Some of Tom's most trusted workmen formed the operating crew, the young inventor taking charge of the pilot-house himself.

"Well she seems to run all right," observed Lieutenant Marbury, as the big craft surged ahead just below a stratum of white, fleecy clouds.

"Yes, but not as fast as I'd like to see her go," Tom replied. "Of course the machinery is new, and it will take some little time for it to wear down smooth. I'll speed her up a little now."

They had been running for perhaps ten minutes when Tom shoved over the hand of an indicator that communicated with the engine-room from the pilot-house. At once the Mars increased her speed.

"She can do it!" cried Ned.

"Bless my-hat! I should say so!" cried Mr. Damon, for he was standing outside the pilot-house just then, on the "bridge," and the sudden increase of speed lifted his hat from his head.

"There you are--caught on the fly!" cried Ned, as he put up his hand just in time to catch the article in question.

"Thanks! Guess I'd better tie it fast," remarked the odd man, putting his hat on tightly.

The aerial warship was put through several evolutions to test her stability, and to each one she responded well, earning the praise of the government officer. Up and down, to one side and the other, around in big circles, and even reversing, Tom sent his craft with a true hand and eye. In a speed test fifty-five miles was registered against a slight wind, and the young inventor said he knew he could do better than that as soon as some of the machinery was running more smoothly.

"And now suppose we get ready for the gun tests," suggested Tom, when they had been running for about an hour.

"That's what I'm mostly interested in," said Lieutenant Marbury. "It's easy enough to get several good types of dirigible balloons, but few of them will stand having a gun fired from them, to say nothing of several guns."

"Well, I'm not making any rash promises," Tom went on, "but I think we can turn the trick."

The armament of the Mars was located around the center cabin. There were two large guns, fore and aft, throwing a four-inch projectile, and two smaller calibered quick-firers on either beam. The guns were mounted on pedestals that enabled the weapons to fire in almost any direction, save straight up, and of course the balloon bag being above them prevented this. However, there was an arrangement whereby a small automatic quick-firer could be sent up to a platform built on top of the gas envelope itself, and a man stationed there could shoot at a rival airship directly overhead.

But the main deck guns could be elevated to an angle of nearly forty-five degrees, so they could take care of nearly any hostile aircraft that approached.

"But where are the bombs I heard you speaking of?" asked Ned, as they finished looking at the guns.

"Here they are," spoke Tom, as he pointed to a s.p.a.ce in the middle of the main cabin floor. He lifted a bra.s.s plate, and disclosed three holes, covered with a strong wire netting that could be removed. "The bombs will be dropped through those holes," explained the young inventor, "being released by a magnetic control when the operator thinks he has reached a spot over the enemy's city or fortification where the most damage will be done. I'll show you how they work a little later. Now we'll have a test of some of the guns."

Tom called for some of his men to take charge of the steering and running of the Mars while he and Lieutenant Marbury prepared to fire the two larger weapons. This was to be one of the most important tests.

Service charges had been put in, though, of course, no projectiles would be used, since they were then flying over a large city not far from Shopton.

"We'll have to wait until we get out over the ocean to give a complete test, with a bursting sh.e.l.l," Tom said.

He and Lieutenant Marbury were beside a gun, and were about to fire it, when suddenly, from the stern of the ship, came a ripping, tearing sound, and, at the same time, confused shouts came from the crew's quarters.

"What is it?" cried Tom.

"One of the propellers!" was the answer. "It's split, and has torn a big hole in the gas bag!"

"Bless my overshoes!" cried Mr. Damon. "We're going down!"

All on board the Mars became aware of a sudden sinking sensation.

CHAPTER XVI

TOM IS WORRIED

"Steady, all!" came in even tones from Tom Swift. Not for an instant had he lost his composure. For it was an accident, that much was certain, and one that might endanger the lives of all on board.

Above the noise of the machinery in the motor room could be heard the thrashing and banging of the broken or loose propeller-blade. Just what its condition was, could not be told, as a bulge of the gas bag hid it from the view of those gathered about the gun, which was about to be fired when the alarm was given.

"We're sinking!" cried Mr. Damon. "We're going down, Tom!"

"That's nothing," was the cool answer. "It is only for a moment. Only a few of the gas compartments can be torn. There will soon enough additional gas in the others to lift us again."

And so it proved. The moment the pressure of the lifting gas in the big oiled silk and aluminum container was lowered, it started the generating machine, and enough extra gas was pumped into the uninjured compartments to compensate for the loss.

"We're not falling so fast now," observed Ned.

"No, and we'll soon stop falling altogether," calmly declared Tom. "Too bad this accident had to happen, though."

"It might have been much worse, my boy!" exclaimed the lieutenant.

"That's a great arrangement of yours--the automatic gas machine."

"It's on the same principle as the air brakes of a trolley car,"

explained Tom, when a look at the indicators showed that the Mars had ceased falling and remained stationary in the air. Tom had also sent a signal to the engine-room to shut off the power, so that the two undamaged propellers, as well as the broken one, ceased revolving.

"In a trolley car, you see," Tom went on, when the excitement had calmed down, "as soon as the air pressure in the tanks gets below a certain point, caused by using the air for a number of applications of the brakes, it lets a magnetized bar fall, and this establishes an electrical connection, starting the air pump. The pump forces more air into the tanks until the pressure is enough to throw the pump switch out of connection, when the pump stops. I use the same thing here."

"And very clever it is," said Mr. Damon. "Do you suppose the danger is all over, Tom?"

"For the time being, yes. But we must unship that damaged propeller, and go on with the two."

The necessary orders were given, and several men from the engine-room at once began the removal of the damaged blades.

As several spare ones were carried aboard one could be put on in place of the broken one, had this been desired. But Tom thought the accident a good chance to see how his craft would act with only two-thirds of her motive force available, so he did not order the damaged propeller replaced. When it was lowered to the deck it was carefully examined.

"What made it break?" Ned wanted to know.

"That's a question I can't answer," Tom replied. "There may have been a defect in the wood, but I had it all carefully examined before I used it."

The propeller was one of the "built-up" type, with alternate layers of ash and mahogany, but some powerful force had torn and twisted the blades. The wood was splintered and split, and some jagged pieces, flying off at a tangent, so great was the centrifugal force, had torn holes in the strong gas bag.

"Did something hit it; or did it hit something?" asked Ned as he saw Tom carefully examining the broken blades.

"Hard to say. I'll have a good look at this when we get back. Just now I want to finish that gun test we didn't get a chance to start."

"You don't mean to say you're going to keep on, and with the balloon damaged; are you?" cried Mr. Damon, in surprise.

"Certainly--why not?" Tom replied. "In warfare accidents may happen, and if the Mars can't go on, after a little damage like this, what is going to happen when she's fired on by a hostile ship? Of course I'm going on!"