Geology
About 1833, through meeting Adam Sedgwick at Barmouth and joining him in several excursions, he became intensely interested in geology. From then on, in papers published by the Cambridge Philosophical Society and the Geological Society of London, he defined the discipline of physical geology, making mathematical investigations dealing with the effects that an elevatory force, acting from below, would produce on a portion of the Earth's crust, in fissures and faults. In this way he discussed the elevation and denudation of the Lake District, the Wealden area, and the Bas Boulonnais.
Hopkins conceived of a largely solid but dynamic Earth threaded with cavities whereby hot vapours or fluids could create locally elevatory pressures. Such a model was at odds with the ideas of Charles Lyell whose theory was of a "steady state" with a largely liquid terrestrial interior, inside a solid crust no more than 100 miles thick. Hopkins presented a series of papers at the Royal Society between 1838 and 1842 analysing the Earth's rotation, including its precession and nutation, and using observations to support his theory, contending that they were inconsistent with a fluid interior. He also interpreted earthquakes and volcanoes through the same model in an 1847 British Association report.
As part of his investigations, Hopkins sought to quantify the effects of enormous pressures on the melting point and thermal conductivity of various substances. With the support of a grant from the Royal Society, he invoked the assistance of Thomson, James Prescott Joule and William Fairbairn to make measurements which he interpreted as supporting his theory. He further asserted that the cooling of the Earth had had no real impact on climate. He read a paper to the Geological Society On the Causes which may have produced changes in the Earth's superficial Temperature (1851). In his second address as president of the Geological Society of London (1853) he criticized Elie de Beaumont's theory of the elevation of mountain-chains and the imperfect evidence on which he saw it as resting.
Ultimately, it was Thomson who tactfully pointed out that, though Hopkins's conclusions about the Earth's structure were correct, his mathematical and physical reasoning was unsound.
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