In preferring the parish of Ruthwell to the better position and wider field of ministerial usefulness presented at Lochmaben, Mr Duncan was influenced by the consideration, that the population of the former parish was such as would enable him to extend the pastoral superintendence to every individual of his flock. In this respect he realised his wishes; but not content with efficiently discharging the more sacred duties of a parochial clergyman, he sought with devoted assiduity, the amelioration of the physical condition of his people. Relieving an immediate destitution in the parish, by a supply of Indian corn brought on his own adventure, he was led to devise means of preventing the recurrence of any similar period of depression. With this intention, he established two friendly societies in the place, and afterwards a local bank for the savings of the industrious. The latter proved the parent of those admirable institutions for the working classes, known as _Savings' Banks_, which have since become so numerous throughout Europe and the United States of America. The Ruthwell Savings' Bank was established in 1810. Numerous difficulties attended the early operation of the system, on its general adoption throughout the country, but these were obviated and removed by the skill and promptitude of the ingenious projector. At one period his correspondence on the subject cost him in postages an annual expenditure of one hundred pounds, a sum nearly equal to half the yearly emoluments of his parochial cure. The Act of Parliament establishing Savings' Banks in Scotland, which was passed in July 1819, was procured through his indomitable exertions, and likewise the Act of 1835, providing for the better regulation of these institutions.
At Ruthwell, Dr Duncan introduced the system of popular lectures on science, which has since been adopted by Mechanics' Institutes. Further to extend the benefits of popular instruction and entertainment, he edited a series of tracts entitled "The Scottish Cheap Repository," one of the first of those periodicals devoted to the moral improvement of the people. A narrative designated "The Cottager's Fireside," which he originally contributed to this series, was afterwards published separately, and commanded a wide circulation. In 1809, Dr Duncan originated the _Dumfries and Galloway Courier_, a weekly newspaper which he conducted during the first seven years of its existence. He was a frequent contributor to "The Christian Instructor," and wrote the articles "Blair" and "Blacklock" for the _Edinburgh Encyclopædia_. At the request of Lord Brougham, he composed two treatises on Savings' Banks and Friendly Societies, for publication by the "Society for the Diffusion of Useful Knowledge." In 1819, he published the "Young Country Weaver," a tale calculated to disseminate just political views among the manufacturing classes; and in 1826 a tale of the times of the Covenant in three volumes, with the title of "William Douglas, or the Scottish Exiles." Deeply interested in the question of Slave Emancipation, he contributed a series of letters on the subject to the _Dumfries Courier_, which, afterwards published in the form of a pamphlet, excited no inconsiderable attention. His most valuable and successful publication, the "Sacred Philosophy of the Seasons" appeared in 1836-7 in four duodecimo volumes.