1. C. Babbage, letter to Mrs. James (approximate date 20 February 1831), sold at Sotheby's Sale, Lot 378, 29 October 1968.
2. Sur l'emploi plus ou moins fréquent des mèmes lettres les différentes langues;Babbage;Corresp. Math. Phys.,1831
3. C. Babbage, letter to the Duke of Somerset, 20 March 1846, correspondence of the Edward Adolphus Seymour (St. Mawr), 11th Duke of Somerset: 13 letters (1830–1849) from Babbage to the Duke, The Bulstrode Collection, Buckinghamshire Record Office, Aylesbury.
4. C. Babbage, letter to J.W.F. Herschel, 24 March 1846, correspondence with members of the Herschel Family, History of Science Collection, Harry Ransom Humanities Research Centre, Univ. Texas, Austin, TX.
5. Slg. Darmst. H 1830 (5): Babbage, Charles;Babbage,1846