Mar. i3,1916 
A New Irrigation Weir 
ii 33 
at the same angle as the other part of the weir box (fig. 2), the discharge 
will be increased about one-third of 1 per cent for i-foot head and 
decreased about 1 per cent for 0.2-foot head, as indicated for the 3-foot 
weir in Nos. 19 and 33 in Table I. 
In addition to the experiments with regular weir notches, three sets 
of experiments were made with 90° triangular notches having sup¬ 
pressed bottom contraction and different end contractions. The results 
are represented by Nos. 35, 36, and 37 in Table I. The logarithmic 
discharge curve for the 90° triangular notch with complete end and 
bottom contractions is a perfect straight line represented by the equation 
g=2.487^ 2,4805 . Suppression of the bottom contraction, No. 35 in 
Table I, resulted in 
changing the logarith¬ 
mic discharge curve 
from a straight line to 
a curved line, and in¬ 
creased the discharge. 
An average straight 
line drawn through the 
discharge data, repre¬ 
sented by the equation 
Q = 2.541 h 2A92 > agrees 
with the experimental 
data for medium 
heads, but is about 1 
per cent low for high 
and low heads. 
The second set of 
experiments, No. 36 in 
Table I, also gave a 
logarithmic plot which 
was a curved line. 
The average straight line for these data was about 1 per cent low for 
heads of 0.3 and 1.3 feet, and about 2 per cent high for heads of ap¬ 
proximately 0.8 foot. This indicates the curvature of the discharge 
plot to be increased by a decrease in end-contraction distances. 
The third set of experiments, No. 37 in Table I, was made under 
conditions which practically amounted to making the weir box 10 feet 
shorter than in the previous case, having the sides of the carrying channel 
parallel in both cases, but closer together in this set of experiments. 
This had little effect upon the discharge in the aggregate, but changed 
the slope of the discharge curve slightly. 
The 90° triangular notch with full contractions is one of the most 
accurate and reliable measuring devices for small quantities of water. 
27466°—16-2 
Fig. ii.—P lan of experimental weir box for No. 35, Table I. 
Fig. 13.—Plan of experimental weir box for No. 37, Table I. 
