"Come on!" yelled Ned. "We'll see how this experiment came out!" and he started to run from beneath the shelter of the hill.

"Hold on!" shouted Tom, laying a restraining hand on his chum's shoulder.

"Why, what's the matter?" asked Ned in surprise.

"Some of that powder may not have exploded," went on the young inventor. "From the sound made I should say the gun burst, and, if it did, that gelatin is bound to be scattered about. There may be a mass of it burning loose somewhere, and it may go off. It ought not to, if my theory about it being harmless in the open is correct, but the trouble is that it's only a theory. Wait a few seconds."

Anxiously they lingered, the echoes of the blast still in their ears, and a peculiar smell in their nostrils.

"But there's no smoke," said Mr. Damon. "Bless my spyglass! I always thought there was smoke at an explosion."

"This is a sort of smokeless powder," explained Tom. "It throws off a slight vapor when it is ignited, but not much. I guess it's safe to go out now. Come on!"

He dropped the pushbutton connected with the igniting battery, and, followed by the others, raced to the scene of the experiment. A curious sight met their eyes.

A great hole had been torn in the hillside, and another where the improvised gun had stood. The gun itself seemed to have disappeared.

"Why--why--where is it?" asked Ned.

"Burst to pieces I guess," replied Tom. "I was afraid that charge was a bit too heavy."

"No, here it is!" shouted Mr. Damon, circling off to one side. "It's been torn from the carriage, and partly buried in the ground," and he indicated a third excavation in the earth.

It was as he had said. The terrific blast had sheared the gun from its temporary carriage, thrown it into the air, and it had come down to bury itself in the soft ground. The carriage had torn loose from the concrete base, and was tossed off in another direction.

"Is the gun shattered?" asked Tom, anxious to know how the weapon had fared. It was, in a sense, a sort of small model of the giant cannon he intended to have cast.

"The breech is cracked a little," answered Mr. Damon, who was examining it; "but otherwise it doesn't seem to be much damaged."

"Good cried Tom. "Another steel jacket will remedy that defect. I guess I'm on the right road at last. But now to see what became of that armor plate."

"Dinner plate not here," spoke Koku, who could not understand how there could be two kind of plates in the world. "Dinner plate gone, but big hole here, and he indicated one in the side of the hill.

"I expect that is where the armor plate is," said Tom, trying not to laugh at the mistake of his giant servant. "Take a look in there, Koku, and, if you can get hold of it, pull it out for us. I'm afraid the piece of nickel-steel armor proved too much for my projectile. But we'll have a look."

Koku disappeared into the miniature cave that had been torn in the side of the bill. It was barely large enough to allow him to go in. But Tom knew none other of them could hope to loosen the piece of steel, imbedded as it must be in the solid earth.

Presently they heard Koku grunting and groaning. He seemed to be having quite a struggle.

"Can you get it, Koku?" asked Torn. "Or shall I send for picks and shovels."

"Me get, Master," was the muffled answer.

Then came a shout, as though in anger Koku had dared the buried plate to defy him. There was a shower of earth at the mouth of the cave, and the giant staggered out with the heavy piece of armor plate. At the sight of it Tom uttered a cry.

"Look!" he shouted. "My projectile went part way through and then carried the plate with it into the side of the hill. Talk about a powerful explosive! I've struck it, all right!"

It was as he had said. The projectile, driven with almost irresistible force, had bitten its way through the armor plate, but a projection at the base of the shell had prevented it from completely passing through. Then, with the energy almost unabated, the projectile had torn the plate loose and hurled it, together with its own body, into the solid earth of the hillside. There, as Koku held them up, they could all see the shell imbedded in the plate, the point sticking out on the other side, as a boy might spear an apple with a sharp stick.

"Bless my spectacle case!" cried Mr. Damon. "This is the greatest ever!"

"It sure is," agreed Ned. "Tom, my boy, I guess you can now make the longest shots on record."

"I can as soon as I get my giant cannon, perhaps," admitted the young inventor. "I think I have solved the problem of the explosive. Now to work on the cannon."

An examination of the gauges, which, being attached to the cannon and plate by electric wires, were not damaged when the blast came, showed that Tom's wildest hopes had been confirmed. He had the most powerful explosive ever made--or at least as far as he had any knowledge, and he had had samples of all the best makes.

Concerning Tom's powder, or explosive, I will only say that he kept the formula of it secret from all save his father. All that he would admit, when the government experts asked him about it, later, was that the base was not nitro-glycerine, but that this entered into it. He agreed, however, in case his gun was accepted by the government, to disclose the secret to the ordnance officers.

But Tom's work was only half done. It was one thing to have a powerful explosive, but there must be some means of utilizing it safely--some cannon in which it could be fired to send a projectile farther than any cannon had ever sent one. And to do this much work was necessary.

Tom figured and planned, far into the night, for many weeks after that. He had to begin all over again, working from the basis of the power of his new explosive. And he had many new problems to figure out.

But finally he had constructed--on paper--a gun that was to his liking. The most exhaustive figuring proved that it had a margin of safety that would obviate all danger of its bursting, even with an accidental over-charge.

"And the next thing is to get the gun cast," said Tom to Ned one day.

"Are you going to do it in your shops?" his chum asked.

"No; it would be out of the question for me. I haven't the facilities. I'm going to give the contract to the Universal Steel Company. We'll pay them a visit in a day or two."

But even the great facilities of the steel corporation proved almost inadequate for Tom's giant cannon. When he showed the drawings, on which he had already secured a patent, the manager balked.

"We can't cast that gun here!" he said.

"Oh, yes, you can!" declared Tom, who had inspected the plant. "I'll show you how."

"Why, we haven't a mould big enough for the central core," was another objection.

"Then we'll make one," declared Tom "We'll dig a pit in the earth, and after it is properly lined we can make the cast there."

"I never thought of that!" exclaimed the manager. "Perhaps it can be done."

"Of course it can!" cried Tom. "Do you think you can shrink on the jackets, and rifle the central tube?"

"Oh, yes, we can do that. The initial cast was what stumped me. But we'll go ahead now."

"And you can wind the breech with wire, and braze it on; can't you?" persisted Tom.

"Yes, I think so. Are you going to have a wire-wound gun?"

"That, in combination with a steel-jacketed one. I'm going to take no chances with 'Swiftite'!" laughed Tom, for so he had named his new explosive, in honor of his father, who had helped him with the formula.

"It must be mighty powerful," exclaimed the manager.

"It is," said Tom, simply.

I am not going to tire my readers with the details leading up to the casting of Tom's big cannon. Sufficient to say that the general plan, in brief, was this: A hole would be dug in the earth, in the center of the largest casting shop--a hole as deep as the gun was to be long. This was about one hundred feet, though the gun, when finished, would be somewhat shorter than this. An allowance was to be made for cutting.

In the center of this hole would be a small "core" made of asbestos and concrete mixed. Around this would be poured the molten steel from great caldrons. It would flow into the hole. The sides of earth--lined with fire-clay--would hold it in, and the middle core would make a hole throughout the length of the central part of the gun. Afterward this hole would be bored and rifled to the proper calibre.

After this central part was done, steel jackets or sleeves would be put on, red-hot, and allowed to shrink. Then would come a winding of wire, to further strengthen the tube, and then more sleeves or jackets. In this way the gun would be made very strong.

As the greatest pressure would come at the breech, or in the powder chamber there, the gun would be thickest at this point, decreasing in size to the muzzle.

It took many weary weeks to get ready for the first cast, but finally Tom received word that it was to be made, and with Ned, and Mr. Damon, he proceeded to the plant of the steel concern.

There was some delay, but finally the manager gave the word. Tom and his friends, standing on a high gallery, watched the tapping of the combined furnaces that were to let the molten steel into the caldrons. There were several of these, and their melted contents were to be poured into the mould at the same time.

Out gushed the liquid steel, giving off a myriad of sparks. The workers, as well as the visitors, had to wear violet-tinted glasses to protect their eyes from the glare.

"Hoist away!" cried the manager, and the electric cranes started off with the caldrons of liquid fire, weighing many tons.

"Pour!" came the command, and into the pit in the earth splashed the melted steel that was to form the big cannon. From each caldron there issued a stream of liquid metal of intense heat. There were numerous explosions as the air bubbles burst-- explosions almost like a battery in action.

"So far so good!" exclaimed the manager, with a sigh of relief as the last of the melted stuff ran into the mould. "Now, when it cools, which won't be for some days, we'll see what we have."

"I hope it contains no flaws," spoke Tom, "That is the worst of big guns--you never can tell when a flaw will develop. But I hope--"

Tom was interrupted by the sound of a dispute at one of the outer doors of the shop.

"But I tell you I must go in--I belong here in!" a voice cried. It had a German accent, and at the sound of it Tom and Ned looked at each other.

"Who is there?" asked the manager sharply of the foreman..

"Oh, a crazy German. He belongs in one of the other shops, and I guess he's mixed up. He thinks he belongs here. I sent him about his business."

"That is right," remarked the manager. "I gave orders, at your request," he said to Tom, "that no one but the men in this part of the plant were to be present at the casting. I cant understand what that fellow wanted."

"I think I can," murmured Tom, to himself.