Close to the keg was an empty box.

Beyond these things the cavern was empty.
"I thought it was full of powder and dynamite," apologized Peggy.
"You see, it's like this," Blackton began.

"We put the powder and dynamite down there, and pack it over solid with rock and cement.

If we didn't leave this big air-chamber above it there would be only one explosion, and probably two thirds of the explosive would not fire, and would be lost.
This chamber corrects that.

You heard a dozen explosions last night, and you'll hear a dozen this afternoon, and the biggest explosion of all is usually the fourth or fifth.