METHODS OI^ APPLYINC^ WATER. 



169 



Subsidiary Canals. — Where the supply canal is 

 large and the banks thick, it is well to divert the water 

 from it in only one place. A shallow subsidiary canal 

 may be made parallel with it, into which sufficient 

 water is allowed to flow to supply the laterals. It is 

 very easy for a stream to get beyond the control of the 



FIG. 47 — LATERAL BULKHEAD. 



irrigator, and he must watch the aperture in the canal 

 bank closely and take measures to prevent this. In 

 the most primitive forms of irrigation the shovel is 

 relied upon entirely for regulating the flow of water ; 

 but a step in advance is made by putting in wooden 

 boxes at such places, with a simple gate board sliding 

 between upright cleats. In this way the exa<ft quan- 

 tity of water desired may be diverted, without danger 



