Mr. Bilverstone paid out the lugsail sheet an inch or two and perched himself on the windward gunwale.
"As a matter of fact," he said, "I believe they know already. I didn't tell you; but an hour ago, while you were having a sleep under my writing-table, we had a report from the two marines patrolling Alderwick beach. At about two o'clock they saw an electric light signal flashed from the foreshore, near one of the groins. There were no ships in sight, and no answering signal was seen. Still the light kept on flashing. The two marines crept up, one on either side of the groin. They got so near that one of them called out a challenge. As there was no response he fired. The light went out then. There was no cry, no sound, no movement. Nobody was shot; yet nobody ran away. The two marines and two Territorial sentries searched, but found absolutely no trace of the chap who had been signalling. He had vanished as completely as if the tide had come up and swallowed him."
"That's queer!" murmured Seth. "Very queer. There must have been somebody working the hand-lamp, sir."
"Not necessarily a hand-lamp," Mr. Bilverstone smiled. "None of the patrol thought of it, but it's easy to imagine how a tricky German, such as Fritz Seligmann, could plant an electric bulb in the sand or shingle, or even among the timbers of the groin, and work the switch from the top of the cliff by means of a long-distance connection. A spy was caught three nights ago signalling from the air. He flew a kite with an electric current running through the string. Spies wouldn't be much good if they weren't tricky."
Slowly the dawning light in the eastern sky grew brighter, changing from steel grey to gold, tinged with a rosy glow. Again and again Mr. Bilverstone put out the gear. No one seeing the two occupants of the little boat, with its brown lugsail, would have believed them to be anything else than ordinary shrimpers. They both wore tanned canvas overalls and oilskin sou'-westers, and their manner of working contributed to their disguise.
Twice they passed along the leeward fringe of the shoal. Seth Newruck's eyes searched the ruffled water where the waves broke here and there above the shallows; but he saw nothing unusual.
"I'm afraid we shall have only the shrimps for our trouble, sir," he remarked with a shiver, for the morning was very cold.
"Don't be impatient," nodded his companion, opening a Thermos flask. "We haven't finished our job yet. Here, have a drink of warm tea; and there are some biscuits in the locker behind you. Come forward here, and I will take the tiller for a spell."
He took the boat outward, as if he were making for the lightship, leaving the shoal in his wake.
"Don't look round, sir," Seth whispered agitatedly. "I can see two periscopes, close together. And there's a sort of commotion in the water round about them, as if the submarine were rising."