Hardly had the door to Mr. Henderson’s office closed behind them before Frank commenced to dance and caper wildly about.

“Hurrah!” he shouted. “This is great! We’re real detectives and working for Uncle Sam!”

“Yes, but don’t make such a row,” cautioned Tom. “We don’t want every one in the place to know it and they’ll think you’re crazy. Come on, let’s hurry and tell Henry.”

When they reached Gramercy Square and dashed into Henry’s room and told him of their talk with Mr. Henderson, he was as excited and pleased as Frank.

“Say, it was funny we didn’t think of that fellow using a telephone!” he exclaimed, when the boys had told him of Mr. Henderson’s theory. “And he’s right about that capacity effect of a fellow near a phone. I was a fool not to have thought of it. Why, Jim told me about that long ago. He even said his brother Ed showed him with his set on the San Jacinto. But I guess it must have been because we were so intent on the messages that we couldn’t think of anything else. I’ll bet we can hear folks on the phone through my set right now.”

“That is funny!” declared Tom, when, a moment later, the boys were listening to a telephone conversation coming to them through Henry’s set. “Say,” he continued, “there isn’t much privacy nowadays, is there? Why, if you could amplify that enough, every one could hear everything that was going on over the telephones.”

“Yes, and to think we were so close to getting that other chap’s talk and never realized it,” said Frank. “Mr. Henderson must think we are great radio fans! I’ll bet he had a mighty good laugh at our expense after we left.”

“Well, we’ll not be fooled again,” declared Tom. “If that fellow begins talking to-night we’ll nail him, too.”

“But we can’t locate him,” objected Henry. “So what good will it do?”

“That’s so,” admitted Tom. “But the main thing is to hear what he says. Then perhaps we can make sense out of it.”

“Say,” suddenly exclaimed Henry, “did you fellows notice that every time we heard those messages the fellow mentioned a flower? First ’twas ‘Azalia’ and then ‘Magnolia’ and then ‘Hibiscus’ and last time ’twas ‘Frangi Pani.’ I’d like to know what that meant.”

“I hadn’t thought of that,” said Tom. “Of course Azalia and Magnolia and Hibiscus are flowers, but what’s Frangi Pani—sounds like some sort of Japanese thing to me. I guess this fellow must be talking about boats. Lots of ships are named after flowers, you know.”

“Well, he must have a whole fleet then,” said Henry.

“Perhaps it’s perfumes or he may be in the flower business,” suggested Frank with a laugh.

“Perhaps we’ll get the answer to that when we hear his mate,” said Tom.

“Hope we hear him to-night,” remarked Henry. “Say, what do you think of this scheme?”

For some time the boys forgot all else in examining a new hook-up which Henry had devised and at last left him with final cautions to be at his instruments that evening and each night thereafter until they again heard the unknown speakers.

But it was several nights before the mysterious messages again greeted their ears. Then Frank and Tom caught them at the same instant and both boys gave a little start and looked at each other in surprise, for the first word they heard was “Tuberose.” Once more the name of a flower had entered into the conversation and mentally wondering what in the world this meant the two boys slipped the receiver of the desk telephone from its hook. Hardly had they done so when they almost jumped, as clear and loud, they heard a human voice; but the next instant their spirits sank to zero and they glanced at each other with disgusted expressions, for instead of the voice of the man they had expected to hear they heard a woman’s voice and her words were: “Number, please?”

With a savage jerk, Tom hung up the receiver.

“Gee!” he exclaimed. “Of course we’d get her. I’ll bet Mr. Henderson knew that and just tried to jolly us. Now what are we going to do? If we—Hello! What’s that?”

Clearly to his ears, and interrupting the words of the mysterious man whom they had almost forgotten in their disappointment, came another voice, evidently that of a woman, and pitched in high tones. “Oh, yes!” it exclaimed. “I’m so glad, my dear. Do you know—” Tom drew his hand from the desk phone on which it had been resting and the words trailed off into a faint indistinct buzz. Tom and Frank grinned.

“Well, it works!” ejaculated Frank. “Of course it doesn’t make any difference if the receiver is off or not—we aren’t getting waves over wires. Henry kept the receiver on to-day, didn’t he?”

“I don’t know,” replied Tom. “But say, we’ve got to get busy. That chap’s been talking for the last five minutes and we haven’t put down a thing he’s said.”

Trying to make up for lost time, the two boys jotted down the words that came in, now and then placing a hand on the desk phone to see if they could hear the other party to the conversation, but each time the nasal voice of the woman, gossiping with a friend, was all that came to them. Then the man’s voice ceased and after a few moments’ wait the boys rose from their seats.

“Darn that old hen!” exclaimed Tom, petulantly. “How the dickens could a fellow expect to hear anything with her tongue going like a house afire?”

“Just think what it’ll be when every one’s talking by radio,” chuckled Frank. “And won’t the women have the time of their lives hearing all their neighbors’ gossip?”

“Government’ll have to license ’em to talk, I guess,” muttered Tom. “Come on, let’s go over to Henry’s and see if he had any better luck.”

But Henry had nothing to tell them. He had heard no conversation over the phone except some man talking business with a friend, but he had written down all the words the mysterious man had spoken and showed them to the boys who had explained how they had forgotten to get the greater part of the conversation.

“Tuberose,” Tom read. “We’ll begin next week. Getting stocked up. I’ll bet it’ll wake things up. Too bad we didn’t know then. Might have been a different tale, eh? Oh, Oscar’s all right. Yes, same old place. Nothing doing, old man. Never a suspicion. Oh, it’s a cinch. I don’t know. Some kids, I expect. Got to see him to-night. So long, old man.”

“Just the same old stuff,” commented Tom when he had finished. “Only no figures this time.”

“And another flower,” added Henry.

“Jim would swear he was crazy if he noticed that,” chuckled Frank. “I’m beginning to think that may be it myself.”

For three consecutive nights the boys heard the conversation and despite all efforts failed to hear anything of interest over the ordinary phones while the radio words were coming in, although they heard various scraps of conversations between other persons.

“Mr. Henderson was off that time,” declared Tom, when the boys rose from their sets on the third night. “His theory was wrong. The other chap’s not talking on a telephone, I’ll bet.”

“Doesn’t look that way at any rate,” agreed Frank. “Let’s go down to-morrow and tell him.”

Accordingly, the three boys visited Mr. Henderson the next day and reported the results of their experiments.

“That does puzzle me,” exclaimed Mr. Henderson as they finished. “If you heard others it’s pretty conclusive evidence he’s not on a wire. Did you hear those buzzing sounds or words again?”

“I did,” said Henry, “and I heard ’em just as plain and no plainer when I was a long way from the phone as when I was touching it.”

“Well, we’ve drawn a blank there,” smiled Mr. Henderson. Then, after a moment’s thought, he exclaimed, “Boys, I’m going to take a chance. I’m pretty well convinced something’s going on that’s crooked and I’m going to send some men out and search every building in that block from cellar to garret. You understand, of course, this is a profound secret. No one will know who they are or what they’re after. It must be a surprise visit so don’t even talk it over among yourselves. But I want you to help us a bit. I’m going to start the men out at eight o’clock sharp, to-night. You must be at your sets and listening. If the fellow’s talking, you’ll know when my men find him, either by what he says or the way he shuts off, and if he goes on talking without interruption for half an hour you’ll know you’ve made some mistake and he’s not in that block. Meet me here to-morrow at about this time and we’ll have something to report—or nothing.”

“Oh, and there’s something else,” announced Tom as the boys turned to leave. “Henry called attention to those names of flowers yesterday. We’d almost forgotten about them. Every time that fellow talks he gets a new name of a flower. Have you noticed it?”

Mr. Henderson chuckled. “You’re getting a pretty good training at this, boys,” he replied. “Yes, I’ve noticed that—that’s one thing that influences me more than anything else. There’s some code to those names, I think, and they may prove the key to the whole thing. We’ll find out sometime probably.”

Remembering Mr. Henderson’s injunction about discussing the proposed raid the boys refrained from mentioning it to one another, but could scarcely restrain their impatience until the time came for them to be at their instruments.

Eight o’clock came and, excited and expectant, the boys listened, hoping to hear the message coming in and to learn from its words or its abrupt ending of the success of the raid. But the minutes ticked by, the hands of the clock pointed to half-past eight, and nine o’clock came and went without a word from the source they so longed to hear.

Anxious to learn the result of the search, the boys hurried to Mr. Henderson’s office the following day.

“Another blank, boys,” he announced when they entered his office. “There wasn’t a sign of a wireless outfit in that block. Did you hear anything last night?”

The boys admitted that they had heard nothing.

“But—but there must be a set there,” insisted Tom, utterly unable to believe that they had been mistaken. “Why, we were all around there with our loops and we got cross bearings and knew he was there.”

“It’s a bit mysterious, I grant,” replied Mr. Henderson. “I fully expected we’d locate it, but my men will swear there isn’t even a piece of radio apparatus in the block. They went through it with a fine-tooth comb. Either you boys were mistaken or else the fellow’s moved away. If you hear him again you’ll know whether he’s changed his location. I’m afraid you’ll never locate him by your instruments, though. I’ve used those loops as direction finders at sea and to some extent ashore and I admit I can’t see how you went wrong, but we’ve got to face the fact that he’s not there—at least not now.”

Thoroughly disappointed and discouraged, the boys left the office and for hours discussed the matter with one another, but at the end of the time were no nearer a solution than ever.

“Oh, bother the old thing, anyhow!” exclaimed Tom at last. “We’ve had our fun and now let’s do something else. Dad’s leaving Nassau to-morrow and we can try sending to him when he gets nearer. Wonder what he’ll say about this thing.”

“Yes, but it gets my goat to think that Mr. Henderson will think we’re such dubs,” said Frank. “He thinks we’ve made some big mistake and put him to all that trouble for nothing.”

“Well, let’s forget it,” suggested Henry, and this seeming the best advice the boys followed it and were soon so busy experimenting along new lines that the mysterious conversations almost slipped from their minds, and as no further messages were heard from the same source they decided that by some coincidence the sender had moved bag and baggage from his former location just in time to escape detection by the men Mr. Henderson had sent on the search.

Tom and Frank were overjoyed when, a day before Mr. Pauling’s ship docked, they succeeded in getting a message to him.

“That’s pretty near 300 miles,” declared Tom jubilantly, “and our set’s only supposed to send 100. Say, that’s a real freak message.”

But when, a few moments later, they heard some one calling their letters and this was followed by a question as to their location and the information that the inquirer was the government operator at Fort Randolph, Canal Zone, Panama, the two boys could only stare at each other in utter amazement.

“Jehoshaphat!” exclaimed Frank at last. “We were heard clear down in Panama! Why that’s pretty near 2000 miles!”

“Almost as good as that fellow over in Jersey who was heard in Scotland and Honduras!” cried Tom. “Hurrah, Frank! Let’s try again.”

But despite every effort the boys failed to get a reply from any one more than fifty or sixty miles distant and realized that, by some peculiar atmospheric condition, their dots and dashes had been carried through the ether for twenty times and more their normal sending range.

“That’s something to tell Dad,” declared Tom, and rushing down the stairs he excitedly told his mother of the wonderful feat.

“I suppose it is remarkable, if you say so,” said Mrs. Pauling, “but really, I can’t see why you should not talk to Balboa or Europe or any other point if you can talk to your father’s ship out at sea. One is just as wonderful as the other to me. But I’m proud of you just the same, Tom.”

When, the next day, Mr. Pauling arrived, Tom could scarcely wait to relate the story of his freak message and his father was enthusiastic enough to satisfy any boy.

“Marvelous!” he declared. “And the operator on the San Jacinto tells me you’ve improved a lot since he first talked to you. Says you can send well and had no trouble in getting his message at regular speed. I’m mighty glad you’ve done so well, Son. Just as soon as I have a chance I’m coming up to see that wonder set of yours. How many have you built since I’ve been gone?”

Then Tom told his father of the mysterious messages and what had come of their attempts to locate the sender.

Mr. Pauling laughed heartily. “Well, if you got old Henderson interested he must have believed there was something in it. I don’t know but what there was. I’ll talk it over with him. But I can imagine your disappointment, and his too—when nothing came of it. No, Son, I can’t offer any explanation and we’re as much in the dark as ever about the smugglers. By the way, I met a chap down at Nassau that was just about as keen on experiments as you boys only he’s not a radio fan. No, he’s a diver. He’s invented a new type of diving suit—self-contained he calls it. Just a sort of rubber cloth shirt and a khaki-colored helmet and lead-soled shoes. He goes down without ropes or life lines or air hose. Gets his air from a little box or receptacle strapped to his body. I don’t know what is in it, but it’s some chemical which produces oxygen and he can walk about where he pleases on the bottom. It’s the weirdest thing I’ve ever seen to watch him wade out into the water and disappear and then, half an hour or two hours later, have him bob up somewhere else.”

“Gosh, I’d love to see that,” declared Tom. “Suppose he wants to come up from deep water without walking ashore, how does he manage?”

“He just produces more oxygen so he floats up,” replied Mr. Pauling. “And you’ll have a chance to talk with him next week. He’s returning to New York and I’ve asked him to call and see us. Nice young chap, name’s Rawlins. The only trouble with his outfit is that he can’t communicate with others ashore or on the boats. Of course he can take down a line or even a telephone, but then he at once destroys one of the great advantages of his invention. A trailing line or wire is as liable to be caught or tangled in a wreck or in coral as an air pipe or any other rope or line and it means some one must be stationed in a boat over him. He claims one big advantage of his suit will be the fact that as no boat or air pump is needed, no one can tell where he is. That would be a fine thing in time of war, of course. Think you’ll take a great fancy to him, Tom.”

For a moment, Tom was silent and then he suddenly let out a yell like an Indian.

“I have it!” he fairly screamed. “Radio! Submarine radio! I’ll bet it’ll work.”

Then, filled with enthusiasm, he started to explain his ideas to his father.

“All right! All right!” cried Mr. Pauling, laughing and holding up his hands in protestation. “I’ll take your word for the technical end of it. Wait and tell Rawlins about it. But honestly I don’t know but what there may be something in it. You and Rawlins can work it out.”

So filled with his new idea was Tom, that he fairly rushed to tell Frank when the latter arrived, and for the next ten days the two were ceaselessly at work, drawing plans and diagrams, making and discarding instruments, purchasing countless rolls of wire and knock-down apparatus, as they strove to put into concrete form the vision in Tom’s brain.

But they found innumerable difficulties to be overcome and were almost discouraged when one evening Rawlins called.

He was such an enthusiastic and interesting man that the boys took a huge liking for him and as soon as Tom told him of his idea he at once fell in with the boys’ plans.

“I do believe it can be done!” he declared, when Tom had shown him the plans and had described his ideas fully. “I don’t know much about radio, but if you are right about the matter there’s no reason I can see why you shouldn’t get it to work. I tell you what, Tom, we’ll fit up a workshop and laboratory down at my father’s dock—it’s down near the foot of 28th St. and we don’t use it except for storage. The old gentleman’s gone out of the wrecking business and has sold all his outfit except the things stored there. It’s a fine place to work and experiment. There are tools and a machine lathe and about ten tons of odds and ends that may come in handy. My father had his office and workshop there—did all his repairing of pumps, diving suits and tugs there, and never threw anything away. I learned to dive there—my father and grandfather were deep-sea divers, too—and there’s a trapdoor where the divers went down to test their suits and pumps. I made my suits and even my under-sea motion picture outfit there and it’s private and no one will disturb us. The only way we can test out this idea of yours is by actual trial under water. If we do get it, it will be a mighty big thing—greatest improvement in sub-sea work ever. I’ll get the place ready and cleaned up a bit to-morrow. I’m just as crazy as you are to try it out.”

Mr. Henderson also was deeply interested in the boys’ new experiments and declared he believed their ideas might be worked out successfully.

“You’ll run across a lot of unexpected and unforeseen difficulties,” he warned them. “One never knows what new laws and phenomena one may run up against in a thing of this sort. During the war our government and the Allies, and no doubt Germany also, carried on a good many experiments with under-water radio, but as far as I know they never came to much. Radio had not progressed so far then and there were more important things to be done and not enough men to attend to it. We did use vacuum tubes and amplifiers for detecting submarines, however. By the way, I have a few things that may be of help to you boys and I’ll be glad to let you have them. Among them is a remarkable tuning device of German make and I don’t think it has ever been tried out. You’ll need something that is simple and accurate and easy to control and this may do the trick.”

By the end of a week a snug little laboratory had been set up on Rawlins’ dock and the boys and their diver friend spent every available moment of their time there.

Tom and Frank were as interested in seeing Rawlins go down in his odd suit as he was in their radio work, and the first time he put it on to demonstrate it to the boys they became tremendously excited. Rawlins carefully explained all about it, pointing out its various parts and showing them how the oxygen generator worked.

“You have to be careful about this,” he said, “if a drop of water gets into it, it blazes or flames up and may kill a fellow. That’s the only danger about it. If a man forgets and takes the mouthpiece from his lips to speak without shutting it off and water gets in, he’ll have a red hot flame inside his helmet. It’s easy to get accustomed to it though—comes as natural as breathing, after a bit of practice.”

But even now that it had been explained to them it seemed a most remarkable feat for Rawlins to don the shirtlike suit and helmet and, with only these over his ordinary garments and with no rubber trousers covering his legs, descend the ladder and disappear in the water without lines, pipes or ropes trailing after him. Both Tom and Frank were crazy to go down, but Rawlins refused to permit it until he had made the suits “fool proof” as he put it. Even then, the boys’ parents objected until they had visited the workshop and Rawlins had proved to their satisfaction that the boys were perfectly safe in shallow water when he accompanied them.

“We’ll have to go down to test out the radio,” argued Tom, “so we might as well learn right away.”

At last the fathers gave in and Tom went down first with Rawlins. For a week afterwards he could think or talk of nothing else and never tired relating his sensations and experiences to his parents and his boy friends, and Frank did the same. But after the first few times the novelty wore off and the boys soon became quite accustomed to going to the bottom of the river. Rawlins, however, never allowed them to stay down more than a few minutes at a time and after the first few descents the boys found little fun in it. They had expected to find a smooth, hard bottom and to see fishes swimming about and to be able to look up and see passing boats overhead. To their surprise, they found they could not walk upright, but leaned far forward and had a peculiar dreamy sensation when they attempted to walk, their feet seeming to half-drag, half-float behind them and that, despite the fact that the bottom of the river was soft and muddy, they did not sink into the bottom to any extent. As Tom put it, it was like trying to hurry in a dream when one’s feet seem tied to something and one can’t possibly run. Moreover, they found the water dark and so filled with sediment that they could see but a few feet and even near-by objects, such as the spiles and abutments of the dock, the ladder down which they descended and the figure of their companion were scarcely visible a yard distant and took on strange, hazy, indistinct and distorted forms. Indeed, Rawlins always held their hands when they went down, explaining that should they stray a few yards away they might be lost or might be swept off in some current.

But they were glad of the experience and realized that in order to carry on their experiments with any hopes of success they must learn to use the suits, for Rawlins had not yet mastered the details of radio.

In the meantime, however, they worked at the radio devices and at last Tom announced that he had a set which he believed might work.

“It’s only an experimental set,” he explained to Rawlins. “And it won’t stand up long under water, but if the idea’s all right and we get any results we can go to work and make a good outfit on the same principle.”

Rawlins was almost as excited as the boys when the day came to test the new device and at Tom’s suggestion was to go down alone with the receiver in his helmet while the boys remained on the dock and attempted to communicate with him.

“We’ll try receiving under water first,” said Tom. “If it works we’ll get it into good shape and then get busy on the under-water sending set.”

So, with the compact but complicated little set inside his helmet, which was specially made to accommodate it, and with the receivers clamped over his ears, Rawlins backed down the ladder while the boys, feeling like explorers about to set foot on some new and unknown land, watched his head disappear beneath the surface of the river.

It was little wonder that they were wildly excited for now, in a few moments, they would know beyond question whether their ideas had been right and whether all their work and trouble had been thrown away or they had made an advance in radio which might revolutionize under-sea work.

At first the boys had not fully realized what the success of their efforts would mean and had gone into it enthusiastically merely as something new and strange.

But as soon as Rawlins had explained the possibilities which a successful under-sea radio telephone would open up, they understood how much might hinge on the triumph or failure of their plans.

“Why,” Rawlins had exclaimed, “think what it will do if it works! A man can go down and walk about any place he chooses and yet can talk back and forth with men on a ship or on shore. In wrecking, he could go all through a ship with no danger of getting his life-line or air-hose tangled and he could direct the fellows on the tug or lighter, telling them just where to lower chains or tackle or anything else. And think what it would mean in time of war! Why, a man could walk out from shore anywhere, go under a ship and fasten a mine to her and blow her up and hear all that was going on aboard the enemy’s ship. And just think what a dangerous sort of spy a man would be—out of sight under the sea and yet able to hear all the talk and messages of the enemy! I tell you, boys, up to now diving’s been like blind man’s work—mostly feeling and signaling by jerks on a line. Of course the ordinary phone was a big advance, but with that you still had to trail a wire along and there was a visible connection between the diver and the surface. With my suits and your radio the country that owned the secrets would be mighty near masters of the sea, I’ll say.”