"Tom, however did you do it?" gasped Ned at length.

"By hard work," was the modest reply. "I've been at this for a longer time than you'd suppose, working on it at odd moments. I had a lot of help, too, or I never could have done it. And now it is nearly all finished, as far as the ship itself is concerned. The only thing that bothers me is to provide for the recoil of the guns I want to carry.

Maybe you can help me with that. Come on, now, I'll explain how the affair works, and what I hope to accomplish with it."

In brief Tom's aerial warship was a sort of German Zeppelin type of dirigible balloon, rising in the air by means of a gas container, or, rather, several of them, for the section for holding the lifting gas element was divided by bulkheads.

The chief difference between dirigible balloons and ordinary aeroplanes, as you all know, is that the former are lifted from the earth by a gas, such as hydrogen, which is lighter than air, while the aeroplane lifts itself by getting into motion, when broad, flat planes, or surfaces, hold it up, just as a flat stone is held up when you sail it through the air. The moment the stone, or aeroplane, loses its forward motion, it begins to fall.

This is not so with a dirigible balloon. It is held in the air by means of the lifting gas, and once so in the air can be sent in any direction by means of propellers and rudders.

Tom's aerial warship contained many new features. While it was as large as some of the war-type Zeppelins, it differed from them materially.

But the details would be of more interest to a scientific builder of such things than to the ordinary reader, so I will not weary you with them.

Sufficient to say that Tom's craft consisted first of a great semi-rigid bag, or envelope, made of specially prepared oiled silk and aluminum, to hold the gas, which was manufactured on board. There were a number of gas-tight compartments, so that if one, or even if a number of them burst, or were shot by an enemy, the craft would still remain afloat.

Below the big gas bag was the ship proper, a light but strong and rigid framework about which were built enclosed cabins. These cabins, or compartments, housed the driving machinery, the gas-generating plant, living, sleeping and dining quarters, and a pilot-house, whence the ship could be controlled.

But this was not all.

Ned, making a tour of the Mars, as she swayed gently in the big shed, saw where several aluminum pedestals were mounted, fore and aft and on either beam of the ship.

"They look just like places where you intend to mount guns," said Ned to Tom.

"And that's exactly what they are," the young inventor replied. "I have the guns nearly ready for mounting, but I can't seem to think of a way of providing for the recoil. And if I don't take care of that, I'm likely to find my ship coming apart under me, after we bombard the enemy with a broadside or two."

"Then you intend to fight with this ship?" asked Ned.

"Well, no; not exactly personally. I was thinking of offering it to the United States Government. Foreign nations are getting ready large fleets of aerial warships, so why shouldn't we? Matters in Europe are mighty uncertain. There may be a great war there in which aerial craft will play a big part. I am conceited enough to think I can build one that will measure up to the foreign ones, and I'll soon be in a position to know."

"What do you mean?"

"I mean I have already communicated with our government experts, and they are soon to come and inspect this craft. I have sent them word that it is about finished. There is only the matter of the guns, and some of the ordnance officers may be able to help me out with a suggestion, for I admit I am stuck!" exclaimed Tom.

"Then you're going to do the same with this aerial warship as you did with your big lantern and that immense gun you perfected?" asked Ned.

"That's right," confirmed Tom. My former readers will know to what Ned Newton referred, and those of you who do not may learn the details of how Tom helped Uncle Sam, by reading the previous volumes, "Tom Swift and His Great Searchlight," and "Tom Swift and His Giant Cannon."

"When do you expect the government experts?" Ned asked.

"Within a few days, now. But I'll have to hustle to get ready for them, as this fire has put me back. There are quite a number of details I need to change. Well, now, let me explain about that gun recoil business. Maybe you can help me."

"Fire away," laughed Ned. "I'll do the best I can."

Tom led the way from the main shed, where the aerial warship was housed, to a small private office. As Ned entered, the door, pulled by a strong spring, swung after him. He held back his hand to prevent it from slamming, but there was no need, for a patent arrangement took up all the force, and the door closed gently. Ned looked around, not much surprised, for the same sort of door-check was in use at his bank. But a sudden idea came to him.

"There you are, Tom!" he cried. "Why not take up the recoil of the guns on your aerial warship by some such device as that?" and Ned pointed to the door-check.

CHAPTER VII

WARNINGS

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 pa.s.s. 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 sh.e.l.ls in a horizontal or upward direction. For a downward direction we won't need any guns, we can simply drop the bombs, or sh.e.l.ls, 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?"