The boy looked at the face till it seemed to be photographed on his mind. He would know it among a thousand faces, he thought. It should not escape him, and he would give a certain person a full description of it. In a moment, as it were, the face vanished. Billy turned again to the dead woman, but looked now and then toward the window. He saw that the old woman had been killed, for the rent in her throat told where the dagger had found her life and put an end to her varied career. As yet the murder was his secret and the murderer’s. Mulberry Billy remained in the little room some time, or until he had composed his nerves. One does not discover a terrible crime every day, not even in New York. He wanted to think the matter over a little; he wanted to decide just what to do. “I’ll see Patsy again, that’s best,” he said aloud, though addressing himself. “Patsy Garvan once befriended me, and he’ll tell Mr. Carter about this, and I know Mr. Carter’s the man to take charge of this matter and avenge Mother Flintstone.” With this the street Arab slipped from the house and went out upon the street again. In a few minutes he ran up a flight of steps leading to Nick’s downtown den, where he had captured Brockey, and knocked at a door. Footsteps crossed the room beyond and the door was opened. “You, boy! Come in.” Billy entered, looking at the person who had opened the door, and who now stood in the middle of the room looking at him with a smile on his face. He had expected to find Chick Carter or Patsy there, and he was surprised to meet the great detective himself already on the trail once more. “What’s happened, boy?” asked Nick. “They’ve got Mother Flintstone at last, sir.” “Who have?” “That’s for you to find out, Mr. Carter.” “You don’t mean that the old lady’s dead, Billy?” “Don’t I?” “Where?” “In the crib.” “Do you know who saw her last?” “Yes, sir; the man who did it.” Carter smiled at the answer and took a seat at the table. “Give me the story,” he said. Billy did so. He omitted nothing, but he dwelt a long time on the face at the window. The famous detective seemed to think that face an important matter, and he made the boy describe it half a dozen times. Presently he arose and put on an overcoat, for the night was cold, and perhaps he wanted to protect his face with the ample collar. The pair left the room together, and Billy piloted the detective to the scene of the crime. “You can go now,” said Carter, when he had taken a survey of the apartment. “I will need you to-morrow, Billy. Don’t go far. You can take my lounge if you want a snooze till then.” The urchin went away, leaving Carter in the hovel where Mother Flintstone lay. Nick went over the old place with his keen eyes and eager hands. If he found anything that let some light upon the mystery he did not divulge the secret, and just as day was breaking over the spires of Gotham he came out of the place and walked away.