He pointed skywards. Less than five hundred feet up, yet sufficiently far from the tank-vessel to enable the latter to screen her from the unterseeboot, was a large naval sea-plane. It was to deaden the noise of her motors that the ship's steam-pipe was continually blowing off steam from the time that U75 made her peremptory demand.
The eyes of the Leutnant and his six men followed the direction indicated by the British skipper's outstretched hand.
At that instant the sea-plane was visible above the towering sides of the British vessel.
U75 was still forging slowly ahead. In a trice Kapitan Schwalbe decided how to act. Ordering the men on deck to their diving stations, he dropped agilely into the conning-tower and gave the word for the helm to be ported.
Thus, while the quick-firers were being housed, the submarine had drawn close under the oil-tank's quarter. Here she was comparatively safe from the sea-plane, as the latter could not drop any bombs without risk of exploding the highly inflammable cargo of the British vessel.
In ten seconds the sea-plane was over and beyond her quarry. She had then to turn and circle overhead, awaiting the chance of shattering her enemy as she dived.
U75 was already disappearing beneath the waves.
She dived at a very oblique angle, steeper than she had ever done before.
Ross and Vernon, unaware of what was taking place, thought for a moment that the submarine was plunging headlong to the bed of the Bristol Channel. They had to cling desperately to the nearest object to hand to prevent themselves from sliding violently against a transverse bulkhead.
Even as they clung they heard two muffled detonations in quick succession, followed by a distinct quiver of the submarine's hull—a movement that bore a marked difference to the vibrations under the pulsations of the motors.