 In June 1792 the French astronomers Jean Baptiste Joseph Delambre and Pierre François André Méchain set out to measure the meridian arc distance from Dunkirk to Barcelona, two cities lying on approximately the same longitude as each other and also the longitude through Paris.The fortress on Montjuïc was chosen as the reference point in Barcelona.After protracted negotiations (France and Spain were technically at war) Méchain made his measurements from the fortress on 16 March 1794.