1. Le Calcul Simplifié, M. d’Ocagne, 1905, pages 24–31.
2. Origin of Modern Calculating Machines, J. A. V. Turck, 1921, pages 11–13.
3. United States patent number 1,514,954 (Crompton), claim number 7.
4. Mathematics I—Calculating Machines and Instruments, D. Baxandall. Catalogue of the collections in the Science Museum of South Kensington (London, England), 1926, pages 3, 14–16.
5. Le Calcul Simplifé, M. d’Ocagne, 1905, page 30.