“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.