Colby's Compensation Bars, as used on the Lough Foyle Base, 1827-1828
Colby's Compensation Bars, as used on the Lough Foyle Base, Londonderry in 1827-1928. Two bars, each ten feet long are joined at the centre but allowed to expand and contract freely depending on changes in temperature. Base-line measurement was taken with three sets of bars, with the distance between a measuring mark, or compensation point, on the metal tongue of the bar being read by microscopes.
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- Ordnance Survey
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