224 "Practical Remarks on Hydrometry. 
to forget my previously expressed intention of contributing 
such a paper ; so that, having ascertained, at the last moment, 
that the paper was announcel for this meeting, I must crave 
the indulgence of the members now present for submitting 
to them the following cursory and hastily written remarks. 
As questions ef moment, in reference to water-power or 
supply, are often dependent upon the accuracy with which 
steam guagings are conducted, I have been surprised at the 
unsatisfactory manner in which experienced engineers have 
frequently, in Great Britain, performed tae simple operation 
of measuring the discharge of a stream or river. 
Little discrimination, for instance, has been displayed in 
the selection of the site for determining the sectional area of 
a stream, notwithstanding tha: the surface velocity was 
derived from observations on a float; yet, when a float was 
employed, no near approach to accuracy could be attained, 
unless not only the sectional areas but also the cross profiles, 
differed so little within the longitudinal limits assigned to the 
observations on the float as to have caused the stream to 
approximate closely thereabouts to that condition aptly termed 
by French writers “ Regime uniforme.” ‘The size, and some- 
times even the specific gravity of the float have also been 
considered of immaterial importance, and the mean velocity 
has been almost universally deduced from Dubuat’s Formula, 
which makes, when expressed in inches, the bottom velocity 
equal to the square of the difference between the square root 
of the surface velocity and unity, and the mean velocity 
equal to half the sum of this bottom velocity and the surface 
velocity. 
I protest against the employment of this formula, which, 
when applied to very small or very great surface velocities, 
is productive of grave errors. 
I am, however, aware, that this formula of Dubuat’s 
(originally promulgated near the close of the last century), 
has been adopted without question as to its accuracy in 
various standard English publications; for instance, the 
hydrometrical table contained in the last edition of the Ency- 
clopedia Britannica is based thereupon, also, the table in 
Stevenson’s Hydrometry, as well as those given in several 
engineering manuals. But on the continent of Europe, 
where the hydraulic investigations since the publication of 
Dubuat’s “ Principes d@ Hydrauliques” have been most 
extensively and accurately conducted, on profound scientific 
principles, Dubuat’s formula has been superseded by more 
accurate although less simple rules. 
