68 MINE VENTILATION. $25. tity of air passing in a mine is according to the square root of the water-gauge. Problem. Suppose we have 30,000 cubic feet of air passing when the water-gauge is 1.6 inches, what quantity will pass with 2.5 inches of water-gauge? or V1.6: √2.5:: 30000: x 1.2649: 1.5811 :: 30000: 37500 (nearly) cubic feet. CHAPTER VII. VENTILATION OF SINGLE PITS OR DRIFTS. 26. DURING the sinking of shafts, or driving of drifts, the ventilation is caused by the diffusion of gases, and by the temperature of the atmosphere or outer air and that of the mine. It can hence only be temporary, becoming insufficient as soon as certain variable and often very small depth is attained, unless recourse be had to some other means. The simplest mode of attaining this end consists in dividing the pit by a wooden brattice, so that the warm air may ascend one side, while the cool air will descend the other. If this does not produce circulation, it can generally be attained by connecting one side of the brattice with a high chimney, which, by establishing a difference. between the levels of the orifices, causes a natural circulation of air. If compressed air or steam be used for drills, or if pumps be suspended in the pit, the chimney will be unnecessary. A fire may be placed at the bottom of the pit to warm the air on one side the brattice, or a small hand centrifugal fan be employed to advantage. In drifts, the brattice is placed parallel to the axis, and disposed in a horizontal or vertical 70 MINE VENTILATION. $ 26. When placed in the plane as better suits the case. vertical plane, one of the compartments may be used as a tramway and intake for the air: the other, communicating with the upcast pit, is used entirely as a return. When a horizontal brattice is used, it may be at the feet or head of the workmen. In this case the brattice is used to facilitate the ventilation of drifts, and also the driving and sinking of internal drifts, by putting the airway one side of the brattice in communication with the downcast. Within the last fifteen years, miners have superseded this method, wherever practicable, by "brattice-cloth" nailed to a series of props. The cloth is made imperme able to the air by being covered with tar, or dipped in a solution of soda silicate. Treated in the latter manner, it is rendered incombustible, as well as impermeable to the air. Brattice-cloth is only used to secure temporary ventilation, and should never be permitted, except at the face, or where it is subjected to constant scrutiny, as it may decay; and inattention to its repair or renewal might cause serious accidents. Another method of conducting air into drifts is by a continuous wooden box. This box may be placed in any position where it will be out of the way, with one of its extremities connected with a chimney, if it be driven from the day; or with the return airway, if the sinking or drift be made in the mine. Air-boxes, whose use is consistent with economy, if of a great length, will not supply sufficient volume of air, owing to the great absorption of the "motive-column" by friction. Tubes are made of other materials than wood, such as zinc, canvas, and paper; but they have never come into great demand. These air-boxes or tubes may be used to convey fresh air to the face of the workings, or return the foul air. It is preferable to employ them for the first method, rather than for the second; for more air is thrown upon the face of the workings, because, when suction is employed, the air will leak through the box before. it reaches the mouth of the box at the upcast, also the contraction of the air-vein at the entry of the box must be greater in the suction than in the forcing principle. Again the air will be purer when delivered to the face fresh from the day, for by suction it must necessarily bring to the face gases and foul air met with along its route. It is also evident that the diffusion of fire-damp by the mechanical action of the air will be better by the forcing-system. When the excavation has attained the extreme point where air-boxes and brattice act with sufficiency, we must replace these modes of ventilation (purely temporary) by other more perfect means, and, above all, not employ air-boxes in order to drive winning-places, because the ventilation 72 MINE VENTILATION. $ 27. would be insufficient, owing to the length which would be required; and impure air would injure the miner's health. Again: should an explosion take place, the boxes would be destroyed, ventilation stopped, and the afterdamp would spread through the workings, and cut off all chance of escape to the uninjured, and all hope of relief to the injured. As soon as the shaft wins the coal, it should be connected with a neighboring shaft, so as to produce a regular and continuous circulation of air. This is usually effected by driving two ways, forming a single conduit with two sides of the pit, so that the air must pass around the face of the drift. Sometimes a single drift is driven, divided by a brattice; sometimes two parallel drifts, separated by a wall of coal, cut through or holed at equal distances by crossdrifts or stentings, which are afterwards closed up, only leaving the one nearest the face open. After communication is made, the brattice should be removed. MINES WITH TWO ORIFICES. 27. A colliery, which, over a small area, has many openings to the day of great sectional area but of little length, requires little or no mechanical ventilation. When the pits are less numerous, and the galleries are longer, we should exercise great care in the distribution |