"Let me make one suggestion to you," said the Commissioner, turning again to the boys. "Before you attempt to do any detective work make yourselves familiar with the city. Get some maps and study them until you know every street and alley. Take your maps and go over the city on foot. Put several days in 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.