Duvall, upon discovering that the address of Miss Marcia Ford was on West 57th Street, but two doors from the building in which the Morton apartment was located, began to feel that he was on the right track. He had known, ever since his first day upon the case, that the mysterious messages found in Ruth Morton's bedroom had been placed there by some ingenious but perfectly natural means. The apparition that had so startled the girl upon her last night at the flat was capable, of course, of some reasonable explanation. When he left Mr. Baker in the morning his plan had been to go to Mrs. Morton's apartment and once more investigate all possible means of entrance, hoping that, by finding out how the messages were delivered, he might also be able to find out by whom. It was for this reason that he had asked Mrs. Morton for the key to the apartment. Now the question seemed in a fair way to being answered for him. The fact that this girl's room was located so near to the Mortons' apartment could not be a mere coincidence. There must be, between her room and the Morton flat some means of communication, although of what nature he could not now surmise. Fully convinced, however, that he might very soon find out, he hurried up to Fifty-seventh Street and walked along until he reached No. 162. The house was, like that which immediately adjoined the apartment building, an old-fashioned one, of brown stone, with a high front stoop. It presented an appearance which, if not exactly dilapidated, was yet in strong contrast to the neat appearance of its neighbors. A printed card in one of the lower front windows indicated that roomers were wanted. It was just the sort of place that Duvall had expected to find--just the sort of place in which a working girl like Marcia Ford would live. Located in a very excellent neighborhood, surrounded by apartment buildings and houses of the best type, it still could afford to rent rooms at the moderate figure that one of her class could pay. He went up the front steps and rang the bell. "Is Miss Ford in? Miss Marcia Ford?" he asked. The servant who came to the door, a neatly dressed German girl, shook her head. "No, Miss Ford is not in. She usually gets back about half past six." Duvall glanced at his watch. It was not yet three o'clock. He realized that he had a long wait before him. "Will you leave any message?" the girl asked. "No. It is not important. I will come back." Descending the steps he walked slowly in the direction of the apartment building, two doors away. Entering, he made his way to Mrs. Morton's apartment. The place was just as they had left it, two days before. The windows had all been tightly closed and fastened, and there were no further mysterious messages lying about. Once more Duvall went to Ruth Morton's room, and opening the two windows looked out.