his method has long been used in Europe and
elsewhere for deep boring. In the United States it is rarely employed
for depths greater than 200 or 300 ft. The usual form of cutting tool
or drill is shown in fig. 3. The iron rods are from 1 to 2 in. square,
in long lengths with screw joints (fig. 4). Wooden rods are
occasionally used. For shallow holes (50 to 75 ft.) the work is done
by hand, one or two cross-bars being clamped to the rod. The men
alternately raise and drop the drill, meanwhile slowly walking around
and around to rotate the bit and so keep the hole true. The cuttings
are cleaned out by a bailer, as for drive pipes.

In boring by hand, the practical limit of depth is soon reached, on
account of the increasing weight of the rods. For going deeper a
"spring-pole" may be used. This is a tapering pole, say 30 ft. long
and 5 or 6 in. diameter at the small end. It rests in an inclined
position on a fulcrum set about 10 ft. from the butt, the latter being
firmly fixed. The rods are suspended from the end of the pole, which
extends at a height of several feet over the mouth of the hole. With
the aid of the spring of the pole the strokes are produced by a slight
effort on the part of the driller. Average speeds of 6 to 10 ft. per
10 hours are easily made, to depths of 200 to 250 ft.

[Illustration: FIG. 5. Sliding Link.]

For deep boring the rod system requires a more elaborate plant. The
rods are suspended from a heavy "walking beam" or lever, usually
oscillated by a steam engine. By means of a screw-feed device, the
rods, which are rotated slightly after every stroke, are gradually fed
down as the hole is deepened, length after length being added. A tall
derrick carries the sheaves and ropes by which the rods and tools are
manipulated. The drill bit cannot be attached rigidly to the rods as
in shallow boring, because the momentum of the heavy moving parts,
transmitted directly to the bit as the blow is struck, would cause
excessive vibration and breakage. It becomes necessary, therefore, to
introduce a sliding-link joint between the rods and bit. One form of
link is shown in fig. 5. On striking its blow, the bit comes to rest,
while the rods continue to descend to the end of the stroke, the upper
member of the link sliding down upon the lower. Then, on the up stroke
the lower link, with the bit, is raised for delivering another blow.
For lar