"But, Tom, I don't see how in the world you can ever hope to make a bigger gun than that."

"I think it can be done, Ned," was the quiet answer of the young inventor. He looked up from some drawings on the table in the office of one of his shops. "Now I'll just show you--"

"Hold on, Tom. You know I have a very poor head for figures, even if I do help you out once in a while on some of your work. Skip the technical details, and give me the main facts."

The two young men--Ned Newton being Tom's special chum--were talking together over Tom's latest scheme.

It was several days after Tom's accident in the airship, when he had been saved by the prompt action of Mr. Peterson. That fortune-hunter, once he had the promise of Mr. Swift to invest in his somewhat visionary plan of locating a lost opal mine near the Panama Canal, had left the Swift homestead to arrange for fitting out the expedition of discovery. He had tried to prevail on Tom to accompany him, and, failing in that, tried to work on Mr. Damon.

"Bless my watch chain!" exclaimed that odd gentleman. "I would like to go with you first rate. But I'm so busy--so very busy-- that I can't think of it. I have simply neglected all my affairs, chasing around the country with Tom Swift. But if Tom goes I-- ahem! I think perhaps I could manage it--ahem!"

"I thought you were busy," laughed Tom.

"Oh, well, perhaps I could get a few weeks off. But I'm not going--no, bless my check book, I must get back to business!"

But as Mr. Damon was a retired gentleman of wealth, his "business" was more or less of a joke among his friends.

So then, a few days after the departure of Mr. Peterson, Tom and Ned sat in the former's office, discussing the young inventor's latest scheme.

"How big is the biggest gun ever made, Tom?" asked his chum. "I mean in feet, in inches, or in muzzle diameter, however they are measured."

"Well," began Tom, "of course some nation may, in secret, be making a bigger gun than any I have ever heard of. As far as I know, however, the largest one ever made for the United States was a sixteen-inch rifled cannon--that is, it was sixteen inches across at the muzzle, and I forget just how long. It weighed many tons, however, and it now lies, or did a few years ago, in a ditch at the Sandy Hook proving grounds. It was a failure."

"And yet you are figuring on making a cannon with a muzzle thirty inches across--almost a yard--and fifty feet long and to weigh--"

"No one can tell exactly how much it will weigh," interrupted Tom. "And I'm not altogether certain about the muzzle measurement, nor of the length. It's sort of in the air at present. Only I don't see why a larger gun than any that has yet been made, can't be constructed."

"If anybody can invent one, you can, Tom Swift!" exclaimed Ned, admiringly.

"You flatter me!" exclaimed his chum, with a mock bow.

"But what good will it be?" went on Ned. "Making big guns doesn't help any in war, that I can see."

"Ned!" exclaimed Tom, "you don't look far enough ahead. Now here's my scheme in a nutshell. You know what Uncle Sam is doing down in his big ditch; don't you?"

"You mean digging the Panama Canal?"

Yes, the greatest engineering feat of centuries. It is going to make a big change in the whole world, and the United States is going to become--if she is not already--a world-power. Now that canal has to be protected--I mean against the possibility of war. For, though it may never come, and the chances are it never will, still it may.

"Uncle Sam has to be ready for it. There never was a more true saying than 'in time of peace prepare for war.' Preparing for war is, in my opinion, the best way not to have one.

"Once the Panama Canal is in operation, and the world-changes incidental to it have been made, if it should pass into the hands of some foreign country--as it very possibly might do--the United States would not only be the laughing-stock of the world, but she would lose the high place she holds.

"Now, then, to protect the canal, several things are necessary. Among them are big guns--cannon that can shoot a long distance-- for if a foreign nation should send some of their new dreadnaughts over here--vessels with guns that can shoot many miles--where would the canal be once a bombardment was opened? It would be ruined in a day--the immense lock-gates would be destroyed. And, not only from the guns aboard ships would there be danger, but from siege cannon planted in Costa Rica, or some South American country below the canal zone.

"Now, to protect the canal against such an attack we need guns that can shoot farther, straighter and more powerfully than any at present in use, and we've got to have the most powerful explosive. In other words, we've got to beat the biggest guns that are now in existence. And I'm going to do it, Ned!"

"You are?"

"Yes, I'm going to invent a cannon that will make the longest shots on record. I'm going to make a world-beater gun; or, rather, I'm going to invent it, and have it made, for I guess it would tax this place to the limit.

"I've been thinking of this for some time, Ned. I've been puttering around inventing new magnetos, potato-parers and the like, but this is my latest hobby. The Panama Canal is a big thing--one of the biggest things in the world. We need the biggest guns in the world to protect it.

"And, listen: Uncle Sam thinks the same way. I understand that the best men in the service--at West Point, Annapolis and Sandy Hook, as well as elsewhere--are working in the interest of the United States to perfect a bigger cannon than any ever before made. In fact, one has just been constructed, and is going to be tried at the Sandy Hook proving grounds soon. I'm going to see the test if I can.

"And here's another thing. Foreign nations are trying to steal Uncle Sam's secrets. If this country gets a big cannon, some other nation will want a bigger one. It's a constant warfare. I'm going to devote my talents--such as they are--to Uncle Sam. I'm going to make the biggest cannon in the world--the one that will shoot the farthest and knock into smithereens all the other big guns. That's the only way to protect the canal. Do you understand, Ned?"

"Somewhat, Tom. Since I gave up my place in the bank, and became a sort of handy-lad for you, I know more about your work. But isn't it going to be dangerous to make a cannon like that?"

"Well, in a way, yes, Ned. But we've got to take chances, just as father did when he invested ten thousand dollars in that opal mine. He'll never see his money again."

"Don't you think so?"

"No, Ned."

"And when do you expect to start on your gun, Tom?"

"Right away. I'm making some plans now. I'm going down to Sandy Hook and witness the test of this new big cannon. You can come along, if you like."

"Well, I sure will like. When is it?"

"Oh, in about a week. I'll have to look--"

"'Scuse me, Massa Tom," broke in Eradicate, as he put his head through the half-opened office door. "'Scuse me, but dere's a express gen'men outside, wif his auto truck, an' he's got some packages fo' yo' all, marked 'dangerous--explosive--an' keep away fom de fire.' He want t' know what he all gwine t' do wif 'em, Massa Tom?"

"Do with 'em? Oh, I guess it's that new giant powder I sent for. Why, Eradicate, have him bring 'em right in here."

"Yais, sah, Massa Tom. Dat's all right; but he jest can't bring 'em in," and Eradicate looked behind him somewhat apprehensively.

"Can't bring 'em in? Why not, I'd like to know?" exclaimed Tom. "He's paid for it."

"'Scuse me, Massa Tom," said the colored man, "but dat express gen'men can't bring dem explosive powder boxes in heah, 'case as how his autermobile hab done ketched fire an' he cain't get near it nohow. Dat's why, Massa Tom!"

"Caesar's ghost!" yelled the young inventor. "The auto on fire, and that powder in it! Come on Ned!" and he made a rush for the door.