"Yes," I urged, "let him down gently. He does his best." "What profession does the owner of that knife follow?" Hilderman and I laughed. "We may as well count that answer as read," he said. "There's a catch there, Dennis," I warned him. "The legal designation is 'mariner.'" "I don't think it is," said my friend. "We won't quarrel about terms," laughed our host graciously. "Sailor or seaman or deckhand will do just as well." "No," said Dennis, "it won't. The owner of this knife is not a sailor by profession." "But," Fuller protested, "it must belong to one of my crew, and it is obviously a seaman's knife." "In that case," Dennis answered, "I think you'll find that you have a man on board who is not a professional seaman in the ordinary use of the term. I'll tell you what I think of this knife, shall I?" "By all means," urged Hilderman and his friend together, and I began to take a keen interest in this curious discussion, for I could see that Dennis was no longer playing. He turned the knife over in his hand, and looked up at Fuller. "Mr. Fuller," he said quietly, "the owner of this knife is not a sailor by profession. He is probably a schoolmaster. I can't be sure of that, but I can say this definitely: he is a professional man of some sort, possibly an engineer, but, as I say, more probably a mathematical master. He is left-handed, has red hair, a wife, and at least one child." I shouted with laughter when I realised how thoroughly my friend had pulled my leg, but I broke off abruptly when Hilderman sat bolt upright, and his chair and Fuller's cigar fell unheeded on to the deck. But in a second they took their cue from me, and roared with laughter. "Oh, excellent, Mr. Burnham," said Hilderman between his guffaws. "But you forgot to mention that his sister married a butcher's assistant." "Ah, but I don't admit she did," Dennis protested. "I'm very much indebted to you for exposing this masquerader," said Fuller. "I shall have the matter inquired into. But seriously, Mr. Burnham, you made one extraordinary fluke in your deductions, which almost took my breath away. I have a man on board with red hair, and when the boat came into the harbour he was working about here. I saw him leave his work to come ashore for us. I shouldn't be at all surprised to find that the knife belonged to him." "Oh, well," Dennis laughed, "one shot right is not a bad average for a beginner, you know." "No," said Hilderman, puffing a cloud of smoke, and dreamily following its ascent with his eyes, "not bad at all. Not bad at all." And then, the joke of the clasp-knife being played out, we admired the scenery, and conversed of less speculative subjects till we arrived at Glasnabinnie. We were pulled ashore by the man with the red hair, and when our host confronted him with the knife he promptly claimed it.