Hodges, Henry
Penguin Press, 1970
Buku Teks
Glover, T. R.
Penguin Books, 1944
Buku Teks
prepared by the Special Publications Division, National Geographic Society, Washington, D.C.
The National Geogarphic Society, 1986
Buku Referensi
Mellersh, H.E.L.
Simon & Schuster, 1994
Buku Referensi
Caldwell, Wallace Everett
Holt, Rinehrt and Winston, 1963
Buku Teks