46 BULLETIN 194, U. S. DEPARTMENT OF AGRICULTURE. 
as possible the correct value of n and then designing for whatever 
overload is deemed conservative. Another method which is some- 
times followed is to choose a relatively high value of n and then design 
for the desired discharge. This method is not to be recommended and 
in some cases may be even dangerous for the reason that the velocity 
may so increase beyond a safe figure for the containing material that 
the canal can be eventually operated at but a part of its designed 
capacity. This fact is brought out in the case of a prominent canal 
in western Nebraska. A value of n of 0.025 was chosen as applying 
to such a canal in a moderate state of operative efficiency. As 
determined by numerous experiments, the material cemented into 
a smooth, hard canal bed with an efficiency almost equal to concrete; 
that is, n is actually about 0.016, and when operated to about one- 
half its depth the velocity becomes so high that there is great danger 
of scouring the channel. Thus a large portion of the excavated 
area of the canal can not be used unless lined with some material 
which would withstand scouring or else checks installed to reduce the 
grade. Had the correct value of n been chosen at the time of con- 
struction, the slope might have been far less than it is and additional 
territory made irrigable or a different-shaped cross section might 
have been chosen which would have had about the same yardage 
per given length and the same ultimate carrying capacity as was 
desired for the present canal. 
In the second case cited above, where the engineer must estimate 
the carrying capacity of a canal already constructed, there is room 
for divergence in the estimates of two men equally competent and 
using data collected under the same conditions. 
If the canal is carrying its capacity at the time of inspection the 
results of two men should agree quite closely, as it is a mere matter of 
making a discharge measurement. 
If the canal is about half full a careful measurement may be made 
similar to the ones for the experiments described in this publication. 
From this experiment the value of n holding for the given condition 
may be determined. If the canal is straight and there is no influence 
from structures it may be safely assumed that the surface slope will 
remain quite constant up to capacity. Any material change in the 
value of n must be determined by the influence of the two banks 
above the surface of the water flowing at the time of test. A clean 
concrete, wood, or steel channel will have a slightly lower value 
of n. An earth channel rarely has the same character of material 
throughout the length of the wetted perimeter, and due allowance 
must be made for changes. 
This involves the judgment of the engineer, and two men wall 
disagree more or less at this point. The general elevation of where the 
water surface will be when the canal is full is another place where the 
