or extracting the oil. This industry has become of great importance in
many parts of the United States, in southern Russia and elsewhere. Rock
salt deposits are sometimes worked through bore-holes, by introducing
water and pumping out the solution of brine for further treatment. The
sinking of artesian wells is another application of boring. They are
often hundreds, and sometimes thousands, of feet in depth. A well in St
Louis, Missouri, has a depth of 3843 ft.

Boring is useful in mines themselves for a variety of purposes, such as
exploring the deposit ahead of the workings, searching for neighbouring
veins, and sounding the ground on approaching dangerous inundated
workings. In the coal regions of Pennsylvania, bore-holes are often sunk
for carrying steam pipes and hoisting ropes underground at points remote
from a shaft.

Several of the methods of boring in soft ground are employed in
connexion with civil engineering operations; as for ascertaining the
depth below the surface to solid rock, preparatory to excavating for and
designing deep foundations for heavy structures, and for estimating the
cost of large scale excavations in earth and rock.

Lastly, a number of deep holes have been bored for geological
exploration or for observing the increase of temperature in depth in the
earth's crust; for example, at Paruschowitz, Silesia, about 6700 ft.
deep; at Leipzig, Germany, 6265 ft.; near Pittsburg, Pennsylvania, 5532
ft.; and at Wheeling, West Virginia, nearly 5000 ft. The two last
mentioned were intended to obtain as complete a knowledge as possible of
the bituminous coal and oil-bearing formations.

There are five methods of boring, viz.: by (1) earth augers, (2) drive
pipes, (3) long, jointed rods and drop drill, (4) the rope system, in
which the rods are replaced by rope, (5) rotary drills. The first two
methods are adapted to soft or earthy soils only; the others are for
rock.

1. _Earth augers_ comprise spiral and pod augers. The ordinary spiral
auger resembles the wood auger commonly used by carpenters. It is
attached to the rod or stem by a socket joint, successive sections of
rod being added as the hole is deepened. The auger is rotated by means
of horizontal levers, clamped to the rod--by hand for holes of small
diameter (2 to 6 in.), the larger sizes (8 to 16 in.) by horse power.
Clayey, cohesive soils, containing few stones, are readily bored;
stony ground with difficulty. The operati