For a moment or two Tom Swift did not seem to comprehend what Ned had said. He remained staring, first at his chum, who stood pointing, and from him Tom's gaze wandered to the top of the door. It may have been, and probably was, that Tom was thinking of other matters at that instant. But Ned said again:

"Wouldn't that do, Tom? Check the recoil of the gun with whatever stuff is in that arrangement!"

A sudden change came over Tom's face. It was lighted up with a gleam of understanding.

"By Jove, Ned, old man!" he cried. "I believe you've struck it! And to think that has been under my nose, or, rather, over my head, all this while, and I never thought of it. Hurray! That will solve the problem!"

"Do you think it will?" asked Ned, glad that he had contributed something, if only an idea, to Tom's aerial warship.

"I'm almost sure it will. I'll give it a trial right away."

"What's in that door-check?" Ned asked. "I never stopped before to think what useful things they are, though at the bank, with the big, heavy doors, they are mighty useful."

"They are a combination of springs and hydrostatic valves," began Tom.

"Good-night!" laughed Ned. "Excuse the slang, Tom, but what in the world is a hydrostatic valve?"

"A valve through which liquids pass. In this door-check there may be a mixture of water, alcohol and glycerine, the alcohol to prevent freezing in cold weather, and the glycerine to give body to the mixture so it will not flow through the valves too freely."

"And do you think you can put something like that on your guns, so the recoil will be taken up?" Ned wanted to know.

"I think so," spoke Tom. "I'm going to work on it right away, and we'll soon see how it will turn out It's mighty lucky you thought of that, for I sure was up against it, as the boys say."

"It just seemed to come to me," spoke Ned, "seeing how easily the door closed."

"If the thing works I'll give you due credit for it," promised Tom. "Now, I've got to figure out how much force a modified hydrostatic valve check like that will take up, and how much recoil my biggest gun will have."

"Then you're going to put several guns on the Mars?" asked Ned.

"Yes, four quick-firers, at least, two on each side, and heavier guns at the bow and stern, to throw explosive shells in a horizontal or upward direction. For a downward direction we won't need any guns, we can simply drop the bombs, or shells, from a release clutch."

"Drop them on other air craft?" Ned wanted to know.

"Well, if it's necessary, yes. Though I guess there won't be much chance of doing that to a rival aeroplane or dirigible. But in flying over cities or forts, explosive bombs can be dropped very nicely. For use in attacking other air craft I am going to depend on my lateral fire, from the guns mounted on either beam, and in the bow and stern."

"You speak as though you, yourself, were going into a battle of the air," said Ned.

"No, I don't believe I'll go that far," Tom replied. "Though, if the government wants my craft, I may have to go aloft and fire shots at targets for them to show them how things work.

"Please don't think that I am in favor of war, Ned," went on Tom earnestly. "I hate it, and I wish the time would come when all nations would disarm. But if the other countries are laying themselves out to have aerial battleships, it is time the United States did also. We must not be left behind, especially in view of what is taking place in Europe."

"I suppose that's right," agreed Ned. "Have you any of your guns ready?"

"Yes, all but the mounting of them on the supports aboard the Mars. I haven't dared do that yet, and fire them, until I provided some means of taking up the recoil. Now I'm going to get right to work on that problem."

There was considerable detailed figuring and computation work ahead of Tom Swift, and I will not weary you by going into the details of higher mathematics. Even Ned lost interest after the start of the problem, though he was interested when Tom took down the door-check and began measuring the amount of force it would take up, computing it on scales and spring balances.

Once this had been done, and Tom had figured just how much force could be expected to be taken up by a larger check, with stronger hydrostatic valves, the young inventor explained:

"And now to see how much recoil force my guns develop!"

"Are you really going to fire the guns?" asked Ned.

"Surely," answered Tom. "That's the only way to get at real results. I'll have the guns taken out and mounted in a big field. Then we'll fire them, and measure the recoil."

"Well, that may be some fun," spoke Ned, with a grin. "More fun than all these figures," and he looked at the mass of details on Tom's desk.

This was the second or third day after the fire in the red shed, and in the interim Tom had been busy making computations. These were about finished. Meanwhile further investigation bad been made of clues leading to the origin of the blaze in the shed, but nothing had been learned.

A photo-telephone had been installed near Eradicate's quarters, in the hope that the mysterious stranger might keep his promise, and come to see about the mule. In that case something would have been learned about him. But, as Tom feared, the man did not appear.

Ned was much interested in the guns, and, a little later, he helped Tom and Koku mount them in a vacant lot. The giant's strength came in handy in handling the big parts.

Mr. Swift strolled past, as the guns were being mounted for the preliminary test, and inquired what his son was doing

"It will never work, Tom, never!" declared the aged inventor, when informed. "You can't take up those guns in your air craft, and fire them with any degree of safety."

"You wait, Dad," laughed Tom. "You haven't yet seen how the Newton hydrostatic recoil operates."

Ned smiled with pleasure at this.

It took nearly a week to get all the guns mounted, for some of them required considerable work, and it was also necessary to attach gauges to them to register the recoil and pressure. In the meanwhile Tom had been in further communication with government experts who were soon to call on him to inspect the aerial warship, with a view to purchase.

"When are they coming?" asked Ned, as he and Tom went out one morning to make the first test of the guns.

"They will be here any day, now. They didn't set any definite date. I suppose they want to take us unawares, to see that I don't 'frame-up' any game on them. Well, I'll be ready any time they come. Now, Koku, bring along those shells, and don't drop any of them, for that new powder is freakish stuff."

"Me no drop any, Master," spoke the giant, as he lifted the boxes of explosives in his strong arms.

The largest gun was loaded and aimed at a distant hill, for Tom knew that if the recoil apparatus would take care of the excess force of his largest gun, the problem of the smaller ones would be easy to solve.

"Here, Rad, where are you going?" Tom asked, as he noticed the colored man walking away, after having completed a task assigned to him.

"Where's I gwine, Massa Tom?"

"Yes, Rad, that's what I asked you."

"I--I'se gwine t' feed mah mule, Boomerang," said the colored man slowly. "It's his eatin' time. jest now, Massa Tom."

"Nonsense! It isn't anywhere near noon yet."

"Yais, sab, Massa Tom, I knows dat," said Eradicate, as he carefully edged away from the big gun, "but I'se done changed de eatin' hours ob dat mule. He had a little touch ob indigestion de udder day, an' I'se feedin' him diff'rent now. So I guess as how yo'll hab t' 'scuse me now, Massa Tom."

"Oh, well, trot along," laughed the young inventor. "I guess we won't need you. Is everything all right there, Koku?"

"All right, Master."

"Now, Ned, if you'll stand here," went on Tom, "and note the extreme point to which the hand on the pressure gauge goes, I'll be obliged to you. Just jot it down on this pad."

"Here comes someone," remarked the bank clerk, as he saw that his pencil was sharpened. He pointed to the field back of them.

"It's Mr. Damon," observed Tom. "We'll wait until he arrives. He'll be interested in this."

"Bless my collar button, Tom! What's going on?" asked the eccentric man, as he came up. "Has war been declared?"

"Just practicing," replied the young inventor. "Getting ready to put the armament on my aerial warship."

"Well, as long as I'm behind the guns I'm all right, I suppose?"

"Perfectly," Tom replied. "Now then, Ned, I think we'll fire."

There was a moment of inspection, to see that nothing had been forgotten, and then the big gun was discharged. There was a loud report, not as heavy, though, as Ned had expected, but there was no puff of smoke, for Tom was using smokeless powder. Only a little flash of flame was observed.

"Catch the figure, Ned!" Tom cried.

"I have it!" was the answer. "Eighty thousand!"

"Good! And I can build a recoil check that will take up to one hundred and twenty thousand pounds pressure. That ought to be margin of safety enough. Now we'll try another shot."

The echoes of the first had hardly died away before the second gun was ready for the test. That, too, was satisfactory, and then the smaller ones were operated. These were not quite so satisfactory, as the recoil developed was larger, in proportion to their size, than Tom had figured.

"But I can easily put a larger hydrostatic check on them," he said. "Now, we'll fire by batteries, and see what the total is."

Then began a perfect bombardment of the distant hillside, service charges being used v, and explosive shells sent out so that dirt, stones and gravel flew in all directions. Danger signs and flags had been posted, and a cordon of Tom's men kept spectators away from the hill, so no one would be in the danger zone.

The young inventor was busy making some calculations after the last of the firing had been completed. Koku was packing up the unfired shells, and Mr. Damon was blessing his ear-drums, and the pieces of cotton he had stuffed in to protect them, when a tall, erect man was observed strolling over the fields in the direction of the guns.

"Somebody's coming, Tom," warned Ned.

"Yes, and a stranger, too," observed Tom. "I wonder if that can be Eradicate's Frenchman?"

But a look at the stranger's face disproved that surmise. He had a frank and pleasant countenance, obviously American.

"I beg your pardon," he began, addressing everyone in general, "but I am looking for Tom Swift. I was told he was here."

"I am Tom Swift," replied our hero.

"Ah! Well, I am Lieutenant Marbury, with whom you had some correspondence recently about--"

"Oh, yes, Lieutenant Marbury, of the United States Navy," interrupted Tom. "I'm glad to see you," he went on, holding out his hand. "We are just completing some tests with the guns. You called, I presume, in reference to my aerial warship?"

"That is it--yes. Have you it ready for a trial flight?"

"Well, almost. It can be made ready in a few hours. You see, I have been delayed. There was a fire in the plant

"A fire!" exclaimed the officer in surprise. "How was that? We heard nothing of it in Washington."

"No, I kept it rather quiet," Tom explained. "We had reason to suspect that it was a fire purposely set, in a shed where I kept a quantity of explosives."

"Ha!" exclaimed Lieutenant Marbury. "This fits in with what I have heard. And did you not receive warning?" he asked Tom.

"Warning? No. Of what?"

"Of foreign spies!" was the unexpected answer. "I am sorry. Some of our Secret Service men unearthed something of a plot against you, and I presumed you had been told to watch out. If you had, the fire might not have occurred. There must have been some error in Washington. But let me tell you now, Tom Swift--be on your guard!"