Throughout his logical writings De Morgan was led by the idea that the followers of the two great branches of exact science, logic and mathematics, had made blunders,--the logicians in neglecting mathematics, and the mathematicians in neglecting logic. He endeavoured to reconcile them, and in the attempt showed how many errors an acute mathematician could detect in logical writings, and how large a field there was for discovery. But it may be doubted whether De Morgan's own system, "horrent with mysterious spiculae," as Hamilton aptly described it, is fitted to exhibit the real analogy between quantitative and qualitative reasoning, which is rather to be sought in the logical works of Boole.
Perhaps the largest part, in volume, of De Morgan's writings remains still to be briefly mentioned; it consists of detached articles contributed to various periodical or composite works. During the years 1833-1843 he contributed very largely to the first edition of the _Penny Cyclopaedia_, writing chiefly on mathematics, astronomy, physics and biography. His articles of various length cannot be less in number than 850, and they have been estimated to constitute a sixth part of the whole _Cyclopaedia_, of which they formed perhaps the most valuable portion. He also wrote biographies of Sir Isaac Newton and Edmund Halley for Knight's _British Worthies_, various notices of scientific men for the _Gallery of Portraits_, and for the uncompleted _Biographical Dictionary_ of the Useful Knowledge Society, and at least seven articles in Smith's _Dictionary of Greek and Roman Biography_. Some of De Morgan's most interesting and useful minor writings are to be found in the _Companions to the British Almanack_, to which he contributed without fail one article each year from 1831 up to 1857 inclusive. In these carefully written papers he treats a great variety of topics relating to astronomy, chronology, decimal coinage, life assurance, bibliography and the history of science. Most of them are as valuable now as when written.
Among De Morgan's miscellaneous writings may be mentioned his _Explanation of the Gnomonic Projection of the Sphere_, 1836, including a description of the maps of the stars, published by the Useful Knowledge Society; his _Treatise on the Globes, Celestial and Terrestrial_, 1845, and his remarkable _Book of Almanacks_ (2nd edition, 1871), which contains a series of thirty-five almanacs, so arranged with indices of reference, that the almanac for any year, whether in old style or new, from any epoch, ancient or modern, up to A. D. 2000, may be found without difficulty, means being added for verifying the almanac and also for discovering the days of new and full moon from 2000 B. C. up to A. D. 2000. De Morgan expressly draws attention to the fact that the plan of this book was that of L. B. Francoeur and J. Ferguson, but the plan was developed by one who was an unrivalled master of all the intricacies of chronology. The two best tables of logarithms, the small five-figure tables of the Useful Knowledge Society (1839 and 1857), and Shroen's Seven Figure-Table (5th ed., 1865), were printed under De Morgan's superintendence. Several works edited by him will be found mentioned in the _British Museum Catalogue_. He made numerous anonymous contributions through a long series of years to the _Athenaeum_, and to _Notes and Queries_, and occasionally to _The North British Review_, _Macmillan's Magazine_, &c.