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 water-resistant cement (a technology which had been lost since the fall of Rome) by John Smeaton (1724-1792) that sparked the second great era of the lighthouse.
With the increased use of lighthouses, it became clear that advances needed to be made in the light itself. Simple wood fires, which were used until the eighteenth century, were not bright enough to penetrate thick fog, nor could they aim light in any specific direction. Coal fires, used after about 1550, were brighter but could consume almost a ton of coal a day; moreover, their thick smoke quickly darkened the lantern panes, obscuring the light. The English scientist William Hutchinson introduced the parabolic reflector in 1752, a device which directed the better part of a candle's light along a straight path. The first lighthouse lens was designed by the eminent French optician Augustin-Jean Fresnel (1788-1827); this lens' face was cut into staggered planes, so that the light would be converted into a bright, straight beam.
At the insistence of David Brewster, English lighthouses were equipped with lenses of Fresnel's design. Aimé Argand invented an oil-burning lamp in 1782 which produced a near-smokeless flame, and whose integrity and range was increased through the use of metallic reflectors. In 1862 British lighthouses began using the first forms of electric light.
Modern lighthouses are the product of centuries of design and improvements. Most use xenon arc lamps as a light source. The light passes through a compound lens almost identical in construction to Fresnel's. The lens, part of which is screened, revolves around the lamp, causing the light to flash intermittently. The arclamp is surrounded by prisms, which focus any stray light into the main lens; some of the largest lighthouses have as many as 200 of these prisms. The lens-and-prism system is housed on a revolving frame that functions electrically, either by rolling or floating devices. Many modern lighthouses also use radio beacons to warn navigators instead of the loud blasts of sound that were designed to penetrate through thick fog. Sailors recognize a specific lighthouse they are seeing by reading its radio signal. By comparing the signal to a list published by the United States Coast Guard, navigators can easily identify any lighthouse. However, recent advances in shipboard navigation, such as satellite positioning systems, have reduced the need for lighthouses; those that remain are mostly automated through the use of computers and other electronic equipment.
This is the complete article, containing 549 words
(approx. 2 pages at 300 words per page).
