The operation was complicated; the visit entailed the necessity of cleaning; hence it was necessary to cleanse and at the same time, to proceed; to note the entrances of water, to count the gratings and the vents, to lay out in detail the branches, to indicate the currents at the point where they parted, to define the respective bounds of the divers basins, to sound the small sewers grafted on the principal sewer, to measure the height under the keystone of each drain, and the width, at the spring of the vaults as well as at the bottom, in order to determine the arrangements with regard to the level of each water-entrance, either of the bottom of the arch, or on the soil of the street. They advanced with toil. The lanterns pined away in the foul atmosphere. From time to time, a fainting sewerman was carried out. At certain points, there were precipices. The soil had given away, the pavement had crumbled, the sewer had changed into a bottomless well; they found nothing solid; a man disappeared suddenly; they had great difficulty in getting him out again. On the advice of Fourcroy, they lighted large cages filled with tow steeped in resin, from time to time, in spots which had been sufficiently disinfected.