These compensation bars, designed by Major General Thomas Colby, were an important development in the measurement of base lines. Previously the accuracy of triangulation was jeopardised by the affect of temperature on the length of the chains or bars used for the measurement of the base line. Colby's bars one made from brass and one iron, are joined together at their centre but allowed to expand or contract freely. Measurements taken were compensated for changes in temperature. These bars were first used in 1827-1828 to measure the Lough Foyle base line for the ordnance survey in Ireland.
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