d so on. I'll mark the disc that way, anyhow."

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."

For some time he sat studying the scratches on the dollar. The four
boys were quiet as mice, each trying to solve the problem that stood
between them and complete mastery of the cipher.

"You said that the metal disc resembled a spider's web," began Captain
Hardy, talking more to himself than to the boys. "We know what the
straight lines--the spokes--are for. The concentric circles must be to
indicate the order of the letters. Let me see." Again he studied the
dollar closely. "Some of these marks are near the centre of the disc,
some half-way between centre and circumference, and some close to the
outer edge. I believe the secret lies there."

"Listen!" cried Willie of a sudden. "When a spider spins a web, she
begins at the centre and works outward. Maybe these spies write their
messages in the same way."

"Willie," cried Captain Hardy, "you've hit it exactly. You're as good
a reasoner as you are an observer. Now we'll begin at the centre and
spin this message outward. What's the first letter?"

"T," said four voices together.

The captain took his