How a Scottish mountain helped scientists weigh the entire planet in 1774

Nature · Published 3 October 2026 Swipe more like this →

In short: In 1774, the Royal Society sent the Astronomer Royal to a remote peak in Perthshire to conduct an experiment of attraction. By measuring how a mountain pulled a plumb line, they aimed to calculate the density of the Earth, effectively weighing our world and debunking the popular Hollow Earth theory.

The Schiehallion experiment was born from a desire to test Isaac Newton’s theory of gravitation. Newton had speculated that a large mass, such as a mountain, might exert a measurable gravitational pull on a plumb line, causing it to deviate from the true vertical. While Newton doubted such a tiny effect could be detected, by the 1770s, the Royal Society was ready to try. Astronomer Royal Nevil Maskelyne proposed the project, arguing it would bring honor to the nation and provide a way to determine the Earth's mean density.

Finding the right site was a rigorous process. After surveyor Charles Mason scouted various locations, he selected Schiehallion in the Scottish Highlands. The mountain was chosen for its isolation and symmetrical shape, which simplified the complex gravitational calculations. Maskelyne spent over 16 weeks on the mountain in 1774, using high-precision instruments to measure the deflection of his pendulum against the stars. To fund this ambitious endeavor, the Royal Society had to liquidate assets, including selling shares in the East India Company and leasing property.

The success of the experiment relied on the mathematical ingenuity of Charles Hutton. To determine the mountain's volume, Hutton processed thousands of survey measurements, inventing the use of contour lines to visualize terrain relief. His calculations allowed the team to compare the mountain's mass to that of the Earth. They concluded that the planet was roughly twice as dense as the mountain's surface rocks, a result that remains within 20% of modern values. Beyond the science, the expedition left a mark on local folklore, including a legendary fire that destroyed a ghillie's fiddle—an instrument Maskelyne allegedly replaced with a Stradivarius.

This achievement was significant for more than just its numerical accuracy. It provided empirical evidence that the Earth was not a hollow shell, a concept still held by some thinkers of the era. By establishing the Earth's density, scientists could also begin to estimate the masses of the Sun and other planets, which had previously been known only through relative ratios. The experiment remains a landmark in the history of geophysics, demonstrating how a singular, well-executed observation could fundamentally alter our understanding of the planet's interior.

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