Alexander Arbuthnot from 1574 to 1596 – also Principal of King's College, Aberdeen, Moderator in 1573 and 1577 David Rait 1598 to 1632, Moderator of Synod and Moderator of the Aberdeen General Assembly of 1605 Alexander Scrogie 1621 to 1640 William Strachan 1640 to 1653 John Seaton 1656 to 1662 Alexander Scrogie (secundus) 1659 to 1661 Alexander Middleton 1661 to 1665 became Principal of King's College, Aberdeen Robert Reynold 1665 to 1670 also Rector of King's College, Aberdeen George Strachan (son of William) 1672 to 1678 George Chalmers 1729 to 1745 also Principal of King's College, Aberdeen Patrick Forbes second charge 1816 to 1847, Moderator in 1829 Conservation and restoration There has been considerable investment in recent years in restoration work and the improvement of the display of historic artefacts at the cathedral. The battlements of the western towers, incomplete for several centuries, have been renewed to their original height and design, greatly improving the appearance of the exterior. Meanwhile, within the building, a number of important stone monuments have been displayed to advantage. These include a possibly 7th–8th-century cross-slab from Seaton (the only surviving evidence from Aberdeen of Christianity at such an early date); a rare 12th century sanctuary cross-head; and several well-preserved late medieval effigies of cathedral clergy, valuable for their detailed representation of contemporary dress. A notable modern addition to the cathedral's artistic treasures is a carved wooden triptych commemorating John Barbour, archdeacon of Aberdeen (d. 1395), author of The Brus. In 1987, bells from the deconsecrated St Stephen's Church, Ealing were restored by Eayre & Smith and installed in St Machar's. It is now one of the few churches in Scotland to have a set of bells designed for change ringing. In 2020 the cathedral embarked on a £1.85m project to re-slate the roof, clean the heraldic ceiling, and repair some of the stained glass windows. Work, which was delayed by Covid-19 began mid-year and is expected to take until the end of 2020. Stained Glass Scots law and religious convention only allowed the re-introduction of stained glass in 1866 and there were no manufacturers then to fill the skills gap so the earliest windows are of English creation. The range and quality of stained glass in St Machar's is exceptional. Main west window, by Clayton and Bell 1870 – a set of seven narrow linear windows Main east window, by William Wilson 1953 – flanked