ge ground over
the heavier machines for stoping and for most lateral development.
All piston types are somewhat cumbersome and the heavier types
require at least four feet of head room. The "baby" type can be
operated in less space than this, but for narrow stopes they do
not lend themselves with the same facility as the third class.

The third class of drills is still in process of development, but
it bids fair to displace much of the occupation of the piston types
of drill. Aside from being a one-man drill, by its mobility it
will apparently largely reproduce the advantage of hand-drilling
in ability to place short holes from the most advantageous angles
and for use in narrow places. As compared with other drills it
bids fair to require less time for setting up and removal and for
change of bits; to destroy less steel by breakages; to dull the
bits less rapidly per foot of hole; to be more economical of power;
to require much less skill in operation, for judgment is less called
upon in delivering speed; and to evade difficulties of fissured
ground, etc. And finally the cost is only one-half, initially and
for spares. Its disadvantage so far is a lack of reliability due to
lightness of construction, but this is very rapidly being overcome.
This type, however, is limited in depth of hole possible, for,
from lack of positive reverse movement, there is a tendency for
the spoil to pack around the bit, and as a result about four feet
seems the limit.

The performance of a machine-drill under show conditions may be
anything up to ten or twelve feet of hole per hour on rock such
as compact granite; but in underground work a large proportion of
the time is lost in picking down loose ore, setting up machines,
removal for blasting, clearing away spoil, making adjustments,
etc. The amount of lost time is often dependent upon the width of
stope or shaft and the method of stoping. Situations which require
long drill columns or special scaffolds greatly accentuate the loss
of time. Further, the difficulties in setting up reflect indirectly
on efficiency to a greater extent in that a larger proportion of
holes are drilled from one radius and thus less adapted to the
best breaking results than where the drill can easily be reset from
various angles.

The usual duty of a heavy drill per eight-hour shift using two men
is from 20 to 40 feet of hole, depending upon the rock, facilities
for setting up, etc., etc.[*] The lighter drills