20 Messrs. Faraday and Lyell on 



according to circumstances, a fall being sometimes many tons 

 in weight. The pillars are not removed indifferently, but 

 those next the mass of ruins already formed are taken away 

 first, so that the first jud which is drawn produces a heap of 

 broken strata, and this increasing with every succeeding jud 

 that is removed, forms the goaf. 



These goafs grow to a great size*. There are three in the 

 Haswell Little Pit; two are small as yet; the largest has an 

 extent of 13 acres. At the edges they are very loose and open, 

 having accidental cavities and passages for air running into 

 them, as might be expected from the falling of rocks Irom a 

 height of five feet one upon another. There is every reason 

 to believe, that the falling goes on towards the middle of the 

 goaf, but how high the heap of broken strata and the vault in- 

 closing it extend, is not known in a large goaf, or, as far as 

 we are aware, even in a small one. The goaf may be consi- 

 dered as a heap of rocky fragments rising up into the vault or 

 cavity from which it has fallen, perhaps nearly compact in the 

 parts which are the oldest, lowest, and nearest the middle, 

 but open in structure towards and near its surface, whether at 

 the centre of the goaf or at the edges; and the vault or con- 

 cavity of the goaf may be considered as an inverted basin, ha- 

 ving its edge coincident with the roof of the mine, all round 

 the goaf. Within this basin there must be air space (as long 

 as the surface of the country above has not sunk), either in 

 the space between it and the goaf, or in the cavities of the goaf 

 itself, nearly equal to the bulk of coal removed ; this in a goaf 

 of 13 acres, and a seam in which 5^- feet of coal, including the 

 top and bottom, are taken away, is equal to a vault 5| feet 

 high and 13 acres in extent. 



Let us now consider this goaf as a receptacle for gas or fire- 

 damp, a compound of hydrogen and carbon, known as light 

 hydro-carbonate, and by other names. The weight of pure 

 fire-damp is little more than half that of air; it gradually and 

 spontaneously mixes with air, and the weight of any mixture 

 is proportionate to the quantities of air and fire-damp. Any 

 gas that may be evolved in the goaf, or that may gradually 

 creep into it along the roof of the workings, against which it 

 will naturally flow, will ascend into the goaf vault, and will 



* Goafs vary in size: tliat at the Meadows Flat Little Pit is 13 acres; 

 the goaf of the High Brockley Whin is 1^ acre, and the one at the Low 

 Brockley Whin li acre. In the North Way of the Little Pit at Haswell, 

 there is a goaf of 35 acres, and in the Engine or Great Pit, one of 17 acres. 

 Perhaps the greatest goaf is that at Fuelling, near Newcastle-upon-Tyne ; it 

 is in the same seam as the Haswell, and has an extent of upwards of 100 

 acres. 



