358 



HYDRAULICS AND ITS APPLICATIONS 



FIG. 159. 



ART. 100. ESTIMATION OF VELOCITY BY FLOATS. 



These are liberated at a series of points in a long straight reach 

 (Captain Cunningham, from experiments on the Ganges Canal 1 recom- 

 mends that this length should not he less than 200 feet) and the time 

 occupied in covering a measured distance is noted. 



Floats may be divided into four 

 classes : 



(1) Surface floats. 



(2) Sub-surface floats. 



(3) Twin floats. 



(4) Velocity rods. 



(1) Surface Floats. These consist 

 of any easily seen masses of light 

 material, painted cork or discs of 

 light wood for example, of small size 

 so as to move along with, and register, 

 the velocity of the surface filaments. A series of trials are necessary to 

 get the maximum surface velocity r 8 of the stream, from which the mean 

 velocity of flow may be estimated from Baziii's formula (p. 342). It is 

 preferable, however, to deduce the mean velocity in each of a number 

 of sections of the stream from repeated observations of the surface 

 velocity in each of these sections. The sections may be marked in a 

 stream of moderate dimensions by ropes hanging from a bridge or 

 temporary support. 



In a large river, observations with the theodolite are necessary to 

 determine the track of the float. This may be satisfactorily carried out 

 as follows : A base line A B (Fig. 159) is chained out parallel to the 

 river for a length of about 250 feet, depending on the width of the river. 

 At the two ends stakes are erected, while second stakes are erected in 

 lines ranged perpendicularly to the base line, as at Si- SV An observer 

 with a theodolite is stationed near the centre of the base line at C, and an 

 observer is stationed at each stake, Si, 82- The float being liberated up 

 stream, the theodolite observer keeps the line of collination of his instru- 

 ment on this. As it passes the line of sight Si A, the observer at Si 

 gives an audible signal and the theodolite observer notes the angle 

 A C 1\. On passing the line S<2 B a second signal is given at the angle 

 B C l\ noted. The line l\ P 2 can then be plotted. With a stream of 

 moderate velocity the same observer may give the signals both, at 



" Proceedings Inst. Civil Engineers," 1882, vol. 71. 



