An observation balloon is a type of balloon that is employed as an aerial platform for intelligence gathering and artillery spotting. Use of observation balloons began during the French Revolutionary Wars, reaching their zenith during World War I, and they continue in limited use today …
British observation balloon from 1908, typical of pre-WWI observation balloons
The battle of Fleurus, 26 June 1794, saw the first military use of an aircraft (L'Entreprenant).
German Parseval-Siegsfeld type balloon at Équancourt (September 1916). The rear "tail" fills with air automatically through an opening facing the wind.
Caquot type kite balloon, used by the Allies in the mid-latter part of WWI