One day has elapsed since the visit of Frank to Miss Edna Callister at the house in Cottage Place, upon which occasion, the reader will remember, the missing parchment containing the secret of the hiding-place of the treasure buried by his grandfather was strangely placed in his hands. Upon further examination, the parchment placed by the mysterious woman upon the window-sill proved to be the document named in the will of Jeremiah Mansfield. Bidding farewell to the faithful girl--not, however, without having promised to see her again at the very first opportunity afforded him--Frank hurried to the Police Headquarters, where at this hour in the evening it sometimes happened that Detective Hook could be found. The parchment, which he studied carefully, gave the most minute instructions as to where the buried treasure could be found. With his soul filled with triumph, Frank hurried through the broad corridors of the Mulberry street building and entered the office where Mr. Hook was usually to be found when not engaged on duty elsewhere. He was all eagerness to tell of his discovery and ask what, under the circumstances, had best be done. But Frank was doomed to disappointment. Caleb Hook had not been seen at headquarters since the night before. Nor did he appear next day. Frank, employing the messenger boys of the district telegraph freely, was unable to find the slightest trace of his whereabouts. Mr. Callister was at his office as usual, and appeared more sleek and urbane than ever before. He made no reference to the little affair at the elevator, treating the disguised Mr. Maxwell with even more politeness and consideration than was ordinarily the case. Considerably alarmed by the unexplained absence of Detective Hook--doubly so from the fact that he had neglected several most important engagements that day--Frank resolved to confide the situation in part to Barney the Bootblack, and assisted by the Bats, whom he had learned to trust fully as the rough but honest lads that they were, to investigate the truth of the statements contained in the parchment for himself. After the close of the day's business, therefore, he paid a visit to the vault and arranged to start upon their expedition that very night. Thus it happened that we find the "Bats" leaving the wall in the manner described just as the clock of old Trinity rings out the hour of twelve. "Now, then, where's de place?" asked Barney, as the four boys hurried along New Church street in the direction of the Rector street station of the Sixth avenue elevated road. "It's at Fort Washington," replied Frank--"a good mile beyond the 155th street station. Up the steps, boys, and we'll start at once. Remember, Barney, if I succeed in this undertaking and in clearing my name before the world the 'Bats in the Wall' will never have occasion to regret their kindness to me."