DUTY A>sD MKASL'KEMENT OF WATER. 121 



urement must therefore be made by other methods, one 

 of the simplest of which is to ascertain the mean velocity 

 of the stream in feet per minute. Next ascertain the 

 area or cross section of the stream in square feet. Hav- 

 ing learned these two quantities their product will give 

 the required amount afforded by the stream. The ve- 

 locity can be estimated by throwing floating bodies into 

 the stream, these bodies having nearly the same specific 

 gravity or weight as the water. The time of their pas- 

 sage can be accurately rated in passing a given distance ; 

 it must be remembered, however, that the velocity is 

 the greatest in the center of the stream and near the sur- 

 face, and that it is least near the bottom and sides. It 

 is usually best to ascertain the velocity at the center, and 

 from this the mean velocity can be estimated, as it has 

 been accurately and reliably ascertained by experiments 

 that the actual mean velocity will be 83 per cent, or 

 about four-fifths of the velocity of the surface. The 

 cross section may be estimated by measuring the depth 

 of the stream at a number of points at equal distances 

 apart, the depth being measured at each of these points 

 and all of these added together, and multiplying their 

 sum by the distance in feet between any two of the points. 

 In driving these stakes or points, the first one o'n each 

 side should be half the distance from the edgs of the 

 water to the stake that any one of the other spaces will 

 measure, the two end or half spaces together amounting 

 to one whole space. Having obtained the cross section of 

 the stream in square feet, and also the mean velocity of 

 the stream in feet per minute, the product of these two 

 gives the quantity of water that the stream affords in 

 cubic feet per minute. 



The Current Meter. The current meter now 

 GO generally used to ascertain with precision the velocity 

 of currents in rivers, irrigating canals and smaller 

 streams, gives the moan velocity of a given filament of 



