in some cases parallel with the surfaces, and in other cases at right angles.

The Field tube is used extensively for the purpose of increasing the heating-surface; in its original form it consisted of a tube with a closed end projecting downward and expanded into the boiler-shell; into this extended another tube which did not reach quite to the bottom, and was held in position by an internal perforated support, as shown in Fig. 126. This is used in heating-boilers with various modifications both projecting downward and horizontally. When used projecting downward, it is termed a drop-tube, and is supplied either with an internal tube, as shown, or a partition; when used horizontally the internal tube is frequently supplied from a compartment separated from that to which the external tube is. attached. Fig. 127 illustrates a type of heating-boiler which is quite extensively used for both hot water and steam, and is built by different manufacturers, either of steel or cast iron. The heater consists of a cylindrical drum, the lower surface of

FIG. 127.-DROP-TUBE SURFACE. FIG. 128.-DROP-TUBE AND COIL-HEATER. which is covered with tubes of the type described which project downward. The tubes directly over the fire and over the fire door are short, while those around the fire are sufficiently long to form the external walls of the heater. The return water is received in one of the long pipes near the bottom of the heater, and the steam or heated water is taken off at the top. The drum in one of these heaters is provided with a baffle-plate connected to the diaphragm in the drop-tube, so

that the circulation must take place in a vertical direction in the tube.

Fig. 128 shows a heater in which the surface is made up partly of pipe-coils and partly of drop-tubes. The return water is received in the lower concentric drum, and as it is warmed passes to the top drum of the heater, from which it flows to the building; a type of heater in many respects similar is made without drop-tubes, the whole surface being obtained by use of pipe-coils, made either with return bends or with branch tees.

Sectional Boilers.-The greater number of cast-iron boilers are made by joining either horizontal or vertical sections. These sections are joined in some instances by a screwed nipple, in other cases by a packed or faced joint, and are held in place with bolts. The sections generally contain water and

steam, and the heated gases circulate around the sections in flues provided for that purpose. The joints in the flues are usually made tight enough to prevent the escape of smoke by the use of an asbestos or similar cement.

Horizontal Sections.--Fig. 129 represents a type of heater in which the various sections are horizontal, the surface being increased to any amount by adding sections. This form is used extensively in a number of hot-water heaters. Fig. 130 shows another form of boiler made in a similar manner, but with the sections of such form as to produce both an up and down circulation within the heater. The up circulation takes place over the hottest portion of the fire, the down circulation in special external passages which are not heated.

W. L.

Vertical Sections.-Boilers with vertical sections are made in the same manner in many respects, the sections being united by internal or external connections. When united by external connections, screwed nipples connecting the sections to outside drums, of the general form as shown in Fig. 131, are usually employed. In this case the return-water is received into horizontal drums AA, which extend the full length of the heater, and flows into the lower part of each section. The steam or hot water is drawn off from a similar drum, B, which extends over the top of the heater and is connected with each section by a screwed nipple. Fig. 130 shows methods of attaching steam- and water gauges. This form is used quite extensively in steamheating and to some extent for hot-water heating.

FIG. 131.

83. Heating-boilers with Magazines.-Nearly all of the heating-boilers are manufactured as required with or without a magazine to hold a supply of coal. The magazine in most cases consists of a cylindrical tube opening at or near the top of the heater and ending eight to twelve inches above the grate. The magazine is filled with coal, which descends as combustion takes place at the lower end, and provides fuel for further combustion (see Fig. 121). The magazine works successfully with anthracite coal, which is that ordinarily employed in domestic heating, but it takes up useful space in the heater, decreases the effective heating surface for a given size, and in

that respect is objectionable. The writer's own experience would lead him to believe that the magazine heater, except in very small sizes, requires as much attention as the surface. burner, and consequently has no special advantage.*

84. Heating-boilers for Soft Coal. It is quite probable that no furnace, either for power or heating boilers, has yet been produced which will consume soft coal without more or less black smoke. This smoke is due principally to the imperfect combustion of the hydrocarbons contained in the coal. The hydrogen burning out after the gases have left the fire leaves solid carbon in the form of small particles, which float with and discolor the products of combustion. The amount of loss as found by experiment in Sibley College,† even when dense black smoke is produced, seldom reaches one per cent, and is of no economical importance. The sooty matter produced in the combustion of this coal is likely to adhere to the water-heating surfaces, and if these are minutely divided it will be certain to choke the passages for the gases of combustion. For the combustion of soft coal those heaters have been the most successful which have a grate with small openings, and with an area 50 to 70 per cent as large as that needed for anthracite coal, also with the heating-surface of comparatively simple form and arranged so as to be easily cleaned. It is considered important that the air-flues be so arranged as to keep the products of combustion as hot as possible. This coal is likely to swell when first heated, and cannot be fed successfully by a magazine.

*Magazine heaters have been constructed with a magazine set obliquely above and to the side of the grate, and in that position are not open to all the objections stated.

+ See Table XII, page 390.

CHAPTER VIII.

SETTINGS AND APPLIANCES-METHODS OF OPERATING BOILERS AND HEATERS.

85. Brick Settings for Boilers.-Horizontal tubular boilers and a few heating-boilers require to be set in brickwork, of which the general arrangement is shown in Fig. 132. The horizontal tubular boiler is usually supported from cast-iron flanges which are riveted to the sides of the shell, and which rest

FIG. 132. PERSPECTIVE VIEW OF TUBULAR BOILER SET IN BRICKWORK. directly on the walls of brickwork, or are supported by suspension-rods from above. In some instances the boiler-lugs rest on cast-iron columns embedded within the brickwork, and of such a length that all the brickwork above the grates can be removed without affecting the setting. In setting the boiler