The 1774 Experiment
In 1772, England's Astronomer Royal, Nevil Maskelyne, proposed an experiment to the Royal Society to determine the mean density of the Earth. Based on an idea first suggested by Isaac Newton, the plan was to measure the tiny gravitational pull of a large mountain. By measuring how much a mountain could deflect a pendulum's plumb line from its true vertical position, scientists could calculate the ratio of the mountain's mass to the Earth's.
The Royal Society's "Committee of Attraction" dispatched astronomer and surveyor Charles Mason to find a suitable mountain. After a summer search in 1773, he selected Schiehallion in the Scottish Highlands. The mountain's isolation from other peaks, its symmetrical shape, and its steep northern and southern slopes made it an excellent natural site. Its east-west orientation also helped the experimental design.
In the summer of 1774, Maskelyne and his team set to work. They established small observatories on the north and south flanks of the mountain. For over 16 weeks, through frequent mist and rain, Maskelyne made hundreds of astronomical observations. He used a zenith sector, a precise telescopic instrument, to measure the apparent angle of specific stars relative to a plumb line. The difference in this angle between the north and south stations, after accounting for the actual difference in latitude, showed the mountain's gravitational influence. The final measured deflection was a mere 11.6 arcseconds—about 0.00322 degrees.
Calculating the Planet
With the deflection measured, the next task fell to mathematician Charles Hutton: calculating the volume and mass of Schiehallion itself. This was a laborious process that took him two years to complete. To determine the mountain's volume from the thousands of survey measurements, Hutton devised a new method. He drew lines on a plan connecting points of equal elevation, creating a visual representation of the terrain's form. This was the first documented use of what are now known as contour lines, a technique used in modern cartography.
Hutton calculated that the Earth's density was approximately 1.8 times that of the mountain. Using an estimated density for Schiehallion's rock of 2,500 kg/m³, he calculated the Earth's mean density to be 4,500 kg/m³. This result is within 20% of the modern accepted value of 5,515 kg/m³.
The Schiehallion experiment provided the first reliable measurement of the Earth's mass and density. The results demonstrated that the Earth was not hollow and its core must be composed of a much denser material than its surface rock, which Hutton correctly suggested was likely metallic. The experiment also confirmed Newton's law of universal gravitation.