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.