Describes his laborious journey to the summit of Mt. Etna; from his barometric readings, concludes that its altitude is 10,000 or 11,000 ft. Asks about the progress of Johann Pfaff's translation of William Herschel's papers; JH mentions that he wrote to Pfaff from Cattagione, Sicily. [Letter continued 20 July from Naples and 16 Aug. from Florence.]