—27— 
is two or three times the depth on the weir, there seems to be 
no further change in the contraction with the inci eased 
tance. In such case, the contraction is said to be complete. 
From the case of complete contraction there may be all de- 
grees of contraction down to no contraction. • 
g The amount of this contraction, when complete lncrea 
with the depth of water flowing over the weir 's ditt.cu t 
and unreliable to measure the amount directly. But we agai 
have recourse to the experiments of Francs, from w ‘ is 
determined that with complete contraction and the same for¬ 
mula, that if an allowance be made, equivalent to a shorten 
ing of the weir equal to one-tenth of the depth of the wat 
flowing over it, for each complete contraction, the discharge 
Figure 7. 
will be given, other conditions being correct, within i percent. 
Thus, we may take an example from the abstract ot 
Francis experiments, given on page 26. 
In the case where the depth is 1.56 feet, there are two 
contractions, hence the effective length of the weir, or t re 
length to be taken in the calculation of the discharge, is not 
ten feet, but ten feet shortened by two (the number of con¬ 
tractions) times one-tenth of 1.56 feet; or .31' es ® 
feet. The effective length is accordingly 9.69 feet. With the 
same weir, but a depth of .80 feet, the effective lengt i, or e 
V 
