BOILER. 238 BOILER. quantities of waste fuel inatorial, such as saw- dust, can be had practically without cost. Jlodcrn steam boilers may be classified ac- cordintr to their form and construction, or ae- cordinu to the character of their employment. Under~the first division we have horizontal and vertical boilers, internally and externally fired boilers, shell and' sectional boilers, tin-tulie and water-tube boilers, or boilers coiubiiiinj; two or more of these several features. Under the second division are included stationary boilers, locomo- tive boilers, marine boilers, and various forms of portable or semi-portable boilers. The variety of forms and method.s of construction of station- ary boilers are very great, but the locomotive ho'iler is practically limited to a single form, and marine boilers are usually of the drum ty)>e known as the Scotch boiler, although water- tube l)oilcrs of various forms are (■oming into use more and niore. Recently considerable at- tention has been devoted to the develoimient of a boiler for automobiles. See Automobile. The most familiar form of- boiler is, perhaps, the cylindrical tubular boiler. It consists of a cylindrical shell closed at the ends by circular plates, and has a number of tubes extending lengthwise of the boiler and terminating in the end plates. These tubes are open at both ends and through them pass the gases of combustion. The grate or furnace is placed underneath the front" end of the boiler-shell, and from it the gases pass back under the shell to its rear end, thence up and forward through the tubes to the front end. and thence out of the chimney. In this passage the hot gases give up a portion of their heat to the water surrounding the tubes inside of the shell. Technically classified, this boiler is a horizontal, externally fired, fire-tube shell boiler. Another form of horizontal shell boiler frequentlv used has the numerous fire- tubes replaced 'by two large cylindrical flvies. This type of boiler is less efficient than the tubular boiler, but its greater simplicity and its accessibility for cleaning give it advantages, especiallv where the feed-water is bad. The flue boiler has several modifications which are known by special names. The Lancashire boiler has two flues which are crossed at intervals by conical-shaped tul)es whose purpo.se is to ])rovide an internal bracing for the flues and to add to the heating surface. In this boiler the grates are placed inside the flues at the front: or, technically classified, it is an internally fired, horizontal, double-fiue, shell boiler. The Gallo- way boiler has two flues at its front end like the I-ancashire boiler, liut a short distance back of the grate these flues converge into erne large flue which is crossed by water-tubes like those used in the Lancashire boiler. The Cornish boiler has one large instead of two smaller flues. Both the Galloway and Cornish boilers are internally fired. Vertical boilers consist of a cylindrical shell set on end, with the grate at the bottom and fire-tubes ninning vertically. In large stationary boilers of this type the boiler is mounted on a masonry foundation, but in small portable boil- ers, such as are commonly used for hoisting- engines, the boiler is set "on a cast-iron base which contains the grate and ash-pit complete. Some of the larger stationary vertical boilers built at present are from 20 feet to 30 feet hih. Fire-engine boilers are always of the vertical type. See Fiue-Engines. The Scotch marine boiler is a cylindrical shell boiler, but little, if any, longer than its diame- ter, resembling a large drum in shape. It is set horizontal. In the lower half of the boiler there are from two to four flues which termi- nate in a cimibustion chamber at the rear, and in the upper part there are a large nuiubir of fire-tubes terminating in the combustion chamber and in the uptake to the smokestack at the front. Double-ended boilers of this type have furnaces at each end, and resemble two single- ended boilers placed back to back. Sometimes o) SCOTCH MARINE BOILER. END VIEW. a common combustion chamber is used for both sets of furnaces in a double-ended Scotch boiler, but it is considered to be better practice to have a separate combustion chamber for each set of furnaces. In recent years, as has previously been stated, water-tube boilers of various forms have been substituted for Scotch boilers, particu- larly for naval vessels. The locomotive boiler consists essentially of a rectangular fire-box and a cylindrical barrel thi-ough which numerous fire-tubes extend from the fire-box to the smoke- box. The latter forms a continuation of the barrel from which the products of combustion p.-iss up the smokestack. . the boilers so far described have been of the fire-tube type — that is. the tubes and flues jiassing through the boiler-shell have been for the conveyance of hot gases from the furnace. 'J'he idea of using tubes and flues is to provide a greater heating surface, and thus use the heat from the fuel more thoroughly and generate steam more rapidly. To economize heat and increase the rapidity of raising steam still fiir- ther, the water-tube boiler has been developed, and is now being extensively used. In water-tube
