Eighty years ago today on "Black Tuesday,"� the stock market collapsed, ushering in the worst economic slump in American history. The 1930s were a national nightmare"”an era of great suffering, smashed dreams, and wasted opportunities.

What we often forget when we think about the Depression, however, is that at several points the contemporary equivalent of "green shoots"� led many to believe that the gloom had passed. A recently published economic journal of the period"”The Great Depression: A Diary"”reminds us just how wrongheaded predictions of prosperity can be, a sobering lesson for today.

For nearly two-and-a-half years, from September 1934 to February 1937, the Dow Jones Industrial Average climbed steadily with...