This essay formed the basis of lectures George Howard Darwin delivered before the Royal Institution of Great Britain on January 25, 1889. It was a popular account of a paper he read before the Royal Society the previous November ("On the Mechanical Conditions of a Swarm of Meteorites, and on Theories of Cosmogony," Proc. 45:3-16).

This is a small, miscellaneous collection of letters written to Alexander Brownlee, C. Scott, F. W. Surman, and Otto Zacharias. There are references to lunar tidal waves and mentions of Charles and Erasmus Darwin.