3 j O.N THE HEAT EVOLVED 



or, in other cases, to drop the fire into a hole, or chimney, lei'f 

 in the centre of the pile which extends to the bottom, or ground : 

 and by giving air holes at the sides of the pit, to use the lan- 

 guage of the colliers, the fire is said to be " drawn to the sides 

 of the pit." 



It is very true, that the fire does eventually extend to the 

 sides of the pit ; but a much more uniform and speedy process, 

 and by which less loss would be sustained, would be to place 

 the fire in the first instance in a number of holes at the sides, 

 near the bottom, leaving an opening at the top by which the 

 heat generated at the sides would be communicated to the 

 wood in the interior, and facilitate the uniform ignition of the 

 whole mass, and the moment this is effected, let the holes at 

 the sides be closed, and that at the top may be lessened, but 

 should not be wholly closed, until the extrication of hydrogen 

 gas has nearly ceased, which, from its prodigious expansion, 

 sometimes bursts the pits, and as this generally occurs when 

 the wood is well covered, and sometimes produces very inju- 

 rious effects, by firing the adjacent woods, (as the column of 

 flame has been known to extend from twenty to thirty feet,) 

 it has probably led many colliers into the belief that the proper 

 remedy is to give the wood a slight covering, by which nu- 

 merous escapes are allowed for the gas ; but in effecting this 

 object, as the holes at the sides are left open, a very strong 

 current is produced through the pit by the slight covering, 

 and another evil is produced, that of burning through the sides 

 of the pit. 



In those instances where pits have been known to burst, 

 when well covered, the cause may probably be traced to having 

 closed the chimney at the top too soon, this being generally 

 done in about fifteen minutes, and having left those open at 

 the sides too long, as the gas will make its escape in some 

 manner, which should be provided for, and this provision is as 

 necessary to a coal pit, as the safety valve is to a steam boiler. 



Both the objections which have been alhided to against piling 

 the wood two or three tiers high, may in part be remedied by 

 changing the manner of igniting the wood as proposed, and if 



