155 
within a short time is often greater ; and they take for a 
basis the Haswell goaf of thirteen acres, instead of the 
Felling, or the Jarrow Bensham seam, of 100 acres each. 
More than all, they assume the goaf mixture to be itself 
simply explosive, instead of being capable of rendering 
explosive many times its own bulk of previously pure air. 
This latter must more often be the case. 
Keeping in view these facts, to ‘‘ sweep the sides” of a 
goaf, or carry off what “ oozes” from it, may carry an 
explosive mixture to naked lights at a distance from the 
goaf, but cannot prevent its formation, or its discharge. 
And to pick up every stone from the floor, as held out at 
Jarrow, incomprehensible as it seems — or to pierce the 
fault bounding the south waste at the Oaks — would leave 
the ffoaf cavities untouched. It is not clear at what levels 
the permanent windways over the gobs” at Risca* are 
placed. If they are at the top of the highest cavity in 
that weak shale, the first part of Faraday and Lyall’s 
recommendation is effected ; it is, therefore, practicable. 
What is further wanting is to carry off the goaf air sepa- 
rately, instead of into the air courses of the workings. 
If they are only at the level of the roof before its breaking 
down, they also leave the hollows untouched. Sir H. De 
la Beche and Mr. Smyth say, The height to which it 
(the gas) might be mixed with the broken roof could not 
be estimated.”! 
As to sinking of the surface, I have heard persons 
engaged in coal mining converse on this matter as if, very 
soon after the last procurable coal was extracted, and the 
‘^juds” drawn, the roof settled down quietly, evenly, and 
entirely from the top, to meet the floor, or creeps in it, 
* Report, p. 57 . 
+ And Faraday and Lyall (p. 20) — How high the heap of broken strata, 
and the vaults inclosing it, extend, is not known in a large goaf, nor, as far as we 
are aware, even in a small one.” 
