“Let in numbers two and four,” commanded the Master, suddenly, into the engine-room telephone.  “In five seconds after we start, hook up one and three; and five later, the other two.”

“Aye, aye, sir,” came back the voice of Auchincloss, chief engineer.  “Ready, sir!”

Almost at once, the vibration of the engines altered, grew more marked, seemed to be taking hold of something with strong but easy effort.  Another trembling made itself felt, as two of the giant screws, connected by reducing-gears with the engine-shafting—­all three engines being geared to one shaft, but any one being capable of separate running—­began to revolve.

From astern, a dull, droning hum mounted, rose, grew rapidly in volume and power.  And, as two more screws began to whirl, the Eagle of the Sky shook herself slightly.  She awoke from slumber.  Steadily, smoothly on her air-cushions she began to move forward down the long, sloping trackway to the brink of the cliff.

“Lord above!” breathed Bohannan, chewing at his nails.  “We’re off!”

Neither the Master nor Captain Alden moved, spoke, manifested any excitement whatever.  Both might have been graven images of coolness.  The Celt, however, got up and leaned at the window-jamb, unable to keep still.  He turned suddenly to Alden.

“Come, man!” he exclaimed, half angrily.  “Got no heart in you, eh?  No interest?  Come along out of that, now, and see what’s what!”

He laid hold on the captain, and drew him to the window as the airship accelerated her plunge along the rails.  The hum of the propellers had now risen to a kind of throaty roar; the craft was shaking with strange quivers that no doubt would cease if she but once could launch herself into the air.  Under her, in and in, the shining metal rails came running swiftly and more swiftly still, gleaming silver-like under the vivid beam of the searchlight.

Wind began to rise up against the glass of the pilot-house; the wind of Nissr’s own making.

Cool as if in his own easy-chair in the observatory, the Master sat there, hand on wheel.  Then all at once he reached for the rising-plane control, drew it over, and into the telephone spoke sharply: 

“Full speed ahead, now!  Give her all she’s got!”

A shout, was it?  Many shouts, cries, execrations!  But where?  Over the roar of the propellers, confused sounds won to the men in the pilot-house.  And all at once, by the dim aura of diffused light reflected from the huge beam, the major saw dim figures running, off there to the left, among the buildings of the stockade.

“For the Lord’s sake!” he cried, amazed, with drooping jaw.  “Men—­after us!  Look there—­look!”

The Master remained utterly impassive, eyes keen on the in-rushing track, now close to its abrupt ending over the vacancy of space.  Captain Alden’s pupils narrowed, through the mask-holes, but he said nothing.  Bohannan gripped the captain’s shoulder painfully, then reached for the pistol in his own holster.