## 2.4 Void set categorisation

To assist in understanding their significance, and to clarify understanding of the broch layout as well as simplify descriptions of void set areas, the collection of voids apparent in the inner walls are described as four void sets labelled (from west clockwise) A, B, C, and D (see 7) and align to the polar co-ordinates shown in Table 1 (There are no voids in high gallery 6. The voids of void set D at gallery levels 2, 3 and 4 do not penetrate the wall.). The details of this approach are shown in Table 1. Dryden (1890) raised the possibility that voids could be regarded as sets and in describing the voids at Mousa broch the author recognised and reproduced his idea. Paterson's illustrations of Mousa broch show cross-sectional plans of the six galleries of Mousa broch. From both these diagrams, we see that 'voids' penetrate the internal courtyard wall and open into circular stone passages/galleries.

Table 1: Details of void sets showing measurements of gallery chambers relative to ground/wall head and orientation of void sets

 Mousa broch location Latitude: 59° 59' 45.37" N Longitude: 1° 10' 50.46" Construction date estimated 100BC–150 AD Orientation of void sets A, B, C, D Where North = 0 degrees East = 90 South = 180 West = 270 Void set label A B C D Gallery 1 273-281 Stair opening 110-122 160-166 Gallery 2 273-283 42-50 112-121 160-166 Gallery 3 273-280 42-50 112-117 160-166 Gallery 4 273-280 42-50 112-116 160-166 Gallery 5 273-275 none none none Gallery 6 none none none none Top of void set distance from ground in metres (approx.) 8.6 9.8 9.3 9.9 Number of openings in void set 12? 16? 8 15/17? Opposite wall mutual linear alignments in degrees 183-185 222-230 292-301 350-355 Mutual linear alignments All measurements estimated error ±3 degrees 273-280 42-50 112-121 160-166

Table 1 shows a vertical relationship between the four void sets. The appearance of co-ordinates based upon common linear alignments does not appear to be random. Void sets are visible as vertical slatted slits in the walls. The vertical relationship extends from gallery level 1 up to gallery 5 and may continue below the stone scarcement.