— 23 — 
feet of this may be lessened by making the width of the box 
greater for the greater discharges. 
By making the box longer, so as to increase the length 
of the edge over which the water spills the device may be 
made more sensitive. Mr. J. C. Ulrich, C. E., the successor 
of Mr. Graves as Chief Engineer of several of the large ca¬ 
nals in the San Luis valley, and of the North Poudre Canal, 
has in some places adopted on the latter canal a form where 
the box is placed so that it spills on both sides, and the sharp 
edge is made on both sides of the box. 
THE WEIR MODULES. 
But of all forms of modules, or that which best satisfies 
the first condition of accuracy, is the form of opening known 
as the weir, or ov^v{2i\\{¥vQnc\\, Deversoir; Italian, Stramaszo; 
German, Ueberfall ). It is not intrinsically more accurate than 
many other forms of openings, but as it is so simple that the 
conditions for accuracy maybe readily met, and because there 
is a vast fund of experimental knowledge regarding its be¬ 
havior under different conditions, no other form of opening 
can compare with it in accuracy. 
Because of these facts and the growing importance of 
accuracy, the coming module will be based upon the weir. It 
is gradually displacing other types. Australia is using it, ex¬ 
clusively, we think; India, to a large extent, and in Italy, the 
originator of most of our measures, the newer canals are 
using it to the exclusiomof the Milanese module. 1 he old ca¬ 
nals will probably continue the use of the old module, for 
rights have become vested in measurement by them, and 
consumers are jealous of change. A large portion of the 
newer canals in Colorado provide that measurement shall be 
made over a weir. So far as learned no canal has abandoned 
its use. Cippoletti, who was commissioned by the Canale 
Villoresi to propose a new module in obedience to the re¬ 
quirement of the Italian government, says in regard to the 
weir: 
“It is indisputably demonstrated that in weirs with complete contraction, 
constructed and observed with the necessary accuracy, ihe coefficient of contraction 
remains constant, and Francis’ formula guarantees the exactness of the discharge 
with an error not greater than one-half of ore per ce?it for depths of water from 3 
to 24 inches; providing the length of the weir is not less than three—or better yep 
four—times the depth of water flowing over it.”—[Cippoletti, Canale Villoresi, 
Modulo per la dispensa delle acqua, Milano, 1886, p. 135. 
Two forms will be considered, and tables given for their 
discharge—the rectangular weir, whose sides are vertical, 
which is the one ordinarily meant when weir is spoken of, and 
the one which has been the subject of experiment; and the 
trapezoidal weir proposed by Cippoletti, after a thorough in- 
