Albert Einstein (2016). “The Albert Einstein Collection: Essays in Humanism, The Theory of Relativity, and The World As I See It”, p.185, Open Road Media
United States. President (1929-1933 : Hoover), Herbert Hoover (1974). “Herbert Hoover: Containing the Public Messages, Speeches, and Statements of the President”
"A Library of American Literature from the Earliest Settlement to the Present Time Vol. V". Book by Edmund Clarence Stedman and Ellen Mackay Hutchinson, 1888.