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 Water ford," 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," asserted 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 shore, 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 glasses. 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 button that released the shells 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.

"All ready," answered Ned.

"Do you think we'd better put on life preservers, Tom?" asked Mr. Damon.

"Nonsense! What for?"

"In case--in case anything happens."

"Nothing will happen. Look out now, I'm going to fire."

The guns were to be fired simultaneously by means of an electric current, when Tom pressed a button.

"Here they go!" exclaimed the young inventor.

There was a moment of waiting, and then came a thundering roar. The Mars trembled, but she did not shift to either side from an even keel. From one barrel of the guns shot out the explosive projectiles, and from the other spurted a jet of water, sent out by a charge of powder, equal in weight to that which forced out the shot.

As the projectile was fired in one direction, and the water in one directly opposite, the two discharges neutralized one another.

Out flew the pointed steel shells, to fall harmlessly into the sea, where they exploded, sending up columns of water.

"Well!" cried Tom as the echoes died away. "How was it?"

"Couldn't have been better," declared Lieutenant Marbury. "There wasn't the least shock of recoil. Tom Swift, you have solved the problem, I do believe! Your aerial warship is a success!"

"I'm glad to hear you say so. There are one or two little things that need changing, but I really think I have about what the United States Government wants."

"I am, also, of that belief, Tom. If only--" The officer stopped suddenly.

"Well?" asked Tom suggestively.

"I was going to say if only those foreign spies don't make trouble."

"I think we've seen the last of them," Tom declared. "Now we'll go on with the tests."

More guns were fired, singly and in batteries, and in each case the Mars stood the test perfectly. The double barrel had solved the recoil problem.

For some little time longer they remained out over the sea, going through some evolutions to test the rudder control, and then as their present object had been accomplished Tom gave orders to head back to Shopton, which place was reached in due time.

"Well, Tom, how was it?" asked Mr. Swift, for though his son had said nothing to his friends about the prospective test, the aged inventor knew about it.

"Successful, Dad, in every particular."

"That's good. I didn't think you could do it. But you did. I tell you it isn't much that can get the best of a Swift!" exclaimed the aged man proudly. "Oh, by the way, Tom, here's a telegram that came while you were gone," and he handed his son the yellow envelope.

Tom ripped it open with a single gesture, and in a flash his eyes took in the words. He read:

"Look out for spies during trial flights."

The message was signed with a name Tom did not recognize.

"Any bad news?" asked Mr. Swift.

"No--oh, no," replied Tom, as he crumpled up the paper and thrust it into his pocket. "No bad news, Dad."

"Well, I'm glad to hear that," went on Mr. Swift. "I don't like telegrams."

When Tom showed the message to Lieutenant Marbury, that official, after one glance at the signature, said:

"Pierson, eh? Well, when he sends out a warning it generally means something."

"Who's Pierson?" asked Tom.

"Head of the Secret Service department that has charge of this airship matter. There must be something in the wind, Tom."

Extra precautions were taken about the shops. Strangers were not permitted to enter, and all future work on the Mars was kept secret. Nevertheless, Tom was worried. He did not want his work to be spoiled just when it was about to be a success. For that it was a success, Lieutenant Marbury assured him. The government man said he would have no hesitation in recommending the purchase of Tom's aerial warship.

"There's just one other test I want to see made," he said.

"What is that?" Tom inquired.

"In a storm. You know we can't always count on haying good weather, and I'd like to see how she behaves in a gale."

"You shall!" declared the young inventor.

For the next week, during which finishing touches were put on the big craft, Tom anxiously waited for signs of a storm. At last they came. Danger signals were put up all along the coast, and warnings were sent out broadcast by the Weather Bureau at Washington.

One dull gray morning Tom roused his friends early and announced that the Mars was going up.

"A big storm is headed this way," Tom said, "and we'll have a chance to see how she behaves in it."

And even as the flight began, the forerunning wind and rain came in a gust of fury. Into the midst of it shot the big aerial warship, with her powerful propellers beating the moisture-laden air.