OPEN DRAINS. 



15S 



While circular curves may be used to describe ap- 

 proximately the curvature that should be given, the 

 true form should not be geometrical, but rather what 

 may be termed natural, or such as is used in laying 

 out artificial streams and roads in parks, in which geo- 

 metrical lines are ignored. The difference between the 

 two is shown in Fig. 29, which is a 12-degree curve 



Fig. 29. — ^Proper Curve for Open Ditches. 



(radius 478 feet) varied so as to subject the bank against 

 which the stream strikes when first deflected to the 

 least possible erosion. The reason for this is well illus- 

 trated by Fig. 30, in which the stream is represented 

 as being divided into filaments, each having a velocity 

 imparted to it by the flow, and striking the opposite 

 bank as an individual force. According to the well- 

 known law of force, the angles of incidence and reflec- 

 tion are equal when a force meets a resisting plane. 

 Hence in the case under consideration, the reflected 



