"Well, sir," Seth continued, "he went by a round-about way, and I lost sight of him for a long time and couldn't move for fear he should see me. While I waited, a very queer thing happened, sir. There were no ships anchored in the Roads, and of course there were none under weigh; and yet when I looked out to sea, I noticed a tiny, green light somewhere about the middle of Alderwick shoal. It disappeared as suddenly as it came. And then, sir, there was a curious grunting noise from the same spot. Are you listening, sir?"
"Yes. What sort of a noise do you say it was?"
"I said grunting, sir; but if it had come from deeper water I should have said that it was the sound made by a submarine emptying or filling her ballast tanks. Do you think it could have been?"
"Wait. Let me consider." There was a long pause. "It's just possible. You've to remember that secret store of petrol. There is no doubt that Heinrich Hilliger intended it to be used by a German submarine. In that case it's not wildly improbable that a German submarine is hanging round with the intention of lifting it, not knowing that it has already been removed. But they can't very well come ashore for it while the sentries are patrolling the beach. Neither can Seligmann do any digging, unless he's desperate enough to shoot the sentries first, and so get them out of the way. Now, if it was indeed a submarine that you heard—a German submarine—and if she is short of petrol, she will wait there, submerged. In that case we may be able to drop on her. Do you understand?"
"Yes, sir," returned Seth. "And I understand, or rather guess, something else. I guess that if she has come to fill her petrol tanks from Alderwick beach, she must have been piloted there by some one who knows every fathom of that shoal. Don't you think it's likely that Max Hilliger is aboard of her, sir?"
"Listen!" Mr. Bilverstone's voice responded. "I will send a couple of marines along to keep watch, while you cut home and dress yourself up as a fisher boy and come down to me here at the naval base."
"A fisher boy, sir?" Seth inquired in wonder.
"Yes," came the answer. "At the first gleam of daylight you are going out with me in a shrimping boat, to fish for shrimps round about Alderwick shoal. You understand? Right."
CHAPTER XVIII.
A SHRIMPING ADVENTURE.