CLXXX. GAUGING OF FLOW OF STREAMS AND ARTESIAN BORES. 
even in such a channel as the Sydney Water Supply Upper 
Canal with constant cross section, regular grade, and 
cement lined, the author has observed the water boiling 
and swirling, some particles moving up, others down, but 
all having a general motion down stream. This is even 
more noticeable in a natural channel or river with ever 
varying cross section and constantly altering grade. 
Any engineer engaged in observing the flow of rivers 
must have a knowledge of the above phenomena, as, if only 
a few observations of velocity are made, these may be 
nearly all at a time of maximum impulse and thus the 
measured mean velocity be too large or vice versa, and if 
the true mean be obtained it will only be by accident. 
Methods of Measuring Velocity. 
Only two methods of measuring the velocity of streams 
will be considered at any length, viz :—rod floats and cur- 
rent meters; as in a large stream, they are the only 
methods that have any pretentions to accuracy. Weirs 
are impossible except for small streams owing to expense; 
these will be dealt with later on under artesian bores. 
Surface floats have been used in a few instances, but they 
are very unreliable, from the fact that the surface velocity 
of a vertical does not bear a constant ratio to the mean 
velocity at that vertical, sometimes it is greater than the 
mean, sometimes equal to it and sometimes less. Hven if 
it did bear a constant ratio, any wind blowing would vitiate 
the results. 
Double floats have also been used, but in the author’s 
opinion the results are not reliable, in fact he considers 
that the double float is even worse than the surface float. 
Cunningham and others speak highly of this form of float, 
but other skilled observers including D. F. Henry, U.S. 
Assistant Engineer, who had charge of the field work of 
the gauging of the outlets of the Great Lakes, and who | 
——s Se lO Oe 
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