Gary Knell, who has headed NPR for less than two years, is departing to become president of the National Geographic Society...That leaves the nation's flagship public radio syndicate without a chief executive for the second time in two years. Knell's predecessor, Vivian Schiller, was forced to resign in the spring of 2011 after a series of damning allegations about NPR's liberal bias, which she had seemingly fueled with injudicious statements and decisions, including the firing of Juan Williams.