FLOATS 359 



A and 13. The effect of wind on the surface velocity, however, together 

 with the tendency of the floats to follow every variable cross current and 

 to be affected by every surface eddy, renders the results obtained by this 

 method unreliable except as approximations to the truth. 



(2) Sub -surface Floats. These consist of bodies having surfaces of 

 large area, as illustrated for example in Fig. 160, attached to small surface 

 floats for ease of observation, the length of connection being adjusted so 

 as to allow the true float to remain at any given depth. The velocity of 

 the float will then be approximately that of the current at the required 

 depth. The figure shows the float used in the Connecticut Eiver survey 

 in 1874. The sub-surface float was a hollow annulus of tin 8J inches 

 high, 8^ inches outside diameter, and 

 7J inches inside diameter. This was j ^ 



weighted with 28 oz. of lead. The ,____ rj^te.^__ _i 



surface float was an ellipsoid of tin = r ^- z =~ := r~:\^. -^= 

 6 inches in diameter and 1*5 inches 

 deep, the connecting cord being '036 

 inches in diameter. A series of such 

 floats liberated at different points in 

 the cross section of a stream and 

 at different depths may be used to 

 give by their mean velocity the mean 

 of that of the stream, or by arrang- 

 ing a single row, the depth of each 

 being f that of the stream at the 



point of introduction, these may be 



. . , ... FIG. 160. 



taken as giving the mean velocities 



in their respective sections. While this type is m6re reliable than the 

 surface float it suffers from the disadvantage that it is impossible to 

 determine the exact position or depth of the lower float, for while the 

 position of the upper float may be known, that of the lower float varies 

 with the direction and velocity of the wind and with the length of cord 

 connecting the two floats. Also the upper float may either drag or be 

 dragged by the lower, and the upper is on this account likely to retard 

 the lower where the latter is above the filament of maximum velocity, and 

 to accelerate it when it is at a greater depth than this. As this latter 

 effect extends over a greater proportion of the depth than the former, it 

 would tend to make the velocities of flow recorded by the floats too 

 high. Experiments made by T. G. Ellis, 1874, on the Connecticut 

 River (mean velocity 2'1 ft. per sec.) with current meters and with 



