Soak 1 pound lentils in cold water to cover for 3 hours or overnight. In a large soup kettle saute 1/4 pound bacon diced, and 2 medium onions and 2 carrots, all thinly sliced, until the onions are golden. Add the lentils, rinsed and drained, 1 cup sliced celery, 2 bay leaves, 3 teaspoons salt, 1/4 each of dried thyme and black pepper, and 2 quarts water. Grate a potato, peeled, into the mixture and add a meaty ham bone. Cover the kettle and simmer the soup for 4 hours. Discard the bay leaves. Cut the meat from the ham bone and return the meat to the kettle. It is even better the second day.