Robert Maynard Hutchins, editor in chief
Encyclopaedia Britannica, Inc., 1952
Buku Referensi
Artikel Jurnal
edited by Harold C. Gardiner
Devin-Adair Co., 1949
Buku Teks
Keller, Helen Rex
McMillan Company, 1959
Buku Referensi
Campbell, Oscar James
Crofts, 1947
Buku Teks