room for a machine to pass along the face, whilst there is plenty of room for the miner. When the coal has been got down in a longwall face, rails are laid along it for the waggon into which the coal is filled; but as soon as all the coal has been sent out, the rails in ordinary course are pulled up, and their place occupied

with packs and props. But if a coal-cutting machine is to be used, then the packs and props must remain at least 4 or 5 feet back from the face, and in some seams the roof may break down. Whatever precautions are taken, the roof is liable to break down, and if a machine is there it may be buried, and some expense

FIG. 414.-Longitudinal section of the Harrison coal-cutter.

has to be incurred in digging it out. It is also difficult to adjust the machine to cut in the dirt underneath the coal.

Coal-cutting machines have been chiefly used where the holing was in a bed of dirt about 18 inches above the bottom. There is a difficulty in moving heavy machines from one working place to another. These difficulties have made the profit of working coal-cutters in many cases so doubtful as to deter coal-mine

owners from incurring the outlay of capital necessary for their use. In some cases where holing is done in the dirt by manual labour, the collier is able to pull out the fire-clay in large lumps instead of hacking it all to pieces; this saves a great deal of labour. The machine cannot do this, but has to cut the holing, whether it is dirt or coal, to dust.

Electrical Machines.-Great expectations were raised by the introduction of electricity, but hitherto electrical coal-cutting machines have made but small progress in Great Britain, although said to be experimentally successful in several cases.

The writer has heard it reported that they are extensively used in the United States of America, but has not had the opportunity of verifying the report. It is probable that, as a rule, the use of coal-cutting machines will be reserved for places where the holing is exceptionally hard, and for very thin seams of coal.

Since this chapter was first written, there has been a considerable development in experimentalization and in the practical adoption of holing-machines. Mr. W. E. Garforth, of Normanton, has perfected a disc coal-cutter of the Gillott and Copley type, which undercuts to a depth of 5 feet 6 inches. The improvements consist chiefly in the mode of attaching the cutters to the disc, so that they can be expeditiously changed when blunt, and in increasing the strength of every part by using cast and wrought steel. The disc is made in two parts, so that it can be taken along narrow gate roads. The machine will hole at the rate of I square yard a minute, the average speed for a whole shift, of course, being only a small fraction of that speed. One machine working in a 3-foot seam, holing partly in hard dirt and partly in coal, will undercut 1000 tons of coal in a week for many successive weeks. These machines are generally driven by compressed air, but Mr. Garforth has lately introduced electrically-driven machines.

Messrs. Clarke, Steavenson and Co., of Hoyland, near Barnsley, have also introduced an improved electrically-driven machine, also of the Gillott and Copley type, and have about sixteen of these at work. These machines are constructed of steel; perhaps no English firm has more experience of electricallydriven coal cutters.

The Jeffrey Co., represented by John Davis and Son, Derby, has also brought out a similar electrically-driven machine, of which the distinguishing feature is that it is carried on one rail.

The Hurd machine is a bar cutter, similar to that described as Bower and Blackburn's (the original bar cutter was invented long before any of the modern machines). This machine has several ingenious arrangements, including a revolving head by which the position of the bar can be altered from its normal position, when

cutting, at right angles to the rails, to a line parallel with the rails; it can also be laid horizontal or at an angle with a horizontal line. The bar is of great strength, fitted with cutters that can be quickly replaced when blunt. The machine is electrically driven. It can be made to cut as far under as required, say 5 feet 6 inches or 6 feet.

The rate at which the coal-cutters will work of course depends on the hardness of the material they cut and the power supplied.

The power actually required seems to vary from 15 H.P. to 30 H.P. at the machine, and the work done is in all cases about I square yard a minute, when the cutters are sharp and ground moderately hard, and the actual number of square yards holed in a nine-hour shift varies from 80 to 250.

At a rough estimate, it may be said that in the year 1899, of the coal got in Great Britain, a quantity not exceeding 2 per cent. of the total is got with the aid of holing-machines, and in the United States perhaps 7 per cent. of the total quantity mined is undercut by machines.

321

CHAPTER XV.

TIMBERING: WOODEN AND STEEL BARS AND PROPS.

THE timber used varies with each locality according to the price of each particular kind of wood. On the eastern sea-board of

England, Norwegian and Baltic fir are almost exclusively used; in the Midland and western counties this is used together with English timber, such as larch, oak, etc. In South Wales a good deal of pine of a heavy kind is imported from the south of France, which is used in addition to other kinds. English larch and French timber are generally used with the bark on, Baltic has generally but little bark. Where, owing to the nature of the ground, the bars will have to bear a heavy weight, round trees are used from 6 to 10 inches in diameter 1 (see Fig. 415); and where the weight is slight, the trees are often sawn down the middle, the flat side being placed against the roof (see Fig. 416). Square

timber is used sometimes, similar to railway sleepers. Steel bars are often substituted for wooden; they are sometimes supported by wooden props, and occasionally by steel props (see Fig. 417). They are often fixed in the stone sides of the road, and sometimes are carried by brick walls. The bars generally employed are girders, similar to those used for ordinary building purposes, as this form will sustain the greatest pressure for a given weight of steel. A rolled steel girder can be bent, but can hardly be broken; it is therefore a very safe kind of bar. 1 Roughly squared hewn trees are also used.

Y

But steel rails are liable to snap and fly under a breaking load, and it is therefore not so safe to use rails for bars. The steel is very much more costly, varying, according to the circum

stances, from twice to six times the cost of the timber. But it is no more costly in labour to erect, and, when erected, will generally endure much longer. There are many places where steel bars once fixed would form a permanent support, because they will not decay; whilst, in the same place, a timber bar would rapidly decay, and within twelve months would have to be replaced, the labour of replacing being more costly than the wood replaced. In such cases there is a manifest economy in the use of steel. In roads which only serve a temporary purpose wood is to be preferred; and in roads where, owing to the excessive pressure or the unusual tenderness of the strata, bars, whether of timber or steel, will be rapidly broken or bent out of shape, timber is also probably the cheaper.

The usual way of fixing a bar on to props is shown in Fig. 415. The diameter of bars used for the timbering in mines in this country varies from about 3 up to 12 inches; occasionally, no doubt, bars of greater diameter are used. The timber for props also varies in this country from about 3 inches to about 10 or 12 inches in diameter. In some mines, however, which the writer has visited, much thicker timber is used; for instance, in the Calumet and Hecla Mine, on Lake Superior, timber props are set 3 feet in diameter.

In order that a bar may not be easily knocked off the prop, it is generally tightened by a wedge at the top; sometimes the end of the bar is notched and the prop also, as shown in Fig. 418. In other cases an iron spike is simply knocked into the bar in front

FIG. 418.-Props and FIG. 419.-Props and FIG. 420.-Props in heading. bars, notched.

bars, spiked.

of the prop, as shown in Fig. 419. In many cases props only are used, as shown in Fig. 420; in other cases props are dispensed