The Mischief Act 1765 (5 Geo III c.39), formally titled "An Act for more effectually preventing the mischief arising to the revenue and commerce of Great Britain and Ireland from the illicit and clandestine trade to and from the Isle of Man", authorised Crown customs officers to search ships in Manx harbours and make seizures on land or water. Introduced in January 1765 as a threat to compel the Duke of Atholl to negotiate, it received Royal Assent on 15 May, five days after the Revestment Act.