28 
MEASUREMENT AND DIVISION OF WATER. 
with uniform velocity without internal agitation or eddies. 
In ordinary cases fiftv or sixty feet is sufficient. 
«y %j 
2. Only by making the contraction complete in the 
whole perimeter can a definite value of the numerical co¬ 
efficient not subject to doubt he attained, and for this it is 
necessary, 
(a) that the opening of the weir be made in a 
plane surface, perpendicular to the course of the 
water; 
(b) that the opening itself have a sharp edge on 
the up-stream face, and the walls 'cut away so 
that their thickness at the point of discharge 
shall not be above 1-10 the depth for depths be¬ 
low 5 inches, nor above J the depth for depths 
from 5 to 24 inches ; 
(c) that the distance of the sill of the weir from 
the bottom of the canal be at least three times 
the depth on the weir, and at least twice the 
depth on the weir from the sides; 
(d) that the lateral contraction remaining undis¬ 
turbed, the length of the weir shall be three, or 
better lour, times the depth of the water; 
(e) 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 one meter (40 in.) long and depth of 12 inches it 
ought not to be greater than G inches per second; for 
weirs of two meters (6J ft.) 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: For the other 
conditions, if c and d of Xo. 2 he fulfilled, they are also 
in the present case. 
4. The layer of falling water should be perfectly free 
from the walls below the weir, in order that air may 
circulate underneath. For short weirs it is suf¬ 
ficient that the lateral walls of the lower canal be free 
