PROCESSES FOR GAUGING STREAMS. 219 



inches diameter were used, loaded to float flush with the 

 water surface, and provided with a stem. Bits of solid 

 wood, and bottles filled with water until nearly submerged, 

 have often been used for surface floats. Boilean proposes 

 balls of soft wax, on account of their adhesive properties. 

 In Captain Cunningham's observations, the floats were thin 

 circular disks of English deal, 3 inches diameter and \ inch 

 thick. For observations near the banks, floats 1 inch diam- 

 eter and | inch thick were used. To render them visible, a 

 tuft of cotton wool was used, loosely fixed in a hole at the 

 centre. 



The velocity is obtained by allowing the float to be car- 

 ried down, and noting the time of passage over a measured 

 length of the stream. If t is the time in which the float 

 passes over a length I, which has been previously measured, 



and staked off on the shore, then the velocity v is v = 



v 



To mark out distinctly the length of stream over which the 

 floats pass, two ropes may be stretched across the stream at 

 a distance apart, which varies usually from 50 to 250 feet, 

 according to the size and rapidity of the river. To mark 

 the precise position at which the floats cross the ropes, Capt. 

 Cunningham, in his experiments, used short white rope 

 pendants, hanging so as nearly to touch the water. In this 

 case the streams were 80 to 180 feet wide. 

 In wider streams the use of ropes to 

 mark the length of run is impossible ; in j 

 such cases, recourse must be had to some B 



such method as the following : Let AB ^ B 



be the measured length = /, on one side Fifl 6I 



of the river. Put two rods C and D, by 

 means of a suitable instrument, in such a position upon the 

 other side of the river that the lines CA and DB shall be 

 perpendicular to AB. Then the observer, placed behind A, 

 notes by his watch the instant the float E, which has been 

 placed in the water some distance above, arrives at the line 



