Caldwell, Wallace Everett, author
Holt, Rinehart and Winston, 1963
Buku Teks
Hodges, Henry, author
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., author
Simon & Schuster, 1994
Buku Referensi