Then the worker arose with a slight movement of impatience and walked straight up to a glass behind which the blows were struck by a hand, or by some invisible mechanism.

It was a large glass let into a panel.

Three other glasses, exactly similar to it, completed the symmetry of the apartment.

Nothing distinguished that one from the others.

Without doubt, these reiterated knocks were a signal; for, at the moment Fouquet approached the glass listening, the same noise was renewed, and in the same measure.