Georges Ifrah (2001). The Universal History of Computing: From the Abacus to the Quatum Computer, p. 171, Trans. E.F. Harding, John Wiley & Sons, Inc. (See [1]Archived 2006년 10월 8일 - 웨이백 머신)
Georges Ifrah (2001). The Universal History of Computing: From the Abacus to the Quatum Computer, p. 171, Trans. E.F. Harding, John Wiley & Sons, Inc. (See [1]Archived 2006년 10월 8일 - 웨이백 머신)
A. Lehr (1981), De Geschiedenis van het Astronomisch Kunstuurwerk, p. 227, Den Haag. (See odur.let.rug.nlArchived 2010년 10월 25일 - 웨이백 머신)
