crosspieces for the support of guides by installing these at longer intervals than usual and making them V shaped (inverted) on top to give sloping surfaces that offer no lodging place for coal dust.

VENTILATING SHAFTS OR COMPARTMENTS

Ventilating shafts or compartments should always be smooth lined. Tests made at Butte, Mont., in cooperation with the Bureau of Mines, demonstrated 33b that smooth-lined shafts save a considerable percentage of the ventilating power required for rough-lined shafts. If the shafts have projecting frame sets, it pays to use concrete slabs between the frames or a gunited lining to minimize resistance to the flow of air.

The square turn from the fan drift at the surface into the shaft and another square turn at the bottom of the shaft into a single airway cause great loss of ventilating power at most mines. These turns should be carefully designed, with suitable curves that aid the flow of air. Experience in tunnel ventilation indicates it would sometimes pay to use curved deflectors.

Wherever possible, the air should be split into two parts at the bottom of the shaft, with easing curves to avoid the loss of energy that commonly results when the air current is throttled.

ARRANGEMENTS FOR VENTILATION AT SLOPE AND DRIFT MINES

Entrances or arrangements for ventilation usually are made more easily at a slope or drift mine than at a shaft mine. On the other hand, experience has shown that this very advantage has fostered negligent planning. Entrances at many mines have been made so close together that serious difficulties arose at the time of an explosion or fire, as smoke and fumes covered both openings. Moreover, there has been a tendency to make the coal pillars between the intake and return so thin that air leaks through crevices and the efficiency of ventilation is impaired.

Too often rooms have been driven close to the slopes, drifts, or main entries, or have been turned directly off them for the sake of easily obtained coal, so that in later work it has been difficult or impossible to drive the additional parallel airways needed for ventilating a large mine. Slope and drift openings should be separated by not less than 50 feet of rock or coal; at the points of entrance it is better to have 100 feet at least. Slope and drift entrances should be of concrete or of brick arched for at least 100 feet. No wooden building on the surface should be within 75 feet of any mine opening.

38b McElroy, G. E., and Richardson, A. S., Resistance of Metal-Mine Airways: Bull. 261, Bureau of Mines, 1927, 149 pp.

FANS

Some mine fans are arranged as pressure and others as exhaust fans; during recent years the latter have been more common. In the best practice, that necessary in gassy mines, the intake air enters, or downcasts, through the hoisting shaft or slope and the main haulage roads, and it returns, or upcasts, through the air shaft.

Many State laws now contain the commendable requirement that fans must be constructed to permit quick reverse of the ventilating currents to meet emergencies due to fires or explosions; they also require that fan casings have explosion relief doors and that fans be offset from the air shaft or air drift at least 25 feet from the projection of the nearest side, so that they will not be damaged by an explosion in the mine. Fireproof construction of fans and casings should be required.

Fans of larger capacity are being installed as mines become larger, more men are employed in a single mine, and the danger from inflammable gas increases as workings are deepened or are extended under thick cover.

Fans at gassy mines should have an auxiliary means of driving, such as a steam or gasoline engine stand-by; if they are electrically driven, a separate source of electrical power should be provided.

Fans with capacities of more than 250,000 cubic feet of air a minute against a pressure head of about 4 or 5 inches of water gauge have been installed at some coal mines in the United States. A few fans with double this capacity requirement have been erected at deep, extensive, gassy coal mines in Europe. There they usually serve to ventilate workings in a number of coal beds developed from the same air and hoisting shafts.

UNDERGROUND FANS

After electric power began to replace steam or compressed air in coal mining there was an increasing tendency to place fans underground. Some mining companies went so far as to place their main fans underground, but explosions that wrecked these fans have brought about general abandonment of the practice, and most State regulations or inspection departments require the main fan to be on the surface and in such a place as to avoid serious damage by explosions or fires. The usual requirement is to offset the fan 15 to 25 feet from the side of the shaft or drift and to provide either explosion-relief doors or a weakly constructed conduit. Well-designed explosion-relief doors are preferable to the latter.

ELECTRICALLY DRIVEN BOOSTER AND AUXILIARY FANS

Booster and auxiliary fans, first used extensively and appropriately in metal mines, have been installed underground in coal mines,

especially of late with the introduction of certain types of concentrated mining methods. Many of these subsurface installations in coal mines are open to severe criticism, for coal mining is attended by dangers from which metal mining, with rare exceptions, is free. Booster fans. Booster fans are those that raise the pressure (positive or negative) of the air current or a split of the current at and from a point where the pressure imparted by the main fans (on the surface) is insufficient to force the air current into distant parts of the mine or through some restricted airways. Use of a booster fan is nearly always an indication that the main fan is not powerful enough or the airways are blocked or are too small. Under very exceptional conditions, however, a booster fan is a proper means of supplementary ventilation, but it should always be placed on the intake air and the entry should be fireproofed 50 feet from the fan in each direction. As a booster fan is always motor driven, the motor should be of sturdy construction, preferably inclosed, and the power cable leading to it should be armored. The fan motor should not have a self-starter; a fire boss should make an examination before an idle booster fan is started. It should run continuously unless stopped for repairs.

Auxiliary fans.-Auxiliary fans are small portable fans; virtually all are blowers and are electrically driven. They are used with canvas tubes or metal pipes to ventilate headings or narrow workings that are without crosscuts or cut-throughs or have them more than the normal distance apart. State regulations fix the maximum interval between crosscuts in entries, but some States do not specify the interval in rooms.

Fans and tubes have been substituted for line brattices in headings and rooms in mines supposed to be nongassy. As a matter of fact, they have frequently been used in headings and rooms that give off dangerous amounts of inflammable gas. So far the electric motors driving booster and auxiliary blower fans have not been flame proof, and hence they may readily ignite inflammable gas. Up to December, 1927, no fan motor had been approved by the Bureau of Mines.

Certain publications and trade journals have referred to the wide use of underground blowers and tubes in European coal mines, but one must remember that those blowers are driven by compressed air exclusively. Furthermore, in Europe no other workings are permitted to be turned off headings or slopes ventilated by compressed-air blowers and metal tubes until an air connection has been made with another heading or slope to establish an air circuit. Even though explosion-proof electric motor or compressed-air motors are used to drive blowers it is still questionable whether

blowers should be used in headings or rooms which give off methane in appreciable amounts. As now used many such blowers are shut off at the quitting time of the day shift and are started automatically or by hand-thrown switches the next morning. If methane accumulates during the night, forcing the body of gas into the air current of that part of the mine involves the danger of ignition through either a "short" or arc of a power wire, the failure of some piece of apparatus or some worker's error or careless act.

Many experienced mining engineers and coal-mine operators believe that this method of ventilation by blowers and tubes is necessary under exceptional conditions only, provided the workings aré properly laid out. In a gassy mine an electrically driven blower creates a dangerous hazard. Exceptional conditions can be met most safely by using a blower driven by compressed air, the compressor being placed well back from the face in pure intake air. In any case the blower should be operated continuously to avoid gas accumulating. When a blower is used, a tight line-brattice is desirable instead of tubing or in addition to it, so that there may be some circulation of air when the blower is shut down. A blower should not be used if the recirculation of air in the working place is more than 10 per cent of the current entering it. Recirculation is best prevented by requiring the split from which the blower intakes to have three or four times the volume forced by the blower, and the volume of the split should be at least 10,000 cubic feet a minute.

Electric motors, whether permissible or open, should not be used where there is more than 0.25 per cent of inflammable gas in the air current. Moreover, electric self-starters should not be used; then there will be no possibility of the blower starting until the place has been inspected for gas. If gas is present, the motor or a short circuit of the wiring may ignite it.

Policy of Bureau of Mines regarding auxiliary fans.-The Bureau of Mines recommends that auxiliary fans or blowers should not be used in coal mines as a substitute for methods of regular and continuous coursing of the air to every face of the mine.

COURSING THE AIR IN A COAL MINE

Good ventilation in coal mines can be assured only by having the airways in proper condition and keeping the velocity of the air moderate-that is, less than 1,800 linear feet a minute (20 miles an hour) in smooth-lined shafts and air courses, less than 900 linear feet a minute in ordinary rough-ribbed airways, and less than 600 feet a minute in haulage ways and manways. To keep down the velocity, the airways must be large, or a number of parallel entries must be used.

Such restrictions on velocity keep the frictional resistance low, allow a continuous saving in power cost, lessen leakage, and make possible the ventilation of distant workings without resort to booster fans or a reduction in the amount of air.

On the other hand, the velocity of the air current of any one split should not be less than 200 linear feet a minute, and the volume of the split should not be less than 10,000 cubic feet a minute in order to dilute and to sweep away gases and provide the men with air that is within the limits of purity for health.

LAYOUT OF VENTILATION SYSTEM

MAIN ENTRIES OR SLOPES

Good practice requires at least three parallel main entries or slopes, even in a mine of small output; the middle entry, for intaking air, should be the haulage road; the others should be the main return entries for each side. Even better practice is a minimum of four parallel entries, the inner pair to be intakes (one for haulage and the other for a manway) and the outer pair to be main returns. One merit of this plan is that each pair can be driven with crosscuts or cut-throughs at the intervals specified by State laws. The pairs of entries need be connected only where branch entries start off, and the manway will therefore be more isolated from the haulage way which, because of the coal dust deposited in it, is nearly always the entry that is traversed most violently by an explosion.

The best practice in long-lived mines of large output is to use not less than three pairs of parallel entries for main entries, thus providing three entries for the intake and three for the return air.

BRANCH ENTRIES OR LEVELS IN PITCHING BEDS

Each main branch should have at least three parallel entries if they are to be long. In the Pittsburgh (Pa.), Fairmont (W. Va.), and Ohio fields some large mines that work flat-lying beds use four parallel entries as main branch entries, thus having two intakes and two returns.

Butt entries, panel entries, or panel slopes may have two parallel entries only where they are comparatively short, say, less than 1,200 feet long.

In pitching beds, panel-slopes to the dip and narrow rooms on the strike (that is, level) afford the best arrangement for ventilation, because the return air current can always be ascending. This layout is required in France, Germany, and Belgium, because methane, which is lighter than air, will be swept away by the