Suddenly a sound came to his ears from across the water, but it was not what he expected, although it was in Jack's voice:
"Frank! Look out! They are after you!"
Instantly, the lad understood the situation. He drew his revolver with his uninjured arm and sprang to one side of the aeroplane. As he did so, a figure reached up and grabbed him by the hand so that he could not fire. At the same time a second figure clambered aboard the craft from the opposite side. Frank raised a cry:
"Hurry, Jack!"
Jack needed no urging. He was swimming through the water as fast as possible.
With a sudden move, Frank jerked his hand loose from the grip that held him and turned just in time to encounter the second German. Frank raised his revolver and fired quickly; but the German ducked, and before Frank could fire again, he had come up close to Frank and grappled with him. In vain Frank sought to release his arm so that he could bring the weapon down on his opponent's head. The man clung tightly.
A sudden lurching of the hydroplane told Frank that the second German was coming aboard. Unmindful of his wounded shoulder, Frank struggled on. With a sharp kick of his right foot he succeeded in knocking the first German's legs from beneath him; and again the lad tried to raise his revolver to shoot the second German, who now advanced.
But the latter was too quick for him. Closing with the lad, the man knocked the revolver from the boy's hand with a quick blow. The weapon spun into the sea.
The first German returned to the attack.
"Get him quick!" he shouted. "There is another one around here some place."