ll hole, leaving a funnel-shaped cavity. No rock,
however, is of uniform mineralogical and physical character, so that in
practice there is only a rough approximation to the conical crater, even
under the most favourable conditions. Generally, the shape of the mass
blasted out is extremely irregular, because of the variable texture of
the rock and the presence of cracks, fissures and cleavage planes. The
ultimate or resultant useful effect of the explosion of a confined
charge is in the direction where the least resistance is presented. In
the actual work of rock excavation it is only by trial, or by deductions
based on experience, that the behaviour of a given rock can be
determined and the quantity of explosive required properly proportioned.

Blasting, as usually carried on, comprises several operations: (1)
drilling holes in the rock to be blasted; (2) placing in the hole the
charge of explosive, with its fuze; (3) tamping the charge, i.e.
compacting it and filling the remainder of the hole with some suitable
material for preventing the charge from blowing out without breaking the
ground; (4) igniting or detonating the charge; (5) clearing away the
broken material. The holes for blasting are made either by hand, with
hammer and drill or jumper, or by machine drill, the latter being driven
by steam, compressed air, or electricity, or, in rare cases, by
hydraulic power. Drill holes ordinarily vary in diameter from 1 to 3
in., and in depth from a few inches up to 15 or 20 ft. or more. The
deeper holes are made only in surface excavation of rock, the shallower,
to a maximum depth of say 12 ft., being suitable for tunnelling and
mining operations.

_Hand Drilling_.--The work is either "single-hand" or "double-hand."
In single-hand drilling, the miner wields the hammer with one hand,
and with the other holds the drill or "bit," rotating it slightly
after every blow in order to keep the hole round and prevent the drill
from sticking fast; in double-hand work, one man strikes, while the
other holds and rotates the drill. For large and deep holes, two
hammermen are sometimes employed.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.--Sledge-hammer.]

A miner's drill is a steel bar, occasionally round but generally of
octagonal cross-section, one end of which is forged out to a cutting
edge (fig. 1). The edge of the drill is made either straight, like
that of a chisel, or with a convex curve, th