COA INING MACHINES, The principal inducement to operators to use coal-cutting machinery in preference to mining by hand-labor is naturally due to a reduction in the cost of getting out the coal to be gained the former method. With it. it is possible to effect a larger saving of coal than is possible by hand-labor. dne to the small height of the undercut ; also the number of men which have to be employed can be materially reduced. To get out the same amount of coal it is not necessary to keep as many working-places open in mines using machinery as it would be when employing hand labor, Hills making it possible to have the working-places more concentrated, and thereby to save a large amount of expense in the form of dead-work, such as keeping open gangways. To give an approximate idea of the cost of milling with machinery as compared with hand-labor, it can be stated that a coal-cutter in the Hocking Valley is capable of giving an output of SO to 85 tons a day. The price now paid for cutting coal by machines in rooms is 8 cents per ton; the price paid for loading coal after the is 35 cents per ton, A miner can mine and load on an average 3 tons per day, being paid 70 cents per ton. This shows a cost of 43 cents per ton of coal mined by machines. against 70 cents mined by hand. To the former will have to be added wages for one engineer, fuel, interest and depreciation, and wear and tear of the plant. By working the machines day and night., however, these last items can be redneed to a minimum. This policy is being followed in most mines using machinery. as it enables a comparatively small machine plant to give a large daily output. For example, should an output of b00 tons per day be required, and the machines be worked during the day only. ten coal-cutters (with the neces sary engines), etc., would be required. By working day aud night.. five coal-cutters would be sufficient, as well as engines, generators or compressors, and ducts of half the size. The work of loading and hauling would lie done during lite day only. There arc at present two general styles of coal-cutti•rs in use ; those using rotary cutters and those using reciprocating cutters. both of which have special features, which make it advisable to use one or the other, according to the nature of the coal.
Rotary imil-t'utters.—The general features of rotary coal-cutters are as follows: the under cut is !nude by means of revolving tools, the axis around which they revolve being either a horizontal line parallel with the coal-cutter (cutter-bar), it horizontal line at right angles with the coal (augers), or a vertical line (chain-machine), The machines in general consist of a stationary bed, upon which slides a movable frame hearing the elating devices. The hitter is gradually fed into the coal as the knives or tools
mit the coal away in front of it. The motor (either compassed air or electric) is attached to t he movable frame or to t he stationary bed, suitable gearing transmitting the power to the cutting devices. The feed is mitornatie, and consists either of a screw and not or rack and pinion. The best speed fir feeding seems to be from one ninth to one tenth of an inch per revolution of the cutting devices; although for some coal this speed might he increased with advantage. An important [valuta: of this style of coal-cut ters is a proper device for withdrawing the coal dirt or slack from the cut. to prevent the knives from beeoming clogged.
In the room and pillar work in use in this country the coal is generally undereut the entire width of the room to a depth equal to the height of the vein. It takes about nine or tell eats to neeomplish this in a room 30 ft. wkly. After the undercut is made. from three to four holes are drilled in the coal ahont two Ihinds of the height from the Boor, lint varying wit h the condition of the vein. These holes are tilled wit h powder, net the (and shot down. After having It1:1?41.41 down, the coal is hauled into the mine-ears by a set of miners, and the room is I•lenned up for another set of cuts. While the process if drilling, blasting. and load ing is going on, the eoal-ent ter is taken into another room for it. and there again undercuts t he coal t he entim length if the room. The best part of the veal is generally at the bottom of vein, Hod it is therefore desirable to sitn• as much if this as 'MS:4NC. .Fur reason the or Nit, is often made in the fire-clay underlying the coal, if this is not to gritty, or in at slate-part ing in the coal. If the latter is high up in the vein, the machines can be worked from the bench—in other words, if the coal underlying the parting is allowed to remain down for a sufficient distance from the face of the room to allow the machines to rest on it while making the new cut. When undercutting in fire-clay, care is generally taken to cut partially in the coal, as the white clay adhering to the hitter would decrease its value in the market. Wherever neither a suitable parting in the coal nor a fire clay bottom exists, and it is desirable to get out the largest amount of lump-coal possible (especially in seine of the small veins), the height of the cut has to be made us small as possible ; it is, however, not advisable to reduce it below 3i in., as otherwise it may not allow the coal to tumble over properly when shot down.