at it. Become acquainted with the water-front, the piers, the surface cars, the subways, the ferries. Learn the city so that you can get around rapidly. Make the acquaintance of as many policemen, wireless operators, secret service men, and other persons as you can. Don't forget that a kind deed or a thoughtful act will help you to make friendships quicker than anything else; and make all the friends you can. In police work you never know who will be of assistance to you. And above all things don't talk. Don't tell a living soul about your purpose or your plans. Let Captain Hardy do that if it is necessary. Secrecy is absolutely essential to the success of your work. Unless you can get along without betraying yourselves you may as well go right back home. Remember the spies you are after are also after you. If they learn what you are, they might even take your lives."

"Commissioner Woods," said Captain Hardy, after a pause, "I have been wondering whether or not these boys should have some kind of passes that will enable them to get through the police lines. There may come times when it is of the highest importance that nothing shall interfere with them. What do you think about it?"

The Commissioner considered for a moment. "If I were sure they could be trusted with——"

"They can," interrupted Captain Hardy. "Absolutely."

"Very well then."

The Commissioner pressed a button on his desk. A clerk entered the room.

"Make out special police cards for Captain Hardy and these four lads," he said, naming the boys.

Again he turned to the young spy hunters. "The cards you are about to get," he said, "will pass you by any policeman or put you through any police line. Do not let any one know you have them and never use them unless you absolutely must. It is best that not even the police should know who you are. Be very careful not to lose your cards."

"We will make some little cloth bags," said Henry, "and carry the cards in them inside of our underclothes."

"I see that you are resourceful," smiled the Commissioner.

The clerk returned with the cards and handed them to Captain Hardy.

"Before you go," said the Commissioner, "perhaps you would like to see our wireless department and get acquainted with Sergeant Pearce who is in charge of it."

He summoned a patrolman to guide them to the wireless rooms and wished the boys success.

A few moments later Sergeant Pearce was showing them the apparatus. Two operators sat at a wonderful Marconi outfit with receivers clamped to their ears. In another room various instruments were installed here and there, the walls were covered with diagrams of wireless instruments and outfits, and lines of men were sitting at long tables with receivers at their ears. It was the police wireless school. High above the roof the aerial hung, suspended between the main dome and a smaller dome at one end of the building.

"We are going to equip every station-house with wireless," said Sergeant Pearce, "and the men you saw at work in the school are being trained for operators. We have put wireless outfits on some of the patrol-wagons and on the police boat Patrol, so you see we can get into touch instantly with any precinct or with the Patrol no matter in what part of the harbor she may be. And when you have as big a harbor as we have, with several hundred miles of waterfront, that means something."

From Police Headquarters the little party went directly to the Post Office Building, near the Brooklyn Bridge, to see Chief Flynn. He was a large, heavy man, with black hair and eyes and a short mustache. He shook hands with each of the party, and gave each a searching look. He spoke quietly but right to the point.

"I had word from Washington about you," he said. "Do you know anything about the city?"

The boys admitted their ignorance.

"Then your first job is to get acquainted with New York. Get some maps and guide-books. While you are getting your bearings you can establish a wireless watch. I have a number of outfits in different parts of the city. For the next week or two, while you are getting acquainted with the city, I want you to maintain a twenty-four-hour watch at a place I shall send you to. Divide the time among you so that some one is listening in all the time. Here are the call signals of all the legitimate plants you will hear, either on land or water. Pay particular attention to call signals. If you catch one not in this list, be sure to get every word sent and let me hear from you at once. We have other operators listening in for messages of the usual commercial wave lengths and for very long wave lengths, so you need watch only for messages of less than three hundred meters."

He wrote an address on a slip of paper and gave it to Captain Hardy. "Go there," he directed. "A wireless outfit has been installed and accommodations await you."

He took the slip of paper from Captain Hardy and wrote some figures on it. "That," said he, "is my private telephone number. But do not bother me unless you get hold of something important."

In another moment the wireless party found itself in the rush and roar of lower Broadway.

CHAPTER V

THE MESSAGE IN CIPHER

The house to which Chief Flynn had directed the wireless patrol proved to be a private residence on a side street that ran between Central Park and the Hudson River. It was a tall house, standing two stories higher than any other structure in the block. Like most of its neighbors it had evidently seen better days. In places the brownstone front was cracked and great chips had flaked off. The broken stones in the long flight of steps that led up to the first floor were patched with colored cement that had faded so the patches stood out baldly. The brass handrail above the stone balustrade was battered and dirty. Altogether it was not a very attractive looking place.

The old lady who opened the door eyed them sharply.

"A gentleman named Flynn recommended me to your place," said Captain Hardy. "We shall need accommodations for quite a while."

"You must be the gentleman from Washington that he 'phoned me about. You are Captain Hardy?"

"I am."

"Come in," said the landlady cordially. "Any friends of Mr. Flynn's are welcome. Your rooms are ready for you. Mr. Flynn said you wanted to be together, so I have given you the entire top floor."

She led the way up one narrow stairway after another until the party reached the top floor. There she threw open the door to the front room and withdrew.

An exclamation of pleasure burst from the lips of the four boys. The shabby exterior of the house and the dim and dingy hallways through which they had come gave no hint of the cozy comfort that awaited them. The room they now entered was of generous size, with soft gray wallpaper and white woodwork. Along one side ran low, well-filled book-shelves. In the middle of the opposite wall, with fire-making materials already piled in it, was a small open grate, surmounted by an attractive mantel of white woodwork. There were a writing-table, a comfortable couch, and easy chairs. And what was most unusual for a city house, the room possessed windows on three sides—two overlooking the street and one giving a view over the housetops on either side. A door at the rear opened into a second room that was equipped as a writing room, with a broad table and several straight-backed chairs. Here, too, was an open grate set in a white mantel. In the room behind this were a number of cots. Back of all was the bath room. A snugger and more comfortable place it would have been hard to find. But nowhere was there anything that suggested a wireless outfit.

The boys looked at one another questioningly. "He said there was an outfit here," said Lew, "so there must be. But I don't see where it can be."

"It would be somewhere by itself," said Roy, "so that the operator wouldn't be