The Slaves of Paris by Emile Gaboriau

The writer of the anonymous communication had only known the secret too well, for the Duchess de Champdoce was awaiting a visit that evening from George de Croisenois; this was, however, the first time. Step by step she had yielded, and at length had fallen into the snare laid for her by the treacherous woman whom she believed to be her truest friend. The evening before this eventful night she had been alone in Madame de Mussidan's drawing-room with George de Croisenois. She had been impressed by his ardent passion, and had listened with pleasure to his loving entreaties.

"I yield," said she. "Come tomorrow night, at half-past ten, to the little door in the garden wall; it will only be kept closed by a stone being placed against it inside; push it, and it will open; and when you have entered the garden, acquaint me with your presence by clapping yours hands gently once or twice."