Copy letter, from Sir John Herschel to James Thomas Walker, dated at Collingwood Copy of a letter. Received Indian survey chart. John Frederick William Herschel's son John wrote of higher-order ellipsoidal triangles with sides based on geodesical triangles. John Frederick William Herschel objects that these violate principle of diagonal bracing. Proposes different triangles and names sites in India as apexes. Received G. B. Airy's paper on reduction. 10 December 1864