Accident from compression of the Air. 39 
arclied cavities. 'The whole mine is commonly sunk to one base, 
namely, just above the level where water would naturally stand. But, 
when by its natural situation, or by artificial draming, the mine is 
carried very deep, and the stratum of coal is thick and extensive, a 
second tummelbau is made under the other, after the former has be-+ 
come exhausted. 
In the mine there are two apertures, one of them for the passage 
to and from it, and the other, called the wind shaft, for ventilation. 
These two shafts at the bottom are united by a passage, connecting 
them at right angles. Upon this passage the mining is begun by form- 
ing a tummel, which is arched above like a bee hive, and its bottom 
is continued on the level of the passage between the two shafts. A 
tummel is commonly from three to six fathoms in diameter, and from 
two to five in height. The coal itself serves to support the highest 
part of the tummel, because, by the pressure of its sides it settles of 
itself to a certain extent, the excavation still preserving its arched 
form. When the tummel is carried so high as to reach the top of 
the layer of coal, or the level of the tummel is itself reached by a 
second mine that has been sunk under it, by degrees, it commonly 
breaks and becomes filled, by which means, the walls or parts subject 
to the pressure, are again brought together and consolidated. By 
the filling up of the tummel, many funnel shaped apertures on the 
surface are formed. A second or third tummel can at any time be 
formed upon a passage by removing the surrounding coal, or by ma- 
king a new excavation; and in the present instance, upon the right 
and left of the main excavation, and at the shafts, new passages had 
been formed, and new tummels had appeared, so that the mine was 
continually enlarging, and receding farther backwards. Finally, near 
the wind shaft, and likewise by the main tummel, pillars were erected 
to make the works as strong as possible. A new tummel had been 
sunk below the passage between the two shafts, and the shafts them- 
selves were continued down to the level of its base, and a passage 
formed for connecting them. ‘The old tummel did not fill again as 
is common after a new one is formed below, because from orte third 
to one fifth of the superincumbent coal had been left for support, in 
consequence of the extreme caution of the workmen. But it was 
difficult to secure it altogether, as the coal was very light in some 
places, and the timber was small as a matter of economy, in conse- 
sequence of the moderate profits of the mine. 
