The Smithsonian Institution, an Establishment for the “increase & diffusion of knowledge” was founded from a bequest to the United States by the British scientist James Smithson (1765-1829). Mr. Smithson named his nephew as beneficiary in his last will and testament and stipulated that if the nephew should die without heirs the estate should go to the United States of America.  James Smithson died in 1829, his nephew; Henry James Hungerford died without heirs in 1835. The President of the United States at that time, Mr. Andrew Jackson informed Congress of this bequest, which amounted to 100,000 gold sovereigns, when recoined is US currency; over 500,000$ (today with inflation, approximately 10,100,000.00$).  http://siarchives.si.edu/history/exhibits/historic/history.htm#sib

As to why Smithson gave all his estate remains unknown, he never traveled to the United States and no one seemed to have correspondence with him. After a debate as to whether the federal government had the authority to accept the gift, Congress accepted the legacy bequeathed to the nation and pledged the faith of the United States to the charitable trust July 1, 1836.  Eight years later, Congress passed an act establishing the Smithsonian Institution, a public/private partnership, and the act was signed into law on August 10, 1846 by James Polk.

The architecture of the Smithsonian Institution Building has resulted in its informal name “The Castle”, it was completed in 1855 by architect James Renwick, Jr.  Many of the Institution’s other buildings are historical and architectural landmarks.

The Museums  http://www.si.edu/museums/

• African Art Museum, Washington DC
• Air and Space Museum, Washington DC
• Air and Space Museum, Udzar-Hazy Center, Chantilly VA
• American Art Museum and its Renwick Gallery, Washington DC
• American History Museum, Washington DC
• American Indian Museum, Washington DC and the Heye Center, NYC
• Anacostia Community Museum, Washington DC
• Cooper-Hewitt, National Design Museum, NYC
• Freer and Sackler Galleries (Asian art), Washington DC
• Hirshhorn Museum and Sculpture Garden, Washington DC
• National Zoo, Washington DC
• Natural History Museum, Washington DC
• Portrait Gallery, Washington DC
• Postal Museum, Washington DC
• Smithsonian Institution Building, the Castle, Washington DC
• African American History and Culture Museum (Opening in 2015)
• Arts and Industries Building, Washington DC (Closed in preparation for renovation)

Research Centers http://www.si.edu/research/

• Archives of American Art
• Astrophysical Observatory (SAO)
• Conservation Biology Institute (SCBI)
• Environmental Research Center (SERC)
• Marine Station at Fort Pierce
• Museum Conservation Institute (MCI)
• Smithsonian Institution Archives
• Smithsonian Institution Libraries
• Tropical Research Institute (STRI)

Besides the Museums and Research Centers, the Smithsonian shares artifacts and other programs with 160 museums and educational organizations. https://affiliations.si.edu/MainPage.asp 

The Smithsonian Institution, the world’s largest museum complex and research organization!