The latter feature puzzled the detective greatly. He could not form any hypothesis to account for it. If the Ford woman, as indicated by the presence of the handkerchief, marked with an "F," had been on the fire-escape, why were there no tell-tale marks to indicate it? And if she had not been there, why was her handkerchief found there, knotted in this peculiar way? Had it formed part of some apparatus, some device, made of a pole and a cord, for inserting the threatening letters through the window? If so, it might, of course, have become detached while the device was being used. Duvall remembered that he had not examined the fire escape on the night when the astonishing apparition had appeared beside Ruth Morton's bed, _because the window opening on the fire escape had been closed and locked_. Had the handkerchief been left there then? He sat for a long time in the deserted library, trying to hit upon some reasonable theory to explain the matter, but his efforts resulted in failure. Not the least confusing feature of the affair was the fact that the woman, Marcia Ford, _was not the woman he was seeking_. He had seen her at the studio that morning, and knew that she was not the one who had escaped from the cab the night before. Were there then two working together? If so, he would, through the Ford girl, in all probability be able to trace her confederate. He waited patiently until the waning afternoon light told him that it was time to begin his watch before the house at number 162. Across the street a residence, closed for the summer, its front entrance boarded up, afforded him a convenient place to wait. He sat down upon the steps, and pretended to be occupied with a newspaper. His eyes, however, sought constantly the doorway opposite. A number of persons entered the place, during the next two hours, but Marcia Ford was not amongst them. As the darkness began to approach, and lights in the streets and houses flared up, Duvall rose, crossed the street, and stationed himself at a nearer point, from which he might the more certainly identify anyone entering the house. Miss Ford, however, failed to appear. From the sign in the window, to the effect that roomers were wanted, Duvall concluded that the Ford girl did not take her meals in the house. His watch showed him that it was nearly seven. Doubtless she had arranged to dine before returning home. In a flash it came to him that his opportunity to make an examination of her room was now at hand.