dividers and found the scratch nearest the centre
of the disc. In the same sector with this scratch were three other
scratches in a line.

"It's a T," he announced, "just as it should be."

With his dividers he found the letter next nearest to the centre. It
stood alone. "That's a W," he announced.

Rapidly he located the scratch the third nearest to the centre. "And
that's an O," he said, looking up with flashing eyes. "We need go no
farther. We have the entire secret. We have deciphered their cipher."

A cry of exultation arose.

"To-morrow," continued Captain Hardy, "we will get a piece of
transparent celluloid and make a disc like their own. We can ink in
the circles and the radius lines and our disc will be almost a
duplicate of theirs, except that our disc will be solid while their
discs have open spaces between the circles. But that is only a detail.
We can read their cipher as well as if we had one of their own discs."

"Wait," cried Willie, as his comrades started to cheer again. "What is
the scratch on the milled edge of the dollar for?"

"That," replied Captain Hardy, "is to indicate how the disc is to be
placed on the dollar. That scratch is exactly between the Z and the A
sectors. It shows where the alphabet begins. Now we have their entire
secret."

CHAPTER XV

ANOTHER MYSTERY UNRAVELED

A piece of transparent celluloid, furnished by their host from a broken
side curtain of his automobile, supplied Captain Hardy with the
material needed for making the disc that was to be the key to future
communications of the enemy. Carefully he cut the celluloid the size
of a dollar, then marked the exact centre of it. Next he clamped the
disc on the captured coin. Between the rows of letters he scratched in
the straight radius lines--the spokes of a wheel. Then Captain Hardy
put the end of one arm of his dividers in the dot at the centre of his
disc, and swept the other arm around, scratching a circle just outside
the first letter in the message--the innermost T. Examination showed
that this circle fell just inside of the second letter in the
message--the W. Adjusting his calipers, the draughtsman made a second
circle, just outside of this second letter. A third circle fell
between the first O and the second T of the message. So Captain Hardy
continued, each succeeding circle falling just outside of the
succeeding letter in the message. When he had finished, his disc
contained tw