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and perpendicular to it; this straight reach to be of such 
length that the water flows with uniform velocity, without in¬ 
ternal agitation or eddies. This should be not less than fifty 
or sixty feet, more if possible. 
2. Only by making the contraction complete on both sides 
and bottom can the coefficient a in the formula have a value 
free from uncertainty, and to secure complete contraction, 
it is necessary: 
that the opening of the weir be made in a plane sur¬ 
face, perpendicular to the course of the water; 
(b) that the opening itself have a sharp edge on the up¬ 
stream face, and its w^alls cut away so that their 
thickness at the point of discharge shall not be 
above i-io the depth for depths below 5 inches, nor 
above yi the depth for depths from 5 to 24 inches; 
{c) that the distance of the sill of the weir from the bot¬ 
tom of the canal be at least three times the depth on 
• the weir; 
{d) that the distance of the sill of the weir from the 
sides of the channel, be at least twice the depth of 
the water flowing over the weir; 
{e) that the lateral contraction remaining undisturbed, 
the length of the weir shall be three, or better four, 
times the depth of the water flowing over; 
(/) that the depth of water flowing over the weir shall 
not be less than 3 inches. 
3. The velocity of approach must be very small; for 
weirs three feet long and depth of 12 inches, it ought not to 
be greater than 6 inches per second; for weirs of six feet long 
and depth of 24 inches it ought not to be above 8 inches per 
second. In all these cases the cross-section of the canal of 
approach ought to be at least seven times that of the weir. 
Other conditions affecting the velocity of approach are in¬ 
cluded in c, d, and e, respecting complete contraction.* 
4. The layer of falling water should be perfectly free 
from the walls below the weir, in order that air may freely 
circulate underneath. For short weirs it. is sufficient that 
the lateral walls of the lower canal be free from the sides of 
the weir. In such case, when air freely passes underneath, 
the level of the water in the lower canal has no influence on 
the discharge of the weir, unless it reaches or exceeds the 
level of the crest. 
5. The depth of the water should be measured with ac¬ 
curacy where the suction of the flow does not affect the 
*Table I. will aid in obtaining proper proportion between the channel of ap¬ 
proach and the weir. 
