Letter, from Henri Victor Regnault to Sir John Herschel, dated at Sèvres Autograph letter signed by sender. Talked to [L.-F.-C.?] Breguet for John Frederick William Herschel. Discusses work [on hygrometer?/metallic thermometer?] in detail, including diagrams. Regrets not having seen John Frederick William Herschel's daughters in Paris. Says he is recuperating from the 'catastrophe' and can still use his left eye. 1861