the coin from his pocket. That driver was the
same one I saw the other day."

"How do you know?" interrupted Captain Hardy. "You didn't see him
to-day?"

"But Roy saw him. He's a tall man with black hair and with a scar on
his left cheek. That's the man I saw, and it's the man who drove up to
the house on the cliff the other day. I knew that I had seen him, but
I couldn't remember just where." For a moment he stood silent, fairly
panting with excitement.

"Well?" said Lew. "What about him? The grocer could owe him a dollar
as well as anybody."

"But he didn't owe him a dollar," cried Willie. "Don't you see? The
spy in the house below gave that dollar to the errand boy. He gave it
to the grocer. He gave it to the motor driver. It's the same dollar.
He didn't put it in the till with the other coins. He kept it in his
pocket separate. That automobile driver is the man who carried the
messages to the wireless. The messages are on the dollars."

CHAPTER IX

A FRESH START

Amazed, the members of the little patrol looked at one another silently.

"How could they send a message on a dollar?" demanded Lew at last.
"They'd have to engrave it, and then they'd never dare to use the
dollar again. Besides, it would be too dangerous. If the message were
on paper, the paper could be burned or chewed up and swallowed, and the
evidence of crime destroyed. But they couldn't erase the engraving on
a dollar."

"I don't know how they do it," said Willie, "but I'm sure they write
their messages on those dollars."

"Willie is doubtless right," said Captain Hardy. "We don't know how
they do it, but the evidence leads directly to the conclusion Willie
has come to. The spy in the house below us writes his messages on
dollars and sends them through this grocer's boy and the motor-car
driver to the various secret wireless plants the Germans evidently
possess near New York. I think that is plain. And it indicates new
lines of action for us. We must not only continue to listen in for
messages and watch this spy's nest, but we shall have to follow this
motor-car driver and also learn the secret of the dollars."

"Hurrah!" cried Roy, his eyes shining. "Now there'll be something
doing." Then he struck a tragic attitude and declaimed, "Little do the
treacherous hawks in yonder nest realize that the eagles of the law are
about to swoop down on them."

"Some orator, Roy," said Lew. "If we're eagles, we must