Jean-François Pilâtre de Rozier the autumn of 1784, but the attempt was not launched until after another Frenchman, Jean-Pierre Blanchard, and American companion, Dr John Jeffries, flew across the English Channel in a hydrogen gas balloon on 7 January 1785, from England to France. Despite several attempts, De Rozier and his companion, Pierre Romain, were not able to set off from Boulogne-sur-Mer until 15 June 1785. After making some progress, a change of wind direction pushed them back to land some 5 km from their starting point. The balloon suddenly deflated (without the envelope catching fire) and crashed near Wimereux in the Pas-de-Calais, from
Hot air balloon indicators would be useless) and direction. The combined mass of an average system can be calculated as follows: using a density of 0.9486 kg/m³ for dry air heated to 210 °F (99 °C). Increasing the air temperature inside the envelope makes it less dense than the surrounding (ambient) air. The balloon floats because of the buoyant force exerted on it. This force is the same force that acts on objects when they are in water and is described by Archimedes' principle. The amount of lift (or buoyancy) provided by a hot air balloon depends primarily upon the difference between the