"That was fine work!" he said, when he had heard a description of Dick's rescue. "Jack, you seem to be around every time one of us gets into trouble and needs help!"
Then Dick told of the things he had overheard–the mysterious references to Von Wedel and to things that were to be done to the barracks at Ealing and Houndsditch. Harry got out a pencil and paper then, and made a careful note of every name that Dick mentioned. Then he took a paper from his pocket.
"Remember this, Dick?" he asked. "It's the thing I spoke of that I forgot until I came across it in my pocket this morning."
"What is it, Harry?"
"Don't you remember that we watched them heliographing some messages, and put down the Morse signs? Here they are. Now the thing to do is to see if we can't work out the meaning of the code. If it's a code that uses words for phrases we're probably stuck, but I think it's more likely to depend on inversions."
"What do you mean, Harry?" asked Jack. "I'm sorry I don't know anything about codes and ciphers."
"Why, there are two main sorts of codes, Jack, and, of course, thousands of variations of each of those principal kinds. In one kind the idea is to save words–in telegraphing or cabling. So the things that are likely to be said are represented by one word. For instance Coal, in a mining code, might mean 'Struck vein at two hundred feet level.' In the other sort of code, the letters are changed. That is done in all sorts of ways, and there are various tricks. The way to get at nearly all of them is to find out which letter or number or symbol is used most often, and to remember that in an ordinary letter E will appear almost twice as often as any other letter–in English, that is."
"But won't this be in German?"
"Yes. That's just why I wanted those names Dick heard. They are likely to appear in any message that was sent. So, if we can find words that correspond in length to those, we may be able to work it out. Here goes, anyhow!"
For a long time Harry puzzled over the message. He transcribed the Morse symbols first into English letters and found they made a hopeless and confused jumble, as he had expected. The key of the letter E was useless, as he had also expected. But finally, by making himself think in German, he began to see a light ahead. And after an hour's hard work he gave a cry of exultation.