MEASUREMENT AND DIVISION OF WATER. 
27 
Cippoletti’s improvement consists principally in auto¬ 
matically overcoming, by the shape of the opening, this 
contraction, so that the effective length of the same weir 
remains the same, and the discharge from two weirs for 
the same depth will be in the ratio of their lengths—one 
twice as long discharging twice as much. 
The weir which he adopted is of the trapezoidal 
shape, the sides being inclined at one-fourth horizontal to 
cue vertical, as shown in fig. 7. This inclination was 
adopted from theoretical and experimental considerations, 
so that with complete contraction the added area of the 
triangles would just make up for the contraction; conse¬ 
quently the discharge through the weir would be 
the same as through the rectangular weir of the same 
length, but without contraction. The actual length of 
the weir is therefore used, and simplifies calculations very 
much. 
The conditions Cippoletti attempted to meet, then, 
were : 
1. To place the weir in such condition that all causes 
affecting its discharge shall not cause an error greater 
than one-half per cent. 
2. To eliminate automaticallv the lateral contraction, 
so as to preserve the simple formula : 
3 
Q =a L Id *. 
(a) with the coefficient a, constant for all depths 
(/;) and so that the discharge shall be rigorously 
proportioned to the length of the weir. 
In order that no cause shall produce an error greater 
than one-half per cent., it is necessary: 
1. That the weir be preceded by a straight channel 
of constant cross-section, with its axis passing through 
the middle of the weir, and perpendicular to it ; this 
straight reach to be of such length that the water goes 
