Manhattan's amazing grid system makes navigating the city a breeze for locals and tourists alike—except when you find yourself in an area where the numbers suddenly vanish. Take Harlem. The streets running east to west are all numbered, but things get a bit trickier when it comes to the avenues. Over the years, three in the neighborhood's center—Sixth, Seventh and Eighth Avenues—have been renamed after prominent African Americans. Visitors may find the sudden lack of number