These experimental and mathematical analyses were applied to several areas of geophysics : Earth 's shape , density , and gravity field ( Pierre Bouguer , Alexis Clairaut and Henry Cavendish ) , Earth 's magnetic field ( Alexander von Humboldt , Edmund Halley and Carl Friedrich Gauss ) , seismology ( John Milne and Robert Mallet ) , and the Earth 's age , heat and radioactivity ( Arthur Holmes and William Thomson , 1st Baron Kelvin ) .