internal and external pressures p 2 p 6 represent the increments of these pressures due to p x and head of metal respectively if desired it may be possible to decrease the external fluid pressures near the bottom of the pressure vessel 57 because of the increasing thickness of the skin 55 with reference to fig2 each of the pressure levels p 2 p 6 is represented by a vertical line 104 the vertical lines 104 representing p 2 p 5 are interconnected by sloping lines 105 each of which represents the pressure transition between each of these pressure levels physically each of the pressure transitions represented by the lines 105 occurs across one of the transverse walls 63 separating adjacent pressure chambers 65 the axial dimension of one of the transverse walls 63 and or of any seal carried thereby affects the rate of pressure change between the pressure chambers 65 on the opposite sides of such transverse wall the axial dimension of any of the transverse walls 63 can be varied in accordance with the desired rate of pressure change between adjacent pressure chamber 63 thus by significantly increasing the axial dimensions of the transverse walls 63 the rate of external pressure change from the top to the bottom of the pressure vessel 57 can be made nearly linear if desired in continuous casting operations.