Application The Third and a Half Order lens was used almost exclusively on the Great Lakes, and naturally enough filled a void between the Third and Fourth order lenses. These lenses were used in coastal lights and for marking sounds. Of the twelve such lenses incorporated in US Lighthouses, seven were installed in Lights on the Great Lakes. These seven were Eagle Bluff, Detour Reef, St. Helena Island, Michigan Island, Huron Island, Grays Reef and Toledo Harbor. Specifications
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| Lens height: | 40" (estimate) | |
| Focal length: | 14.7" | |
| Weight of assembly: | 1,000 lbs. (estimate) | |
| Relative brightness (over sixth order): | X3 (approx) | |
| Useful range: | Up to 17 miles. | |
| Number in US Lighthouses in 1884: | 11 | |
| Number in US Lighthouses in 1922: | 25 | |
| Oil consumption per hour | 6 Ounces (estimate) | |
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References Annual Report of the Lighthouse Board, 1884 The United States Lighthouse Service, Dept of Commerce, 1915 Aids to Navigation Manual, US Coast Guard, 1945 Comparative Table of Lens Orders, Mabel A. Baiges, 1988 Letter from Helen & Al Gademsky, 07/02/01 |
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