1918.] The N.Z. Journal of Science and Technology. 277 
comparatively small the method of generating electrically appears to be the 
most economical; but where a large quantity of air is required and it is 
possible to install economical steam-driven compressors, then, owing to the 
conversion losses in the electric compressors, probably the steam-driven type 
is preferable. Against this it may be stated that the compressors provide a 
very good load for the power-station, and tend to increase the load-factor. 
In the large generating-stations in South Africa for supplying power to the 
mines compressed air is generated by coupling rotary compressors direct to 
steam turbines and supplying the compressed air in bulk to the various 
mines, where it is metered. Some of the mines still retain their existing 
steam compressors, which in several cases have been electrified. 
Up to the present time no electrically driven rock-drills have been 
a practical success, and compressed-air drills possess so many obvious 
advantages that there is little doubt that the}' will continue to be the 
most suitable machine for this class of work. The exhaust from these 
machines materially assists in the ventilation of the mine. 
Compressed air is also extensively used for winches for sinking shafts 
and winzes. For pumping its use is confined practically to sinking, and 
draining sumps. 
Ventilation .—This is most effectively carried out by an exhaust fan on 
the surface and by pressure or exhaust fans underground. The most suitable 
method of driving the fans is by means of an electric motor; but great care 
has to be exercised in installing an electrically driven fan underground, 
because these fans are often placed in exposed positions, and the cables, 
switch gear, and motor must be efficiently protected and earthed. Under 
some conditions it is therefore advisable to resort to fans driven by com¬ 
pressed-air motors, or to jets of compressed air arranged so as to induce air 
from the downcast shafts and deliver to the various faces. 
Underground Haulage .—Underground haulage is not carried out in gold¬ 
mines to anything like the extent that it is in coal-mines, and the reason 
is that whereas a coal-mine has usually only one level, a gold-mine has 
numerous levels, and the drives on the reefs are often very irregular ; also, 
the various levels, owing to the workings in the vicinity, are often in a state 
of movement. Compressed-air-driven rope haulage is sometimes adopted, 
owing to its simplicity. In the Waihi Mine an electric locomotive is in 
use in one of the levels for the purpose of filling the depleted stopes with 
mullock. This locomotive has a run of about 1,000 ft., and is driven by a 
direct-current motor receiving current from an overhead trolly-wire at 
220 volts, and has proved to be very satisfactory in practice. 
The Mill. 
The application of electricity for driving the various machines in a 
battery and treatment plalit is in most cases not difficult, but calls for some 
skill in the arrangement of the circuits. The various sections of a treat¬ 
ment plant are so intimately connected that care has to be taken that the 
failure of a small motor does not mean shutting down a considerable section 
of plant in addition to portions driven by the said motor. Alternating 
current has very considerable advantages in mill work. Practically the 
whole of the machinery requires to be run at a constant speed, and in 
certain sections of the plant it is absolutely essential that speed be con¬ 
stant to ensure efficient conditions. In this class of work the advantages 
o 
of the alternating-current motors are undisputed. The simplicity of the 
alternating-current motors, and the small number of wearing-parts as 
