ng around Fort Slocum in the darkness.

But this was all the secret service ever learned as to the operations
of this gang of wireless spies. The prisoners refused to name those
higher up, and the Chief could only guess whose might be the master
minds behind the plot.

As for the various wireless agents who were relaying the spy messages
to Mexico, several were caught by decoy messages and shared the fate of
Sanders and the others.

Even the mystery of the sudden flight of Sanders and his crew was
cleared up. Following the Chief's orders, his men on the border had
taken the three silver coins away from the Mexican. And, sure enough,
the coins contained messages. One was the message from New York
concerning the sailing of transports. The other messages were about
army secrets, and it is not yet known where they came from or how they
got into the hands of the Mexican.

The latter protested violently when asked for his silver dollars, and
they had to be taken from him by force. The next day one of his guards
discovered that the left cuff of his shirt was missing. The shirt had
been intact when he was arrested. No trace of the cuff could be found
anywhere. The window of the room where the Mexican was confined
overlooked a public street. And it was believed by the secret service
men that the spy had written a message on his cuff in some way and
dropped it out of the window to a confederate. Thereupon a warning had
been flashed back along the wireless line--a warning message in the new
cipher that had so puzzled the lads of the wireless patrol.

"It's all clear enough now," sighed Willie, when the story had been put
together, "but when you have only one piece of a jig-saw puzzle you
can't make much out of it. And one piece was about all we had for a
long time. I see it all now, but there's one thing I don't yet
understand. Why didn't they use a more difficult cipher?"

"I suppose," explained the Chief, "that this very pursuit and capture
of the spies answers that question. They knew that if the secret
service picked up their messages, we could sooner or later decipher
anything they sent. But even a very simple cipher might baffle one
unaccustomed to such things. Always there was the danger that some one
would pick up their messages. So they chose ciphers that would bother
the ordinary man but that they themselves could read readily. They
didn't dare use a cipher that would require a long time to unrav