before many years assume large proportions. Ocean freights are the present great drawback.
Mining of Coal. The presence of coal in paying quantities having been determined by prospecting and geological surveys, the next consideration is to extract this coal from seams. No definite rules can be given for the selection of a method of mining that will cover all conditions; each mine furnishes a distinct and separate problem. Every system of mining, however, aims to extract the maximum amount of the deposit in the best marketable shape and at a minimum cost and danger. Speaking broadly, all methods of mining come under the head of either open working or closed working. Open working is employed when the deposits have no overburden of barren rock or earth, or where this overburden is of such small depth that it can be easily and cheaply removed, leaving the coal deposit exposed. The mining of such exposed seams of coal is really a process of excavation or quarrying, and the machines used in making open-pit excavations and in quarrying are applicable to the work. Closed working is adopted when the depth of the overburden is so great that the mining must be conducted underground. The first task in opening up underground coal-seams is to secure access to the seam by means of shafts, slopes, or tunnels. Shafts are vertical openings from the ground surface to the coal-seams. In the United States shafts are usually made square or rectangular in form. This practice is largely due to the fact that timber is used for lining shafts. In Europe round or oval shafts are frequently employed with linings of brick, iron, or masonry.
Generally the shafts are divided into two or more compartments, in each of which is installed an elevator for hoisting the coal-cars to the surface. The number of compartments in a shaft and their arrangements depend upon the particular use to which the shaft is to be put, the number of shafts employed, and their depths. Where the seams are comparatively near the surface, it is usually cheaper to sink a number of two or three compartment shafts than it is to haul all the ore to one large shaft; while, when the shafts are very deep, it is preferable to sink a smaller number of four or six compartment shafts and extend the underground haulage to a single shaft over a great area of the workings. Where timber lining is employed, a stronger construction is obtained by placing the compartments side by side in a long, narrow shaft than by grouping them in a square shaft. In shallow mines separate shafts are often employed for hoisting and for pumping, ventilation and ladder-ways. One of the largest coal-mine shafts in America is situated at Wilkesbarre, Pa.; it is 1039 feet deep, 12 × 52 feet in size, and has five compartments. The methods of sinking mine shafts are essentially the same as those used in sinking shafts for tunnels. (See Tunnel.) Slopes are openings begun at the outcrop of an inclined seam, which they follow down into the earth. Slopes are usually made with three compartments side by side, two of which are used as hoistways and the third for the traveling-way, piping, etc. When the dip of the slope is under 40 degrees the slope is made about seven feet high, but when the dip exceeds 40 degrees cages have to be used and a great height is necessary. Slopes are usually lined with timber. Tunnels are nearly horizontal passageways beginning on the side of a hill or mountain and extending into the earth until they meet the coal-seam; they are built for both haulage and drainage purposes, and are constructed like railway tunnels, except that the cross-section is usually much smaller, and that it is lined with timber instead of with permanent masonry. The forms of timbering used in coal-mining are various, and are of interest chiefly to the practical miner; special treatises should be consulted by those interested in the details. In a general way, it may be said that timber used for underground support in mines should be of a light and elastic variety of wood. Oak, beech, and similar woods are heavy and have great strength, but when they do break it is suddenly and without warning, thus bringing disaster to the miners who might escape if a tough wood were employed which gives warning of rupture by bending and cracking. It is a very common practice to employ preserved timber in mining work. See Forestry.
The systems of working the coal-seams after access is attained to them by the means described are two, known as the room-and-pillar and the long-wall systems. The room-and-pillar method—also known as the pillar-and-chamber or board-and-pillar method, which may include the pillar-and-stall system—is the oldest of the systems, and the one very generally used in the United States. By this system, coal is first mined from a number of comparatively small places, called rooms, chambers, stalls, boards, etc., which are driven either square from or at an angle to the haulageway. Pillars are left to support the roof. In the long-wall method the whole face of the coal-seam is taken out, leaving no coal behind, and the roof is allowed to settle behind as the excavation progresses, care being taken to preserve haulageways through the falling material. Both the room-and-pillar and the long-wall methods are employed in various modifications, for the details of which special treatises on coal-mines should be consulted. The coal is cut from the seam by hand or by some form of coal-cutting machine. In America machine cutting is used extensively. There are four general types of machines in general use: Pick machines, chain-cutter machines, cutter-bar machines, and long-wall machines; the machines most used in America are pick machines and chain-cutter machines. Both compressed air and electricity are used for operating coal-cutting machines. Pick machines are very similar to a rock-drill; chain-cutter machines consist of a low metal bed-frame upon which is mounted a motor that rotates a chain to which suitable cutting teeth are attached. The ventilation of the workings, owing to the presence of gases, is a very important feature of coal-mining, and great care is taken to lay out the workings so as to facilitate ventilation. Mechanical ventilation by means of fans and blowers (see Blowing Machines) is usually employed. Hoisting in mines is accomplished by means of cages running up and down the shafts, and operated by large hoisting engines on the surface. There are two general systems of hoisting in use—hoisting without attempt to balance the load, in which the cage and its load are hoisted by the engine and lowered by gravity, and hoisting in balance, in which the descending cage or a spe-
