connected by curved tubes, and also a series of down-circulating tubes connecting the same drums, but not exposed to the heat of the fire. In the Stirling boiler, the feed-water is intro

duced in the top drums; in the Hogan boiler, into a special heater and purifier arranged as a part of the downward circulation.

The Harrison boiler consists of an aggregation of spheres of cast iron or steel connected by necks, forming what is to be considered rather as a sectional, than a water-tube boiler. These

spheres are held in place by bolts, which will stretch and act as safety-valves in case of excessive pressure.

In addition to the water-tube boilers for power purposes which have been mentioned here, there are many others which cannot be described in the space at our command, but of which we may name the National, Campbell & Zell, and the Caldwell as worthy of notice.

All the water-tube boilers are provided with mud-drums, which are usually cast-iron cylinders removed from the circu

FIG. 120-HEINE BOILER.

lation and intended to receive any deposits of scale or material which is loosened in the process of circulation.

81. Hot-water Heaters.-Hot-water heaters differ essentially from steam-boilers, principally in the omission of a reservoir or space for steam above the heating surface. The steam-boiler might answer as a heater for hot water, but the large capacity left for the steam would tend to make its operation. slow and quite unsatisfactory.

The passages in a hot-water heater need not extend so directly from bottom to top as in a steam-heater, since the problem of providing for the early liberation of the steambubbles does not have to be considered. In general, the heat from

HOT-WATER HEATER.

the furnace should strike the surfaces FIG. 121.-VERTICAL MAGAZINE in such a manner as to increase the natural circulation, and not act to produce a backward circulation. This may be accomplished in a certain measure by ar

ranging the heating-surface so that a large proportion of the direct heat will be absorbed near the top of the heater.

There is a great difference of opinion as to the relative merits of horizontal and vertical heating-surfaces for this purpose, but the writer cannot find that any experiments have been made which satisfactorily decide this question. Where the surface is very much divided, and the fire is maintained at a high temperature, considerable steam is likely to be formed, and this always acts in a certain measure to increase circulation in the heating-pipes and diminish it in the heater; it is likely also to produce a disagreeable crackling noise.

Practically, the boilers for low-pressure steam and for hot water differ from each other very little as to the character of the heating-surface, and in describing the general classes which are in use no attempt will be made to make any distinction as to whether the apparatus will be used for hot-water or steam heating. If designed for steam-heating, a reservoir or chamber connected with the circulating system is in every case provided, containing water in its lower part and considerable steam capacity above the water-line, also sufficient area of water-surface to permit the separation of the steam from the water without noise and violent ebullition.

82. Classes of Heating-boilers and Hot-water Heaters -Plain-surface Boilers.-There are probably no boilers or heaters built at the present time with a plain surface, either spherical or cylindrical, since the expense of a given amount of surface in that form would practically preclude its use.

Extended-surface Heaters (Figs. 122 and 123).-Heaters of this class with extended and irregular surface, are used quite extensively in hot-water heating, and with the addition of domes are used to some extent in steam-heating. In these heaters the water is received at the lowest point, as at A, and is heated as it gradually rises, receiving the effect of the fire at various projections, and is finally discharged at B. The grate is at G, the smoke being discharged at S. The smoke. and heated gases move in nearly a direct line in Fig. 122, and in a sinuous course in Fig. 123.

A form which is in extensive use, and in which water and smoke are each grouped in one body, is shown in

Fig. 124. In this case the extended surface is produced by the wedge-shaped hollow prisms extending over the fire-space. The heated gases have a return circulation around the lower portion of the heater, and also come in contact with a top dome from which the heated water is drawn off.

Heaters belonging to the extended-surface class made with vertical cylinders, into which are connected either straight horizontal tubes with closed end, as shown on the right-hand side of Fig. 125, or U-shaped projections of pipe either horizontal or slightly inclined, are in use for both water- and steam-heat

ing. In case they are used for steam-heating the water-line is carried at sufficient distance from the top of the cylinder to give the required steam-space, and the heater is supplied with both pressure- and water-gauges. The heated gases pass around the cylindrical part of the boiler and may be made to circulate among the projections by means of baffle-plates.

Tubular Boilers.-Heating-boilers with fire-tubes and with a steel shell similar in construction to the horizontal and vertical tubular boiler described in Articles 76 and 78, are in use for heating to considerable extent in the forms already described. Modifications of these, with return flues arranged so that the heat passes both upward and downward, and also with two or more short cylindrical shells connected together by tubes filled with water, are in extensive use. Very few horizontal tubular boilers, or boilers of the locomotive type, are used for the heating of small buildings.

Water-tube Boilers.-Water-tube boilers of all classes and various modifications are in extensive use for heating. The tubes are made of either cast-iron or wrought-iron pipe. The pipe-boilers which are in the market are arranged with nearly

every form of heating-surface; some are built with heatingsurface in the form of the pipe-coil, as shown in Fig. 92, page 108, and others in the form of a manifold coil, as shown in Fig. 93, page 108. Still other boilers have the pipe arranged in the form of a spiral connecting with a receiving-drum below and a steam-drum above. The heated gases are arranged to move