Slowly he turned the disc around, putting a letter at the bottom of each sector. When he had finished, he had completed the alphabet. About him clustered his four comrades, too deeply interested to speak. They hardly even breathed.
"Take this paper, Roy," said Captain Hardy, "and tell me how many times each letter in the message appears."
Roy took the paper on which Captain Hardy had made his numerical enumeration. "Three A's," he said.
Captain Hardy made three marks in the A sector.
"No B's, no C's, and two D's."
The D's were scored. So they went through the alphabet. When they were done, the markings on the disc were practically a duplicate of those on the dollar, for Captain Hardy studied the dollar each time before marking the paper disc.
"That's it," cried Willie. "That's it exactly."
"It's right so far as it goes, Willie," said their leader, "but we haven't all of it yet. Suppose I hand you a disc with four T's, three S's, two Z's, three L's, and so on. Could you make a message out of it?"
Willie studied the disc on the desk. "No," he said, "I couldn't. I shouldn't know how to arrange the letters to make words out of them."
"Neither would anybody else," continued Captain Hardy. "Those spies have some way of knowing how to tell the order in which to read these letters."