Thirty seconds later came the dull muffled sound of an explosion. A thin wreath of vapour issued from the open conning-tower.
"Not much of a bust-up that," exclaimed Sefton contemptuously. "It would not have flicked a fly from her deck. Well, I suppose I must take the beggars into the boat."
The lightness of the explosion had also astonished the German officers. Adopting their usual procedure they had fixed three detonators in the hull of the stranded vessel, and upon the approach of the Calder's boat the second time they had lighted the four-minute time-fuses.
Sefton, guessing rightly what had been done, had resolved to give the Huns, not a bad quarter of an hour, but a worse three minutes. He, too, expected to see the submarine's hull disintegrated by a terrific explosion.
On the boat's return to the destroyer with the rest of the prisoners, Sefton made his report to the lieutenant-commander.
"Can't blame them," declared Crosthwaite. "In similar circumstances we would have done the same, but with better results, I hope. Send that petty officer aft; I want to speak to him."
The man indicated was, as luck would have it, the fellow responsible for lighting the fuses. Putting on his fiercest expression, Lieutenant-Commander Crosthwaite sternly taxed him with attempting to destroy the submarine after she had surrendered.
Taken aback, the man admitted that it was so.
"How many detonators?" asked Crosthwaite.
"Three, Herr Kapitan."