32 
BFTXETIX 906, T. S. DEPAETVTEXT OF AGEIC"TLTI"EE. 
miner 's-inch boxes, automatic registers, and other mechanical record- 
ing devices. 
Venturi meters. 
These make use of the Venturi principle by forcing water through 
a small throat in a pipe, thereby converting most of the static into 
velocity head. By keeping a continuous record of the normal static 
pressure and also the pressure at the throat the flow of water at any 
time can be com- 
puted for a meter of 
a given size. Meters 
of this type are in- 
stalled on main pipe 
lines when it is de- 
sired to keep a con- 
tinuous record of the 
total quantity of 
water delivered to 
any system. They 
might also be in- 
stalled a d v a n t a - 
geously on lateral 
pipe lines were it not 
for their first cost, 
since they have no 
moving parts liable 
to be obstructed or 
clogged by sand, 
leaves, or other ma- 
terial carried by the 
water. 
Weirs. 
The weir is the 
most commonly used 
device for measuring water in open ditches and canals. It is more 
difficult to adapt it to pipe systems, and when this has been attempted 
due consideration has seldom been given to the effects produced on 
the accuracy of the measurement by seemingly trivial changes from 
the standard specifications. One of the essential requirements of 
the weir is that the water shall approach the weir notch at a slow 
velocity and unaffected by eddies or cross-currents. These condi- 
tions are seldom fulfilled when water issues from a pipe directly 
above the weir. 
Fig. 10. — Twelve-in cli overflow stand with 6-inch pipe inside. 
