a 4d illustrate several of these ways fig4 a schematically illustrates an arrangement for sealing an assembly 30 by disposing the assembly within a sealed covering of flexible fluid impermable membrane 40 e g a plastic bag in this arrangement the regions between successive layers are sealed in relation to an external pressure vessel preferably the air is removed from the bag before applying pressure fig4 b shows an alternate sealing arrangement wherein the assembly 30 is sealed by a peripheral sealing clamp 61 which can be in the form of a hollow elastic torroid sealing can be assisted by providing one of the layers with a protruding region 62 extending around the region to be bonded in use the clamp and pressurized fluid will press the protruding region 62 into the layers sealing the region to be bonded fig4 c illustrates a sealing arrangement in which the assembly 30 is sealed by applying a peripheral tube or weight 63 which lightly presses the periphery of the layers together a peripheral protruding region 62 can assist sealing fig4 d shows an alternative sealing arrangement wherein the assembly 30 is sealed by a sealing o ring 64 between successive layers preferably the.