Within the breech of the gun was disclosed a chamber enclosing a small cylinder of steel. This was ribbed by metallic strips connected with electric wires and capable of being superheated by electrical current. Inside this chamber was placed the explosive projectile which it was desired to launch.

This done, a small amount of water was admitted to the electrically connected chamber, and a switch turned which caused the metal to become superheated. In a flash steam, at terrific pressure, was formed, and by a twist of a handle it could be released when desired. Simple as the device appeared, Mr. Dancer informed the boys that in some experiments that he had made it had proved most effective.

With the inspection of the gun their survey of the craft practically was over, except for the exhibition by Mr. Dancer of the anchoring device and other minor details.

When they stood once more on the top of the curved deck Jack exclaimed with enthusiasm:

“You have the finest craft of its kind I have ever seen or read of, Mr. Dancer.”

But, far from seeming elated, the inventor only sighed.

“It appears all right, I know,” he said, “and it cost me almost all my fortune to build it; but there is one fatal defect in it: the diving devices do not work properly.”

The boys regarded the gray-haired scientist with astonished eyes.

“It won’t dive?” asked Jack, at length.

“No; that is, not properly. You see, I had devised a sort of double skin for it in parts, and I imagined that I could fill this with water and make the craft sink when I so desired, and then pump out the water when I wished to rise.”

“And you did not do so?” queried Jack.

“Yes, I equipped it with the tanks all right; but I found that I would have to install such large pumps that it would be impracticable to work them with the power I had; so that now, as I told you some time ago, you find me at a standstill.”

“You mean that you cannot think of any other plan of making your craft ascend and descend in the water?”

“That’s just it. I’m up against a stone wall. They call you the ‘Boy Inventors.’ I’ve heard how you have aided other inventors in trouble. Can you think of a way to make the White Shark dive?”

“Not off-hand,” declared Jack positively; “but I promise you we’ll give the matter thought and do our best to help you. And now, Mr. Dancer, we should be getting back. It is late and my father, for whom we ran into town to purchase some electric apparatus, will be worrying about us.”

“But the wreck of the car has blocked the road and I have no vehicle handy that you can use.”

“I thought I noticed a wireless apparatus on the White Shark; is it working?” asked Jack.

“Yes; but its radius is limited. You see, I had to install the aërials inside the hull of the submarine; but with the powerful current I can command I can send a message up to twenty miles, or even more, under favorable conditions.”

“If you don’t mind, then, I’ll send a message to High Towers asking Jupe, that’s our colored man, to come right over with the automobile.”

“What, you have a colored man who can take wireless messages?”

“Yes indeed, Jupe learned all of that on our trip to the Gulf of Mexico.”

“True, I recall now reading about the colored man in some magazine account of your adventures. You must have had a stirring trip and some exciting times.”

“We did, indeed,” was Jack’s reply.

Readers of “The Boy Inventors’ Wireless Triumph,” the first volume of this series, will agree with him. This story told of the finding of Tom’s father, an explorer long lost in the mysterious land of Yucatan, and also related the odd quest of Prof. Chadwick, including the astonishing adventures of the two young inventors in a wonderful craft of their own designing.

After returning from this exciting trip they encountered, and aided materially, the inventor of a vanishing gun, designed to fight airships. Unscrupulous men tried to steal the plans of the gun, and finally succeeded, but through the boys’ pluck and cleverness their purposes were ultimately foiled. These experiences form a part of the story entitled, “The Boy Inventors and the Vanishing Gun.”

We now find them on the threshold of even stranger adventures than have already befallen them and, having made this necessary digression, let us follow our enterprising lads once more within the hull of the White Shark, the diving craft that so far had not dived.

Jack found the wireless of the usual type and lost no time in sending out his call for High Towers. After some delay, Jupe answered. Jack told him to bring the small runabout to the place, which he described, as soon as possible.

The colored man agreed to be with them in half an hour, and, much relieved, the boys sauntered out of the shed with Mr. Dancer to await the arrival of the auto.

They were standing in the road outside the gates of the carefully secluded workshop, when a man on a high-powered motorcycle suddenly appeared from the direction of the grade down which the runaway car had dashed.

Mr. Dancer uttered an exclamation as he saw him.

“It’s Adam Duke!” he exclaimed, in a rather perturbed tone.

The words had hardly left his lips before the motorcycle chugged up to where the little group was standing, and the rider swung himself from his seat.

When he pushed up his goggles, after alighting, the boys saw that the newcomer was a tall, well-built man of middle age. But what might have been a clever, good-looking face was marred by an expression of fixed sullenness and aggression.

“Well, what’s all this?” he muttered rather gruffly, as he stared at the two lads. As for Mr. Dancer, even if his exclamation of recognition had not told them, the boys would have known that he was no stranger to the new arrival.

“What do you want?” he exclaimed, as the man motioned inquiringly toward the two boys.

“A few words with you alone, Mr. Dancer.”

Then, as the inventor hesitated:

“Come; I’m in no mood to be trifled with.”

Under the tan that overspread his rather wizened features the inventor turned pale.

“You must excuse me a minute,” he said, turning to the boys.

Then he and the newcomer turned, the latter having leaned his motorcycle against the fence, and they entered the territory beyond the forbidding palings that marked the dwelling place of the White Shark.