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STONE: NATURAL AND ARTIFICIAL.
89
point of incision, an adjunct which is of decided advantage in all forms of
drilling, whether by hand or mechanism. In this respect hydraulic motive
power may serve a double purpose, the waste water from the pressure cylinder
acting also as a lubricant and dust preventer.
Percussion drills, which also have a subsidiary rotary movement, conform
to the principle of the manual drill
in that they are driven forcibly
against the rock by steam or other
pressure. The essential parts are
a cylinder and piston, the latter of
which receives the pressure alter-
nately on eacli of its faces and acts
as a combined hammer and drill,
or perhaps more closely resembles
the jumper. The drill rod proper
is solid throughout and attached
to the end of the piston. It is
provided with a cutting edge or bit,
of I, X,1 or Z shape. The bit
requires sharpening every 2 to 4
feet of penetration. The pressure
employed is about 60 to 70 Ibs.
per square inch. About 300 blows
are delivered per minute, and the
rate of progress ranges from 3 to
10 feet per hour when the diameter
of the hole lies between 1 and 2
inches. One man suffices to operate
a machine, which may comprise
several drills, but two or three
men are required to transport it,
Fro. 76.-—Ingersoll Percussive Drill.
and two are generally in attendance. In granite, two men working a steam
Fiö. 77. —Section of Ingersoll Drill.
drill can do about three times the amount of work which would be done by
hand in the same time.
1 An X shape is preferable to an exact cross, as it affords less likelilrood of the same
grooves being struck repeatedly.