In this particular domain invention has been comparatively slow. There are novel pumps, it is true, for handling the water, such as the Humphrey Gas Pump, which the Metropolitan Water Board (London) have installed for filling their great reservoirs at Chingford. In these an explosion of gas is the motive force. Water flows by gravitation into a huge iron pipe closed at the top but open at the bottom. It is so arranged that a quantity of gas shall be entrapped in the upper end, which, being exploded by an electric spark, drives the mass of water out. Some of it, together with a quantity of fresh water, presently comes surging back, entrapping a fresh supply of gas and causing a new explosion; and so it goes on over and over again. The particular pumps at the waterworks referred to discharge about fourteen tons of water at each explosion, of which there are nine every minute. The special effect of these machines, however, is not to improve the public health so much as to relieve the public pocket, for their chief feature is that they work more economically than any other kind of pump. The filters, by which the water is purified, are simply layers of sand, much the same as have been in use for many years, although in some cases chemistry is brought in and the work of the filters aided by the action of precipitants. These are substances which combine in some way with the impurities in the water, and carry them to the bottom of the tank or reservoir, while the pure water remains to be drawn off from the top. This is also the most usual method by which water is softened. Hardness in water is due to the presence of certain salts which are dissolved out of the ground as the water percolates through it, and which are absent from rain-water. To get rid of these the hard water has chemicals mixed with it in a tank, from which it flows slowly through another tank. The effect of the added chemicals is to convert the soluble salts in the water into insoluble particles, which then tend to fall down to the bottom of the containing vessel. The slow passage through the second tank is intended to give the particles time to settle. [Illustration: SECTIONAL VIEW OF HYDRAULIC BUFFER AND RUNNING-OUT PRESSES OF A 60-POUNDER GUN]