66 HYDROLOGY 



the middle of the stream, noting the time it takes for the same to 

 pass over this 100-foot stretch. This is repeated a number of 

 times and the average taken. As the stream-flow at the surface is 

 greater than at the bottom, the average must be taken which is 

 about 83 per cent of the surface velocity. It is, therefore, con- 

 venient to lay off the distance as 120 feet and reckon it as 100 feet, 

 using the surface velocity. 



Government Records. The Water Supply and Irrigation 

 papers of the United States Geological Survey furnishes the chief 

 source of information relating to stream-flow measurements, and 

 a complete list on these may be had by applying to the Director, 

 U. S. Geological Survey, Washington, D. C. 



The U. S. Weather Bureau also issues annual reports on the 

 flow of the principal rivers of the country, while the War Depart- 

 ment from time to time issues reports dealing with special investi- 

 gations undertaken by~the engineers for determining the navi- 

 gation facilities of certain rivers and the possibilities of their 

 improvement. 



In addition to the above Federal Reports, numerous investi- 

 gations are also made every year by different States and these can, 

 as a rule, be obtained from the Geological Survey Departments 

 of these States. 



It is thus seen that there is an abundant amount of data on 

 stream-flows in the different sections of the country. These 

 records are, however, scattered around in so many different publi- 

 cations, that it is a difficult matter to find the desired information. 

 An excellent system of indexing such data on stream-flow and 

 rainfall has been devised and is used by H. M. Byllesby & Co., 

 Chicago. It is described in Engineering Record for January 31, 

 1914. 



6. ENERGY OF FLOWING WATER 



The energy of flowing water is entirely due to its position, or 

 head. It follows in general the same laws as falling bodies so 

 that, assuming a 100 per cent efficiency, its potential energy 

 depending on the position must be equal to its kinetic energy 

 depending on the velocity. That is 



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