"What are you going to do, Tom?" cried Ned, as he, with the others, worked the hand gear that shifted the big gun. When it was permanently mounted electricity would accomplish this work. "What's your game, Tom?"

"Don't you remember, Ned? When we were talking about the chance of the dam bursting, I said if the current of suddenly released water could be turned into the other valley, the people below us would be saved."

"Yes."

"Well, that's what I'm going to do. I'm going to fire a bursting shell at the point where the two valleys come together. I'll break down the barrier of rock and stone between them."

"Bless my shovel and hoe!" cried Mr. Damon.

"If we can turn enough of the water into the other valley, where no one lives, and where it can escape into the big river there, the amount that will flow down this valley will be so small that only a little damage will be done."

"That's right!" declared the steel foreman, as he caught Tom's idea. "It's the only way it could be done, too, for there won't be time to make the necessary excavation any other way. Is the gun swung around far enough, Mr. Swift?"

"No, a little more toward me," answered Tom, as he peered through the telescope sights. "There, that will do. Now to get the proper elevation," and he began to work the other apparatus, having estimated the range as well as he could.

In a few seconds the giant cannon was properly trained on the white rock. Meanwhile the horseman, with his red flag, had continued on down the valley. In spite of his warning of the night before, it developed that a number had disregarded it, and had remained in their homes. Most of the inhabitants, however, had fled to the hills, to stay in tents, or with such neighbors as could accommodate them. Some lingered to move their household goods, while others fled with what they could carry.

It was to see that the town was deserted by these late-stayers that the messenger rode, crying his warning as did the messenger at the bursting of the Johnstown dam twenty-odd years ago.

"The projectile!" cried Tom, as he saw that all was in readiness. "Lively now! I can see the top of the dam beginning to crumble," and he laid aside the telescope he had been using.

The projectile, with a heavy charge of bursting powder, was slung into the breech of the gun.

"Now the powder, Koku!" called Tom. "Be quick; but not so fast that you drop any of it."

"Me fetch," responded the giant, as he hastened toward the small cave where the explosive was kept. As the big man brought the first lot, and Ned was about to insert it in the breech of the gun, behind the projectile, Tom exclaimed:

"Just let me have a look at that. It's some that I first made, and I want to be sure it hasn't gone stale."

Critically he looked at the powerful explosive. As he did so a change came over his face.

"Here, Koku!" the young inventor said. "Where did you get this?"

"In cave, Master."

"Is there any more left?"

"Only enough for this one shoot."

"By Jove!" muttered Tom. "There's been some trick played here!" and he set off on a run toward the bomb-proof.

"What's the matter?" cried Ned, as he noticed the agitation of his chum.

"The powder has been doped!" yelled Tom. "Something has been put in it to make it nonexplosive. It's no good. It wouldn't send that shell a thousand yards, and it's got to go five miles to do any good. My plan won't work."

"Doped the powder?" gasped Ned. "Who could have done it?"

"I don't know. There must have been some spy at work. Quick, run and ask the foreman if any of his men are missing. I'll see if there's enough of the good powder left to break down the barrier!"

Ned was away like a shot, while the others, not knowing what to make of the strange conduct of the two lads, looked on in wonder. Tom raced toward the cave where the powder was stored, Koku following him.

"Bless my shoe laces!" cried Mr. Damon. "Look at the dam now

They gazed to where he pointed. In several places the concrete spillway had crumbled down to a ragged edge, showing that the solid wall was giving way. The amount of water flowing over the dam was greater now. The creek was steadily rising. Down the valley the horseman with the red flag was but a speck in the distance.

"What can I do? What can I do?" murmured Tom. "If all the powder there is left has been doped, I can't save the town! What can I do? What can I do?"

Ned had reached the foreman, who, with his helpers, was standing about the big gun.

"Have any of your men left recently?" yelled Ned.

"Any of my men left? What do you mean?

"Schlichter went yesterday," said the timekeeper. "I thought he was in quite a hurry to get his money, too."

"Schlichter gone!" exclaimed the foreman. "He was no good anyhow. I think he was a sort of Anarchist; always against the government, the way he talked. So he has left; eh? But what's the matter, Ned?"

"Something wrong with the powder. Tom can't shoot the cannon and turn aside the water to save the town. Some of his enemies have been at work. Schlichter leaving at this time, and in such hurry, makes it look suspicious."

"It sure does! And, now I recall it, I saw him yesterday near your powder magazine. I called him down for it, for I knew Tom Swift had given orders that only his own party was to go near it. So the powder is doped; eh?"

"Yes! It's all off now."

He turned to see Tom approaching on the run.

"Any good powder left?" asked Ned.

"Not a pound. Did you hear anything?"

"Yes, one man has disappeared. Oh, Tom, we've got to fail after all! We can't save the town!"

"Yes, we can, Ned. If that dam will only hold for half an hour more."

"What do you mean

"I mean that I have another supply of good powder in the village. I secreted some there, you remember I told you. If I can go get that, and get back here in time, I can break down the barrier with one shot, and save Preston."

"But you never can make the trip there and back in time, with the powder, Tom. It's impossible. The dam may hold half an hour, or it may not. But, if it does, you can't do anything!"

"I can't? Well, I'm going to make a big try, Ned. You stay on the job here. Have everything ready so that when I get back with the new explosive, which I hope hasn't been tampered with, I can shove it into the breech, and set it off. Have the wires, primers and button all ready for me."

Then Tom set off on the run.

"Where are you going?" gasped his chum. "You can never run to Preston and back in time."

"I don't intend to. I'm going in my airship. Koku, never mind bringing the rest of the powder from the cave. It's no good. Run out the Humming Bird. I'm going to drive her to the limit. I've just got to get that powder here on time!"

"Bless my timetable!" gasped Mr. Damon. "That's the only way it can be done. Lucky Tom brought the airship along!"

The young inventor, pausing only to get some cans for the explosive, and some straps with which to fasten them in the monoplane, leaped into the speedy craft.

The motor was adjusted; Koku whirled the propeller blades. There was a staccato succession of explosions, a rushing, roaring sound, and then the craft rose like a bird, and Tom circled about, making a straight course for the distant town, while below him the creek rose higher and higher as the dam continued to crumble away.