FLOATS 361 



Of all floats the velocity rod gives the best results, and for channels of 

 moderate and uniform depth, encumbered with floating weeds and grass, 

 this is probably the best method of obtaining the velocity. In a series of 

 experiments on the Loch Katrine Aqueduct ; concreted surface ; concave 

 bottom ; width 9' 1" ; radius of curvature 20' 10J" ; hydraulic mean depth 

 2*87 feet; velocity rods 2' 2" long, gave results which agreed within J of 1 

 per cent, with results as obtained by weir measurement, while the velocity 

 as obtained from the maximum surface velocity, and an application of the 

 formula v = v s - 25'4 V in i, was 18 per cent, too low. 



An elaborate series of experiments was carried out in 1856 by 

 J. B. Francis on the Lowell Canal 1 to determine the relative accuracy of 

 weir and rod float measurements, this canal being 27 '1 5 feet wide where 

 the first 63 experiments were made and 14 feet wide where the remaining 

 52 were made. , The length of run was 70 feet, the floats being loaded tin 

 tubes, 2 inches in diameter. From these it was found that the mean 

 difference in the discharge as obtained by the two methods was less than 

 2 per cent, in all but three of the experiments, the mean difference being 

 about 1 per cent. The mean velocity in these experiments varied from 

 5 to 5 feet per second. 



Experiments in 1900 at Cornell University 2 showed about the same 

 degree of accuracy in the case of flow in a canal 16 feet wide and with 

 depths of water ranging from 5 to 10 feet, and velocities of flow up to 

 2'07 feet per second. The immersed portions of the rods varied from 

 75 per cent, to 95 per cent, of the depth of the stream, and the length of 

 measured run varied from 7 to 25 feet, depending on the velocity. In 

 every case the float velocity was slightly less than that given by the 

 weir. 



Other Methods of Measuring the Velocity. 



. 



ART. 101. EIPPLE FORMATION. 



An ingenious method of obtaining the surface velocity at various points 

 in the cross section of a stream was described by Mr. E. C. Thrupp 

 (" Proc. Inst. C.E.," vol. 167, 1907, p. 217). This depends upon the fact 

 that if a small obstruction cut the surface of a stream, ripples are formed 

 if the velocity exceeds about 9 inches per second, while the angle of 

 divergence of these ripples appears to bear a definite relation to the 

 surface velocity. To overcome the difficulty of accurately measuring this 

 angle Mr. Thrupp constructed a velocity meter consisting of two vertical 





1 Lowell, Hydraulic Experiments, p. 170. 



2 " Trans. Am. Soc. C.E.," vol. 12, p. 301. 



