Large airtight bag filled with hot air or a lighter-than-air gas such as helium or hydrogen that can rise and float in the atmosphere. Experimental attempts may have begun by 1709, but not until 1783 did J.-M. and J.-É. Montgolfier develop a fabric-bag balloon that would rise when filled with hot air. Balloons provided military aerial observation sites in the 19th century and were used in the 20th century by scientists such as Auguste Piccard to gather high-altitude data. The first round-the-world balloon flight was achieved in 1999 by Bertrand Piccard and Brian Jones. See alsoairship.