"Bless my necktie!" e.j.a.c.u.l.a.t.ed 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 ma.s.ses 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 s.p.a.ce, 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 alt.i.tude, 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.

CHAPTER XVII

AN OCEAN FLIGHT

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 b.a.l.l.s 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 a.s.sistants, 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.