gear-wheels, the feed-nut is
caused to revolve a little faster than the screw-shaft, so that the
drill-rod is fed downward a small fraction of an inch for each
revolution. To vary the rate of feed, as suitable for different rocks,
three pairs of gears with different ratios of teeth are provided. The
screw-shaft and gearing are carried by a swivel-head, which can be
rotated in a vertical plane, for boring holes at an angle.

[Illustration: FIG. 11. Core Lifter and Barrel.]

The _hydraulic feed_ is an improvement on the above, in that the rate
of feed is independent of the rotative speed of the rods and can be
adjusted with the utmost nicety. There are either one or two feed
cylinders, supplied with water from the pump. The rod, while rotating
freely, is supported by the feed cylinder piston and caused to move
slowly downward by allowing the water to pass from the lower to the
upper part of the cylinder. A valve regulates the passage of the water
and hence the rate of feed.

The bit (fig. 10 and fig. 11, B) is of soft steel, set with six to
eight or more diamonds according to its diameter. The diamonds,
usually from 1-1/2 to 2-1/2 carats in size, are carefully set in the
bit, projecting but slightly from its surface. Two kinds of diamonds
are used, "carbons" and "borts." The carbons are opaque, dark in
colour, tougher than the brilliant, and have no cleavage planes. They
are therefore suitable for drilling in hard rock. Borts are rough,
imperfect brilliants, and are best used for the softer rocks. As the
bit wears, the stones must be reset from time to time. The wear of
carbons in a well-set bit is small, though extremely variable. Above
the bit are the core-lifter and core-barrel. The core-lifter (fig. 11,
A) is a device for gripping and breaking off the core and raising it
to the surface. The barrel, 3 to 10 ft. long, fits closely in the hole
and is often spirally grooved for the passage of the water and debris.
It serves partly as a guide, tending to keep the hole straight, partly
for holding and protecting the core.

Diamond drills do not work satisfactorily in broken, fissured rock, as
the carbons are liable to be injured, loosened or torn from their
settings. In these circumstances, and for soft rocks, the diamond bit
may be replaced by a steel toothed bit. Another apparatus for
core-drilling is the Davis Calyx drill. For hard rock it has an