Whitaker, Arthur P.
Harvard University Press, 1948
Buku Teks
Risjord, Norman K.
Prentice-Hall, 1985
Buku Teks
Stuart, Graham Henry, 1887-
Appleton-Century-Crofts, 1955
Buku Teks
Rand McNally and Company.
Rand Mcnally & Company, 2006
Buku Teks
Washington U.S. Department of Health, Education, and Welfare 1955
Buku Teks