The first aerial crossing of the South Atlantic was made by the Portuguese naval aviators Gago Coutinho and Sacadura Cabral in which year, to mark the centennial of Brazil's independence, although the North Atlantic had already been traversed in a non-stop flight by John Alcock and Arthur Brown in 1919, Coutinho and Cabral's flight remains notable as a milestone in transatlantic aviation, and for its use of new technologies such as the artificial horizon?