and above it was an attic, with which we need not concern ourselves. The plan of the ground-floor only, sketched roughly, is what I here submit to the reader.
1. The Yellow Room, with its one window and its one door opening into the laboratory.
2. Laboratory, with its two large, barred windows and its doors, one serving for the vestibule, the other for The Yellow Room.
3. Vestibule, with its unbarred window and door opening into the park.
4. Lavatory.
5. Stairs leading to the attic.
6. Large and the only chimney in the pavilion, serving for the experiments of the laboratory.
The plan was drawn by Rouletabille, and I assured myself that there was not a line in it that was wanting to help to the solution of the problem then set before the police. With the lines of this plan and the description of its parts before them, my readers will know as much as Rouletabille knew when he entered the pavilion for the first time. With him they may now ask: How did the murderer escape from The Yellow Room?
Before mounting the three steps leading up to the door of the pavilion, Rouletabille stopped and asked Monsieur Darzac point blank:—
"What was the motive for the crime?"
"Speaking for myself, Monsieur, there can be no doubt on the matter," said Mademoiselle Stangerson's fiancé, greatly distressed. "The nails of the fingers, the deep scratches on the chest and throat of Mademoiselle Stangerson show that the
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