Bob Jannoni and Lou Cook at the Burial Hill monument to the General Arnold casualties. (Emily Wilcox/Globe Correspondent) The brigantine General Arnold was heading south out of Boston, carrying supplies and reinforcements to struggling Revolutionary War troops in the Carolinas, when, on Dec. 25, 1778, a northeaster hit the New England coast. Hurricane-force winds and blinding snow forced Captain James Magee to seek shelter in Plymouth Harbor. It was a mistake. The ship ran aground on White Flat, a treacherous sandbar half a mile from shore and safety. There, as the storm raged on over the long Christmas weekend,...