There were murmurs throughout the throng about the big gun, as the officer approached Tom Swift and shook hands with him.

"What have you in mind now, Tom, that you come to Sandy Hook?" the much-medaled officer asked.

"Nothing much, Admiral," answered our hero.

"Oh, yes, you have!" returned Admiral Woodburn, head of the naval forces of Uncle Sam. "You've got some idea in your head, or you wouldn't come to see this test of my friend's gun. Well, if you can invent anything as good for coast defense, or even interior defense, as your submarines, it will be in keeping with what you have done in the past. I congratulate you, General Waller, on having Tom Swift here to give you the benefit of some of his ideas."

"I--I haven't had the pleasure of meeting Mr. Swift before," said the gun inventor, stiffly. "I did not recognize his name when I countersigned his pass."

It was plain that the greeting of Tom by Admiral Woodburn had had a marked effect in changing sentiment toward our hero. Captain Badger smiled as he noticed with what different eyes the gun inventor now regarded the lad.

"Well, if Tom Swift gives you any points about your gun, you want to adopt them," went on the Admiral. "I thought I knew something about submarines, but Tom taught me some things, too; didn't you, Tom?"

"Oh, it was just a simple matter, Admiral," said Tom, modestly. "Just that little point about the intake valves and the ballast tanks."

"But they changed the whole matter. Yes, General, you take Tom's advice--if he gives you any."

"I don't know that I will need any--as yet," replied General Waller. "I am confident my gun will be a success as it is at present constructed. Later, however, if I should decide to make any changes, I will gladly avail myself of Mr. Swift's counsel," and he bowed stiffly to Tom. "We will now proceed with the test," he went on. "Kindly send a wireless to the patrol ships that we are about to fire, and ask them to note carefully where the projectile falls."

"Very good, sir," spoke the officer in immediate charge of the matter, as he saluted. Soon from the aerials snapped the vicious sparks that told of the wireless telegraph being worked.

I might explain that near the spot where the projectile was expected to fall into the sea--about fifteen miles from Sandy Hook--several war vessels were stationed to warn shipping to give the place a wide berth. This was easy, since the big gun had been aimed at a spot outside of the steamship lanes. Aiming the rifle in a certain direction, and giving it a definite angle of inclination, made it practically certain just where the shot would fall. This is called "getting the range," and while, of course, the exact limit of fire of the new gun was not known, it had been computed as nearly as possible.

"Is everything ready now?" asked General Waller, while Tom was conversing with his friends, Captain Badger and Admiral Woodburn, Ned taking part in the conversation from time to time.

"All ready, sir," was the assurance. The inventor was plainly nervous as the crucial moment of the test approached. He went here and there upon the barbette, testing the various levers and gear wheels of the gun.

The projectile and powder had been put in, the breech-block screwed into place, the primer had been inserted, and all that remained was to press the button that would make the electrical connection, and explode the charge. This act of firing the gun had been intrusted to one of the soldiers, for General Waller and his brother officers were to retire to a bomb-proof, whence they would watch the effect of the fire, and note the course of the projectile.

"It seems to me," remarked Ned, "that the soldier who is going to fire the gun is in the most danger."

"He would be--if it exploded," spoke Tom, for his officer friends had joined their colleagues, most of whom were now walking toward the shelter. "But I think there is little danger.

"You see, the electric wires are long enough to enable him to stand some distance from the gun. And, if he likes, he can crouch behind that concrete wall of the next barbette. Still, there is some chance of an accident, for, no matter how carefully you calculate the strain of a bursting charge of powder, and how strongly you construct the breech-block to stand the strain, there is always the possibility of a flaw in the metal. So, Ned, I think we'll just go to the bombÄproof ourselves, when we see General Waller making for the same place."

"I suppose," remarked Ned, "that in actual warfare anyone who fired one of the big guns would have to stand close to it--closer than that soldier is now."

"Oh, yes--much," replied Tom, as he watched General Waller giving the last instructions to the private who was to press the button. "Only, of course, in war the guns will have been tested, and this one has not. Here he comes; I guess we'd better be moving."

General Waller, having assured himself that everything was as right as possible, had given the last word to the private and was now making his way toward the bomb-proof, within which were gathered his fellow-officers and friends.

"You had better retire from the immediate vicinity of the gun," said its inventor to Tom and Ned, as he passed them. "For, while I have absolute confidence in my cannon, and I know that it is impossible to burst it, the concussion may be unpleasant at such close range."

"Thank you," said Tom. "We are going to get in a safe place."

He could not refrain from contrasting the general's manner now with what it had been at first.

As for Ned, he could not help wondering why, if the inventor had such absolute faith in his weapon, he did not fire it himself, even at the risk of a "concussion."

How it happened was never accurately known, as the soldier declared positively--after he came out of the hospital--that he had not pressed the button. The theory was that the wires had become crossed, making a short circuit, which caused the gun to go off prematurely.

But suddenly, while Tom, Ned and General Waller were still some distance away from the bomb-proof, there was a terrific explosion. It seemed as if the very foundations of the fortifications would be shattered There was a roaring in the air --a hot burst of flame, and instantly such a vacuum was created that Tom and Ned found themselves gasping for breath.

Dazed, shaken in every bone, with their muscles sore, they picked themselves up from the ground, along which they had been blown with great force in the direction of the bomb-proof. Even as Tom struggled to his feet, intending to run to safety in fear of other explosions, he realized what had happened.

"What--what was it?" cried Ned, as he, too, arose.

"The gun burst!" yelled Tom.

He looked to the left and saw General Waller picking himself up, his uniform torn, and blood streaming from a cut on his face. At the same instant Tom was aware of the body of a man flying through the air toward a distant grass plot, and the young inventor recognized it as that of the soldier who had been detailed to fire the great cannon.

Almost instantaneously as everything happened, Tom was aware of noticing several things, as though they took place in sequence. He looked toward where the gun had stood. It was in ruins. The young inventor saw something, which he took to be the projectile, skimming across the sea waves, and he had a fleeting glimpse of the greater portion of the immense weapon itself sinking into the depths of the ocean.

Then, coming down from a great height in the air, he saw a dark object. It was another piece of the cannon that had been hurled skyward.

"Look out!" Tom yelled, instinctively, as he staggered toward the bomb-proof, Ned following.

He saw a number of officers running out to assist General Waller, who seemed too dazed to move. Many of them had torn uniforms, and not a few were bleeding from their injuries. Then the air seemed filled with a rain of small missilesÄstones, dirt, gravel and pieces of metal.