five o’clock
`` If I did not reason as I do in regard to this gravel , '' he went on , `` I should have to assume a balloon ! -- My dear fellow , the science of the aerostation of dirigible balloons is not yet developed enough for me to consider it and suppose that a murderer would drop from the clouds ! So do n't say a thing is possible , when it could not be otherwise . We know now how the man entered by the window , and we also know the moment at which he entered , -- during the five o'clock walk of the professor and his daughter . The fact of the presence of the chambermaid -- who had come to clean up `` The Yellow Room '' -- in the laboratory , when Monsieur Stangerson and his daughter returned from their walk , at half-past one , permits us to affirm that at half-past one the murderer was not in the chamber under the bed , unless he was in collusion with the chambermaid . What do you say , Monsieur Darzac ? '' Monsieur Darzac shook his head and said he was sure of the chambermaid 's fidelity , and that she was a thoroughly honest and devoted servant .