The external walls of Mousa are in outline like 'truncated cones, the outer face of the wall "batters" or inclines inwards' (Dryden 1890, 200). In contrast, the horizontal cross-section of the internal walls show that they are not cone-shaped, nor are they vertical (cylinder-shaped) but might be described as similar to the contours of an Iron Age (Beltic) pot. Prof. M. Walker (pers.comm.) concurs that prediction of the effect of the curved walls on sound or light rays inside the broch is difficult because the construction of the broch is not uniform. The internal walls of the broch are asymmetrical and the diameter of the space inside the broch varies from 19 feet to 26 feet (5.8m to 8m). Light or sound waves inside the broch may be reflected around the wall surfaces but these reflections cannot be traced using the laws of reflection. Closer investigation of the internal wall located on the east side of the broch shows the most extreme concave element; this feature may be the result of stress creating shifts in the wall. As well as the shape of the internal wall, the nature of the stone used in its construction influences sound reflection. 'The stones are of a laminated texture, commonly micaceous schist, roughly shaped; laid in courses at the inner and outer faces as regularly as the materials admitted without mortar' (Dryden 1890, 200). Diffuse reflection of sound or light (discussed in 5.1) depends on the roughness or smoothness of the surface of the wall, and may be reduced when the stone is wet.
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