Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/87

Rh CUTTING MACHINES.] COAL 67 porarily along the face. These are placed 5 feet apart, the props of the back row alternating with those in front. The props used are preferably of small oak or English larch, but large quantities of fir props, cut to the right length, aro also imported from the north of Europe. As the work proceeds onwards, the props are withdrawn and replaced in advance, except those that may be crushed by the pressure or buried by sudden falls of the roof. In Yorkshire hollow square pillars, formed by piling up short blocks of wood or chocks, are often used instead of props formed of a single stem. Iron pit props have been proposed at different times, but their use has not become general. When the coal has been under-cut for a sufficient length, the struts are withdrawn, and the overhanging mass is allowed to fall during the time that the workmen are out of the pit, or it may be brought down by driving wedges, or if it be of a compact character a blast of gunpowder in a bore hole near tho roof may be required. Sometimes, but rarely, it happens that it is necessary to cut vertical grooves in the face to determina the limit of the fall, such limits being usually dependent upon the cleet or divisional planes in the coal, especially when the work is carried perpen dicular to them or on the end. The substitution of machinery for hand labour in cut- Coal- ting coal has long been a favourite problem with iuven- c tors, the earliest plan being that of Menzies, in 1761, who proposed to work a heavy pick underground by power transmitted from an engine at the surface, through the agencies of spear-rods and chains passing over pulleys; but none of the methods suggested proved to be prac tically successful until the general introduction of com pressed air into mines furnished a convenient motive power, susceptible of being carried to considerable distances without any great loss of pressure. This agent has of late years been applied in various ways, in machines which either imitate the action of the collier by cutting with a pick or make a groove by rotating cutters attached to an endless chain or a revolving disc or wheel. The most successful of the first class, or pick machines, is that of Mr William Firth of Sheffield, represented in fig. 13. It consists esseu- FIG. 13. Firth s Coal-cutting Machine. tially of a horizontal piston and cylinder engine fixed upon a platform carried upon four wheels, which are coupled to gether by side rods, so that on motion being communi cated by means of a mitre wheel in the hind axle, it can be moved forward by hand. On the forward end of the frame are two bosses forming the centres for a pair of bell cranks or bent levers placed close to the ground, and facing in opposite directions, either one of which can be con nected with the piston rod. The outer arm of each lever carries a square socket, into which is fixed the pick, which has two cutting heads, one placed a little in front of the other so as to cut to the whole depth at one operation. In the older forms picks of different length were used, and it was necessary to go over the work a second or third time, in order to hole to the full depth. The cutting- points are loose, being secured by cotters to the pick head, so that broken or blunted ones can be readily replaced without removing the pick arm. The power used is air, at about 40 to 60 ft&amp;gt; above atmospheric pressure. It is con ducted from the reservoir connected with a compressing engine at the surface, through iron pipes fixed in the pit, and along the main roads to the working face, where thick vulcanized india-rubber pipes are used, sufficient length of pipe lying loose on the ground to allow the engine to move freely, the connection being made by a screwed joint at the back of the slide-valve chest. The valve is worked by tappets on the piston-rod, so as to be perfectly self- acting when properly adjusted ; it can also be moved by hand. The pick holders face in opposite directions, in order that the machine may be worked from either side. The size of the machine as ordinarily made is about 4 feet in length, 2 feet 2 inches high, and from 18 to 24 inches gauge of rails. The weight is about 15 cwt. The working speed is from 60 to 90 strokes per minute, corresponding to a length of from 10 to 20 yards, cut to a depth of 3 feet per hour. At the former rate, or 60 yards per shift of 6 hours, the work done corresponds to that of twelve average men. The width of the groove is from 2 to 3 inches at the face, diminishing to 1^ inches at the back, the proportion of waste being very considerably diminished as compared with the system of holing by hand. The use of this machine has allowed a thin seam of cannel, from 10 to 14 inches in thickness, to be worked to profit, which had formerly been abandoned as too hard to be worked by hand-labour. An earlier form of the second class of machine, in which the cutters have a continuous motion like those of a slotting machine, is that invented by Mr William Peace in the Wigan district, which is reproduced from the last edition as illustrating the principle which has since been carried out by other inventors in a more convenient and simplified form. It is represented in Plate V., figs, 1, 2, and 3. AAA is the frame, upon which are fixed one or more cylinders B, arranged so as to turn a crank shaft C, fixed to the frame, as is also another shaft D. This latter is capable of being turned by the former, by means of mitre or bevel wheels EEE ; upon the lower end of the latter shaft D is placed a wheel termed the driving wheel, having upon its periphery a groove with