Schiehallion experiment

The 1774 experiment measured how Schiehallion’s gravitational pull deflected plumb lines to estimate Earth’s mean density, providing an early measurement of Earth’s mass.

Plumb line: A suspended weight that indicates the direction of local gravity. Its direction shifted toward Schiehallion because the mountain exerted a sideways gravitational pull.
Nevil Maskelyne: An English astronomer who served as Astronomer Royal from 1765 to 1811. He led the field observations and interpreted the plumb-line measurements.
Astronomical transit: The passage of a celestial object across a telescope’s field of view or a reference line. Maskelyne used stellar observations to establish the vertical directions at sites around the mountain.
Earth's mass: The total mass of Earth, inferred from its gravitational effects and other measurements. Combining Earth’s volume with the experiment’s density estimate gave an estimate of its mass.
Gravitational attraction: The mutual force by which masses attract one another. The mountain’s attraction was compared with Earth’s pull to infer their relative masses.
Charles Mason: An English astronomer and surveyor known for geodetic work in Britain and America. His earlier survey of Schiehallion helped identify the mountain as a possible test site.
Plumb-line deflection at Schiehallion: The measured angular shift of plumb lines toward Schiehallion during the 1774 experiment. These opposing shifts were the experiment’s direct observational result.
Isostasy: The approximate gravitational balance of Earth’s crust, with elevation compensated by variations in underlying mass. Later gravity studies had to account for crustal compensation absent from the mountain’s simple model.
Pendulum: A mass suspended from a pivot that swings under gravity. The pendulum’s alignment provided a practical way to measure the plumb line’s deflection.
Henry Cavendish: An English natural philosopher whose experiments included measuring Earth’s density. His later torsion-balance experiment refined the same broad measurement goal.

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