As soon as Rawlins was out of sight the boys commenced to talk, Tom speaking through the transmitter while Frank wrote down what he said, for of course they could not know if Rawlins heard them, and the only means of determining if he had received all the words was to keep a record for comparison when he came up. They were busily engaged at this and tremendously interested and excited, when the telephone bell rang. Telling Rawlins to wait a moment, and explaining the reason, Tom ceased speaking while Frank answered the call.

“Hello, Frank,” came Henry’s voice. “I just rang up to be sure you were there. How’s everything going?”

“Fine!” replied Frank, “come on down, we’re just testing it out for the first time. When did you get back?”

“Last evening—but didn’t have a chance to run around to see you. I called up, but the maid said you were out with Tom. Didn’t she tell you? I’ll be right down, you bet. Say, I’ve some news for you. So long.”

“I’m glad he’s back from that trip with his father and is coming down,” said Tom, “Won’t he be interested and surprised if this works? Wonder what the news is.”

Then, turning to his set, he continued his interrupted talk, or attempt to talk, with Rawlins until, five minutes later, Henry was pounding at the door.

“Gee, but you’ve a fine place here!” he cried as he glanced about the little laboratory, “and you’ve diving suits and helmets and everything. Say, I was just crazy to get back when I got your letter telling about your experiments and everything. Where’s the diver fellow? Oh say, you’re not really talking to him under water! Crickety! Isn’t that wonderful to think he can hear you down under the river!”

Tom laughed. “Don’t know if he can,” he replied. “We’ll have to wait for him to come up and tell. You see we haven’t got an under-sea sending set rigged up yet and the one he’s got is just a sort of makeshift for experimenting.”

“Have you fellows heard anything more of that mystery chap?” cried Henry, suddenly changing the subject.

“Not a word,” Tom assured him.

“Well, I have then,” declared Henry triumphantly. “I heard him last night and I got him again to-day just before I called you fellows. He was in the same old place, too.”

“Honest? Say, that is funny!” exclaimed Frank. “What was he saying?”

“Don’t know,” replied Henry, “He was talking some foreign lingo that I couldn’t make out, but I got one word. Bet you couldn’t guess what ’twas—another flower—Oleander this time.”

The boys were so interested in Henry’s news that they had temporarily forgotten their under-water companion until Henry uttered a half surprised exclamation and jumped away from the square opening in the floor over the river.

“Gosh, there he comes!” he cried, as overcoming his first surprise at a gurgling splash he glanced through the trapdoor and saw the diver’s helmet appearing. “Don’t he look like a regular sea monster?”

A moment later, Rawlins was removing his suit and helmet.

“Did you hear us?” cried Tom the moment Rawlins’ face was visible.

“Did I!” exclaimed the diver. “Did I! Let me tell you I wished I had cotton stuffed in my ears. You must think I’m deaf,—yelling like that. Did you think you had to shout loud enough to have your voice go through the water? And I’ll tell you I thought a tornado’d struck the place when your friend here arrived. I even heard the telephone bell.”

Tom and Frank fairly danced with delight. “Hurrah! It works! It’s a success! We’ve solved it! It’s under-sea radio!” shouted the excited boys.

“I’ll say it works!” declared Rawlins. “But what the deuce were you trying to talk Dutch for?”

“Talk Dutch?” cried Tom in a puzzled tone. “We weren’t talking Dutch or anything but United States.”

It was Rawlins’ turn to be amazed. “Well, who in thunder was then?” he asked. “I heard some one jabbering Dutch or some other foreign language—don’t know what ’twas except it wasn’t French or Spanish.”

Henry gave a whoop. “It was that other fellow!” he cried excitedly. “I’ll bet ’twas. He was talking just before I rang up as I told you. Jehoshaphat! Mr. Rawlins must have heard him under water.”

“I guess that’s it,” agreed Tom. “Funny it didn’t occur to me. Of course there’s no reason why he shouldn’t have been heard under water. We’re using a tiny little wave length and so’s he, and he’s close to here, you know. Did you hear him loudly, Mr. Rawlins?”

“Well, not so as to deafen me the way you did,” replied the diver with a grin, “but if I’d understood his lingo I could have told what he was talking about. The only word that sounded like sense to me was something like Oleander.”

“Then ’twas him!” fairly yelled Henry ungrammatically. “That’s the name he was using when I heard him.”

“Well, it just proves this new thing is a peacherino,” declared Tom. “Now let’s get busy and fix it up in good shape and make a sending set to try out.”

Now that the boys’ first experiment had been such a huge success they were more enthusiastic and excited than ever. They had been confident that the diver would be able to hear sounds or that he might even distinguish words under water, but they had not dared to hope that their very first efforts would result in the sound being carried to the ears of the man beneath the water as clearly and loudly as though he had been present in the same room with the speaker.

“I’ll bet water carries electromagnetic waves better than air,” declared Tom. “Why, if this little set can respond to these short five watt waves in this way, think what it would mean to a submarine with big amplified sets and getting messages sent with hundreds of watts. Why a fellow could sit in Washington and talk to submarines and divers all over the Atlantic.”

“You’ve hit on a wonderful possibility,” Rawlins assured him. “Of course I was pretty close—I didn’t go over a hundred yards from the dock and it’s shoal water. I’m anxious to try it down a hundred feet or so and a mile or two from the sender. We’ll do that after we get things right—go down to my hangout in the Bahamas and give it a real honest-to-goodness tryout.”

“It’s all in that new amplifying arrangement and that single control tuner Frank hit upon,” said Tom. “And we’re not really responsible for either. Mr. Henderson gave us the idea for the tuner and a friend of Dad’s invented the tube, but couldn’t get any one interested. You see, Henry, this tube is just about 400 times as much of an amplifier as the other tubes, and we get a detector and amplifier all in one. Look here—it’s the smallest bulb you ever saw—about the size of a peanut and we operate it on a flashlight battery with a special little dry cell for the filament. Of course they don’t last long, but a fellow can’t stay down more than an hour or two anyway and the batteries will run the set steadily for five hours. For under-sea work the cost don’t count. What we’re up against now is to make the sending set to go with it. The receiver was easy. That fits in this special helmet all right and don’t have to be waterproof, but the sending set’ll have to be outside and it’ll be an awful job to keep the water from short circuiting it.”

As he talked, Tom was showing Henry the set and pointing out its many novel features.

“This single tuner is great,” he continued. “It’s fixed so it’s set at a certain spot for the normal wave lengths sent from the diver’s home station. See, here in the middle at zero. Then, if he wants to get a shorter wave he turns it to the left which gives him a range down to half his normal wave length, or for longer waves he turns it to the right and gets twice his normal length. If he wants to go to long wave lengths—for example, if he was a spy or something and wanted to get the big sending stations—he’d turn the knob clear to the left and then back to the right and around to opposite the zero point. Then he’d be on about 2500 meters and that being his utmost length he just has to tune slowly towards zero again. And the rheostat works automatically with it and so does the variable condenser and it’s not very complicated either.”

“But what does he do for an aËrial?” queried Henry.

“Doesn’t use one,” replied Tom. “Just has this sort of wire cage sticking from his helmet, like a loop, but made of two grids set at right angles to each other. But gosh! I never thought about there being interferences under water.”

“I suppose Henry understands all that,” interrupted Rawlins laughingly, “but it means about as much to me as that Dutch talk I heard. Somehow or other I can’t get on to this radio a little bit. When you get that sending outfit rigged you’ll have to go down and test it. I’d probably bungle something. I didn’t even dare meddle with this gadget for tuning. I tried it once and when your voice stopped I just shoved her back and let it go at that. That’s when I heard that Dutchman.”

“Then he’s on a different wave length and it proves we can tune out under water,” declared Tom gleefully. “That’s another feather in our caps.”

Henry quickly grasped the boys’ ideas and together the three worked diligently until sundown while Rawlins busied himself devising the fittings for his suit to accommodate the sending apparatus and helped the boys tremendously with suggestions for rendering a set watertight and with advice as to mechanical and other details.

By the time they were obliged to stop their work the plans for the under-sea transmission set were well worked out and, with high hopes and flushed with the success of their achievements, they locked up the workshop and walked up town discussing plans for the morrow.

The following day they went to the dock right after breakfast, for school was over for the season and they had all their time to themselves. Rawlins was already there and before they left that night they had the set nearly completed and Tom declared they would be able to give it a test the next day.

Mr. Pauling was of course deeply interested and enthusiastic over the boys’ work and promised to go down himself as soon as the instruments were perfected. He listened to the boys’ glowing accounts of their work and their success and later, when Mr. Henderson called, he too became most optimistic regarding their under-sea radio.

“It’s merely a question of experimenting, boys,” he declared. “We were on the right track during the war, but radio’s jumped ahead a lot since then and whatever the government experts accomplish is kept mighty quiet. I’m glad that single control works out so well. We’ll have to thank the Huns for that. We found one on a captured U-boat, but as far as I know the government never took it up seriously—don’t know why unless it was because there was no particular need of it. We never did find out what the Germans used it for—for all we know they may have been experimenting along under-sea lines too. And if that new tube of Michelson’s proves good he’ll make a fortune and have you boys to thank for it. I’m coming down to see your outfit just as soon as we get a breathing space. We’re rushed to death just now.”

With nothing else to do the boys amused themselves listening at their sets which, with so many other interests, had been sadly neglected of late, and, out of pure curiosity and never expecting to hear anything, Tom turned his loop aËrial to the southeast and tuned for the short wave lengths used by the mysterious talker they had once followed and tried to locate so persistently. To his surprise, the sound of words came clearly over the set.

“There he is again!” Tom exclaimed to Frank who was listening to a broadcasted speech. “Get him and we’ll see what he says.”

But despite the fact that the boys could both hear the man plainly his words were meaningless, for he was speaking some guttural, harsh-sounding tongue.

“Oh, pshaw!” ejaculated Tom disgustedly after a few minutes of this. “Who cares what he’s saying. I guess it’s some crazy foreigner.”

So saying, he again picked up the broadcasting station and forgot all about the incident in his interest as he listened to a lecture on new developments in radio.

“Some night we’ll be listening to that fellow talking about the new under-sea radio,” chuckled Frank as the talk ceased and the boys laid aside their receivers. “Say, won’t it be sport to hear him telling about us and know all the fellows are listening to it?”

“Well, we won’t count our chickens just yet,” declared Tom sagely. “Just because that receiving set works isn’t any proof the sending set will. And without being able to talk back a diver isn’t any better off—or at least much better off—if he can hear what’s going on in the air.”

But Tom might have been far more confident, for the following day when the test was made it worked much better than their most sanguine expectations had led them to think possible. To be sure, their experiments came to an abrupt ending right in the midst of the test, for the sending set on Tom’s suit leaked and, with a feeble buzz and sputter, his words trailed off to nothingness.

But when, upon reaching the surface, Rawlins reported that he had heard everything Tom had said and Frank and Henry in the shop had also heard him, the boys knew that their plans and the principles of the outfit were all right and that only the question of making the set absolutely watertight remained to be solved.

“I don’t see why it should not be inside the suit,“ declared Rawlins, as the boys were discussing the matter and were at a loss to know how to accomplish their aims. “You say these wireless waves go through everything and we get them through the suit in the receiving set so why shouldn’t they go out through everything just as well. Look here, I was thinking over this last night and here’s my idea.”

As the boys gathered about, the diver rapidly sketched his plan of a new suit in which the sending set could be placed within a receptacle full of compressed air.

“I believe that would work,” cried Tom when he grasped Rawlins’ scheme. “I don’t see why compressed air should affect the outfit any and it’s easy enough to make watertight fittings where the wires come out and there’s no tuning to do, We can always use a special wave length and if several men were talking under water each one could have his own wave length. Yes. I’ll bet you’ve solved the puzzle, Mr. Rawlins.”

Keen on the new plan the boys started a new set, or rather two new sets, for they wished to make a test to determine if two men under water could converse, while Rawlins busied himself on the special suits and air pockets to be used.

“We’ll have to balance the weight of the set against the increased buoyancy of this compressed air,” he remarked as he worked. “But I see where that’s an advantage. One of your troubles has been the weight of batteries and by this air caisson arrangement weight won’t cut any figure under water.”

“But suppose the air pocket springs a leak?” queried Frank. “We’d be just as badly off as before.”

“Well, I don’t calculate to have it leak,” replied Rawlins, “but if you make the sets as near watertight as you can, they’d still go on working for some time before they got soaked. And if I can’t make a little caisson that’ll hold a hundred pounds of air for ten or twelve hours I’ll give up diving and drive a taxi.”

Several days, however, were required to get the set and the air pocket suits ready and when, after a test in the workshop, everything seemed in perfect working order, Tom and Rawlins donned their suits and prepared to descend the ladder through the trapdoor.

Just before his head dipped beneath the surface of the water Tom spoke into his mouthpiece and Frank, listening at his instruments, gave a start as his chum’s voice came clearly to his ears.

“So long, old man,” came Tom’s cheery voice, which somehow Frank had expected would sound muffled. “Keep your ear glued to the set and be ready for great news. I’ll bet we give you a surprise.”

The next instant only a few bubbles marked the spot where Tom had sunk beneath the surface of the water, and little did he or the others dream how much truth was in his parting words or what an amazing surprise was awaiting not only Frank but himself.