You could not mistake this city for anything but the capital of a great republic. Everything about it smacks of government. Monuments and ministries abound. The architecture leans to the temples of ancient Greece and Rome: Washington Monument, Lincoln Memorial, Jefferson Memorial, the White House, Capitol Hill and Library of Congress. Along The Mall, a 3-Km long avenue that goes from the Capitol to Lincoln Memorial, one can visit the Smithsonian museums. They constitute the world's largest museum complex. Among them is the National Air and Space Museum. Worth visiting as well are the US Naval Observatory, FBI Headquarters, Ford's Theatre and Arlington National Cemetery across the Potomac River, in the state of Virginia. Also here are the headquarters of several professional societies.