SLOPES OF CANALS. 



241 



The slope of some of the largest irrigating canals in 

 Europe is from . 13 to 200 feet in 10,000. The slope in 

 the canals of the Tyrol and other localities in the Alps is 

 frequently as great as six feet to the thousand. In these 

 cases the sides of the canals are of masonry or timber. 

 The rule upon which the average fall of canals is indicated 

 is as follows, viz. : For 

 a bed which consists of 

 fine mud, 16 feet in 

 100,000 ; for soft clay, 

 45 in 100,000 ; for sand, 

 136 in 100,000; for 

 gravel, 433 in 100,000, 

 and for solid clay, 570 in 100,000. With greater in- 

 clinations than these there is a probability that the sub- 

 stances of which the bed is formed will be taken in sud- 

 pension and transported by the water. 



It is obvious that in those cases in which the fall is at 

 the minimum, the size of the canal must be enlarged 

 proportionately to pass a required amount of water. The 

 velocity may be hastened without enlarging the size in 

 certain cases. For instance, it is a rule in hydraulic 

 engineering, that the velocity is in proportion to the mean 



Fig. 121. A DEEP CANAL. 



Fig. 122. A SHALLOW CANAL. 



radius or diameter of the canal, other things being equal. 

 Thus the water in the canal deep in proportion to its 

 width, as illustrated in fig. 121, meets with less resistance 

 from the surface of the bed and sides, (called by engineers 

 the "wet perimeter") than that in the shallow canal 

 seen in fig. 122, and its velocity being therefore greater 

 than in one of a contrary character, a larger quantity of 

 water is passed through it in a given time. 



