Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/240

228 HORTICULTURE [HEATING. Hot wtiter. a source of heat, should be carried along the front of the house, returning near the back ; by the time it lias run thus far most of the heat taken up from the fuel will be in course of transmission to the enclosed atmosphere of the house to be heated. There should be a gradual rise for some distance after leaving the furnace, which should be from 1 to 2 feet below the level of the front flue ; and there should be no sharp angles or turnings. Earthenware pipes may be substituted when appearance is not a consideration. Smoke flues should be cleaned out at least once a year. When properly constructed, tln-y answer their purpose sufficiently well ; but this mode of heating is now virtually superseded in all gardens of note by the hot-water system. Hot Water. The diffusion of heat in plant houses by causing hot water to circulate in iron pipes or vessels was brought into notice in 1827, and has gradually superseded all other modes. The apparatus is more durable than flues, occupies less space, can be placed in situations where flues cnnnot, is more elegant in appear ance, gives out a more steady and uniform heat, and can seldom be overheated. The true cause of the circulation of the water in the pipes may be explained by Jig. 29. When the water in the boiler FIG. 29. Diagram of Tank Boiler showing Circulation in Pipes. a is heated up, it expands and so becomes lighter than that in the cistern b ; the pressure at z in the horizontal pipe d is thus less than at y, so that the water flows through d from b to a, and b is kept supplied from the pipe c. In this arrangement, which represents the common tank boiler, with a cistern at the extremity of the pipes, the process of heating was slow, and many changes have consequently been made. The The Furnace. The most perfect furnace is that in which the furnace, combustion is most complete. On this account the tire should be surrounded by slow conductors of heat, such as Welsh lumps or other kinds of fire-brick ; and the sides of the furnace should not be formed by any part of the boiler, nor should the furnace bars be tubular and connected with the boiler, though this latter plan is sometimes adopted to prevent the burning out of the bars. All the air necessary for maintaining combustion should enter from below, that is, through the ash-pit, and not through the door or sides of the furnace ; but to produce this effect the furnace should be fitted with double doors and ash-pit registers, for thus only can Fio. 30. Sliding Furnace Doors. the stoker have control over his furnace. By shutting the ash pit and furnace door closely combustion is lessened, and the fire may be kept almost in a state of quiescence for many hours together ; while, on opening either the door or ash-pit register, air is admitted and combustion goes on. Sylvester s furnace doors or doors of .similar sliding form (fig. 30) are preferable to the ordinary hinged doors, because they can be opened to any extent, and are less liable to get out of order. These doors are faced with fire brick, and run on rollers on an iron rod, or slide by means of a ledge. Mr Tapliii recommended, in the Florist and Pomoloyist (1868, p. 32), that, to prevent the burning away of the furnace bars and the formation of clinkers, the ash-pit should be kept filled with water, and states that the bars will then last three times as long as with the ordinary dry ash-pit. The ash-pit may be built in cement for this purpose, or fitted with a cast-iron pan for the water. Hot- Water Boilers. Only a few of the principal types of the Boi very diverse kinds of boilers can be noticed here. The simpler the form and the less complicated the whole apparatus is the better. It is essential that a large area of the boiler surface should be brought within the direct action of the fire. Rogers s Conical Boiler (fig. 31), which has long been in use, is Coi very suitable for heating pits and small houses, since it is econo- boi FIG. 31. Rogers s Conical Boiler. I mical of fuel, and gives out a steady heat for a long time 15 to 20 , hours. It is formed of two truncated concentric cones, with a, space of 2 or 3 inches between them for the water, the furnace i being in the inner cone, and the fuel supplied from the top. It was originally surrounded with brickwork, but several improve ments have been introduced. In fig. 31 the boiler is placed in a cast-iron stand, with ground circular furnace, and register ash-pit , doors, a being the furnace, b the boiler, c flow and return pipes, i d the furnace door, e smoke-pipe to the vent, /ash-pit, g grating, gas cinders, and anthracite ; but common coal which does not cake very much is also suitable, as it is soon formed into coke. Saddle Boilers. The saddle boiler is a very efficient form, steady Sac and sure in its working. In its improved forms it is, perhaps, the boi best for general purposes. It should be set over a furnace, the sides and end of which should be of fire-brick. It may be quite plain, or with the inner surface ribbed or corrugated (fig. 32), a modification adopted by Mr Gray of Chelsea. In all saddle boilers, Fio. 32. Gray s Saddle Boiler. which should be deep from front to back, and set well back from the doors of the furnace, the full force of the fire strikes the dome of the boiler directly, and so long as the fire continues this is the part most directly influenced by its heat. The flow pipe is shown at a, and the return pipe at b, while c represents an opening (one on each side) for the passage of the smoke into the flues, the end being a water-way, and forming part of the boiler. The boiler is emptied by a small tap affixed to the pipe shown in the front. There are many forms of what are called Flued Saddle Boilers, all of them good and thoroughly efficient if properly set. One of
 * h hole for cleaning the furnace. The best kinds of fuel are coke,