by means of an electromagnet the detent or stop means described are merely one technical possibility among many others of achieving the same goal fig3 a shows schematically in cross section supplementing fig3 one possibility of connecting the surfaces of the two structural shells or panels 41 for instance by detent or stop means in the end position imparted to them in the pressure housing for an unlimited period of time for this purpose there are welded onto the inner side of the lower structural shell or panel 41 two correspondingly large square pipes 75 detent or stop elements 76 are inserted against the action of compression springs 77 between universal guides 78 in said pipes said detent or stop elements 76 propel and can be pushed in to an extent which is limited by stops they are provided with backwardly extending bevels the distance between the two detent or stop element carriers 75 is such that said elements snap and engage into solid body 79 which is welded to the inside of the opposite structural shell or panel 41 and which is provided with incisions 80 corresponding to the triangular shape of the detent or stop elements 76 and their distances apart from each other under spring pressure 77 upon a corresponding change in position i e reduction of the distance between the structural shells or panels thus they permit further movement up to the end position only in direction towards a reduction of the distance between the structural shells or panels such means which connect the inner surfaces permanently against the inner.