o keep out water which may be struck in passing through the
various strata, it is protected by plank or wood tubbing, or the shaft is
bricked over, or sometimes even cast-iron segments are sunk. In many
shafts which, owing to their great depth, pass through strata of every
degree of looseness or viscosity, all three methods are utilised in turn.
In Westphalia, where coal is worked beneath strata of more recent
geological age, narrow shafts have been, in many cases, sunk by means of
boring apparatus, in preference to the usual process of excavation, and
the practice has since been adopted in South Wales. In England the usual
form of the pit is circular, but elliptical and rectangular pits are also
in use. On the Continent polygonal-shaped shafts are not uncommon, all of
them, of whatever shape, being constructed with a view to resist the
great pressure exerted by the rock around.

[Illustration: FIG. 31.--Engine-House and Buildings at head of a
Coal-Pit.]

If there be one of these shafts at one end of the mine, and another at a
remote distance from it, a movement of the air will at once begin, and a
rough kind of ventilation will ensue. This is, however, quite
insufficient to provide the necessary quantity of air for inhalation by
the army of workers in the coal-mine, for the current thus set up does
not even provide sufficient force to remove the effete air and impurities
which accumulate from hundreds of perspiring human bodies.

It is therefore necessary to introduce some artificial means, by which a
strong and regular current shall pass down one shaft, through the mine in
all its workings, and out at the other shaft. This is accomplished in
various ways. It took many years before those interested in mines came
thoroughly to understand how properly to secure ventilation, and in
bygone days the system was so thoroughly bad that a tremendous amount of
sickness prevailed amongst the miners, owing to the poisonous effects of
breathing the same air over and over again, charged, as it was, with more
or less of the gases given off by the coal itself. Now, those miners who
do so great a part in furnishing the means of warming our houses in
winter, have the best contrivances which can be devised to furnish them
with an ever-flowing current of fresh air.

Amongst the various mechanical appliances which have been used to ensure
ventilation may be mentioned pumps, fans, and pneumatic screws. There is,
as we have said, a certai