design as well as construction of the Lithgow No. 2, Katoomba and Medlow dams. The thirteen dams of this type were built of concrete (not reinforced), curved in plan, reliance for stability being placed only on the capacity of the material in the wall and sides of the valley to resist compression. In designing these dams, the complex question of the exact stresses that may occur and the assistance that may be afforded by the weight of the wall was disregarded; as it was considered that all practical requirements would be met if in theory the dams were treated simply as sections of rigid cylinders subject to exterior water pressure. The radius of this cylinder depended upon the natural features of the sites, it being stated elsewhere in the paper that the use of such curved walls was restricted to comparatively narrow valleys and gorges. This local (and as it turned out, entirely successful) design followed an earlier and perhaps even bolder 1884 precedent at the Bear Valley Dam in California but it led, as had the Californian work, to a storm of criticism from the engineering profession of the day, not least, because it had proved to be successful in the face of orthodox theory, which at that time concentrated on gravity dam walls. Comments of members of the Institution in 1909 included: Even the eminent Sir Alexander Binnie, Past President of the Institution, was moved to remark that "to look at the cross sections produced a blood-curdling sensation". He went on, however, to support the design as a practical achievement: it was not a matter of cavilling at theory or formulas; the dams were built and were standing. The problem that now arose was for mathematicians to show how the strains were accommodated in such apparently narrow walls. The entire justification of this then-daring and unorthodox design was economic. As stated by its originator, Mr Darley: small towns could not afford expensive gravity dams, and therefore it was a case of either building a cheap dam or not giving a water supply. Description Medlow Dam is a small water storage dam which utilises a thin-arch concrete wall under high stress. Its design capacity for the reservoir is , with a wall height (maximum) of . The radius of curvature for the dam wall is , the length of the wall is and the thickness of the wall at