50 W. & L. E. GURLEY. TROY. NijW YORK 



observed, depending on the velocity of the water, should be one 

 of iliM-- lur \\hirh the meter rating table has been prepared. 

 A check on the work is made by repeating the observation. If 

 the run is not repeated, a check can be obtained by noting men- 

 tally the time for each five revolutions. A stop-watch is used 

 for observing the time, and the record is made to the nearest V* 

 second. The observations should be recorded on properly 

 prepared forms. See pages 54, 55, and 60. 



In discharge measurements, the mean horizontal velocity in 

 a vertical at the measuring point is desired. Various methods 

 are used for this determination, among which the following four 

 are most common: (a) Vertical velocity -curve method, (b) two- 

 tenth and eight-tenth-depth method, (c) six-tenth-depth method, 

 and (d) sub-surface method. 



(a). By the vertical velocity-curve method, measurements 

 of horizontal velocity are usually made just beneath the surface, 

 at 0.5 ft. below the surface, and at each fifth to each tenth of 

 the depth from the surface to the bed of the stream, and as near 

 the bottom as possible. These measured velocities, when 

 plotted with depths as ordinates and velocities as abscissas, 

 define for each vertical, the vertical velocity-curve which shows 

 the velocity at every point in the vertical, and from which the 

 mean velocity can be determined by dividing the area bounded 

 by the curve, the top and bottom ordinates, and the axis of depth 

 by the total depth. The area may be found by planimeter, or 

 by Simpson's rule or Durand's rule*, which will be found in 

 books on elementary mechanics. 



Studies of vertical velocity -curves taken on many streams 

 under various conditions of depth, velocity, and roughness of 

 bed, show that these vertical velocity-curves have approximately 

 the form of a parabola in which the axis, coinciding with the 

 filament of maximum velocity, is parallel with the surface and 

 is in general situated between the surface and one-third of the 

 depth of the water. From the maximum the velocity decreases 

 gradually upward to the surface and downward nearly to the 

 bottom, where it changes more rapidly on account of the friction 

 on the bed. As the depth and velocity increase, the curve 

 approaches a vertical line as its limiting position. 



MprhnnicH for Engineers. 



