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 PL III, 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 
fine as direct as possible and thus avoid increased construction. 
Feeder flumes do not have to be as substantially constructed as the 
main flume fines, 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 
surve} r , 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 
