while drilling. The speed of
rope-boring is therefore but little affected by increase of depth,
while with rod-boring it falls off rapidly. In its simplest form the
so-called "string of tools," suspended from the rope, is composed of
the bit or drill, jars and rope-socket. The jars are a pair of sliding
links, similar to those used for rod-boring, but serving a different
purpose, viz. to produce a sharp shock on the upward stroke, as the
jars come together, for loosening the bit should it tend to stick fast
in the hole. A heavy bar (auger stem) is generally inserted between
the jars and bit, for increasing the force of the blow. The weight of
another bar above the jars (sinker-bar) keeps the rope taut. The
length of stroke and feed are regulated by the "temper-screw" (fig.
7), a feed device resembling that used for rod-boring. Clamped to it
is the drill rope, which is let out at intervals, as the hole is
deepened. The bits usually range from 3 to 8 in. diameter, the speed
of boring being generally between 20 and 40 ft. per 24 hours,
according to the kind of rock. A great variety of special "fishing
tools" are made, for use in case of breakage of parts in the hole or
other accident.

[Illustration: FIG. 7. Temper Screw.]

5. _Diamond Drill._--The methods described above are capable of boring
holes vertically downward only. By the diamond drill, holes can be
bored in any direction, from vertically downward to vertically upward.
It has the further advantage of making an annular hole from which is
obtained a core, furnishing a practically complete cross-section of
the strata penetrated; the thickness and character of each stratum are
shown, together with its depth below the surface. Thus, the diamond
drill is peculiarly well adapted for prospecting mineral deposits from
which samples are desired. The first practical application of diamonds
for drilling in rock was made in 1863 by Professor Rudolph Leschot, a
civil engineer of Paris.

The apparatus consists essentially of a line of hollow rods, coupled
by screw joints, an annular steel bit or crown, set with diamonds,
being attached to the lower end. By means of a small engine on the
surface the rods are rapidly rotated and fed down automatically as the
hole deepened. The speed of rotation is from 300 to 800 revolutions
per minute, depending on the character of the rock and diameter of the
bit. Whil