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 explosion itself. The terrible effects of fire-damp have led to the adoption of elaborate systems of ventilation, as the most effective safeguard against these explosions is the dilution and removal of the fire-damp as promptly and completely as possible. Very large volumes of air are necessary for this purpose, so that in such mines other sources of vitiation are adequately provided against and need not be considered. In metal mines, however, artificial ventilation is rarely attempted, and natural ventilation often fails to furnish a sufficient quantity of air. The examination of the air of metal mines has shown that in most cases it is much worse than the air of crowded theatres or other badly ventilated buildings. This has a serious effect on the health and efficiency of the workmen employed, and in extreme cases may even result in increased cost of mining operations. The ventilation of a mine must in general be produced artificially. In any case whether natural or artificial means be employed, a mine can only be ventilated properly when it has at least two distinct openings to the surface, one an intake or “downcast,” the other a chimney serving as an “upcast” Two compartments of a shaft may be utilized for this purpose, but greater safety is ensured by two separate openings, as required by law in most mining countries.

The air underground remains throughout the year at nearly the same temperature, and is warmer in winter and cooler in summer than the outside air. If the two openings to the mine are at different levels the difference in weight of the inside and outside air due to difference in temperature causes a current, and in the winter months

large volumes of air will be circulated through the mine from this cause alone. In summer there will be less movement of air and the current will frequently be reversed. In a mine with shafts opening at, the same level, natural ventilation once established will be effective during cold weather, as the downcast will have the temperature of the outside air, while the upcast will be filled with the warm air of the mine. In summer this will occur only on cool days and at night. When the temperature of outside and inside air becomes equal or nearly so natural ventilation ceases or becomes insignificant. In a mine with two shafts a ventilating current may result from other conditions creating a difference in the temperature of the air in either shaft—for example, the cooling effect of dropping water or the heating effect of steam pipes. Natural ventilation is impracticable in flat deposits worked by drifts and without shafts.

Ventilation may be produced by heating the air of the mine, as for example, by constructing a ventilating furnace at the bottom of an air shaft. The efficiency of such ventilating furnaces is low, and they cannot safely be used in mines producing fire-damp. They are sometimes the cause of underground fires, and they are always

a source of danger when by any chance the ventilating current becomes reversed, in which case the products of combustion, containing large quantities of carbon dioxide, will be drawn into the mine to the serious danger of the men. On account of their dangerous character furnaces are prohibited by law in many countries.

Positive blowers and exhausting apparatus of a great variety of forms have been used in mines for producing artificial ventilation. About 1850, efficient ventilators of the centrifugal type were first introduced, and are now almost universally employed where the circulation of large volumes of air is necessary, as in collieries. The typical

mine fan consists of a shaft upon which are mounted a number of vanes enclosed in a casing; the air entering a central side inlet is caught up by the revolving vanes and thrown, out at the periphery by the centrifugal force thus generated. “Open-running” fans have no peripheral casing, and discharge freely throughout their entire circumference; in “closed” fans the revolving part is completely enveloped by a spiral casing opening at one point into a discharge chimney. Fans either force air into or exhaust it from the mine. The inlet opening of the pressure fan is in free communication with the outside air,

the discharge connecting with the mine air-way; in the more generally used exhaust fan the inlet is connected with the airway, the fan discharging into the atmosphere. Among the exhaust fans most widely employed is the Guibal. Many others have been introduced, such as the Capell (fig. 19), Rateau, Schiele, Pelzer, Hanarte, Ser, Winter, Kley, and Sirocco fans.

The Waddle may be instanced as an example of the open fans. Slow-speed fans are sometimes of large dimensions, up to 30 and even 45 ft. diameter, discharging hundreds of thousands of cubic feet of air per minute. Occasionally, at very gassy and dangerous collieries, two fans and driving engines are erected at the same air shaft, and in case of accident to the fan in operation the other can be started within a few minutes.

Opposed to the motive force producing the air current is the frictional resistance developed in passing through the mine workings. This resistance is equal to the square of the velocity of the current in feet per minute, multiplied by the total rubbing or friction surface of the air-ways in square feet and by the coefficient of

friction. The latter, determined experimentally, varies with different kinds of surfaces of mine workings, whether rough or smooth, timbered or unlined; it ranges from 0·000000001872 to per sq. ft., the latter being the value usually adopted. A certain pressure of air is required to maintain circulation against the resistance, and for a given volume per minute the smaller and more irregular the mine openings the greater must be the pressure. The pressure is measured by a “Water-gauge” and the velocity of flow by an “anemometer.” The power required to circulate the air through a mine increases as the cube of the velocity of the air current. To decrease the velocity, when large volumes of air are required, the air passages are made larger, and the mine is divided into sections and the air current subdivided into a corresponding number of independent circuits. This splitting of the air not only lessens the cost of ventilating, but greatly increases its efficiency by permitting the circulation of much larger volumes, and has the added advantage that the effect of an explosion or other accident vitiating the air current is often confined to a single division of the mine, and affects but a small part of the working force. The adjustment of the air currents in the different splits is affected by regulators which are placed in the return air-ways, and act as throttle valves to determine the volume of air in each case. The circulation of air in any given division of the mine is further controlled and its course determined by temporary or permanent partitions (“brattices”), by the erection of stoppings, or by the insertion of doors in the mine passages and by the use of special air-ways (see ). In devising a system of it is customary to subdivide the workings so that the resistance to the ventilating current in each split shall be nearly equal, or so that the desired amount of air shall be circulated in each without undue use of regulating appliances which add to the friction and increase the cost of removing the air. In addition to this it is desirable to take advantage of the natural ventilation, that is, to circulate the air in the direction that it goes naturally, as otherwise the resistance to the movement of the air may be