“Yes, I see it is. See here, he is in trouble somewhere, and trying to send us word. Don’t you think we had better get out and try and find some balloon that has dropped on land, or chase one and run it down?”
“Well, that might be a good way,” replied Tom slowly, as though he was thinking deeply on some matter. “But perhaps we can do it easier.”
“How?”
“By trying to decipher the writing on this card.”
“But you can’t!” exclaimed Ben half impatiently, as he held up the dripping pasteboard. “You can’t read it. Try for yourself. Might as well try to read in the dark.”
“I know you can’t read it now,” assented Tom, “for the water has about soaked off the black marks of the pencil. But there may be a way of bringing back the writing.”
“How? Do you think Harry used some kind of invisible ink? I’ve read of prisoners sending secret messages to their friends written with some chemical that would not show unless it was heated, or something like that. Say!” he cried with sudden interest, “do you mean that way, Tom?”
“Well, no, not exactly. Harry didn’t use ink. He used a common lead pencil, from all appearances, and the water has soaked the black marks off. But you know when you use a pencil on paper, it always makes little depressions in the surface, corresponding to the shape of the letters. Did you ever put a piece of paper on top of another piece, and write on the top sheet?”
“Of course I have.”
“Then you’ve probably noticed that on the second sheet there would be marks by which the writing could be read, even though the black pencil characters did not show.”