296 



Messrs. Scott and Galloway on the Connexion [Apr. 18, 



The cause of this prolongation of the dangerous period is that when 

 fire-damp issues in greater quantity than usual from cavities and fissures 

 into the workings, and more especially into places where the air is stagnant 

 and already more or less foul by admixture of gas, the volume of the 

 explosive portion of this mixture will increase in consequence of the in- 

 creased rapidity of diffusion, or, in other words, the explosive boundary 

 will extend itself. This extension of the explosive boundary is gradual, 

 and in some cases a considerable time may elapse before the boundary 

 has reached its extreme limits and begins again to recede. During 

 all this period the mine will be in an abnormally dangerous state. 



Meanwhile, although the pressure of the atmosphere rises, and a current 

 sets in backwards* into the cavities whence the pure gas has just issued, 

 yet if the entrance to such a cavity be at a lower level than the highest 

 portion of the space occupied by such escaped gas, which rises, owing to 

 its low density, it is evident that what is driven back into the cavity will 

 be a mixture of gas and air, and that no portion of the gas which lies 

 above the level of the aperture to the cavity can be driven back into it. 

 Accordingly a certain volume of this pure gas remains, diffusing itself freely 

 and fouling the surrounding air. 



It is evident from these considerations that, in the case of continued un- 

 steadiness of pressure and repeated violent oscillations of the barometer, 

 we need not expect that each of these reductions of pressure will cause the 

 efflux of a quantity of gas proportionate to the extent of such reduction. 

 If the successive falls of the mercury are of less magnitude than the first, 

 or than any previous one in the series, the quantity of gas given off cannot 

 possibly be as great on each occasion as if that fall had been preceded by 

 a period of high pressure. If, however, any of the later oscillations be 

 more serious than their predecessors, a certain fresh supply of pure gas 

 will be given off. Hence we see that, as a general rule, we do not find a 

 succession of explosions at a time when the barometer is in a state of 

 continued violent oscillation. 



Recurring to what has already been said about temperature, we would 

 here remark that in cold weather the ventilation of the pits is exceedingly 

 active, many collieries being ventilated easily by natural means without any 

 extraneous agency whatever. In the height of summer, however, it is 

 different ; for then the temperature of the air in the downcast shaft is higher, 

 and the ventilation can only be kept up by the help of the furnace. 

 In some cases, then, a sudden rise of temperature may catch the miner un- 

 prepared, and where an active current would have remained safe, a sluggish 

 one may become foul, and possibly an explosion may occur. 



We shall now proceed to a discussion of the facts represented on 

 the Plate for the years 1868, 1869, 1870. 



* Parliamentary E\idence, 1855, p. 110. Transactions Nat. Hist. Soc. of Northum- 

 berland &c. 1831, p. 184. 



