36 THE IMPROVEMENT OF RIVERS. 



The theory of the movement of this class of floats is that the lower one will be carried 

 along by the water in which it is situated, while that at surface will travel fast or 

 slow in proportion as the current there is faster or slower than that below. This method 

 is applicable to large streams as well as to those of great depth or with rapid currents, 

 but it gives only the approximate velocity of that portion of the water through which 

 the lower float travels. To obtain a velocity at a point between the upper and the 

 lower floats it will be necessary to shorten the connecting cord and make a new obser- 

 vation. 



For reasons mentioned, double floats have not proved very satisfactory in use 

 and the opinion of experienced observers is that they can rarely be relied on within 

 10 per cent of the truth. Individual results have been known to vary 25 per cent on 

 similar observations. Moreover, in high water or swift currents the effects of eddies 

 and of cross-currents in changing the velocities of the floats is very noticeable. 



By Current-meters. Current-meters have to a great extent succeeded floats for the 

 gauging of streams, but they have not proved wholly satisfactory. Much of the dissatis- 

 faction with them undoubtedly comes from their mechanical construction and from care- 

 less use and rating. The rating is done by moving the meter through quiet water, at a 

 depth of about 5 feet, and at a uniform rate of speed. Its purpose is to obtain the ratio 

 between a revolution of the wheel and the velocity of the current. These ratings should 

 be tested frequently. The meter is well adapted to streams of good size which are com- 

 paratively free from drift, and in which the current is neither too sluggish for certain 

 action nor too swift to secure proper anchorage. The principle is that of obtaining the 

 velocity of the current by counting the number of revolutions it will produce on a 

 wheel, screw, or vanes, properly arranged. The revolutions may be recorded by a 

 system of toothed wheels in the instrument, or by an electrical apparatus protected 

 from the water and usually placed on shore. The meter is operated from a boat 

 anchored where desired, and is weighted in order to lower it to the depth required. 



To find the mean velocity in a vertical plane on the cross-section the meter is 

 lowered at a uniform rate of speed from the surface to the river-bed, and then raised 

 again to the surface, taking the reading for the bottom and for both surface posi- 

 tions. Each of these is divided by the time of movement and a mean of the results 

 taken. 



By Gauge-tubes. Pitot used a glass tube, open at both ends, and having a short 

 horizontal and a long vertical arm connected by a short bend. By immersing the 

 tube with its long arm upright, the end of the horizontal portion being presented to 

 the current, the pressure will raise the water in the tube and thus furnish a measure 

 of the velocity. An improvement was made upon this apparatus by Darcy, who 

 employed two tubes connected at the top by a copper tube containing a stop-cock, 

 and provided at their bottoms with bent copper tubes with mouth-pieces and a double- 

 acting stop-cock. Before placing the apparatus in the water the upper cock is closed 

 and the tubes are then immersed to the depth desired, with one mouth-piece to the cur- 



