19G 
Italian Irrigation. 
better to commence with any measure — any rule, however im- 
perfect — than with uncontrolled service. In fixing the mode of 
charge for water, the choice clearly lies between looking to the 
area irrigated, or to the size and form of the outlet through which 
the water flows. The former standard is found in many respects 
objectionable : — 1st. The area irrigated will vary from year to 
year, and will yearly have to be measured, whereby a door is 
opened for fraudulent collusion on the part of subordinate officials. 
2ndly. The cultivator will for some crops risk the chances of a 
rainy season, and in a gambling spirit defer the proper applica- 
tion of water. It is best, then, to regulate the charge by the size 
and form of the outlet. 
But this regulation of the outlet has been found to be a 
more difficult problem than may at first sight be imagined. 
At a very early period it was discovered that the size of the 
aperture was an insufficient test, and that account must be taken 
of the water's velocity. In 1570, when the scientific principles 
of hydraulics were scarcely known, Soldati invented his moduloy 
constructed on purely empirical data ; and in 1643 Torricelli 
published his theorem for determining the velocity of fluids ; 
" but, in truth, there is no branch of physics in which the theo- 
retical correspond less with the observed results than in hydrau- 
lics." Slight variations in the force of gravity itself; in the 
resistance of the air ; in that of the water into which the discharge 
takes place ; modifications in the form of material or inclination 
of the chamber of supply- ; or in the thickness and perimeter of 
the outlet itself; in the freedom of the water's escape from that 
outlet, as well as in the velocity with which it enters the supply- 
chamber : all these exercise disturbing influences, even when the 
head of water is fixed. 
Already in the sixteenth century, in the days of Soldati, the 
conditions of perfect success were laid before that engineer by 
the inagistracy of jNIilan, and of these conditions Colonel Baird 
Smith writes, " it is clear that to satisfy them all perfectly would 
be impracticable, even with the increased knowledge of the 
present da}'." 
The conditions were briefly these: — "1st. To establish a just 
and exact unity of measure. 2nd. To devise a form of apparatus 
for the outlets, which would be injurious neither to the state, nor 
to the consumers of the water, nor to the navigation of the canal, 
ord. To protect the apparatus from all risk of alteration in its 
essential parts by the cultivators. 4th. That precautions should 
be taken against infiltraticm from the main canal into the private 
channels. 5th. To regulate the velocity of the stream passing 
through the outlet, so as to render it as far as possible independent 
of tlie velocitv o{ the main canal. Oth. To ensuic the same dis- 
