The Portland Observatory, built in 1807 at Portland, Maine, is the last surviving maritime signal tower in the United States. Using both a telescope and signal flags, two-way communication between ship and shore was possible several hours before an incoming vessel reached the docks.
The 86-foot (26 m) tall observatory (7 stories) is octagonal and lighthouse-shaped, with a fieldstone base, and stands 222 feet (68 m) above sea level. The observatory's 'lantern' (cupola) included a P & J Dolland Achromatic Refracting Telescope, which could identify ships 30 miles (48 km) to sea. That telescope disappeared from the observatory in 1939.
Greater Portland Landmarks, a non-profit organization, maintains the building. Guided tours with discussion about the history of the building and the neighborhood are provided by volunteer docents from Memorial Day to Columbus Day for a small fee.