"artillery observation balloon" . "observation balloons" . "reconnaissance balloon" . "Observation balloon" . "lighter-than-air branch of aviation" . "balloon busting" . "observation" . "Observation balloon" . "observation balloon hangar" . "Kite Balloon" . . . . "balloon observer" . "balloons" . "manned balloons" . . "observation balloon" . "balloonists" . "Balloon" . . "kite" . "reconnaissance balloons" . "observer kite balloons" . . "kite balloons" . "balloons for observation" . "balloon unit" . "kite balloon" . . . "an observation balloon" . "balloon" .