Image: National Museum of Science and Industry
Early hot-air balloons had to be fueled on the ground (or carry dangerous open fires aloft) and, rather inconveniently, they came back down when they cooled. So they were soon replaced by envelopes filled with buoyant gases, such as hydrogen and helium. These balloons continued to rack up records for long-duration flights.
Balloons were used by the military as observation posts (and a few fairly futile attempts to drop bombs) and have a rich history of carrying scientific payloads into the upper atmosphere. The National Aeronautics and Space Administration operates a Scientific Ballooning Program that has collected data about cosmic rays in the upper atmosphere.
But when it comes to controlled long-distance flight, gas-filled balloons are also limited. As the sun heats the gas in the envelope, the balloon rises uncontrollably and so gas has to be vented; when the gas cools, the balloon sinks and pilots must drop ballast to maintain altitude. Sooner or later, they run out of gas or ballast and the journey is over.
The problem was solved in the 1960s when Edward Yost equipped a balloon with an onboard propane burner. This device allows pilots to control the balloon's buoyancy by changing the amount of heat injected into the envelope. His innovation created the present boom in sport ballooning and opened the prospect of an around-the-world flight.