Filtering rainwater in a barrel is actually a really good idea if you store rain water. If you want filtered water right from a rain barrel this is for you. For gardening, rainwater is, naturally, best unfiltered. But for household use, the vintage book says the following instructions yield a cheap and easy way to make a filter just as good as a patent filter costing 10 times as much.
On the next page see how to use a 100-year-old method to filter rainwater through a barrel.