Florence Nightingale, Michael D. Calabria, Janet A. Macrae (1994). “Suggestions for Thought by Florence Nightingale: Selections and Commentaries”, p.41, University of Pennsylvania Press
Leonhard Euler, C. Truesdell (1980). “The Rational Mechanics of Flexible Or Elastic Bodies 1638 - 1788: Introduction to Vol. X and XI”, p.200, Springer Science & Business Media