437 
1908-9.] Crushing of Stones. 
crushing, and must therefore be greater in the original specimen, since the 
bar is appreciably shortened under the load. The general practice is to 
make the length from 1 J to 2 times the diameter. Shorter bars fracture 
as shown in fig. 3. 
5. Stone, Brick, and Concrete. 
The coefficient of friction for these materials is extremely variable, and 
only rough results can be given. For stones, fx usually lies between 0*4 
and 0'6, the lower value corresponding with the harder and finer stones ; 
for masonry and brickwork an average value of /ul is from 06 to 0 65. The 
value of ft, the coefficient of friction between stone and steel, or stone and 
cast iron, may be taken as varying from 025 to 05. With these extreme 
values the following results are obtained : — 
For 
[x = 04, and // = 025 ; 
<f> = 22°, </>' = 14°, = 34°, and /?' = 27°. 
For 
jx — 065, and fx = 0*5 ; 
C P = 33°, C j>' = 27°, /3 = 28J*, ft = 15°. 
The inclination of sheared surfaces measured from broken stone test- 
pieces is apt to vary considerably. This is to be expected if one considers 
the variability and heterogeneity of most kinds of stone, even when 
samples are taken close together from the same quarry. The bedding of 
the stone gives rise to surfaces of minimum resistance other than those 
considered here. A crushed stone is usually much disintegrated, and the 
angles may be altered by the removal of loose material. Only such surfaces 
