¬ 
siderable portion of land. Its waters may be 
collected and allowed to accumulate in a pond, 
or reservoir, and let out occasionally, so that 
none be lost or run to waste. If there is water 
only at particular seasons of the year, and at a 
time when it would not be of much use to the 
land, it may be thus kept in ponds, and will 
lose none of its qualities by exposure to the air. 
If animal or vegetable matter, in a partial state 
of decomposition, be added to this water, it will 
much improve its quality, and by a proper dis¬ 
tribution of it over the land, a 
great benefit will follow. 
The supply of water must come 
from natural lakes and streams, 
or from artificial wells and ponds, 
in which it is collected in suffi¬ 
cient quantity to disperse itself 
over a given surface. As it must 
flow over the land, or in channels 
through it, the supply of water 
must be above the level of the 
land to be irrigated. This is 
one of the principal objects to 
be considered. If no water can 
be conducted to a reservoir above 
the level of the land, it cannot be 
self-irrigated ; but there must also 
be a ready declivity, or descent, 
for the water to escape, and 
therefore the land must not be 
so low as the natural level of the 
final receptacle, whether it be a 
lake, river, or sea. 
Along the banks of running 
streams, nature points out the 
declivity. A channel that re¬ 
ceives the water at a point high¬ 
er than that to which the stream 
flows, may be dug with a gentler 
declivity than that of the bed of 
the stream, and made to convey 
the water much higher along the 
sides of the valley, than the nat¬ 
ural banks. It may thence be 
distributed so as to descend slow¬ 
ly, and water a considerable ex¬ 
tent of ground on its way to re¬ 
join the stream below the fall. This is by far 
the most common mode of irrigation, and the 
form, size, and direction of the channels are 
regulated by the nature of the surface and other 
circumstances, which vary in almost every sit¬ 
uation. Let us suppose, for instance, that a 
river running with a rapid current between two 
distant hills, as denoted in Fig. 61. At the point 
A, of its course, a dam is constructed, and a por¬ 
tion of the water diverted into the feeders /,/, 
dug along the hillsides, with a slight declivity. 
The water in these canals will flow with less 
rapidity than that in the stream, but will main¬ 
tain nearly the same level as that part of the 
river directly above the dam, at A. Thus the 
water may be carried over lands which are sit¬ 
uated considerably above the bed of the stream, 
further down, and it is obvious that all the lands 
between these canals and the river, may be irri¬ 
gated, if there be a sufficient supply of water. 
With a given quantity of water at command, 
it may be conducted from these canals, or feed¬ 
ers, to smaller channels, lower down the sides of 
the valley, so as to irrigate the whole equally. 
These lower channels, b , c, b, c, d , should be nearly 
horizontal, in order that the water may overflow 
their sides, and be equally distributed over the 
land directly below them. Each channel should 
have a corresponding drain below it, running 
nearly parallel, to carry off the water; other¬ 
wise it might stop and stagnate. When the 
f, 
Irrigation—Vertical Section.—Fig. 62. 
Irrigation—Ground Plan.—Fig. 63. 
water has run 20 or more feet, according to 
the declivity, over the land situated below the 
“feeder,” or the channel which brings the wa¬ 
ter from above the dam, it should be collected in 
a drain to be carried off, unless it can be used 
to irrigate lands that lie still lower down, and 
finally discharge itself into the river from 
which it was taken at a lower point of its 
course. 
Instances may occur, however, where there 
is not sufficient fall, or declivity, in the river or 
stream to enable the water to flow to any con¬ 
siderable elevation along the sides of the valley 
or hills. In such cases, if a fall of a few feet is 
at command, a portion of the stream can be ele¬ 
vated at a proper height and distance to irrigate 
