Cast-iron boilers as commonly used for heating dwellings are made in two definite styles. The small sizes are cylindrical in form and are used for either steam or hot-water heating. The larger sizes are made as illustrated in Figs. 16 and 17, the former being an outside view, and the latter showing the internal arrangement of the same boiler. The fire-box, water space and smoke passages are easily recognized. Each division represents a separate section which assembled as that in the figures makes a complete boiler with a common opening as shown at the top of Fig. 17. These boilers are used for residences of large size and for buildings of less than 10,000 feet of radiating surface. For large buildings, the steam is most commonly generated in boilers built for high pressure. In small plants, intended for either steam or hot-water heating, the cylindrical style of boiler shown in Fig. 18 is commonly used. As constructed by different manufacturers, the parts differ quite materially but Fig. 18 shows all of the essential features and serves to illustrate the different working parts. The sections into which the boiler is divided are indicated on the left-hand side of the figure by the numbers 1 to 6. The parts from 1 to 5 are screwed together with threaded nipples, joining the central column. The part 6 contains the grate and the ash-pit, with the draft and clean-out doors. [Illustration: FIG. 16. FIG. 17. FIG. 16.--Sectional cast-iron boiler for steam or hot-water heating. FIG. 17.--Interior view of the boiler shown in Fig. 16.] The drawing shows the boiler cut through the middle lengthwise and exposes to view all of the essential features. The fire-box and the spaces occupied by the steam and water are easily recognized. It will be seen that the water space surrounds the fire-box except at the bottom and that the space above the fire-box presents a large amount of heating surface to the flame and heated gases as they pass to the chimney. The arrows show their course; first through the openings near the center, then through those further away. The object being to keep the heat as long as possible in contact with the heating surfaces without interfering with the draft. [Illustration: FIG. 18.--Sectional view of the cylindrical type of cast-iron, sectional boiler.]