ere did
they see anything to arouse their suspicion.

"We may as well go back, boys," their leader said at last. "We shall
have to depend upon our ears rather than our eyes if we are to catch
these villains. But we have made progress. We know where they are.
We have limited our field of observation to one place. Now we shall
have to do as we did at Elk City. We shall have to get two portable
sets with compact detectors and begin a watch in Hoboken. We'll have
to find this hidden wireless by triangulation, just as we caught the
dynamiters. But we haven't enough of a force to maintain two watches.
We shall likely have to send for more of the boys to come on."

They recrossed the river and made their way back to their headquarters.
Lew had heard nothing. He was relieved by Henry.

The others went down to dinner, and food was sent up to the lone
watcher. But when his trick was ended, he made the same report that
Lew had rendered. He, too, had heard nothing.

"Doubtless," said Captain Hardy, "they use their wireless seldom for
fear of discovery. Probably they send a message only when troop ships
have actually sailed. That is likely the reason it was such a long
time before we caught the first message. And it may be just as long
before we hear another. But when it comes, we must be ready with our
two detectors. I'll see Chief Flynn about them in the morning. And
I'll tell him what we have learned in addition to what the cipher
message told us."

"I wonder," said Roy, "how the secret service men ever unraveled that
cipher. I could never have done it. I was looking for something like
the code message we caught at Camp Brady."

"It probably was not very difficult, Roy," replied Captain Hardy, "or
it could not have been fathomed so soon. I believe that most cipher
messages to-day are like the one you caught at Camp Brady. Apparently
they are innocent messages but they have a hidden meaning. The most
difficult cipher messages, I have heard, are of the substitution kind,
where many alphabets are used. It is pretty difficult to decipher such
messages unless you have the key word."

"Then why didn't the Germans use a substitution cipher when they sent
this message about the transports?" asked Willie. "Then we might never
have been able to tell what they said."

"It was hardly worth while, Willie. They know the authorities are
listening for their messages. It made no particular difference if the
cont