Galileo Galilei (1967). “Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican, Second Revised Edition”, p.59, Univ of California Press
Henry Ward Beecher (1887). “Beecher : Christian Philosopher, Pulpit Orator, Patriot and Philantropist: A Volume of Representative Selections from the Sermons, Lectures, Prayers, and Letters of Henry Ward Beecher”