HYDROGRAPHIC SURVEYS. 37 



rent. The other mouth-piece is at right angles to it. The lower stop-cock is then 

 opened and the water rises in the tube which points to the current. The cock is then 

 closed. The water will stand at a higher level in this tube, and this is the measure 

 desired, and is represented by the formula V = ftV2gh, where h is the height of the 

 column, and fi is a coefficient to be determined by experiment. 



By Hydrodynamometer. The engineer Gros de Perrodil designed an instrument 

 for measuring the current by means of the torsion produced by the pressure against 

 a disc upon a wire placed vertically in the water. The velocity was obtained by the 

 equation V =c\/a, in which a = the angle of torsion, the value of c being found by 

 experiment. 



Sediment. The determination of the quantity of sediment carried in suspension 

 is often of great importance in the study of river improvements, particularly where 

 a decision is to be made between fixed and movable dams, as when silt travels in great 

 quantities its deposit in the upper and shallow parts of pools formed by stationary 

 dams will often seriously interfere with navigation, and may even permanently raise 

 the river-bed. It is also of importance in the construction of works of contraction. 

 Formerly the plan of improvement of the Mississippi was considered to be wholly 

 dependent upon the understanding of the laws connected with the suspension and 

 transportation of the sediment. 



It is necessary in these observations to secure samples of water from various 

 depths below the surface and at different points of the section. These are allowed 

 to settle and the sediment in each is then weighed, a proportion being thus established. 

 For this purpose a tin can may be used attached to a graduated pole and having a 

 valve at the bottom which is opened by a string leading to the top of the pole, and 

 which closes by its own weight. For great depths it is necessary to weight the can 

 and use a cord instead of a rod. 



Some interesting data upon the subject of sediment are given in connection with 

 the Arkansas River, in a pamphlet by S. C. Branner, as follows:* 



"The matter in suspension is greatest during a sudden high rise; but after the 

 water in the stream stands at any high mark for a few days, the decrease of the amount 

 of suspended matter it carries is very marked. The amount of sediment carried by 

 the river varies widely also with the same gauge-reading at any stage, being greater 

 with a rising and less with a falling river. 



" The greatest amount of sediment found in the water during the year under con- 

 sideration was 225 grains to the gallon (5^5) when the river stood at 17 feet on the 

 gauge, and after protracted rains. (Extreme high water at Little Rock is about 28 

 feet.) It should be added, however, that while this high water may be taken as a type 

 of the ordinary rises, there are times when there is but little or no rise, no increase in 

 the volume of water discharged, but a very marked increase in the amount of mechan- 

 ically suspended matter. In October, 1891, occurred one of these so-called 'red rises' 



* Observations upon the Erosion in the Hydrographic Basin of the Arkansas River, 



