A manual of the principles and practice of road-making: comprising the location, construction, and improvement of roads, (common, macadam, paved, plank, etc.) and rail-roads. By W. M. Gillespie ...

164 THE CONSTRUCTION OF ROADS.
When the rock is stratified, Fig. 77.
having beds and seams, as in
the figure, holes bored parallel to the joints will produce
much greater effect than the
usual vertical ones. \,^
When a rocky surface is to FAX\ x\\,
he cut down to a line AB, the holes should be oblique, as
Fig. 78.
in the figure. In some cases, a horizontal one, from B
towards A would be advantageous.
On a high face of rock a'system of undermining may
be usefully employed, by blowing out a mass below, and
removing the remaining overhanging portion by crowbars.
wedges, &c.
The crater, or cavity formed by an explosion, is assumed to be a truncated cone, which has its inner or smaller
diameter equal to half the diameter of the mouth of the
crater. It has been found by experiment that the outer
diameter of the crater may be increased, in ordinary soils,
by excessive charges, to three times the length of the' line of least resistance," but not much beyond this; and
that within this limit this diameter increases nearly in the
ratio of the square root of the charge.
The most unfavorable situation for a charge is where a
re-entering angle is to be blown out, as the rock all around
it exerts a powerful resisting pressure. The charge needs