of the wall.

"Shadow the couple that has just left here," said Byrnes. "You are under Mr. Carter's orders until dismissed by him."

Musgrave turned to Nick.

"I have no special instructions," said Nick, "except that you keep your eyes on the woman."

The officer saluted, and vanished almost as quickly as he had come in.

At half-past seven o'clock that evening Musgrave was on guard outside the flat, the address of which had been given to Nick by Jones.

An old man selling papers came along the street, calling "Extra!" in a cracked voice.

Musgrave bought a paper.

"Well," said the newsman, in Nick Carter's voice, "what have you to report?"

"From headquarters they went to an employment agency on Sixth avenue. They engaged a colored girl as a servant.

"They then came straight here, and the girl followed them. Mr. and Mrs. Jones have not been out since."

"Are you sure of that?"

"Perfectly. There is no way to get out of that house from the rear."

"How about the fire-escape?"

"There is only that one on the side which you can see. The little yard back of the house is walled in by buildings."

"So Mr. and Mrs. Jones must be inside?"

"Yes."

"And the girl?"

"She is out. She has been going on errands half a dozen times, but usually to the grocer's or the butcher's around the corner. I don't know where she has gone this time. She's been out about a quarter of an hour."

"All right. I'm going over there."

Nick changed his disguise to that in which Jones had seen him. He did it in the hall of the flat house, while waiting for the door to be opened in answer to his ring.

Jones met him on the upper landing.

"Look here," said Jones, when he recognized Nick, "isn't this going a little too far? What do you want now?"

"I would like to ask Mrs. Jones a few questions if you have no objections."

"I object very seriously."

"Will you ask her if she is willing to see me?"

"No; I won't."

"Then I shall have to use my authority."

"Don't do that. Come now, be a good fellow. Amy is sick with all this worry. She's just gone to bed. Let her alone until to-morrow."

"I will," said Nick. "Good-night."

He descended the stairs and rejoined Musgrave, who was standing in a dark place on the opposite side of the street.

"Have you seen a light in that window?" asked Nick, pointing to the flat.

"No."

"Then Jones lied to me a minute ago when he said that his wife had just gone to bed. That window is in the principal bedroom of the flat."

"There's been no light there."

"Then they've fooled you, Musgrave."

"What do you mean?"

"I mean that Mrs. Jones is out."

"It can't be possible."

"It's true. She's gone out disguised as her own servant."

"I can't believe it. Why, the girl's black as your hat."

"That's why they engaged her, in my opinion. It made the trick easier. A black face is a good disguise. But I'm going to be sure about it."

"How?"

"I'm going to see whether the colored girl is in the flat."

"How can you get in?"

"I'm going down the air shaft. The servant's room opens on that shaft. They'll have made her go in there so that her light won't show, as it would if she were in the kitchen."

Nick went to an engine-house near by, where he secured a coil of knotted rope.

He wished to make his investigations secretly, so as not to put Jones on his guard. It would not have been safe to get into the flat by the ordinary methods.

By using the fire escape of the building next door to the flat house, Nick got to the roof.

The top of the air shaft was covered with a framework, in which large panes of glass were set.

Nick removed one of them. Then he made his rope fast, and crept through the space where the glass had been.

The Jones' flat was next to the top, so Nick had a short descent.

But there was an awful stretch of empty air under him as he hung there.

The shaft went to the basement floor, about seventy feet below the level of the window which opened into the room occupied by the Jones' new servant.

He found that window readily. One glance through it was enough to satisfy him.

There sat the colored girl, reading a book. Nick's suspicions had been correct.

Naturally he did not delay very long in the air shaft. He had a hard climb to make, hand over hand, to the roof.

The instant that his eyes rested on the girl, he began the ascent.

He had gone up less than six feet when the rope suddenly gave way, and he found himself plunging downward through the shaft.