Charlotte Bronte, Emily Bronte, Anne Bronte (2009). “The Bronte Sisters: Three Novels: Jane Eyre; Wuthering Heights; and Agnes Grey (Penguin Classics Deluxe Edition)”, p.373, Penguin
James Joseph Sylvester (1877). “Address Delivered by J.J. Sylvester, F.R.S. (corresponding Member of the Institute of France), Professor of Mathematics, at Johns Hopkins University on Commemoration Day, February 22, 1877”