d and transmitting
its power through a flexible shaft. Although rather frequently used,
electric percussion drills cannot yet be considered entirely
successful, at least for mine service, in competition with compressed
air machines. Another type of electric drill, however, has been
successfully used in collieries, viz. rotary auger drills, mounted on
light columns and driven through gearing by diminutive motors. These
are intended for boring in coal, slate or other similar soft material.
Hand augers resembling a carpenter's brace and bit are also often used
in collieries.

Whatever may be the method of drilling, after the hole has been
completed to the depth required, it is finally cleaned out by a
scraper or swab; or, when compressed air drills are used, by a jet of
air directed into the hole by a short piece of pipe connected through
a flexible hose with the compressed air supply pipe. The hole is then
ready for the charge.

[Illustration: FIG. 7.]

[Illustration: FIG. 8.]

_Location and Arrangement of Holes._--For hand drilling in mining the
position of the holes is determined largely by the character and shape
of the face of rock to be blasted. The miner observes the joints and
cracks of the rock, placing the holes to take advantage of them and so
obtain the best result from the blast. In driving a tunnel or drift,
as in figs. 7 and 8, the rock joints can be made of material
assistance by beginning with hole No. 1 and following in succession by
Nos. 2, 3 and 4. Frequently the ore, or vein matter, is separated from
the wall-rock by a thin, soft layer of clay (D, D, fig. 8). This would
act almost as a free face, and the first holes of the round would be
directed at an angle towards it, for blasting out a wedge; after which
the positions of the other holes would be chosen.

[Illustration: FIG. 9.]

[Illustration: FIG. 10.]

When machine drills are employed, less attention is given to natural
cracks or joints, chiefly because when the drill is once set up
several holes at different angles can be drilled in succession by
merely swinging the cylinder of the machine into a new position with
respect to its mounting. According to one method, the holes are placed
with some degree of symmetry, in roughly concentric rings, as shown by
figs. 9 and 10. The centre holes are blasted first, and are followed
by the others in one or more volleys as