It is of very great importance in many ways that astronomers and surveyors know as exactly as possible the dimensions of the spheroid. Many men have made estimates based upon astronomical facts, pendulum experiments and careful surveys, as to the equatorial and polar diameters of the earth. Perhaps the most widely used is the one made by A. R. Clarke, for many years at the head of the English Ordnance Survey, known as the Clarke Spheroid of 1866.
It is upon this spheroid of reference that all of the work of the United States Geological Survey and of the United States Coast and Geodetic Survey is based, and upon which most of the dimensions given in this book are determined. In 1878 Mr. Clarke made a recalculation, based upon additional information, and gave the following dimensions, though it THE FORM OF THE EARTH 30 is doubtful whether these approximations are any more nearly correct than those of 1866.