"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!"
"Bless my necktie!" ejaculated the odd man.
"That's the way to talk!" exclaimed Lieutenant Marbury. "I'm with you."
There really was very little danger in proceeding. The Mars was just as buoyant as before, for more gas had been automatically made, and forced into the uninjured compartments of the bag. At the same time enough sand ballast had been allowed to run out to make the weight to be lifted less in proportion to the power remaining.
True, the speed would be less, with two propellers instead of three, and the craft would not steer as well, with the torn ends of the gas bag floating out behind. But this made a nearer approach to war conditions, and Tom was always glad to give his inventions the most severe tests possible.
So, after a little while, during which it was seen that the Mars was proceeding almost normally, the matter of discharging the guns was taken up again.
The weapons were all ready to fire, and when Tom had attached the pressure gauges to note how much energy was expended in the recoil, he gave the word to fire.
The two big weapons were discharged together, and for a moment after the report echoed out among the cloud masses every soul on the ship feared another accident had happened.
For the big craft rolled and twisted, and seemed about to turn turtle. Her forward progress was halted, momentarily, and a cry of fear came from several of the members of the crew, who had had only a little experience in aircraft.
"What's the matter?" cried Ned. "Something go wrong?"
"A little," admitted Tom, with a rueful look on his face. "Those recoil checks didn't work as well in practice as they did in theory."
"Are you sure they are strong enough?" asked Lieutenant Marbury.
"I thought so," spoke Tom. "I'll put more tension on the spring next time."
"Bless my watch chain!" cried Mr. Damon. "You aren't going to fire those guns again; are you, Tom?"
"Why not? We can't tell what's the matter, nor get things right without experimenting. There's no danger."
"No danger! Don't you call nearly upsetting the ship danger?"
"Oh, well, if she turns over she'll right herself again," Tom said. "The center of gravity is low, you see. She can't float in any position but right side up, though she may turn over once or twice."
"Excuse me!" said Mr. Damon firmly. "I'd rather go down, if it's all the same to you. If my wife ever knew I was here I'd never hear the last of it!"
"We'll go down soon," Tom promised. "But I must fire a couple of shots more. You wouldn't call the recoil checks a success, would you?" and the young inventor appealed to the government inspector.
"No, I certainly would not," was the prompt answer. "I am sorry, too, for they seemed to be just what was needed. Of course I understand this is not an official test, and I am not obliged to make a report of this trial. But had it been, I should have had to score against you.
"I realize that, and I'm not asking any favors, but I'll try it again with the recoil checks tightened up. I think the hydrostatic valves were open too much, also."
Preparations were now made for firing the four-inch guns once more. All this while the Mars had been speeding around in space, being about two miles up in the air. Tom's craft was not designed to reach as great an elevation as would be possible in an aeroplane, since to work havoc to an enemy's fortifications by means of aerial bombs they do not need to be dropped from a great height.
In fact, experiments in Germany have shown that bombs falling from a great height are less effective than those falling from an airship nearer the earth. For a bomb, falling from a height of two miles, acquires enough momentum to penetrate far into the earth, so that much of the resultant explosive force is expended in a downward direction, and little damage is done to the fortifications. A bomb dropped from a lower altitude, expending its force on all sides, does much more damage.
On the other hand, in destroying buildings, it has been found desirable to drop a bomb from a good height so that it may penetrate even a protected roof, and explode inside.
Once more Tom made ready to fire, this time having given the recoil checks greater resistance. But though there was less motion imparted to the airship when the guns were discharged, there was still too much for comfort, or even safety.
"Well, something's wrong, that's sure," remarked Tom, in rather disappointed tones as he noted the effect of the second shots. "If we get as much recoil from the two guns, what would happen if we fired them all at once?"
"Don't do it! Don't do it, I beg of you!" entreated Mr. Damon. "Bless my toothbrush—don't do it!"
"I won't—just at present," Tom said, ruefully. "I'm afraid I'll have to begin all over again, and proceed along new lines."
"Well, perhaps you will," said the lieutenant. "But you may invent something much better than anything you have now. There is no great rush. Take your time, and do something good."
"Oh, I'll get busy on it right away," Tom declared. "We'll go down now, and start right to work. I'm afraid, Ned, that our idea of a door-spring check isn't going to work."
"I might have known my idea wouldn't amount to anything," said the young bank clerk.
"Oh, the idea is all right," declared Tom, "but it wants modifying. There is more power to those recoils than I figured, though our first experiments seemed to warrant us in believing that we had solved the problem."
"Are you going to try the bomb-dropping device?" asked the lieutenant.
"Yes, there can't be any recoil from that," Tom said. "I'll drop a few blank ones, and see how accurate the range finders are."
While his men were getting ready for this test Tom bent over the broken propeller, looking from that to the recoil checks, which had not come up to expectations. Then he shook his head in a worried and puzzled manner.
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