Copy letter, from Sir John Herschel to [J. S. C.] Schweigger, dated at Slough Contemporary copy in sender's hand, signed by sender. [Responding to Johann Schweigger's letter dated 1822, i.e., #655,] acknowledges receipt of mail. Regrets delay but wished to examine the differences in electrical results between [J. W. A.] Pfaff and J 5 July 1822