360 HYDRAULICS AND ITS APPLICATIONS 



double floats 1 showed that the mean velocity as found by floats was 

 from 6 to 26 per cent, greater than by meter, the difference increasing 

 with the velocity. Marr October, 1879, on the Mississippi, 2 the width 

 being about 2,000 feet and the depth 16'4 feet (mean velocity 2'6 ft. per 

 sec.) found the mean float velocity to be about 3'5 per cent, greater than 

 the meter velocity, while Henry 1869 on the St. Glair River 3 45 feet 

 deep and with a mean velocity of 3*4 feet per second, found the mean 

 float velocity about 10 per cent, greater than the meter velocity. In this 

 case the float velocity was less than the meter velocity to a depth of 

 about 7 feet. Below that depth the float velocity was the greater, the 

 difference increasing with the depth. 



(3) Twin Floats. These consist of two masses of equal size, usually 

 spheres coupled together by means of a wire, the lower of which is 

 weighted so as to remain vertically below the upper, which floats at the 

 surface. The velocity of the float then gives the mean of the velocities at 

 the surface and at the depth of the lower mass. If this is adjusted so as 

 to just clear the bottom, the velocity of the float will be approximately the 

 mean velocity for the vertical in which the instrument floats. 



(4) Velocity Rods. The velocity rod, or rod float, consists of a light 

 wooden rod or tin tube about 1 inch in diameter, and made in adjust- 

 able lengths. The lower end of the bottom length is weighted and the 

 length adjusted until the rod floats vertically with its lower end clearing 

 the bottom by a few inches. In a large river and where these are not 

 likely to interfere with navigation, logs of wood about 12 inches in 

 diameter, having their lower ends weighted with iron and their upper ends 

 painted white, may be used. 



The velocity of the rod is approximately the same as the mean over its 

 depth, and gives the mean velocity over the vertical in which it floats. The 

 difficulty in using the rod lies in its tendency to drag over shoals and 

 weeds, and to obviate this its- lower end may be arranged to float at a 

 height h l above the bed of the stream. 



For such a case Francis gives the empirical formula 



- '116 



giving the mean velocity in the vertical containing the rod in terms of the 

 velocity of the rod (v r ), h 1 , and h the depth of the stream. Here h 1 should 

 be less than *25 h. 



1 Report Chief Eng., U.S.A., 1878, Appendix B. 



2 McKenzie, A. Report on Current Meter Observations. Burlington, 1884. 

 ' " Journal Franklin Inst.," vol. 62, p. 322. 



