Smith, Charles Edward
Appleton-Century-Crofts, 1939
Buku Teks SO
Hodges, Henry
Penguin Press, 1970
Buku Teks SO
prepared by the Special Publications Division, National Geographic Society, Washington, D.C.
The National Geogarphic Society, 1986
Buku Referensi
Glover, T. R.
Penguin Books, 1944
Buku Teks SO
Glover, T. R.
Penguin Books, 1944
Buku Teks SO