s. The rope, which
is guided upon sheaves between the rails, is taken twice round the head
pulley. It is also customary to use a stretching pulley to keep the rope
strained when the pull of the load diminishes. This is done by passing a
loop at the upper end round a pulley mounted in a travelling frame, to
which is attached a weight of about 15 cwt. hanging by a chain. This
weight pulls directly against the rope; so if the latter slacks, the
weight pulls out the pulley frame and tightens it up again. The tubs are
usually formed into sets of from 2 to 12, the front one being coupled up
by a short length of chain to a clamping hook formed of two jaws moulded
to the curve of the rope which are attached by the "run rider," as the
driver accompanying the train is called. This system in many respects
resembles the tail rope, but has the advantage of working with one-third
less length of rope for the same length of way.

The endless rope system overhead is substantially similar to the endless
chain. The wagons are attached at intervals by short lengths of chain
lapped twice round the rope and hooked into one of the links, or in some
cases the chains are hooked into hempen loops on the main rope. In mines
that are worked from the outcrop by adits or day levels traction by
locomotives driven by steam, compressed air or electricity is used to
some extent. The most numerous applications are in America.

Ventilation.

One of the most important branches of colliery work is the management of
the ventilation, involving as it does the supply of fresh air to the men
working in the pit, as well as the removal of inflammable gases that may
be given off by the coal. This is effected by carrying through the
workings a large volume of air which is kept continually moving in the
same direction, descending from the surface by one or more pits known as
intake or downcast pits, and leaving the mine by a return or upcast pit.
Such a circulation of air can only be effected by mechanical means when
the workings are of any extent, the methods actually adopted being--(1)
The rarefaction of the air in the upcast pit by a furnace placed at the
bottom; and (2) Exhaustion by machinery at the surface. The former plan,
being the older, has been most largely used, but is becoming replaced by
some form of machine.

The usual form of ventilating furnace is a plain fire grate placed under
an arch, and communicating with the upcast shaft by an inclined d