Friday, April 29, St. George Island- The St.
George Lighthouse Association is pleased to announce a major addition to the
collection at the Keeper’s House Museum.
A life-size
reproduction of the Cape S
The museum is closed on Thursdays.
Docents will be on hand to explain the science and
history behind the new exhibit.
The original lighthouse lens was lost. It is known
to have been deinstalled in 1949 and moved to a warehouse somewhere in New Orleans for storage,
but there the records end.
To quote association board member Bob Heide, “We
felt that the lighthouse without the lens was like an engagement ring without
the diamond. It just wasn’t right.”
After attempting to discover the whereabouts of
the 2000-pound third order glass Fresnel lens for about ten years, the board of
the lighthouse association decided to go to “Plan B” and began searching for
somebody who could create a reproduction.
They found engineer Dan Spinella of Artworks
Florida.
In his spare time, he constructs lighthouse lenses
and he has produced them for lighthouses all over the US including two in Hawaii .
Spinella subcontracts the work of machining and
polishing the prisms and brass framework.
He labored for eight mont hs on the lens for the Cape St. George
Light, which is an assembly of 149 prisms set in a brass framework based on
photographs of standard lenses. Installing a single prism can take up to two
hours.
The model stands 68-inches tall and 42-inches in
diameter and produces light for all 360 degrees. It contains an electric bulb
in a replica brass lamp which imparts a warm yellow glow to the museum exhibit.
Like the original, the model includes a door that would have been used to allow
the keeper to tend the kerosene lamp.
The reproduction of the Fresnel lens is
constructed of acrylic unlike the original, which was glass.
Glass lenses cost about ten times as much as
acrylic and a single prism can have a $20,000 price tag. Glass is also much
heavier than acrylic. The original lens weighed about a ton. The model tips the
scales at around 600 pounds.
The Fresnel lens was a technological wonder at the
time of its invention.
By the middle of the eighteenth century, the
"state of the art" in lighthouse optics consisted of single or
multiple whale oil burning lamps placed in the lantern at the top of a tower.
Only three percent of the light ended up being visible at any given point at
sea.
By 1820, US lighthouse optics had progressed
to the use of a silvered-metal parabolic reflector placed behind the oil lamp,
known as the Argand Lamp.
While this was a vast improvement over the simple
lamp, only 39 percent of the light was transmitted in the desired direction.
The visible distance was still limited to a maximum of 15 to 20 miles in clear
conditions.
With shipping increasing throughout the world, an
optical system was desperately needed to cast light many miles out to sea,
providing ample advanced warning of hazards.
In 1819, the French government commissioned 34
year old Augustin Jean Fresnel to develop an improved lighting system for
French lighthouses. Fresnel began investigating ways that glass lenses could be
used to concentrate the light source. Since a single lens of sufficient
strength would be too large to be practical, Fresnel began looking at multiple
lenses that would surround the light source and capture the light rays emitted
and direct them into a narrow horizontal beam.
With Fresnel's optic array, output was increased
dramatically from the old reflector systems, with as much as eighty percent of
the light transmitted over twenty miles out to sea.
The first Fresnel lens was used in 1823 in
the Cordouan Lighthouse at the mouth of the Gironde
estuary in France.
Our own island light was reconstructed on Cape St.
George just 29 years later in 1852, and the third order Fresnel lens was added
in 1857.
As most county
residents know, the lighthouse originally stood on Little St. George Island but
was felled after years of shoreline erosion when Hurricane Wilma was churning
in the Gulf of Mexico on Friday, October
21, 2005. A group of lighthouse enthusiasts salvaged material from the wreckage
and raised the funds to erect the tower in its current place of honor at island
center on St. George Island
For more information, call (850) 927-7745.
http://live.oysterradio.com/
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