THE OX AND THE DAIRY. 163 



It may thence be distributed so as to descend slowly, and 

 water a considerable extent of ground in its way to rejoin the 

 stream. This is by far the most common mode of irrigation ; 

 and the shape, size, and direction of the channels are regu- 

 lated by the nature of the surface and other circumstances, 

 which vary in almost every situation. 



We shall suppose a river to run with a rapid current be- 

 tween 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 

 may be carried over lands which are situated considerably 

 above the bod of the river farther down. All the lands 

 between this canal arid the river may be irrigated if there is a 

 sufficient supply of water. The canal may be carried to a 

 considerable distance from tbe 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 

 river, in order 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 water at the time wben it is advan- 

 tageous to irrigate tbe 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 which the water 

 may be stopped or made to flow as may be required. It must 

 be remembered, that to carry water to a considerable dis- 

 tance, and in great quantity, a larger channel and more rapid 

 declivity are required ; and it is a matter of calculation whe- 

 ther it is most advantageous to bring a smaller quantity to a 

 higher point, or a greater abundance somewhat lower. Having 

 a certain command of water, it may be carried from the main 

 channel by smaller branches to different points, so as to irri- 

 gate 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 wbich brings water over the land should have a 

 corresponding channel below to carry it off; for the water 

 must never be allowed to stop and stagnate. When it. has 

 run fifteen or twenty feet, according to the declivity, over the 

 land situated below the feeder, or the channel which brings 

 the water, it should be collected into a drain, to be carried off, 

 unless it can be used to irrigate lands which lie still lower. 



