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88
HARBOUR ENGINEERING.
and slowly rotated as before. One drawback of the jumper is its liability to
deflection from its assigned direction if it happens to corne across a vein of
harder material. Guidance is practically absent at the moment of impact.
The Hand-drill is a short steel bar of octagonal section, manipulated either
by one man, who holds the bar with one hand while he strikes it on the head
with a hammer held in the other, or by two, or even three men, one of whom
acts as holder and the others as strikers. The command over a hand-drill is
more effective in maintaining its alignment than it is in the case of a jumper.
The limiting amount of useful penetration by the hand-drill is about 2
feet and it is chiefly used for making the short plug-holes, some few inches in
depth, which enable large blocks to be split up into smaller pieces. The
jumper may be effectively used for holes of from 3 to 4 feet in depth. The
rate of progress in either case depends, of course, mainly on the hardness of
the rock, and, in the second place, on the diameter of the hole, but it may be
ta en, on an average, at from 1 foot to 5 feet per hour; the former rate for holes
of 2 inches diameter in granité, and the latter for IJ-inch holes in limestone.
At the Kirkmabreck granite quarry, the following rates of work obtain •
viz., three men will bore about 7 feet per day of 2j-inch hole, and 84 feet per
day of 2-inch hole. Of plug-holes 9 inches deep, three men will drive 24 feet
per day to 1J inches diameter, and 32 feet per day of holes f inch diameter
and 4 inches deep. One man alone can do 14 feet per day of «-inch plug-
holes 3 inches deep. 8 r a
Machine drills are either rotary or percussive in action, and are actuated
vanously by steam, compressed air, water under pressure, and electricity.
Purely rotary drille generally take the form of a tube with an annular
cutting edge, formed either with hardened steel teeth or with a row of
Fig. 74.—Core-bit or Cutting Edge of
Rotary Drill, set with Diamonds.
Fig. 75. —Core Lifter.
diamonds. In the Brandt drill, steel teeth are forced against the surface of
the rock under enormous hydraulic pressure, while the tube makes from live
to eight revolutions per minute. In the ordinary diamond drill, the periphery
of the “core-bit,” as it is termed, has a number of diamonds embedded in it
and rotation is much more rapidly performed—from 200 to 400 revolutions per
minute. The core, which results from the action of the tube, is subsequently
broken off and withdrawn by a “core-lifter,” which forms part of the internal
mechanism of the drill.
The annular form of such drills lends itself to the supply of water to the