Vanished from history is the story of the ‘Mariner's Calculator’, invented and patented at the Great Seal Patent Office, London, by Mrs Janet Taylor, in 1834. Dismissed by the Admiralty, it had no commercial future and only one instrument is known to remain in existence. The article traces the invention from its inception and provides relevant biographical details of its inventor. The authors then analyse the assessment by the Admiralty to determine if it was fair and outline the endeavour in 2004 to reassess the achievement by a reconstruction of the Mariner's Calculator from its original patent.