406 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



boiler ; the lower side being vaulted. The flame, after having heated 

 the concave lower surface directly, returned upon itself by lateral 

 flues. Later on, this form was employed in the first steam-boats, but 

 then there was added an inner flue, through which the gases of combus- 

 tion passed before entering the lateral ones. The sides of the vaulted 

 boiler were of a bad form for resisting pressure, and the history of 

 accidents in steam-engines shows that the greater number of explosions 



occurred to boilers constructed on 

 this system. They are now gone 

 out of use almost everywhere. 



An interesting and original 

 arrangement is that of lateral 

 heaters in Farcot's boiler. In 

 this system (Fig. 282) the prin- 

 cipal cylindrical body, A, is heated 

 directly by the fire. Four heaters 

 are placed vertically one above 

 the other in a side compartment 

 of brickwork, divided into four 

 compartments or flues, through 



which the gaS6S of COmbllStion 



are compelled to pass succes- 



sively before reaching the chimney. The lowermost heater, A', 

 receives the fresh water. As the gases travel from above downwards, 

 while the water follows an opposite path to go from A' to the boiler, 

 it follows that the hottest portions of the gas are in contact with 

 the hottest parts of the sides of the boilers, and the cooler parts give 

 up their heat to warm the still colder water before escaping up the 

 chimney. 



Suppose that the cylindrical body of a boiler incloses an inner tube 

 of sufficient diameter entirely surrounded by water, and that we place 

 the fire in this tube, instead of making it simply a flue like that of the 

 boiler described above, we should then have a boiler with an inside 

 fire. In this system the heat of the fire is entirely used and em- 

 ployed in the direct heating of the metallic sides of the boiler, with- 

 out being absorbed by brickwork. But the heating surface will 

 still not be large 1 enough, unless the boiler be enveloped by flues on 

 the outside, and then the inconvenience of a fire necessarily restricted 



Fir,. 282.-Bmler wJ 



heaters. 



