placed steel discs with a sharp edge on the periphery. These discs cut away the rock, and a circular tunnel was made 4 feet 6 inches in diameter in a hard slate rock. As there was no blast to shake the ground and leave loose projecting pieces, a tunnel made by this machine in such ground requires no lining to make it safe. Another machine, invented by Colonel Beaumont, shown in Fig. 400, was used for the exploring heading for the Channel tunnel, which was driven in the chalk under the sea near Dover. A revolving head carried steel teeth which scraped away the chalk, the fragments of which were carried to the rear of the machine, and thus cleared away. The chalk is a very suitable material for such a machine, being so soft that it can be cut with a penknife, whilst, on the other hand, it is sufficiently tough to stand without lining. The diameter of the heading was 7 feet, and the rate of advance when in actual work inch per minute, which is equal to 3 feet an hour; this rate, of course, could not be maintained for long, having regard to the numerous stoppages necessary for the adjustment of the machine. The above machine was driven by compressed air. Another and similar machine, but much stronger, was used by Colonel Beaumont in tunneling under the Mersey, between Birkenhead and Liverpool, cutting a circular heading 7 feet 4 inches in diameter, and advancing on an average 17 yards a week, with a maximum of 34 yards a week; but, afterwards improved, it attained a speed of 54 to 65 yards a week, whilst the speed of driving by hand a heading 10 feet by 8 feet in the same rocks was from 10 to 13 yards a week.1 The rock in this case was a sandstone of the New Red formation. Colonel Beaumont also used a similar machine at one or two other tunnels in different rock. A somewhat similar machine has been patented by Messrs. Stanley (see Fig. 401) for driving in coal; the machines are made suitable for driving tunnels either 5 feet, 6 feet, or 7 feet diameter as required. These machines have a revolving head, on to which are fixed projecting teeth or cutters upwards of 2 feet long, which cut a circular groove about 3 inches wide, leaving a solid block of coal in the centre of the heading. As the boring-head revolves, it is automatically advanced from the frame by a screw. After boring the depth of the teeth, the centre block is broken off and cleared away; another cut is then commenced, and so on as far as the traverse of the central screw permits. The frame, which is carried on two centre wheels, is then moved forward by reversing the movement of the screw, and cutting recommenced. The centre block often bursts off when only 9 to 10 inches has been cut. This machine will work at the rate of 3 feet an hour; the average speed

1 The machine was inspected by the author whilst at work, but the particulars of speed attained are taken from a paper read at the Inst. Civil Engineers, May 4, 1886, on the "Mersey Tunnel," by Mr. Francis Fox, M.I.C.E.

FIG. 400.-Section of the present heading under the sea at Dover, showing the air-boring machine of Colonels Beaumont and English in isometrical perspective.

of heading attained is usually much less, but that is simply a matter of cost and organization. A speed of from 30 to 50 yards a week has been obtained in practice. The engines are driven by compressed air. By the use of these machines the shattering effects produced by gunpowder are avoided, and in some cases the headings will stand secure. The size of headings driven, however, is in many cases too small, and two machines have to be employed side by side (see Fig. 402), the cuts from

FIG. 402.-Stanley's two tunnels.

which leave a little centre rib at top and bottom, which has to be got down. A two-headed machine is also made by Stanley, the circles made by the two revolving heads intersecting each other, so that no central pillar of coal is left between the two cuts. The head may be also widened in the manner described in the chapter on " Opening Out." Owing to the nature of beds of coal, fire-clay, and shale, the headings cut by these machines do not always stand without timbering, for which extra height may have to be got down. Where it is necessary to advance a pair of headings with great speed, and the ground is not too hard, there is no doubt that this machine will do good service; on the other hand, it must be borne in mind that if a heading is driven in fiery coal, at the rate of 20 yards a day, the amount of gas encountered may be unusually large, and require careful ventilation. The compressed air used for driving the machines will suffice to clear the heading, but the manager must remember that when this compressed air is shut off, from any cause, the heading may be rapidly filled with fire-damp.1

Messrs. Stanley have recently introduced a new form of their heading-machine, in which the coal or stone is all cut up into little pieces, so that the fragments can be gathered up, and carried behind by means of machinery, and thus the advance of the machine may be continuous; and they claim, for this reason, for the new machine twice the speed usually averaged with the annular groove machine.

Electric Drills. Some electric power drills have been recently made, and were exhibited at the Frankfort Exhibition of 1891; one of these was the application of an ordinary motor to gearing, the movement of a crank being used to lift the drill, the blow being struck by the force of a spring. Another used the solenoid, by which a reciprocating movement is given to a bar of iron. As these have not yet come into general use, it will be unnecessary to give a further description.

As it is undesirable ever to have an accumulation of fire-damp, it is necessary to provide for the constant ventilation of the heading independently of the cutting-machine.

CHAPTER XIV.

COAL-CUTTING MACHINERY.

PERHAPS the most arduous part of the work of a miner is holing or undercutting, called "kirving" in the north of England. In order to get the coal, it is frequently necessary to cut away either some of the lower part of the seam or some of the under

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clay for a distance of from I foot up to, in some cases, 8 feet, under the coal. The height of the excavation so made varies from 12 inches up to 2 feet 6 inches at the face, tapering down to a FLOOR couple of inches at the back (see Fig. 403). This undercutting is sometimes done only for a short distance, say 2 yards, in the case of stalls in pillar-and-stall mines. One side of the stall is then nicked, and the other side of the piece of coal that has been holed is broken down by a shot. Sometimes the whole width of the stall is holed at once.

FIG. 403.-Section of holing.

In longwall stalls it is customary to hole a considerable length, say from 10 yards up to as much as 100 yards. Usually the holing only extends the length of one-half the stall, say 20 or 30 yards. During the process of holing, the coal is supported by sprags fixed at intervals of not less than 6 feet. When the sprags

are withdrawn, the coal often tumbles down. There are generally joints in the coal parallel to the cleavage, and others at right angles to the cleavage, and if the holing is made as far in as one of these joints, there may be nothing to prevent the coal from falling when the sprags are withdrawn. In other cases the coal sticks to the roof, and has to be either cut, wedged, or blown down. The holing is sometimes done in dirt above the seam of coal, and sometimes it is done in the middle of the seam, especially where the seam of coal is divided by a band of dirt.

It is a long time since inventors first began to attempt to make machines to do some of the work usually done by the collier in