CLXXXVI. GAUGING OF FLOW OF STREAMS AND ARTESIAN BORES. 
reliable. Captain Cunningham gives the following advan- 
tages :—' 
(1) They interfere less with the natural flow of the water. 
(2) They measure velocity direct. 
(3) They can be used in a stream of any size. 
(4) They are not affected by silt and weeds. 
(5) They measure forward velocity. 
(6) They can be made by common workmen. 
(7) They are cheap. 
It is quite true that the floats interfere very little with 
the flow, but this is also true of a well designed current 
meter with a screw fan. They do not measure mean 
velocity direct, but each rod only measures the velocity of 
a particular impulse. Cunningham himself admits that 
about 50 rods must pass one vertical to obtain the mean 
velocity of that vertical. It is a common practice to send 
a few floats past each vertical, and only make use of those 
that agree together and cast out those that disagree. This 
is totally wrong; they should all be made use of and the 
mean taken. They certainly cannot in the author’s opinion 
be used with any degree of accuracy when the depth is 
greater than 20 feet. They are not affected by silt and 
weeds, they do measure forward velocity and they can be 
made by common workmen, but when many gaugings have 
to be taken they most certainly are not cheap, when it is 
considered that it takes twice as long to make a gauging 
with the rods as with the current meter, even when only 
two or three floats are sent past each vertical, and there- 
fore we only get the mean velocity of two or three impulses, 
and again on the New South Wales rivers, when high, the 
water level fluctuates so rapidly that there is only time to 
send one rod past each vertical, and therefore, as stated 
in the beginning of this paper the computed discharge may 
be too high or too low. 
1 Proc. Inst. C. E., Vol. uxx1. 
= Oe eo eee 
