tral station; and also because, by means
of the storage cells or "accumulators," the amount of available
electrical energy can be subdivided into different and subordinate
circuits, thus obviating the necessity for the employment of currents
of very high voltage and eluding the only imperfectly-solved problem
of dividing a current traversing a wire as conveniently as lighting
gas is divided by taking small pipes off from the gas mains.

Compressed air for the storage of power has hitherto been best
appreciated in mining operations, one of the main reasons for this
being that the liberated air itself--apart from the power which it
conveyed and stored--has been so great a boon to the miner working in
ill-ventilated stopes and drives. The cooling effects of the
expansion, after close compression, are also very grateful to men
labouring hard at very great depths, where the heat from the country
rock would become, in the absence of such artificial refrigeration,
almost overpowering. For underground railway traffic exactly the same
recommendations have, at one period during the fourth quarter of the
nineteenth century, given an adventitious stimulus to the use of
compressed air.

Yet it is now undoubted that, even in deep mining, the engineer's best
policy is to adopt different methods for the conveyance and storage of
power on the one hand, and for the ventilation of the workings on the
other. Few temptations are more illusory in the course of industrial
progress than those presented by that class of inventions which aim at
"killing two birds with one stone". If one object be successfully
accomplished it almost invariably happens that the other is
indifferently carried out; but the most frequent result is that both
of them suffer in the attempt to adapt machinery to irreconcilable
purposes.

The electric rock-drill is now winning its way into the mines which
are ventilated with comparative ease as well as into those which are
more difficult to supply with air. It is plain, therefore, that on its
merits as a conveyer and storer of power the electric current is
preferable to compressed air. The heat that is generated and then
dissipated in the compression of any gas for such a purpose represents
a very serious loss of power; and it is altogether an insufficient
excuse to point to the compensation of coolness being secured from the
expansion. Fans driven by electric motors already offer a better
solution of the ventilation