Jenks, Jeremiah Whipple
Doubleday, Page & Company, 1919
Buku Teks
Adams, Walter, 1922-1998
Macmillan, 1957
Buku Teks
Gunton, George
Appelton and Company , 1899
Buku Teks
Stocking, George W., author
The Twentieth Century Fund, 1948
Buku Teks
edited by Edwin Mansfield
W.W. Norton, 1964
Buku Teks