rift.
It is separated from the coal by a narrow passage walled and arched in
brickwork on both-sides. The size of the grate varies with the
requirements of the ventilation, but from 6 to 10 ft. broad and from 6
to 8 ft. long are usual dimensions. The fire should be kept as thin and
bright as possible, to reduce the amount of smoke in the upcast. When
the mine is free from gas, the furnace may be worked by the return air,
but it is better to take fresh air directly from the downcast by a
scale, or split, from the main current. The return air from fiery
workings is never allowed to approach the furnace, but is carried into
the upcast by a special channel, called a dumb drift, some distance
above the furnace drift, so as not to come in contact with the products
of combustion until they have been cooled below the igniting point of
fire-damp. Where the upcast pit is used for drawing coal, it is usual to
discharge the smoke and gases through a short lateral drift near the
surface into a tall chimney, so as to keep the pit-top as clear as
possible for working. Otherwise the chimney is built directly over the
mouth of the pit.

Mechanical ventilation may be effected either by direct exhaustion or
centrifugal displacement of the air to be removed. In the first method
reciprocating bells, or piston machines, or rotary machines of varying
capacity like gas-works exhausters, are employed. They were formerly
used on a very large scale in Belgium and South Wales, but the great
weight of the moving parts makes it impossible to drive them at the high
speed called for by modern requirements, so that centrifugal fans are
now generally adopted instead. An early and very successful machine of
this class, the Guibal fan, is represented in fig. 12. The fan has eight
arms, framed together of wrought iron bars, with diagonal struts, so as
to obtain rigidity with comparative lightness, carrying flat
close-boarded blades at their extremities. It revolves with the smallest
possible clearance in a chamber of masonry, one of the side walls being
perforated by a large round hole, through which the air from the mine is
admitted to the centre of the fan. The lower quadrant of the casing is
enlarged spirally, so as to leave a narrow rectangular opening at the
bottom, through which the air is discharged into a chimney of gradually
increasing section carried to a height of about 25 ft. The size of the
discharge aperture can be varied by means of a flexib