"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, aft closed up, save a hole for the fuse, and what do you have? An explosion. That's the principle ot 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, se-
