HYDROGRAPHIC SURVEYS. 35 



Measurement of Velocity. By Floats. Two methods have been in general use for 

 measuring the velocity at any point, that by floats and that by current-meters. For 

 ascertaining the mean velocity in a vertical plane rod-floats are employed, while the 

 velocity at the surface may be obtained by any ordinary floating substance. 



Although the method of floats has long been employed, its use has been condemned 

 by certain authorities, especially as regards double floats, because the movement of 

 the lower float is not accurately indicated by that of the one at the surface, and the 

 motion of the lower float, being influenced by a different portion of the current, is 

 modified by the upper one, while invisible eddies may further affect the lower one with- 

 out indication above. 



Surface-floats may be very simple; an orange, apple, cork, piece of wood, or 

 partly filled bottle will answer the purpose when there is no wind stirring. With any 

 disturbance in the atmosphere, however, the floats should be submerged to such extent 

 as to be just visible. 



Rod-floats which must be loaded at the bottom, may be made of tin, or may be 

 simple wooden rods. They travel with their tops slightly above the water and their 

 bottoms as near as practicable to the river-bed; but owing to the irregularities which 

 usually exist in the latter, it is not always possible for the lower end to reach the level 

 desirable. Again, since the pressure of a fluid upon a body moving through it varies 

 as the square of its relative velocity, the float will travel slower than the topmost fila- 

 ments and faster than those at the bottom. This difference being greatest at the latter 

 point, the velocity of the float will be held back toward the bottom, and will be some- 

 what slower than the mean of the plane ; but for measurements in which extreme 

 accuracy is not required this character of float answers very well. It is necessary 

 to make it in sections, which can be fastened together, in order to adapt it to the 

 varying depths of the river. 



On certain rivers in America the passage of floats has been recorded electrically.* 

 The apparatus consisted of a wire stretched from bank to bank, and held in position 

 at frequent intervals by short wires fastened to anchors of stone resting on the bottom. 

 A skiff moved across the river, held by the wire and provided at the stern with a strip 

 of wood 20 feet long, placed normal to the current. From each end of this strip a wire 

 about 100 feet long and supported on buoys trailed down stream, having its other end 

 fastened to a similar strip, this being provided with a slack copper ribbon. The floats 

 were started from the skiff, and on striking the ribbon they closed a circuit and rang 

 a bell in the skiff. They were then caught by a man in another skiff and returned. 



Double Floats are of several forms and are made of wood, tin, galvanized iron, 

 or other materials. A simple one is made of two jugs connected with a string. The 

 idea is to attach to a surface-float, by a fine cord or otherwise, a submerged float, which 

 maybe adjusted to various depths. The bottom float should be of such a weight as will 

 keep the connecting cord tight without drawing the surface-float wholly out of view. 



*' Annual Report, Chief of Engineers, U. S. A., 1883. 



