Heldon Foyle was very silent as they trudged side by side out of Whitechapel into the silent City streets--for there are no taxicabs to be found in the East End at such hours. The case was developing; but though he was beginning to have a hazy glimpse into some of its workings, there was much that remained a mystery to him. His questionings of Israels had satisfied him that the man who had escaped was neither Grell nor Ivan. He could not blame himself for not effecting an arrest. Looking back over the night's events, he could not see that he could have taken further precaution. If he had taken more men the escape would have occurred just the same over the roofs, for he would still have felt it his duty to question Israels. He could not have foreseen that the ready-witted Lola was there, nor that she should have so ingeniously given the alarm. The luck had been against him. Nevertheless he had gained an important fact. Lola was in London and was obviously acting in concert with Grell. It was easier to look for two persons than one. Sooner or later he would lay hands on them and solve the mystery of the murder. He clenched his fists resolutely as his thoughts carried him away. Meanwhile there was the cipher. If that could be de-coded it might be valuable. Green's voice broke in upon his thoughts. "We didn't find anything bearing on Waverley." "Waverley?" repeated Foyle. "Oh yes, I had almost forgotten him." For an hour after they had reached Scotland Yard the superintendent laboured at his desk, collecting reports and writing fresh chapters in the book which held all the facts in relation to the crime, so far as he knew them. He slipped the result of his labours at last in an envelope and left them over to be dealt with by the inspector in charge of the Registry, which is a department that serves as official memory to Scotland Yard. "That is all right," he said, and stretched himself. Some one knocked at the door. The handle turned and an erect man with his right arm carried in a black silk handkerchief improvised into a sling entered the room. It was Detective-Inspector Waverley. CHAPTER XVIII Heldon Foyle was on his feet in a second, and he pushed a chair towards his subordinate. Detective-Inspector Waverley sat down and drummed nervously on his knees with the fingers of his left hand. "Well, you've got back," said the superintendent in a non-committal tone. "We were beginning to wonder what had happened to you. I hope that arm of yours is not badly hurt. What has been the trouble?" The inspector winced and sat bolt upright in his chair.