In the mid 1800s, British surveyors discovered a discrepancy of 150 m in horizontal distance between 2 stations in India 625 km apart, as measured by surveying and as calculated from astronomical readings. J.H. Pratt, a British physicist, explained the discrepancy by arguing that the mass of the Himalaya Mountains would deflect the plumb line (a way of measuring vertical) northward at each station, but more at the station closer to the Himalayas. However, Pratt's estimate of how much the mass of the Himalayas should deflect the plumb line was 3 times the amount actually measured. G.B. Airy, another British scientist, explained the difference by theory of isostasy. He proposed that the Earth's crust is floating on a dense, plastic substratum and that the extreme elevation of the Himalayas is supported by a root of low-density rocks, much as an iceberg is supposed by its underwater mass. Pratt later agreed that the crust is in a state of flotational balance, but hypothesized that topography is supported by a crust having a uniform thickness below sea-level, but varying density (e.g., a balsa wood plank will float higher than an oak plank).
Evidence supporting the idea of isostasy comes from Scandanavia and the Hudson Bay lowlands of North America, where the land is slowly rising. During the Pleistocene ice ages, the land there was once covered by a sheet of ice 3 kilometers thick, weighing down the crust. The ice melted some tens of thousands of years ago, and the crust is still rising slowly because isostatic balance has not yet been attained.
Worldwide gravity measurements indicate that some form of isostatic compensation exists over most of the Earth's surface, although the density distribution within the crust is far more complicated than suggested by either the Airy or Pratt models.
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