enty-three concentric circles, between which could plainly
be seen the bright dots or scratches in the dingy dollar.

"Whew!" said Captain Hardy, as he laid down his dividers. "That's
pretty fine work--twenty-three circles within a space of an inch and a
half. I'll wager a watchmaker made their pattern for them. The solid
parts of their metal discs can't be much larger than these lines I have
scratched on the celluloid. You were right when you named it, Willie.
The parts of it must be just about as thick as a spider-web."

The boys passed the dollar and its superimposed disc from hand to hand,
examining them with eager interest.

"Suppose they wanted to send a message with more than twenty-four
letters in it," said Roy. "How could they do it? I'm sure some of the
messages we intercepted had more than twenty-four letters in them."

Captain Hardy picked up the disc-covered dollar and studied it
intently. "I suppose," he said after a time, "that they would put more
than one dot in the same circle, and the dots would be read in the same
way they are now. The one to be read first would be nearest the centre
of the coin, and so on. Or they could write on several coins, each
coin being numbered in some way, and corresponding to a paragraph in a
composition."

Again he studied the dollar closely. "Clever!" he said admiringly.
"Mighty clever! Who would ever dream that those tiny scratches meant
anything? Many a time I've seen a dollar scratched and nicked a deal
worse than this one is, though they've evidently chosen a battered one
so that their own marks will be less noticeable. Why, that coin might
have passed through our hands a hundred times, and if we had not
actually seen it marked we should never dream it said anything other
than 'In God We Trust.' We've had a great stroke of luck, boys."

He paused and meditated. "I wonder if it is luck," he went on. "May
not the motto on that dollar explain our good fortune? Perhaps it is
Providence rather than blind luck that has guided us. At any rate let
us hope so. Now I'm going to report to the Chief. Won't he get a
surprise?"

And Captain Hardy left his subordinates, chuckling at the prospect of
the Chief's astonishment.

But it was Captain Hardy who had the surprise. Instead of the stern,
silent, brusque man he had become accustomed to, Captain Hardy found
the Chief smiling and talkative. As his eye fell on Captain Hardy, the
Chief rubbed his hands