holing into it from the face. The forward
feed is given by a chain winding upon a drum, which hauls upon a pulley
fixed to a prop about 30 yds. in advance. This is one of the most
compact forms of machine, the smaller size being only 20 in. high. With
an air pressure of from 35 to 40 lb. per sq. in., a length of from 300
to 350 ft. of coal is holed, 2 ft. 9 in. deep, in the shift of from 8 to
10 hours. The chain machine has been largely developed in America in the
Jeffrey, Link Bell, and Morgan Gardner coal cutters. These are similar
in principle to the Baird machine, the cutting agent being a flat link
chain carrying a double set of chisel points, which are drawn across the
coal face at the rate of about 5 ft. per second; but, unlike the older
machines, in which the cutting is done in a fixed plane, the chain with
its motor is made movable, and is fed forward by a rack-and-pinion
motion as the cutting advances, so that the cut is limited in breadth
(3-1/2 to 4 ft.), while its depth may be varied up to the maximum travel (8
ft.) of the cutting frame. The carrying frame, while the work is going
on, is fixed in position by jack-screws bearing against the roof of the
seam, which, when the cut is completed, are withdrawn, and the machine
shifted laterally through a distance equal to the breadth of the cut and
fixed in position again. The whole operation requires from 8 to 10
minutes, giving a cutting speed of 120 to 150 sq. ft. per hour. These
machines weigh from 20 to 22 cwt., and are mostly driven by electric
motors of 25 up to 35 h.p. as a maximum. By reason of their intermittent
action they are only suited for use in driving galleries or in
pillar-and-stall workings.

[Illustration: FIG. 11.--Winstanly & Barker's Coal-cutting
Machine--Plan.]

A simple form of the saw or spur wheel coal-cutting machine is that of
Messrs Winstanly & Barker (fig. 11), which is driven by a pair of
oscillating engines placed on a frame running on rails in the usual way.
The crank shaft carries a pinion which gears into a toothed wheel of a
coarse pitch, carrying cutters at the ends of the teeth. This wheel is
mounted on a carrier which, being movable about its centre by a screw
gearing worked by hand, gives a radial sweep to the cutting edges. When
at work it is slowly turned until the carrier is at right angles to the
frame, when the cut has attained the full depth. The forward motion is
given by a chain winding upon a crab placed in fron