ings away from the main circuits, which was largely advocated at one
time, has lost its popularity, but a useful substitute has been found in
the induced draught produced by jets of compressed air or high-pressure
water blowing into ejectors. With a jet of 1/200 in. area, a pipe
discharging 1-2/3 gallon of water per minute at 165 lb pressure per sq.
in., a circulation of 850 cub. ft. of air per minute was produced at the
end of a level, or about five times that obtained from an equal volume
of air at 60 lb pressure. The increased resistance, due to the large
extension of workings from single pairs of shafts, the ventilating
currents having often to travel several miles to the upcast, has led to
great increase in the size and power of ventilating fans, and engines
from 250 to 500 H.P. are not uncommonly used for such purposes.

Lighting.

The lighting of underground workings in collieries is closely connected
with the subject of ventilation. In many of the smaller pits in the
Midland districts of England, and generally in South Staffordshire, the
coals are sufficiently free from gas, or rather the gases are not liable
to become explosive when mixed with air, to allow the use of naked
lights, candles being generally used. Oil lamps are employed in many of
the Scotch collieries, and are almost universally used in Belgium and
other European countries. The buildings near the pit bottom, such as the
stables and lamp cabin, and even the main roads for some distance, are
often in large collieries lighted with gas brought from the surface, or
in some cases the gas given off by the coal is used for the same
purpose. Where the gases are fiery, the use of protected lights or
safety lamps (q.v.) becomes a necessity.

Composition of gas evolved by coal.

The nature of the gases evolved by coal when freshly exposed to the
atmosphere has been investigated by several chemists, more particularly
by Lyon Playfair and Ernst von Meyer. The latter observer found the
gases given off by coal from the district of Newcastle and Durham to
contain carbonic acid, marsh gas or light carburetted hydrogen (the
fire-damp of the miner), oxygen and nitrogen. A later investigation, by
J. W. Thomas, of the gases dissolved or occluded in coals from South
Wales basin shows them to vary considerably with the class of coal. The
results given below, which are selected from a much larger series
published in the _Journal of the Chemical Society_, were