"Whew, how it rains!" exclaimed Ned, as he looked out of the window.

"And it doesn't seem to show any signs of letting up," remarked Tom. "It's been at it nearly a week now, and it is likely to last a week longer."

"It's beastly," declared his chum. "How can you test your gun in this weather?"

"I can't. I've got to wait for it to clear."

"Bless my rubber boots! it's just got to stop some time," declared Mr. Damon. "Don't worry, Tom."

"But I don't like this delay. I have heard that General Waller has perfected a new gun--and it's a fine one, from all accounts. He has the proving grounds at Sandy Hook to test his on, and I'm handicapped here. He may beat me out."

"Oh, I hope not, Tom!" exclaimed Ned. "I'm going to see what the weather reports say," and he went to hunt up a paper.

It was several weeks after the completion of Tom's giant cannon. In the meanwhile the gun had been moved by the steel company to a little-inhabited part of New York State, some miles from the plant. The gun had been mounted on an improvised carriage, and now Tom and his friends were waiting anxiously for a chance to try it.

The work was not complete, for the steel company employees had been hampered by the rain. Never before, it seemed, had there been so much water coming down from the clouds. Nearly every day was misty, with gradations from mere drizzles to heavy downpours. There were occasional clear stretches, however, and during them the men worked.

A few more days of clear weather would be needed before the gun could be fastened securely to the carriage, and then Tom could fire one of the great projectiles that had been cast for it. Not until then would he know whether or not his cannon was going to be a success.

Meanwhile nothing more had been heard or seen of the spy. He appeared to have given up his attempts to steal Tom's secret, or to spoil his plans, if such was his object.

The place of the test, as I have said, was in a deserted spot. On one side of a great valley the gun was being set up. Its muzzle pointed up the valley, toward the side of a mountain, into which the gigantic projectile could plow its way without doing any damage. Tom was going to fire two kinds of cannon balls--a solid one, and one containing an explosive.

The gun was so mounted that the muzzle could be elevated or depressed, or swung from side to side. In this way the range could be varied. Tom estimated that the greatest possible range would be thirty miles. It could not be more than that, he decided, and he hoped it would not be much less. This extreme range could be attained by elevating the gun to exactly the proper pitch. Of course, any shorter range could, within certain limits, also be reached.

The gun was pointed slantingly up the valley, and there was ample room to attain the thirty-mile range without doing any damage.

At the head of the valley, some miles from where the giant cannon was mounted, was an immense dam, built recently by a water company for impounding a stream and furnishing a supply of drinking water for a distant city. At the other end of the valley was the thriving village of Preston. A railroad ran there, and it was to Preston station that Tom's big gun had been sent, to be transported afterward, on specially made trucks, drawn by powerful autos, to the place where it was now mounted.

Tom had been obliged to buy a piece of land on which to build the temporary carriage, and also contract for a large slice of the opposite mountain, as a target against which to fire his projectiles.

The valley, as I have said, was desolate. It was thickly wooded in spots, and in the centre, near the big dam, which held back the waters of an immense artificial lake, was a great hill, evidently a relic of some glacial epoch. This hill was a sort of division between two valleys.

Tom, Ned, Mr. Damon, with Koku, and some of the employees of the steel company, had hired a deserted farmhouse not far from the place where the gun was being mounted. In this they lived, while Tom directed operations.

"The paper says 'clear' tomorrow," read Ned, on his return. "'Clear, with freshening winds.'"

"That means rain, with no wind at all," declared Tom, with a sigh. "Well, it can't be helped. As Mr. Damon says, it will clear some time."

"Bless my overshoes!" exclaimed the odd gentleman. "It always has cleared; hasn't it?"

No one could deny this.

There came a slackening in the showers, and Tom and Ned, donning raincoats, went out to see how the work was progressing. They found the men from the steel concern busy at the great piece of engineering.

"How are you coming on?" asked Tom of the foreman.

"We could finish it in two days if this rain would only let up," replied the man.

"Well, let's hope that it will," observed Tom.

"If it doesn't, there's likely to be trouble up above," went on the foreman, nodding in the direction of the great dam.

"What do you mean?"

"I mean that the water is getting too high. The dam is weakening, I heard."

"Is that so? Why, I thought they had made it to stand any sort of a flood."

"They evidently didn't count on one like this. They've got the engineer who built it up there, and they're doing their best to strengthen it. I also heard that they're preparing to dynamite it to open breeches here and there in it, in case it is likely to give way suddenly."

"You don't mean it! Say, if it does go out with a rush it will wipe out the village."

"Yes, but it can't hurt us," went on the foreman. "We're too high up on the side of the hill. Even if the dam did burst, if the course of the water could be changed, to send it down that other valley, it would do no harm, for there are no settlements over there," and he pointed to the distant hill.

It was near this hill that Tom intended to direct his projectiles, and on the other side of it was another valley, running at right angles to the one crossed by the dam.

As the foreman had said, if the waters (in case the dam burst) could be turned into this transverse valley, the town could be saved.

"But it would take considerable digging to open a way through that side of the mountain, into the other valley," went on the man.

"Yes," said Tom, and then he gave the matter no further thought, for something came up that needed his attention.

"Have you your explosive here?" asked the foreman of the young inventor the next day, when the weather showed signs of clearing.

"Yes, some of it," said Tom. "I have another supply in a safe place in the village. I didn't want to bring too much here until the gun was to be fired. I can easily get it if we need it. Jove! I wish it would clear. I want to get out in my Humming Bird, but I can't if this keeps up." Tom had brought one of his speedy little airships with him to Preston.

The following day the clouds broke a little, and on the next the sun shone. Then the work on the gun went on apace. Tom and his friends were delighted.

"Well, I think we can try a shot tomorrow!" announced Tom with delight on the evening of the first clear day, when all hands had worked at double time.

"Bless my powder-horn!" exclaimed Mr. Damon. "You don't mean it!"

"Yes, the gun is all in place," went on the young inventor. "Of course, it's only a temporary carriage, and not the disappearing one I shall eventually use. But it will do. I'm going to try a shot tomorrow. Everything is in readiness."

There came a knock on the door of the room Tom had fitted up as an office in the old farmhouse.

"Who is it?" he asked.

"Me--Koku," was the answer.

"Well, what do you want, Koku?"

"Man here say him must see Master."

Tom and Ned looked at each other, suspicion in their eyes.

"Maybe it's that spy again," whispered Ned.

"If it is, we'll be ready for him," murmured his chum. "Show him in, Koku, and you come in too."

But the man who entered at once disarmed suspicion. He was evidently a workman from the dam above, and his manner was strangely excited.

"You folks had better get out of here!" he exclaimed.

"Why?" asked Tom, wondering what was going to happen.

"Why? Because our dam is going to burst within a few hours. I've been sent to warn the folks in town in time to let them take to the hills. You'd better move your outfit. The dam can't last twenty-four hours longer!"