338 THE DIAMOND MINES OF SOUTH AFRICA
Other Electric Service
Electric
bells are in use throughout the mines, and have very greatly promoted
the rapidity of hoisting through the shafts. Owing to instantaneous
communication between the man in charge of loading the skips and the
engine-driver, hunÂdreds of loads more are sent to the surface daily
than could be forwarded under the old " pull bell " system. For instant
additional communication between the surface and the underÂground work
telephones have been installed, and the same rapid communication
extends to the depositing floors, concentration works, and offices of
the company.
Natural Ventilation
The
Kimberley mine is ventilated in a somewhat peculiar manner. The rock
shafts at both De Beers and Kimberley mines are downcast, i.e., the
air for ventilation goes down these shafts, along the bottom tunnels
and thence up through the various levels, and it is fortunate for the
men working in the mine that it is so, for the cool air comes in at the
bottom and ventilates the mine much better than if the rock shaft drew
the heated air down through all the lower workings. The upcast in
Kimberley mine is through the Harvey shaft, the top of which is 300
feet below the top of the rock shaft. This shaft, with which the
various levels of the mine are connected, extends down to the 1200-foot
level, and a similar shaft or winze situated near it extends from the
1200-foot to the levels below. As the top levels in the mine are the
hottest, the current of air ascends through the Harvey shaft. The usual
direction of air currents in mines with two shafts and natural
ventilation is down the shorter shaft and up the shaft the mouth of
which is situated at the greater height on the surface. The reverse is
the case at Kimberley mine. The quantity of air which passes down De
Beers rock shaft was 33,300 cubic feet per minute until 1898, when the
enlargement of the upcast shaft was completed,
