26 to each side of it adapted to span chords of the ring 19 a pair of cross arms 27 and 28 are pivotably connected in the manner of a pantograph to the three cross bars 24 to 26 as shown in fig1 the cross bars 24 to 26 are of a suitable cross section such as that of an i beam and each of them and the cross arms 27 and 28 is preferably of aluminum at one end of the cross bars 24 to 26 hereinafter referred to as their lower end the bars are hinged to mountings 29 to 31 respectively at the other end of the cross bars 24 to 26 hereinafter referred to as their upper end the cross bars are attached without hinges to mountings 32 to 34 respectively lower angle brackets 35 and 36 and upper angle brackets 37 and 38 extend from the cross bars 25 and 26 respectively as shown in fig1 releasable attachment means are provided for anchoring the support frame 23 with respect to the ring 19 in particular the lower mounting 29 of the central cross bar 24 has bayonet pin means 40 the lower mounting 30 on the outer cross bar 25 has double bayonet pin means 41 and 42 and the lower mounting 31 on the other outer cross bar 26 has double bayonet means 43 and 44 at the upper end of the central cross arm 24 the mounting 32 has single bayonet pin means 45 at the upper end of the outer cross arm 25 the mounting 33 has double bayonet pin means 46 and 47 and at the upper end of the other outer cross arm 26 the mounting 34 has double bayonet pin means 48.