"What has that to do with it?" Will wanted to know.
"Because I think these diamonds are the proceeds of some robbery," went on the girl. "As you say, the stones are wrapped in a paper just as though they had come from a jewelry store. It might be that those men broke into a store, took the diamonds and hid them in this secret part of the box, which one of them owned. They are probably from some big robbery in New York, or Boston, seeing we're nearer Boston than we are New York, up here."
"I don't remember any such robbery lately," Roy said, and he was a faithful reader of the newspapers. "But of course we've been pretty busy lately. I'll get some back numbers of the papers."
"Ha! What's going on now?" asked the voice of Mr. Nelson. He had come in from the station, having run up to Boston on business.
"Oh, Daddy!" cried Betty. "Such news! You'll never guess!"
"You've solved the cipher!" he hazarded.
"No. We didn't need to. We solved the mystery of the box, and look——"
She spread the sparkling stones out before him.
"Whew!" he whistled. "I should say that was news. Where did you get those?"
"In a hidden compartment of the black box. I stumbled on the secret spring by accident when I was measuring it. Are they diamonds, Father?"