The driver of a coal wagon, which had drawn up before an imposing structure devoted to the storage of army supplies, and supposed to contain an immense supply of ammunition, suddenly conceived the notion that he was doing overtime duty. At least such was his manner when one of the Big Ben clocks overhead ding-donged the hour of six. Perhaps, too, the movement of gathering up reins and whip had its measure of prompting in the appearance of the driver's mate from some underground space in the big building. At any rate, the old nags dragging the heavy vehicle were given the full benefit, and without warning, of a long and knotted whip-lash, and covered several city blocks at a lively gait before they realized that they were traveling out of their class. The heaver who had emerged from the building in response to the clock summons showed tremor of the hands when he lifted them to draw the cape of his greatcoat closer about his throat. "It's set for eight," he hoarsely whispered; "I turned the key when I heard the strokes outside." Strangely enough, the wagon kept a course directly to a residence section at once fashionable and quiet, and hardly the possible location of a coal yard or the home, either, of a humble employee thereof. One of the men in the wagon, the fellow with the hoarse whisper, left the vehicle in a square marked by a tall column with a statue on top, while the driver continued the urging of his horses up the ever ascending street. Gaining the level above, the horses were given their own heads, which meant a snail's pace. Close at hand were two towers of considerable height. While the horses plodded on the highroad stretching to the west, pressure on their bits was lacking. The wagon was empty. Two figures appeared on the terrace back of the twin towers, these terraces rising in tiers from the bank of the fast-flowing river below. "You left Ricker in the square?" This question put by the man who evidently had just returned from a mission that did not include a ride in a coal wagon. "He left me, rather," replied the late driver, with a touch of grim humor. The first speaker held a watch in his hand, consulting it frequently, holding it closer and closer to his eyes as the light faded before the advance of night's shadows. "Seven o'clock," he announced. "Another hour." This was the last notation of time by the watch holder. There was an explosion that, notwithstanding the distance, seemed to shake the everlasting hills to their very foundations. The men on the terrace stared aghast, each at the other. "The die is cast," cried the one with the commanding voice, "and an hour too soon!" CHAPTER XXIII. A LEAP FOR LIBERTY. THE Boy Aviators had just left the mess hall, and were proceeding to the hangars where the No. 3's were housed, fully intending to carry out their compromise plan of giving Roque and Schneider the means to escape, and return themselves as hostages for the honor of the aviation chief.