\fdO'^ - Old Forge Coal Mines

RAILROAD CONSTRUCTION. 285
(850) The friction of the backing against the wall adds
considerably to its stability.
(851)
(852)
See Art. 1481.
See Art. 1482.
(853) See Art. 1483 and Fig. 401.
(854)
See Art. 1484.
(855) The line d c in Fig. 73 forms an angle of 33° 41'
with the horizontal d h and is the natural slope of earth.
I
5^--i
Fig. 73.
The line df bisects the angle ode. The angle o df is called
the angle, the slope df is called the slope, and the triangular
prism o df \s called the prism of maximum pressure.
(856) See Art. 1486.
(857) See Art. 1486.
(858) From Fig. 73, given above, we have, by applying
formula 106, Art. 1486,
perpendicular pressure;/ P =
the weight o f the tri angle of earth odfx of
the vertical height od