hould be as large as possible, so that expansion may be adopted to a large
extent; and with any given speed of piston, the power of an engine either
to draw heavy loads, or achieve high velocities, will be increased with
every increase of the dimensions of the cylinder. The framing of
locomotives, to which the boiler and machinery are attached, and which
rests upon the springs situated above the axles, is formed generally of
malleable iron, but in some engines the side frames consist of oak with
iron plates riveted on each side. The guard plates are in these cases
generally of equal length, the frames being curved upward to pass over the
driving axle. Hard cast iron blocks are riveted between the guard plates to
serve as guides for the axle bushes. The side frames are connected across
the ends, and cross stays are introduced beneath the boiler to stiffen the
frame sideways, and prevent the ends of the connecting or eccentric rods
from falling down if they should be broken.

504. _Q._--What is the nature and arrangement of the springs of
locomotives?

_A._--The springs are of the ordinary carriage kind, with plates connected
at the centre, and allowed to slide on each other at their ends. The upper
plate terminates in two eyes, through each of which passes a pin, which
also passes through the jaws of the bridle, connected by a double threaded
screw to another bridle, which is jointed to the framing; the centre of the
spring rests upon the axle box. Sometimes the springs are placed between
the guard plates, and below the framing which rests upon their extremities.
One species of springs which has gained a considerable introduction,
consists of a number of flat steel plates with a piece of metal or other
substance interposed between them at the centre, leaving the ends standing
apart. It would be preferable, perhaps, to make the plates of a common
spring with different curves, so that the leaves, though in contact at the
centre, would not be in contact with the ends with light loads, but would
be brought into contact gradually, as the strain conies on: a spring would
thus be obtained that was suitable for all loads.

505. _Q._--What is the difference between inside and outside cylinder
engines?

_A._--Outside cylinders are so designated when placed upon the outside of
the framing, with their connecting rods operating upon pins in the driving
wheels; while the inside cylinders are situated within the framing, and the
con