An Aqueduct Ventilator — Harper's Magazine (1860) Engraving of a ventilation tower along the Croton Aqueduct. These distinctive stone towers, spaced at intervals along the 41-mile route, allowed air circulation in the underground tunnel. They became landmarks along the aqueduct trail and several survive today. From Harper's, December 1860. Date: December 1860 Type: engraving Subjects: Croton Aqu…