"Tom! Tom!" cried Ned, as he watched the disappearing figure of his chum. "Come back here! If there's going to be an explosion we ought to run out of the back door!"

"I'm not running away!" flashed back Tom. "I'm going to get that powder out of the auto before it goes up! If it does we'll be blown to kingdom come, back door or front door! Come on!"

"Bacon and eggs!" yelled Ned. "He's running an awful risk! But I can't let him go alone! I guess we're in for it!"

Then he, too, rushed from the office toward the front of the shop, before which, in a sort of private road, stood the blazing auto. And Ned, who had now lost sight of Tom, because of our hero having turned a corner in the corridor, heard excited shouts coming from the seat of trouble.

"If that's some new kind of powder Tom's sent for, to test for his new big gun, and it goes up," Ned said to himself, as he rushed on, "this place will be blown to smithereens. All Tom's valuable machinery and patents will be ruined!"

Ned had now reached the front door of the shop. He had a glimpse of the burning auto--a small express truck, well loaded with various packages. And, through the smoke, which from the odor must have been caused by burning gasoline, Ned could see several boxes marked in red letters:

DANGEROUS            EXPLOSIVE

     KEEP AWAY FROM FIRE

"Keep away from fire!" murmured the panting lad. "If they can get any nearer fire I don't see how."

"Oh, mah golly!" gasped Eradicate, who had lumbered on behind Ned. "Oh, mah golly! Oh, good land ob massy! Look at Massa Tom!"

"I've got to help him!" cried Ned, for he saw that his chum had rushed to the rear of the auto, and was endeavoring to drag one of the powder boxes across the lowered tail-board. Tom was straining and tugging at it, but did not seem able to move the case. It was heavy, as Ned learned later, and was also held down by the weight of other express packages on top of it.

"Oh, mah golly!" cried Eradicate. "Git some watah, somebody, an' put out dat fire!"

"No--no water!" yelled Tom, who heard him. "Water will only make it worse--it'll scatter the blazing gasoline. The feed pipe from the tank must have burst. Throw on sand--sand is the only thing to use!"

"I'll git a shubble!" cried Eradicate. "I'll git a sand- shubble!" and he tottered off.

"Wait, Tom, I'll give you a hand!" cried Ned, as he saw his chum step away from the end of the auto for a moment, as a burst of flame, and choking smoke, driven by the wind, was blown almost in his face. "I'll help you!"

"We've got to be lively, then, Ned!" gasped Tom. "This is getting hotter every minute! Where's that Koku? He could yank these boxes out in a jiffy!"

And indeed a giant's strength was needed at that moment.

Ned glanced around to see if he could catch a glimpse of the big man whom Tom had brought from Giant Land, but Koku was not in sight.

"Let's have another try now, Ned!" suggested Tom, when a shift in the wind left the rear of the auto comparatively free from smoke and flame.

"You fellows had better skip!" cried the expressman, who had been throwing light packages off his vehicle from in front, where, as yet, there was no fire. "That powder'll go up in another minute. Some of the boxes are beginning to catch now!" he yelled. "Look out!"

"That's right!" shouted Tom, as he saw that the edge of one of the wooden cases containing the powder was blazing slightly. "Lively, Ned!"

Ned held back only for a second. Then, realizing that the time to act was now or never, and that even if he ran he could hardly save himself, he advanced to Tom's side. The smoke was choking and stifling them, and the flames, coming from beneath the auto truck, made them gasp for breath.

Together Tom and Ned tugged at the nearest case of powder--the one that was ablaze.

"We--we can't budge it!" panted Tom.

"It--it's caught somewhere," added Ned. "Oh, if Koku were only here!"

There was a sound behind the lads. A voice exclaimed:

"Master want shovel, so Eradicate say--here it is!"

They turned and saw a big, powerful man, with a simple, child- like face, standing calmly looking at the burning auto.

"Koku!" cried Tom. "Quick! Never mind the shovel! Get those powder boxes out of that cart before they go up! Yank 'em out! They're too much for Ned and me! Quick!"

"Oh, of a courseness I will so do!" said Koku, to whom, even yet, the English language was somewhat of a mystery. He dropped the shovel, and, heedless of the thick smoke from the burning gasoline, reached over and took hold of the nearest box. It seemed as though he pulled it from the auto truck as easily as Tom might have lifted a cork.

Then, carrying the box, which was now burning quite fiercely on one corner, over toward Tom and Ned, who had moved back, the giant asked:

"What you want of him, Master?"

"Put it down, Koku, and get out all the others! Lively, now, Koku!"

"I do," was the simple answer. The giant put the box on the grass and ran back toward the auto.

"Quick, Ned!" shouted Tom. "Throw some sand on this burning box! That will put out the fire!"

A few handfuls of earth served to extinguish the little blaze, and by this time Koku had come back with another box of powder.

"Get 'em all, Koku, get 'em all! Then we can put out the fire on the auto."

For the giant it was but child's play to carry the heavy boxes of powder, and soon he had them all removed from the truck. Then, with the danger thus narrowly averted, they all, including the expressman, turned in and began throwing sand on the fire, which now had a good hold on the body of the auto. The shovel, which Eradicate had sent by Koku, who could use more speed than could the aged colored man, came in handy.

Soon the fire was out, though not before the truck had been badly damaged, and some of its load destroyed. But, beyond a charring of some of the powder boxes, the explosive was intact.

"Whew! That was a lucky escape," murmured Tom, as he sat down on one of the boxes, and wiped the smoke and sweat from his face. "A little later and there'd only been a hole in the ground to tell what happened. hot work; eh, Ned?"

"I guess yes, Tom."

"I thought of the powder as soon as I saw that the truck was on fire," explained the expressman; "but I didn't know what to do. I was kinder flustered, I guess. This is the second time this old truck has caught fire from a leaky gasoline pipe. I guess that will be the last--it will for me, anyhow. I'll resign if they don't give me another machine. Will you sign for your stuff?" he asked Tom, holding out the receipt book, which had escaped the flames.

"Yes, and I'm mighty glad I'm here to sign for it," replied the young inventor. "Now, Koku, I guess you can take that stuff up to the shop; but be careful where you put it."

"I do, Master," replied the giant.

"What sort of powder is that, Tom?" asked Ned a little later, when they were again back in the office, the excitement having calmed down. The expressman had gone back to town afoot, to arrange about getting another vehicle for what remained of his load. "Is it the kind they use in big guns?"

"One of the kinds," replied Tom. "I sent for several samples, and this is one. I'm going to conduct some tests to see what kind I'll need for my own big gun. But I expect I'll have to invent an explosive as well as a cannon, for I want the most powerful I can get. Want to look at some of this powder?"

"Yes, if you think it's safe."

"Oh, it's safe enough if you treat it right. I'll show you," and working carefully Tom soon had one of the boxes open. Reaching into the depths he held up a handful of something that looked like sticks of macaroni. "There it is," he said.

"That powder?" cried Ned. "That's a queer kind. I've seen the kind they use in some guns on the battleships. That powder was in hexagonal form, about two inches across, and had a hole in the centre. It was colored brown."

"Well, powder is made in many forms," explained Tom. "A person who has only seen black gunpowder, with its little grains, would not believe that this was one grain of the new powder."

"That macaroni stick a grain of powder?" cried Ned.

"Yes, we'll call it a grain," went on the young inventor, "just as the brown, hexagonal cube you saw was a grain. You see, Ned, the idea is to explode all the powder at once--to get instantaneous action. It must all burn up at once as soon as it is detonated, or set off.

"To do that you have to have every grain acted on at the same moment, and that could not be done if the powder was in one solid chunk, or closely packed. For that reason they make it in different shapes, so it will lie loose in the firing chamber, just as a lot of jack-straws are piled up. In fact, some of the new powder looks like jack-straws. Some, as this, for instance, looks like macaroni. Other is in cubes, and some in long strings."

As he spoke Tom struck a match and held the flames near the end of one of the "macaroni" sticks.

"Caesar's grandmother!" yelled Ned. "Are you crazy, Tom?" as he started to leap for a window.

"Don't get excited," spoke Tom, quietly. "There's no danger," and he actually set fire to the stick of queer powder, which burned like some wax taper.

"But--but--" stammered Ned.

"It is only when powder is confined that it explodes," Tom explained. "If it can burn in the open it's as harmless as water, provided you don't burn too much at once. But put it in something where the resulting gases accumulate and can't escape, and then-- why, you have an explosion--that's all."

"Yes--that's all," remarked Ned, grimly, as he nervously watched the burning stick of powder. Tom let it flame for a few seconds, and then calmly blew it out.

"You know what a little puff black gunpowder gives, if you burn some openly on the ground," went on Tom; "don't you, Ned?"

"Sure, I've often done that."

"But put that same powder in a tight box, and set fire to it, and you have a bang instead of a puff. It's the same way with this powder, only it doesn't even puff, for it burns more slowly.

"An explosion, you see, is the sudden liberation at one time of the gases which result when the powder is burned. If the gases are given off gradually, and in the open, no harm is done. But put a stick like this in, say, a steel box, all closed up, save a hole for the fuse, and what do you have? An explosion. That's the principle of all guns and cannon.

"But say, Ned, I'm getting to be a regular lecturer. I didn't know I was running on so. Why didn't you stop me?"

"Because I was interested. Go on, tell me some more."

"Not now. I want to get this powder in a safe place. I'm a little nervous about it after that fire. You see if it had caught, when tightly packed in the boxes, there would have been a terrific explosion, though it does burn so harmlessly in the open air. Now let me see--"

Tom was interrupted by the postman's whistle, and a little later Eradicate came in with the mail that had been left in the box at the shop door. Tom rapidly looked over the letters.

"Here's the note I want, I think," he said, Selecting one. "Yes, this is it. 'Permission is hereby granted,' he read, 'to Thomas Swift to visit,' and so on, and so on. This is the stuff, Ned!" he cried.

"What is it?"

"A permit to visit the government proving grounds at Sandy Hook, Ned, and see 'em test that new big gun I was telling you about. Hurray! We'll go down there, and I'll see how my ideas fit in with those of the government's experts."

"Did you say 'we' would go down, Tom?"

"I sure did. You'll go with me; won't you?"

"Well, I hadn't thought very much about it, but I guess I will. When is it?"

"A week from today, and I'm going to need all that time to get ready. Now let's get busy, and we'll arrange to go to Sandy Hook. I've had trouble enough to get this permit--I guess I'll put it where it won't get lost," and he locked it in a secret drawer of his desk.

Then the lads stored the powder in a safe place, and soon were busy about several matters in the shop.