Portal:Scotland/Selected article/Week 46, 2009

The Schiehallion experiment was an eighteenth-century experiment to determine the mean density of the Earth. Funded by a grant from the Royal Society, it was conducted in the summer of 1774 around the Scottish mountain of Schiehallion, Perthshire. The experiment involved measuring the tiny deflection of a pendulum due to the gravitational attraction of a nearby mountain. Schiehallion was considered the ideal location after a search for candidate mountains, thanks to its isolation and almost symmetrical shape.

The experiment had previously been considered, but rejected, by Isaac Newton as a practical demonstration of his theory of gravitation. However, a team of scientists, notably Nevil Maskelyne, the Astronomer Royal, were convinced that the effect would be detectable and undertook to conduct the experiment. The deflection angle depended on the relative densities and volumes of the Earth and the mountain: if the density and volume of Schiehallion could be ascertained, then so could the density of the Earth. Once this was known, then this would in turn would yield approximate values for those of the other planets, their moons, and the Sun, previously known only in terms of their relative ratios. As an additional benefit, the experiment provided cartographers with contour lines, devised to simplify the process of surveying the mountain.