496 



THE STEAM-ENGINE. 



carry off with them a considerable quantity of the heat. Provision should 

 therefore be made to keep them in contact with the boiler such a length of 

 time as will enable them to impart such a portion of the heat which they have 

 absorbed from the fuel, as will still leave them at a temperature sufficient, and 

 not more than sufficient, to produce the necessary draught in the chimney. 



The forms of boiler which have been proposed as the most convenient for 

 the attainment of all these requisite purposes have been very various. If 

 strength alone were considered, the spherical form would be the best ; and the 

 early boilers were very nearly hemispheres, placed on a slightly concave base. 

 The form adopted by Watt, called the wagon-boiler, consists 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 did not 

 exceed the pressure of the external atmosphere by more than from 3 to 5 Ibs., 

 per square inch ; and the flat sides and ends, though unfavorable to strength, 

 could 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 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 boiler of this form is represented in fig. 52. 



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. 53, and across section in fig. 54. 



Fig. 53 



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

 the water in the boiler, showing the course of the flues, is given in fig. 55. 

 The corresponding parts in all the figures are marked by the same letters. 



