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 certain, 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.