—i7— 
epth or pressure. While the variation is not as much, meas- 
red in inches, in the regulating chamber, as in the main 
hannel, it is the same relatively, and the discharge, with a 
ariation of level, will increase in the same ratio as it would 
•om an orifice discharging direct from the main canal and 
ithout the regulating chamber. In the latter case, if the 
riftce gave the same discharge as the one from the regulat- 
ig chamber it would be smaller, of course. 
It follows from this that the value of this as a regu- 
itor is apparent rather than real, for the amount of water 
elivered will vary as much as if the opening was in the ca- 
al and subject to the greater variation of water which there 
; in the canal. 
It does afford a means of keeping the variation of head 
bove the orifice within smaller limits, though a larger open- 
lg is required, and this is sometimes desirable. But it should 
e kept clearly in mind that unless regulated with every variation 
'i the level of the supplying canal it does not assure a constant 
low, which has been its supposed principal merit. In order 
lat the discharge shall be in proportion to the size of the 
penings, which this method implies, it is evidently necessary 
fiat the water shall pass through with the same velocity, 
rhich not only requires that the pressure or head shall be the 
ame, but also that disturbing influences of other causes, as 
f velocity in the water from its passage through the first op- 
ning, waves, etc, shall be eliminated. Accordingly the mod- 
le consists generally of two parts, that already described for 
ttempting to maintain a constant pressure, and. other appa- 
atus intended to still the waves in the regulating chamber 
nd assure that the water shall reach the'opening without 
elocity. A variety of means has been adopted mostly by 
arying the length and breadth of the regulating chamber, 
n the case of the Naviglio Grande, the canal where troubles 
f measurement caused the original introduction of the mod¬ 
ule, the boxes which I examined when in Italy had also a 
over for the purpose of stilling the fluctuations of the sur- 
ace and insuring that the water should leave with the veloc- 
ty due to the simple pressure. 
This method is used with many variations throughout 
taly. The openings in the different provinces have different 
izesand shapes, and different pressures are used above the 
•rifice. All are called “oncia” or inches. Some of them are 
ircular. No less than ten are given in Carton & Marcolon- 
ro’s “ Anuale del Ingegnere Agronomo,” 1888. 
In this State the same principle has been extensively 
ised in the Max Clark box, as it is called, named from its in- 
