Mapping out the origins of contour lines

It was 250 years ago that Nevil Maskelyne, the astronomer royal, stood on Schiehallion, a lonely mountain in Perthshire, in a quest to find out how much the Earth weighed. It was called the Schiehallion experiment and Maskelyne first needed to work out the mass of the mountain by calculating its volume and estimating its mean density based on what sort of rock it contained. Once the mountain’s mass was calculated, the results could be scaled up to estimate the Earth’s mass.
Maskelyne and other scientists realised that if you could get close enough to the centre of the mass of a mountain, it might be possible to measure its gravity. That meant finding a mountain with steep slopes that was far enough away from surrounding mountains that could distort the measurements, and Schiehallion was chosen by his fellow astronomer and surveyor Charles Mason because of its isolated location and its beautiful symmetry, with a sharp ridge running from east to west and steep slopes tapering up to a sharp peak.
Observation stations were built on Schiehallion’s slopes to hang a pendulum. Maskelyne was looking for a tiny deflection of the pendulum away from its true vertical position, which is pulled by the Earth’s gravity, and for that Maskelyne first measured the true vertical by tracking the transit of 43 different stars from each observation station. From measuring this, he discovered a clear deviation of the pendulum close to the centre of the mass of the mountain.
Maskelyne was also helped by Charles Hutton, a mathematician and surveyor who mapped thousands of precise geographical readings on the mountain needed for the calculations. Hutton also had the ingenious idea of drawing lines of equal altitudes on a map of the mountain, forming a series of imperfect rings, incidentally inventing map contour lines.
The surveying took two years, often in atrocious weather, and the final results of the Schiehallion experiment were published in 1778. The estimation of the mass of the Earth was so good it was within 20 per cent of modern estimates.