With the discovery of fire, man conquered cold. Beating the heat was a more daunting challenge. In ancient Rome, patricians simply fled to the Alban Hills to wait out the sultry summer months. Caliph Mahdi of Baghdad was more ambitious. In A.D. 775, he began cooling his garden by packing the hollow walls around it with snow from nearby mountains. Some 700 years later, Leonardo da Vinci devised a water-driven fan for a patron's home. Not to be outdone, one 19th-century inventor built a Venetian blind "suit" of clothes that permitted ventilation but blocked out direct sunlight. Only with the development of the mechanical air conditioner in the 20th century did man come up with a practical form of summer relief. As historian Raymond Arsenault points out, the air conditioner has done more than lower indoor temperatures; it has also contributed to social and economic change, notably the "Americanization of Dixie.”