400 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



According to a rule given by Darcet, if the chimney have a height of 

 twenty or thirty metres, the section ought to contain .-% to ^ as 

 many square centimetres as it is required to burn kilogrammes of coal 

 per hour. So that a chimney twenty metres high ought to have a 

 section of y^, or forty square decimetres if the furnace is to consume 

 180 kilogrammes of coal per hour. Its interior diameter, if it is round, 

 must be '07 m., and if square, '63 m. 



Under certain circumstances the draught must be moderated. This 

 is easily accomplished by means of a damper or movable valve, 

 which is seen at R in Fig. 276, and by the aid of which the opening 

 into the chimney for the smoke and gases of combustion may be 

 diminished at pleasure. 



The form and dimensions of the bars, and the spaces between them, 

 afford elements of great importance in the good performance of the 

 furnace, in the activity of the fire, and consequently in the vaporiza- 

 tion of the water in due proportion to the consumption of fuel. All 

 this must be calculated, arranged, and constructed according to the facts 

 of science and the teachings of experience. 



To conclude our account of the furnace of a steam-engine, we may 

 say one word upon a question which has attracted some atten- 

 tion in industrial quarters : we refer to the possibility of obtaining 

 what is called a smoke-consuming furnace. The true question is this, 

 to make a furnace in which no smoke is produced, or, to speak more 

 correctly, in which the gases, disengaged from the fuel, may be burnt 

 as completely as possible. When the draught does not furnish a 

 sufficient quantity of air, the incompletely burned hydrocarbons 

 escape in the form of thick and black smoke, a very disagreeable and 

 undesirable substance but which manufacturers wish to retain for a 

 much more important reason, namely, that it is the best part of the 

 coal that is thus lost without having produced any heat. 



But this great disadvantage of incomplete combustion may be still 

 produced even when there is no smoke. Tor coal, besides the hydro- 

 carbons just mentioned, which are first decomposed, as soon as the 

 combustion commences, contains a quantity of carbon, which the 

 oxygen transforms into carbonic oxide, and then into carbonic acid, if 

 the draught furnishes a sufficient supply of air. If the draught is 

 bad, the carbonic oxide escapes without having been completely 

 burned, and it is possible in this way to lose a considerable amount of 



