305
AIRS.
for prisoners to cook individually for themselves would be
superseded, much to their advantage.
On the other hand, it must also be observed, that if close
stoves acting on this principle were adopted, Count Rum
ford’s objections to the introduction of fresh air would be
obviated with regard to any room in which they should be
m action, provided the opening through which it entered
was made on a level with the ceiling.
Air entering at this level would in the absence of open
fires be acted upon by no other draft than the mouth of the
funnel in the ceiling, and could not descend in currents to
the lower region of the room.
In a room so filled with company as to vitiate the air
within it, the atmospheric air entering being specifically
heavier would indeed descend and be replacedi by the
ascending impure air, but as it would not descend by a
stronger impulse than its difference of specific weight, it must
be slow in its motion, and would produce no sensible current.
Ventilation of Mines.
The ventilation of mines is founded, in general, on the
same principles as those of buildings.
The air chauged by the respiration of the labourers, their
fires and lights, is allowed to pass off by pits and galleries
made for that purpose, and which, for the most.part, serve
also for the passage of men and the produce of the mines.
When a sufficient velocity of the air can be obtained by
these ordinary means, the current is increased by making a
fire at the bottom of the pit, or at the top, to cause a strong
draught of air through it. Or the air is drawn out or driven
in by machinery contrived for that purpose, and having
channels made of boards reaching to the place where the
men are at work.
Besides the change produced in air by the respiration of
the miners and their lights, some subterraneous works are
infested with gases of a different nature, which must be
got rid of before the works can be extended.
The first and most common of these gases is the choke
damp, or carbonic acid gas, which, being heavier than air,
lies at the bottom of the works, and instantly drowns those
that enter into it. This damp is frequently met with in opening
old dried-up wells, and even on the floors of cellars which
are seldom opened. Fortunately, this gas is very quickly
absorbed by water, and it soon mixes with the air, and
there is no danger to be apprehended out of its immediate