David McCullough (2011). “David McCullough Library E-book Box Set: 1776, Brave Companions, The Great Bridge, John Adams, The Johnstown Flood, Mornings on Horseback, Path Between the Seas, Truman, The Course of Human Events”, p.1000, Simon and Schuster
Isaac Barrow (1734). “The Usefulness of Mathematical Learning Explained and Demonstrated: Being Mathematical Lectures Read in the Publick Schools at the University of Cambridge”, p.32