Peel the potatoes, and boil them with the loose hops tied in a thin muslin bag. Use enough water to cover them well. When the potatoes are tender strain off the water. Mash the potatoes, return them to the water in which they were boiled, and mix them well together. Add the flour, granulated sugar, and salt. Cook it for a few minutes, adding sufficient flour to make a thin batter. Set it aside until lukewarm; then add the yeast-cake. Mix it well and place in a stone jar. Let it stand for twelve hours in a warm place. Stir it three times during this period. Place a weight on the lid of the jar, and set it in a cool place.