by girders resting on the
brickwork. Bolted joints were discarded, with the exception of those
connecting the headers to the front and rear ends of the drums and the
bottom of the rear headers to the mud drum. Even such joints, however,
were found objectionable and were superseded in subsequent construction
by short lengths of tubes expanded into bored holes.

[Illustration: No. 16]

No. 16. In this design, headers were tried which were made in the form
of triangular boxes, in each of which there were three tubes expanded.
These boxes were alternately reversed and connected by short lengths of
expanded tubes, being connected to the drum by tubes bent in a manner to
allow them to enter the shell normally. The joints between headers
introduced an element of weakness and the connections to the drum were
insufficient to give adequate circulation.

[Illustration: No. 17]

No. 17. Straight horizontal headers were next tried, alternately shifted
right and left to allow a staggering of tubes. These headers were
connected to each other and to the drums by expanded nipples. The
objections to this boiler were almost the same as those to No. 16.

[Illustration: No. 18]

[Illustration: No. 19]

Nos. 18 and 19. These boilers were designed primarily for fire
protection purposes, the requirements demanding a small, compact boiler
with ability to raise steam quickly. These both served the purpose
admirably but, as in No. 9, the only provision made for the securing of
dry steam was the use of the steam dome, shown in the illustration. This
dome was found inadequate and has since been abandoned in nearly all
forms of boiler construction. No other remedy being suggested at the
time, these boilers were not considered as desirable for general use as
Nos. 21 and 22. In Europe, however, where small size units were more in
demand, No. 18 was modified somewhat and used largely with excellent
results. These experiments, as they may now be called, although many
boilers of some of the designs were built, clearly demonstrated that the
best construction and efficiency required adherence to the following
elements of design:

1st. Sinuous headers for each vertical row of tubes.

2nd. A separate and independent connection with the drum, both front and
rear, for each vertical row of tubes.

[Illustration: No. 20A]

[Illustration: No. 20B]

3rd. All joints between parts of the boiler proper to be made without
bolts or screw plates.