FLUMES AND THEIR STRUCTURE. 



97 



to bear the water pressure. The standards and floor 

 beams are boxed in and bolted to the outside braces, the 

 whole forming the foundation for the. sheath ing or box- 

 ing. The third form, employed more generally on large 

 flumes, consists in framing the floor beams and stringers 

 in cross yokes to receive the boxing. 



A verygocd representation of a flume provided with 

 a waste gate is portrayed in Figure 28. It is customary 



FIG. 29. FLUME ACROSS A VALLEY. 



to place a waste gate in each flume, because the struc- 

 ture furnishes a cheap mode of introducing an escape, 

 and furthermore it is desirable to be able to empty the 

 canal immediately in case the structure should neecl 

 repair. Where flumes are built on trestles the latter are 

 usually supported on piles, though in cases where the 

 bed of the drainage is of sufficiently firm nature, they 

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