Perhaps it may seem as if the boys had met with success too easily and had accomplished far more in a short time than would be possible. But as a matter of fact they had encountered innumerable difficulties, had made numbers of mistakes, had been faced with failure or negative results time after time and would have given up in despair had it not been for the encouragement of Mr. Pauling and Mr. Henderson and the never-ceasing optimism of Rawlins. Indeed, Rawlins had done fully as much to make the under-sea radio a success as had the boys.

Although he did not or could not become an adept at radio and insisted that it was all Greek to him, yet he was a born inventor and a mechanical genius. He had been diving since he was a mere boy, his father and grandfather had made deep-sea diving their profession, and he felt as much at home under water as on land. Hence, to him, there was nothing mysterious or baffling about the depths and he could see no valid reason why anything that could be accomplished on shore should not be accomplished equally well under water. He had distinguished himself by devising a submarine apparatus for taking motion pictures at the bottom of the sea and it was while engaged in making a sub-sea film that he had invented and perfected his remarkable self-contained diving suit. To him, with his experience, the shortcomings of the suit—the danger of the chemicals flaming up if they came in contact with water—were of no moment, for, as he had explained to the boys, he automatically shut the valve if for any reason he removed his lips from the breathing tube, the action being as natural and unconscious as holding one’s breath when swimming under water.

But he at once realized that if the suits were to become a commercial or practical thing, or if the under-sea radio was to be used, it would be necessary to make the apparatus absolutely safe and fool proof. He therefore set to work at once to devise an entirely new system and absolutely refused to allow the boys to don suits and go down until he had thoroughly tested out and proved the new equipment. It was not an easy matter, but in the end he succeeded, and, risking his own life in the experiment, he gave the safety suit a most severe tryout. It fulfilled his greatest expectations and feeling sure that no matter how careless or inexperienced the wearer might be there could be no accident, as far as the suit and oxygen generator were concerned, he was satisfied.

He freely expressed his satisfaction and his indebtedness to the boys, insisting that if it had not been for them and their radio he never would have improved the suit and made it practical for any one to use without danger. In addition, there were innumerable other changes and alterations which had to be made to adapt the suits to radio work, and so, by the time the boys were ready to make their tests, they were using suits which bore but little resemblance to those Rawlins had first shown them.

Upon the helmets were the odd grids of wire at right angles like some great crown; the compressed air receptacles containing the sending sets were attached to the shoulders like old-fashioned knapsacks, and the front of the helmet resembled some grotesque monster’s head with the protuberance which contained the compact little receiving set like a huge goiter. Indeed, as Henry had remarked when he first saw Rawlins appearing dripping from the river, they looked like weird and fearful sea monsters. So, if the reader imagines that the boys and Rawlins had had an easy time or that their success was of the phenomenal kind which occurs only in fiction, he is greatly mistaken and the impression is due wholly to the fact that their failures and troubles have not been chronicled.

And now, having explained this, let us return to the boys when, their sub-sea sending set complete, the test was about to take place. As Tom sank beneath the water and slowly descended the ladder he was more excited and thrilled than ever before, for he was about to try an experiment which, if successful, would mark a new era in radio telephony and he was keyed up to a high pitch when at last he dropped from the final rung of the ladder and settled, half-floating like some big, ungainly fish upon the river bottom. Through the half opaque green water he could see the irregular, grotesquely distorted and hazy form of Rawlins appearing gigantic and phantomlike. He might have been fifteen or fifty feet away, for despite the fact that Tom had been down several times he could never accustom himself to the deceptive effects of distance under water and when he stretched his hand towards the indistinct figure he gave an involuntary start when he found Rawlins within arm’s length. As his hand touched the clammy rubber surface he uttered an exclamation of surprise and the next instant gave a joyful yell, for at his ejaculation he had heard Rawlins’ voice in his ears asking, “What’s wrong?”

“For heaven’s sake, don’t yell so!” came Rawlins’ words in response to Tom’s, “Hurrah, it’s working!”

“I’ll tell the world it’s working!” continued the diver, “but don’t shout. I’m talking in my lowest tones. Here, how do you like this?”

Tom’s ears were almost split as a thunderous bellow filled his helmet, and involuntarily he clapped his hands to the outside of his helmet over his ears.

“That’s a lesson,” he said in his lowest tones. “Sorry I didn’t know, Mr. Rawlins. It won’t happen again. I guess these helmets act like sounding boards or something. Hello, there’s Frank’s voice.”

Clear and distinct they could hear Frank asking if there was trouble and Tom barely checked another outburst as he realized that the boys on shore could talk with them and could hear what was going on under the water.

“We can hear everything you say,” went on Frank’s voice. “Can you hear us and each other?”

“Gee, you bet we can!” replied Tom. “Isn’t this just great?”

“Say, are you whispering?” inquired Frank. “I can hardly hear your voice.”

“No, but don’t shout so,” answered Tom. “Down here everything just roars. We have to talk low or we’ll deafen each other. I’ll bet we don’t need head phones on our ears under water.”

“Henry’s going to talk with you,” Frank announced, “he’s just crazy to try.”

For the next half hour the boys talked back and forth between the workshop and the bottom of the river and then Rawlins and Tom ascended the ladder and removed their suits.

For fully five minutes, the boys pranced, danced, hurrahed, yelled, laughed and made such a racket celebrating their success that it was a wonder the river police did not break in thinking a horde of Indians had taken possession of the dock. And if the truth must be told, Rawlins was just about as excited and acted as crazily as the youngsters.

But at last they calmed down and Frank, mad to go down, donned Tom’s suit.

“Try it without the phones,” Tom advised him. “Then you can talk loudly enough to be heard up here without deafening Mr. Rawlins.”

To Tom, listening at the set on the dock, it seemed little short of uncanny to hear Rawlins and Frank talking from under the water, and indeed, it impressed him as even more remarkable than hearing those on shore when he was below the surface.

Both Rawlins and Frank assured him that the sets worked far better without the receivers on their heads, and even when Frank spoke in his loudest tones Rawlins replied that it did not deafen him as before.

“Now let’s try tuning, Frank,” said Tom. “I’m going to vary my wave length and see if you can pick it up. Then change yours and I’ll see if I can get you.”

As Tom spoke, he altered the sending waves slightly and breathlessly waited. Presently Frank’s voice came in.

“Got it!” he exclaimed. “Had a bit of trouble at first, but it’s all right now. Now see if you can get this.”

As he spoke his words ended in a high, shrill squeal, but an instant later, as Tom turned the knob on his tuner, the words suddenly returned in a most startling way, the squeal seeming to change magically into words.

“Hurrah, I got it!” cried Tom jubilantly. “Come on up, Frank, Henry wants a chance.”

“You’ve certainly struck a wonderful thing here,” declared Rawlins, when he and Henry came up and had removed their suits. “How far do you suppose it will work?”

“That’s something we’ll have to find out,” replied Tom. “But the sounds come so loudly I’ll bet it’s good for a long distance. Somehow or other we get sound a lot louder inside a helmet than outside. I don’t just get the reason, but I expect it’s either because the whole air vibrates to the diaphragm of the receivers inside the helmet and no sound waves are lost or else because the helmet itself acts like a sounding board or maybe there are some sort of amplified waves set up.”

“I guess it’s the air being inclosed,” said Rawlins. “When I used to wear a regular suit and used an ordinary phone under water it was the same way, but I never thought of it in connection with radio. The whole thing gets me, there’s millions in this if we can patent it. Let’s go down once more and give her a real tryout. We’ll take a hike down river a few hundred yards and see if the boys get us. If they don’t we’ll come back and keep trying and if they do we’ll go on down as far as we can. Then, if we find it’s O. K. we’ll try to get your folks to let you go down to Nassau and we’ll show the world, I’ll bet.”

“That’s a good idea,” agreed Tom. “You keep listening and now and then talking, Frank, and as soon as we lose your voice we’ll send and then walk back until we get you again. That way we can find if we can hear farther than you can or whether it’s the other way about.”

Donning their suits, Tom and Rawlins once more descended the ladder and half-floating, half-walking turned downstream. Rawlins had already cautioned Tom to keep close to his side and to hold to his hand, for, with the mud stirred up by their feet and carried by the current with them, it was impossible to see more than a few feet and Rawlins knew the danger that lay in becoming separated.

Even with the radio connecting them with the boys on the surface Tom might easily get confused and hopelessly lost if he strayed or was carried from sight of Rawlins and while Tom knew that, by turning on more oxygen, he could bob to the surface, yet danger lurked in this as he might emerge in the path of some steamer or motor boat and be run down or torn to pieces with the propellers. As long as they kept close to shore, following the docks and piers, there was no danger, for the only vessels in the vicinity were canal boats and barges which were not in use, the piers for several hundred yards having been used merely for storage and as warehouses for some time. Moreover, by keeping under the docks they were perfectly safe and Rawlins had no intention of going out into the channel with its swift currents and constantly passing tugs, ferryboats and small craft. So, half feeling his way and moving by the diver’s intuitive sixth sense of direction and holding to Tom’s hand, Rawlins moved slowly down the river.

Frank’s words were constantly in their ears and now and then they replied, and somehow to Tom there was a most remarkable sensation of making no progress whatsoever. There was nothing visible by which to gauge their motion and, as the voice through the set continued to sound exactly the same and did not grow fainter with distance, he seemed to be standing still, although exerting himself and constantly stepping or rather pushing himself forward. He was so intent on this and so interested in the novel experience that he scarcely realized that Frank’s voice had suddenly grown faint and was interrupted by an odd buzzing sound which instantly brought back the memory of the sounds they had heard when listening to the mysterious speaker with their loop aËrials. He was just about to speak and ask Frank if he could hear when he felt Rawlins jerk his arm. He floundered forward and the next instant was dragged between the spiles of a dock where the water was dark with shadows.

“What,—what—” he began, but instantly checked his words as a low “Ssh!” from Rawlins reached his ears. Not knowing what had happened or why Rawlins had suddenly acted in this strange manner, confused and bewildered, Tom peered about through the murky water. At first he saw nothing save the surrounding spiles, seeming to move and sway in dim, shadowy forms—the bottom of a canal boat with yard-long streamers of sea weeds waving from its barnacle-encrusted planks; a piece of trailing, rusty cable; a few rotting, water-soaked timbers protruding from the mud; and a shapeless mass which might have been almost any piece of jetsam cast into the river. Then like phantom shapes, so indistinct, hazy and formless that he was not sure they were not shadows in the water, he saw two figures—two moving things that, for a brief instant, he thought must be huge, dull-green fish nosing about the mud. And then, as he gazed fixedly at them from between the spiles, a strange unreasoning fear clutched at his heart and he felt an odd, prickly sensation on his scalp and at the back of his neck, for the moving, sinister, unnatural things were approaching, moving noiselessly, slowly, but certainly towards him as though they had scented his presence and were bent on hunting him out.

What were they? What strange, unknown, impossible sea monsters were these? He was frightened, shaking, and in his terror had forgotten completely about the radio outfit. Glad, indeed, was Tom that Rawlins was beside him, that the diver was armed—for Rawlins, he knew, never went down without a hatchet in his belt ready for use in case of an emergency such as fouling a rope or timber. But why didn’t Rawlins speak? Why had he ordered him to be silent? The sea monsters could not hear; what was the reason?

And then, so suddenly that it came as a shock, he realized that the approaching forms, the grotesque shapes, were no sea creatures, no gigantic savage fish, but men! Men in diving suits much like their own. Men walking in the odd, half-sprawling, half-floating, forward-leaning posture he knew so well. But great as was Tom’s relief at this discovery his wonderment was doubly increased. Who were they and what were they doing here? Why had Rawlins drawn him into hiding? What did it all mean? Then, just as he was about to disregard Rawlins’ whispered orders and ask, the two figures disappeared. Without reason, without warning, they vanished from sight as if by magic.

So dumbfounded was Tom that involuntarily he uttered an ejaculation of surprise and fairly jumped when, faint but clear, he heard Frank ask, “What’s that you said?”

But before he could reply, Rawlins was speaking. “Come on!” he whispered, his voice being as low as if he feared the others might hear and, quite forgetting that he was under water, cut off from all conversation with other human beings save the boys. “Come on, I don’t know who they are, but there’s something funny. They’ve got suits like mine and the Lord knows who they are or how they got ’em. I’m going to find out where they went.”

Slipping between the spiles with their slimy, weed-grown surfaces, Rawlins, holding to Tom’s hand, struggled forward into the lighter water. Beside them rose a dark wall of masonry and reaching this Rawlins proceeded to feel his way along it. Before they had traveled ten feet the diver uttered a sharp ejaculation. Beside them in the wall, loomed a huge, black hole, the mouth of a great sewer.

“They went in here,” whispered Rawlins. “Come on!”

A moment later they were in utter blackness, feeling their way forward along the walls.

And now, very thin and faint, Tom heard Frank’s voice again. “What on earth’s the matter?” he asked. “I haven’t heard a word from you two for five minutes. Can you hear me?”

Tom was about to answer for they were evidently at nearly the limit of receiving range and his mouth opened, his lips formed the words of his reply, but no sound issued from them. Clear, loud and harsh, guttural words rang in Tom’s ears. This was not Frank’s voice nor Henry’s; the words were not even English. Amazed and uncomprehending Tom was speechless and then, among the incomprehensible foreign syllables, came a word he recognized, the one word “Oleander!”

Instantly he knew that by some strange freak, by some mystifying coincidence he was again hearing that unknown man to whom he had so often listened. It seemed strange, weird, uncanny to have it coming to his ears here in the old disused sewer, but after all, he reflected, why not? Rawlins had heard it once before, there was nothing remarkable about it and he was on the point of asking his companion if he had heard and of trying to tell Frank, when once more his words were stayed. Before him the stygian darkness suddenly grew light, a brilliant beam stabbed down from overhead and through the strangely illuminated water Tom saw the two men in diving suits standing beneath a square opening down which a ladder was being thrust. But why, he vaguely wondered, was the water so transparent? How was it that he could now see clearly for many yards? And then, with a start, it dawned upon him that he was not looking through water, that there was nothing between him and the trap save air. He was standing with head and shoulders out of water.

And now the gruff, guttural words were once more beating in his ears and the next instant he saw the strange divers seize a dangling rope, tipped with a great iron hook, dip it under the water and then, as the hook again ascended, he saw a dripping, cigar-shaped object like a torpedo slowly rise from the water and disappear in the opening above. Close behind it the two divers followed up the ladder, the ladder was drawn up, the light snapped out and the next instant Tom and Rawlins were once more in absolute darkness.

“What does it all mean?” exclaimed Tom, finding his voice at last.

“What does what—” commenced Frank’s voice, only to be overwhelmed and drowned out by Rawlins’ louder words.

“Search me!” replied the latter. “Something rotten going on here. Don’t know what, but I intend to find out. Did you hear them talking?”

“Hear them?” replied Tom not understanding. “Of course not. But I heard that same chap you heard the other day—talking Dutch or something.”

“That was them!” announced Rawlins decidedly. “Tom, they’ve got under-sea radio, too. It’s those chaps we’ve been hearing, I’m beginning to get it. That word Oleander. That’s a password—a countersign. Just as soon as they spoke it the door opened. There’s some deep mystery here. What the deuce that torpedolike affair was I don’t know. Perhaps they’re trying to blow up some building. This sewer is under a busy part of the city. Hear those trucks and surface cars overhead?”

Absolutely dumbfounded, heedless of Frank’s insistent but weak voice in his ears, striving to grasp all this astounding statement of Rawlins’, Tom stood speechless for a moment. And then an idea flashed through his mind.

“Gosh!” he exclaimed. “Say, Mr. Rawlins, they’ll find us. If they’ve got radio they can hear us too! Say, perhaps they’re listening to us now. Come on, let’s get out of here.”

Rawlins’ surprised whistle came shrilly to Tom’s ears.

“You’re right!” replied the diver. “We’re in a dangerous place. Come on. Let me go first.”

Crowding past Tom, Rawlins hurried as fast as the constantly deepening water and the darkness would permit and presently, though to Tom it seemed hours, a lighter space appeared ahead and a few moments later they once more were standing at the bottom of the river.

They had turned to retrace their steps towards their own dock and were following along the old wall when once more they were halted in their tracks. Again to their ears, borne to them by the radio waves, came the harsh foreign words.

So close did the words sound in their ears that instinctively, without stopping to think that the speakers might be hundreds of feet or even yards distant, the two crouched back in a recess of the masonry, flattening themselves against the slime-covered, weed-draped stones and gazing apprehensively towards the spot where the old sewer pierced the wall.