13.2 How do we know ... that mountains have roots?
Fig 13.7
Surveyors in the 1800s used the angle made by sighting on the North Star and a plumb bob to determine latitude. The plumb bob was presumed to always point toward the center of Earth as the center of mass—a reasonable assumption. However, survey errors in northern India in the mid-1800s were of
concern. John Pratt, a British mathematician, hypothesized that the mass of the Himalaya Mountains was deflecting the plumb bob. However, the calculations, based on masses derived from elevations, did not agree with the deflections encountered.