Explosions in Coal Mines. 21 



find its place higher, in proportion to its freedom from air; 

 and this will go ort continually, the goaf vault forming the 

 natural basin into which all gas will drain (upwards), from 

 parts inclining to the gonf, just as a concavity on the side of 

 a gentle hill will receive water draining downwards from its 

 sides, and from the parts above inclining towards it. 



The gas which may thus be expected to collect in the goaf 

 of a mine, where there is any fire-damp at all, cannot escape 

 at the top of the goaf vault ; instead of passing away there, the 

 whole surface of the vault may rather be viewed as having a 

 tendency, more or less, to evolve gas from the upper broken 

 and bruised coal measures (often containing small seams not 

 worked) into the space beneath ; and the only escape for the 

 gas is by the flowing of it under the edge of the goaf vault into 

 the workings of the mine. Two main circumstances tend to 

 this effect; the one, the continued accumulation of gas in the 

 Upper part of tiie goaf vault; the other, the continual tendency 

 to mix with the air beneath, and consequent formation of mix- 

 tures larger and heavier than the gas itself. As Sir Humphry 

 Davy has stated, any mixture containing from one-fifth to 

 one-sixteenth of the gas will explode. These mixtures are of 

 course from six to seventeen times greater in volume than the 

 fire-damp in them, and evidently not much lighter than air 

 (0"91 and 0*96). Except, therefore, in the almost impossible 

 case of a goaf quite filled with fire-damp, it will be these or 

 weaker mixtures that underflow the edge of the vault, unless 

 upon extraordinary occasions. 



The underflow will not take place all round the edge of the 

 goaf basin, but at that point which is highest ; for just as water 

 takes its level in a pond on the side of a hill, and flows over 

 the lower edge, so here, air strata of equal density will be ho- 

 rizontal. Coal seams are rarely quite horizontal ; in the Little 

 Haswell Pit, the rise is about 1 in 24, and the coal very re- 

 gular. At the lower edge of such a goaf, nothing but pure 

 air might be present in the air space, and also for a consider- 

 able distance up into the vault; yet at the upper edge, a mix- 

 ture of gas and air, and even a highly explosive mixture might 

 be escaping. 



Thus goafs are evidently, in mines subject more or less to 

 fire-damp, reservoirs of the gas and explosive mixtures ; giving 

 out their gas into the workings of the mines by a gradual un- 

 derflow in smaller or larger quantities under ordinary circum- 

 stances, or suddenly, and in great proportion on extraordinary 

 occasions: and they may either supply that explosive mixture 

 which first takes fire, as appears to have been the case at the 

 spot called Williamson's jW, close to the goaf of the Mea- 



