10 provides a greater understanding of the configuration and therefore working of the portion 12 in the illustrated embodiment circumferential lines 11 and 13 indicate areas of higher bending stress where the portion 12 will tend to yield under compressive load at the selected threshold level fig3 illustrates a side cut away view of the seal assembly 10 the force transmission portion 12 has an outer diameter at the center of the force transmission portion 12 that is greater than the outer diameter of the ends of the force transmission portion 12 the main axis of the assembly 10 is represented by line 38 fig4 a 4c illustrate an example of the operation of an array of seal assemblies 10 in an arrangement 400 referring to fig4 a a force 4 is transmitted through the tubing string 2 through the seal assemblies 10 a and 10 b such that the force 4 actuates a downhole tool 6 which could be another compression set seal or some other tool responsive to compressive load the axial force 4 may be applied mechanically by for example decreasing support of the weight of a tubing string 2 by for example a derrick at the.