"Well, Mr. Peterson," remarked Tom, after a pause, "I'm sure I hope you will succeed in your quest. You must have met disappointment so far."

"I have, Tom. But I'm not going to give up. Can't you come over and see me before you go back North?"

"I'll try. Just where is your island?"

"Off in that direction," responded the fortune-hunter, pointing to the northeast. "It's a little farther from here than I thought it was at first--about thirty miles. But I have a little second- hand steam launch that my pardners and I use. I'll come for you, take you over and bring you back any time you say."

"After my gun has been tested," said Tom, with a smile. "Better stay and see it."

"No, I must get back to the island. I have some new information that I am sure will enable me to locate the lost mine."

"Well, good-bye, and good luck to you," called Tom, as the fortune-hunter started away.

"Do you think he'll ever find the opals, Tom?" asked Ned.

His chum shook his head.

"I don't believe so," he answered. "Alec has always been that way--always visionary--always just about to be successful; but never quite getting there."

"Then your father's ten thousand dollars will be lost?"

"Yes, I suppose so; but, in a way, dad can stand it. And if I make good on this gun test, ten thousand dollars won't look very big to me. I guess dad gave it to Alec from a sort of sentimental feeling, anyhow."

"You mean because he saved you from the live wire?"

"That's it, Ned. It was a sort of reward, in a way, and I guess dad won't be broken-hearted if Alec doesn't succeed. Only, of course, he'll feel badly for Alec himself. Poor old man! he won't be able to do much more prospecting. Well, Ned, let's get to work on that ammunition hoist. It still jams a little on the ways, and I want it to work smoothly. There's no use having a hitch--even a small one--when the big bugs assemble to see how my cannon shoots."

"That's right, Tom. Well, start off, I'm with you."

The two youths labored for some time, being helped, of course, by the workmen provided by the government, and some from the steel concern.

There were many little details to look after, not the least of which was the patrolling of the stretch of ocean over which the great projectiles would soar in reaching the far-off targets at which Tom had planned to shoot. No ships were to be allowed to cross the thirty-mile mark while the firing was in progress. So, also, the zone where the shots were expected to fall was to be cleared.

But at last all seemed in readiness. The gun had been tried again and again on its carriage. The projectiles were all in readiness, and the terribly powerful ammunition had been stored below the gun in a bomb-proof chamber, ready to be hoisted out as needed.

Because the gun had been fired so many times with a charge of powder heavier than was ordinarily called for, and had stood the strain well, Tom had no fear of standing reasonably close to it to press the button of the battery. There would be no retreating to the bombproof this time.

The German officer was occasionally seen about the place where the gun was mounted, but he appeared to take only an ordinary interest in it. Tom began to feel more than ever that perhaps his suspicions were unfounded.

Some officials high in government affairs had arrived at Colon in anticipation of the test, which, to Tom's delight, had attracted more attention than he anticipated. At the same time he was a bit nervous.

"Suppose it fails, Ned?" he said.

"Oh, it can't!" cried his chum. "Don't think about such a thing.''

Plans had been made for a ship to be stationed near the zone of fire, to report by wireless the character of each shot, the distance it traveled, and how near it came to the target. The messages would be received at a station near the barbette, and at once reported to Tom, so that he would know how the test was progressing.

"Well, today tells the tale!" exclaimed the young inventor, as he got up one morning. "How's the weather, Ned?"

"Couldn't be better--clear as a bell, Tom."

"That's good. Well, let's have grub, and then go out and see how my pet is."

"Oh, I guess nothing could happen, with Koku on guard."

"No, hardly. I'm going to keep him in the ammunition room until after the test, too. I'm going to take no chances."

"That's the ticket!"

The gun was found all right, in its great tarpaulin cover, and Tom had the latter taken off that he might go over every bit of mechanism. He made a few slight changes, and then got ready for the final trials.

On an improvised platform, not too near the giant cannon, had gathered the ordnance board, the specially invited guests, a number of officers and workers in the canal zone, and one or two representatives of foreign governments. Von Brunderger was there, but his "familiar," as Ned had come to call the stolid German servant, was not present.

Tom took some little time to explain, modestly enough, the working of his gun. A number of questions were asked, and then it was announced that the first shot, with only a practice charge of powder, would be fired.

"Careful with that projectile now. That's it, slip it in carefully. A little farther forward. That's better. Now the powder--Koku, are you down there?" and Tom called down the tube into the ammunition chamber.

"Me here, Master," was the reply.

"All right, send up a practice load."

Slowly the powerful explosive came up on the electric hoist. It was placed in the firing chamber and the breech dosed.

"Now, gentlemen," said Tom, "this is not a shot for distance. It is merely to try the gun and get it warmed up, so to speak, for the real tests that will follow. All ready?"

"All ready!" answered Ned, who was acting as chief assistant.

"Here she goes!" cried Tom, and he pressed the button.

Many were astonished by the great report, but Tom and the others, who were used to the service charges, hardly noticed this one. Yet when the wireless report came in, giving the range as over fourteen thousand yards, there was a gasp of surprise.

"Over eight miles!" declared one grizzled officer; "and that with only a practice charge. What will happen when he puts in a full one?'

"I don't know," answered a friend.

Tom soon showed them. Quickly he called for another projectile, and it was inserted in the gun. Then the powder began to come up the hoist. Meanwhile the young inventor had assured himself that the gun was all right. Not a part had been strained.

This time, when Tom pressed the button there was such a tremendous concussion that several, who were not prepared for it, were knocked back against their neighbors or sent toppling off their chairs or benches. And as for the report, it was so deafening that for a long time after it many could not hear well.

But Tom, and those who knew the awful power of the big cannon, wore specially prepared eardrum protectors, that served to reduce the shock.

"What is it?" called Tom to the wireless operator, who was receiving the range distance from the marking ship.

"A little less than twenty-nine miles."

"We must do better than that," said Tom. "I'll use more powder, and try one of the newer shells. I'll elevate the gun a trifle, too."

Again came that terrific report, that trembling of the ground, that concussion, that blast of air as it rushed in to fill the vacuum caused, and then the vibrating echoes.

"I think you must have gone the limit this time, Tom!" yelled Ned, as he turned on the compressed air to blow the powder fumes and unconsumed bits of explosive from the gun tube.

"Possibly," admitted Tom. "Here comes the report." The wireless operator waved a slip of paper.

"Thirty-one miles!" he announced.

"Hurray!" cried Mr. Damon. "Bless my telescope! The longest shot on record!"

"I believe it is," admitted the chief of the ordnance department. "I congratulate you, Mr. Swift."

"I think I can do better than that," declared Tom, after looking at the various recording gauges, and noting the elevation of the gun. "I think I can get a little flatter trajectory, and that will give a greater distance. I'm going to try."

"Does that mean more powder, Tom?" asked Ned.

"Yes, and the heaviest shell we have--the one with the bursting charge. I'll fire that, and see what happens. Tell the zone-ship to be on the lookout," he said to the wireless operator, giving a brief statement of what he was about to attempt.

"Isn't it a risk, Tom?" his chum asked.

"Well, not so much. I'm sure my cannon will stand it. Come on now, help me depress the muzzle just a trifle," and by means of the electric current the big gun was raised at the breech a few inches.

As is well known, cannon shots do not go in straight lines. They leave the muzzle, curve upward and come down on another curve. It is this curve described by the projectile that is called the trajectory. The upward curve, as you all know, is caused by the force of the powder, and the downward by the force of gravitation acting on the shot as soon as it reaches its zenith. Were it not for this force the projectiles could be fired in straight lines. But, as it is, the cannon has to be elevated to send the shot up a bit, or it would fall short of its mark.

Consequently, the flatter the trajectory the farther it will go. Tom's object, then, was to flatten the trajectory, by lowering the muzzle of the gun, in order to attain greater distance.

"If this doesn't do the trick, we'll try it with the muzzle a bit lower, and with a trifle more powder," he said to Ned, as he was about to fire.

The young inventor was not a little nervous as he prepared to press the button this time. It was a heavier charge than any used that day, though the same quantity had been fired on other occasions with safety. But he was not going to hesitate.

Coincident with the pressure of Tom's fingers there seemed to be a veritable earthquake. The ground swayed and rocked, and a number of the spectators staggered back. It was like the blast of a hundred thunderbolts. The gun shook as it recoiled from the shock, but the wonderful disappearing carriage, fitted with coiled, pneumatic and hydrostatic buffers, stood the strain.

Following the awful report, the terrific recoil and the howl of the wind as it rushed into the vacuum created, there was an intense silence. The projectile had been seen by some as a dark speck, rushing through the air like a meteor. Then the wireless operator could be seen writing down a message, the telephone-like receivers clamped over his ears.

"Something happened, all right!" he called aloud. "That shot hit something."

"Not one of the ships!" cried Tom, aghast.

"I don't know. There seems to be some difficulty in transmitting. Wait--I'm getting it: now."

As he ceased speaking there came from underneath the great gun the sound of confused shouts. Tom and Ned recognized Koku's voice protesting:

"No--no--you can't come in here! Master said no one was to come in."

"What is it, Koku?" yelled Tom, springing to the speaking tube connecting with the powder magazine, at the same time keeping an eye on the wireless operator. Tom was torn between two anxieties.

"Someone here, Master!" cried the giant. "Him try to fix powder. Ah, I fix you!" and with a savage snarl the giant, in the concrete chamber below, could be heard to attack someone who cried out gutturally in German:

"Help! Help! Help!"

"Come on, Ned!" cried Tom, making a dash for the stairs that led into the magazine. There was confusion all about, but through it all the wireless operator continued to write down the message coming to him through space.

"What is it, Koku? What is it?" cried Tom, plunging down into the little chamber.

As he reached it, a door leading to the outer air flew open, and out rushed a man, badly torn as to his clothes, and scratched and bleeding as to his face. On he ran, across the space back of the barbette, toward the lower tier of seats that had been erected for the spectators.

"It's von Brunderger's servant!" gasped Ned, recognizing the fellow.

"What did he do, Koku?" demanded the young inventor.

"Him sneak in here--have some of that stuff you call 'dope.' I sent up powder, and I come back here to see him try to put some dope in Master's ammunition."

"The scoundrel!" cried Tom. "They're trying to break me, even at the last minute! Come on, Ned."

They raced outside to behold a curious sight. Straight toward von Brunderger rushed the man as if in a frenzy of fear. He called out something in German to his master, and the latter's face went first red, then white. He was observed to look about quickly, as though in alarm, and then, with a shout at his servant, the German officer rushed from the stand, and the two disappeared in the direction of the barracks.

"What does it mean?" cried Ned.

"Give it up," answered Tom, "except that Koku spoiled their trick, whatever it was. It looks as if this was the end of it, and that the mystery has been cleared up."

"Mr. Swift! Where's Mr. Swift?" shouted the wireless operator. "Where are you?"

"Yes; what is it?" demanded Tom, so excited that he hardly knew what he was doing.

"The longest shot on record!" cried the man. "Thirty-three miles, and it struck, exploded, and blew the top off a mountain on an island out there!" and he pointed across the sun-lit sea.