"Then why didn't the Germans use a substitution cipher when they sent this message about the transports?" asked Willie. "Then we might never have been able to tell what they said."
"It was hardly worth while, Willie. They know the authorities are listening for their messages. It made no particular difference if the contents of this message were known. But when they send out an order for a spy to do something, I have no doubt they use the most difficult code they can devise, or at least one that they believe only the spy will understand. So we may expect to catch messages in different codes before we are through with our work."
Captain Hardy rose and began to look along the shelves of books. "Here is a volume," he said presently, "that will tell us a great deal about cipher messages."
He had just laid open the book when Roy rushed in from the wireless room. "I've got another message," he said, holding out a paper on which was a long string of letters.
"I wasn't expecting another message so soon," said Captain Hardy in surprise. Slowly he read the letters on the paper Roy had given him:
"FTSTITEIAFTDLLTNSYWTORPSLHVNRLEEYLIOTEIHUAOSEIEGGEVNCENDRRTERNRADSNLEEITOCGEOSHM."
"It looks like the same cipher used before," he went on. "If it is, we can unravel this message without bothering the secret service. At any rate we'll make a try at it. Where's that other message, Willie?"
The first message was brought. Captain Hardy spread it on the table and the group bent over it.
"The letters divide evenly into four lines, you notice," said the leader. "Let's see if this message will do the same."
He counted the letters with his pencil. "Eighty," he announced. "That would make four lines of twenty letters each. We'll try it."