CHAPTER XVI
TOM IS WORRIED
"Steady, all!" came in even tones from Tom Swift. Not for an instanthad he lost his composure. For it was an accident, that much wascertain, and one that might endanger the lives of all on board.
Above the noise of the machinery in the motor room could be heard thethrashing and banging of the broken or loose propeller-blade. Justwhat its condition was, could not be told, as a bulge of the gas baghid it from the view of those gathered about the gun, which was aboutto 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 afew of the gas compartments can be torn. There will soon enoughadditional gas in the others to lift us again."
And so it proved. The moment the pressure of the lifting gas in the bigoiled silk and aluminum container was lowered, it started thegenerating machine, and enough extra gas was pumped into the uninjuredcompartments 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. "Toobad 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 hadceased falling and remained stationary in the air. Tom had also sent asignal to the engine-room to shut off the power, so that the twoundamaged propellers, as well as the broken one, ceased revolving.
"In a trolley car, you see," Tom went on, when the excitement hadcalmed down, "as soon as the air pressure in the tanks gets below acertain point, caused by using the air for a number of applications ofthe brakes, it lets a magnetized bar fall, and this establishes anelectrical connection, starting the air pump. The pump forces more airinto the tanks until the pressure is enough to throw the pump switchout 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 isall 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-roomat once began the removal of the damaged blades.
As several spare ones were carried aboard one could be put on in placeof the broken one, had this been desired. But Tom thought the accidenta good chance to see how his craft would act with only two-thirds ofher motive force available, so he did not order the damaged propellerreplaced. 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 adefect in the wood, but I had it all carefully examined before I usedit."
The propeller was one of the "built-up" type, with alternate layers ofash and mahogany, but some powerful force had torn and twisted theblades. The wood was splintered and split, and some jagged pieces,flying off at a tangent, so great was the centrifugal force, had tornholes in the strong gas bag.
"Did something hit it; or did it hit something?" asked Ned as he sawTom carefully examining the broken blades.
"Hard to say. I'll have a good look at this when we get back. Just nowI 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 balloondamaged; 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 isgoing to happen when she's fired on by a hostile ship? Of course I'mgoing 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 asbuoyant as before, for more gas had been automatically made, and forcedinto the uninjured compartments of the bag. At the same time enoughsand ballast had been allowed to run out to make the weight to belifted 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 gasbag floating out behind. But this made a nearer approach to warconditions, and Tom was always glad to give his inventions the mostsevere tests possible.
So, after a little while, during which it was seen that the Mars wasproceeding almost normally, the matter of discharging the guns wastaken up again.
The weapons were all ready to fire, and when Tom had attached thepressure gauges to note how much energy was expended in the recoil, hegave the word to fire.
The two big weapons were discharged together, and for a moment afterthe report echoed out among the cloud masses every soul on the shipfeared 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 camefrom several of the members of the crew, who had had only a littleexperience in aircraft.
"What's the matter?" cried Ned. "Something go wrong?"
"A little," admitted Tom, with a rueful look on his face. "Thoserecoil 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 nexttime."
"Bless my watch chain!" cried Mr. Damon. "You aren't going to firethose guns again; are you, Tom?"
"Why not? We can't tell what's the matter, nor get things rightwithout 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 positionbut 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 allthe same to you. If my wife ever knew I was here I'd never hear thelast of it!"
"We'll go down soon," Tom promised. "But I must fire a couple of shotsmore. You wouldn't call the recoil checks a success, would you?" andthe 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 thisis not an official test, and I am not obliged to make a report of thistrial. 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 againwith the recoil checks tightened up. I think the hydrostatic valveswere open too much, also."
Preparations were now made for firing the four-inch guns once more. Allthis while the Mars had been speeding around in space, being about twomiles up in the air. Tom's craft was not designed to reach as great anelevation as would be possible in an aeroplane, since to work havoc toan enemy's fortifications by means of aerial bombs they do not need tobe dropped from a great height.
In fact, experiments in Germany have shown that bombs falling from agreat height are less effective than those falling from an airshipnearer the earth. For a bomb, falling from a height of two miles,acquires enough momentum to penetrate far into the earth, so that muchof the resultant explosive force is expended in a downward direction,and little damage is done to the fortifications. A bomb dropped from alower altitude, expending its force on all sides, does much more damage.
On the other hand, in destroying buildings, it has been found desirableto drop a bomb from a good height so that it may penetrate even aprotected roof, and explode inside.
Once more Tom made ready to fire, this time having given the recoilchecks greater resistance. But though there was less motion imparted tothe airship when the guns were discharged, there was still too much forcomfort, or even
safety.
"Well, something's wrong, that's sure," remarked Tom, in ratherdisappointed tones as he noted the effect of the second shots. "If weget as much recoil from the two guns, what would happen if we firedthem all at once?"
"Don't do it! Don't do it, I beg of you!" entreated Mr. Damon. "Blessmy toothbrush--don't do it!"
"I won't--just at present," Tom said, ruefully. "I'm afraid I'll haveto begin all over again, and proceed along new lines."
"Well, perhaps you will," said the lieutenant. "But you may inventsomething much better than anything you have now. There is no greatrush. 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 adoor-spring check isn't going to work."
"I might have known my idea wouldn't amount to anything," said theyoung 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 firstexperiments seemed to warrant us in believing that we had solved theproblem."
"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 fewblank ones, and see how accurate the range finders are."
While his men were getting ready for this test Tom bent over the brokenpropeller, looking from that to the recoil checks, which had not comeup to expectations. Then he shook his head in a worried and puzzledmanner.