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!"