The boiler is the most important and costly part of a steam locomotive,
representing one-fourth to one-third of the total cost. A poorly built
or designed boiler will produce a poor locomotive no matter how well
made the remainder of mechanism. The boiler of the _Pioneer_ is of the
wagon-top, crownbar, fire-tube style and is made of a 5/16-inch thick,
wrought-iron plate. The barrel is very small, in keeping with the size
of the engine, being only 27 inches in diameter. While some readers may
believe this to be an extremely early example of a wagon-top boiler, we
should remember that most New England builders produced few locomotives
with the Bury (dome) boiler and that the chief advocates of this later
style were the Philadelphia builders. By the early 1850's the Bury
boiler passed out of favor entirely and the wagon top became the
standard type of boiler with all builders in this country.
Sixty-three iron tubes, 1-7/8 inches by 85 inches long are used. The
original tubes may have been copper or brass since these were easier to
keep tight than the less malleable iron tubes. The present tube sheet is
of iron but was originally copper. Its thickness cannot be conveniently
measured, but it is greater than that of the boiler shell, probably
about 1/2 to 5/8 inch. While copper tubes and tube sheets were not much
used in this country after about 1870, copper was employed as recently
as 1950 by Robert Stephenson & Hawthorns, Ltd., on some small industrial
locomotives.
The boiler shell is lagged with wooden tongue-and-groove strips about
2-1/2 inches wide (felt also was used for insulation during this
period). The wooden lagging is covered with Russia sheet iron which is
held in place and the joints covered by polished brass bands. Russia
sheet iron is a planish iron having a lustrous, metallic gray finish.
[Illustration: Figure 14.--THE "FURY," BUILT FOR THE Boston and
Worcester Railroad in 1849 by Wilmarth. It was known as a "Shanghai"
because of its great height. (Smithsonian Chaney photo 6443.)]
[Illustration: Figure 15.--THE "NEPTUNE," BUILT FOR THE Boston and
Worcester in 1847 by Hinkley and Drury. Note the similarity of this
engine and the _Fury_.]
[Illustration: Figure 16.--THE "PIONEER" AS FIRST EXHIBITED in the Arts
and Industries building of the Smithsonian Institution prior to
restoration of the sandbox. (Smithsonian photo 48069D.)]
The steam dome (fig. 18) is located directly over the firebox, inside
the cab. It is lagged and jacketed in an identical manner to the boiler.
The shell of the dome is of 5/16-inch wrought iron, the top cap is a
cast-iron plate which also serves as a manhole cover offering access to
the boiler's interior for inspection and repair.
[Illustration: Figure 17.--"PIONEER" locomotive. (Drawing by J. H.
White.)]
[Illustration: Figure 18.--"PIONEER" LOCOMOTIVE, (1) Safety valve, (2)
spring balance, (3) steam jet, (4) dry pipe, (5) throttle lever, (6)
throttle, (7) crown bar, (8) front tube sheet, (9) check valve, (10) top
rail, (11) rear-boiler bracket, (12) pedestal, (13) rocker bearing, (14)
damper, (15) grate, (16) bottom rail, (17) pump heater valve, (18)
cylinder lubricator, (19) reversing lever, (20) brake shoe, (21) mud
ring, (22) blowoff cock, (23) ashpan. (Drawing by J. H. White.)]
A round plate, 20 inches in diameter, riveted on the forward end of the
boiler, just behind the bell stand, was found when the old jacket was
removed in May 1963. The size and shape of the hole, which the plate
covers, indicate that a steam dome or manhole was located at this point.
It is possible that this was the original location of the steam dome
since many builders in the early 1850's preferred to mount the dome
forward of the firebox. This was done in the belief that there was less
danger of priming because the water was less agitated forward of the
firebox.
The firebox is as narrow as the boiler shell and fits easily between the
frame. It is a deep and narrow box, measuring 27 inches by 28 inches by
about 40 inches deep, and is well suited to burning wood. A deep firebox
was necessary because a wide, shallow box suitable for coal burning,
allowed the fuel to burn so quickly it was difficult to fire the engine
effectively. With the deep, narrow firebox, wood was filled up to the
level of the fire door. In this way, the fire did not burn so furiously
and did not keep ahead of the fireman; at the same time, since it burned
so freely, a good fire was always on hand. The _Pioneer_ burned oak and
hickory.[14] For the firebox 5/16-inch thick sheet was used, for heavier
sheet would have blistered and flaked off because of the intense heat of
the fire and the fibrous quality of wrought-iron sheet of the period.
Sheet iron was fabricated from many small strips of iron rolled together
while hot. These strips were ideally welded into a homogeneous sheet,
but in practice it was found the thicker the sheet the less sure the
weld.
The fire grates are cast iron and set just a few inches above the bottom
of the water space so that the water below the grates remains less
turbulent and mud or other impurities in the water settle here. Four
bronze mud plugs and a blowoff cock are fitted to the base of the
firebox so that the sediment thus collected can be removed (figs. 17,
18).
The front of the boiler is attached to the frame by the smokebox, which
is a cylinder, bolted on a light, cast-iron saddle (not part of the
cylinder castings nor attached to them, but bolted directly to the top
rail of the frame; it may be a hastily made repair put on at the shops
of the C.V.R.R.). The rear of the boiler is attached to the frame by two
large cast-iron brackets, one on each side of the firebox (fig. 18).
These are bolted to the top rail of the frame but the holes in the
brackets are undoubtedly slotted, so that they may slide since the
boiler will expand about 1/4 inch when heated. In addition to the crown
bars, which strengthen the crown sheet, the boiler is further
strengthened by stay bolts and braces located in the wagon top over the
firebox, where the boiler had been weakened by the large hole necessary
for the steam dome. This boiler is a remarkably light, strong, and
compact structure.