FIGURE 2 is a plan view of the chimney of FIGURE 1, with certain details omitted for clarity; FIGURE 3 is a section on the line III-III of FIGURE 1, to an enlarged scale; FIGURE 4 is a section on the line IVIV of FIG- URE 3; FIGURE 5 is a section on the line VV of FIGURE 3; FIGURE 5a shows an enlarged detail of FIGURE 5; FIGURE 6 is a vertical section, to an enlarged scale, of the upper portion of