e latter shape being best
for very hard rock. For hard rock the cutting edge should be rather
thicker and blunter, and therefore stronger, than for soft rock.
Drills are made of high-grade steel, as they must be tempered
accurately and uniformly. The diameter of drill steel for hand work is
usually from 3/4 to 1 in., and the length of cutting edge, or gauge,
of the drill is always greater than the diameter of the shank, in the
proportion of from 7.4 to 4.3. Holes over 10 or 12 in. deep generally
require the use of a set of drills of different lengths and depending
in number on the depth required. The shortest drill, for starting the
hole, has the widest cutting edge, the edges of the others being
successively narrower and graduated to follow each other properly, as
drill after drill is dulled in deepening the hole. Thus the hole
decreases in diameter as it is made deeper. The miner's hammer (fig.
2) ranges in weight from 3-1/2 to 4-1/2 lb. for single-hand drilling,
up to 8 or 10 lb. for double-hand. If the hole is directed downward, a
little water is poured into it at intervals, to keep the cutting edge
of the drill cool and make a thin mud of the cuttings. From time to
time the hole is cleaned out by the "scraper" or "spoon," a long
slender iron bar, forged in the shape of a hollow semi-cylinder, with
one end flattened and turned over at right angles. If the hole is
directed steeply upward and the rock is dry, the cuttings will run out
continuously during the drilling; otherwise the scraper is necessary,
or a small pipe with a plunger like a syringe is used for washing out
the cuttings. The "jumper" is a long steel bar, with cutting edges on
one or both ends, which is alternately raised and dropped in the hole
by one or two men. In rock work the jumper is rarely used except for
holes directed steeply downward, though for coal or soft shale or
slate it may be employed for drilling holes horizontally or upward.
Other tools used in connexion with rock-drilling are the pick and gad.

Holes drilled by hand usually vary in depth from say 18 to 36 in.,
according to the nature of the rock and purpose of the work, though
deeper holes are often made. For soft rock, single-hand drilling is
from 20 to 30% cheaper than double-hand, but this difference does not
hold good for the harder rocks. For these double-hand drilling is
preferable, and may even be essential,