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 outspoken and benevolent. Many anecdotes are told of his frankness of speech at court, and of his generous actions. He holds a well-defined place in French literature, as the first who reduced its versification to rule, and taught the value of workmanship for its own sake. His influence on English literature, through Pope and his contemporaries, was not less strong, though less durable. After much undue depreciation Boileau’s critical work has been rehabilitated by recent writers, perhaps to the extent of some exaggeration in the other direction. It has been shown that in spite of undue harshness in individual cases most of his criticisms have been substantially adopted by his successors.

 BOILER, a vessel in which water or other liquid is heated to the boiling point; specifically, the apparatus by which steam is produced from water, as one step in the process whereby the potential energy of coal or other fuel is converted into mechanical work by means of the steam-engine. Boilers of the latter kind must all possess certain essential features, whilst of other qualities that are desirable some may not be altogether compatible with the special conditions under which the boilers are to be worked. Amongst the essentials are a receptacle capable of containing the water and the steam produced by its evaporation, and strong enough continuously to withstand with safety the highest pressure of steam for which the boiler is intended. Another essential is a furnace for burning the fuel, and a further one is the provision of a sufficiency of heating surface for the transmission of the heat produced by the combustion of the fuel to the water which is required to be evaporated. Desirable qualities are that the arrangements of the furnaces should be such that a reasonably perfect combustion of the fuel should be possible, and that the heating surfaces should be capable of transmitting a large proportion of the heat produced to the water so as to obtain a high evaporative efficiency. Further, the design generally should be compact, not too heavy or costly, and such that the cleaning necessary to maintain the evaporative efficiency can be easily effected. It should also be such that the cost of upkeep will be small, and that only an average amount of skill and attention will be required under working conditions. It is for providing these qualities in different degrees according to the special requirements of various circumstances that the very different designs of the various types of boilers have been evolved.

Classes of Boilers.—Boilers generally may be divided into two distinct classes, one comprising those which are generally called “tank” boilers, containing relatively large quantities of water, and the other those which are generally called “water-tube” boilers, in which the water is mainly contained in numerous comparatively small tubes. There are, however, some types of boiler which combine to some extent the properties of both these classes. Each class has its representatives amongst both land and marine boilers. In “tank” boilers the outer shell is wholly or partially cylindrical, this form being one in which the necessary strength can be obtained without the use of a large number of stays. The boilers are generally internally fired, the furnace plates being surrounded with water and forming the most efficient portion of the heating surfaces. On leaving the furnace the products of combustion are led into a chamber and thence through flues or through numerous small tubes which serve to transmit some of the heat of combustion to the water contained in the boiler. In “water-tube” boilers the fire is usually placed under a collection of tubes containing water and forming the major portion of the heating surface of the boiler. Both the fire and the tubes are enclosed in an outer casing of brickwork or other fire-resisting substance. In some forms of water-tube boiler the fire is entirely surrounded by water-tubes and the casing is in no part exposed to the direct action of the fire. In “tank” boilers generally no difficulty is experienced in keeping all the heating surfaces in close contact with water, but in “water-tube” boilers special provision has to be made in the design for maintaining the circulation of water through the tubes. (For “flash” boilers see, and for domestic hot-water boilers .)

Tank Boilers.—Of large stationary boilers the forms most commonly used are those known as the “Lancashire” boiler, and its modification the “Galloway” boiler. These boilers are made from 26 to 30 ft. long, with diameters from 6 to 8 ft., and have two cylindrical furnace flues which in the

“Lancashire” boiler extend for its whole length (see fig. 3). The working pressure is about 60 ℔ per sq. in. in the older boilers, from 100 ℔ to 120 ℔ per sq. in. in those supplying steam to compound engines, and from 150 to 170 ℔ where triple expansion engines are used. In some cases they have been constructed for a pressure of 200 ℔ per sq. in. The furnace flues are usually made in sections from 3 to 3 ft. long. Each section consists of one plate bent into a cylindrical form, the longitudinal joint being welded, and is flanged at both ends, the various pieces being joined together by an “Adamson” joint (fig. 1.). It will be seen that these joints do not expose either rivets or double thickness of plate to the action of the fire; they further serve as stiffening rings to prevent collapse of the flue. In most of these boilers the heating surface is increased by fitting in the furnace flues a number of “Galloway” tubes. These are conical tubes, made with a flange at each end, by means of which they are connected to the furnace plate. They are so proportioned that the diameter of the large end of the tube is slightly greater than that of the flange of the small end; this enables them to be readily removed and replaced if necessary. These tubes not only add to the heating surface, but they stiffen the flue, promote circulation of the water in the boiler, and by mixing up the flue gases improve the evaporative efficiency.

In the “Galloway” boiler the two furnaces extend only for about 9 or 10 ft. into the boiler, and lead into a large chamber or flue in which a number of “Galloway” tubes are fitted, and which extends from the furnace end to the end of the boiler. A cross section of this flue showing the distribution of the Galloway tubes is shown in fig. 2. When boilers less than about 6 ft. in diameter are needed, a somewhat similar type to the Lancashire boiler is used containing only one furnace. This is called a “Cornish” boiler.

2.—Galloway Boiler: Section beyond the Bridge.

In all three types of boiler the brickwork is constructed to form one central flue passing along the bottom of the boiler and two side flues extending up the side nearly to the water-level. A cross section of the brickwork is shown in fig. 2. The usual arrangement is for the flue gases to be divided as they leave the internal flue; one-half returns along each side flue to the front of the boiler, and the whole then passes downwards into the central flue, travelling under the bottom of the boiler until the gases again reach the back end, where they pass into the chimney. In a few cases the arrangement is reversed, the gases first passing along the bottom flue and returning along the side flues. This latter arrangement, whilst promoting a more rapid circulation of water, has the disadvantage of requiring two dampers, and it is not suitable for those cases in which heavy deposits form on the bottoms of the boilers.

Where floor space is limited and also for small installations, other forms of cylindrical boilers are used, most of them being of the vertical type. That most commonly used is the simple vertical boiler, with a plain vertical fire-box, and an internal smoke stack traversing the steam space. The fire-box is made slightly

tapering in diameter, the space between it and the shell being filled with water. In all but the small sizes cross tubes are generally fitted. These are made about 9 in. in diameter of -in. plate flanged at each end to enable them to be riveted to the fire-box plates. They are usually fitted with a slight inclination to facilitate water circulation. 