Anderson, Wallace L.
Holt, Rinehart and Winston, 1966
Buku Teks SO
edited by Duddley Bailey
W.W. Norton, 1965
Buku Teks SO
Encyclopaedia Britannica, Inc. ; Funk & Wagnalls Co., 1961
Buku Referensi
Encyclopaedia Britannica, Inc. ; Funk & Wagnalls Co., 1961
Buku Referensi
St. Martin's press, 1981
Buku Teks SO