The timing of this story from the Wessex Tales (1888) depends upon the date of the hanging. "The Withered Arm" begins about six years before when Rhoda's son — we never learn his Christian name — is "twelve or thereabout" (in April) according to Part I. He is "only just turned eighteen" at the time of his hanging (July) as the hangman Davies tells Gertrude during their conversation in the cottage in Part VIII. Also "Half a dozen years passed away..." (Part VI opening). At the end of Part VI Conjuror Trendle says (the winter before the — first — March assizes): "The last I sent was in '13 — near twelve years ago." Probably on this basis Kathryn R. King dates the story from 1818 to 1825 although she is clearly well-versed on the 1830s unrest [offsite link].
Claire Tomalin's biography of Hardy mentions that an account was "told to [Hardy] by [Hardy Senior] which must have gone back to the troubled times of the 1830s. [Hardy Senior] said he had seen four men hanged only for being with some others who set fire to a rick" (p. 22). Her statement is based on the record of an interview with Hardy in his seventies (1910-20). Tomalin must have the Swing Riots in mind — see the historyhome site.
Hardy Senior was born in 1811 making him 14 in 1825 and 19 in 1830. His recollection of the event seems to have made a deep impression on his son who, by the time he was writing, was living in an England where capital punishment had been abolished for all but murder, treason, espionage, arson in royal dockyards and piracy with violence. Public hangings (mainly for murder) continued until 1868. Hardy witnessed two in the 1860s and the injustice of the hanging of the young men described by Hardy Senior must have left a deep impression on Hardy — the sort of thing he could save up for one of his very best tales.
But for Trendle's reference to dates there would be a case for fixing 1830 for the hanging, since the level of unrest in 1830 fits in with Davies' remark; ("...there having been so much destruction of property that way lately.") On the other hand it would be surprising if the level of agrarian unrest in 1830 had not had an impact on the lives of Gertrude and her husband Farmer Lodge and perhaps factored into their travel arrangements — Gertrude, a timid traveller at best, is not beset by any concerns in her second visit to Trendle or journey to the assizes. In the scenes in Casterbridge there is no suggestion that the locals — always hungry for news and gossip — are in the grip of riots. I should expect Hardy to have had a clear sense of the date when the incident his father described took place. It is the sort of attention to detail that Hardy gave from an early age. Therefore (1) perhaps Hardy Senior was describing events in 1825 or some other (specified or unspecified) time after all or (2) Hardy realised Hardy senior's description related to 1830 but chose to time "The Withered Arm" so as to avoid the complications of the 1830s unrest. A multiple hanging would have resulted in a rather crowded prison morgue and diluted the climactic focus on Rhoda's son.
Tomlin, Claire. Thomas Hardy, The Time-Torn Man. Penguin Press: 2006.
Last modified 6 January 2009