the plain imprint of a diamond
tread. Instantly Roy and Willie dropped to their knees and began to
feel along the line of the tire mark.

Henry and Lew, meantime, searched to right and left of them. "Here's
his other wheel track," suddenly cried Lew, and there, sure enough, was
a distinct impression of a chain tread tire.

They proceeded in the direction in which the car had been moving.
"Here's where he turned," cried Henry.

The turf before him was torn and ragged. Distinctly they could see the
impression made by the driving wheels as they gripped the ground in
starting.

"Then here is where he stood," said Captain Hardy. "It is immediately
behind the wood. Your mark pointed straight enough, Henry, but your
man was farther away than you thought. Probably he ran behind this
grove to make doubly sure he would not be seen from the highway. The
hidden station must be in this end of the grove. We'll search again."

Once more they plunged into the wood. Again they examined every tree.
Up one trunk after another shinned Roy and Lew, who were born climbers.
But hunt as they would, search as they might, they found nothing to
indicate a secret wireless. At last, completely baffled, they gave up
the search.

"It's here," insisted Captain Hardy. "Our experience at the Elk City
reservoir makes me sure of that. They're too clever for us. But we'll
get them yet. We'll follow that roadster so closely next time that we
can't miss the secret. It's too bad, boys, but don't be discouraged.
We have done much to come so close. Now we'll go back to the
road-house. It's long past time for luncheon."

CHAPTER XII

ANOTHER OBSTACLE

Time wore on. Now that there was something definite to work on, the
secret service began to take a more active part in the spy hunt.

"You have helped us greatly," said the Chief to Captain Hardy, one day.
"My men were so rushed with work that they simply could not take the
time to go hunting round for clues. But now that the wireless patrol
has furnished those clues, we shall be able to follow them up. But we
want you to continue at work just the same. You can still help us."

But the members of the wireless patrol, and especially Henry, found
small satisfaction in the Chief's praise. They had not come to New
York merely to furnish the secret service with clues. They had come to
uncover the system by which spies were betraying the movements of our
transports. At the Elk