THE advertisement warns of speculative financial bubbles. It mocks a group of gullible Frenchmen seduced into a silly, 18th-century investment scheme, noting that the modern shareholder, armed with superior information, can avoid the pitfalls of the past. “How different the position of the investor today!” the ad enthuses. It ran in The Saturday Evening Post on Sept. 14, 1929. A month later, the stock market crashed.