Lightning Bread

Two cups of potatoes mashed fine

From 1911:

Make yeast as follows:
Two cups of potatoes mashed fine, one tablespoon of sugar, two tablespoons of salt, four cups of boiling water. When cool add starter; let raise about 4 or 5 hours.
Take two and a half or three sifters-ful of flour, mix up with the yeast, but don’t add any more water. Put in baking pans, let raise about 3 or 4 hours, then bake about one hour.