330 STEAM BOILER used by George Stephenson contemporaneous- ly, in 1828 or 1829. Fig. 3 is a common form of marine steam boiler. It is used very exten- sively in the United States when the steam pressure does not exceed 40 or 45 Ibs. to the square inch. The gases, leaving the fur- nace F, pass to the " back connection " through large flues; there turning, as FIG. 2. Locomotive Boiler. shown by the arrow, they return to the front through the tubes, and from the "front con- nection " rise into the chimney. Large steam space is secured in this boiler by a steam drum, as in the locomotive boiler; but here the chimney passes through the steam drum, and thus this "steam chimney," as it is called, is made more useful in drying the steam and in economizing heat. The circular shell, the well stayed surfaces of the fire box and the ends, the convenient distribution of parts, and their excellent relative proportions, make this one of the most satisfactory of all types of boilers which are suitable for moderate pres- sure. Fig. 4 is a type of marine tubular boiler which is in most extensive use in sea- going steamers for moderate pressure, and particularly for naval vessels. Here the gases pass directly into the back connection from Fio. 8. Marine Flue and Tube Boiler. the fire, and thence forward again, through horizontal tubes, to the front connection and up the chimney. In naval vessels the steam chimney is omitted, as it is there necessary to keep all parts of the boiler as far below the water line as possible. Steam is taken from the boiler by pipes which are carried from end to end of the steam space, near the top of the boiler, the steam entering these pipes through small holes drilled on the upper side. Steam is thus taken from the boiler " wet," but no large quantity of water can usually be "en- trained" by the steam. A marine boiler has been quite extensively introduced into the Uni- ted States navy, in which the gases are led from the back con- nection through a tube box around and among a set of upright water tubes, which are filled with water, circula- tion taking place free- ly from the water space immediately above the crown sheet of the furnace up through these tubes into the water space above them. These " water-tubular " boilers have a slight advan- tage over the "fire-tubular" boilers already described in compactness, in steaming capaci- ty, and in economical efficiency. They have a very marked advantage in the facility with which the tubes may be scraped, or freed from the deposit when a scale of sulphate of FIG. 4. Marine Fire-tubular Boiler. lime or other salt has formed within them by precipitation from the water. The fire-tubu- lar boiler excels in convenience of access for plugging up leaking tubes, and is much less costly than the water-tubular. The water-tube class of boilers still remain in extensive use in the United States naval steamers. They have never been much used in the merchant service, although introduced by Montgomery in the United States and by Lord Dundonald in Great Britain twenty years ago. Opinion still remains divided among engineers in re- gard to their relative value. They are grad- ually reassuming prominence by their intro-
