1900s: By the end of the decade, America boasts more than forty thousand millionaires, many of whom earned their fortunes in industry.

Today: America has countless millionaires and several billionaires. Industry, most notably information technology, is the major source of this income for some. However, an increasing number of sports stars, entertainers, writers, and other public figures—who often earn advances, sponsorship deals, and other supplemental income— join the ranks of the extremely wealthy.

1900s: New York contains some of the tallest skyscrapers in the world, including the twentystory, steel-frame Flatiron Building. Skyscrapers begin to include electric elevators.

Today: On September 11, 2001, terrorists attack and destroy the twin towers of the World Trade Center in New York City. Each of the two 110- story buildings contained ninety-seven passenger elevators.

1900s: New York is the hub of the theatre industry, represented by Broadway, and...