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.
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