Mortimer J. Adler, editor in chief
Encyclopedia Britanica, 1994
Buku Referensi
Artikel Jurnal
edited by Harold C. Gardiner
Devin-Adair Co., 1949
Buku Teks
Keller, Helen Rex, author
McMillan Company, 1959
Buku Referensi
Campbell, Oscar James, author
Crofts, 1947
Buku Teks