[Illustration: _Sonar works actively and passively. In active sonar, the transducer in the dome transmits sound pulses through the water. When the pulses reach an object (or the bottom), they are reflected and received by the transducer as echoes. Distance is determined by time elapsed between transmission and echo. In passive sonar, hydrophones pick up noises generated by underwater sources._] [Illustration: Installing a sonar dome] Part 2 Visiting Charlestown Navy Yard [Illustration: _U.S.S._ Cassin Young _at Pier 1_.] The Yard Today The yard offers the visitor a captivating glimpse into the activities that for nearly two centuries supported the United States Navy. Here on the home front, thousands of civilian workers and navy personnel built, repaired, and supplied warships from the majestic sailing vessels of the early 19th century to the powerful steel navy of the 20th century. In 1974, the year of its closing, 30 acres of the historic yard were set aside for the National Park Service as a living museum of the Navy’s activities here. The remaining 100 acres continue to be developed as part of the revitalization of Boston’s waterfront. Nineteenth-century buildings, docks, and piers reflect the yard’s 174-year history. The commandant’s 1805 hilltop mansion overlooked the activities below. Within view are Dry Dock 1, used by U.S.S. _Constitution_ as early as 1833 and as late as 1995, the 1842 Carpenter Shop, the 1852 Pitch House (Building 10) for caulking wooden vessels, the 1833 Officers’ Quarters, the 1813 Navy Stores (Building 5), and the 1811 Marine Barracks. Two venerable warships, the 1797 frigate U.S.S. _Constitution_ and the powerful 1943 destroyer, U.S.S. _Cassin Young_, float alongside the working piers, illustrating the changing United States Navy. Both the _Constitution_, an active duty Navy warship, and _Cassin Young_, maintained by National Park Service rangers and volunteers, offer free tours daily. [Illustration: Tours of the yard] A National Park Service exhibit, “Serving the Fleet,” and tours of the yard (_above_) provide visitors with opportunities to explore the history of the site. The USS Constitution Museum offers a rich collection of artifacts, paintings, and models relating to the history of “Old Ironsides.” The museum, located inside the Dry Dock 1 Pumphouse, is open daily to visitors. [Illustration: Key to map] Historic structure Historic building name Historic building letter or number Non-historic structure Freedom Trail (route subject to change) National Park Service boundary Public open space Parking [Illustration: CHARLESTOWN NAVY YARD]