1066 VENTILATION OF MIKES 



ing from 25 to 30 feet in thickness, are well worthy of consideration ; since a solid 

 mass of that magnitude must be peculiarly difficult to drain of its imprisoned gas. In 

 excavating such coal large masses must be detached, and pockets or hollows must be 

 formed, which are immediately filled with carburetted hydrogen ; whilst a thin rein, 

 for which a level roof can be generally secured, can be kept tolerably free from such 

 accumulations. 



According to the ordinary system adopted in the collieries of the South Staffordshire 

 district, two shafts are sunk, near together, about 7 to 7J feet in diameter, each to the 

 bottom of the coal, say about 180 yards depth, the two shafts commencing at the same 

 level, and terminating at the same level. One of these becomes the ' downcast pit ' 

 down which the air descends, and the other the ' upcast pit' up which the air ascends, 

 when a communication is made between them at the bottom ; but the only determining 

 causes for the motion of the air being accidental, it is unknown beforehand what 

 direction the current will take, and which will become the downcast pit. It is always 

 found that a current of air does take place without any other means being employed ; 

 but the determining power is so faint, that, issuing from the upcast pic with such 

 trifling velocity, it is liable to be deranged by the action of the wind, or by atmospheric 

 changes ; and it sometimes happens that the air becomes quiescent, or an unsteady 

 column, alternately ascending and descending the same shaft ; and then, in miner's 

 language, the pits ' fight,' and the air will neither ascend nor descend with regularity 

 in one direction. 



When the two pits are sunk down through the stratum of coal 30 ft. in thickness, a 

 ' gate-road ' or horse-way is next driven in the bottom of the coal, from 8 to 9 ft. high, 

 and about the same width, commencing from the bottom of the downcast pit. 



At the same time an air-head is driven about the middle of the coal, or 15 feet high 

 from the ' floor ' or the bottom of the coal, commencing from the downcast pit. The 

 gate-road and air-head are then driven in parallel lines, at the same level upon which 

 they commence, for the distance of 100 to 500 yards, or more, according to the quantity 

 of coal intended to be cleared by the pits. 



A series of ' spouts ' or openings are driven upwards from the gate-road into the 

 air-head, at intervals of 10 or 15 yards to carry off the gas formed, and produce a 

 current of air for the workmen, each spout being closed up when a new one is made 

 in advance. The excavation of the whole thickness of the stratum of coal, 30 feet 

 thick, is then proceeded with, by opening right and left from the end of the gate-road, 

 and excavating a ' side of work,' which forms a rectangular cavity, say about 90 yards 

 long by 50 yards wide, or about an acre, the whole of the coal being taken away as 

 far as practicable, excepting the pillars of coal (generally 10 yards square and 10 yards 

 distant from each other) which are left to support the superincumbent strata. 



The air descending the downcast pit, and travelling along the gate-road into the 

 workings, ascends to the air-head, and traversing that, ascends the upcast pit, carrying 

 with it the gas and impure vapours, as far as such imperfect and interrupted means 

 will effect, and delivering them into the open air. 



By this plan the mine is ventilated, until the lower 15 feet of the coal is excavated; 

 but where the whole thickness of the coal above the air-head has been removed, by 

 undergoing the coal from the bottom, and dropping it down in large masses, the upper 

 portion of the cavity, being above the level of the air-head, forms a reservoir for gas, 

 which gradually accumulates, and has no means of escape, a reservoir of the capacity 

 of some hundred thousand of cubic feet, which may be wholly or in part occupied by 

 gas. An accidental change in the direction of the current of air would turn the course 

 of the air along the air-head into this reservoir of gas, and from thence into the gate- 

 road, and render an explosion very probable. After the coal is extracted, a solid wall 

 or 'rib' of coal, from 6 to 10 yards thick, which is commonly termed a 'fire-rib,' is left 

 all round the chamber, separating it from the next workings ; and the entrance from 

 the gate-road is securely walled up, to exclude the air, and prevent spontaneous com- 

 bustion, which would otherwise, in a short period, take place. When an explosion 

 occurs, it is generally followed by a second, or more, as portions of the gas become 

 successively charged with the due proportions of air ; and the liability to these terrible 

 explosions will always remain in mines thus worked, till, by some efficient means, the 

 gas can be allowed a continuous escape, and a current of air can be ensured to move 

 always in one direction, with sufficient power to overcome all extraneous disturbing 

 forces, either of the wind or any atmospheric changes. 



In fig. 2082 the system adopted and carried into operation by the late Benjamin Gib- 

 bons is shown. One pit a, is sunk, instead of two ; and in the side of the shall a smaller 

 shaft b is cut, to form an 'air-chimney,' and is afterwards separated from the main 

 shaft ; this air-chimney is circular, and may be made about three feet diameter inside, 

 or more, as may be required. The air-chimney is bricked at the same time with the 



