The proportion of methane in the air in a mine depends not so much on the amount given off by the coal as on the volume of air used for ventilating the workings. Therefore for comparisons it is best to consider the volume of methane in a unit of time. This depends upon the amount of coal mined hut with notable variations from place to place and in different mines and districts. There has been consider able discussions as to what constitutes a gas eous mine but most authorities agree that 2 per cent of methane in the return air places the mine well within the category, especially if there is liability of a sudden increase. In certain mines in the northern anthracite basin of Pennsylvania with total discharge of 2,000 to 3,000 cubic feet of methane a minute, the per centage approaches 2. The return air from a group of mines in Prussia carried from .13 to 1.5 per cent of methane and some of the more gaseous ones gave off 256,500 to 897,600 cubic feet of that gas a day. Some Austrian mines carried from .68 to 1.90 per cent of methane in their total returns, Russian mines .13 to 2.92 per cent and mines in Scotland .05 to .5 per cent.
Mode of Entry.— Gas escapes from coal mostly through pores and minute fissures but in places °feeders* or "blowers" occur which yield large volumes of gas. These are not al ways in the most gaseous mines. A persistent blower in the Pelham mine in England in 1847 for a while discharged 47,000 cubic feet a minute. Some blowers continue for many years. The gas is mostly methane with 5 to 20 per cent of nitrogen and a small amount of carbon di oxide. In a few mines in Europe blowers of carbon dioxide occur. Sometimes in the course of mining and especially through "runs" or "falls" of coal, "squeezes" or roof caving, large bodies of gas come suddenly .into a mine and such ((outbursts') have caused some very serious explosions. In places gas accumulates in pockets and this is rapidly liberated when large volumes of coal are shattered. Many runs of coal are caused by gas pressure. An outburst in a mine six miles west of Pottsville, Pa., liberated a body of gas estimated at 50,000 to 200,000 cubic feet; at Agrappe Colliery, Belgium, in 1879 an outburst of 85,000 cubic feet caused the death of 121 miners; at •esseges, Belgium, in 1890 an outburst of 28,000 cubic feet exploded killing 131 miners, the outflow continuing 12 hours. At Morrissey in British Columbia in 1904 an outburst threw out 3,500 tons of coal with emission of about 5,000,000 cubic feet of gas. At Abercarne mine in south Wales the face blew out and about 1,000,000 cubic feet of gas were emitted. The great squeezes at Pittston, Pa., in 1904 and at Warrior Run, near Wilkes Barre, Pa., two years later, gave off vast vol umes of explosive gas. It is estimated that 357 of the explosions in Belgium mines from 1850 to 1908 with large fatalities were due to gas. Doubtless some of the great dust explosions in coal mines have been started by ignition of a gas outburst.
The amount of methane given off in mining coal differs greatly in various regions and mines and parts of mines. In 23 mines in the Saarbrucken Basin, Germany, it ranged from 16 to 1,060 cubic feet to the ton of coal mined or from to 30 times the volume of the coal and in one case the volume of gas was 2,160 cubic feet to the ton (a ton of coal is about 20 cubic feet). The total ema nation from these 23 mines ranged from 8,870 to 732,566 cubic feet a day, with a yearly total of about two and one-quarter billion cubic feet or 44,000 tons. Certain mines near Wilkes
Barre, Pa., producing about 2,000 tons of coal a day each, give off 2,000 to 3,000 cubic feet a minute or 1,500 to 2,500 cubic feet of methane to the ton of coal mined. Other mines in the same basin with equal output gave off only 15 to 75 cubic feet to the ton of coal produced.
In gaseous districts in Austria the methane emanation averaged 7,479 cubic feet to the ton of coal extracted or 370 cubic feet of the gas to a cubic foot of coal, and the average in the less gaseous mines was 26 cubic feet to the ton. The Prussian Commission found 30 to 120 cubic feet to the cubic foot of coal removed and in one district the amount was 14 cubic feet. It was found by Darton that in the north ern anthracite coal field of Pennsylvania the average amount was 140 cubic feet and in mines in the bituminous field of central Illinois the amount ranged from 14 to 10 cubic feet of gas to the cubic foot of coal removed. Generally the amount is less in old mines because much of the gas has escaped. In most cases also the cessation of mining on Sundays and other times showed material diminution in the gas emanation. Some comparisons made after a shut down of one month caused by a strike in Pennsylvania and Illinois mines showed a marked decrease in parts of most mines but in a few places there was no change or a slight increase. The general decrease in a group of Illinois mines was 32 per cent.
The gas confined in coal beds must exert a pressure proportionate to its rela tive volume and•as the volume is variable the pressure varies accordingly. Observations made with tubes sunk in the coal faces in various mines showed pressures from almost inappreci able amounts to several hundred pounds to the square inch. The cause of the variation is difficult to understand. Pressure is manifested in many places by the disposition of the coal to break out of the face in mining and often men are killed by this cause. Naturally, how ever, some of the evident pressure in the face is due to stress from the roof and other sources. The pressure observations gave varied results. In a group of English mines the pressures ranged from 28 to 461 pounds to the square inch, in Belgium mines the range was 187 to 555 pounds, in Lievin, France, 64 to 96 pounds while at Saint Etienne it ranged from to 44 pounds and in Austria 82 to 142 pounds. In a mine near Wilkes-Barre, Pa., a pressure of 45 pounds was observed and in several mines in central Illinois the pressure ranged from a few ounces to 33 pounds. At most places where the pressures were notable several days were required for the maximum amount to be mani fested in the gages used in the tests. The pres sure shown was seldom closely related to the amount of gas escaping. In a given locality the highest pressure was usually found in the deepest test holes and compactness of the coal has much influence because the gas escapes from the coal near the face and the pressure rapidly declines, especially in the more permeable coal. It is well known that considerable gas escapes through permeable rocks adjoining the coal, in some areas reaching the surface of the ground. Much gas also flows into the mine workings by this means.