it, the train rolled into a tunnel, dived
under a great river, and emerged again in a huge yard far below the
level of the streets, that was filled with many tracks and closed in
with enormous walls of cement. Then the train ran into a great shed
and came to rest. The boys left the coach, mounted a long flight of
iron steps and found themselves in the city of their dreams--New York.

And there, at the gateway, was their beloved captain. They swarmed
about him and grasped his hand. Then Captain Hardy led them to a
corner of the waiting-room that offered a little privacy, and there
they sat down in a group, close to one another, to talk over the
business that had brought them again together.

"As I wrote you in my letter, Henry," said Captain Hardy, "I was not at
all hopeful that your plan would meet with official encouragement. But
I had promised you that I would mention it to the Chief of the Radio
Service and I did so. It didn't take him a minute to decide on it. To
my surprise he said he wanted you. 'I haven't a bit of doubt,' said
he, 'that the country's full of secret German wireless outfits. They
are probably of small sending power and operate in unusual wave
lengths. It is almost impossible for our regular service to detect
them. In fact I don't know how we are ever going to locate them unless
we organize the amateurs all over the country so that they can listen
in and catch practically everything that goes through the air. We are
not able to do that yet, but I shall be very glad to have the help of
your boys. I've been mighty interested in the way they handled that
affair at Elk City. They are experienced and have good sense. They
should be very useful to Uncle Sam.'" Dr. Hardy paused and smiled.
"You see," he went on, "the Chief has kept pretty close watch of you
boys. He knows all about the affair at Elk City." And Captain Hardy
smiled affectionately at his charges.

"What are the Radio Chief's instructions?" asked Roy. "What are we to
do?"

"The Radio Service," replied Captain Hardy, "has no agencies for making
arrests and detecting crime. So we shall work under the direction of
the secret service and in cooperation with the police. And our first
duty is to make ourselves known to both."

"If the Chief of the Radio Service wanted the wireless patrol," said
Roy, "why did you telegraph for just the four of us? And why are we in
New York instead of Washington?"

"You couldn't be of a