The 20th century will remain as one of the richest period in geophysics. Among others we can cite the discovery of the internal structure of the Earth by seismologists, followed by the first realistic model of geomagnetic field resulting from internal dynamo processes in the iron rich liquid core, and of course the establishment of plate tectonics. However, none of that would have been possible without the precursory works of scientists at the end of the 19th century. Indeed, the second half of the 19th century is marked by the introduction of fundamental concepts and theoretical development that form the basis of modern geophysicists tool set. The internal structure of the Earth was one of the greatest remaining challenges that animated the scientific community at that time. Several models including purely solid, partially molten or even mostly liquefied interior were suggested. However, without further discriminant observations it was impossible to quantify the validity of these speculations. D'Alembert in the introduction of his 1749 treatise pointed out that the knowledge of the Earth's internal structures was necessary to achieve a correct theory of nutation, leading to the idea that astronomical observations may be used to probe the Earth's interior. Revisiting the precursory work of W. Hopkins (1839), H. Poincaré, Lord Kelvin, T. Sloudsky and S. Hough investigated the role of an internal liquid cavity on the expected nutation and precession of the Earth. Although their final conclusions suggested a completely solid Earth rather than a partially liquid interior, the mathematical and physical framework they established remains valid and widely used in modern planetary dynamics.