"All right," assented his chum. "I hope it amounts to something." With strained ears they listened. Now they heard steps coming along the corridor. Curious, shuffling steps they were, not hard, honest heel-and-toe steps--rather those of someone treading softly, as on soles of rubber. "It's him all right this time!" whispered Andy in Dunk's ear. "I guess so--yes. Shall we follow him?" "Yes. Take off your shoes." Silently they removed them, and waited. The steps were nearer now, and a long shadow was thrown athwart the place where Andy and Dunk were hiding. They could not recognize it, however. The shadow came nearer, flickering curiously as the swaying of an electric lamp threw it in black relief on the corridor floor. Then a figure came past the recess where the two lads were concealed. They hardly breathed, and, peering out they beheld Mortimer Gaffington stealing into Wright Hall. It was only what they had expected to see, but, nevertheless, it gave them both a shock. Mortimer moved on. They could see now why he could walk so silently. He had on rubbers over his shoes. The same trick used by the thief who had entered Frank's room. Mortimer looked all around. He stood in a listening attitude for a moment, and then, as if satisfied that the coast was clear, started up the stairs toward the corridor from which opened the room of Andy and Dunk. The two waited until he was out of sight, and then followed, making no more noise than the thief himself. They timed their movements by his. When he advanced they went forward, and when he stopped to listen, they stopped also. It was like some game--a very grim sort of game, though. There was only a dim light in the upper corridor, and, coming to a halt where the shadows were deepest, Andy and Dunk watched. They saw Mortimer stop before a student's door, try it and then came the faint tinkle of a bunch of keys. "Skeletons," whispered Dunk. Andy nodded in assent. The manipulation of the lock by means of a false key seemed to come easy to Mortimer. In a moment he was inside the room. What he did there Andy and Dunk could not see, but he remained but a few minutes, and came out, softly closing the door after him. "I wonder what he got?" whispered Dunk. "We'll soon know," was Andy's answer. Mortimer went softly down the corridor. He did not try every door, but only went in certain rooms, and these, the two watchers noticed, were those where well-to-do students lived. Mortimer made four or five visits, and then moved towards the apartment of Andy and Dunk. "It's our turn now," whispered the latter. Silently they turned a corner, just in time to see Mortimer enter their room. "Now we've got him!" exulted Andy. "Not yet; we've got to nab him," whispered Dunk. "Oh, Andy, this is fierce! To think that we're spying on a Yale man! To think that a Yale man should turn out to be a common thief! It makes me sick!"