"What is it that you wish me to do?" asked Jane wonderingly. "You'll
have to tell me how to go about it."

"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