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.

His investigations, however, told him no more than he had learned before. The three dormer windows in the home next door gazed vacantly down at him, their windows covered with cobwebs and dust. The impossibility of anyone making their way from even the nearest of them, to the window where he stood, was manifest. And that a long rod or pole could have been utilized to introduce the letters into the girl's room was even more impossible. He shook his head, then turned to the other window, that facing upon the fire escape.

Here, as on the occasion of his previous examination, the smooth glossy surface of the freshly dried paint showed no marks, except those he had himself made during his former visit. And yet, as his eyes searched the grated surface, he saw that there was something there, something that had not been there before. He reached out and picked it up.

It was a woman's handkerchief, a tiny square of lace-edged linen, of an inexpensive variety. But it was not the mere presence of the handkerchief that so interested him. It might readily have belonged to Miss Morton herself, and have been accidentally dropped from the window. There were two things about this particular handkerchief, however, that marked it as a clue of the utmost value. One was the fact that in its corner was embroidered an initial, the letter "F." The other was that two of the corners of the handkerchief were knotted together, as though it had been tied about someone's wrist, for what reason, he could not imagine.

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.

To secure entrance to the room by the usual channels was clearly out of the question. The people at the boarding house would, of course, not permit it. But could he discover the means of communication, whatever they were, between Miss Morton's apartment and the girl's room, he might be able to enter the latter unknown and unobserved. He had thought of attempting this during the afternoon, but realized that he could not hope to accomplish it, in broad daylight, without being seen by the occupants of the neighboring buildings, and perhaps arrested as a burglar or sneak thief.

With a last glance down the street, he hastened back to the apartment building and made his way to Mrs. Morton's flat. Passing quickly through Ruth Morton's bedroom, he climbed out upon the fire escape and looked about.

Below him were the rear yards of the houses fronting on the next street. To the right he could see the bulk of the apartment building, blocking his view of the avenue beyond. To the left were the rear buildings of the adjoining houses. It was quite dark, the sky was starless, but all about him gleamed the lights in the windows of the neighboring buildings.

Neither to the right, nor to the left was there any possible way by which access to the point where he now stood could be gained. From below, it was possible, although his previous examination had showed him both the fact that the newly painted surface of the fire escape was unmarred, and that the ladder at the lower floor was drawn up some nine or ten feet from the ground. He felt certain that Miss Ford had not reached Ruth's room in that way.

He glanced upward. The fire escaped stopped at the level of the floor above. To ascend from it to the roof was impossible.

Remembering that the top apartment was vacant, Duvall re-entered the building and hunting up the janitor, told him that he desired to get out on the roof.

The man remembered him, from his first visit, and the inquiries he had then made about the tenants of the apartment above.

"I am making some special inquiries on Mrs. Morton's behalf," he explained. "You can go with me, if you like, to see that I do nothing I shouldn't."

The janitor joined in his laugh.

"I'm not worrying," he rejoined, "but I'll go along, just the same, to show you the way." He led the detective up one flight of stairs and, going to the end of the outer hall, unlocked and opened a small door beside the elevator shaft. A short spiral staircase was disclosed.

Snapping on an electric light, the man ascended the steps, and, after fumbling for a moment with the catch, threw open a trapdoor leading to the roof. In a moment both he and Duvall had climbed out upon the tiled surface. Duvall went to the edge which overlooked the house adjoining, and peered down. He at once saw something that interested him.

The house with the dormer windows consisted, as has been previously mentioned, of four stories and an attic. Its roof rose several feet above the level of the window of Ruth's room, which was on the fourth floor of the apartment building. But Duvall saw at once that this elevation of the adjoining house did not extend all the way back, but, in fact, stopped a little beyond the point where it joined the apartment. From here to the rear of the lot the building had no attic, its rear extension being but four stories high. In this position on the apartment-house roof, the roof of the back building was at least fifteen feet below him.

Another thing that he noticed at once was the fact that the second house, No. 162, was of almost the same design as the first, that is, it consisted of a main building with an attic, and a rear extension, reaching to the same level as that of the house between. It was clear that if anyone living in the second house could obtain access to the roof of the back building, he would be able to walk across that of the first or adjoining house, and reach a point directly beneath where he stood.

But, granting the possibility of this, of what use would it be? A person on the roof below him would in no conceivable way be able to reach either of the windows of Ruth Morton's room. Was it possible that an opening had been made through the wall of the apartment building itself? He thought it unlikely, but determined to investigate.

"I must get down on that roof below," he informed his companion. The janitor grinned.

"How are you going to do it?" he asked.

"Haven't you a ladder—a rope?"

The man thought a moment.

"I've got a short ladder in the cellar, only about eight feet long, I guess. I'm afraid it would not do."

"Yes it would," replied Duvall, pointing to the roof of the attic portion of the house below. "I'll get down to the roof of the main part of the house first, and from there to the roof of the back building. An eight-foot ladder will be long enough for that. Bring it up, will you?"

The man hesitated.

"I don't just like this idea of going on other people's roofs," he said.

"You don't need to go. I've got to. I'm a detective, and I'm working for Mrs. Morton on a most important case." As he spoke, he took a bill from his pocket and pressed it into the man's hand.

The janitor responded at once.

"I'll fetch it up, sir," he said. "Wait for me here."

Duvall occupied the few moments consumed by the janitor's absence in examining, by means of his pocket electric torch, the surface of the roof on which he stood. The smooth flat terra cotta tiles showed no distinguishing marks. Here and there spots of paint, marred by footprints, indicated where the painters at work on the building had set their buckets, no doubt while painting the wooden portions of the trapdoor, and the metal chimney-pots on the roof.

The man returned in a few moments with the ladder, and Duvall, lowering it to the level of the main portion of the adjoining house, saw that it was of sufficient length to permit his descent. In a moment he had slipped off his shoes, and was cautiously descending the ladder.

Once on the surface of the main roof of the house, he had intended to take down the ladder and, by means of it, descend the remaining six or seven feet to the roof of the back building, but he found that means for this descent already existed. A rough but permanent wooden ladder led from the higher level to the lower. Duvall judged that it had been placed there to provide easy communication between the upper roof and the lower. Leaving the ladder where it stood, he made his way down to the roof of the back building. It was covered with tin, and he walked softly in his stockinged feet to avoid being overheard.

His first act was to go to the wall of the apartment house which faced him, and make a thorough examination of it by the light of his electric torch. He judged that in the position in which he now stood he was about on a level with the floor of Ruth's room. The brick wall of the apartment building facing him was blank, that is, it contained no windows. After a minute examination, Duvall was forced to the conclusion that no entrance to the girl's bedroom had been made through it. The bricks were solid, immovable, the cemented joints firm and unbroken. A moment later he turned to the left.

Here the rising wall of the attic story of the house faced him, reaching to a point above his head. Two dusty and long unopened dormer windows, similar to those facing on the court, confronted him. He remembered that the servant of the house next door had informed him, earlier in the week, that the attic was, and long had been, unoccupied.

Whether the attic was tenanted or not, however, had no bearing on the problem which confronted him. The windows might serve as a means whereby anyone could reach the roof of the back building from the house proper, but they did not suggest any means whereby anyone might reach the windows of Ruth's bedroom. And by ascending to the point on the attic roof where his ladder stood, the problem was no nearer a solution, for a person standing there was on the edge of the court between the buildings, seven feet or more above the girl's bedroom window, and as many away from it. He turned away, and approaching the rear edge of the back building, looked over.

To his left, some eight feet away, was the fire escape before the rear window of the girl's bedroom. Standing on that sharp edge, he realized that in no way could he reach the railing of the fire escape, except by jumping, a feat that an expert gymnast might have hesitated to attempt, at that height above the ground. And could it be done successfully, what about the crash, the noise which must inevitably result from such a performance? What about the damage to the paint upon the fire escape's iron surface? And yet it would seem that a young girl had accomplished this feat, without noise, without making the least mark to register her passage. He thought of the tell-tale handkerchief, which he had found on the fire escape earlier in the evening, then turned back with a feeling of annoyance. The thing was, he realized, an impossibility.

A sudden sense of the passage of time made him hurry to the roof of the rear building of the house at No. 162. Like its neighbor, it was built with an attic story, and in the rear were two dormer windows opening in the same way upon the lower roof. Could these windows, by any chance, be those of the room of Marcia Ford? It seemed highly probable, since, if she had operated from the roof, they could afford an easy way to reach it. Very cautiously he crept up to the nearer of the two windows and looked in.

The room before him was in total darkness, and the very faint radiance from without was not sufficient to enable him to distinguish anything within it. The window, however, he saw to his delight was open, and the opening, although small, was quite large enough to enable him to crawl in. Holding his electric torch in one hand, he crept into the room.

The beam of light from his torch, although powerful, was, of course, very concentrated. He swept it about the room, to make sure that it was unoccupied. It was a small room, long and narrow, with the single dormer window, by which he had just entered, at one end, and a similar one at the side, in the slanting mansard roof. It contained a small bed, a chiffonier and dresser, a table, some chairs and a trunk. It was a woman's room; one glance at the dresser told him that, and a handkerchief lying crumpled on the latter's top proved to be identical with the one he had found on the fire escape, both in its general character, and in the initial "F" in one of its borders. Beyond any doubt, he was now in Marcia Ford's room.

Had he been inclined to doubt it, two photographs upon the wall would have convinced him. One was a picture of the Ford girl herself. The other was a portrait of the woman of the cab, the one that Duvall fully believed to be the author of the attacks upon Ruth Morton.

He examined the various articles about the room with the utmost care, but nothing of any interest rewarded his search. It had been his hope that he might find something of definite value—the typewriter, perhaps, upon which the threatening letters had been written, the black sealing wax, used in making the death's-head seals, the paper employed by the writer. None of these things was in evidence; there was no typewriter, the table contained a small bottle of ink, a couple of pens, and some cheap envelopes and a writing tablet of linen paper quite different from that upon which the warning letters had been written. There was nothing, absolutely nothing, in the place to connect its occupant with the sending of the letters, except the room's location, in such close proximity to that of Ruth Morton, and the photograph of the woman of the cab, hanging upon the wall.

Duvall, greatly disappointed, was about to take his departure, when he observed at the far end of the room a door. Whether it led to another room, or to a bathroom, or merely to a closet, he did not, of course, know. There was danger, he fully realized, that Marcia Ford might return at any moment. There was equal danger that, upon opening the door, he might find himself in another room, possibly an occupied one. He thought at one time that he heard sounds on the far side of the door, but when he paused and stood listening he could distinguish nothing, and concluded that he had been mistaken. Shutting off the light of his pocket torch for the moment, in order that, should the entrance lead to another room, its rays might not betray his presence, Duvall grabbed the door knob, and, turning it softly, opened the door.

For a moment he had a glimpse of a black cavern, and then, with incredible swiftness, something struck him a heavy blow in the face. What it was he was too much surprised and stunned to realize. His electric lamp fell from his hand, and clattered to the floor.

Realizing his helplessness in the almost total darkness, he bent down, groping about in an unsuccessful effort to recover the searchlight. And then, with a loud cry, a heavy body projected itself upon him, grasping wildly at his hair. An arm, clothed in some silken material, encircled his throat. He felt himself choking. And at the same moment a strange and irrational terror seized him. He seemed in the grasp of something uncanny, something inhuman, in spite of its very human cries. With a shudder he sprang to his feet, unable to locate the missing electric torch, and shaking the shrieking figure from him, plunged toward the window by which he had entered. It was not alone the surprise, the nameless terror of the thing, that sent Duvall headlong from the room. He fully realized that the noise of the encounter, the shrieks of his assailant, would quickly bring the other inmates of the house to the room. He had no wish to be discovered there—his entrance had been too irregular, too illegal, for that. With extraordinary rapidity he flung himself through the window and without waiting to observe the results of his intrusion, sped swiftly across the roofs of the two buildings, up the steps to the attic roof, and from there, by means of the ladder, to the roof of the apartment building. The janitor sat where he had left him, smoking a pipe. Duvall looked back. Lights were visible in the room he had just left. He saw a figure, one that closely resembled Marcia Ford, cross the lighted area of the window. There was a second figure with her—smaller, shorter, he thought. Who—what was it that had attacked him? He stood in a daze, unable to grasp the meaning of the experience through which he had just passed.

The janitor took his pipe from his mouth and rose.

"Find what you were looking for?" he asked with a grin. Duvall shook his head.

"No," he said. "Not exactly. But I'm on the track of it."

"Want the ladder any more?"

"No, not to-night." He assisted the man to draw it up to the roof.

A few moments later he had reached the sidewalk. He glanced at his watch. It was just eight o'clock. As he walked toward the entrance of the house at No. 162, the front door opened, and a woman came out.

Duvall quickened his pace, but the woman was also apparently in a great hurry. She ran swiftly across the sidewalk, and sprang into a cab which stood beside the curb. Duvall was able to get but a fleeting glance at her, but that glance was enough to convince him that she was the mysterious prisoner who had so neatly given him the slip while in the cab the night before. He sprang forward with a cry, but before he had come within ten feet of the cab, the vehicle dashed off and proceeded at a rapid rate up the street.

A second cab came along at almost the same moment. Duvall hailed it, but the driver shook his head, indicating that he had a fare. In a moment the second cab had passed, apparently in hot pursuit of the first. There were no other cabs in sight. With a growl of anger and annoyance Duvall turned back to the door of No. 162.

Should he ring the bell and ask for Miss Ford? he wondered. Of what use would it be, to request an interview? Yet there seemed to be nothing else that he could do. Miss Ford had not left the house, although the other woman, apparently her confederate, had done so. He stood in the shadow of the apartment building, trying to decide what move he should make next.