Like children clinging to their parents, stock market traders turned to their central banks last week as they sought protection from the frightening economic figures coming out of China. Surely, they asked, the central banks would ward off the approaching bogeymen, as they had so many times since the 2008 crash. The US Federal Reserve came up with the goods. William Dudley, president of the bank’s New York branch, hinted that the interest rate rise many had expected next month was likely to be delayed. A signal that borrowing costs would remain at rock bottom was all it took. After...