MEASUREMENT AND DIVISION OF WATER. 
23 
that it is safe to apply it only within the conditions of the 
experiments on which it is based. 
CONDITIONS OF TIIE RECTANGULAR WEIR. 
If the weir be placed so as to meet the following con¬ 
ditions, the formula above given and the tables attached 
to this bulletin, may be used with confidence that the 
result is correct within 1 per cent: 
First—That the water shall not exceed 24 nor be less 
than 6 inches in depth. Experiments subsequent show 
that it is practically safe to 3 inches. 
Second—That the depth of water on the crest shall 
not be more than one-third the length of the weir. 
Third—The crest of the weir itself should be horizon¬ 
tal ; the sides vertical; with both crest and sides brought 
to a sharp edge on the up-stream face. The least round¬ 
ing increases the discharge. The up-stream face should 
be vertical. 
It is also necessary to secure : 
Complete contraction. 
Free discharge. 
And the approach of water to the weir without 
perceptible velocity, cross currents or eddies. 
Hence the following additional conditions : 
Fourth—The distance from the side walls to the crest 
should be at least equal to the depth on the weir, in order 
to secure contraction. 
Fifth—The distance of the crest above the bottom of 
the channel should be at least twice the depth of the 
water flowing over it, in order to avoid the effect of the 
bottom on the crest contraction. 
Sixth—The air must have free access under the fall¬ 
ing sheet. 
