FIG. 4b is a side view of the embodiment of FIG. 4a; FIG. 4c is a curve of resistance vs. deflection for the embodiment of FIGS. 4a and 4 b; FIG. 4d is a curve of force vs. deflection for the embodiment of FIGS. 4a and 4 b; FIG. 5a is a pictorial view of another embodiment of the invention having a curved wall;