ments for this purpose vary, of course, with the amount
of work to be done with one fixing of the machinery; where it is likely
to be used for a considerable time, the drum and brake are solidly
constructed, and the ropes of steel or iron wire carefully guided over
friction rollers, placed at intervals between the rails to prevent them
from chafing and wearing out on the ground. Where the load has to be
hauled up a rising gradient, underground engines, driven by steam or
compressed air or electric motors, are used. In some cases steam
generated in boilers at the surface is carried in pipes to the engines
below, but there is less loss of power when compressed air is sent down
in the same way. Underground boilers placed near the up-cast pit so that
the smoke and gases help the ventilating furnace have been largely used
but are now less favourably regarded than formerly. Water-pressure
engines, driven by a column of water equal to the depth of the pit, have
also been employed for hauling. These can, however, only be used
advantageously where there are fixed pumps, the fall of water generating
the power resulting in a load to be removed by the expenditure of an
equivalent amount of power in the pumping engine above that necessary
for keeping down the mine water.

The principal methods in which power can be applied to underground
traction are as follows:--

1. Tail rope system.
2. Endless chain system.
3. Endless rope system on the ground.
4. Endless rope system overhead.

The three last may be considered as modifications of the same principle.
In the first, which is that generally used in Northumberland and Durham,
a single line of rails is used, the loaded tubs being drawn "out bye,"
i.e. towards the shaft, and the empty ones returned "in bye," or towards
the working faces, by reversing the engine; while in the other systems,
double lines, with the rope travelling continuously in the same
direction, are the rule. On the tail rope plan the engine has two drums
worked by spur gearing, which can be connected with, or cast loose from,
the driving shaft at pleasure. The main rope, which draws out the loaded
tubs, coils upon one drum, and passes near the floor over guide sheaves
placed about 20 ft. apart. The tail rope, which is of lighter section
than the main one, is coiled on the second drum, passes over similar
guide sheaves placed near the roof or side of the gallery round a pulley
at the bottom of the plane, an