Copy of letter, from Sir John Herschel to William Thomson, dated at Collingwood Copy of letter, copied as part of the correspondence project led by Colonel John Herschel FRS following the death of his father. Thanks William Thomson for paper on rigidity of the earth; agrees tidal action affects not only fluid motion of earth, but the solid as well. Mentions John Frederick William Herschel's paper of the previous year suggesting that tidal waves cause earthquakes. Raises questions about the idea of the earth as a magnet and about William Thomson's dynamical theory of heat. 25 August 1864