Inquiry 7: How Does It Work? Lighthouses
Though many ancient peoples built fires on hills and mountainsides to bring sailors home from the sea, the first great lighthouse was built on an island in the harbor of Alexandria, Egypt. The Pharos tower, built around 280 BC, was 450 feet high, and the light produced by a fire kept blazing on its roof could be seen from as far away as 29 miles out in the Mediterranean. Sailors needed the Pharos light because the city of Alexandria was on the flat Nile Delta, so there were no mountains or other natural features to help them find the city. The Pharos tower attracted sailors from far distances and, as a result, Alexandria became one of the busiest ports in the world.
Before the development of the lighthouse it was extremely difficult for sailors to navigate over long distances. By day, it was best for them to remain in sight of the land so that they could use features of the land to determine their location. Most of the travel, however, was local because sailors knew their own waters best and could spot where the water swirled around hidden rocks. At night, however, the open ocean was a better place to be in order that they might avoid underwater reefs and rugged coastlines. The light from the stars and the moon helped them to navigate.
An open fire produced the light that emanated from early lighthouses. Since the fire was not protected, wind direction was an important factor, causing the light to be more or less bright on the side facing the sea. Heavy rains also created problems, which were later solved by using covered lanterns or enclosing lanterns with glazing. As a consequence, however, the intensity of the light was greatly reduced, especially if the lighthouse keeper did not keep the glazing clean.
Fuel for the early fires was wood, and forests were often destroyed to keep the light burning. After 1500 AD coal became the primary fuel used in lighthouses. This method was quite effective at producing light except when it was windy, since the flames of the fire could become hot enough to melt the grate holding the coal. Nevertheless, coal-burning lighthouses continued to be used well into the nineteenth century. Some lighthouses also used oil lamps, which consisted of small containers filled with oil and a floating wick. Later a cresset, a stone bowl containing oil and a fixed wick, was used. The type of oil preferred was sperm whale oil. Petroleum oil or kerosene was used around the 1870's in regular lamps. In the early 1900's the incandescent oil vapor lamp came into use, which produced a bright light that was used with the Fresnel lens.
By 1820 the United States had 55 coastal and harbor lighthouses. The lighting system used at the time was the Argand lamp that consisted of a lens and a parabolic reflector. By 1850 however, the government authorized use of the multi-prismed lens invented in France by Augustin Fresnel in 1822. Fresnel lenses are shaped like a beehive, with concentric rings of prisms around a lens, which produce parallel beams of light. The design enables the focusing of scattered light from a lamp or bulb into a tight beam. In the nineteenth century, each lens cost $12,000 plus shipping costs from France. The lenses were ranked in six "orders." The order of the lens was determined by the distance of the flame from the lens, known as the focal distance. The weakest, ranked sixth, was used to light lakes and harbors. The largest, first-order lens, made up of over 1000 prisms, stood 10 to 12 feet tall, measured six feet in circumference, and weighed up to three tons. When these lenses were mounted at 100 feet above sea level the light could be seen for up to 18 miles at sea.
K-W-L Chart - Ask students to create a K-W-L chart on lighthouses in cooperative groups. After they have created their charts, have them plan how they will answer their questions. Encourage students to answer the questions and share their findings with the rest of the group.
Art - Have students design and draw what they think a lighthouse looks like. Encourage them to put as much detail into their drawings as possible and discuss features that they think are necessary to create a working facility. This will entail some research on the student's part about different types of lighthouses.
Parabolic Reflector Model - Parabolic reflectors were used to increase the amount of light produced by a light source in lighthouses. They were made out of curved metal, usually copper, and coated with a silvery compound. Have students design and create a working parabolic reflector. Provide them with any items they could use to concentrate or increase the amount of light coming from a source. Cardboard, candles, flashlights, aluminum pie plates, and lenses would all be suitable items.
Diorama - Have students design a diorama showing what a lighthouse and surrounding area would look like along the Florida coast or along the California or Oregon coast. This will require some research. For instance, your students will need to know that, generally speaking, lighthouses found on the Pacific coastline were not very tall because of the rocky terrain, whereas the ones found along southeast coastline were usually quite tall.
Poetry - After reading the poem below, have students either find other poems about lighthouses, or write a poem of their own with a lighthouse as the theme.
The rocky ledge runs far into the sea,
Even at this distance I can see the tides,
And as the evening darkens, lo! how bright,
No one alone: from each projecting cape
Like the great giant Christopher it stands
And the great ships sail outward and return
They come forth from the darkness, and their sails
The mariner remembers when a child,
Steadfast, serene, immovable, the same,
It sees the ocean to its bosum clasp
The startled waves leap over it; the storm
The sea-bird wheeling round it, with the din
A new Prometheus, chained upon the rock,
"Sail on!" it says: "sail on, ye stately ships!
Game - The light keepers on many of the earlier lighthouses often remained isolated for long periods of time. Students in cooperative groups could make a list of things that a light keeper might do to for entertainment. Have students share their ideas and design a game or activity for the child of a lighthouse keeper. Students could then play the games and imagine what it would be like to live in a lighthouse. Students should build a list of all the items they would need, were they planning an extended stay at a lighthouse. Have students keep in mind that living space in lighthouses is limited, and that they only have room for one suitcase.
Writing - Have students write a story about life in a lighthouse. Have them think about what they would miss most, what meals would be like, and how they would get to school.
Life Science - Many light keepers or their wives became involved in gardening to beautify the lighthouse. Have students design and draw a plan that would beautify the lighthouse grounds. When choosing their plant-life, encourage students to consider the types of conditions the plants would have to endure to flourish near a lighthouse. Should some students wish to include a vegetable garden, have them research the varieties of vegetables that would most aptly suit the environment.
Art - Lighthouses along coastlines were often used as "daymarks." A daymark is a landmark used during the day by sailors. However, lighthouses along some coastlines were identical in design. Therefore, they had to be painted in different ways so that they could be used as daymarks. Have students draw several lighthouses of the same design and color them in ways that would be visible from far away. Once these are completed, display them in class. Students will be surprised at the variations.
Geography/Mathematics - Have students research the geography and history of the lighthouses along the Florida coast. Students could take a map of Florida and locate the lighthouses that still exist on the map. Many of these lighthouses are open for tours and belong to the National Parks Service. Have students plan a trip to one of the open lighthouses close to them and calculate how much money the trip would cost.
Journal Writing - Light keepers usually kept a journal or log documenting the events of the day. Have students pretend they are keeping a log and record some of the things they might encounter during a two-week stay at a lighthouse. Ask them to include any ships they might see and any activities they would participate in. You may want to have students discuss in small groups the kinds of things they could write about before they begin.
Research - There are Fresnel lenses found in other things besides lighthouses. Have students find out what some of them are and be able to explain how they help us. For example, Fresnel lenses are now created out of plastic and are used for solar energy collection.
Lighthouse Model - Using print and electronic media, students should design a model or draw a picture of a lighthouse that exists somewhere in the United States. The name of the lighthouse, its location, and the date it was constructed should be on this project. Encourage students to research the type of light or lens used and to include historical information. Have them look into whether or not the lighthouse has been relocated due to coastal erosion, if it is part of the National Parks system, and whether or not it was ever used as a personal residence.
Current Issues - Even though many lighthouses are not currently operational, there are conservationists who believe these should be preserved. In some locations, erosion has caused the land on which the structures rest to become unstable. Divide students into groups to debate whether or not lighthouses should be preserved, restored, or moved to another location. Students should justify their ideas and opinions with facts.
Reading/Social Studies - Have students read, or read together as a class, A Light In The Storm by Karen Hesse (ISBN 0590567330). This is a civil war diary of a young woman who is part of a family responsible for the lighthouse on Fenwick Island, Delaware. This story not only provides a literature connection to light and optics, but it can also provide an introduction to the study of the life during the Civil War.
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