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