HOW MUCH IS A STREAM WORTH? 



79 



be run by it. It would be possible to construct a turbine so 

 large that the stream would not suffice to turn the wheel ; for 

 this reason, the power of a stream is carefully determined 

 before machine construction is begun, and the size of the 

 machinery depends upon the estimates of the water power 

 furnished by expert engineers. 



A rough estimate of the volume of a stream may be made 

 by the method described below : 



Suppose we allow a stream of water to flow through a 

 rectangular trough ; the speed with which the water flows 



FIG. 123. Estimating the quantity of water which flows through the trough each 



second. 



through the trough can be determined by noting the time 

 required for a- chip to float the length of the trough; if the 

 trough is 10 feet long and the time required is 5 seconds, 

 the water has a velocity of 2 feet per second. 



The quantity of water which flows through the trough each 

 second depends upon the dimensions of the trough and the 

 velocity of the water. Suppose the trough is 5 feet wide and 

 3 feet high, or has a cross section of 15 square feet. If the 

 velocity of the water were I foot per second, then 1 5 cubic feet 



