An Early Geophysical Estimate of the Mean Density of the Earth: Schehallien, 1774

The Schehallien experiment, begun in 1774, represented one of the earliest attempts, after Bouguer's work in the Viceroyalty of Peru, to determine the mean density of the earth from the observed effects of topography on the direction of the plumb-line. The site chosen was a hill in Perthshire in the Scottish Highlands, and the work involved Nevil Maskelyne, the Astronomer Royal, who carried out the survey; Charles Hutton, the mathematician, who in 1778 published a method to compute the effect of a mass of known shape of the plumb-line; and John Playfair, who in 1811 published a detailed account of the lithology of the hill. Hutton's and Playfair's work foreshadowed in many respects modern aspects of gravity surveys such as terrain correction and density sampling.