2108. The modes of constructing flues are various. The original practice was to build them on the naked earth, like drains or conduits; or in the solid walls of the backs and fronts of the pits, like the flues of dwelling-houses. The first improvement seems to have been that of detaching them from the soil by building them on flag-stones, or tiles supported by bricks; and the next was, probably, that of detaching them from every description of wall, and building their sides as thin as possible. A subsequent amelioration consisted in not plastering them within, but in making their joints perfect by lime-putty, by which means the bricks were left to exert their full influence in giving out the heat of the smoke to the house.