Practical Remarks on Hydrometry. 



227 



The float was then thrown into the main thread of the 

 stream^ a few yards above the upper line, and the number 

 of seconds observed to elapse in its passage from the upper 

 to the lower line. The number of seconds consumed in this 

 passage was observed about a dozen times, and that observa- 

 tion which gave the least number of seconds was obviously 

 adopted as the most correct. With this number of seconds, 

 the distance of fifty feet, and the table, the mean velocity 

 corresponding to the portion of stream under examination 

 \vas computed. 



Then this mean velocity, multiplied by the mean sectional 

 area of the stream within the specified limits, gave the discharge 

 of the stream much more accurately than if one cross section 

 only had been measured. 



If the observer be provided with Brunning's, Woltman's, 

 or any other accurate tachometer, then Mr. Stevenson's 

 mode of determining the discharge of a river, by subdividing 

 the cross section into a number of trapezoids, and determining 

 the mean velocities for each of them as multipliers for the 

 corresponding areas thereof, is a very correct one. I have 

 followed this method when I formerly had occasion to deter- 

 mine the quantities of water that passed to, and through, 

 opposite a town on a tidal estuary, during the flow and ebb 

 of a tide that rose and fell a certain number of feet and 

 inches at a tide-guage. I tried Pitot's tube during the opera- 

 tion, but found it unsatisfactory for great velocities ; also the 

 hydrometrical pendulum, which was not adapted for deter- 

 mining the great variations that occurred in the surface velo- 

 city ; for when the ball of the latter instrument worked well • 

 for moderate velocities, it proved quite unmanageable for 

 high velocities, whilst for low velocities it showed no appre- 

 ciable deflexion from the vertical. I subsequently con- 

 structed a hydrometrical pendulum, furnished with four balls, 

 of gradually increasing specific gravity, so that whilst the 

 lio^htest ball was very sensible to the influence of very small 

 surface velocities, the heaviest was not too violently acted 

 upon, by the highest velocities that occurred in ordinary 

 rivers. 



The co-efficients for each ball were respectively determined 

 by direct experiment ; and the numbers affixed to the balls 

 and corresponding coefficients were registered on the in- 

 strument. 



Then, in the measurement of the surface velocity of a 

 stream, that ball was employed which proved on trial to be 

 the best adapted for the measurement of such velocity, by 



