must of necessity be hand work, which is never the equal of
machine riveting. The use of water-leg construction ordinarily requires
the flanging of large plates, which is difficult, and because of the
number of heats necessary and the continual working of the material, may
lead to the weakening of such plates.

In vertical or semi-vertical water-tube boilers utilizing flat-stayed
surfaces under pressure, these surfaces are ordinarily so located as to
offer a convenient lodging place for flue dust, which fuses into a hard
mass, is difficult of removal and under which corrosion may be going on
with no possibility of detection.

Where stayed surfaces or water legs are features in the design of a
water-tube boiler, the factor of safety of such parts must be most
carefully considered. In such parts too, is the determination of the
factor most difficult, and because of the "rule-of-thumb" determination
frequently necessary, the factor of safety becomes in reality a factor
of ignorance. As opposed to such indeterminate factors of safety, in the
Babcock & Wilcox boiler, when the factor of safety for the drum or drums
has been determined, and such a factor may be determined accurately, the
factors for all other portions of the pressure parts are greatly in
excess of that of the drum. All Babcock & Wilcox boilers are built with
a factor of safety of at least five, and inasmuch as the factor of the
safety of the tubes and headers is greatly in excess of this figure, it
applies specifically to the drum or drums. This factor represents a
greater degree of safety than a considerably higher factor applied to a
boiler in which the shell or any riveted portion is acted upon directly
by the fire, or the same factor applied to a boiler utilizing
flat-stayed surface construction, where the accurate determination of
the limiting factor of safety is difficult, if not impossible.

That the factor of safety of stayed surfaces is questionable may perhaps
be best realized from a consideration of the severe requirements as to
such factor called for by the rules and regulations of the Board of
Supervising Inspectors, U. S. Government.

In view of the above, the absence of any stayed surfaces in the Babcock
& Wilcox boiler is obviously a distinguishing advantage where safety is
a factor. It is of interest to note, in the article on the evolution of
the Babcock & Wilcox boiler, that staybolt construction was used in
several designs, found unsatis