“That’s another part of the story,” Nick replied, smiling. “One of them evidently is on the way here. Some one whose initials are S. D.,” added Belden, glancing at the message. “If you can identify him and find Gus Dewitt——” “I shall certainly do the latter,” Nick interposed. “But you are wrong in regard to the other.” “How so?” “S. D. does not, in all probability, refer to a man.” “A woman?” “No.” “To what, then?” “To a special-delivery letter,” said Nick confidently. “Oh, by thunder!” Belden exclaimed. “That must be right, too. You have nailed every point in both of these messages.” “And the next step, Belden, is to nail the special-delivery letter,” Nick declared. “It presumably is coming from Philadelphia, and most likely sent by this man Martin. Do you know whether a mail from Philadelphia has arrived here since ten this morning?” “There has not,” said Belden promptly. “I know all about the mails. One is due here from Philadelphia at two o’clock.” “Very good. Let me use your telephone to talk with one of my assistants. I want him to meet me at the post office.” “Certainly. Go as far as you like.” “In the meantime, Belden, kindly make me a copy of each of these messages,” Nick added, turning to the telephone. “I then will be off to intercept that special-delivery letter. I may yet succeed, I think, in putting something over on Martin, Dalton, and Dewitt.” Belden hastened to comply. Nick called up the Shelby House, in the meantime, and quickly got in communication with Chick Carter and Patsy Garvan, his two assistants, both of whom he directed to meet him in disguise at the local post office. Then, having again cautioned Belden to absolute secrecy, Nick hastened away to keep the appointment. It was half past one when he entered the post office, where he found Chick and Patsy awaiting him. Without delaying to explain the situation, he at once led the way to the private office of the postmaster, Adam Holden, who readily gave him an interview. Nick then made himself known, introducing Chick and Patsy, after which he exhibited the two telegrams, confiding his suspicions to Holden and stating what he required of him. “But that is decidedly against the law, Mr. Carter, the intercepting and opening of another person’s letter,” Holden forcibly objected. “I don’t see how I can consent to let you do so. It is a very serious offense.” “Not nearly as serious as the circumstances,” Nick forcibly argued. “When dealing with offenders against the law, with a gang of criminals engaged in we know not what, nor have other means of learning, an unlawful step in order to foil them and serve the law may very properly be taken.” “Possibly. I do not feel, nevertheless, that I can permit——” “Now, Holden, you wait one moment,” Nick interrupted. “It is absolutely necessary that I shall see that letter. I will assume all of the responsibility.” “But——”