No, having assumed an attitude of ignorance, I saw I was now compelled to retain it. Kirk, clever, crafty, and far-seeing, had most ingeniously sealed my lips. Yet why, if he were the actual criminal, had he taken me, a perfect stranger, into his confidence? And again, what connection could the Eckhardt tyre have with the strange affair? Who were those two mysterious callers who had followed his visit, and whom Pelham had seen? What could have been their object? I stood in the large drawing-room listening to the discussion between the searchers, who had now returned there disappointed. "I can only repeat, sir," remarked the inspector, addressing Langton, "that you must have been mistaken regarding the light in the window of the next room." "I'm certain I was not," replied the young man doggedly. "Someone was in this house--someone who, when I rang, extinguished the light and escaped!" "But how could he have escaped?" queried the officer. "Ah! that's the mystery. By the roof, perhaps." "The trap-door is bolted on the inside," declared the constable; "I examined it, sir." "Or by a window leading out on to some leads somewhere?" I suggested. "There are no windows unfastened by which anyone could have escaped," the sergeant exclaimed; "I've looked at them all." "Well," exclaimed the young man with a puzzled air, "nothing will ever convince me that I've brought you all here upon a fool's errand. I still maintain that something unusual has happened. Why has Antonio fled to France?" "We must ask the Professor," replied the inspector. "He may have been sent by his master upon perfectly legitimate business. He was entirely trusted, you say." "But he saw me in the buffet at Calais, and, turning, hurried away," Langton said. "In other circumstances he would certainly have raised his hat in greeting; he is a most polite, tactful man." "Well, sir," laughed the officer, "I don't think we can assist you any further. Just go out, 403," he added, turning to the constable, "and tell the two men in the park that we've finished, and they can go back to their beats." "Very well, sir," responded the man, replacing his truncheon as he left the room. Both inspector and sergeant soon followed him, leaving Langton and myself alone. After the front door had closed, we returned to the big dining-room. "Well," he exclaimed, "I don't know what your theory is, Mr. Holford, but I'm absolutely certain that something has happened here. There is some crooked circumstance," and I saw deep lines of thought upon his shrewd, clever, clean-shaven countenance. Why dare not Kirk meet him? "The absence of everybody is certainly mysterious," I admitted. "Doubly mysterious when one takes into consideration the fact that the doors leading into the laboratory have been forced," he remarked quickly. "Three persons dined here to-night. The Professor entertained a man-friend. Who was he?" "That we can only discover when the servants return," I said.