"On the horizon float the bright yellow, sunlit observation-balloons"

French WW1 Observation Balloon
Public DomainFrench WW1 Observation Balloon - Credit: Ray Williams
Observation balloons were used by both Allied and German forces to spot artillery fire and gather intelligence. They were generally stationed a few miles behind the front lines. The balloons were made of fabric and were filled with hydrogen. It was not unusual for soldiers manning these balloons to have to parachute out to escape the possibility of an explosion.