Engraving showing a hot-air balloon being filled. Hot-air balloons are based on the simple scientific principle that hot air rises. Hot air is lighter than cool air because it has les mas per unit of volume. Two French paper makers, Joseph Michel and Jacques-Etienne Montgolfier, were amongst the first to succesfully exploit this idea, experimenting by inflating paper balloons using the hot air produced by straw fires. However, even they were unaware what made the balloon rise, crediting this miracle to the discovery of an unknown gas, the product of burning wool and straw, which became known as 'Montgolfier's gas'. On 21 November 1783, the Montgolfiers staged the first manned free flight in a hot-air balloon from the Tuilleries Gardens in Paris, France.
© Science Museum / Science & Society Picture Library