NEW YORK -- It is a city in constant motion. The sidewalks of Midtown are ringing with white noise at noon. Traffic cops brazenly wade into the streets and direct the crush of taxis, buses, bicycles and pedestrians. Vendors hawk Broadway posters, Central Park pictures and famously salty pretzels. Tourists tilt upward to snap pictures of Times Square, Rockefeller Center and Trump Tower. Seventy blocks south, past the iconic Empire State and Chrysler buildings, the skyline crescendos again around the yawning gap where the World Trade Center towers once marked the top of the financial world. Lower Manhattan's tall buildings are a rhapsody on 20th-century American ambition and bravado, changing tastes and fortunes. Such buildings are at once permanent and fleeting, for men in hard hats stir at their foundations. Skeletal steel frames will soon tell a new century's story.