The first step is efficiently performed by nature, in the case of well and spring water, by subsidence and a long period of filtration through the earth; in the case of river water supplied by the various companies, it is carried out in immense settling ponds and filter beds of sand and gravel. This suffices for water destined for many purposes. The second and third steps are essential for all drinking-water, and are the aim of every domestic filter. The construction of water filters may now be discussed according to the nature of the filtering medium. Gravel and Sand.--The usual plan adopted by the water companies is to build a series of tunnels with bricks without mortar; these are covered with a layer of fine gravel 2 ft. thick, then a stratum of fine gravel and coarse sand, and lastly a layer of 2 ft. of fine sand. The water is first pumped into a reservoir, and after a time, for the subsidence of the coarser impurities, the water flows through the filter beds, which are slightly lower. For the benefit of those desirous of filtering water on a large scale with sand filtering beds, it may be stated that there should be 1½ yd. of filtering area for each 1000 gal. per day. For effective work, the descent of the water should not exceed 6 in. per hour. This simple means of arresting solid impurities and an appreciable portion of the matters in solution, may be applied on a domestic scale, in the following manner. Procure an ordinary wooden pail and bore a number of ¼ in. holes all over the bottom. Next prepare a fine muslin bag, a little larger than the bottom of the pail, and about 1 in. in height. The bag is now filled with clean, well-washed sand, and placed in the pail. Water is next poured in, and the edges of the bag are pressed against the sides of the pail. Such a filter was tested by mixing a dry sienna colour in a gallon of water, and, passing through, the colour was so fine as to be an impalpable powder, rendering the water a deep chocolate colour. On pouring this mixture on to the filter pad and collecting the water, it was found free of all colouring matter. This was a very satisfactory test for such a simple appliance, and the latter cannot be too strongly recommended in cases where a more complicated arrangement cannot be substituted. The finest and cleanest sand is desirable, such as that to be purchased at glass manufactories.