Measurement and Division oe Water. 31 
clination is down or up stream. For inclination of 45 °, the effect 
is to cause a decrease of 7% if it be up stream, and an increase of 
10% if down stream. For less inclinations, the correction is cor¬ 
respondingly less, being less than 4% if the slant is not over 18 0 . 
It is easy to maintain practically vertical. 
If the box or channel of approach is narrower than required 
by condition 2f, the flow is increased. If the width is reduced to 
1 1/2 times the depth, the flow is increased by about one-tlurd of 
one per cent, and still more if narrower. 
If the condition 2c is not complied with, the increase amounts 
to one-half of 1 per cent, if the distance of the sill from the bottom 
is reduced to 2.5 times the depth of the water over the weir; and the 
increase amounts to 1 per cent, if this height is reduced to two times 
the depth; 1 1/2 per cent, if it is equal to the depth; and 2 per 
cent, if it is one-half the depth. This is the height of the sill above 
the bottom on the up stream side. If filled in with sand, the surface 
of the sand should be considered as the bottom. Table II. in the 
appendix gives means for estimating the effect for any other depth. 
The temperature of the water has a slight effect, but for all 
ordinary conditions may be neglected. An increase of tempera¬ 
ture seems to increase the discharge through action of the surface 
tension of the liquid. With large openings, the effect is less than 
with small openings. 
The velocity of approach is, all things considered, the most 
difficult to reduce within reasonable limits. These errors are the 
most difficult to allow for and they are the source of the most 
considerable errors under ordinary conditions. It is not possible 
to entirely prevent some velocity in the approaching water. It is 
desirable, however, to cause it to flow from rest as near as possible. 
The velocity is reduced by increasing the size of the approaching 
channel or by making a basin in front of the weir. This is suf- 
ficently reduced by making the cross-section of the basin seven 
times as great as the opening of the weir. 
When this is the case, the effect of velocity of approach may 
be neglected. For many cases this cannot be entirely prevented. 
Often the greatest trouble comes from sediment in the water, which 
is deposited in front of the weir board. Thus, even if the weir has 
been placed originally under proper conditions, the effect of the 
silt may be to change those conditions and cause the weir to be 
unreliable. In some streams the trouble is serious, and almost 
prevents the use of the weir. It can usually be met by arranging 
the weir board so that it may be raised and flush out the sediment. 
The ditch rider should understand the effect of the silt, and that 
the discharge under those conditions is greater than would be in¬ 
dicated by the tables. 
