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