18 BULLETIN 87, U. S. DEPARTMENT OF AGRICULTURE. 



the different grades. A volume of water that would be ample to 

 handle or float the material satisfactorily on a long stretch of flat 

 grade might not be sufficient to furnish an adequate volume to 

 prevent the material from rubbing or striking on the bottom in a 

 section of the flume where the descent was more rapid; and it is also 

 usually necessary to have feeders coming in at different points along 

 the line in order to replace the water that has been lost as the result 

 of leakage or slopping at the abrupt curves. The side feeders are 

 usually brought in from small side creeks by means of a short line 

 of V or square flume constructed from a small dam made in a side 

 creek at some point where there is sufficient descent to carry the 

 water down into the main flume. (See PI. Ill, fig. 2.) The number 

 of feeders, the distance apart, and the points where they shall be 

 brought in are determined by the necessities of each case. 



Where a main flume fine is following closely the bed of a creek 

 having considerable drop, it is usually advisable to take the water 

 from the creek itself by means of a feeder flume having a slight down 

 grade until it intersects the main flume. This can be done by 

 blocking up under the feeder flume, either by trestles or by any other 

 economical and convenient form of foundation, until its end will 

 reach the top of the V. It is usually advisable to make the feeder 

 line as direct as possible and thus avoid increased construction. 

 Feeder flumes do not have to be as substantially constructed as the 

 main flume lines, since nothing except water is conducted in them, 

 nor is the form of construction as important for the same reason. 

 Therefore the single-thickness square-box type of flume is often used 

 for this purpose. Any method of blocking in under a feeder flume 

 that will furnish a substantial foundation is permissible. Here again, 

 however, as in the trestling under the main timber or lumber flume, 

 too cheap construction, trestling, or blocking is false economy, as the 

 work should always be stable enough to be lasting. 



TUNNELING. 



Tunneling through such obstacles as sharp ridges or projecting 

 bluffs has sometimes been found advisable, economical, and necessary 

 in order to maintain a proper and desirable grade, shorten distance, 

 and prevent too abrupt curvature. It is sometimes cheaper to tunnel 

 for a short distance through an obstruction than to trestle for a long 

 distance in order to raise the line over it or to put in a long curve to 

 get around it. It is also sometimes cheaper to tunnel through a ridge 

 or projecting point of rocks than it is to make an open cut. This 

 has been demonstrated by the practical experience of a number of 

 operators. (See PI. IV.) A tunnel should be carefully located by 

 survey, so as to be certain that it will be very close to the desired 

 grade in order to reduce the amount of necessary excavation to the 



