John Playfair on Schiehallion, 1801–1811

Abstract John Playfair first visited the Scottish mountain, Schiehallion, during Nevil Maskelyne's 1774 plumbline deflection experiment, which was conducted to measure the density of the Earth. The mathematician Charles Hutton analysed the survey data from the experiment, reporting the mean Earth specific gravity as 4.5 in 1778. Playfair undertook a lithological mapping exercise in 1801, to improve the accuracy of Hutton's estimate, and reported a range of 4.56–4.87 in 1811. The computation of the gravitational effect of topography with variable subsurface density effectively made him the creator of the first geophysical model. As such, not only was Playfair's Schiehallion contribution pioneering in itself, but it was representative of his more significant works in both mathematics and geology, in that he built on existing benchmark work with novel and valuable additions of his own. Although Playfair's map of the extent of the Schiehallion quartzite was quite accurate, the Society's fourth President, John MacCulloch, having visited Schiehallion, was dismissive of Playfair's representation of the subsurface density variation. MacCulloch spent several years searching Scotland for a more favourable site for a plumbline experiment, travels that allowed him to compile the data for his 1836 geological map of Scotland.