The effect of nearness of floor is to increase the dis¬ 
charge. When the depth below the crest is three times the 
depth over the weir the increase is insensible; if 2.5 times the 
depth, is less than one-half of 1 per cent., if 2 times the 
depth nearly 1 per cent., if equal to the depth is 1.5 P ei cent< 
and if but one-half the depth, over 2 per cent. 1 he amount 
of this varies with other conditions. # r 
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 
laro-e 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 e 
size of the channel to the opening of the weir, the ve oci y 
may be reduced to such limits that its effects may e 
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 m front ot 
the weir under most conditions is liable to fill up. 1 he water 
being thus confined to a smaller cross-section, the velocity is 
augmented, increasing the discharge for the same depth oyer 
the weir It is troublesome to make the computation for the 
allowance for velocity of approach the better way being to 
keep within the bounds indicated by the condhions on page 
3 i 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 ..5 feet the results will be correct For 
greater velocities it seems probable that the corr ® ct10 ? . 1 
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, severs 
teet tromthe crest velocity q{ the water in the channel of 
velocity is v = Q. 
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. ^ I hen 
h = 
64.4 
the denominator being twice the acceleration of gravitation, ^ The correction for 
velocity is then made by using in the weir formula,Q=3.33 LH 2, H -|-_i h instead of 
