1 he effect of nearness of floor is to increase the dis¬ 
charge. When the depth below the crest is three times the 
depth oyer the weir the increase is insensible; if 2.5 times the 
depth, is less than one-half of i per cent., if 2 times the 
depth nearly i per cent., if equal to the depth is 1.5 per cent, 
and if but one-half the depth, over 2 per cent. The amount 
of this varies with other conditions. 
An increase of temperature seems to increase the dis¬ 
charge, and the presence of sediment has the same effect 
through action on the surface tension of the liquid. With 
large openings the effect of the temperature is less than with 
small. Under present conditions they need to be neglected. 
Their influence is small. 
The velocity of approach is, all things considered, the 
most difficult to reduce within reasonable limits, and the 
errors thus introduced in ordinary measurement are the most 
considerable. It is not possible to entirely prevent velocity in 
the approaching water, but by properly proportioning the 
size of the channel to the opening of the weir, the velocity 
may be reduced to such limits that its effects may be 
neglected. A comparison of tables I. and II. for allowing for 
velocity of approach will show this. As the water for irriga¬ 
tion is liable at times to carry sediment, the space in front of 
the weir under most conditions is liable to fill up. The water 
being thus confined to a smaller cross-section, the velocity is 
augmented, increasing the discharge for the same depth over 
the weir. It is troublesome to make the computation for the 
all owance for velocity of approach, the better wa}^ being to 
keep within the bounds indicated by the conditions on page 
31, or within limits indicated by study of tables I. and II; but 
where necessary the following method may be used ; with 
velocities of less than i .5 feet the results will be correct. For 
greater velocities it seems probable that the correction is not 
quite sufficient. It should also be remembered that this cor¬ 
rection is only for the additional head due to the velocity: 
Let H — the head passing over the weir, measured in quiet water, several 
feet from the crest. 
Ji the head which would give the velocity of the water in the channel of 
apjjroach. This velocity may be found by determining the quantity passing over 
the weir, by reference to the tables, without correcting for velocity. Then the 
velocitv is Q 
A 
Where A is the area of the section of the channel above the weir in square feet, and 
Q is the quantity in cubic feet per second. Then 
h = TA 
64.4 
the denominator being twice the acceleration of gravitation, The correction for 
3 
velocity is then made by using in the weir formula,Q~3.33 LH 2, H h instead of 
