156 IRRIGATION FARMING. 



u<5l will give the required amount afforded by the 

 stream. The velocity can be CvStimated by throwing 

 floating bodies into the stream, these bodies having 

 nearly the same specific gravity or weight as the water. 

 The time of their passage can be accurately rated in 

 passing a given distance ; it must be remembered, how- 

 ever, that the velocity is the greatest in the center of 

 the  stream and near the surface, and that it is least 

 near the bottom and sides. It is usually best to ascer- 

 tain the velocity at the center, aitd from this the mean 

 velocity can be estimated, as it has been accurately and 

 reliably ascertained by experiments that the a(5lual 

 mean velocity will be 83 per cent. , or about four-fifths 

 of the velocity of the surface. The cross-sedlion 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 dis- 

 tance in feet between any two of the points. In driv- 

 ing these stakes or points, the first one on each side 

 should be half the distance from the edge of the water 

 to the stake that any one of the other spaces will 

 measure, the two end or half spaces together amount- 

 ing to one whole space. Having obtained the cross- 

 sec5lion of the stream in square feet, and also the mean 

 velocity of the stream in feet per minute, the produdl 

 of these two gives the quantity of water that the stream 

 affords in cubic feet per minute. 



The Current Meter.— The current meter now 

 so generally used to ascertain with precision the veloc- 

 ity of currents in rivers, irrigating canals, and vSmaller 

 streams gives the mean velocity of a given filament of 



