e boring a stream of water is forced down the hollow rods by
a pump, passing back to the surface through the annular space between
the rods and the walls of the drill hole. The cuttings are thus
carried to the surface, leaving the bottom of the hole clean and
unobstructed. For recovering the core and inspecting the bit and
diamonds, the rods are raised at every 3 to 8 ft. of depth. This is
done by a small drum and rope, operated by the driving engine.

[Illustration: FIG. 8.--Little Champion Rock Drill.]

[Illustration: FIG. 9.]

Diamond drills of standard designs (fig. 8) bore holes from 1-9/16 to
2-3/4 in. diameter, yielding cores of 1 to 1-15/16 in. diameter, and
are capable of reaching depths of a few hundred to 4000 ft. or more.
They require from 8 to 30 boiler horse-power. Large machines will bore
shallower holes up to 6, 9 or even 12 in. diameter. For operating in
underground workings of mines, small and compact machines are
sometimes mounted on columns (fig. 9). They bore 1-1/4 to 1-9/16 in.
holes to depths of 300 to 400 ft., cores being 7/8 to 1 in. diameter.
Hand-power drills are also built. In the South African goldfields
several diamond drill holes from 4500 to 5200 ft. deep have been
successfully bored. Rates of advance for core-drilling to moderate
depths range usually from 2 to 3 ft. per hour, including ordinary
delays, though in favourable rock much higher speeds are often
attained. In deep holes the speeds diminish, because of time consumed
in raising and lowering the rods. If no core is desired a "solid bit"
is used. The drilling then proceeds faster, as it is only necessary to
raise the rods occasionally, for examining the condition of the bit.

[Illustration: FIG. 10. Diamond Drill Bit.]

The driving engine has two inclined cylinders, coupled to a
crank-shaft, by which, through gearing, the drill-rod is rotated. The
rods are wrought iron or steel tubes, in 5 to 10 ft. lengths. For
producing the feed two devices are employed, the differential screw
and hydraulic cylinder. For the _differential feed_ (fig. 9) the
engine has a hollow left-hand threaded screw-shaft, to which the rods
are coupled. This shaft is driven by a spline and bevel gearing and is
supported by a threaded feed-nut, carried in the lower bearing. Geared
to the screw-shaft is a light counter-shaft. By properly proportioning
the number of teeth in the system of