They mounted swiftly into the face of the moon, higher, ever higher. Beneath them lay a cold world making ready for sleep. Behind them glowed the lights of the city, of the city going about its affairs, unconscious of the calamity that threatened. Richly gowned men and women occupied theater chairs; cafÉs, restaurants with their gaudy cabarets, were preparing to receive their guests. Lights were blinking out in the homes of working-men and of millionaire as the inmates retired, conscious of security, for the night. In many homes were dancing and gaiety; in other homes—a scattering few—nightly family prayers were being said. As on any other nights, the down-town streets were quick with moving motor-cars, or black with automobiles parked and waiting until their owners should issue from theater, from club, from innocent amusement, or from squalid place of license and artificial joy.

Detroit lay spread out in great panorama, sleeping, spending, each inhabitant flying high or flying low, according to his means and his desires ... all defenseless against the thing that threatened; defenseless save for that frail craft of the air that mounted and mounted on fragile wings until it was but a speck against the illuminated heavens!

Steadily, swiftly it sped to take its place as sentinel at those high gates; to bar the roads of the sky; to ride the clouds a challenger, a champion, a winged guardian, steadfast, courageous, eagerly dauntless....

Potter felt the splendor of it; it uplifted him. Soaring there above the lake, he knew a strange sense of detachment from the fetters of the flesh. He felt, not like a man, liable to weaknesses, ills, death, but like an immortal upon a mission for Olympus. He was not afraid of the issue. He was confident. His exhilaration was not of the sort that makes men shout and sing; it was as if some great, glowing dignity rested upon him, to be worn with worthiness. The fate of a city was in his hands, and he rejoiced to have it there....

On he flew, steadily mounting as to some appointed watch-tower. He would mount to a height from which his view would be limited only by the power of his eyes. Nothing could move upon that lake, or in the air above that lake, that could escape him.

He rejoiced in his motor. It did not falter, it was true; if great events must depend upon a tiny mechanism, then this mechanism was worthy of the responsibility.... Almost alone he had created it and given it to his country. It was right that he should be proud.

He hovered and circled, his eyes ever toward that distant island, ever watching, vigilant. Hildegarde sat as vigilant as he, as thrilled as he. She was almost happy.... How distant seemed the squalid events of her life, the tragedy of the day! She, too, was an immortal, experiencing the privileges of immortality. Her heart was high, her soul was confident. It was a better, a greater thing than mere happiness....

The night was still, clear, cold. Ten thousand feet above the earth the cold was indescribable; it gnawed through their garments, froze the very moisture in their eyes, yet, somehow, they were glad to endure it; to endure was part of the service demanded of them.... So they hovered and watched....

Hildegarde touched Potter’s arm and pointed. His eyes had seen it too—that speck against the luminous dome of the sky. They watched it, uncertain if it were the thing they awaited.... Potter shut off his motor, and the stillness left them aghast. It was unreal, uncanny.... They listened. Faintly, a shade this side of inaudibility, was wafted to them a rhythmic splutter, growing stronger, stronger.... It told its story.

Again the roar of the motor engulfed them, and Potter swerved toward the oncoming speck, flying high, maintaining a position thousands of feet above his unsuspecting prey. Presently the other aeroplane became distinct, a black silhouette, the ghost of an aeroplane traversing a worldless sky.... On it came. But Potter knew he was rushing to meet it with twice the speed of its own approach.... He had not been seen. The roar of Cantor’s motor concealed the fury of Potter’s machine. Now Cantor was below them, almost directly below them.... The moment had arrived!

Potter turned his nose downward, swooping like a preying eagle, and as he volplaned he swerved. He was flying on a level with Cantor’s machine, approaching it from the rear. Hildegarde saw the blue shape of a revolver in Potter’s hand and bent forward, tense, breathless for the event.

Cantor, unconscious that he did not occupy the heavens alone, sped onward with singleness of purpose. He was unaware of danger, unconscious of near-flying menace. Potter did not want it so. He could not attack a man, an unconscious man, from behind. What he wanted was combat, man to man conflict. Even a spy bent on treacherous mission of destruction he could not destroy without warning.

Swiftly his ’plane drew alongside Cantor’s machine, passed it, and as it passed Potter looked at his adversary, saw his wide eyes fixed upon him with unbelieving astonishment. Then Cantor was left behind.

Potter mounted, turned, approached again. Flashes of fire spat out from Cantor’s machine, bullets droned harmlessly by.... It was as Potter wished it. He did not fire himself. He was not ready yet, but passed Cantor, circled again and came up from the rear. He was capable, he knew, of circling Cantor’s machine. For every mile Cantor could traverse Potter traversed two.

Now he drew to a position at the side of Cantor’s ’plane again, wing-tip almost touching wing-tip. He could see Cantor’s face distorted by rage and apprehension; saw Cantor lift his arm, saw the spurt of fire from his revolver. The bullet plowed through Potter’s coat behind his shoulders.... Almost simultaneously he answered the fire with his automatic, once, twice, thrice.... He turned in his seat as he passed, but Cantor was still erect, his machine still under control.

Again he repeated the maneuver. Again Cantor fired at point-blank range—and missed. Once more Potter’s automatic spluttered.... Cantor lurched forward, his machine veered, seemed to wabble in the air—and Potter sped away, mounting, circling. As he returned he saw Cantor’s machine careening crazily; saw it dip, plunge, tip perilously with no intelligence to hold it in control.... Then, suddenly, it pointed its nose downward and plunged.... Potter held his breath. He found himself counting, “One—two—three—four—five—six—”

Not like an arrow, not like a plummet, but like a weighted leaf, the aeroplane fell.... Potter leaned far over to watch. Hildegarde, awed in the face of this happening, peered over also, fascinated.... The ’plane seemed to merge with the earth—with the ice of the lake. Then there was a mighty burst of flame, a gigantic, cataclysmic sound which dwarfed the roar of the motor; which seemed to tear the very air into shreds and to threaten to rock the moon in its distant sky.... Potter’s machine rocked and dipped as the blast of air surged upward.... It righted itself and moved onward, smoothly, peacefully—the sole occupant of the sky.

The bombs with which Cantor had meant to devastate a city had let loose their awful force ... harmlessly to the sleeping city.... There would be no need to look for Cantor, for Cantor’s ’plane. They had vanished, been snuffed out by that awful force, wiped clean from existence as if they had never been.

It was a homing flight now, back to a city saved from a peril of which it had been unconscious, of which the major part of it might forever remain unconscious. The thing was done; the high gates had not been forced; the winged guardian had blocked the roars of the sky....

They were descending now, nearing their place of alighting.... Potter, thankful for the moonlight, pointed for the field, the earth leaped up to meet them, touched them, they bounded along its frozen surface....

Potter lifted Hildegarde from her place, carried her into the hangar, himself stiff, aching with the cold. She lay quiet in his arms, her eyes closed, but she was not unconscious; she was keenly conscious.

“I saw it,” she said, her voice a breath. “I was there. I was a part of it.”