I 
MEASUREMENT AND DIVISION OF WATER. 29 
from the sides of the weir. The level of the water in the 
lower canal has no influence on the discharge of the weir, 
unless it may reach the plane of the sill. 
5. The depth of the water should be measured with 
accuracy where the suction of the flow does not affect the 
height and where it is free from influences which can af¬ 
fect the true level of the water, as the wind or the move¬ 
ment of the water. The height should be read to within 
1-300 of the depth in order that the error may be within 
J per cent, 
G. The weir ought to be constructed with care and 
carefully located. It should not vary more than 4° from 
being perpendicular to the channel. Its sill should be 
horizontal. 
Effect of Disturbing Causes. 
All the disturbing causes may be divided into three 
categories : First—Those which may increase or decrease . 
the discharge. Second—Those which always tend to in¬ 
crease the discharge. Third—Those which always tend 
to decrease the discharge. 
The height of water measured may be too great or 
small, and, as it is as likely to be one as the other, the 
errors due to this may be neglected. 
The effect of obliquity of the weir, or eddies, is to de¬ 
crease the flow. 
Velocity of approach, nearness of sides and bottom to 
the crest, of incomplete contraction, of a crest not per¬ 
fectly sharp, of air not having free access beneath the 
sheet of falling water, etc.—the effect of each of these is to 
increase the discharge. 
The causes tending to increase the discharge evidently 
outnumber those decreasing it, and are, all things being 
taken into account, more difficult to overcome. The com¬ 
bined effect of all taken together is evidently to increase 
