[farmer] determination OF COEFFICIENT OF DISCHARGE 



63 



As regards the comparative effects at different velocities of the 

 impact on a particular form of vane, the second table gives precisely the 

 same results as the first. 



It is for the comparison of the different forms of vane in their effi- 

 ciency in converting the momentum of the stream into a force that the 

 second table is useful. 



An instructive diagram is obtained by setting out the coefficients for 

 various angles of deflection, as is done for the vanes of circular section in 

 Diagram IX. From this diagram it is seen that the extra efffect obtained 

 by increasing the angle through which the water is deflected is, at each 

 velocity at which experiments w^ere made, counterbalanced by the reduc- 

 tion of the final velocity, owing to friction and other resistances of the 

 surface passed o.ver. As it is known, however, that the value for a flat 

 plate is a little below unitv. it is evident that there must be a maximum 



1.13 



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i' ' ' 



ooJt- 



Diaorrani IX. 



value of this coefficient betw^een the values 0° and 43° of the angle. 

 With the values known for various angles, and also knowing the value 

 for the angle 0° to be approximately 1, it is possible to form an approx- 

 imate idea of the curves which the values of the coefficient would ti'ace, 

 as indicated by the broken lines in the diagram. Having sketched the 

 curves, the positions of the maximum values can be marked, and though 

 the data are not sufficient to fix values for the angle and the coefficients 

 at the maximum value of the coefl[icient, the following general results 

 are indicated. 



(1) That the maximum value of the comparative coeflScient of impact 

 corresponds to a greater angle of deflection of the stream, the greater the 

 velocity. 



• (2) That the maximum coeflficient increases with the velocity up to 

 a certain point, and after that point decreases. The velocity at which the 



