Two captive (tethered) military balloons being manoeuvred into position as French troops attack the citadel of Hong Hoa in April, 1884, against the Chinese patriot Liu Yongfu's Black Flag army and his Vietnamese allies, as part of the Tonkin campaign. The purpose of these military captive, or tethered, balloons, was reconnaissance in order to direct gunfire etc. The balloons are restrained by one or more tethers attached to the ground so they cannot float freely. Winding the tether around the drum of steam-powered winches mounted on wagons made it possible to raise and lower the balloons as necessary. The Tonkin campaign was an armed conflict fought by the French against the Vietnamese and Chinese armies in order to occupy Tonkin, a district of Northern Vietnam, and establish a French protectorate there. Painting by an Annonite