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 a p 
t= —J— - 
For wrought iron, f = 17800 lbs. 
a p 
1 = 4060 ' 
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 lbs. per circular inch. Many lives were lost 
by this explosion ; and the boiler was thrown to a distance of 150 feet, to a place 30 feet above the 
level of its former seat. The upper plates were undoubtedly too weak.