"Stop!" exclaimed Vernon as Ross began to strike out towards the buoy. "There'll be trouble if we get mixed up in that oil. It's much lighter than water. I doubt whether we could swim in it. Do you think the Tremendous will put back?"
"Not likely," replied Trefusis.
He looked in the direction of the fast-vanishing battleship, half hoping that she would slow down and lower a boat. As he did so, something caught his eye: a cloud of grey smoke apparently issuing from the sea.
"What's that?" he asked, pointing in that direction.
"Torpedo, perhaps; one that has finished her run," suggested Vernon; but his chum waved aside the explanation.
"If U77 did fire a torpedo, you can bet your bottom dollar it wasn't one with a dummy head!" he said. "Only practice torpedoes send up a calcium light when their compressed air has given out. By Jove, I believe it's one of those patent buoys! Let's make for it."
The lads swam strongly, making powerful and comparatively slow breast-strokes. The water was warm. They were in no immediate danger of cramp.
As they skirted the patch of oil they noticed that the seaman holding on to the buoy had turned round. His face was now in their direction. The man was Hans Koppe.
"Are you all right, Hans?" shouted Ross.
"Yes, mein herr," replied the man. "I've found a buoy."