In the spring of 1879 died the venerable Principal Harper.  Dr. Cairns felt the loss very keenly, for Dr. Harper had been a loyal and generous friend and colleague, on whose clear and firm judgment he had been wont to rely in many a difficult emergency.  Besides, as his biographer has truly said, “he was habitually thankful to have someone near him whom he could fairly ask to take the foremost place."[18] Now that Dr. Harper was gone, there seemed to be no doubt that that foremost place would be thrust upon him.  These expectations were fulfilled by the Synod of that year, which unanimously and enthusiastically appointed him Principal of the College.  His friend Dr. Graham, who, as a corresponding member from the Synod of the Presbyterian Church of England, supported the appointment, gave voice to the universal feeling when he described him as “a man of thought and labour and love and God, who had one defect which endeared him to them all—­that he was the only man who did not know what a rare and noble man he was.”

[Footnote 18:  Life and Letters, p. 661.]

In the following year (1880) Principal Cairns delivered the Cunningham Lectures.  These lectures were given on a Free Church foundation, instituted in memory of the distinguished theologian whose name it bears; and now for the first time the lecturer was chosen from beyond the borders of the Free Church.  Dr. Cairns highly appreciated the compliment that was thus paid him, regarding it as a happy augury of the Union which he was sure was coming.  He had chosen as his subject “Unbelief in the eighteenth century as contrasted with its earlier and later history”; and, although it was one in which he was already at home, he had again worked over the familiar ground with characteristic diligence and thoroughness.  Thus, in preparing for one of the lectures, he read through twenty volumes of Voltaire, out of a set of fifty which had been put at his disposal by a friend.  The first lecture dealt with Unbelief in the first four centuries, which he contrasted in several respects with that of the eighteenth.  Then followed one on the Unbelief of the seventeenth century, then three on the Unbelief of the eighteenth century, in England, France, and Germany respectively; and, finally, one on the Unbelief of the nineteenth century, from whose representatives he selected three for special criticism as typical, viz.  Strauss, Renan, and John Stuart Mill.  These lectures, while not rising to the level of greatness, impress one with his mastery of the immense literature of the subject, and are characterised throughout by lucidity of arrangement and by sobriety and fairness of judgment.  They were very well received when they were delivered, and were favourably reviewed when they were published a year later.[19]

[Footnote 19:  In the following year (1882) he received the degree of LL.D. from Edinburgh University.]