SECT, vii.] STEAM ENGINES. 251 



If a boiler flue rise from the fire, and then descend again before it enters the 

 chimney, it will in particular states of the fire be liable to fill with inflammable 

 gas, which takes fire and explodes. The effect of such an explosion in the flues of 

 a boiler must cause an impulsive strain on the boiler, under which it may fail. 



The danger may be avoided by making the flues lead off to the chimney without 

 depression, and constructing the damper so that it cannot be perfectly closed ; and 

 it should either rise so as to close the upper part of the aperture last, or move 

 horizontally. 



Hydrogen gas may be, and frequently is, formed in steam boilers, through the 

 water being in contact with a part of the boiler which is red hot ; and it seems to 

 be regularly produced during the formation of steam at very high temperatures : 

 and though it appears to me that it would not add to the risk of an explosion 

 happening, it undoubtedly would render it more destructive if it should take 

 place. 1 



BOILERS FORMED OF PLATES. 



524. Having determined the resistance of plates of any curvature, it is easy 

 to apply these rules to rectangular boilers ; remarking, that it is indifferent whe- 

 ther the curve be convex or concave to the pressure, provided it have either abut- 

 ments as an arch, or forms a complete circle. I doubt the efficacy of the usual 

 abutments, and I think the fact that boilers fail round the seats is greatly owing to 

 the strain and motion of the parts at every change of force or temperature. 



A rectangular boiler may be considered as a cylinder, taking the greatest dia- 

 gonal line of its section for the diameter ; and for strength the thickness will be, 

 (art. 521.) 



3-81 ap 



t '^ = - ' ^ . 



For wrought iron, f= 17800 Ibs. 



t = ap 



4660' 



1 In a letter I received from Mr. W. Williams, of Cyrfartha Iron Works, he attributes the 

 destructive effects of an accident in that neighbourhood to an accumulation of hydrogen inflaming 

 when the boiler burst. The boiler was constructed of the old spherical form, twenty feet in dia- 

 meter ; the thickness of the plates when new was, top plates a full quarter of an inch, bottom 

 plates half an inch ; load on the safety valve 7 Ibs. per circular inch. Many lives were lost 

 by this explosion ; and the boiler was thrown to a distance of 1 50 feet, to a place 30 feet above the 

 level of its former seat. The upper plates were undoubtedly too weak. 



