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 course) stated in the previous section. Liftoff would require a slightly greater temperature, depending on the desired rate of climb. In reality, the air contained in the envelope is not all the same temperature, as the accompanying thermal image shows, and so these calculations are based on averages. For typical atmospheric conditions (20 °C, 68 °F), a hot air balloon heated to (99 °C, 210 °F) requires about 3.91 m³ of envelope volume to lift 1 kilogram (62.5 ft³/lb). The precise amount of lift provided depends not only upon the internal temperature mentioned above, but the external temperature, altitude above