For the next few days the boys were very busy perfecting their instruments and, when Mr. Pauling bade Tom and his mother good-by and sailed southward, Tom assured him that he would be able to pick up any messages he sent.

“Maybe I’ll surprise you by sending a message,” he declared. “I’m going to apply for a license next week and make a sending set. Of course it won’t be able to send clear to Cuba or Nassau, but freak messages do go long distances sometimes and anyway, I can get in touch with your ship before you reach port coming back.”

“Great!” exclaimed his father heartily. “And don’t forget about stray messages—you may help us out yet. I spoke to Henderson about your idea that the bootleggers were using radio and he says he should not be a bit surprised. They’re right up to date in their methods, you know.”

That evening, Tom and Frank hurried to their sets promptly at 7:30 accompanied by Mrs. Pauling who seemed as interested as the boys in the result of their first attempt to pick up a message intended for them. She was rather disappointed, however, when Tom clamped on his phones and told her she wouldn’t be able to hear anything.

“You see,” he explained, “if the message comes in, it will be just code signal—dots and dashes in International Morse—and wouldn’t mean anything to you and I might miss it if I used the loud speaker.”

Slowly the minutes slipped by. From out of the silent air came various sounds to the boys’ impatient ears—little buzzing dots and dashes from local stations; the faint sounds of a phonograph from some amateur’s radiophone; fragments of speech from a broadcasting station. Carefully the two waiting, expectant boys tuned their instruments, for they had taken the precaution of asking the wireless operator on the ship what wave length he used and with their sets tuned as nearly to this as possible they cut out the amateur senders with their short wave lengths and the broadcasting stations with their evening entertainments on 360 meter waves and heard only the meaningless or uninteresting Morse messages passing from ships to shore or vice versa.

Over and over Tom and Frank glanced anxiously at the little nickel-plated clock ticking merrily on its shelf, until at last the hands pointed to 7:45 and the boys fairly thrilled with excitement. Would they hear the message from the speeding ship? Would they pick up that one message that they were expecting? Would they, in a moment more, be listening to the dots and dashes that represented Mr. Pauling’s words? Neither boy was yet expert at reading Morse if sent rapidly, but the wireless man aboard the Havana had laughingly agreed to send Mr. Pauling’s messages slowly and the boys were not worried on that score.

Suddenly, to Tom’s ears, came a sharp buzz—faint and blurred, and with trembling fingers he tuned his set, adjusted the variable condenser and as the short, staccato sounds grew sharp, loud and clear he knew that the long-hoped-for message was coming to his ears. “Dah, dah dah dah, dah dah, dee dah dah dee, dee dah, dee dee dah, dee dah dee dee, dee dee, dah dee, dah dah dee,” came the dots and dashes, sent slowly as if by an amateur and mentally Tom translated them. Yes, there was no doubt of it, TOM PAULING were the words the dots and dashes spelled and Tom’s heart beat a trifle faster and his face flushed with excitement as he heard his own name coming out of space and realized that, across a hundred miles and more of tossing sea, his father was talking to him and steadily he jotted down the letters as they buzzed in dots and dashes through the air from the distant ship.

“Hurrah!” he fairly yelled, as with the final “dee dah dee dah dee” the operator signified that the message was finished. “Hurrah! I got it. See, here ’tis, Mother!”

Frank also had received the message on his set and the two compared the letters they had written down.

“Of course we made some mistakes,” explained Tom as his mother puzzled over the unpunctuated, apparently meaningless letters. “See,” he continued, “you have to separate the letters into words and sentences and this one should be an “N” instead of an “A” and I guess this is a “D” instead of a “B,” Frank’s got it that way. One’s a dash and three dots and the other’s a dash and two dots.”

As he spoke, Tom was busily copying the letters and forming words and presently showed his mother the finished message. “That’s it,” he announced proudly. “Just think of Dad talking to us—and he’ll do it every night all the way down and after he gets there. Gosh! It’s funny to think we can hear from him that way. Say, isn’t radio great?”

“But I thought you could hear him talking,” said his mother in rather disappointed tones. “He could send messages that way by the regular radio companies or by cable.”

“Of course he could,” agreed Tom somewhat disturbed because his mother was not more enthusiastic over his achievement. “But you see the fun is in getting it ourselves this way. It wouldn’t be any sport to have the messages brought in an envelope like ordinary telegrams. Gee! I just wish we could hear him talk over the phones. Some of the ships have talked with the shore farther away than he is, but I guess the Havana’s radio isn’t up-to-date.”

“I think it’s fine and splendid of you boys to be able to do this,” declared his mother. “What I meant was, that I had expected to hear your father’s voice and I really was disappointed when I found it was so different.”

“Well, I’m going to fix a set to talk back to him,” said Tom. “And just as soon as I get the sending set done we’ll get to work and make a better receiving set, won’t we, Frank?”

“You bet!” agreed Frank. “Perhaps by the time your father is on the way back we can really talk to him.”

“Now let’s have some music,” suggested Tom, and for the next hour they all listened to the broadcasting station’s program as the loud speaker filled the room with the sounds of music, singing, speeches and news.

For the next three nights the two boys picked up Mr. Pauling’s messages regularly and were as proud as peacocks when they managed to get the first message from Havana telling of his safe arrival in Cuba. And by their enthusiastic studies and the practice they gained by deciphering the messages, the boys were successful in passing the required examination and proudly exhibited their license to maintain and operate a sending station.

It was a red letter day in their lives when they at last had the transmitting set in working order and flashed a message into the night, to have it promptly answered by an unknown boy in Garden City. Each night, too, they sent out messages directed to their father in the vain hope that, by some chance or by the same mysterious combination of conditions which had wafted other messages to vast distances beyond the range of the instruments, their words might be picked up in Havana or Nassau; but no reply came and at last they gave up in despair.

Then, their sending set being no longer a novelty, the boys set diligently to work on other matters and worked early and late.

“What on earth is that?” asked Tom’s mother, when finally the new idea had assumed concrete form and she was invited to witness a demonstration. “It looks like some sort of a huge birdcage,” she continued as she seated herself and glanced at the wooden framework wound with wire that stood on a small table.

“Well, I don’t suppose you can understand,” replied Tom, with the superior air of one who is master of an art beyond ordinary comprehension, “but I’ll try to explain. That’s a loop aËrial.”

“But I thought the aËrial was that wire clothesline-like affair on the roof,” objected Mrs. Pauling. “You see,” she laughed, “I am beginning to learn a little.”

Tom grinned, “Oh, yes, that’s an aËrial, too,” he replied. “But this is another kind. With this we don’t need any ground or lead-ins or lightning switches. And it’s directional too. That is,” he hastened to explain, “by turning it one way or another we can pick up signals from certain directions and not from others. Some people call them compass aËrials and they’re used on ships for locating other vessels or for finding their way. And besides, they cut out a lot of static.”

“Now please, Tom, what is all this you’re talking about? What is static?”

“Well that’s mighty hard to explain,” said Tom, scratching his head reflectively. “It’s a sort of electricity in the air—lots of it around when there are thunderstorms and lightning.”

“Lightning!” exclaimed his mother. “Do be careful, fooling with all these things, Tom. I’m always afraid you’ll get a fearful shock or something.”

“Nonsense,” laughed Tom. “Static doesn’t hurt any one and lightning won’t do any harm. An aËrial is just like a lightning-rod and if it’s struck the lightning is just carried down to the ground harmlessly; but this loop aËrial’s different. Now let’s hear how it works.”

Adjusting the instruments and attaching the loud-speaker, Tom slowly turned the cagelike affair about and suddenly, as it faced the west, the sounds of music burst out from the horn.

“There ’tis!” cried Tom, exultantly. “That’s Newark. Now, see here.” As he spoke, he swung the loop aËrial to one side, and instantly, the music died out. “Now, listen carefully,” he continued and turned the loop slowly around until, somewhat fainter, the sounds of a human voice came from the loud-speaker. “That’s Pittsburgh,” declared Tom. “Now you see how it works. If it’s turned towards Newark we get Newark and if towards Pittsburgh we get that.”

“Yes, it’s all very interesting,” admitted his mother. “But what advantage is it? You used to hear both Newark and Pittsburgh with the aËrial on the roof.”

“Oh, it’s no advantage for ordinary work,” replied Tom. “But it’s a fine thing in some ways. Now, for instance, if we heard a fellow’s message and didn’t know where it came from we could tell by turning this back and forth until we got his direction. Then, if we wanted to locate him exactly, we could put it up somewhere else and in that way we could find out just where he was. Frank and I have a particular scheme in hand, but that’s a secret and I’m not ready to tell it yet.”

His mother laughed. “I’m not a bit curious,” she declared. “I suppose some day I’ll wake up to find you two boys have astonished the world.”

But had Frank and Tom told Mrs. Pauling what their secret was she would have been both curious and surprised. Several times within the preceding weeks the boys, listening at their instruments, had received messages which they could not locate. At first they had given no heed to these, thinking they were merely from some amateur, but when, after repeated requests for the unknown’s call letters, no answer was received and the messages abruptly ceased, the two boys began to be curious.

“There’s something mighty funny about him,” declared Frank. “Every time we answer him or ask a question he shuts up like a clam. Say, Tom, maybe he’s a crook or a bootlegger.”

“More likely some amateur sending without a license and afraid the government inspector will get after him,” suggested Tom. “But I would like to find out who it is.”

A few days later Frank, who was poring over the latest issue of a radio magazine, uttered an exclamation. “Gosh! here’s the scheme,” he cried. “Now we can find out who that mysterious chap is.”

“What’s the big idea?” queried Tom, who was busy making a new vario-coupler.

“Loop aËrial,” replied his chum. “Here’s an article all about it. It says they’re used aboard ships to find the location of other vessels and are called compass aËrials.”

Tom dropped his work and hurried to Frank’s side.

“Well,” he remarked, after a few moments’ study of the article and the diagrams, “I don’t see how that would work in our case. It says one ship can find another or can work its way into port by using the loop aËrial like a compass, but the trouble is the ship’s moving and so the thing will work, but we can’t go running around New York City or the state with a set in one hand and a big loop aËrial in the other.”

“No,” admitted Frank rather regretfully, “but we can tell in which direction his station is.”

“Yes, and it will be fun to make one and experiment with it,” agreed Tom, “especially as the article says the thing cuts out static and interferences and it’s getting on towards warm weather now when the air will be full of static.”

“Well, let’s make one then,” suggested Frank.

As a result, the boys had constructed their loop aËrial and a special set to go with it and the very first time they tested the odd affair they were overjoyed at the result. Again they had picked up the messages which had aroused their curiosity and, by turning the loop one way and then another, they were soon convinced that the sender had a station to the southeast of their own.

“Well, that’s settled,” announced Tom, “and the only things southeast of here are the East Side, the river and Brooklyn. That fellow is not far away—he’s using a very short wave and his messages are strong. I’ll bet he’s right here in New York.”

“I guess you’re right,” agreed Frank, “but that doesn’t do much good. There’s an awful lot of the city southeast from here.”

“Sure there is,” said Tom, “but, after all, what do we care. I still think he’s just some unlicensed chap—probably some kid over on the East Side who can’t pass an examination or get a license and is just having a little fun on the quiet.”

This conversation took place two days before Tom received his father’s message telling of his safe arrival in Cuba and no more messages from the mysterious stranger were heard until the day after Mr. Pauling’s message had been received.

Then, as Tom was listening at the loop aËrial set and idly turned the aËrial about, he again picked up the well-known short-wave messages. Heretofore the messages had been meaningless sentences in code, dots and dashes which the boys out of curiosity had jotted down only to find them devoid of any interest—items regarding shipping which Tom had declared had been culled from the daily shipping lists and were being sent merely for practice—and so now, from mere habit, Tom wrote down the letters as they came to him over the instruments. Suddenly he uttered a surprised whistle.

“Gee Whittaker!” he exclaimed in low tones. “Come here, Frank.”

The other hurried to him and as he glanced at the pad on the table beside Tom he too gave an ejaculation of surprise. The letters which Tom had jotted down were as follows: LEAR P IN HAVANA ARRIVED YESTERDAY GET BUSY.

“They are rum runners!” cried Tom as the signals ceased.

“Gosh, I believe they are!” agreed Frank. “But of course,” he added, “it may not mean your father by ‘P’ and we don’t know the first part of the message. Maybe they were just talking about a ship—that ‘lear’ might have been something about a ship clearing for some place.”

“You are a funny one,” declared Tom. “Here you’ve been insisting all along that there was some deep mystery or plot behind these messages and I’ve said it was just some amateur and nothing to it and now, just as soon as we get a message which really means something, you shift around and say it’s only about some boat.”

“Well, if it’s anything secret why do they talk plain English?” asked Frank. “That’s what makes me change my views. When they were sending things that sounded like nonsense I thought they might be code messages, but now that they send things that are so plain it doesn’t seem mysterious.”

“Yes, there’s sense in that argument, I admit,” replied Tom. “But perhaps there was just as much sense in the others—if they are bootleggers. Of course as you say, they may not mean anything about Dad, but it would be a mighty funny coincidence if any one or anything else beginning with ‘P’ arrived in Havana yesterday and it happened to come in with this message and with a ‘get busy’ after it. I’ll bet you, Frank, they’re smugglers and that’s a message to some boat or something that the coast’s clear and to unload their stuff. Let’s go down and tell Mr. Henderson about it.”

“No,” Frank advised. “He’d probably laugh at us and it wouldn’t be any use to him anyhow. We’ll keep the message and all others we hear and if anything else is going on we’ll get some more messages, you can bet. And I’ve a scheme, Tom. I know a fellow down at Gramercy Park and we can go down there and set up a loop aËrial and see if this chap that’s talking is still southeast of there.”

“That’s a bully scheme!” cried Tom with enthusiasm. “We can turn radio detectives—that’ll be great! And if we find he’s north or west or east of Gramercy Square we can try some other place. Probably your friend knows fellows who have sets all around that part of the city.”

The next day they visited Frank’s friend and after making him promise secrecy they divulged a part of their plan, omitting, at Tom’s suggestion, any reference to their suspicions of the messages coming from a gang of bootleggers. Henry fell in readily with the idea of locating the messages, which he had also heard repeatedly, and was deeply interested in the loop aËrial. He had an excellent set and numerous instruments and supplies and the three boys soon rigged up a compass set in Henry’s home.

“Now, you listen with this and try to pick him up,” instructed Frank. “Keep turning the aËrial about in this way and, as soon as you hear him, write down what he says. We’ll listen too, whenever we have a chance, and will let you know. Then, if you haven’t picked him up, you can turn the loop until you do. Too bad you haven’t a sending set so you could tell us.”

“But he’ll hear you and quit,” objected Henry, “and how can I hear you if I don’t happen to have the loop pointed your way or am listening to this fellow?”

Frank looked puzzled. “Gee!” he ejaculated, “I hadn’t thought of that.

“Oh, that’s easy,” declared Tom. “You’ll hear us over the other set with the loud-speaker you have. That works with a regular aËrial and is entirely separate from this set. And we’ll arrange a code so he won’t know what we’re talking about. Let’s see, I guess we’d better use the phone and not send dot and dash, we’ll just say ‘we’ve got the message’ and you’ll know what it means.”

“No, that’s no good,” declared Frank. “That’s not a bit mysterious or exciting. We’re radio detectives, you know. We must have something like a password or code or something. Say, let’s begin with ‘loop,’ then Henry’ll know we mean him. We’ll say ‘loop, be ready to receive.’”

“Yes, and have him know something’s wrong when we don’t begin to send anything,” said Tom.

“I have it!” exclaimed Henry, “Say, ‘loop, coming over,’ and then any one’ll think you are telling me you are coming over here. But say, how’ll I get your message if I don’t sit at my set and tune to you?”

“That’s easy,” said Frank. “Just as soon as we get home to Tom’s we’ll begin to send and you listen and tune until you get us good and loud and then mark your knobs so you can set ’em whenever you want to hear us. Then ring us by regular phone and tell us it’s O. K.”

Thus, all being arranged, Tom and Frank went up town and as soon as they reached Tom’s room began to send calls for Henry as they had agreed. Very soon the telephone bell rang and Tom ran to the instrument.

“It’s all right, Frank,” he announced as he returned to the room. “Henry says he got our calls finely and has marked his knobs. He’s going to turn them about and then set them back at the marks and we’re to call him again. Then if he gets us right off he’ll know he won’t miss us next time.”

When, a few minutes later, the phone rang again and Henry told Tom that the message had come in on the adjusted set the boys felt sure that their fellow conspirator would not miss any calls they might send him. So, having nothing else to do, they worked at another step of amplification for their new set, and listened for any signals or messages that might come in from the person whom they were endeavoring to trail by means of radio.

Evidently, however, the mysterious stranger had no business to transact and no message from him was received. When at last they were obliged to leave for dinner they phoned to Henry who reported that he had been listening all the afternoon, but had heard nothing.

“We’ll get at it again to-night,” said Tom. “Most of the messages we’ve heard come in just when the broadcasting stations are giving their concerts. I’d bet he takes that time so nobody will hear him, or pay attention to him. If they’re all tuned to 360 meters they’d never know he was talking, you see, and if they just chanced to hear him they’d be too busy with the music to bother with him.”

As Tom had suspected, the mysterious messages did come in that night and so interesting and exciting did they prove to the boys’ imaginative and suspicious minds that they were thankful they had foregone the pleasure of hearing the concert on the chance of the supposed smugglers talking.