5921944 A German observation balloon, 1917 circa (b/w photo) by Unknown photographer, (20th century); National Army Museum, London; (add.info.: A German observation balloon, 1917 circa. Photograph, World War One, Air Operations (1914-1918), 1917 circa. Observation balloons were used for intelligence gathering and artillery spotting. Observers would be carried in baskets or containers underneath the balloon. The balloons were usually heavily defended by anti-aircraft guns and fighter aircraft. From an album of 105 photographs, 1916-1918.); by National Army Museum ; out of copyright.