In 1932, Clarence Mackay was forced to sell some of his most valuable and beloved art. In 1933, Clarence and his wife Anna were forced to close the Harbor Hill mansion and reduce saleries and staff. In November 1938, at the time of Mr. Mackay's death, his estate was valued at only 2-3 million dollars (at one time his net worth was 50-100 times that amount).
For an excellent explanation of why the 1929 stock market crashed, click here: Stock-Market-Crash.
