In 1858, he became the first person to take aerial photographs. This was done using the wet plate collodion process, and since the plates had to be prepared and developed (a process that required a chemically neutral setting) while the basket was aloft, Nadar experienced imaging problems as gas escaped from his balloons. After Nadar invented a gas-proof cotton cover and draped it over his balloon baskets, he was able to capture stable images. He also pioneered the use of artificial lighting in photography, working in the catacombs of Paris. He was thus the first person to photograph from the air with his balloons, as well as the first to photograph underground, in the Catacombs of Paris. In 1867, he published the first magazine to focus on air travel: "L'Aéronaute". In 1863, Nadar commissioned the prominent balloonist Eugène Godard to construct an enormous balloon, high and with a capacity of , and named "Le Géant" (The Giant). On his visit to Brussels with "Le Géant", on 26 September 1864, Nadar erected mobile barriers to keep the crowd at a safe distance. Crowd control barriers are still known in Belgium as "Nadar barriers". "Le Géant" was badly damaged at the end of its second flight, but Nadar rebuilt the gondola and the envelope, and continued his flights. In 1867, he was able to take as many as a dozen passengers aloft at once, serving cold chicken and wine. "Le Géant" (The Giant), inspired Jules Verne's "Five Weeks in a Balloon". Nadar was the inspiration for the character of Michael Ardan in Verne's "From the Earth to the Moon". In 1862, Verne and Nadar established a "Société pour la recherche de la navigation aérienne", which later became "La Société d'encouragement de la locomotion aérienne au moyen du plus lourd que l'air" (The Society for the Encouragement of Aerial Locomotion by Means of Heavier than Air Machines). Nadar served as president and Verne as secretary. During the Siege of Paris in 1870–71, Nadar was instrumental in organising balloon flights carrying mail to reconnect the besieged Parisians with the rest of the world, thus establishing the world's first airmail service. In April 1874, he lent his photo studio to a group of painters to present the first exhibition of the Impressionists.