IRRIGATION. 



will be thrown on the practical part 

 of irrigation, which, having been long 

 established by experience, before 

 these principles were thought of, de- 

 pends not on their correctness, but 

 only confirms their truth. 



" The whole art of irrigation may 

 be deduced from two simple rules, 

 which are. first, to give a sufficient 

 supply of water during all the time 

 the plants are growing ; and, second- 

 ly, never to allow it to accumulate so 

 long as to stagnate. We shall see, 

 hereafter, one apparent exception to 

 this last rule, but it will be readily 

 explained. 



"The supply of water must come 

 from natural lakes and rivers, or from 

 artificial wells and ponds, in which it 

 is collected in sufficient quantity to 

 disperse it over a certain surface. 

 As the water must flow over the land, 

 or in channels through it, the supply 

 must be above the level of the land 

 to be irrigated. This is generally the 

 principal object to be considered. If 

 no water can be conducted to a reser- 

 voir above the level of the land, it 

 cannot be irrigated ; but there must 

 also be a ready exit for the water, 

 and therefore the land must not be 

 so low as the natural level of the 

 common receptacle of the waters, 

 whether it be a lake or the sea to 

 which they run. The taking of the 

 level is therefore the first step to- 

 wards an attempt to irrigate any 

 lands. 



" Along the banks of running 

 streams nature points out the decliv- 

 ity. A channel, which receives the 

 water at a point higher than that to 

 which the river flows, may be dug 

 with a much smaller declivity than 

 that of the bed of the river, and made 

 to carry the water much higher than 

 the natural banks ; it may thence be 

 distributed so as to descend slowly, t 

 and water a considerable extent of 

 ground in its way to rejoin the streain. ; 

 This is by far the most common mode 

 of irrigation, and the shape, size, and 

 direction of the channels are regula- 

 ted by the nature of the surface and 

 other circumstances, which vary in 

 almost every situation. A few ex- 

 436 



amples will give to those who are 

 not acquainted with the best modes 

 of irrigating land, a pretty accurate 

 notion of the system. 



" We shall suppose a river to run 

 with a rapid current between high 

 banks : at some point of its course a 

 portion of the water is diverted into 

 a canal dug along the bank with a 

 very small declivity. The water in 

 this canal will flow with less rapidity 

 than the river, but will keep the same 

 level as that part of the river where 

 it has its origin. Thus the water 

 t may be carried over lands which are 

 j situated considerably above the bed 

 of the river farther down. All the 

 lands between this canal and the riv- 

 j er may be irrigated, if there is a suf- 

 [ ficient supply of water. The canal 

 may be carried to a considerable dis- 

 tance from the river. The size of 

 the canal and its declivity depend on 

 the quantity of water which may be 

 made to flow into it. A dam is often 

 constructed across a stream, in or- 

 der that as much of its water as is 

 possible may be diverted, and the 

 original channel is often laid quite 

 dry, to take advantage of all the wa- 

 ter at the time when it is advanta- 

 geous to irrigate the land. To have 

 [ an entire command of the water, 

 there are flood-gates on the main 

 channel and on the lesser branches ; 

 by opening or shutting these, the wa- 

 ter may be stopped or made to flow 

 as may be required. It must be re- 

 membered that, to carry water to a 

 considerable distance, and in great 

 quantity, a larger channel and more 

 rapid declivity are required ; and it 

 is a matter of calculation whether it 

 is most advantageous to bring a 

 smaller quantity to a higher point, or 

 a greater abundance somewhat low- 

 er. Having a certain command of 

 water, it may be carried from the 

 main channel by smaller branches to 

 different poiiits, so as to irrigate the 

 whole equally. These branches should 

 be nearly horizontal, that the water 

 may overflow the sides of them, and 

 be equally distributed over the land 

 immediately below. Every branch 

 which brings water over the land 



