- 15 “ 
growth of hydraulic science the amount of water may be 
measured with more accuracy than the early users knew 
how, and because our conditions are different. In our prac¬ 
tice it is rarely attempted to make the discharges constant. In¬ 
stead, as the canals usually have largely different quanti¬ 
ties of water during the season, it is far more important that 
the water shall be cut down in each smaller ditch in the same 
ratio. 
The devices for maintaining a constant flow may be di¬ 
vided into two classes: 
1. Those which attempt to maintain the pressure of 
water above the opening constant, the orifice remaining un¬ 
changed. 
2. Those which vary the size of the opening with differ¬ 
ent heads of water, in such manner as to keep the discharge 
constant. 
Nearly all modules are of the first class, as the difficulties 
are simpler. The module of the Isabella Canal of Spain, de¬ 
scribed in the latter pages, and an excellent module, is of the 
second class. 
But even if the head may be kept constant, the accuracy 
of the measure in those of the first class depends upon the 
kind of opening. The module proper consists of the orifice, 
the remainder being simply accessory. 
The old Italian modules are mostly based on an orifice 
of a determinate size, the head above the orifice remaining 
constant. The miners’ inch of the west and the statute inch of 
Colorado are sdch modules. Some of the Italian modules are 
circular, but most are rectangular of various dimensions. 
Those of the same kind had the same height of orifice and 
the same head. When different quantities of water were to 
be given, the orifice varied in length. But in all these cases 
it was assumed that the discharge was in proportion to the 
length of the opening. The prevailing form of measurement 
on the older canals of Colorado has essentially the same idea 
involved. It is generally attempted to deliver water through 
orifices of the same depths, but with variable widths according 
to the quantity of water to be furnished. Butin cases where the 
elevation of the surface is such that the ordinary depth can 
not be given, it is not uncommon to make the depth less but 
the width greater, keeping the area uniform. Among these 
Italian modules, the Milanese module, orsometimes called the 
Soldati module, has been the most celebrated, as it has been 
the best. This dates from 1585. The canals of the province of 
Milan had no good system of measure, though having been 
in operation for several centuries. There was much trouble 
