How Schehallion Helped Weigh the Earth

Transcript

In the Schehallion experiment, Nevil Maskelyne used a mountain as a giant natural test mass. Schehallion’s gravity was expected to pull a plumb-line slightly away from the true vertical, and the attached source reports that the combined deflection on the two sides of the hill was 11.6 seconds. The source does not carry the calculation through to Earth’s density, but the historical idea was straightforward: estimate the mountain’s pull from its surveyed size and rock density, compare that pull with the much larger force of Earth acting at the observing stations, and use that ratio to infer the mountain’s mass relative to Earth. From that, the Earth’s mean density could be found, and once mean density was known, multiplying it by Earth’s volume gave the planet’s mass. The attached source stops before those later steps, so the exact numerical conversion is not shown there.