Talk about mysteries: why does the British public continue to lap up books on real Victorian crime? Surely it is not simply because writers now find the necessary research material, from The Times reports to Metropolitan Police archives, easily accessible on the web? And it is not as though these stories serve some warped paternalistic social purpose, along the lines of Dr Johnson’s argument for public executions: ‘Sir,’ he declared (so we are reminded by Kate Colquhoun), ‘executions are intended to draw spectators. If they do not draw spectators, they don’t answer their purpose.’

Perhaps in this uncertain age we need a sense, even when a historical murder is solved, of the triumph of reason over wanton passion. Or perhaps it is the reverse: we are indeed still voyeurs and thrill-seekers. Probably it is a mixture of both. Or, more likely, we