Gravity surveying in early geophysics; I, From the time-keeping to figure of the Earth

This two-part review of the development of the measurement of the Earth's gravity field, and its application to geology, up to the early 1960s, is intended primarily for an earth-science readership. The focus here is on the pendulum, which played the dominant role in measurement of the intensity of gravity (g), both in absolute (at national observatories) and relative terms (at field stations), until the early twentieth century. Following discovery of the properties of the pendulum and its incorporation in time-keepers, early post-Newtonian investigations used the length of a pendulum beating seconds as a proxy for g. The goal was to obtain ever-improved knowledge of the "Figure of the Earth," initially encapsulated in determination of the degree of flattening of the oblate ellipsoid used as a model for the geometry of the globe. Developments in theory went hand-in-hand with both improvements in pendulum design and the establishment of a constantly expanding network of astrogeodetic stations as a basis for national cartographic surveys. By the late nineteenth century, results from astrogeodetic determinations of the length of a degree of arc and those derived from gravity determinations (by means of Clairaut's theorem) had converged to an inverse flattening of 298, and emphasis switched to determination of the geoid. However, by the 1840s, discrepancies between observed and model-fitted values were providing increasingly strong evidence that geological factors also affected the local value of g. This would give rise to the use of gravity determination as a geological exploration tool in the twentieth century.