STEAM GAUGE FUEIsTACE. 



13. The most important appendage of the boiler is the furnace, 

 which consists of a grate, upon which the fuel is maintained in 

 combustion, a system of flues, by which the flame and heated 

 gases proceeding from the fuel in combustion are conducted in 

 contact with the boiler, so as to impart more or less of their 

 heat to the boiler, and, in fine, a chimney by which these gases 

 escape into the atmosphere, and which maintains the draft neces- 

 sary to give effect to the combustion. 



The explanation of the furnace and its appendages, as well as 

 that of the boiler already given, will be rendered much more easily 

 intelligible by the aid of the figures 7, 8, 9, and 10, which, though 

 they represent a particular form of boiler, indicate those provisions 

 and arrangements which are most generally used in boilers of all 

 forms. 



The form here represented is called the waggon-boiler, and con- 

 sists of a semi- cylindrical top, flat perpendicular sides, flat ends, 

 and a slightly concave bottom. The steam intended to be used in 

 boilers of this description does not exceed the pressure of the 

 external atmosphere by more than from 3 to, 5lbs. per square 

 inch ; and the flat sides and ends, though unfavourable to strength, 

 can be constructed sufficiently strong for this purpose. In a 

 boiler of this sort, the air and smoke passing through the flues 

 that are carried round it, are in contact at one side only with the 

 boiler. The brickwork, or other materials forming* a the flue, must 

 therefore be non-conductors of heat, that they may not absorb 

 any considerable portion of heat from the air passing in contact 

 with them. 



A perspective view of the boiler and furnace is presented in 

 fig. 7. The grate and a part of the flues are rendered visible by 

 the removal of a portion of the surrounding masonry in which the 

 boiler is set. The interior of the boiler is also shown by cutting 

 off one half of the semi-cylindrical roof. A longitudinal vertical 

 section is shown in fig. 8, and a cross section in fig. 9. A 

 horizontal section taken above the level of the grate, and below 

 the level of the water in the boiler, shewing the course of the 

 flues, is given in fig. 10. The corresponding parts in all the figures 

 are marked by the same letters. 



14. The door by which fuel is introduced upon the grate is repre- 

 sented at A, and the door leading to the ash-pit at B. The fire 

 bars at c slope downwards from the front at an angle of about 25, 

 giving a tendency to the fuel to move from the front towards the 

 back of the grate. The ash-pit D is constructed of such a magni- 

 tude, form, and depth, as to admit a current of atmospheric air to 

 the grate-bars, sufficient to sustain the combustion. The form 

 of the ash-pit is usually wide below, contracting towards the top. 



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