“As easy as that!”
“It’s a wonder we didn’t think of that!”
“Two papers—one with our numbers on, and one with his!”
“That’s the whole secret,” explained Joe. “That is, all but the stone. Of course if I had had a slate to use that would have been a little different.”
“That’s what I don’t understand,” observed Charlie. “That professor last night passed the slate around for inspection, and there wasn’t any number written on it.”
“Oh, yes there was,” said Joe with a smile. “Only you didn’t see it. It was a trick slate. On one side, covered by a piece of black stiff paper, which looked almost like the slate, was the number written in chalk—a number that was the sum of three figures previously known to the professor, and on the piece of paper he gave out to be added up.
“When he took back the slate, after having passed it around for inspection, he walked up on to the stage and quietly slipped out the piece of black paper. That left the chalk sum exposed. He could either do that before he covered the slate with the handkerchief and gave it to some one to hold, or afterward, as he took it from the person and raised the handkerchief covering. In his case he did it before, since he let the person holding the slate lift the handkerchief.”
“Then the number was there all the while!” cried Tom.
“Yes.”
“And if the one who held the slate had lifted the handkerchief it would have been seen?”