Lady Mary wears a traveling costume called a Brunswick gown https://ru.wikipedia.org/wiki/%D0%91%D0%B0%D1%82%D0%BE%D0%BD%D0%B8,_%D0%9F%D0%BE%D0%BC%D0%BF%D0%B5%D0%BE#/media/File:Batoni_lady_mary_fox.jpg