Campbell, Oscar James, author
Crofts, 1947
Buku Teks
Johns, Anthony Hearly, author
Australian National University Press , 1979
Buku Teks
Watt, Homer A., author
Scott, Foresman and company , 1925
Buku Teks
Merriam-Webster, 1995
Buku Referensi
Gillespie, Sheena, author
Allyn and Bacon , 1994
Buku Teks