Tom Swift was something like a fireman. He had lived so long in an atmosphere of constant alarms and danger, that he was always ready for almost any emergency. His room was equipped with the end in view that he could act promptly and effectively.
So, when he heard Eradicate's alarm, though he wondered what the old colored man was doing out of bed at that hour, Tom did not stop to reason out that puzzle. He acted quickly.
His first care was to throw on the main switch, connected with a big storage battery, and to which were attached the wires of the lighting system. This at once illuminated every shop in the plant, and also the grounds themselves. Tom wanted to see what was going on. The use of a storage battery eliminated the running of the dynamo all night.
And once he had done this, Tom began pulling on some clothes and a pair of shoes. At the same time he reached out with one hand and pressed a button that sounded an alarm in the sleeping quarters of Koku, the giant, and in the rooms of some of the older and most trusted men.
All this while Eradicate was shouting away, down in the yard.
"Massa Tom! Massa Tom!" he called. "Hurry! Hurry! Dey is killin' Koku!"
"Killing Koku!" exclaimed Tom, as he finished his hasty dressing. "Then my giant must already be in the fracas. I wonder what it's all about, anyhow."
"What's up, Tom?" came Ned's voice from the adjoining room. "I thought I heard a noise."
"Your thoughts do you credit, Ned!" Tom answered. "If you listen right close, you'll hear several noises."
"By Jove! You're right, old man!"
Tom could hear his chum bound out of bed to the floor, and, at the same time, from the big shed where Tom was building his aerial warship came a series of yells and shouts.
"That's Koku's voice!" Tom exclaimed, as he recognized the tones of the giant.
"I'm coming, Tom!" Ned informed his chum. "Wait a minute."
"No time to wait," Tom replied, buttoning his coat as he sped down the hall.
"Oh, Tom, what is it?" asked Mrs. Baggert, the housekeeper, looking from her room.
"I don't know. But don't let dad get excited, no matter what happens. Just put him off until I come back. I think it isn't anything serious."
Mr. Damon, who roomed next to Ned, came out of his own apartment partially dressed.
"Bless my suspenders!" he cried to Tom, those articles just then dangling over his hips. "What is it? What has happened? Bless my steam gauge, don't tell me it's a fire!"
"I think it isn't that," Tom answered. "No alarm has rung. Koku seems to be in trouble."
"Well, he's big enough to look after himself, that's one consolation," chuckled Mr. Damon. "I'll be right with you."
By this time Ned had run out into the hall, and, together, he and Tom sped down the corridor. They could not hear the shouts of Eradicate so plainly now, as he was on the other side of the house.
But when the two young men reached the front porch, they could hear the yells given with redoubled vigor. And, in the glare of the electric lights, Tom saw Eradicate leading along Boomerang, the old mule.
"What is it, Rad? What is it?" demanded the young inventor breathlessly.
"Trouble, Massa Tom! Dat's what it am! Trouble!"
"I know that—but what kind?"
"De worstest kind, I 'spects, Massa Tom. Listen to it!"
From the interior of the big shed, not far from the house, Tom and Ned heard a confused jumble of shouts, cries and pleadings, mingled with the rattle of pieces of metal, and the banging of bits of wood. And, above all that, like the bellowing of a bull, was noted the rumbling voice of Koku, the giant.
"Come on, Ned!" Tom cried.
"It's suah trouble, all right," went on Eradicate. "Mah mule, Boomerang, had a touch ob de colic, an' I got up t' gib him some hot drops an' walk him around, when I heard de mostest terrific racket-sound, and den I 'spected trouble was comm."
"It isn't coming—it's here!" called Tom, as he sped toward the big shop. Ned was but a step behind him. The big workshop where the aerial warship was being built was, like the other buildings, brilliantly illuminated by the lights Tom had switched on. The young inventor also saw several of his employees speeding toward the same point.
Tom was the first to reach the small door of the shed. This was built in one of the two large main doors, which could be swung open when it was desired to slide the Mars in from the ground, and not admit it through the roof.
"Look!" cried Tom, pointing.
Ned looked over his chum's shoulder and saw the giant, Koku, struggling with four men—powerful men they were, too, and they seemed bent on mischief.
For they came at Koku from four sides, seeking to hold his hands and feet so that he could not fight them back. On the floor near where the struggle was taking place was a coil of rope, and it was evident that it had been the intention of the men to overcome Koku and truss him up, so that he would not interfere with what they intended to do. But Koku was a match for even the four men, powerful as they were.
"We're here, Koku!" cried Tom. "Watch for an opening, Ned!" he called to his chum.
The sound of Tom's voice disconcerted at least two of the attackers, for they looked around quickly, and this was fatal to their chances.
Though such a big man, Koku was exceptionally quick, and no sooner did he see his advantage, as two of the men turned their gaze away from him, than he seized it.
Suddenly tearing loose his hands from the grip of the two men who had looked around, Koku shot out his right and left fists, and secured good hold on the necks of two of his enemies. The other two, at his back, were endeavoring to pull him over, but the giant's sturdy legs still held.
So big was Koku's hands that they almost encircled the necks of his antagonists. Then happened a curious thing.
With a shout that might have done credit to some ancient cave-dweller of the stone age, Koku spread out his mighty arms, and held apart the two men he had grasped. In vain they struggled to free themselves from that terrible grip. Their faces turned purple, and their eyes bulged out.
"He's choking them to death!" shouted Ned.
But Koku was not needlessly cruel.
A moment later, with a quick and sudden motion he bent his arms, bringing toward each other the two men he held as captives. Their heads came together with a dull thud, and a second later Koku allowed two limp bodies to slip from his grip to the floor.
"He's done for them!" Tom cried. "Knocked them unconscious. Good for you, Koku!"
The giant grunted, and then, with a quick motion, slung himself around, hoping to bring the enemies at his back within reach of his powerful arms. But there was no need of this.
As soon as the other two ruffians had seen their companions fall to the floor of the shop they turned and fled, leaping from an open window.
"There they go!" cried Ned.
"Some of the other men can chase them," said the young inventor. "We'll tie up the two Koku has captured."
As he approached nearer to the unconscious captives Tom uttered a cry of surprise, for he recognized them as two of the new men he had employed.
"What can this mean?" he asked wonderingly.
He glanced toward the window through which the two men had jumped to escape, and he was just in time to see one of them run past the open door. The face of this one was under a powerful electric light, and Tom at once recognized the man as Feldman, the worker who had had so much trouble with the trip-hammer.
"This sure is a puzzle," marveled Tom. "My own men in the plot! But why did they attack Koku?"
The giant, bending over the men he had knocked unconscious by beating their heads together, seemed little worse for the attack.
"We tie 'em up," he said grimly, as he brought over the rope that had been intended for himself.
Little time was lost in securing the two men who had been so effectively rendered helpless by Koku's ready, if rough, measures. One of them was showing signs of returning consciousness now, and Tom, not willing to inflict needless pain, even on an enemy, told one of his men, summoned by the alarm, to bring water. Soon the two men opened their eyes, and looked about them in dazed fashion.
"Did—did anything hit me?" asked one meekly.
"It must have been a thunderbolt," spoke the other dreamily. "But it didn't look like a storm."
"Oh, dere was a storm, all right," chuckled Eradicate, who, having left his mule, Boomerang outside, came into the shed. "It was a giant storm all right."
The men put their hands to their heads, and seemed to comprehend. They looked at the rope that bound their feet. Their forearms had been loosened to allow them to take a drink of water.
"What does this mean—Ransom—Kurdy?" asked Tom sternly, when the men seemed able to talk. "Did you attack Koku?"
"It looks as though he had the best of us, whether we did or not," said the man Tom knew as Kurdy. "Whew, how my head aches!"
"Me sorry," said Koku simply.
"Not half as sorry as we are," returned Ransom ruefully.
"What does it mean?" asked Tom sternly. "There were four of you. Feldman and one other got away."
"Oh, trust Feldman for getting away," sneered Kurdy. "He always leaves his friends in the lurch."
"Was this a conspiracy?" demanded Tom.
The two captives looked at one another, sitting bound on the floor of the shop, their backs against some boxes.
"I guess it's all up, and we might as well make a clean breast of it," admitted Kurdy.
"Perhaps it would be better," said Tom quietly. "Eradicate," he went on, to the colored man, "go to the house and tell Mrs. Baggert that everything is all right and no one hurt."
"No one hurt, Massa Tom? What about dem dere fellers?" and the colored man pointed to the captives.
"Well, they're not hurt much," and Tom permitted himself a little smile. "I don't want my father to worry. Tell him everything is all right."
"All right, Massa Tom. I'se gwine right off. I'se got t' look after mah mule, Boomerang, too. I'se gwine," and he shuffled away.
"Who else besides Feldman got away?" asked Tom, looking alternately at the prisoners.
They hesitated a moment about answering.
"We might as well give up, I tell you," spoke Kurdy to Ransom.
"All right, go ahead, we'll have to take our medicine. I might have known it would turn out this way—going in for this sort of thing. It's the first bit of crooked business I ever tried," the man said earnestly, "and it will be the last—believe me!"
"Who was the fourth man?" Tom repeated.
"Harrison," answered Kurdy, naming one of the most efficient of the new machinists Tom had hired during the rush.
"Harrison, who has been working on the motor?" cried the young inventor.
"Yes," said Ransom.
"I'm sorry to learn that," Tom went on in a low voice. "He was an expert in his line. But what was your object, anyhow, in attacking Koku?"
"We didn't intend to attack him," explained Ransom, "but he came in when we were at work, and as he went for us we tried to stand him off. Then your colored man heard the racket, and—well, I guess you know the rest."
"But I don't understand why you came into this shed at night," went on Tom. "No one is allowed in here. You had no right, and Koku knew that. What did you want?"
"Look here!" exclaimed Kurdy, "I said we'd make a clean breast of it, and we will. We're only a couple of tools, and we were foolish ever to go in with those fellows; or rather, in with that Frenchman, who promised us big money if we succeeded."
"Succeeded in what?" demanded the young inventor.
"In damaging your new aerial warship, or in getting certain parts of it so he could take them away with him."
Tom gave a surprised whistle.
"A frenchman!" he exclaimed. "Is he one of the—?"
"Yes, he's one of the foreign spies," interrupted Ransom. "You'd find it out, anyhow, if we didn't tell you. They are after you, Tom Swift, and after your machines. They had vowed to get them by fair means or foul, for some of the European governments are desperate."
"But we were only tools in their hands. So were Feldman and Harrison, but they knew more about the details. We were only helping them."
"Then we must try to capture them," decided Tom. "Ned, see if the chase had any results. I'll look after these chaps—Koku and I."
"Oh, we give in," admitted Kurdy. "We know when we've had enough," and he rubbed his head gently where the giant had banged it against that of his fellow-conspirator.
"Do you mean that you four came into this shop, at midnight, to damage the Mars?" asked Tom.
"That's about it, Mr. Swift," replied Kurdy rather shamefacedly. "We were to damage it beyond repair, set fire to the whole place, if need be, and, at the same time, take away certain vital parts.
"Harrison, Feldman, Ransom and I came in, thinking the coast was clear. But Koku must have seen us enter, or he suspected we were here, for he came in after us, and the fight began. We couldn't stop him, and he did for us. I'm rather glad of it, too, for I never liked the work. It was only that they tempted me with a promise of big money."
"Who tempted you?" demanded Tom.
"That Frenchman—La Foy, he calls himself, and some other foreigners in your shops."
"Are there foreigners here?" cried Tom.
"Bless my chest protector!" cried Mr. Damon, who had come in and had been a silent listener to this. "Can it be possible?"
"That's the case," went on Kurdy. "A lot of the new men you took on are foreign spies from different European nations. They are trying to learn all they can about your plans, Mr. Swift!"
"Are they friendly among themselves?" asked Tom.
"No; each one is trying to get ahead of the other. So far the Frenchman seems to have had the best of it. But to-night his plan failed."
"Tell me more about it," urged Tom.
"That's about all we know," spoke Ransom. "We were only hired to do the rough work. Those higher up didn't appear. Feldman was only a step above us."
"Then my suspicions of him were justified," thought Tom. "He evidently met La Foy in the woods to make plans. But Koku and Eradicate spoiled them."
The two captives seemed willing enough to make a confession, but they did not know much. As they said, they were merely tools, acting for others. And events had happened just as they had said.
The four conspirators had managed, by means of a false key, and by disconnecting the burglar alarm, to enter the airship shed. They were about to proceed with their work of destruction when Koku came on the scene.
The giant's appearance was due to accident. He acted as a sort of night watchman, making a tour of the buildings, but he entered the shed where the Mars was because, that day, he had left his knife in there, and wanted to get it. Only for that he would not have gone in. When he entered he surprised the four men.
Of course he attacked them at once, and they sprang at him. Then ensued a terrific fight. Eradicate, arising to doctor his mule, as he had said, heard the noise, and saw what was going on. He gave the alarm.
"Well, Ned, any luck?" asked Tom, as his chum came in.
"No, they got away, Tom. I had a lot of your men out helping me search the grounds, but it wasn't of much use."
"Particularly if you depended on some of my men," said Tom bitterly.
"What do you mean?"
"I mean that the place is filled with spies, Ned! But we will sift them out in the morning. This has been a lucky night for me. It was touch and go. Now, then, Koku, take these fellows and lock them up somewhere until morning. Ned, you and I will remain on guard here the rest of the night."
"I'm with you, Tom."
"Will you be a bit easy on us, considering what we told you?" asked Kurdy.
"I'll do the best I can," said Tom, gently, making no promises.
The two captives were put in secure quarters, and the rest of the night passed quietly. During the fight in the airship shed some machinery and tools had been broken, but no great amount of damage was done. Tom and Ned passed the remaining hours of darkness there.
A further search was made in the morning for the two conspirators who had escaped, but no trace of them was found. Tom then realized why Feldman was so anxious to be placed in the aeroplane department—it was in order that he might have easier access to the Mars.
A technical charge was made against the two prisoners, sufficient to hold them for some time. Then Tom devoted a day to weeding out the suspected foreigners in his place. All the new men were discharged, though some protested against this action.
"Probably I am hitting some of the innocent in punishing those who, if they had the chance, would become guilty," Tom said to his chum, "but it cannot be helped—I can't afford to take any chances."
The Mars was being put in shape for her first flight. The guns, fitted with the recoil shock absorbers, were mounted, and Lieutenant Marbury had returned to go aloft in the big aerial warship. He congratulated Tom on discovering at least one plot in time.
"But there may be more," he warned the young inventor. "You are not done with them yet."
The Mars was floated out of her hangar, and made ready for an ascent. Tom, Ned, Lieutenant Marbury, Mr. Damon, and several workmen were to be the first passengers. Tom was busy going over the various parts to see that nothing had been forgotten.
"Well, I guess we're ready," he finally announced. "All aboard!"
"Bless my insurance policy!" exclaimed Mr. Damon. "Now that the time comes I almost wish I wasn't going."
"Nonsense!" exclaimed Tom. "You're not going to back out at the last minute. All aboard! Cast off the ropes!" he cried to the assistants.
A moment later the Mars, the biggest airship Tom Swift had ever constructed, arose from the earth like some great bird, and soared aloft.
"Well, Tom, we're moving!" cried Ned Newton, clapping his chum on the back, as he stood near him in the pilot-house. "We're going up, old sport!"
"Of course we are," replied Tom. "You didn't think it wouldn't go up, did you?"
"Well, I wasn't quite sure," Ned confessed. "You know you were so worried about—"
"Not about the ship sailing," interrupted Tom. "It was only the effect the firing of the guns might have. But I think we have that taken care of."
"Bless my pin cushion!" cried Mr. Damon, as he looked over the rail at the earth below. "We're moving fast, Tom."
"Yes, we can make a quicker ascent in this than in most aeroplanes," Tom said, "for they have to go up in a slanting direction. But we can't quite equal their lateral speed."
"Just how fast do you think you can travel when you are in first-class shape?" asked Lieutenant Marbury, as he noted how the Mars was behaving on this, the first trip.
"Well, I set a limit of seventy-five miles an hour," the young inventor replied, as he shifted various levers and handles, to change the speed of the mechanism. "But I'm afraid we won't quite equal that with all our guns on board. But I'm safe in saying sixty, I think."
"That will more than satisfy the government requirements," the officer said. "But, of course, your craft will have to come up to expectations and requirements in the matter of armament."
"I'll give you every test you want," declared Tom, with a smile. "And now we'll see what the Mars can do when put to it."
Up and up went the big dirigible aerial warship. Had you been fortunate enough to have seen her you would have observed a craft not unlike, in shape, the German Zeppelins. But it differed from those war balloons in several important particulars.
Tom's craft was about six hundred feet long, and the diameter of the gas bag, amidships, was sixty feet, slightly larger than the largest Zeppelin. Below the bag, which, as I have explained, was made up of a number of gas-tight compartments, hung from wire cables three cabins. The forward one was a sort of pilot-house, containing various instruments for navigating the ship of the air, observation rooms, gauges for calculating firing ranges, and the steering apparatus.
Amidships, suspended below the great bag, were the living and sleeping quarters, where food was cooked and served and where those who operated the craft could spend their leisure time. Extra supplies were also stored there.
At the stern of the big bag was the motor-room, where gas was generated to fill the balloon compartments when necessary, where the gasoline and electrical apparatus were installed, and where the real motive power of the craft was located. Here, also, was carried the large quantity of gasoline and oil needed for a long voyage. The Mars could carry sufficient fuel to last for over a week, provided no accidents occurred.
There was also an arrangement in the motor compartment, so that the ship could be steered and operated from there. This was in case the forward pilot-house should be shot away by an enemy. And, also, in the motor compartment were the sleeping quarters for the crew.
All three suspended cabins were connected by a long covered runway, so that one could pass from the pilot-house to the motor-room and back again through the amidship cabin.
At the extreme end of the big bag were the various rudders and planes, designed to keep the craft on a level keel, automatically, and to enable it to make headway against a strong wind. The motive power consisted of three double-bladed wooden propellers, which could be operated together or independently. A powerful gasoline engine was the chief motive power, though there was an auxiliary storage battery, which would operate an electrical motor and send the ship along for more than twenty-four hours in case of accident to the gasoline engine.
There were many other pieces of apparatus aboard, some not completely installed, the uses of which I shall mention from time to time, as the story progresses. The gas-generating machine was of importance, for there would be a leakage and shrinking of the vapor from the big bag, and some means must be provided for replenishing it.
"You don't seem to have forgotten anything, Tom," said Ned admiringly, as they soared upward.
"We can tell better after we've flown about a bit," observed the young inventor, with a smile. "I expect we shall have to make quite a number of changes."
"Are you going far?" asked Mr. Damon.
"Why, you're not frightened, are you?" inquired Tom. "You have been up in airships with me before."
"Oh, no, I'm not frightened!" exclaimed the odd man. "Bless my suspenders, no! But I promised my wife I'd be back this evening, and..."
"We'll sail over toward Waterford," broke in Tom, "and I'll drop you down in your front yard."
"No, don't do that! Don't! I beg of you!" cried Mr. Damon. "You see—er—Tom, my wife doesn't like me to make these trips. Of course, I understand there is no danger, and I like them. But it's just as well not to make her worry-you understand!"
"Oh, all right," replied Tom, with a laugh. "Well, we're not going far on this trip. What I want to do, most of all, is to test the guns, and see if the recoil check will work as well when we are aloft as it did down on the ground. You know a balloon isn't a very stable base for a gun, even one of light caliber."
"No, it certainly is not," agreed Lieutenant Marbury, "and I am interested in seeing how you will overcome the recoil."
"We'll have a test soon," announced Tom.
Meanwhile the Mars, having reached a considerable height, being up so far, in fact, that the village of Shopton could scarcely be distinguished, Tom set the signal that told the engine-room force to start the propellers. This would send them ahead.
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 space in the middle of the main cabin floor. He lifted a brass 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 shell," 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.
"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.
Dropping bombs from an aeroplane, or a dirigible balloon, is a comparatively simple matter. Of course there are complications that may ensue, from the danger of carrying high explosives in the limited quarters of an airship, with its inflammable gasoline fuel, and ever-present electric spark, to the possible premature explosion of the bomb itself. But they seem to be considered minor details now.
On the other hand, while it is comparatively easy to drop a bomb from a moving aeroplane, or dirigible balloon, it is another matter to make the bomb fall just where it will do the most damage to the enemy. It is not easy to gauge distances, high up in the air, and then, too, allowance must be made for the speed of the aircraft, the ever-increasing velocity of a falling body, and the deflection caused by air currents.
The law of velocity governing falling bodies is well known. It varies, of course, according to the height, but in general a body falling freely toward the earth, as all high-school boys know, is accelerated at the rate of thirty-two feet per second. This law has been taken advantage of by the French in the present European war. The French drop from balloons, or aeroplanes, a steel dart about the size of a lead pencil, and sharpened in about the same manner. Dropping from a height of a mile or so, that dart will acquire enough velocity to penetrate a man from his head all the way through his body to his feet.
But in dropping bombs from an airship the damage intended does not so much depend on velocity. It is necessary to know how fast the bomb falls in order to know when to set the time fuse that will explode it; though some bombs will explode on concussion.
At aeroplane meets there are often bomb-dropping contests, and balls filled with a white powder (that will make a dust-cloud on falling, and so show where they strike) are used to demonstrate the birdman's accuracy.
"We'll see how our bomb-release works," Tom went on. "But we'll have to descend a bit in order to watch the effect."
"You're not going to use real bombs, are you, Tom?" asked Ned.
"Indeed not. Just chalk-dust ones for practice. Now here is where the bombs will be placed," and he pointed to the three openings in the floor of the amidship cabin. The wire nettings were taken out and one could look down through the holes to the earth below, the ground being nearer now, as Tom had let out some of the lifting gas.
"Here is the range-finder and the speed calculator," the young inventor went on as he indicated the various instruments. "The operator sits here, where he can tell when is the most favorable moment for releasing the bomb."
Tom took his place before a complicated set of instruments, and began manipulating them. One of his assistants, under the direction of Lieutenant Marbury, placed in the three openings bombs, made of light cardboard, just the size of a regular bomb, but filled with a white powder that would, on breaking, make a dust-cloud which could be observed from the airship.
"I have first to determine where I want to drop the bomb," Tom explained, "and then I have to get my distance from it on the range-finder. Next I have to know how fast I am traveling, and how far up in the air I am, to tell what the velocity of the falling bomb will attain at a certain time. This I can do by means of these instruments, some of which I have adapted from those used by the government," he said, with a nod to the officer.
"That's right—take all the information you can get," was the smiling response.
"We will now assume that the bombs are in place in the holes in the floor of the cabin," Tom went on. "As I sit here I have before me three buttons. They control the magnets that hold the bombs in place. If I press one of the buttons it breaks the electrical current, the magnet no longer has any attraction, and it releases the explosive. Now look down. I am going to try and drop a chalk bomb near that stone fence."
The Mars was then flying over a large field and a stone fence was in plain view.
"Here she goes!" cried Tom, as he made some rapid calculations from his gauge instruments. There was a little click and the chalk bomb dropped. There was a plate glass floor in part of the cabin, and through this the progress of the pasteboard bomb could be observed.
"She'll never go anywhere near the fence!" declared Ned. "You let it drop too soon, Tom!"
"Did I? You just watch. I had to allow for the momentum that would be given the bomb by the forward motion of the balloon."
Hardly had Tom spoken than a puff of white was seen on the very top of the fence.
"There it goes?" cried the lieutenant. "You did the trick, Swift!"
"Yes, I thought I would. Well, that shows my gauges are correct, anyhow. Now we'll try the other two bombs."
In succession they were released from the bottom of the cabin, at other designated objects. The second one was near a tree. It struck within five feet, which was considered good.
"And I'll let the last one down near that scarecrow in the field," said Tom, pointing to a ragged figure in the middle of a patch of corn.
Down went the cardboard bomb, and so good was the aim of the young inventor that the white dust arose in a cloud directly back of the scarecrow.
And then a queer thing happened. For the figure seemed to come to life, and Ned, who was watching through a telescope, saw a very much excited farmer looking up with an expression of the greatest wonder on his face. He saw the balloon over his head, and shook his fist at it, evidently thinking he had had a narrow escape. But the pasteboard bomb was so light that, had it hit him, he would not have been injured, though he might have been well dusted.
"Why, that was a man! Bless my pocketbook!" cried Mr. Damon.
"I guess it was," agreed Tom. "I took it for a scarecrow."
"Well, it proved the accuracy of your aim, at any rate," observed Lieutenant Marbury. "The bomb dropping device of your aerial warship is perfect—I can testify to that."
"And I'll have the guns fixed soon, so there will be no danger of a recoil, too," added Tom Swift, with a determined look on his face.
"What's next?" asked Mr. Damon, looking at his watch. "I really ought to be home, Tom."
"We're going back now, and down. Are you sure you don't want me to drop you in your own front yard, or even on your roof? I think I could manage that."
"Bless my stovepipe, no, Tom! My wife would have hysterics. Just land me at Shopton and I'll take a car home."
The damaged airship seemed little the worse for the test to which she had been subjected, and made her way at good speed in the direction of Tom's home. Several little experiments were tried on the way back. They all worked well, and the only two problems Tom had to solve were the taking care of the recoil from the guns and finding out why the propeller had broken.
A safe landing was made, and the Mars once more put away in her hangar. Mr. Damon departed for his home, and Lieutenant Marbury again took up his residence in the Swift household.
"Well, Tom, how did it go?" asked his father.
"Not so very well. Too much recoil from the guns."
"I was afraid so. You had better drop this line of work, and go at something else."
"No, Dad!" Tom cried. "I'm going to make this work. I never had anything stump me yet, and I'm not going to begin now!"
"Well, that's a good spirit to show," said the aged inventor, with a shake of his head, "but I don't believe you'll succeed, Tom."
"Yes I will, Dad! You just wait."
Tom decided to begin on the problem of the propeller first, as that seemed more simple. He knew that the gun question would take longer.
"Just what are you trying to find out, Tom?" asked Ned, a few nights later, when he found his chum looking at the broken parts of the propeller.
"Trying to discover what made this blade break up and splinter that way. It couldn't have been centrifugal force, for it wasn't strong enough."
Tom was "poking" away amid splinters, and bits of broken wood, when he suddenly uttered an exclamation, and held up something. "Look!" he cried. "I believe I've found it."
"What?" asked Ned.
"The thing that weakened the propeller. Look at this, and smell!" He held out a piece of wood toward Ned. The bank employee saw where a half-round hole had been bored in what remained of the blade, and from that hole came a peculiar odor.
"It's some kind of acid," ventured Ned.
"That's it!" cried Tom. "Someone bored a hole in the propeller, and put in some sort of receptacle, or capsule, containing a corrosive acid. In due time, which happened to be when we took our first flight, the acid ate through whatever it was contained in, and then attacked the wood of the propeller blade. It weakened the wood so that the force used in whirling it around broke it."
"Are you sure of that?" asked Ned.
"As sure as I am that I'm here! Now I know what caused the accident!"
"But who would play such a trick?" asked Ned. "We might all have been killed."
"Yes, I know we might," said Tom. "It must be the work of some of those foreign spies whose first plot we nipped in the bud. I must tell Marbury of this, but don't mention it to dad."
"I won't," promised Ned.
Lieutenant Marbury agreed with Tom that someone had surreptitiously bored a small hole in the propeller blade, and had inserted a corrosive acid that would take many hours to operate. The hole had been varnished over, probably, so it would not show.
"And that means I've got to examine the other two blades," Tom said. "They may be doctored too."
But they did not prove to be. A careful examination showed nothing wrong. An effort was made to find out who had tried to destroy the Mars in midair, but it came to nothing. The two men in custody declared they knew nothing of it, and there was no way of proving that they did.
Meanwhile, the torn gas bag was repaired, and Tom began working on the problem of doing away with the gun recoil. He tried several schemes, and almost was on the point of giving up when suddenly he received a hint by reading an account of how the recoil was taken care of on some of the German Zeppelins.
The guns there were made double, with the extra barrel filled with water or sand, that could be shot out as was the regular charge. As both barrels were fired at the same time, and in opposite directions, with the same amount of powder, one neutralized the other, and the recoil was canceled, the ship remaining steady after fire.
"By Jove! I believe that will do the trick!" cried Tom. "I'm going to try it."
"Good luck to you!" cried Ned.
It was no easy matter to change all the guns of the Mars, and fit them with double barrels. But by working day and night shifts Tom managed it. Meanwhile, a careful watch was kept over the shops. Several new men applied for work, and some of them were suspicious enough in looks, but Tom took on no new hands.
Finally the new guns were made, and tried with the Mars held on the ground. They behaved perfectly, the shooting of sand or water from the dummy barrel neutralizing the shot from the service barrel.
"And now to see how it works in practice!" cried Tom one day. "Are you with me for a long flight, Ned?"
"I sure am!"
The next evening the Mars, with a larger crew than before, and with Tom, Ned, Mr. Damon and Lieutenant Marbury aboard, set sail.
"But why start at night?" asked Ned.
"You'll see in the morning," Tom answered.
The Mars flew slowly all night, life aboard her, at about the level of the clouds, going on almost as naturally as though the occupants of the cabins were on the earth. Excellent meals were served.
"But when are you going to try the guns?" asked Ned, as he got ready to turn in.
"Tell you in the morning," replied Tom, with a smile.
And, in the morning, when Ned looked down through the plate glass in the cabin floor, he uttered a cry.
"Why, Tom! We're over the ocean!" he cried.
"I rather thought we'd be," was the calm reply. "I told George to head straight for the Atlantic. Now we'll have a test with service charges and projectiles!"