NEW YORK, NY - DECEMBER 09: Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge sign an interactive digital memorial message installation attached to the remnant of an original column from the South Tower during a visit the National September 11 Memorial & Museum on December 09, 2014 in New York City.  The couple, who are traveling without their son Prince George, are on a three-day US east coast visit. This is the Duke and Duchess' first official visit to New York City. (Photo by Jonathan Brady - Pool/Getty Images)