d no longer suppose it accidental. Kevima being
near us, I asked him what it meant.

"Ventilation," he answered. "The air in these tunnels would be foul
and stagnant, perhaps unbreathable, if we did not drive a constant
current of air through them. You did not notice, a few yards from the
entrance, a wheel which drives a large fan. One of these is placed at
every half mile, and drives on the air from one end of the tunnel to
the other. They are reversed twice in a zyda, so that they may create
no constant counter-current outside."

"But is not the power exerted to drive so great a body of air
exceedingly costly?"

"No," he answered. "As you are aware, electricity is almost our only
motive power, and we calculate that the labour of two men, even
without the help of machines, could in their working zydau [eight
hours] collect and reduce a sufficient amount of the elements by which
the current is created to do the work of four hundred men during a
whole day and night."

"And how long," I inquired, "has electricity had so complete a
monopoly of mechanical work?"

"It was first brought into general use," he replied, "about eight
thousand years ago. Before that, heated air supplied our principal
locomotive force, as well as the power of stationary machines wherever
no waterfall of sufficient energy was at hand. For several centuries
the old powers were still employed under conditions favourable to
their use. But we have found electricity so much cheaper than the
cheapest of other artificial forces, so much more powerful than any
supplied by Nature, that we have long discontinued the employment of
any other. Even when we obtain electricity by means of heat, we find
that the gain in application more than compensates the loss in the
transmutation of one force into another."

In the course of little more than half an hour we emerged from the
tunnel, whose gloom, when once the attraction of novelty was gone, was
certainly unpleasant to myself, if not by any means so frightful as
Eveena still found it. There was nothing specially attractive or
noticeable in the valley through which our course now ran, except the
extreme height of its mountain walls, which, though not by any means
perpendicular, rose to a height of some 3000 feet so suddenly that to
climb their sides would have been absolutely impossible. Only during
about two hours in the middle of the day is the sun seen from the
level of the stream; and it is dark in the