"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 b.u.t.ton.

"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 sh.e.l.ls, 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 a.s.sured 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 having 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.

CHAPTER XIX

QUEER HAPPENINGS

"Say, Tom, are you sure you're all right?"

"Of course I am! What do you mean?"