17 
depth or pressure. While the variation is not as much, meas¬ 
ured in inches, in the regulating chamber, as in the main 
channel, it is the same relatively, and the discharge, with a 
variation of level, will increase in the same ratio as it would 
from an orifice discharging direct from the main canal and 
without the regulating chamber. In the latter case, if the 
orifice gave the same discharge as the one from the regulat¬ 
ing chamber it would be smaller, of course. 
It follows from this that the value of this as a regu¬ 
lator is apparent rather than real, for the amount of water 
delivered will vary as much as if the opening was in the ca¬ 
nal and subject to the greater variation of water which there 
is in the canal. 
It does afford a means of keeping the variation of head 
above the orifice within smaller limits, though a larger open¬ 
ing is required, and this is sometimes desirable. But it should 
be kept clearly in mind that unless regulated with every variation 
in the level of the supplying canal it does not assure a constant 
flow, which has been its supposed principal merit. In order 
that the discharge shall be in proportion to the size of the 
openings, which this method implies, it is evidently necessary 
that the water shall pass through with the same velocity, 
which not only requires that the pressure or head shall be the 
same, but also that disturbing influences of other causes, as 
of velocity in the water from its passage through the first op¬ 
ening, waves, etc, shall be eliminated. Accordingly the mod¬ 
ule consists generally of two parts, that already described for 
attempting to maintain a constant pressure, and^ other appa¬ 
ratus intended to still the waves in the regulating chamber 
and assure that the water shall reach the opening without 
velocity. AAariety of means has been adopted mostly by 
varying the length and breadth of the regulating chamber. 
In the case of the Naviglio Grande, the canal where troubles 
of measurement caused the original introduction of the mod¬ 
ule, the boxes which I examined when in Italy had also a 
covQr for the purpose of stilling the fluctuations of the sur¬ 
face and insuring that the water should leave with, the veloc- 
itydue to the simple pressure. 
This method is used with many variations throughout 
Italy. The openings in the different provinces have different 
sizes and shapes, and different pressures are used above the 
orifice. All are called “oncia” or inches. Some of them are 
circular. No less than ten are given in Carton & Marcolon- 
go’s “ Anuale del Ingegnere Agronomo,” 1888. 
In this State the same principle has been extensively 
used in the Max Clark box, as it is called, named from its in- 
