Abraham Lincoln, Don Edward Fehrenbacher (1977). “Abraham Lincoln, a Documentary Portrait Through His Speeches and Writings”, p.39, Stanford University Press
Second lecture on discoveries and inventions, delivered to the Phi Alpha Society of Illinois College at Jacksonville, Illinois, (February 11, 1859); "The Collected Works of Abraham Lincoln, Vol. 3" edited by Roy P. Basler, (p. 357), 1953.