Elmer calculated to a nicety just how far they ought to allow their chum to get before starting to follow. It was important that they should be concealed from the eyes of the four in ambush; and yet, on the other hand, he did not want to drop back to such a distance that they might be cheated out of hearing what happened when the surprise came. In order to maintain a certain distance in the rear he had instructed the one ahead to keep up a steady whistle. Lil Artha was known to be a whistler, and often amused his chums by his accomplishment in this line. It was a gift, such as an occasional boy finds himself in possession of. And more than once had Elmer told his friend that he would make a good woodsman if only he turned his talent toward imitating the various clear sweet notes of wild birds. They could hear him easily now, and Elmer fixed the sound in his mind. As he had cautioned Lil Artha to keep up a steady flow, it would become apparent that they were either diminishing the distance or adding to it, if that whistle became louder or softer in volume. Five minutes passed. Elmer caught a big sigh close beside him, which he knew must proceed from the impetuous Nat. Doubtless every sixty seconds that dragged by seemed like an age to the Scott boy; who fancied that after all their trouble perhaps they were going to be cheated out of their fun, and that the plotters had weakened at the last round. Not so Elmer, who estimated things at their true value, and not by the rapid pulsations of an excited heart. "Cheer up, Nat," he whispered in the ear of the other; "it's going to come pretty soon now." "Oh, I hope so!" sighed the one who loved action above all things. "He's stopped whistling, Elmer!" whispered Toby, excitedly. "No, there he starts again," replied the leader, who in truth suspected what the little break in Lil Artha's melody might signify. Possibly he had caught some suspicious rustling sound, and unconsciously held his breath for just five seconds in order to listen better. Was it a false alarm, or would the music begin immediately? Warned by this suggestive hint, Elmer waited, fully expecting to hear a loud voice suddenly break forth from some point ahead. Since this was not "Out West" where lawless desperadoes held sway, it would hardly come in a hoarse demand to "throw up your hands," but in some milder fashion. And presently Elmer realized that his guess had hit the mark. The whistle suddenly ceased. Then they heard a voice call out in the most familiar way possible: "Hello, there, Lil Artha! Hold up a bit, won't you?" CHAPTER XIII. FRIENDS IN TIME OF NEED. A NERVOUS hand gripped the sleeve of Elmer's jacket.