The torpedo was fired by the ignition of a small charge of petrol gas, and could be aimed with considerable accuracy. At the head of the weapon was a small fan, the use of which was to prevent premature explosion of the charge on impact with the surface of the water. The depth at which the torpedo exploded could be regulated by adjusting the fan to a certain position on its threaded axis.
The sea-planes had been up for less than ten minutes when a wireless was received reporting the position of the quarry. The U-boat was "sounding" at a depth of twelve fathoms—too deep for the aerial torpedoes to reach with accuracy. Her grey hull could be discerned by the airmen with tolerable ease as she lay upon the sandy bottom.
It was now the Fleetwing's task to get the submarine to bestir herself. The German captain would be too wily to attempt to rise to the surface with the churning of four high-speed propellers sounding over his head. So the patrol-boats eased down while the sea-plane carrier forged ahead, thrashing the water with her twin screws, the sea-planes describing vast circles over the spot where the U-boat lay.
Presently another message was received that the submarine was moving. She was about to take stock of the apparently solitary vessel. If she did rise to the surface the patrol-boats could almost with certainty destroy her, either by gun-fire or by ram. On the other hand, if she exposed the tips of her periscopes only, such tactics would not be likely to result in definite destruction.
Breathlessly Osborne and Webb awaited developments, ready at the first warning to urge No. 0916 at full speed towards the enemy.
Still the sea-planes circled. It was the only means of keeping in touch with their prey, for the former were travelling through the air at fifty miles an hour, compared with the latter's maximum submerged speed of fifteen knots. Trained downwards, and only a few degrees from the perpendicular, were the grey-painted torpedo-tubes of each tri-plane.
Presently the upward movement of the U-boat ceased. Her periscopes rippled the surface. Something glistening shot from the sea-plane nearest overhead. Like a silvery dart the object plunged seawards, struck the water with hardly a splash, and disappeared.
Almost simultaneously a column of foam was hurled skywards, to the accompaniment of a muffled detonation.
"Missed!" was the laconic wireless message from the air-craft that had discharged the missile. "She's heading nor'east."
Two more aerial torpedoes were fired, with no better result than to send the U-boat scurrying off at a depth of ten fathoms. It was now the patrol-boats' turn to take up the pursuit.