In his book, "Structures: Or Why Things Don't Fall Down" (Pelican 1978–1984), Professor J.E. Gordon considers the height of the Tower of Babel. He wrote, "brick and stone weigh about 120 lb per cubic foot (2,000 kg per cubic metre) and the crushing strength of these materials is generally rather better than 6,000 lbs per square inch or 40 mega-pascals. Elementary arithmetic shows that a tower with parallel walls could have been built to a height of before the bricks at the bottom were crushed. However, by making the walls taper towards the top they ... could well have been built to a height where the men of Shinnar would run short of oxygen and had difficulty in breathing before the brick walls crushed beneath their own dead weight."