12 DEPARTMENT BULLETIN 1110. 
16 inch weir, an upper head of 64 inches and a lower head of 44 
inches. To find the discharge, it is first necessary to convert the 
heads in inches into feet. This conversion is readily made with the 
aid of Table 3, which shows that 64 inches equals 0.542 foot and 44 
inches equals 0.354 foot. Of course, if both heads are measured in 
feet, no conversion is necessary. In order to find the difference of 
head, subtract 0.354 from 0.542 and get 0.188 foot, or approximately 
0.54 and 0.19, respectively, for the upper head and the difference of - 
head. Next turn to Figure 8, which is the discharge diagram for a 
2 by 16 inch weir 1 foot long. On the left of the diagram find the 
horizontal line for the 0.19 difference of head, follow it to the right 
until the 0.5-upper-head line is reached, estimate the position of the 
0.54-line, and then find where the 0.19 difference-of-head line cuts the 
0.54-upper-head line, and directly beneath this point read 0.98 cubic 
feet per second as the discharge for a 1-foot flume. Then for a 3-foot 
flume, the discharge is 3 times 0.98 or 2.94 cubic feet per second 
under the conditions given above. These diagrams should not be 
used for flumes less than 1 foot nor more than 4 feet in width. 
It will be observed that the diagrams do not cover the complete 
range of differences possible for each upper head; in fact, only the 
small differences are shown except for the small upper heads. This is 
due to the difficulty in obtaining these heads in practice. Where large 
flows with large differences in head are measured, the velocity of the 
water is so great below the crest that there is very little tendency to 
submerge the weir until the retardation below is quite great. It is 
doubtful even, if these large differences of head are possible except 
for very high weirs which tend to reduce the velocity of the water 
below the crest. 
SPECIAL TESTS. 
The results of the special tests to determine the effect of changes 
in the flume are given in Table 4. The results of these tests show that — 
neither the changes in the crest nor the changes in the approach con- 
dition cause errors greater than 3.3 per cent. For submerged condi- 
tions, the errors would probably be less on account of the lower veloci- 
ties of flow. This indicates that the changes which ordinarily occur 
in the field would not cause serious errors in the measurements of 
discharges from this type of measuring device. 
