disturbed. It must be on another floor."

"But if we are to keep a twenty-four-hour watch," argued Henry, "it ought to be right in our apartment."

"Let's look at the aerial, anyway," suggested Lew.

A door at the end of the hallway quite evidently led to the roof. They had noticed it as they followed their landlady up the stairs. Willie led the way through it and the boys found themselves on the roof, which, like the roofs of most city houses, was flat. Like its neighbors, also, this roof was encumbered with a number of long, wire clothes-lines, but the boys found nothing that suggested an aerial to them. Puzzled, they returned to their apartment.

Presently there was a rap at the door. Captain Hardy opened it and a man dressed as a waiter, whom they had seen in the hallway as they entered, stepped into the room.

"I came to show you your outfit," he said.

Stepping into the writing room, he grasped the corners of the mantel and gave a sharp pull. The entire upper half of the mantel swung outward and came to rest on the writing-table, revealing a compact but wonderfully well-equipped wireless outfit, including even a wireless detector for telling the direction a wireless message came from. The boys stared in astonishment while the waiter grinned.

"What kind of a boarding-house is this, anyway?" asked Lew.

"This ain't no boardin'-house," replied the man. "This is a sort of headquarters for secret service men from out of town."

"Where's your aerial?" demanded Willie.

"If you go on the roof you'll see it—that is you will if your eyes are sharp enough."

"I'll bet it's those wire clothes-lines," said Willie.

"Nothin' wrong with your eyes," said the waiter with a smile. "But I guess there wouldn't be, if the Chief sent you here."

Naturally each of the boys was eager to test the outfit before them. They crowded round it, sliding the coil, shifting the condenser, examining this and that, and voicing their approval and pleasure in the different instruments.

"We may as well begin our watch at once," said Captain Hardy. "Each of you will have to listen in six hours a day. If we divide the watches into two tricks of three hours each, it will be easier for you."

The matter was arranged accordingly, and the first trick given to the most experienced operator, Henry. After the others had seen him take his seat and adjust his receivers to his head, they withdrew from the wireless room.

But Henry was far from being in solitude. Sitting apparently alone, he was listening to a multitude of voices; for before beginning his vigil he wanted to test out his instruments and see how well they worked and how sharply they would register sounds. So he sat at his table, tuning now to this wave length and now to that, now catching a land message and now one from the sea. Distinctly he caught the signal NAA from the great navy wireless plant at Arlington. He recognized it before the operator had finished sending his call signal. Night after night with his home-made outfit at Central City, Henry had heard this station send forth the time signals at ten o'clock; and during his brief period as radio man for Uncle Sam he had often talked with Arlington, both sending and receiving messages from the great station. But though he recognized the voice, he did not know the language he heard; for Arlington was flinging abroad a message in the secret code of the navy. Press messages and commercial communications were buzzing through the air like swarms of bees. Orders to departing steamships came surging over his line. Suddenly a strong whining note filled the air, drowning out all other notes, and Henry knew the Brooklyn Navy Yard was talking. He caught messages from the Waldorf, from the Wanamaker station, from the police wireless. Never had he heard so many messages or imagined that the air could be so filled with talk. And had he not been a very able operator, he would have been so confused by the babel that he would have understood none of it clearly. But he tuned sharply, shutting out interfering vibrations, and caught clearly message after message. But every message that he intercepted was sent by a regularly licensed station.

After he had sufficiently tested his instruments, and assured himself of their ability to register even the faintest sounds sharply and distinctly, Henry shifted his coils and condensers again and began to listen in for messages of less than three hundred meters' wave length. Instantly the room that had hummed with voices grew silent as a cave. No message, no vibrations, no whisper of sound came to his waiting ears. For three hours he sat, continually shifting his coils, but he heard nothing. As well might he have sat three hours by a rock, waiting for it to speak. And well he knew that this was only the first of many long weary watches that would be kept ere the voice they looked for would come out of the air.

Vividly Henry recalled the long vigils at Camp Brady, when he sat for many hours at a time listening for the call of the dynamiters. He remembered how irksome that had been. He remembered the chill of the night and the silence of the great forest. Here the watchers would be more comfortable, but the vigil was likely to be as tedious and trying as their watch in the Pennsylvania mountains had proved. But that watch had been rewarded. The dynamiters had been located and captured. And Henry never doubted that this vigil, too, would meet with success. So he schooled himself to patience and keyed his ear and his instrument to the keenest pitch.

Meantime his companions had lost not a moment in beginning their study of the city. When Captain Hardy emerged from the wireless room, he ran his eye over the contents of the bookshelves; and one section he discovered was filled with maps and guide-books and local histories, not only of New York but also of other American cities. He found a large-scale map of the metropolis and spread it out on the table, true to the indicated compass points. Clustered about this outspread map, the other members of the patrol followed with eager eyes and retentive minds their instructor's every word.

Dr. Hardy called their attention to the contour of Manhattan Island, long and tongue-shaped, and running almost north and south. He showed them the main thoroughfares, the great arteries of north and south traffic. He traced for them the routes of subway, surface, and elevated car lines. Together they located the tunnels and the ferries. They studied the harbor and the different shipping districts, coming quickly to know where the transatlantic liners docked, where the coastwise steamers were berthed, and where tramp steamers could find safe anchorages. They examined the harbor and adjacent waterways. They studied the locations of police stations and hospitals, of passenger stations and freight depots. They noted the location of the forts. They identified the sites of the largest buildings.

When they had finished with Manhattan, they studied one by one the other boroughs—the Bronx, the boroughs east of Manhattan, Staten Island, and finally the Jersey shore, searching always for what would lend itself to spying or the use of a secret wireless. Especially they studied all that related to ships that cross the Atlantic.

Not in one evening or in one day was this accomplished, but through the long hours of many days, as one boy after another took his turn at the wireless. And between tricks at listening in or studying maps and guide-books, they roamed the streets, traveled on subway and surface and elevated trains, crossed the ferries, rode in the sightseeing motors, visited the bridges, the museums, the public buildings, and within a short time knew more about the topography and geography of the city than nine-tenths of the people who lived in it. As they became accustomed to the noise and the confusion and were able to find their way about with ease, they scraped acquaintances on every side, and soon knew a multitude of newsies, porters, policemen, truck drivers, car-conductors, and