HIERRO or FERRO, an island forming part of the Span ish archipelago of the Canary islands (q.v.). Pop. (1920) 8,344; area 107 sq.m. Hierro is the smallest island of the group. Its length is about 18 m., its greatest breadth about 15 m., and its circumference 5o m. It lies 92 m. W.S.W. of Teneriffe. Fresh water is scarce, but there is a sulphurous spring, with a tempera ture of F. The once celebrated and almost sacred Til tree, which was reputed to be always distilling water in great abun dance from its leaves, no longer exists. Valverde (pop. about 5,000) is the principal town. Geographers were formerly in the habit of measuring all longitudes from Ferro, the most westerly land known to them. The longitude assigned at first has, how ever, turned out to be erroneous; and the so-called "Longitude of Ferro" does not coincide with the actual longitude of the island.