Score corn ears lengthwise down the middle of each row of kernels with a sharp knife. Cut kernels from cobs. Discard cobs. Try out salt pork until crisp. Dip out browned pork and set aside. Saute onions in pork fat. In soup kettle, alternate layers of corn, onion, and potatoes. Sprinkle each layer with salt, pepper, and flour. Pour pork fat over all, and add hot water to cover well. Let simmer for 30 minutes, or until potatoes are done. Soak crackers in cold water for 3 minutes. Place on top of chowder and stir in milk. Heat through, sprinkle with crisp pork bits, and serve.