Leaving the Pennsylvania mills the aeroflyer, traveling at high speed landed the party within a very short time on one of the tall landing buildings in New York. Ralph and Alice made their way down to the elevated roadway, where, at Ralph's suggestion they put on their power skates, for, as he explained smilingly, it was but a short distance to his home and the exercise would do them both good and give them an appetite for luncheon. When they were but a little way from their destination Ralph became conscious of a faint hissing sound close behind them. Twice he glanced over his shoulder, but the roadway at that hour—it was just before noon—was deserted.
Yet the sibilant sound persisted, seeming to be getting closer and closer, like some persevering insect about to alight.
Alice apparently heard nothing, or perhaps she thought it merely one of the noises of the street, for she chattered on in the gay animated fashion that was one of her charms, oblivious to the fact that the man at her side was so preoccupied that he scarcely replied to her.
For Ralph had now satisfied himself that there was nothing anywhere around them which could cause that[Pg 119] untiring pursuant hiss. Then from what secret invisible source did it emanate—and why?
To the scientist, accustomed to explaining the unexplainable, it was ominous—menacing—
Again he turned to look behind him, along the deserted way, and at that moment he heard a stifled cry from the girl beside him. He whirled to face her, and faced—nothing! He was alone in the empty street!
Unbelieving, doubting the evidence of his eyes, he stared about him, too astounded for the moment, by this mystifying and amazing disappearance to think collectively.
Above him the sun shone in a clear blue sky, before and behind him stretched the still roadway. Then he was aware of the silence, the deadly quiet. For the hissing had receded into nothingness, and with it, Alice.
As the full force of the catastrophe struck him, something akin to panic seized him. Danger to himself he could have faced with the calm courage of a brave man, but this unseen and unexpected blow from an invisible source aimed at the girl so close and dear to his heart smote him with a chill terror that for an instant held him powerless in its grip.
That he should have been careless when she was in danger—but this was no time for self-reproaches. To act, and to act at once—that was vital.
Thoughts of high frequency radio waves—of X-rays—of Fernand—
"Fernand!" he exclaimed aloud, and with the name coherent thought returned. Putting on all possible speed he covered the distance to his home in a few seconds and dashed up to his laboratory, the while his swiftly-working[Pg 120] brain attacked the greatest personal problem that it had ever been called upon to solve.
Having experimented with ultra-short waves, he knew that it was possible to create total transparency of any object if the object could be made to vibrate approximately at the same rate as light. He was familiar with the theory, and although he had worked on it at times, he had never seen a practical demonstration of it.[7] He realized a machine was in the hands of someone, intent on kidnapping Alice. He knew, too, that a police description would be flashed within a radius of thousands of miles instantly, it would be necessary for the abductor to keep Alice invisible for some time to come, for fear of some one seeing and recognizing her. All this flashed through his mind as he assembled a detecting apparatus consisting of a portable aerial and a small box containing a few radio instruments and a pair of headphones.
The aerial, by being rotated, could determine the point from which the waves emanated. In ten minutes Ralph had the apparatus rigged up and began rotating the aerial, until a roaring noise was heard in the telephones. He knew that this must be the apparatus producing the invisibility, and within a few seconds he had dashed from the house on his power skates, carrying the detector in front of him. Two of his assistants accompanied him.
The pursuit was on. As they approached the kidnapper the sounds in the telephones became stronger. They sped along Broadway, while the hastily notified police kept the way open. The rising sound in the 'phones clearly indi[Pg 121]cated they were headed directly toward the abductor.
They gained steadily on him while the rolling, flying police cleared Ralph's way with their shrieking sirens, while the kidnapper had to pick his way slowly through crowds.
The chase led them into a narrow street on the outskirts of the city.
The sound that came through the telephones was now exceedingly loud, indicating that the quarry was near by. But this very nearness was confusing to Ralph, for the volume of sound prevented him from exactly locating the invisible kidnapper and the girl. In vain he turned the aerial in all directions, seeking one point from which it came louder than another that would determine the course of his pursuit. For the moment he was halted, and, like some hound baffled by the cunning of the fox, he cast about him eagerly, waiting for what he knew must come, the next move of the pursued man.
And then it came—a deepening tone in the telephones, a gradation of sound that to the trained ear of the scientist told him all that he wished to know. With an exultant cry he sprang forward, and dashed through the entrance of a small store.
The proprietor, whose state of mind may best be described by the word "flabbergasted," struggled for some moments in vain for speech while Ralph and his men, with outstretched hands eagerly swept from wall to wall.
"Here, here, you fellows," he finally managed to gasp, "what are you after? What are you trying to do? You'll knock something over in a minute. Hey, look out there—there it goes!"
For Ralph had reached around a tailor's dummy,[Pg 122] knocking it over as his hands closed upon something behind it, something invisible and yet warm and firm; something that quivered and shrunk away at his touch.
The proprietor, rushing forward to pick up the dummy, stopped short, gaping. Ralph's hands, at the moment of contact, vanished into thin air. But in an instant they re-appeared, as he drew towards him, out of the influence of the ultra-short waves what he knew must be the bound and gagged form of Alice.
Once away from the influence of the apparatus she became visible again. A sack had been tied over her head and shoulders and her hands were tightly bound to her sides. She was still on her roller skates, and her feet had been left free, the sack being sufficient to render her almost wholly helpless, and unable to make any effective resistance.
As Ralph removed the fastenings and rel............