242 IRRIGATION. 



In soils that do not admit of rapid currents, and in 

 cases where a greater fall is unavoidable, it is customary 

 to construct the canal in sections, joined by chutes of 

 stonework or timber. The water passes through these 

 chutes with great velocity, and accomplishes the fall with- 

 out injury to the canal. 



The inclination of the banks depends upon the con- 

 sistency of the soil. The angles of repose, or the slopes 

 at which various kinds of soil will cease to slide down a 

 declivity, are as follows : Wet clay, 16 degrees from the 

 horizontal ; dry clay, 45 degrees ; coarse gravel, 40 de- 

 grees ; compact earth, 50 degrees ; arable loam or mucky 

 earth, 28 degrees ; wet sand, 22 degrees ; dry sand, 38 

 degrees ; fine gravel, 40 degrees. It depends upon the 

 position of the canal as regards the surface as well as 



. 128. SLOPE OF "ONE FOOT IN ONE." 



upon the nature of the soil, what inclination is necessary 

 to be given to the banks. "When the canal is excavated 

 wholly beneath the surface, an angle of 45 degrees, or a 

 slope having one foot of vertical hight to one of horizontal 

 base, is generally chosen. This is shown in fig. 123, in 

 wjiich the dotted lines show that the slope is the diago- 

 nal diameter of a perfect square, and therefore one 

 of 45 degrees, or of " one foot in one." When the 

 canal is deeply excavated, the slope should be broken 

 by a narrower bank, slightly above the level of the water, 

 shown in fig. 124, and the upper slope above the bank 

 should be increased. The width of the bank should be 

 proportioned to the hight of the upper slope ; its purpose 

 is to prevent earth loosened from the slope falling into 



