t, by which it is
hauled slowly forward. With 25 lb pressure it will hole 3 ft. deep, at
the rate of 30 yds. per hour, the cut being only 2-3/4 in. high, but it
will only work on one side of the carriage. This type has been greatly
improved and now is the most popular machine in Great Britain,
especially in long-wall workings. W. E. Garforth's Diamond coal cutter,
one of the best known, undercuts from 5-1/2 to 6 ft. In some instances
electric motors have been substituted for compressed-air engines in such
machines.

Another class of percussive coal-cutters of American origin is
represented by the Harrison, Sullivan and Ingersoll-Sergeant machines,
which are essentially large rock-drills without turning gear for the
cutting tool, and mounted upon a pair of wheels placed so as to allow
the tool to work on a forward slope. When in use the machine is placed
upon a wooden platform inclining towards the face, upon which the miner
lies and controls the direction of the blow by a pair of handles at the
back of the machine, which is kept stationary by wedging the wheels
against a stop on the platform. These machines, which are driven by
compressed air, are very handy in use, as the height and direction of
the cut may be readily varied; but the work is rather severe to the
driver on account of the recoil shock of the piston, and an assistant is
necessary to clear out the small coal from the cut, which limits the
rate of cutting to about 125 sq. ft. per hour.

Coal-wedging machines.

Another kind of application of machinery to coal mining is that of
Messrs Bidder & Jones, which is intended to replace the use of blasting
for bringing down the coal. It consists of a small hydraulic press,
which forces a set of expanding bits or wedges into a bore-hole
previously bored by a long screw augur or drill, worked by hand, the
action of the press being continued until a sufficient strain is
obtained to bring down the coal. The arrangement is, in fact, a
modification of the plug and feather system used in stone quarrying for
obtaining large blocks, but with the substitution of the powerful
rending force of the hydraulic press for hand-power in driving up the
wedges. This apparatus has been used at Harecastle in North
Staffordshire, and found to work well, but with the disadvantage of
bringing down the coal in unmanageably large masses. A method of wedging
down coal sufficiently perfected to be of general application would add
greatly