An analysis of the air of these sewers gives surprising results. The proportion of carbonic acid is somewhat greater than in the air of the streets overhead, that of ammoniacal azote is much more considerable, and that of bacteria only half as great. Consequently, not only does the personnel of this underground labyrinth traverse it constantly without danger, but visitors from the upper world find amusement in exploring it. Every fortnight, on the second and fourth Wednesdays of the month, the Préfet of the Seine, or the Chief Engineer of the _Service de l'Assainissement de Paris_, grants permits for these visits to a certain number of applicants,--the visitors are transported through the collecteurs of the Châtelet to the Place de la Concorde, under the Boulevard Sebastopol and the Rue de Rivoli, in little vehicles forming two trains, drawn each by an electric engine; then from the Concorde to the Madeleine, under the Rue Royale, in boats drawn by an electric tug. The trip takes about an hour, and can be made in either direction; the sewers are open to this invasion from Easter to the end of October, excepting in case of storms, when the water in the canals is apt to rise rapidly over the banquettes and drive the workmen to the _regards_ or places of ascent provided every fifty or a hundred mètres apart. The danger of asphyxia, which was formerly very serious, is now practically abolished, the ventilation being assured by numerous openings in the street gutters under the curb-stones, which are kept free from floating materials and obstructions by a special corps of égoutiers.