The Penny Cyclopædia of the Society for the Diffusion of Useful Knowledge was a multi-volume encyclopedia edited by George Long and published by Charles Knight alongside the Penny Magazine. Twenty-seven volumes and three supplements were published from 1828 to 1843.
The contributors to the Penny Cyclopædia were not individually credited with the articles they created, although a list of their names appears in volume 27. The contributors included many notable figures of the period, including the librarian Henry Ellis, the biblical scholar John Kitto, the publisher Charles Knight, the critic George Henry Lewes, the mathematician Augustus De Morgan, the surgeon James Paget, the statistician George Richardson Porter, the sanitary reformer Thomas Southwood Smith, and the art historian Ralph Nicholson Wornum.
The novelist Herman Melville is known to have used the Penny Cyclopædia while writing Moby-Dick and other novels, and scholar Paul McCarthy has suggested that the encyclopedia's coverage of moral insanity and monomania may have influenced Melville's characters in Moby-Dick and other writings.
- Society for the Diffusion of Useful Knowledge (1843). "List of Contributors". The Penny Cyclopaedia of the Society for the Diffusion of Useful Knowledge: Wales–Zygophyllaceae. 27. Knight. pp. v–vii.
- "The International Plant Names Index". Retrieved 27 February 2015.
- Baker, William (September 1974). "George Henry Lewes and the 'Penny Cyclopaedia': Twenty-Seven Unattributed Articles". Victorian Periodicals Newsletter. 7 (3): 15–18. Retrieved 6 December 2013.
- McCarthy, Paul (1987). "Forms of Insanity and Insane Characters in Moby-Dick". Colby Quarterly. 23 (1): 1–14. Retrieved 6 December 2013.