224 



Practical RcmarJis on Hydrometry, 



to forget my previously expressed intention of contributing 

 such a paper ; so that, having ascertained, at the last moment, 

 tliat the paper was announced for this meeting, I must crave 

 tlic indulgence of the members now present for submitting 

 to them the following cursory and hastily written remarks. 



As questions of moment, in reference to water-power or 

 supply, arc often dependent upon the accuracy with which 

 steam guagings are conducted, I have been surprised at the 

 unsatisfactory manner in wliich experienced engineers have 

 frequently, in Great Britain, performed the simple operation 

 of measuring the discharge of a stream or river. 



Little discrimination, for instance, lias been displayed in 

 the selection of the site for determining the sectional area of 

 a stream, notwithstanding that 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, 

 diflered so little Avith^n the longitudinal limits assigned to the 

 observations on the float as to liave caused the stream to 

 approximate closely thereabouts to that condition aptly termed 

 by French writers " Regime uniformer 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 diflerencc between tlic square root 

 of the surface velocity and unity, and tlie mean velocity 

 equal to half the sum of this bottom velocity and the surface 

 velocity. 



I protest against the employment of this formula, wliich, 

 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 proimdgated near the close of the last century), 

 has been adopted without question as to its accuracy in 

 various standard English publications; for instance, the 

 hydroinctrical table contained in the last edition of the Ency- 

 ciopaidia 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 ^nce the publication of 

 Du))uat's " rrincipes d' Ilydrauliques " have been most 

 extensively and accurately conducted, on profound scientific 

 principles, Dubuat's formula has been superseded by more 

 accurate although less simple rules. 



