Tom looked at Ned in dismay. After all their work and planning, to be thus thwarted, and by a mere technicality! As they stood there, hardly knowing what to do, the sound of a tremendous explosion came to their ears from behind the big pile of earth and concrete that formed the bomb-proof around the testing ground.

"What's that?" cried Ned, as the earth shook.

"Just trying some of the big guns," explained the sentry, who was not a bad-natured chap. He had to do his duty. "You'd better move on," he suggested. "If anything happens the government isn't responsible, you know."

"I wish there was some way of getting in there," murmured Tom.

"You can see General Waller after the test, and he will probably countersign the permit," explained the sentry.

"And we won't see the test of the gun I'm most interested in," objected Tom. "If I could only--"

He stopped as he noticed the sentry salute someone coming up from the rear. Tom and Ned turned to behold a pleasant-faced officer, who, at the sight of the young inventor, exclaimed:

"Well, well! If it isn't my old friend Tom Swift! So you got here on my permit after all?"

"Yes, Captain Badger," replied the lad, and then with a rueful face he added: "But it doesn't seem to be doing me much good. I can't get into the proving grounds."

"You can't? Why not?" and he looked sharply at the sentry.

"Very sorry, sir," spoke the man on guard, "but General Wailer has left orders, Captain Badger, that no outsiders can enter the proving grounds when his new gun is being tested unless he countersigns the permits. And he's engaged just now. I'm sorry, but--"

"Oh, that's all right, Flynn," said Captain Badger. "It isn't your fault, of course. I suppose there is no rule against my going in there?" and he smiled.

"Certainly not, sir. Any officer may go in," and the guard stepped to one side.

"Let me have that pass, Tom, and wait here for me," said the Captain. "I'll see what I can do for you," and the young officer, whose acquaintance Tom had made at the tests when the government was purchasing some aeroplanes for the army, hurried off.

He came back presently, and by his face the lads knew he had been successful.

"It's all right," he said with a smile. "General Waller countersigned the pass without even looking at it. He's so excited over the coming test of his gun that he hardly knows what he is doing. Come on in, boys. I'll go with you."

"Then they haven't tested his gun yet?" Asked Tom, eagerly, anxious to know whether he had missed anything.

"No, they're going to do so in about half an hour. You'll have time to look around a bit. Come on," and showing the sentinel the counter-signed pass, Captain Badger led the two youths into the proving grounds.

Tom and Ned saw so much to interest them that they did not know at which to look first. In some places officers and firing squads were testing small-calibre machine guns, which shot off a round with a noise like a string of firecrackers on the Chinese New Year's. On other barbettes larger guns were being tested, the noise being almost deafening.

"Stand on your tiptoes, and open your mouth when you see a big cannon about to be fired," advised Captain Badger, as he walked alongside the boys.

"What good does that do?" inquired Ned.

"It makes your contact with the earth as small as possible-- standing on your toes," the officer explained, "and so reduces the tremor. Opening your mouth, in a measure, equalizes the changed air pressure, caused by the vacuum made when the powder explodes. In other words, you get the same sort of pressure down inside your throat, and in the tubes leading to the ear--the same pressure inside, as outside.

"Often the firing of big guns will burst the ear drums of the officers near the cannon, and this may often be prevented by opening the mouth. It's just like going through a deep tunnel, or sometimes when an elevator descends quickly from a great height. There is too much outside air pressure on the ear drums. By opening your mouth and swallowing rapidly, the pressure is nearly equaled, and you feel no discomfort."

The boys tried this when the next big gun was fired, and they found it true. They noticed quite a crowd of officers and men about a certain large barbette, and Captain Badger led them in that direction.

"Is that General Wailer's gun?" asked Tom.

"That's where they are going to test it," was the answer.

Eagerly Tom and Ned pressed forward. No one of the many officers and soldiers grouped about the new cannon seemed to notice them. A tall man, who seemed very nervous and excited, was hurrying here and there, giving orders rapidly.

"How is that range now?" he asked. "Let me take a look! Are you sure the patrol vessels are far enough out? I think this projectile is going farther than any of you gentlemen have calculated."

"I believe we have correctly estimated the distance," answered someone, and the two entered into a discussion.

"That excited officer is General Wailer," explained Captain Badger, in a low voice, to Tom and Ned.

"I guessed as much," replied the young inventor. Then he went closer to get a better look at the big cannon.

I say big cannon, and yet it was not the largest the government had. In fact, Tom estimated the calibre to be less than twelve inches, but the cannon was very long--much longer in proportion than guns of greater muzzle diameter. Then, too, the breech, or rear part, was very thick and heavy.

"He must be going to use a tremendous lot of powder," said Tom.

"He is," answered Captain Badger. "Some of us think he is going to use too much, but he says it is impossible to burst his gun. He wants to make a long-range record shot, and maybe he will."

"That's a new kind of breech block," commented Tom, as he watched the mechanism being operated.

"Yes, that's General Waller's patent, too. They're going to fire soon."

I might explain, briefly, for the benefit of you boys who have never seen a big, modern cannon, that it consists of a central core of cast steel. This is rifled, just as a small rifle is bored, with twisted grooves throughout its length. The grooves, or rifling, impart a twisting motion to the projectiles, and keep them in a straighter line.

After the central core is made and rifled, thick jackets of steel are "shrunk" on over the rear part of the gun. Sometimes several jackets are put on, one over the other, to make the gun stronger.

If you have ever seen a blacksmith put a tire on a wheel you will understand what I mean. The tire is heated, and this expands it, or makes it larger. It is put on hot, and when it cools it shrinks, getting smaller, and gripping the rim of the wheel in a strong embrace. That is what the jackets of steel do to the big guns.

A big rifled cannon is loaded from the rear, or breech, just as is a breech-loading shotgun or rifle. That is, the cannon is opened at the back and the projectile is put in by means of a derrick, for often the projectiles weigh a thousand pounds or more. Next comes the powder--hundreds of pounds of it--and then it is necessary to close the breech.

The breech block does this. That block is a ponderous piece of steel, quite complicated, and it swings on a hinge fastened to one side of the rear of the gun. Once it is swung back into place, it is made fast by means of screw threads, wedges or in whatever way the inventor of the gun deems best.

The breech block must be very strong, and held firmly in place, or the terrific force of the powder would blow it out, wreck the gun and kill those behind it. You see, the breech block really stands a great part of the strain. The powder is between it and the projectile, and there is a sort of warfare to see which will give way--the projectile or the block. In most cases the projectile gracefully bows, so to speak, and skips out of the muzzle of the gun, though sometimes the big breech block will be shattered.

With eager eyes Tom and Ned watched the preparations for firing the big gun. The charge of powder was hoisted out of the bomb- proof chamber below the barbette, and then the great projectile was brought up in slings. At the sight of that Tom realized that the gun was no ordinary one, for the great piece of steel was nearly three feet long, and must have weighed nearly a thousand pounds. Truly, much powder would be needed to send that on its way.

"I'm afraid, General, that you are using too much of that strong powder," Tom heard one officer say to the inventor of the gun. "It may burst the breech."

"Nonsense, Colonel Washburn. I tell you it is impossible to burst my gun--impossible, sir! I have allowed for every emergency, and calculated every strain. I have a margin of safety equal to fifty per cent."

"Very well, I hope it proves a success."

"Of course it will. It is impossible to burst my gun! Now, are we ready for the test."

The gun was rather crude in form, not having received its final polish, and it was mounted on a temporary carriage. But even with that Tom could see that it was a wonderful weapon, though he thought he would have put on another jacket toward the muzzle, to further strengthen that portion.

"I'm going to make a gun bigger than that," said Tom to Ned. He spoke rather louder than he intended, and, as it was at a moment when there was a period of silence, the words carried to General Waller, who was at that moment near Tom.

"What's that?" inquired the rather fiery-tempered officer, as he looked sharply at our hero.

"I said I was going to make a larger gun than that," repeated Tom, modestly.

"Sir! Do you know what you are saying? How did you come in here, anyhow? I thought no civilians were to be admitted today! Explain how you got here!"

Tom felt an angry flush mounting to his cheeks.

"I came in here on a pass countersigned by you," he replied.

"A pass countersigned by me? Let me it."

Tom passed it over.

"Humph, it doesn't seem to be forged," went on the pompous officer. "Who are you, anyhow?"

"Tom Swift."

"Hum!"

"General Waller, permit me to introduce Tom Swift to you," spoke Captain Badger, stepping forward, and trying not to smile. "He is one of our foremost inventors. It is his type of monoplane that the government has adopted for the coming maneuvers at Panama, you may recall, and he was very helpful to Uncle Sam in stopping that swindling on the border last year--Tom and his big searchlight. Mr. Swift, General Waller," and Captain Badger bowed as he completed the introduction.

"What's that. Tom Swift here? Let me meet him!" exclaimed an elderly officer coming through the crowd. The others parted to make way for him, as he seemed to be a person of some importance, to judge by his uniform, and the medals he wore.

"Tom Swift here!" he went on. "I want to shake hands with you, Tom! I haven't seen you since I negotiated with you for the purchase of those submarines you invented, and which have done such splendid service for the government. Tom, I'm glad to see you here today."

The face of General Waller was a study in blank amazement.