“What do you make of it all, captain?” he asked, in a conciliatory tone. “You were on the ground, and ought to be able to form a better judgment than any of the rest of us.” “It’s gumshoe work,” Grail answered; “a trick of some of those foreign spies who have been hanging around ever since Colonel Vedant started on his present series of experiments. They thought, no doubt, that, with a hurry call of this sort, they might catch him with some of the papers on his person.” “Then, you believe that Schilder is----” Grail shook his head. “Too obvious,” he objected. “Whatever else Schilder may be, he is not a fool.” “But whom else can we suspect, under the circumstances?” queried Appleby. “Have you any theory at all, captain, that will account for the mystery?” The adjutant hesitated a moment. “I think I will wait to answer those questions, major, until after we have examined the colonel’s note.” “Ah, true!” assented the other. “That must naturally be our starting point. And here we are!” The automobile turned in from the tree-shaded street, and sped down the roadway past officers’ row. It halted in front of headquarters, and the four passengers piled hurriedly out. Grail, abstractedly acknowledging the salute of the soldier on guard, pressed forward in the lead, and, unlocking the door, swung it open. There was no need to switch on the lights, as the room was already sufficiently illuminated by a night bulb which hung in front of the safe. The adjutant, closely followed by the others, advanced to the desk, then paused, with a little gasp of bewilderment. “Why,” he exclaimed, “the note is gone! I am positive I left it here.” He turned to the colonel’s “striker,” who lounged sleepily in the adjoining room, to inquire if any one had been there in his absence. “Not a soul, sir,” was the answer. “Then, have you yourself been in here, or touched any of the papers on the desk?” “Haven’t stirred from my seat, sir, since you and the colonel went.” That seemed to settle pretty well the question of outside interference, for, with the guard outside and this man seated where he could command the whole interior of the place, no person could have entered undetected. Yet the note was indubitably gone. The drawers of the desk were ransacked, the files gone over, even the floor thoroughly searched, without revealing the slightest trace of it. With all the doors and windows closed, there was no chance of it having been carried away by some frolicsome breeze. Major Appleby regarded Grail with a portentous frown. “Captain,” he said stiffly, “this is very, very strange.” CHAPTER III. UNDER SUSPICION.