to secure a reasonable speed of
work.

[Illustration: Fig. 3.--Ingersoll-Sergeant Mining Drill.]

_Machine Drills._--The introduction of machine drills in the latter
part of the 19th century exerted an important influence on the work of
rock excavation in general, and specially on the art of mining. By
their use many great tunnels and other works involving rock excavation
under adverse conditions have been rapidly and successfully carried
out. Before the invention of machine drills such work progressed
slowly and with difficulty. Nearly all machine drills are of the
reciprocating or percussive type, in which the drill bit is firmly
clamped to the piston rod and delivers a rapid succession of strong
blows on the bottom of the hole. The ordinary compressed air drill
(which may, for surface work, be operated also by steam) may be taken
as an illustration. The piston works in a cylinder, provided with a
valve motion somewhat similar to that of a steam-engine, together with
an automatic device for producing the necessary rotation of the piston
and drill bit. While at work the machine is mounted on a heavy tripod
(fig. 3); or, if underground, sometimes on an iron column or bar,
firmly wedged in position between the roof and floor, or side walls,
of the tunnel or mine working. As the hole is deepened, the entire
drill head is gradually fed forward on its support by a screw feed, a
succession of longer and longer drill bits being used as required.

Among the numerous types and makes of percussion drill may be named
the following:--Adelaide, Climax, Darlington, Dubois-Francois,
Ferroux, Froelich, Hirnant, Ingersoll, Jeffrey, Leyner, McKiernan,
Rand, Schram, Sergeant, Sullivan and Wood.

[Illustration: Figs. 4 and 5.--Darlington's Rock Drill.]

One of the simplest of the machine drills is the Darlington (figs. 4
and 5): a is the cylinder; b, piston rod; c, bit; d, d, air inlets,
either being used according to the position of the drill while at
work; h, piston; j, rifle-bar for rotating piston and bit; k, ratchet
attached to j; l, brass nut, screwed into h, and in which j works; f,
chuck for holding drill-bit; n, air port communicating between ends of
cylinder, front and back of piston; o, exhaust port. This machine has
no valve. From its construction, the compressed air (or steam) is
always acting on the annular shoulder round the forward end of the
pi