He passed into the shop as he spoke, and the Professor could hear the sound of his footsteps as he climbed the stairs. The Professor, left alone in the room, proceeded to examine the titles of the books in the shelves. They were mainly botanical works, such as one might expect to find in the library of a herbalist. But here and there among them were a few volumes of general scientific interest. Dr. Priestley smiled as he saw, lying inconspicuously at the end of one of the shelves, a work he had published a year or so before, _Fact and Fallacy_. No doubt this had given Ludgrove the cue to his own interest in a problem so intricate as that of the Praed Street murders. Lying next to it was a book bearing the name of an author well-known in the scientific world, which the Professor had never heard of. He took it up and began to turn over the pages with interest. His eye caught a passage dealing with one of the subtler mathematical problems raised by a particular aspect of relativity. He became absorbed in this, forgetting all about Mr. Ludgrove and his promised demonstration. So he remained for several minutes, until he had read the passage from beginning to end. Then, all at once, he became conscious that he was no longer alone in the room, that somebody was standing just inside the door, awaiting his pleasure. The Professor laid the book down, and turned on his heel, an apology upon his lips. But the words died in his throat, and he stood for an instant staring at the figure with unbelieving eyes. It was the Black Sailor. Chapter XXI An Explanation The Professor’s first startled thought concerned the manner in which the man had entered the room. He had heard no sound of his entry, and he remembered that Mr. Ludgrove had been careful to lock the door of the shop when they had come in together. His second was a sudden thrust of anxiety for Mr. Ludgrove’s safety. In the complete silence of the little room, into which no sound penetrated but the faint low rumble of the traffic outside, he listened for the sound of his footsteps above. But, even to his keen ears, the house seemed as quiet as the grave. It was the Black Sailor who broke the silence. “Sit down, Dr. Priestley,” he said gravely. “I have much to say to you. You will recall that this is our second meeting.” But Dr. Priestley took a step forward, and stared into the man’s face, at his jet-black beard and hair, at the scar which gleamed angrily upon his cheek. He wore a woollen cap, drawn over his face, and a blue pilot coat over his jersey. But for these details, he was dressed as he had been when the Professor had met him upon the Isle of Purbeck. And the tones of his rasping voice sent a thrill of fear through even the Professor’s determined heart. “How did you get in here? Where is Mr. Ludgrove?” The Professor found his voice at last.