220 PROCESSES FOR GAUGING STREAMS. 



AC, and then, passing down to B, he observes the instant 

 that the float arrives at the line BD. By subtracting the 

 time of the first observation from that of the second, he 

 obtains the time t in which the space / is described. 



For measuring the velocity below the surface, double 

 floats * are used. They are of various kinds, usually con- 

 sisting of small surface floats, supporting by cords larger 

 submerged bodies. Suppose two equal and similar floats, 

 connected by a string, wire, or thin wire chain. Let one 

 float be a little heavier, and the other a little lighter than 

 water, so that only a small portion of the latter will project 

 above the surface of the water. We first determine by a 

 single float the surface velocity v s ; we then determine the 

 velocity of the connected floats, which will be the mean of 

 the surface velocity and the velocity at the depth at which 

 the heavier float swims. If Vj is the velocity at the depth 

 to which the lower float sinks, we have, calling v the mean 

 velocity, 



_ 



/. v d = 2v v g . (1) 



By connecting the floats successively by longer and longer 

 pieces of wire, we obtain in this way the velocities at greater 

 and greater depths. 



To obtain the mean velocity in a perpendicular, a floating 

 staff or rod is often employed. This consists of a cylindrical 

 rod, loaded at the lower end so as to float nearly vertical in 

 water. A wooden rod, with a metal cap at the bottom in 

 which shot can be placed, so as to prevent more than the 

 head from projecting above the water surface, answers well, 

 and sometimes the wooden rod is made of short pieces which 

 can be screwed together so as to suit streams of different 

 depths. A tuft of cotton wool at the top serves to make 



First used by da Vinci. 



