n, though slight, movement of the air in the two
columns which constitute the upcast and the downcast shafts, but in order
that a current may flow which shall be equal to the necessities of the
miners, some means are necessary, by which this condition of almost
equilibrium shall be considerably disturbed, and a current created which
shall sweep all foul gases before it. One plan was to force fresh air
into the downcast, which should in a sense push the foetid air away by
the upcast. Another was to exhaust the upcast, and so draw the gases in
the train of the exhausted air. In other cases the plan was adopted of
providing a continual falling of water down the downcast shaft.

These various plans have almost all given way to that which is the most
serviceable of all, namely, the plan of having an immense furnace
constantly burning in a specially-constructed chamber at the bottom of
the upcast. By this means the column of air above it becomes rarefied
under the heat, and ascends, whilst the cooler air from the downcast
rushes in and spreads itself in all directions whence the bad air has
already been drawn. On the other hand, to so great a state of perfection
have ventilating fans been brought, that one was recently erected which
would be capable of changing the air of Westminster Hall thirty times in
one hour.

Having procured a current of sufficient power, it will be at once
understood that, if left to its own will, it would take the nearest path
which might lie between its entrance and its exit, and, in this way,
ventilating the principal street only, would leave all the many
off-shoots from it undisturbed. It is consequently manipulated by means
of barriers and tight-fitting doors, in such a way that the current is
bound in turn to traverse every portion of the mine. A large number of
boys, known as trappers, are employed in opening the doors to all comers,
and in carefully closing the doors immediately after they have passed, in
order that the current may not circulate through passages along which it
is not intended that it should pass.

The greatest dangers which await the miners are those which result, in
the form of terrible explosions, from the presence of inflammable gases
in the mines. The great walls of coal which bound the passages in mines
are constantly exuding supplies of gas into the air. When a bank of coal
is brought down by an artificial explosion, by dynamite, by lime
cartridges, or by some other age