She seemed to be adding and subtracting, and then she suddenly cried:
"I thought so! I thought so! It is the only way to account for the thickness of it."
"The thickness of what?" asked Allen.
"The bottom of that box!" went on Betty. "It has a false bottom. I'm sure of it. Look here! It is seven inches deep on the outside, and only five inches deep inside. Where are those two missing inches except in a false bottom?"
In her excitement Betty tapped on the inside of the bottom of the box with the ruler, and then a strange thing happened.
There was a clicking, springing sound, and the bottom of the iron box seemed to rise up in two parts, like the twin doors of a sidewalk elevator hatchway. The false bottom had been found, and as it swung up out of the way there was disclosed an opening in which lay a package wrapped in white tissue paper.
"Oh! Oh!" cried Betty, staring at the box "I—I've found it—the treasure!"