Longwall retreating. — The system of working just described is known as longwall advancing, because the coal is worked away from the shafts towards the boundary of the holding. In some cases the main roads are first driven to the boundary, and the coal opened up there and worked backwards to the shaft. This system is known as longwall retreating. With this method the waste ground is left behind, and no roads have to be maintained through it. Perhaps on the whole this method is the safest and most economical, but is rarely adopted because of the long time required to attain complete development. This means deferred profits, and the modern investor prefers quick returns, even if they should be rather less than he would get by waiting some years.

Pillar and Stall or Stoop and Room. This system of working is usually adopted in seams of over 4 feet thick, where the depth from the surface is not excessive. The coal is removed in two separate operations, the first known as working in the whole, the second as working in the broken. In Scotland the first operation is usually described as working in the fast, and the second as stooping. In the first stage the coal is divided into rectangular or square blocks or pillars by places driven at right angles to each other. The places driven parallel to the plane of the coal are known as walls, while those driven at right angles are known as 'bords,' In Scotland the places driven parallel to the plane of the coal are called planes, and those driven at right angles arc called ends. The width of these places varies with the nature of the roof, being as little as 10 feet where the roof is weak, to as much as 18 feet where the roof is strong. A common width is about 12 feet. The size of pillars or stoops varies from 4 yards by 4 yards up to as much as 60 yards by 60 yards. These pillars are frequently rectangular in place of being square, and a not uncommon size is 30 yards by 40 yards. The actual size depends upon depth of seam from the surface, nature of the coal, nature of the floor, and nature of the roof. This latter condition affects the case indirectly, as a good roof usually means wider rooms, and wide rooms mean proportionately larger pillars if the same percentage of coal has to be left for the second working. In general about 30 per cent. of the coal is taken out in the narrow work, the remaining 70 per cent. being recovered when the pillars are worked. The rooms are usually worked in two sets, one parallel to the plane of the coal, the other parallel to the end. These are usually called ends and planes. In moderate inclinations it is usually best to drive the places in this fashion, but with steep inclinations the places may require to be driven half-end, half-plane in order to facilitate haulage. The pillar and stall system is applicable in seams of 4 feet and over, where the roof and floor are hard, and where the seam is clean. If the seam contains bands of shale or other useless material to the extent of 15 per cent, or more of its total thickness, then the longwall system is likely to give better results. The conditions that affect the size of the pillars have already been noticed, but it is desirable to bear in mind, that nothing is gained by leaving too small pillars, as with a soft floor 'creep' may be set up, or with a hard floor thrust.'

Creep is a general movement throughout a district caused by too small pillars being left where the floor is soft, the pillars being forced down into the floor, thus causing it to rise between the pillars and fill up the roadways. Under similar conditions, but with a hard floor, the coal in the pillars gets badly crushed, or thrust. Both conditions cause considerable trouble, and add to the expense of the second working, and in certain cases may cause the complete loss of the pillars in the area affected.

Another point to notice is that pillars may be large enough to prevent anything of the above nature taking place during the first working, but once the taking out of the pillars is commenced, they may be found insufficient to resist the added pressure due to the extraction of several of them in a section before the first roof break takes place. To get the best results when taking out pillars they should be of ample size, as large pillars furnish the greatest proportion of round coal, and are more easily taken out without loss than small ones.

The general appearance of the plan of a stoop and room working is shown in Fig. 40.

Taking out the pillars or stooping may be done in sections as the working of the seam proceeds, or it may be delayed until the places are driven to the boundary, the pillars being worked back towards the pit bottom. Like longwall retreating, this latter method brings the best return, but it involves larger capital outlay and deferred profits. Whatever method may be adopted, care must be taken to have as little loss of coal as possible. It is also desirable to avoid having parts subjected to very heavy pressure, and thereby crushed to such an extent as to reduce the market value of the coal, because of the amount of small it contains. The usual method of working a pillar is first to split it across then take cuts up the side from bottom to top. These cuts are usually about 15 feet wide, and as the coal is removed, timber is set to carry the roof. As soon as the cut is up the timber is withdrawn, and the roof allowed to fall in. Cut after cut is taken in this way until the remaining part of the pillar forms a square. When this stage is arrived at cuts are taken both uphill and across until the whole of the coal is removed. In each case the timber is drawn as soon as the coal is out. A diagram showing the position of a cut in process is given in Fig. 41.

It will be observed from the diagram that the road is carried next the coal, the timber being placed next the waste side. Plenty of timber should be used, as it distributes the weight over a larger number of props, and makes their withdrawal easier. Odd props should be left in to give warning that the roof is beginning to give way. Drawing timber is a dangerous occupation, and should be done, or at least superintended, by experienced men.

In all cases when stooping, the cuts should be so worked as to go through on fallen waste — this prevents loss of coal and adds to the safety of the miner. Another golden rule in stooping is to have as small an area of roof as possible hanging on timber. If too large an area is held on timber a break may occur, and the greater part of the timber be lost. In carrying back a section of stooping the various cuts should follow each other in the different lines of pillars in successive order: by this means the waste break is kept running in a straight line and the best conditions of working secured. This line should lie diagonally across the successive rows of pillars and present as straight a front to the oncoming weight as possible.

The chief objections to the pillar and stall method of working are difficulties of ventilation, the amount of coal lost, and the large percentage of small coal produced. In spite of these objections there are many cases where this system is the best and most economical that can be adopted to meet the conditions of the seam.

Use of Machinery at the Coal face. — During recent years many of the finest seams in various parts of our coalfields have been worked out, and thinner seams have had to be attacked. High costs prevented many of these thin seams from being worked at a profit, and those interested in the industry had to turn their attention to methods of working that could be applied at less cost than the ordinary method of digging or hewing the coal by hand. This was done, and gradually two separate types of machinery designed to aid the miner in his work of coal getting were evolved. These are coal cutters for holing or undercutting the coal, and coal conveyors for removing the fallen coal from the face to the road head and loading it automatically into hutches.

Coal-Cutting Machines. — The use of coal-cutting machines for holing or undercutting coal-seams has made rapid progress, especially in Scotland, during recent years. It is claimed that the employment of such machines is accompanied by saving in the cost of production, which may arise from various causes, such as decreased expense in getting coal compared with manual labour; larger percentage of lump coal and therefore a higher average selling price; smaller cost for timber due to rapid advance of the coal face; reduced upkeep for roads as output is obtained from a smaller area; reduced cost of blasting where such is necessary; decreased number of accidents due to coal falling on the miner when holing; increased output per man employed; ability to continue working thin seams during periods of bad trade because of cheaper production; and more rapid development of the mine, and an earlier return on capital outlay.

All seams are not suitable for coal-cutters, so much depends on conditions, but generally, it may be said that the best results are likely to be obtained in seams of less than 3½ feet thick, where the inclination is moderate, and where both roof and floor are good. Machines are used for undercutting both in pillar and stall and longwall workings, but by far the larger number are employed in longwall. The reason is that thin seams are invariably worked on the longwall system, and since the amount of coal obtained per square yard of holing is proportional to the thickness of the seam, holing represents a large part of the work. This is the only part that the coal-cutter is capable of doing, and it is thus more likely to be used with success where holing represents the greatest portion of the work. In longwall working it is customary to use machines to cut back and forward on a face of from 80 to 150 yards. The machine drags itself forward while cutting by means of a wire rope attached to a drum, the rope being passed round a wheel anchored to a prop fixed some distance ahead, and brought back and fixed to the frame. Cutting is usually done during the night, the coal being stripped off and filled during the day, and the ripping or brushing being done between the filling and cutting shifts.

In pillar and stall workings the seams are usually of fair thickness, and holing here represents a smaller part of the work of coal getting, so that coal-cutters are less frequently used. Another drawback is the shifting of the machine from one stall to another, and the consequent loss of time caused thereby. Various types of coal-cutting machines are in use, and they are usually classified by the nature of their cutting appliances. They consist of (1) Bar Machines, (2) Disc Machines, (3) Chain Machines, and (4) Percussive or Punching Machines. In addition, machines specially designed for driving narrow headings and for shearing are also used to a limited extent.

The work of a coal-cutter consists of undercutting the seam, and in performing this work it has to move along the face besides being moved forward with each cut. The power supply must therefore be of such a nature that it can be readily adapted to these movements. The methods of transmitting power are practically limited to two: either Compressed Air or Electricity. Of these two powers electricity is the more efficient and convenient, but requires greater precautions to give the same degree of safety. There is little to say in favour of compressed air further than that it is the safer of the two powers, and for this reason is frequently adopted in fiery mines. Since the majority of mines do not fall within this class, electricity is far more extensively used for power transmission underground than compressed air. Two classes of current are available: Continuous Current and Alternating Current. The former is the more suitable of the two for driving coal-cutters, but both kinds are in use.