Captain Horst was seated at the table, his head in his hands, his back to them. Spread out before him was the chart with the little red crosses. He sat motionless, staring at it, as though absorbed in reverie. The three cups of cocoa were steaming on the table. His was untouched. For one wild moment Lyngstrand thought he might be able to surprise a glance at the chart. He turned the handle of the door as stealthily as he could. Slight as the sound had been, however, Captain Horst had heard it. When they entered he was stuffing something into his breast pocket, and the chart was no longer on the table. They drank their cocoa in silence, Horst staring moodily at the floor, Jensen and Lyngstrand risking a glance of mutual comprehension. Suddenly two loud, sharp knocks broke the stillness--knocks that seemed to be on the charthouse wall. Captain Horst raised his head. “_Herein!_” he cried, automatically, obviously without thinking. Jensen shot a swift look at his friend, eyebrows raised at this German permission of entry. Horst bit his lip, suddenly self-conscious. He repeated the authorization in Swedish. No one entered. Expectation was just passing into a vague surprise, when the knocks were repeated--three heavy blows, obviously deliberate, upon the after-wall of the charthouse. Horst sprang up, with a savage curse of exasperation. He was self-controlled enough, however, to utter his thought in Swedish. “I’ll teach them!” he exclaimed, as he flung open the charthouse door. “Fooling around here!” He disappeared into the night and they heard the tramp of his heavy sea-boots as he ran round the charthouse. But no other sound woke upon his passage. The circuit completed, they heard his angry yell to the look-out man on the bridge above, heard the quietly normal response, the surprised denial. The interior of the charthouse was a hushed stillness where Jensen and Lyngstrand sat exchanging a smile of malicious enjoyment. Horst vituperated the stammering look-out man in a flood of ugly oaths that were plainly a break-down of nervous control. The door opened again for his entry. “Extraordinary thing!” he scowled across at them. “No one there! You heard them, didn’t you?” He seated himself with an angry grunt. Before they could answer, the knocks recommenced in a sudden vehemence--not slow and deliberate this time, but in a rapid succession which quickened to a fast and furious fusillade from origins that seemed to play, flitting arbitrarily, all over the walls and roof. The charthouse reverberated with them. Their intensity varied at every moment from sharp, hammer-like blows to rapid, nervous taps from what might have been a feverishly agitated pencil. The wild and uncanny tattoo culminated in three crashing blows that seemed to be on the underside of the table itself. There was silence. “What are you playing at?” cried Horst, glaring at them in fierce suspicion of a hoax.