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. As a man of science, the name of Dr Duncan is associated with the discovery of footprints of four-footed animals in the New Red-Sandstone. He made this curious geological discovery in a quarry at Corncocklemuir, about fifteen miles distant from his parochial manse. In 1823, he received the degree of D.D. from the University of St Andrews. In 1839, he was raised to the Moderator's chair in the General Assembly. In church politics, he had early espoused liberal opinions; at the Disruption in 1843, he resigned his charge and united himself to the Free Church. He continued to minister in the parish of Ruthwell, till the appointment of an assistant and successor a short time before his decease. Revisiting the scene of his ministerial labours after a brief absence, he was struck with paralysis while conducting service at a prayer-meeting, and two days afterwards expired. He died at Comlongon, the residence of his brother-in-law Mr Phillips, on the 12th February 1846, and his remains were committed to the church-yard of Ruthwell, in which he had ministered during an incumbency of upwards of forty-six years.