“That accounts for how Graves knew,” he said, with much satisfaction.  “What happened then?”

When he heard of how they had thought too late of calling Colonel Throckmorton by telephone he sighed.

“If you’d only got that message through before Graves did his work!” he said.  “He’d have had to believe you then, of course.  How unlucky!”

“I know,” said Jack.  “We were frightfully sorry.  And then we went out to find where the wire was cut, and then got Dick.  But I got away, and I managed to stay fairly close to them.  I followed them when they left Dick in a little stone house, as a prisoner, and I heard this — I heard them talking about getting a big supply of petrol.  Now what on earth do they want petrol for?  They said there would still be plenty left for the automobiles — and then that they wouldn’t need the cars any more, anyhow!  What on earth do you make of that, Harry?”

“Tell me the rest, then I’ll tell you what I think,” said Harry.  “How did you get Dick out?  And did you hear them saying anything that sounded as if it might be useful, Dick?”

“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 what 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’ve probably stuck, but I think its 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?”