188 



HYDRAULICS AND ITS APPLICATIONS 



the particular disc and casing under consideration. The speed was 1,500 

 revolutions per minute throughout. 



From these results it appears that in practically every case the 

 minimum resistance is obtained with the minimum clearance, and that 

 relative increased resistance with an increase in side clearance is much 

 more marked with a smooth than with a rough disc. With a rough 

 cast-iron disc indeed, and with clearances from f inch to 2J inches, the 



EBSISTANCES WITH VARIOUS SURFACES. 



resistance appears to vary very little with the clearance, whether with a 

 rough or a smooth casing. One rather peculiar feature of the results 

 obtained with a smooth disc in a smooth casing, and to a smaller extent 

 with the rougher discs and with the rougher casing, is worthy of note. 

 This is, that as the clearance is increased from J inch, the resistance at 

 first increases, then diminishes, attaining a minimum value with a clear- 

 ance of about If inch, and afterwards appears to increase rapidly as the 

 clearance is increased to 2J inches. The following is offered as a possible 

 explanation of this phenomenon : 



In considering the simple theory of disc-resistance it is tacitly assumed 

 that the water dragged around with the disc travels in concentric paths, 

 each particle remaining at a constant distance from the axis of the disc. 

 In a closed casing and with side clearances so small as to lead to a non- 

 sinuous motion of the water this would be the case ; the index n would be 

 unity, and the resistance would be due to simple viscous shear of 



