The Eiffel Tower in Paris was once the tallest structure in the world until the Empire State Building was constructed in New York City about four decades later. It is of course considered to be one of the most romantic places on Earth and many weak-kneed men have often proposed to their future wives under this colossal structure. The original designer, also named Eiffel, was no stranger to the United States as he also designed the wrought-iron pylon inside the Statue of Liberty. Eiffel was the leading European authority on the aerodynamics of high frames and wrote a famous book entitled "The Resistance of the Air" in 1913. The curve of the base pylons in the Eiffel Tower were calculated exactly so as to withstand the bending and shearing forces of the wind. These forces were progressively transformed into forces of compression so that the structure of the tower could withstand them more effectively. Eiffel's genius helped to conquer even in the strongest winds and it is commonly known that the Eiffel Tower never sways more than 4-1/2 inches. Ever since the construction of the Eiffel Tower, other buildings have been designed in much the same way. The World Trade Center, for example, utilized many of the same ideas as the Eiffel tower. Today, the "Tour Eiffel", as it is known to Parisians, is internationally recognized as one of the great romantic symbols of Paris itself.