198 



STRENGTH OF MATERIALS 



P F and P yt respectively parallel and perpendicular to DE. Then 

 P F = P N tan co, and comparing this with the relation P F = kP y , 



k = tan a) ; 



that is to say, the coefficient of friction is equal to the tangent of the 

 angle of repose. 



The following table gives the numerical values of the angles of 

 repose and coefficients of friction for various materials, and also the 

 weight in pounds of one cubic foot of each material.* 



160. Bearing power of piles. The custom of driving piles into the 

 soil to increase its bearing power is of very ancient origin, and is still 

 frequently used because of its cheapness and efficiency. Until quite 



* See Fanning, Treatise on Hydraulic and Wafer N //./>/*/ /Xv/'"' r""/. 1 "th <<!., 1002, 

 p. 345; Trautwine, Engineer's Pocket-Book, 190i, pp. 4O7-411: Sniithtnniaii VI 

 Tables, 1896, Table 95 ; also the results compiled by Rankine from experiments by Gen- 

 eral Morin and others, ibid., Table 149. 



