The Problem of the Old Gristmill

"The Problem of the Old Gristmill" (1975) is a mystery short story by Edward D. Hoch. It is the second story featuring Dr. Sam Hawthorn, and one of the few that does not contain an impossible murder, although an "impossible crime" does occur.

Plot summary
Old Dr. Sam tells the story of how everyone in town is sad to see Henry Cordwainer leave. Cordwainer, who lived in the old gristmill owned by Seth Hawkins and his mother, is a full-bearded naturalist, who has been writing a book, A Year in Snake Creek. Now that year is up and he is leaving. When he first came, he was cold and uncaring, but when winter began his whole personality changed, and he became helpful, evan helping at the ice house. Sam and Seth help Cordwainer pack up his stuff, including his lock boxes with three dozen journals in them, and Seth reveals that his mother wants him to start up the gristmill (and also that Seth does not want to start it up). Sam does not think much about this, even when he sees Seth at the local cockfights that night.

The next morning everyone learns that the gristmill has burned down, and Cordwainer has had his head bashed in. Sheriff Lens is brought in and suspicion falls on Seth -- but Seth was at the cockfight the night before. To complicate matters, Cordwainer's journals that were mailed to Boston in locked strong boxes, have disappeared. The strong boxes arrived, with a very small hole in the bottom, completely empty except for some sawdust. Sam attempts to prove Seth's innocence by injecting him with an experimental truth drug, "scopolamine". Sam proves that Seth could not have committed the murder, and begins to wonder about the sawdust that was found on the bottom of the strong boxes. When Sam works out the solution, Sam and Lens than drive to Albernathy, on Sam's urging. Here, they find the man they thought was Cordwainer, alive and well -- who is actually an escaped convict named Delos. Delos killed Cordwainer before winter, which explains the change in personality. He then pretended to be him. The journals in the strong boxes were revealed to be blocks of ice, which explains the small holes and the traces of sawdust. The gristmill was set on fire in order to thaw Cordwainer's body, which had been frozen since winter.