Two semi-circular arches each made up of twin ¾ inch plywood sandwiched screwed and glued together were made of a radius measured to meet the upper base ring at such point as to leave a distance between them of some 68 cms.
This was the desired aperture width of the dome window and hatch.
The same technique as that used for the horizontal circular beams was used, namely using sandwiched staggered sectors of plywood cut to the right curve by means of a router strapped to a ‘compass’ jig. See previous description.
On the outer face of each arched beam a single ⅝ inch thick arch of narrower diameter was added in such a way as to leave the inner arch projecting out by about 10 cms all around.
This added arch would serve for seating and fixing down the metal cladding.
The back side of the arches was bridged across by transversely placed sections of 4 x 2 inch red deal that were glued and fixed on the inside surface to the side of the arches by means of galvanized steel brackets. The front and back ends of both arched beams were also bolted and fixed to the upper face of the circular base by two sets of two brackets each at either end. Though hardly sufficient for permanent stability of the rising beams, these brackets provided useful anchorage until the outer ribs and shell were completed.