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XXXVII. 



ON A NEW METHOD OF GAUGING THE DISCHAEGE OF 



EIVEES. 



By J. JOLY, Sc.D., F.E.S., F.T.C.D. 



[Reail February 28. Published April 13, 1922.] 



The new method is based on the principle underlying " chemical hydrometry." 

 This principle may be simply expressed as follows :— 



Suppose it were required to estimate the quantity of water contained in a 

 large tank the volume of which it was difficult to determine by mere geometric 

 measurements. By means of chemical hydrometry the quantity of water is 

 easily found. A known weight of any suitable soluble salt is stirred into the 

 tank, so that there is uniform distribution of the salt throughout the water. 

 We then abstract a known volume V of the water, and by titration or other 

 means estimate how much of the salt is contained in this volume of water. 



Now, if this amouirts to -th of the salt put into the tank, the volume of 



water in the tank must be n x V. 



This method has been applied to the discharge of rivers. A tank is 

 erected at some convenient point on the river. In this tank a large volume 

 of water is stored, and into it is stirred a heavy charge of the salt; generally 

 chloride of sodium. A pipe is led from the tank into the river, and the salt 

 solution is fed slowly, and at a uniform rate, from the tank, escaping from 

 perforations in the pipe and intermingling with the current. At a point 

 some distance down stream samples of water are taken at intervals. These 

 samples are titrated ; and, knowing the rate at which the salt has been fed 

 into the river, the rate of discharge can be calculated. 



Very accurate results are obtained by this method. The error can be 

 reduced below 1 per cent. It is said to be superior in accuracy to weir- 

 measurements, and very much superior in accuracy to measurements based on 

 current meters. A very full account of the method is given by B. F. Groat, 

 M.AM.SOC.C.E., in the Proceedings of the American Society of Civil Engineers, 

 vol. xli, November, 1915. 



A study of Mr. Groat's paper reveals the advantages and disadvantages of 

 the method. Chief of the latter is the cumbersome nature of the prepara- 

 tions which have to be made, due to the large amount of salt required. 



