SAT R&W 44: Central Ideas and Details — Mixed Practice

Roman concrete harbour works have stood in seawater for two thousand years while modern piers crumble in fifty. The Roman mix used volcanic ash, and seawater reacting with it grows crystals inside the cracks that form. Modern concrete is designed to keep water out, because water reaching the steel inside it causes rust. The ancient material has no steel to protect and is therefore free to let the sea repair it.

Between 1890 and 1914 the number of public libraries in Britain rose sharply, funded largely by a single donor who required each town to commit to a rate for the upkeep before he would pay for the building. Towns that refused the condition got nothing. The condition was the point: a building with no committed running cost tends to close within a generation.

Sevara had promised to say nothing, and she said nothing for nine days. On the tenth her sister mentioned the wedding in passing, as settled, and Sevara heard her own voice ask what would happen to the flat. The sentence was out before she had decided to say it. In the silence that followed she understood that she had been waiting for somebody to ask her a question she could answer honestly, and that nobody was going to.

A trial found that a drug reduced the rate of a rare complication from four cases per thousand patients to two. The result was reported as halving the risk, which it did. It was also reported as preventing two cases in every thousand people treated, which it also did. The two sentences describe one number and produce different decisions in almost everyone who reads them.

Ice cores are read like tree rings: each year's snowfall makes a layer, and the layers can be counted down. Near the surface the counting is reliable. Deeper down the weight of the ice above squeezes the layers thin, and below a certain depth two years can no longer be told apart. Dates from the deepest ice are therefore matched to other records rather than counted directly.

Rice terraces on steep hillsides hold soil that would otherwise wash into the valley, and the walls that retain them need repair after every heavy season. In villages where the young have gone to the cities, the terraces fail in a particular order: the highest first, because they are furthest from the houses and the last to be walked to. What the hillside loses is not labour in general but the walk.