They left the bustling water-front and passed to the higher ground where stood the city proper. It was like most other American municipalities—dirty, dingy, and unattractive, a hotchpotch of buildings with no architectural unity. But it had one feature possessed by few cities—an outlook on a great and busy harbor.
As the boys stood looking at the rolling Hudson below them, watching the ferry-boats come and go, like huge shuttles in a giant loom, following the movements of steamers, and tugs and tow-boats, and tracing the circling flight of the gulls, they forgot entirely the errand that had brought them. Presently their leader broke the silence.
"We shall have to get to work," he said.
Starting at one end of the street, they walked slowly along its entire length, studying every house that fronted on the river. They saw at once that their task was hopeless. Square after square the houses stretched in unbroken blocks. A hundred spies might be living in those houses and no one be the wiser. A hundred wireless outfits might be flashing messages among the clothes-lines on the roofs and only a roof to roof survey would reveal the fact. But it was not necessary to run even so slender a risk of discovery. As the wireless patrol knew only too well, an aerial would work with great efficiency even though it were strung in a chimney or erected entirely within doors. Yet the little party continued its investigation until dusk, scanning every window whence a glass might be directed toward the river, and threading alleys and scrutinizing the wires of roofs and yards. But nowhere 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."