and 78 to form tracks 32 and rack 70 recess 44 in circular walls 16 has a pair of secondary recesses 80 and 82 which have fixed pairs of opposed rails 84 and 86 respectively recess 44 also has further rails 88 and 90 normal one to the other rails 84 86 88 and 90 together form track 42 fig1 and 12 show the arrangement of trucks 28 associated with barrel vault 20 not every truck 28 is needed to move vault 20 and the drive trucks are sequentially spaced between slave trucks 28a which are of the same construction as the drive trucks but do not carry motors 64 as seen in fig1 5 and 13 dome 12 when open has end segment 24 vault 20 and vault 22 stacked above fixed end segment 26 in nesting position to close dome 12 into the position shown in fig2 6 and 14 end segment 24 and vaults 20 22 are moved as seen in fig7 more particularly end segment 24 is rotated on track 42 in the direction of arrow 116 about any imaginary centre of rotation 118 vault 20 is moved linearly on track 36 in the direction of arow 120 and vault 22 is moved linearly on track 32 in the direction of arrow 122 in the structure as seen in fig1 it will be noted that vaults 20 22 and end segments 24 26 carry interengaging means to seal dome 12 in its closed position in particular end segment 24 carries at its.