An unprecedented wave of sell orders shook the New York Stock Exchange. Stock prices tumbled. As prices fell, brokers required investors who had bought stock on margin to put up money to cover their loans. To raise money, many investors dumped stocks for whatever price they could fetch. During the first three hours of trading stock values plunged by $11 billion. At noon, a group of prominent bankers met at the offices of J.P. Morgan and Co. The bankers’ pool agreed to buy stocks well above the market. At 1:30 p.m. they put their plan into...