er engine _Jenny Lind_ (fig. 22) was equipped with identical driving
wheels. The present tires are very thin and beyond their last turning.
They are wrought iron and shrunk to fit the wheel centers. Flush rivets
are used for further security. The left wheel, shown in figure 17, is
cracked at the hub and is fitted with an iron ring to prevent its
breaking.

The truck wheels, of the hollow spoke pattern, are cast iron with
chilled treads. They were made by Asa Whitney, one of the leading
car-wheel manufacturers in this country, whose extensive plant was
located in Philadelphia. Made under Whitney's patent of 1866, these
wheels may well have been added to the _Pioneer_ during the 1871
rebuilding. Railroad wheels were not cast from ordinary cast iron, which
was too weak and brittle to stand the severe service for which they were
intended, but from a high-quality cast iron similar to that used for
cannons. Its tensile strength, which ranged from 31,000 to 36,000 psi,
was remarkably high and very nearly approached that of the best
wrought-iron plate.

The cylinders are cast iron with an 8-1/2-inch bore about half the size
of the cylinders of a standard 8-wheel engine. The cylinders are bolted
to the frame but not to the saddle, and are set at a 9 deg. angle to clear
the leading wheels and at the same time to line up with the center of
the driving-wheel axle. The wood lagging is covered with a decorative
brass jacket. Ornamental brass jacketing was extensively used on
mid-19th-century American locomotives to cover not only the cylinders
but steam and sand boxes, check valves, and valve boxes. The greater
expense for brass (Russia iron or painted sheet iron were a cheaper
substitute) was justified by the argument that brass lasted the life of
the engine, and could be reclaimed for scrap at a price approaching the
original cost; and also that when brightly polished it reflected the
heat, preventing loss by radiation, and its bright surface could be seen
a great distance, thus helping to prevent accidents at grade crossings.
The reader should be careful not to misconstrue the above arguments
simply as rationalization on the part of master mechanics more intent on
highly decorative machines than on the practical considerations
involved.

The valve box, a separate casting, is fastened to the cylinder casting
by six bolts. The side cover plates when removed show only a small
opening suitable for inspection and adjustment of the v