The polymath Luigi Ferdinando Marsili (1658–1730) is generally regarded as the founder of scientific oceanography and marine geology. He also founded the renowned Istituto delle Scienze of Bologna in 1711 (a successor to the Florentine Accademia del Cimento). Marsili's major scientific concern and unfulfilled ambition was a Treatise on the Structure of the Earthy Globe (Trattato sulla Struttura del Globo Terreo), a work-in-progress, containing some 200 sheets with more than 35 water-colored plates and some 50 pen drawings, kept in the Main Library of the Bologna University.

Out of this material, Marsili's manuscript 90, A, 21 (dated 1728) is published for the first time as an appendix to this paper. It is an introduction to the planned treatise, accompanied by a summary and detailed index of its contents. This manuscript is useful in understanding the meaning of some recently published illustrations contained in the same collection. Such illustrations, together with their captions and the text of manuscript 90, A, 21, may be considered the earliest suggestions of the principle of isostasy, as well as of the concept of roots of mountain chains. Also expressed in Marsili's drawings are the concepts of Earth's spheroid-ellipsoid surface and the difference in thickness of “marine” (oceanic) and “mountainous” (continental) crust. His hemiglobal, water-colored section shows a balance of mountain-peak height and seafloor depth compared to sea level. This setting is formulated in the text as a general principle describing an isostatic condition. Different colors indicate three different types of crust beneath the deep seas, the low-elevation continental plains, and the mountain chains, respectively.