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.