ON THE SETTING OP STEAM BOILERS. 



137 



boiler is carried on iron stools, with a dividing wall only 

 toucliing and not supporting it, tlius securing all tlie advan- 

 tages of the wheel draught without its disadvantages.] 



The principle adopted, is that of successive expanding 

 areas. When air or other fluid passes through an orifice 

 forming the inlet to a gradually expanding tube or funnel, 

 its velocity through the ori/lce will bo increased by reason 

 of its meeting witli loss resistance, and it will pass through 

 the tube with a constantly diminishing velocity, because of 

 the increase of area, which velocity will, of course, bo still 

 further diminished when the fluid is of such a nature as to 

 contract in its passage, because of its taking a longer time 

 to fill up the tube. 



On this principle M. Livet constructs his flues. The 

 back combustion-chamber is larger in area than the flue tube 

 or tubes— the first side flue is also larger than the flue tubes, 

 the cross section of the front chamber is larger than the 

 first side flue, and the second side fluo is larger again than 

 the front chamber. Thus these several parts of the flues, 

 each being of larger area than its predecessor, make up a 

 continuously expanding whole, the proportions of each part 

 being carefully calculated according to the dimensions of the 

 boiler, the area of the grate, and the work to be done. 



The retardation of the gases due to this cause of course 

 allows more time for the heat to pass into the water while 

 the initial velocity over the bridge, or the draught, is in- 

 tensified by the diminution of resistance to the gases in the 

 expanding flues. 



These two results are of the greatest importance in 

 economical steam generation, and it is impossible to obtain 

 botli of them together in any other way. A quick draught 

 may be obtained by a tall chimney or by a forced blast, but 

 if the flues do not act in the way now indicated, the gases 



