Adjoining the Maria Mitchell homestead is a memorial astronomical observatory and library, containing the collections of Miss Mitchell and of her brother, Professor Henry Mitchell (1830-1902), a distinguished hydrographer.
Another brother was Alexander Dalrymple (1737-1808), the first admiralty hydrographer, who distinguished himself in the East India Company's service and as a geographer.
The Hydrographic Department of the British Admiralty, established in 1795, undertakes the making of charts for the admiralty, and is under the charge of the hydrographer to the admiralty (see Chart).
