Melissa Tierney, age 13, of Watertown, N.Y., for her question:
HOW DOES A LIGHTHOUSE WORK?
The lantern that houses the light is the most important part of a lighthouse. It is the complicated mechanism that enables the light beam to carry far out to sea.
A lighthouse's lens is usually shaped somewhat like a barrel. It may be eight feet high and six feet in diameter. These large lights can be seen 30 miles away. Most of them send a flashing light which can be seen every few seconds.
The light is made to flash by revolving the lens around the lamp. Part of the lens is blacked out by a screen so that the light is hidden for several seconds each time the lens revolves. The lens and lamp are sheltered in a glass and steel enclosure known as "the lamp room."
Each lens face has a large plane convex section of its own surrounded by several rings of prisms. The prisms reflect light along the path of focus and make the main light stronger. Some of the largest lenses have 200 prisms.
The entire lens is placed on a frame that either revolves on rollers or floats in a tank of mercury. It is operated by a clocklike device that is kept in motion by a suspended weight or by an electric motor. The flashing light can be seen only when the lens is directly facing the person who is looking at it.
When any side is covered with colored glass, colored light passes through. Then a flashing light may give out a series of flashes, of different colors.
There are also many small, automatic lights placed along rivers and lakes, and at hundreds of points along the coasts where building a lighthouse would be too expensive. These lights have small lenses made of pressed glass. Almost all of these lights are operated by electric batteries.
Many lighthouses are equipped with radio beacons that send out signals that can be picked up by shipboard radio direction finders.
Fully automatic lighthouses are replacing many older lighthouses. Most of the new lighthouses operate without keepers. Many of the old lighthouses are being preserved as historic monuments.
Modern lighthouses are built at ports and harbors, on isolated rocks and some on capes and headlands. Others are some distance from shore, with their foundations sunk beneath the water. They must be very strong for they are exposed to furious ocean gales and driving waves.
Some lighthouses are tall, round, stone towers that are narrow at the top. Others are made of iron frames, cast iron or reinforced concrete.
Lighthouses give out one of five basic types of signals: fixed, which is a steady light; flashing, which has periods of darkness that are longer than the periods of light; occulting, with the flashes of light lasting longer than the dark periods; group flashing of two or more flashes at regular intervals; and group occulting, where a fixed light with two or more periods of darkness appear at regular intervals.
There are several variations of these five basic patterns.