There are two systems of filtering - by high pressure and low pressure. The first-named system is very objectionable, as in the filtration of any liquid it is essential that it neither be forced through the filtering material by pressure nor by suction; the very essence of all filtration is that the surface exposed should be as large as possible, so that slow percolation only through the filtering material takes place, as by no other means is it possible to approach purity by filtration. The filters described farther on are constructed on the low-pressure principle, in which the filtration takes place by gravitation. However, a few high-pressure filters we have also appended for the benefit of those who favor or employ them for certain industrial purposes. Effectiveness of Upward Filtration Questioned. - The Sanitary Bra says on this subject: For separating pure water from fine foreign particles and even sediment, as some kind of filters claim to do, there needs no revelation but common sense to show that the upward mode has no adaptation. If the filtering material be movable, like sand and other granulated substances, it is constantly stirred up by forcing the water through, and the impurities, being lighter and finer than the sand, are forced through it with the water. If they lodge temporarily, they cannot fall back against the current, but must be gradually forced upward, until all get through into the water; and so the very object fondly sought, namely, a constant dejection of the exfiltered impurities, is exactly reversed. If the filtering material be of a fixed character, such as paper, flannel, cotton batting, etc., the undesired result will come more slowly but no less surely, from the constant working upward of the finer particles through the fabric, unless it be renewed almost daily, as it needs to be when used in downward filtration. There can be no continuous filtration but by daily and thorough washing and repacking.