"Not a boat," replied Tom, "but I think I have a means of sending out a call for help!"

"Oh, Tom--Mr. Swift--how?" exclaimed Mrs. Nestor. "Do you mean we can send a message to my Mary?"

"Well, not exactly to her," answered the young inventor, though he wished that such a thing were possible. "But I think I can summon help."

"How?" demanded Mr. Hosbrook. "Have you managed to discover some cable line running past the island, and have you tapped it?"

"Not exactly." was Tom's calm answer, "but I have succeeded, with the help of Mr. Damon and Mr. Fenwick, in building an apparatus that will send out wireless messages!"

"Wireless messages!" gasped the millionaire. "Are you sure?"

"Wireless messages!" exclaimed Mr. Jenks. "I'll give--" He paused, clasped his hands on his belt, and turned away.

"Oh, Tom!" cried Mrs. Nestor, and she went up to the lad, threw her arms about his neck, and kissed him; whereat Tom blushed.

"Perhaps you'd better explain," suggested Mr. Anderson.

"I will," said the lad. "That is the secret we have been engaged upon--Mr. Damon, Mr. Fenwick and myself. We did not want to say anything about it until we were sure we could succeed."

"And are you sure now?" asked Captain Mentor.

"Fairly so."

"How could you build a wireless station?" inquired Mr. Hosbrook.

"From the electrical machinery that was in the wrecked WHIZZER,"

spoke Tom. "Fortunately, that was not damaged by the shock of the fall, and I have managed to set up the gasolene engine, and attach the dynamo to it so that we can generate a powerful current. We also have a fairly good storage battery, though that was slightly damaged by the fall."

"I have just tested the machinery, and I think we can send out a strong enough message to carry at least a thousand miles."

"Then that will reach some station, or some pa.s.sing ship," murmured Captain Mentor. "There is a chance that we may be saved."

"If it isn't too late," gloomily murmured the scientist. "There is no telling when the island will disappear beneath the sea."

But they were all so interested in Tom's announcement that they paid little attention to this dire foreboding.

"Tell us about it," suggested Mr. Nestor. And Tom did.

He related how he had set up the dynamo and gasolene engine, and how, by means of the proper coils and other electrical apparatus, all of which, fortunately, was aboard the WHIZZER, he could produce a powerful spark.

"I had to make a key out of strips of bra.s.s, to produce the Morse characters," the lad said. "This took considerable time, but it works, though it is rather crude. I can click out a message with it."

"That may be," said Mr. Hosbrook, who had been considering installing a wireless plant on his yacht, and who, therefore, knew something about it, "you may send a message, but can you receive an answer?"

"I have also provided for that," replied Tom. "I have made a receiving instrument, though that is even more crude than the sending plant, for it had to be delicately adjusted, and I did not have just the magnets, carbons, coherers and needles that I needed.

But I think it will work."

"Did you have a telephone receiver to use?"

"Yes. There was a small interior telephone arrangement on Mr.

Fenwick's airship, and part of that came in handy. Oh, I think I can hear any messages that may come in answer to ours."

"But what about the aerial wires for sending and receiving messages?" asked Mr. Nestor.

"Don't you have to have several wires on a tall mast?"

"Yes, and that is the last thing to do," declared Tom. "I need all your help in putting up those wires. That tall tree on the crest of the island will do," and he pointed to a dead palm that towered gaunt and bare like a ship's mast, on a pile of rocks in the centre of Earthquake Island.

CHAPTER XXI

MESSAGES INTO s.p.a.cE

Tom Swift's announcement of the practical completion of his wireless plant brought hope to the discouraged hearts of the castaways. They crowded about him, and asked all manner of questions.

Mr. Fenwick and Mr. Damon came in for their share of attention, for Tom said had it not been for the aid of his friends he never could have accomplished what he did. Then they all trooped up to the little shack, and inspected the plant.

As the young inventor had said, it was necessarily crude, but when he set the gasolene motor going, and the dynamo whizzed and hummed, sending out great, violet-hued sparks, they were all convinced that the young inventor had accomplished wonders, considering the materials at his disposal.

"But it's going to be no easy task to rig up the sending and receiving wires," declared Tom. "That will take some time."

"Have you got the wire?" asked Mr. Jenks.

"I took it from the stays of the airship," was Tom's reply, and he recalled the day he was at that work, when the odd man had exhibited the handful of what he said were diamonds. Tom wondered if they really were, and he speculated as to what might be the secret of Phantom Mountain, to which Mr. Jenks had referred.

But now followed a busy time for all. Under the direction of the young inventor, they began to string the wires from the top of the dead tree, to a smaller one, some distance away, using five wires, set parallel, and attached to a wooden spreader, or stay. The wires were then run to the dynamo, and the receiving coil, and the necessary ground wires were installed.

"But I can't understand how you are going to do it," said Mrs.

Nestor. "I've read about wireless messages, but I can't get it through my head. How is it done, Mr. Swift?"

"The theory is very simple," said the young inventor. "To send a message by wire, over a telegraph system, a battery or dynamo is used. This establishes a current over wires stretched between two points. By means of what is called a 'key' this current is interrupted, or broken, at certain intervals, making the sounding instrument send out clicks. A short click is called a dot, and a long click a dash. By combinations of dots, dashes, and s.p.a.ces between the dots and dashes, letters are spelled out. For instance, a dot and a s.p.a.ce and a dash, represent the letter 'A' and so on."

"I understand so far," admitted Mrs. Nestor.

"In telegraphing without wires," went on Tom, "the air is used in place of a metallic conductor, with the help of the earth, which in itself is a big magnet, or a battery, as you choose to regard it.

The earth helps to establish the connection between places where there are no wires, when we 'ground' certain conductors."

"To send a wireless message a current is generated by a dynamo. The current flows along until it gets to the ends of the sending wires, which we have just strung. Then it leaps off into s.p.a.ce, so to speak, until it reaches the receiving wires, wherever they may be erected. That is why any wireless receiving station, within a certain radius, can catch any messages that may be flying through the air--that is unless certain apparatus is tuned, or adjusted, to prevent this."

"Well, once the impulses, or electric currents, are sent out into s.p.a.ce, all that is necessary to do is to break, or interrupt them at certain intervals, to make dots, dashes and s.p.a.ces. These make corresponding clicks in the telephone receiver which the operator at the receiving station wears on his ear. He hears the code of clicks, and translates them into letters, the letters into words and the words into sentences. That is how wireless messages are sent."

"And do you propose to send some that way?" asked Mrs. Anderson.

"I do," replied Tom, with a smile.