IRRIGATION. 



water on water moailows is just be- 

 fore spring, and it may continue to 

 flow over the land as long as the 

 frost lasts ; in mild weather it may 

 be turned ofT during the day and put 

 on again at night, until the frost is 

 gone. The grass will soon begin to 

 grow, and be ready to be fed off. 

 When this is done, the water is im- 

 mediately let on for a short time, and 

 turned off again to allow the ground 

 to dry after a few days' flooding, and 

 the water is let on again at short in- 

 tervals. The warmer the air is, the 

 shorter time must the water be al- 

 lowed to cover the meadows. As 

 soon as the grass is five or six inch- 

 es long it must be left dry entirely 

 till it is mowed or fed ofT. In sum- 

 mer the floodings must be very short, 

 seldom more than twenty-four hours 

 at a time, but frequent. Thus a great 

 weight of grass may be obtained, year 

 after year, without any manure being 

 put on the land, care being taken that, 

 where the surface is not quite even, 

 the hollows be filled up with earth 

 brought from another place, or dug 

 out of the drain, if that should be par- 

 tially filled up with the soil which the 

 ■water has carried into it. We allu- 

 ded before to a case where water 

 may remain a considerable time on 

 the land without injury ; this is when 

 there are inundations from rivers 

 which rise above their beds in spring, 

 and cover the low meadows which lie 

 along their banks. In this case, the 

 grass, which had not yet sprung up, 

 is protected from the cold ; and if 

 there is a deposite from the water, 

 there is a considerable advantage ; 

 but when it subsides it must be made 

 to run off entirely, without leaving 

 small pools, by which the grass would 



" Fig. 2. 



b 



invariably be injured. Small ditches 

 or clianncls are usually dug, by which 

 all the water may run off, unless 

 where the subsoil is very i)orous, or 

 the land is well under-drained, which 

 is seldom the case in these low mead- 

 ows, for the drains would be apt to 

 be choked by the earthy deposite 

 from the water. These inimdations 

 can sometimes be regulated by means 

 of dikes and flood-gates, in which 

 case they partake of the advantages 

 of irrigation, and also of 1 hat deposi- 

 tion of fertilizing mud which is called 

 warping. See War-ping. 



" The opposite plan {Fig. 1) will ex- 

 plain what has been said respecting 

 the difTerent modes of irrigating land. 

 A A is a river, which has a consider- 

 able fall, and then flows through a 

 level plain. Channels are cut at B B, 

 where there is a rapid fall over a nat- 

 ural or artificial dam. The channels 

 are carried round a hill, and supply a 

 series of channels, C, C, C, placed 

 below each other, forming catch- 

 work along a declivity. A portion of 

 the water goes on to D, where it sup- 

 plies the feeders of a regular set of 

 ridges, or beds, made as before descri- 

 bed, from which the water returns 

 into the river by a main trench, into 

 which all the drains run. E E repre- 

 sent flood-gates, to direct the water 

 into different channels. 



" On the other side of the river, 

 where the slopes lie somewhat dif- 

 ferently, there are several examples 

 of catch-work, the black lines repre- 

 senting the drains which receive the 

 water after it has flowed over the 

 surface, and carry it into the river be- 

 low. It is evident that all the feed- 

 ers are nearly horizontal, to allow 

 the water to flow over their sides." 



Fig. 2 is the section of calch-work. a, a are the feeders; 6, the drains; c, c, c, c. interme- 

 diate channels, which act as feeders and drains. 



Fis. 3. 



Ridge-worh. 

 Fig. 3 is the section of two adjoining ridges, a, o the feeders ; i, 4, 6 the drains. 

 440 



