Tuesday, March 23, 2010

Science Makes a Better Lighthouse Lens

State of the art, one of the seven wonders of the Ancient World, the great lighthouse of Alexandria, built around 280 B.C., towered some 450 feet above Egypt's greatest harbor. At that height, it was the second tallest structure in the world, after another of the seven — the Great Pyramid of Giza. The light within, also state of the art, was an open flame.

From that time until the 18th century, the lights that warned ships that they were approaching land improved hardly at all. Some burned coal. Others stuck with wood. Oil lamps backed by mirrors eventually offered a bit more candlepower. Still, every coastline in the world remained littered with the ribs of broken ships whose captains didn't see the lighthouse until it was too late. Then, in 1822, a frail scientist with a passion for optics made a revolutionary breakthrough. His name was Augustin Jean Fresnel.

An example of his greatest creation, a large object that looks like a crystalline beehive, may be seen inside the Hall of American Maritime Enterprise at the National Museum of American History. It towers above the museum's displays of ship models and marine artifacts. This is the Fresnel lens, not one lens really, but numerous tiers of prisms. Lenses like this one turned simple flames into beams upon which sea captains could take their bearings, avoiding risk to their ships and to the lives of their passengers.

As big as it seems, by lighthouse standards the Smithsonian glass beehive is just above average, measuring nearly six feet high and more than three feet wide. It was installed in the famous Bolivar Lighthouse overlooking Galveston, Texas, sometime around 1907. This lighthouse served as sentry, and for some as sanctuary, during the deadliest hurricane in American history. The lens is visible proof that unlike the sea, light can be mastered by human ingenuity.

For thousands of years, lighthouses created their lights with open flames which, naturally, were ineffective during high winds or rain. Then, in the 1690s, the glass lantern room was invented and first installed in England's Eddystone Lighthouse. Candles placed in the lantern room's glass lantern burned more brightly and securely than the earlier open fires. Another improvement came when pieces of mirrors placed in huge, round, wooden bowls served as crude reflectors that helped to direct the light. But throughout the 18th century, as maritime traffic increased, shipwrecks multiplied. The search was under way for a stronger, more reliable light.

There is some controversy over who first placed parabolic mirrors behind flames to boost candlepower. L. Reynaud, an 18th-century chronicler and public works official, credited Swiss scientist Aimé Argand with the first installation of an apparatus using reflectors, in 1783. Argand also invented, in the 1780s, a long-burning smokeless oil lamp that removed the necessity of stoking the flames all night. But the greatest innovation was to come from Augustin Fresnel.
As a child, he was a slow learner who showed little interest in language studies or in tests of memory. By the age of 8, he could barely read. Yet his boyhood friends, for whom he studiously determined how to increase the power of popguns and bows, called him "the genius." When applied to optics, his genius proved to be real and considerable. Where others had improved existing lighthouse technology, Fresnel leapt forward by studying the behavior of light itself. His studies both advanced the understanding of the nature of light and produced the most important breakthrough in lighthouse lights in 2,000 years.

Fresnel worked out a number of formulas to calculate the way light changes direction, or refracts, while passing through glass prisms. Working with some of the most advanced glassmakers of the day, he produced a combination of prism shapes that together made up a lens. The Fresnel lighthouse lens used a large lamp at the focal plane as its light source. It also contained a central panel of magnifying glasses surrounded above and below by concentric rings of prisms and mirrors, all angled to gather light, intensify it and project it outward.

The various reflector systems installed in lighthouses during the 40 years preceding the introduction of the Fresnel lens certainly had been improvements over the open fires or candles in lantern rooms. Still, they could trap only a small percentage of the light. All prior systems paled by comparison with the Fresnel lens.

The first Fresnel lens, installed in the elegant Cardovan Tower lighthouse on France's Gironde River in 1822, was visible to the horizon, more than 20 miles away. Sailors had long romanticized lighthouses. Now scientists could rhapsodize, too. "Nothing can be more beautiful than an entire apparatus for a fixed light," one engineer said of Fresnel's device. "I know of no work of art more beautifully creditable to the boldness, ardor, intelligence, and zeal of the artist."

Fresnel lenses soon shone along the ragged coastlines of Europe, but surprisingly, America was slower to see the light. As mariners came to depend on Europe's powerful new lights, they complained bitterly about the puny lamps lighting America's coasts. Despite the clear superiority of Fresnel lenses, the parsimonious bureaucrat in charge of federal lighthouses, Stephen Pleasanton, considered the cost prohibitive. Finally, the uproar became so great that in 1838 Congress launched an investigation. It was not until then that Congress coughed up the cash to import a few Fresnel lenses.

Only after 1852, when the United States created a Lighthouse Board made up of eminent scientists and mariners, including Joseph Henry of the Smithsonian and Alexander Bache of the U.S. Coast Survey, did the great lenses really begin to light America's coastline. By the Civil War, nearly all lighthouses in the United States had Fresnel lenses. It was shortly after the Civil War, however, before a beehive of prisms first shone from the Bolivar Light watching over Galveston. A Fresnel lens, similar to the one on display at NMAH, was still in use there in September 1900, when waves from the Gulf of Mexico began pounding the coast.

Late in the morning of September 7, the U.S. Weather Station in Galveston learned by telegraph that a hurricane had just ripped across Florida and was somewhere over the Gulf. The next day, a telegrapher wired Washington, D.C. that Galveston was going under. Thousands died. Among the survivors were 125 people who found safety in the lighthouse. Keeper H. C. Claiborne exhausted a month's supply of food feeding the crowd. When the tower swayed in the wind, disabling the machinery that rotated the lens, Claiborne turned the lens by hand crank and kept the Bolivar Light shining through that terrible night.

With time, Galveston recovered. The Bolivar Light served the city until 1933, then was replaced by another light on the south side of Galveston. The museum's lens served through the Galveston hurricane of 1915, then retired with the lighthouse and was stored by the U.S. Department of the Interior until it was transferred to the Smithsonian in 1976. By then, Augustin Jean Fresnel, who lived only a short while after inventing his great device, had been dead for nearly a century and a half. Along with the lens, Fresnel left behind his theories of light, which form the basis of modern optics. Today, the principle behind the Fresnel lens is used in the headlights of cars and in the flashing lights on police and emergency vehicles. And in a few older lighthouses around the country, and the world, Fresnel's elegant beehives still shine.

References: Smithsonian.com

Saturday, March 13, 2010

Navesink Light, NJ

The Highlands of Navesink overlooks the entrance to New York Bay and, as suggested by its name, is one of the highest points along the eastern seaboard of the United States. Due to its geography, the Highlands has through the years been used in many diverse ways to preside over shipping traffic entering New York Harbor.

During the War of Austrian Succession (1740-1748), a beacon that served as an early warning system was established on the Highlands near the site of the present lighthouse. The English colonists, fearing an attack by the French, devised the system in 1746 where lighted kegs of oil at night or large balls during the day would be hoisted if enemy ships were spotted entering the harbor. Observers across the bay in New York were to alert the City of New York when the beacon was activated. One night in September of 1746, the beacon was accidentally lit, but no alarm was raised in the city. This failure destroyed confidence in the system and evoked reprimand of the negligent observers in New York.

A lighthouse on Sandy Hook, just four miles north of the Highlands, was established in 1764. While there are some clues that a lighthouse was established at Navesink around this same time, the evidence is inconclusive, and a pair of beacons, built on the highlands in 1828, is considered the first Navesink Lighthouse. Congress appropriated funds for the lights on May 18, 1826, allowing 2 ¾ acres of land to be purchased from Nimrod Woodward for $600. Instead of a single tower, two octagonal ones, constructed of blue split stone and separated by 320 feet, were built on the summit. Charles Smith of Stonington, CT erected the towers and a dwelling, located midway between them, for the cost of $8,440, while David Melville of Newport, RI supplied the necessary lamps and reflectors for $1,850. When Keeper Joseph Doty first lit the Twin Lights of Navesink, the north tower exhibited a fixed, white light and the south tower a flashing, white light.

The twin lights were one of seven such pairings that were used in the United States.
Commodore Matthew C. Perry was dispatched in 1838 to examine the state of lighthouses in England and France and to arrange for the shipment of two Fresnel lenses to the United States: a first-order, fixed lens, and a second-order, revolving lens. In a letter to the chairman of the Committee on Commerce, Stephen Pleasonton, Fifth Auditor of the Treasury and the person responsible for the country’s lighthouses, provided details on the purchase and installation of the lenses at Navesink’s Twin Lights.

$72,941 was appropriated by Congress on June 20, 1860 for a new lighthouse at Navesink, and Joseph Lederle was selected as the architect. Lederle’s plans called for a castle-like structure, built of brownstone, with an octagonal tower on its north end and a square tower, 228 feet away, on the opposite end. A two-story residence for the principal keeper and his first assistant was centered between the two towers, while the living space for the second and third assistant keepers, along with workshops and oil rooms, were located in the wings that attached the towers to the two-story dwelling. Each tower was outfitted with a first-order Fresnel lens capable of producing 8,000 candlepower, making Navesink the most powerful lighthouse in the U.S. at the time.

The seven-ton lens was designed to be used with an electric arc lamp. As no electricity was available at Navesink, a temporary wooden building was built behind the south tower to house a generator. When activated on June 20, 1898, the lens and arc lamp produced a whopping 25,000,000 candlepower, making Navesink the first coastal light to use electricity and, once again, the most powerful beacon in the country. The revolving lens produced one flash every five seconds that could be seen from over twenty-two miles at sea. The reflection of the light off clouds was reportedly seen at a distance of seventy-five miles.

Residents living near Navesink didn’t have the same admiration for the piercing beacon as the Lighthouse Service did. Neighbors complained that after the new light was installed, they could not sleep, their chickens wouldn’t lay eggs, and their cows refused to give milk. Panels were soon placed on the landward side of the south tower’s lantern room to placate the locals and pacify their animals. The powerful light affected the lives of the keepers as well, as they had to wear special goggles, similar to those worn by welders, when working near the light to protect their eyes.

Navesink’s powerful light was active for just a few years following World War II before it was extinguished for good in 1949. A minor optic was installed outside the lantern room at that time. Rockette retired from his new employer, the Coast Guard, in 1951 but was permitted to stay in the lighthouse until it was closed for good the following year. The large lens was dismantled and shipped to the Boston Museum of Science, which placed it on exhibit.

The Borough of Highlands received ownership of the Twin Lights in 1954 after the property was declared surplus. Unable to maintain the lighthouse and grounds, the small community passed control of the site to the State of New Jersey in 1962. The State Park Service, Twin Lights Historical Society and Rumson Garden Club managed to raise nearly one million dollars to fund the renovation of the lighthouse and establishment of a museum in several rooms near the north tower.

Today, the thousands of visitors that come to see one of the country’s most unique lighthouses can climb the north tower, where a sixth-order Fresnel lens is in use, for a spectacular panoramic view of the area. The bivalve lens was purchased for $5,000 and returned to Navesink in 1979. This important piece of the station’s history is prominently displayed in the brick generator building, which at one time fed power to the impressive lens.

Location: Located in Highlands, in the northeast corner of New Jersey.

Latitude: 40.39624

Longitude: -73.98572


"The Twin Lights of Navesink," Kim M. Ruth, The Keeper’s Log, Fall, 1991