FLUMKS AND THEIR STRUCTURE. 1 25 



inches beyond the posts, to which side braces are nailed 

 to strengthen the strudlure. Where flumes are not 

 supported on trestles, but rest on an excavated ledge, 

 it is desirable still to use the stringers, which should be 

 placed just outside the posts, so that water leaking 

 from the sides will drop clear of them. Main supports, 

 such as trestles, are placed eight or more feet apart. 

 Planking should be of pine, redwood, or hemlock. 

 The cross-secftion of a flume should be no narrower than 

 the bottom of the ditch, for if not built in this way 

 there would necessarily be a contradlion that would 

 check the free flow of water. This leads to the question 

 of velocity. The best flumes are built with a vertical 

 drop of from two to four feet at the upper end of the 

 strucfture and these drops have come to take the place 

 of the inclined aprons, which were formerly much in 

 vogue. There should also be a similar drop at the 

 lower end of the flume to make a water cushion, by 

 which the velocity is broken and. washing out is pre- 

 vented. Most engineers agree that the more velocity 

 a flume has without dropping the grade the better it 

 will be, provided arrangements are made to take care 

 of the water at the discharge. If a flume is narrowed 

 in toward its discharge the sides should be raised pro- 

 portionately, in order to provide the proper carrying 

 capacity and at the same time prevent slopping over. 

 Construction. — There are wooden and iron flumes, 

 each built in various forms. In building wooden 

 flumes they are so superficial at best that they should 

 be well made and no expense should be spared in their 

 construdlion. The best material only should be used, 

 and the writer has found seasoned and surfaced lum- 



