The filters already noticed are those that act by the fluid descending through the media; but in some cases the reverse method is employed, and the liquid filters upward instead of downwards. These are called ascending filters, and are ofter preferable to those on the descending principle, because the suspends matters that require removal by filtration usually sink, and thus a portion escapes being forced into the pores of the filter. They are also more con venient when pressure is employed. The construction depends upon the same principle as the common filter, and merely requires that the feeding vessel should be higher than the upper surface of the filtering media. Oils are conveniently filtered in this way, because of their little specific gravity. By fixing a small filter on this principle into the head of a cask, and pouring in water through a funnel, whose neck reaches nearly to the bottom of the cask, the oil will float up and pass the filter, leaving the sediment behind. In cold weather hot water may be employed.

In some cases the upward and downward systems of filtration are united in the same apparatus, and this plan is advantageous where the space for operating is limited. For this purpose it is merely necessary to connect the bottom of an ascending filter with the top of a descending one, or the reverse; the proper pressure being in either case applied.