"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 ofthe 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 nowThey 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.