258 



WELLS'S NATURAL PHILOSOPHY. 



What is the 

 construction of 

 a flue-boiler? 



A very great improvement was FiG. 216. 



effected in the construction of steam- 

 boilers by placing a cylindrical fur- 

 nace within a cylindrical boiler, thus surrounding the 

 heated surfaces with water upon all sides. By this 

 method, all the heat, except what escapes up the 

 chimney, is communicated to the water. Such boilers 

 are known as " flue-boilers." Their general form and 

 plan of construction are represented in Fig. 216. 

 ___ , The requirements of a boiler suit- 



peculiarities of able for a locomotive are, that 



boiier'?™°"^^' *^® greatest possible quantity of water should be evapor- 

 ated with the greatest rapidity in the least possible space. 

 The quantity of fuel consumed is a secondary consideration, as this can be 

 carried in a separate vehicle. The principle by which this has been accom- 

 plished, and the invention of which may be said to have made the present 

 railway system, consists in carrjdng the hot product of the fire through the 

 water in numerous small parallel flues or tubes, thus dividing the heated 

 matter, and as it were filtering it through the water to be heated. In this 

 manner the surfaces, by which the water and the heating gases communicate, 

 axe immensely increased, the whole having a resemblance to the mechan- 



FiG. 217. 



ism of the lungs of animals, in 

 which the air and the blood are 

 divided and presented to each 

 other at as many points, and 

 with as little intervening matter 

 between them, as is consistent 

 with their separation. Fig. 217 

 represents the interior of the fire- 

 box of a locomotive, showing 

 the opening of the tubes, which 

 extend through the whole length 

 of the boiler, and are surrounded 

 with water. The smoke and 

 other products of combustion pass 

 through these tubes, and finally 

 escape up the smoke-pipe. It 

 will be furfher observed by the 

 examination of the figure that 

 the fire-box is double-walled, or rather walled and roofed with a layer of 

 water, leaving onty the bottom vacant, which receives the grate-bars. 



. 582. The safety-valve is generally a conical lid fitted 



safety-valve. i^^o the boiler, and opening outward ; it is kept down by a 



weight, acting on the end of a lever, equal to the pressure 



which the boiler is capable of sustaining without danger from the steam 



generated within. If the amount of steam at any time exceeds the pressure, 



