A small filter for purifying water may be made in this way. Procure a large earthenware flower pot, well clean it, and fix a piece of glass tube in the hole at the bottom. Put in a layer of very small gravel (flint pebbles for preference); upon this place a layer of fine clean sand, and over this a layer of granular animal charcoal about 4 in. deep. Above all place another layer of clean sand. The filter may be supported on a large jug or other suitable receptacle, and the water run in at the top. Plenty of water should first be run through the filter so that the sand and charcoal may settle down properly and the filter become efficient; it will be working at its best when the water falls only in drops.