CD’s letter is written on the cover of an offprint of Jules Carret’s paper on the causes of polar displacement (Carret 1878), read at a meeting of the Société Savoisienne d’histoire et d’archéologie on 23 May 1878, and sent to CD by the author; no other communication between CD and Carret has been found. In his paper, Carret referred to calculations of changes in the earth’s axis by George Howard Darwin and others. George was writing a series of papers modelling the possible physical composition of the earth and the effects of gravitational forces over time (G. H. Darwin 1878b, 1878d, and 1878e).