The first manned flight by a lighter-than-air contraption was that of a hot air balloon at the Bois de Boulogne, Paris on 21st November 1783. It was made by Joseph-Michel and Jacques-Étienne Montgolfier. The envelope was made from paper-lined linen. In 1766 Sir Henry Cavendish had discovered the light gas hydrogen and the Montgolfier brothers’ success revived interest in the possibility of using hydrogen for “lift”. But hydrogen was able to permeate most envelope materials until brothers Anne-Jean and Marie-Noel Robert revealed their formula for rubberised silk which they had used in their illegal contraceptives business. The first manned flight using hydrogen took place 10 days after Montgolfier’s first flight. The balloon age immediately launched the quest for a form of transport that was less dependent on wind direction and speed and more amenable to going where it was needed to go. Whilst balloons had their uses (military reconnaissance in "captive" form; escape from the 1870-71 Siege of Paris when 66 balloons carried 168 people out over the heads of the investing Prussin armies) their inability to be directed limited their use almost completely. “Dirigible” means “Capable of being directed or guided” and the process of achieving this in the air required a combination of envelope construction and mechanical power. The use of balloons at the Siege of Paris led the French government later to establish the first official aeronautical research facility in the world at Chalais-Meudon near Paris. The Giffard demonstrated the potential of dirigible flight in still air but it was the Lebaudy's "La Jaune" that paved the way for the short age of dirigible airships in which the Zeppelin was to become the most important player.