“And even learnt his name, Chief:  Hubert Lautier, of the Avenue du Roule.  Only he decamped from there six months ago, leaving his furniture behind him and taking nothing but two trunks.”

“What about the post-office?”

“We have been to the post-office.  One of the clerks recognized the description which we supplied.  Our man calls once every eight or ten days to fetch his mail, which never amounts to much:  just one or two letters.  He has not been there for some time.”

“Is the correspondence in his name?”

“No, initials.”

“Were they able to remember them?”

“Yes:  B.R.W.8.”

“Is that all?”

“That is absolutely all that I have discovered.  But one of my fellow officers succeeded in proving, from the evidence of two detectives, that a man carrying a silver-handled ebony walking-stick and a pair of tortoise-shell glasses walked out of the Gare d’Auteuil on the evening of the double murder and went toward Renelagh.  Remember the presence of Mme. Fauville in that neighbourhood at the same hour.  And remember that the crime was committed round about midnight.  I conclude from this—­”

“That will do; be off!”

“But—­”

“Get!”

“Then I don’t see you again?”

“Meet me in half an hour outside our man’s place.”

“What man?”

“Marie Fauville’s accomplice.”

“But you don’t know—­”

“The address?  Why, you gave it to me yourself:  Boulevard Richard-Wallace, No. 8.  Go!  And don’t look such a fool.”

He made him spin round on his heels, took him by the shoulders, pushed him to the door, and handed him over, quite flabbergasted, to a footman.

He himself went out a few minutes later, dragging in his wake the detectives attached to his person, left them posted on sentry duty outside a block of flats with a double entrance, and took a motor cab to Neuilly.

He went along the Avenue de Madrid on foot and turned down the Boulevard Richard-Wallace, opposite the Bois de Boulogne.  Mazeroux was waiting for him in front of a small three-storied house standing at the back of a courtyard contained within the very high walls of the adjoining property.

“Is this number eight?”

“Yes, Chief, but tell me how—­”

“One moment, old chap; give me time to recover my breath.”

He gave two or three great gasps.

“Lord, how good it is to be up and doing!” he said.  “Upon my word, I was getting rusty.  And what a pleasure to pursue those scoundrels!  So you want me to tell you?”

He passed his arm through the sergeant’s.

“Listen, Alexandre, and profit by my words.  Remember this:  when a person is choosing initials for his address at a poste restante he doesn’t pick them at random, but always in such a way that the letters convey a meaning to the person corresponding with him, a meaning which will enable that other person easily to remember the address.”