242 
pounds; of stale or bind, 160 pounds; and of Coal, 82 
pounds. Mr. Fowler calculates that the " Average weight of 
these in different localities may be taken at 144 pounds per 
" cubic foot of the Coal measures, which represents a pressure 
"of one pound per square inch for each vertical foot of 
"strata. A Coal seam, therefore, 1,000 feet deep, will have 
" a dead loading of 1,000 pounds per square inch upon it as 
" it lies untouched." 
I think these calculations to be as near as it is possible to 
arrive at, not forgetting that there would be an increased local 
pressure, varying according to the system of Coal mining 
employed; it would therefore be impossible to preserve any pil- 
lars of Coal, however large they might be, if once disturbed ; 
and even if it were possible to get such pillars, it would only 
be at a loss; hence "LongwaU" suggests the removing of this 
pressure from the Coal on to a well constructed artificial 
goaf, built as the workings proceed. 
The writer is convinced that "LongwalF' will pave the way 
for a revolution in the detail of Coal mining, which must 
come sooner or later, viz., the extensive use of machinery 
in Coal getting. The public have before them at the present 
time, a great variety of inventions, aiming to do this success- 
fully : some arranged for baring, and some for breaking down 
the Coal, and some aspiring to do both operations. The motive 
power employed is often hydraulic, and in some instances 
compressed air. Mr. Grrafton Jones's breaking-down ma- 
chine, and the machines of the West Ardsley Coal Company, 
are the most successful representations of such inventions that 
have appeared up to the present time, but as Mr. Jones's 
machine is only a substitute in most instances for the use of 
powder, and although it may be very valuable in some seams 
as a mechanical wedge for breaking down the Coal where the 
use of powder damages the same, or creates danger by the 
probable existence of fire-damp, stiU the baring of the Coal 
