happens that it
is necessary to cut vertical grooves in the face to determine the limit
of the fall, such limits being usually dependent upon the cleet or
divisional planes in the coal, especially when the work is carried
perpendicular to them or on the end.

Coal-cutting machines.

The substitution of machinery for hand labour in cutting coal has long
been a favourite problem with inventors, the earliest plan being that of
Michael Meinzies, in 1761, who proposed to work a heavy pick underground
by power transmitted from an engine at the surface, through the agencies
of spear-rods and chains passing over pulleys; but none of the methods
suggested proved to be practically successful until the general
introduction of compressed air into mines furnished a convenient motive
power, susceptible of being carried to considerable distances without
any great loss of pressure. This agent has been applied in various ways,
in machines which either imitate the action of the collier by cutting
with a pick or make a groove by rotating cutters attached to an endless
chain or a revolving disk or wheel. The most successful of the first
class, or pick machines, that of William Firth of Sheffield, consists
essentially of a horizontal pick with two cutting arms placed one
slightly in advance of the other, which is swung backwards and forwards
by a pair of bell crank levers actuated by a horizontal cylinder engine
mounted on a railway truck. The weight is about 15 cwt. At a working
speed of 60 yds. per shift of 6 hours, the work done corresponds to that
of twelve average men. The width of the groove cut is from 2 to 3 in. at
the face, diminishing to 1-1/2 in. at the back, the proportion of waste
being very considerably diminished as compared with the system of holing
by hand. The use of this machine has allowed a thin seam of cannel, from
10 to 14 in. in thickness, to be worked at a profit, which had formerly
been abandoned as too hard to be worked by hand-labour. Pick machines
have also been introduced by Jones and Levick, Bidder, and other
inventors, but their use is now mostly abandoned in favour of those
working continuously.

In the Gartsherrie machine of Messrs Baird, the earliest of the flexible
chain cutter type, the chain of cutters works round a fixed frame or jib
projecting at right angles from the engine carriage, an arrangement
which makes it necessary to cut from the end of the block of coal to
the full depth, instead of