“All a good detective needs,” said Mr. Fleck, “is, let us say, three things—­observation, addition and common sense.  You must observe everything closely, be able to put two and two together and use your common sense.  Do you know the Hoffs by sight?”

“Only by sight.”

“They live in the next apartment on your floor, do they not?”

“Yes.  Young Mr. Hoff’s bedroom is the room next to mine.”

“Good,” cried Mr. Fleck.  “Can you hear anything from the next apartment, any conversations?”

“No, only muffled sounds.”

“The windows overlook the river and the transports, do they not?”

“Yes, the windows of Mr. Hoff’s bedroom and the room next.  Their apartment is a duplicate of ours.”

Mr. Fleck sprang up and crossed to the big safe.  Opening an inner drawer he took out a small metal disk and handed it to her.  Jane looked at it curiously.  It bore no wording save the inscription “K-19.”

“That,” said Mr. Fleck, “is the only thing I can give you in the way of credentials.  Keep it somewhere safely concealed about your clothing and never exhibit it except in case of extreme necessity.  If ever you are in peril any police officer will recognize it at once and will promptly give you all the assistance possible.”

“But,” protested the girl, “I don’t know yet what I am to do.”

“For the present I am trusting to your resourcefulness to make opportunities to help us.  We are watching the house closely from the outside.  Carter will identify you to the other operatives.  Once a day I will expect you to call me up, not from your home but from a public ’phone.  Here is my number.  Say ‘this is Miss Jones speaking,’ and I will know who it is.  I can communicate with you by note without arousing suspicion?”

“Oh, yes, certainly.”

“If at any time I have to call you on the ’phone, or if any of the other operatives want to communicate with you the password will be ’I am speaking for Miss Jones.’”

“Isn’t that exciting—­a secret password,” cried Jane enthusiastically.

“If you can manage it without compromising yourself too seriously, I wish you would make the young man’s acquaintance.”

“That will be simple,” said Jane, remembering the admiring way in which he had raised his cup in her direction as she left the hotel.

“If possible find out who their visitors are in the apartment and keep your eyes open for any sort of signalling to the transports.  If ever there is an opportunity to get hold of notes or mail delivered to either of them, don’t hesitate to steam it open and copy it.”

“Must I?” said Jane.  “That hardly seems right or fair.”

“Of course it’s right,” cried Mr. Fleck warmly.  “Think of the lives of our soldiers that are at stake.  The devilish ingenuity of these German spies must be thwarted at all costs.  They seem to be able to discover every detail of our plans.  Only two days ago one of our transports was thoroughly inspected from stem to stern.  Two hours later twenty-six hundred soldiers were put aboard her on their way to France.  Just by accident, as they were about to sail, a time-bomb was discovered in the coal bunkers, a bomb that would have sent them all to kingdom come.”