Nor did he mention the little fact--and this the reader does not know--of his having followed the respectable Mr. Elijah Callister to the very door of his Fifth avenue mansion before allowing him to pass from before his eyes. For reasons best known to himself Jerry Buck was silent in all these points, neither Frank nor the detective, as a matter of course, dreaming of the knowledge he thus held in reserve. But whatever had been the motive of Caleb Hook in thus penetrating the retreat of the Bats in the Wall--the entrance to which let us say right here, his keen eyes had detected from the suspicious circumstances of the snow being cleared away around the great flat tombstone by the side of which he had come to a halt, when in company with Frank he had followed the strange apparition through the Trinity church-yard--whatever had been his motive, we repeat, it was evident that he had satisfied it now, for he leaped from the table and moved toward the foot of the ladder leading to the church-yard above. "Well, good-bye, boys," he exclaimed, as carelessly as though his unexpected visit had been an every-day affair. "I'll call upon you fellows when I want your testimony. Never mind opening the door; I can raise the stone myself. Frank Mansfield, I want you. Come along with me." He sprang up the ladder, Frank following him. Completely carried away by the manner of the man, not one of the Bats even thought of interfering. The flat tombstone was raised and lowered as they passed out. It was of sandstone, much decayed, and only a light affair, after all. What Caleb Hook had to say to Frank as they once more vaulted the fence, this time walking up Broadway together, we do not know, but it is a fact that the boy, who, had he fallen into the hands of a detective of the average density of skull, would at that moment, in all probability have been an inmate of the Tombs, spent the short remainder of the night upon a lounge in Detective Hook's own room, as comfortably as you pleased. CHAPTER XVI. THE THREE OAKS. The Three Oaks was haunted. At least every one said so, and what every one says is supposed to be true. To be sure, it would have been difficult to have found any person who had actually seen the ghost. Nor was this necessary. Strange lights moving about the old house at night, flitting from window to window, from the great parlor panes upon the first floor to the little diamond-shaped panes up under the roof, had been seen by the passers-by on the Fort Washington road at night, over and over again, and was quite enough for the neighbors who dwelt outside its crumbling walls. But what was Three Oaks and where was it? These are both proper questions, and should be answered at once.