ng or
rotating screens into a great number of sizes, which are cleaned by
washing in continuous current or pulsating jigging machines, where the
lighter coal rises to the surface and is removed by a stream of water,
while the heavier waste falls and is discharged at a lower level, or
through a valve at the bottom of the machine. The larger or "nut" sizes,
from 1/4 in. upwards, are washed on plain sieve plates, but for
finer-grained duff the sieve is covered with a bed of broken felspar
lumps about 3 in. thick, forming a kind of filter, through which the
fine dirt passes to the bottom of the hutch. The cleaned coal is carried
by a stream of water to a bucket elevator and delivered to the storage
bunkers, or both water and coal may be lifted by a centrifugal pump into
a large cylindrical tank, where the water drains away, leaving the coal
sufficiently dry for use. Modern screening and washing plants,
especially when the small coal forms a considerable proportion of the
output, are large and costly, requiring machinery of a capacity of 100
to 150 tons per hour, which absorbs 350 to 400 H.P. In this, as in many
other cases, electric motors supplied from a central station are now
preferred to separate steam-engines.

Anthracite coal in Pennsylvania is subjected to breaking between toothed
rollers and an elaborate system of screening, before it is fit for sale.
The largest or lump coal is that which remains upon a riddle having the
bars 4 in. apart; the second, or steamboat coal, is above 3 in.; broken
coal includes sizes above 2-1/2 or 2-3/4 in.; egg coal, pieces above
2-1/4 in. sq.; large stove coal, 1-3/4 in.; small stove, 1 to 1-1/2 or
1-1/3 in.; chestnut coal, 2/3 to 3/4 in.; pea coal, 1/2 in.; and buckwheat
coal, 1/3 in. The most valuable of these are the egg and stove sizes,
which are broken to the proper dimensions for household use, the larger
lumps being unfit for burning in open fire-places. In South Wales a
somewhat similar treatment is now adopted in the anthracite districts.

Depth of working.

With the increased activity of working characteristic of modern coal
mining, the depth of the mines has rapidly increased, and at the present
time the level of 4000 ft., formerly assumed as the possible limit for
working, has been nearly attained. The following list gives the depths
reached in the deepest collieries in Europe in 1900, from which it will
be seen that the larger number, as well as the deepest, are in