to the security of colliers.

Underground conveyance.

The removal of the coal broken at the working face to the pit bottom may
in small mines be effected by hand labour, but more generally it is done
by horse or mechanical traction, upon railways, the "trams" or "tubs,"
as the pit wagons are called, being where possible brought up to the
face. In steeply inclined seams passes or shoots leading to the main
level below are sometimes used, and in Belgium iron plates are sometimes
laid in the excavated ground to form a slide for the coal down to the
loading place. In some instances travelling belts or creepers have been
adopted, which deliver the coal with a reduced amount of breakage, but
this application is not common. The capacity of the trams varies with
the size of the workings and the shaft. From 5 to 7 cwt. are common
sizes, but in South Wales they are larger, carrying up to one ton or
more. The rails used are of flat bottomed or bridge section varying in
weight from 15 to 25 lb to the yd.; they are laid upon cross sleepers in
a temporary manner, so that they can be easily shifted along the working
faces, but are carefully secured along main roads intended to carry
traffic continuously for some time. The arrangement of the roads at the
face is shown in the plan, fig. 10. In the main roads to the pit when
the distance is not considerable horse traction may be used, a train of
6 to 15 vehicles being drawn by one horse, but more generally the
hauling or, as it is called in the north of England, the leading of the
trains of tubs is effected by mechanical traction.

In a large colliery where the shafts are situated near the centre of the
field, and the workings extend on all sides, both to the dip and rise,
the drawing roads for the coal may be of three different kinds--(1)
levels driven at right angles to the dip, suitable for horse roads, (2)
rise ways, known as jinny roads, jig-brows, or up-brows, which, when of
sufficient slope, may be used as self-acting planes, i.e. the loaded
waggons may be made to pull back the empty ones to the working faces,
and (3) dip or down-brows, requiring engine power. A road may be used as
a self-acting or gravitating incline when the gradient is 1 in 30 or
steeper, in which case the train is lowered by a rope passing over a
pulley or brake drum at the upper end, the return empty train being
attached to the opposite end of the rope and hauled up by the descending
load. The arrange