14 
6. Surreptitious attempts to alter its discharge should 
leave traces easy to recognize. 
7. It should be simple enough to be operated by ordi¬ 
narily intelligent men. 
8. Calculation ought not to be required in order to regu¬ 
late the discharge of different modules, or to determine how 
much they are discharging. 
9. It should occupy but small space. 
10. The discharge should not be affected by variations 
in the level of water in the supplying canal, or, in other words, 
it should be self-regulating. 
*11. Its cost should be small, and it ought not to require 
much fall. 
These conditions are evidently not of equal importance. 
The most of the conditions have been recognized for several 
centuries. Those unmarked are essentially the same as 
those given some centuries ago. 
Condition i, on which depends the accuracy of the meas¬ 
ure, becomes day by day increasingly important, and is the 
one which with the passage of time may be considered the 
most important. If the first condition is met, Nos. 2, 3, and 4 
which are practically included in it are also met. 
The question of expense, mentioned in No. ii, is a rela¬ 
tive one, and may or may not be of importance. It becomes 
of less importance as the development of the irrigated sec¬ 
tion becomes greater and the needs for accuracy become 
more generally felt. But the requirement that the fall re¬ 
quired shall be small, is a physical condition which it is often 
absolutely necessary that modules for special locations must 
meet. 
Condition 8 becomes of less importance, with the in¬ 
crease in intelligence of those whose duty it is to distribute 
water, and is not objectionable against such nVodules as the 
weir, where tables of discharge may be prepared which en¬ 
ables the discharge to be determined without computation by 
the user. 
No. lo is the condition which to the early users is almost 
always the most important. The reason has been partl}^ one 
of the stage of hydraulic science, in which it has not been 
known how to measure the quantity of water passing except 
by passing the water through orifices. In order to make the 
velocities through these orifices the same, it has been neces¬ 
sary to make the head of the water equal in these different 
places, and consequently to secure a measure it has been de¬ 
sirable to keep the heads over the openings constant. The 
condition is less important with us, both because with the 
