AIRS. 
367 
direction, but that in which it is necessary it should traverse. 
in order to sweep, as it were, in its course, every avenue 
and corner of the mine. The current of air which enters thé 
downcast shaft has thus sometimes been forced to travel twen 
ty-seven miles in an area of only six hundred yards square 
a distance which it will readily be supposed requires a con 
siderable power to force an effective current through its 
whole extent ; in fact, it seldom happens that the velocity 
of this current exceeds two miles an hour. 
From this it appears that the only method at present used 
for preventing accidents by explosion is, a mechanical ap 
plication of the atmospheric air to the removing or sweep 
ing away of the inflammable gas, as it is generated in the 
working of collieries. 
This method involves the dangerous expedient of the 
furnace, placed near the bottom of the upcast pit, which 
is supposed to produce the requisite current by the rarefac 
tion of the air within it. Over this furnace all the air of 
the mine, including of course all the hydrogen gas, mixed 
in various proportions with atmospheric air, is compelled 
to pass. 
Every coal-mine on working is confined to the depth of 
one stratum, or, in cases where the strata are thin, of two 
or more, so as not to exceed, in general, eight feet in depth 
from the roof to the floor of the mine. The roof and the 
floor, or thrill, as it is called, although they are nearly equi 
distant, are never perfectly horizontal, but incline or dip 
from that position in different angles. From this last cir 
cumstance it is evident that one board will perform a de 
scending course whilst the next ascends. Thus, alternately 
changing its direction at every turn, and generally leaving 
a gradually accumulating portion of inflammable gas, or 
fire damp, at the commencement of every descent, mixed 
with or diluted by atmospheric air, but thus rendered more 
and more susceptible of explosion on the approach of flame. 
In Felling colliery, for instance, a colliery in which several 
dreadful accidents have occurred, causing the loss of nume 
rous lives, the fire-damp is calculated to descend, in its ser 
pentine course, not less than two hundred and fifty feet 
perpendicular height, in each revolution of the air through 
the boards andheadways. Now as this gas is so much lighter 
than common air as to float upon the latter as oil does 
upon water, it is evident that the fire-damp will accumulate 
in the higher part of every turn or winding of its course, pre 
cisely in the same way as the air which lodges in the upper