23 Name: Botanic Gardens Location: First Street and Maryland Avenue SW Building Stones: Indiana limestone; patio, Pennsylvania sandstone Remarks: The present conservatory was built when the Botanic Gardens was relocated on the Mall in 1933.

24 Name: National Air and Space Museum, Smithsonian Institution Location: Independence Avenue between 4th and 7th Streets, SW Building Stones: Tennessee marble Remarks: This museum, opened in 1976, is one of the world's most popular museums. The interior is designed to resemble an airplane hangar, with huge open spaces and high ceilings to display aircraft.

25 Name: Smithsonian Institution Main Building (the Castle) Location: 1000 Jefferson Drive SW Building Stones: Seneca, Md., sandstone Remarks: The Smithsonian Institution was founded in 1829 with a bequest of James Smithson, a British mineralogist and chemist, who had never been to this country. This building, now the administrative headquarters, is the oldest of the Smithsonian buildings on the Mall. It was designed by James Renwick in the Gothic Revival or Romantic style and was completed in 1855.