Used for thousands of years, lighthouses function primarily to guide navigators and warn them of various hazards. The first known lighthouse was constructed on the island of Pharos, near Alexandria, Egypt, during the reign of Ptolemy II (308-246 b.c.), whose rule began in 285 b.c. One of the seven wonders of the ancient world, the lighthouse at Pharos measured about 350 ft. (110 m) high. Lit by a wood fire bright enough to be seen from sea, this architectural wonder stood until 1302, when it was finally toppled during an earthquake. The traders and conquerors of imperial Rome built lighthouses at nearly every port; after the fall of the empire, however, the use of lighthouses dwindled with the decline of commerce in the Dark Ages. As traders began to cross the seas again during the eleventh century, the old lighthouses were once again lit. However, it was the rediscovery of...