ovide
for maintenance of pressure against moments when all drills or
motors in the mine synchronize in heavy demand for air, and thus
lower the pressure at certain periods.

AIR-DRILLS.--Air-drills are from a mechanical point of view broadly
of two types,--the first, in which the drill is the piston extension;
and the second, a more recent development for mining work, in which
the piston acts as a hammer striking the head of the drill. From an
economic point of view drills may be divided into three classes.
First, heavy drills, weighing from 150 to 400 pounds, which require
two men for their operation; second, "baby" drills of the piston type,
weighing from 110 to 150 pounds, requiring one man with occasional
assistance in setting up; and third, very light drills almost wholly
of the hammer type. This type is built in two forms: a heavier
type for mounting on columns, weighing about 80 pounds; and a type
after the order of the pneumatic riveter, weighing as low as 20
pounds and worked without mounting.

The weight and consequent mobility of a drill, aside from labor
questions, have a marked effect on costs, for the lighter the drill
the less difficulty and delay in erection, and consequent less
loss of time and less tendency to drill holes from one radius,
regardless of pointing to take best advantage of breaking planes.
Moreover, smaller diameter and shorter holes consume less explosives
per foot advanced or per ton broken. The best results in tonnage
broken and explosive consumed, if measured by the foot of drill-hole
necessary, can be accomplished from hand-drilling and the lighter
the machine drill, assuming equal reliability, the nearer it
approximates these advantages.

The blow, and therefore size and depth of hole and rapidity of
drilling, are somewhat dependent upon the size of cylinders and
length of stroke, and therefore the heavier types are better adapted
to hard ground and to the deep holes of some development points.
Their advantages over the other classes lie chiefly in this ability
to bore exceedingly hard material and in the greater speed of advance
possible in development work; but except for these two special
purposes they are not as economical per foot advanced or per ton
of ore broken as the lighter drills.

The second class, where men can be induced to work them one man per
drill, saves in labor and gains in mobility. Many tests show great
economy of the "baby" type of piston drills in avera