from the apparatus 2 the sealing at surface 15 around the enclosed void 16 will now be described in more detail with reference to fig2 it can be seen from fig2 that the two parts 6a 6b of the lower enclosure part 6 are shaped so that their adjacent surfaces 31 32 are inclined relative to each other this inclination is exaggerated for the sake of clarity in fig2 but results in those two surfaces 31 32 meeting at a line 33 when the enclosed void 16 is subjected to high pressure the part 6a of the upper enclosure part 6 deforms slightly in the direction of arrows 34 relative to the part 6b with the surface 31 pivoting about line 33 which thus acts as fulcrum hence the upper parts of part 6a move slightly outwardly and downwardly as the surface 31 pivots about line 33 this deformation causes a lip 35 on the upper surface of part 6a to be forced outwardly in the direction of arrows 36 and hence to press firmly into the upper enclosure part 7 at the sealing surface 15 hence the strength of the seal is increased in proportion to the increased pressure in the enclosed void 16 in order for the correct.