This section is from the book "Philadelphia Cook Book: A Manual Of Home Economies", by Sarah Tyson Heston Rorer. Also available from Amazon: Philadelphia Cook Book.
Pare four good-sized potatoes, and let them He in cold water for a half-hour. Put one quart of boiling water in a saucepan. Now grate the potatoes quickly and stir them into the boiling water; stir over the fire for five minutes, then take from the fire, add a half-cupful of sugar, and two tablespoonfuls of salt, turn into a stone jar or bowl, and let stand until lukewarm; then add one cupful of good yeast, cover and ferment three or four hours; stir it down every time it comes to the top of the vessel; then put it into a jar or large bottle, or something you can cover tightly, and stand it in a place where it will keep very cold, but not freeze. It will keep two weeks. Save one cupful of this yeast to start with next time.
This is the simplest and best yeast that can be made.
Pare and boil four good-sized potatoes; when done, mash them fine. Put a half-cupful of dried hops into one quart of water, and boil fifteen minutes. Put one pint of flour into a bowl, strain over it the boiling hop water, add the mashed potatoes, and beat until smooth; then add a half-cup of sugar, and two heaping tablespoonfuls of salt, and finish the same as Yeast No. 1.