ich the angle irons would communicate the strain. The whole of the long
stays within a boiler should be firmly riveted to the shell, as if built
with and forming a part of it; as, by the common method of fixing them in
by means of cutters, the decay or accidental detachment of a pin or cutter
may endanger the safety of the boiler. Wherever a large perforation in the
shell of any circular boiler occurs, a sufficient number of stays should be
put across it to maintain the original strength; and where stays are
intercepted by the root of the funnel, short stays in continuation of them
should be placed inside.

BOILER EXPLOSIONS.

306. _Q._--What is the chief cause of boiler explosions?

_A._--The chief cause of boiler explosions is, undoubtedly, too great a
pressure of steam, or an insufficient strength of boiler; but many
explosions have also arisen from the flues having been suffered to become
red hot. If the safety valve of a boiler be accidentally jammed, or if the
plates or stays be much worn by corrosion, while a high pressure of steam
is nevertheless maintained, the boiler necessarily bursts; and if, from an
insufficiency of water in the boiler, or from any other cause, the flues
become highly heated, they may be forced down by the pressure of the steam,
and a partial explosion may be the result. The worst explosion is where the
shell of the boiler bursts; but the collapse of a furnace or flue is also
very disastrous generally to the persons in the engine room; and sometimes
the shell bursts and the flues collapse at the same time; for if the flues
get red hot, and water be thrown upon them either by the feed pump or
otherwise, the generation of steam may be too rapid for the safety valve to
permit its escape with sufficient facility, and the shell of the boiler
may, in consequence, be rent asunder. Sometimes the iron of the flues
becomes highly heated in consequence of the improper configuration of the
parts, which, by retaining the steam in contact with the metal, prevents
the access of the water: the bottoms of large flues, upon which the flame
beats down, are very liable to injury from this cause; and the iron of
flues thus acted upon may be so softened that the flues will collapse
upward with the pressure of the steam. The flues of boilers may also become
red hot in some parts from the attachment of scale, which, from its
imperfect conducting power, will cause the iron to be unduly heated; and if
the scale