John Playfair on Schiehallion, 1801–1811
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.
Figures & Tables
The Making of the Geological Society of London
Founded in 1807, the Geological Society of London became the world’s first learned society devoted to the Earth sciences. In celebration of the Society’s 200-year history, this book commemorates the lives of the Society’s 13 founders and sets geology in its national and European context at the turn of the nineteenth century. In Britain, geology was emerging as a subject in its own right from three closely related disciplines — chemistry, mineralogy and medicine — disciplines that reflect the principal professions and interests of the founders. The tremendous energy and cooperation of these 13 men, about whom little was previously known, quickly mobilized like-minded men around the country and fuelled the nation’s passion for geology; an enthusiasm that soon spread to America and Australia. Two previously unpublished works from this period, essential to understanding the founding of the Society, are reproduced here for the first time. The book closes with a review of the Society’s 2007 Bicentenary celebrations.