m, categorically, at this
moment that I do know him. The mathematical idea I have of the murderer
gives results so frightful, so monstrous, that I hope it is still
possible that I am mistaken. I hope so, with all my heart!"

"Five minutes ago, you did not know the murderer; how can you say that
you expect him this evening?"

"Because I know that he must come."

Rouletabille very slowly filled his pipe and lit it. That meant an
interesting story. At that moment we heard some one walking in the
gallery and passing before our door. Rouletabille listened. The sound of
the footstep died away in the distance.

"Is Frederic Larsan in his room?" I asked, pointing to the partition.

"No," my friend answered. "He went to Paris this morning,--still on
the scent of Darzac, who also left for Paris. That matter will turn out
badly. I expect that Monsieur Darzac will be arrested in the course of
the next week. The worst of it is that everything seems to be in league
against him,--circumstances, things, people. Not an hour passes without
bringing some new evidence against him. The examining magistrate is
overwhelmed by it--and blind."

"Frederic Larsan, however, is not a novice," I said.

"I thought so," said Rouletabille, with a slightly contemptuous turn
of his lips, "I fancied he was a much abler man. I had, indeed, a great
admiration for him, before I got to know his method of working. It's
deplorable. He owes his reputation solely to his ability; but he lacks
reasoning power,--the mathematics of his ideas are very poor."

I looked closely at Rouletabille and could not help smiling, on hearing
this boy of eighteen talking of a man who had proved to the world that
he was the finest police sleuth in Europe.

"You smile," he said? "you are wrong! I swear I will outwit him--and in
a striking way! But I must make haste about it, for he has an enormous
start on me--given him by Monsieur Robert Darzac, who is this evening
going to increase it still more. Think of it!--every time the murderer
comes to the chateau, Monsieur Darzac, by a strange fatality, absents
himself and refuses to give any account of how he employs his time."

"Every time the assassin comes to the chateau!" I cried. "Has he
returned then--?"

"Yes, during that famous night when the strange phenomenon occurred."

I was now going to learn about the astonishing phenomenon to which
Rouletabille had made allusion half an hour earlier without giving me
any expla