1090 
Journal of Agricultural Research 
Vol. V, No. 23 
Notches with angles made as precisely as those used in the test would 
not be practicable for field use, and consequently a maximum thickness 
of yi inch probably would be safer than % inch where heads as low as 
0.2 foot will be used. While no experiments were made, edges as thick 
as % inch probably can be used where the minimum head will be 1 foot. 
The edges of the weir notches must be straight, true, and rigid. These 
conditions are best insured by using angle irons or similar material that 
can be securely fastened to the bulkheads, as wood edges become splin¬ 
tered and warped, and thin sheet-metal weir plates buckle and bend 
easily. Regardless of the material used, the notches will be more 
permanent and reliable if the upstream comers of the notches are made 
definitely angular and the edges are left as thick as possible and still 
permit a free flow. 
DISTANCE FROM NOTCH AT WHICH HEAD SHOULD BE MEASURED 
In connection with the experiments with notches of different types, 
measurements were made to determine the transverse and longitudinal 
curves of the water surface upstream from the weirs when different heads 
were being used. These measurements showed that the extent of the 
curves backward from and to the sides of the notches depends upon the 
length of the crest and the head being used. Plots of the data obtained 
show that measurements of head should be made either at a distance of 
at least 4# upstream from the notch or at a distance of at least 2 H side- 
wise from the end of the crest of the notch. 
Table XII gives the errors and the percentage of error made in com¬ 
puting discharges for notches of different shapes and sizes with different 
heads caused by errors of 0.01 foot in reading the heads. 
Table XII .—Errors and percentage of error in computed discharges caused by 0.01-foot 
error in reading the heads 
RECTANGULAR WEIRS 
Error. 
Correct 
head. 
i-foot crest. 
15^-foot crest. 
2-foot crest. 
3-foot crest. 
4-foot crest. 
Cu. ft. 
Cu.ft. 
Cu.ft. 
Cu.ft. 
Cu. ft. 
Feet. 
per sec. 
Per ct. 
per sec. 
Per ct. 
per sec. 
Per ct. 
per sec. 
Per ct. 
per sec. 
Per ct. 
d. 20 
0.021 
7.22 
0.033 
7-52 
0. 044 
7.48 
0 .067 
7 * 55 
0.09 
7 * 56 
•30 
.026 
4 .94 
.04 
5-03 
•05 
4.67 
.08 
4*97 
. 10 
4-63 
.40 
.029 
3.61 
■05 
4-13 
. 06 
3-68 
.09 
3. 66 
. 12 
3*64 
•50 
.04 
3- 60 
■05 
2.98 
.07 
3-10 
. 10 
2. 92 
• 14 
3*06 
. 60 
.04 
2. 76 
•05 
2. 27 
.07 
2. 36 
. 11 
2.46 
. 14 
2 - 33 
. 70 
.04 
2. 20 
.06 
2.17 
.07 
1. 89 
. 12 
2.14 
. 16 
2.13 
,80 
.04 
1.81 
. 06 
1-79 
.08 
1.77 
. 12 
1. 76 
•17 
1.86 
,90 
.05 
1.91 
.07 
1. 76 
.09 
i.68 
•13 
1.60 
.18 
1.65 
1.00 
•05 
1.63 
.07 
i-Si 
. 09 
1.44 
.14 
1.48 
.19 
1*49 
1.10 
.07 
1.31 
.09 
1.25 
.14 
1. 28 
.19 
1-30 
1.20 
•07 
1.16 
. 10 
1.23 
*15 
1. 21 
. 20 
1. 20 
I. 30 
A* 
.16 
1 .15 
. 21 
1. xa 
1.40 
.16 
1.03 
. 22 
1.05 
1* CO 
. 16 
» 2% 
1.00 
A* 
* y.5 
