"Well, sir, we was following a woman, sir. She went to a hotel on Seventy-second Street, and Mrs. Duvall told me to watch for her. I did, and tracked her to the place at 96th Street. Then I telephoned to Mrs. Duvall to come, and she did." "What time was that?" "About half-past nine, sir." "All right. Go on." "Mrs. Duvall came, sir, in another taxi. I pointed out the place where the woman went in, and Mrs. Duvall went in after her. She didn't say I was to wait, but I guess she expected me to, because she had sent the other cab away. I waited over two hours, and then, when she didn't come out, I supposed she had returned to her hotel, so I came back, too. She wasn't there, though. That's why I left the note." "How did you think Mrs. Duvall could have gotten back to her hotel, if you were watching the door of the apartment house all the time?" "I wasn't watching it all the time, sir. I went into the drug store once, sir, and got a cigar. And then, later on, I went to a saloon a piece down the Avenue and got a glass of beer. Mrs. Duvall didn't say I was to watch the place, sir. I thought when she got through what she had to do, she would come back to the cab. But she didn't. Do you think I ought to have waited, sir?" The man seemed greatly distressed. "No use talking about that now," Duvall remarked, shortly. "I want to drive there at once. Get on the box, with the chauffeur, and point out the place to him." "Yes, sir." A moment later they had started on their way uptown. Knowing as he did Grace's impetuous nature, Duvall could only conclude that her pursuit of the woman had led her into some trap. What danger she might at this moment be facing, he could only surmise. The apartment building, when they finally reached it, presented a grim and forbidding appearance. Not a light broke the darkness of any of its windows. The drug store on the opposite corner, too, was closed for the night. The whole locality was dark and silent. "There's the place, sir," Leary exclaimed, as they drew up to the corner. "Tell the driver to stop a few doors up the block--not right in front of the building." Leary nodded. Presently the cab stopped, and he and Duvall got out. The detective's first move was to ascertain whether or not the building had any rear exit, by which Grace might have left, without being seen by Leary. He walked down the avenue to its rear wall, only to find that it abutted against the wall of the next building. There was no rear entrance. If, then, Grace had not left the place during the past hour, she must still be in one of the ten flats that formed the five floors of the building. But which one? That, apparently, was the problem he had to solve. It would be useless, he felt, to inquire at the doors of the various apartments at this hour of the morning. Admission, at least on the part of those he sought, would certainly be refused. Yet he felt that there was no time to be lost.