Here's why it's significant:
* The Crime: The story revolves around the gruesome discovery of a severed thumb, found in a lathe's chuck.
* The Evidence: The thumb's presence in the lathe is crucial evidence for Sherlock Holmes. It suggests the victim was either using the machine or was murdered near it.
* The Mystery: The machine itself becomes part of the mystery, as Holmes uses it to deduce information about the victim's profession and the possible motives behind the crime.
The lathe's presence adds a unique element to the story, highlighting the industrial context of Victorian England and the potential for violence even in seemingly mundane settings.