S.

Our engravings illustrate an improved compound car-spring, which
appears to possess all the requisites of a first-class spring,
combining in its construction extreme simplicity with great strength,
and a feature whereby the power of the spring increases with increase
of the load, and _vice versa_, so that its flexibility remains nearly
constant for all loads.

Fig. 1 is a perspective view of this spring, with a portion of the
side of the case broken out to show the interior arrangement of the
spiral springs. Fig. 2 is a section of the compressing plate. Fig. 3
is a plan view, showing the arrangement of the tubes which enclose the
springs.

[Illustration: POTT'S' SPIRAL CAR SPRING FOR RAILWAY CARRIAGES.
_Fig. 1 Fig. 2 Fig. 3_]

The case is cast in two pieces. Its vertical wall is cast in a single
piece, and has at the top a flange or bead extending inwardly, against
which the compressing plate abuts when the spring is not compressed,
as shown in Fig. 2. A bottom plate completes the case.

The spiral components of the spring are inclosed in tubes, as shown in
Figs. 1 and 3. It is not deemed essential that these tubes should be
seamless, or that their edges, brought together in bending, should be
soldered, brazed, or welded. They act merely as guides to compel the
component springs to expand or contract in vertical lines, and need
only be strong enough for that purpose.

The compressing plate is formed with concentric steps or ledges,
as shown in Fig. 2, so that with light loads, only a portion of the
component spirals act. With a heavier load a new series of spirals is
brought into action, and so on, till the spring is loaded to its full
capacity. This feature is novel, and as important as novel, as it
gives the spring a far more easy and flexible carriage, with light
loads, than would be the case if all the spirals were permitted to
act.

In putting the spring together, the vertical part of the case is
inverted. The compressing plate is then placed within the case,
resting upon the inner flange of the case above described. The tubes
with their inclosed springs are then arranged in position, as shown in
the plan view, Fig. 3. The bottom plate of the case is then placed in
position, and held to its place by lugs and rivets, as shown in Fig.
1; the spring is then ready for use.

The employment of tubes in the manner described, enables springs of
the greatest practical length to be used, without