32 The Colorado Experiment Station. 
It is troublesome to compute the allowance for velocity of ap¬ 
proach. The better way is to avoid the necessity by keeping within 
the bounds indicated by condition No. 3, page 29. If correction 
needs to be made then use tables I. and II. of the appendix which 
are prepared to aid in making the correction. 
The following method due to Fteley and Stearns I have found 
to be satisfactory and easily used. 
The correction for velocity of approach may be made by 
using HT 8 -h in place of the measured depths in the weir for¬ 
mula, Q=3.33 LH 3 ' 2 instead of H as the depth to be taken. 
Where h=V 2 -^- 64 . 4 ; V being the average velocity in the canal 
found by dividing the quantity by the area. All expressed in feet. 
It gives a much larger correction for velocity than is furnished 
by the Francis correction, but it agrees much better with measure¬ 
ments I have made for high velocities. The experiments on which 
Francis’ formula was based were limited to velocities of 2.5 feet per 
second. Table II. gives the per cent, increase in discharge caused 
by different velocities. It will be seen how great this correction be¬ 
comes, sometimes causing an increase of several hundred per cent., 
and consequently shows the importance of keeping the velocity 
within low limits. 
To aid in the practical allowance for velocity of approach, two 
tables have been prepared and are printed as tables I. and II., of the 
appendix. Table II. shows the allowance in per cent, to be made to 
the quantities given in tables III. and IV. The increase with a given 
velocity varies with the depth of water over the weir, being greater 
for small depths. A velocity of one foot per second increases the 
discharge over a weir when the water is flowing three inches deep, 
over 14 per cent.; if flowing over the weir one foot deep, only 3.5 per 
cent. Table I. shows the average velocity as the water passes 
through the weir, or it shows what is the velocity in the channel 
if the section is the same as that of the weir, as it frequently is. A 
comparison of the two tables will show the proper section to give 
the channel in order that the resulting error shall be within reason¬ 
able limits. 
If condition No. 4 is not met, there is an appreciable increase 
in flow. 
The effects of the conditions already given are sufficient to 
show that the conditions increasing the discharge are more num¬ 
erous and more difficult to remedy than those which have the oppo¬ 
site effect. If there has been carelessness or ignorance in placing 
the weir, it would commonly F shown in making the weir too low 
or making the opening too lor - compared with the width of the 
basin or canal above the weir, r : n allowing it to silt up, the effect 
of all of these is to increase t ! lischarge. Likewise, if the crest 
