CANAL CONvSTRUCTlON. 



73 



diversion dam. This system was worked out on the 

 theory that it is useless to undertake the expense of 

 raising water above its natural level in order to divert 

 it from the bed of the stream, for it is more simple to 



FIG. 12 — HEADGATE OF A CANAL 



have it flow out by its own gravity and volition. To 

 do this a gallery is opened alongside and parallel with 

 the stream, and the water is thus allowed to flow over 

 and into it. This gallery is the real intake of the canal 

 and is built of heavy planking with double floor, laid 

 on mudsills set in concrete, so as to remain firm and 

 fast. The river side of the gallery is faced with an 

 apron, over which the water flows into the gallery, the 

 floor of which is a foot or so lower than the bed of the 



