98 



DAMS AND HEADWORKS 



strains to which it is subjected. At each end the cylinder is pro- 

 vided with a specially designed gear engaging a rack laid in an 

 inclined recess in the abutment or pier. By means of a sprocket 

 chain wrapped around one end of the cylinder and connecting with 



the operating mechanism the dam can be rolled up or down as 

 desired, see Figs. 49 and 49 A 



For larger lifts and moderate spans, the cylindrical part of the 

 weir is often much smaller in diameter than the height of the weir, 

 the upstream side of the gate being provided with a metal shield 

 connected by strong braces to the cylindrical body. 



This type of gate is a comparatively new invention and, while 



