It was no easy matter to change all the guns of the Mars, and fit them with double barrels. But by working day and night shifts Tom managed it. Meanwhile, a careful watch was kept over the shops. Several new men applied for work, and some of them were suspicious enough in looks, but Tom took on no new hands.

Finally the new guns were made, and tried with the Mars held on the ground. They behaved perfectly, the shooting of sand or water from the dummy barrel neutralizing the shot from the service barrel.

"And now to see how it works in practice!" cried Tom one day. "Are you with me for a long flight, Ned?"

"I sure am!"

The next evening the Mars, with a larger crew than before, and with Tom, Ned, Mr. Damon and Lieutenant Marbury aboard, set sail.

"But why start at night?" asked Ned.

"You'll see in the morning," Tom answered.

The Mars flew slowly all night, life aboard her, at about the level of the clouds, going on almost as naturally as though the occupants of the cabins were on the earth. Excellent meals were served.

"But when are you going to try the guns?" asked Ned, as he got ready to turn in.

"Tell you in the morning," replied Tom, with a smile.

And, in the morning, when Ned looked down through the plate gla.s.s in the cabin floor, he uttered a cry.

"Why, Tom! We're over the ocean!" he cried.

"I rather thought we'd be," was the calm reply. "I told George to head straight for the Atlantic. Now we'll have a test with service charges and projectiles!"

CHAPTER XVIII

IN A STORM

Surprise, for the moment, held Mr. Damon, Ned and Lieutenant Marbury speechless. They looked from the heaving waters of the ocean below them to the young pilot of the Mars. He smiled at their astonishment.

"What--what does it mean, Tom?" asked Ned. "You never said you were going to take a trip as far as this."

"That's right," chimed in Mr. Damon. "Bless my nightcap! If I had known I was going to be brought so far away from home I'd never have come."

"You're not so very far from Waterford," put in Tom. "We didn't make any kind of speed coming from Shopton, and we could be back again inside of four hours if we had to."

"Then you didn't travel fast during the night?" asked the government man.

"No, we just drifted along," Tom answered. "I gave orders to run the machinery slowly, as I wanted to get it in good shape for the other tests that will come soon. But I told George, whom I left in charge when I turned in, to head for New York. I wanted to get out over the ocean to try the guns with the new recoil arrangement."

"Well, we're over the ocean all right," spoke Ned, as he looked down at the heaving waters.

"It isn't the first time," replied Tom cheerfully. "Koku, you may serve breakfast now," for the giant had been taken along as a sort of cook and waiter. Koku manifested no surprise or alarm when he found the airship floating over the sea. Whatever Tom did was right to him. He had great confidence in his master.

"No, it isn't the first time we've taken a water flight," spoke Ned. "I was only surprised at the suddenness of it, that's all."

"It's my first experience so far out above the water," observed Lieutenant Marbury, "though of course I've sailed on many seas. Why, we're out of sight of land."

"About ten miles out, yes," admitted Tom. "Far enough to make it safe to test the guns with real projectiles. That is what I want to do."

"And we've been running all night?" asked Mr. Damon.

"Yes, but at slow speed. The engines are in better shape now than ever before," Tom said. "Well, if you're ready we'll have breakfast."

The meal was served by Koku with as much unconcern as though they were in the Swift homestead back in Shopton, instead of floating near the clouds. And while it was being eaten in the main cabin, and while the crew was having breakfast in their quarters, the aerial warship was moving along over the ocean in charge of George Watson, one of Tom's engineers, who was stationed in the forward pilot-house.

"So you're going to give the guns a real test this time, is that it, Tom?" asked Ned, as he pushed back his plate, a signal that he had eaten enough.

"That's about it."

"But don't you think it's a bit risky out over the water this way.

Supposing something should--should happen?" Ned hesitated.

"You mean we might fall?" asked Tom, with a smile.

"Yes; or turn upside down."

"Nothing like that could happen. I'm so sure that I have solved the problem of the recoil of the guns that I'm willing to take chances. But if any of you want to get off the Mars while the test is being made, I have a small boat I can lower, and let you row about in that until--"

"No, thank you!" interrupted Mr. Damon, as he looked below. There was quite a heavy swell on, and the ocean did not appear very attractive.

They would be much more comfortable in the big Mars.

"I think you won't have any trouble," a.s.serted Lieutenant Marbury. "I believe Tom Swift has the right idea about the guns, and there will be so small a shock from the recoil that it will not be noticeable."

"We'll soon know," spoke Tom. "I'm going to get ready for the test now."

They were now well out from sh.o.r.e, over the Atlantic, but to make certain no ships would be endangered by the projectiles, Tom and the others searched the waters to the horizon with powerful gla.s.ses.

Nothing was seen and the work of loading the guns was begun. The bomb tubes, in the main cabin, were also to be given a test.

As service charges were to be used, and as the projectiles were filled with explosives, great care was needed in handling them.

"We'll try dropping bombs first," Tom suggested. "We know they will work, and that will be so much out of the way."

To make the test a severe one, small floating targets were first dropped overboard from the Mars. Then the aerial warship, circling about, came on toward them. Tom, seated at the range-finders, pressed the b.u.t.ton that released the sh.e.l.ls containing the explosives. One after another they dropped into the sea, exploding as they fell, and sending up a great column of salt water.

"Every one a hit!" reported Lieutenant Marbury, who was keeping "score."

"That's good," responded Tom. "But the others won't be so easy. We have nothing to shoot at."

They had to fire the other guns without targets at which to aim. But, after all, it was the absence of recoil they wanted to establish, and this could be done without shooting at any particular object.

One after another the guns were loaded. As has been explained, they were now made double, one barrel carrying the projectile, and the other a charge of water.

"Are you ready?" asked Tom, when it was time to fire. Lieutenant Marbury, Ned and Mr. Damon were helping, by being stationed at the pressure gauges to note the results.