f cages and engines. In such cases the engines may be placed in line on
opposite sides of the pit, or at right angles to each other. It is said
that the output of single shafts has been raised by this method to 3500
and 4500 tons in the double shift of sixteen hours. It is particularly
well suited to mines where groups of seams at different depths are
worked simultaneously. Some characteristic figures of the yield for
British collieries in 1898 are given below:--

Albion Colliery, South Wales 551,000 tons in a year for one shaft
and one engine.

Silksworth Colliery, 535,000 tons in a year for shaft
Northumberland 580 yds. deep, two engines.

Bolsover Colliery, Derby 598,798 tons in 279 days, shaft 365
yds. deep.

Denaby Main Colliery, Yorkshire 629,947 tons in 281 days, maximum per
day 2673 tons.

At Cadeby Main colliery near Doncaster in 1906, 3360 tons were drawn in
fourteen hours from one pit 763 yds. deep.

The tub when brought to the surface, after passing over a weigh-bridge
where it is weighed and tallied by a weigher specially appointed for the
purpose by the men and the owner jointly, is run into a "tippler," a
cage turning about a horizontal axis which discharges the load in the
first half of the rotation and brings the tub back to the original
position in the second. It is then run back to the pit-bank to be loaded
into the cage at the return journey.

Coal as raised from the pit is now generally subjected to some final
process of classification and cleaning before being despatched to the
consumer. The nature and extent of these operations vary with the
character of the coal, which if hard and free from shale partings may be
finished by simple screening into large and nut sizes and smaller slack
or duff, with a final hand-picking to remove shale and dust from the
larger sizes. But when there is much small duff, with intermixed shale,
more elaborate sizing and washing plant becomes necessary. Where
hand-picking is done, the larger-sized coal, separated by 3-in. bar
screens, is spread out on a travelling band, which may be 300 ft. long
and from 3 to 5 wide, and carried past a line of pickers stationed along
one side, who take out and remove the waste as it passes by, leaving the
clean coal on the belt. The smaller duff is separated by vibrati