The letter is dated ‘1859’, but it is not certain that this is in CD’s hand. The year is established by the reference to Main ed. 1859 (see n. 2, below). CD returned to Down from Moor Park hydropathic establishment on 26 July 1859 (‘Journal’; Appendix II).

CD refers to Main ed. 1859. CD had contributed the chapter on geology to John Frederick William Herschel’s earlier edition of the same work, the Manual of scientific enquiry (see Correspondence vol. 4). Herschel’s edition had been published in 1849, followed by a second edition in 1851. The supervision of a third edition was transferred to Main, who was chief assistant at the Royal Observatory and president of the Royal Astronomical Society in 1859. CD’s copy of his chapter (apparently unrevised) is in DAR 132 (7). It is separately paginated and has the title ‘Manual of geology … Extracted from the Admiralty Manual of Scientific Enquiry, third edition, 1859’. The book was published early in September (Publishers’ Circular, 15 September 1859, p. 461).