THE FIRE DAMP. 167 



towers which frequently fringed the loftiest rocks, on one side was to be 

 seen a cultivated expanse covered with numberless habitations, and on 

 the other all the \\ildness of the desert, that seemed never to have been 

 trod by human footsteps, but abounded with all sorts of wild beasts. 

 The view from the wall itself must be equally imposing as it traverses one 

 vast plain after another, and strides over lofty mountains its numerous 

 towers here entire, and there falling into ruins, the sides of the walls here 

 free and open, there overgrown with creeping plants and garlanded with 

 hardy trees, that shoot from their interstices, or that spring from their 

 base; the whole, to appearance, stretching out as if it were to girdle th 

 globe, or as if it had no end. 



THE FIRE DAMP. 



This is the most frigh tful accident to which coal mines are exposed ; it is the 

 most frequent, and by far the most calamitous, and is thus caused : All 

 coal, even in the charcoal-like variety, called Anthracite, appears to 

 contain in its natural state, 'while underground, a considerable quantity 

 of free uncombined gas, which it parts with when exposed to the air, or 

 when it is relieved from great superincumbent pressure. The gas is evolved 

 from the coal in great quantity at the ordinary temperature of the mines; and 

 instances have been known of explosions on board ships laden with 

 fresh worked coals. Coals lying deep give out more gas than those near 

 the surface, because there are openings at the top by which it escapes ; 

 but in the deep mines it cannot have such an outlet, and therefore it 

 accumulates in all the fissures of the stone above the coal ; and this sort 

 of natural distillation is constantly going on. The fissures of the roof are 

 in some places very great, and there are sometimes miles of communica- 

 tion from one fissure to another : they may be considered as natural 

 gasometers, and having no outlet, and as the process of distillation is 

 constantly going on, the gas becomes accumulated in them in a very 

 highly condensed state, the degree of condensation depending on the 

 thickness of the surrounding rock and quantity poured in. In the course 

 of pursuing the workings, the miners sometimes cut across one of these 

 fissures, or approach so near to it, that the intervening rock becomes too 

 weak to resist the elastic force of the compressed gas; it gives way, and 

 then, in either case, the gas rushes out with immense force. These 

 blowers, as they are called, emit sometimes as much as seven hundred 

 hogsheads of gas in a minute, and continue in a state of activity for 



