Ballard, Robert D.
National Geographic Society, 1983
Buku Referensi
Hunt, Kellogg W.
Houghton Miffiln, 1967
Buku Teks SO
edited by Robert L. McDougal
University of Toronto Press in association with Carleton University, 1959
Buku Teks
Unger, Peter
Oxford University Press , 1996
Buku Teks
editor, Clifton Fadiman
Doubleday, 1990
Buku Teks SO